SECRETION OF GLYCOSYLATED PROTEINS USING A TISSUE PLASMINOGEN ACTIVATOR PRO-SEQUENCE

Abstract: DNA constructs, host cells and production methods are disclosed for the expression and recovery of polypeptides, especially those altered to have one or more glycosylation sites added or deleted. The DNA constructs, host cells and methods provided herein employ a DNA segment corresponding to a mammalian tissue plaminogen activator signal and/or pro-peptide.

Description: SECRETION OF GLYCOSYLATED PROTEINS USING A TISSUE PLASMINOGEN ACTIVATOR PRO-SEQUENCE Background of the Invention Field of the Invention The present invention relates generally to the field of protein production by cell culture. In particular aspects, the invention provides methods and compositions for the production of polypeptides, especially by eukaryotic cell culture, which enhance secretion and facilitate recovery of the polypeptides. Description of Related Disclosures Many eukaryotic cell surface- and secreted proteins are post-translationally processed to incorporate N-linked and O-linked carbohydrate (Kornfeld and Kornfeld (1985) Annu. Rev. Biochem. 54:631-64; Rademacher et al., (1988) Annu. Rev. Biochem. 57:785-838). Protein glycosylation is thought to subserve a variety of functions including: augmentation of protein folding, inhibition of protein aggregation, regulation of intracellular trafficking to organelles, increasing resistance to proteolysis, modulation of protein antigenicity, and mediation of intercellular adhesion (Fieldler and Simons ( 1995) Cell 81 :309-312; Helenius (1994) Mol. Biol. of the Cell 5:253-265; Olden et al.., (1978) Cell, 13:461-473; Caton et al.., (1982) Cell 37:417-427; Alexander and Elder (1984) Science 226: 1328-1330; Flack et al.. (1994 J. Biol. Chem. 269:14015-14020). In higher organisms, the nature and extent of glycosylation can markedly affect the circulating half-life and bio-availability of secreted proteins by mechanisms involving receptor mediated uptake and clearance (Ashwell and Morrell (1974) Adv. Enzymol. 41 :99-128; Ashwell and Harford (1982) Ann. Rev. Biochem. 51:531-54). Receptor systems have been identified that are thought to play a major role in the clearance of serum proteins through recognition of various carbohydrate structures on the glycoproteins (Stockert (1995) Physiol. Rev. 75:591-609; Kery et al.., (1992) Arch. Biochem. Biophys.298:49-55). Thus, production strategies resulting in incomplete attachment of terminal sialic acid residues might provide a means of shortening the bioavailability and half-life of secreted glycoproteins by promoting clearance by the hepatic asialoglycoproteiπ receptor. Conversely, expression strategies resulting in saturation of terminal sialic acid attachment sites might lengthen protein bioavailability and half-life by preventing uptake by hepatic receptors. In the development of recombinant glycoproteins for use as pharmaceutical products, it has been speculated that the pharmacodynamics of recombinant proteins can be modulated by the addition or deletion of glycosylation sites from a glycoprotein's primary structure (Berman and Lasky (1985a) Trends in Biotechnol.3 :51 -53). However, studies have reported that the deletion of N-linked glycosylation sites often impairs intracellular transport and results in the intracellular accumulation of glycosylation site variants (Machamer and Rose (1988) J. Biol. Chem. 263:5955-5960; Gallagher et al., (1992) J. Virology 66:7136- 7145; Collier et al.., (1993) Biochemistry 32: 7818-7823; Claffey et al., (1995) Biochemica et Biophysica Acta 1246 :l-9; Dube et al.., (1988) J. Biol. Chem. 263:17516-17521). While glycosylation site variants of secreted proteins can be expressed intracellularly, it has proved difficult to recover useful quantities from growth conditioned cell culture medium. It has been shown that impaired secretion of the HIV-1 envelope glycoprotein containing N-linked carbohydrate structures can be partially overcome by replacing the native signal sequence of the glycoprotein with the signal sequence and 5' untranslated region of the herpes simplex virus type 1 glycoprotein D (HSV -1- gD-1) (Lasky et al , (1986) Science 233 209-212. Berman et al , (1989) J Virol 63 3489-3498) Studies have reported the use of the native secretory peptide of human tissue type plasmmogen activator in cell culture production of non-t-PA glycoprotems (International Publication No WO 96/17067. Krasnev and Young (1992) Cytokme 4 134-143, Rhodes etal , (1994) J Gen Virol 75 207-213) None have reported a strategy for the secretion and recovery of proteins wherein one or more native glycosylation sites have been added to or deleted from the protein's primary structure Summary of the Invention The present invention provides methods and compositions useful in the production of proteins by recombmant cell culture The methods and compositions overcome intracellular retention of proteins and facilitate extracellular recovery of the produced proteins In particular embodiments, the invention provides methods and compositions for the production of polypeptides other than tissue-type plasmmogen activators (t-PA's) utilizing a precursor peptide corresponding to the ammo terminal signal and/or pro peptides naturally associated with a mammalian t-PA which act to direct the secretion of the mature t-PA In particular embodiments, the invention provides for the export and secretion of polypeptides other than t-PA's For example, the invention provides methods and compositions useful in the recombmant production and extracellular recovery of novel chimeπc or fusion proteins such as immunoadhesins and especially those with poor secretion kinetics In particular embodiments the invention provides for the production and recovery of glycoprotems that have been altered from their native sequence to add or remove one or more glycosylation sites Advantageously, the invention improves and in preferred embodiments restores the export and secretion of such glycosylation site variants It is a further advantage of the present invention that greater quantities of the polypeptides produced utilizing the compositions and methods of the present invention can be recovered from the extracelluar medium of the cultured cells from which they are produced Accordmgly, in particular embodiments, the invention provides nucleic acids, expression systems and host cells, as well as methods for the production of non t-PA polypeptides, such as immunoadhesins and especially glycosylation site variant polypeptides. in cell culture In one embodiment of the present invention a DNA construct is provided for the export and secretion of such polypeptides comprising a first DNA segment encodmg a precursor peptide corresponding to all or a portion of the ammo terminal signal and/or pro peptides naturally associated with a mammalian and preferably a human t-PA The first DNA segment is operably linked, in frame, to a second DNA segment encodmg the heterologous (non-t-PA) polypeptide In particular aspects, the first DNA segment encodes a peptide corresponding to a mammalian and preferably a human t-PA pro-sequence as defined herein According to this aspect of the present invention, the first DNA segment is operably linked to a second DNA segment encodmg a heterologous polypeptide Certain embodiments additionally compπse a DNA segment encoding a peptide corresponding to a mammalian signal and or pro-peptide upstream of and operably linked to the first DNA segment For example, when the heterologous polypeptide is an immunoadhesm, such as a TNFR-IgG chimera, the first DNA segment may be preceded by a DNA segment encodmg a mammalian t-PA or alternatively, a heterologous signal and or pro-sequence According to this aspect of the invention the mammalian t-PA signal sequence or other heterologous signal and or pro-sequence and the pro-peptide ammo acid sequence compπse the precursor peptide of the invention In preferred embodiments the first DNA segment encodes a native or naturally occurring signal and pro peptide of a mammalian t-PA and especially a human t-PA The second DNA sequence preferably encodes a polypeptide other than a mammalian t-PA such as, for example, an immunoadhesm Accordmg to further aspects of the invention, the second DNA sequence encodes a naturally occurring or chimeπc polypeptide other than a mammalian t-PA wherein one or more glycosylation sites have been added to or deleted from the polypeptide's native sequence The mvention further provides a DNA construct comprising one or more additional DNA segments operablv linked to the first and second DNA segments The mvention further provides a cultured eukaryotic host cell compπsmg a DNA construct having a first DNA segment encodmg a precursor peptide the precursor peptide corresponding to all or a portion of a mammalian t-PA signal and or pro peptide and a second DNA segment operably linked to the first DNA sequence, the second DNA sequence encodmg a polypeptide other than a mammalian t-PA and preferably a polvpeptide other than a mammalian t-PA wherein one or more glycosvlation sites have been added to or deleted from the polypeptide The invention further provides methods of producing a heterologous polypeptide especially a polypeptide which has been altered to delete one or more native N-linked glycosylation sites compπsmg the steps of (a) cultuπng a eukaryotic host cell compπsmg a DNA construct, the DNA construct compπsmg a first DNA segment encodmg a precursor peptide corresponding to all or a portion of a mammalian tissue plasmmogen activator signal and or pro peptide. and a second DNA segment operably linked to the first DNA sequence, the second DNA sequence encodmg a polypeptide other than t-PA wherein one or more native N-linked glycosylation sites of the polypeptide have been deleted from the poiypeptide where the eukaryotic host cell expresses the first and second DNA segments and the polypeptide is secreted from the cell, and (b) recovermg the polypeptide so produced Brief Description of the Drawings Figure 1 Diagram of a tumor necrosis factor immunoglobulm chimeπc molecule (TNFR-IgG 1 ) and signal sequences TNFR-IgG 1 is a chimeπc protein consisting of the extracellular domam of the p55 TNF receptor fused to the hmge and Fc domain of an immunoglobulm heavy chain TNFR-IgG 1 is secreted as a homodimer with four N-linked glycosylation sites (squares) per monomer The proteins expressed m this study were synthesized usmg the wild type TNFR signal sequence contaming 29 amino acids (SEQ ID NO 2), or a combination of the TNFR signal sequence and the signal and/or pro-sequence of human tissue plasmmogen activator (tPA) (SEQ ID NO 1) Figures 2A-2D Effect of the signal sequence on the secretion kinetics of TNFR-IgG 1 and tPA TNFR-IgG 1 Parallel cultures of human embryonic kidney cells were transfected with plasmids encodmg either TNFR-IgG 1 