Abstract:
The present invention relates to discovery of the ectopic expression of EDEM2 in a production cell to improve the yield of a useful multi-subunit protein. Thus, the present invention provides for production cell lines, such as the canonical mammalian biopharmaceutical production cell—the CHO cell, containing recombinant polynucleotides encoding EDEM2. Also disclosed is a production cell containing both an EDEM2-encoding polynucleotide as well an XBP1-encoding polynucleotide. Improved titers of antibodies produced by these cell lines are disclosed, as well as the improved cell densities attained by these cells in culture.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of PCT International Application No. PCT/US2013/043116, filed 29 May 2013, which claims the benefit of U.S. patent application Ser. No. 13/904,587, filed 29 May 2013, and claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/652,549 filed 29 May 2012, all of which are herein specifically incorporated by reference in their entirety. 
    
    
     SEQUENCE LISTING 
     This application incorporates by reference the sequence listing submitted in computer readable form as file 8150U502_ST25.txt created on Nov. 26, 2014 (206,400 bytes). 
     FIELD OF THE INVENTION 
     The invention relates to a cell or cells expressing a recombinant stress-response lectin for the improved production of a multi-subunit protein. Specifically, the invention provides a mammalian cell and cell-line derived therefrom containing a gene encoding EDEM2, and which yields antibody at a high titer. 
     BACKGROUND OF THE INVENTION 
     The manufacture of therapeutically active proteins requires proper folding and processing prior to secretion. Proper folding is particularly relevant for proteins, such as antibodies, which consist of multiple subunits that must be properly assembled before secretion. Eukaryotic cells have adapted a system that ensures the proper folding of proteins and the removal of misfolded proteins from the secretory pathway. This system is called the unfolded protein response (UPR) pathway, and it is triggered by the accumulation of misfolded proteins in the endoplasmic reticulum (ER). 
     An early event of the UPR is the activation of the transcription factor Xbp1, which in turn activates the transcription of endoplasmic reticulum degradation-enhancing alpha-mannosidase-like protein 2 (EDEM2), a member of the endoplasmic reticulum associated degradation (ERAD) pathway. EDEM2 facilitates the removal of misfolded proteins. The ERAD pathway comprises five steps: (1) chaperone-mediated recognition of malformed proteins, (2) targeting of malformed proteins to the retrotranslocation machinery or E3-ligases, which involves EDEM2, (3) initiation of retrotranslocation; (4) ubiquitylation and further retrotranslocation; and (5) proteosome targeting and degradation. 
     Antibodies are multi-subunit proteins comprising two heavy chains and two light chains, which must be properly folded and associated to form a functional heterotetramer. Any improvement in the efficient and accurate processing of the heavy and light chains to improve the yield or titer of functional antibody heterotetramers is desired. 
     SUMMARY OF THE INVENTION 
     Applicants made the surprising discovery that the ectopic expression of EDEM2 in a protein-manufacturing cell line increases the average output of protein per cell, increases the titer of protein secreted into the media, and increases the integrated cell density of production cell lines. 
     Thus, in one aspect, the invention provides a cell containing (a) a recombinant polynucleotide that encodes a stress-induced mannose-binding lectin and (b) a polynucleotide that encodes a multi-subunit protein. In some embodiments, the stress-induced mannose-binding lectin is an EDEM2 protein, non-limiting examples of which are provided in Table 1, and the multi-subunit protein is an antibody. In other embodiments, the cell also contains a polynucleotide that encodes the active spliced form of XBP1, non-limiting examples of which are provided in Table 2. In one embodiment, the cell is a mammalian cell, such as a CHO cell used in the manufacture of biopharmaceuticals. 
     In another aspect, the invention provides a cell line derived from the cell described in the previous aspect. By “derived from”, what is meant is a population of cells clonally descended from an individual cell and having some select qualities, such as the ability to produce active protein at a given titer, or the ability to proliferate to a particular density. In some embodiments, the cell line, which is derived from a cell harboring the recombinant polynucleotide encoding a stress-induced mannose-binding lectin and a polynucleotide encoding a multi-subunit protein, is capable of producing the multi-subunit protein at a titer of at least 3 grams per liter of media (g/L), at least 5 g/L, or at least 8 g/L. In some embodiments, the cell line can attain an integrated cell density (ICD) that is at least 30% greater, at least 50% greater, at least 60% greater, or at least 90% greater than the integrated cell density attainable by a cell line derived from what is essentially the same cell but without the recombinant polynucleotide encoding the stress-induced mannose-binding lectin. 
     In another aspect, the invention provides an isolated or recombinant polynucleotide comprising a nucleic acid sequence encoding an EDEM2 protein, which is operably linked (cis) to a constitutive and ubiquitously expressed mammalian promoter, such as the ubiquitin C promoter. In some embodiments, the EDEM2 protein has the amino acid of SEQ ID NO: 8, or an amino acid sequence that is at least 92% identical to any one of SEQ ID NO: 1-7. In some embodiments, the polynucleotide comprises a nucleic acid sequence of SEQ ID NO: 16. In one particular embodiment, the polynucleotide consists of a nucleic acid sequence of SEQ ID NO: 14; and in another particular embodiment, SEQ ID NO: 15. 
     In another aspect, the invention provides an isolated or recombinant polynucleotide comprising a nucleic acid sequence encoding an XBP1 protein, which is operably linked to (in cis) a constitutive and ubiquitously expressed mammalian promoter, such as the ubiquitin C promoter. In some embodiments, the XBP1 protein has the amino acid of SEQ ID NO: 13, or an amino acid sequence that is at least 86% identical to any one of SEQ ID NO: 9-12. In some embodiments, the polynucleotide comprises a nucleic acid sequence of SEQ ID NO: 18. In one particular embodiment, the polynucleotide consists of a nucleic acid sequence of SEQ ID NO: 17. 
     In another aspect, the invention provides a cell that contains an EDEM2-encoding polynucleotide, as described in the prior aspect, and a polynucleotide that encodes a multi-subunit protein, such as an antibody. In some embodiments, the cell also contains an XBP1-encoding polynucleotide, as described in the preceding aspect. In one embodiment, the multi-subunit protein is an antibody, and the heavy chain of the antibody comprises an amino acid sequence of SEQ ID NO: 43 and SEQ ID NO: 44, and the light chain of the antibody comprises an amino acid sequence of SEQ ID NO: 45 and SEQ ID NO: 46. In this and several embodiments, each polypeptide subunit of the multi-subunit protein is encoded by a separate polynucleotide. Thus, for example, a polynucleotide encoding an antibody may include a polynucleotide encoding a heavy chain and a polynucleotide encoding a light chain, hence two subunits. In some embodiments, the cell is a Chinese hamster ovary (CHO) cell. 
     In one embodiment, the encoded multi-subunit protein is an anti-GDF8 antibody having a heavy chain variable region amino acid sequence of SEQ ID NO: 20 and a light chain variable region amino acid sequence of SEQ ID NO: 22. In one embodiment, the anti-GDF8 antibody comprises a heavy chain having an amino acid sequence of SEQ ID NO: 19 and a light chain having an amino acid sequence of SEQ ID NO: 21. In one embodiment, the polynucleotide that encodes the heavy chain of the anti-GDF8 antibody comprises a nucleic acid sequence of SEQ ID NO: 23; and the polynucleotide that encodes the light chain of the anti-GDF8 antibody comprises a nucleic acid sequence of SEQ ID NO: 25. In one embodiment, the polynucleotide that encodes the heavy chain of the anti-GDF8 antibody consists of a nucleic acid sequence of SEQ ID NO: 24; and the polynucleotide that encodes the light chain of the anti-GDF8 antibody consists of a nucleic acid sequence of SEQ ID NO: 25. 
     In another embodiment, the encoded multi-subunit protein is an anti-ANG2 antibody having a heavy chain variable region amino acid sequence of SEQ ID NO: 28 and a light chain variable region amino acid sequence of SEQ ID NO: 30. In one embodiment, the anti-ANG2 antibody comprises a heavy chain having an amino acid sequence of SEQ ID NO: 27 and a light chain having an amino acid sequence of SEQ ID NO: 29. In one embodiment, the polynucleotide that encodes the heavy chain of the anti-ANG2 antibody comprises a nucleic acid sequence of SEQ ID NO: 31; and the polynucleotide that encodes the light chain of the anti-ANG2 antibody comprises a nucleic acid sequence of SEQ ID NO: 33. In one embodiment, the polynucleotide that encodes the heavy chain of the anti-ANG2 antibody consists of a nucleic acid sequence of SEQ ID NO: 32; and the polynucleotide that encodes the light chain of the anti-ANG2 antibody consists of a nucleic acid sequence of SEQ ID NO: 34. 
     In another embodiment, the encoded multi-subunit protein is an anti-ANGPTL4 antibody having a heavy chain variable region amino acid sequence of SEQ ID NO: 36 and a light chain variable region amino acid sequence of SEQ ID NO: 38. In one embodiment, the anti-ANGPTL4 antibody comprises a heavy chain having an amino acid sequence of SEQ ID NO: 35 and a light chain having an amino acid sequence of SEQ ID NO: 37. In one embodiment, the polynucleotide that encodes the heavy chain of the anti-ANGPTL4 antibody comprises a nucleic acid sequence of SEQ ID NO: 39; and the polynucleotide that encodes the light chain of the anti-ANGPTL4 antibody comprises a nucleic acid sequence of SEQ ID NO: 41. In one embodiment, the polynucleotide that encodes the heavy chain of the anti-ANGPTL4 antibody consists of a nucleic acid sequence of SEQ ID NO: 40; and the polynucleotide that encodes the light chain of the anti-ANGPTL4 antibody consists of a nucleic acid sequence of SEQ ID NO: 42. 
     In another aspect, the invention provides a method of manufacturing a multi-subunit protein, by culturing a cell of the previous aspect in a medium, wherein the multi-subunit protein is synthesized in the cell and subsequently secreted into the medium. In some embodiments, the multi-subunit protein is an antibody, such as for example anti-GDF8, anti-ANG2, anti-ANGPTL4, or an antibody having a heavy chain sequence of SEQ ID NO: 43 and 44, and a light chain sequence of SEQ ID NO: 45 and 46. In some embodiments, the multi-subunit protein attains a titer of at least 3 g/L, at least 5 g/L, at least 6 g/L, or at least 8 g/L. In some embodiments, the cell proliferates in the medium and establishes an integrated cell density of about ≧5×10 7  cell-day/mL, about ≧1×10 8  cell-day/mL, or about ≧1.5×10 8  cell-day/mL. 
     In another aspect, the invention provides a multi-subunit protein, which is manufactured according to the method described in the preceding aspect. In one embodiment, the manufactured protein is an antibody. In some embodiments, the antibody consists of a heavy chain, which comprises an amino acid sequence of SEQ ID NO: 43 and SEQ ID NO: 44, and a light chain, which comprises an amino acid sequence of SEQ ID NO: 45 and SEQ ID NO: 46. In one specific embodiment, the manufactured multi-subunit protein is an anti-GDF8 antibody having a heavy chain variable region amino acid sequence of SEQ ID NO: 20 and a light chain variable region amino acid sequence of SEQ ID NO: 22. In another specific embodiment, the manufactured multi-subunit protein is an anti-ANG2 antibody having a heavy chain variable region amino acid sequence of SEQ ID NO: 28 and a light chain variable region amino acid sequence of SEQ ID NO: 30. In yet another specific embodiment, the manufactured multi-subunit protein is an anti-ANGPTL4 antibody having a heavy chain variable region amino acid sequence of SEQ ID NO: 36 and a light chain variable region amino acid sequence of SEQ ID NO: 38. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows consistently higher protein titer (in mg/L) for EDEM2-expressing clonal cell-lines (gray circles) compared to XBP1-expressing clonal cell-lines (black circles). 
         FIG. 1B  depicts the integrated cell density (cell-days/mL) for EDEM2-expressing clones (gray circles) compared to XBP1-expressing clones (black circles). Each clone (#1-24) expresses the same gene of interest (antibody), under the same regulatory conditions, and expressing either XBP1 (RGC91) or EDEM2 (RGC92) at a transcriptionally active locus. 
         FIG. 2A  shows the FACS scans (flow cytometry-based autologous secretion trap (FASTR)) of several clones expressing XBP1. Inconsistency in the peaks is indicative of unstable growth (i.e. a variable or heterogeneous mixture) of the cells in the clones tested. 
         FIG. 2B  shows the FACS scans of several clones expressing EDEM2, having little to no variation in the peaks representative of clonal stability in the clones tested.  FIG. 2B  depicts the clonal stability of several clonal cell-lines expressing EDEM2 and an antibody of interest. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Before the present invention is described, it is to be understood that this invention is not limited to particular methods and experimental conditions described, as such methods and conditions may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. As used herein, the term “about”, when used in reference to a particular recited numerical value or range of values, means that the value may vary from the recited value by no more than 1%. For example, as used herein, the expression “about 100” includes 99 and 101 and all values in between (e.g., 99.1, 99.2, 99.3, 99.4, etc.). 
     Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference in their entirety. 
     As used herein, the term “recombinant polynucleotide”, which is used interchangeably with “isolated polynucleotide”, means a nucleic acid polymer such as a ribonucleic acid or a deoxyribonucleic acid, either single stranded or double stranded, originating by genetic engineering manipulations. A recombinant polynucleotide may be a circular plasmid or a linear construct existing in vitro or within a cell as an episome. A recombinant polynucleotide may be a construct that is integrated within a larger polynucleotide molecule or supermolecular structure, such as a linear or circular chromosome. The larger polynucleotide molecule or supermolecular structure may be within a cell or within the nucleus of a cell. Thus, a recombinant polynucleotide may be integrated within a chromosome of a cell. 
     As used herein, the term “stress-induced mannose-binding lectin” refers to a mannose-binding protein, which means a protein that binds or is capable of binding mannose, derivatives of mannose, such as mannose-6-phosphate, or a glycoprotein that expresses mannose or a mannose derivative in its glycocalyx; and whose activity is upregulated during stress. Cellular stress includes inter alia starvation, DNA damage, hypoxia, poisoning, shear stress and other mechanical stresses, tumor stress, and the accumulation of misfolded proteins in the endoplasmic reticulum. Exemplary stress-induced mannose-binding lectins include the EDEM proteins EDEM1, EDEM2 and EDEM3, Yos 9, OS9, and XTP3-B (see Vembar and Brodsky, Nat. Rev. Mol. Cell. Biol. 9(12): 944-957, 2008, and references cited therein). 
     As used herein, the term “EDEM2” means any ortholog, homolog, or conservatively substituted variant of endoplasmic reticulum degradation-enhancing alpha-mannosidase-like protein. EDEM2 proteins are generally known in the art to be involved endoplasmic reticulum-associated degradation (ERAD), being up-regulated by Xbp-1 and facilitating the extraction of misfolded glycoproteins from the calnexin cycle for removal. (See Mast et al., Glycobiology 15(4): 421-436, 2004; Olivari and Molinari, FEBS Lett. 581: 3658-3664, 2007; Olivari et al., J. Biol. Chem. 280(4): 2424-2428, 2005; and Vembar and Brodsky 2008, which are herein incorporated by reference.) Exemplary EDEM2 sequences are depicted in Table 1, which is cross-referenced to the Sequence Listing. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Animal 
                 SEQ ID NO: 
                 % id human 
                 % id mouse 
                 % id hamster 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Mouse 
                 1 
                 93 
                 100 
                 96 
               
