METHOD OF INCREASING EXPRESSION OF VIRAL PROTEINS

Abstract: Methods for the recombinant expression and secretion of viral proteins are disclosed. The methods involve the use of compatible escorts to shuttle the proteins to the cell surface. In this way, egress of recombinantly produced proteins out of the cell is facilitated, resulting in increased yields and easier purification of the desired protein.

Description: METHOD OF INCREASING EXPRESSION OF VIRAL PROTEINS Background of the Invention Technical Field The present invention relates generally to recombinant production of proteins. More particularly, the invention relates to methods for facilitating egress of recombinantly produced proteins from a host cell using escorts. Background of the Invention A desirable method for the production of proteins involves recombinant expression. However, commercially useful quantities of proteins can be difficult to obtain. This is particularly true with viral glycoproteins where recombinant production does not always mimic expression during viral infection. Specifically, some viral proteins are expressed at the cell surface during viral infection but are expressed intracellularly in recombinant systems. Thus, additional purification steps must be taken in order to isolate the proteins. Additionally, the proteins are susceptible to degradation by intracellular enzymes prior to purification. Such intracellular expression has been observed in herpesvirus systems with respect to glycoprotein H (gH) expression (Gompels and Minson, J. Virol. (1989) £3:4744-4755) ; Spaete et al. , Progress in Cytomecralovirus Research (M.P. Landini, ed. , 1991) pp. 133-136) . Chaperones bind newly synthesized polypeptides and appear to stabilize these polypeptides until they are assembled into their proper native structure or until they are transported to another cellular compartment, i.e., for secretion. Sambrook and Gething, Nature (1989) 342:224-225. Gompels and Minson, supra, postulate that additional virus gene products are required for transport of herpes simplex virus type 1 (HSV-1) gH through the endoplasmic reticulum-Golgi network to the cell surface. Similarly, the existence of a viral function which escorts cytomegalovirus (CMV) gH to the cell surface was proposed in Spaete et al. , supra . However, CMV UL56, the homologue of herpesvirus ICP18.5 which has been implicated in the egress of viral glycoproteins to the cell surface (Pancake et al., J. Virol. (1983) 42:568-585) , failed to promote movement of both truncated and full-length forms of CMV gH to the cell surface. As with gH, cell lines transfected with the gene encoding the human immunodeficiency virus (HIV) type 1 envelope glycoprotein, gp 160, have also failed to process or export the protein. Haffar et al. , J. Virol. (1990) £4:3100-3103, speculated that this was due to the absence of other viral proteins during recombinant production. A new HSV glycoprotein, gL, has now been identified (Hutchinson et al. , J. Virol. (1992) £6:2240-2250, and Hutchinson et al. , Abstract, XVI International Herpesvirus Workshop, July 7-12, 1991) . This protein is encoded by the UL1 gene of HSV-1. Like gH, gL is not properly processed when expressed in the absence of other HSV polypeptides. However, when HSV gL and HSV gH are coexpressed, the proteins are antigenically similar to those found in infected cells and the proteins are processed and transported to the cell surface. The experimenters postulated that formation of an HSV gL/gH complex was therefore a prerequisite for the processing and transport of both molecules. However, the use of a gL/gH complex as a tool for increasing the recombinant expression of either of the proteins was not addressed. The fibroblast growth factors (FGFs) are a family of structurally related polypeptides that regulate the growth and differentiation of a diverse number of cell types. Currently, seven distinct gene products have been identified. These include acidic and basic FGFs (Jaye et al. , Science (1986) 233 :541- 545; Abraham et al. , Science (1986) 23 . 3:545-548; Abraham et al . , EMBO J (1986) 5_:2523-2528) , the product of the int-2 oncogene (Moore et al. , EMBO J (1986) 5 . :919-924; Jakobovits et al. , Proc. Natl . Acad. Sci. USA (1986) £1:7806-7810) , a growth factor identified from Kaposi's sarcoma DNA, known as hst-1 or KS-FGF (Bovi et al. , Cell (1987) £0:729-737; Taira et al., Proc. Natl. Acad. Sci. USA (1987) 84:2980- 2984) , FGF-5 (Zahn et al. , Mol . Cell. Biol . (1988) £:3487-3495) , FGF-6 (Maries et al. , Oncogene (1989) 4:335-340) and keratinocyte growth factor, KGF or FGF- 7 (Finch et al., Science (1989) 245:752-755. FGF receptors appear to mediate the effects of the various FGFs on cells. Two classes of receptors have been identified for the acidic and basic FGFs and a number of FGF receptors have now been cloned. See, e.g., Kaner et al., Science (1990) 24 :1410-1413; Mansukhani et al. , Proc. Natl. Acad. Sci. USA (1990) £7:4378-4382; Dionne et al. , EMBO J (1990) 9 . :2685-2692; Mirda and Williams, Clin. Res. (1990) 3_8:310A; and Kiefer et al . , Growth Factors (1991) £:115-127. Kiefer et al . , supra, cloned an FGF receptor from a human cell line cDNA library. The cDNA encodes a three-immunoglobulinlike-domain FGF receptor and is capable of binding both acidic and basic FGFs. A soluble, extracellular domain form of this FGF receptor was produced in a baculovirus expression system and termed EC-FGF. An FGF receptor has been implicated as a cellular receptor for HSV-1 (Kaner et al . , Science (1990) 24£:1410-1413) . The initial HSV-1 virion attachment to cells requires an interaction with heparin like cell associated glycosaminoglycans and may be mediated by HSV envelope glycoproteins gB and gC. WuDunn and Spear, J. Virol. (1989) £1:52-58. gB and gD are essential for the secondary interactions at the cell surface that lead to virus entry into cells (Cai et al., J. Virol. (1988) £1:2596-2604; Fuller and Spear Proc. Natl. Acad. Sci. USA (1987) 84:5454-5458; Ligas and Johnson J. Virol. (1988) 62:1486-1494; Johnson et