utilizing the native signal sequence or the signal and pro-sequence of t-PA (tPA TNFR- IgG 1) for pulse labeling experiments Two days post-transfection the culture, medium was removed and replaced (after washing two times) with methionme- and cysteine-free DMEM supplemented with ( J S)- labeled methionme and cysteine Cells were labeled at 37°C (5% C0 2 ) for the times mdicated The labeling reaction was terminated by washing the cells immediately in chilled (4°C) PBS followed by detergent extraction Cell lysates and cell culture supernatants were precipitated by the addition of Protein A SEPHAROSE™ The Protein A TNFR-IgG 1 complexes were pelleted by centπfugation. washed repeatedly and eluted m SDS-PAGE sample buffer contammg mercaptoethanol The eluted protein was resolved on 10% SDS-PAGE gels and visualized by autoradiography Figure 2A, intracellular (l e cell associated) TNFR- IgG 1 Figure 2B. Secreted TNFR-IgG 1 , Figure 2C, intracellular tPA TNFR IgG, Figure 2D, secreted tPA TNFR IgG Figures 3A-3F Secretion kmetics of TNFR-IgG 1 signal sequence and glycosylation mutants Plasmids encodmg fully glycosylated (Figure 3A) or mutated forms of TNFR-IgG 1 (Figures 3B-3F) were transfected into 293 cells and analyzed by pulse-chase analysis as described in Figure 2 Autoradiographs were analyzed with a scanning densitometer and the optical density values for supernatants (o) and cell lysates ( ) were normalized and plotted as a function of time (0-24 hr) The glycosvlation site mutants are identified usmg the single letter amino acid code with reference to the ammo acid residue present at each of the four N- linked glycosylation sites (ammo acid positions 14, 105, 111, and 248) Thus, the designation NNNN represents the wild type TNFR-IgG 1, whereas NQNN indicates TNFR-IgG 1 where glutamine replaced asparagine at the second glycosylation site (position 105) Figure 3 A, TNFR-IgG 1 , Figure 3B tPA TNFR- IgGl, Fιgure 3C. TNFR-IgG 1 mutant lacking 2 native glycosylation sites (NNQQ), Figure 3D, tPA NNQQ, Figure 3E, QSNQ, Figure 3F, tPA QSNQ Figures 4A-4C Immunoprecipitation of TNFR-IgG 1 glycosylation mutants Plasmids encodmg either TNFR-IgG 1 or TNFR-IgG glycosylation site mutants were transfected into 293 cells using calcium phosphate precipitated DNA Two days post-transfection the culture, medium was removed and replaced 35 (after washing two times) with methionme- and cysteine-free DMEM supplemented with ( S)-labeled methionme and cvsteine Cells were labeled overnight at 37°C (5% C0 2 ) Cell Ivsates and cell culture supernatants were precipitated by the addition of Protem A SEPHAROSE™ The Protein A TNFR-IgG 1 complexes were pelleted by centπfugation, washed repeatedly and eluted in SDS-PAGE sample buffer contammg mercaptoethanol The eluted protein was resolved on 10% SDS-PAGE gels and visualized by autoradiography The glycosylation site mutants were exammed m four separate experiments (Figures 4 A-4D) where transiently expressed TNFR-IgG 1 (TNFR-IgG 1) expressed in 293 cells or a stable CHO cell line expressmg TNFR-IgG (TRY+) were used as positive controls Background binding was determined in experiments with cells transfected with thrombopoeitm (TPO-) The mobilities of molecular weight markers are mdicated at the left margins Figures 5 A and 5B Effect of signal sequence replacement on the secretion of TNFR-igGl glycosylation site mutants Plasmids encoding TNFR-IgG 1 glycosylation site mutants contammg the tPA signal/pro sequence (tPA) (SEQ ID NO 1) or the TNFR signal sequence (TNFR) (SEQ ID NO 2) were transfected mto 293 cells, metabo cally labeled with ( 35 S) methionme and cysteine, and immunoprecipitated from cell culture supernatants (S) or cell lysates (L) by the addition of Protem A SEPHAROSE™ as descπbed -4- in Figure 4 The Protein A TNFR-IgG 1 complexes were pelleted bv centπfugation, resolved by SDS-PAGE, and visualized by autoradiography The mobilities of molecular weight markers are indicated at the left margins Figure 6 Characterization of TNFR-IgG 1 glycosylation mutants purified for receptor binding studies TNFR-IgG 1 glycosylation site mutants were purified by protem A affinity chromatography from growth conditioned cell culture supernatants of stabile transfected CHO cells The purified protems were treated with SDS-PAGE sample buffer with- (lanes 8- 12) or without (lanes 3-6) added 2-mercaptoethanol and resolved by SDS-PAGE The resulting gel was stained with Coomassi blue Each lane contained 3 ug of punfied protein Lanes 3 and 8 contamed fully glycosylated TNFR-IgG 1. lanes 4 and 9 contained the NNQQ glycosylation site mutant, lanes 5 and 10 contamed the NSNQ glycosylation site mutant, lanes 6 and 1 1 contained the QSNQ glycosylation site mutant Lane 1 contamed molecular weight standards (indicated at left margin), lanes 2,7, and 12 contamed sample buffer alone 125 Figures 7A-7D Competitive binding of ( I)-labeled TNF to TNFR-IgG 1 glycosylation mutants Punfied TNFR-IgG 1 and TNFR-IgG 1 glycosylation site mutants were captured onto micotiter plates coated with affinity purified goat antibodies to human IgG Fc domain The captured receptor chimeras were reacted 125 with ( I)-labeled TNF along with varying concentrations of unlabeled TNF The bindmg of TNF to fully glycosylated TNFR-IgG 1 is shown in Figure 7A, bmdmg to the NNQQ glycosylation mutant is shown in Figure 7B, binding to the NSNQ glycosylation mutant is shown in Figure 7C, and bmdmg to the QSNQ glycosvlation mutant is shown in Figure 7D Figure 8A-C Secretion kmetics of TNFR-IgG 1 signal sequence variants Plasmids encoding TNFR 1 -IgG 1 utilizing the signal sequence of TNFR 1 (SEQ ID NO 2)(Fιgure 8 A), the signal-pro sequence of tPA (SEQ ID NO l)(Fιgure 8B) or the signal sequence of TNFR 1 (SEQ ID NO 2) and a pro-sequence (SEQ ID NO 7) of human t-PA (SEQ ID NO 8) (Figure 8C) were transfected mto CHO cells and analyzed by pulse-chase analysis as described m Figure 2 Autoradiographs were analyzed with a scanning densitometer and the optical density values for supernatants (□) and cell lysates ( ) were normalized and plotted as a function of time (0-24 hr) Detailed Description of the Preferred Embodiments Definitions The terms "DNA sequence encodmg", "DNA encoding" and "nucleic acid encodmg" refer to the order or sequence of deoxyribonucleotides along a strand of deoxyribonucleic acid The order of these deoxyribonucleotides determines the order of ammo acids along the polypeptide chain The DNA sequence thus codes for a particular ammo acid sequence of a polypeptide The terms "t-PA" and "tissue plasmmogen activator" refer to a naturally occurring extrinsic (tissue- type) plasmmogen activator having fibπnolytic activity and typically havmg a structure with five domains (finger, growth factor, krmgle-l, krmgle-2, and protease domams) Naturally occurring mammalian t-PA mcludes human species as well as other animal species such as rabbit, rat, porcine, non human primate, equine, muπne, and ov e t-PA Nucleic acids encodmg t-PA from human and non-human species is known -5- in the art For example, human t-PA is encoded by the cDNA sequence reported m U S Patent Number 4 766.075. issued 23 August 1988 A "precursor peptide" as used in the context of the present invention and as more fully described herem below, is used to refer to a polypeptide havmg an ammo acid sequence corresponding to all or a portion of a naturally occurring mammalian t-PA signal and/or pro peptide which participates in the secretion of t-PA under native conditions "Pre-pro" or "signal-pro ' peptide as used in the context of the present mvention is meant to refer to an ammo acid sequence such as that naturally associated with a mammalian t-PA which acts to direct the secretion of a mature polypeptide, for example, a mammalian t-PA, from a cell As used herem. the term 'signal-pro peptide" includes the "pre-" or "signal" sequence such as that naturally associated with a mammalian t-PA which functions to bind to the signal-recognition particle and direct the protem to the lumen of the endoplasmic reticulum (ER) A "signal" sequence is an amino acid sequence, charactenstically hydrophobic in nature, cleaved by signal peptidases in the ER For example, the signal sequence of t-PA is generallv removed from the nascent t-PA co-translationally In addition to a signal sequence, some mammalian protems are associated post-translationally with a "pro" sequence such as the signal-pro sequence of a mammalian t-PA The "pro-" sequence serves to target the nacsent polypeptide, for example a mature t-PA, to the Golgi apparatus (GA) and is cleaved post-translationally The pro-sequence is charactenstically positively charged and commonly contams golgi peptidase cleavage sequences The ammo acid sequences of the mammalian t-PA signal-pro peptides are generally known or obtamable through conventional techniques Signal-pro peptides from human and non-human t-PAs are known in the art and have been disclosed in, for example U S Patent 4,766,075, Rickles e α/ . (1988) J Biol Chem 263 1563-1560 and Feng et a , (1990) J Biol Chem 265 2022-2027 The term native signal-pro peptide specifically encompasses naturally-occurnng pre- and prepro- sequences as defined and mcludes naturally-occurring variant forms (e , alternatively spliced forms) and naturally-occurring alle c variants of the t-PA signal-pro peptide DNA sequences encoding mammalian t-PA amino terminal signal-pro peptides can be cloned as cDNA or genomic molecules accordmg to techniques that are standard in the art or can be synthesized, preferably usmg automated equipment and the application of conventional synthetic protocols As used herem, "heterologous polypeptide" refers generally to polypeptides and protems regardless of their oπgm other than a t-PA or a variant thereof and generally having more than about ten ammo acids which may or may not have one or more native or synthetic sites for the attachment of a carbohydrate Preferably, heterologous polypeptides are "heterologous glycoprotems", that is a heterologous polypeptide as descnbed above havmg one or more sites for the attachment of a carbohydrate in their native sequence Examples of heterologous glycoprotems mclude molecules such as cytokines and their receptors, as well as chimeπc protems compπsmg cytokines or their receptors, mcludmg, for instance tumor necrosis factor alpha and beta, their receptors (TNFR- 1 , Gray etal , (1990) Proc Natl Acad Sci USA 87 7380-7384, and TNFR- 2, Kohno et al , (1990) Proc Natl Acad Sci USA 87 8331-8335) and their derivatives, renin, a growth hormone, mcludmg human growth hormone, and bovme growth hormone, growth hormone releasmg factor, parathyroid hormone, thyroid stimulating hormone, hpoproteins, alpha- 1-antιtrypsιnjnsulιnA-chaιn. insulin -6- B-chain. proinsuhn. follicle stimulating hormone, calcitonm, luteinizmg hormone, glucagon clottmg factors such as factor VIIIC. factor