Patent Publication Number: US-2022227837-A1

Title: Il-2 compositions and methods of use thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/852,768, filed May 24, 2019, which is incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING THE SEQUENCE LISTING 
     The Sequence Listing associated with this application is provided in text format in lieu of a paper copy, and is hereby incorporated by reference into the specification. The name of the text file containing the Sequence Listing is PRVA_002_01WO_ST25.txt. The text file is about 464 KB, was created on May 18, 2020, and is being submitted electronically via EFS-Web. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an activatable proprotein comprising at least two separate polypeptide chains, the first comprising IL-2 fused to a masking moiety and the second comprising an IL-2 binding protein fused to a masking moiety, and related pharmaceutical compositions and methods of use thereof. 
     Description of the Related Art 
     Interleukin-2 (IL-2) immunotherapy has proven utility in the treatment of cancers such as malignant melanoma and renal cell cancer, and chronic infections such as HIV infections. 
     However, there are certain problems associated with most IL-2 therapies. For example, current forms of IL-2 therapy have a short half-life in circulation and predominantly expand immunosuppressive regulatory T cells, or T regs  (see, for example, Arenas-Ramirez et al., Trends in Immunology. 36: 763-777, 2015). Also, the effects of IL-2 therapy are predominantly systemic, rather than being localized to target tissues, resulting in many severe side effects such as breathing problems, nausea, low blood pressure, loss of appetite, confusion, serious infections, seizures, allergic reactions, heart problems, renal failure, and vascular leak syndrome. Nonetheless, IL-2 therapy can be effective, and there is an unmet need in the art to overcome these and other drawbacks. 
     Embodiments of the present disclosure address these problems and more by providing an activatable proprotein comprising IL-2 that can be activated within a disease tissue, for example, a cancer tissue or tumor. 
     BRIEF SUMMARY 
     Embodiments of the present disclosure include an activatable proprotein, comprising a first polypeptide and a second polypeptide, wherein the first polypeptide comprises a first masking moiety and an IL-2 protein, wherein the first masking moiety comprises a first binding moiety and a first linker that is fused to the IL-2 protein, wherein the second polypeptide comprises a second masking moiety and an IL-2 binding protein, wherein the second masking moiety comprises a second binding moiety and a second linker that is fused to the IL-2 binding protein, wherein the first and second masking moieties bind together via their respective first and second binding moieties, optionally as a dimer, and thereby mask a binding site of the IL-2 protein that binds to an IL-2Rβ/γc chain present on the surface of an immune cell in vitro or in vivo, and wherein at least one of the first linker or the second linker is a cleavable linker. 
     In some embodiments, the IL-2 protein comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to a sequence selected from Table S1 or to amino acids 21-153 of SEQ ID NO: 1 (full-length wild-type human IL-2), optionally comprising a C145X (X is any amino acid) or a C145S substitution as defined by SEQ ID NO: 1. In some embodiments, the IL-2 protein comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 2 (mature human IL-2 with C125S substitution), optionally wherein the IL-2 protein retains the S125 residue as defined by SEQ ID NO: 2. In some embodiments, the IL-2 protein comprises one or more substitutions selected from K35C, R38C, T41C, F42C, E61C, and V69C as defined by SEQ ID NO: 2. In some embodiments, the IL-2 protein forms a disulfide bond with the IL-2 binding protein, optionally via one or more of the cysteines in claim  4  and one or more cysteines in the IL-2 binding protein. In some embodiments, the IL-2 protein comprises one or more amino acid substitutions at position 69, 74, or 128 as defined by SEQ ID NO: 2, optionally wherein the one or more amino acid substitutions are selected from V69A, Q74P, and I128T as defined by SEQ ID NO: 2. In some embodiments, the IL-2 protein comprises one or more amino acid substitutions at position R38, F42, Y45, E62, E68, and/or L72 as defined by SEQ ID NO: 2, optionally wherein the one or more amino acid substitutions are selected from R38A and R38K; F42A, F42G, F42S, F42T, F42Q, F42E, F42N, F42D, F42R, F42K, and F42I; Y45A, Y45G, Y45S, Y45T, Y45Q, Y45E, Y45N, Y45D, Y45R, and Y45K; E62A and E62L; E68A and E68V; and L72A, L72G, L72S, L72T, L72Q, L72E, L72N, L72D, L72R, and L72K, including combinations thereof, optionally a combination selected from F42A, Y45A, and L72G; R38K, F42Q, Y45N, E62L, and E68V; R38K, F42Q, Y45E, and E68V; R38A, F42I, Y45N, E62L, and E68V; R38K, F42K, Y45R, E62L, and E68V; R38K, F42I, Y45E, and E68V; and R38A, F42A, Y45A, and E62A. In some embodiments, the IL-2 protein comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 3 (mature human IL-2 “D10” variant), optionally wherein the IL-2 protein retains any one or more of the Q74H, L80F, R81D, L85V, I86V, and/or I92F substitutions as defined by SEQ ID NO: 3. 
     In some embodiments, the IL-2 binding protein is an IL-2Rα protein, or an antibody or antigen binding fragment thereof that specifically binds to the IL-2 protein, optionally a bi-specific antibody or antigen binding fragment thereof. In some embodiments, IL-2Rα protein comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% to a sequence selected from Table S2 or to amino acids 22-187 of SEQ ID NO: 4 (full-length wild-type human IL-2Rα). In some embodiments, the IL-2Rα protein comprises one or more cysteine substitutions selected from D4C, D6C, N27C, K38C, S39C, L42C, Y43C, I118C, and H120C as defined by SEQ ID NO: 6 (human IL-2Rα Sushi 1 to Sushi 2 domain). In some embodiments, the IL-2Rα protein forms a disulfide bond with the IL-2 protein, optionally via one or more of the cysteines in claim  11  and one or more cysteines in the IL-2 protein, optionally one or more of the cysteines in claim  4 , optionally one or more cysteine pairs selected from IL2-K35C and IL2Rα-D4C, IL2-R38C and IL2Rα-D6C, IL2-R38C and IL2Rα-H120C, IL2-T41C-IL2Rα-I118C, IL2-F42C and IL2Rα-N27C, IL2-E61C and IL2Rα-K38C, IL2-E61C and IL2Rα-S39C, and IL2-V69C and IL2Rα-L42C, wherein disulfide binding between the IL-2 protein and the IL-2Rα protein masks the binding site of the IL-2 protein that preferentially binds to the IL-2Rαβγ chain expressed on T regs . In some embodiments, the IL-2Rα protein comprises an alanine substitution at position 49 and/or 68 as defined by SEQ ID NO: 6. 
     In some embodiments, the antibody or antigen binding fragment thereof that specifically binds to the IL-2 protein is selected from one or more of a whole antibody, Fab, Fab′, F(ab′)2, monospecific Fab2, bispecific Fab2, FV, single chain Fv (scFv), scFV-Fc, nanobody, diabody, camelid, and a minibody, optionally wherein the antibody is NARA1 or an antigen binding fragment thereof. 
     In some embodiments, the first masking moiety and/or the second masking moiety does/do not bind to the IL-2 protein or the IL-2 binding protein. In some embodiments, first masking moiety and/or the second masking moiety bind to the IL-2 protein. 
     In some embodiments, the first and second binding moieties bind together, optionally dimerize, via one at least one non-covalent bond. In some embodiments, the first and second binding moieties bind together, optionally dimerize, via one at least one covalent bond. In some embodiments, the at least one covalent bond comprises at least one disulfide bond. In some embodiments, the first binding moiety and the second binding moiety are selected from Table M1. In some embodiments, the first binding moiety and/or the second binding moiety comprise an antigen binding domain of an immunoglobulin, including antigen binding fragments and variants thereof. In some embodiments, the first binding moiety and/or the second binding moiety comprise a CH1, CH2, CH3, CH1CH3, CH2CH3, CH1CH2CH3, and/or CL domain of an immunoglobulin, including fragments and variants thereof. In some embodiments, the first binding moiety and/or the second binding moiety comprise, in an N- to C-terminal orientation: (1) an antigen binding domain of an immunoglobulin, including antigen binding fragments and variants thereof; and (2) a CH1, CH2, CH3, CH1CH3, CH2CH3, CH1CH2CH3, and/or CL domain of an immunoglobulin, including fragments and variants thereof. In some embodiments, the antigen binding domain comprises a VH or VL domain of an immunoglobulin, including antigen binding fragments and variants thereof. In some embodiments, the first binding moiety and/or the second binding moiety does/do not bind to an antigen. In some embodiments, the first binding moiety comprises a VL and a CL domain of an immunoglobulin, and wherein the second binding moiety comprises a VH and a CH1 domain of an immunoglobulin. In some embodiments, the first binding moiety comprises a VH and a CH1 domain of an immunoglobulin, and wherein the second binding moiety comprises a VL and a CL domain of an immunoglobulin. In some embodiments, the immunoglobulin is from an immunoglobulin class selected from IgG1, IgG2, IgG3, IgG4, IgA, IgD, IgE, and IgM. In some embodiments, the first binding moiety and the second binding moiety each comprise a leucine zipper peptide. In some embodiments, the first and second masking moieties bind together via their respective first and second binding moieties as a heterodimer. In some embodiments, the first and second masking moieties bind together via their respective first and second binding moieties as a homodimer, optionally wherein each of the first and second binding moieties comprise a CH2 domain and a CH3 domain. 
     In some embodiments, the cleavable linker comprises a protease cleavage site, optionally wherein the cleavable linker is selected from Table S4. In some embodiments, the protease cleavage site is cleavable by a protease selected from one or more of a metalloprotease, a serine protease, a cysteine protease, and an aspartic acid protease. In some embodiments, protease cleavage site is cleavable by a protease selected from one or more of MMP1, MMP2, MMP3, MMP4, MMP5, MMP6, MMP7, MMP8, MMP9, MMP10, MMP11, MMP12, MMP13, MMP14, TEV protease, matriptase, uPA, FAP, Legumain, PSA, Kallikrein, Cathepsin A, and Cathepsin B. In some embodiments, the first linker and/or the second linker are about 1-50 1-40, 1-30, 1-20, 1-10, 1-5, 1-4, 1-3 amino acids in length, or about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50 amino acids in length. In some embodiments, the first linker is a cleavable linker, and wherein the second linker is a non-cleavable linker. In some embodiments, cleavage, optionally protease cleavage, of the first linker releases the first masking moiety from the activatable proprotein, and thereby exposes the binding site of the IL-2 protein that binds to the IL-2Rβ/γc chain present on the surface of the immune cell in vitro or in vivo. In some embodiments, the first linker is a non-cleavable linker, and wherein the second linker is a cleavable linker. In some embodiments, cleavage, optionally protease cleavage, of the second linker releases the second masking moiety from the activatable proprotein, and thereby exposes the binding site of the IL-2 protein that binds to the IL-2Rβ/γc chain present on the surface of the immune cell in vitro or in vivo. In some embodiments, the immune cell is selected from one or more of a T cell, a B cell, a natural killer cell, a monocyte, and a macrophage. 
     In some embodiments, the first polypeptide further comprises a protein domain A at the free terminus of the first masking moiety and/or a protein domain B at the free terminus of the IL-2 protein. In some embodiments, the second polypeptide further comprises a protein domain C at the free terminus of the second masking moiety and/or a protein domain D at the free terminus of the IL-2 binding protein. In some embodiments, the protein domains A-D are the same or different, and are optionally selected from one or more of cell receptor targeting moieties optionally bi-specific targeting moieties, antigen binding domains optionally bi-specific antigen binding domains, cell membrane receptor extracellular domains (ECDs), Fc domains, human serum albumin (HSA), Fc binding domains, HSA binding domains, cytokines, chemokines, and soluble protein ligands. 
     In some embodiments, the first polypeptide comprises, in an N- to C-terminal orientation, the first masking moiety and the IL-2 protein. In some embodiments, the first polypeptide comprises, in an N- to C-terminal orientation, the IL-2 protein and the first masking moiety. In some embodiments, the second polypeptide comprises, in an N- to C-terminal orientation, the second masking moiety and the IL-2 binding protein. In some embodiments, the second polypeptide comprises, in an N- to C-terminal orientation, the IL-2 binding protein and the second masking moiety. 
     In some embodiments, the first polypeptide comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 9, 13, 17, 21, 25, 29, 33, 37, 41, 45, 49, 53, 57, 61, 65, 69, 73, 77, 81, 233, 235, 237, 239, 241, 243, or 245, and wherein the second polypeptide, respectively, comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 234, 236, 238, 240, 242, 244, or 246. 
     In particular embodiments, for example, an activatable proprotein comprising additional domains such as immunoglobulin antigen binding domains, such as light chain variable regions and/or heavy chain variable regions (see, for example,  FIGS. 9A-9B ), the first polypeptide comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 247, 250, 253, 256, 259, or 262, the second polypeptide respectively comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 248, 251, 254, 257, or 263, and a third and/or fourth polypeptide respectively comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 249, 252, 255, 258, 261, or 264. In particular embodiments, the additional domains comprise light chain variable regions and/or heavy chain variable regions that specifically bind to an antigen of interest, for example, fibroblast activation protein (FAP). 
     Also included are recombinant nucleic acid molecules encoding an activatable proprotein of described herein, for example, wherein the first polypeptide and the second polypeptide are encoded on the same or on separate recombinant nucleic acid molecules. 
     Also included are vectors comprising the recombinant nucleic acid molecule described herein, for instance, wherein the first polypeptide and the second polypeptide are encoded on the same or on separate recombinant nucleic acid molecules or vectors. Also included are host cells comprising the recombinant nucleic acid molecules or vector described herein. 
     Particular embodiments include methods of producing an activatable proprotein, comprising culturing a host cell described herein under culture conditions suitable for the expression of the activatable proprotein, and isolating the activatable proprotein from the culture. 
     Certain embodiments include pharmaceutical compositions, comprising at least one activatable proprotein described herein, and a pharmaceutically acceptable carrier. 
     Some embodiments include methods of treating disease in a subject, and/or methods of enhancing an immune response in a subject, comprising administering to the subject a therapeutically effective amount of a pharmaceutical composition or at least one activatable proprotein described herein. In some embodiments, the disease is selected from one or more of a cancer, a viral infection, and an immune disorder. In some embodiments, the cancer is a primary cancer or a metastatic cancer, and is selected from one or more of melanoma (optionally metastatic melanoma), kidney cancer (optionally renal cell carcinoma), pancreatic cancer, bone cancer, prostate cancer, small cell lung cancer, non-small cell lung cancer (NSCLC), mesothelioma, leukemia (optionally lymphocytic leukemia, chronic myelogenous leukemia, acute myeloid leukemia, or relapsed acute myeloid leukemia), multiple myeloma, lymphoma, hepatoma (hepatocellular carcinoma), sarcoma, B-cell malignancy, breast cancer, ovarian cancer, colorectal cancer, glioma, glioblastoma multiforme, meningioma, pituitary adenoma, vestibular schwannoma, primary CNS lymphoma, primitive neuroectodermal tumor (medulloblastoma), bladder cancer, uterine cancer, esophageal cancer, brain cancer, head and neck cancers, cervical cancer, testicular cancer, thyroid cancer, and stomach cancer. 
     In some embodiments, following administration, the activatable proprotein is activated through protease cleavage in a cell or tissue, optionally a cancer cell or cancer tissue, which releases the masking moiety comprising the protease cleavage site, exposes the binding site of the IL-2 protein that binds to the IL-2Rβ/γc chain present on the surface of the immune cell in vitro or in vivo, and thereby generates an activated protein. In some embodiments, the activated protein binds via the IL-2 protein to the IL-2Rβ/γc chain present on the surface of an immune cell in vitro or in vivo. In some embodiments, the immune cell is selected from one or more of a T cell, a B cell, a natural killer cell, a monocyte, and a macrophage. In some embodiments, binding between the IL-2 protein and the IL-2 binding protein (optionally disulfide binding between the IL-2 protein and the IL-2Rα protein) in the activated protein masks the binding site of the IL-2 protein that binds to the IL-2Rα/β/γc chain expressed on T regs , and thereby interferes with binding of the activated protein to T regs . 
     In some embodiments, administration and activation of the activatable proprotein increases an immune response in the subject by about or at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000% or more, relative to a control, optionally wherein the immune response is an anti-cancer or anti-viral immune response. In some embodiments, administration and activation of the activatable proprotein increases cell-killing in the subject by about or at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000% or more, relative to a control, optionally wherein the cell-killing is cancer cell-killing or virally-infected cell-killing. 
     In some embodiments, the viral infection is selected from one or more of human immunodeficiency virus (HIV), Hepatitis A, Hepatitis B, Hepatitis C, Hepatitis E, Caliciviruses associated diarrhoea, Rotavirus diarrhoea,  Haemophilus influenzae  B pneumonia and invasive disease, influenza, measles, mumps, rubella, Parainfluenza associated pneumonia, Respiratory syncytial virus (RSV) pneumonia, Severe Acute Respiratory Syndrome (SARS), Human papillomavirus, Herpes simplex type 2 genital ulcers, Dengue Fever, Japanese encephalitis, Tick-borne encephalitis, West-Nile virus associated disease, Yellow Fever, Epstein-Barr virus, Lassa fever, Crimean-Congo haemorrhagic fever, Ebola haemorrhagic fever, Marburg haemorrhagic fever, Rabies, Rift Valley fever, Smallpox, upper and lower respiratory infections, and poliomyelitis, optionally wherein the subject is HIV-positive. 
     In some embodiments, the immune disorder is selected from one or more of type 1 diabetes, vasculitis, and an immunodeficiency. 
     In some embodiments, the pharmaceutical composition is administered to the subject by parenteral administration. In some embodiments, the parenteral administration is intravenous administration. 
     Certain embodiments include the use of a pharmaceutical composition described herein in the preparation of a medicament for treating a disease in a subject, and/or for enhancing an immune response in a subject. Some embodiments include a pharmaceutical composition described herein for use in treating a disease in a subject, and/or for enhancing an immune response in a subject. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows the protein topology of human interleukin 2 (IL-2) and human interleukin 2 receptor alpha chain (IL-2Rα), and  FIG. 1B  shows the quaternary structure of IL-2 in complex with its receptors IL-2Rα (CD25), IL-2Rβ (CD122) and the common gamma chain (CD132) (PDB: 2ERJ). 
         FIG. 2A  illustrates a fusion of the N-terminus of IL-2Rα to the C-terminus of a masking moiety, and a fusion of the N-terminus of IL-2 to the C-terminus of a masking moiety. The interaction sites on IL-2 in the fusion protein with its signaling IL-2Rβ/γc receptors are indicated at IL-2Rβ/γc interaction interface.  FIG. 2B  illustrates a schematic diagram of fusion structure described in  FIG. 2A . IL-2 in this fusion protein is not able to bind to and signal through IL-2Rβ/γc receptors because of the steric hindrance resulted from the masking moiety.  FIG. 2C  illustrates a diagram of the protein sequence motifs and configurations for proteins described in  FIGS. 2A and 2B .  FIG. 2D  illustrates a diagram of a hetero-dimeric fusion structure with a protein domain at the C-terminus of IL-2 on the first polypeptide, a protein domain at the N-terminus of the first masking moiety, a protein domain at the C-terminus of the IL-2 binding protein (e.g., IL-2Rα) on the second polypeptide, and a protein domain at the N-terminus of the second masking moiety.  FIG. 2E  shows a disulfide bond between IL-2 (or variant) and IL-2Rα (or variant) to form tighter IL-2/IL-2Rα complex.  FIG. 2F  illustrates fusion of the N-terminus of IL-2Rα (or variant) to the C-terminus of a masking moiety and fusion of the N-terminus of IL-2 (or variant) to the C-terminus of a masking moiety. 
         FIG. 2G  shows a schematic diagram of activation of an activatable proprotein (or prodrug) with masking moieties through protease cleavage of the substrate linker sequences in the fusion polypeptide. Example of protease substrate sequence for linker 1 is shown. Digestion of the protease substrate sequence in the linker releases the steric hindrance imposed by the masking moiety, and thereby allows the IL-2 in the fusion to bind to and signal through IL2Rβ/γc receptors. 
         FIG. 2H  shows a schematic diagram of activation of an activatable proprotein (or prodrug) with masking moieties and di-sulfide bond between IL-2 (or variant) and IL-2Rα (or variant) through protease cleavage of the substrate linker sequences in the fusion polypeptide. Example of protease substrate sequence for linker 1 is shown. Digestion of the protease substrate sequence in the linker releases the steric hindrance imposed by the masking moiety, and thereby allows the IL-2 in the fusion to bind to and signal through IL2Rβ/γc receptors. 
         FIG. 2I  shows a schematic diagram of activation of an activatable proprotein (or prodrug) with masking moieties through protease cleavage of the substrate linker sequences in the fusion polypeptide. Example of protease substrate sequence for linker 2 is shown. Digestion of the protease substrate sequence in the linker releases the steric hindrance imposed by the masking moiety, and thereby allows the IL-2 in the fusion to bind to and signal through IL2Rβ/γc receptors. 
         FIG. 2J  shows a schematic diagram of activation of an activatable proprotein (or prodrug) with masking moieties and di-sulfide bond between IL-2 (or variant) and IL-2Rα (or variant) through protease cleavage of the substrate linker sequences in the fusion polypeptide. Example of protease substrate sequence for linker 2 is shown. Digestion of the protease substrate sequence in the linker releases the steric hindrance imposed by the masking moiety, and thereby allows the IL-2 in the fusion to bind to and signal through IL2Rβ/γc receptors. 
         FIGS. 3A-3B  show examples of VH-CH1 and VL-CL as masking moieties in an exemplary activatable proprotein. 3A shows fusion of IL-2Rα on the C-terminus of VH-CH1 and fusion of IL-2 on the C-terminus of VL-CL. 3B shows fusion of IL-2Rα on the C-terminus of VL-CL and fusion of IL-2 on the C-terminus of VH-CH1.  FIG. 3C  shows a schematic diagram of activation of the activatable proprotein depicted in  FIG. 3A , and  FIG. 3D  shows a schematic diagram of activation of the activatable proprotein fusion protein depicted in  FIG. 3B . 
         FIGS. 4A-4C  show SDS-PAGE results of purified and cleaved proteins.  FIG. 4A  shows non-reducing SDS-PAGE results and  FIG. 4B  shows reducing SDS-PAGE results.  FIG. 4C  illustrates protease (TEV) cleavage of IL-2 fusion proteins. “M” represents protein standard maker. “1” represents proteins before TEV cleavage and “2” represents proteins after TEV cleavage. 
         FIGS. 5A-5E  illustrate representative HPLC analysis results of the purified proteins. 
         FIGS. 6A-6R  illustrate the activity of IL-2 fusion proteins on M-07e proliferation determined by a colorimetric assay (Cell Counting Kit-8 (CCK-8)). 
         FIGS. 7A-7B  illustrate SDS-PAGE results of purified proteins.  FIG. 7C  illustrates protease (TEV) cleavage of IL-2 fusion proteins. “M” on the figures represents protein standard marker. “1” on the figures represents proteins before TEV cleavage and “2” represents proteins after TEV cleavage.  FIG. 7D  illustrates HPLC analysis results of purified proteins. 
         FIG. 8A  illustrates activity of cleaved and uncleaved P16121613 on M-07e proliferation determined by a colorimetric assay (Cell Counting Kit-8 (CCK-8)).  FIG. 8B  shows activity comparison of cleaved and uncleaved P16121613 with P13591366 on M-07e proliferation determined by a colorimetric assay (Cell Counting Kit-8(CCK-8)). 
         FIGS. 9A-9B  illustrate various structures of the activatable proproteins described herein, including proproteins comprising multiple chains (see also  FIG. 2D ). 
         FIGS. 10A-10B  show SDS-PAGE results of purified proteins.  FIG. 10A  shows non-reducing SDS-PAGE results and  FIG. 10B  shows reducing SDS-PAGE results. “M” represents protein standard maker. 
         FIGS. 10C-10D  show protease cleavage of IL-2 fusion proteins. “M” represents protein standard maker. In  FIG. 10C , “1” represents proteins before TEV cleavage and “2” represents proteins after TEV cleavage. In  FIG. 10D , “1” represents proteins before protease cleavage, “2” represents proteins after uPA cleavage, “3” represents proteins after MMP-2 cleavage, “4” represents proteins after matriptase cleavage and “5” represents proteins after legumain cleavage. 
         FIGS. 11A-11P  illustrate representative HPLC analysis results of the purified proteins. 
         FIGS. 12A-12P  illustrate the activity of IL-2 fusion proteins on M-07e proliferation determined by a colorimetric assay (Cell Counting Kit-8 (CCK-8)). 
     