               
                 Rat 
                 2 
                 94 
                 98 
                 96 
               
               
                 Hamster 
                 3 
                 93 
                 96 
                 100 
               
               
                 Human 
                 4 
                 100 
                 93 
                 93 
               
               
                 Chimpanzee 
                 5 
                 99 
                 94 
                 93 
               
               
                 Orangutan 
                 6 
                 97 
                 92 
                 92 
               
               
                 Zebra fish 
                 7 
                 69 
                 70 
                 69 
               
               
                 Consensus 
                 8 
                 100 
                 100 
                 100 
               
               
                   
               
             
          
         
       
     
     As used herein, the term “Xbp1”, also known as XBP1 or X-box binding protein 1, means any ortholog, homolog, or conservatively substituted variant of Xbp1. Xbp1 is a transcription factor and functional element of the UPR. ER stress activates both (1) the transcription factor ATF6, which in turn upregulates the transcription of Xbp1 mRNA, and (2) the ER membrane protein IRE1, which mediates the splicing of the precursor Xbp1 mRNA to produce active Xbp1. As mentioned above, activated Xbp1 in turn upregulates the activity of EDEM2. (See Yoshida et al., Cell Structure and Function 31(2): 117-125, 2006; and Olivari, 2005.) Exemplary Xbp1 amino acid sequences are depicted in Table 2, which is cross-referenced to the Sequence Listing. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Animal 
                 SEQ ID NO 
                 % id human 
                 % id mouse 
                 % id hamster 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Mouse 
                 9 
                 86 
                 100 
                 92 
               