IX. tissue factor, and von Willebrands factor, anti-clotting factors such as Protem C. atπal natπuretic factor, lung surfactant, a plasmmogen activator other than a tissue-tvpe plasmmogen activator (t-PA), for example a urokmase. bombesin. thrombin. hemopoietic growth factor enkephalinase, RANTES (regulated on activation normally T-cell expressed and secreted), human macrophage inflammatory protem (MIP- 1 -alpha),a serum albumin such as human serum albumin, mulleπan- inhibiting substance, relaxm A-cham, relaxin B-chain, prorelaxin, mouse gonadotropm-associated peptide a microbial protem, such as beta-Iactamase, DNase, mhibm, activm, vascular endothe al growth factor (VEGF), receptors for hormones or growth factors, mtegrm, protein A or D, rheumatoid factors, a neurotrophic factor such as bone-denved neurotrophic factor (BDNF), neurotrophm-3, -4, -5, or -6 (NT-3. NT-4, NT-5, or NT-6) or a nerve growth factor such as NGF-β, platelet-derived growth factor (PDGF), fibroblast growth factor such as aFGF and bFGF, epidermal growth factor (EGF), transforming growth factor (TGF) such as TGF-alpha and TGF-beta, mcludmg TGF-βl, TGF-β2, TGF-β3, TGF-β4, or TGF-β5, insulin-like growth factor-I and -II (IGF-I and IGF-II) des( 1 -3)-IGF-I (bram IGF-I), msulm-like growth factor binding protems CD proteins such as CD-3, CD-4, CD-8. and CD- 19, erythropoietin. osteomductivefactors, immunotoxms. a bone morphogenetic protem (BMP), an interferon such as interferon-alpha, -beta, and -gamma, colony stimulating factors (CSFs), e g , M- CSF, GM-CSF, and G-CSF, mterleukins (ILs), e g , IL-1 to IL-10, superoxide dismutase T-cell receptors, surface membrane protems, decay accelerating factor viral antigen such as. for example, a portion of the HI V- 1 envelope glycoprotein, gp 120, gp 160 or fragments thereof, transport proteins, homing receptors, addressins, regulatory protems, antibodies and chimeπc proteins, such as immunoadhesins Preferably the heterologous glycoproteins are "glycosylation site variants" of any of the heterologous glycoprotems described above More particularly, the glycosylation site variants accordmg to the present mventionare naturally occurring heterologous glycoprotems or variations thereof such as the chimenc protems described above, having at least one site for glycosylation, preferably an N-linked glycosvlation site, that has been added to or deleted from their native sequence Such glycosvlation site variants include both "glycosvlation site addition variants" as well as "glycosylation site deletion variants ' so named herein to delineate heterologous glycoproteins as defined above having one or more sites for glycosvlation preferably N-linked, added to or deleted from their native sequence, respectively The heterologous glycoproteins of the present invention have one or more carbohvdrate structures that occur on the native protem expressed as N-linked or O-linked carbohydrates The N-linked and O-linked carbohydrates differ primarily m their core structures N-linked glycosylation refers to the attachment of the carbohydrate moiety via N-acetylglucosamine (GlcNAc) to an asparagme residue in the peptide cham The ammo acid sequence of the heterologous polypeptide will contain an asparagme-X-seπne. asparagme-X- threonine, or asparagine-X-cysteine, wherem X is any ammo acid except prolme O-linked carbohydrates, by contrast are characterized by a common core structure, which is the N-acetylgalactosamme (GalNAc) attached to the hydroxyl group of a threomne or seπne in the native ammo acid sequence of the heterologous polypeptide -7- The terms tumor necrosis factor receptor" and "TNFR" in the context of the present mvention refer to a polypeptide comprising the ammo acid sequence of a native TNF bmdmg polypeptide, or any combination, derivative or fragment thereof which is capable of bmdmg to tumor necrosis factor-α and/or -β (TNF-α and or -β) This definition mcludes substantially intact ceil surface or soluble type 1 and type 2 TNFR polypeptides (TNFR 1 and TNFR2) from natural sources, synthetically produced in vitro or obtamed by genetic manipulation mcludmg methods of recombinant DNA technology, as well as various cham combmations of such polypeptides Native TNFR mclude human and non-human animal, e g murme, bovine, equme. porcme, etc TNFR polypeptides The non-human mammalian species can. for example, be obtamed by cross-species hybridization, usmg probes obtamed from the human DNA sequence as hybndization probes to isolate TNFR cDNAs from the respective non-human mammalian cDNA libraries The chimeπc gene encoding TNFR 1 -IgG 1 is described in Ashkenazi e/α/ , (1991) Fτoc Natl Acad Sci 88 10535-10539 TNFRl-IgGl is a chimeπc protem constructed by fusing the extracellular domam of the receptor type 1 for TNF alpha with sequences encoding the Fc domain and hmge region of IgGl (Ashkenazi et al , (1991) The chimenc protem (Figure 1 ) contains four N-linked glycosvlation sites three located in the TNFR region (positions 14, 105, and 1 1 1) and one in the Fc domain (position 248) For simplicity, the glycosylation mutants descπbed in this mvention are named usmg the single letter ammo acid code with reference to the ammo acid residue present at each of the four N-linked glycosylation sites at amino acid positions 14, 105, 1 1 1, and 248 (Figure 1) Thus, the designation NNNN represents the fully glycosvlated TNFR 1 -IgGl, whereas NQNN indicates TNFR-IgG 1 where glutamine replaced asparagine at the second glycosvlation site (position 105) As used herein, the term "immunoadhesm" designates antibody-like molecules which combme the "bmdmg domain" of a heterologous protein (an "adhesin", e g a receptor, ligand or enzyme) with the effector functions of immunoglobulm constant domams Structurally, the immunoadhesins comprise a fusion of the adhesin amino acid sequence with the desired bmdmg specificity which is other than the antigen recognition and bmdmg site (antigen combining site) of an antibody (; e is heterologous ') and an immunoglobulm constant domain sequence The immunoglobulm constant domam sequence m the immunoadhesm may be obtamed from any immunoglobulm, such as IgG ] , IgG 2 , IgG 3 or IgG 4 subtypes, IgA, IgE, IgD or IgM Immunoadhesms are described in, for example, U S Patent Nos 5 1 16,964, 5,714, 147 and 5,336,603 the disclosures of which are hereby incorporated by reference Immunoadhesins include CD4 (Capon et αl , (1989) Nature 337 525-531, Traunecker et αl , (1989) Nature 339 68-70. and Byrn et αl , (1990) Nature 344 667-670), L-selectin or homing receptor ( Watson etαl , (\ 990) J Cell Biol 110 2221-2229 and Watson et αl , (1991 ) Nature 349 164-167), CD44 (Aruffo et αl , (1990) Cell 61 1303-1313, CD28 and B7 (Lmsley et αl , (1991) J Exp Med 173 721-730), CTLA-4 (Lisley et αl , J Exp Med 174 561-569), CD22 (Stamenkovιc e/ α/ , Cell 66 1 133-1 144), TNF receptor ( Ashkenazi ef α/ , (l 99 l) Proc Natl Acad Sci USA 88 10535-10539, Lesslauer e α/ , ( 1991) Eur J Immunol 27 2883-2886, and Peppel et αl (1991) J Exp Med 174 1483-1489, Mohler etαl , (1993) J Immunol 151 1548- 1561), NP receptors (Bennett etαl , (1991) J Biol Chem 266 23060-23067, mterferon γ receptor (Kurschner et αl , (1992) J Biol Chem 267 9354- 9360: 4- IBB (Chalupny et al.., ( 1992) PNAS USA 89: 10360-10364) and IgE receptor α (Ridgway and Gorman. (1991) J. Cell. Biol. 1 15, Abstract No. 1448). The terms "host cell" and "host cell line" refer to cells and cell lines derived from a prokaryotic or eukaryotic organism that are capable of growth and survival when placed in either monolayer culture or in suspension culture in a medium containing the appropriate nutrients and growth factors. Suitable host cells for use within the present invention include any type of cell that can be engineered to express a DNA molecule, can be grown in culture, and have a secretory pathway. Although prokaryotic cells such as E. coli cells are capable of secreting protein at least into the periplasmic space, it is preferred within the context of the present invention to use cultured eukaryotic cells, such as fungai cells, insect cells, yeast cells or in particular mammalian cells. The necessary growth factors for a particular cell line are readily determined empirically without undue experimentation, as described for example in Mammalian Cell Culture (Mather, J.P. ed.. Plenum Press, N.Y. [1984]), and Barnes and Sato, (1980) Cell, 22:649. Typically, the cells are capable of expressing and secreting large quantities of a particular glycoprotein of interest into the culture medium. Examples of such cells include SF9 insect cells (Summers and Smith (1987) Texas Agriculture Experiment Station Bulletin, 1555; and Insect Cell Culture Engineering, Goosen Daugulis and Faulkner Eds. Dekker. New York). Examples of suitable mammalian host cells within the context of the present invention may include Chinese hamster ovary cellsADHFR (CHO, Urlaub and Chasin, Proc. Natl. Acad. Sci. USA. 77:4216 [1980]); dpl2.CHO cells (EP 307,247 published 15 March 1989); monkey kidney CV1 line transformed by S V40 (COS-7, ATCC CRL 1651 ); human embryonic kidney line (293 or 293 cells subcloned 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). Preferred host cells include Chinese hamster ovary cellsΛDHFR (CHO. Urlaub and Chasin, Proc. Natl. Acad. Sci. USA, 77:4216 [1980]); dpl2.CHO cells (EP 307,247 published 15 March 1989). "Expression vector" refers to a DNA construct containing a DNA sequence which is operably linked to a suitable control sequence capable of effecting the expression of the DNA in a suitable host. Such control sequences include a promoter to effect transcription, an optional operator sequence to control such transcription, optional RNA splice donor/acceptor sequences, a sequence encoding suitable mRNA ribosome binding sites, and sequences which control termination of transcription and translation. The vector may be a plasmid, a phage particle, a recombinant virus or simply a potential genomic insert. Once transformed into a suitable host, the vector may replicate and function independently of the host genome, or may, in some instances, integrate into the genome itself. In the present specification, "plasmid" and "vector" are sometimes used interchangeably as the plasmid is the most commonly used form of vector at present. However, the invention is intended to include such other forms of expression vectors which serve equivalent functions and which are. or become, known m the art "Operably linked" when describing the relationship between two DNA regions simply means that they are functionally related to each other For example, a signal sequence is operably linked to a peptide if it functions to ensure synthesis on membrane bound πbosomes and to translocate the nascent protem across the lumen of the endoplasmic reticulum as a signal sequence, participating m the secretion of the mature form of the protem