METHODS FOR MAKING PROTEINS CONTAINING FREE CYSTEINE RESIDUES

Abstract: The present invention relates to novel methods of making soluble proteins having free cysteines in which a host cell is exposed to a cysteine blocking agent. The soluble proteins produced by the methods can then be modified to increase their effectiveness. Such modifications include attaching a PEG moiety to form pegylated proteins.

Description: METHODS FOR MAKING PROTEINS CONTAINING FREE CYSTEINE RESIDUES Field of the Invenuon The present invention relates generally to methods of making proteins and more specifically to recombinant proteins containing a "free" cysteme residue that does not form a disulfide bond Background of the Invention There is considerable interest on the part of patients and healthcare providers in the development of low cost, long-acting, "user-friendly" protein therapeutics Proteins are expensive to manufacture and unlike conventional small molecule drugs, are usually not readily absorbed by the body Moreover they are digested if taken orally Therefore, proteins must typically be administered by injection After injecϋon most proteins are cleared rapidly from the body, necessitating frequent, often daily, injecϋons Patients dislike injections, which leads to reduced compliance and reduced drug efficacy Some proteins such as erythropoieUn (EPO) are effective when administered less often (three times per week for EPO) but this is due to the fact that the proteins are glycosylated Glycosylation requires that the recombinant proteins be manufactured using mammalian cell expression systems, which is expensive and increases the cost of protein pharmaceuticals Thus, there is a strong need to develop protein delivery technologies that lower the costs of protem therapeutics to patients and healthcare providers One solution to this problem is the development of methods to prolong the circulating half-lives of protein therapeutics in the body so that the proteins do not have to be injected frequently This solution also satisfies the needs and desires of patients for protem therapeutics that are " user-friendly", l e , protein therapeutics that do not require frequent injections Covalent modification of proteins with polyethylene glycol (PEG) has proven to be a useful method to extend the circulating half-lives of proteins in the body (Abuchowski et al , 1984, Hershfield, 1987, Meyers et al , 1991) Covalent attachment of PEG to a protem increases the protein's effective size and reduces its rate of clearance from the body PEGs are commercially available in several sizes, allowing the circulating half-lives of PEG-modified proteins to be tailored for individual indications through use of different size PEGs Other documented in vivo benefits of PEG modification are an increase in protein solubility, stability (possibly due to protection of the protein from proteases) and a decrease in protein lmmunogemcity (Katre et al , 1987, Katre, 1990) One known method for PEGylating proteins uses compounds such as N-hydroxy succinimide (NHS)-PEG to attach PEG to free amines, typically at lysine residues or at the N-terπunal ammo acid A major limitation of this approach is that proteins typically contain several lysines, in addition to the N-terminal am o acid, and the PEG moiety attaches to the protein non-specifically at any of the available free amines, resulting in a heterogeneous product mixture Many NHS-PEGylated proteins are unsuitable for commercial use because of low specific activities and heterogeneity Inactivation results from covalent modification of one or more lysine residues or the N-terminal amino acid required for biological activity or from covalent attachment of the PEG moietv near the active site of the protein Of particular relevance to this application is the finding that modification of human growth hormone (hGH) with amine-reactive reagents, including NHS-PEG reagents, reduces biological activity of the protein by more than 10-fold (Teh and Chapman, 1988, Clark et al , 1996) GH is a 22 kDa protein secreted by the pituitary gland GH stimulates metabolism of bone, cartilage and muscle and is the body's primary hormone for stimulating somatic growth duπng childhood Recombinant human GH (rhGH) is used to treat short stature resulting from GH inadequacy, Turner's Syndrome and renal failure in children GH is not glycosylated and is fully active when produced m bacteria The protem has a short in vivo half-life and must be adnumstered by daily subcutaneous injection for maximum effectiveness (MacGilhvray et al , 1996) There is considerable interest in the development of long-acting forms of hGH Attempts to create long-acting forms of hGH by PEGylating the protein with amine-reactive PEG reagents have met with limited success due to significant reductions m bioactivity upon PEGvlation Further, the protem becomes PEGylated at multiple sites (Clark et al , 1996) hGH contains mne lvsines, in addition to the N-termmal ammo acid Certain of these lysines are located in regions of the protein known to be critical for receptor binding (Cunningham et al , 1989, Cunningham and Wells, 1989) Modification of these lysine residues significantly reduces receptor binding and bioactivity of the protein (de la Llosa et al , 1985, Martal et al , 1985, Teh and Chapman, 1988, Cunningham and Wells, 1989) hGH is readily modified by NHS-PEG reagents, but biological activity of the NHS-PEG protein is severely compromised, amounting to only 1% of wild type GH biological activity for a GH protein modified with five 5kDa PEG molecules (Clark et al , 1996) The EC 5 o for this multiply PEGylated GH protein is 440 ng/ml or approximately 20 nM (Clark et al , 1996) In addition to possessing significantly reduced biological activity, NHS-PEG-hGH is very heterogeneous due to different numbers of PEG molecules attached to the protein and at different ammo acid residues, which has an impact on its usefulness as a potential therapeutic Clark et al (1996) showed that the circulating half-life of NHS-PEG-hGH in animals is significantly prolonged relative to non-modified GH Despite possessing significantly reduced in vitro biological activity, NHS-PEG-hGH was effective and could be adnumstered less often than non-modified hGH m a rat GH-deficiency model (Clark et al , 1996) However, high doses of NHS-PEG-hGH (60-180 μg per injection per rat) were required for efficacy in the animal models due to the low specific activity of the modified protein There is a clear need for better methods to create PEGylated hGH proteins that retain greater bioactivitv There also is a need to develop methods for PEGylating hGH in a way that creates a homogeneous PEG-hGH product Biological activities of several other commercially important proteins are significantly reduced by amine-reactive PEG reagents EPO contains several lysine residues that are critical for bioactivity of the protein (Boissel et al , 1993, Matthews et al , 1996) and modification of lysine residues in EPO results in near complete loss of biological activity (Wojchowski and Caslake, 1989) Covalent modification of alpha-mterferon-2 with amine-reactive PEGs results in 40-75% loss of bioactivity (Goodson and Katre, 1990, Karasiewicz et al , 1995) Loss of biological activity is greatest with large (e g , 10 kDa) PEGs (Karasiewicz et al , 1995) Covalent modification of G-CSF with amine-reactive PEGs results in greater than 60% loss of bioactivity (Tanaka et al , 1991) Extensive modification of T -2 with amine-reactive PEGs results in greater than 90% loss of bioactivity (Goodson and Katre. 1990) A second known method for PEGylating proteins covalently attaches PEG to cysteme residues using cysteine-reactive PEGs A number of highly specific, cysteine-reactive PEGs with different reactive groups (e g , maleiπude, vinylsulfone) and different size PEGs (2-40 kDa) are commercially available At neutral pH, these PEG reagents selectively attach to "free" cysteine residues, l e , cysteine residues not involved in disulfide bonds Cysteine residues in most proteins participate in disulfide bonds and are not available for PEGylation using cysteine-reactive PEGs Through in vitro mutagenesis using recombinant DNA techniques, additional αsteine residues can be introduced anywhere into the protein The newly added "free" cysteines can serve as sites for the specific attachment of a PEG molecule using cysteine-reactive PEGs The added cvsteine residue can be a substitution for an existing ammo acid in a protein, added preceding the amino-termmus of the protein or after the carboxy-terminus of the protein, or inserted between two ammo acids in the protein Alternatively, one of two cysteines involved in a native disulfide bond may be deleted or substituted with another aπuno acid, leaving a native cysteine (the cysteine residue in the protein that normally would form a disulfide bond with the deleted or substituted cysteine residue) free and available for chemical modification Preferably the am o acid substituted for the cysteine would be a neutral ammo acid such as seπne or alanine Growth hormone has two disulfide bonds that can be reduced and alkylated with lodoacetimide without impairing biological activity (Bewley et al , (1969) Each of the four cysteines would be reasonable targets for deletion or substitution by another ammo acid Several naturally-occurπng proteins are known to contain one or more "free ' cvsteine residues Examples of such naturally-occurπng proteins include human Interleukin (IL)-2, beta mterferon (Mark et al , 1984), G-CSF (Lu et al , 1989) and basic fibroblast growth factor (Thompson, 1992) IL-2, G-CSF and beta mterferon contain an odd nurnber of cysteine residues, whereas basic fibroblast growth factor contains an even number of cysteine residues However, expression of recombinant proteins containing free cysteine residues has been problematic due to reactivity of the free sulfhydryl at physiological conditions Several recombinant proteins containing free cysteines have been expressed as intracelkilar proteins m bacteπa such as E colt Examples include natural proteins such as IL-2, beta mterferon, G-CSF, basic fibroblast growth factor and engineered cysteine muteins of IL-2 (Goodson and Katre, 1990), EL-3 (Shaw et al , 1992), Tumor Necrosis Factor Binding Protein (Tuma et al , 1995), IGF-I (Cox and McDermott. 1994), IGFBP-1 (Van Den Berg et al , 1997) and protease nexin and related proteins (Braxton, 1998) All of these proteins were insoluble when expressed lntracellularly in bacteπa The insoluble proteins could be refolded into their native conformations by performing a seπes of denaturation, reduction and refolding procedures These steps add time and cost to the manufactuπng process for producing the proteins in bacteπa Improved stability and yields of IL-2 (Mark et al , 1985) and beta mterferon (DeChiara et al 1986) have been obtained by substituting another ammo acid, e g , senne, for the free cysteine residue It would be preferable to express the recombinant proteins in a soluble, biologically active form to eliminate these extra steps One known method of expressing soluble recombinant proteins in bactena is to secrete them into the peπplasnuc space or into the media It is known that certain recombinant proteins such as GH are expressed in a soluble active form when they are secreted into the E colt peπplasm, whereas they are insoluble when expressed lntracellularly in E coli Secretion is achieved by fusing DNA sequences encoding growth hormone or other proteins of interest to DNA sequences encoding bacteπal signal sequences such as those deπved from the stll (Fujimoto et al , 1988) and ompA proteins (Ghrayeb et al , 1984) Secretion of recombinant proteins in bacteπa is desirable because the natural N-terπunus of the recombinant protein can be maintained Intracellular expression of recombinant proteins requires that an N-terminal methionine be present at the amino-terπunus of the recombinant protein Methionine is not normally present at the aπuno-terminus of the mature forms of many human proteins For example, the ammo-terminal ammo acid of the mature form of human growth hormone is phenvlalanme An ammo-terminal methionine must be added to the ammo- terminus of a recombinant protein, if a methionine is not present at this position, in order for the protein to be expressed efficiently in bacteπa Typically addition of the ammo-terminal methionine is accomplished by adding an ATG methionine codon preceding the DNA sequence encoding the recombinant protein The added N-terπunal methionine often is not removed from the recombinant protein, particularly if the recombinant protein is insoluble Such is the case with hGH, where the N- terπunal methionine is not removed when the protein is expressed lntracellularly in E coli The added N-terminal methionine creates a "non-natural" protein that potentially can stimulate an immune response in a human In contrast, there is no added methionine on hGH that is secreted into the peπplasmic space using stll (Chang et al , 1987) or ompA (Cheah et al , 1994) signal sequences, the recombinant protein begins with the native aπuno-terminal ammo acid phenylalamne The native hGH protein sequence is maintained because of bacteπal enzymes that cleave the stll-hGH protein (or ompA-hGH protein) between the stll (or ompA) signal sequence and the start of the mature hGH protein While the peπplasmic space is believed to be an oxidizing environment that should promote disulfide bond formation, coexpression of protein disulfide isomerase with bovine pancreatic tryps inhibitor resulted in a six-fold increase in the yield of coπectly folded protein from the E colt peπplasm (Ostermeier et al , (1996) This result would suggest that peπplasmic protein folding can at times be inefficient and is in need of improvement for large scale protein production hGH has four cysteines that form two disulfides hGH can be secreted into the E colt peπplasm using stll or ompA signal sequences The secreted protein is soluble and biologically active (Hsiung et al , 1986) The predominant secreted form of hGH is a monomer with an apparent molecular weight by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of 22 kDa Recombinant hGH can be isolated from the peπplasmic space by using an osmotic shock procedure (Koshland and Botstein, 1980), which preferentially releases peπplasmic, but not intracellular, proteins into the osmotic shock buffer The released hGH protein is then purified by column chromatography ((Hsiung et al . 1986) When similar procedures were attempted to secrete hGH vaπants containing a free cysteine residue (five cysteines, 2N+1), it was discovered that the recombinant hGH vaπants formed multimers and aggregates when isolated using standard osmotic shock and purification procedures developed for hGH Very little of the monomeπc hGH vaπant proteins could be detected by non-reduced SDS- P AGE in the osmotic shock lysates or duπng purification of the proteins by column chromatography Alpha mterferon (IFN-α2) also contains four cysteine residues that form two disulfide bonds IFN-α2 can be secreted into the E coli peπplasm using the stll signal sequence (Voss et al , 1994) The secreted protein is soluble and biologically active (Voss et al , 1994) The predominant secreted form of BFN- 2 is a monomer with an apparent molecular weight by SDS-PAGE of 19 kDa Secreted recombinant IFN-α2 can be purified by column chromatography (Voss et al . 1994) When similar procedures were attempted to secrete IFN- 2 vaπants containing a free cysteine residue (five cysteines. 2N+1), it was discovered that the recombinant IFN- 2 vaπants formed multimers and aggregates when isolated using standard purification procedures developed for IFN-α2 The IFN- 2 vaπants eluted from the columns very differently than IFN-α2 and very little of the monomeπc IFN-α2 vaπant proteins could be purified using column chromatography procedures developed for IFN-α2 An alternative method to synthesizing a protein containing a free cysteine residue is to introduce a thiol group into a protein post-tianslationally via a chemical reaction with succmimidyl 6- [3-2-pyπdyldιthιo)propιonaπudo]hexanoate (LC-SPDP, commerciallv available from Pierce Chemical Company) LC-SPDP reacts with lysine residues to create a free sulfhydryl group Chemically cross- linked dimeπc EPO was prepared using this reagent in conjunction with a maleiπude protein modifying reagent (Sytkowsk et al , 1998) A heterologous mixture of chemically cross-linked EPO proteins was recovered after purification due to non-specific modification of the vaπous lysine residues in EPO Enhanced pharmacokinetics and in vivo potency of the chemically cross-linked EPO proteins were observed Another method that has been used to increase the size of a protem and improve its in vivo potency involves dimenzation of the protein using chemical crosslmking reagents. GH is thought to transduce a cellular signal by cross-linking two GH receptors A GH-GH dimer might facilitate enhanced receptor dimenzation and subsequent amplification of the intracellular signal Chemically cross-linked dimeπc hGH proteins have been descnbed bv Mockπdge et al (1998) Using a water soluble cross-linking reagent l-ethyl-3-(3-dιmethylamιnopropyl)-carbodιιπude (EDC), GH was randomly deπvatized to give predominantly amide-linked dimers but also arrude- linked multimers, depending on the concentration of EDC reagent used While an increase m in vivo potency was observed, the final protem preparation was heterogeneous due to non-specific reaction of the EDC reagent with vaπous aπuno acids in the protein, including lysine, aspartic acid and glutamic acid residues and the am o- and carboxy-termini Injection of such a preparation into humans would be undesirable due to the toxic nature of EDC, potential lmmunogenic response to the unnatural amide bond formed between the proteins Generating consistent batches of a purified protein also would be difficult at the manufactuπng scale Therefore, despite considerable effort, a need still exists for a process for generating homogeneous preparations of long acting recombinant proteins by enhancement of protein molecular weight A need also for methods that allow secretion and recovery of recombinant proteins contaimng free cysteine residues in high yield The present invention satisfies these needs and provides related advantages as well Summary of the Invention The present invention relates to methods for obtaining a soluble protein having a free cysteine The methods are generally accomplished by obtaining a host cell capable of expressing the soluble protein, exposing the host cell to a cysteine blocking agent, and isolating the soluble protein from the host cell In one embodiment in which the protein is not secreted into the media by the host cell, the host cell is disrupted in the presence of the cvsteine blocking agent and the soluble protein is isolated or purified from the soluble fraction of the disrupted host cell In another embodiment in which the soluble protein is secreted by the host cell into the media, the host cell is exposed to the cysteine blocking agent before, duπng or after synthesis of the soluble protein by the host cell Suitable host cells include bacteπa. yeast, insect or mammalian cells Preferably, the host cell is a bacteπal cell, particularly E colt Preferably, the soluble protein produced by the methods of the present invention are recombinant proteins, especially cysteine vaπants or muteins of a protein The methods are useful for producing proteins including, without limitation, human growth hormone, EPO and mterferon, especially alpha mterferon, their deπvatives or antagonists Other proteins include members of the TGF-beta superfamily, platelet deπved growth factor- A, platelet deπved growth factor-B, nerve growth factor, brain deπved neurotophic factor, neurotrophιn-3, neurotroplun-4, vascular endothehal growth factor, or a deπvative or an antagomst thereof Cysteine muteins of heavy or light chain of an lmmunoglobulm or a denvative thereof are also contemplated Useful cysteine blocking agents include any thiol-reactive compound, including for example, cystine, cystaπune, dithioglycolic acid, oxidized glutathione, iodine, hydrogen peroxide, dihydroascorbic acid, tetrathionate, O-iodosobenzoate or oxygen in the presence of a metal ion The present methods further include vanous methods of attaching a PEG moiety to the soluble protem to form pegylated proteins in which the PEG moiety is attached to the soluble protein through the free cysteine Higher order multimeπc proteins involving the coupling of two or more of the soluble proteins are also within the present invention The present invention further includes the soluble proteins and their denvatives, including pegylated proteins, made by the methods disclosed herein Such pegylated proteins include monopegylated hGh, EPO and alpha mterferon The present invention also provides methods for pegylating the soluble proteins obtained by the methods descnbed herein Such methods include purifying the protein, reducing at least partially the protein with a disulfie-reducing agent and exposing the protein to a cysteine-reactive moiety Optionally, the modified cysteine protein can be isolated from unmodified protein Methods of treating conditions treatable by growth hormone, EPO and alpha mterferon are also within the present invention The soluble proteins or their deπvatives, including pegylated deπvatives, are adnumstered to patients suffeπng from conditions in which known growth hormone, EPO or alpha mterferon is effective Bnef Descnption of the Figure Figure 1 is a diagram of mutagenesis by overlap extension m which two separate fragments are amplified from a target DNA segment Descnption of the Invention The present invention provides novel methods of obtaining proteins having free cysteine residues The invention further provides novel proteins, particularly recombinant proteins, produced by these novel methods as well as deπvatives of such recombinant proteins The novel methods for prepaπng such proteins are generally accomplished by (a) obtaining a host cell capable of expressing a protein having a free cysteine, (b) exposing the host cell to a cysteine blocking agent, and (c) isolating the protein from the host cell In one embodiment, the methods include the steps of disrupting the host cell in the presence of the cysteine blocking agent, followed by isolating the protein from the soluble fraction of the disrupted cell In a further embodiment in which the proteins are secreted into the media by prokaryotic or eukaryotic host cells, the methods include the steps of (a) obtaining a host cell capable of expressing a protein having a free cysteine, (b) exposing the host cell to a cysteine blocking agent dunng synthesis, or after synthesis but pnor to purification, of the protein having a free cysteine residue, and (d) isolating the protein from other cellular components in the media As identified above, the first step in these methods is to obtain a host cell capable of expressing a protein having a free cysteine residue Suitable host cells can be prokaryotic or eukaryotic Examples of appropπate host cells that can be used to express recombinant proteins include bacteπa, yeast, insect and mammalian cells Bactena cells are particularly useful, especially As used herein, the term "protein having a free cysteine residue" means any natural or recombinant protem peptide contaimng 2N+1 cysteine residues, where N can be 0 or any integer, and proteins or peptides contaimng 2N cysteines, where two or more of the cysteines do not normally participate in a disulfide bond Thus, the methods of the present invention are useful in enhancing the expression, recovery and purification of any protein or peptide having a free cysteine, particularly cysteine added variant recombinant proteins (refeπed to herein as "cysteine muteins" or "cysteine variants") having one or more free cysteines and/or having 2 or more cysteines that naturally form a disulfide bond. Although the expression, recovery and purification of a natural protein having a free cysteine expressed by its natural host cell can be enhanced by the methods of the present invention, the description herein predominantly refers to recombinant proteins for illustrative purposes only. In addition, the proteins can be derived from any animal species including human, companion animals and farm animals. Accordingly, the present invention encompasses a wide variety of recombinant proteins. These proteins include, but are not limited to, glial-derived neurotrophic factor (GDNF), transforming growth factor-betal (TGF-betal), TGF-beta2, TGF-beta3, inhibin A, inhibin B, bone morphogenetic protein-2 (BMP-2), BMP-4, inhibin alpha, Mullerian inhibiting substance (MIS), OP-1 (osteogenic protein 1), which are all members of the TGF-beta superfamily. The monomer subunits of the TGF- beta superfamily share certain structural features: they generally contain 8 highly conserved cysteine residues that form 4 intramolecular disulfides. Typically a ninth conserved cysteine is free in the monomeric form of the protein but participates in an intermolecular disulfide bond formed during the homodimerization or heterodimerication of the monomer subunits. Other members of the TGF-beta superfamily are described by Massague (1990), Daopin et al. (1992), Kingsley (1994), Kutty et al. (1998), and Lawton et al. (1997), incorporated herein by reference. Immunoglobulin heavy and light chain monomers also contain cysteine residues that participate in intramolecular disulfides as well as free cysteines (Roitt et al., 1989 and Paul, 1989). These free cysteines normally only participate in disulfide bonds as a consequence of multimerization events such as heavy chain homodimerization, heavy chain - light chain heterodimerization, homodimerization of the (heavy chain - light chain) heterodimers, and other higher order assemblies such as pentamerization of the (heavy chain - light chain) heterodimers in the case of IgM. Thus, the methods of the present invention can be employed to enhance the expression, recovery and purification of heavy and/or light chains (or various domains thereof) of human immunoglobulins such as IgGl, IgG2, IgG3, IgG4, IgM IgAl, IgA2, secretory IgA, IgD and IgE. Immunoglobulins from other species could also be similarly expressed, recovered and purified. Proteins genetically fused to immunoglobulins or immunoglobulin domains as described in Chamow & Ashkenazi (1996) could also be similarly expressed, recovered and purified. The present methods can also enhance the expression, recovery and purification of additional recombinant proteins including members of growth hormone superfamily. The following proteins are encoded by genes of the growth hormone (GH) supergene family (Bazan (1990); Bazan (1991); Mott and Campbell (1995); Silvennoinen and Ihle (1996); Martin et al., 1990; Hannum et al.. 1994): growth hormone, prolactin, placental lactogen, erythropoietin (EPO), thrombopoietin (TPO), interleukin-2 (IL-2), IL-3, IL-4, IL-5, IL-6, IL-7, IL-9, IL-10, IL-11, IL-12 (p35 subunit), EL-13, IL- 15, oncostatin M, ciliary neurotrophic factor, leukemia inhibitory factor, alpha interferon, beta interferon, gamma interferon, omega interferon, tau interferon, granulocyte-colony stimulating factor (G-CSF), granulocyte-macrophage colony stimulating factor (GM-CSF), macrophage colony stimulating factor (M-CSF), cardιotrophιn-1 (CT-1), Stem Cell Factor and the flt3/flk2 ligand ("the GH supergene family") It is anticipated that additional members of this gene family will be identified m the future through gene clomng and sequencing Members of the GH supergene family have similar secondary and tertiary structures, despite the fact that they generally have limited ammo acid or DNA sequence identity The shared structural features allow new members of the gene family to be readily identified A group of proteins has been classed as a structural superfamily based on the shared structural motif termed the "cystine knot" The cystine knot is defined by six conserved cysteine residues that form three intramolecular disulfide bonds that are topologically "knotted" (McDonald and Hendnckson, 1993) These proteins also form homo- or heterodimers and in some but not all instances dimenzation involves lntermolecular disulfide formation Members of this family include the members of the TGF-beta superfamily and other proteins such as platelet deπved growth factor- A (PDGF-A), PDGF-B, nerve growth factor (NGF), brain deπved neurotrophic factor (BDNF), neurotrophιn-3 (NT-3), NT-4, and vascular endothehal growth factor (VEGF) Cystine and other cysteine blocking reagents could also enhance expression, recovery and purification of proteins with this structural motif The present methods can also enhance the expression, recovery and purification of other recombinant proteins and/or cysteine added vaπants of those proteins Classes of proteins would include proteases and other enzymes, protease inhibitors, cytokines, cytokine antagonists, allergens, chemokines, gonadotrophms, chemotactins, hpid-binding proteins, pituitary hormones, growth factors, somatomedans, immunoglobulins, lnterleukins. interferons, soluble receptors, vaccines, and hemoglobins Specific