               
                 Hamster 
                 10 
                 86 
                 92 
                 100 
               
               
                 Human 
                 11 
                 100 
                 86 
                 86 
               
               
                 Zebra fish 
                 12 
                 47 
                 47 
                 48 
               
               
                 Consensus 
                 13 
                 100 
                 100 
                 100 
               
               
                   
               
             
          
         
       
     
     As used herein, the term “antibody” is generally intended to refer to immunoglobulin molecules comprising four polypeptide chains, two heavy (H) chains and two light (L) chains inter-connected by disulfide bonds, as well as multimers thereof (e.g., IgM); however, immunoglobulin molecules consisting of only heavy chains (i.e., lacking light chains) are also encompassed within the definition of the term “antibody”. Each heavy chain comprises a heavy chain variable region (abbreviated herein as HCVR or VH) and a heavy chain constant region. The heavy chain constant region comprises three domains, CH1, CH2 and CH3. Each light chain comprises a light chain variable region (abbreviated herein as LCVR or VL) and a light chain constant region. The light chain constant region comprises one domain (CL1). The VH and VL regions can be further subdivided into regions of hypervariability, termed complementary determining regions (CDRs), interspersed with regions that are more conserved, termed framework regions (FR). Each VH and VL is composed of three CDRs and four FRs, arranged from amino-terminus to carboxy-terminus in the following order: FR1, CDR1, FR2, CDR2, FR3, CDR3, FR4. An “isolated antibody” or “purified antibody” may be substantially free of other cellular material or chemicals. 
     The term “specifically binds”, or the like, means that an antibody or antigen-binding fragment thereof forms a complex with an antigen that is relatively stable under physiologic conditions. Specific binding can be characterized by a dissociation constant of at least about 1×10 −6  M or greater. Methods for determining whether two molecules specifically bind are well known in the art and include, for example, equilibrium dialysis, surface plasmon resonance, and the like. An isolated antibody that specifically binds human GDF8 (for example) may, however, have cross-reactivity to other antigens, such as GDF8 molecules from other species (orthologs). 
     Various antibodies are used as examples of multi-subunit proteins secreted by cells harboring the polynucleotide encoding a stress-induced mannose-binding lectin. Those examples include anti-GDF8, anti-ANG2, and anti-ANGPTL4 antibodies. These and similar antibodies are described in US Pat. Apps. No. 20110293630, 20110027286, and 20110159015 respectively, which are incorporated herein by reference. 
     As used herein, the term “cell” refers to a prokaryotic or eukaryotic cell capable of replicating DNA, transcribing RNA, translating polypeptides, and secreting proteins. Cells include animal cells used in the commercial production of biological products, such as insect cells (e.g., Schneider cells, Sf9 cells, Sf21 cells, Tn-368 cells, BTI-TN-5B1-4 cells; see Jarvis, Methods Enzymol. 463: 191-222, 2009; and Potter et al., Int. Rev. Immunol. 10(2-3): 103-112, 1993) and mammalian cells (e.g., CHO or CHO-K1 cells, COS or COS-7cells, HEK293 cells, PC12 cells, HeLa cells, Hybridoma cells; Trill et al., Curr. Opin. Biotechnol. 6(5): 553-560, 1995; Kipriyanov and Little, Mo. Biotechnol. 12(2): 173-201, 1999). In one embodiment, the cell is a CHO-K1 cell containing the described UPR pathway polynucleotides. For a description of CHO-K1 cells, see also Kao et al., Proc. Nat&#39;l. Acad. Sci. USA 60: 1275-1281, 1968. 
     As used herein, the term “promoter” means a genetic sequence generally in cis and located upstream of a protein coding sequence, and which facilitates the transcription of the protein coding sequence. Promoters can be regulated (developmental, tissue specific, or inducible (chemical, temperature)) or constitutively active. In certain embodiments, the polynucleotides that encode proteins are operably linked to a constitutive promoter. By “operably linked”, what is meant is that the protein-encoding polynucleotide is located three-prime (downstream) and cis of the promoter, and under control of the promoter. In certain embodiments, the promoter is a constitutive mammalian promoter, such as the ubiquitin C promoter (see Schorpp et al., Nucl. Acids Res. 24(9): 1787-1788, 1996); Byun et al., Biochem. Biophys. Res. Comm. 332(2): 518-523, 2005) or the CMV-IE promoter (see Addison et al., J. Gen. Virol. 78(7): 1653-1661, 1997; Hunninghake et al., J. Virol. 63(7): 3026-3033, 1989), or the hCMV-IE promoter (human cytomegalovirus immediate early gene promoter) (see Stinski &amp; Roehr, J. Virol. 55(2): 431-441, 1985; Hunninghake et al., J. Virol. 63(7): 3026-3033, 1989). 
     As used herein, the phrase “integrated cell density”, or “ICD” means the density of cells in a culture medium taken as an integral over a period of time, expressed as cell-days per mL. In some embodiments, the ICD is measured around the twelfth day of cells in culture. 
     As used herein, the term “culture” means both (1) the composition comprising cells, medium, and secreted multi-subunit protein, and (2) the act of incubating the cells in medium, regardless of whether the cells are actively dividing or not. Cells can be cultured in a vessel as small as a 25 mL flask or smaller, and as large as a commercial bioreactor of 10,000 liters or larger. “Medium” refers to the culture medium, which comprises inter alia nutrients, lipids, amino acids, nucleic acids, buffers and trace elements to allow the growth, proliferation, or maintenance of cells, and the production of the multi-subunit protein by the cells. Cell culture media include serum-free and hydrolysate-free defined media as well as media supplemented with sera (e.g., fetal bovine serum (FBS)) or protein hydrolysates. Non-limiting examples of media, which can be commercially acquired, include RPMI medium 1640, Dulbecco&#39;s Modified Eagle Medium (DMEM), DMEM/F12 mixture, F10 nutrient mixture, Ham&#39;s F12 nutrient mixture, and minimum essential media (MEM). 
     As used herein, the phrase “conservatively substituted variant”, as applied to polypeptides, means a polypeptide having an amino acid sequence with one of more conservative amino acid substitutions. A “conservative amino acid substitution” is one in which an amino acid residue is substituted by another amino acid residue having a side chain (R group) with similar chemical properties (e.g., charge or hydrophobicity). In general, a conservative amino acid substitution will not substantially change the functional properties of a protein. In cases where two or more amino acid sequences differ from each other by conservative substitutions, the percent or degree of similarity may be adjusted upwards to correct for the conservative nature of the substitution. Means for making this adjustment are well known to those of skill in the art. See, e.g., Pearson (1994) Methods Mol. Biol. 24: 307-331, which is herein incorporated by reference. Examples of groups of amino acids that have side chains with similar chemical properties include 1) aliphatic side chains: glycine, alanine, valine, leucine and isoleucine; 2) aliphatic-hydroxyl side chains: serine and threonine; 3) amide-containing side chains: asparagine and glutamine; 4) aromatic side chains: phenylalanine, tyrosine, and tryptophan; 5) basic side chains: lysine, arginine, and histidine; 6) acidic side chains: aspartate and glutamate, and 7) sulfur-containing side chains: cysteine and methionine. Preferred conservative amino acids substitution groups are: valine-leucine-isoleucine, phenylalanine-tyrosine, lysine-arginine, alanine-valine, glutamate-aspartate, and asparagine-glutamine. Alternatively, a conservative replacement is any change having a positive value in the PAM250 log-likelihood matrix disclosed in Gonnet et al. (1992) Science 256: 1443-45, herein incorporated by reference. A “moderately conservative” replacement is any change having a nonnegative value in the PAM250 log-likelihood matrix. 
     EMBODIMENTS 
     The Cell 
     In one aspect, the invention provides a cell useful in the production of a protein having therapeutic or research utility. In some embodiments, the protein consists of multiple subunits, which must be properly folded and assembled to produce sufficient quantities of active protein. Antibodies are an example of multi-subunit proteins having therapeutic or research utility. In some embodiments, the cell harbors a recombinant genetic construct (i.e., a polynucleotide) that encodes one or more of the individual subunits of the multi-subunit protein. In other embodiments, the genetic construct encoding the individual polypeptide subunits is naturally occurring, such as for example the nucleic acid sequences encoding the subunits of an antibody in a B cell. 