most probably involving cleavage of the signal sequence A promoter is operably linked to a coding sequence if it controls the transcription of the sequence, a πbosome bmdmg site is operably linked to a codmg sequence if it is positioned so as to permit translation Therefore, nucleic acid is ' operably linked" when it is placed mto a functional relationship with another nucleic acid sequence Generally, "operably linked" means that the DNA sequences being linked are contiguous, and, in the case of a secretory leader, contiguous and in readmg phase However, enhancers do not have to be contiguous Linking is accomplished by hgation at convenient restriction sites If such sites do not exist, the synthetic oligonucleotide adaptors or linkers are used in accordance with conventional practice The terms "transfected." "transformed (host) cell" , "transfectant" and "transformed 'and the like refer to the introduction of DNA mto a cell The cell is termed a "host cell" The introduced DNA is usually m the form of a plasmid denved vector containing an inserted piece of DNA The introduced D A sequence may be from the same species as the host cell or a different species from the host cell, or it mav be a hybrid DNA sequence, containing some foreign and some homologous DNA The words transformants and transformed (host) cells include the primary subject cell and cultures derived therefrom, without regard to the number of transfers It is also understood that all progeny may not be precisely identical in DNA content, due to deliberate or inadvertent mutations or amplification of chromosomal fragments Mutant progeny that have the same function or biological property as screened for in the originally transformed cell are included The term ammo acid or ammo acid residue, as used herein, refers to naturallv occurring L ammo acids or residues, unless otherwise specifically indicated The commonlv used one- and three-letter abbreviations for ammo acids are use herein (Lehnmger, A L , Biochemistry 2d ed pp 71-92, Worth Publishers, N Y (1975)) Modes for Carrying out the Invention Studies have demonstrated that deletion of N-linked glycosylation sites from a glycoprotem's primary structure often impairs the mtracellular transport and secretion of the polypeptide Accordmg to the present invention, the defect m secretion of such polypeptides can be overcome by operably linking the nucleic acid sequence encodmg the polypeptide to a nucleic acid sequence encodmg a precursor peptide Surprisingly, the precursor polypeptide increased the secretion efficiency of the parent glycoprotein as well The precursor peptide has an ammo acid sequence which correspond to all or a portion of a naturally occurring mammalian t-PA signal-pro peptide which participates in the secretion of t-PA under native conditions Preferably the precursor peptide of the present invention is "capable of restoring the export and secretion" of a heterologous polypeptide, especially a heterologous glycoprotem such as an immunoadhesm and more preferably a heterologous glycosylation site variant glycoprotem to the extracellular space As will -10- be appreciated by the skilled artisan "capable of restoring the export and secretion" of a polypeptide to the extracellular space is a relative term Thus the term when used to describe the biological activity of the precursor peptide means a peptide that when linked to a heterologous polypeptide produces an increase m the amount of heterologous polypeptide that can be recovered from the extracellular space when compared to the amount recoverable from the extracellular space in the absence of the precursor peptide of the present invention and m the presence of the native signal and/or pro sequence of the heterologous polypeptide To determine whether the precursor peptide improves the export and secretion of the polypeptide. the efficiency of mtracellular transport can be measured by routme experimentation For example, a plasmid can be constructed for transient transfection studies in eukaryotic host cells Pulse-chase experiments similar to those described herein will show that replacement of the native signal sequence associated with the heterologous polypeptide with the precursor peptide of the present invention results in an improvement m secretion efficiency Thus, after transient expression at, for example 24 hours, the amount of heterologous polypeptide secreted into the culture medium can be measured and compared to a control usmg a native or other signal sequence For example, approximately 70-80% and preferably 80-100% of the pulse-labeled heterologous polypeptide can be secreted mto the cell culture medium when a precursor peptide of the present invention is employed in the expression construct, whereas only 50-60% of the heterologous polypeptide is secreted using the wild-type or native signal sequence This represents a 20 to 50% increase in secretion efficiency The increased efficiency of secretion achieved accordmg to the present invention utilizing the precursor peptide is also apparent from the kinetics of secretion of the heterologous polypeptide For example, when the native signal sequence is employed, 40% of the pulse-labeled heterologous polypeptide is secreted in 2-4 hrs after pulse labeling However when the native signal sequence is replaced with the precursor peptide corresponding to a mammalian t-PA signal-pro peptide, approximately 60% of the heterologous polypeptide is secreted in this time frame The precursor peptide of the present invention has an ammo acid sequence which corresponds to all or a portion of the amino acid sequence of a mammalian t-PA signal-pro peptide including native signal-pro peptides of t-PA havmg the same ammo acid sequence as a signal-pro peptide of a mammalian t-PA sequence denved from nature Usmg human t-PA as but an example, the signal-pro peptide precedes the mature t-PA protem sequence and acts to direct export and secretion of the mature protein The approximately 35 amino acid signal pro-peptide of human t-PA consists ofa signal sequence of about 21 (SEQ ID NO 3) or 22 (SEQ ID NO 12) amino acid residues, also called a pre-sequence, and an 1 1 (SEQ ID NO 4) or 10 (SEQ ID NO 13) ammo acid pro-sequence, mcludmg a 3 ammo acid exopeptidase cleavage site (see, for example, Figure 1 infra. Example 8, infra, and Berg and Grmnell ( 1991 ) Biochem Biophys Res Commun 179 31289-1296) However, it is the ability of the precursor peptide to restore or mcrease export and secretion of the heterologous polypeptide and not the length of the precursor peptide that is important Therefore, m one embodiment of the invention, the signal-pro peptide of a mammalian t-PA is a native human sequence comprising ammo acids 1 to 35 of SEQ ID NO 1 numbered as shown below and corresponding to ammo acids -1 1- -35 to -1 of Fig 1 Therefore, in general the present invention makes use of a precursor peptide havmg the sequence 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Met-Asp-Ala-Met-Lys-Arg-Gly-Leu-Cys-Cys-Val-Leu- eu-Leu-Cys-Gly- 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Ala-Val-Phe-Val-Ser-Pro-Ser-Gln-Glu-Ile-His-Ala-Arg-Phe-Arg-Arg- 33 34 35 Gly-Ala-Arg (SEQ ID NO: 1) . In another embodiment of the invention, the precursor peptide comprises a human t-PA pro-sequence as, for example, ammo acids 22 to 32 (SEQ ID NO 4) ofSEQ ID NO 1, or ammo acids 22-35 (SEQ IDNO 5) of SEQ ID NO 1 and more preferably ammo acid residues 23-35 of SEQ ID NO 1 (SEQ ID NO 7) In another embodiment of the invention, the precursor peptide is a human signal-pro sequence compπsmg ammo acids 1 to 32 (SEQ ID NO 6) of SEQ ID NO 1 and preferably amino acid residues 1-35 (SEQ ID NO 1) Such precursor peptides and the DNA encodmg them can be isolated from nature or can be produced by recombinant or synthetic means In a further aspect of the present invention the precursor peptides of the present mvention are homologous ammo acid sequences of mammalian t-PA's signal-pro peptides or homologous ammo acid sequences of the sequence of, for example, SEQ ID NO 1, SEQ ID NO 3, SEQ ID NO 4, SEQ ID NO 5, SEQ ID NO 6, and SEQ ID NO 7 mcludmg homologous in vitro generated variants havmg the qualitative biological activity defined above Homology with respect to the precursor peptides of the present invention is defined as the percentage of amino acid residues m a candidate sequence that are identical with either the ammo acid residues m SEQ ID NO 1 , SEQ ID NO 3, SEQ ID NO 4, SEQ ID NO 5, SEQ ID NO 6, SEQ ID NO 7, the ammo acid sequence of a mammalian t-PA or a composite sequence of a human and other mammalian sequence as defined herein after aligning the sequences and introducing gaps if necessary to achieve the maximum identity No N- or C- terminal extensions or deletions in the candidate sequence shall be construed as reducing identity For example, N- or C-terminal addition of a signal sequence other than that ofa mammalian t-PA to mammalian precursor peptide as defined herein is within the scope of the precursor peptide of the invention "Composite amino acid" within the present invention refers to an alternate ammo acid having the same position in the 35 ammo acid residue structure as human t-PA from another mammalian vertebrate species Therefore, an ammo acid substitution referred to as a composite ammo acid substitution replaces the identified ammo acid with the equivalent or composite amino acid from another mammalian species A composite sequence is defined as havmg at least one ammo acid from the native human t-PA sequence replaced with a composite ammo acid from another mammalian species Therefore, the invention contemplates an precursor peptide having at least the qualitative biological activity as defined above and havmg, for example, at least about 75% ammo acid homology with the polypeptide of SEQ ID NO 1, SEQ ID NO 3, SEQ ID NO 4 or the polypeptide of SEQ ID NO 1 lackmg the 3 carboxyl terminal ammo acid residues, SEQ ID NO 6 and/or the 21 ammo terminal residues, SEQ ID NO 5 The precursor peptide ammo acid sequence preferably will share at least 80%, more preferably, greater -12- than 85% sequence homology with the sequence of SEQ ID NO: 1 or the sequences lacking the 3 carboxyl terminal amino acid residues (SEQ ID NO: 6) and/or the 21 amino terminal residues (SEQ ID NO: 5). However, a precursor peptide may exhibit less than 50% sequence homology with the sequence of SEQ ID NO: 1 and still retain the characteristics of the precursor peptide as defined above. Since the mechanism by which the signal-pro peptide of t-PA overcomes endoplasmic reticulum retention may relate to its complex structure, precursor peptides which comprise a portion of a signal-pro peptide as defined above are also within the scope of the present invention. For example, as noted above, the signal-pro peptide of t-PA is unusual, and contains an approximately 21 -22 amino acid pre- or signal sequence and a 10 or 1 1 amino acid pro-sequence, including a 3 amino acid exopeptidase cleavage site (Figure 1). Previous studies, (Berg and Grinnell (1991) Biochem. Biophys. Res. Commun. 