examples of proteins include, for example, leptin, insulin, insulin-like growth factor 1 (IGF1), superoxide dismutase, catalase, asparaginase, uncase, fibroblast growth factors, arg ase, phenylalanine ammonia, angiostatin, endostatin. Factor VIII, Factor IX, lnterleukm 1 receptor antagomst, protease nexm and anti-thrombin III Other protein vaπants that would benefit from PEGylation and would therefore be reasonable candidates for cysteine added modifications include proteins or peptides with poor solubility or a tendency to aggregate, proteins or peptides that are susceptable to proteolysis, proteins or peptides needing improved mechanical stability, proteins or peptides that are cleared rapidly from the body, or proteins or peptides with undesirable lmmunogenic or antigentic properties If desired, muteins of natural proteins can be generally constructed using site-directed PCR- based mutagenesis as decπbed in general in Methods in Molecular Biology, Vol 15 PCR Protocols Current Methods and Applications edited by White, B A (1993) Humana Press, Inc , Totowa, NJ and PCR Protocols A Guide to Methods and Applications edited by Innis, M A et al (1990) Academic Press, Inc San Diego, C A Typically, PCR pnmer oligonucleotides are designed to incorporate nucleotide changes to the coding sequence of proteins that result m substitution of a cysteine residue for an ammo acid at a specific position within the protein Such mutagenic oligonucleotide pnmers can also be designed to incorporate an additional cysteine residue at the carboxy terminus or ammo terminus of the coding sequence of proteins In this latter case one or more additional ammo acid residues could also be incoφorated ammo terminal and/or carboxy terminal to the added cysteine residue if that were desirable Moreover oligonucleotides can be designed to incorporate cysteine residues as insertion mutations at specific positions within the protein coding sequence if that were desirable Again, one or more additional ammo acids could be inserted along with the cysteine residue and these aπuno acids could be positioned anuno terminal and or carboxy terminal to the cysteine residue The choice of sequences for mutagemc oligos is dictated by the position where the desired cysteine residue is to be placed and the propinquity of useful restπction endonuclease sites Generally it is desirable to place the mutation, 1 e the mis-matched segment near the middle of the oligo to enhance the annealing of the oligo to the template It is also desirable for the mutagemc oligo to span a unique restπction site so that the PCR product can be cleaved to generate a fragment that can be readily cloned into a suitable vector An example would be one that can be used to express the mutein or that provides convement restπction sites for excising the mutated gene and readily cloning it into such an expression vector It is generally desirable to employ mutagemc oligos under 80 bases m length and lengths of 30-40 bases are more preferable Sometimes mutation sites and restπction sites are separated by distances that are greater than that which is desirable for synthesis of synthetic oligonucleotides In such instances, multiple rounds of PCR can be employed to incrementally extend the length of the PCR product such that it includes the desired useful restπction site or genes targeted for mutagenesis can be reengineered or resynthesized to incorporate restπction sites at appropπate positions Alternatively, vaπations of PCR mutagenesis protocols, such as the so-called "Megapnmer Method" (Bank, S pp277-286 in Methods m Molecular Biology, Vol 15 PCR Protocols Cunent Methods and Applications edited by White, B A (1993) Humana Press, Inc , Totowa, NJ) or "Gene Splicing by Overlap Extension" (Horton, R M pp251-261 in Methods m Molecular Biology, Vol 15 PCR Protocols Current Methods and Applications edited by White, B A, (1993) Humana Press, Inc , Totowa, NJ), both incorporated herein by reference, can also be employed to construct such mutations Next, the host cell is exposed to a cysteine blocking agent In one embodiment, the blocking agent is present at the time of cell disruption, and preferably is added pnor to disrupting the cells Cell disruption can be accomplished by, for example, mechanical sheer such as a French pressure cell, enzymatic digestion, sonication, homogenization, glass bead vortexing, detergent treatment, organic solvents, freeze thaw, gπnding with alumina or sand and the like (Bollag et al , 1996) In an alternative embodiment, the cysteine blocking agent can be exposed to the host cell before, duπng or after the host cell is induced to express the desired protein For example, the host cell can be cultured m the presence of the cysteine blocking agent, which might be prefeπed for expression of proteins that are secreted into the media such as erythropoietin, for example Alternatively, pnor to or at the time of, exposing the host cell to the cysteine blocking agent, the host cell can be induced to express the desired protein, either as a secreted protein in the peπplasm or media, or as a cytoplasmic protein Methods known in the art can be used to induce such expression the cytoplasm or to direct secretion depending on cell ongm, including, for example, the methods descnbed in the examples below A wide vanety of signal peptides have been used successfully to transport proteins to the peπplasmic space Examples of these include prokaryotic signal sequences such as ompA, stll, PhoA signal (Denefle et al , 1989), OmpT (Johnson et al , 1996), LamB and OmpF (Hoffman and Wπght, 1985), beta-lactamase (Kadonaga et al , 1984), enterotoxins LT-A, LT- B (Monoka-Fujimoto et al , 1991), and protein A from S aureus (Abrahmsen et al , 1986) A number of non-natural, synthetic, signal sequences that facilitate secretion of certain proteins are also known to those skilled in the art For proteins secreted to the peπplasm, an osmotic shock treatment can be used to selectively disrupt the outer membrane of the host cells with the resulting release of penplasmic proteins The osmotic shock buffer can be any known in the art, including, for example, the osmotic shock buffer and procedures descnbed in Hsiung et al (1986), or as descnbed in the examples below For proteins secreted into the media, preferably the media should contain the cysteine blocking agent duπng the time the cells express and secrete the protein The cysteine blocking agent also could be added to the media following secretion of the protein but pnor to purification of the protein The cysteine blocking agent can be added to the culture media at a concentration in the range of about 0 1 μM to about 100 mM Preferably, the concentration of cysteine blocking agent in the media is about 50 μM to about 5 mM Although not wishing to be bound by any particular theory, it is believed that the cysteine blocking agents used in the present methods covalently attach to the "free" cysteine residue, forming a mixed disulfide, thus stabilizing the free cysteine residue and preventing multimeπzation and aggregation of the protein Alternatively, the presence of an oxidizing agent m the osmotic shock buffers may be augmenting the protein refold process in case incomplete renaturation had occurred following secretion of the protem into the peπplasmic space However, as noted above, peπplasmic protem refolding can be inefficient For this reason it is believed that addition of cystine to osmotic shock buffers also may increase the recovery of recombinant proteins contaimng an even number of cysteine residues, even if the cysteines normally form disulfide bonds In addition, the inventors believe it may be advantageous to add cystine or similar compounds to the fermentation media duπng bactenal growth because these compounds should diffuse into the peπplasm due to the porous nature of the bacteπal outer membrane Protein folding and blocking of the free cysteine can be accomplished before cell recovery and lysis Early protein stabilization protects against proteolysis and can contπbute to higher recovery yields of recombinant proteins A number of thiol-reactive compounds can be used as cysteine blocking agents to stabilize proteins contaimng free cysteines In addition to cystine, blocking agents can also include reagents contaimng disulfide linkages such as cystamme, dithioglycolic acid, oxidized glutathione, 5,5'- dιthιobιs(2-mtrobenzoιc acid (Ellman's reagent), pyndine disulfides, compounds of the type R-S-S- CO-OCH 3 , other deπvatives of cystine such as diformylcystine, diacetylcystine, diglycylcystine, dialanylcystine diglutaminylcystine, cystinyldiglycine, cystinyldiglutamine, dialanylcystine dianhydnde, cystine phenylhydantoin, homocystme, dithiodipropiomc acid , dimethylcystine, or any dithiol or chemical capable of undergoing a disulfide exchange reaction Sulfenyl halides can also be used to prepare mixed disulfides Other thiol blocking agents that may find use in stabilizing cysteine added protein vaπants include compounds that are able to reversibly react with free thiols These agents include certain heavy metals salts or organic deπvatives of zinc, mercury, and silver Other mercaptide forming agents or reversible thiol reactive compounds are descnbed by R Cecil and J R McPhee (1959) and Torchmsku (1971) In the final step of the general method, the desired protem is recovered and purified from the soluble cytoplasmic fraction, the soluble penplasπuc fraction or the soluble fraction of the media Any method for recoveπng and purifying proteins from the media, cytoplasmic or peπplasmic fraction can be used Such recovery and purification methods are known or readily determined by those skilled in the art. including, for example, centπfugation, filtration, dialysis, chromatography, including size exclusion, procedures and the like A suitable method for the recovery and purification of a desired protein will depend, in part, on the properties of the protein and the intended use The present invention further provides novel methods for producing soluble interferon, particularly alpha interferon that results in a significant increase in the percent of the recovered interferon that has been properlv processed These methods include cultunng host cells capable of expressing interferon at a pH range of about 5 to about 6 5, and preferably about 5 5 to about 6 5 Published reports (Voss et al (1994)) using a higher pH only resulted in 50% properly processed interferon, whereas the new methods of the present invention at lower pHs recovered about 80-90% A discovery was made that certain of the monomeπc hGH cysteine vanants formed disulfide- hnked dimenc hGH proteins dunng the chromatography procedures used to purify these proteins The disulfide-lmked hGH dimers formed when cystine was removed from the column buffers New procedures were developed to pirrrfy the disulfide-linked dimeπc hGH proteins because the disulfide- linked dimeπc hGH proteins behaved differently than monomenc hGH proteins duπng the column chromatography steps used to purify the proteins Unexpectedly, it was discovered that the disulfide- hnked dimeπc hGH proteins were biologically active in in vitro bioassays Biologically active, homogeneous, disulfide-linked dimeπc hGH proteins are novel Accordingly, the present invention further relates to these biologically active, homogeneous di-sulfide lined, dimeπc hGH proteins as well Higher order multimers are also contemplated, including tinners, tetramers and the like, as descnbed in the examples below The purified proteins can then be further processed if desired For example, the proteins can be PEGylated at the free cysteine site with vanous cysteine-reactive PEG reagents, and subsequently purified as monoPEGylated proteins The term "monoPEGylated" is defined to mean a protein modified by covalent attachment of a single PEG molecule at a specific site on the protein Any method known to those skilled in the art can be used, including, for example, the methods descnbed in the examples below, particularly Example 11 Braxton (1998) teaches methods for PEGylating cysteine muteins of proteins, and m particular cysteine muteins of GH and erythropoietin Braxton (1998) specifically teaches that the buffers used to PEGylate the cvsteine muteins should not contain a reducing agent Examples of reducing agents provided by Braxton (1998) are beta-mercaptoethanol (BME) and dithiothreitol (DTT) When similar procedures were used to PEGylate cysteine muteins of GH, erythropoietin and alpha mterferon, it was discovered that the cysteine muteins did not PEGylate It has now been discovered that treatment of the purified cysteine muteins with a reducing agent is required for the proteins to be PEGylated Although not wanting to be bound by any particular theory, the inventors believe that the reducing agent is required to reduce the mixed disulfide and expose the free cysteine residue in the protem so that the free cysteine can react with the PEG reagent Thus, the present invention also relates to methods for PEGylating cysteine muteins of GH, erythopoietin, alpha mterferon and other proteins contaimng 2N + 1 cysteine residues, proteins contaimng 2N cysteine residues where two or more of the cysteine residues are free, particularly those muteins and proteins in which the free cysteine residue is blocked by a mixed disulfide The present invention further relates to purified, monoPEGylated protein vaπants produced by the methods disclosed herein that are not only biologically active, but also retain high specific activity in protein-dependent mammalian cell proliferation assays Such protein vaπants include, for example, the following purified, monPEGylated cysteine muteins hGH, EPO and alpha IFN For example, the m vitro biological activities of the monoPEGylated hGH vanants were 10- to 100-fold greater than the biological activity of hGH that has been PEGylated using NHS-PEG reagents In one embodiment of the monoPEGylated hGH, the polyethylene glycol is attached to the C- D loop of hGH and the resulting monoPEGylated hGH has an EC 50 less than about 110 ng/ml (5 nM), preferably less than about 50 ng/ml (2 3 nM) Alternatively, the polyethylene glycol moiety can be attached to a region proximal to the Helix A of hGH and the resulting monoPEGylated hGH has an EC5 0 less than about 110 ng/ml (5nM), preferably less than 11 ng/ml (0 5nM), and more preferably less than about 2 2 ng/ml (0 1 nM) In one embodiment of the monoPEGylated EPO, the polyethylene glycol is attached to the C- D loop of EPO and the resulting monoPEGylated EPO has an EC 50 less than about 1000 ng/ml (21 nM), preferably less than about 100 ng/ml (approximately 6 nM), more preferably less than about 10 ng/ml (approximately 0 6 nM) and most preferably less than about 1 ng/ml (approximately 0 06 nM) Alternatively, the polyethylene glycol moiety can be attached to the A-B loop of EPO and the resulting monoPEGylated EPO has an EC 50 less than about 100 ng/ml (approximately 5nM), preferably less than 20 ng/ml ( approximately 1 nM), and more preferably less than about 1 ng/ml (approximately 0 05 nM) In one embodiment of the monoPEGylated alpha IFN, the polyethylene glycol is attached to the region proximal to Helix A of alpha EFNand the resulting monoPEGylated alpha IFNhas an IC50 less than about 100 pg/ml (approximately 5 pM), more preferably less than about 50 pg/ml (approximately 2 5 pM) and most preferably about 22 ng/ml (approximately 1 2 pM) Alternatively, the polyethylene glycol moiety can be attached to the C-D loop of IFN-α2 and the resulting monoPEGylated IFN-α2 has an EC50 less than about 100 pg/ml (approximately 5 pM) There are over 25 distinct IFN-α genes (Pestka et al , 1987) Members of the IFN-α family share varying degrees of ammo acid homology and exhibit overlapping sets of biological activities Non-natural recombinant IFN-αs, created through joining together regions of different IFN-α proteins are in vaπous stages of clinical development (Hoπsberger and DiMarco, 1995) A non-natural "consensus" interferon (Blatt et al , 1996), which incorporates the most common aπuno acid at each position of IFN-α, also has been descnbed Appropnate sites for PEGylating cysteine muteins of IFN- α2 should be directly applicable to other members of the IFN-α gene family and to non-natural IFN- αs Kinstler et al , (1996) descnbed monoPEGylated consensus mterferon m which the protein is preferentially mono PEGylated at the N-terminal, non-natural methionine residue Bioactivity of the PEGylated protein was reduced approximately 5-fold relative to non-modified consensus mterferon (Kinstler et al , 1996) The present invention further provides protein vanants that can be covalently attached or conjugated to each other or to a chemical group to produce higher order multimers, such as dimers, tπmers and tetramers Such higher order multimers can be produced according to methods known to those skilled in the art or as descnbed in Example 15 below For example, such a conjugation can produce a hGH, EPO or alpha IFN adduct having a greater molecular weight than the coπesponding native protein Chemical groups smtable for coupling are preferably non-toxic and non-immunogenic These chemical groups would include carbohydrates or polymers such as polyols The "PEG moiety" useful for attaching to the cysteine vanants of the present invention to form "pegylated" proteins include any smtable polymer, for example, a linear or branched chained polyol A prefeπed polyol is polyethylene glycol, which is a synthetic polymer composed of ethylene oxide umts The ethylene oxide umts can vary such that PEGylated-protem vaπants can be obtained with apparent molecular weights by size-exclusion chromatography ranging from approximately 30- 500,000 The size of the PEG moiety directly impacts its circulating half-life Yamaoka et al (1994) Accordingly, one could engineer protein vanants with diffenng circulating half-lives for specific therapeutic applications or prefeπed dosing regimes by varying the size or structure of the PEG moietv Thus, the present invention encompasses GH protein vaπants having an apparent molecular weight greater than about 30 kDa, and more preferably greater than about 70 kDa as determined by size exclusion chromatography, with an EC 50 less than about 400 ng/ml (18 nM), preferably less than 10 ng/ml (5 nM), more preferably less than about 10 ng/ml (0 5 nM), and even more preferably less than about 2 2 ng/ml (0 1 nM) The present invention further encompasses EPO protein vaπants having an apparent molecular weight greater than about 30 kDa, and more preferably greater than about 70 kDa as determined by size exclusion chromatography, with an EC 50 less than about 1000 ng/ml (21 nM), preferably less than 100 ng/ml (6 nM), more preferably less than about 10 ng/ml (0 6 nM), and even more preferably less than about 1 ng/ml (0 06 nM) The present invention further encompasses alpha BFN (IFN-α) protein vaπants having an apparent molecular weight greater than about 30 kDa, and more preferably greater than about 70 kDa as determined by size exclusion chromatography, with an IC 50 less than about 1900 pg/ml (100 pM), preferably less than 400 pg/ml (21 pM), more preferably less than 100 pg/ml (5 nM). and even more preferably less than about 38 pg/ml (2 pM) The reactiv e PEG end group for cysteine modification includes but is not limited to vmylsulfone, maleimide and lodoacetyl moieties The PEG end group should be specific for free thiols with the reaction occurring under conditions that are not detπmental to the protein Antagomst hGH vaπants also can be prepared using a cysteine-added vanant GH where chemical denvatization does not interfere with receptor binding but does prohibit the signaling process Conditions that would benefit from the administration of a GH antagomst include acromegaly, vascular eye diseases, diabetic nephropathy, restenosis following angioplasty and growth hormone responsive malignancies As used herein, the term "deπvative" refers to any vanant of a protein expressed and recovered by the present methods Such vaπants include, but are not limited to, PEGylated versions, dimers and other higher order vanants, ammo acid vaπants, fusion proteins, changes in carbohydrate, phosphorylation or other attached groups found on natural proteins, and any other vaπants disclosed herein The compounds produced by the present methods can be used for a vanety of in vitro and in vivo uses The proteins and their denvatives of the present invention can be used for research, diagnostic or therapeutic purposes that are known for their wildtype, natural, or previously-known modified counterparts In vitro uses include, for example, the use of the protein for screemng, detecting and or purifying other proteins For therapeutic uses, one skilled in the art can readily determine the appropπate dose, frequency of dosing and route of administration Factors in making such determinations include, without limitation, the nature of the protein to be adnumstered, the condition to be treated, potential patient compliance, the age and weight of the patient, and the like The compounds of the present invention can also be used as delivery vehicles for enhancement of the circulating half-life of the therapeutics that are attached or for directing delivery to a specific target within the body The following examples are not intended to be limiting, but only exemplary of specific embodiments of the invention Example 1 Development of an in vitro Bioassay for Human Growth Hormone An hGH cell proliferation assay that uses the muπne FDC-P1 cell line stably transfected with the rabbit GH receptor was developed (Rowhnson et al , 1995) The mouse FDC-Pl cell line was purchased from the Amencan Type Culture Collection and routinely propagated m RPMI 1640 media supplemented with 10% fetal calf serum, 50 μg/ml penicillin, 50 μg/ml streptomycin, 2 mM glutamine and 17-170 Units/ml mouse IL-3 (FDC-Pl media) A. Cloning a cDNA Encoding the Rabbit GH Receptor The rabbit GH receptor was cloned by PCR using forward pnmer BB3 (5'- CCCCGGATCCGCCACCATGGATCTCTGG CAGCTGCTGTT-3') (SEQ ID NO l)and reverse pnmer BB36 (5'- CCCCGTCGACTCTAGAGCCATTA GATACAAAGCTCT TGGG-3') (SEQ ID NO 2) BB3 anneals to the DNA sequence encoding the initiator methionine and ammo terminal portion of the receptor BB3 contains an optimized KOZAK sequence preceding the initiator methionine and a Bam HI site for clomng purposes BB36 anneals to the 3 ' untranslated region of the rabbit GH receptor mRNA and contains Xba I and Sal I restnction sites for clomng purposes Rabbit liver poly(A) + mRNA (purchased from CLONTECH, Inc ) was used as the substrate in first strand synthesis of single-stranded cDNA to produce template for PCR amplification First strand synthesis of single-stranded cDNA was accomplished using a 1st Strand cDNA Synthesis Kit for RT-PCR (AMV) from Boehπnger Mannheim Corp Parallel first strand cDNA syntheses were performed using random hexamers or BB36 as the pnmer Subsequent PCR reactions using the products of the first strand syntheses as templates were earned out with pnmers BB3 and BB36 The expected ~ 1 9 kb PCR product was observed m PCR reactions using random hexamer-pnmed or BB36-pπmed cDNA as template The random hexamer-pnmed cDNA was digested with Bam HI and Xba I, which generates two fragments (~ 365 bp and ~ 1600 bp) because the rabbit GH receptor gene contains an internal Bam HI site Both fragments were gel-puπfied The full-length rabbit GH receptor cDNA was then cloned in two steps First the -1600 bp Bam HI - Xba I fragment was cloned into pCDNA3 1(+) (Invitrogen Corporation) that had been digested with these same two enzymes These clones were readily obtained at reasonable frequencies and showed no evidence of deletions as determined by restπction digests and subsequent sequencing To complete the rabbit receptor cDNA clone, one of the sequenced plasnuds contaimng the 1600 bp Bam HI -Xba I fragment was digested with Bam HI, treated with calf alkaline phosphatase, gel-puπfied and ligated with the gel-purified -365 bp Bam HI fragment that contains the 5 ' portion of the rabbit GH receptor gene Transformants from this hgation were picked and analyzed bv restπction digestion and PCR to confirm the presence of the -365 bp fragment and to determine its onentation relative to the distal segment of the rabbit GH receptor gene The sequence for one full length clone was then verified This plasmid, designated pBBTl 18, was used to stably transfect FDC-Pl cells B. Selection of Stably Transfected FDC-Pl Cells Expressing the Rabbit GH Receptor Endotoxin-free pBBTl 18 DNA was prepared using a Qiagen "Endo-Free Plasmid Purification Kit" and used to transfect FDC-Pl cells Mouse IL-3 was purchased from R&D Systems FDC-Pl cells were transfected with plasmid pBBTl 18 using DMRTE-C catiomzed lipid reagent purchased from GIBCO, following the manufacturer's recommended directions Bπefly, 4 μg of plasmid DNA were complexed with 4-30 μl of the DMRTE-C reagent in 1 ml of OptiMEM media (GEBCO) for 45 minutes in six well tissue culture dishes Following complex formation, 2x 10 6 FDC-Pl cells 200 μl of OptiMEM media supplemented with mouse IL-3 were added to each well and the mixture left for 4 h at 37°C The final mouse IL-3 concentration was 17 Units/ml Two ml of FDC-Pl media contaimng 15% fetal bovine serum were added to each well and the cells left overnight at 37°C The next day transfected cells were transfeπed to T-75 tissue culture flasks contaimng 15 ml FDC-Pl media supplemented with IL-3 (17 U/ml), hGH (5 nM) and 10% horse serum rather than fetal calf serum Horse serum was used because of reports that fetal calf serum contains a growth-promoting activity for FDC-Pl cells Three days later the cells were centπfuged and resuspended in fresh FDC- Pl media contaimng 400 μg/ml G418, 17 U/ml IL-3, 5 nM hGH, 10 % horse serum and incubated at 37°C Media was changed every few days The cells from each transfection were split into T-75 tissue culture flasks contaimng fresh media and either mouse IL-3 (17 U/ml) or hGH (5 nM) G418 resistant cells were obtained from both the IL-3- and hGH-containing flasks The transformants used in the bioassays oπginated from flasks contaimng hGH Twelve independent cell lines were selected by limiting dilution Five of the cell lines (GH-R3, -R4, -R5, -R6 and -R9) showed a good proliferative response to hGH Preliminary expenments indicated that the EC 50 for hGH was similar for each cell line, although the magnitude of the growth response vaπed depending upon the line The GH-R4 cell line was stadied in most detail and was used for the assays presented below The cell lines were routinely propagated in RPMI 1640 media contaimng 10% horse serum, 50 Units/ml penicillin, 50 μg/ml streptomycin, 2 mM glutanune, 400 μg ml G418 and 2-5 nM pituitary hGH or rhGH C. Development of an hGH Bioassay Using FDC-Pl Expressing the Rabbit GH Receptor A modified version of the MTT cell proliferation assay descnbed by Rowlmson et al (1995) was developed to measure hGH bioactivity Our assay measures uptake and reduction of the dye MTS, which creates a soluble product, rather than MTT, which creates an insoluble product that must be solubihzed with organic solvents The advantage of using MTS is that absorbance of the wells can be determined without the need to lyse the cells with organic solvents GH-R4 cells were washed three times with phenol red-free RPMI 1640 media and suspended in assay media (phenol red-free RPMI 1640, 10% horse serum, 50 Units/ml penicillin, 50 μg/ml streptomycin, 400 μg/ml G418) at a concentration of lxlO 5 cells/ml Fifty πucroliters of the cell suspension (5xl0 3 cells) were added to wells of a 96 well flat bottomed tissue culture plate Senal three-fold dilutions of protein samples were prepared in assay media and added to microtiter wells in a volume of 50 μl, yielding a final volume of 100 μl per well Protein samples were assayed in tπphcate wells Plates were incubated at 37°C in a humidified 5% C0 2 tissue culture incubator for 66-72 h, at which time 20 μl of an MTS/PES (PES is an electron coupler) reagent mixture (CellTiter 96 Aqueous One solution reagent, Promega Corporation) was added to each well Absorbance of the wells at 490 nm was measured 2-4 h later Absorbance value means +/- standard deviations for the tnplicate wells were calculated Control wells contained media but no cells Absorbance values for the control wells (typically 0 06 - 0 2 absorbance umts) were subtracted from the absorbance values for the sample wells Control expenments demonstrated that absorbance signals coπelated with cell number up to absorbance values of 2 All assays included a human pituitary GH standard Expenments utilizing the parental FDC-Pl cell line were performed as descnbed above except that the assay media did not contain G418 and fetal calf serum was substituted for horse serum Example 2 Clomng and Expression of rhGH A. Cloning a cDNA encoding human Growth Hormone (GH) A human GH cDNA was amplified from human pituitary single-stranded cDNA (commercially available from CLONTECH, Inc , Palo Alto, CA) using the polymerase chain reaction (PCR) technique and pnmers BB1 and BB2 The sequence of BB1 is (5'- GGGGGTCGACCATATGTTCCCAACCATTCCCTTATCCAG-3')(SEQ ID NO 3) The sequence ofBB2 ιs (5'- GGGGGATCCTCACTAGAAGCCACAGCTGCCCTC - 3')(SEQ rD NO 4) Pnmer BB1 was designed to encode an initiator methionine preceding the first armno acid of mature GH, phenvlalalamne, and Sal I and Nde I sites for clomng purposes The reverse pnmer, BB2, contains a Bam HI site for clomng purposes The PCR reactions contained 20 pmoles of each oligo pnmer, IX PCR buffer (Perkin-Elmer buffer contaimng MgCl ), 200 μM concentration of each of the four nucleotides dA, dC, dG and dT, 2 ng of single-stranded cDNA, 2 5 umts of Taq polymerase (Perkin- Elmer) and 2 5 umts of Pfu polymerase (Stratagene, Inc) The PCR reaction conditions were 96°C for 3 minutes, 35 cycles of (95°C, 1 minute, 63°C for 30 seconds, 72°C for 1 minute), followed by 10 minutes at 72°C The thermocycler employed was the Amphtron II Thermal Cycler (Thermolyne) The approximate 600 bp PCR product was digested with Sal I and Bam HI, gel-purified and cloned into similarly digested plasmid pUC19 (commercially available from New England BioLabs, Beverly, MA) The hgation mixture was transformed into E coli strain DHSalpha and transformants selected on LB plates contaimng ampicillin Several colomes were grown overmght in LB media and plasmid DNA isolated using miiuplasmid DNA isolation kits purchased from Qiagen, Inc (Valencia, CA) Clone LB6 was determined to have the coπect DNA sequence For expression in E coli, clone LB6 was digested with Nde I and Eco RI, the approximate 600 bp fragment gel-puπfied, and cloned into plasmid pCYBl (commercially available from New England Biolabs, Beverly, MA) that had been digested with the same enzymes and treated with calf alkaline phosphatase The hgation mixture was transformed into E coli DH5alpha and transformants selected on LB ampicillin plates Plasmid DNA was isolated from several transformants and screened by digestion with Nde I and Eco RI A coπect clone was identified and named pCYB 1 wtGH (pBBT120) This plasmid was transformed into E colt strains JM109 or W3110 (available from New England BioLabs and the Amencan Type Culture Collection) B. Construction of stll-GH Wild type GH clone LB6 (pUC19 wild type GH) was used as the template to construct a GH clone contaimng the E coli stll signal sequence Because of its length, the stll sequence was added in two sequential PCR reactions The first reaction used forward pnmer BB 12 and reverse pnmer BB 10 BB10 has the sequence (5' CGCGGATCCGATTAGAATCCACAGCTCCCCTC 3')(SEQ ID NO 5) BB12 has the sequence (5'-GCATCTATGTTCGTTTTCTCTATCGCTACCAACGCTTACGCA TTCCCAACCATTCCCTTATCCAG-3 ')(SEQ ID NO 6) The PCR reactions were as descnbed for amplifying wild type GH The approximate 630 bp PCR product was gel-puπfied using the Qiaex II Gel Extraction Kit (Qiagen, Inc), diluted 50-fold in water and 2 μl used as template for the second PCR reaction The second PCR reaction used reverse pnmer BB 10 and forward pnmer BB 11 BB 11 has the sequence (5'CCCCCTCTAGACATATGAAGAAGAACATCGCATTCCTGCTGGCATCT ATGTTCGTTTTCTCTATCG-3 ')(SEQ ID NO 7) Pnmer BB 11 contains Xbal and Ndel sites for clomng purposes PCR conditions were as descnbed for the first reaction The approximate 660 bp PCR product was digested with Xbal and BamEl, gel-punfied and cloned into similarly cut plasmid pCDNA3 1(+) ( Invitrogen, Inc Carlsbad, CA) Clone pCDNA3 1(+) stII-GH(5C) or "5C" was determined to have the coπect DNA sequence Clone "5C" was cleaved with Ndel and BamYQ. and cloned into similarly cut pBBT108 (a denvative of pUC19 which lacks a Pst I site, this plasmid is descnbed below) A clone with the coπect insert was identified following digestion with these enzymes This clone, designated pBBTl 11, was digested with Nde I and Sal I, the 660 bp fragment contaimng the stll-GH fusion gene, was gel-punfied and cloned into the plasmid expression vector pCYB 1 (New England BioLabs) that had been digested with the same enzymes and treated with calf alkaline phosphatase A recombinant plasmid contaimng the stll-GH insertion was identified by restnction endonuclease digestions One such isolate was chosen for further studies and was designated pBBTl 14 This plasmid was transformed into E coli strains JM109 or W3110 (available from New England BioLabs and the Amencan Type Culture Collection) C. Construction of ompA-GH Wild type GH clone LB6 (pUC19 wild type GH) was used as the template to construct a GH clone contaimng the E colt ompA signal sequence Because of its length, the ompA sequence was added in two sequential PCR reactions The first reaction used forward pnmer BB7 (5'GCAGTGGC ACTGGCTGGTTTCGCTACCGTAGCGCAGGCCTTCCCAACCATTCCCTTATCCAG 3')(SEQ ID NO 8) and reverse pnmer BB10 (5' CGCGGATCCGATTAGAATCC ACAGCTCCCCTC 3 ')(SEQ ID NO 5) The PCR reactions were as descnbed for amplifying wild type GH except that approximately 4 ng of plasmid LB6 was used as the template rather than single- stranded cDNA and the PCR conditions were 96°C for 3 minutes, 30 cycles of (95°C for 1 minute, 63°C for 30 seconds, 72°C for 1 minute) followed by 72°C for 10 minutes The approximate 630 bp PCR product was gel-puπfied using the Qiaex II Gel Extraction Kit (Qiagen, Inc), diluted 50-fold in water and 2 μl used as template for the second PCR reaction The second PCR reaction used reverse pnmer BB 10 and forward pnmer BB6 (5'CCCCG TCGACACATATGAAGAAGACAGCTATCGCGATTGCAGT GGCACTGGCTGGTTTC 3 ')(SEQ ID NO 9) PCR conditions were as descnbed for the first reaction The approximate 660 bp PCR product was gel-punfied, digested with Sa[ I and Bam HI and cloned into pUC19 (New England BioLabs) which was cut with Sal I and Bam HI or pCDNA3 1(+) (Invitrogen) which had been cut by Xho I and Bam HI (Sal I and Xho I produce compatible single-stranded overhangs) When several clones were sequenced, it was discovered that all pUC19 (8/8) clones contained eπors in the region of the ompA sequence Only one pCDNA3 1(+) clone was sequenced and it contained a sequence ambiguity in the ompA region In order to generate a coπect ompA-GH fusion, gene segments of two sequenced clones, which contained different eπors separated by a convement restπction site were recombmed and cloned into the pUC19-deπvative that lacks the Pst I site (see pBBT108 descnbed below) The resulting plasmid, termed pBBT112, carnes the ompA-GH fusion gene cloned as an Nde I - Bam HI fragment into these same sites in pBBT108 This plasmid is designated pBBTl 12 and is used in PCR-based, site-specific mutagenesis of GH as descnbed below D. Construction of Pst I pUC19 (pBBT 108) To facilitate mutagenesis of the cloned GH gene for construction of selected cysteine substitution and insertion mutations, a deπvative of the plasmid pUC19 (New England BioLabs) lacking a Pst I site was constructed as follows pUC19 plasmid DNA was digested with Pst I and subsequently treated at 75°C with Pfu DNA Polymerase (Stratagene) using the vendor-supplied reaction buffer supplemented with 200 μM dNTPs Under these conditions the polymerase will digest the 3 ' single-stranded overhang created bv Pst I digestion but will not digest into the double- stranded region and the net result will be the deletion of the 4 single-stranded bases which compπse the middle four bases of the Pst I recognition site The resulting molecule has double-stranded, l e "blunt", ends Following these enzymatic reactions the linear monomer was gel-punfied using the Qiaex II Gel Extraction Kit (Qiagen, Inc) This purified DNA was treated with T4 DNA Ligase (New England BioLabs) according to the vendor protocols, digested with st I, and used to transform E coli DH5alpha Transformants were picked and analyzed by restnction digestion with Pst I and Bam HI One of the transformants which was not cleaved by Pst I but was cleaved at the nearby Bam HI site was picked and designated pBBT108 E. Expression of met-hGH and stH-hGH in E.coli pBBT120, which encodes met-hGH, and pBBTl 14, which encodes stll-hGH were transformed into E coli strain W3110 The parental vector pCYBl also was transformed into W3110 The resulting strains were given the following designations BOB130 W3110 (pCYBl) = vector only BOB133 W3110 (pBBT12O) = met-hGH BOB132 W3110 (pBBT114) = stIι-hGH These strains were grown overmght at 37°C in Luna Broth (LB) contaimng 100 μg/ml ampicillin The saturated overmght cultures were diluted to - 0 025 O D s at Agoo in LB contaimng 100 μg/ml ampicillin and incubated at 37°C in shake flasks When culture O D s reached -0 25 - 0 5, IPTG was added to a final concentration of 0 5 mM to induce expression of the recombinant proteins For imtial expenments, cultures were sampled at 0, 1, 3, 5 and -16 h post-mduction Samples of induced and umnduced cultures were pelleted and resuspended in SDS-PAGE sample buffer with the addition of 1% β- mercaptoethanol (BME) when desirable Samples were electrophoresed on precast 14% Tπs-glycine polyacrylamide gels Gels were stained with Coomassie Blue or were analyzed by Western blotting Coomassie staining of whole cell lvsates from strains BOB 133, expressing met-hGH, and BOB 132, expressing stll-hGH, showed a band of -22 kDa that co-migrated with a purified rhGH standard purchased from Research Diagnostics Inc The rhGH band was most prominent in induced cultures following overmght induction Western blots using a polyclonal rabbit anti-hGH antiserum purchased from Umted States Biological, Inc confirmed the presence of rhGH in lysates of induced cultures of BOB 132 and BOB 133 at both 3 and 16 h post-induction No rhGH was detected by Western Blotting of induced cultures of the control strain BOB130 (vector only) at 3 or 16 h post- mduction An induced culture of BOB 132 (expressing stll-hGH) was prepared as descnbed above and subjected to osmotic shock based on the analytical procedure of Koshland and Botstein (1980) This procedure ruptures the E coli outer membrane and releases the contents of the penplasm into the suπounding medium Subsequent centπfugation separates the soluble peπplasmic components (recovered in the supernatant) from cytoplasmic, insoluble penplasπuc, and cell-associated components (recovered in the pellet) Specifically, E coli strain W3110 contaimng the st-Il hGH plasmid was grown at 37°C overmght in LB contaimng 100 μg/ml ampicillin The saturated overmght culture was diluted to 0 03 O D at Aβoo in 25 ml of LB contaimng 100 μg ml ampicillin and incubated at 37°C in a 250 ml shake flask When the culture O D reached approximately 0 4, 100 mM IPTG was added for a final concentration of 0 5 mM to induce expression of the recombinant protem The induced culture was then incubated at 37°C overmght (-16 h) The induced overnight culture reached an O D of 3 3 at Aβoo and 4 O D s (1 2 ml) was centnfuged in a Eppendorf model 5415C microfuge at 14,000 rpm for 5 minutes at 4°C The cell pellet was resuspended to approximately 10 O D in ice cold 20% sucrose, 10 mM Tπs-HCl pH 8 0 by tnturation and vortexmg and EDTA pH 8 0 was added for a final concentration of 17 mM Resuspended cells were incubated on ice for 10 minutes and centnfuged as above The resultant pellet was resuspended at 10 O D s in ice cold water by tnturation and vortexmg and incubated on ice for 10 minutes The resuspended cells were then centnfuged as above and the resultant supernatant (soluble peπplasmic fraction) and cell pellet (insoluble peπplasmic and cell associated components) were analyzed by SDS-PAGE The bulk of the rhGH synthesized by BOB 132 was found to be soluble and localized to the peπplasm The penplasπuc rhGH protein was indistinguishable in size from a purified pituitary hGH standard indicating that the stll signal sequence was removed dunng protein secretion Example 3 Purification and Bioactivity of rhGH A. Purification of wild-type rhGH In order to punfy a significant quantity of wild-type rhGH, a 330 ml culture of BOB132 (expressing stll-hGH) was induced, cultured overmght and subjected to osmotic shock based on the preparative procedure descnbed by Hsiung et al (1986) Specifically, E coli strain W3110 contaimng the st-II hGH plasmid was grown at 37°C overmght in LB contaimng 100 μg/ml ampicillin The saturated overmght culture was diluted to 0 03 O D at Aβoo in 2 X 250 ml (500 ml total volume) of LB contaimng 100 μg/ml ampicillin and incubated at 37°C in 2 L shake flasks When the culture's O D reached approximately 0 4, lOOmM IPTG was added for a final concentration of 0 5 mM to induce expression of the recombinant protein The induced culture was incubated at 37°C overmght (-16 h) The induced overmght cultures reached an O D of approximately 3 8 at Agoo and were centnfuged using a Sorval RC-5 centrifuge and a GSA rotor at 8,000 rpm for 5 minutes at 4°C The cell pellets were combined and resuspended to approximately 47 O D in ice cold 20% sucrose, 10 mM Tns-HCl pH 8 0 by tnturation and EDTA pH 8 0 was added for a final concentration of -25 mM Resuspended cells were incubated on ice for 10 minutes and centnfuged in an IEC Centra MP4R centrifuge with an 854 rotor at 8,500 rpm for 7 minutes at 4°C The resultant pellets were resuspended at 47 O D m ice cold water by tnturation and vortex g and incubated on ice for 30 minutes The resuspended cells were centnfuged in the IEC centrifuge as above and the resultant supernatant (soluble penplasπuc fraction) and cell pellet (insoluble peπplasmic and cell associated components) were analyzed by SDS-PAGE Again the gel showed that rhGH produced was soluble, penplasmic, and indistinguishable in size from the pitutary hGH standard rhGH was purified in a two step procedure based on that descnbed by Becker and Hsiung (1986) The supernatant from the osmotic shock was loaded onto a 5 ml Pharmacia HiTrap Q Sepharose column equilibrated in 10 mM Tns-HCl pH 8 0 and the bound proteins were eluted with a 15 column volume 50-250 mM linear NaCl gradient Column fractions were analyzed by SDS-PAGE Fractions 22-25 eluting at a salt concentration of around 100-125 mM were enπched for hGH, and were pooled, concentrated and further fractionated on a Superdex 200 HR 10/30 sizing column Fractions 34-36 from the Superdex column (representing MWs around 21-22 kDA based on the elution profile of MW standards) contained most of the rhGH, were pooled and stored as frozen aliquots at -80°C The final yield of rhGH, as determined by absorbance at 280 nm and by using a Bradford protein assay kit (Bio-Rad Laboratoπes), was about 2 mg Non-reduced pituitary hGH migrates with a slightly smaller apparent molecular weight than reduced pituitary hGH when analyzed by SDS-PAGE This molecular weight change is indicative of proper disulfide bond formation Our purified rhGH co-migrated with pituitary hGH under both reducing and non-reducmg conditions indicating that the rhGH was properly folded and disulfide-bonded Data presented below indicates that the biological activity of rhGH is indistinguishable from that of pituitary hGH B. Bioassay results for Pituitary hGH and rhGH The parental FDC-Pl cell line shows a strong proliferative response to mouse IL-3, but not to pituitary hGH In the absence of IL-3, the majoπty of FDC-Pl cells die, giving absorbance values less than 0 2 In contrast, FDC-Pl cells transformed with the rabbit growth hormone receptor proliferate in response to pituitary hGH, as evidenced by a dose-dependent increase in cell number and absorbance values The EC 5 o (protein concentration required to achieve half-maximal stimulation) for this effect ranged from 0 75- 1 2 ng/ml pituitary hGH (0 03-0 05 nM) in different experiments, similar to what has been reported in the literature (Rowhnson et al , 1995) A significant difference between the parental FDC-Pl line and FDC-Pl cells transformed with the rabbit growth hormone receptor is that the latter cells survive in the absence of IL-3 or hGH, resulting in higher absorbance values (typically 0.6 - 1.1, depending upon the assay and length of incubation with MTS in the zero growth factor control wells). The initial pool of rabbit growth hormone receptor transformants and all five independent growth hormone receptor cell lines isolated showed the same effect. A similar result was obtained with a second set of independently isolated rabbit growth hormone receptor transfectants. Rowlinson et al. (1995) observed a similar effect, suggesting that EL- 3/GH-independent survival is a consequence of the transformation procedure. Although the growth hormone receptor cell lines did not require EL-3 or hGH for growth, they still showed a robust proliferative response to JL-3 and hGH. The practical effect of the higher absorbance values in the absence of hGH is to decrease the "window" of the hGH response (the difference between the maximum and minimum absorbance values). This window consistently ranged from 40 - 70% of the zero growth factor values, similar to what was reported by Rowlinson et al.(1995). Pituitary GH and wild-type rhGH prepared by us had similar dose response curves in the bioassay with EC50S ranging from 0.6-1.2 ng/ml in different expenments (Table 1). Table 1 Bioactivities of hGH and hGH Cvsteine Muteins N/A, not applicable 1 EC 50 values from individual experiments. An EC 50 range is shown when N>5. Example 4 Construction of hGH cysteine muteins The cysteine substitution mutation T135C was constructed as follows. The mutagenic reverse oligonucleotide BB28 (5>CTGCTTGAAGATCTGCCCACACCG GGGG CTGCCATC>3)(SEQ.ID.NO. 10) was designed to change the codon ACT for threonine at amino acid residue 135 to a TGT codon encoding cysteine and to span the nearby Bgl II site. This oligo was used in PCR along with BB34 (5>GTAGCGCAGGCCTTCCCAACC ATT>3)(SEQ.ID.NO. 11) which anneals to the junction region of the ompA-GH fusion gene and is not mutagenic. The PCR was performed in a 50 μl reaction in IX PCR buffer (Perkin-Elmer buffer containing 1.5 mM MgCl 2 ), 200 μM concentration of each of the four nucleotides dA, dC, dG and dT, with each oligonucleotide primer present at 0.5 μM, 5 pg of pBBTl 12 (described above) as template and 1.25 units of AmpliTaq DNA Polymerase (Perkin-Elmer) and 0.125 umts of Pfu DNA Polymerase (Stratagene). Reactions were performed in a Robocycler Gradient 96 thermal cycler (Stratagene). The program used entailed: 95°C for 3 minutes followed by 25 cycles of [95°C for 60 seconds, 45°C or 50°C or 55°C for 75 seconds, 72°C for 60 seconds] followed by a hold at 6° C. The PCR reactions were analyzed by agarose gel electrophoresis which showed that all three different annealing temperatures gave significant product of the expected size, -430 bp The 45°C reaction was "cleaned up" using the QIAquick PCR Purification Kit (Qiagen) and digested with Bgl II and Pst I The resulting 278 bp Bgl II- Pst I fragment, which includes the putative T135C mutation, was gel-punfied and ligated into pBBT 111 , the pUC 19 deπvative carrying the stll-GH fusion gene (descnbed above) which had been digested with Bgl II and Pst I and gel-puπfied Transformants from this hgation were initially screened by digestion with Bgl II and Pst I and subsequently one clone was sequenced to confirm the presence of the T135C mutation and the absence of any additional mutations that could potentially be introduced by the PCR reaction or by the synthetic oligonucleotides The sequenced clone was found to have the coπect sequence throughout the Bgl II - Pst I segment The substitution mutation S132C was constructed using the protocol descnbed above for T135C with the following differences mutagemc reverse olignucleotide BB29 (5>CTGCTTGAA GATCTGCCCAGTCCGGGGGCAGCCATCTT03)(SEQ ID NO 12) was used instead of BB28 and the PCR reaction with annealing temperature of 50°C was used for clomng One of two clones sequenced was found to have the coπect sequence The substitution mutation T148C was constructed using an analogous protocol but employing a different clomng strategy The mutagemc forward oligonucleotide BB30 (5>GGGCAGATCTT CAAGCAGACCTACAGCAAGTTCGACTGCAACTCACACAAC>3)(SEQ ID NO 13) was used in PCR with the non-mutagenic reverse pnmer BB33 (5>CGCGGTACCCGGGATCCGATTAGAAT CCACAGCT>3)(SEQ ED NO 14) which anneals to the most 3 ' end of the GH coding sequence and spans the Bam HI site immediately downstream PCR was performed as descnbed above with the exception that the annealing temperatures used were 46, 51 and 56°C Following PCR and gel analysis as descnbed above the 46 and 51°C reactions were pooled for clomng These were digested with Bam HI and Bgl II and the resulting 188 bp fragment was gel-punfied and cloned into pBBTl 11 which had been digested with Bam HI and Bgl II, treated with calf alkaline phosphatase (Promega) according to the vendor protocols, and gel-punfied Tranformants from this hgation were analyzed by digestion with Bam HI and Bgl II to identify clones in which the 188 bp Bam HI- Bgl II mutagemc PCR fragment was cloned in the proper oπentation because Bam HI and Bgl II generate compatible ends, this clomng step is not oπentation specific Five of six clones tested were shown to be coπectly oπented One of these was sequenced and was shown to contain the desired T148C mutation The sequence of the remainder of the 188 bp Bam HI- Bgl II mutagemc PCR fragment in this clone was confirmed as coπect The construction of the substitution mutation S144C was identical to the construction of T148C with the following exceptions Mutagemc forward oligonucleotide BB31 (5>GGGCAGATCT TCAAGCAGACCTACTGCAAGTTCGAC>3)(SEQ ED NO 15) was used instead of BB30 Two of six clones tested were shown to be coπectly onented One of these was sequenced and was shown to contain the desired S144C mutation The sequence of the remainder of the 188 bp Bam HI- Bgl II mutagemc PCR fragment in this clone was also confirmed as coπect A mutation was also constructed that added a cysteine residue to the natural carboxyterminus of GH The construction of this mutation termed stpl92C, was earned out using the procedures descnbed above for construction of the T148C mutein but employed different oligonucleotide pnmers The mutagemc reverse oligonucleotide BB32 (5>CGCGGTACCGGATCCTTAGCAGAAGCCACAG CTGCCCTCCAC>3)(SEQ ED NO 16) which inserts a TGC codon for cysteine between the codon for the carboxy terminal phe residue of GH and the TAA translational stop codon and spans the nearby Bam HI site was used along with BB34 which is descnbed above Following PCR and gel analysis as descnbed above the 46°C reaction was used for clomng Three of six clones tested were shown to be coπectly oπented One of these was sequenced and was shown to contain the desired stpl92C mutation The sequence of the remainder of the 188 bp Bam HI- Bgl II mutagemc PCR fragment m this clone was confirmed as coπect The substitution mutation S100C was constructed using mutagemc reverse oligonucleotide BB25 (5>GTCAGAGGCGCCGTACACCAGGCAGTTGGCGAAGAC>3)(SEQ D NO 17) which alters the AGC codon for senne at ammo acid residue 100 to a TGC codon encoding cysteine BB25 also spans the nearby Nar I site PCR reactions using BB25 and BB34 were earned out using the PCR protocol descnbed above for the construction of the T135C mutation Following gel analysis of the PCR products, the product of the 50°C annealing reaction was cleaned up using the QIAquick PCR Purification Kit (Qiagen), with digested with Pst I and Nar I The resulting 178 bp fragment was gel-puπfied and ligated into pBBTl 11 which had been digested with Pst I and Nar I and gel- punfied Plasmids isolated from transformants from this hgation were screened by digestion with Pst I and Nar I and subsequently one plasmid was sequenced to confirm the presence of the S100C mutation and the absence of any other mutations in the 178 bp Pst I - Nar I segment The substitution mutation A98C was constructed using the procedure descnbed above for S100C with the following differences the mutagemc reverse oligonucleotide BB26 (5>GTCAG AGGCGCCGTACACCAGGCTGTTGCAGAAGACACTCCT>3)(SEQ ED NO 18) was used for PCR in place of BB25 and the PCR reaction performed with an annealing temperature of 45°C was used for clomng One clone was sequenced and found to have the coπect sequence The substitution mutation A34C was constructed as follows The mutagemc reverse oligo BB23 (5>GCGCTGCAGGAATGAATACTTCTGTTCCTTTGGGATATAGCATTCTTC AAACTC>3)(SEQ ED NO 19) was designed to change the GCC codon for alamne at amino acid residue 34 to a TGC codon encoding cysteine and to span the Pst I site This oligonucleotide was used in PCR reactions along with BB11 (5>CCCCCTCTAGACAT ATGAAGAAGAACATCGC ATTCCTGCTGGCATCTATGTTCGTTTTCTCTATCG>3)(SEQ D NO 20) which anneals to the 5' end of the coding sequence of the stll leader sequence and spans the Nde I site that overlaps the initiator methionine codon PCR reactions were performed in 50 μl in IX PCR buffer (Promega) contaimng 1 5 mM MgCl 2 , 200 uM concentration of each of the four nucleotides dA, dC, dG and dT, with each oligonucleotide pnmer present at 0 2 μM, 1 ng of pBBTlll (descnbed above) as template and 0 8 umts of Tac DNA Polymerase (Promega) and 0 33 umts of Pfu DNA Polymerase (Stratagene) Reactions were performed in a Robocycler Gradient 96 thermal cycler (Stratagene) The program used entailed 96°C for 3 minutes followed by 25 cycles of [95°C for 60 seconds, 50°C or 55°C or 60°C for 75 seconds, 72°C for 60 seconds] followed by a hold at 6°C The PCR reactions were analyzed by agaraose gel electrophoresis which showed that all three different annealing temperatures gave significant product of the expected size, -220 bp The 50 and 55°C reactions were pooled, "cleaned u ' using the QIAquick PCR Purification Kit (Qiagen) and digested with Nde I and Pst I The resulting 207 bp Nde I- Pst I fragment, which includes the putative A34C mutation, was gel-puπfied and ligated into pBBTl 11, which had been digested with Nde I and Pst I, treated with alkaline phosphatase. and gel-punfied Transformants from this hgation were initially screened by digestion with Nde I and Pst I and subsequently one clone was sequenced to confirm the presence of the A34C mutation and the absence of any additional mutations that could potentially be introduced by the PCR reaction or by the synthetic oligonucleotides The sequenced clone was found to have the coπect sequence throughout the Nde I - Pst I segment The substitution mutation S43C was constructed using the protocol descnbed above for A34C with the following differences mutagemc reverse oligonucleotide BB24 (5 >GCGCTG CAGGAAGCAATACTTCTGTTCCTTTGG>3)(SEQ ED NO 21) was used instead of BB23 One clone was sequenced and shown to contain the coπect sequence The substitution mutation T3C was constructed using two sequential PCR steps The first step created the desired mutation while the second step extended the PCR product of the first reaction to encompass a useful clomng site The mutagemc forward oligonucleotide BB78 (5>GCAT CTATGTTCGTTTTCTCTATCGCTACCAACGCT TACGCATTCCCATGCATTCCCT TATCCAG>3)(SEQ D NO 22) was designed to change the ACC codon for threonine at amino acid residue 3 to a TGC codon encoding cysteine and to span and anneal to the junction of the stll - hGH fusion gene BB78 was used in the first step PCR along with BB33 which is descnbed above The first PCR reaction was performed in 50 μl in IX PCR buffer (Promega) contaimng 1 5 mM MgCl 2 , 200 μM concentration of each of the four nucleotides dA, dC, dG and dT, each oligonucleotide pnmer at 0 2 μM, 1 ng of pBBTlll (descnbed above) as template and 1 5 umts of Tac DNA Polymerase ( Promega) and 0 25 umts of Pfu DNA Polymerase (Stratagene) Reactions were performed in a Robocycler Gradient 96 thermal cycler (Stratagene) The program used entailed 96°C for 3 minutes followed by 25 cycles of [94°C for 60 seconds, 60°C for 75 seconds, 72°C for 60 seconds] followed by a hold at 6°C The PCR reaction was ethanol precipitated and the -630 bp product was gel-punfied and recovered in 20 μl 10 mM Tns-HCl (pH 8 5) An aliquot of this gel- punfied fragment was diluted 100-fold and 2 μl of the diluted fragment was used as template n the second PCR step The second PCR step employed oligonucleotides BB11 and BB33 (both descnbed above) and used the reaction conditions employed in the first step PCR reaction The second step PCR reaction was analyzed by agarose gel electrophoresis and the expected - 670 bp fragment was observed. The PCR reaction was "cleaned up" using the QIAquick PCR Purification Kit (Qiagen) and digested with Nde I and Pst I. The resulting 207 bp Nde I -Pst I fragment, which includes the putative T3C mutation, was gel-purified and ligated into pBBTl 11, which had been digested with Nde I and Pst I, treated with alkaline phosphatase, and gel-purified. Transformants from this ligation were initially screened by digestion with Nde I and Pst I and subsequently one clone was sequenced to confirm the presence of the T3C mutation and the absence of any additional mutations that could potentially be introduced by the PCR reaction or by the synthetic oligonucleotides. The sequenced clone was found to have the coπect sequence throughout the Nde I - Pst I segment. The substitution mutation A105C was constructed using the technique of "mutagenesis by overlap extension" as described in general by Horton, R. M. pp251-261 in Methods in Molecular Biology, Vol. 15: PCR Protocols: Cuπent Methods and Applications edited by White, B. A. (1993) Humana Press, Inc., Totowa,NJ. With this technique two separate fragments are amplified from the target DNA segment as diagrammed in Figure 1. One fragment is produced with primers a and b to yield product AB. The second primer pair, c and d, are used to produce product CD. Primers b and c introduce the same sequence change into the right and left ends of products AB and CD, respectively. Products AB and CD share a segment of identical (mutated) sequence, the "overlap", which allows annealing of the top strand of AB to the bottom strand of CD, and the converse. Extension of these annealed overlaps by DNA polymerase in a subsequent PCR reaction using primers a and d with products AB and CD both added as template creates a full length mutant molecule AD. With the exception of the use of different oligonucleotide primers, the imtial PCR reactions for the A105C construction were performed identically to those described in the construction of T3C above. The primer pairs used were: (BB27 + BB33) and (BB11 + BB79). BB11 andBB33 are described above. BB27 and BB79 are complementary mutagenic oligonucleotides that change the GCC codon for alanine at amino acid residue 105 to a TGC codon encoding cysteine. The sequence of BB27 is (5>AGCCTGGTGTAC GGCTGCTCTGACAGCAACGTO3)(SEQ.ED.N0. 