     To facilitate the proper assembly and secretion of the multi-subunit protein, the cell contains a recombinant polynucleotide that encodes a stress-induced mannose-binding lectin, which in some embodiments is a component of the ERAD. In some embodiments, the stress-induced mannose-binding lectin is an endoplasmic reticulum degradation-enhancing alpha-mannosidase-like protein 2 (EDEM2). It is envisioned that any encoded EDEM2 or conservatively-substituted variant can be successfully employed in the instant invention. Table 1 lists some examples of vertebrate EDEM2 proteins. A multiple pairwise comparison of those protein sequences, which was performed using the Clustal W program of Thompson et al., Nucl. Acids Rev. 22(22): 4673-80, 1994 (see also Yuan et al., Bioinformatics 15(10): 862-3, 1999), revealed that each of the disclosed EDEM2 polynucleotide sequences is at least 69% identical to each other EDEM2 sequence. A Clustal W comparison of the disclosed mammalian EDEM2 sequences revealed that each sequence is at least 92% identical to the other. Thus, in some embodiments, the cell contains a polynucleotide that encodes an EDEM2 polypeptide having a sequence that is at least 92% to any one of a mammalian EDEM2. A consensus EDEM2 amino acid sequence was built by aligning a mouse, rat, hamster, chimpanzee, and human EDEM2 polypeptide amino acid sequences. That consensus sequence is depicted as SEQ ID NO: 8. Thus, in some embodiments, the cell contains a polynucleotide that encodes an EDEM2 polypeptide having an amino acid sequence of SEQ ID NO: 8. 
     In various embodiments, the cell contains a recombinant polynucleotide that encodes an EDEM2 polypeptide having an amino acid sequence that is at least 92% identical to the mouse EDEM2 (mEDEM2) amino acid sequence; and in a particular embodiment, the polypeptide is mEDEM2 or a conservatively substituted variant thereof. 
     In some embodiments, the multi-subunit protein is an antibody, and the cell contains a polynucleotide encoding any one or more of a polypeptide comprising an amino acid sequence of SEQ ID NO: 43, SEQ ID NO: 44, SEQ ID NO: 45, and SEQ ID NO: 46. SEQ ID NO: 43 and 44 each represent consensus sequences of the roughly N-terminal and C-terminal portions, respectively, of particular antibody heavy chains. Thus, the polynucleotide encoding a protein subunit in one embodiment encodes a polypeptide comprising both SEQ ID NO: 43 and SEQ ID NO: 44. SEQ ID NO: 45 and 46 each represent consensus sequences of the roughly N-terminal and C-terminal portions, respectively, of particular antibody light chains. Thus, the polynucleotide encoding a protein subunit in one embodiment encodes a polypeptide comprising both SEQ ID NO: 45 and SEQ ID NO: 46. In some embodiments, in addition to the recombinant polynucleotide encoding the EDEM2 protein, the cell contains at least two polynucleotides, each of which encodes a particular subunit of the multi-subunit protein. For example, and as exemplified below, the cell contains a polynucleotide encoding an antibody heavy chain comprising an amino acid sequence of SEQ ID NO: 43 and SEQ ID NO: 44, and another polynucleotide encoding an antibody light chain comprising an amino acid sequence of SEQ ID NO: 45 and SEQ ID NO: 46. 
     In some embodiments, the cell, in addition to containing the stress-response polynucleotide and one or more polynucleotides encoding a polypeptide subunit, as described above, also contains a polynucleotide that encodes an unfolded protein response transcription factor that operates upstream of EDEM2. The upstream transcription factor is in some cases the spliced form of an XBP1. It is envisioned that any encoded XBP1 can be successfully employed in the instant invention. Table 2 lists some examples of sequences of vertebrate XBP1 spliced-form polypeptides. A multiple pairwise comparison of those polypeptide sequences, which was performed using Clustal W (Thompson 1994; Yuan 1999), revealed that each of the disclosed spliced XBP1 polynucleotide sequences is at least 48% identical to each other XBP1 sequence. A Clustal W comparison of the disclosed mammalian XBP1 sequences revealed that each sequence is at least 86% identical to the other. Thus, in some embodiments, the cell contains a polynucleotide that encodes a spliced-form of an XBP1 polypeptide having a sequence that is at least 86% to any one of a mammalian spliced XBP1. A consensus XBP1 amino acid sequence was built by aligning a mouse, hamster, and human XBP1 amino acid sequences. That consensus sequence is depicted as SEQ ID NO: 13. Thus, in some embodiments, the cell contains a polynucleotide that encodes an XBP1 polypeptide having an amino acid sequence of SEQ ID NO: 13. 
     In various embodiments, the cell contains a polynucleotide that encodes an XBP1 polypeptide having an amino acid sequence that is at least 86% identical to the mouse XBP1 (mXBP1) amino acid sequence (SEQ ID NO: 9); and in a particular embodiment, the polypeptide is mXBP1, or a conservatively substituted variant thereof. 
     The invention envisions that any cell may be used to harbor the lectin-encoding polypeptide for the production of a properly folded and active multi-subunit protein. Such cells include the well-known protein production cells such as the bacterium  Escherichia coli  and similar prokaryotic cells, the yeasts  Pichia pastoris  and other  Pichia  and non- pichia  yeasts, plant cell explants, such as those of  Nicotiana , insect cells, such as Schneider 2 cells, Sf9 and Sf21, and the  Trichoplusia ni -derived High Five cells, and the mammalian cells typically used in bioproduction, such as CHO, CHO-K1, COS, HeLa, HEK293, Jurkat, and PC12 cells. In some embodiments, the cell is a CHO-K1 or a modified CHO-K1 cell such as that which is taught in U.S. Pat. Nos. 7,435,553, 7,514,545, and 7,771,997, as well as U.S. Published Patent Application No. US 2010-0304436 A1, each of which is incorporated herein by reference in its entirety. 
     In some particular embodiments, the invention provides ex vivo a CHO-K1 cell that contains (1) a mEDEM2-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 16, (2) an XBP1-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 18, (3) an antibody heavy chain-encoding polynucleotide comprising a nucleotide sequence that encodes a polypeptide comprising the amino acid sequences of SEQ ID NO: 43 and 44, and (4) an antibody light chain-encoding polynucleotide comprising a nucleotide sequence that encodes a polypeptide comprising the amino acid sequences of SEQ ID NO: 45 and 46. 
     In one particular embodiment, the invention provides ex vivo a CHO-K1 cell that contains (1) a mEDEM2-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 16, (2) an XBP1-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO:18, (3) an antibody heavy chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 23, and (4) an antibody light chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 25. 
     In another particular embodiment, the invention provides ex vivo a CHO-K1 cell that contains (1) a mEDEM2-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 16, (2) an XBP1-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 18, (3) an antibody heavy chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 31, and (4) an antibody light chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 33. 
     In yet another particular embodiment, the invention provides ex vivo a CHO-K1 cell that contains (1) a mEDEM2-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 16, (2) an XBP1-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 18, (3) an antibody heavy chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 39, and (4) an antibody light chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 41. 
     The Cell Line 
     In another aspect, the invention provides a cell line, which comprises a plurality of cells descended by clonal expansion from a cell described above. At least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 98%, at least 99%, or about 100% of the constituent cells of the cell line contain a recombinant polynucleotide that encodes a stress-induced mannose-binding lectin, which in some embodiments is a component of the ERAD. In some embodiments, the stress-induced mannose-binding lectin is an endoplasmic reticulum degradation-enhancing alpha-mannosidase-like protein 2 (EDEM2). It is envisioned that any encoded EDEM2 or conservatively-substituted variant thereof can be successfully employed in the instant invention. Table 1, as discussed in the previous section, lists some examples of vertebrate EDEM2 proteins. In some embodiments, the constituent cell contains a polynucleotide that encodes an EDEM2 polypeptide having a sequence that is at least 92% identical to any mammalian EDEM2. In some embodiments, the constituent cell contains a polynucleotide that encodes an EDEM2 polypeptide having the mammalian consensus amino acid sequence of SEQ ID NO: 8. In some embodiments, the constituent cell contains a recombinant polynucleotide of SEQ ID NO: 1 or a conservatively substituted variant thereof. 
     In some embodiments, the multi-subunit protein that is produced by the cell line is an antibody, and the constituent cell of the cell line contains a polynucleotide encoding any one or more of a polypeptide comprising an amino acid sequence of SEQ ID NO: 43 and SEQ ID NO: 44 (which represent consensus sequences of the N-terminal and C-terminal portions, respectively, of particular antibody heavy chains), and SEQ ID NO: 45 and SEQ ID NO: 46 (which represent consensus sequences of the N-terminal and C-terminal portions, respectively, of particular antibody light chains). In some embodiments, in addition to the recombinant polynucleotide encoding the EDEM2 protein, the constituent cell of the cell line contains at least two polynucleotides, each of which encodes a particular subunit of the multi-subunit protein. For example, the constituent cell contains a polynucleotide encoding an antibody heavy chain comprising an amino acid sequence of SEQ ID NO: 43 and SEQ ID NO: 44, and another polynucleotide encoding an antibody light chain comprising an amino acid sequence of SEQ ID NO: 45 and SEQ ID NO: 46. 
     In some embodiments, the constituent cell, in addition to containing the stress-response polynucleotide and one or more polynucleotides encoding a polypeptide subunit, as described above, also contains a polynucleotide that encodes an unfolded protein response transcription factor, which operates upstream of EDEM2, such as a spliced form of an XBP1. It is envisioned that any encoded XBP1 can be successfully employed in the instant invention. Table 2, as discussed in the preceding section, lists some examples of sequences of vertebrate XBP1 spliced-form polypeptides. Clustal W analysis of those sequences revealed that each of the disclosed spliced XBP1 polynucleotide sequences is at least 48% identical to each other XBP1 sequence; and a comparison of the mammalian XBP1 sequences revealed that each sequence is at least 86% identical to the other. Thus, in some embodiments, the constituent cell of the cell line contains a polynucleotide that encodes a spliced-form of an XBP1 polypeptide having a sequence that is at least 86% to any one of a mammalian spliced XBP1. In some embodiments, the constituent cell contains a polynucleotide that encodes an XBP1 polypeptide having a consensus amino acid sequence of SEQ ID NO: 13. 
     In various embodiments, the cell contains a polynucleotide that encodes an XBP1 polypeptide having an amino acid sequence that is at least 86% identical to the mouse XBP1 (mXBP1) amino acid sequence (SEQ ID NO: 9); and in a particular embodiment, the polypeptide is mXBP1 of SEQ ID NO: 9, or a conservatively substituted variant thereof. 
     The invention envisions that the cell line comprises constituent cells whose parent is selected from a list of well-known protein production cells such as, e.g., the bacterium  Escherichia coli  and similar prokaryotic cells, the yeasts  Pichia pastoris  and other  Pichia  and non- pichia  yeasts, plant cell explants, such as those of  Nicotiana , insect cells, such as Schneider 2 cells, Sf9 and Sf21, and the  Trichoplusia ni -derived High Five cells, and the mammalian cells typically used in bioproduction, such as CHO, CHO-K1, COS, HeLa, HEK293, Jurkat, and PC12 cells. In some embodiments, the cell is a CHO-K1 or a modified CHO-K1 cell, such as that which is taught in U.S. Pat. Nos. 7,435,553, 7,514,545, and 7,771,997, as well as U.S. Published Patent Application No. US 2010-0304436 A1. 
     In some embodiments, the cell line, which is cultured in media, is capable of producing the multi-subunit protein and secreting the properly assembled multi-subunit protein into the media to a titer that is at least 3 g/L, at least 5 g/L, or at least 8 g/L. 
     Furthermore, the constituent cells of the cell line are capable proliferating in culture to such an extent as to attain an integrated cell density that is about 30% greater than the integrated cell density of a cell line that does not contain the recombinant polynucleotide encoding the stress-induced mannose-binding lectin. In some cases, the cell line is able to attain an integrated cell density that is at least about 50% greater, at least 60% greater, or at least 90% greater than the integrated cell density of a cell line that does not contain the recombinant polynucleotide that encodes a stress-induced mannose-binding lectin. In some embodiments, the integrated cell density of the cell line is assessed after about 12 days in culture. 
     In some particular embodiments, the invention provides a cell-line comprising clonally-derived constituent cells, wherein the constituent cell is a CHO-K1 cell that contains (1) a mEDEM2-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 16, (2) an XBP1-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 18, (3) an antibody heavy chain-encoding polynucleotide comprising a nucleotide sequence that encodes a polypeptide comprising the amino acid sequences of SEQ ID NO: 43 and 44, and (4) an antibody light chain-encoding polynucleotide comprising a nucleotide sequence that encodes a polypeptide comprising the amino acid sequences of SEQ ID NO: 45 and 46. 
     In one particular embodiment, the invention provides a cell-line comprising clonally-derived constituent cells, wherein the constituent cell is a CHO-K1 cell that contains (1) a mEDEM2-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 16, (2) an XBP1-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 18, (3) an antibody heavy chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 23, and (4) an antibody light chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 25. 
     In another particular embodiment, the invention provides a cell-line comprising clonally-derived constituent cells, wherein the constituent cell is a CHO-K1 cell that contains (1) a mEDEM2-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 16, (2) an XBP1-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 18, (3) an antibody heavy chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 31, and (4) an antibody light chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 33. 
     In yet another particular embodiment, the invention provides a cell-line comprising clonally-derived constituent cells, wherein the constituent cell is a CHO-K1 cell that contains (1) a mEDEM2-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 16, (2) an XBP1-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 18, (3) an antibody heavy chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 39, and (4) an antibody light chain-encoding polynucleotide comprising a nucleotide sequence of SEQ ID NO: 41. 
     The EDEM2 Polynucleotide 
     In another aspect, the invention provides a polynucleotide that encodes an EDEM2 protein. The EDEM2-encoding polynucleotide is recombinant and can be manufactured, stored, used or expressed in vitro, as in a test tube, or an in vitro translation system, or in vivo, such as in a cell, which can be ex vivo, as in a cell culture, or in vivo, as in an organism. In some embodiments, the EDEM2-encoding polynucleotide is within a gene, meaning that it is under the control of and down stream of a promoter, and up stream of a polyadenylation site. The EDEM2-encoding polynucleotide or gene can be within a plasmid or other circular or linear vector. The EDEM2-encoding polynucleotide or gene can be within a circular or linear DNA construct, which can be within a cell as an episome or integrated into the cellular genome. 
     As described above, the EDEM2-encoding polynucleotide encodes any ortholog, homolog or conservatively substituted EDEM2 polypeptide of Table 1, or an EDEM2 polypeptide having an amino acid sequence that is at least 92% identical to any one of SEQ ID NO: 1-5 and 8, including the mammalian consensus sequence of SEQ ID NO: 8. 
     In some cases, the recombinant or isolated EDEM2-encoding polynucleotide is operably linked to a mammalian promoter. The promoter can be any promoter, but in some cases it is a mammalian promoter, such as for example a ubiquitin C promoter. 
     In a particular embodiment, the EDEM2-encoding polynucleotide essentially consists of, from 5′ to 3′, a promoter, such as a ubiquitin C promoter, followed by an optional intron, such as a beta globin intron, followed by an EDEM2 coding sequence, followed by a polyadenylation sequence, such as an SV40 pA sequence. A specific example, which is also a particular embodiment, of such an EDEM2-encoding polynucleotide is described by SEQ ID NO: 16. Conserved variants of that sequence are also envisioned to be embodiments of the invention. 