179:1289-1296) have shown that the pre- or signal sequence is cleaved co-translationally in the endoplasmic reticulum and that the pro- sequence is removed in the Golgi apparatus (G A) by cleavage at a furin processing site. The fact that the endo H sensitive, intracellular forms of an exemplary heterologous glycoprotein glycosylation site variantexpressed with the amino terminal precursor peptide sequence of the present invention possessed higher molecular weight than the secreted products demonstrates that, like wild type t-PA, removal of the pro-sequence from glycosylation mutants occurs during export through the GA. Based on these observations, the amino terminal pro-sequence of t-PA represents a structural element able to promote ER to GA transport of the heterologous polypeptide, and especially glycosylation mutants. Therefore according to a preferred aspect of the present invention, a precursor peptide comprising all or a portion of the pro-sequence ofa mammalian-tPA is employed to effect the export and secretion of the heterologous polypeptides of the present invention. For example, the pre-sequence of any mammalian pre- protein sequence, for example, that associated with the native heterologous polypeptide, may be employed in conjunction with the pro-sequence of a mammalian t-PA in the precursor peptide of the present invention. Forexample, insertion of the 13 amino acid tPA pro-peptide Ser-Gln-Glu-Ile-His-Ala-Arg-Phe-Arg-Arg-Gly- Ala-Arg (SEQ ID NO: 7) between the TNFR 1 -IgG 1 endogenous signal sequence, i.e., that naturally associated with TNFR1 (Gray et al.., (1990) supra, amino acids -40 to -12 of Figure 1, SEQ ID NO: 2) and the TNFR1- IgGl coding sequence provides a precursor peptide (SEQ ID NO: 8) within the context of the present invention that exhibits an increase in export and secretion of the mature molecule (TNFRl-IgGl). According to another aspect of the present invention the DNA sequence of the t-PA pro-sequence (SEQ ID NO: 5; SEQ ID NO: 7) is modified to introduce or eliminate proteolytic processing sites. Such modified precursor peptides are described in, for example, International publication No. WO 96/17067. Briefly, and while not wishing to be bound by theory it is believed that pro-sequence of mammalian t-PA is cleaved at a site dependent upon a prohormone converting enzyme such as the yeast KEX2 gene product or the mammalian enzymes PC 1 , PC2 and furin. Enzymes of this type recognize cleavage sites characterized by arginine residues in the - 1 and -4 positions. Cleavage is facilitated by a basic amino acid residue e.g. Lys or Arg in the - 1 position. Within the present invention therefore are a pro-sequence having an Arg at residue 35 and 32 and optionally a basic amino acid residue at position 34. -13- As mdicated, the present invention furthercompπsesa second DNA segment encodmg a heterologous polypeptide, l e , a polypeptide other than a mammalian t-PA such as a TNFR immunoadhesm Preferably the heterologous polypeptide lacks or has been modified to delete one or more native glycosvlation sites as descnbed Deletion of glycosylation sites from a heterologous polypeptide may be accomplished by altering the ammo acid sequence of the polypeptide The alteration may be made by, for example, the addition of or substitution by one or more ammo acids to the polypeptide' s native sequence For example, if N-linked glycosvlation is contemplated, the glycosylation site m the variant is a tπpeptidyl sequence of the formula asparagme-X-seπne or asparagme-X-threonme, wherein asparagme is the acceptor and X is any of the twenty genetically encoded ammo acids except prolme (See Struck, D K and Lennarz, W J in The Biochemistry of Glycoprotems and Proteoglycans, W J Lennarz ed , Plenum Press, 1980, p 35, Marshall. R D (1974) Biochem Soc Symp 40 17, and W zler, R J in Hormonal Proteins and Peptides. Li, C I ed , Academic Press, New York, 1973, pp 1-15) The amino acid sequence variants herem are modified bv substituting the appropriate ammo acιd(s) at the proper sιte(s) to effect a change in the native glycosvlation site For example an asparagme residue can be replaced by a glutamme residue to disrupt a native glycosvlation site withm the polypeptide If O-linked glycosylation is to be employed, O-glycosidic lmkage occurs m animal cells between N- acetylgalactosamine, galactose or xylose, and one of several hydroxyamino acids, most commonly serme or threonine, but also in some cases a 5-hydroxyprolιne or 5-hydroxylysιne residue placed m the appropriate region of the molecule Accordmg to the present invention the polypeptides are altered through changes at the DNA level, particularly by mutatmg the DNA encodmg the polypeptide at preselected bases, l e , those encoding N- or O-linked glycosylation sites such that codons are generated that will translate into ammo acids other than those that will typically be present for N- or O-linked glycosylation The variations can be made usmg methods known in the art such as oligonucleotide-mediated (site directed) mutagenesis alanine scannmg and PCR mutagenesis Site directed mutagenesis (Carter etal , (1986) Nucl Acids Res 13 4331 , Soller£?r<a/ , (1987) Nucl Acd Res , 10 6487, cassette mutagenesis (Wells et al , (1985) Gene 34 315), restriction selection mutagenesis ( Wells etal , (\ 986) Philos Trans R Soc London) or other known techniques can be performed on the cloned DNA to produce the glycosylation site mutants that are a subject of the present invention Therefore, accordmg to the present mvention in order to direct the secretion of a heterologous polypeptide and preferably a glycosylation variant as described above, a DNA segment encodmg a precursor peptide is joined to a DNA sequence encodmg the heterologous polypeptide in the correct readmg frame so that the jomed sequences encode a fusion protem Expression vectors for use in the methods and compositions of the present mvention will generally include a promotor capable of directmg the transcription of the first and second DNA of the mvention The term "control sequences" refers to DNA sequences necessary for the expression of an operably linked codmg sequence in a particular host organism The control sequences that are suitable for prokaryotes, for example, mclude a promoter, optionally an operator sequence, and a πbosome bmdmg site Eukaryotic cells are known -14- to utilize promoters, polyadenylation signals, and enhancers. Preferred promtftβfsTKclude viral promoters and cellular promoters. Host cells are transfected or transformed with expression or cloning vectors described herein for heterologous polypeptide production and cultured in conventional nutrient media modified as appropriate for inducing promoters, selecting transformants, or amplifying the genes encoding the desired sequences. The culture conditions, such as media, temperature, pH and the like, can be selected by the skilled artisan without undue experimentation. In general, principles, protocols, and practical techniques for maximizing the productivity of cell cultures can be found in Mammalian Cell Biotechnology: a Practical Approach. M. Butler, ed. (IRL Press, 1991) and Sambrook et al.., supra. Methods of transfection are known to the ordinarily skilled artisan, for example, CaP0 4 and electroporation, as well as the use of commercially available cationic lipid reagents which facilitate transfection. Depending on the host cell used, transformation is performed using standard techniques appropriate to such cells. The calcium treatment employing calcium chloride, as described in Sambrook et al. supra, or electroporation is generally used for prokaryotes or other cells that contain substantial cell-wall barriers. Infection with Agrobacterium tumefaciens is used for transformation of certain plant cells, as described by Shaw et al.., Gene, 23:315 (1983) and WO 89/05859 published 29 June 1989. For mammalian cells without such cell walls, the calcium phosphate precipitation method of Graham and van der Eb, Virology. 52:456-457 (1978) can be employed. General aspects of mammalian cell host system transformations have been described in U.S. Patent No. 4,399,216. Transformations into yeast are typically carried out according to the method of Van Solingen etai.., J. Bact., 130:946 (1977) and Hsiao etai.., Proc. Natl. Acad. Sci. (USA), 76:3829 ( 1979). However, other methods for introducing DNA into cells, such as by nuclear microinjection, electroporation, bacterial protoplast fusion with intact cells, or polycations, e.g. , polybrene, polyornithine, may also be used. For various techniques for transforming mammalian cells, see Keown et al.., Methods in Enzvmology, 185:527-537 (1990) and Mansour et al.., Nature. 336:348-352 (1988). Suitable host cells for the expression of heterologous polypeptide that retain one or more glycosylation sites are derived from multicellular organisms. Examples of invertebrate cells include insect cells such as Drosophila S2 and Spodoptera Sf9, as well as plant cells. Examples of useful mammalian host cell lines include Chinese hamster ovary (CHO) and COS cells. More specific examples include monkey kidney C V 1 line transformed by S V40 (COS-7, ATCC CRL 1651 ); human embryonic kidney line (293 or 293 ceils subcloned for growth in suspension culture, Graham et al.. , J. Gen Virol..36:59(1977)); Chinese hamster ovary cellsΛDHFR (CHO, Urlaub and Chasin, Proc. Natl. Acad. Sci. USA, 77:4216 (1980)); mouse sertoli cells (TM4, Mather, Biol. Reprod., 23:243-251 (1980)); human lung cells (W138, ATCC CCL 75); human liver cells (Hep G2, HB 8065); and mouse mammary tumor (MMT 060562, ATCC CCL51). The selection of the appropriate host cell is deemed to be within the skill in the art. Host cells containing DNA constructs of the present invention are then cultured to produce the heterologous protein. The cells are cultured according to standard methods in a culture medium containing the necessary nutrients for growth of cultured mammalian cells. -15- For the culture of the eukaryotic cells expressing the desired protein and modified as described for the instant invention, numerous culture conditions can be used paying particular attention to the host cell bemg cultured Suitable culture conditions for eukaryotic cells are well known in the art (J Immunol Methods ( 1983)56 221 -234) or can be easily determined by the skilled artisan (see, for example. Animal Cell Culture A Practical Approach 2nd Ed , Rickwood, D and Hames, B D . eds Oxford University Press, New York (1992)), and vary according to the particular host cell selected The eukaryotic cell culture of the present invention is prepared in a medium suitable for the particular cell being cultured Commercially available media such as Ham's Fl 0 (Sigma), Minimal Essential Medium ([MEM], Sigma), RPMI-1640 (Sigma), and Dulbecco's Modified Eagle's Medium ([DMEM]. Sigma) are exemplary nutrient solutions In addition, any of the media described in Ham and Wallace,( 1979) Meth Enz , 58 44 Barnes and Sato,( 1980) Anal Biochem . 