    
    
     DETAILED DESCRIPTION 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the disclosure belongs. Although any methods, materials, compositions, reagents, cells, similar or equivalent similar or equivalent to those described herein can be used in the practice or testing of the subject matter of the present disclosure, preferred methods and materials are described. All publications and references, including but not limited to patents and patent applications, cited in this specification are herein incorporated by reference in their entirety as if each individual publication or reference were specifically and individually indicated to be incorporated by reference herein as being fully set forth. Any patent application to which this application claims priority is also incorporated by reference herein in its entirety in the manner described above for publications and references. 
     Standard techniques may be used for recombinant DNA, oligonucleotide synthesis, and tissue culture and transformation (e.g., electroporation, lipofection). Enzymatic reactions and purification techniques may be performed according to manufacturer&#39;s specifications or as commonly accomplished in the art or as described herein. These and related techniques and procedures may be generally performed according to conventional methods well known in the art and as described in various general and more specific references that are cited and discussed throughout the present specification. Unless specific definitions are provided, the nomenclature utilized in connection with, and the laboratory procedures and techniques of, molecular biology, analytical chemistry, synthetic organic chemistry, and medicinal and pharmaceutical chemistry described herein are those well-known and commonly used in the art. Standard techniques may be used for recombinant technology, molecular biological, microbiological, chemical syntheses, chemical analyses, pharmaceutical preparation, formulation, and delivery, and treatment of patients. 
     For the purposes of the present disclosure, the following terms are defined below. 
     The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” includes “one element”, “one or more elements” and/or “at least one element”. 
     By “about” is meant a quantity, level, value, number, frequency, percentage, dimension, size, amount, weight or length that varies by as much as 30, 25, 20, 15, 10, 9, 8, 7, 6, 5, 4, 3, 2 or 1% to a reference quantity, level, value, number, frequency, percentage, dimension, size, amount, weight or length. 
     The terms “activatable proprotein,” “activatable prodrug”, “prodrug” or “proprotein” are used interchangeably herein and refer to an activatable proprotein comprising at least a masking moiety and an active domain, or derivatives/variants therefrom, as described herein. In one embodiment, the proprotein may also comprise one or more protein domains. 
     The term “antigen” refers to a molecule or a portion of a molecule capable of being bound by a selective binding agent, such as an antibody, and additionally capable of being used in an animal to produce antibodies capable of binding to an epitope of that antigen. An antigen may have one or more epitopes. As used herein, the term “antigen” includes substances that are capable, under appropriate conditions, of inducing an immune response to the substance and of reacting with the products of the immune response. More broadly, the term “antigen” includes any substance to which an antibody binds, or for which antibodies are desired, regardless of whether the substance is immunogenic. For such antigens, antibodies can be identified by recombinant methods, independently of any immune response. 
     An “antagonist” refers to biological structure or chemical agent that interferes with or otherwise reduces the physiological action of another agent or molecule. In some instances, the antagonist specifically binds to the other agent or molecule. Included are full and partial antagonists. 
     An “agonist” refers to biological structure or chemical agent that increases or enhances the physiological action of another agent or molecule. In some instances, the agonist specifically binds to the other agent or molecule. Included are full and partial agonists. 
     As used herein, the term “amino acid” is intended to mean both naturally occurring and non-naturally occurring amino acids as well as amino acid analogs and mimetics. Naturally-occurring amino acids include the 20 (L)-amino acids utilized during protein biosynthesis as well as others such as 4-hydroxyproline, hydroxylysine, desmosine, isodesmosine, homocysteine, citrulline and ornithine, for example. Non-naturally occurring amino acids include, for example, (D)-amino acids, norleucine, norvaline, p-fluorophenylalanine, ethionine and the like, which are known to a person skilled in the art. Amino acid analogs include modified forms of naturally and non-naturally occurring amino acids. Such modifications can include, for example, substitution or replacement of chemical groups and moieties on the amino acid or by derivatization of the amino acid. Amino acid mimetics include, for example, organic structures which exhibit functionally similar properties such as charge and charge spacing characteristic of the reference amino acid. For example, an organic structure which mimics arginine (Arg or R) would have a positive charge moiety located in similar molecular space and having the same degree of mobility as the e-amino group of the side chain of the naturally occurring Arg amino acid. Mimetics also include constrained structures so as to maintain optimal spacing and charge interactions of the amino acid or of the amino acid functional groups. Those skilled in the art know or can determine what structures constitute functionally equivalent amino acid analogs and amino acid mimetics. 
     As used herein, a subject “at risk” of developing a disease, or adverse reaction may or may not have detectable disease, or symptoms of disease, and may or may not have displayed detectable disease or symptoms of disease prior to the treatment methods described herein. “At risk” denotes that a subject has one or more risk factors, which are measurable parameters that correlate with development of a disease, as described herein and known in the art. A subject having one or more of these risk factors has a higher probability of developing disease, or an adverse reaction than a subject without one or more of these risk factor(s). 
     “Biocompatible” refers to materials or compounds which are generally not injurious to biological functions of a cell or subject and which will not result in any degree of unacceptable toxicity, including allergenic and disease states. 
     The term “binding” refers to a direct association between two molecules, due to, for example, covalent, electrostatic, hydrophobic, and ionic and/or hydrogen-bond interactions, including interactions such as salt bridges and water bridges. 
     By “coding sequence” is meant any nucleic acid sequence that contributes to the code for the polypeptide product of a gene. By contrast, the term “non-coding sequence” refers to any nucleic acid sequence that does not directly contribute to the code for the polypeptide product of a gene. 
     Throughout this disclosure, unless the context requires otherwise, the words “comprise,” “comprises,” and “comprising” will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements. 
     By “consisting of” is meant including, and limited to, whatever follows the phrase “consisting of” Thus, the phrase “consisting of” indicates that the listed elements are required or mandatory, and that no other elements may be present. By “consisting essentially of” is meant including any elements listed after the phrase, and limited to other elements that do not interfere with or contribute to the activity or action specified in the disclosure for the listed elements. Thus, the phrase “consisting essentially of” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present depending upon whether or not they materially affect the activity or action of the listed elements. 
     The term “endotoxin free” or “substantially endotoxin free” relates generally to compositions, solvents, and/or vessels that contain at most trace amounts (e.g., amounts having no clinically adverse physiological effects to a subject) of endotoxin, and preferably undetectable amounts of endotoxin. Endotoxins are toxins associated with certain micro-organisms, such as bacteria, typically gram-negative bacteria, although endotoxins may be found in gram-positive bacteria, such as  Listeria monocytogenes . The most prevalent endotoxins are lipopolysaccharides (LPS) or lipo-oligo-saccharides (LOS) found in the outer membrane of various Gram-negative bacteria, and which represent a central pathogenic feature in the ability of these bacteria to cause disease. Small amounts of endotoxin in humans may produce fever, a lowering of the blood pressure, and activation of inflammation and coagulation, among other adverse physiological effects. 
     Therefore, in pharmaceutical production, it is often desirable to remove most or all traces of endotoxin from drug products and/or drug containers, because even small amounts may cause adverse effects in humans. A depyrogenation oven may be used for this purpose, as temperatures in excess of 300° C. are typically required to break down most endotoxins. For instance, based on primary packaging material such as syringes or vials, the combination of a glass temperature of 250° C. and a holding time of 30 minutes is often sufficient to achieve a 3 log reduction in endotoxin levels. Other methods of removing endotoxins are contemplated, including, for example, chromatography and filtration methods, as described herein and known in the art. 
     Endotoxins can be detected using routine techniques known in the art. For example, the Limulus Amoebocyte Lysate assay, which utilizes blood from the horseshoe crab, is a very sensitive assay for detecting presence of endotoxin. In this test, very low levels of LPS can cause detectable coagulation of the limulus lysate due a powerful enzymatic cascade that amplifies this reaction. Endotoxins can also be quantitated by enzyme-linked immunosorbent assay (ELISA). To be substantially endotoxin free, endotoxin levels may be less than about 0.001, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.09, 0.1, 0.5, 1.0, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, 9, or 10 EU/mg of active compound. Typically, 1 ng lipopolysaccharide (LPS) corresponds to about 1-10 EU. 
     The term “half maximal effective concentration” or “EC 50 ” refers to the concentration of an agent (e.g., activatable proprotein) as described herein at which it induces a response halfway between the baseline and maximum after some specified exposure time; the EC 50  of a graded dose response curve therefore represents the concentration of a compound at which 50% of its maximal effect is observed. EC50 also represents the plasma concentration required for obtaining 50% of a maximum effect in vivo. Similarly, the “EC 90 ” refers to the concentration of an agent or composition at which 90% of its maximal effect is observed. The “EC 90 ” can be calculated from the “EC50” and the Hill slope, or it can be determined from the data directly, using routine knowledge in the art. In some embodiments, the EC 50  of an agent (e.g., activatable proprotein) is less than about 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 200 or 500 nM. In some embodiments, an agent will have an EC 50  value of about 1 nM or less. 
     “Immune response” means any immunological response originating from immune system, including responses from the cellular and humeral, innate and adaptive immune systems. Exemplary cellular immune cells include for example, lymphocytes, macrophages, T cells, B cells, NK cells, neutrophils, eosinophils, dendritic cells, mast cells, monocytes, and all subsets thereof. Cellular responses include for example, effector function, cytokine release, phagocytosis, efferocytosis, translocation, trafficking, proliferation, differentiation, activation, repression, cell-cell interactions, apoptosis, etc. Humeral responses include for example IgG, IgM, IgA, IgE, responses and their corresponding effector functions. 
     The “half-life” of an agent such as an activatable proprotein can refer to the time it takes for the agent to lose half of its pharmacologic, physiologic, or other activity, relative to such activity at the time of administration into the serum or tissue of an organism, or relative to any other defined time-point. “Half-life” can also refer to the time it takes for the amount or concentration of an agent to be reduced by half of a starting amount administered into the serum or tissue of an organism, relative to such amount or concentration at the time of administration into the serum or tissue of an organism, or relative to any other defined time-point. The half-life can be measured in serum and/or any one or more selected tissues. 
     The terms “modulating” and “altering” include “increasing,” “enhancing” or “stimulating,” as well as “decreasing” or “reducing,” typically in a statistically significant or a physiologically significant amount or degree relative to a control. An “increased,” “stimulated” or “enhanced” amount is typically a “statistically significant” amount, and may include an increase that is 1.1, 1.2, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100 or more times (e.g., 500, 1000 times) (including all integers and ranges in between e.g., 1.5, 1.6, 1.7. 1.8, etc.) the amount produced by no composition (e.g., the absence of agent) or a control composition. A “decreased” or “reduced” amount is typically a “statistically significant” amount, and may include a 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100% decrease (including all integers and ranges in between) in the amount produced by no composition (e.g., the absence of an agent) or a control composition. Examples of comparisons and “statistically significant” amounts are described herein. 
     The terms “polypeptide,” “protein” and “peptide” are used interchangeably and mean a polymer of amino acids not limited to any particular length. The term “enzyme” includes polypeptide or protein catalysts. The terms include modifications such as myristoylation, sulfation, glycosylation, phosphorylation and addition or deletion of signal sequences. The terms “polypeptide” or “protein” means one or more chains of amino acids, wherein each chain comprises amino acids covalently linked by peptide bonds, and wherein said polypeptide or protein can comprise a plurality of chains non-covalently and/or covalently linked together by peptide bonds, having the sequence of native proteins, that is, proteins produced by naturally-occurring and specifically non-recombinant cells, or genetically-engineered or recombinant cells, and comprise molecules having the amino acid sequence of the native protein, or molecules having deletions from, additions to, and/or substitutions of one or more amino acids of the native sequence. In certain embodiments, the polypeptide is a “recombinant” polypeptide, produced by recombinant cell that comprises one or more recombinant DNA molecules, which are typically made of heterologous polynucleotide sequences or combinations of polynucleotide sequences that would not otherwise be found in the cell. 
     The term “polynucleotide” and “nucleic acid” includes mRNA, RNA, cRNA, cDNA, and DNA. The term typically refers to polymeric form of nucleotides of at least 10 bases in length, either ribonucleotides or deoxynucleotides or a modified form of either type of nucleotide. The term includes single and double stranded forms of DNA. The terms “isolated DNA” and “isolated polynucleotide” and “isolated nucleic acid” refer to a molecule that has been isolated free of total genomic DNA of a particular species. Therefore, an isolated DNA segment encoding a polypeptide refers to a DNA segment that contains one or more coding sequences yet is substantially isolated away from, or purified free from, total genomic DNA of the species from which the DNA segment is obtained. Also included are non-coding polynucleotides (e.g., primers, probes, oligonucleotides), which do not encode a polypeptide. Also included are recombinant vectors, including, for example, expression vectors, viral vectors, plasmids, cosmids, phagemids, phage, viruses, and the like. 
     Additional coding or non-coding sequences may, but need not, be present within a polynucleotide described herein, and a polynucleotide may, but need not, be linked to other molecules and/or support materials. Hence, a polynucleotide or expressible polynucleotides, regardless of the length of the coding sequence itself, may be combined with other sequences, for example, expression control sequences. 
     The term “isolated” polypeptide or protein referred to herein means that a subject protein (1) is free of at least some other proteins with which it would typically be found in nature, (2) is essentially free of other proteins from the same source, e.g., from the same species, (3) is expressed by a cell from a different species, (4) has been separated from at least about 50 percent of polynucleotides, lipids, carbohydrates, or other materials with which it is associated in nature, (5) is not associated (by covalent or non-covalent interaction) with portions of a protein with which the “isolated protein” is associated in nature, (6) is operably associated (by covalent or non-covalent interaction) with a polypeptide with which it is not associated in nature, or (7) does not occur in nature. Such an isolated protein can be encoded by genomic DNA, cDNA, mRNA or other RNA, of may be of synthetic origin, or any combination thereof. In certain embodiments, the isolated protein is substantially free from proteins or polypeptides or other contaminants that are found in its natural environment that would interfere with its use (therapeutic, diagnostic, prophylactic, research or otherwise). 
     In certain embodiments, the “purity” of any given agent (e.g., activatable proprotein) in a composition may be defined. For instance, certain compositions may comprise an agent such as a polypeptide agent that is at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% pure on a protein basis or a weight-weight basis, including all decimals and ranges in between, as measured, for example and by no means limiting, by high performance liquid chromatography (HPLC), a well-known form of column chromatography used frequently in biochemistry and analytical chemistry to separate, identify, and quantify compounds. 
     The term “reference sequence” refers generally to a nucleic acid coding sequence, or amino acid sequence, to which another sequence is being compared. All polypeptide and polynucleotide sequences described herein are included as references sequences, including those described by name and those described in the Tables and the Sequence Listing. 
     Certain embodiments include biologically active “variants” and “fragments” of the proteins/polypeptides described herein, and the polynucleotides that encode the same. “Variants” contain one or more substitutions, additions, deletions, and/or insertions relative to a reference polypeptide or polynucleotide (see, e.g., the Tables and the Sequence Listing). A variant polypeptide or polynucleotide comprises an amino acid or nucleotide sequence with at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more sequence identity or similarity or homology to a reference sequence, as described herein, and substantially retains the activity of that reference sequence. Also included are sequences that consist of or differ from a reference sequences by the addition, deletion, insertion, or substitution of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150 or more amino acids or nucleotides and which substantially retain at least one activity of that reference sequence. In certain embodiments, the additions or deletions include C-terminal and/or N-terminal additions and/or deletions. 
     The terms “sequence identity” or, for example, comprising a “sequence 50% identical to,” as used herein, refer to the extent that sequences are identical on a nucleotide-by-nucleotide basis or an amino acid-by-amino acid basis over a window of comparison. Thus, a “percentage of sequence identity” may be calculated by comparing two optimally aligned sequences over the window of comparison, determining the number of positions at which the identical nucleic acid base (e.g., A, T, C, G, I) or the identical amino acid residue (e.g., Ala, Pro, Ser, Thr, Gly, Val, Leu, Ile, Phe, Tyr, Trp, Lys, Arg, His, Asp, Glu, Asn, Gln, Cys and Met) occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison (i.e., the window size), and multiplying the result by 100 to yield the percentage of sequence identity. Optimal alignment of sequences for aligning a comparison window may be conducted by computerized implementations of algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package Release 7.0, Genetics Computer Group, 575 Science Drive Madison, Wis., USA) or by inspection and the best alignment (i.e., resulting in the highest percentage homology over the comparison window) generated by any of the various methods selected. Reference also may be made to the BLAST family of programs as for example disclosed by Altschul et al., Nucl. Acids Res. 25:3389, 1997. 
     The term “solubility” refers to the property of an agent (e.g., activatable proprotein) provided herein to dissolve in a liquid solvent and form a homogeneous solution. Solubility is typically expressed as a concentration, either by mass of solute per unit volume of solvent (g of solute per kg of solvent, g per dL (100 mL), mg/ml, etc.), molarity, molality, mole fraction or other similar descriptions of concentration. The maximum equilibrium amount of solute that can dissolve per amount of solvent is the solubility of that solute in that solvent under the specified conditions, including temperature, pressure, pH, and the nature of the solvent. In certain embodiments, solubility is measured at physiological pH, or other pH, for example, at pH 5.0, pH 6.0, pH 7.0, pH 7.4, pH 7.6, pH 7.8, or pH 8.0 (e.g., about pH 5-8). In certain embodiments, solubility is measured in water or a physiological buffer such as PBS or NaCl (with or without NaPO 4 ). In specific embodiments, solubility is measured at relatively lower pH (e.g., pH 6.0) and relatively higher salt (e.g., 500 mM NaCl and 10 mM NaPO 4 ). In certain embodiments, solubility is measured in a biological fluid (solvent) such as blood or serum. In certain embodiments, the temperature can be about room temperature (e.g., about 20, 21, 22, 23, 24, 25° C.) or about body temperature (37° C.). In certain embodiments, an agent has a solubility of at least about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 40, 50, 60, 70, 80, 90 or 100 mg/ml at room temperature or at 37° C. 
     A “subject” or a “subject in need thereof” or a “patient” or a “patient in need thereof” includes a mammalian subject such as a human subject. 
     “Substantially” or “essentially” means nearly totally or completely, for instance, 95%, 96%, 97%, 98%, 99% or greater of some given quantity. 
     By “statistically significant,” it is meant that the result was unlikely to have occurred by chance. Statistical significance can be determined by any method known in the art. Commonly used measures of significance include the p-value, which is the frequency or probability with which the observed event would occur, if the null hypothesis were true. If the obtained p-value is smaller than the significance level, then the null hypothesis is rejected. In simple cases, the significance level is defined at a p-value of 0.05 or less. 
     “Therapeutic response” refers to improvement of symptoms (whether or not sustained) based on administration of one or more therapeutic agents. 
     As used herein, the terms “therapeutically effective amount”, “therapeutic dose,” “prophylactically effective amount,” or “diagnostically effective amount” is the amount of an agent (e.g., activatable proprotein, activated protein) needed to elicit the desired biological response following administration. 
     As used herein, “treatment” of a subject (e.g., a mammal, such as a human) or a cell is any type of intervention used in an attempt to alter the natural course of the individual or cell. Treatment includes, but is not limited to, administration of a pharmaceutical composition, and may be performed either prophylactically or subsequent to the initiation of a pathologic event or contact with an etiologic agent. Also included are “prophylactic” treatments, which can be directed to reducing the rate of progression of the disease or condition being treated, delaying the onset of that disease or condition, or reducing the severity of its onset. “Treatment” or “prophylaxis” does not necessarily indicate complete eradication, cure, or prevention of the disease or condition, or associated symptoms thereof. 
     The term “wild-type” refers to a gene or gene product (e.g., a polypeptide) that is most frequently observed in a population and is thus arbitrarily designed the “normal” or “wild-type” form of the gene. 
     Each embodiment in this specification is to be applied to every other embodiment unless expressly stated otherwise. 
     Activatable Proproteins 
     Embodiments of the present disclosure relate to activatable proproteins, or prodrugs, comprising an IL-2 protein that remains relatively inactive in the proprotein form, and which can be activated upon contact with the appropriate environment. The activatable proproteins described herein comprise at least two separate or distinct polypeptide chains, which bind together via non-covalent and/or certain covalent bonds, for example, disulfide bonds, but not via peptide or amide bonds. Generally, at least one polypeptide chain comprises an IL-2 protein, and at least one polypeptide chain comprises an IL-2 binding protein such as an IL-Ra protein. Each polypeptide chain also comprises a masking moiety, which binds to the masking moiety in the other chain and sterically hinders the IL-2 protein from interacting with or binding to its cognate receptor(s) on a cell. Typically, at least one of the masking moieties comprises a cleavable linker, which upon cleavage in a target tissue releases that masking moiety (and its steric hindrance) from the activatable proprotein complex, and restores IL-2 activity by exposing at least one active or binding site of the IL-2 protein. Such allows the IL-2 portion of the now activated protein to interact with or bind to certain of its cognate receptor(s), for example, an IL-2Rβ/γc receptor chain on an immune cell, and thereby effect downstream immune cell-signaling pathways. 
     The activatable proproteins described herein address many of the drawbacks of standard IL-2 therapies, including high initial serum C max , which causes over-activation of the immune system, short PK because of the otherwise small molecular size of IL-2 and/or catabolism by the large number of immune cells that express IL-2 receptors, poor accumulation in the target tissues (e.g., cancers, tumors) because of the short PK and/or ineffective tumor targeting, and undesirable accumulation and immune activation in normal tissues. 
     Embodiments of the present disclosure thus include an activatable proprotein (complex), comprising a first polypeptide (chain) and a second polypeptide (chain), 
     wherein the first polypeptide comprises a first masking moiety and an IL-2 protein, wherein the first masking moiety comprises a first binding moiety and a first linker that is fused to the IL-2 protein, 
     wherein the second polypeptide comprises a second masking moiety and an IL-2 binding protein, wherein the second masking moiety comprises a second binding moiety and a second linker that is fused to the IL-2 binding protein, 
     wherein the first and second masking moieties bind together via their respective first and second binding moieties, optionally as a dimer, and thereby mask a binding site of the IL-2 protein that binds to an IL-2Rβ/γc chain present on the surface of an immune cell in vitro or in vivo, 
     and wherein at least one of the first linker or the second linker is a cleavable linker. 
     In some embodiments, the IL-2 protein and the IL-2 binding protein interact or bind together, for example, via non-covalent or certain covalent bonds (e.g., disulfide bonds). In some instances, the binding of the IL-2 protein to the IL-2 binding protein, for example, an IL-2Rα protein, sterically blocks or hinders binding of the IL-2 protein to its cognate IL-2Rα/β/γc receptor chain expressed on regulatory T-cells (T regs ). In some instances, that binding and steric hindrance is preserved in the activated form of the protein, and can provide the advantage of minimizing the activation of immunosuppressive T regs , and reducing the consumption of the proprotein and the active protein alike. Exemplary IL-2 proteins and IL-2 binding proteins are described elsewhere herein. 
     Typically, as noted above, the first and second masking moieties bind together, for example, dimerize together, via one or bonds. Such binding typically occurs between the binding moieties contained within each masking moiety, rather than the linker. However, a linker can in some instances contribute to the binding between two masking moieties. It is the interaction between the two masking moieties (via their respective binding moieties), which sterically masks or otherwise blocks the binding of the IL-2 protein to its cognate receptor (for example, IL-2Rβ/γc chain) present on the surface of an immune cell in vitro or in vivo, and thereby keeps the activatable proprotein in its relatively inactive form. In some instances, the linker contributes to the steric masking or blocking activity of the masking moiety. 
     More specifically, in some embodiments, the first and second masking moieties dimerize together via at least one non-covalent bond, at least one covalent bond (for example, at least one disulfide bond), or any combination of non-covalent and covalent bonds. Typically, however, the first and second masking moieties do not bind together or dimerize via a peptide or amide bond. In some embodiments, the masking moieties bind together via their respective binding moieties as a heterodimer, that is, a heterodimer composed of two different binding moieties. In some embodiments, the masking moieties bind together as a homodimer, that is, a homodimer composed of two identical or nearly identical binding moieties. Thus, the first and second masking moieties, or the first and second binding moieties, can be the same (or substantially the same) or different. In most instances, the first and second masking moiety do not bind to the IL-2 protein, or the IL-2 binding protein. However, in some instances, one or both of the masking moieties can bind to the IL-2 protein and/or the IL-2 binding protein. 
     As noted above, at least one of the polypeptide chains (i.e., the first polypeptide or the second polypeptide) comprises a cleavable linker, for example, a linker cleavable by a protease. In some instances, the protease is expressed in target tissues or cells, for example, cancer tissues or cancer cells. Cleavage of the linker in that context releases a masking moiety, removes the steric hindrance of the IL-2 protein, and allows selective activation of the IL-2 protein in diseased tissues or cells, relative to normal or healthy tissues or cells. Such selective and localized activation not only reduces needless consumption of administered IL-2, thereby increasing its half-life, but also reduces undesirable systemic effects of IL-2, among other advantages. Exemplary masking moieties, including binding moieties and linkers, are described herein. 
     The various components of each polypeptide chain can be fused in any orientation. However, the linker region is typically located between the IL-2 peptide and the binding moiety portion of the masking moiety, and likewise for the polypeptide chain comprising the IL-2 binding protein. For example, in some embodiments, the first polypeptide comprises, in an N- to C-terminal orientation, the first masking moiety (orientated as the first binding moiety and the first linker) and the IL-2 protein. In some embodiments, the first polypeptide comprises, in an N- to C-terminal orientation, the IL-2 protein and the first masking moiety (oriented as the first linker and the first binding moiety). In certain embodiments, the second polypeptide comprises, in an N- to C-terminal orientation, the second masking moiety (orientated as the second binding moiety and the second linker) and the IL-2 binding protein. In particular embodiments, the second polypeptide comprises, in an N- to C-terminal orientation, the IL-2 binding protein and the second masking moiety (oriented as the second linker and the second binding moiety). 
     Certain activatable proproteins are composed only of two of the foregoing protein chains, that is, they are composed only of a first polypeptide and a second polypeptide, as described herein (see, for example, the various structures in  FIGS. 2A-2J  and  FIGS. 3A-3D ). In some instances, however, certain activatable proproteins comprise multiple chains, for example, where the first and second polypeptide chains form a “core structure” upon which additional or higher-order structures can be built, the various core structures being optionally bound together via additional protein binding domains, as illustrated, for example, in  FIGS. 9A-9B . Examples of additional protein binding domains include immunoglobulin domains, such as light chain variable regions, heavy chain variable regions, and/or Fc regions, the latter optionally comprising a knob and hole structure to improve specific binding between desired pairs (see  FIGS. 9A-9B ). 
     The individual components of the activatable proproteins are described in greater detail herein. 
     IL-2 Proteins. 
     The activatable proproteins described herein comprise at least one “IL-2 protein” (or Interleukin-2 protein), including human IL-2 proteins. IL-2 is a cytokine signals though the IL-2 receptor (IL-2R), a complex composed of up to three chains, termed the α (CD25), β (CD122) and γ (CD132) chains. IL-2 is produced by T-cells in response to antigenic or mitogenic stimulation, and is required for T-cell proliferation and other activities crucial to regulation of the immune response. IL-2 can stimulate B-cells, monocytes, lymphokine-activated killer cells, natural killer cells, and glioma cells, among other immune cells. 
     IL-2 is a 15-16 kDA protein composed of a signal peptide (residues 1-20) and an active mature protein (residues 21-153). Exemplary human IL-2 amino acid sequences are provided in Table S1 below. 
     
       
         
           
               
             
               
                 TABLE S1 
               
             
            
               
                   
               
               
                 Exemplary IL-2 Peptides 
               
            
           
           
               
               
               
            
               
                   
                   
                 SEQ 
               
               
                   
                   
                 ID 
               
               
                 Name 
                 Sequence 
                 NO: 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Human IL-2 
                   MYRMQLLSCIALSLALVTNS APTSSSTKKTQLQLEHLLLDLQMILNGINNYK 
                 1 
               
               
                 FL 
                 NPKLTRMLTFKFYMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRD 
                   
               
               
                 Precursor 
                 LISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKKATE 
                 2 
               
               
                 mature 
                 LKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form C1255 
                 CEYADETATIVEFLNRWITF S QSIISTLT 
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKKATE 
                 3 
               
               
                 mature 
                 LKHLQCLEEELKPLEEVLNLAHSKNFHFDPRDVVSNINVFVLELKGSETTFM 
                   
               
               
                 form (D10) 
                 CEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                 Q74H,L80F, 
                   
                   
               
               
                 R81D,L85V, 
                   
                   
               
               
                 I86V, and 
                   
                   
               
               
                 I92F 
                   
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKKATE 
                 104 
               
               
                 mature 
                 LKHLQCLEEELKPLEE A LNLA P SKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form with 
                 CEYADETATIVEFLNRWITFCQS T ISTLT 
                   
               
               
                 V69A, Q74P 
                   
                   
               
               
                 and I128T 
                   
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLT A KF A MPKKATE 
                 105 
               
               
                 mature 
                 LKHLQCLEEELKPLEEVLN G AQSKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form with 
                 CEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                 F42A, Y45A 
                   
                   
               
               
                 and 
                 L72G 
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLT K MLT Q KF N MPKKATE 
                 106 
               
               
                 mature 
                 LKHLQCLEE L LKPLE V VLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form with 
                 CEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                 R38K, 
                   
                   
               
               
                 F42Q, 
                   
                   
               
               
                 Y45N, E62L 
                   
                   
               
               
                 and E68V 
                   
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLT K MLT Q KF E MPKKATE 
                 107 
               
               
                 mature 
                 LKHLQCLEEELKPLE V VLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form with 
                 CEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                 R38K, 
                   
                   
               
               
                 F42Q, Y45E 
                   
                   
               
               
                 and E68V 
                   
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLT A MLT I KF N MPKKATE 
                 108 
               
               
                 mature 
                 LKHLQCLEE L LKPLE V VLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form with 
                 CEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                 R38A, 
                   
                   
               
               
                 F42I, 
                   
                   
               
               
                 Y45N, E62L 
                   
                   
               
               
                 and E68V 
                   
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLT K MLT K KF R MPKKATE 
                 109 
               
               
                 mature 
                 LKHLQCLEE L LKPLE V VLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form with 
                 CEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                 R38K, 
                   
                   
               
               
                 F42K, 
                   
                   
               
               
                 Y45R, E62L 
                   
                   
               
               
                 and E68V 
                   
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLT K MLT I KF E MPKKATE 
                 110 
               
               
                 mature 
                 LKHLQCLEEE L KPLE V VLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form with 
                 CEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                 R38K, 
                   
                   
               
               
                 F42I, Y45E 
                   
                   
               
               
                 and E68V 
                   
                   
               
               
                   
               
               
                 Human IL-2 
                 APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLT A MLT A KF A MPKKATE 
                 111 
               
               
                 mature 
                 LKHLQCLEE A LKPLEEVLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFM 
                   
               
               
                 form with 
                 CEYADETATIVEFLNRWITFCQSIISTLT 
                   
               
               
                 R38A, 
                   
                   
               
               
                 F42A, Y45A 
                   
                   
               
               
                 and E62A 
               
               
                   
               
            
           
         
       
     
     Thus, in certain embodiments, an IL-2 protein comprises, consists, or consists essentially of an amino acid sequence selected from Table S1, or an active variant or fragment thereof that is at least 80, 85, 90, 95, 98, or 100% identical to a sequence selected from Table S1. In some embodiments, an “active” IL-2 protein or fragment or variant is characterized, for example, by its ability to bind to an IL-2Rβ/γc receptor chain present on the surface of an immune cell in vitro or in vivo, and stimulate downstream signaling activities, absent steric hindrance by the masking moieties described herein. Examples of downstream signaling activities include IL-2 mediated signaling via one or more of the JAK-STAT, PI3K/Akt/mTOR, and MAPK/ERK pathways, including combinations thereof. Altogether, IL-2 signaling stimulates an array of downstream pathways leading to responses that have a significant role in the development, function, and survival of CD4 T cells, CD8 T cells, NK cells, NKT cells, macrophages, and intestinal intraepithelial lymphocytes, among others. 
     In particular embodiments, the IL-2 protein is a mature form of IL-2, or an active variant or fragment thereof, which comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to amino acids 21-153 of SEQ ID NO: 1. In some embodiments, the IL-2 protein comprises a C145X substitution, as defined by SEQ ID NO: 1, wherein X is any amino acid. In specific embodiments, the IL-2 protein comprises a C145S substitution as defined by SEQ ID NO: 1. 
     Certain IL-2 proteins comprise, consist, or consist essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 2 (mature human IL-2 with C125S substitution). In some embodiments, an active variant or fragment of SEQ ID NO: 2 retains the S125 residue as defined therein. 
     Certain IL-2 proteins comprise one or more defined amino acid substitutions relative to the exemplary amino acid sequences in Table S1. For example, some IL-2 proteins comprise one or more amino acid substitutions selected from K35C, R38C, T41C, F42C, E61C, and V69C as defined by SEQ ID NO: 2. In some embodiments, the IL-2 protein forms a disulfide bond with the IL-2 binding protein (e.g., IL-2Ra) via one or more of the cysteine substitutions selected from K35C, R38C, T41C, F42C, E61C, and V69C. Certain IL-2 proteins comprise one or more amino acid substitutions at position 69, 74, and/or 128 as defined by SEQ ID NO: 2, including combinations thereof and including, for example, wherein the one or more amino acid substitutions are selected from V69A, Q74P, and 1128T as defined by SEQ ID NO: 2. Some IL-2 proteins comprise one or more amino acid substitutions at position R38, F42, Y45, E62, E68, and/or L72 as defined by SEQ ID NO: 2, including combinations thereof and including, for example, wherein the one or more amino acid substitutions are selected from R38A and R38K; F42A, F42G, F42S, F42T, F42Q, F42E, F42N, F42D, F42R, F42K, and F42I; Y45A, Y45G, Y45S, Y45T, Y45Q, Y45E, Y45N, Y45D, Y45R, and Y45K; E62A and E62L; E68A and E68V; and L72A, L72G, L72S, L72T, L72Q, L72E, L72N, L72D, L72R, and L72K, including combinations thereof. Specific examples include where the IL-2 protein comprises one or combination of amino acid substitutions selected from F42A, Y45A, and L72G; R38K, F42Q, Y45N, E62L, and E68V; R38K, F42Q, Y45E, and E68V; R38A, F42I, Y45N, E62L, and E68V; R38K, F42K, Y45R, E62L, and E68V; R38K, F42I, Y45E, and E68V; and R38A, F42A, Y45A, and E62A. Thus, an IL-2 protein can comprise any one or more of the foregoing amino acid substitutions, including combinations thereof. 
     Any one or more of the foregoing IL-2 proteins can be combined with any of the other components described herein, for example, IL-2 bindings proteins such as IL-2Rα proteins, masking moieties including binding moieties and linkers, and other optional protein domains, to generate one or more activatable proproteins or larger, multi-chain structures comprising the same. 
     IL-2 Binding Proteins. 
     The activatable proproteins described herein comprise at least one “IL-2 binding protein”. Examples of IL-2 binding proteins include IL-2Rα proteins, including human IL-2Rα proteins, and antibodies and antigen binding fragments thereof that bind to an IL-2 protein described herein. 
     In particular embodiments, the IL-2 binding protein is a human IL-2Rα protein, or a variant or fragment thereof that binds to an IL-2 protein. Exemplary human IL-2Rα amino acid sequences are provided in Table S2 below. 
     