23) and the sequence of BB78 is 5>GACGTTGCTGTCAGAGCAGCCGTACACCAGGCT>3 (SEQ.ED.NO. 24). The (BB27 x BB33) and (BB11 x BB79) PCR reactions were ethanol precipitated, gel-purified, and recovered in 20 μl 10 mM Tris-HCl (pH 8.5). The preparative gel showed that the predominant product from each PCR reaction was of the expected size: -290 bp for the (BB27 x BB33) reaction and ~408 bp for the (BB11 x BB79) reaction. These two mutagenized fragments were then "spliced" together in a subsequent PCR reaction. In this reaction 1 μl of each of gel-purified fragments from the imtial reactions were added as template and BB 11 and BB33 were used as primers. Otherwise, the PCR reaction was performed using the same conditions employed for the initial (BB27 x BB33) and (BB 11 x BB79) reactions. An aliquot of the secondary PCR reaction was analyzed by agarose gel electrophoresis and the expected -670 bp band was observed. The bulk of the PCR reaction was then "cleaned up" using the QIAquick PCR Purification Kit (Qiagen) and digested with Bgl II and Pst I. The resulting 276 bp Bgl II - Pst I fragment, which includes the putative A105C mutation, was gel- purified and ligated into pBBTl 11, which had been digested with Bgl II and Pst I, and gel-purified. Transformants from this ligation were initially screened by digestion with Bgl II and Pst I and subsequently one clone was sequenced to confirm the presence of the A105C mutation and the absence of any additional mutations that could potentially be introduced by the PCR reaction or by the synthetic oligonucleotides The sequenced clone was found to have the coπect sequence throughout the Bgl II - Pst I segment For expression in E coli as proteins secreted to the peπplasmic space, the stll-hGH genes encoding the 11 muteins were excised from the pUC19-based pBBTll l denvatives as ΛWe I -Xba I fragments of -650 bp and subcloned into the pCYBl expression vector that had been used to express rhGH For expression expenments, these plasmids were introduced into E coli W3110 Using procedures similar to those descnbed here and in Example 23, one can construct other cysteine muteins of GH The cysteine muteins can be substitution mutations that substitute cysteine for a natural ammo residue m the GH coding sequence, insertion mutations that insert a cysteine residue between two naturally occurπng amino acids in the GH coding sequence, or addition mutations that add a cysteine residue preceding the first amino acid, FI, of the GH coding sequence or add a cysteine residue following the terminal amino acid residue, F191, of the GH coding sequence. The cysteine residues can be substituted for any amino acid, or inserted between any two amino acids, anywhere in the GH coding sequence Prefeπed sites for substituting or inserting cysteine residues in GH are in the region preceding Helix A, the A-B loop, the B-C loop, the C-D loop and the region distal to Helix D Other prefeπed sites are the first or last three amino acids of the A, B, C and D Helices Prefeπed residues in these regions for creating cysteine substitutions are FI, P2, P5, E33, K38, E39, Q40, Q46, N47, P48, Q49, T50, S51, S55, S57, T60, Q69, N72, N99, L101, V102, Y103, G104, S106, E129, D130, G131, P133, R134, G136, Q137, K140, Q141, T142, Y143, K145, D147, N149, S150, H151, N152, D153, S184, E186, G187, S188, and G190 Cysteine residues also can be inserted immediately preceding or following these amino acids One prefeπed site for adding a cysteine residue would be preceding FI, which is refeπed to herein as *-lC One also can construct GH muteins contaimng a free cysteme by substituting another ammo acid for one of the naturally -occurπng cysteine residues in GH The naturally occurring cysteine residue that normally forms a disulfide bond with the substituted cysteine residue is now free The non-essential cysteine residue can be replaced with any of the other 19 amino acids, but preferably with a senne or alamne residue A free cysteine residue also can be introduced into GH by chemical modification of a naturally occurπng ammo acid using procedures such as those descnbed by Sytkowski et al (1998), incorporated herein by reference Using procedures similar to those descnbed in Examples 1 - 15, one can express the proteins in prokaryotic cells such as E coli, purify the proteins, PEGylate the proteins and measure their bioactivities in an in vitro bioassay The GH muteins also can be expressed in eukaryotic cells such as insect or mammalian cells using procedures similar to those descnbed in Examples 16-19, or related procedures well known to those skilled in the art If secretion is desired, the natural GH signal sequence, or another signal sequence, can be used to secrete the protein from eukaryotic cells Example 5 Cystine Addition Improves Recovery of Cysteine Mutein T3C E coli strain W3110 contaimng the T3C mutation was grown overmght in 3 ml of LB contaimng 100 μg/ml ampicillin The saturated overmght culture was diluted to 0 03 O D. at Aβoo m 300 ml of LB contaimng 100 μg/ml ampicillin and incubated at 37°C m 2 L shake flasks When the culture O D reached 0 420, 1 5 ml of 100 mM EPTG was added for a final concentration of 0 5 mM to induce expression of the recombinant protein The induced culture was incubated at 37°C overmght (-16 h) The induced overmght culture reached an O D of 3 6 at Aβoo and was split into 2 x 135 ml volumes and centnfuged using a Sorval RC-5 centrifuge with a GSA rotor at 8,000rpm for 10 minutes at 4°C The supernatants were discarded and the cell pellets subjected to osmotic shock treatment as follows The cell pellets were resuspended to approximately 49 O D in 10 ml of ice cold Buffer A [20% sucrose, 10 mM Tns-HCl pH 7 5] or Buffer B [20% sucrose, 10 mM Tns-HCl pH 7 5, 5 5 mM cystine (pH adjusted to 7 5-8 0)] The pellets were resuspended by tnturation and vortexmg and 1 ml of 0 25 M EDTA pH 8 0 was added to give a final concentration of -25 mM Resuspended cells were incubated on ice for 30 minutes and centnfuged in an SS-34 rotor at 8.500 rpm for 10 minutes at 4°C The supernatants were discarded and the pellets resuspended by tnturation and vortexmg in 10ml of ice cold Buffer C [5 mM Tns-HCl pH 7 5] or Buffer D [5 mM Tns-HCl pH 7 5, 5 mM cystine (pH adjusted to 7 5-8 0)] and incubated on ice for 30 minutes The resuspended cell pellet was centnfuged in an SS-34 rotor at 8,500 rpm for 10 minutes at 4°C and the resultant supernatant (soluble peπplasmic fraction) was recovered and stored at -80°C SDS-PAGE analysis of the vaπous supernatants revealed that by incoφorating cystine into the osmotic shock procedure, the amount of monomeπc hGH species recovered in the soluble peπplasmic fraction can be significantly improved In the sample treated with cystine, we observed that a majoπty of the hGH ran as a single band of about 22 kDa on a non-reduced SDS-PAGE gel and that band co-migrated with pituitary hGH In the absence of cystine, a number of different molecular weight species were observed in the monomenc range Larger molecular weight protein aggregates are visible when the gel is developed using Western blotting Example 6 General Method for Expression and Purification of GH Cysteine Added Variants A standard protocol for isolation of the hGH cyteme muteins is as follows Cultures of W3110 E colt strains contaimng the stll-hGH mutein plasmids were grown to saturation in LB contaimng 100 μg/ml ampicillin at 37°C The overmght cultures were typically diluted to 0 025-0 030 O D. at Aβoo m LB contaimng 100 μg/ml ampicillin and grown at 37°C in shake flasks When culture O.D.'s reached 0 300-0 500 at Agoo , IPTG was added to a final concentration of 0 5mM to induce expression The induced cultures were incubated at 37°C overmght Induced overmght cultures were pelleted by centnfugation m a Sorval RC-5 centrifuge at 8,000-10,000 x g for 10 minutes at 4°C and the resultant pellets subjected to osmotic shock based on the procedures of Koshland and Botstein (1980) or Hsiung et al , (1986) depending on the size of the culture For osmotic shock, cell pellets were resuspended at 25-50 O D in 20% sucrose, 10 mM Tns-HCl pH7 5 In certain instances 5 or 25 mM EDTA pH 8 0 and or 5 mM cystine (pH adjusted to 7 5-8 0) were included in the resuspension buffer The resuspended pellets were incubated on ice for 15-30 minutes and centnfuged as above The supernatants were discarded and the pellets resuspended at 25-50 O D at Aβoo in 5 or 10 mM Tns- HCl pH 7 5 In certain instances 5 mM EDTA pH 8 0 and / or 5 mM cystine (pH adjusted to 7 5-8 0) were included in the resuspension buffer The resuspended pellets were incubated on ice for 30 minutes and centnfuged as above The resulting supernatant contains the soluble peπplasmic components and the pellet is compnsed of insoluble penplasnuc. and cell associated components Addition of cystine to the osmotic shock buffer resulted in significantly improved recovenes and stabilization of many of the cysteine muteins In the presence of cystine the cysteine muteins were largely monomeπc. whereas in the absence of cystine, a mixture of hGH monomers, dimers and higher order aggregates were observed when the samples were analyzed by non-reducing SDS-PAGE and Western blots Presumably the higher order aggregates are a consequence of the added cysteine residues in the proteins since they were greatly reduced or absent with wild-type rhGH Addition of cystine to the osmotic shock buffer largely solved this problem We believe cystine reacts with the free cysteine residue in the muteins to form a stable mixed disulfide, thus preventing disulfide shuffling and aggregation We have also tested a second dithiol, cystamine and have seen a similar stabilizing effect on cysteine muteins of hGH Presumably other dithiols such as oxidized glutathione would also lead to improved recovenes of proteins contaimng a free cysteine Of the 11 hGH muteins analyzed, six expressed at levels sufficient for isolation and purification Non-reduced SDS-PAGE analysis of the osmotic shock supernatants for the A34C, S43C, A98C, S100C and S132C muteins showed little or no protein present at the coπect molecular weight Addition of cystine did not discermbly improve recovery of these proteins The relatively low expression levels of these muteins makes it difficult to observe improved yields Preliminary analyses of whole cell lysates indicated that these mutant proteins were expressed, but insoluble The T3C, A105C, T135C, S144C, T148C and stpl92C muteins showed moderate to good expression levels Five of these cysteine muteins (T3C, T135C, S144C, T148C and stpl92C) showed significantly improved recovenes when cystine was present in the osmotic shock buffer A105C was not tested in the absence of cystine General protein purification protocols are descnbed below WT-rhGH was punfied by ion exchange and gel filtration chromatography The cysteine muteins were punfied by an assortment of chromatograpmc procedures including ion exchange, hydrophobic interaction (EDC), metal chelation affinity chromatographies (EMAC), Size Exclusion Chromatography (SEC) or a combination of these techniques Generally, the hGH mutein was captured from the fresh osmotic shock supernatant using a Q-Sepharose fast flow resin (Pharmacia) equilibrated in 20 mM Tns-HCl, pH 8 0 The column was washed with 20 mM Tns-HCl, pH 8 0 and bound proteins eluted with a linear 10 volume increasing salt gradient from 0 to 250 mM NaCl in 20 mM Tns-HCl, pH 8 0 Fractions contaimng the hGH muteins were identified by SDS-PAGE and Western blotting These fractions were pooled and stored frozen Alternative resins can be used to capture hGH mutems from the osmotic shock mixture These include HIC, cation ion exchange resms or affinity resins For hydrophobic interaction chromatography (HIC), the Q column pool was thawed to room temperature and NaCl added to a final concentration of 2 M The pool was loaded onto the Butyl- Sepharose fast flow resin previously equilibrated in 2 M NaCl, 20 mM sodium phosphate, pH 7 5 hGH muteins were eluted from the resin using a reverse salt gradient form 2 M to 0 M NaCl in 20 mM phosphate, pH 7 5 Fractions contaimng the hGH muteins were identified by SDS-PAGE and Western blotting, and pooled In some instances, the HIC pool was subsequently loaded directly onto a nickel chelating resin (Qiagen) equilibrated in 10 mM sodium phosphate, 0 5 M NaCl, pH 7 5 Following a wash step, muteins were recovered using a 0 - 30 mM imidizole gradient in 10 mM sodium phosphate, 0 5 M NaCl, pH 7 5 hGH has a high affinity for nickel, presumably through the divalent metal-bmding site formed by H18. H21 and E174 As a result, hGH can be obtained in highly pure form using a metal chelation column (Maisano et al , 1989) All of the muteins tested bound tightly to the mckel column and eluted at similar lmidazole concentrations (around 15 mM) as wild-type rhGH Example 7 Purification of T3C The soluble penplasrmc fraction prepared in the presence of cystine (Example 5) was loaded onto a 1 ml HiTrap Q-Sepharose column (Pharmacia Biotech, Uppsala, Sweden) equilibrated in 10 mM Tns-HCl pH 8 0 and the bound proteins were eluted with a 20 column volume linear gradient of 0-250 mM NaCl The column load and recovered fractions were analyzed by 14% SDS-PAGE T3C eluted at a salt concentration around 170-200 mM which was signficantly later than WT- hGH Surpπsingly, T3C mutein was present in a monomenc form when loaded on the column, but was recovered predominantly as a stable disulfide linked homodimer following elution from the Q column When reduced, the T3C mutein co-migrated on a SDS-PAGE gel with wild type hGH Dimer formation occuπed dunng the Q column purification step Cystine was not present in the Q column buffer or the buffers used for the other column steps The T3C dimer remained intact throughout any further concentration or purification procedures The pool from the Q column was adjusted to a final NaCl concentration of 0 5 M and loaded onto 1 ml Ni-agarose column (Qiagen) previously equilibrated in 10 mM sodium phosphate, 0 5 M NaCl, pH 7 5 Following a wash step, highly punfied T3C dimers were recovered using a 0 - 30 mM imidizole gradient in 10 mM sodium phosphate, 0 5 M NaCl, pH 7 5 T3C dimers were active in the bioassay (Example 10 and Table 1) The A105C, T135C and stpl92C muteins also were recovered from the Q column as disulfide-linked dimers It appears that certain muteins are capable of forming stable disulfide-linked dimers, presumably through the added cysteine residue, once cystine is removed We believe monomeπc forms of these proteins could be stabilized by including cystine or other dithiol compounds and/or cysteine blocking agents m all the column buffers and/or maintaining the pH of the buffers below 7 to prevent disulfide reaπangement Example 8 Purification of T148C E coli strain W3110 expressing the T148C mutein was grown overmght in 3 ml of LB contaimng 100 μg/ml ampicillin The saturated overmght culture was diluted to 0 025 O D at Aβoo in 500 ml of LB contaimng 100 μg ml ampicillin and divided into 2 x 250 ml volumes and incubated at 37°C in 2 L shake flasks When the culture O D reached approximately 0 35, 1 25 ml of 100 mM EPTG was added to each flask for a final concentration of 0 5 mM to induce expression of the recombinant protein The induced culture was incubated at 37°C overnight (-16 h) The induced overnight cultures reached an O D of approximately 3 5 at Aβoo and were centnfuged using a Sorval RC-5 centrifuge with a GSA rotor at 8,000rpm for 10 minutes at 4°C The supernatants w ere discarded and the cell pellets subjected to osmotic shock treatment as follows The cell pellets were resuspended to approximately 43 O D m ice cold 20% sucrose, 10 mM Tns-HCl pH 7 5, 5 0 mM EDTA pH 8 0, 5 0 mM cystine (pH adjusted to 7 5-8 0) by tnturation and vortexmg Resuspended cells were incubated on ice for 15 minutes, centnfuged in Sorval centrifuge with an SS- 34 rotor at 8,500 rpm for 10 minutes at 4°C The supernatants were discarded and the pellets resuspended to approximately 43 O D in ice cold ImM Tns-HCl pH 7 5, 5 mM EDTA pH 8 0, 5 mM cystine (pH adjusted to 7 5-8 0) by tnturation and vortexmg Resuspended cells were incubated on ice for 30 minutes and centnfuged in an SS-34 rotor at 8,500 rpm for 10 minutes at 4°C and the resultant supernatant ( soluble penplasnuc fraction) was recovered and stored at -80°C A second culture of T148C was prepared in a similar manner with the following exceptions 1) the induced culture volume was 200 ml and reached an O D of approximately 4 0 at Aβoo after overmght incubation at 37°C. and 2) The cell pellet resuspensions were done at approximately 30 O D s at Aeoo The soluble peπplasmic fraction from the above preparations were combined and dialyzed against 1 L of 10 mM Tns-HCl pH 8 0 overnight at 4°C The dialysis retentate was loaded onto a 5 ml HiTrap Q-Sepharose column (Pharmacia Biotech, Uppsala, Sweden) eqmlibrated m 10 mM Tπs- HCl pH 8 0 and the bound proteins were eluted with a 20 column volume linear gradient of 0-250 mM NaCl Column fractions were analyzed by 14% SDS-PAGE (Novex, San Diego, CA ) T148C mutein eluted as a sharp peak at a salt concentration around 60-80 mM The appropπate fractions were pooled and concentrated on Centncon 10 concentrators (Amicon, Inc , Beverly, MA) The concentrated pool was further punfied on a Superdex 200 HR 10/30 column (Pharmacia Biotech Inc , Uppsala, Sweden ) eqmlibrated in 20 mM sodium phosphate pH 7 5, 150 mM NaCl The column fractions were again analyzed by SDS-PAGE and the T148C mutein contaimng fractions were pooled We observed that the T148C remained monomeπc after recovery from the Q-Sepharose column and eluted from the Superdex column at a molecular weight similar to wild type hGH T148C prepared by Q-Sepharose and size exclusion chromatography is refeπed to as Lot A Alternatively the Q-pool can be loaded directly onto a nickel chelating resin (Qiagen) equilibrated in 10 mM sodium phosphate, 0.5 M NaCl, pH 7.5. Following a wash step, T148C was recovered using a 0 - 30 mM imidizole gradient in 10 mM sodium phosphate, 0.5 M NaCl, pH 7.5. T148C protein prepared by Q-Sepharose and nickel affinity chromatography is refeπed to as Lot B. An osmotic shock T148C supernatant was prepared in the absence of cystine and purification over a Q-Sepharose column was attempted. The identical protocol was followed as described above except cystine was absent from the osmotic shock buffers. The T148C protein product eluted from the Q column over a broad range of salt concentrations, recoveries were lower and the protein preparation was substantially less pure than the cystine treated T148C sample. Example 9 Purification of S144C E. coli strain W3110 containing the S144C mutation was grown overnight in 3 ml of LB containing 100 μg/ml ampicillin . The saturated overnight culture was diluted to 0.03 O.D.s at Aβoo in 250 ml of LB containing 100 μg/ml ampicillin and incubated at 37°C in a 2 L shake flask. When the culture O.D. reached approximately 0.3, 1.25 ml of 100 mM IPTG was added for a final concentration of 0.5 mM to induce expression of the recombinant protein. The induced culture was incubated at 37°C overnight (-16 h). The induced overnight culture reached an O.D of approximately 3.3 at Aβoo and was centrifuged using a Sorval RC-5 centrifuge with a GSA rotor at 8,000rpm for 10 minutes at 4°C. The supernatant was discarded and the cell pellet subjected to osmotic shock treatment as follows. The cell pellet was resuspended to approximately 33 O.D. in ice cold 20% sucrose, 10 mM Tris-HCl pH 7.5, 5.0 mM cystine (pH adjusted to 7.5-8.0) by trituration and vortexing and 0.25 M EDTA pH 8.0 was added to a final concentration of 25 mM. Resuspended cells were incubated on ice for 30 minutes, centrifuged in the Sorval RC-5 centrifuge with an SS-34 rotor at 8,500 rpm for 10 minutes at 4°C. The supernatants were discarded and the pellets resuspended to approximately 33 O.D.s in ice cold 5 mM Tris-HCl pH 7.5, 5 mM cystine (pH adjusted to 7.5-8.0) by trituration and vortexing. Resuspended cells were incubated on ice for 30 minutes and centrifuged in an SS-34 rotor at 8,500 rpm for 10 minutes at 4°C and the resultant supernatant (soluble periplasmic fraction) was recovered and stored at -80°C. The soluble periplasmic fraction from the above preparation was loaded onto a 5 ml HiTrap Q-Sepharose column (Pharmacia Biotech, Uppsala, Sweden) equilibrated in 10 mM Tris-HCl pH 8.0 and the bound proteins were eluted with a 20 column volume linear gradient of 0-250 mM NaCl. Column fractions were analyzed by 14% SDS-PAGE (Novex, San Diego, CA.) and S144C mutein- containing fractions, 125-150 mM NaCl, were pooled and frozen. The S144C mutein was monomeric. Example 10 Bioactivities of hGH Cysteine Muteins The biological activities of the purified T3C, S144C and T148C cysteine muteins were measured using the cell proliferation assay described in Example 1. Protein concentrations were determined using a Bradford assay. All three muteins were biologically active. All of the muteins reached the same level of maximum stimulation as pituitary hGH, within the eπor of the assay. The mean EC 50 s for the T3C, S144C and T148C muteins were similar to that of pituitary hGH and rhGH (Table 1). Two independent preparations of the T148C mutein and one preparation each of the S144C and T3C muteins (all prepared in the presence of cystine) have been assayed multiple times (Table 1). The partially purified A105C, T135C and stpl92C muteins also have been assayed and are biologically active. Example 11 General Methods for Pegylation and Purification of Cysteine Muteins GH muteins can be PEGylated using a variety of cysteine-reactive PEG-maleimide (or PEG- vinylsulfone) reagents that are commercially available. Generally, methods for PEGylating the proteins with these reagents will be similar to those described in WO 9412219 (Cox and McDermott) and WO 9422466 (Cox and Russell), both incorporated herein by reference, with minor modifications. The recombinant proteins are generally partially reduced with dithiothreitol (DTT), Tris (2- carboxyethyl) phosphine-HCl (TCEP) or some other reducing agent in order to achieve optimal PEGylation of the free cysteine. The free cysteine is relatively unreactive to cysteine-reactive PEGs unless this partial reduction step is performed. The amount of reducing agent required to partially reduce each mutein can be determined empirically, using a range of reducing agent concentrations at different pHs and temperatures. Reducing agent concentrations typically vary from 0.5 equal molar to 10-fold excess. Prefeπed temperatures are 4°C to 37°C. The pH can range from 6.5 to 9.0 but is prefeπably 7.5 to 8.5. The optimum conditions will also vary depending on the reductant and time of exposure. Under the proper conditions, the least stable disulfides (typically intermolecular disulfides and mixed disulfides) are disrupted first rather than the more thermodynamically stable native disulfides. Typically, a 5-10 fold molar excess of DTT for 30 minutes at room temperature is effective. Partial reduction can be detected by a slight shift in the elution profile of the protein from a reversed- phase column. In the case of a dimeric hGH, a shift in molecular weight is visible by SDS-PAGE analysis run under non-reducing condition. Care must be taken not to over-reduce the protein and expose additional cysteine residues. Over-reduction can be detected by reversed phase-HPLC (the over-reduced protein will have a retention time similar to the fully reduced and denatured protein) and by the appearance of GH molecules containing two PEGs following the PEGylation reaction (detectable by a molecular weight change on SDS-PAGE). Wild type GH can serve as a control since it should not PEGylate under conditions that do not reduce the native intramolecular disulfides. Excess reducing agent can be removed prior to PEGylation by size exclusion chromatography or by dialysis. TCEP need not be removed before addition of the PEGylation reagent as it is does not contain a free thiol group. The partially reduced protein can be reacted with various concentrations of PEG-maleimide or PEG-vinylsulfone (typically PEG: protein molar ratios of 1:1, 5:1,10:1 and 50:1) to determine the optimum ratio of the two reagents. PEGylation of the protein can be monitored by a molecular weight shift for example, using SDS-PAGE. The lowest amount of PEG that gives significant quantities of mono-pegylated product without giving di-pegylated product is typically considered optimum (80% conversion to mono-pegylated product is generally considered good). Generally, mono-PEGylated protein can be purified from non-PEGylated protein and unreacted PEG by size-exclusion, ion exchange, affinity, reversed phase, or hydrophobic interaction chromatography. Other purification protocols such as 2-phase organic extraction or salt precipitation can be used. The purified PEGylated protein can be tested in the cell proliferation assay described in Example 1 to determine its specific activity. Experiments can be performed to confirm that the PEG molecule is attached to the protein at the proper site. This can be accomplished by chemical or proteolytic digestion of the protein, purification of the PEGylated peptide (which will have a large molecular weight) by size exclusion, ion exchange or reversed phase chromatography, followed by amino acid sequencing. The PEG- coupled amino acid will appear as a blank in the amino acid sequencing run. Example 12 Preparation and Purification of PEG-T3C A preliminary titration study was performed to determine appropriate reducing agent and PEG reagent concentrations and to avoid over-reduction of the protein. We monitored partial reduction of the protein by conversion of dimer to non-reduced monomer species on non-reduced SDS-PAGE. One μg aliquots of purified T3C dimer were incubated with increasing concentrations of TCEP for 60 minutes at room temperature. The reactions were immediately analyzed by non- reducing SDS-PAGE. The amount of TCEP that yielded significant amounts of monomer T3C without oveπeducing and denaturing the protein was used for subsequent experiments. TCEP is a convenient reducing agent for small scale experiments because it does not interfere with the PEGylation reaction; thus the proteinTCEP mixture did not have to be dialyzed prior to PEG addition. At a larger scale inexpensive reducing agents such as DTT are prefeπed for reducing proteins. Generally, the protein is treated with a reducing agent for an optimal amount of time. The pH of the reaction is then adjusted to 6.5 or below to limit disulfide rearrangements. The reducing agent is removed by dialysis or liquid chromatography. The pH is then readjusted to greater than 6.5 or preferably 7.5 to 8.5 and the PEG reagent is added. The titration experiments at pH 7.5 indicated that a five-fold molar excess of TCEP for 60 minutes at room temperature converted the majority of the T3C dimer species into properly disulfide- bonded monomer without over reducing the protein. Control experiments indicated that, as expected, the T3C dimer needed to be reduced with TCEP to be PEGylated. These reaction conditions were then scaled to 100 μg reaction scale. A 10-fold molar excess of 5 kDa maleimide-PEG (Fluka) was added to the T3CTCEP mixture after 10 minutes and the PEGylation reaction was allowed to proceed for 60 minutes at room temperature. The sample was loaded quickly onto a 1 ml Q-Sepharose column equilibrated in 20 mM Tris-HCl, pH 8.0. The column was washed with 20 mM Tris-HCl, pH 8.0 and bound proteins eluted with a linear 10 volume increasing salt gradient from 0 to 250 mM NaCl in 20 mM Tris-HCl, pH 8.0. Fractions containing mono-PEGylated T3C (a single PEG molecule attached to the T3C monomer) were identified by SDS-PAGE and Western blotting. These fractions were pooled and stored frozen. The presence of the PEG moiety decreases the protein's affinity for the resin, allowing the PEGylated protein to be separated from the non-PEGylated protein. Example 13 PEGylation of S144C and other Cysteine Muteins A preliminary titration study was also performed for S144C to determine appropriate reducing agent, PEG reagent concentrations and to avoid over-reduction of the protein as described in Example 12 for T3C. The larger