102 255, U S Patent Nos 4,767,704, 4,657,866.4.927,762, 5,122.469 or 4 560.655, International Publication Nos WO 90/03430, and WO 87/00195, the disclosures of all of which are incorporated herein by reference, may be used as culture media Any of these media may be supplemented as necessary with hormones and/or other growth factors (such as insulin, transferπn, or epidermal growth factor), salts (such as sodium chloride, calcium, magnesium, and phosphate), buffers (such as HEPES), nucleosides (such as adenosine and thymidine), antibiotics (such as Gentamyαn™ drug), trace elements (defined as inorganic compounds usually present at final concentrations in the micromolar range) hpids (such as hnoleic or other fatty acids) and their suitable carriers, and glucose or an equivalent energy source Any other necessary supplements may also be included at appropriate concentrations that would be known to those skilled in the art In a particular embodiment, the mammalian host cell is a CHO cell, preferably a DHFR-CHO cell and a suitable medium contains a basal medium component such as a DMEM/HAM F-12 based formulation (for composition of DMEM and HAM F12 media, see culture media formulations in American Type Culture Collection Catalogue of Cell Lmes and Hybπdomas, Sixth Edition. 1988, pages 346-349) (the formulation of medium as described m U S Patent 5, 122,469 are particularly appropriate) with modified concentrations of some components such as amino acids, salts, sugar, and vitamins, and optionally contammg glycine. hypoxanthme, and thymidine, recombinant human insulin, hydrolyzed peptone, such as Pπmatone HS or Pπmatone RL (Sheffield, England), or the equivalent, a cell protective agent, such as Pluronic F68 or the equivalent pluronic polyol, Gentamycin, and trace elements For the production of the sought after glycoprotems, production by growing the host cells of the present invention under a variety of cell culture conditions is typical For instance, cell culture procedures for the large or small scale production of proteins are potentially useful withm the context of the present invention Procedures including, but not limited to, a fluidized bed bioreactor, hollow fiber bioreactor. roller bottle culture, or stirred tank bioreactor system may be used, in the later two systems, with or without microcarners, and operated alternatively in a batch, fed-batch, or continuous mode Following the polypeptide production phase, the polypeptide of interest is recovered from the culture medium using techniques which are well established in the art -16- The polypeptide of interest preferably is recovered from the culture medium as a secreted polypeptide As a first step, the culture medium or lysate is centπfuged to remove paniculate cell debris The polypeptide thereafter is purified from contaminant soluble proteins and polypeptides with the following procedures being exemplary of suitable purification procedures by fractionation on immunoaffinity or lon- exchange columns, ethanol precipitation, reverse phase HPLC, chromatography on silica or on a cation- exchange resin such as DEAE, chromatofocusmg, SDS-PAGE, ammonium sulfate precipitation, gel filtration usmg for example, Sephadex G-75. and protem A SEPHAROSE™ columns to remove contaminants such as IgG A protease inhibitor such as phenyl methyl sulfonyl fluoride (PMSF) also may be useful to inhibit proteolvtic degradation during purification The following examples are offered by way of illustration and not by way of limitation The disclosures of all citations in the specification are expressly incorporated herein by reference EXAMPLES Introduction The following are general methods used in the Examples that follow Methods DNA and Constructions The chimeπc gene encodmg TNFR-IgG 1 is described (Ashkenazi et al , (1991) Proc Natl Acad Sci 88 10535-10539) Site directed mutagenesis was carried out following the mismatched primer method ofKunkel. etai (1985) Proc Natl Acad Sci 82 488-492) using the Muta-Gene M 13 Kit (Bio-Rad Inc . Hercules, CA) Mutations were verified by dye terminator cycle sequencing usmg the automated ABI 310 Genetic Analyzer (Applied Biosystems, Foster City, CA) For simplicity, the glycosvlation mutants described in this paper are named using the smgle letter amino acid code with reference to the amino acid residue present at each of the four N-linked glycosylation sites at ammo acid positions 14, 105 1 1 1, and 248 (Figure 1) Thus, the designation NNNN represents the fully glycosylated TNFR-IgG 1, whereas NQNN indicates TNFR-IgG 1 where glutamme replaced asparagme at the second glycosylation site (position 105) TNFR-IgG 1 glycosylation variants were cloned into the expression vector pRK5 (Eaton et al . ( 1986) Biochem 291 8343-8347), that enabled transient expression in the human embryonic kidney 293 cell line The pRK5 chimeπc transcription unit includes a cytomegalovirus (CMV) immediate early promoter and a simian virus 40 (SV-40) polyadenylation site For expression in CHO cells, gene encoding TNFR-IgG 1 vanants were cloned into the closely related expression vector pSVI6B5 that differs from pRK5 in that transcnption is directed by SV-40 promoter and enhancer elements rather than by the CMV promoter Cells and Transfections 293 cells were cultured in 100 mm culture plates contammg a mixture of Dulbecco s modified Eagles medium (DMEM) and Ham's F12 medium (F12) (Gibco BRL. Grand Island, NY) supplemented with 10% whole fetal bovine serum (FBS) and mcubated at 37° C in an atmosphere contammg 5% C0 2 Confluent plates of 293 cells were passaged at a ratio of 1 5. and were transfected at 60% confiuency with plasmids containing TNFR-IgG 1 glycosylation variants by the calcium phosphate method (Graham and Van Der Eb (1973) Virol 52 456-467) For expression of TNFR-IgG 1 variants m CHO cell lines, the genes encoding TNFR-IgG 1 glycosylation site mutants were cloned into the pSVI6B5 expression vector These plasmids were then co-transfected into CHO cells deficient in the production of dihydrofolate -17- reductase (dhfr) along with a plasmid (pFDl 1 ) containing a cDNA encodmg muπne dhfr The transfected cells were selected for the ability to grow in culture medium deficient in the production of glycine, hypoxanthine and thymidine Resulting colonies were picked and then selected for growth m varying concentration of methotrexate Stable cell lines expressmg TNFR-IgG 1 glycosylation site mutants were grown m 10 liter mini-fermentors, and growth conditioned cell culture medium was harvested for affinity purification (described below) Metabolic Labeling and Immunoprecipitation Two days post-transfection. the culture medium was removed from the transfected 293 cells and the cell monoiayer washed twice with phosphate buffered saline (PBS) Cells were incubated in methionme and cysteme free DMEM and supplemented with ProMix 35S- Cell Labelling Mix (Amersham, Arlington Hts, IL) (100(Cι/mL) The cells were labeled 8-16 hours at 37 °C m an atmosphere of 5% C02 After labeling, the cell supernatants were removed, centπfuged at 4000 rpm for 5 minutes, and 1 mL was a quoted for immunoprecipitation experiments The labeled cells were washed three times with PBS, lysed directly on the culture dishes with cell lysis buffer (PBS contammg 3% NP-40). and centπfuged at 14.000 x g for 5 mmutes The lysate was transferred to a fresh tube, and 200 μL was removed for immunoprecipitation Immunoprecipitation of both supernatants and lysates was accomplished by addition of 30 μL of S aureus Protein A (Pharmacia Inc , Piscataway, NJ) After a short incubation period on ice (10-15 minutes) the Protein A/TNFR-IgGl complexes were sedimented by centπfugation at 14,000 x g for 1 minute, washed in wash buffer (PBS, 1% NP-40, 0 1% sodium dodecyl sulfate (SDS)), and resuspended in 2X sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS- PAGE) sample buffer contammg 5% 2-mercaptoethanol (2-ME) The immunoprecipitated protems were resolved by SDS-PAGE using 10% Tπs-Glycine polyacrylamide gels (Novex, Inc . San Diego, C A) Proteins were visualized by autoradiography and mobilities were calculated with reference to C-Methy latedRainbow Colored Protem Molecular Weight Markers (Amersham. Arlington Heights, IL) Proteins were quantitated by densitometπc scanning of autoradiographs usmg the Personal Densitometer (Molecular Dynamics, Sunnyvale, CA) Pulse Chase Analysis 48 hours post-transfection, cells were pulse-labeled with ProMix S-Cell Labeling Mix (500(Cι /mL) for 15 minutes The labeling medium was then removed and the cell monoiayer was washed at room temperature with PBS Fresh medium (DMEM/F12 + 10% FBS) was then added, and cells were harvested at various time-pomts For harvesting, plates were chilled on ice, supernatants were removed, and the cell monoiayer was washed once with PBS Cells were then lysed with lmL of lysis buffer Lysate and supernatant samples were ahquoted and immunoprecipitated as described above Purification of TNFR-IgG glycosylation site mutants Growth conditioned culture medium from cultures at 10 L CHO cells expressing TNFR-IgG 1 glycosylation site mutations were purified by affinity chromatography on immobilized S aureus protein A using a method described previously (Chamow et al (1994) J Immunol 153 4268-4280) Four TNFR-IgG 1 variants were purified NNNN, NNQQ, NSNQ, and QSNQ Briefly, purification was involved the following steps (I) Prior to sample loading, the Protein A column was equilibrated with 20 mM Tπs buffer, 15 mM NaCl (pH 7 4) After the growth conditioned cell culture medium was loaded onto the column, the column was sequentially washed with the following buffers -18- 20 mM Tπs buffer, 150 mM NaCl (pH 7 4). 20 mM Tπs buffer. 