       
         
           
               
             
               
                 TABLE S2 
               
             
            
               
                   
               
               
                 Exemplary IL-2Rα Proteins 
               
            
           
           
               
               
               
            
               
                   
                   
                 SEQ 
               
               
                   
                   
                 ID 
               
               
                 Name 
                 Sequence 
                 NO: 
               
               
                   
               
               
                 Human IL- 
                 MDSYLLMWGLLTFIMVPGCQAELCDDDPPEIPHATFKAMAYKEGTMLNCECK 
                 4 
               
               
                 2Rα FL 
                 RGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKER 
                   
               
               
                   
                 KTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQCVQGY 
                   
               
               
                   
                 RALHRGPAESVCKMTHGKTRWTQPQLICTGEMETSQFPGEEKPQASPEGRPE 
                   
               
               
                   
                 SETSCLVTTTDFQIQTEMAATMETSIFTTEYQVAVAGCVFLLISVLLLSGLT 
                   
               
               
                   
                 WQRRQRKSRRTI 
                   
               
               
                   
               
               
                 Human IL- 
                 ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTGNSSH 
                 5 
               
               
                 2Rα-ECD 
                 SSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASLPGHC 
                   
               
               
                 (22-240) 
                 REPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHGKTRW 
                   
               
               
                   
                 TQPQLICTGEMETSQFPGEEKPQASPEGRPESETSCLVTTTDFQIQTEMAAT 
                   
               
               
                   
                 METSIFTTEYQ 
                   
               
               
                   
               
               
                 Human IL- 
                 ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTGNSSH 
                 6 
               
               
                 2Rα-sushi 
                 SSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASLPGHC 
                   
               
               
                 (22-187) 
                 REPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHGKTRW 
                   
               
               
                   
                 TQPQLICTGE 
               
               
                   
               
            
           
         
       
     
     Thus, in certain embodiments, an IL-2Rα protein comprises, consists, or consists essentially of an amino acid sequence selected from Table S2, or an active variant or fragment thereof that is at least 80, 85, 90, 95, 98, or 100% identical to a sequence selected from Table S2, and which binds to an IL-2 protein. In some embodiments, the IL-2Rα protein comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% to amino acids 22-187 or 22-240 of SEQ ID NO: 4 (full-length wild-type human IL-2Rα). 
     Certain IL-2Rα proteins comprise one or more defined amino acid substitutions relative to the exemplary amino acid sequences in Table S2. For example, in some instances the IL-2Rα protein comprises one or more cysteine substitutions selected from D4C, D6C, N27C, K38C, S39C, L42C, Y43C, I118C, and H120C as defined by SEQ ID NO: 6 (human IL-2Rα Sushi 1 to Sushi 2 domain). In some instances, the IL-2Rα protein comprises an alanine substitution at position 49 and/or 68 as defined by SEQ ID NO: 6. Thus, an IL-2Rα protein can comprise any one or more of the foregoing amino acid substitutions, including combinations thereof. 
     In certain of these and related embodiments, the IL-2Rα protein forms at least one disulfide bond with the IL-2 protein via one or more of the foregoing cysteines and one or more cysteines in the IL-2 protein. In specific embodiments, the IL-2Rα and IL-2 protein form disulfide at least one disulfide bond between one or more cysteine pairs selected from IL2-K35C and IL2Rα-D4C, IL2-R38C and IL2Rα-D6C, IL2-R38C and IL2Rα-H120C, IL2-T41C-IL2Rα-1118C, IL2-F42C and IL2Rα-N27C, IL2-E61C and IL2Rα-K38C, IL2-E61C and IL2Rα-S39C, and IL2-V69C and IL2Rα-L42C. In particular embodiments, as noted above, the binding (for example, disulfide binding) between the IL-2 protein and the IL-2Rα protein masks or sterically hinders the binding site of the IL-2 protein that preferentially binds to the IL-2Rαβγ chain expressed on T regs . In some instances, the active or activated form of the protein, following cleavage of at least one linker and release of the corresponding masking moiety, retains the binding between the IL-2 protein and the IL-2Rα protein, and thus does not preferentially bind to the IL-2Rαβγ chain expressed on T regs . 
     As noted above, in certain embodiments, the IL-2 binding protein comprises an antibody or antigen binding fragment thereof that specifically binds to the IL-2 protein. Examples include a whole antibody, Fab, Fab′, F(ab′)2, monospecific Fab2, bispecific Fab2, FV, single chain Fv (scFv), scFV-Fc, nanobody, diabody, camelid, and a minibody. In specific embodiments, the antibody is NARA1 or an antigen binding fragment thereof (see, for example, Arenas-Ramirez et al., Science Translational Medicine. 8: 367ra166, 2016; and U.S. Application No. 2019/0016797, herein incorporated by reference). In particular embodiments, and likewise to above, the binding (for example, disulfide binding) between the IL-2 protein and the anti-IL-2 antibody (or antigen binding fragment thereof) masks or sterically hinders the binding site of the IL-2 protein that preferentially binds to the IL-2Rαβγ chain expressed on T regs . In some instances, the active or activated form of the protein, following cleavage of at least one linker and release of the corresponding masking moiety, retains the binding between the IL-2 protein and the IL-2Rα protein, and thus does not preferentially bind to the IL-2Rαβγ chain expressed on T regs . 
     As used herein, the term “antibody” encompasses not only intact polyclonal or monoclonal antibodies, but also fragments thereof (such as dAb, Fab, Fab′, F(ab′)2, Fv), single chain (ScFv), synthetic variants thereof, naturally occurring variants, fusion proteins comprising an antibody portion with an antigen-binding fragment of the required specificity, humanized antibodies, chimeric antibodies, and any other modified configuration of the immunoglobulin molecule that comprises an antigen-binding site or fragment (epitope recognition site) of the required specificity. Certain features and characteristics of antibodies (and antigen-binding fragments thereof) are described in greater detail herein. 
     An antibody or antigen-binding fragment can be of essentially any type. As is well known in the art, an antibody is an immunoglobulin molecule capable of specific binding to a target, such as an immune checkpoint molecule, through at least one epitope recognition site, located in the variable region of the immunoglobulin molecule. 
     The term “antigen-binding fragment” as used herein refers to a polypeptide fragment that contains at least one CDR of an immunoglobulin heavy and/or light chain that binds to the antigen of interest. In this regard, an antigen-binding fragment of the herein described antibodies may comprise 1, 2, 3, 4, 5, or all 6 CDRs of a V H  and V L  sequence from antibodies that bind to a target molecule. 
     The binding properties of antibodies and antigen-binding fragments thereof can be quantified using methods well known in the art (see Davies et al., Annual Rev. Biochem. 59:439-473, 1990). In some embodiments, an antibody or antigen-binding fragment thereof specifically binds to a target molecule, for example, an IL-2 protein or an epitope or complex thereof, with an equilibrium dissociation constant that is about or ranges from about ≤10 −7  M to about 10 −8  M. In some embodiments, the equilibrium dissociation constant is about or ranges from about ≤10 −9  M to about ≤10 −10  M. In certain illustrative embodiments, an antibody or antigen-binding fragment thereof has an affinity (Kd or EC 50 ) for an IL-2 protein (to which it specifically binds) of about, at least about, or less than about, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 40, or 50 nM. 
     A molecule such as a polypeptide or antibody is said to exhibit “specific binding” or “preferential binding” if it reacts or associates more frequently, more rapidly, with greater duration and/or with greater affinity with a particular cell, substance, or particular epitope than it does with alternative cells or substances, or epitopes. An antibody “specifically binds” or “preferentially binds” to a target molecule or epitope if it binds with greater affinity, avidity, more readily, and/or with greater duration than it binds to other substances or epitopes, for example, by a statistically significant amount. Typically one member of the pair of molecules that exhibit specific binding has an area on its surface, or a cavity, which specifically binds to and is therefore complementary to a particular spatial and/or polar organization of the other member of the pair of molecules. Thus, the members of the pair have the property of binding specifically to each other. For instance, an antibody that specifically or preferentially binds to a specific epitope is an antibody that binds that specific epitope with greater affinity, avidity, more readily, and/or with greater duration than it binds to other epitopes. It is also understood by reading this definition that, for example, an antibody (or moiety or epitope) that specifically or preferentially binds to a first target may or may not specifically or preferentially bind to a second target. The term is also applicable where, for example, an antibody is specific for a particular epitope which is carried by a number of antigens, in which case the specific binding member carrying the antigen-binding fragment or domain will be able to bind to the various antigens carrying the epitope; for example, it may be cross reactive to a number of different forms of a target antigen from multiple species that share a common epitope 
     Immunological binding generally refers to the non-covalent interactions of the type which occur between an immunoglobulin molecule and an antigen for which the immunoglobulin is specific, for example by way of illustration and not limitation, as a result of electrostatic, ionic, hydrophilic and/or hydrophobic attractions or repulsion, steric forces, hydrogen bonding, van der Waals forces, and other interactions. The strength, or affinity of immunological binding interactions can be expressed in terms of the dissociation constant (Kd) of the interaction, wherein a smaller Kd represents a greater affinity. Immunological binding properties of selected polypeptides can be quantified using methods well known in the art. One such method entails measuring the rates of antigen-binding site/antigen complex formation and dissociation, wherein those rates depend on the concentrations of the complex partners, the affinity of the interaction, and on geometric parameters that equally influence the rate in both directions. Thus, both the “on rate constant” (Kon) and the “off rate constant” (Koff) can be determined by calculation of the concentrations and the actual rates of association and dissociation. The ratio of Koff/Kon enables cancellation of all parameters not related to affinity, and is thus equal to the dissociation constant Kd. As used herein, the term “affinity” includes the equilibrium constant for the reversible binding of two agents and is expressed as Kd or EC 50 . Affinity of an antibody for an IL-2 protein or epitope can be, for example, from about 100 nanomolar (nM) to about 0.1 nM, from about 100 nM to about 1 picomolar (pM), or from about 100 nM to about 1 femtomolar (fM). As used herein, the term “avidity” refers to the resistance of a complex of two or more agents to dissociation after dilution. 
     Antibodies may be prepared by any of a variety of techniques known to those of ordinary skill in the art. See, e.g., Harlow and Lane, Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratory, 1988. Monoclonal antibodies specific for a polypeptide of interest may be prepared, for example, using the technique of Kohler and Milstein, Eur. J. Immunol. 6:511-519, 1976, and improvements thereto. Also included are methods that utilize transgenic animals such as mice to express human antibodies. See, e.g., Neuberger et al., Nature Biotechnology 14:826, 1996; Lonberg et al., Handbook of Experimental Pharmacology 113:49-101, 1994; and Lonberg et al., Internal Review of Immunology 13:65-93, 1995. Particular examples include the VELOCIMMUNE® platform by REGENEREX® (see, e.g., U.S. Pat. No. 6,596,541). 
     In certain embodiments, antibodies and antigen-binding fragments thereof as described herein include a heavy chain and a light chain CDR set, respectively interposed between a heavy chain and a light chain framework region (FR) set which provide support to the CDRs and define the spatial relationship of the CDRs relative to each other. As used herein, the term “CDR set” refers to the three hypervariable regions of a heavy or light chain V region. Proceeding from the N-terminus of a heavy or light chain, these regions are denoted as “CDR1,” “CDR2,” and “CDR3” respectively. An antigen-binding site, therefore, includes six CDRs, comprising the CDR set from each of a heavy and a light chain V region. A polypeptide comprising a single CDR, (e.g., a CDR1, CDR2 or CDR3) is referred to herein as a “molecular recognition unit.” Crystallographic analysis of a number of antigen-antibody complexes has demonstrated that the amino acid residues of CDRs form extensive contact with bound antigen, wherein the most extensive antigen contact is with the heavy chain CDR3. Thus, the molecular recognition units are primarily responsible for the specificity of an antigen-binding site. 
     As used herein, the term “FR set” refers to the four flanking amino acid sequences which frame the CDRs of a CDR set of a heavy or light chain V region. Some FR residues may contact bound antigen; however, FRs are primarily responsible for folding the V region into the antigen-binding site, particularly the FR residues directly adjacent to the CDRs. Within FRs, certain amino residues and certain structural features are very highly conserved. In this regard, all V region sequences contain an internal disulfide loop of around 90 amino acid residues. When the V regions fold into a binding-site, the CDRs are displayed as projecting loop motifs which form an antigen-binding surface. It is generally recognized that there are conserved structural regions of FRs which influence the folded shape of the CDR loops into certain “canonical” structures-regardless of the precise CDR amino acid sequence. Further, certain FR residues are known to participate in non-covalent interdomain contacts which stabilize the interaction of the antibody heavy and light chains. 
     The structures and locations of immunoglobulin variable domains may be determined by reference to Kabat, E. A. et al., Sequences of Proteins of Immunological Interest. 4th Edition. US Department of Health and Human Services. 1987, and updates thereof. 
     Also include are “monoclonal” antibodies, which refer to a homogeneous antibody population wherein the monoclonal antibody is comprised of amino acids (naturally occurring and non-naturally occurring) that are involved in the selective binding of an epitope. Monoclonal antibodies are highly specific, being directed against a single epitope. The term “monoclonal antibody” encompasses not only intact monoclonal antibodies and full-length monoclonal antibodies, but also fragments thereof (such as Fab, Fab′, F(ab′)2, Fv), single chain (ScFv), variants thereof, fusion proteins comprising an antigen-binding portion, humanized monoclonal antibodies, chimeric monoclonal antibodies, and any other modified configuration of the immunoglobulin molecule that comprises an antigen-binding fragment (epitope recognition site) of the required specificity and the ability to bind to an epitope. It is not intended to be limited as regards the source of the antibody or the manner in which it is made (e.g., by hybridoma, phage selection, recombinant expression, transgenic animals). The term includes whole immunoglobulins as well as the fragments etc. described above under the definition of “antibody.” 
     The proteolytic enzyme papain preferentially cleaves IgG molecules to yield several fragments, two of which (the F(ab) fragments) each comprise a covalent heterodimer that includes an intact antigen-binding site. The enzyme pepsin is able to cleave IgG molecules to provide several fragments, including the F(ab′)2 fragment which comprises both antigen-binding sites. An Fv fragment for use according to certain embodiments can be produced by preferential proteolytic cleavage of an IgM, and on rare occasions of an IgG or IgA immunoglobulin molecule. Fv fragments are, however, more commonly derived using recombinant techniques known in the art. The Fv fragment includes a non-covalent VH::VL heterodimer including an antigen-binding site which retains much of the antigen recognition and binding capabilities of the native antibody molecule. See Inbar et al., PNAS USA. 69:2659-2662, 1972; Hochman et al., Biochem. 15:2706-2710, 1976; and Ehrlich et al., Biochem. 19:4091-4096, 1980. 
     In certain embodiments, single chain Fv (scFV) antibodies are contemplated. For example, Kappa bodies (Ill et al., Prot. Eng. 10:949-57, 1997); minibodies (Martin et al., EMBO J 13:5305-9, 1994); diabodies (Holliger et al., PNAS 90: 6444-8, 1993); or Janusins (Traunecker et al., EMBO J 10: 3655-59, 1991; and Traunecker et al., Int. J. Cancer Suppl. 7:51-52, 1992), may be prepared using standard molecular biology techniques following the teachings of the present application with regard to selecting antibodies having the desired specificity. 
     A single chain Fv (scFv) polypeptide is a covalently linked VH::VL heterodimer which is expressed from a gene fusion including VH- and VL-encoding genes linked by a peptide-encoding linker. Huston et al. (PNAS USA. 85(16):5879-5883, 1988). A number of methods have been described to discern chemical structures for converting the naturally aggregated-but chemically separated-light and heavy polypeptide chains from an antibody V region into an scFv molecule which will fold into a three dimensional structure substantially similar to the structure of an antigen-binding site. See, e.g., U.S. Pat. Nos. 5,091,513 and 5,132,405, to Huston et al.; and U.S. Pat. No. 4,946,778, to Ladner et al. 
     In certain embodiments, the antibodies or antigen-binding fragments described herein are in the form of a “diabody.” Diabodies are multimers of polypeptides, each polypeptide comprising a first domain comprising a binding region of an immunoglobulin light chain and a second domain comprising a binding region of an immunoglobulin heavy chain, the two domains being linked (e.g., by a peptide linker) but unable to associate with each other to form an antigen-binding site: antigen-binding sites are formed by the association of the first domain of one polypeptide within the multimer with the second domain of another polypeptide within the multimer (WO94/13804). A dAb fragment of an antibody consists of a VH domain (Ward et al., Nature 341:544-546, 1989). Diabodies and other multivalent or multispecific fragments can be constructed, for example, by gene fusion (see WO94/13804; and Holliger et al., PNAS USA. 90:6444-6448, 1993)). 
     Minibodies comprising a scFv joined to a CH3 domain are also included (see Hu et al., Cancer Res. 56:3055-3061, 1996). See also Ward et al., Nature. 341:544-546, 1989; Bird et al., Science. 242:423-426, 1988; Huston et al., PNAS USA. 85:5879-5883, 1988); PCT/US92/09965; WO94/13804; and Reiter et al., Nature Biotech. 14:1239-1245, 1996. 
     Where bispecific antibodies are to be used, these may be conventional bispecific antibodies, which can be manufactured in a variety of ways (Holliger and Winter, Current Opinion Biotechnol. 4:446-449, 1993), e.g., prepared chemically or from hybrid hybridomas, or may be any of the bispecific antibody fragments mentioned above. Diabodies and scFv can be constructed without an Fc region, using only variable domains, potentially reducing the effects of anti-idiotypic reaction. 
     Bispecific diabodies, as opposed to bispecific whole antibodies, may also be particularly useful because they can be readily constructed and expressed in  E. coli . Diabodies (and many other polypeptides such as antibody fragments) of appropriate binding specificities can be readily selected using phage display (WO94/13804) from libraries. If one arm of the diabody is to be kept constant, for instance, with a specificity directed against antigen X, then a library can be made where the other arm is varied and an antibody of appropriate specificity selected. Bispecific whole antibodies may be made by knobs-into-holes engineering (Ridgeway et al., Protein Eng., 9:616-621, 1996). 
     In certain embodiments, the antibodies or antigen-binding fragments described herein are in the form of a UniBody®. A UniBody® is an IgG4 antibody with the hinge region removed (see GenMab Utrecht, The Netherlands; see also, e.g., US20090226421). This antibody technology creates a stable, smaller antibody format with an anticipated longer therapeutic window than current small antibody formats. IgG4 antibodies are considered inert and thus do not interact with the immune system. Fully human IgG4 antibodies may be modified by eliminating the hinge region of the antibody to obtain half-molecule fragments having distinct stability properties relative to the corresponding intact IgG4 (GenMab, Utrecht). Halving the IgG4 molecule leaves only one area on the UniBody® that can bind to cognate antigens (e.g., disease targets) and the UniBody® therefore binds univalently to only one site on target cells. For certain cancer cell surface antigens, this univalent binding may not stimulate the cancer cells to grow as may be seen using bivalent antibodies having the same antigen specificity, and hence UniBody® technology may afford treatment options for some types of cancer that may be refractory to treatment with conventional antibodies. The small size of the UniBody® can be a great benefit when treating some forms of cancer, allowing for better distribution of the molecule over larger solid tumors and potentially increasing efficacy. 
     In certain embodiments, the antibodies and antigen-binding fragments described herein are in the form of a nanobody. Minibodies are encoded by single genes and are efficiently produced in almost all prokaryotic and eukaryotic hosts, for example,  E. coli  (see U.S. Pat. No. 6,765,087), molds (for example  Aspergillus  or  Trichoderma ) and yeast (for example  Saccharomyces, Kluyvermyces, Hansenula  or  Pichia  (see U.S. Pat. No. 6,838,254). The production process is scalable and multi-kilogram quantities of nanobodies have been produced. Nanobodies may be formulated as a ready-to-use solution having a long shelf life. The Nanoclone method (see WO 06/079372) is a proprietary method for generating Nanobodies against a desired target, based on automated high-throughput selection of B-cells. 
     Also included are heavy chain dimers, such as antibodies from camelids and sharks. Camelid and shark antibodies comprise a homodimeric pair of two chains of V-like and C-like domains (neither has a light chain). Since the VH region of a heavy chain dimer IgG in a camelid does not have to make hydrophobic interactions with a light chain, the region in the heavy chain that normally contacts a light chain is changed to hydrophilic amino acid residues in a camelid. VH domains of heavy-chain dimer IgGs are called VHH domains. Shark Ig-NARs comprise a homodimer of one variable domain (termed a V-NAR domain) and five C-like constant domains (C-NAR domains). 
     In camelids, the diversity of antibody repertoire is determined by the complementary determining regions (CDR) 1, 2, and 3 in the VH or VHH regions. The CDR3 in the camel VHH region is characterized by its relatively long length averaging 16 amino acids (Muyldermans et al., 1994, Protein Engineering 7(9): 1129). This is in contrast to CDR3 regions of antibodies of many other species. For example, the CDR3 of mouse VH has an average of 9 amino acids. Libraries of camelid-derived antibody variable regions, which maintain the in vivo diversity of the variable regions of a camelid, can be made by, for example, the methods disclosed in U.S. Patent Application Ser. No. 20050037421, published Feb. 17, 2005 
     In certain embodiments, the antibodies or antigen-binding fragments thereof are humanized. These embodiments refer to a chimeric molecule, generally prepared using recombinant techniques, having an antigen-binding site derived from an immunoglobulin from a non-human species and the remaining immunoglobulin structure of the molecule based upon the structure and/or sequence of a human immunoglobulin. The antigen-binding site may comprise either complete variable domains fused onto constant domains or only the CDRs grafted onto appropriate framework regions in the variable domains. Epitope binding sites may be wild type or modified by one or more amino acid substitutions. This eliminates the constant region as an immunogen in human individuals, but the possibility of an immune response to the foreign variable region remains (LoBuglio et al., PNAS USA 86:4220-4224, 1989; Queen et al., PNAS USA. 86:10029-10033, 1988; Riechmann et al., Nature. 332:323-327, 1988). Illustrative methods for humanization of antibodies include the methods described in U.S. Pat. No. 7,462,697. 
     Another approach focuses not only on providing human-derived constant regions, but modifying the variable regions as well so as to reshape them as closely as possible to human form. It is known that the variable regions of both heavy and light chains contain three complementarity-determining regions (CDRs) which vary in response to the epitopes in question and determine binding capability, flanked by four framework regions (FRs) which are relatively conserved in a given species and which putatively provide a scaffolding for the CDRs. When nonhuman antibodies are prepared with respect to a particular epitope, the variable regions can be “reshaped” or “humanized” by grafting CDRs derived from nonhuman antibody on the FRs present in the human antibody to be modified. Application of this approach to various antibodies has been reported by Sato et al., Cancer Res. 53:851-856, 1993; Riechmann et al., Nature 332:323-327, 1988; Verhoeyen et al., Science 239:1534-1536, 1988; Kettleborough et al., Protein Engineering. 4:773-3783, 1991; Maeda et al., Human Antibodies Hybridoma 2:124-134, 1991; Gorman et al., PNAS USA. 88:4181-4185, 1991; Tempest et al., Bio/Technology 9:266-271, 1991; Co et al., PNAS USA. 88:2869-2873, 1991; Carter et al., PNAS USA. 89:4285-4289, 1992; and Co et al., J Immunol. 148:1149-1154, 1992. In some embodiments, humanized antibodies preserve all CDR sequences (for example, a humanized mouse antibody which contains all six CDRs from the mouse antibodies). In other embodiments, humanized antibodies have one or more CDRs (one, two, three, four, five, six) which are altered with respect to the original antibody, which are also termed one or more CDRs “derived from” one or more CDRs from the original antibody. 
     In certain embodiments, the antibodies are “chimeric” antibodies. In this regard, a chimeric antibody is comprised of an antigen-binding fragment of an antibody operably linked or otherwise fused to a heterologous Fc portion of a different antibody. In certain embodiments, the Fc domain or heterologous Fc domain is of human origin. In certain embodiments, the Fc domain or heterologous Fc domain is of mouse origin. In other embodiments, the heterologous Fc domain may be from a different Ig class from the parent antibody, including IgA (including subclasses IgA1 and IgA2), IgD, IgE, IgG (including subclasses IgG1, IgG2, IgG3, and IgG4), and IgM. In further embodiments, the heterologous Fc domain may be comprised of CH2 and CH3 domains from one or more of the different Ig classes. As noted above with regard to humanized antibodies, the antigen-binding fragment of a chimeric antibody may comprise only one or more of the CDRs of the antibodies described herein (e.g., 1, 2, 3, 4, 5, or 6 CDRs of the antibodies described herein), or may comprise an entire variable domain (VL, VH or both). 
     Any one or more of the foregoing IL-2 binding proteins can be combined with any of the other components described herein, for example, IL-2 proteins, masking moieties including binding moieties and linkers, and other optional protein domains, to generate one or more activatable proproteins or larger, multi-chain structures comprising the same. 
     Masking Moieties. 
     As noted above, the activatable proproteins described herein comprise a first polypeptide and a second polypeptide, each of which comprises a “masking moiety”. That is, the first polypeptide comprises a first masking moiety, and the second polypeptide comprises a second masking moiety. The first and second masking moieties in any given activatable proprotein can be the same (or substantially the same) or different. 
     In some instances, a masking moiety dimer masks the active domain of the IL-2 protein. Hence, within the context of an activatable proprotein provided herein, when an “active domain” comprising an IL-2/IL-2 binding protein complex (for example, IL-2/IL-2Rα complex) is modified by the addition of at least one masking moiety (via one or more cleavable linkers) and is in the presence of a target (for example, IL-2Rβ/γc receptor chain), binding of the active domain to its target is blocked, reduced, or inhibited, relative to the specific binding of an equivalent active domain that is not modified by the addition of a masking moiety. 
     In some embodiments, the masking moiety allosterically inhibits the binding of the activatable proprotein to its target, for example, a cognate IL-2Rβ/γc receptor chain on the surface of an immune cell. In these and related embodiments, the activatable proprotein shows no binding or substantially no binding to its target, or no more than 0.001%, 0.01%, 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or 50% binding to its target, as compared to the binding of the active domain or the IL-2 protein alone, optionally for at least 2, 4, 6, 8, 12, 28, 24, 30, 36, 48, 60, 72, 84, 96 hours, or 5, 10, 15, 30, 45, 60, 90, 120, 150, 180 days, or 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 months or greater, optionally as measured in vivo or in a Target Displacement in vitro assay available in the art. 
     The masking moieties described herein each comprise a “binding moiety”, which facilitates the binding between the first and second masking moieties, and a “linker”, which separates each binding moiety from its respective IL-2 protein or IL-2 binding protein (e.g., IL-2Rα protein). At least one of the linkers in each activatable proprotein is a cleavable linker, which after cleavage releases at least one of the masking moieties and thereby exposes an active or binding site of the IL-2 protein, optionally at a tumor site or in a cancer tissue. The first and second binding moieties can be the same or different, and the first and second linkers can be the same or different. In some embodiments, the first masking moiety and/or the second masking moiety does/do not bind to the IL-2 protein or the IL-2 binding protein. 
     The structural properties of the binding moieties will vary according to a variety of factors, such as the minimum amino acid sequence required for interference with IL-2 binding to its target, the length of the linker between the binding moiety and the IL-2 protein or the IL-2 binding protein, the presence or absence of a cysteine within or flanking the IL-2 protein or the IL-2 binding protein that is suitable for providing dissociation of a cysteine-cysteine disulfide bond, and the like. 
     General examples of binding moieties are provided in Table M1 below. 
     