scale PEGylation was carried out at pH 7.5 at room temperature for 2 h using a 2-fold molar excess of TCEP and a 10-fold molar excess of 5 kDa maleimide- PEG. SDS- PAGE analysis of the reaction mixture showed some species with two or more PEGs present. These were separated from the mono-PEGylated S144C using a Q-Sepharose column as described in Example 12. Separately we performed a PEGylation reaction using 5 kDa vinylsulfone-PEG (Fluka) which resulted in monopegylated S144C under identical reducing conditions. We have also performed small scale PEGylation reactions on T148C, stpl92C and T135C, all of which yielded mono-PEGylated protein with 5 kDa maleimide-PEG and/or 5 kDa vinylsulfone- PEG. Example 14 Bioactivity of the PEG-T3C and PEG-S144C hGH Proteins The biological activity of the PEG-T3C and PEG-S144C proteins were measured in the cell proliferation assay (Example 1). The PEG-T3C protein showed a similar dose-response curve as pituitary hGH and non-PEGylated T3C protein and reached the same level of maximal stimulation. The mean EC 50 value for PEG-T3C was 1.6 ng/ml (0.07 nM) (values of 1.3, 1.5, 1.7, 1.8 ng/ml in four experiments). Bioactivity of the PEG-T3 C protein is at least 100-fold greater than that of rhGH that has been PEGylated using non-specific NHS- PEG reagents (EC 50 of 440 ng/ml (20 nM) as described in Clark et al., (1996). The mean EC 50 value for PEG-S144C was 43 ng/ml (2 nM) (40 and 45 ng/ml in two experiments). Bioactivity of the PEG-S144C protein is approximately 10-fold greater than that of rhGH that has been PEGylated using non-specific NHS- PEG reagents (EC 50 of 440 ng/ml (20 nM); Clark et al., 1996). Example 15 Construction of Disulfide-linked T rimers and Disulfide-linked Higher Order Multimers of hGH Additional hGH vanants having more than one "free" cysteine could be constructed and used to create higher order disulfide-linked multimers of hGH These multimers could include tnmers, tetramers, pentamers, hexamers, septamers, octamers, and any higher order multimers For example, an hGH vanant having two "free" cysteine residues could be constucted by using recombinant DNA technology to recombine in vitro DNA plasmid vectors carrying individual "free" cysteine mutations Alternatively, mutagenesis of an hGH cysteine vaπant could be employed to add an additional "free" cysteine mutation Further iterations of either of these two procedures could be used to construct hGH vanants having three or more "free" cysteines An hGH vanant having two free cysteine residues could be used to generate hGH tnmers and higher order multimers as follows Such a vaπant would be expressed in E colt and recovered as a monomer in the supernatant of an osmotic shock lysate as disclosed in Examples 5 - 9 herein Subsequent processing steps could then be employed to induce di-sulfide bond formation, e g Q Sepharose chromatography as descnbed m Examples 6 - 9 herein Under such conditions some hGH vaπants having one free cysteine, such as T3C and stpl92C, are converted virtually quantitatively to disulfide-linked dimers Under the same or similar conditions intermolecular disulfide formation by an hGH vaπant having two free cysteines, e g a double mutant that combined T3C and stpl92C, would result in a polymeπzation of hGH molecules and the chain length of such polymers would in pnnciple be unlimited The chain length could be limited and to some extent controlled by addition to the polymeπzation reaction of hGH molecules having only one free cysteine such as the T3C vaπant and / or the stpl92C vanant Disulfide bond formation between the growing polymer and a molecule having only one free cysteine will "cap" or prevent further extension of one of the two polymenzation sites in the nascent polymer A subsequent reaction of a second hGH molecule that has only one free cysteine with the other polymeπzation site of that nascent polymer terminates polymenzation and fixes the length of that polymenc molecule The average polymer length could be controlled by the stoichiometry of the reactants, l e the ratio of hGH molecules with two free cysteines to hGH molecules with one free cysteine Average shorter polymers would be favored by lower ratios and average longer polymers would be favored by higher ratios More complex "branched" polymers could be constructed from reactions involving hGH vaπants with 3 or more free cysteines with hGH vanants having only one free cysteine Discrete size classes of certain polymers could subsequently be punfied by chromatographic methods such as size exclusion chromatography, ion exchange chromatography, hydrophobic interaction chromatography, and the like Similar procedures to those descnbed for GH could be used to create disulfide-linked dimers and higher order multimers of EPO and alpha interferon EXAMPLE 16 Cloning, Expression and Purification of Baculovirus (BV)-Expressed Recombinant Human Erythropoietin (rEPO) A. Cloning a cDNA encoding EPO. A cDNA encoding human EPO was cloned by PCR using forward pnmer BB45 (5> CCCGGAT CCATGGGGGTGCACGAATGTCCTG >3)(SEQ D NO 25)and reverse pnmer BB47 (5> CCCGA ATTCTATGCCCAGGTGGACACACCTG >3)(SEQ D NO 26) BB45 anneals to the DNA sequence encoding the initiator methionine and amino terminal portion of the EPO signal sequence and contains a Bam HI site for clomng purposes (Jacobs et al , 1985, Lin et al , 1985) BB47 anneals to the 3' untranslated region of the EPO mRNA immediately downstream of the tianslational stop signal and contains an Eco RI restπction site for clomng purposes Total RNA isolated from the human liver cell line Hep3B was used in first strand synthesis of single-stranded cDNA for PCR For preparation of total cellular RNA, Hep3B cells (Amencan Type Culture Collection) were grown in Delbecco's Modified Eagle's media (DMEM) supplemented with 10% fetal bovine serum (FBS) EPO expression was induced by treating the cells for 18 h with 130 μM Deferoxamme or 100 μM cobalt chlonde Both compounds have been shown to induce EPO mRNA and protein expression in Hep 3B cells (Wang and Semenza, 1993) RNA was isolated from the cells using an RNeasy Mini kit (Qiagen, Inc ), following the manufacturer's directions Approximately 320 μg of total RNA was isolated from 1 4 xlO 7 cells treated with cobalt chlonde and 270 μg of total RNA isolated from 1 4 xlO 7 cells treated with Deferoxamme First strand synthesis of single-stranded cDNA was accomplished using a 1st Strand cDNA Synthesis Kit for RT-PCR (AMV) from Boehnnger Mannheim Corp and random hexamers were used as the pnmer Subsequent PCR reactions using the products of the first strand syntheses as templates were earned out with pnmers BB45 and BB47 The expected - 600 bp PCR product was observed when reaction products were run out on an agarose gel Both RNA preparations yielded an EPO PCR product The PCR product was digested with Bam HI and Eco RI and cloned into vector pUC19 that had been cut with Bam HI and Eco RI and treated with alkaline phosphatase DNA sequencing identified a clone contaimng the coπect coding sequence for the EPO gene This plasmid was designated pBBT131 and used in the construction of EPO vaπants by site directed mutagenesis as descnbed in Example 17 B. Expression of BV rEPO in Insect Cells For expression in insect cells the EPO cDNA in pBBT131 was modified at both the 5' and 3' ends At the 5 ' end, the sequence CAAA was added immediately upstream of the initiator ATG to enhance translation This sequence compnses a proposed consensus translational initiation sequence for baculovirus (Ayres et al , 1994, Ranjan and Hasnain, 1995) At the 3 ' end, DNA encoding the 8 ammo acid FLAG epitope sequence (asp-tyr-lys-asp-asp-asp-asp-lys)(SEQ ID NO 27) was added to provide a purification system The FLAG epitope was fused to the EPO gene via a flexible linker ser- gly-gly-ser-gly-gly-ser (SEQ ID NO 28) These modifications were made via PCR using oligonucleotide pnmers that incorporated the desired additions to the EPO sequence Oligonucleotide pnmer BB63 (5> CGCGGATCCAAAATGGGGGTGCAC GAATGTCCT>3)(SEQ ID NO 29) was used to modify' the 5 end of the gene BB63 adds the CAAA sequence upstream of the ATG, anneals to the DNA sequence encoding the initiator methionine and amino terminal portion of the EPO signal sequence, and contains a Bam HI site for clomng purposes The linker and FLAG sequences were added in two sequential PCR reactions using reverse pnmers BB60 (5> GTCTETGTAGTCCGAGCCTCCGCTTCCGCCCGATCT GTCC CCTGTCCTGCA >3)(SEQ ED NO 30) and BB61 (5> CGCGAATTCTTATTTATCGTCATCGTCTTTGTAGT CCGAGCCTCC >3)(SEQ ED NO 31) BB60 anneals to 3' of the EPO coding sequence and contains the fused peptide linker sequence and a portion of the FLAG sequence BB61 overlaps a segment of the BB60 sequence annealing to the junction of the linker-FLAG sequence and adds the remainder of the FLAG sequence followed by a translational stop codon (TAA) and an Eco RI site for clomng purposes The modified EPO cDNA was cloned as a Bam HI - Eco RI fragment into pUC19 and the DNA sequence of this construct was confirmed The resulting plasmid was designated pBBT132 and used in the construction of EPO vaπants as descnbed in Example 17 For expression in baculovirus, the "FLAG-tagged ' EPO cDNA was excised from pBBT132 as - 630 bp Bam HI - Eco RI fragment, gel punfied, and cloned into the baculovirus transfer vector pBlueBac4 5 (Invitrogen) which had been cut with Bam HI and Eco RI and treated with alkaline phosphatase One clone was picked for further use and designated pBBT138 pBBT138 DNA was used to cotransfect Spodoptera frugtperda deπved cell line Sf 9 along with hneanzed (Bsu36 I digested) Bac-N-Blue™ (Invitrogen Corporation) baculovirus DNA The Bac-N-Blue™ genome is engineered so that formation of plaque-forming viral particles requires recombination between the linear Bac-N-Blue™ DNA and the pBlueBac4 5 vector, resulting in the incorporation of the cloned EPO gene into the baculovirus genome This obligate recombination also results in incorporation of a functional β-galactosidase gene into the baculovirus genome The co- transfection was performed according to the Invitrogen "Bac-N-Blue™ Transfection Kit" protocols using 2 x 10 6 Sf 9 cells to generate a - 1 ml supernatant Dilutions of this supernatant were assayed on Sf 9 cells at 27° C for blue-plaque formation Ten blue plaques were picked and subcultured Each plaque was used to inoculate 2 5 x 10 6 Sf 9 cells in a T25 flask contaimng 5 ml of Grace's Insect Media supplemented with 10% FBS After 5 days the supernatants from these infected cells (the "PI" stocks) were collected and assayed by Western Blot for EPO expression The ten resultant supernatants were prepared m SDS sample buffer with the addition of 1% β mercaptoethanol (BME) and electrophoresed on precast 14% Tns-glycine polyacrylamide gels (Novex) Uninfected SF9 cell supernatant was included as a negative control Following electrophoresis, the proteins were transfeπed onto 0 45 μm nitrocellulose (Novex) The nitrocellulose membrane was blocked in Tns Buffered Saline (TBS) with 005% Tween 20 and 4% powered milk (blocking buffer) Anti-FLAG M2 mouse IgG, monoclonal antibody (Eastman KODAK) was used at 1 1500 or 1 2500 dilution in blocking buffer and the blot routinely incubated overmght at 4°C Alkaline phosphatase conjugated Sheep Anti-Mouse IgGi (The Binding Site Limited) was diluted 1 1000 in blocking buffer and the blot incubated for lhour at room temperature The Western blot was developed using NBTλBCIP color development substrate (Promega) Nine of the ten isolates were positive for EPO expression The molecular weight of the BV rEPO protein was approximately 30 kDa under reducing conditions and consisted on one major and one to two minor bands in this molecular weight range, which is consistent with a vanably glycosylated protein Two of the positive supernatants were tested in the m vitro EPO bioassay descnbed in Example 16 D below Both supernatants stimulated proliferation of the EPO-dependent cell line in a dose-dependent manner, indicating that they contained active EPO Control supernatants of mock infected Sf 9 cells and the one baculovirus supernatant that was negative for EPO expression by Western blot showed no detectable activity in the EPO bioassay In order to produce and purify larger amounts of wild type B V rEPO, one positive recombinant baculovirus, termed bvBBT138A, was chosen for further amplification A 500 ml high titer viral stock was prepared by subculruπng the PI stock of isolate 138A at 27° C in a 500 ml spinner flask culture of Sf 9 cells in Grace's Insect Media supplemented with 10% FBS Grace's Insect Media contains approximately 100 μM L-cystine The supernatant from this culture was harvested after 7 days and found to have a titer of - 10 8 plaque-formmg-units / ml An aliquot of this lysate, termed the "P2" stock, was subsequently used to infect a 500 ml culture for larger scale production of wild type BV rEPO A 500 ml culture of Sf 9 cells in Grace's Insect Media supplemented with 10% FBS was grown in a spinner flask to a titer of 1 0 x 10 6 / ml and then infected with bvBBT138A at a multiplicity of infection of 1 After 3 days the supernatant from this culture was harvested and wild type BV rEPO-138A protein punfied as descnbed in Example 16 C C. Affinity Purification of Wild Type Baculovirus-Produced rEPO The cell supernatant was clarified by centπfugation and 0 2μM filtration Expression of wild type BV rEPO was confirmed by Western blot analysis Wild type BV rEPO was punfied in a single step procedure using Anti-FLAG M2 Affinity Gel (Eastman KODAK) Bnefly, 5ml of the M2 affinity gel was washed with 5 column volumes of 50mM Tπs pH 7 4, 150mM NaCl (TBS), 5 column volumes of 0 1M glycine pH 3 5, then eqmlibrated in TBS The clarified baculoviral cell supernatant was adjusted to 150mM NaCl and the eqmlibrated resin was added Batch loading was allowed to continue at 4°C overmght on a roller bottle apparatus After overmght incubation, the resin was recovered using a Pharmacia XK 16/20 FPLC column and washed with TBS until the A280 reached baseline The bound protem was eluted with 0 1M glycme pH 3 5 and fractions were collected and neutralized with 1 0M Tns pH 9 0 Column fractions were prepared in SDS-PAGE sample buffer with the addition of 1% BME when desirable and electrophoresed on precast 14% Tπs-glycme polyacrylamide gels Fractions from the M2 affinity column that contained most of the BV rEPO were pooled and concentrated on a Centncon 10 spin concentrator (Amicon) The final yield of wild type BV rEPO, as determined using a Bradford protein assay kit (BIO-RAD Laboratoπes, Inc ) and bovine serum albumin (BSA) as the standard, was approximately 360μg The protem was estimated to be greater than 90% pure by Coomassie Blue staining of SDS gels D. In Vitro Bioactivity of Wild Type Baculovirus rEPO A cell proliferation assav using the human UT7/epo cell line (Komatsu et al , 1991) was developed to measure bioactivity of wild type B V rEPO The human UT7/epo cell line was obtained from Dr F Bunn of Harvard Medical School This cell line is dependent upon EPO for cell proliferation and survival (Boissel et al , 1993) The cells were maintained in Iscove's Modified Delbecco's Media (EMDM) supplemented with 10% FBS, 50 units/ml penicillin, 50 μg/ml streptomycin and 1 umt/ml rEPO (CHO (Chinese Hamster Ovary) cell-expressed, purchased from R&D Systems, Inc ) For bioassays, the cells were washed three times with EMDM media and resuspended at a concentration of lxlO 5 cells/ml in EMDM media contaimng 10% FBS, 50 units/ml penicillin and 50 μg/ml streptomycin Fifty μl (5xl0 3 cells) of the cell suspension were aliquotted per test well of a flat bottom 96 well tissue culture plate Senal 3 -fold dilutions of the protein samples to be tested were prepared in phenol red-free EMDM media contaimng 10% FBS, 50 units/ml penicillin and 50 μg/ml streptomycin Fifty μl of the diluted protein samples were added to the test wells and the plates incubated at 37°C in a humidified 5% C0 2 tissue culture incubator Protein samples were assayed in tnplicate wells After 60-72 h, 20 μl of CellTiter 96 AQueous One Solution Reagent (Promega Corporation) was added to each well and the plates incubated at 37°C in the tissue culture incubator for 1-4 h Absorbance of the wells was read at 490 nm using a πucroplate reader Control wells contained media but no cells Mean absorbance values for the tnplicate control wells were subtracted from mean values obtained for each set of tnplicate test wells Senal dilutions of CHO cell-expressed rEPO or wild type BV rEPO were analyzed in parallel The UT7/epo cell line shows a strong proliferative response to rEPO, as evidenced by a dose- dependent increase in cell number and absorbance values Absorbance is proportional to cell number up to values of 2 0 (Promega Corporation product specifications) In the absence of rEPO, the majonty of UT7/epo cells die, giving absorbance values less than 0 1 Commercial CHO cell- expressed rEPO and wild type BV rEPO prepared by us reached the same maximal level of stimulation, withm the eπor of the assay, and had similar mean EC 50 s in the bioassay of approximately 0 4-0 5 ng/ml (Table 2) EC 50 values for these proteins ranged from 0 21 to 0 65 ng/ml in assays performed on different days (Table 2), therefore compaπsons between protem samples were made on samples analyzed on the same day These EC 50 values agree with values reported in the R&D Systems specifications for CHO rEPO (0 05 - 0 1 umt/ml or approximately 0 4-0 8 ng/ml) EXAMPLE 17 Construction, Expression, Purification and Bioactivity of EPO Cysteine Muteins A. Construction of EPO Cysteine Muteins. Eight mutant EPO genes were constructed using site-directed PCR-based mutagenesis procedures similar to those used to construct the Growth Hormone muteins descnbed in Example 4 (Innis et al ,1990, Horton et al , 1993) We constructed four muteins at or near the two N-hnked glycosylation sites in the A-B loop (N24C, T26C, N38C and T40C), two muteins at or near the N-hnked glycosylation site in the B-C loop (N83C and S85C), one mutein at the O-lmked glycosylation site in the C-D loop (S126C) and one mutein (*167C), that adds a cysteine between the natural carboxyterminal R166 residue and the 15 ammo acid carboxyterminal extension consisting of the peptide linker and FLAG sequences The template for the mutagemc PCR reactions was plasmid pBBT131, in which the unmodified EPO gene is cloned as an Bam HE - Eco RI fragment into pUC19 PCR products were digested with appropπate restnction endonucleases, gel-puπfied and ligated with pBBT132 vector DNA that had been cut with appropnate restnction enzymes, alkaline phosphatase treated, and gel-punfied As detailed above, pBBT132 is a pUC19 denvative carrying the cloned modified (FLAG tagged) EPO gene Transformants from these hgations were grown up and plasmid DNAs isolated and sequenced The sequence of the entire cloned mutagenized PCR fragment was determined to verify the presence of the mutation of interest, and the absence of any additional mutations that potentially could be introduced by the PCR reaction or by the synthetic oligonucleotide pnmers The cvsteine substitution mutation N24C was constructed as follows The mutagemc forward oligonucleotide BB64 (5> GAGGCCAAGGAGGCCGAGTGTATCACGACGGGCTGTGCT >3)(SEQ ID NO 32) was designed to change the codon AAT for asparagine at position 24 to a TGT encoding cysteine and to span the nearby Stv I site This oligo was used in PCR with the reverse, non- mutagenic, pnmer BB47 (5> CCCGAATTCTGGTGGATATGCCCAGGTGGAC >3)(SEQ ID NO 33) which anneals to DNA sequences 3 ' to the coding sequence of EPO A 50 μl PCR reaction was performed m IX Promega PCR buffer contaimng 1 5 mM MgCl 2 , each pnmer at 0 2μM, each of dATP, dGTP, dTTP and dCTP at 200 μM, 1 ng of template plasmid pBBT131 (descnbed in Example 16). 1 5 umts of Taq Polymerase (Promega), and 0 25 umts of Pfu Polymerase (Stratagene) Reactions were performed in a Robocycler Gradient 96 thermal cycler (Stratagene) The reaction program entailed 96°C for 3 minutes, 25 cycles of [95°C for 1 minute, 60°C for 1 25 minutes, 72°C for 1 minute] followed by a hold at 6°C A 5 μl aliquot of the PCR reaction was analyzed by agarose gel electrophoresis and found to produce a single fragment of the expected size - 470 bp The remainder of the reaction was "cleaned up" using the QIAquick PCR Purification (Qiagen) according to the vendor protocol, digested with Sty I and Bsr GI (New England BioLabs) according to the vendor protocols, ethanol-precipitated, resuspended m 20 μl of 10 mM Tns-HCl pH 8 5 and run out on a 2% agarose gel The -400 bp Stv I - Bsr GI fragment of interest was gel punfied using a QIAEX II Gel Extraction Kit (Qiagen) according to the vendor protocol This fragment was ligated with pBBT132 (descnbed in Example 16) that had been cut with Sty I and Bsr GI, treated with calf intestinal alkaline phosphatase (New England BioLabs) and gel punfied The ligation reaction was used to transform E colt and plasmids from resulting transformants were sequenced to identify a clone contaimng the N24C mutation and having the coπect sequence throughout the remainder of the -400 bp Stv I - Bsr GI segment The substitution mutation T26C was constructed and its sequence verified using the protocol descnbed above for N24C except that mutagemc forward oligo BB65 (5> GAGGCCAAGGA GGCCGAGAAATCTGTACGGGCTGTGCT >3)(SEQ ID NO 34) was used instead of BB64 The substitution mutation N38C was constructed using the technique of "mutagenesis by overlap extension" as descnbed in Example 4 With the exception of the use of different oligonucleotide pnmers, the imtial, or "pnmary" PCR reactions for the N38C construction were performed identically to those descnbed in the construction of N24C above The pnmer pairs used were (BB66 + BB47) and (BB67 + BB45) BB47 is descnbed above The forward, non-mutagemc, pnmer BB45 (5> CCCGGATCCATGGGGGTGCACGAATGTCCTG >3)(SEQ ED NO 35) anneals to the EPO sequence encoding the first seven amino acids of EPO BB66 and BB67 are complementary mutagemc oligonucleotides that change the AAT codon for N38 to a TGT codon for cysteine The sequence of BB66 is (5> AGCTTGAATGAGTGTATCACTGTCCCAGACACC >3)(SEQ D NO 36) and the sequence of BB67 is (5> GGTGTCTGGGACAGTGATACACTCATT CAAGCT >3)(SEQ ID NO 37) The (BB66 x BB47) and (BB67 x BB45) PCR reactions gave products of the expected sizes -420 bp for (BB66 x BB47) and -220 bp for (BB67 x BB45) The PCR products were ethanol - precipitated, gel-puπfied and recovered in 20 μl 10 mM Tns-HCl as detailed above These two mutagemzed fragments were then "spliced" together in the subsequent, or "secondary" PCR reaction In this reaction lμl of each of the gel-puπfied PCR products of the pnmary reactions were used as template and BB45 and BB47 were used as pnmers Otherwise, the reaction conditions identical to those used in the pnmary reactions An aliquot of the secondary PCR was analyzed by agarose gel electrophoresis and the expected band of -630 bp was obseπ'ed The bulk of the secondary PCR reaction was "cleaned up" using the QIAquick PCR Purification (Qiagen) according to the vendor protocol, digested with Stv I and Bsr GI (New England BioLabs) according to the vendor protocols ethanol-precipitated, resuspended m 20 μl of 10 mM Tns-HCl pH 8 5 and run out on a 2% agarose gel The -400 bp Stv I - Bsr GI fragment of interest was gel punfied using a QIAEX II Gel Extraction Kit (Qiagen) according to the vendor protocol This fragment was ligated with pBBT132 (descnbed in Example 16) that had been cut with Sty I and Bsr GI, treated with calf intestinal alkaline phosphatase (New England BioLabs) and gel punfied The ligation reaction was used to transform E colt and plasmids from resulting transformants were sequenced to identify a clone contaimng the N38C mutation and having the conect sequence throughout the -400 bp Stv I - Bsr GI segment The substitution mutation T40C was constructed and sequence verified using the procedures descnbed above for N38C except that complementary mutagemc pnmers BB68 (5> AGCTTG AATGAGAATATCTG TGTCCCAGACACC >3)(SEQ ID NO 38) and BB69 (5> GGTGTCTGGGACACAGATATTCTC ATTCAAGCT >3)(SEQ ID NO 39), which change the ACT codon for T40 to a TGT codon for cysteine, replaced BB66 and BB67 respectively in the pπmarv PCR reactions The substitution mutation N83C was constructed and sequence verified using the procedures descnbed above for N38C except that complementary mutagemc pnmers BB70 (5> GCCCTGT TGGTCTGCTCTTCCCAGCCGTGGGAGCCCCTG >3)(SEQ ED NO 40) and BB71 (5> CAGGGGCTCCCACGGCTGG GAAGAGCAGACCA ACAGGGC >3)(SEQ ED NO 41). which change the AAC codon for N83 to a TGC codon for cysteine, replaced BB66 and BB67 respectively, in the pπmarv PCR reactions The sizes of the products of the pnmary PCR reactions were also different The (BB70 x BB47) reaction gave, as predicted, a product of -300 bp and the (BB71 x BB45) reaction gave, as predicted, a product of -360 bp The substitution mutation S85C was constructed and sequence verified using the procedures descnbed above for N38C except that complementary mutagemc pnmers pnmers BB72 (5> GCC CTGTTGGTCAAC TCTTGCCAGCCGTGGGAGCCCCTG >3)(SEQ ID NO 42) and BB73 (5> CAGGGGCTCCCACG GCTGGCAAGAGTTGACCAACAGGGC >3)(SEQ ID NO 43), which change the TCC codon for S85 to a TGC codon for cysteine, replaced BB66 and BB67 respectively m the pnmary PCR reactions The sizes of the products of the pnmary PCR reactions were also different The (BB72 \ BB47) reaction gave, as predicted, a product of -300 bp and the (BB73 \ BB45) reaction gave, as predicted, a product of -360 bp The substitution mutation S126C was constructed using the procedures descnbed above for N38C except that complementary mutagemc pnmers BB74 (5> CCAGATGCGGCCTGTGCTGC TCCACTC >3)(SEQ ID NO 44) and BB75 (5> GAGTGGAGCAGCACAGGCCGCATCTGG >3)(SEQ ID NO 45), which change the TCA codon for S126 to a TGT codon for cysteine, replaced BB66 and BB67 respectively in the pnmary PCR reactions The sizes of the products of the pnmary PCR reactions were also different The (BB74 x BB47) reaction gave, as predicted, a product of- 175 bp and the (BB75 x BB45) reaction gave, as predicted, a product of- 480 bp A mutation was also constructed that added a cysteine following the carboxyterminal amino acid of the EPO coding sequence This mutant, termed * 167C was constructed as follows A PCR reaction was earned out under the conditions descnbed above for the construction of the N24C mutant, but employing oligonucleotides BB45 (see above) and reverse mutagemc oligonucleotide BB77 (5> TTTGTAGTCCGAG CCTCCGCTTCCGCCCGAACA TCTGTCCCCTGTCCTGCA>3)(SEQ ID NO 46) and using 2 5 umts of Taq Polymerase and 0 5 umts of Pfu Polymerase BB77 anneals to the terminal 21 residues of EPO coding sequence and adds a TGT codon for cysteine following the AGA codon for R166, which is the terminal ammo acid in the EPO coding sequence BB77 also adds sequences encoding the seven amino acid linker -ser-gly-gly- ser-gly-gly-ser- (SEQ ID NO 27), and a portion of the FLAG epitope sequence The -630 bp product of this PCR reaction was gel punfied and used as template in a subsequent PCR reaction employing the same reaction conditions but using pnmers BB47 and BB61 (5> CGCGAATTCTTATTTATCGTCATCGTCTTTGTAGTCCGAGCC TCC >3)(SEQ ID NO 30), which adds the remainder of the FLAG epitope sequence followed by a TAA stop codon and an Eco RI clomng site The -675 bp product of this PCR reaction was "cleaned up" using the QIAquick PCR Purification (Qiagen) according to the vendor protocol, digested with Stv I and Eco RI (New England BioLabs) according to the vendor protocols, ethanol-precipitated, resuspended in 20 μl of 10 mM Tns-HCl pH 8 5 and run out on a 2% agarose gel The -86 bp Eco RI - Bsr GI fragment of interest was gel punfied using a QIAEX II Gel Extraction Kit (Qiagen) according to the vendor protocol This fragment was ligated with pBBT132 (descnbed in Example 16) that had been cut with Eco RI and Bsr GI, treated with calf intestinal alkaline phosphatase (New England BioLabs) and gel punfied The ligation reaction was used to transform E colt and plasmids from resulting transformants were sequenced to identify a clone contaimng the * 167C mutation and having the coπect sequence throughout the -86 bp Eco RI - Bsr GI segment For expression in baculovirus, the EPO genes encoding the 8 muteins were excised from the pUC19-based pBBT132 denvatives as Bam H I - Eco RI fragments of- 630 bp and subcloned into the pBlueBac4 5 baculovirus transfer vector used to express wild type EPO Using procedures similar to those descnbed here, one can construct other cysteine muteins of EPO The cysteine muteins can be substitution mutations that substitute cysteine for a natural amino residue in the EPO coding sequence, insertion mutations that insert a cysteine residue between two naturally occurπng ammo acids in the EPO coding sequence, or addition mutations that add a cysteine residue preceding the first ammo acid, A 1, of the EPO coding sequence or add a cysteine residue following the terminal amino acid residue, R166, of the EPO coding sequence. The cysteine residues can be substituted for any ammo acid, or inserted between any two amino acids, anywhere in the EPO coding sequence Prefeπed sites for substituting or inserting cysteine residues m EPO are in the region preceding Helix A, the