500 mM TMAC (pH 7 4). and 20 mM Tπs buffer (pH 7 4) The glycosylation site mutants were eluted with 50 mM citric acid. 20% (w/v) glycerol (pH 3 0) and the elution pools were subsequently adjusted to pH 6 0 using 1 0 M sodium citrate Finally, the eluates were buffer exchanged by gel filtration chromatography mto PBS (pH 7 4) The purified mutants were analyzed using SDS-PAGE ( 12.5%) with Coomassie blue staining TNF binding assay The bmdmg of TNF to TNFR-IgG 1 and TNFR-IgG 1 glycosylation site mutants was determined using a competitive binding immunoassay similar to that described by Ashkenazi, et al ( 1991 ) supra Briefly, microtiter plates were coated with affinity purified goat antibodies to human IgG Fc domain Purified TNFR-IgG 1 or TNFR-IgG 1 glycosylation site mutants were captured on the plates and then reacted with varying concentrations of unlabeled TNF (0 8 to 800 nM) and a fixed amount of ( 125 I)-labeled TNF (0 05 nM) After washmg the amount of TNF remammg bound to the wells of the microtiter plates was determined with a gamma counter Data were fit using a four parameter curve The effective concentration of unlabeled TNF that resulted m half maximal (50%) bmdmg of ( 125 I)-labeled TNF to TNFR-IgGl glycosvlation mutants was reported as EC 50 values EXAMPLE 1 TNFR-IgGl is a chimeπc protein constructed by fusing the extracellular domam of the receptor for TNF alpha with sequences encodmg the Fc domain and hmge region of IgG 1 (Ashkenazi etai , ( 1991 ) supra) The chimeπc protem (Figure 1) contams four N-linked glycosylation sites three located in the TNFR region (positions 14, 105, and 1 11) and one in the Fc domain (position 248) Previous studies have shown that TNFR-IgGl is secreted as a homodimer, binds to TNF alpha with high affinity, and has potent anti- lnflammatory activity in vivo Pulse-chase experiments were carried out to characterize the secretion efficiency of TNFR-IgG 1 In these studies, 293 cells were transiently transfected with a calcium phosphate precipitated plasmid (pRK TNFR-IgG 1 ) and cultured for 2 days The cells were pulse-labeled with ( J S)- methionme. and samples were collected at various time points Immunoprecipitation studies (Figure 2) indicated that TNFR-IgG 1 first appeared in the cell culture medium approximately 1 hr after pulse labeling, and that the amount of secreted protein progressively increased over the entire 24 hr observation period These studies also showed that a significant fraction of the TNFR-IgGl synthesized during a 15 minute pulse labeling was retained inside the cells Densitometπc analysis of these data (Figure 3 A) demonstrated that only about 50% of the pulse-labeled protem was secreted from the cell during a 24 hr period, and that labeled precursor appeared to remam in a stable mtracellular pool for more than 24 hr The kmetics and pattern of secretion observed for TNFR-IgGl were similar to those previously reported for two other secreted variants of membrane glycoprotems (Berman et ai , ( 1989) J Virol 63 3489-3498) suggesting that secretion of TNFR-IgGl was similarly inefficient Endoglycosidase digestion and lmmunofluorescence studies similar to those described by Machamer and Rose (1988) J Biol Chem 263 5955-5960) showed that the intracellular TNFR-IgGl could be localized to the ER and was sensitive to endoglycosidase H digestion (endoH) These studies further showed that secreted TNFR-IgG 1 was resistant to endo H digestion and sensitive to neuraminidase digestion Together these results suggested that the intracellular TNFR-IgG 1 accumulated in the endoplasmic reticulum (ER) and -19- possessed the characteristic high mannose form of N-linked carbohvdrate, whereas the secreted protein possessed terminal sialation acquired in the trans-Golgi apparatus (tGA) EXAMPLE 2 Deletion of glycosylation sites can impair secretion of TNFR-IgGl To examine the effect of sequential deletion of glycosylation sites on the secretion of TNFR-IgG 1 , further experiments were carried out with a series of glycosylation mutants (Table 1) Table 1 Summary of TNFRl-IgGl Glycosylation Mutants Ammo acid position Secretion Efficiency 14 105 1 11 248 N N N N +-H- Q N N N -H- N Q Q N N N N Q +++ N N Q N + N Q N N - Q N N Q + N N Q Q + Q Q Q N +/- Q Q Q Q - N s N N +++ N N S N + N D D N + N K K N ++ N s S N - N R R N + N T T N +/- Q s N Q - N D D Q + N K K Q ++ In these studies, mutagenesis primers were designed to replace the codon specifying asparagme (N) in the N- linked carbohydrate attachment seqon, N-X-S/T, with codons specifying glutamme (Q) Because glycosvlation sites 2 and 3 in TNFR-IgGl are only a few ammo acids apart (residues 105 and 1 1 1), a single mutageneis primer could be used to mutate both sites simultaneously We found that all of the glycosylation mutants were expressed, but there were significant differences in secretion efficiency (Figure 4A) Removal of the fourth glycosylation site (residue 248), located in the Fc domain of TNFR-IgGl (NNNQ) had no effect -20- on secretion, relative to wild type TNFR-IgG 1 (NNNN) In contrast, removal of glycosvlation sites 2 and 3 (NQQN) resulted m a nearly complete blockade of secretion Deletion of the first glycosylation site (QNNN) also blocked secretion, however m this case secretion was approximately 35% of that seen with the fully glycosylated protem Removal of the first three glycosylation sues (QQQN) completely inhibited secretion as did removal of all four glycosylation sites (QQQQ) Further mutants were constructed (Table 1) to study, mdependently, the influence of glycosylation sites 2 and 3 on secretion It was found that removal of glycosylation site 3 alone (NNQN) allowed for secretion at approximately 10% of level seen with the fully glycosylated protein, whereas deletion of glycosylation site 2 alone (NQNN) completely abolished secretion (Fιgure4B) Deletion of glycosvlation sites 3 and 4 (NNQQ) resulted in a secretion efficiency (Figure 4C) that was similar to the NNQN mutant but lower than the NNNQ mutant (Figure 4B) Deletion of glycosylation sites 1 and 4 (QNNQ) resulted in significantly lower levels of secretion (Figure 4C) than was observed with the QNNN and NNNQ mutants alone (Figure 4A) EXAMPLE 3 Nature of the ammo acid side chain used to mutate N lmked glycosylation site can effect secretion efficiency Additional studies were carried out to determine if the nature of the ammo acid side cham used for replacement of the N residue in the sequence creatmg an N-linked glycosylation site affected secretion efficiency Each N-linked oligosacchaπde cha can possess two to four sialic acid residues, dependmg on the antennaπty of the high mannose precursor We reasoned that replacement of N at N-linked glycosylation sites with charged residues (e g aspartic acid, glutamic acid, lysine, arginme) might have less effect on conformation than replacement with hydrophihc uncharged side-chains (e g glutamine, threon e, serme) We found that replacement of the N residues at glycosylation sites 2 and 3 with acidic, aspartic acid residues (NDDN) or basic, lysine residues (NKKN) allowed for better secretion (Figure 4B) than the uncharged glutamine residues in the (NQQN) construct (Figure 4A), however, the amount of protein secreted was far less than that observed with the wild-type TNFR-IgG 1 protein Replacement of asparagme at sites 2 and 3 with arginine (NRRN), also gave better secretion than the NQQN mutant, but the total amount of secreted protein appeared to be less compared to the NDDN or NKKN mutants (Figure 4D) We next considered the effect of uncharged, hydrophihc side-chains, serme and threonine. on the secretion efficiency of TNFR-IgG 1 We found that replacement of glycosylation sites 2 and 3 with seπne (NSSN) or threonine (NTTN), like the glutamine mutants (NQQN) completely inhibited secretιon(Fιgure 4D) However, smgle substitutions of serme for asparagme at glycosylation sites 2 and 3 with serme gave different results (Figure 4D) The replacement of serme for asparagme at glycosylation site 2 (NSNN) allowed for better secretion than any of the other ammo acid replacements examined at this position In contrast, replacement of serme for asparagme at glycosylation site 3 (NNSN) resulted in secretion similar to that observed in the NNQN mutant (Figure 4B) -21- EXAMPLE 4 Triple deletion mutants. To investigate the effect of glycosylation site 4 in combination with sites 1-3. a series of mutants were constructed where three of the four glycosylation sites were deleted (Table 1). It was found that the NDDQ, the NKKQ, the NDDN and NKKN mutants were all secreted from transfected cells (Figure 4C). However, in all four cases the amount of protein secreted was a small fraction (approximately 10%) of the amount secreted by fully glycosylated TNFR-IgG l . Thus, removal of glycosylation site 4 did not alter the secretion efficiency of variants where asparagine residues at glycosylation sites 2 and 3 were replaced by lysine or aspartic acid. A final mutant where three of the four glycosylation sites were deleted (QSNQ) was also examined (Table 1, Figure 5). This variant accumulated intracellularly and was not secreted. EXAMPLE 5 Secretion efficiency of immunoadhesins can be improved by altering the signal sequence. Studies were performed to demonstrate that the yield and secretion efficiency of a representative immunoadhesm. TNFR-IgGl, from stably transfected CHO cells can be improved by replacement of the TNFR-IgGl signal sequence with that of human tissue plasminogen activator (tPA). In these studies the signal sequences of herpes simplex virus type 1 glycoprotein D (HSV gD- 1 ), Met-Gly-Gly-Thr-Ala-Ala-Arg-Leu-Gly-Ala-Val-Ile- Leu-Phe-Val-Val-Ile-Val-Gly-Leu-His-Gly-Val-Arg-Gly (SEQ ID NO: 9), recombinant human DNase, Met-Arg-Gly-Lys-Leu-Leu-Gly-Ala-Leu-Leu-Ala-Leu-Ala-Ala-Leu-Leu-Gln-Gly-Ala-Val-Ser (SEQ ID NO: 10), HER-2, Met-Gly-Trp-Ser-Cys-Ile-Ile-Leu-Phe-Leu-Val-Ala-Thr-Ala-Thr-Gly-Val-His-Ser (SEQ ID NO: 1 1 ), TNFR, Met-Gly-Leu-Ser-Thr-Val-Pro- Asp-Leu-Leu-Leu-Pro-Leu- Val-Leu-Leu-Glu-Leu- Leu-Val-Gly-Ile-Tyr-Pro-Ser-Gly-Val-Ile-Gly (SEQ ID NO: 2) (Ashkenazi et al., (1991) Proc. Natl. Acad. Sci. 88:10535- 10539), and t-PA (SEQ ID NO: 1 ) were operably linked to the nucleic acid sequence encoding TNFR 1 -IgG 1 and the secretion kinetics were followed. The results indicated an increase in amount of TNFR- IgG 1 that could be recovered from the supernatant as well as an improvement in the secretion kinetics (2 hrs and 24 hrs). To determine whether the tPA signal/pro-sequence improved yield by increasing the efficiency of intracellular transport, a plasmid (pRK.tPA-TNFR-IgG 1 ) was constructed for transient transfection studies in 293 cells. Pulse-chase studies (Figure 2, Figure 3A and B) showed that replacement of the TNFR signal sequence with the tPA signal/pro sequence resulted in a significant improvement in secretion efficiency. Thus, after 24 hours, approximately 70-80% of the pulse-labeled TNFR-IgGl was secreted into the cell culture medium when the tPA signal/pro sequence was included, whereas only 50-60% of the protein was secreted using the wild-type TNFR signal sequence. The increased efficiency of secretion achieved with the tPA signal/pro sequence was apparent from the kinetics of secretion of TNFR-IgG 1 (Figures 2. Figures 3 A and B). When the TNF signal/pro sequence was used, only 40% of the pulse-labeled TNFR-IgG 1 was secreted in 2-4 hr after pulse labeling. However when the TNF signal sequence was replace with the tPA signal/pro sequence approximately 60% of the TNFR-IgG synthesized with the TNFR signal sequence was secreted in this time frame. -22- Studies of the biosynthesis of tPA have shown that the N terminal processing of tPA is complex and involves co-translational cleavage