     In some cases, the recombinant EDEM2-encoding polynucleotide is part of a plasmid, which can be linear, circular, episomal, integrated, a static DNA construct, or a vector for delivering the EDEM2 gene or expressing the EDEM2 protein. In one particular embodiment, the plasmid contains (1) an EDEM2 gene, which is under the control of a ubiquitin C promoter and terminates with an SV40 polyadenylation signal, and (2) a selectable marker, such as a polynucleotide encoding a polypeptide that confers resistance to zeocin or a polynucleotide encoding a polypeptide that confers resistance to neomycin, under the control of a promoter, such as an SV40 promoter, and terminated with a polyadenylation sequence, such as a PGK pA sequence. In one particular embodiment, the plasmid comprises, in a circular format running in a 5′ to 3′ direction, a ubiquitin C promoter, a beta globin intron, an EDEM2 coding sequence, an SV40 pA sequence, an SV40 promoter, a neomycin-resistance coding sequence, and a PGK pA sequence. A specific example of this embodiment is exemplified by a plasmid having the sequence of SEQ ID NO: 14. In another particular embodiment, the plasmid comprises, in a circular format running in a 5′ to 3′ direction, a ubiquitin C promoter, a beta globin intron, an EDEM2 coding sequence, an SV40 pA sequence, an SV40 promoter, a zeocin-resistance coding sequence, and a PGK pA sequence. A specific example of this embodiment is exemplified by a plasmid having the sequence of SEQ ID NO: 15. 
     The XBP1 Polynucleotide 
     In another aspect, the invention provides a polynucleotide that encodes an XBP1 protein. The XBP1-encoding polynucleotide is recombinant and can be manufactured, stored, used or expressed in vitro, as in a test tube, or an in vitro translation system, or in vivo, such as in a cell, which can be ex vivo, as in a cell culture, or in vivo, as in an organism. In some embodiments, the XBP1-encoding polynucleotide is within a gene, meaning that it is under the control of and down stream of a promoter, and up stream of a polyadenylation site. The XBP1-encoding polynucleotide can be within a plasmid or other circular or linear vector. The XBP1-encoding polynucleotide or gene can be within a circular or linear DNA construct, which can be within a cell as an episome, or integrated into the cellular genome. 
     As described above, the XBP1-encoding polynucleotide encodes any ortholog, homolog or conservatively substituted XBP1 polypeptide of Table 2, or an XBP1 polypeptide having an amino acid sequence that is at least 86% identical to any one of SEQ ID NO: 9, 10, and 11, including the mammalian consensus sequence of SEQ ID NO: 13. 
     In some cases, the recombinant or isolated XBP1-encoding polynucleotide is operably linked to a mammalian promoter. The promoter can be any promoter, but in some cases it is a mammalian promoter, such as for example a ubiquitin C promoter. 
     In a particular embodiment, the XBP1-encoding polynucleotide essentially consists of, from 5′ to 3′, a promoter, such as a ubiquitin C promoter, followed by an optional intron, such as a beta globin intron, followed by an XBP1 coding sequence, followed by a polyadenylation sequence, such as an SV40 pA sequence. SEQ ID NO: 18 describes an example of an XBP1-encoding polynucleotide. Conserved variants of that exemplary sequence are also envisioned to be embodiments of the invention. 
     In some cases, the recombinant XBP1-encoding polynucleotide is part of a plasmid, which can be linear, circular, episomal, integrated, a static DNA construct, or a vector for delivering the XBP1 gene or expressing the spliced and active XBP1 protein. In one particular embodiment, the plasmid contains (1) an XBP1 gene, which is under the control of a ubiquitin C promoter and terminates with an SV40 polyadenylation signal, and (2) a selectable marker, such as a polynucleotide encoding a polypeptide that confers resistance to zeocin or a polynucleotide encoding a polypeptide that confers resistance to neomycin, under the control of a promoter, such as an SV40 promoter, and terminated with a polyadenylation sequence, such as a PGK pA sequence. In one particular embodiment, the plasmid comprises, in a circular format running in a 5′ to 3′ direction, a ubiquitin C promoter, a beta globin intron, an XBP1 coding sequence, an SV40 pA sequence, an SV40 promoter, a zeocin-resistance coding sequence, and a PGK pA sequence. A specific example of this embodiment is exemplified by a circular plasmid having the sequence of SEQ ID NO: 17. 
     The Antibody Heavy and Light Chain-Encoding Polynucleotides 
     In another aspect, the invention provides a polynucleotide that encodes an antibody heavy chain polypeptide (HC). The HC-encoding polynucleotide is recombinant and can be manufactured, stored, used or expressed in vitro, as in a test tube, or an in vitro translation system, or in vivo, such as in a cell, which can be ex vivo, as in a cell culture, or in vivo, as in an organism. In some embodiments, the HC-encoding polynucleotide is within a gene, meaning that it is under the control of and down stream from a promoter, and up stream of a polyadenylation site. The HC-encoding polynucleotide may be within a plasmid or other circular or linear vector. The HC-encoding polynucleotide or gene may be within a circular or linear DNA construct, which may be within a cell as an episome or integrated into the cellular genome. 
     In some cases, the recombinant or isolated HC-encoding polynucleotide is operably linked to a mammalian promoter. The promoter can be any promoter, but in some cases it is a mammalian promoter, such as for example a ubiquitin C promoter or an hCMV-IE promoter. 
     In a particular embodiment, the HC-encoding polynucleotide is an HC gene, which essentially comprises, from 5′ to 3′, a promoter, for example an hCMV-IE promoter, followed by an optional intron, such as a beta globin intron, followed by a heavy chain coding sequence, such as for example a sequence encoding an amino acid sequence of SEQ ID NO: 43 and 44, SEQ ID NO: 19, SEQ ID NO: 27, or SEQ ID NO: 35, followed by a polyadenylation sequence, for example an SV40 pA sequence. A specific example of an HC gene is described by SEQ ID NO: 23, SEQ ID NO: 31, or SEQ ID NO: 39. Conserved variants of any one of these sequences are also envisioned to be embodiments of the invention. 
     In some cases, the recombinant HC-encoding polynucleotide is part of a plasmid, which can be linear, circular, episomal, integrated, a static DNA construct, or a vector for delivering the heavy chain gene or expressing the heavy chain sububunit. In one particular embodiment, the plasmid contains (1) an HC gene, which is under the control of an hCMV-IE promoter and terminates with an SV40 polyadenylation signal, and (2) a selectable marker, such as a polynucleotide encoding a polypeptide that confers resistance to hygromycin, under the control of a promoter, such as an SV40 promoter, and terminated with a polyadenylation sequence, such as a PGK pA sequence. In one particular embodiment, the plasmid comprises, in a circular format running in a 5′ to 3′ direction, an hCMV-IE promoter, a beta globin intron, an antibody heavy chain coding sequence (which encodes a HC having an amino acid of SEQ ID NO: 43 and 44, SEQ ID NO: 19, SEQ ID NO: 27, or SEQ ID NO: 35), an SV40 pA sequence, an SV40 promoter, a hygromycin-resistance coding sequence, and a PGK pA sequence. A specific example and particular embodiment of such a plasmid containing an HC gene is described by SEQ ID NO: 24, SEQ ID NO: 32, or SEQ ID NO: 40. Conserved variants of any one of these sequences are also envisioned to be embodiments of the invention. 
     In another aspect, the invention provides a polynucleotide that encodes an antibody light chain polypeptide (LC). The LC-encoding polynucleotide is recombinant and can be manufactured, stored, used or expressed in vitro, as in a test tube, or an in vitro translation system, or in vivo, such as in a cell, which can be ex vivo, as in a cell culture, or in vivo, as in an organism. In some embodiments, the LC-encoding polynucleotide is within a gene, meaning that it is under the control of and down stream from a promoter, and up stream of a polyadenylation site. The LC-encoding polynucleotide or gene may be within a plasmid or other circular or linear vector. The LC-encoding polynucleotide or gene may be within a circular or linear DNA construct, which may be within a cell as an episome or integrated into the cellular genome. 
     In some cases, the recombinant or isolated LC-encoding polynucleotide is operably linked to a mammalian promoter. The promoter can be any promoter, but in some cases it is a mammalian promoter, such as, e.g., a ubiquitin C promoter or an hCMV-IE promoter. 
     In a particular embodiment, the LC-encoding polynucleotide is an LC gene, which essentially comprises, from 5′ to 3′, a promoter, for example an hCMV-IE promoter, followed by an optional intron, such as a beta globin intron, followed by a light chain coding sequence, such as for example a sequence encoding an amino acid sequence of SEQ ID NO: 45 and 46, SEQ ID NO: 21, SEQ ID NO: 29, or SEQ ID NO: 37, followed by a polyadenylation sequence, such as an SV40 pA sequence. A specific example and particular embodiment of such an LC gene is described by SEQ ID NO: 25, SEQ ID NO: 33, or SEQ ID NO: 41. Conserved variants of any one of these sequences are also envisioned to be embodiments of the invention. 