A-B loop, the B-C loop, the C-D loop and the region distal to Helix D Other prefeπed sites are the first or last three amino acids of the A. B, C and D Helices Prefeπed residues in these regions for creating cysteine substitutions are Al, P2, P3, R4, L5, D8, S9, 125, T27, G28, A30, E31, H32, S34, N36, 139, D43, T44, K45, N47, Y49, A50, K52, R53, M54, E55, G57, Q58, G77, Q78, A79, S84, Q86, W88, E89, T107, R110, Alll, G113, A114, Q115, K116, El 17, A118, S120, P121, P122, D123, A124, A125, A127, A128, R131, T132, K154, Y156, T157, G158, E159, A160, T163, G164. D165, R166 Cysteine residues also can be inserted immediately preceding or following these ammo acids Another prefeπed site for adding a cysteine residue would be preceding Al, which we refer to as *-lC One also can construct EPO muteins contaimng a free cysteine by substituting another amino acid for one of the naturally-occurnng cysteine residues in EPO The naturally-occurπng cysteine residue that normally forms a disulfide bond with the substituted cysteine residue is now free The non-essential cysteine residue can be replaced with any of the other 19 amino acids, but preferably with a senne or alamne residue A free cysteine residue also can be introduced into EPO by chemical modification of a naturally occurring amino acid using procedures such as those descnbed by Sytkowski et al (1998) Using procedures similar to those descnbed in Examples 16 - 19, one can express the proteins in eukaryotic cells (e g , insect cells or mammalian cells), punfy the proteins, PEGylate the proteins and measure their bioactivities in an in vitro bioassay The EPO muteins also can be expressed in prokaryotic cells such as E colt using procedures similar to those descnbed in Examples 1-15, or related procedures well known to those skilled m the art B. Insect Cell Expression of EPO-Cys Muteins. For expression expenments, the eight plasmids encoding mutant EPO genes were used to cotransfect Sf 9 cells along with hneaπzed Bac-N-Blue™ DNA using the procedures descnbed above for plasmid pBBT138, which encodes wild type EPO The transfection supernatants were assayed on Sf 9 cells for blue-plaque formation For each mutant, ten blue plaques were picked and subcultured as descnbed in Example 16 Five of the ten resulting supernatants were screened by SDS-PAGE and Western blot to detect expression of the EPO muteins Western blot analysis was earned out using the procedure descnbed in Example 16 for wild type EPO Western results showed that at least 3 of the 5 supernatants screened from each clone contained FLAG-tagged EPO protein Western blot results also revealed that plasmids encoding muteins that should prevent glycosylation at one of the N-hnked glycosylation sites (N24C, T26C, N38C, T40C, N83C and S85C) yielded proteins with molecular weights approximately 2,200-2,800 daltons smaller than wild type BV rEPO In contrast, the S126 mutein at the O-linked glycosylation site, and * 167C (C-terπunal cysteine addition) yielded EPO proteins that co-migrated with wild type BV rEPO These results are consistent with the observation that insect cells typically perform N-hnked glycosylation and that sugar groups attached to O-linked glycosylation sites are generally small and cause minimal increases in a protein's molecular weight Insect cells are reported to perform O-linked glycosylation (Davies, 1995) In order to punfy larger amounts of the EPO muteins one positive recombinant baculovirus isolate that encoded each mutein was chosen for further amplification A 500 ml high titer viral stock was prepared from each of the mutant PI stocks by subcultuπng as descnbed above for the wild type isolate An aliquot of each of these P2 lysates was subsequently used to infect a 500 ml culture for larger scale production of each of the EPO muteins using the procedures descnbed for wild type EPO in Example 16 The 500 ml supernatants from baculovirus infected cells were subsequently stenle filtered and stored at 4°C rEPO-Cys muteins were punfied using an anti-FLAG M2 Affinity Gel column (Eastman Kodak), using a different batch of resin for each mutein to ensure no cross- contamination 5 ml of resin was first washed with 50 mM Tπs, 150 mM NaCl, pH 7 4 (TBS), followed by 0 1 M Glycine pH 3 0, and finally re-equilibrated in TBS Sodium chlonde was added to the baculovirus culture filtrate to a final concentration of 0 15 M before the affinity resin was added The resin was batch-loaded on a roller bottle apparatus overmght at 4°C The next morning the resin was captured using an XK16 Pharmacia column and washed with TBS until the A280 reached baseline The rEPO-Cys muteins were eluted with 0 1 M Glycine, pH 3 0 Fractions (1 5 ml) were collected into tubes contaimng 20 μL of 1 M Tπs Base, pH 9 0 to neutralize the solution Based on the chromatograms and SDS-PAGE analyses, fractions contaimng predominantly pure rEPO-Cys proteins were pooled and frozen Protein recovenes from the 500 ml supernatant cultures vaπed from 75 to 800 μg, depending upon the mutein (Table 2) The punfied rEPO-Cys muteins migrated with apparent molecular weights of 27-30 kDa under non-reducing conditions, consistent with the proteins being monomenc The apparent molecular weights of the rEPO-Cys muteins fell into two classes, presumably due to glycosylation differences rEPO-Cys proteins contaimng mutations at the O-glycosylation site (S126C) and at the C-terminus (* 167C) migrated at a position similar to wild type BV rEPO Proteins contaimng mutations that should prevent glycosylation of one of the N-hnked glycosylation sites (N24C. T26C, N38C, T40C, N83C and S85C proteins) migrated with apparent molecular weights of 27-28 kDa, slightly smaller than wild type BV rEPO Surpπsingly, the S85C protein migrated as a doublet under both reducing and non-reducing conditions, the major band of the doublet was the expected size for a protein lacking glycosylation at one of the N-hnked glycosylation sites, while the minor band was the size expected for fully glycosylated wild-type B V rEPO All of the oπginal recombinant Baculovirus plaques isolated for the S85C mutem yielded rEPO proteins that migrated as doublets, suggesting that the doublet is due to partial glycosylation at N83 (or another ammo acid) rather than contamination of S85C with wild type B V EPO or S 126C or * 167C proteins Several of the muteins, e g , S 126C, *167C, had small amounts of a second protein that migrated with an apparent molecular weight of 45-55 kDa, depending upon the mutem This is the molecular weight expected for disulfide-linked rEPO dimers These bands reacted with the anti-FLAG antibody m Western blots and were absent when the SDS gels were run under reducing conditions These data indicate that the 45-55 kDa bands represent disulfide-linked rEPO-Cys dimers C. Bioactivities of EPO Cysteine Muteins. Biological activities of the punfied rEPO- Cys muteins were measured in the UT7/epo cell proliferation assay descnbed in Example 16 Protein concentrations were determined using a Bradford assay Because of the large number of samples the muteins were divided into two groups for analysis muteins T26C, T40C, N83C and S126C compnsed one group and muteins N24C, N38C, S85C and * 167C compnsed the second group Muteins within a group were analyzed on the same day Wild type BV rEPO and CHO rEPO were analyzed in parallel on the same days to control for mterday vanabil lty in the assays All of the muteins stimulated proliferation of UT7/epo cells and had EC 50 s that were within three-fold of the wild type rEPO control proteins Mean EC 50 s for the N24C, T26C, N83C, S85C. S126 and *167C muteins were similar to the mean EC 50 s for wild type BV rEPO and CHO rEPO. averaging 0 3-0 8 ng/ml (Table 2) Mean EC 50 s for the N38C and T40C proteins were approximately 1 ng/ml (Table 2) Bill et al , (1995) reported expression of the EPO cysteine muteins. N24C, N38C and N83C, in E coli In contrast to our results, they reported that bioactivities of these cysteine muteins were significantly reduced relative to wild type EPO Bioactivity of the N38C mutein was reduced to less than 20% of wild type EPO bioactivity and bioactivities of the N24C and N83C muteins were reduced to less than 5% of wild type EPO bioactivity EC 50 s were not reported Bill et al (1995) postulated that the reduced activities of the cysteine muteins were due to improper folding due to incoπect formation of disulfide bπdges resulting from the extra cysteine residues introduced into the proteins They did not employ cystine or other cysteme-blocking agents m the solutions used to punfy the cysteine muteins Based upon the results of Bill et al (1995), it was surpπsing that our punfied N24C, N38C and N83C muteins had biological activities equal to, or within 3 -fold of, wild type EPO The data indicate that our methods for expression and purification of the EPO cysteine muteins results in proteins with supenor in vitro biological activities compared to the methods employed by Bill et al (1995) 1 Data from individual expenments. A range is shown when N > 3. EXAMPLE 18 PEGylation of EPO Cysteine Muteins A. Small-Scale PEGylation of EPO Cysteine Muteins Several cysteine muteins and wild type EPO were tested for their ability to be PEGylated with a 5 kDa cysteine-reactive PEG following treatment with the reducing agent TCEP (Tris (2- carboxyethyl) phosphine-HCl). A titration study was performed with the T26C mutein to identity appropπate TCEP and PEG reagent concentrations for PEGylation, while avoiding over-reduction of the protein. We momtored partial reduction of the protein by SDS-PAGE, using wild type BV rEPO as a control. Increasing concentrations of TCEP (0.5 M to 6.0 M excess) in the presence of a 10-40- fold molar excess of either vinylsulfone or maleimide 5kDa PEG (Fluka) were tested. The reactions were analyzed by non-reducing SDS-PAGE Wild type BV rEPO was treated m parallel as a control to identify partial reduction conditions that yielded significant amounts of monoPEGylated EPO-Cys muteins (a single PEG molecule attached to the EPO-Cys mutein), but no PEGylation of wild type BV rEPO. A 5-fold molar excess of TCEP and a 30-fold molar excess of 5 kDa vinylsulfone-PEG yielded significant amounts of monoPEGylated T26C protem without PEGylation of wild type B V rEPO Based on our findings with the T26C mutein, the following conditions were used to PEGylate several other cysteine muteins. Aliquots (1-2 μg) of punfied BV rEPO and EPO-Cys muteins were incubated for 1 hr with a 5X molar excess of TCEP and a 3 OX molar excess of 5 kDa vinylsulfone PEG at pH 8 0 at room temperature. The reactions were stopped by dilution into SDS sample buffer (without reducing reagent) and analyzed by SDS-PAGE. Four cysteine muteins (N24C, T26C, S126C and *167C) were readily monoPEGylated under these conditions, as evidenced by the appearance of a new protein band migrating at approximately 35 kDa. The 35 kDa species is the size expected for mono-PEGylated EPO; no diPEGylated species, which are expected to have molecular weights of approximately 40 kDa, were detected for any of the EPO-cys muteins under these reaction conditions. Control expenments indicated that the cysteine muteins needed to be reduced with TCEP in order to be PEGylated. Wild type BV EPO did not PEGylate under identical partial reducing conditions. These data indicate that the PEG molecule is attached to the free cysteine introduced into the N24C, T26C, S126C and *167C EPO-Cys muteins B. Preparation of PEG-T26C and PEG-S 126C for Bioactivity Measurements We PEGylated larger quantities of the T26C and S 126C proteins so that the PEGylated proteins could be punfied for bioactivity measurements PEGylation reactions were scaled to include 75 μg of each protein and the same molar ratios of TCEP and 5kDa-PEG used in the smaller 1 μg reactions After 1 hour, the PEGylation mixture was diluted 10X with 20 mM Tns-HCl, pH 8 0, 20% glycerol (Buffer A) and loaded immediately onto a 1 ml Q-Sepharose column eqmlibrated in Buffer A The column was washed with eqmlibration buffer and bound proteins eluted with a linear 10 volume increasing salt gradient from 0 to 150 mM NaCl m 20 mM Tns-HCl, pH 8 0, 20% glycerol The presence of the PEG moiety decreased the protein's affinity for the resin, allowing the PEGylated protein to be separated from the non-PEGylated protein Fractions contaimng monoPEGylated protein were identified bv SDS-PAGE, followed by Coomassie Blue staining Fractions contaimng PEG-T26C but no visible undenvatized protein, were stored frozen at -80°C and subsequently used in bioassays Similar procedures were used to obtain punfied PEG-S126C A Western blot was performed to assess the punty of the PEG-T26C and PEG-S 126C preparations used in bioassays The Western blot was performed as descnbed in Example 16. The Western blot gave strong signals at the sizes expected for PEG-T26C and PEG-S 126C, but failed to detect any protein migrating at the positions expected for the unPEGylated S 126C and T26C proteins From these results, we conclude there is little (<5 %) unPEGylated protein present in the punfied PEGylated muteins C. Bioactivities of PEG-T26C and PEG-S126C Cysteine Muteins The biological activities of the punfied PEG-T26C and PEG-S 126C proteins were measured in the UT7/epo cell proliferation assay descnbed in Example 16 Protem concentrations of the PEG- EPO-Cys muteins were quanutated using a human EPO ELISA kit (R & D Systems), following the manufacturer's suggested directions Protein concentrations of the non-PEGylated muteins and wild type BV rEPO also were quantitated by ELISA The ELISA assay was performed on all the proteins on the same day Senal three-fold dilutions of the protein samples were prepared and analyzed in the bioassay Unused mateπal from each senal dilution was stored frozen at -80°C and later analyzed the ELISA to determine an accurate protein concentration for the starting mateπal Several of the senal dilutions for each protein sample were analyzed to ensure that the protein concentration in at least one test sample fell within the linear range of the standard ELISA curve, which is 0 0025 - 0 2 Umts/ml or approximately 0 02 - 1 6 ng/ml At least two of the senal dilutions for each sample fell withm this linear range Biological activities for the PEG-T26C and PEG-S 126C muteins were similar to those of the non-modified T26C and S126C proteins and wild type BV rEPO Mean EC 50 s for the PEG-T26C and PEG-S 126C muteins were similar to the mean EC 5 o values determined for the non-PEGylated T26C and S126C muteins and wild type BV rEPO, ranging from 0 48 - 0 82 ng/ml (Table 3) Biologically active, PEGvlated EPO proteins have not been descnbed previously Table 3. Bioactivities of PEG-Cvs EPO Muteins Data from three expenments Example 19 Expression and purification of EPO cysteine muteins in mammalian cells. The EPO cvsteine muteins also can be expressed in mammalian cells and punfied from the conditioned media The EPO cysteine muteins can be expressed by transient transfection of mammalian cells or by constructing stably transformed mammalian cell lines expressing the EPO cysteine mutems For therapeutic applications in humans, it is preferable that the EPO cvsteine muteins be expressed without the linker and FLAG sequences Monkey COS cells (available from the Amencan Tvpe culture Collection) can be used for transient transfection expenments, using procedures ell known to those of skill in the art A number of commercial sources , e g , GfBCO/BRL sell lipid transfection reagents and provide detailed protocols that can be used to express the EPO c> steme muteins by transient transfection Wild type EPO is manufactured for use in humans using stably transformed Chinese hamster ovary (CHO) cells expressing the protein Stably transfected CHO cell lines are widely used for high-level expression of recombinant proteins (Geisse et al 1996 Tπll et al 1995) In CHO cells, high level expression of chromosomallv integrated heterologous genes can be achieved by gene amplification Typically the gene of interest is linked to a marker gene for which amplification is selectable A vanety of genes which provide selections for amplification have been descnbed (Kaufman 1990) but munne dihvdrofolate reductase (dhfr) is frequently employed Amplification of this gene confers resistance to the folate analog methotrexate (MTX) and the level of resistance increases with dhfr gene copy number (Alt et al , 1978) Utility of MTX selection is enhanced by the availability of CHO cell lines that are deficient in dhfr (Urlaub and Chasm, 1980) One skilled in the art can construct expression vectors for the EPO cvsteine muteins that incorporate the munne dhfr gene into the commercially available pCDNA3 1 expression vector (Invitrogen), which includes the neomycin phosphotransferase (NPT) gene that confers resistance to G418 The munne dhfr expression vector pdhfr2 9 is available from the Amencan Type Culture Collection (catalogue No 37165) This dhfr gene is selectable in dhfr CHO cell lines and can be amplified by standard selections for MTX resistance (Crouse et al 1983) The dhfr coding sequence can be excised from pdhfr2 9 as a - 900 bp Bgl II fragment and cloned into the umque Bam HI site of the polyhnker of the expression vector pREP4 (Invitrogen) This construct will position the gene downstream of the strong RSV promoter, which is known to function in CHO cells (Tnll et al, 1995) and upstream of a polyadenylation site denved from S V40 This dhfr expression cassette can then be excised from pREP4 as a Sal I fragment since Sal I sites closely flank the promoter and polyA addition site Using oligonucleotide linkers this Sal I fragment can be cloned into the umque Bgl II sιte ofpCDNA3 1 For expression in mammalian cells such as COS or CHO cells, the EPO cysteine mutein cDNAs can be modified by PCR-based mutagenesis At the 5' end one can add a Kozak consensus sequence pnor to the translational initiator ATG codon m order to enhance translation in mammalian cells At the 3 ' end, the linker and FLAG sequences can be removed and the natural coding sequence and translation termination signal restored These modified EPO cDNAs can then be cloned as Bam HI - Eco RI fragments into the polylinker of pCDNA3 1 for transient transfection expenments or into the polyhnker of the the pCDNA3 1 dhfr vector for stable expression in mammalian cells For mammalian cell expression, the mutant EPO genes encoding the cysteine muteins would be modified at the 5' end to incorporate a Kozak consensus sequence (GCCACC) immediately preceding the translational lnitiiatior ATG codon in order to enhance translation in mammalian cells At the 3 ' end the linker and FLAG sequences would be removed and the natural coding sequence restored These modifications could be accomplished by a vanety of mutagensis techniques that are well known to those skilled in the art PCR-based mutagensis procedures based on those descnbed in Examples 4 and 17 could be employed PCR pnmers BB302 (5> CGCGGATCCGCCACCATGGGGGTGCAC GAATGTCCT >3)(SEQ ID NO 47) and BB303 (5> CGCGAATTCTCATCTGTCCCCTGTCCTGCAGCC >3)(SEQ ID NO 48) could be used to PCR the mutated EPO genes cloned in pUC19 or pBlueBac4 5 as templates Forward pnmer BB302 anneals to the EPO gene sequence encoding the first seven amino acids of the EPO secretion signal, adds a Kozak consensus sequence, GCCACC, immediately preceding the translational initiator ATG codon, and includes a Bam HE site for clomng purposes The reverse pnmer BB303 anneals to sequences encoding the carboxyterminal seven ammo acids of the EPO coding sequence, adds a TGA translational stop codon following the EPO coding sequence and includes a Eco RI site for clomng purposes The products of the individual PCR reactions using these pnmers and any mutant EPO gene template that has a mutation between these two pnmers, e g amino acid substitutions at amino acids 1 through 159 of the mature EPO coding sequence, will produce PCR products that can be punfied, digested with Bam HI and Eco RI and cloned into a vector smtable for expression in mammalian cells such as pCDNA3 1(+) (Invitrogen) PCR pnmers 302 and 303 also could be used to modify EPO cysteine muteins in which a cysteine residue is added preceding the first amino acid, A 1, of the mature EPO coding sequence, but distal to the EPO signal sequence Mutations m the EPO sequence that encode the carboxyterminal seven amino acids, or the * 167C mutation descnbed in Example 17, would require the use of alternative reverse pnmers m place of BB303 These pnmers would be individually designed to include the mutated cysteine codon, at least 21 nucleotides at the 3' end of the oligo that exactly match the template target, and the Eco RI site for clomng purposes For example the reverse pnmer BB304 (5> CGCGAATTCTCAACATCT GTCCCCTGTCCTG CAGCC >3)(SEQ ID NO 49) and BB302 could be used in a PCR reaction with the mutated EPO *167C gene cloned in pUC19 or pBlueBac4 5 as template to generate a modified mutant EPO gene encoding the *167C mutein that would be smtable for expression in mammalian cell expression systems. Endotox free plasmid DNAs are prefeπed for fransfecting mammalian cells such as COS or dhfr " CHO cells Dhfr " CHO cell lines can be obtained from a number of sources such as Dr. L Chasm at Columbia University (CHO Kl DUKX Bl 1) or from the Amencan Type Culture Collection (CHO duk " , ATCC No CRL-9096) The cells can be cultured m F12 DMEM medium supplemented with 10% FBS, glutamine, glycine, hypoxanthine, and thymidine (Lucas et al , 1996). Transfections can be earned out by electroporation or by using transfection reagents well known to those of skill in the art such as LipofectAMINE (Gibco BRL), using the vendor protocols and / or those descnbed in the literature (Kaufman, 1990) One can select for dhfr + trasfectants in F12/DMEM supplemented with 7% dialyzed FCS and lacking glutamine, glycine, hypoxanthine, and thymidine (Lucas et al , 1996) Alternatively one can select for G418 resistance (encoded by the NPT gene of pCDNA3 1) and subsequently screen transfectants for the dhfr + phenotype Dhfr + clones can be expanded in selection medium and culture supernatants screened for EPO cysteine mutein production using commercially available EPO ELISA kits (available from R & D Systems) or by Western blot using anti-EPO antisera (available from R&D Systems) Clones expressing the EPO cysteine mutein can then be pooled and subjected to multiple rounds of selection for MTX resistance at increasing drug concentration as descnbed by Kaufman (1990) After each round of MTX selection, individual clones can be tested for EPO cysteine mutein production These procedures are well descnbed in the literature and have been used to express a vanety of heterologous protein in CHO cells (reviewed m Kaufman, 1990) Preferably, the media used to grow the CHO cells expressing the EPO cysteine muteins should contain cystine or another cysteine blocking agent The EPO cysteine muteins can be punfied from the conditioned medium of CHO cells using protocols similar to those descnbed by Imai et al (1990) After removal of the CHO cells by centnfugation, the EPO cysteine muteins can be punfied from the supernatant by column chromatography using techniques well known to those of skill in the art The column chromatography steps employed for the purification of the EPO cysteine muteins could include Blue Sepharose, hydroxyapatite, reversed phase, hydrophobic interaction, size exclusion and ion-exchange chromatography EXAMPLE 20 Cloning, Expression and Purification of IFN-α.2. A. Cloning DNA sequences encoding IFN- 2. There are at least 25 distinct EFN- genes which encode proteins that share 70% or greater ammo acid identity (Blatt et al ,1996) Due to the high degree of DNA sequence homology between EFN- species, the EFN-α2 gene was cloned in two steps First, the EFN-α2 gene was amplified by PCR from human genoπuc DNA using pnmers coπesponding to umque sequences upstream and downstream of the EFN-α2 gene This PCR product was cloned and sequenced to confirm that it encodes the EFN-α2 gene Subsequently, the EFN-α2 coding sequence was modified by PCR and subcloned for expression in E coli and site-directed mutagenesis DNA encoding EFN-α.2 was amplified by PCR from human genomic DNA (CLONTECH) PCR reactions were earned out with BB93 (5>CGCGAATTCGGATATGTAAA TAGATACACAGTG>3) (SEQ ID NO 50) and BB94 (5>CGCAAGCTTAAAAGATTTAAATCGTGTCATGGT>3) (SEQ ID NO 51) BB93 anneals to genomic sequences -300 bp upstream (I e 5' to) of the EFN-α2 coding sequence and contains an Eco RI site for clomng purposes BB94 anneals to genomic sequences -100 bp downstream (i e 3 ' to) of the EFN- 2 coding sequence and contains a Hind III site for clomng purposes The resulting - 1 kb PCR product was digested with Eco RI and Hind III and cloned into similarly digested, and alkaline phosphatased, pCDNA3 1(+) (Invitrogen) A clone having the coπect DNA sequence for EFN-α2 (Henco et al, 1985) was identified and designated pBBT160 For cytoplasmic expression in E coli the cloned EFN-α2 gene of pBBT160 was modified by PCR to incorporate a methionine codon immediately pnor to the first residue (CI) of the mature EFN- α.2 protein A TAA stop codon was added following the carboxy-terπunal residue, E165 At the same time, Xba I and Sal I sites were added and a Bgl II site was eliminated in order to provide convement restnction sites for subsequent mutagenesis In this reaction pBBT160 was used as template and amplified by pnmers BB99 5>CGCAAGCTTCATATGTGTGATCTGCCTCAAACCCACAGCCTG GGTTCTAGAAGGACCTTGATGCT03) (SEQ ID NO 52) and BB100 (5>CGCGAATTCTTATT CCTTACTTCTTAAACTTTCTTGCAAGTTTGTCGACAAAGAAAAGGATCTCATGAT>3) (SEQ ID NO 53) BB99 anneals to the 5 ' end of the coding sequence of mature EFN-α2 and encodes an initiator methionine preceding the first amino acid of mature IFN-α2 BB99 introduces an Xba I site - 30 bp downstream of the initiator ATG, but the amino acid sequence of the protein is unaltered A Hind III site and an Nde I site, which overlaps the ATG, were included for clomng purposes The reverse pnmer, BB100, anneals to the 3' end of the coding sequence and adds a TAA stop codon BB 100 introduces a Sal I site -30 bp upstream of the TAA codon and eliminates a naturally occurπng Bgl II site located -15 bp further upstream As a result, the naturally occurnng Bgl II site located -185 bp downstream of the initiator ATG becomes a umque site None of these alterations changed the amino acid sequence An Eco RI site was added immediately downstream of the stop codon for clomng purposes The -520 bp PCR product was digested with Hind III and Eco RI, gel punfied and cloned into similarly digested plasmid pCDNA3 1(+) One clone was determined to have the coπect DNA sequence This plasmid was designated pBBT164 For cytoplasmic expression in E coli, the -520 bp Nde I - Eco RI fragment of pBBT164 was cloned into similarly digested expression vector, pCYBl, (New England Biolabs) The plasmid vector pCYBl allows genes to be expressed as unfused proteins or as fusion proteins, this construct was created so that the protein is expressed as an unfused protein The resulting plasmid was termed pBBT 170 and encodes met-EFN-α.2 The ΛWe l- Eco RI fragment of pBBT164 contaimng the met-EFN-α2 sequence also was subcloned into Nde I-Eco Rl-digesled pUC18 to generate plas id pBBT168 EFN-α2 also can be expressed in an active form in E colt by secretion into the peπplasmic space (Voss et al , 1994) Secreted IFN-α2 lacks an N-terminal methionine and has an amino acid sequence identical to naturally occurπng EFN-α2 In order to express a secreted form of EFN- 2, the leader sequence of the E coli heat-stable enterotoxin (STII) gene (Picken et al , 1983) was fused to the coding sequence for mature EFN-α2 via PCR Because of its length, the STII sequence was added in two sequential PCR reactions The first reaction used forward pnmer BB 101 (5>GCATCTATGTT CGTTTTCTCTATCGCTACCAACGCTTACGCATGTGATCT GCCTCAAACCCAC AG03) (SEQ ID NO 54) and reverse pnmer BB100 with pBBT164 (descnbed above) as template The 3 ' end (21 bp) of BB 101 anneals to the 5 ' end of the coding sequence of mature EFN-α.2 The 5 ' segment (39 nucleotides) of BB101 encodes a portion of the STII leader peptide The -550 bp PCR product of this reaction was gel purified and used as template for the second PCR reaction The second PCR reaction used reverse pnmer BB100 and forward pnmer BB11 (5'-CCCCCTCTAGACA TATGAAG AAGAACATCGCATTCCTGCTGGCAT CTATGTTCGTTTTCTC TATCG-3') (SEQ ID NO 7) BB 11 adds the remainder of the STII leader peptide and contains an Nde I site overlapping the initiator ATG of the STII leader The -590 bp product of this reaction was digested with Nde I andXba I The -100 bp Nde I - Xbal fragment contaimng the STII leader sequence and amino-termmal -30 bp of EFN-α2, was gel-punfied and ligated with pBBT168 [pUC18 met-EFN-α2] that had been digested with Nde l and Xba I, treated with alkaline phosphatase and gel punfied In this step the —30 bp Nde I - Xba I ammo-terminal segment of the met-EFN- 2 gene is replaced with the PCR denved -100 bp Nde I -Xba I ammo-terminal segment of the STII-fFN-α2 PCR product The sequence of the resulting pUC18 STII-fFN-α2 construct was confirmed and that plasmid was designated pBBT177 For expression in £ coli, pBBT177 was digested with Nde I and Eco