of a 21 ammo acid signal sequence, post-translation cleavage of an 1 1 ammo acid pro-sequence at a furin cleavage site, and extracellular cleavage of a 3 amino acid. N-terminal peptide by an undefined exopeptidase (Figure 1) This complex strategy forN terminal processmg of TNFR- IgG 1 mutants is supported by our results showmg that intracellular TNFR-IgG 1 variants possessmg the tPA signal/pro sequence possessed higher molecular weights than the corresponding secreted protems (Figure 5 A) Since furm is known to be localized m the trans-GA, the pro-sequence would not be expected to be removed until the tPA-TNFR precursor has been transported from the ER to the trans-GA Endoglycosidase digestion studies and immunofluorescence studies have provided data consistent with an ER localization of intracellular vaπants of tPA TNFR-IgG 1 EXAMPLE 6 Secretion efficiency of glycosylation site mutants can be improved by altering the signal sequence To determine whether signal sequence exchange could improve the secretion efficiency of the TNFR-IgGl glycosvlation site mutants described m Figure 4. the TNFR signal sequence was deleted and replaced by the tPA signal/pro sequence It was found (Table 2. Figure 5) that the signal sequence exchange resulted m a marked increase in secretion efficiency of these glycosylation site mutants Table 2 Effect of tPA Signal Sequence or Secretion of TNFR 1 -IgG l Glycosylation Mutants % Secretion Efficiency* EC50 for TNF Binding! Mutant TNFR ss tPA ss (nM) N N N N 50 70 6 66 +/- 0 73 N N Q Q 20 70 4 74 +/- 0 48 N S N Q <5 60 6 94 ι-/- 0 83 N K K Q 10 40 ND Q S N Q <5 65 2 34 +/- 0 22 Q Q Q Q <5 <5 ND * Data represent results from pulse-chase experiments (e g Figure 3) where the percentage of pulse-labeled protem secreted in a 24 hr period was measured by scanning densitometry 125 JData represent EC50 values for the displacement of ( I)-labeled TNF by TNFR-IgG 1 glycosylation site mutants (Figure 7) For example, only 10-20 % of the NNQQ mutant was secreted usmg the TNFR signal sequence, whereas 60-70% of the NNQQ mutant contammg the tPA signal pro sequence was secreted (Figures 3C and D, Figure 5) Similarly, little or none of the QSNQ mutant was secreted contammg the TNFR signal sequence, but approximately 60% was secreted from the tPA signal pro sequence containing variant (Figures 3E and F, Figure 5) Similar improvements in secretion efficiency were observed for the NKKQ and NSNQ mutants (Figure 5) Densitometπc analysis of pulse chase experiments (Figure 3) showed that attachment of the tPA signal/pro sequence accelerated the kinetics of intracellular transport as well as increasing the total amount -23- of secreted protein. Although replacement of the tPA /pro sequence could overcome the blockade of protein secretion for many of the glycosylation mutants, this strategy was not effective for the fully glycosylated QQQQ glycosylation site mutant (Figure 5). EXAMPLE 7 Structure and TNF binding activity of TNFR-IgG 1 glycosylation mutants. Although the signal sequence exchange strategy described above provided a method that allowed for the secretion of TNFR-IgG 1 glycosylation site mutants, we wondered whether the secreted proteins were properly folded. To answer this question, stable CHO cell lines, expressing several of the TNFR-IgG 1 glycosylation mutants, were constructed in order to produce sufficient quantities for ligand binding studies. Recombinant proteins representing the NNQQ, NSNQ, and QSNQ mutants were purified from growth conditioned cell culture medium using protein A affinity chromatography. It was found (Figure 6) that the glycosylation site mutants, like TNFR-IgGl, associated to form covalent disulfide bonded dimers that were stable in SDS and could be dissociated by the addition of reducing agents (e.g. 2-mercaptoethanol). As expected, the molecular mass of the glycosylation variants correlated with the number of intact glycosylation sites. Thus the variants with two glycosylation sites deleted (NNQQ and NSNQ) were smaller in molecular weight than the wild type (NNNN) proteins and larger in molecular mass than the QSNQ variant the possessed only one glycosylation site. TNF binding studies were carried out to determine whether the secreted glycosylation variants were folded into a functionally relevant conformation. For this purpose, the three glycosylation mutants illustrated in Figure 6 were evaluated in a binding assay (Figure 7) using ( 125 I)-labeled TNF by a method similar to that 125 described by Ashkenazi et al.. All three mutants bound the ( I)-labeled TNF with EC 50 values (Table 2) comparable to that observed for fully glycosylated TNFR-IgGl . These studies suggested that neither glycosylation sites nor the tPA signal/pro sequence affected the interaction of TNFR-IgGl with it's ligand. EXAMPLE 8 To determinethe influence of the tPA propeptide on secretion kinetics of tPA/TNFR-IgG 1 a construct was generated containing the 29aa TNFR signal sequence (SEQ ID NO: 2) and a 13 amino acid t-PA propeptide (SEQ ID NO: 7) having the sequence Met-Gly-Leu-Ser-Thr-Val-Pro-Asp-Leu-Leu-Leu-Pro-Leu- Val-Leu-Leu-Glu-Leu-Leu-Val-Gly-Ile-Tyr-Pro-Ser-Gly-Val-Ile-Gly-Ser-Gln-Glu-Ile-His-Ala-Arg-Phe-Arg- Arg-Gly-Ala-Arg(SEQ ID NO: 8) was prepared. From RIP experiments SEQ ID NO: 8 was shown to have 1 / m τ\ v similar t and percent secretion at 24 hours to that of tPA/TNFR-IgG 1 (Figure 8 A-C). Endo H digest also reveal similar secretion kinetics to that of tPA/TNFR-IgG 1 , where approximately 1 /3 of the lysate is endo H resistant between 10 and 30 minutes and is probably secreted within 30 minutes, as opposed to the wild type TNFr-IgG in which very little of the lysate is Endo H resistant and the band does not decrease overtime. This 11/ fnfϊ v evidence strongiy indicates that the decrease in t and percent secretion at 24 hours for tPA/TNFR-IgG 1 is due to the presence of the 13 amino acid propeptide. This is supported by transient expression data, wherein 14 yields similar total expression as well as specific productivity. -24-

Claims: WHAT IS CLAIMED IS: 1. A DNA construct comprising: a first DNA segment encoding a precursor polypeptide comprising a pro-sequence ofa mammalian t-PA; and a second DNA segment operably linked to the first DNA sequence, the second DNA sequence encoding a heterologous glycoprotein. 2. The DNA construct of claim 1 wherein the heterologous glycoprotein is an immunoadhesm. 3. The DNA construct of claim 2 wherein the immunoadhesin is a TNF receptor immunoadhesm. 4. The DNA construct of claim 3 wherein the TNF receptor immunoadhesin is TNFRl-IgGl . 5. The DNA construct of claim 1 wherein the DNA encoding the mammalian t-PA pro-sequence is operably linked to a pre-sequence other than a mammalian t-PA pre-sequence. 6. The DNA construct of claim 5 wherein the heterologous glycoprotein is and immunoadhesin. 7. The DNA construct of claim 6 wherein the immunoadhesin is a TNF receptor immunoadhesin. 8. The DNA construct of claim 7 wherein the TNF receptor immunoadhesin is TNFRl-IgGl . 9. The DNA construct of claim 5 wherein the mammalian t-PA pro-sequence is operably linked a pre- sequence associated with the native heterologous polypeptide. 10. The DNA construct of claim 9 wherein the heterologous glycoprotein is a TNF receptor immunoadhesin and the pre-sequence is a pre-sequence of a mammalian TNF receptor. 11. The DNA construct of claim 10 wherein the mammalian t-PA pro-sequence is SEQ ID NO: 7. 12. The DNA construct of claim 11 wherein the pre-sequence is SEQ ID NO: 8. 13. The DNA construct of claim 12 wherein the TNF receptor immunoadhesin is TNFRl-IgGl. 14. A DNA construct comprising: a first DNA segment encoding a precursor peptide; and -25- a second DNA segment operably linked to the first DNA sequence, the second DNA sequence encoding a heterologous glycosylation site variant glycoprotein. 15. The DNA construct of claim 14 further comprising one or more additional DNA segments operably linked to the first and second DNA segments. 16. The DNA construct of claim 14 wherein the precursor peptide comprises the pro-sequence of a mammalian t-PA. 17. The DNA construct of claim 16 wherein the pro-sequence is a human t-PA pro-sequence. 18. The DNA construct of claim 17 wherein the pro-sequence is SEQ ID NO: 5. 19. The DNA construct of claim 16 further comprising a pre-sequence of a mammalian t-PA. 20. The DNA construct of claim 19 wherein the pre-sequence is a human t-PA pre-sequence. 21. The DNA construct of claim 20 wherein the pre-sequence is SEQ ID NO: 3. 22. The DNA construct of claim 21 wherein the precursor peptide is SEQ ID NO 1. 23. The DNA construct of claim 14 wherein the heterologous glycosylation site variant is a glycosylation site addition variant. 24. The DNA construct of claim 14 wherein the heterologous glycosylation site variant is a glycosylation site deletion variant. 25. The DNA construct of claim 24 wherein the heterologous glycosylation site variant is an immunoadhesin. 26. The DNA construct of claim 25 wherein the immunoadhesin is TNFR-IgG. 27. The DNA construct of claim 26 wherein the TNFR-IgG is TNFR 1 -IgG 1. 28. The DNA construct of claim 27 wherein the TNFRl -IgG 1 has an N-linked glycosylation site selected from the group consisting of amino acid positions 14, 105, 1 11 and 248 deleted. 29. The DNA construct of claim 28 wherein the TNFRl-IgG 1 has the N-linked site at 14 deleted. -26- 30. A cultured eukaryotic host cell comprising a DNA construct comprising: a first DNA segment encoding a precursor peptide corresponding to a mammalian tissue plasminogen activator secretory peptide; and a second DNA segment operably linked to the first DNA sequence, the second DNA sequence encoding a heterologous glycosylation site variant. 31. The cultured eukaryotic host cell of claim 30 wherein the host cell is a rodent host cell. 32. The cultured eukaryotic host cell of claim 31 which is a CHO cell. 33. A method of producing a polypeptide which has been altered to delete one or more native N-linked glycosylation sites comprising the steps of (a) culturing a eukaryotic host cell comprising a DNA construct comprising: a first DNA segment encoding a precursor peptide corresponding to a mammalian tissue plasminogen activator signal-pro peptide; and a second DNA segment operably linked to the first DNA sequence, the second DNA sequence encoding a heterologous glycosylation site deletion variant polypeptide; wherein the eukaryotic host cell express the first and second DNA segments and the polypeptide is secreted from the cell; and (b) recovering the polypeptide so produced. -27-

Inventors: ASHKENAZI AVI J, BERMAN PHILLIP W, BROUSSEAU DAVID, ETCHEVERRY TINA, ASHKENAZI, AVI, J., BERMAN, PHILLIP, W., BROUSSEAU, DAVID, ETCHEVERRY, TINA
Applicants: GENENTECH INC, GENENTECH, INC.
Application year: 1999
Main classification: C12N15
IPC codes: C07K14, C12N5, C12N9, C12N15, C12P21