       
         
           
               
             
               
                 TABLE M1 
               
               
                   
               
               
                 Exemplary Binding Moieties 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 Short peptide 
               
               
                 Leucine zipper peptide 
               
               
                 VH 
               
               
                 VL 
               
               
                 VH-CH1 
               
               
                 VL-CL 
               
               
                 VH-CL 
               
               
                 VL-CH1 
               
               
                 CH3 
               
               
                 CH2CH3 
               
               
                 Fab-CH3 
               
               
                 Fab-CH2CH3 
               
               
                 Antigen binding domain-CH3 
               
               
                 Antigen binding domain-CH2CH3 
               
               
                 CH3 variant 
               
               
                 CH2CH3 variant 
               
               
                 Fab-CH3 variant 
               
               
                 Fab-CH2CH3 variant 
               
               
                 Antigen binding domain-CH3 variant 
               
               
                 Antigen binding domain-CH2CH3 variant 
               
               
                   
               
            
           
         
       
     
     Thus, in certain embodiments, the first binding moiety and the second binding moiety are selected from Table M1. 
     In particular embodiments, the first binding moiety and/or the second binding moiety comprise an antigen binding domain of an immunoglobulin, including antigen binding fragments and variants thereof, such as a VL domain and/or a VH domain. In some embodiments, the antigen binding domain does not bind to an antigen, for example, a human antigen. In some embodiments, the antigen binding domain binds to an antigen, for example, a human antigen. 
     In some embodiments, the first binding moiety and/or the second binding moiety comprise a constant domain of an immunoglobulin, or a fragment or variant thereof. For example, in certain embodiments the first and/or second binding moiety comprise a CH1, CH2, CH3, CH1CH3, CH2CH3, CH1CH2CH3, and/or CL domain of an immunoglobulin, including fragments and variants thereof, and combinations thereof. In some instances, the light chain (CL) is a lambda or kappa chain. In some embodiments, the constant domains present in a masking moiety or binding moiety of an activatable proprotein provided herein is glycosylated. In some embodiments, the glycosylation is N-glycosylation. In some embodiments, the glycosylation is O-glycosylation. In certain embodiments, the masking moieties comprise a knob and hole structure to improve specific binding between desired masking moiety pairs (see  FIGS. 9A-9B ). For example, in specific embodiments, the CH3 domains of a masking moiety pair comprise a knob and hole structure (see, for example, Shatz et al., MAbs. 5(6):872-881, 2013). 
     In specific embodiments, the first binding moiety and/or the second binding moiety comprise, in an N- to C-terminal orientation: (1) an antigen binding domain of an immunoglobulin, including antigen binding fragments and variants thereof; and (2) an immunoglobulin constant domain, including fragments and variants thereof, for example, a CH1, CH2, CH3, CH1CH3, CH2CH3, CH1CH2CH3, and/or CL domain of an immunoglobulin, including combinations thereof. 
     The immunoglobulin domains used herein (antigen binding domains, constant domains) optionally comprise IgG domains. However, certain embodiments comprise alternate immunoglobulins such as IgM, IgA, IgD, and IgE. Furthermore, all possible isotypes of the various immunoglobulins are also encompassed within the current embodiments. Thus, IgG1, IgG2, IgG3, etc., are all possible molecules in the binding domains. In addition to choice in selection of the type of immunoglobulin and isotype, certain embodiments comprise various hinge regions (or functional equivalents thereof). Such hinge regions provide flexibility between the different domains of the proproteins described herein. In some embodiments, the immunoglobulin portion of the binding domain (or larger masking moiety) is from an immunoglobulin class selected from IgG1, IgG2, IgG3, IgG4, IgD, IgA, and IgM. 
     As noted above, in some embodiments, the first binding moiety and the second binding moiety are different, for instance, wherein the first and second masking moieties bind together via their respective first and second binding moieties as a heterodimer. For example, in some embodiments, the first binding moiety comprises a VL and a CL domain of an immunoglobulin, and the second binding moiety comprises a VH and a CH1 domain of an immunoglobulin. In some embodiments, the first binding moiety comprises a VH and a CH1 domain of an immunoglobulin, and the second binding moiety comprises a VL and a CL domain of an immunoglobulin. Exemplary structures of this type are illustrated in  FIGS. 3A-3D . 
     In particular embodiments, the first binding moiety and the second binding moiety are the same or substantially the same, for instance, wherein the first and second masking moieties bind together via their respective first and second binding moieties as a homodimer. As one example, in some embodiments, each of the first and second binding moieties comprise a CH2 domain and a CH3 domain (see, e.g., far left structure in  FIG. 9B ). 
     Illustrative examples of exemplified masking moieties are provided in Table S3 below. 
     
       
         
           
               
             
               
                 TABLE S3 
               
             
            
               
                   
               
               
                 Exemplary Masking Moieties 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 SEQ ID 
               
               
                   
                 Name 
                 Sequence 
                 NO: 
               
               
                   
               
               
                   
                 Herceptin 
                 DIQMTQSPSSLSASVGDRVTITCRAS 
                 7 
               
               
                   
                 light 
                 QDVNTAVAWYQQKPGKAPKLLIYSAS 
                   
               
               
                   
                 chain with 
                 FLYSGVPSRFSGSRSGTDFTLTISSL 
                   
               
               
                   
                 C214S 
                 QPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                   
                 mutation 
                 EIKRTVAAPSVFIFPPSDEQLKSGTA 
                   
               
               
                   
                   
                 SVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                   
                   
                 GNSQESVTEQDSKDSTYSLSSTLTLS 
                   
               
               
                   
                   
                 KADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                   
                 FNRGES 
                   
               
               
                   
               
               
                   
                 Herceptin 
                 EVQLVESGGGLVQPGGSLRLSCAASG 
                 8 
               
               
                   
                 Fd 
                 FNIKDTYIHWVRQAPGKGLEWVARIY 
                   
               
               
                   
                 fragment 
                 PTNGYTRYADSVKGRFTISADTSKNT 
                   
               
               
                   
                   
                 AYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                   
                   
                 YAMDYWGQGTLVTVSSASTKGPSVFP 
                   
               
               
                   
                   
                 LAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                   
                   
                 VTVSWNSGALTSGVHTFPAVLQSSGL 
                   
               
               
                   
                   
                 YSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                   
                 PSNTKVDKKVEPK 
               
               
                   
               
            
           
         
       
     
     As noted above, in certain embodiments the masking moiety comprises a linker, that is, a peptide linker. In some embodiments, at least one of the linkers is a cleavable linker, for example, a cleavable linker that comprises a protease cleavage site. In some embodiments, at least one of the linkers is a non-cleavable linker, that is, a physiologically-stable linker. 
     In some embodiments, the first linker and/or the second linker are about 1-50 1-40, 1-30, 1-20, 1-10, 1-5, 1-4, 1-3 amino acids in length, or about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50 amino acids in length. In particular embodiments, the first linker is a cleavable linker, and the second linker is a non-cleavable linker. In some embodiments, the first linker is a non-cleavable linker, and the second linker is a cleavable linker. 
     In some embodiments, a cleavable linker comprises at least one protease cleavage site. Suitable protease cleavages sites and self-cleaving peptides are known to the skilled person (see, e.g., Ryan et al., J. Gener. Virol. 78:699-722, 1997; and Scymczak et al., Nature Biotech. 5:589-594, 2004). In some embodiments, the protease cleavage site is cleavable by a protease selected from one or more of a metalloprotease, a serine protease, a cysteine protease, and an aspartic acid protease. In particular embodiments, the protease cleavage site is cleavable by a protease selected from one or more of MMP1, MMP2, MMP3, MMP4, MMP5, MMP6, MMP7, MMP8, MMP9, MMP10, MMP11, MMP12, MMP13, MMP14, TEV protease, matriptase, uPA, FAP, Legumain, PSA, Kallikrein, Cathepsin A, and Cathepsin B. 
     Examples of cleavable linkers are provided in Table S4 below. 
     
       
         
           
               
             
               
                 TABLE S4 
               
             
            
               
                   
               
               
                 Exemplary cleavable linkers 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 SEQ 
               
               
                   
                   
                   
                 ID 
               
               
                   
                 Name 
                 Sequence 
                 NO: 
               
               
                   
               
               
                   
                 CTMX 
                 GSLSGRSDNHGS 
                 112 
               
               
                   
               
               
                   
                 P51 
                 GSLGGSGRSANAGS 
                 113 
               
               
                   
               
               
                   
                 PS2 
                 GGSLSGRSANAGGS 
                 114 
               
               
                   
               
               
                   
                 PS3 
                 GPLGLAGRSANAGS 
                 115 
               
               
                   
               
               
                   
                 PS4 
                 PLGLSGRSANAGPA 
                 116 
               
               
                   
               
               
                   
                 PS5 
                 PLGLAGRSANAGPA 
                 117 
               
               
                   
               
               
                   
                 PS6 
                 GPLGLSGRSANAGPASG 
                 118 
               
               
                   
               
               
                   
                 PS7 
                 GPLGLAGRSANAGPASG 
                 119 
               
               
                   
               
               
                   
                 PS8 
                 SGPLGLAGRSANAGPAS 
                 120 
               
               
                   
               
               
                   
                 PS9 
                 SGPASGRSANAPLGLAG 
                 121 
               
               
                   
               
               
                   
                 PS10 
                 GSGPASGRSANAPLGLAGS 
                 122 
               
               
                   
               
               
                   
                 PS11 
                 GPLGLAGRSANPGPASG 
                 123 
               
               
                   
               
               
                   
                 PS12 
                 GPLGLAGRSDNHGPASG 
                 124 
               
               
                   
               
               
                   
                 PS13 
                 GPLGLAGRSDNPGPASG 
                 125 
               
               
                   
               
               
                   
                 PS14 
                 GPLGLAGRSENPGPASG 
                 126 
               
               
                   
               
               
                   
                 PS15 
                 GPLGLAGRSDNLGPASG 
                 127 
               
               
                   
               
               
                   
                 PS16 
                 GPLGLAGRNAQVGPASG 
                 128 
               
               
                   
               
               
                   
                 PS17 
                 GSLSGRSDNAGS 
                 129 
               
               
                   
               
               
                   
                 PS18 
                 GSLSGRSDNDGS 
                 130 
               
               
                   
               
               
                   
                 PS19 
                 GSLSGRSDNEGS 
                 131 
               
               
                   
               
               
                   
                 PS20 
                 GSLSGRSDNFGS 
                 132 
               
               
                   
               
               
                   
                 PS21 
                 GSLSGRSDNGGS 
                 133 
               
               
                   
               
               
                   
                 PS22 
                 GSLSGRSDNIGS 
                 134 
               
               
                   
               
               
                   
                 PS23 
                 GSLSGRSDNKGS 
                 135 
               
               
                   
               
               
                   
                 PS24 
                 GSLSGRSDNLGS 
                 136 
               
               
                   
               
               
                   
                 PS25 
                 GSLSGRSDNMGS 
                 137 
               
               
                   
               
               
                   
                 PS26 
                 GSLSGRSDNNGS 
                 138 
               
               
                   
               
               
                   
                 PS27 
                 GSLSGRSDNPGS 
                 139 
               
               
                   
               
               
                   
                 PS28 
                 GSLSGRSDNQGS 
                 140 
               
               
                   
               
               
                   
                 PS29 
                 GSLSGRSDNRGS 
                 141 
               
               
                   
               
               
                   
                 PS30 
                 GSLSGRSDNSGS 
                 142 
               
               
                   
               
               
                   
                 PS31 
                 GSLSGRSDNTGS 
                 143 
               
               
                   
               
               
                   
                 PS32 
                 GSLSGRSDNVGS 
                 144 
               
               
                   
               
               
                   
                 PS33 
                 GSLSGRSDNWGS 
                 145 
               
               
                   
               
               
                   
                 PS34 
                 GSLSGRSDNYGS 
                 146 
               
               
                   
               
               
                   
                 PS35 
                 GSLSGRSANDGS 
                 147 
               
               
                   
               
               
                   
                 PS36 
                 GSLSGRSANEGS 
                 148 
               
               
                   
               
               
                   
                 PS37 
                 GSLSGRSANFGS 
                 149 
               
               
                   
               
               
                   
                 PS38 
                 GSLSGRSANGSS 
                 150 
               
               
                   
               
               
                   
                 PS39 
                 GSLSGRSANHGS 
                 151 
               
               
                   
               
               
                   
                 PS40 
                 GSLSGRSANIGS 
                 152 
               
               
                   
               
               
                   
                 PS41 
                 GSLSGRSANKGS 
                 153 
               
               
                   
               
               
                   
                 PS42 
                 GSLSGRSANLGS 
                 154 
               
               
                   
               
               
                   
                 PS43 
                 GSLSGRSANMGS 
                 155 
               
               
                   
               
               
                   
                 PS44 
                 GSLSGRSANNGS 
                 156 
               
               
                   
               
               
                   
                 PS45 
                 GSLSGRSANPGS 
                 157 
               
               
                   
               
               
                   
                 PS46 
                 GSLSGRSANQSS 
                 158 
               
               
                   
               
               
                   
                 PS47 
                 GSLSGRSANRGS 
                 159 
               
               
                   
               
               
                   
                 PS48 
                 GSLSGRSANSGS 
                 160 
               
               
                   
               
               
                   
                 PS49 
                 GSLSGRSANTGS 
                 161 
               
               
                   
               
               
                   
                 PS50 
                 GSLSGRSANVGS 
                 162 
               
               
                   
               
               
                   
                 PS51 
                 GSLSGRSANWGS 
                 163 
               
               
                   
               
               
                   
                 PS52 
                 GSLSGRSANYSS 
                 164 
               
               
                   
               
               
                   
                 PS12b 
                 GPLGLAGRSDNHSG 
                 165 
               
               
                   
               
               
                   
                 PS53 
                 PLGLAGSGRSDNR 
                 166 
               
               
                   
               
               
                   
                 PS54 
                 PLGLAGSGRSDNRGS 
                 167 
               
               
                   
               
               
                   
                 PS55 
                 GSPLGLAGSGRSDNRGS 
                 168 
               
               
                   
               
               
                   
                 PS103 
                 PLGLAGSGRSDNRGA 
                 169 
               
               
                   
               
               
                   
                 PS104 
                 PLGLAGSGRSDNQGA 
                 170 
               
               
                   
               
               
                   
                 PS105 
                 PLGLAGSGRSDNYGA 
                 171 
               
               
                   
               
               
                   
                 PS106 
                 GPLGLAGSGRSDNQG 
                 172 
               
               
                   
               
               
                   
                 PS107 
                 GSPLGLAGSGRSDNQGA 
                 173 
               
               
                   
               
               
                   
                 PS108 
                 GGSPLGLAGSGRSDNQGGA 
                 174 
               
               
                   
               
               
                   
                 PS109 
                 GGGSPLGLAGSGRSDNQGGGA 
                 175 
               
               
                   
               
               
                   
                 P5110 
                 GGSGSPLGLAGSGRSDNQGGGGA 
                 176 
               
               
                   
               
               
                   
                 PS111 
                 GGSGGSPLGLAGSGRSDNQGGSGGA 
                 177 
               
               
                   
               
               
                   
                 PS112 
                 GPLGLAGSGRSDNRG 
                 178 
               
               
                   
               
               
                   
                 PS113 
                 GSPLGLAGSGRSDNRGA 
                 179 
               
               
                   
               
               
                   
                 PS114 
                 GGSPLGLAGSGRSDNRGGA 
                 180 
               
               
                   
               
               
                   
                 PS115 
                 GGGSPLGLAGSGRSDNRGGGA 
                 181 
               
               
                   
               
               
                   
                 PS116 
                 GGSGSPLGLAGSGRSDNRGGGGA 
                 182 
               
               
                   
               
               
                   
                 PS117 
                 GGSGGSPLGLAGSGRSDNRGGSGGA 
                 183 
               
               
                   
               
               
                   
                 PS118 
                 GGSGGSPLGLAGSGRSDNHGGSGGA 
                 184 
               
               
                   
               
            
           
         
       
     
     Thus, in certain embodiment, a cleavable linker is selected from Table S4. Additional examples of cleavable linkers include an amino acid sequence cleaved by a serine protease such as thrombin, chymotrypsin, trypsin, elastase, kallikrein, or subtilisin. Illustrative examples of thrombin-cleavable amino acid sequences include, but are not limited to: -Gly-Arg-Gly-Asp-(SEQ ID NO:185), -Gly-Gly-Arg-, -Gly-Arg-Gly-Asp-Asn-Pro-(SEQ ID NO:186), -Gly-Arg-Gly-Asp-Ser-(SEQ ID NO: 187), -Gly-Arg-Gly-Asp-Ser-Pro-Lys-(SEQ ID NO:188), -Gly-Pro-Arg-, -Val-Pro-Arg-, and -Phe-Val-Arg-. Illustrative examples of elastase-cleavable amino acid sequences include, but are not limited to: -Ala-Ala-Ala-, -Ala-Ala-Pro-Val-(SEQ ID NO: 189), -Ala-Ala-Pro-Leu-(SEQ ID NO: 190), -Ala-Ala-Pro-Phe-(SEQ ID NO:191), -Ala-Ala-Pro-Ala-(SEQ ID NO:192), and -Ala-Tyr-Leu-Val-(SEQ ID NO:193). 
     Cleavable linkers also include amino acid sequences that can be cleaved by a matrix metalloproteinase such as collagenase, stromelysin, and gelatinase. Illustrative examples of matrix metalloproteinase-cleavable amino acid sequences include, but are not limited to: -Gly-Pro-Y-Gly-Pro-Z-(SEQ ID NO:194), -Gly-Pro-, Leu-Gly-Pro-Z-(SEQ ID NO:195), -Gly-Pro-Ile-Gly-Pro-Z-(SEQ ID NO: 1960, and -Ala-Pro-Gly-Leu-Z-(SEQ ID NO: 197), where Y and Z are amino acids. Illustrative examples of collagenase-cleavable amino acid sequences include, but are not limited to: -Pro-Leu-Gly-Pro-D-Arg-Z-(SEQ ID NO: 198), -Pro-Leu-Gly-Leu-Leu-Gly-Z-(SEQ ID NO: 199), -Pro-Gln-Gly-Ile-Ala-Gly-Trp-(SEQ ID NO: 200), -Pro-Leu-Gly-Cys(Me)-His-(SEQ ID NO: 201), -Pro-Leu-Gly-Leu-Tyr-Ala-(SEQ ID NO: 202), -Pro-Leu-Ala-Leu-Trp-Ala-Arg-(SEQ ID NO: 203), and -Pro-Leu-Ala-Tyr-Trp-Ala-Arg-(SEQ ID NO: 204), where Z is an amino acid. An illustrative example of a stromelysin-cleavable amino acid sequence is -Pro-Tyr-Ala-Tyr-Tyr-Met-Arg-(SEQ ID NO: 205); and an example of a gelatinase-cleavable amino acid sequence is -Pro-Leu-Gly-Met-Tyr-Ser-Arg-(SEQ ID NO: 206). 
     Cleavable linkers also include amino acid sequences that can be cleaved by an angiotensin converting enzyme, such as, for example, -Asp-Lys-Pro-, -Gly-Asp-Lys-Pro-(SEQ ID NO: 207), and -Gly-Ser-Asp-Lys-Pro-(SEQ ID NO: 208). Cleavable linkers also include amino acid sequences that can be degraded by cathepsin B, such as, for example, Val-Cit, Ala-Leu-Ala-Leu-(SEQ ID NO: 209), Gly-Phe-Leu-Gly-(SEQ ID NO: 210) and Phe-Lys. 
     In particular embodiments, a cleavable linker has a half life at pH 7.4, 25° C., for example, at physiological pH, human body temperature (e.g., in vivo, in serum, in a given tissue), of about or less than about 30 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 12 hours, 18 hours, 24 hours, 36 hours, 48 hours, 72 hours, or 96 hours, or any intervening half-life. 
     Typically, at least one of the first or second linker is a non-cleavable linker. Exemplary non-cleavable linkers include those disclosed in Maratea et al., Gene 40:39-46, 1985; Murphy et al., PNAS USA. 83:8258-8262, 1986; U.S. Pat. Nos. 4,935,233 and 4,751,180. Particular non-cleavable linker sequences contain Gly, Ser, and/or Asn residues. Other near neutral amino acids, such as Thr and Ala may also be employed in the peptide linker sequence, if desired. 
     Certain exemplary non-cleavable linkers include Gly, Ser and/or Asn-containing linkers, as follows: [G] x , [S] x , [N] x , [GS] x , [GGS] x , [GSS] x , [GSGS] x (SEQ ID NO:211), [GGSG] x (SEQ ID NO: 212), [GGGS] x (SEQ ID NO: 213), [GGGGS] x (SEQ ID NO: 214), [GN] x , [GGN] x , [GNN] x , [GNGN] x (SEQ ID NO: 215), [GGNG] x (SEQ ID NO: 216), [GGGN] x (SEQ ID NO: 217), [GGGGN] x  (SEQ ID NO: 218) linkers, where  x  is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 or more. Other combinations of these and related amino acids will be apparent to persons skilled in the art. 
     Additional examples of non-cleavable linkers include the following amino acid sequences: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 219) 
               
               
                   
                 Gly-Gly-Gly-Gly-Ser-Gly-Gly-Gly- 
               
               
                   
               
               
                   
                 Gly-Ser-Gly-Gly-Gly-Gly-Ser-; 
               
               
                   
               
               
                   
                 (SEQ ID NO: 220) 
               
               
                   
                 Gly-Ser-Gly-Gly-Gly-Gly-Ser-Gly- 
               
               
                   
               
               
                   
                 Gly-Gly-Gly-Ser-Gly-Gly-Gly-Gly- 
               
               
                   
               
               
                   
                 Ser-Gly-Gly-Gly-Gly-Ser-; 
               
               
                   
               
               
                   
                 (SEQ ID NO: 221) 
               
               
                   
                 Gly-Gly-Gly-Gly-Ser-Gly-Gly-Gly- 
               
               
                   
               
               
                   
                 Gly-Ser-Gly-Gly-Gly-Gly-Ser-Gly- 
               
               
                   
               
               
                   
                 Gly-Gly-Gly-Ser-Gly-Gly-Gly-Gly- 
               
               
                   
               
               
                   
                 Ser-Gly-Gly-Gly-Gly-Ser-; 
               
               
                   
               
               
                   
                 (SEQ ID NO: 222) 
               
               
                   
                 Asp-Ala-Ala-Ala-Lys-Glu-Ala-Ala- 
               
               
                   
               
               
                   
                 Ala-Lys-Asp-Ala-Ala-Ala-Arg-Glu- 
               
               
                   
               
               
                   
                 Ala-Ala-Ala-Arg-Asp-Ala-Ala-Ala- 
               
               
                   
               
               
                   
                 Lys-; 
               
               
                   
                 and 
               
               
                   
               
               
                   
                 (SEQ ID NO: 223) 
               
               
                   
                 Asn-Val-Asp-His-Lys-Pro-Ser-Asn- 
               
               
                   
               
               
                   
                 Thr-Lys-Val-Asp-Lys-Arg-. 
               
            
           
         
       
     
     Further non-limiting examples of non-cleavable linkers include DGGGS (SEQ ID NO: 224); TGEKP (SEQ ID NO: 225) (see, e.g., Liu et al., PNAS. 94:5525-5530, 1997); GGRR (SEQ ID NO: 226) (Pomerantz et al. 1995); (GGGGS) n  (SEQ ID NO: 214) (Kim et al., PNAS. 93:1156-1160, 1996); EGKSSGSGSESKVD (SEQ ID NO: 227) (Chaudhary et al., PNAS. 87:1066-1070, 1990); KESGSVSSEQLAQFRSLD (SEQ ID NO: 228) (Bird et al., Science. 242:423-426, 1988), GGRRGGGS (SEQ ID NO: 229); LRQRDGERP (SEQ ID NO: 230); LRQKDGGGSERP (SEQ ID NO: 231); LRQKd(GGGS) 2  ERP (SEQ ID NO: 232). In specific embodiments, the linker comprises a Gly3 linker sequence, which includes three glycine residues. In particular embodiments, flexible linkers can be rationally designed using a computer program capable of modeling both DNA-binding sites and the peptides themselves (Desjarlais &amp; Berg,  PNAS.  90:2256-2260, 1993; and PNAS. 91:11099-11103, 1994) or by phage display methods. 
     In some embodiment, a linker comprises an immunoglobulin (Ig)/antibody hinge region or fragment thereof, for example, a hinge region obtained or derived from an IgG1 antibody. In some embodiments, the term Ig “hinge” region refers to a polypeptide comprising an amino acid sequence that shares sequence identity, or similarity, with a portion of a naturally-occurring Ig hinge region sequence, which optionally includes the cysteine residues at which the disulfide bonds link the two heavy chains of the immunoglobulin. Sequence similarity of the hinge region linkers of the present invention with naturally-occurring immunoglobulin hinge region amino acid sequences can range from at least 50% to about 75-80%, and typically greater than about 90%. 
     In some embodiments, the linker comprises a spacer element and a cleavable element so as to make the cleavable element more accessible to the enzyme responsible for cleavage. 
     Any one or more of the foregoing linkers can be combined with any one or more of the binding moieties described herein, to form a masking moiety, which can be combined with any one or more of the IL-2 proteins and/or IL-2 binding proteins described herein, to form an activatable proprotein of the disclosure. 
     Additional Domains. 
     Certain activatable proproteins comprise one or more additional domains, for example, binding domains (see, for example,  FIG. 2D  and  FIGS. 9A-9B ). In some embodiments the first polypeptide further comprises a protein domain A at the free terminus of the first masking moiety and/or a protein domain B at the free terminus of the IL-2 protein. In some embodiments, the second polypeptide further comprises a protein domain C at the free terminus of the second masking moiety and/or a protein domain D at the free terminus of the IL-2 binding protein. 
     In some embodiments, the protein domains A-D are the same or different. In particular embodiments, the protein domains A-D are selected from one or more of cell receptor targeting moieties optionally bi-specific targeting moieties, antigen binding domains optionally bi-specific antigen binding domains, cell membrane receptor extracellular domains (ECDs), Fc domains, human serum albumin (HSA), Fc binding domains, HSA binding domains, cytokines, chemokines, and soluble protein ligands 
     In some embodiments, the one or more additional protein domains can be used to form complexes of two, three, four, five, or more activatable proproteins, which are bound to together via the additional domain(s). Examples of such complexes are provided in  FIGS. 9A-9B . 
     Illustrative examples of activatable proproteins and their expected cleavage products are provided in Table S5 below (see also the Examples). 
     