RI and the -570 bp fragment contaimng the STII-EFN-α2 gene was gel-punfied and cloned into the expression vector pCYB 1 under the control of the tac promoter The resulting plasmid was designated pBBT178 B. Expression of rTJFN-α2 in E. coli. pBBT170, which encodes met-fFN-α.2. and parental vector, pCYBl, were transformed into E colt JM109 Expenments with these strains resulted in expression of met-IFNα-2 Secreted EFN-α2 is preferable to cytoplasmic met-IFN- 2 in that secreted EFN-α2 has the same ammo acid sequence as naturally occurnng IFN-α2 For expression of secreted rEFN-α2, pBBT178 [pCYBl STII-EFN-α2] and the parental vector pCYBl were transformed into E coli W3110 The resulting strains were designated BOB202 W3110(pBBT178) and BOB130 W3110(pCYBl) We performed a senes of expenments testing growth and rEFN-α2 expression of BOB202 in phosphate- or MES- buffered LB media at imtial pHs ranging from 5 0 to 7 0 Saturated overmght cultures were diluted to - 0 025 O D at Aβoo in buffered LB contaimng 100 μg/ml ampicillin and incubated at 37°C in shake flasks When culture O D s reached -0 3 - 0 5, EPTG was added to a final concentration of 0 5 mM to induce expression of rfFN- α.2 For imtial expenments, cultures were sampled at 0, 1, 3, 5 and -16 h post-mduction Samples of induced and unmduced cultures were analyzed by SDS-PAGE on precast 14% Tπs-glycme polyacrylamide gels stained with Coomassie Blue In addition to the expected - 19 kDa processed form of EFN-α.2, we observed a higher than expected molecular weight (-21 5 kDa) form of the protein The higher molecular weight form could result from lack of proteolytic processing of the STII leader peptide, the molecular weight of the leader peptide is consistent with this hypothesis Our results indicated that lower pH enhanced accumulation of the coπectly sized rfFN-α2 band We observed that at or above pH 6 5 the 21 5 kDa form was predominant, while at or below pH 6 0 a band of -19 kDa, which comigrated with an E co/z-expressed rEFN- 2 standard (Endogen, Inc ), was the predominant form of the protem At or below pH 6 0, the 19 kDa band accounted for at least 80% and probably greater than 90% of the EFN-α2 expressed by the E coli cells Based on these findings, further expression expenments used LB medium buffered with 100 mM MES to a pH of 5 5 Voss et al (1994) also reported secretion of IFN- 2 to the E colt penplasm using the STII leader sequence They also observed that the proportion of rIFN-α,2 present in the 21 5 kDa band was reduced, and the proportion migrating at 19 kDa was increased when culture pH was maintained at 67 as compared to 7 0 At pH 7 0 Voss et al (1994) reported that 10-30% of the STII EFN-α2 fusion protein was processed to Meld the 19 kDa mature IFN-α2 protein The percentage of coπectly processed 19 kDa EFN-α.2 protein could be increased to 50-60% by growing the E coli at pH 6 7 However, even at this pH, a substantial amount (40-50%) of the STII EFN-α.2 fusion protein remained unprocessed, reducing the yield of coπectly processed 19 kDa EFN-α2 Voss et al (1994) suggested that pH 6 7 was optimal for maximizing the amount of secreted rEFN-α2 migrating at 19 kDa Voss et al (1994) vaπed several growth parameters to attempt to increase the percentage of coπectly processed 19 kDa EFN-α2 protein to greater than 50-60%, but were unsuccessful Our data indicate that lowenng the pH to below 6 5, and preferably to 5 5 to 6 0, maximizes the ratio of cleaved (19 kDa) to uncleaved (21 5 kDa) rEFN-α2 product At these lower pHs the percent of conectly processed 19 kDa EFN-ot2 is increased to at least 80% and probably 90-100% of the total EFN-α2 synthesized bv the cells Cultures expressing rEFN-α2 were subjected to osmotic shock based on the procedure of Koshland and Botstein (1980) This procedure ruptures the E coli outer membrane and releases the contents of the peπplasm into the suπounding medium Subsequent centnfugation separates the soluble peπplasrmc components (recovered in the supernatant) from cytoplasmic, insoluble peπplasmic, and cell-associated components (recovered in the pellet) Approximately 25 - 50 % of the 19 kDa rEFN-α2 synthesized by BOB202 was recovered in the supernatant None of the 21 5 kDa form of rEFN-α2 was observed m the soluble peπplasmic fraction C Large-Scale Expression and Purification of rD N- 2 in E. coli: In order to punfy the wild type rEFN-α2 protein, fresh saturated overmght cultures of BOB202 were inoculated at - 0 02 OD @ Aβoo m LB lOOmM MES (pH5 5) contaimng 100 μg / ml ampicillin Typically, a 325 ml culture was grown in a 2 liter shake flask at 37°C in a gyrotory shaker water bath at -160-200 rpm When cultures reached a density of - 0 3 - 0 4 OD, IPTG was added to a final concentration of 0 5 mM The induced cultures were then incubated for -16 h Cultures were subjected to osmotic shock based on the procedure of Hsuing et al (1986) The cells were pelleted by centnfugation and resuspended at -25 OD/ml in ice cold 20 % sucrose. 10 mM Tns-HCl (pH 8 0) Resuspended cells were incubated on ice for 15 nun and centnfuged at 9500 x g for 10 nun Pellets were then resuspended in ice cold 10 mM Tns-HCl pH (8 0), incubated on ice for 15 nun and centnfuged at 9500 x g for 10 nun The resulting supernatant (the osmotic shock lysate) was either processed immediately or stored at -80°C rEFN-α2 was punfied as follows The pH of the supernatant from the osmotic shock was adjusted to 3, centnfuged to remove any precipitate, and loaded onto a 5 ml Pharmacia HiTrap S- Sepharose column eqmlibrated in 20 mM MES pH 5 0 (Buffer A) The bound proteins were eluted with a linear salt gradient from 0-100% Buffer B (500 mM NaCl, 20 mM MES, 10% ethylene glycol) Column fractions were analyzed by non-reducing SDS-PAGE rfFN-α2 eluted at approximately 225-235 mM NaCl Fractions that were enπched for rfFN-α2 were pooled and further fractionated on a 1 mL CA ' EMAC (Immobilized Metal Affinity Chromatography) Hi Trap column previously eqmlibrated in 40 mM sodium phosphate pH 6 0, 1 M NaCl, 0 1% Tween 20 rfFN-α.2 was eluted with a reverse pH gradient from 5 5 to 4 1 in 40 mM sodium phosphate, 1 M NaCl, 0 1% Tween 20 rfFN-α2 eluted after the gradient reached 100% buffer B. when the pH of the eluate finally reached pH 4 1 Fractions from the Cu ^ EMAC column that contained punfied, properly folded rEFN-α.2 were pooled and stored as frozen ahquots at -80°C A minor rfFN- 2 vanant was detectable in some of the earlier eluting fractions This vaπant, which results from incomplete disulfide formation and is biologically active, has been descnbed previously (Khan and Rai, 1990) Fractions contaimng this vaπant were not added to the final pool of punfied rIFN-α2 The final yield of rfFN-α2, as determined by absorbance at 280 nm and by Bradford analysis, was about 400 μg from 250 ml of culture Reduced EFN- 2 migrates with a slightly larger apparent molecular weight than non-reduced EFN-α2 when analyzed by SDS-PAGE (Morehead et al , 1984) This apparent molecular weight change is due to the reduction of the native disulfides in EFN-α2 Our rEFN-α2 comigrated with the commercial rEFN-α2 standard under both reducing and non-reducing conditions D In Vitro Bioactivity of Wild Type rIFN-α2. EFN-α bioactivity can be measured using in vitro antiviral assays or cell proliferation inhibition assays We developed a cell growth inhibition assay to measure bioactivity of wild type rEFN-α2 The human Daudi B cell line (Amencan Type Culture Collection) is sensitive to the growth inhibiting properties of EFN-α and is routinely used to measure bioactivity of EFN-α (Horoszewicz et al , 1979, Ev ger and Pestka, 1981) Daudi cells were maintained in RPMI 1640 media supplemented with 10% FBS, 50 umts/ml penicillin and 50 μg/ml streptomycin For bioassays, the cells were washed three times with RPMI 1640 media and resuspended at a concentration of 4xl0 5 cells/ml in RPMI 1640 media contaimng 10% FBS, 50 umts/ml penicillin and 50 μg/ml streptomycin Fifty μl (2xl0 4 cells) of the cell suspension were aliquotted per test well of a flat bottom 96 well tissue culture plate Senal 3-fold dilutions of the protein samples to be tested were prepared in RPMI 1640 media contaimng 10% FBS, 50 umts/ml penicillin and 50 μg/ml streptomycin Fifty μl of the diluted protem samples were added to the test wells and the plates incubated at 37°C in a humidified 5% C0 2 tissue culture incubator Protein samples were assayed in tnplicate wells After 4 days, 20 μl of CellTiter 96 AQueous One Solution Reagent (Promega Corporation) was added to each well and the plates incubated at 37°C in the tissue culture incubator for 1-4 h Absorbance was read at 490 nm using a microplate reader Control wells contained media but no cells Mean absorbance values for the tnplicate control wells were subtracted from mean values obtained for each set of tnplicate test wells Senal dilutions of E co/z-expressed rfFN-α.2 (Endogen, Inc ) were analyzed in parallel IC 50 s (the concentration of protein reqmred for half maximal growth inhibition) were calculated for each sample and used to compare bioactivities of the proteins Proliferation of the Daudi cell line is strongly inhibited by rfFN-α2, as evidenced by a dose- dependent decrease in absorbance values Commercial rEFN-α2 (Endogen) and wild type rEFN-α2 prepared by us reached the same maximal level of growth inhibition, within the eπor of the assay, and had similar mean IC 5 0S of 13-16 pg ml (Table 4) IC 50 values for these proteins ranged from 7-29 pg/ml in assays performed on different days (Table 4), therefore compaπsons between proteins were made on samples analyzed on the same day EXAMPLE 21 Construction, Expression, Purification and Bioactivity of EFN-α2 Cysteine Muteins A. Construction of IFN-α2 Cysteine Muteins. Seventeen mutant fFN-α.2 genes were constructed using site-directed PCR-based mutagenesis procedures similar to those descnbed m Example 4 We constructed one mutein in the amino-terminal region proximal to helix A [Q5C], six muteins in the A-B loop [N45C, Q46C, F47C, Q48C, A50C and 43C44 (an insertion of a cysteine between residues 43 and 44), one mutein [D77C] in the short, two residue, BC loop, four muteins in the CD loop [Q101C, T106C, E107C, and T108C], three muteins in the carboxy-terminal region distal to the E helix [S163C. E165C, and *166C (the addition of a cysteine residue to the natural carboxy-terminus)] We also constructed muteins CIS and C98S, which eliminate the naturally occurπng, but non-essential, C1-C98 disulfide (Lydon et al , 1985, Morehead et al , 1994) The CIS substitution in the amino-termmal region proximal to helix A generates a free cysteine in the C helix (C98), whereas the C98S substitution in the C helix generates a free cysteine in the region proximal to helix A (CI) For mutagenesis, PCR pnmer oligonucleotides were designed to incorporate nucleotide changes that resulted in the incorporation of a cysteine residue at the chosen position within the EFN- 2 coding sequence Where feasible, the mutagemc oligo also was designed to span a nearby restnction site that could be used to clone the mutagenized PCR fragment into an appropπate plasmid When no useful restπction site was located sufficiently near the position of the mutation, the technique of "mutagenesis by overlap extension" was employed (Horton et al , 1993) The template used for the mutagemc PCR reactions was plasmid pBBT177 (descnbed in Example 20) in which the STII-EFN-α2 gene is cloned as an Nde 1 - Eco l fragment into pUC18 The PCR products were digested with appropnate restπction endonucleases, gel-punfied and ligated with pBBT177 vector DNA that had been cut with those same restπction enzymes, alkaline phosphatase treated, and gel- puπfied Transformants from these hgations were grown up and plasmid DNAs isolated and sequenced The sequence of the entire cloned mutagenized PCR fragment was determined to verify the presence of the mutation of interest, and the absence of any additional mutations that potentially could be introduced by the PCR reaction or by the synthetic oligonucleotide pnmers The substitution mutation Q5C was constructed using the technique of "mutagenesis by overlap extension" as descnbed in Example 4 The imtial, or "pnmary" PCR reactions for the Q5C construction were performed in a 50 μl reaction volume IX Promega PCR buffer contaimng 1 5 mM MgCl 2 , each pnmer at 0 2μM, each of dATP, dGTP, dTTP and dCTP at 200 μM, 1 ng of template plasmid pBBT177 (descnbed in Example 20), 1 5 umts of Taq Polymerase (Promega), and 0 25 umts of Pfu Polymerase (Stratagene) Reactions were performed in a Robocycler Gradient 96 thermal cycler (Stratagene) The reaction program entailed 96°C for 3 minutes, 25 cycles of [95°C for 1 minute, 60°C for 1 25 minutes, 72°C for 1 minute] followed by a hold at 6°C The pnmer pairs used were [BB125 x BB130] and [BB126 x BB129] BB125 (5> CTATGC GGCATCAGAGCAGATA >3)(SEQ ID NO 55) anneals to the pUC18 vector sequence -20 bp upstream of the cloned EFN-α2 sequence BB 126 (5> TGTGGAATTGTGAGCGGATAAC >3)(SEQ ID NO 56) anneals to the pUC18 vector sequence ~40 bp downstream of the cloned EFN-α2 sequence BB129 and BB130 are complementary mutagemc oligonucleotides that change the CAA codon for Q5 to a TGT codon for cysteine The sequence of BB129 is (5> TGTGATCTGCCTTGTACCCACAGCCTG >3)(SEQ ID NO 57) and the sequence of BB130 is (5> CAGGCTGT GGGTACAAGGCAGATCACA >3)(SEQ ID NO 58) The [BB125 x BB130] and [BB126 x BB129] PCR reactions gave products of the expected sizes -140 bp for [BB125 x BB130] and -560 bp for [BB 126 x BB 129] The PCR products were "cleaned up" using the QIAquick PCR Purification Kit (Qiagen) according to the vendor protocol, run out on a 2% agarose gel, gel-puπfied using a QIAEX II Gel Extraction Kit (Qiagen) according to the vendor protocol and recovered in 20 μl 10 mM Tns-HCl (pH 8 5) These two mutagenized fragments were then "spliced" together in the subsequent, or "secondary" PCR reaction In this reaction 2μl of each of the gel-punfied PCR products of the pnmary reactions were used as template and BB125 and BB126 were used as pnmers The reaction volume was 100 μl and 2 5 umts of Taq Polymerase and 0 5 umts of Pfu Polymerase were employed Otherwise, the reaction conditions were identical to those used in the pnmary reactions An aliquot of the secondary PCR was analyzed by agarose gel electrophoresis and the expected band of -670 bp was observed The bulk of the secondary PCR reaction was "cleaned up" using the QIAquick PCR Purification (Qiagen), digested with Nde I and Xba I (New England BioLabs) according to the vendor protocols, "cleaned up" using the QIAquick PCR Purification Kit and run out on a 2% agarose gel The -100 bp Nde I - Xba I fragment of interest was gel punfied usmg a QIAEX II Gel Extraction Kit (Qiagen) according to the vendor protocol This fragment was ligated with pBBT177 (descnbed in Example 20) that had been cut with Nde I and Xba I, treated with calf intestinal alkaline phosphatase (New England BioLabs) and gel punfied The ligation reaction was used to transform E coli and plasmids from resulting transformants were sequenced to identify a clone contaimng the Q5C mutation and having the coπect sequence throughout the -100 bp Nde I - Xba I segment The substitution mutation CIS was constructed and sequence verified using the protocols detailed above for Q5C except that complementary mutagemc pnmers BB128 (5> AGGCAGATC AGATGCGTAAGC >3)(SEQ ID NO 59) and BB127 (5> GCTTACGCATCTGATCTGCCT >3)(SEQ ID NO 60), which change the TGT codon for CI to a TCT codon for senne, replaced BB130 and BB129 respectively m the pnmary PCR reactions The [BB125 x BB1128] and [BB126 x BB127] PCR reactions gave products of the expected sizes -120 bp for [BB125 \ BB128] and -570 bp for [BB126 x BB127] The substitution mutation N45C was constructed and sequence verified using the protocols detailed above for Q5C with the following differences Complementary mutagemc pnmers BB134 (5> CTTTTG GAACTGGCAGCCAAACTCCTC >3)(SEQ ED NO 61) and BB133 (5> GAGGAGTTTGGCTGCCAGTTCCAAAAG >3)(SEQ ID NO 62), which change the AAC codon for N45 to a TGC codon for cysteine, replaced BB130 and BB129 respectively in the pnmary PCR reactions The [BB125 x BB134] and [BB126 x BB133] PCR reactions gave products of the expected sizes -255 bp for [BB125 x BB134] and -440 bp for [BB126 x BB133] The product of the secondary PCR reaction was "cleaned up" using the QIAquick PCR Purification (Qiagen), digested with Bgl II and Xba I (New England BioLabs) according to the vendor protocols, "cleaned up" using the QIAquick PCR Purification Kit and run out on a 2% agarose gel The -155 bp Bgl II - Xba I fragment of interest was gel punfied using a QIAEX II Gel Extraction Kit (Qiagen) according to the vendor protocol This fragment was ligated with pBBT177 that had been cut with Bgl II and Xba I, treated with calf intestinal alkaline phosphatase (New England BioLabs) and gel punfied The ligation reaction was used to transform E colt and plasmids from resulting transformants were sequenced to identify a clone contaimng the N45C mutation and having the coπect sequence throughout the -155 bp Bgl II - Xba I segment The substitution mutation F47C was constructed and sequence verified using the protocols detailed above for N45C with the following differences Complementary mutagemc pnmers BB136 (5> TTCAGC CTTTTGGCACTGGTTGCCAAA >3)(SEQ ID NO 63) and BB135 (5> TTTGGCAACCAGTGCCAAAAGGCTGAA >3)(SEQ ED NO 64), which change the TTC codon for F47 to a TGC codon for cysteine, replaced BB134 and BB133 respectively in the pnmary PCR reactions The [BB125 x BB136] and [BB126 x BB135] PCR reactions gave products of the expected sizes -260 bp for [BB125 x BB136] and -435 bp for [BB126 x BB135] The insertion mutation 43C44 was constructed and sequence verified using the protocols detailed above for N45C with the following differences Complementary mutagemc pnmers BB132 (5> TTCAGCCTT TTGGCACTGGTTGCCAAA >3)(SEQ ED NO 65) and BB131 (5> TTTGGCAACCAGTGCCAAAAGGCTGAA >3)(SEQ ED NO 66), which insert a TGC codon for cysteine between the codons encoding ammo acid residues 43 and 44, replaced BB134 and BB133 respectively in the pnmary PCR reactions The [BB125 x BB132] and [BB126 x BB131] PCR reactions gave products of the expected sizes -250 bp for [BB125 x BB132] and -445 bp for [BB126 x BB131] The substitution mutation Q46C was constructed and sequence verified using the protocols detailed above for N45C with the following differences Complementary mutagemc pnmers BB154 (5> AGCCTT TTGGAAACAGTTGCCAAACTC >3)(SEQ ID NO 67) and BB153 (5> GAGTTTGGCAACTGTTTCCAAAAGGCT >3)(SEQ ID NO 68), which change the CAG codon for Q46 to a TGT codon for cysteine. replaced BB134 and BB133 respectively in the pnmary PCR reactions The pnmary reactions were performed in a Perkin-Elmer GeneAmp® PCR System 2400 thermal cycler The reaction program entailed 95°C for 5 minutes, 30 cycles of [95°C for 30 seconds, 62°C for 30 seconds, 72°C for 1 minute] followed by 72°C for 7 minutes and a hold at 4°C The [BB125 x BB154] and [BB126 x BB153] PCR reactions gave products of the expected sizes -260 bp for [BB125 x BB154] and -440 bp for [BB126 x BB153] The secondary PCR reaction was also performed in a Perkin-Elmer GeneAmp® PCR System 2400 thermal cycler This reaction program entailed 96°C for 5 minutes, 25 cycles of [95°C for 30 seconds, 60°C for 30 seconds, 72°C for 1 minute] and a hold at 4°C Following digestion with Bgl II and Xba I, the products of this reaction were cleaned up using the QIAquick PCR Purification Kit but not gel punfied pnor to ligation The substitution mutation Q48C was constructed and sequence verified using the protocols detailed above for Q46C with the following differences Complementary mutagemc pnmers BB156 (5> GGTTTCA GCCTTACAGAACTGGTTGCC >3)(SEQ ID NO 69) and BB155 (5> GGCAACCAGTTCTGTAAGGCTGAAACC>3)(SEQ ID NO 70), which change the CAA codon for Q48 to a TGT codon for cysteine, replaced BB154 and BB153 respectively in the pnmary PCR reactions The [BB125 x BB156] and [BB126 x BB155] PCR reactions gave products of the expected sizes -265 bp for [BB125 x BB156] and -435 bp for [BB126 x BB155] The substitution mutation A50C was constructed and sequence verified using the protocols detailed above for Q46C with the following differences Complementary mutagemc pnmers BB158 (5> AGGGATGGT TTCACACTTTTGGAACTG >3)(SEQ D NO 71) and BB157 (5> CAG TTCCAAAAGTGTGAAAC CATCCCT >3)(SEQ ED NO 72), which change the GCT codon for A50 to a TGT codon for cysteine, replaced BB154 and BB153 respectively in the pnmary PCR reactions The [BB 125 x BB 158] and [BB 126 x BB 157] PCR reactions gave products of the expected sizes -270 bp for [BB125 \ BB156] and -440 bp for [BB126 x BB155] The substitution mutation D77C was constructed and sequence verified using the protocols detailed above for Q5C with the following differences Complementary mutagemc pnmers BB138 (5> TAGGAG GGTCTCACACCAAGCAGCAGA >3)(SEQ ID NO 73) and BB137 (5> TCTGCTGCTTGGTGTGAGACCCTCCTA >3)(SEQ ID NO 74), which change the GAT codon for D77 to a TGT codon for cysteine, replaced BB130 and BB129 respectively in the pnmary PCR reactions. The [BB125 x BB138] and [BB126 x BB137] PCR reactions gave products of the expected sizes -350 bp for [BB125 x BB138] and -345 bp for [BB126 x BB137] The product of the secondary PCR reaction was "cleaned up" using the QIAquick PCR Purification (Qiagen), digested with Bgl II and Sal I (New England BioLabs) according to the vendor protocols, "cleaned up" using the QIAquick PCR Purification Kit and nm out on a 2% agarose gel The -275 bp Bgl II - Sal I fragment of interest was gel punfied using a QIAEX II Gel Extraction Kit (Qiagen) according to the vendor protocol This fragment was ligated with pBBT177 that had been cut with Bgl II and Sal I, treated with calf intestinal alkaline phosphatase (New England BioLabs) and gel punfied The ligation reaction was used to transform E coli and plasmids from resulting transformants were sequenced to identify a clone contaimng the D77C mutation and having the coπect sequence throughout the -275 bp Bgl II - Sal I segment The substitution mutation T106C was constructed and sequence verified using the protocols detailed above for D77C with the following differences Complementary mutagemc pnmers BB140 (5> CAGGGGAGTCTCACACACCCCCACCCC >3)(SEQ ID NO 75) and BB139 (5> GGGGTGGGGGTGTGTGAGACTCCCCTG >3)(SEQ ID NO 76), which change the ACA codon for T106 to a TGT codon for cysteine, replaced BB138 and BB137 respectively in the pnmary PCR reactions The [BB125 x BB140] and [BB126 x BB139] PCR reactions gave products of the expected sizes -435 bp for [BB125 x BB140] and -260 bp for [BB126 x BB139] The substitution mutation T108C was constructed and sequence verified using the protocols detailed above for D77C with the following differences Complementary mutagemc pnmers BB142 (5> CTTCAT CAGGGGACACTCTGTCACCCC >3)(SEQ ID NO 78) and BB141 (5> GGGGTGACAGAGTGTCCCCTGATGAAG >3)(SEQ ID NO 79), which change the ACT codon for T 108 to a TGT codon for cysteine, replaced BB 138 and BB 137 respectively in the pnmary PCR reactions The [BB125 x BB142] and [BB126 x BB141] PCR reactions gave products of the expected sizes -440 bp for [BB125 x BB142] and -250 bp for [BB126 x BB141] The substitution mutation Q101C was constructed and sequence verified using the protocols detailed above for D77C with the following differences Complementary mutagemc pnmers BB162 (5> CACCCC CACCCCACATATCACACAGGC >3)(SEQ ID NO 80) and BB161 (5> GCCTGTGTGATATGTGGGGTGGGGGTG>3)(SEQ ID NO 81), which change the CAG codon for Q 101 to a TGT codon for cysteine, replaced BB138 and BB137 respectively in the pnmary PCR reactions The pnmary reactions were performed in a Perkin-Elmer GeneAmp® PCR System 2400 thermal cycler The reaction program entailed 95°C for 5 minutes, 30 cycles of [95°C for 30 seconds, 62°C for 30 seconds, 72°C for 1 mmute] followed by 72°C for 7 minutes and a hold at 4°C The [BB125 x BB162] and [BB126 x BB161] PCR reactions gave products of the expected sizes - 425 bp for [BB125 x BB162] and -275 bp for [BB126 x BB161] The secondary PCR reaction was also performed in a Perkin-Elmer GeneAmp® PCR System 2400 thermal cycler This reaction program entailed 96°C for 5 minutes. 25 cycles of [95°C for 30 seconds, 60°C for 30 seconds, 72°C for 1 minute] and a hold at 4°C Following digestion with Bgl II and Sal I, the products of this reaction were cleaned up using the QIAquick PCR Purification Kit but not gel punfied pnor to ligation The substitution mutation E107C was constructed and sequence verified using the protocols detailed above for Q10 IC with the following differences Complementary mutagemc pnmers BB 164 (5> CAT CAGGGGAGTACATGTCACCCCCAC >3)(SEQ ED NO 81) and BB163 (5> GTGGGGGTGACATGTACTCCCCTG ATG >3)(SEQ ID NO 82), which change the GAG codon for E107 to a TGT codon for cysteine, replaced BB162 and BB161 respectively in the pnmary PCR reactions The [BB125 x BB164] and [BB126 x BB163] PCR reactions gave products of the expected sizes - 440 bp for [BB125 x BB164] and -255 bp for [BB126 x BB163] The substitution mutation C98S was constructed and sequence verified using the protocols detailed above for Q101C with the following differences Complementary mutagemc pnmers BB160 (5> CCC CTGTATCACAGAGGCTTCCAGGTC >3)(SEQ ID NO 83) and BB159 (5> GACCTGGAAGCCTCTGTGATACA GGGG >3)(SEQ ID NO 84), which change the TGT codon for C98S to a TCT codon for senne, replaced BB162 and BB161 respectively m the pnmary PCR reactions The [BB 125 x BB 160] and [BB 126 x BB 159] PCR reactions gave products of the expected sizes - 415 bp for [BB125 x BB160] and - 285 bp for [BB126 x BB159] The cysteine substitution mutation S163C was constructed as follows The mutagemc reverse oligonucleotide BB143 (5> CGCGAATTCTTATTCCTTACATCTTAAACTTTC >3)(SEQ ID NO 85) was designed to change the codon AGT for senne at position 163 to a TGT encoding cysteine and to span the nearby Eco RI site This oligo was used in PCR with the forward, non-mutagenic, pnmer BB125 A 50 μl PCR reaction was performed in IX Promega PCR buffer contaimng 1 5 mM MgCl 2 , each pnmer at 0 2μM, each of dATP, dGTP, dTTP and dCTP at 200 μM, 1 ng of template plasmid pBBT131 1 5 umts of Taq Polymerase (Promega), and 0 25 umts of Pfu Polymerase (Stratagene) Reactions were performed in a Robocycler Gradient 96 thermal cycler (Stratagene) The reaction program entailed 96°C for 3 minutes, 25 cycles of [95°C for 1 minute, 60°C for 1 25 minutes, 72°C for 1 minute] followed by a hold at 6°C A 5 μl aliquot of the PCR reaction was analyzed by agarose gel electrophoresis and found to produce a single fragment of the expected size ~ 610 bp The remainder of the reaction was "cleaned up" using the QIAquick PCR Purification (Qiagen) according to the vendor protocol, digested with Sal I and Eco RI (New England BioLabs) according to the vendor protocols, ethanol-precipitated, resuspended in 20 μl of 10 mM Tns-HCl pH 8 5 and run out on a 2% agarose gel The -42 bp Sal I - Eco RI fragment of interest was gel punfied using a QIAEX II Gel Extraction Kit (Qiagen) according to the vendor protocol This fragment was ligated with pBBT132 that had been cut with Sal I and Eco RI, treated with calf intestinal alkaline phosphatase (New England BioLabs) and gel punfied The ligation reaction was used to transform E colt and plasmids from resulting transformants were sequenced to identify a clone contaimng the E107C mutation and to have the coπect sequence throughout the -42 bp Sal I - Eco RI segment A mutation was also constructed that added a cvsteine following the carboxyterminal ammo acid of the fFN-α.2 coding sequence This mutant, termed * 167C was constructed using the protocols descnbed above for the construction of the S163C mutant with the following differences The mutagemc reverse oligonucleotide BB144 (5> CGCGAATTCTTAACATTCCTTACTTCTTAAACTTTC >3)(SEQ ID NO 86) which adds a TGT codon for cysteine between the GAA codon for E 165 and the TAA stop codon and spans the nearby Eco RI site was used m the PCR reaction in place of BB143 The substitution mutation E165C was constructed and sequence verified using the protocols detailed above for S163C with the following differences The mutagemc reverse oligonucleotide BB165 (5> CGCGAATTCTTA ACACTTACTTCTTAAACT >3)(SEQ ID NO 87) which changes the GAA codon for E165 to a TGT codon for cysteine and spans the nearby Eco RI site was used m the PCR reaction in place of BB143 The PCR reaction was performed in a Perkin-Elmer GeneAmp® PCR System 2400 thermal cycler The reaction program entailed 95°C for 5 minutes. 