     In some cases, the recombinant LC-encoding polynucleotide is part of a plasmid, which may be linear, circular, episomal, integrated, a static DNA construct, or a vector for delivering the light chain gene or expressing the light chain sububunit. In one particular embodiment, the plasmid contains (1) an LC gene, which is under the control of an hCMV-IE promoter and terminates with an SV40 polyadenylation signal, and (2) a selectable marker, such as a polynucleotide encoding a polypeptide that confers resistance to hygromycin, under the control of a promoter, such as an SV40 promoter, and terminated with a polyadenylation sequence, such as a PGK pA sequence. In one particular embodiment, the plasmid comprises, in a circular format running in a 5′ to 3′ direction, an hCMV-IE promoter, a beta globin intron, an antibody light chain coding sequence (which encodes a LC having an amino acid of SEQ ID NO: 45 and 46, SEQ ID NO: 21, SEQ ID NO: 29, or SEQ ID NO: 37), an SV40 pA sequence, an SV40 promoter, a hygromycin-resistance coding sequence, and a PGK pA sequence. A specific example and particular embodiment of such a plasmid containing an LC gene is described by SEQ ID NO: 26, SEQ ID NO: 34, or SEQ ID NO: 42. Conserved variants of any one of these sequences are also envisioned to be embodiments of the invention. 
     Methods of Manufacturing Multi-subunit Proteins 
     In another aspect, the invention provides a method for manufacturing a multi-subunit protein by culturing a cell, or a constituent cell of a cell line, which is capable of producing and secreting relatively large amounts of a properly assembled multi-subunit protein, in a medium, wherein the multi-subunit component is secreted into the medium at a relatively high titer. The cell utilized in this manufacturing process is a cell described in the foregoing aspects, which contains an ERAD lectin-encoding polynucleotide described herein. 
     Methods of culturing cells, and in particular mammalian cells, for the purpose of producing useful recombinant proteins is well-known in the art (e.g., see De Jesus &amp; Wurm, Eur. J. Pharm. Biopharm. 78:184-188, 2011, and references cited therein). Briefly, cells containing the described polynucleotides are cultured in media, which may contain sera or hydrolysates, or may be chemically defined and optimized for protein production. The cultures may be fed-batch cultures or continuous cultures, as in a chemostat. The cells may be cultured in lab bench size flasks (˜25 mL), production scale-up bioreactors (1-5 L), or industrial scale bioreactors (5,000-25,000 L). Production runs may last for several weeks to a month, during which time the multi-subunit protein is secreted into the media. 
     The subject cell has an enhanced ability to produce and secrete properly assembled multi-subunit proteins. In some embodiments, the multi-subunit protein, for example an antibody, is secreted into the media at a rate of at least 94 ρg/cell/day, at least 37 ρg/cell/day, or at least 39 ρg/cell/day. In some embodiments, the multi-subunit protein attains a titer of at least at least 3 g/L, at least 5 g/L, at least 6 g/L, or at least 8 g/L after about twelve days of culture. 
     Furthermore, the subject cell has an enhanced ability to proliferate and attain a relatively high cell density, further optimizing productivity. In some embodiments, the cell or cell-line seed train attains an integrated cell density in culture of at least 5×10 7  cell-day/mL, at least 1×10 8  cell-day/mL or at least 1.5×10 8  cell-day/mL. 
     Optionally, the secreted multi-subunit protein is subsequently purified from the medium into which it was secreted. Protein purification methods are well-known in the art (see e.g., Kelley, mAbs 1(5):443-452). In some embodiments, the protein is harvested by centrifugation to remove the cells from the liquid media supernatant, followed by various chromatography steps and a filtration step to remove inter alia viruses and other contaminants or adulterants. In some embodiments, the chromatography steps include an ion exchange step, such as cation-exchange or anion-exchange. Various affinity chromatographic media may also be employed, such as protein A chromatography for the purification of antibodies. 
     Optionally, the manufacturing method may include the antecedent steps of creating the cell. Thus, in some embodiments, the method of manufacturing the multi-subunit protein comprises the step of transfecting the cell with the vector that encodes the stress-induced mannose-binding lectin, as described above, followed by selecting stable integrants thereof. Non-limiting examples of vectors include those genetic constructs that contain a polynucleotide that encodes an EDEM2 having an amino acid sequence of any one of SEQ ID NO: 1-8, an amino acid sequence that is at least 92% identical to any one of SEQ ID NO: 1-8, or any one of a conservatively substituted variant of SEQ ID NO: 1-8. Useful vectors also include, for example, a plasmid harboring the gene of SEQ ID NO: 16, the plasmid of SEQ ID NO: 15, and the plasmid of SEQ ID NO: 14. One should keep in mind that the plasmid sequences (e.g., SEQ ID NO: 14, 15, 17, 24, 26, 32, 34, 40, and 42) are circular sequences described in a linear manner in the sequence listing. Thus, in those cases, the 3-prime-most nucleotide of the written sequence may be considered to be immediately 5-prime of the 5-prime-most nucleotide of the sequence as written. In the example of the plasmid of SEQ ID NO: 14, transformants are selected through resistance to neomycin; for SEQ ID NO: 15, by selection through ZEOCIN resistance. 
     Detailed methods for the construction of polynucleotides and vectors comprising same, are described in U.S. Pat. Nos. 7,435,553 and 7,771,997, which are incorporated herein by reference, and in, e.g., Zwarthoff et al., J. Gen. Virol. 66(4):685-91, 1985; Mory et al., DNA. 5(3):181-93, 1986; and Pichler et al., Biotechnol. Bioeng. 108(2):386-94, 2011. 
     The starting cell, into which the vector that encodes the stress-induced mannose-binding lectin is placed, may already contain the constructs or genetic elements encoding or regulating the expression of the subunits of the multi-subunit protein, or XBP1 for those embodiments utilizing XBP1. Alternatively, the vector that encodes the stress-induced mannose-binding lectin may be put inside the cell first, and followed by the other constructs. 
     Multi-Subunit Proteins Manufactured by the Process 
     In another aspect, the invention provides a multi-subunit protein that is made according to the process disclosed herein. Given the inclusion of one or more elements that facilitate the proper folding, assembly, and post-translational modification of a multi-subunit protein, such as an antibody, one of ordinary skill in the art would reasonably expect such a protein to have distinct structural and functional qualities. For example, an antibody manufactured by the disclosed process is reasonably believed to have a particular glycosylation pattern and a quantitatively greater proportion of non-aggregated heterotetramers. 
     EXAMPLES 
     The following examples are presented so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use the methods and compositions of the invention, and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers used (e.g., amounts, temperature, etc.) but some experimental errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by mole, molecular weight is average molecular weight, percent concentration (%) means the mass of the solute in grams divided by the volume of the solution in milliliters times 100% (e.g., 10% substance X means 0.1 gram of substance X per milliliter of solution), temperature is in degrees Centigrade, and pressure is at or near atmospheric pressure. 
     Example 1 
     Cell Lines 
     CHO-K1 derived host cell line was transfected with two plasmids encoding heavy and light chain of a human antibody. Both plasmids contain the hph gene conferring resistance to hygromycin B (Asselbergs and Pronk, 1992, Mol. Biol. Rep., 17(1):61-70). Cells were transfected using LIPOFECTAMIN reagent (Invitrogen cat.#18324020). Briefly, one day before transfection 3.5 million cells were plated on a 10 cm plate in complete F12 (Invitrogen cat.#11765) containing 10% fetal bovine serum (FBS) (Invitrogen cat.#10100). On the day of transfection the cells were washed once and medium was replaced with OPTIMEM from (Invitrogen cat.#31985). DNA/Lipofectamin complexes were prepared in OPTIMEM medium and then added to the cells. The medium was changed again to the complete F12 with 10% FBS 6 hours later. The stable integration of the plasmids was selected using hygromycin B selection agent at 400 μg/ml. Clonal antibody expressing cell lines were isolated using the FASTR technology (described in the U.S. Pat. No. 6,919,183, which is herein incorporated by reference). 
     The antibody expressing lines were then re-transfected with the EDEM2 encoding plasmid. EDEM2 plasmids contained either neomycin phosphotransferase (plasmid construct designated “p3”) or sh ble (plasmid “p7”) genes to confer resistance to either G418 or zeocin respectively. The same transfection method was used. Depending on the selectable marker, cells were selected with either G418 or zeocin at 400 μg/ml or 250 μg/ml, respectively. The clonal cell lines were then isolated using FASTR technology. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Cell Lines 
               