       
         
           
               
               
               
             
               
                 TABLE S5 
               
               
                   
               
               
                   
                   
                 SEQ ID 
               
               
                 Name 
                 Sequence 
                 NO: 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 Exemplary Activatable Proproteins and Expected Cleavage Products 
               
               
                 P13541362 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 9 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGESGSAPTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITFSQSIISTLTGGGGSHHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 85 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL-2Rcx 
                 PSNTKVDKKVEPKGGSENLYFQGGGSELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                   
                 NCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13541362 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 10 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGESGSAPTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITFSQSIISTLTGGGGSHHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 11 
               
               
                 Herceptin- 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 _ 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTG 
                 12 
               
               
                 gggs-5l- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2 Rex 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13551363 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 13 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-K35C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGESGSAPTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPCLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITFSQSIISTLTGGGGSHHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 86 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPKGGSENLYFQGGGSELCCDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-D04C 
                 NCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13551363 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 14 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-K35C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNP C LTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 15 
               
               
                 Herceptin- 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCCDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTG 
                 16 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-D04C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13561364 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 17 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-R38C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTCMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 87 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDD C PPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-D06C 
                 NCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13561364 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 18 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKKRTVAAPSVFIFPPSDEQLKSGTASWCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-R38C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTCMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 19 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    EL C DDCPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTG 
                 20 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-D06C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13561371 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 21 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-R38C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTCMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 88 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK    ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-H120C 
                 NCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYCFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13561371 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 22 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-R38C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEH1LLDLQMILNGINNYKNPKLTCMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 23 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTG 
                 24 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-H120C 
                 PGHCREPPPWENEATERIYCFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13571370 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 25 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-T41C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLCFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 89 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPKGGSENLYFQGGGSELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rcc-I118C 
                 NCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERCYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13571370 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 26 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-T41C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLCFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 27 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTG 
                 28 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-I118C 
                 PGHCREPPPWENEATERCYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13581365 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 29 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-F42C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTCKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 90 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGT1VTV3SASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPKGGSEN1YFQGGGSELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rcx-N27C 
                   C CECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13581365 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 30 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-F42C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTCKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 31 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLCCECKRGFRRIKSGSLYMLCTG 
                 32 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-N27C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13581369 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 33 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-F42C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTCKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 91 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-Y43C 
                 NCECKRGFRRIK S    LCMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13581369 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 34 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-F42C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTCKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 35 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPKGGSENLYFQ 
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLCMLCTG 
                 36 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-Y43C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591366 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 37 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLECELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 92 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  GGGSELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-K38C 
                 NCECKRGFRRICSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591366 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 38 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 39 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRICSGSLYMLCTG 
                 40 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-K38C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591367 Activatable Proprotein 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 41 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGESGSAPTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLECELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 93 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-S39C 
                 NCECKRGFRRIKCGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591367 Cleaved Proteins (TEV Cleavage) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 42 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 43 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKCGSLYMLCTG 
                 44 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-S39C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13601368 Activatable Proprotein 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 45 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-V69C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEE C LNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT    HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 94 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rcx-L42C 
                 NCECKRGFRRIKSGSCYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13601368 Cleaved Proteins (TEV Cleavage) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 46 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-V69C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLEEELKPLEECLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 47 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSCYMLCTG 
                 48 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-L42C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13611362 Activatable Proprotein 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 49 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-V69A- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 Q74P- 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 I128T- 
                 YMPKKATELKHLQCLEEELKPLEE A LNLA P SKNFHLRPRDLISNINVIVLEL 
                   
               
               
                 GGGGSHHHHH 
                 KGSETTFMCEYADETATIVEFLNRWITF S QS T ISTLT  HHHHHH 
                   
               
               
                 H 
                   
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 95 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL-2Rα 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                   
                 NCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13611362 Cleaved Proteins (TEV Cleavage) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 50 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-V69A- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 Q74P- 
                 FNRGESGSAPTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 I128T- 
                 YMPKKATELKHLQCLEEELKPLEE A LNLA P SKNFHLRPRDLISNINVIVLEL 
                   
               
               
                 GGGGSHHHHH 
                 KGSETTFMCEYADETATIVEFLNRWITF S QS T ISTLT  HHHHHH 
                   
               
               
                 H 
                   
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 51 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTG 
                 52 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
            
           
           
               
            
               
                 P15071366 Activatable Proprotein 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 53 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GSGS- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 IL-2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTF 
                   
               
               
                 H 
                 KFYMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVL 
                   
               
               
                   
                 ELKGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 96 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-K38C 
                 NCECKRGFRRI C SGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P15071366 Cleaved Proteins (TEV Cleavage) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 54 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GSGS- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 IL-2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTF 
                   
               
               
                 H 
                 KFYMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVL 
                   
               
               
                   
                 ELKGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 55 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRICSGSLYMLCTG 
                 56 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-K38C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P15081366 Activatable Proprotein 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 57 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GGSGGS- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 IL-2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRML 
                   
               
               
                 H 
                 TFKFYMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVI 
                   
               
               
                   
                 VLELKGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 97 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  GGGSELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-K38C 
                 NCECKRGFRRICSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P15081366 Cleaved Proteins (TEV Cleavage) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 58 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GGSGGS- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 IL-2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRML 
                   
               
               
                 H 
                 TFKFYMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVI 
                   
               
               
                   
                 VLELKGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 59 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRICSGSLYMLCTG 
                 60 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-K38C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P15091366 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 61 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 GGGSGGGGS- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 IL-2-E61C- 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTR 
                   
               
               
                 GGGGSHHHHH 
                 MLTFKFYMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNIN 
                   
               
               
                 H 
                 VIVLELKGSETTFMCEYADETATIVEFLNRWITF S QS11STLT  HHHH 
                   
               
               
                   
                 HH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 98 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-K38C 
                 NCECKRGFRRI C SGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P15091366 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 62 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 GGGSGGGGS- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 IL-2-E61C- 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTR 
                   
               
               
                 GGGGSHHHHH 
                 MLTFKFYMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNIN 
                   
               
               
                 H 
                 VIVLELKGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHH 
                   
               
               
                   
                 HH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 63 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRICSGSLYMLCTG 
                 64 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-K38C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P15101366 Activatable Proprotein 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 65 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 ElKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 D10- 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 GGGGSHHHHH 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLA H SKNFH FD PRD VV SNINV F VLEL 
                   
               
               
                 H 
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 99 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-K38C 
                 NCECKRGFRRI C SGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                   
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P15101366 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 66 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 D10- 
                 FNRGESGSAPTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 GGGGSHHHHH 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLA H SKNFH FD PRD VV SNINV F VLEL 
                   
               
               
                 H 
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 67 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRICSGSLYMLCTG 
                 68 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-K38C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P16121613 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 69 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 GGSENLYFQG 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGS-IL- 
                 FNRGE S    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRG 
                   
               
               
                 2Rα-K38C 
                 FRRI C SGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKT 
                   
               
               
                   
                 TEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRA 
                   
               
               
                   
                 LHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 100 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd-GS-IL- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 2-E61C- 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGGGSHHHHH 
                 PSNTKVDKKVEPK  APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLT 
                   
               
               
                 H 
                 RMLTFKFYMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNI 
                   
               
               
                   
                 NVIVLELKGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHH 
                   
               
               
                   
                 HHH 
                   
               
               
                   
               
            
           
           
               
            
               
                 P16121613 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 70 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 GGSENLYFQ 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGE S     
                   
               
               
                   
               
               
                 Chain 2 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRI C SGSLYMLCTG 
                 71 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-K38C 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                   
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chain 3 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 72 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd-GS-IL- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 2-E61C- 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGGGSHHHHH 
                 PSNTKVDKKVEPK  APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLT 
                   
               
               
                 H 
                 RMLTFKFYMPKKATELKHLQCLECELKPLEEVLNLAQSKNFHLRPRDLISNI 
                   
               
               
                   
                 NVIVLELKGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHH 
                   
               
               
                   
                 HHH 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591627 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 73 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 101 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-K38C- 
                 NCECKRGFRRI C SGSLYMLCTG A SSHSSWDNQCQCTSSATRNTTKQVTPQPE 
                   
               
               
                 N49A 
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591627 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 74 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 75 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRI C SGSLYMLCTG 
                 76 
               
               
                 GGGS-IL- 
                   A SSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rcx-K38C- 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                 N49A 
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591628 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 77 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 102 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-K38C- 
                 NCECKRGFRRI C SGSLYMLCTGNSSHSSWDNQCQCTSSATR A TTKQVTPQPE 
                   
               
               
                 N68A 
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591628 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 78 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGESGSAPTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLECELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 79 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRICSGSLYMLCTG 
                 80 
               
               
                 GGGS-IL- 
                 NSSHSSWDNQCQCTSSATR A TTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-K38C- 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                 N68A 
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591629 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 81 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 103 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQG 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                 GGS-IL- 
                 PSNTKVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTML 
                   
               
               
                 2Rα-K38C- 
                 NCECKRGFRRI C SGSLYMLCTG A SSHSSWDNQCQCTSSATR A TTKQVTPQPE 
                   
               
               
                 N49A-N68A 
                 EQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQ 
                   
               
               
                   
                 CVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P13591629 Cleaved Proteins (TEV Cleavage) 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSAS 
                 82 
               
               
                 Herceptin 
                 FLYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKV 
                   
               
               
                 LC-GS-IL- 
                 EIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQS 
                   
               
               
                 2-E61C- 
                 GNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                 GGGGSHHHHH 
                 FNRGE S    APTSSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKF 
                   
               
               
                 H 
                 YMPKKATELKHLQCLE C ELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLEL 
                   
               
               
                   
                 KGSETTFMCEYADETATIVEFLNRWITF S QSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIY 
                 83 
               
               
                 Herceptin 
                 PTNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGF 
                   
               
               
                 Fd- 
                 YAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEP 
                   
               
               
                 GGSENLYFQ 
                 VTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHK 
                   
               
               
                   
                 PSNTKVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRI C SGSLYMLCTG 
                 84 
               
               
                 GGGS-IL- 
                   A SSHSSWDNQCQCTSSATR A TTKQVTPQPEEQKERKTTEMQSPMQPVDQASL 
                   
               
               
                 2Rα-K38C- 
                 PGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHG 
                   
               
               
                 N49A-N68A 
                 KTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 Exemplary Activatable Proproteins 
               
               
                 P16841362 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSASF 
                 233 
               
               
                 Herceptin 
                 LYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKVEI 
                   
               
               
                 LC-GS-IL- 
                 KRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNS 
                   
               
               
                 2-T3A- 
                 QESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRG 
                   
               
               
                 V69A-Q74 P- 
                 ES  AP A SSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKK 
                   
               
               
                 I128T - 
                 ATELKHLQCLEEELKPLEE A LNLA P SKNFHLRPRDLISNINVIVLELKGSETT 
                   
               
               
                 GGGGSHHHHH 
                 FMCEYADETATIVEFLNRWITFSQS T ISTLT  HHHHHH 
                   
               
               
                 H 
                   
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIYP 
                 234 
               
               
                 Herceptin 
                 TNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGFYA 
                   
               
               
                 Fd- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVTV 
                   
               
               
                 GGSENLYFQG 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT 
                   
               
               
                 GGS-IL-2Rα 
                 KVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTMLNCECK 
                   
               
               
                   
                 RGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERK 
                   
               
               
                   
                 TTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRA 
                   
               
               
                   
                 LHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P16841687 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSASF 
                 235 
               
               
                 Herceptin 
                 LYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKVEI 
                   
               
               
                 LC-GS-IL- 
                 KRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNS 
                   
               
               
                 2-T3A- 
                 QESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRG 
                   
               
               
                 V69A-Q74 P- 
                 ES  AP A SSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKK 
                   
               
               
                 I128T - 
                 ATELKHLQCLEEELKPLEE A LNLA P SKNFHLRPRDLISNINVIVLELKGSETT 
                   
               
               
                 GGGGSHHHHH 
                 FMCEYADETATIVEFLNRWITFSQS T ISTLT  HHHHHH 
                   
               
               
                 H 
                   
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIYP 
                 236 
               
               
                 Herceptin 
                 TNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGFYA 
                   
               
               
                 Fd- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVTV 
                   
               
               
                 GGSENLYFQG 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT 
                   
               
               
                 GGS-IL- 
                 KVDKKVEPK  ELCDDDPPEIPHATFKAMAYKEGTMLNCECK 
                   
               
               
                 2Rα- 
                 RGFRRIKSGSLYMLCTG N SSHSSWDNQCQCTSSATR N TTKQVTPQPEEQKERK 
                   
               
               
                 SSDKTHTCPP 
                 TTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRA 
                   
               
               
                 CP-Fc- 
                 LHRGPAESVCKMTHGKTRWTQPQLICTGESSDKTHTCPPCPAPE AA GGPSVFL 
                   
               
               
                 L234A- 
                 FPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTKPREEQ 
                   
               
               
                 L235A- 
                 YNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKAL A AP1EKTISKAKGQPREPQV 
                   
               
               
                 P329A 
                 YTLPPSRDELTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSD 
                   
               
               
                   
                 GSFFLYSKLTVDKSRWQQGNVFSCSVMHEALHNHYTQKSLSLSPGK 
                   
               
               
                   
               
            
           
           
               
            
               
                 P16851363 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSASF 
                 237 
               
               
                 Herceptin 
                 LYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKVEI 
                   
               
               
                 LC-GS-IL- 
                 KRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNS 
                   
               
               
                 2-T3A- 
                 QESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRG 
                   
               
               
                 K35C-V69A- 
                 ES  AP A SSSTKKTQLQLEHLLLDLQMILNGINNYKNPCLTRMLTFKFYMPKK 
                   
               
               
                 Q74P- 
                 ATELKHLQCLEEELKPLEE A LNLA P SKNFHLRPRDLISNINVIVLELKGSETT 
                   
               
               
                 I128T- 
                 FMCEYADETATIVEFLNRWITFSQS T ISTLT  HHHHHH 
                   
               
               
                 GGGGSHHHHH 
                   
                   
               
               
                 H 
                   
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIYP 
                 238 
               
               
                 Herceptin 
                 TNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGFYA 
                   
               
               
                 Fd- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVTV 
                   
               
               
                 GGSENLYFQG 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT 
                   
               
               
                 GGS-IL- 
                 KVDKKVEPK  ELC C DDPPEIPHATFKAMAYKEGTMLNCECK 
                   
               
               
                 2Rα-D4C 
                 RGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERK 
                   
               
               
                   
                 TTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRA 
                   
               
               
                   
                 LHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
            
           
           
               
            
               
                 P16931694 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSASF 
                 239 
               
               
                 Herceptin 
                 LYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKVEI 
                   
               
               
                 LC-GS-IL- 
                 KRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNS 
                   
               
               
                 2-T3A- 
                 QESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRG 
                   
               
               
                 GGGGSHHHHH 
                 ES  AP A SSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKK 
                   
               
               
                 H 
                 ATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLELKGSETT 
                   
               
               
                   
                 FMCEYADETATIVEFLNRWITFSQSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIYP 
                 240 
               
               
                 Herceptin 
                 TNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGFYA 
                   
               
               
                 Fd- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVTV 
                   
               
               
                 GGSENLYFQG 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT 
                   
               
               
                 GGS-NARA1- 
                 KVDKKVEPK  AIRLTQSPSSFSASTGDRVTITCKASQSVDY 
                   
               
               
                 scFv 
                 QGDSYMNWYQQKPGKAPKLLIYSASNLESGVPSRFSGSGSGTDFTLTISSLQS 
                   
               
               
                   
                 EDFATYYCQQSNEDPYTFGGGTKVEIK  EVQLVQSGAEV 
                   
               
               
                   
                 POPV) 
                   
               
               
                   
                 KKPGESLKISCKGSGYAFTNYLIEWVRQMPGKGLEWMGVINPGSGGTOYNEKF 
                   
               
               
                   
                 KGQVTISADKSISTAYLQWSSLKASDTAMYYCARWRGEGYYAYYDVWGQGTTV 
                   
               
               
                   
                 TVSS 
                   
               
               
                   
               
            
           
           
               
            
               
                 P16931695 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSASF 
                 241 
               
               
                 Herceptin 
                 LYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKVEI 
                   
               
               
                 LC-GS-IL- 
                 KRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNS 
                   
               
               
                 2-T3A- 
                 QESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRG 
                   
               
               
                 GGGGSHHHHH 
                 ES  AP A SSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKK 
                   
               
               
                 H 
                 ATELKHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLELKGSETT 
                   
               
               
                   
                 FMCEYADETATIVEFLNRWITFSQSIISTLT  HHHHHH 
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIYP 
                 242 
               
               
                 Herceptin 
                 TNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGFYA 
                   
               
               
                 Fd- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVTV 
                   
               
               
                 GGSENLYFQG 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT 
                   
               
               
                 GGS-NARA1- 
                 KVDKKVEPK  AIRLTQSPSSFSASTGDRVTITCKASQSVDY 
                   
               
               
                 scFv- 
                 QGDSYMNWYQQKPGKAPKLLIYSASNLESGVPSRFSGSGSGTDFTLTISSLQS 
                   
               
               
                 SSDKTHTCPP 
                 EDFATYYCQQSNEDPYTFGGGTKVEIKGGGGSGGGGSGGGGSEVQLVQSGAEV 
                   
               
               
                 CP-Fc- 
                 KKPGESLKISCKGSGYAFTNYLIEWVRQMPGKGLEWMGVINPGSGGTNYNEKF 
                   
               
               
                 L234A- 
                 KGQVTISADKSISTAYLQWSSLKASDTAMYYCARWRGEGYYAYYDVWGQGTTV 
                   
               
               
                 L235A- 
                 TVSSSSDKTHTCPPCPAPE AA GGPSVFLFPPKPKDTLMISRTPEVTCVVVDVS 
                   
               
               
                 P329A 
                 HEDPEVKFNWYVDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYK 
                   
               
               
                   
                 CKVSNKAL A APIEKTISKAKGQPREPQVYTLPPSRDELTKNQVSLTCLVKGFY 
                   
               
               
                   
                 PSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNVFSCS 
                   
               
               
                   
                 VMHEALHNHYTQKSLSLSPGK 
                   
               
               
                   
               
            
           
           
               
               
            
               
                 P17081710 Activatable Proprotein 
                   
               
            
           
           
               
               
               
            
               
                 Chains 1 
                 AP A SSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKKATEL 
                 243 
               
               
                 and 2 
                 KHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFMCE 
                   
               
               
                 IL2_T3A- 
                 YADETATIVEFLNRWITFSQSIISTLT  AIRLTQ 
                   
               
               
                 (GGGGS)4- 
                 SPSSFSASTGDRVTITCKASQSVDYQGDSYMNWYQQKPGKAPKLLIYSASNLE 
                   
               
               
                 NARAl_LC 
                 SGVPSRFSGSGSGTDFTLTISSLQSEDFATYYCQQSNEDPYTFGGGTKVEIKR 
                   
               
               
                   
                 TVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNSQE 
                   
               
               
                   
                 SVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRGEC 
                   
               
               
                   
               
               
                 Chains 3 
                 EVQLVQSGAEVKKPGESLKISCKGSGYAFTNYLIEWVRQMPGKGLEWMGVINP 
                 244 
               
               
                 and 4 
                 GSGGTNYNEKFKGQVTISADKSISTAYLQWSSLKASDTAMYYCARWRGEGYYA 
                   
               
               
                 NARA1-VH- 
                 YYDVWGQGTTVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVT 
                   
               
               
                 huG1HC- 
                 VSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSN 
                   
               
               
                 L234A- 
                 TKVDKKVEPKSCDKTHTCPPCPAPE AA GGPSVFLFPPKPKDTLMISRTPEVTC 
                   
               
               
                 L235A- 
                 VVVDVSHEDPEVKFNWYVDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWL 
                   
               
               
                 P32 9A 
                 NGKEYKCKVSNKAL A APIEKTISKAKGQPREPQVYTLPPSRDELTKNQVSLTC 
                   
               
               
                   
                 LVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQG 
                   
               
               
                   
                 NVFSCSVMHEALHNHYTQKSLSLSPGK 
                   
               
               
                   
               
            
           
           
               
               
            
               
                 P17091710 Activatable Proprotein 
                   
               
            
           
           
               
               
               
            
               
                 Chains 1 
                 AP A SSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKKATEL 
                 245 
               
               
                 and 2 
                 KHLQCLEEELKPLEEVLNLAQSKNFHLRPRDLISNINVIVLELKGSETTFMCE 
                   
               
               
                 IL2_T3A- 
                 YADETATIVEFLNRWITFSQSIISTLT  AIRLTQ 
                   
               
               
                 GGSPLGLAGS 
                 SPSSFSASTGDRVTITCKASQSVDYQGDSYMNWYQQKPGKAPKLLIYSASNLE 
                   
               
               
                 GRSDNRGGGA- 
                 SGVPSRFSGSGSGTDFTLTISSLQSEDFATYYCQQSNEDPYTFGGGTKVEIKR 
                   
               
               
                 NARAl_LC 
                 TVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNSQE 
                   
               
               
                   
                 SVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRGEC 
                   
               
               
                   
               
               
                 Chains 3 
                 EVQLVQSGAEVKKPGESLKISCKGSGYAFTNYLIEWVRQMPGKGLEWMGVINP 
                 246 
               
               
                 and 4 
                 GSGGTNYNEKFKGQVTISADKSISTAYLQWSSLKASDTAMYYCARWRGEGYYA 
                   
               
               
                 NARA1-VH- 
                 YYDVWGQGTTVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVT 
                   
               
               
                 huG1HC- 
                 VSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSN 
                   
               
               
                 L234A- 
                 TKVDKKVEPKSCDKTHTCPPCPAPE AA GGPSVFLFPPKPKDTLMISRTPEVTC 
                   
               
               
                 L235A- 
                 VVVDVSHEDPEVKFNWYVDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWL 
                   
               
               
                 P32 9A 
                 NGKEYKCKVSNKAL A APIEKTISKAKGQPREPQVYTLPPSRDELTKNQVSLTC 
                   
               
               
                   
                 LVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQG 
                   
               
               
                   
                 NVFSCSVMHEALHNHYTQKSLSLSPGK 
                   
               
               
                   
               
            
           
           
               
            
               
                 P1453182124 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chains 1 
                 DIQMTQSPSSLSASVGDRVTITCRASKSVSTSAYSYMHWYQQKPGKAPKLLIY 
                 247 
               
               
                 and 2 FAP- 
                 LASNLESGVPSRFSGSGSGTDFTLTISSLQPEDFATYYCQHSRELPYTFGQGT 
                   
               
               
                 L2-huIgkLC 
                 KLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQ 
                   
               
               
                   
                 SGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGEC 
                   
               
               
                   
               
               
                 Chain 3 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWFHP 
                 248 
               
               
                 FAP-H2- 
                 GSGSIKYNEKFKDRVTMTADTSTSTVYMELSSLRSEDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 S228P- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 delK- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 GPLGLAGSGR 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 SDNQG-IL- 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                 2Rα-K38C 
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  ELCDDDPPEIPHATFK 
                   
               
               
                   
                 AMAYKEGTMLNCECKRGFRRICSGSLYMLCTGNSSHSSWDNQCQCTSSATRNT 
                   
               
               
                   
                 TKQVTPQPEEQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFV 
                   
               
               
                   
                 VGQMVYYQCVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chain 4 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWFHP 
                 249 
               
               
                 FAP-H2- 
                 GSGSIKYNEKFKDRVTMTADTSTSTVYMELSSLRSEDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 S228P- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 delK-GA- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 IL2-T3A- 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 V69A-Q74P- 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                 I128T 
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  AP A SSSTKKTQLQLEHLLLDLQMILNGIN 
                   
               
               
                   
                 NYKNPKLTRMLTFKFYMPKKATELKHLQCLE C ELKPLEE A LNLA P SKNFHLRP 
                   
               
               
                   
                 RDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQS T ISTLT 
                   
               
               
                   
               
            
           
           
               
            
               
                 P1453182730 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chains 1 
                 DIQMTQSPSSLSASVGDRVTITCRASKSVSTSAYSYMHWYQQKPGKAPKLLIY 
                 250 
               
               
                 and 2 FAP- 
                 LASNLESGVPSRFSGSGSGTDFTLTISSLQPEDFATYYCQHSRELPYTFGQGT 
                   
               
               
                 L2-huIgkLC 
                 KLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQ 
                   
               
               
                   
                 SGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGEC 
                   
               
               
                   
               
               
                 Chain 3 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWFHP 
                 251 
               
               
                 FAP- 
                 GSGSIKYNEKFKDRVTMTADTSTSTVYMELSSLRSEDTAVYYCARHGGTGRGA 
                   
               
               
                 H2_huG4HC- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 hole- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 Y349C- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 del K- 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 GPLGLAGSGR 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQV C TLPPSQEEMTKNQVSL S C A VKG 
                   
               
               
                 SDNQG- 
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFL V SRLTVDKSRWQEGNVFS 
                   
               
               
                 IL2R_K38C 
                 CSVMHEALHNHYTQKSLSLSLG  ELCDDDPPEIPHATFK 
                   
               
               
                   
                 AMAYKEGTMLNCECKRGFRRI C SGSLYMLCTGNSSHSSWDNQCQCTSSATRNT 
                   
               
               
                   
                 TKQVTPQPEEQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFV 
                   
               
               
                   
                 VGQMVYYQCVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chain 4 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWFHP 
                 252 
               
               
                 FAP-H2- 
                 GSGSIKYNEKFKDRVTMTADTSTSTVYMELSSLRSEDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 knob- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 S354C- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 delK-GA- 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 IL2-T3A- 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPP C QEEMTKNQVSL W CLVKG 
                   
               
               
                 E61C-V69A- 
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                 Q74P-I128T 
                 CSVMHEALHNHYTQKSLSLSLG  AP A SSSTKKTQLQLEHLLLDLQMILNGIN 
                   
               
               
                   
                 NYKNPKLTRMLTFKFYMPKKATELKHLQCLECELKPLEE A LNLA P SKNFHLRP 
                   
               
               
                   
                 RDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQS T ISTLT 
                   
               
               
                   
               
            
           
           
               
            
               
                 P2492962158 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 253 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GA-IL2- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 T3A-K35C 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                   
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  AP A SSSTKKTQLQLEHLLLDLQMILNGIN 
                   
               
               
                   
                 NYKNP C LTRMLTFKFYMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRP 
                   
               
               
                   
                 RDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT 
                   
               
               
                   
               
               
                 Chain 2 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 254 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GPLGLAGSGR 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 SDNQG- 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 IL2Rα-DO4C 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  ELC C DDPPEIPHATFK 
                   
               
               
                   
                 AMAYKEGTMLNCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNT 
                   
               
               
                   
                 TKQVTPQPEEQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFV 
                   
               
               
                   
                 VGQMVYYQCVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chains 3 
                 EIVLTQSPGTLSLSPGERATLSCRASQSVSTSAYSYMHWYQQKPGQAPRLLIY 
                 255 
               