30 cycles of [95°C for 30 seconds, 62°C for 30 seconds, 72°C for 1 minute] followed by 72°C for 7 minutes and a hold at 4°C Following digestion with Eco RI and Sal I, the products of this reaction were cleaned up using the QIAquick PCR Purification For expression in E coli as proteins secreted to the peπplasmic space, the STEI-EFN-α2 genes encoding the 17 muteins were excised from the pUC18-based pBBT177 deπvatives as Nde I - Eco RI fragments of -590 bp and subcloned into the pCYBl expression vector that had been used to express wild type SΗI-EFNα-2 For expression expenments, these plasmids were introduced into E colt W3110 Using procedures similar to those descnbed here, one can construct other cysteine muteins of EFN- 2 The cvsteine muteins can be substitution mutations that substitute cysteine for a natural amino residue in the EFN-α2 coding sequence, insertion mutations that insert a cysteine residue between two naturally occurπng amino acids in the EFN-α2 coding sequence, or addition mutations that add a cysteine residue preceding the first amino acid, CI, of the EFN-α2 coding sequence or add a cysteine residue following the terminal amino acid residue, E165, of the fFN-α.2 coding sequence The cysteine residues can be substituted for any amino acid, or inserted between any two amino acids, anywhere in the EFN-α2 coding sequence Prefeπed sites for substituting or inserting cysteine residues in fFN-α.2 are in the region preceding Helix A, the A-B loop, the B-C loop, the C-D loop, the D-E loop and the region distal to Helix E Other prefeπed sites are the first or last three amino acids of the A, B, C, D and E Helices Prefeπed residues in these regions for creating cysteine substitutions are D2, L3, P4, T6, H7, S8, Q20, R22, K23, S25, F27, S28, K31, D32, R33, D35, G37, F38, Q40, E41, E42, F43, G44. K49, T52, N65, S68, T69. K70, D71, S72, S73, A74, A75, D77, E78, T79, Y89, Q90, Q91, N93. D94. E96, A97, G102, V103, G104, V105, P109, Mil l, K112, E113. D114, S115, K131, E132. K133, K134, Y135, S136, A139, S152, S154, T155, N156, L157, Q158, E159, S160, L161, R162, and K164 Cysteine residues also can be inserted immediately preceding or following these amino acids Another prefeπed site for adding a cysteine residue would be preceding CI, which we refer to as *-lC One also can construct EFN-α2 muteins contaimng a free cysteine by substituting another ammo acid for one of the naturally-occurπng cysteine residues in EFN-α.2 The naturally-occurπng cysteine residue that normally forms a disulfide bond with the substituted cysteine residue is now free The non-essential cysteine residue can be replaced with any of the other 19 amino acids, but preferably with a senne or alamne residue A free cysteine residue also can be introduced into IFN-α2 by chemical modification of a naturally occurnng amino acid using procedures such as those descnbed by Sytkowski et al (1998) Using procedures similar to those descnbed in Examples 20 - 22, one can express the proteins in E coli, punfy the proteins, PEGylate the proteins and measure their bioactivities in an in vitro bioassay The EFN-α.2 muteins also can be expressed in eukaryotic cells such as insect or mammalian cells, using procedures similar to those descnbed in Examples 16-20, or related procedures well known to those skilled m the art B E. coli Expression of rD?N-α2 Cysteine Muteins. To assess expression, cultures of the rIFN-α2 muteins were grown and induced as descnbed above for wild type rfFN-α2 Typically, 45 ml cultures were grown in 250 ml shake flasks at 37°C in a gyrotory shaker water bath at -180-200 rpm We observed that vigorous aeration of shake flask cultures results in reduced levels of EFN-α2 protein in the supernatants of the osmotic shock lysates Therefore we routinely used conditions that were sub-optimal for aeration but preferable for soluble rIFN-α2 and rEFN-α2 mutein production The induced cultures were incubated for —16 h, harvested and subjected to osmotic shock as detailed above for wild-type rEFN-α.2 with the exception that cystine was added to a final concentration of 5 mM to the buffers used for the osmotic shock procedure Adding cystine to the osmotic shock buffers resulted in significantly improved chromatographic properties for the first two muteins analyzed, Q5C and S163C Interferon muteins not treated with cystine consistently eluted from the S-Sepharose column as broad bands, which when analyzed by non-reducing SDS-PAGE, showed multiple molecular weight species at and around the expected monomer molecular weight These species most likely represent πusfolded or incompletely folded mterferon vaπants In contrast, interferon muteins treated with cystine duπng the osmotic shock procedure eluted from the S-Sepharose column as sharp bands, which, when analyzed by non- reducing SDS-PAGE, consisted of only one mterferon species that co-migrated with the interferon wild type standard Recovenes of the punfied mterferon muteins from the S-Sepharose column also were 1 5- to 2-fold greater when cystine was included in the osmotic shock buffers Based upon these results, 5 mM cystine was added to the osmotic shock buffers used to punfv all the cysteine muteins SDS-PAGE analysis of the osmotic shock supernatants of the muteins showed most to have reduced (as compared to wild type) levels of the 19 kDa rIFN- 2 band Two muteins, Q5C and S163C, were expressed at levels equivalent to wild type interferon, and several hundred micrograms of each of these muteins were readily punfied as detailed below Eight muteins (CIS, 43C44, N45C, Q46C, Q48C, A50C, D77C and T108C) were essentially undetectable in osmotic shock supernatants The remaining seven muteins (F47C, C98S, Q101C, T106C, E107C, E165C, *166C) were detected at varymgly reduced levels (as compared to wild type) in osmotic shock supernatants Some of these muteins (T106C, C98S, E107C. and Q101C) were punfied from osmotic shock supernatants, but only small quantities of the pure proteins were recovered Certain muteins, C98S, Q101C, T106C, E107C and * 166C, were expressed at relatively high levels but accumulated pπmanlv in an insoluble form, presumably in the peπplasm These proteins comigrated with wild type rfFN-α.2 standards under reducing conditions indicating that the STII leader had been removed Qualitative assessments of relative expression levels of the muteins are summanzed in Table 4 C. Purification of rD?N-α2 cysteine muteins. In order to purify the rfFN-α2 muteins, typically, a 325 ml culture in a 2 liter shake flask, or a 500 ml culture in a 2 liter baffled shake flask, were grown at 37°C in a gyrotory shaker water bath (New Brunswick Scientific) -170-220 rpm. Cultures were grown, induced, harvested, and subjected to osmotic shock as described in Example 20. Resulting osmotic shock supernatants were processed immediately or stored at -80°C. The soluble IFN-α2 muteins in the osmotic shock supernatants were purified using S- Sepharose and Cu^ EMAC chromatography as detailed above for purification of wild type rfFN- 2. All of the muteins tested bound tightly to the copper column and eluted under conditions similar to wild-type rFFN-α2. This result suggests that the conformations of the cysteine muteins are similar to that of native rEFN-α2, at least in the regions that comprise the metal-binding pocket. Non-reducing SDS-PAGE analysis of the purified Q5C, C98S, Q101C, T106C, E107C, S163C, and *166C cysteine muteins showed that the muteins were recovered predominantly as monomers, migrating at the expected molecular weight of - 19 kDa. C98S migrated with a slightly higher molecular weight than the other rINF-α2 muteins due to the absence of the native Cysl-Cys-98 disulfide bond. Some of the purified muteins contained small amounts of disulfide-linked rEFN-α.2 dimers. The molecular weights of the dimer species were approximately 37-38 kDa. D. Bioactivities of rIFN-α2 Cysteine Muteins. Biological activities of the purified Q5C and S163C rfFN-α.2 cysteine muteins were measured in the Daudi growth inhibition assay descnbed in Example 20 Protein concentrations were determined using Bradford or BCA protein assay kits (Bio-Rad Laboratories and Pierce). Commercial wild type rfFN-α2 and rEFN-α2 prepared by us were analyzed in parallel on the same days to control for mterday variability in the assays. The muteins inhibited proliferation of Daudi cells to the same extent as the wild type rEFN-α2 control proteins, within the eπor of the assay. The mean IC 50 for the Q5C mutein was 13 pg/ml, which is similar to the mean IC 50 s of the wild type rfFN-α proteins. The mean IC 50 for the S163C protein was 27 pg/ml. These data are summarized in Table 4. Table 4. Expression and in vitro Bioactivities of IFN-α.2 Cysteine Muteins Portion of the EFN-α2 protein in the osmotic shock supernatant, estimated from SDS-PAGE gels 3 ICso values from individual expenments 4 Commercial wild type rfFN-α.2 (Endogen, Inc ) 5 Wild type rEFN-α2 prepared by Bolder Biotechnology, Inc 6 Mutation creates a free cysteine (C98) in the C-helix 7 Mutation creates a free cysteine (CI) in the N-terminal region EXAMPLE 22 PEGylation, Purification and Bioactivity of PEG-Q5C and PEG-S163C A. PEGylation of DFN-α Cysteine muteins. A small-scale PEGylation expeπment was performed with the punfied rIFN-α2 cysteine muteins to identify conditions that allowed the proteins to be monoPEGylated at the free cysteine residue Over-reduction of the proteins was monitored by non-reducmg SDS-PAGE, looking for a shift to a higher than expected apparent molecular weight as a result of protein unfolding, or for the appearance of multiple PEGylated species generated as the result of native disulfide reduction Imtial titration expenments were performed with the Q5C protein One μg aliquots of punfied Q5C were incubated with increasing concentrations of TCEP [Tns (2-carboxyethyl) phosphine] -HCl at room temperature in 100 mM Tns, pH 8 5 m the presence of varying amounts of excess 5 kDa maleimide- PEG After 60 nun, the reactions were stopped and immediately analyzed by non-reducmg SDS- PAGE The amounts of TCEP and PEG reagent that yielded significant amounts of monoPEGylated Q5C protein (molecular weight of approximately 28 kDa by non-reducing SDS-PAGE), without modifying wild type rfFN-α2, were used for further expenments The titration expenments indicated that a 10-fold molar excess of TCEP and 20-fold excess of 5 kDa maleimide PEG gave around 60% monoPEGylated protein without detectable di or tn-PEGylated protein, or modification of wild type rfFN-α2 These conditions also were used to PEGylate several other rEFN-α.2 muteins One μg aliquots of punfied wild type and the rEFN-α2 muteins (Q5C, T106C, E107C, S163C) were incubated for 1 hour with a 10-fold molar excess TCEP and a 20-fold molar excess of 5 kDA maleimide PEG at pH 8 5 at room temperature The four muteins were monoPEGylated to varying degrees (estimated to be from 30-60%) based on SDS-PAGE analysis of the reaction mixtures Wild-type rIFN- 2 showed no detectable PEGylation under these conditions Control expenments indicated that the Q5C, T106C, E107C and S163C cysteine muteins needed to be reduced with TCEP to be PEGvlated These data indicate that the PEG molecule is attached to the cysteine residue introduced into the Q5C, T106C, E107C and S163C proteins B. Preparation and Purification of PEG-Q5C IFN-α2 and PEG-S163C: Larger quantities of the Q5C and S163 muteins were PEGylated so that biological activities of the PEGylated proteins could be measured For the Q5C protein, the PEGylation conditions used for the small-scale expenments were scaled to 140 μg protein to give sufficient matenal for purification and characteπzation The larger PEGylation reaction was performed for 1 hr at room temperature, diluted 10X with 20 mM MES, pH 5 0, adjusted to pH 3 0, and then loaded quickly onto an S-Sepharose column using conditions similar to those descnbed for imtial purification of the rfFN- α.2 muteins The presence of the PEG moiety decreased the protein's affinity for the resin, allowing the PEGylated protein to be separated from the non-PEGylated protein The chromatogram from the S-Sepharose column showed two major protein peaks eluting at approximately 190 mM NaCl and 230 mM NaCl The earlv eluting major peak (eluting at approximately 190 mM NaCl) was determined to be mono-PEGylated Q5C by SDS-PAGE The apparent molecular weight of monoPEgylated Q5C is approximately 28 kDa by SDS-PAGE The later eluting major peak (eluting at approximately 230 mM NaCl) was determined to be unreacted Q5C protein Fractions from the early eluting peak contaimng predominantly PEG-Q5C, were pooled and used for bioactivity measurements The S163C cysteine mutant was PEGylated at a 90 μg scale and punfied using protocols essentially identical to those descnbed for PEG-Q5C C. Bioactivities of PEG-Q5C and PEG-S163C Cysteine Muteins: Biological activity of the punfied PEG-Q5C protein was measured in the Daudi cell assay descnbed in Example 20 Concentration of the protein was determined using a Bradford dve binding assay The PEG-Q5C protein showed a similar dose-response curve and reached the same level of maximal growth inhibition as wild type rfFN-α2 and the non-modified Q5C protein, withm the eπor of the assay The mean IC 50 for the PEG-Q5C mutein was - 22 pg/ml, which is within 2-fold of the ICso values determined for wild type fFN-α.2 and the unmodified Q5C proteins analyzed on the same days (Table 5) Bioactivity of the PEG-Q5C protein is significantly greater than that of rEFN- 2 that has been PEGylated with non-specific, amine-reactive PEG reagents The latter protein has an IC 50 of 164 pg/ml in the Daudi cell assay (Monkarsh et al , 1997) These data are summanzed in Table 5 Bioactivity expenments also were performed with the PEG-S163C protein The PEG-S163C protein also was biologically active and inhibited Daudi cell proliferation to the same extent as wild type rEFN-α.2, within the eπor of the assay The average IC 50 for the PEG-S163C protem was about 42 pg/ml, which is better than the amine-Pegylated EFN-α Table 5. Bioactivit of PEG- 5C EFN-c 1 Data from three expenments 2 rEFN-α2 prepared by Bolder Biotechnology, Inc. 3 Data from Monkarsh et al. (1997) Example 23 hi vivo efficacy of the PEGylated GH cysteine muteins can be tested in hypophysectomized (HYPOX) rats. This is a well-charactenzed model of GH deficiency (Cox et al., 1994; Clark et al., 1996). GH stimulates body weight gain and bone and cartilage growth in HYPOX rats (Cox et al., 1994; Clark et al., 1996). Hypophysectomized Sprague-Dawley rats can be purchased from a commercial supplier such as Charles River (Wilmington, MA). Typically, rats are hypophysectomized between 40 and 50 days of age and weigh approximately 120g. Groups of 8 rats should receive subcutaneous injections of rhGH, PEG-Cys-GH or placebo (vehicle solution) at specified intervals and weight gam measmed daily over a 10 day penod. Rats should be weighed daily at the same time per day to eliminate possible vaπables associated with feeding. In addition to overall weight gain, bone growth (tibial epiphysis width) can be measured. At time of sacrifice, the right and left proximal tibial epiphyses can be removed and fixed in formalin The fixed tibias can be split at the proximal end in a saggital plane, stained with silver nitrate and exposed to a strong light (Greenspan et al., 1949) The width of the cartilaginous epiphseal plate can be measmed using a stereomicroscope equipped with a micrometer eyepiece Ten measurements should be made for each epiphysis and the means +/- SEM for the combined values for the left and πght tibias should be calculated Compansons between groups can be made using a Students T test for single comparisons and one-way analysis of vanance for multiple compansons. P< 0.05 should be considered significant. Efficacy of the GH cysteine muteins modified with 10 kDa or 20 kDa PEGs can be tested by admimstenng the proteins to the rats daily, every other day, every third day, every fourth day or following a single injection. Five μg of non-PEGylated hGH adnumstered twice a day (10μg BID) by subcutaneous injection gives a strong growth response in the HYPOX rat model (Cox et al., 1994; Clark et al., 1996). In initial expenments different groups of rats should receive subcutaneous injections of 0.08, 0.4, 2, 10. or 50 μg of the PEGylated Cys-GH proteins/injection/rat. Control rats should receive vehicle solution only. Additional control groups should receive non-PEGylated rhGH (10 μg BED) and 10 μg non-PEGylated hGH using the same dosing regimen as the PEGylated Cys- GH proteins. Administration of the PEGylated GH cysteine muteins to the HYPOX rats should result in an increase in body weight gam and tibial epyphysis width growth compared to the vehicle-treated group. Efficacy of the PEGylated GH cvsteine muteins also can be tested in rodent models of cachexia Dexamethasone (DEX) can be adnumstered to the rats to induce weight loss Groups of normal Sprague-Dawley rats (200 - 225g) should receive daily subcutaneous injections of dexamethasone (200 μg rat, approximately 1 mg/kg) This amount of dexamethasone should induce a loss of approximately 5-6g over an 8d penod Vehicle or varying doses of the PEGylated GH cysteine muteins can be adnumstered to the rats once, daily, every other day, every third day or every fourth day in different expenments Different groups of rats should receive subcutaneous injections of 0 08, 0 4, 2, 10, or 50 μg of the PEGylated Cys-GH proteins/injection/rat Additional controls should include a group of rats that will receive no DEX or injections, a group of rats that receives DEX and non-PEGylated rhGH (10 μg BED) and a group of rats that receives DEX and non-PEGylated rhGH (10 μg daily, every other day, every third day, or every fourth day, depending upon the expenment, i e , frequency that the PEGylated GH cysteine mutein is adnumstered) Animals should be weighed dai Food and water consumption should be momtored daily At time of sacrifice, internal organs should be weighed Statistical analyses should be performed as descnbed for the HYPOX rat studies Animals treated with the PEGylated GH cysteine muteins should lose less weight than the vehicle- treated animals In vivo efficacy of the PEGylated EPO cysteine muteins can be measured in normal rats by demonstrating that the proteins stimulate increases in hemocπt and erythropoiesis compared to vehicle-treated animals EPO stimulates a significant increase in hematocnt in rats when dosed on a daily basis (Matsumoto et al , 1990, Vaziπ et al , 1994, Baldwin et al , 1998) Sprague-Dawley rats can be purchased from a commercial supplier such as Charles River (Wilmington, MA) Groups of 5 rats should receive subcutaneous injections of BV rEPO, PEGylated EPO cysteine mutein or placebo (vehicle solution) at specified intervals for up to five days On day 6 the animals should be sacrificed and blood samples collected for hematocπt and complete blood cell count (CBC) analysis, which can be performed by a commercial laboratory Hematopoietic tissues (liver and spleen) should be collected, weighed and fixed in formalin for histopathologic analyses to look for evidence of increased erythropoiesis Bone maπow should be removed from vanous long bones and the sternum for unit particle preparations and histopathologic analysis to look for evidence of increased erythropoiesis Compansons between groups should be made using a Students T test for single compansons and one- way analysis of vaπance for multiple compansons P< 0 05 should be considered significant The PEGylated EPO cysteine muteins should stimulate increases in hematocπt and erythropoiesis m the rats compared to the vehicle-injected animals Efficacy of the PEGylated EPO cysteine muteins modified with 10 kDa or 20 kDa PEGs can be tested when adnumstered once, every other day or every third day 100 IU/kg (~ 800 ng/kg) of non-PEGylated EPO adnumstered once per day (160 ng SfD/200g rat) by subcutaneous injection gives a significant increase in hematocπt (Matsumoto et al , 1990. Vaziπ et al , 1994, Baldwin et al , 1998) In imtial expenments different groups of rats should receive subcutaneous injections of 0 32, 1 6, 8, 40 or 160 ng of the PEGylated EPO cysteine mutems Control rats should receive vehicle solution only Additional control groups should receive non- PEGylated rEPO (160 ng/SED) and 160 ng non-PEGylated rEPO using the same dosing regimen as the PEGylated EPO cysteine muteins Efficacy of the PEGylated EPO muteins also can be tested m chemotherapy-induced anemia models Cisplatin-mduced anemia is a well-charactenzed rodent model of chemotherapy-induced anemia and has direct relevance to the human clinical setting rEPO reverses the anemia in this model when adnumstered at daily doses of 100 Units/kg (Matsumoto et al , 1990, Vaziπ et al , 1994, Baldwin et al 1998) Sprague-Dawley rats (~200g) should be treated on day 0 with an mtrapeπtoneal injection of Cisplatin (3 5mg/kg) to induce anemia and randomized to vanous treatment groups Efficacy of the PEGylated EPO cysteine muteins modified with 10 kDa or 20 kDa PEGs can be tested when adnumstered once (on day 1), every other day or every third day Different groups of rats should receive subcutaneous injections of 0 32, 1 6, 8, 40 or 160 ng/mjection of the PEGylated EPO cysteine muteins Rats should be injected with the test compounds for up to 8 days One control group of rats should receive daily subcutaneous injections of rEPO (100 Units/kg) Another control group should not receive the imtial Cisplatin injection but should receive injections of saline using the same dosing schedules used for the PEGylated EPO cysteine muteins On day 9 the rats should be sacrificed and blood and tissue samples obtained for comprehensive CBC and histopathology analyses The PEGylated EPO cysteine muteins should stimulate increases in hemocnt and erythropoiesis in the rats compared to the vehicle-injected control group EFN-α biological activity is relatively species-specific, which limits the range of prechmcal ammal models that can be stadied One model that can be used to measure in vivo efficacy of PEGylated EFN-α2 cysteine muteins is inhibition of human tumor xenograft growth in athymic nude mice Human EFN- 2 is not active on mouse cells and inhibition of human tumor xenograft growth in nude mice occurs through a direct antiproliferative effect on the human tumor cells EFN-α2 inhibits growth of a vaπetv of pnmary human tumor xenografts and human tumor cell lines in athymic mice (Balkwill et al , 1985, Balkwill, 1986, Johns et al , 1992, Lindner and Borden. 1997) The pnmary endpomt for the studies should be tumor volume in treated mice We expect to find that the administration of the PEGylated JFN-α2 cysteine muteins inhibits tumor growth (as measured by tumor volume) in the mice relative to vehicle-treated animals Athymic nude mice can be purchased from a commercial vendor such as Charles River Each mouse should be injected with 2 x 10 δ NEH- OVCAR-3 or MCF-7 tumor cells (the cell lines are available from the Amencan Type Culture collection) on day 0 and randomly assigned to test groups, consisting of ten mice each The tumor cells should be injected into the derrms overlying the mammary gland nearest the axillae The different test groups should receive subcutaneous injections of varying doses of wild type rEFN-α2, 10 kDa-PEGylated EFN-α2 cysteine mutein, 20 kDa-PEGylated EFN-α2 cysteine mutein or placebo (vehicle solution) at specified intervals every day (S D), every other day (EOD) or every third day (ETD) Tumor volumes should be determined at 4 day intervals by measuπng the length and width of the tumors with calipers, as descnbed by Linder and Borden (1997) At time of sacrifice, the tumors should be excised and weighed Mean tumor volumes +/- SEM for each test group should be calculated for each sampling point Compansons between groups should be made using a Students T test for single compansons and one-way analysis of vanance for multiple compansons Five μg of non-PEGylated EFN- 2 adnumstered once per day by subcutaneous injection inhibits growth of NEH- OVCAR-3 cells and MCF-7 cells in athymic mice by 80% after 6 weeks (Lindner and Borden, 1997) Either cell line can be used for these studies The NEH-OVCAR-3 line (available from the ATCC) does not reqmre estrogen for growth, as do the MCF-7 cells Xenograft expenments with MCF-7 cells reqmre that the mice be oophorectoπuzed and implanted with estrogen pellets (Lindner and Borden, 1997) In imtial expenments. different groups of mice should receive subcutaneous injections of 1 or 5 μg per injection of rFFN-α2, 10-kDa-PEGylated EFN- 2 cysteine mutein or 20-kDa- PEGylated EFN-α2 cysteine mutein using every day, every other day or every third day dosing schedules Dosing should begin on day 2 following injection of the tumor cells into the mice Control mice should receive vehicle solution only In the every other day and every third day dosing expenments, an additional positive control 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J. Physiol. 266: F360-366. While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. It is to be expressly understood, however, that such modifications and adaptations are within the scope of the present invention.

Claims: Claims 1 A method for obtaining a soluble protein having a free cysteine compπsing the step of (a) obtaining a host cell capable of expressing the soluble protein, (b) exposing the host cell to a cysteine blocking agent, and (c) isolating the soluble protein from the host cell 2 The method of claim 1, further compπsing disrupting the host cell m the presence of the cysteine blocking agent and isolating the protein from the soluble fraction of the disrupted host cell 3 The method of claim 1, wherein exposing the host cell to a cysteine blocking agent occurs before, dunng or after synthesis of the soluble protein by the host cell and wherein the soluble protem is secreted from the host cell 4 The method of claims 1 to 3, wherein said host cell is a bacteπa. yeast, insect or mammalian cell 5 The method of claims 1 to 3, wherein said host cell is a bacteπa cell 6 The method of claim 5, wherein said host cell is E coli 7 The method of claims 1 to 3, wherein said soluble protein is a recombinant protem 8 The method of claim 7, wherein said recombinant protein is a cysteine mutein of a member of the growth hormone supergene family, a denvative or an antagomst thereof 9 The method of claim 8, wherein said member is growth hormone 10 The method of claim 8, wherein said member is erythropoietin 11 The method of claim 8, wherein said mterferon is alpha interferon alpha (EFN-α) 12 The method of claim 8, wherein said alpha interferon is interferon alpha 2 (EFN-α2) 13 The method of claim 7, wherem said recombinant protein is a cysteine mutein of a member of the TGF-beta superfamily, platelet deπved growth factor-A, platelet denved growth factor-B, nerve growth factor, brain deπved nemotophic factor, neurotroplun-3, neurotroplun-4, vascular endothelial growth factor, or a deπvative or an antagomst thereof 14 The method of claim 7, wherein said recombinant protem is a cysteine mutein of a heavy or light chain of an immunoglobulin or a deπvative thereof 15 The method of claim 1, where said cysteine blocking agent is a thiol-reactive compound 16 The method of claim 15, wherein said thiol-reactive compound is cystine, cystanune, dithioglycohc acid, oxidized glutathione, iodine, hydrogen peroxide, dihydroascorbic acid, tetrathionate, O-iodosobenzoate or oxygen in the presence of a metal ion 17 The method of claim 15, wherein said thiol-reactive compound is cystine 18 The method of claims 1 to 3, further comrpsimg attaching a cysteine-reactive moiety to said isolated protem to form a cysteine modified protein 19 The method of claims lto 3, further compπsing attaching a polyethylene glycol to said isolated protein to form a pegylated protein 20 A pegylated human growth hormone (hGH) or a denvative thereof having an EC 50 of less than about 110 ng/ml 21 The pegylated hGH ofclaim 20, wherein the PEG moiety is attached to a C-D loop or a region proximal to Helix A of said hGH 22 A pegylated erythropoietin (EPO) or a deπvative thereof having an ECso of less than about 1000 ng/ml 23 The pegylated EPO of claim 22, wherein the PEG moiety is attached to a C-D loop or an A-B loop of EPO 24 A pegylated alpha interferon (EFN-α2) or a deπvative having an EC 5 o of less than about 100 pg/ml 25 The pegylated EFN-α2 of claim 24, wherein the PEG moiety is attached to a region proximal to Helix A or a C-D loop of EFN-α2 26 A multimenc protein compπsing at least two proteins each having a free cysteine and wherein said proteins are attached through said free cysteines 27. A method of treating a condition treatable with growth hormone, EPO or an alpha interferon, comprising administering to a patient in need thereof an effective amount of a cysteine variant of growth hormone, EPO or alpha interferon, or a derivative thereof to treat said condition. 28. The method of claim 27, wherein said derivative is a pegylated cysteine variant of growth hormone, EPO or alpha interferon. 29. A method for covalently modifying a protein produced according to claims 1 to 3, comprising the steps of: (a) purifying the soluble protein; (b) reducing the protein with a disulfide-reducing agent; and (c) exposing the protein to a cysteine-reactive moiety to obtain a cysteine-modified protein. 30. The method of claim 29, further comprising isolating the cysteine-modified protein from the unmodified protein. 31. The method of claim 29, wherein the cysteine-reactive moiety is a polyethylene glycol.

Inventors: COX GEORGE N, DOHERTY DANIEL H, ROSENDAHL MARY S, COX, GEORGE, N., DOHERTY, DANIEL, H., ROSENDAHL, MARY, S.
Applicants: BOLDER BIOTECHNOLOGY INC, COX GEORGE N, DOHERTY DANIEL H, ROSENDAHL MARY S, BOLDER BIOTECHNOLOGY INC., COX, GEORGE, N., DOHERTY, DANIEL, H., ROSENDAHL, MARY, S.
Application year: 2000
Main classification: C12N15
IPC codes: A61K38, A61K47, A61P5, A61P7, A61P43, C07K14, C12N15, C12P21, C12R1