             
          
           
               
                 Name 
                 Enhancers 
                 Constructs 
                 Protein 
               
               
                   
               
               
                 C1 
                 EDEM2 + XBP1 
                 HC/LC = p1/p2 
                 αAng2 
               
               
                 C2 
                 XBP1 
                 EDEM2 = p3 
               
               
                   
                   
                 XBP1 = p4 
               
               
                 C3 
                 EDEM2 + XBP1 
                 HC/LC = p5/p6 
                 αGDF8 
               
               
                 C4 
                 XBP1 
                 EDEM2 = p7 
               
               
                 C5 
                 EDEM2 
                 XBP1 = p4 
               
               
                 C6 
                 EDEM2 + XBP1 
                 HC/LC = p8/p9 
                 αAngPtl4 
               
               
                 C7 
                 XBP1 
                 EDEM2 = p3 
               
               
                   
                   
                 XBP1 = p4 
               
               
                   
               
             
          
         
       
     
     Example 2 
     The antibody production was evaluated in a scaled-down 12-day fed batch process using shaker flasks. In this method the cells were seeded in a shaker flask at the density of 0.8 million cells per mL in the production medium (defined media with high amino acid). The culture was maintained for about 12 days, and was supplemented with three feeds as well as glucose. The viable cell density, and antibody titer were monitored throughout the batch. 
     To determine the effect of mEDEM2 on enhanced protein production, the production of proteins by CHO cell lines containing mEDEM2 and mXBP1 were compared to production by control cells that contained mXBP1, but not mEDEM2. Protein titers were higher in those cell lines expressing mEDEM2 versus those cell lines that did not express mEDEM2. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 TITERS 
               
             
          
           
               
                   
                   
                 Production rate 
                 Titre g/L 
                   
               
               
                 Cell Line 
                 Enhancers 
                 (ρg/cell/day) 
                 (% increase) 
               
               
                   
               
             
          
           
               
                 C1 
                 EDEM2 + XBP1 
                 39 
                 8.1 
                 (93) 
               
             
          
           
               
                 C2 
                 XBP1 
                 39 
                 4.2 
               
             
          
           
               
                 C3 
                 EDEM2 + XBP1 
                 37 
                 5.9 
                 (55) 
               
             
          
           
               
                 C8 
                 XBP1 
                 32 
                 3.8 
               
             
          
           
               
                 C6 
                 EDEM2 + XBP1 
                 94 
                 5.3 
                 (152) 
               
             
          
           
               
                 C7 
                 XBP1 
                 52 
                 2.1 
               
             
          
           
               
                 C5 
                 EDEM2 
                 29 
                 3.1 
                 (343) 
               
             
          
           
               
                 C9 
                 — 
                 9 
                 0.7 
               
               
                   
               
             
          
         
       
     
     Example 3 
     Integrated Cell Days 
     Integrated Cell Density (“ICD”) is a phrase used to describe the growth of the culture throughout the fed batch process. In the course of the 12-day production assay, we monitored viable cell density on days 0, 3, 5, 7, 10, and 12. This data was then plotted against time. ICD is the integral of viable cell density, calculated as the area under the cell density curve. EDEM2 transfected lines have higher ICD in a 12-day fed batch process (see Table 5). 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 INTEGRATED CELL DENSITIES 
               
             
          
           
               
                   
                   
                 ICD 10 6  cell-day/mL 
                   
               
               
                 Cell Line 
                 Enhancers 
                 (% increase) 
               
               
                   
               
             
          
           
               
                 C1 
                 EDEM2 + XBP1 
                 205 
                 (93) 
               
             
          
           
               
                 C2 
                 XBP1 
                 106 
               
             
          
           
               
                 C3 
                 EDEM2 + XBP1 
                 157 
                 (34) 
               
             
          
           
               
                 C4 
                 XBP1 
                 117 
               
             
          
           
               
                 C6 
                 EDEM2 + XBP1 
                 56 
                 (51) 
               
             
          
           
               
                 C7 
                 XBP1 
                 37 
               
             
          
           
               
                 C5 
                 EDEM2 
                 116 
                 (59) 
               
             
          
           
               
                 C9 
                 — 
                 73 
               
               
                   
               
             
          
         
       
     
     Example 4 
     Anti-GDF8 Antibody Production 
     The effect of ectopic expression of EDEM2, XBP1, or both on the production of an anti-GDF8 antibody having a heavy chain sequence of SEQ ID NO: 19 and a light chain sequence of SEQ ID NO: 21 was examined. Individual cell-lines were examined for titer and integrated cell density and placed into “bins”, or ranges of values. Ectopic expression of EDEM2 significantly increased the number of cell lines that express antibody in the 5-6 g/L titer range. The combination of XBP1 and EDEM2 showed more than an additive effect toward the increase in high titer cell lines. The expression of EDEM2 in the antibody secreting cells also significantly increased the number of cell lines that attain a high ICD (see Table 6). 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 con- 
                 Titre Bins (g/L) 
                 ICD Bins (10 6  cell-day/mL) 
               
             
          
           
               
                 struct 
                 &lt;1 
                 1-3 
                 3-5 
                 5-6 
                 30-50 
                 50-100 
                 100-200 
               
               
                   
               
               
                 E + X 
                  0% 
                 33.3% 
                 44.4% 
                 22.2% 
                 11.1% 
                   50% 
                 38.9% 
               
               
                 X 
                  0% 
                 37.5% 
                   54% 
                  8.3% 
                 14.3% 
                 85.7% 
                   0% 
               
               
                 E 
                  0% 
                   33% 
                   60% 
                   7% 
                   0% 
                   27% 
                   73% 
               
               
                 — 
                 82% 
                   18% 
                   0% 
                   0% 
                   13% 
                   67% 
                   21% 
               
               
                   
               
             
          
         
       
     
     Example 5 
     Productivity and Stability of EDEM2-Exppressing Cells 
     The effect of ectopic expression of XBP1 or EDEM2 on the production of a monospecific antibody of interest (identified as clonal cell lines RGC91 or RGC92, respectively) was examined. Individual cell lines were examined for protein titer and integrated cell density, as well as stability. 
     Modified CHO K1 host cells stably expressing XBP1 (RGC91) or EDEM2 (RGC92) at a transcriptionally active locus (US8389239B2, issued Mar. 5, 2013) were transfected with a recombinant plasmid vector comprising the antibody gene of interest and a hygromycin resistance gene (hyg). 
     400 μg/mL hygromycin was used for selection of transfected cells. Positive integrants expressing the antibody of interest (randomly integrated in the CHO genome), and also stably expressing either XBP1 or EDEM2, were confirmed and isolated by fluorescence-activated cell sorting (FACS) analysis. The isolated clones were expanded in suspension cultures in serum-free production medium. Clones were isolated from selected pools and were subjected to a 12 day fed batch productivity assay, and the protein titer of the antibody of interest was determined. Integrated cell density is calculated by measuring viable cell count on a given day in the production assay (counts are taken every 3 days and plotted on a curve against cell count). As shown in  FIG. 1A , the average protein titers for clones isolated from RGC91 and RGC92 was 4.2 and 5.2, respectively (for 24 representative antibody-expressing clones). Ectopic expression of EDEM2 increased the number of clones that attain antibody titer above 5 g/L ( FIG. 1A ) compared to XBP1-expressing clones. Clonal cell lines expressing EDEM2 isolated from the RGC92 host also maintained higher (25%-100% higher) integrated cell densities when compared to clones isolated from XBP1-expressing RGC91 host (see  FIG. 1B ). EDEM2 clones established an ICD greater than 100 ( FIG. 1B ) in most clones tested. Clones isolated from EDEM2 expressing RGC92 host also resulted in significantly higher stability ( FIG. 2B ), as observed in flow cytometry-based autologous secretion trap (FASTR) scans (for reference, US6919183B2, issued Jul. 19, 2005) showing a homogenous producing population, in the representative sample of 24 clonal cell lines tested. Many of the clones isolated from XBP1 expressing RGC91 host appear to have a non-producing heterogeneous cell population ( FIG. 2A ). Without being bound to any one theory, EDEM2 facilitated the removal of misfolded proteins in the high expressing clones, thereby reducing stress in the cell during protein production and resulting in a more stable cell population.