               
                 and 4 FAP- 
                 LASNLESGIPDRFSGSGSGTDFTLTISRLEPEDFAVYYCQQSRELPYTFGQGT 
                   
               
               
                 L7-huIgkLC 
                 KLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQ 
                   
               
               
                   
                 SGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGEC 
                   
               
               
                   
               
            
           
           
               
            
               
                 P2492972158 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 256 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GA-IL2- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 T3A-K35C 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                   
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  AP A SSSTKKTQLQLEHLLLDLQMILNGIN 
                   
               
               
                   
                 NYKNPCLTRMLTFKFYMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRP 
                   
               
               
                   
                 RDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT 
                   
               
               
                   
               
               
                 Chain 2 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 257 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GPLGLAGSGR 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 SDNQG- 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 IL2Rα- 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                 DO4C-N49Q 
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  ELC C DDPPEIPHATFK 
                   
               
               
                   
                 AMAYKEGTMLNCECKRGFRRIKSGSLYMLCTG Q SSHSSWDNQCQCTSSATRNT 
                   
               
               
                   
                 TKQVTPQPEEQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFV 
                   
               
               
                   
                 VGQMVYYQCVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chains 3 
                 EIVLTQSPGTLSLSPGERATLSCRASQSVSTSAYSYMHWYQQKPGQAPRLLIY 
                 258 
               
               
                 and 4 FAP- 
                 LASNLESGIPDRFSGSGSGTDFTLTISRLEPEDFAVYYCQQSRELPYTFGQGT 
                   
               
               
                 L7-huIgkLC 
                 KLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQ 
                   
               
               
                   
                 SGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGEC 
                   
               
               
                   
               
            
           
           
               
            
               
                 P2492982158 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 259 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GA-IL2- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 T3A-K35C 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                   
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  AP A SSSTKKTQLQLEHLLLDLQMILNGIN 
                   
               
               
                   
                 NYKNPCLTRMLTFKFYMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRP 
                   
               
               
                   
                 RDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT 
                   
               
               
                   
               
               
                 Chain 2 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 260 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GPLGLAGSGR 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 SDNQG- 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 IL2Roc- 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                 DO4C-N68Q 
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  ELCCDDPPEIPHATFK 
                   
               
               
                   
                 AMAYKEGTMLNCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATR Q T 
                   
               
               
                   
                 TKQVTPQPEEQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFV 
                   
               
               
                   
                 VGQMVYYQCVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chains 3 
                 EIVLTQSPGTLSLSPGERATLSCRASQSVSTSAYSYMHWYQQKPGQAPRLLIY 
                 261 
               
               
                 and 4 FAP- 
                 LASNLESGIPDRFSGSGSGTDFTLTISRLEPEDFAVYYCQQSRELPYTFGQGT 
                   
               
               
                 L7-huIgkLC 
                 KLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQ 
                   
               
               
                   
                 SGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGEC 
                   
               
               
                   
               
            
           
           
               
            
               
                 P2493962158 Activatable Proprotein 
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 262 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GA- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 IL2 T3A K3 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 5C-V69A- 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                 Q74P-I128T 
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  AP A SSSTKKTQLQLEHLLLDLQMILNGIN 
                   
               
               
                   
                 NYKNPCLTRMLTFKFYMPKKATELKHLQCLEEELKPLEE A LNLAPSKNFHLRP 
                   
               
               
                   
                 RDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQS T ISTLT 
                   
               
               
                   
               
               
                 Chain 2 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 263 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC_S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GPLGLAGSGR 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 SDNQG- 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                 IL2RocDO4C 
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  ELCCDDPPEIPHATFK 
                   
               
               
                   
                 AMAYKEGTMLNCECKRGFRRIKSGSLYMLCTGNSSHSSWDNQCQCTSSATRNT 
                   
               
               
                   
                 TKQVTPQPEEQKERKTTEMQSPMQPVDQASLPGHCREPPPWENEATERIYHFV 
                   
               
               
                   
                 VGQMVYYQCVQGYRALHRGPAESVCKMTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chains 3 
                 EIVLTQSPGTLSLSPGERATLSCRASQSVSTSAYSYMHWYQQKPGQAPRLLIY 
                 264 
               
               
                 and 4 FAP- 
                 LASNLESGIPDRFSGSGSGTDFTLTISRLEPEDFAVYYCQQSRELPYTFGQGT 
                   
               
               
                 L7-huIgkLC 
                 KLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQ 
                   
               
               
                   
                 SGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGEC 
                   
               
               
                   
               
            
           
           
               
            
               
                 Exemplary cleavage products 
               
               
                 P16841362 Cleaved proteins (TEV cleavage) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSASF 
                 265 
               
               
                 Herceptin 
                 LYSGVPSRFSGSRSGTDFTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKVEI 
                   
               
               
                 LC-GS-IL- 
                 KRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNS 
                   
               
               
                 2-T3A- 
                 QESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRG 
                   
               
               
                 V69A-Q74P- 
                 ES  AP A SSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTRMLTFKFYMPKK 
                   
               
               
                 I128T- 
                 ATELKHLQCLEEELKPLEE A LNLA P SKNFHLRPRDLISNINVIVLELKGSETT 
                   
               
               
                 GGGGSHHHHH 
                 FMCEYADETATIVEFLNRWITFSQS T ISTLTGGGGSHHHHHH 
                   
               
               
                 H 
                   
                   
               
               
                   
               
               
                 Chain 2 
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIYP 
                 266 
               
               
                 Herceptin 
                 TNGYTRYADSVKGRFTISADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGFYA 
                   
               
               
                 Fd- 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAALGCLVKDYFPEPVTV 
                   
               
               
                 GGSENLYFQ 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT 
                   
               
               
                   
                 KVDKKVEPK   
                   
               
               
                   
               
               
                 Chain 3 
                    ELCDDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTGN 
                 267 
               
               
                 GGGS-IL- 
                 SSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASLPG 
                   
               
               
                 2Rα 
                 HCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHGKTR 
                   
               
               
                   
                 WTQPQLICTGE 
                   
               
            
           
           
               
               
            
               
                 P2492962158 cleaved proteins (14MP-2 or 14MP-9 cleavage) 
                   
               
               
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 268 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC_S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GA-IL2- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 T3A-K35C 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                   
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  AP A SSSTKKTQLQLEHLLLDLQMILNGIN 
                   
               
               
                   
                 NYKNP C LTRMLTFKFYMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRP 
                   
               
               
                   
                 RDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT 
                   
               
               
                   
               
               
                 Chain 2 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 269 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GPLG 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                   
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                   
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG   
                   
               
               
                   
               
               
                 Chain 3 
                    LCCDDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSL 
                 270 
               
               
                 LAGSGRSDNQ 
                 YMLCTGNSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPV 
                   
               
               
                 G-IL2Rα- 
                 DQASLPGHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCK 
                   
               
               
                 DO4C 
                 MTHGKTRWTQPQLICTGE 
                   
               
               
                   
               
               
                 Chains 4 
                 EIVLTQSPGTLSLSPGERATLSCRASQSVSTSAYSYMHWYQQKPGQAPRLLIY 
                 271 
               
               
                 and 5 FAP- 
                 LASNLESGIPDRFSGSGSGTDFTLTISRLEPEDFAVYYCQQSRELPYTFGQGT 
                   
               
               
                 L7-huIgkLC 
                 KLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQ 
                   
               
               
                   
                 SGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGEC 
                   
               
               
                   
               
            
           
           
               
               
            
               
                 P2492962158 cleaved proteins (uPA or Matriptase cleavage) 
                   
               
            
           
           
               
               
               
            
               
                 Chain 1 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 272 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GA-IL2- 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                 T3A-K35C 
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                   
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG  AP A SSSTKKTQLQLEHLLLDLQMILNGIN 
                   
               
               
                   
                 NYKNP C LTRMLTFKFYMPKKATELKHLQCLEEELKPLEEVLNLAQSKNFHLRP 
                   
               
               
                   
                 RDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT 
                   
               
               
                   
               
               
                 Chain 2 
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTENIIHWVRQAPGQGLEWMGWIHP 
                 273 
               
               
                 FAP-H6- 
                 GSGSIKYAQKLQGRVTMTTDTSTSTAYMELRSLRSDDTAVYYCARHGGTGRGA 
                   
               
               
                 huG4HC S22 
                 MDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTV 
                   
               
               
                 8P-delK- 
                 SWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNT 
                   
               
               
                 GPLGLAGSGR 
                 KVDKRVESKYGPPCP P CPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
                   
               
               
                   
                 VSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKE 
                   
               
               
                   
                 YKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTKNQVSLTCLVKG 
                   
               
               
                   
                 FYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFS 
                   
               
               
                   
                 CSVMHEALHNHYTQKSLSLSLG   
                   
               
               
                   
               
               
                 Chain 3 
                    ELC C DDPPEIPHATFKAMAYKEGTMLNCECKRGFRRIKSGSLYMLCTG 
                 274 
               
               
                 SDNQG- 
                 NSSHSSWDNQCQCTSSATRNTTKQVTPQPEEQKERKTTEMQSPMQPVDQASLP 
                   
               
               
                 IL2Rα-D04C 
                 GHCREPPPWENEATERIYHFVVGQMVYYQCVQGYRALHRGPAESVCKMTHGKT 
                   
               
               
                 RWTQPQLICTGE 
                   
                   
               
               
                   
               
               
                 Chains 4 
                 EIVLTQSPGTLSLSPGERATLSCRASQSVSTSAYSYMHWYQQKPGQAPRLLIY 
                 275 
               
               
                 and 5 FAP- 
                 LASNLESGIPDRFSGSGSGTDFTLTISRLEPEDFAVYYCQQSRELPYTFGQGT 
                   
               
               
                 L7-huIgkLC 
                 KLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQ 
                   
               
               
                   
                 SGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKS 
                   
               
               
                   
                 FNRGEC 
                   
               
               
                   
               
               
                 * Any of the foregoing sequences can include or exclude the histidine tag 
               
            
           
         
       
     
     Thus, in certain embodiments, an activatable proprotein comprises a first polypeptide that comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 9, 13, 17, 21, 25, 29, 33, 37, 41, 45, 49, 53, 57, 61, 65, 69, 73, 77, 81, 233, 235, 237, 239, 241, 243, or 245 and a second polypeptide that respectively comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 234, 236, 238, 240, 242, 244, or 246. 
     In certain embodiments, an activatable proprotein comprises a first polypeptide that comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 247, 250, 253, 256, 259, or 262, a second polypeptide that respectively comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 248, 251, 254, 257, or 263, and a third and fourth polypeptide (e.g., an immunoglobulin domain such as a light chain variable region or a heavy chain variable region) that respectively comprises, consists, or consists essentially of an amino acid sequence that is at least 80, 85, 90, 95, 98, or 100% identical to SEQ ID NO: 249, 252, 255, 258, 261, or 264. In specific embodiments, an activatable proprotein comprises light chain variable regions and/or heavy chain variable regions that specifically bind to an antigen of interest, for example, fibroblast activation protein (FAP). 
     In certain embodiments, the TEV protease cleavage site of any one or more of the foregoing sequences (from Table S5) is replaced with a human protease cleavage site, that is, a cleavage site cleavable by a human protease, for example, a human protease expressed in a cancer tissue or cancer cell. 
     Methods of Use and Pharmaceutical Compositions 
     Certain embodiments include methods of treating, ameliorating the symptoms of, and/or reducing the progression of, a disease or condition in a subject in need thereof, comprising administering to the subject at least one activatable proprotein, as described herein. Also included are methods of enhancing an immune response in a subject comprising administering to the subject at least one activatable proprotein, as described herein. In particular embodiments, the disease is selected from one or more of a cancer, a viral infection, and an immune disorder. 
     In some embodiments, following administration, the activatable proprotein is activated through protease cleavage in a cell or tissue, which releases the masking moiety comprising the protease cleavage site, exposes the binding site of the IL-2 protein that binds to the IL-2Rβ/γc chain present on the surface of the immune cell in vitro or in vivo, and thereby generates an activated protein. In particular embodiments, the protease cleavage occurs in a cancer cell or cancer tissue, or a virally-infected cell or virally-infected tissue. Typically, the activated protein has at least one immune-stimulating IL-2 activity, for example, by binding to the IL-2Rβ/γc chain present on the surface of an immune cell in vivo, and thereby stimulating the immune cell. In particular embodiments, the immune cell is selected from one or more of a T cell, a B cell, a natural killer cell, a monocyte, and a macrophage. 
     In some embodiments, administration and activation of the activatable proprotein, to generate an activated protein, increases an immune response in the subject by about or at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000% or more, relative to a control. In some instances, the immune response is an anti-cancer or anti-viral immune response. In some embodiments, administration and activation of the activatable proprotein, to generate an activated protein, increases cell-killing in the subject by about or at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000% or more, relative to a control. In some embodiments, wherein the cell-killing is cancer cell-killing or virally-infected cell-killing. 
     In some embodiments, administration and activation of the activatable proprotein, to generate an activated protein, does not significantly increase binding of the activated protein to the IL-2Rα/β/γc chain expressed on regulatory T cells (T regs ). For example, in certain activated proteins, the binding between the IL-2 protein and the IL-2 binding protein (for example, disulfide binding between the IL-2 protein and the IL-2Rα protein) is maintained following linker cleavage, masks the binding site of the IL-2 protein that binds to the IL-2Rα/β/γc chain expressed on T regs , and thereby interferes with binding of the activated protein to T regs . Thus, in certain embodiments, the activated protein does not significantly stimulate or enhance the proliferation and/or activation of (T regs ), relative to the activatable proprotein. 
     In some embodiments, the disease is a cancer, that is, the subject in need thereof has or is suspected of having a cancer. Certain embodiments thus include methods of treating, ameliorating the symptoms of, or inhibiting the progression of, a cancer in a subject in need thereof, comprising administering to the subject at least one activatable proprotein, as described herein. In particular embodiments, the cancer is a primary cancer or a metastatic cancer. In specific embodiments, the cancer is selected from one or more of melanoma (optionally metastatic melanoma), kidney cancer (optionally renal cell carcinoma), pancreatic cancer, bone cancer, prostate cancer, small cell lung cancer, non-small cell lung cancer (NSCLC), mesothelioma, leukemia (optionally lymphocytic leukemia, chronic myelogenous leukemia, acute myeloid leukemia, or relapsed acute myeloid leukemia), multiple myeloma, lymphoma, hepatoma (hepatocellular carcinoma), sarcoma, B-cell malignancy, breast cancer, ovarian cancer, colorectal cancer, glioma, glioblastoma multiforme, meningioma, pituitary adenoma, vestibular schwannoma, primary CNS lymphoma, primitive neuroectodermal tumor (medulloblastoma), bladder cancer, uterine cancer, esophageal cancer, brain cancer, head and neck cancers, cervical cancer, testicular cancer, thyroid cancer, and stomach cancer 
     In some embodiments, as noted above, the cancer is a metastatic cancer. Further to the above cancers, exemplary metastatic cancers include, without limitation, bladder cancers which have metastasized to the bone, liver, and/or lungs; breast cancers which have metastasized to the bone, brain, liver, and/or lungs; colorectal cancers which have metastasized to the liver, lungs, and/or peritoneum; kidney cancers which have metastasized to the adrenal glands, bone, brain, liver, and/or lungs; lung cancers which have metastasized to the adrenal glands, bone, brain, liver, and/or other lung sites; melanomas which have metastasized to the bone, brain, liver, lung, and/or skin/muscle; ovarian cancers which have metastasized to the liver, lung, and/or peritoneum; pancreatic cancers which have metastasized to the liver, lung, and/or peritoneum; prostate cancers which have metastasized to the adrenal glands, bone, liver, and/or lungs; stomach cancers which have metastasized to the liver, lung, and/or peritoneum; thyroid cancers which have metastasized to the bone, liver, and/or lungs; and uterine cancers which have metastasized to the bone, liver, lung, peritoneum, and/or vagina; among others. 
     The methods for treating cancers can be combined with other therapeutic modalities. For example, a combination therapy described herein can be administered to a subject before, during, or after other therapeutic interventions, including symptomatic care, radiotherapy, surgery, transplantation, hormone therapy, photodynamic therapy, antibiotic therapy, or any combination thereof. Symptomatic care includes administration of corticosteroids, to reduce cerebral edema, headaches, cognitive dysfunction, and emesis, and administration of anti-convulsants, to reduce seizures. Radiotherapy includes whole-brain irradiation, fractionated radiotherapy, and radiosurgery, such as stereotactic radiosurgery, which can be further combined with traditional surgery. 
     Certain embodiments thus include combination therapies for treating cancers, including methods of treating ameliorating the symptoms of, or inhibiting the progression of, a cancer in a subject in need thereof, comprising administering to the subject at least one activatable proprotein described herein in combination with at least one additional agent, for example, a chemotherapeutic agent, a hormonal therapeutic agent, and/or a kinase inhibitor. In some embodiments, administering the at least one activatable proprotein enhances the susceptibility of the cancer to the additional agent (for example, chemotherapeutic agent, hormonal therapeutic agent, and or kinase inhibitor) by about or at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000% or more relative to the additional agent alone. 
     Certain combination therapies employ one or more chemotherapeutic agents, for example, small molecule chemotherapeutic agents. Non-limiting examples of chemotherapeutic agents include alkylating agents, anti-metabolites, cytotoxic antibiotics, topoisomerase inhibitors (type 1 or type II), an anti-microtubule agents, among others. 
     Examples of alkylating agents include nitrogen mustards (e.g., mechlorethamine, cyclophosphamide, mustine, melphalan, chlorambucil, ifosfamide, and busulfan), nitrosoureas (e.g., N-Nitroso-N-methylurea (MNU), carmustine (BCNU), lomustine (CCNU), semustine (MeCCNU), fotemustine, and streptozotocin), tetrazines (e.g., dacarbazine, mitozolomide, and temozolomide), aziridines (e.g., thiotepa, mytomycin, and diaziquone (AZQ)), cisplatins and derivatives thereof (e.g., carboplatin and oxaliplatin), and non-classical alkylating agents (optionally procarbazine and hexamethylmelamine). 
     Examples of anti-metabolites include anti-folates (e.g., methotrexate and pemetrexed), fluoropyrimidines (e.g., 5-fluorouracil and capecitabine), deoxynucleoside analogues (e.g., ancitabine, enocitabine, cytarabine, gemcitabine, decitabine, azacitidine, fludarabine, nelarabine, cladribine, clofarabine, fludarabine, and pentostatin), and thiopurines (e.g., thioguanine and mercaptopurine); Examples of cytotoxic antibiotics include anthracyclines (e.g., doxorubicin, daunorubicin, epirubicin, idarubicin, pirarubicin, aclarubicin, and mitoxantrone), bleomycins, mitomycin C, mitoxantrone, and actinomycin. Examples of topoisomerase inhibitors include camptothecin, irinotecan, topotecan, etoposide, doxorubicin, mitoxantrone, teniposide, novobiocin, merbarone, and aclarubicin. 
     Examples of anti-microtubule agents include taxanes (e.g., paclitaxel and docetaxel) and vinca alkaloids (e.g., vinblastine, vincristine, vindesine, vinorelbine). 
     The various chemotherapeutic agents described herein can be combined with any one or more of the activatable proproteins described herein, and used according to any one or more of the methods or compositions described herein. 
     Certain combination therapies employ at least one hormonal therapeutic agent. General examples of hormonal therapeutic agents include hormonal agonists and hormonal antagonists. Particular examples of hormonal agonists include progestogen (progestin), corticosteroids (e.g., prednisolone, methylprednisolone, dexamethasone), insulin like growth factors, VEGF derived angiogenic and lymphangiogenic factors (e.g., VEGF-A, VEGF-A145, VEGF-A165, VEGF-C, VEGF-D, PIGF-2), fibroblast growth factor (FGF), galectin, hepatocyte growth factor (HGF), platelet derived growth factor (PDGF), transforming growth factor (TGF)-beta, androgens, estrogens, and somatostatin analogs. Examples of hormonal antagonists include hormone synthesis inhibitors such as aromatase inhibitors and gonadotropin-releasing hormone (GnRH)s agonists (e.g., leuprolide, goserelin, triptorelin, histrelin) including analogs thereof. Also included are hormone receptor antagonist such as selective estrogen receptor modulators (SERMs; e.g., tamoxifen, raloxifene, toremifene) and anti-androgens (e.g., flutamide, bicalutamide, nilutamide). 
     Also included are hormonal pathway inhibitors such as antibodies directed against hormonal receptors. Examples include inhibitors of the IGF receptor (e.g., IGF-IR1) such as cixutumumab, dalotuzumab, figitumumab, ganitumab, istiratumab, and robatumumab; inhibitors of the vascular endothelial growth factor receptors 1, 2 or 3 (VEGFR1, VEGFR2 or VEGFR3) such as alacizumab pegol, bevacizumab, icrucumab, ramucirumab; inhibitors of the TGF-beta receptors R1, R2, and R3 such as fresolimumab and metelimumab; inhibitors of c-Met such as naxitamab; inhibitors of the EGF receptor such as cetuximab, depatuxizumab mafodotin, futuximab, imgatuzumab, laprituximab emtansine, matuzumab, modotuximab, necitumumab, nimotuzumab, panitumumab, tomuzotuximab, and zalutumumab; inhibitors of the FGF receptor such as aprutumab ixadotin and bemarituzumab; and inhibitors of the PDGF receptor such as olaratumab and tovetumab. 
     The various hormonal therapeutic agents described herein can be combined with any one or more of the various activatable proproteins described herein, and used according to any one or more of the methods or compositions described herein. 
     Certain combination therapies employ at least one kinase inhibitor, including tyrosine kinase inhibitors. Examples of kinase inhibitors include, without limitation, adavosertib, afanitib, aflibercept, axitinib, bevacizumab, bosutinib, cabozantinib, cetuximab, cobimetinib, crizotinib, dasatinib, entrectinib, erdafitinib, erlotinib, fostamitinib, gefitinib, ibrutinib, imatinib, lapatinib, lenvatinib, mubritinib, nilotinib, panitumumab, pazopanib, pegaptanib, ponatinib, ranibizumab, regorafenib, ruxolitinib, sorafenib, sunitinib, SU6656, tofacitinib, trastuzumab, vandetanib, and vemuafenib. 
     The various kinase inhibitors described herein can be combined with any one or more of the various activatable proproteins described herein, and used according to any one or more of the methods or compositions described herein. 
     In some embodiments, the methods and pharmaceutical compositions described herein increase median survival time of a subject by 4 weeks, 5 weeks, 6 weeks, 7 weeks, 8 weeks, 9 weeks, 10 weeks, 15 weeks, 20 weeks, 25 weeks, 30 weeks, 40 weeks, or longer. In certain embodiments, the methods and pharmaceutical compositions described herein increase median survival time of a subject by 1 year, 2 years, 3 years, or longer. In some embodiments, the methods and pharmaceutical compositions increase progression-free survival by 2 weeks, 3 weeks, 4 weeks, 5 weeks, 6 weeks, 7 weeks, 8 weeks, 9 weeks, 10 weeks or longer. In certain embodiments, the methods and pharmaceutical compositions described herein increase progression-free survival by 1 year, 2 years, 3 years, or longer. 
     In certain embodiments, the methods and therapeutic compositions described herein are sufficient to result in tumor regression, as indicated by a statistically significant decrease in the amount of viable tumor, for example, at least a 10%, 20%, 30%, 40%, 50% or greater decrease in tumor mass, or by altered (e.g., decreased with statistical significance) scan dimensions. In certain embodiments, the methods and therapeutic compositions described herein are sufficient to result in stable disease. 
     In some embodiments, the disease is a viral disease or viral infection. In certain embodiments, the viral infection is selected from one or more of human immunodeficiency virus (HIV), Hepatitis A, Hepatitis B, Hepatitis C, Hepatitis E, Caliciviruses associated diarrhoea, Rotavirus diarrhoea,  Haemophilus influenzae  B pneumonia and invasive disease, influenza, measles, mumps, rubella, Parainfluenza associated pneumonia, Respiratory syncytial virus (RSV) pneumonia, Severe Acute Respiratory Syndrome (SARS), Human papillomavirus, Herpes simplex type 2 genital ulcers, Dengue Fever, Japanese encephalitis, Tick-borne encephalitis, West-Nile virus associated disease, Yellow Fever, Epstein-Barr virus, Lassa fever, Crimean-Congo haemorrhagic fever, Ebola haemorrhagic fever, Marburg haemorrhagic fever, Rabies, Rift Valley fever, Smallpox, upper and lower respiratory infections, and poliomyelitis. In specific embodiments, the subject is HIV-positive. In some embodiments, the methods and pharmaceutical compositions described herein increase an anti-viral immune response by about or at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000% or more, relative to a control. 
     In some embodiments, the immune disorder is selected from one or more of type 1 diabetes, vasculitis, and an immunodeficiency. In some embodiments, the methods and pharmaceutical compositions described herein improve immune function in the subject, for example, by about or at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000% or more, relative to a control. 
     In certain embodiments, the methods and therapeutic compositions described herein are sufficient to result in clinically relevant reduction in symptoms of a particular disease indication known to the skilled clinician. 
     For in vivo use, as noted above, for the treatment of human or non-human mammalian disease or testing, the agents described herein are generally incorporated into one or more therapeutic or pharmaceutical compositions prior to administration, including veterinary therapeutic compositions. 
     Thus, certain embodiments relate to pharmaceutical or therapeutic compositions that comprise at least one activatable proprotein, as described herein. In some instances, a pharmaceutical or therapeutic composition comprises one or more of the activatable proproteins described herein in combination with a pharmaceutically- or physiologically-acceptable carrier or excipient. Certain pharmaceutical or therapeutic compositions further comprise at least one additional agent, for example, a chemotherapeutic agent, a hormonal therapeutic agent, and/or a kinase inhibitor as described herein. 
     Some therapeutic compositions comprise (and certain methods utilize) only one activatable proprotein. Certain therapeutic compositions comprise (and certain methods utilize) a mixture of at least two, three, four, or five different activatable proproteins. 
     In particular embodiments, the pharmaceutical or therapeutic compositions comprising at least one activatable proprotein is substantially pure on a protein basis or a weight-weight basis, for example, the composition has a purity of at least about 80%, 85%, 90%, 95%, 98%, or 99% on a protein basis or a weight-weight basis. 
     In some embodiments, the activatable proproteins described herein do not form aggregates, have a desired solubility, and/or have an immunogenicity profile that is suitable for use in humans, as known in the art. Thus, in some embodiments, the therapeutic composition comprising an activatable proprotein is substantially aggregate-free. For example, certain compositions comprise less than about 10% (on a protein basis) high molecular weight aggregated proteins, or less than about 5% high molecular weight aggregated proteins, or less than about 4% high molecular weight aggregated proteins, or less than about 3% high molecular weight aggregated proteins, or less than about 2% high molecular weight aggregated proteins, or less than about 1% high molecular weight aggregated proteins. Some compositions comprise an activatable proprotein that is at least about 50%, about 60%, about 70%, about 80%, about 90% or about 95% monodisperse with respect to its apparent molecular mass. 
     In some embodiments, the activatable proprotein are concentrated to about or at least about 0.1 mg/ml, 0.2 mg/ml, 0.3 mg/ml, 0.4 mg/ml, 0.5 mg/ml, 0.6, 0.7, 0.8, 0.9, 1 mg/ml, 2 mg/ml, 3 mg/ml, 4 mg/ml, 5 mg/ml, 6 mg/ml, 7 mg/ml, 8 mg/ml, 9 mg/ml, 10 mg/ml, 11, 12, 13, 14 or 15 mg/ml and are formulated for biotherapeutic uses. 
     To prepare a therapeutic or pharmaceutical composition, an effective or desired amount of one or more agents is mixed with any pharmaceutical carrier(s) or excipient known to those skilled in the art to be suitable for the particular agent and/or mode of administration. A pharmaceutical carrier may be liquid, semi-liquid or solid. Solutions or suspensions used for parenteral, intradermal, subcutaneous or topical application may include, for example, a sterile diluent (such as water), saline solution (e.g., phosphate buffered saline; PBS), fixed oil, polyethylene glycol, glycerin, propylene glycol or other synthetic solvent; antimicrobial agents (such as benzyl alcohol and methyl parabens); antioxidants (such as ascorbic acid and sodium bisulfite) and chelating agents (such as ethylenediaminetetraacetic acid (EDTA)); buffers (such as acetates, citrates and phosphates). If administered intravenously (e.g., by IV infusion), suitable carriers include physiological saline or phosphate buffered saline (PBS), and solutions containing thickening and solubilizing agents, such as glucose, polyethylene glycol, polypropylene glycol and mixtures thereof. 
     Administration of agents described herein, in pure form or in an appropriate therapeutic or pharmaceutical composition, can be carried out via any of the accepted modes of administration of agents for serving similar utilities. The therapeutic or pharmaceutical compositions can be prepared by combining an agent-containing composition with an appropriate physiologically acceptable carrier, diluent or excipient, and may be formulated into preparations in solid, semi-solid, liquid or gaseous forms, such as tablets, capsules, powders, granules, ointments, solutions, suppositories, injections, inhalants, gels, microspheres, and aerosols. In addition, other pharmaceutically active ingredients (including other small molecules as described elsewhere herein) and/or suitable excipients such as salts, buffers and stabilizers may, but need not, be present within the composition. 
     Administration may be achieved by a variety of different routes, including oral, parenteral, nasal, intravenous, intradermal, intramuscular, subcutaneous or topical. Preferred modes of administration depend upon the nature of the condition to be treated or prevented. Particular embodiments include administration by IV infusion. 
     Carriers can include, for example, pharmaceutically- or physiologically-acceptable carriers, excipients, or stabilizers that are non-toxic to the cell or mammal being exposed thereto at the dosages and concentrations employed. Often the physiologically-acceptable carrier is an aqueous pH buffered solution. Examples of physiologically acceptable carriers include buffers such as phosphate, citrate, and other organic acids; antioxidants including ascorbic acid; low molecular weight (less than about 10 residues) polypeptide; proteins, such as serum albumin, gelatin, or immunoglobulins; hydrophilic polymers such as polyvinylpyrrolidone; amino acids such as glycine, glutamine, asparagine, arginine or lysine; monosaccharides, disaccharides, and other carbohydrates including glucose, mannose, or dextrins; chelating agents such as EDTA; sugar alcohols such as mannitol or sorbitol; salt-forming counterions such as sodium; and/or nonionic surfactants such as polysorbate 20 (TWEEN™) polyethylene glycol (PEG), and poloxamers (PLURONICS™), and the like. 
     In some embodiments, one or more agents can be entrapped in microcapsules prepared, for example, by coacervation techniques or by interfacial polymerization (for example, hydroxymethylcellulose or gelatin-microcapsules and poly-(methylnethacylate)microcapsules, respectively), in colloidal drug delivery systems (for example, liposomes, albumin microspheres, microemulsions, nano-particles and nanocapsules), or in macroemulsions. Such techniques are disclosed in Remington&#39;s Pharmaceutical Sciences, 16th edition, Oslo, A., Ed., (1980). The particle(s) or liposomes may further comprise other therapeutic or diagnostic agents. 
     The precise dosage and duration of treatment is a function of the disease being treated and may be determined empirically using known testing protocols or by testing the compositions in model systems known in the art and extrapolating therefrom. Controlled clinical trials may also be performed. Dosages may also vary with the severity of the condition to be alleviated. A pharmaceutical composition is generally formulated and administered to exert a therapeutically useful effect while minimizing undesirable side effects. The composition may be administered one time, or may be divided into a number of smaller doses to be administered at intervals of time. For any particular subject, specific dosage regimens may be adjusted over time according to the individual need. 
     Typical routes of administering these and related therapeutic or pharmaceutical compositions thus include, without limitation, oral, topical, transdermal, inhalation, parenteral, sublingual, buccal, rectal, vaginal, and intranasal. The term parenteral as used herein includes subcutaneous injections, intravenous, intramuscular, intrasternal injection or infusion techniques. Therapeutic or pharmaceutical compositions according to certain embodiments of the present disclosure are formulated so as to allow the active ingredients contained therein to be bioavailable upon administration of the composition to a subject or patient. Compositions that will be administered to a subject or patient may take the form of one or more dosage units, where for example, a tablet may be a single dosage unit, and a container of a herein described agent in aerosol form may hold a plurality of dosage units. Actual methods of preparing such dosage forms are known, or will be apparent, to those skilled in this art; for example, see  Remington: The Science and Practice of Pharmacy,  20th Edition (Philadelphia College of Pharmacy and Science, 2000). The composition to be administered will typically contain a therapeutically effective amount of an agent described herein, for treatment of a disease or condition of interest. 
     A therapeutic or pharmaceutical composition may be in the form of a solid or liquid. In one embodiment, the carrier(s) are particulate, so that the compositions are, for example, in tablet or powder form. The carrier(s) may be liquid, with the compositions being, for example, an oral oil, injectable liquid or an aerosol, which is useful in, for example, inhalatory administration. When intended for oral administration, the pharmaceutical composition is preferably in either solid or liquid form, where semi-solid, semi-liquid, suspension and gel forms are included within the forms considered herein as either solid or liquid. Certain embodiments include sterile, injectable solutions. 
     As a solid composition for oral administration, the pharmaceutical composition may be formulated into a powder, granule, compressed tablet, pill, capsule, chewing gum, wafer or the like. Such a solid composition will typically contain one or more inert diluents or edible carriers. In addition, one or more of the following may be present: binders such as carboxymethylcellulose, ethyl cellulose, microcrystalline cellulose, gum tragacanth or gelatin; excipients such as starch, lactose or dextrins, disintegrating agents such as alginic acid, sodium alginate, Primogel, corn starch and the like; lubricants such as magnesium stearate or Sterotex; glidants such as colloidal silicon dioxide; sweetening agents such as sucrose or saccharin; a flavoring agent such as peppermint, methyl salicylate or orange flavoring; and a coloring agent. When the pharmaceutical composition is in the form of a capsule, for example, a gelatin capsule, it may contain, in addition to materials of the above type, a liquid carrier such as polyethylene glycol or oil. 
     The therapeutic or pharmaceutical composition may be in the form of a liquid, for example, an elixir, syrup, solution, emulsion or suspension. The liquid may be for oral administration or for delivery by injection, as two examples. When intended for oral administration, preferred composition contain, in addition to the present compounds, one or more of a sweetening agent, preservatives, dye/colorant and flavor enhancer. In a composition intended to be administered by injection, one or more of a surfactant, preservative, wetting agent, dispersing agent, suspending agent, buffer, stabilizer and isotonic agent may be included. 
     The liquid therapeutic or pharmaceutical compositions, whether they be solutions, suspensions or other like form, may include one or more of the following adjuvants: sterile diluents such as water for injection, saline solution, preferably physiological saline, Ringer&#39;s solution, isotonic sodium chloride, fixed oils such as synthetic mono or diglycerides which may serve as the solvent or suspending medium, polyethylene glycols, glycerin, propylene glycol or other solvents; antibacterial agents such as benzyl alcohol or methyl paraben; antioxidants such as ascorbic acid or sodium bisulfite; chelating agents such as ethylenediaminetetraacetic acid; buffers such as acetates, citrates or phosphates and agents for the adjustment of tonicity such as sodium chloride or dextrose. The parenteral preparation can be enclosed in ampoules, disposable syringes or multiple dose vials made of glass or plastic. Physiological saline is a preferred adjuvant. An injectable pharmaceutical composition is preferably sterile. 
     A liquid therapeutic or pharmaceutical composition intended for either parenteral or oral administration should contain an amount of an agent such that a suitable dosage will be obtained. Typically, this amount is at least 0.01% of the agent of interest in the composition. When intended for oral administration, this amount may be varied to be between 0.1 and about 70% of the weight of the composition. Certain oral therapeutic or pharmaceutical compositions contain between about 4% and about 75% of the agent of interest. In certain embodiments, therapeutic or pharmaceutical compositions and preparations are prepared so that a parenteral dosage unit contains between 0.01 to 10% by weight of the agent of interest prior to dilution. 
     The therapeutic or pharmaceutical compositions may be intended for topical administration, in which case the carrier may suitably comprise a solution, emulsion, ointment or gel base. The base, for example, may comprise one or more of the following: petrolatum, lanolin, polyethylene glycols, bee wax, mineral oil, diluents such as water and alcohol, and emulsifiers and stabilizers. Thickening agents may be present in a therapeutic or pharmaceutical composition for topical administration. If intended for transdermal administration, the composition may include a transdermal patch or iontophoresis device. 
     The therapeutic or pharmaceutical compositions may be intended for rectal administration, in the form, for example, of a suppository, which will melt in the rectum and release the drug. The composition for rectal administration may contain an oleaginous base as a suitable nonirritating excipient. Such bases include, without limitation, lanolin, cocoa butter, and polyethylene glycol. 
     The therapeutic or pharmaceutical composition may include various materials, which modify the physical form of a solid or liquid dosage unit. For example, the composition may include materials that form a coating shell around the active ingredients. The materials that form the coating shell are typically inert, and may be selected from, for example, sugar, shellac, and other enteric coating agents. Alternatively, the active ingredients may be encased in a gelatin capsule. The therapeutic or pharmaceutical compositions in solid or liquid form may include a component that binds to agent and thereby assists in the delivery of the compound. Suitable components that may act in this capacity include monoclonal or polyclonal antibodies, one or more proteins or a liposome. 
     The therapeutic or pharmaceutical composition may consist essentially of dosage units that can be administered as an aerosol. The term aerosol is used to denote a variety of systems ranging from those of colloidal nature to systems consisting of pressurized packages. Delivery may be by a liquefied or compressed gas or by a suitable pump system that dispenses the active ingredients. Aerosols may be delivered in single phase, bi-phasic, or tri-phasic systems in order to deliver the active ingredient(s). Delivery of the aerosol includes the necessary container, activators, valves, subcontainers, and the like, which together may form a kit. One of ordinary skill in the art, without undue experimentation may determine preferred aerosols. 
     The compositions described herein may be prepared with carriers that protect the agents against rapid elimination from the body, such as time release formulations or coatings. Such carriers include controlled release formulations, such as, but not limited to, implants and microencapsulated delivery systems, and biodegradable, biocompatible polymers, such as ethylene vinyl acetate, polyanhydrides, polyglycolic acid, polyorthoesters, polylactic acid and others known to those of ordinary skill in the art. 
     The therapeutic or pharmaceutical compositions may be prepared by methodology well known in the pharmaceutical art. For example, a therapeutic or pharmaceutical composition intended to be administered by injection may comprise one or more of salts, buffers and/or stabilizers, with sterile, distilled water so as to form a solution. A surfactant may be added to facilitate the formation of a homogeneous solution or suspension. Surfactants are compounds that non-covalently interact with the agent so as to facilitate dissolution or homogeneous suspension of the agent in the aqueous delivery system. 
     The therapeutic or pharmaceutical compositions may be administered in a therapeutically effective amount, which will vary depending upon a variety of factors including the activity of the specific compound employed; the metabolic stability and length of action of the compound; the age, body weight, general health, sex, and diet of the subject; the mode and time of administration; the rate of excretion; the drug combination; the severity of the particular disorder or condition; and the subject undergoing therapy. In some instances, a therapeutically effective daily dose is (for a 70 kg mammal) from about 0.001 mg/kg (i.e., ˜0.07 mg) to about 100 mg/kg (i.e., ˜7.0 g); preferably a therapeutically effective dose is (for a 70 kg mammal) from about 0.01 mg/kg (i.e., ˜0.7 mg) to about 50 mg/kg (i.e., ˜3.5 g); more preferably a therapeutically effective dose is (for a 70 kg mammal) from about 1 mg/kg (i.e., ˜70 mg) to about 25 mg/kg (i.e., ˜1.75 g). In some embodiments, the therapeutically effective dose is administered on a weekly, bi-weekly, or monthly basis. In specific embodiments, the therapeutically effective dose is administered on a weekly, bi-weekly, or monthly basis, for example, at a dose of about 1-10 or 1-5 mg/kg, or about 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 mg/kg. 
     The combination therapies described herein may include administration of a single pharmaceutical dosage formulation, which contains an activatable proprotein and an additional therapeutic agent (e.g., chemotherapeutic agent, hormonal therapeutic agent, kinase inhibitor), as well as administration of compositions comprising an activatable proprotein and an additional therapeutic agent in its own separate pharmaceutical dosage formulation. For example, an activatable proprotein and additional therapeutic agent can be administered to the subject together in a single oral dosage composition such as a tablet or capsule, or each agent administered in separate oral dosage formulations. Similarly, an activatable proprotein and additional therapeutic agent can be administered to the subject together in a single parenteral dosage composition such as in a saline solution or other physiologically acceptable solution, or each agent administered in separate parenteral dosage formulations. As another example, for cell-based therapies, an activatable proprotein can be mixed with the cells prior to administration, administered as part of a separate composition, or both. Where separate dosage formulations are used, the compositions can be administered at essentially the same time, i.e., concurrently, or at separately staggered times, i.e., sequentially and in any order; combination therapy is understood to include all these regimens. 
     Also included are patient care kits, comprising (a) at least one activatable proprotein, as described herein; and optionally (b) at least one additional therapeutic agent (e.g., chemotherapeutic agent, hormonal therapeutic agent, kinase inhibitor). In certain kits, (a) and (b) are in separate therapeutic compositions. In some kits, (a) and (b) are in the same therapeutic composition. 
     The kits herein may also include a one or more additional therapeutic agents or other components suitable or desired for the indication being treated, or for the desired diagnostic application. The kits herein can also include one or more syringes or other components necessary or desired to facilitate an intended mode of delivery (e.g., stents, implantable depots, etc.). 
     In some embodiments, a patient care kit contains separate containers, dividers, or compartments for the composition(s) and informational material(s). For example, the composition(s) can be contained in a bottle, vial, or syringe, and the informational material(s) can be contained in association with the container. In some embodiments, the separate elements of the kit are contained within a single, undivided container. For example, the composition is contained in a bottle, vial or syringe that has attached thereto the informational material in the form of a label. In some embodiments, the kit includes a plurality (e.g., a pack) of individual containers, each containing one or more unit dosage forms (e.g., a dosage form described herein) of an activatable proprotein and optionally at least one additional therapeutic agent. For example, the kit includes a plurality of syringes, ampules, foil packets, or blister packs, each containing a single unit dose of an activatable proprotein and optionally at least one additional therapeutic agent. The containers of the kits can be air tight, waterproof (e.g., impermeable to changes in moisture or evaporation), and/or light-tight. 
     The patient care kit optionally includes a device suitable for administration of the composition, e.g., a syringe, inhalant, dropper (e.g., eye dropper), swab (e.g., a cotton swab or wooden swab), or any such delivery device. In some embodiments, the device is an implantable device that dispenses metered doses of the agent(s). Also included are methods of providing a kit, e.g., by combining the components described herein. 
     Expression and Purification Systems 
     Certain embodiments include methods and related compositions for expressing and purifying an activatable proprotein described herein. Such recombinant activatable proproteins can be conveniently prepared using standard protocols as described for example in Sambrook, et al., (1989, supra), in particular Sections 16 and 17; Ausubel et al., (1994, supra), in particular Chapters 10 and 16; and Coligan et al., Current Protocols in Protein Science (John Wiley &amp; Sons, Inc. 1995-1997), in particular Chapters 1, 5 and 6. As one general example, activatable proproteins may be prepared by a procedure including one or more of the steps of: (a) preparing one or more vectors or constructs comprising one or more polynucleotide sequences that encode a first polypeptide (i.e., a first polypeptide chain comprising an IL-2 protein and a masking moiety), and a second polypeptide (i.e., a second polypeptide chain comprising an IL-2 binding protein and a masking moiety), which are operably linked to one or more regulatory elements; (b) introducing the one or more vectors or constructs into one or more host cells; (c) culturing the one or more host cell to express the first and second polypeptides, which bind together to form an activatable proprotein; and (d) isolating the activatable proprotein from the host cell. Alternatively, the first and second polypeptides can be produced in separate host cells, isolated separately, and then combined to form an activatable proprotein. 
     To express a desired polypeptide, a nucleotide sequence encoding a first and/or second polypeptide chain of an activatable proprotein may be inserted into appropriate expression vector(s), i.e., vector(s) which contain the necessary elements for the transcription and translation of the inserted coding sequence. Methods which are well known to those skilled in the art may be used to construct expression vectors containing sequences encoding a polypeptide of interest and appropriate transcriptional and translational control elements. These methods include in vitro recombinant DNA techniques, synthetic techniques, and in vivo genetic recombination. Such techniques are described in Sambrook et al., Molecular Cloning, A Laboratory Manual (1989), and Ausubel et al., Current Protocols in Molecular Biology (1989). 
     A variety of expression vector/host systems are known and may be utilized to contain and express polynucleotide sequences. These include, but are not limited to, microorganisms such as bacteria transformed with recombinant bacteriophage, plasmid, or cosmid DNA expression vectors; yeast transformed with yeast expression vectors; insect cell systems infected with virus expression vectors (e.g., baculovirus); plant cell systems transformed with virus expression vectors (e.g., cauliflower mosaic virus, CaMV; tobacco mosaic virus, TMV) or with bacterial expression vectors (e.g., Ti or pBR322 plasmids); or animal cell systems, including mammalian cell and more specifically human cell systems. 
     The “control elements” or “regulatory sequences” present in an expression vector are those non-translated regions of the vector—enhancers, promoters, 5′ and 3′ untranslated regions--which interact with host cellular proteins to carry out transcription and translation. Such elements may vary in their strength and specificity. Depending on the vector system and host utilized, any number of suitable transcription and translation elements, including constitutive and inducible promoters, may be used. For example, when cloning in bacterial systems, inducible promoters such as the hybrid lacZ promoter of the PBLUESCRIPT phagemid (Stratagene, La Jolla, Calif.) or PSPORT1 plasmid (Gibco BRL, Gaithersburg, Md.) and the like may be used. In mammalian cell systems, promoters from mammalian genes or from mammalian viruses are generally preferred. If it is necessary to generate a cell line that contains multiple copies of the sequence encoding a polypeptide, vectors based on SV40 or EBV may be advantageously used with an appropriate selectable marker. 
     In bacterial systems, a number of expression vectors may be selected depending upon the use intended for the expressed polypeptide. For example, when large quantities are needed, vectors which direct high level expression of fusion proteins that are readily purified may be used. Such vectors include, but are not limited to, the multifunctional  E. coli  cloning and expression vectors such as BLUESCRIPT (Stratagene), in which the sequence encoding the polypeptide of interest may be ligated into the vector in frame with sequences for the amino-terminal Met and the subsequent 7 residues of β-galactosidase so that a hybrid protein is produced; pIN vectors (Van Heeke &amp; Schuster, J. Biol. Chem. 264:5503 5509 (1989)); and the like. pGEX Vectors (Promega, Madison, Wis.) may also be used to express foreign polypeptides as fusion proteins with glutathione S-transferase (GST). In general, such fusion proteins are soluble and can easily be purified from lysed cells by adsorption to glutathione-agarose beads followed by elution in the presence of free glutathione. Proteins made in such systems may be designed to include heparin, thrombin, or factor XA protease cleavage sites so that the cloned polypeptide of interest can be released from the GST moiety at will. 
     Certain embodiments employ  E. coli -based expression systems (see, e.g., Structural Genomics Consortium et al., Nature Methods. 5:135-146, 2008). These and related embodiments may rely partially or totally on ligation-independent cloning (LIC) to produce a suitable expression vector. In specific embodiments, protein expression may be controlled by a T7 RNA polymerase (e.g., pET vector series). These and related embodiments may utilize the expression host strain BL21(DE3), a λDE3 lysogen of BL21 that supports T7-mediated expression and is deficient in lon and ompT proteases for improved target protein stability. Also included are expression host strains carrying plasmids encoding tRNAs rarely used in  E. coli , such as ROSETTA™ (DE3) and Rosetta 2 (DE3) strains. Cell lysis and sample handling may also be improved using reagents sold under the trademarks BENZONASE® nuclease and BUGBUSTER® Protein Extraction Reagent. For cell culture, auto-inducing media can improve the efficiency of many expression systems, including high-throughput expression systems. Media of this type (e.g., OVERNIGHT EXPRESS™ Autoinduction System) gradually elicit protein expression through metabolic shift without the addition of artificial inducing agents such as IPTG. Particular embodiments employ hexahistidine tags (such as those sold under the trademark HIS⋅TAG® fusions), followed by immobilized metal affinity chromatography (IMAC) purification, or related techniques. In certain aspects, however, clinical grade proteins can be isolated from  E. coli  inclusion bodies, without or without the use of affinity tags (see, e.g., Shimp et al., Protein Expr Purif. 50:58-67, 2006). As a further example, certain embodiments may employ a cold-shock induced  E. coli  high-yield production system, because over-expression of proteins in  Escherichia coli  at low temperature improves their solubility and stability (see, e.g., Qing et al., Nature Biotechnology. 22:877-882, 2004). 
     Also included are high-density bacterial fermentation systems. For example, high cell density cultivation of  Ralstonia eutropha  allows protein production at cell densities of over 150 g/L, and the expression of recombinant proteins at titers exceeding 10 g/L. 
     In the yeast  Saccharomyces cerevisiae , a number of vectors containing constitutive or inducible promoters such as alpha factor, alcohol oxidase, and PGH may be used. For reviews, see Ausubel et al. (supra) and Grant et al., Methods Enzymol. 153:516-544 (1987). Also included are  Pichia pandoris  expression systems (see, e.g., Li et al., Nature Biotechnology. 24, 210-215, 2006; and Hamilton et al., Science, 301:1244, 2003). Certain embodiments include yeast systems that are engineered to selectively glycosylate proteins, including yeast that have humanized N-glycosylation pathways, among others (see, e.g., Hamilton et al., Science. 313:1441-1443, 2006; Wildt et al., Nature Reviews Microbiol. 3:119-28, 2005; and Gerngross et al., Nature-Biotechnology. 22:1409-1414, 2004; U.S. Pat. Nos. 7,629,163; 7,326,681; and 7,029,872). Merely by way of example, recombinant yeast cultures can be grown in Fernbach Flasks or 15 L, 50 L, 100 L, and 200 L fermentors, among others. 
     In cases where plant expression vectors are used, the expression of sequences encoding polypeptides may be driven by any of a number of promoters. For example, viral promoters such as the 35S and 19S promoters of CaMV may be used alone or in combination with the omega leader sequence from TMV (Takamatsu, EMBO J. 6:307-311 (1987)). Alternatively, plant promoters such as the small subunit of RUBISCO or heat shock promoters may be used (Coruzzi et al., EMBO J. 3:1671-1680 (1984); Broglie et al., Science 224:838-843 (1984); and Winter et al., Results Probl. Cell Differ. 17:85-105 (1991)). These constructs can be introduced into plant cells by direct DNA transformation or pathogen-mediated transfection. Such techniques are described in a number of generally available reviews (see, e.g., Hobbs in McGraw Hill, Yearbook of Science and Technology, pp. 191-196 (1992)). 
     An insect system may also be used to express a polypeptide of interest. For example, in one such system,  Autographa californica  nuclear polyhedrosis virus (AcNPV) is used as a vector to express foreign genes in  Spodoptera frugiperda  cells or in  Trichoplusia  cells. The sequences encoding the polypeptide may be cloned into a non-essential region of the virus, such as the polyhedrin gene, and placed under control of the polyhedrin promoter. Successful insertion of the polypeptide-encoding sequence will render the polyhedrin gene inactive and produce recombinant virus lacking coat protein. The recombinant viruses may then be used to infect, for example,  S. frugiperda  cells or  Trichoplusia  cells in which the polypeptide of interest may be expressed (Engelhard et al., Proc. Natl. Acad. Sci. U.S.A. 91:3224-3227 (1994)). Also included are baculovirus expression systems, including those that utilize SF9, SF21, and  T. ni  cells (see, e.g., Murphy and Piwnica-Worms, Curr Protoc Protein Sci. Chapter 5:Unit5.4, 2001). Insect systems can provide post-translation modifications that are similar to mammalian systems. 
     In mammalian host cells, a number of viral-based expression systems are generally available. For example, in cases where an adenovirus is used as an expression vector, sequences encoding a polypeptide of interest may be ligated into an adenovirus transcription/translation complex consisting of the late promoter and tripartite leader sequence. Insertion in a non-essential E1 or E3 region of the viral genome may be used to obtain a viable virus which is capable of expressing the polypeptide in infected host cells (Logan &amp; Shenk, Proc. Natl. Acad. Sci. U.S.A. 81:3655-3659 (1984)). In addition, transcription enhancers, such as the Rous sarcoma virus (RSV) enhancer, may be used to increase expression in mammalian host cells. 
     Examples of useful mammalian host cell lines include monkey kidney CV1 line transformed by SV40 (COS-7, ATCC CRL 1651); human embryonic kidney line (293 or 293 cells sub-cloned for growth in suspension culture, Graham et al., J. Gen Virol. 36:59 (1977)); baby hamster kidney cells (BHK, ATCC CCL 10); mouse sertoli cells (TM4, Mather, Biol. Reprod. 23:243-251 (1980)); monkey kidney cells (CV1 ATCC CCL 70); African green monkey kidney cells (VERO-76, ATCC CRL-1587); human cervical carcinoma cells (HELA, ATCC CCL 2); canine kidney cells (MDCK, ATCC CCL 34); buffalo rat liver cells (BRL 3A, ATCC CRL 1442); human lung cells (W138, ATCC CCL 75); human liver cells (Hep G2, HB 8065); mouse mammary tumor (MMT 060562, ATCC CCL51); TRI cells (Mather et al., Annals N.Y. Acad. Sci. 383:44-68 (1982)); MRC 5 cells; FS4 cells; and a human hepatoma line (Hep G2). Other useful mammalian host cell lines include Chinese hamster ovary (CHO) cells, including DHFR-CHO cells (Urlaub et al., PNAS USA 77:4216 (1980)); and myeloma cell lines such as NSO and Sp2/0. For a review of certain mammalian host cell lines suitable for protein production, see, e.g., Yazaki and Wu, Methods in Molecular Biology, Vol. 248 (B. K. C Lo, ed., Humana Press, Totowa, N.J., 2003), pp. 255-268. Certain preferred mammalian cell expression systems include CHO and HEK293-cell based expression systems. Mammalian expression systems can utilize attached cell lines, for example, in T-flasks, roller bottles, or cell factories, or suspension cultures, for example, in 1 L and 5 L spinners, 5 L, 14 L, 40 L, 100 L and 200 L stir tank bioreactors, or 20/50 L and 100/200 L WAVE bioreactors, among others known in the art. 
     Also included is the cell-free expression of proteins. These and related embodiments typically utilize purified RNA polymerase, ribosomes, tRNA and ribonucleotides; these reagents may be produced by extraction from cells or from a cell-based expression system. 
     Specific initiation signals may also be used to achieve more efficient translation of sequences encoding a polypeptide of interest. Such signals include the ATG initiation codon and adjacent sequences. In cases where sequences encoding the polypeptide, its initiation codon, and upstream sequences are inserted into the appropriate expression vector, no additional transcriptional or translational control signals may be needed. However, in cases where only coding sequence, or a portion thereof, is inserted, exogenous translational control signals including the ATG initiation codon should be provided. Furthermore, the initiation codon should be in the correct reading frame to ensure translation of the entire insert. Exogenous translational elements and initiation codons may be of various origins, both natural and synthetic. The efficiency of expression may be enhanced by the inclusion of enhancers which are appropriate for the particular cell system which is used, such as those described in the literature (Scharf. et al., Results Probl. Cell Differ. 20:125-162 (1994)). 
     In addition, a host cell strain may be chosen for its ability to modulate the expression of the inserted sequences or to process the expressed protein in the desired fashion. Such modifications of the polypeptide include, but are not limited to, post-translational modifications such as acetylation, carboxylation, glycosylation, phosphorylation, lipidation, and acylation. Post-translational processing which cleaves a “prepro” form of the protein may also be used to facilitate correct insertion, folding and/or function. Different host cells such as yeast, CHO, HeLa, MDCK, HEK293, and W138, in addition to bacterial cells, which have or even lack specific cellular machinery and characteristic mechanisms for such post-translational activities, may be chosen to ensure the correct modification and processing of the foreign protein. 
     For long-term, high-yield production of recombinant proteins, stable expression is generally preferred. For example, cell lines which stably express a polynucleotide of interest may be transformed using expression vectors which may contain viral origins of replication and/or endogenous expression elements and a selectable marker gene on the same or on a separate vector. Following the introduction of the vector, cells may be allowed to grow for about 1-2 days in an enriched media before they are switched to selective media. The purpose of the selectable marker is to confer resistance to selection, and its presence allows growth and recovery of cells which successfully express the introduced sequences. Resistant clones of stably transformed cells may be proliferated using tissue culture techniques appropriate to the cell type. Transient production, such as by transient transfection or infection, can also be employed. Exemplary mammalian expression systems that are suitable for transient production include HEK293 and CHO-based systems. 
     Any number of selection systems may be used to recover transformed or transduced cell lines. These include, but are not limited to, the herpes simplex virus thymidine kinase (Wigler et al., Cell 11:223-232 (1977)) and adenine phosphoribosyltransferase (Lowy et al., Cell 22:817-823 (1990)) genes which can be employed in tk- or aprt-cells, respectively. Also, antimetabolite, antibiotic or herbicide resistance can be used as the basis for selection; for example, dhfr which confers resistance to methotrexate (Wigler et al., Proc. Natl. Acad. Sci. U.S.A. 77:3567-70 (1980)); npt, which confers resistance to the aminoglycosides, neomycin and G-418 (Colbere-Garapin et al., J. Mol. Biol. 150:1-14 (1981)); and als or pat, which confer resistance to chlorsulfuron and phosphinotricin acetyltransferase, respectively (Murry, supra). Additional selectable genes have been described, for example, trpB, which allows cells to utilize indole in place of tryptophan, or hisD, which allows cells to utilize histinol in place of histidine (Hartman &amp; Mulligan, Proc. Natl. Acad. Sci. U.S.A. 85:8047-51 (1988)). The use of visible markers has gained popularity with such markers as green fluorescent protein (GFP) and other fluorescent proteins (e.g., RFP, YFP), anthocyanins, P-glucuronidase and its substrate GUS, and luciferase and its substrate luciferin, being widely used not only to identify transformants, but also to quantify the amount of transient or stable protein expression attributable to a specific vector system (see, e.g., Rhodes et al., Methods Mol. Biol. 55:121-131 (1995)). 
     Also included are high-throughput protein production systems, or micro-production systems. Certain aspects may utilize, for example, hexa-histidine fusion tags for protein expression and purification on metal chelate-modified slide surfaces or MagneHis Ni-Particles (see, e.g., Kwon et al., BMC Biotechnol. 9:72, 2009; and Lin et al., Methods Mol Biol. 498:129-41, 2009)). Also included are high-throughput cell-free protein expression systems (see, e.g., Sitaraman et al., Methods Mol Biol. 498:229-44, 2009). 
     A variety of protocols for detecting and measuring the expression of polynucleotide-encoded products, using binding agents or antibodies such as polyclonal or monoclonal antibodies specific for the product, are known in the art. Examples include enzyme-linked immunosorbent assay (ELISA), western immunoblots, radioimmunoassays (RIA), and fluorescence activated cell sorting (FACS). These and other assays are described, among other places, in Hampton et al., Serological Methods, a Laboratory Manual (1990) and Maddox et al., J. Exp. Med. 158:1211-1216 (1983). 
     A wide variety of labels and conjugation techniques are known by those skilled in the art and may be used in various nucleic acid and amino acid assays. Means for producing labeled hybridization or PCR probes for detecting sequences related to polynucleotides include oligolabeling, nick translation, end-labeling or PCR amplification using a labeled nucleotide. Alternatively, the sequences, or any portions thereof may be cloned into a vector for the production of an mRNA probe. Such vectors are known in the art, are commercially available, and may be used to synthesize RNA probes in vitro by addition of an appropriate RNA polymerase such as T7, T3, or SP6 and labeled nucleotides. These procedures may be conducted using a variety of commercially available kits. Suitable reporter molecules or labels, which may be used include radionuclides, enzymes, fluorescent, chemiluminescent, or chromogenic agents as well as substrates, cofactors, inhibitors, magnetic particles, and the like. 
     Host cells transformed with one or more polynucleotide sequences of interest may be cultured under conditions suitable for the expression and recovery of the protein from cell culture. Certain specific embodiments utilize serum free cell expression systems. Examples include HEK293 cells and CHO cells that can grown on serum free medium (see, e.g., Rosser et al., Protein Expr. Purif. 40:237-43, 2005; and U.S. Pat. No. 6,210,922). 
     An activatable proprotein produced by a recombinant cell may be secreted or contained intracellularly depending on the sequence and/or the vector used. As will be understood by those of skill in the art, expression vectors containing polynucleotides may be designed to contain signal sequences which direct secretion of the encoded polypeptide through a prokaryotic or eukaryotic cell membrane. Other recombinant constructions may be used to join sequences encoding a polypeptide of interest to nucleotide sequence encoding a polypeptide domain which will facilitate purification and/or detection of soluble proteins. Examples of such domains include cleavable and non-cleavable affinity purification and epitope tags such as avidin, FLAG tags, poly-histidine tags (e.g., 6×His), cMyc tags, V5-tags, glutathione S-transferase (GST) tags, and others. 
     The protein produced by a recombinant cell can be purified and characterized according to a variety of techniques known in the art. Exemplary systems for performing protein purification and analyzing protein purity include fast protein liquid chromatography (FPLC) (e.g., AKTA and Bio-Rad FPLC systems), high-pressure liquid chromatography (HPLC) (e.g., Beckman and Waters HPLC). Exemplary chemistries for purification include ion exchange chromatography (e.g., Q, S), size exclusion chromatography, salt gradients, affinity purification (e.g., Ni, Co, FLAG, maltose, glutathione, protein A/G), gel filtration, reverse-phase, ceramic HYPERD® ion exchange chromatography, and hydrophobic interaction columns (HIC), among others known in the art. Also included are analytical methods such as SDS-PAGE (e.g., Coomassie, silver stain), immunoblot, Bradford, and ELISA, which may be utilized during any step of the production or purification process, typically to measure the purity of the protein composition. 
     Also included are methods of concentrating activatable proproteins, and composition comprising concentrated soluble activatable proproteins. In some aspects, such concentrated solutions of at least tone activatable proprotein comprise proteins at a concentration of about or at least about 5 mg/mL, 8 mg/mL, 10 mg/mL, 15 mg/mL, 20 mg/mL, or more. 
     In some aspects, such compositions may be substantially monodisperse, meaning that an activatable proprotein exists primarily (i.e., at least about 90%, or greater) in one apparent molecular weight form when assessed for example, by size exclusion chromatography, dynamic light scattering, or analytical ultracentrifugation. 
     In some aspects, such compositions have a purity (on a protein basis) of at least about 90%, or in some aspects at least about 95% purity, or in some embodiments, at least 98% purity. Purity may be determined via any routine analytical method as known in the art. 
     In some aspects, such compositions have a high molecular weight aggregate content of less than about 10%, compared to the total amount of protein present, or in some embodiments such compositions have a high molecular weight aggregate content of less than about 5%, or in some aspects such compositions have a high molecular weight aggregate content of less than about 3%, or in some embodiments a high molecular weight aggregate content of less than about 1%. High molecular weight aggregate content may be determined via a variety of analytical techniques including for example, by size exclusion chromatography, dynamic light scattering, or analytical ultracentrifugation. 
     Examples of concentration approaches contemplated herein include lyophilization, which is typically employed when the solution contains few soluble components other than the protein of interest. Lyophilization is often performed after HPLC run, and can remove most or all volatile components from the mixture. Also included are ultrafiltration techniques, which typically employ one or more selective permeable membranes to concentrate a protein solution. The membrane allows water and small molecules to pass through and retains the protein; the solution can be forced against the membrane by mechanical pump, gas pressure, or centrifugation, among other techniques. 
     In certain embodiments, an activatable proprotein in a composition has a purity of at least about 90%, as measured according to routine techniques in the art. In certain embodiments, such as diagnostic compositions or certain pharmaceutical or therapeutic compositions, an activatable proprotein composition has a purity of at least about 95%, or at least about 97% or 98% or 99%. In some embodiments, such as when being used as reference or research reagents, activatable proproteins can be of lesser purity, and may have a purity of at least about 50%, 60%, 70%, or 80%. Purity can be measured overall or in relation to selected components, such as other proteins, e.g., purity on a protein basis. 
     Purified activatable proproteins can also be characterized according to their biological characteristics. Binding affinity and binding kinetics can be measured according to a variety of techniques known in the art, such as Biacore® and related technologies that utilize surface plasmon resonance (SPR), an optical phenomenon that enables detection of unlabeled interactants in real time. SPR-based biosensors can be used in determination of active concentration, screening and characterization in terms of both affinity and kinetics. The presence or levels of one or more biological activities can be measured according to cell-based assays, including those that utilize at least one IL-2 receptor, which is optionally functionally coupled to a readout or indicator, such as a fluorescent or luminescent indicator of biological activity, as described herein. 
     In certain embodiments, as noted above, an activatable proprotein composition is substantially endotoxin free, including, for example, about 95% endotoxin free, preferably about 99% endotoxin free, and more preferably about 99.99% endotoxin free. The presence of endotoxins can be detected according to routine techniques in the art, as described herein. In specific embodiments, an activatable proprotein composition is made from a eukaryotic cell such as a mammalian or human cell in substantially serum free media. In certain embodiments, as noted herein, an activatable proprotein composition has an endotoxin content of less than about 10 EU/mg of activatable proprotein, or less than about 5 EU/mg of activatable proprotein, less than about 3 EU/mg of activatable proprotein, or less than about 1 EU/mg of activatable proprotein. 
     In certain embodiments, an activatable proprotein composition comprises less than about 10% wt/wt high molecular weight aggregates, or less than about 5% wt/wt high molecular weight aggregates, or less than about 2% wt/wt high molecular weight aggregates, or less than about or less than about 1% wt/wt high molecular weight aggregates. 
     Also included are protein-based analytical assays and methods, which can be used to assess, for example, protein purity, size, solubility, and degree of aggregation, among other characteristics. Protein purity can be assessed a number of ways. For instance, purity can be assessed based on primary structure, higher order structure, size, charge, hydrophobicity, and glycosylation. Examples of methods for assessing primary structure include N- and C-terminal sequencing and peptide-mapping (see, e.g., Allen et al., Biologicals. 24:255-275, 1996)). Examples of methods for assessing higher order structure include circular dichroism (see, e.g., Kelly et al., Biochim Biophys Acta. 1751:119-139, 2005), fluorescent spectroscopy (see, e.g., Meagher et al., J. Biol. Chem. 273:23283-89, 1998), FT-IR, amide hydrogen-deuterium exchange kinetics, differential scanning calorimetry, NMR spectroscopy, immunoreactivity with conformationally sensitive antibodies. Higher order structure can also be assessed as a function of a variety of parameters such as pH, temperature, or added salts. Examples of methods for assessing protein characteristics such as size include analytical ultracentrifugation and size exclusion HPLC (SEC-HPLC), and exemplary methods for measuring charge include ion-exchange chromatography and isolectric focusing. Hydrophobicity can be assessed, for example, by reverse-phase HPLC and hydrophobic interaction chromatography HPLC. Glycosylation can affect pharmacokinetics (e.g., clearance), conformation or stability, receptor binding, and protein function, and can be assessed, for example, by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. 
     As noted above, certain embodiments include the use of SEC-HPLC to assess protein characteristics such as purity, size (e.g., size homogeneity) or degree of aggregation, and/or to purify proteins, among other uses. SEC, also including gel-filtration chromatography (GFC) and gel-permeation chromatography (GPC), refers to a chromatographic method in which molecules in solution are separated in a porous material based on their size, or more specifically their hydrodynamic volume, diffusion coefficient, and/or surface properties. The process is generally used to separate biological molecules, and to determine molecular weights and molecular weight distributions of polymers. Typically, a biological or protein sample (such as a protein extract produced according to the protein expression methods provided herein and known in the art) is loaded into a selected size-exclusion column with a defined stationary phase (the porous material), preferably a phase that does not interact with the proteins in the sample. In certain aspects, the stationary phase is composed of inert particles packed into a dense three-dimensional matrix within a glass or steel column. The mobile phase can be pure water, an aqueous buffer, an organic solvent, or a mixture thereof. The stationary-phase particles typically have small pores and/or channels which only allow molecules below a certain size to enter. Large particles are therefore excluded from these pores and channels, and their limited interaction with the stationary phase leads them to elute as a “totally-excluded” peak at the beginning of the experiment. Smaller molecules, which can fit into the pores, are removed from the flowing mobile phase, and the time they spend immobilized in the stationary-phase pores depends, in part, on how far into the pores they penetrate. Their removal from the mobile phase flow causes them to take longer to elute from the column and results in a separation between the particles based on differences in their size. A given size exclusion column has a range of molecular weights that can be separated. Overall, molecules larger than the upper limit will not be trapped by the stationary phase, molecules smaller than the lower limit will completely enter the solid phase and elute as a single band, and molecules within the range will elute at different rates, defined by their properties such as hydrodynamic volume. For examples of these methods in practice with pharmaceutical proteins, see Bruner et al., Journal of Pharmaceutical and Biomedical Analysis. 15: 1929-1935, 1997. 
     Protein purity for clinical applications is also discussed, for example, by Anicetti et al. (Trends in Biotechnology. 7:342-349, 1989). More recent techniques for analyzing protein purity include, without limitation, the LabChip GXII, an automated platform for rapid analysis of proteins and nucleic acids, which provides high throughput analysis of titer, sizing, and purity analysis of proteins. In certain non-limiting embodiments, clinical grade activatable proproteins can be obtained by utilizing a combination of chromatographic materials in at least two orthogonal steps, among other methods (see, e.g., Therapeutic Proteins: Methods and Protocols. Vol. 308, Eds., Smales and James, Humana Press Inc., 2005). Typically, protein agents (e.g., activatable proprotein) are substantially endotoxin-free, as measured according to techniques known in the art and described herein. 
     Protein solubility assays are also included. Such assays can be utilized, for example, to determine optimal growth and purification conditions for recombinant production, to optimize the choice of buffer(s), and to optimize the choice of activatable proproteins and variants thereof. Solubility or aggregation can be evaluated according to a variety of parameters, including temperature, pH, salts, and the presence or absence of other additives. Examples of solubility screening assays include, without limitation, microplate-based methods of measuring protein solubility using turbidity or other measure as an end point, high-throughput assays for analysis of the solubility of purified recombinant proteins (see, e.g., Stenvall et al., Biochim Biophys Acta. 1752:6-10, 2005), assays that use structural complementation of a genetic marker protein to monitor and measure protein folding and solubility in vivo (see, e.g., Wigley et al., Nature Biotechnology. 19:131-136, 2001), and electrochemical screening of recombinant protein solubility in  Escherichia coli  using scanning electrochemical microscopy (SECM) (see, e.g., Nagamine et al., Biotechnology and Bioengineering. 96:1008-1013, 2006), among others. Activatable proprotein with increased solubility (or reduced aggregation) can be identified or selected for according to routine techniques in the art, including simple in vivo assays for protein solubility (see, e.g., Maxwell et al., Protein Sci. 8:1908-11, 1999). 
     Protein solubility and aggregation can also be measured by dynamic light scattering techniques. Aggregation is a general term that encompasses several types of interactions or characteristics, including soluble/insoluble, covalent/noncovalent, reversible/irreversible, and native/denatured interactions and characteristics. For protein therapeutics, the presence of aggregates is typically considered undesirable because of the concern that aggregates may cause an immunogenic reaction (e.g., small aggregates), or may cause adverse events on administration (e.g., particulates). Dynamic light scattering refers to a technique that can be used to determine the size distribution profile of small particles in suspension or polymers such as proteins in solution. This technique, also referred to as photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS), uses scattered light to measure the rate of diffusion of the protein particles. Fluctuations of the scattering intensity can be observed due to the Brownian motion of the molecules and particles in solution. This motion data can be conventionally processed to derive a size distribution for the sample, wherein the size is given by the Stokes radius or hydrodynamic radius of the protein particle. The hydrodynamic size depends on both mass and shape (conformation). Dynamic scattering can detect the presence of very small amounts of aggregated protein (&lt;0.01% by weight), even in samples that contain a large range of masses. It can also be used to compare the stability of different formulations, including, for example, applications that rely on real-time monitoring of changes at elevated temperatures. Accordingly, certain embodiments include the use of dynamic light scattering to analyze the solubility and/or presence of aggregates in a sample that contains an activatable proprotein of the present disclosure. 
     Although the foregoing embodiments have been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to one of ordinary skill in the art in light of the teachings of this disclosure that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. The following examples are provided by way of illustration only and not by way of limitation. Those of skill in the art will readily recognize a variety of noncritical parameters that could be changed or modified to yield essentially similar results. 
     EXAMPLES 
     Example 1 
     Engineering of LC-IL-2/Fd-IL-2Rα Fusion Proteins 
     To reduce the toxicity of IL-2 related therapeutic drugs, LC-linker-IL-2/Fd-linker-IL-2Rα fusion proteins were generated as prodrugs (i.e., as activatable proproteins as described elsewhere herein). The prodrugs have very low activities. To restore the activity of IL-2, TEV protease cleavage site was used to prove the concept and introduced into the linker between Fd and IL-2Rα. Full activities can be restored upon protease cleavage of the designed protease specific linker sequence within the prodrugs. The designations and corresponding sequences identifiers for each activatable proprotein are provided in Table E1 below. 
     
       
         
           
               
               
               
             
               
                 TABLE E1 
               
               
                   
               
               
                 Activatable Proprotein 
                 SEQ ID NO: of 
                 SEQ ID NO: of 
               
               
                 Designation 
                 IL-2 fusion 
                 IL-2Rα fusion 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 P13541362 
                 9 
                 85 
               
               
                 P13551363 
                 13 
                 86 
               
               
                 P13561364 
                 17 
                 87 
               
               
                 P13561371 
                 21 
                 88 
               
               
                 P13571370 
                 25 
                 89 
               
               
                 P13581365 
                 29 
                 90 
               
               
                 P13581369 
                 33 
                 91 
               
               
                 P13591366 
                 37 
                 92 
               
               
                 P13591367 
                 41 
                 93 
               
               
                 P13601368 
                 45 
                 94 
               
               
                 P13611362 
                 49 
                 95 
               
               
                 P15071366 
                 53 
                 96 
               
               
                 P15081366 
                 57 
                 97 
               
               
                 P15091366 
                 61 
                 98 
               
               
                 P15101366 
                 65 
                 99 
               
               
                 P16121613 
                 69 
                 100 
               
               
                 P13591627 
                 73 
                 101 
               
               
                 P13591628 
                 77 
                 102 
               
               
                 P13591629 
                 81 
                 103 
               
               
                 P16841362 
                 233 
                 234 
               
               
                 P16851363 
                 237 
                 238 
               
               
                 P16841687 
                 235 
                 236 
               
               
                   
               
            
           
         
       
     
     To confirm disulfide formation between IL-2 and IL-2Rα, the disulfide bond in Herceptin Fab was eliminated by mutating the C-terminal cysteine on the light chain to serine and excluding the cysteine in the hinge region. The C125S point mutation was introduced into IL-2 to avoid unwanted IL-2 interchain disulfide formation. An illustrative protein is P13541362. 
     To form a tighter IL-2/IL-2Rα complex, a single cysteine mutation was introduced into IL-2 or IL-2Rα separately to force disulfide formation between IL-2 and IL-2Rα. Cysteine mutations designed in IL-2 were as follows: K35C, R38C, T41C, F42C, E61C and V69C. Cysteine mutations designed in IL-2Rα were as follows: D4C, D6C, N27C, K38C, S39C, L42C, Y43C, 1118C and H120C. Illustrative proteins are P13551363, P13561364, P13561371, P13571370, P13581365, P13581369, P13591366, P13591367, and P13601368. 
     A triple mutein of IL-2 (V69A, Q74P, and 1128T substitutions) with higher binding affinity towards IL-2Rα was tested in prodrug format. The illustrative proteins are P13611362, P16841362, and P16851363. 
     Different linker lengths between the LC and IL-2 were also designed to evaluate the effect on IL-2 activity after protease cleavage. Illustrative proteins are P15071366, P15081366, and P15091366. 
     IL-2 superkine (D10) was also prepared in the prodrug format. Illustrative protein is P15101366. 
     Potential N-glycosylation sites in IL-2Rα were mutated to alanine to eliminate glycosylation. Illustrative proteins are P13591627, P13591628, and P13591629. 
     The LC-linker-IL-2/Fd-linker-IL-2Rα-Fc format was also tested. Illustrative protein is P16841687. 
     Plasmids coding for Herceptin-LC-linker-IL-2 (or variants) and Herceptin-Fd-TEV-IL-2Rα (or variants) were constructed by standard gene synthesis, followed by sub-cloning into pTT5 expression vector. 
     Production, Purification, and Physical Characterization. 
     Fab-IL-2/IL-2Rα fusion proteins were produced by transient transfection in Expi293 cells, and purified by a two-step purification process comprising CaptureSelect™ IgG-CH1 Affinity Matrix (ThermoFisher) and size exclusion chromatography (Superdex 200, GE Healthcare). 
     Purified proteins were characterized by SDS-PAGE for purity assessment and showed good purity (see  FIGS. 4A-4B  and  FIGS. 10A-10B ). No di-sulfide bond was formed between IL-2 and IL-2Rα in P13541362 and P13611362. Di-sulfide bond was partially formed between IL-2 and IL-2Rα in proteins: P13561364, P13571370, P13581365, P13581369, and P13601368. Di-sulfide bond was formed completely between IL-2 and IL-2Rα in proteins: P13551363, P13561371, P13591366, P13591367, P15071366, P15081366, P15091366, P15101366, P13591627, P13591628, and P13591629. Purified proteins could be cleaved by TEV completely or partially as shown in  FIG. 4C  and  FIG. 10C . 
     Purified proteins were also characterized by high performance liquid chromatography (HPLC) for homogeneity assessment. HPLC analysis was performed using Nanofilm SEC-250 column (Sepax) and Agilent 1260 according to the manufacturer&#39;s instructions. Representative HPLC results are shown in  FIGS. 5A-5E  and  FIGS. 11A-11F . Some of the fusion proteins were a mixture of dimer and monomer as shown in  FIGS. 5A-5C . Some of the fusion proteins showed a main peak of monomer and low left shoulder of dimer as shown in  FIGS. 5D-5E . 
     Functional Proliferation Assays. Proliferation assays were performed for purified proteins before and after TEV cleavage. M-07e cells (IL-2Rβ/γc) were cultured in RPMI 1640 supplemented with 20% fetal bovine serum (FBS), 1% non-essential amino acids (NEAA), and 10% of 5637 cell culture supernatant. To measure cytokine-dependent cell proliferation, M-07e cells were harvested in the logarithmic growth phase and washed twice with PBS. 90 μl of cell suspension (2×10 4  cells/well) was seeded into 96-well plate and incubated for 4 hours in assay medium (RPMI 1640 supplemented with 10% FBS and 1% NEAA) for cytokine starvation at 37° C. and 5% CO 2 . IL-2 and purified protein samples used in assays were prepared in assay medium to an initial concentration of 300 nM, followed by 1/3 serial dilutions. 101 diluted protein was added into corresponding wells and incubated at 37° C. and 5% CO 2  for 72 hours. Colorimetric assays using a Cell Counting Kit-8 (CCK-8, Dojindo, CK04) were performed to measure the amount of live cells. The results are shown in  FIGS. 6A-6R  and  FIGS. 12A-12F . The Fab-IL-2/IL-2Rα activatable proproteins showed very low or no functional activity before TEV cleavage, and showed significantly increased activity after TEV cleavage. 
     Example 2 
     Engineering of LC-IL-2Rα/Fd-IL-2 Fusion Proteins 
     An activatable proprotein having the LC-IL-2Rα/Fd-IL-2 format was designed and tested. TEV protease cleavage site was used to prove the concept and introduced into the linker between the LC and IL-2Rα. To form a disulfide bond between IL-2 and IL-2Rα, an E61C mutation was introduced into IL-2 and a K38C was introduced into IL-2Rα. Illustrative protein is P16121613. 
     Plasmids coding for Herceptin-LC-linker-IL-2 Ra (or variants) and Herceptin-Fd-TEV-IL-2 (or variants) were constructed by standard gene synthesis, followed by sub-cloning into pTT5 expression vector. 
     Production, Purification, and Physical Characterization. 
     Activatable proproteins were produced and characterized as in Example 1. Purified P16121613 showed good purity on SDS-PAGE ( FIGS. 7A-7B ) and could be cleaved by TEV ( FIG. 7C ). P16121613 showed one main peak of monomer and a low left shoulder of dimer ( FIG. 7D ). 
     Functional Proliferation Assays. 
     Proliferation assay was performed for purified P16121613 before and after TEV cleavage as described in Example 1. The results are summarized in  FIGS. 8A-8B , which includes a comparison to P13591366 (8B). P16121613 showed low activity before TEV cleavage, and almost full activity after TEV cleavage, as compared to wild type IL-2. Compared with P13591366, P16121613 showed somewhat higher background activity before TEV cleavage, and higher activity after cleavage. 
     Example 3 
     Engineering of IL-2 and Anti-IL-2 Antibody Fusion Proteins 
     An activatable proprotein having the LC-IL-2/Fd-anti-IL-2-scFv format was designed and tested. TEV protease cleavage site was used to prove the concept and introduced into the linker between the Fd and anti-IL-2-scFv. Illustrative proteins are P16931694 and P16931695 (with Fc at the C-terminal of anti-IL-2-scFv). 
     An activatable proprotein having IL-2 at the N-terminal of light chain of an anti-IL-2 antibody was designed and tested. Protease cleavage site was introduced into the linker between IL-2 and light chain. Illustrative proteins are P17081710 and P17091710. 
     Plasmids coding for LC-IL-2/Fd-anti-IL-2-scFv format and IL-2-anti-IL-2-antibody format were constructed by standard gene synthesis, followed by sub-cloning into pTT5 expression vector. 
     Production, Purification, and Physical Characterization. 
     Activatable proproteins were produced and characterized as in Example 1. Purified proteins showed good purity on SDS-PAGE ( FIGS. 10A-10B ) and could be cleaved by protease ( FIGS. 10C-10D ). P16931694 and P16931695 showed good homogeneity ( FIGS. 11G-11H ). P17081710 and P17091710 showed one main monomer peak and a low left shoulder of dimer ( FIGS. 11I-11J ). 
     Functional Proliferation Assays. 
     Proliferation assay was performed for purified proteins before and after protease cleavage as described in Example 1. The results are summarized in  FIGS. 12G-12J . P16931694 and P16931695 showed low activity before TEV cleavage and partial activity after TEV cleavage. P17081710 and P17091710 showed low activity as a prodrug. 
     Example 4 
     Engineering of HC1-IL-2/HC2-IL-2Rα Fusion Proteins 
     An activatable proprotein having the HC1-IL-2/HC2-IL-2Rα format was designed and tested. Protease cleavage site was introduced into the linker between the heavy chain and IL-2Rα. Illustrative proteins are P1453182124, P1453182730, P2492962158, P2492972158, P2492982158, and P2493962158. 
     Plasmids coding for HC1-IL-2/HC2-IL-2Rα fusion proteins were constructed by standard gene synthesis, followed by sub-cloning into the pTT5 expression vector. 
     Production, Purification, and Physical Characterization. 
     Activatable proproteins were produced and characterized as in Example 1. Purified proteins showed good purity on SDS-PAGE ( FIGS. 10A-10B ) and could be cleaved by protease partially or completely ( FIG. 10D ). Purified proteins showed one main monomer peak and a low left shoulder of dimer ( FIGS. 11K-11P ). 
     Functional Proliferation Assays. 
     Proliferation assay was performed for purified proteins before and after protease cleavage as described in Example 1. The results are summarized in  FIGS. 12K-12P . Purified proteins showed low activity before protease cleavage and higher activity after protease cleavage. 
     All publications, patent applications, and issued patents cited in this specification are herein incorporated by reference as if each individual publication, patent application, or issued patent were specifically and individually indicated to be incorporated by reference.