al., J. Virol. (1990) £4:2569-2576) , and gH is probably also involved in the viral penetration (Fuller et al. , J. Virol. (1989) £1:3435-3443) . The use of an FGF receptor as an escort for increasing recombinant expression of a desired protein has not heretofore been suggested. Disclosure of the Invention The present invention is based on the discovery that escort proteins can significantly enhance the expression of viral glycoproteins at the cell surface. Accordingly, proteins are secreted from the transformed cell, thereby dramatically increasing protein yields without further extensive manipulation and purification required. Accordingly, in one embodiment, the present invention is directed to a method for recombinantly producing a protein comprising coexpressing in a host cell a first gene encoding the protein with a second gene encoding an escort, under conditions whereby the protein is secreted from the host cell. In still another embodiment, the invention is directed to a method for recombinantly producing CMV gH comprising coexpressing in a host cell a first gene encoding the gH with a second gene encoding soluble FGF receptor, under conditions whereby the gH is secreted from the host cell. In another embodiment, the subject invention is directed to a method for recombinantly producing an immunologically reactive truncated CMV gH wherein the truncated gH lacks all or a portion of a transmembrane binding domain which is present in native human CMV gH. The method comprises coexpressing in a host cell a first gene encoding the truncated gH with a second gene encoding soluble FGF receptor, under conditions whereby the truncated gH is secreted from the host cell . In yet another embodiment, the invention is directed to a method for recombinantly producing CMV gH comprising coexpressing in a host cell a first gene encoding the gH with a second gene encoding UL115, under conditions whereby the gH is secreted from the host cell. In another embodiment, the subject invention is directed to a method for recombinantly producing an immunologically reactive truncated CMV gH wherein the truncated gH lacks all or a portion of a transmembrane binding domain which is present in native human CMV gH, the method comprising coexpressing in a host cell a first gene encoding the truncated gH with a second gene encoding UL115, under conditions whereby the truncated gH is secreted from the host cell. In still another embodiment, the invention pertains to a complex comprising a recombinant, immunologically reactive herpesvirus polypeptide and an escort. These and other embodiments of the subject invention will readily occur to those of skill in the art in light of the disclosure herein. Brief Description of the Drawings Figure 1 is the nucleotide sequence of the CMV Towne gH gene. The DNA sequence and the predicted amino acid sequence are shown. The putative TATA, CAT and polyadenylation sequences have been underlined. Potential N-linked glycosylation sites are overlined and the predicted signal sequence and transmembrane domain are boxed. The locations of p86 tryptic peptides used in the initial identification of gH are indicated by the broken lines (Gin-,-, to Pro.-- and Gln 370 to Gln 377 ) . Figure 2 is a diagram of pCMAdgH6, a CMV gH mammalian expression vector encoding a truncated gH gene. Figure 3 is a diagram of pACgH2, a CMV gH baculovirus transfer vector in which the gH coding sequence has been spliced into the Ba Hl site of pAc373. Figure 4 is a diagram of the truncated gH transfer vector (pACgH6) in which the CMV gH coding sequence has been spliced into the BamHl site of pAc373. Figure 5 depicts a schematic diagram of the human FGF receptor cDNA (f " 2gr5) and sequencing strategy. The translated regions are boxed, and various shaded domains are indicated: S, signal peptide; 1-3, immunoglobulinlike-domains 1-3; ARR, acidic amino acid rich region; TM, transmembrane region; TK, tyrosine kinase domains. Potential Asn- linked glycosylation sites are also indicated (♦) as are the Bglll (G) and EcoRI (E) restriction endonuclease sites. Although shown, the location of the most carboxyl-terminal consensus glycosylation site most likely precludes its use. Sequences were obtained by using M13 primers and specific internal primers. Arrows indicate the direction and extent of individual sequencing runs. The DNA sequence is in the Genbank and EMBL data bases, and accession numbers are available from these organizations. Figure 6 depicts an amino acid sequence comparison of the six different human FGF receptor forms. Sequences have been aligned for maximum identity and those that differ or are deleted have been boxed. Various domains (abbreviations as in Figure 5) and regions used for PCR primers (P1-P4) are indicated above sequence 1 [flg5 ) . The putative signal peptidase cleavage site . is also indicated (1) . Sequence 2 was from A. Isacchi et al. , Nucleic Acids Res . (1990) 11:1906, and sequences 3-6 were from D.E. Johnson et al . , Mol . Cell. Biol . (1990) 10:4728-4736. Figure 7 is a comparison of the nucleotide sequence of the UL115 open reading frame (ORF) from CMV strain Towne with strain AD169. The Towne sequence is shown in its entirety along with the predicted single letter amino acid code translation product. Nucleotide differences in the AD169 strain sequence are shown above the Towne sequence in lower case letters. Asterisked base changes indicate those changes that result in an amino acid change in the AD169 gene product. Figure 8 is a representation of an autoradiogram showing that coexpression of the gene product of the UL115 open reading frame with gH results in secretion of gH. A stable gH expressing CHO cell line 171 was transfected with: (1) plasmid pFGFrtpaneo expressing truncated FGF r to generate cell line 171-3-16 (lane 2) , (2) vector control plasmid pSV2neo to generate cell lines 171-neo-l, 171-neo-2, 171-neo-3, 171-neo-5, 171-neo-6, 171-neo-8 (lanes 3- 8) , and (3) plasmid pMCUL115neo expressing the gene product of the UL115 ORF from CMV strain Towne to generate cell line 171-UL115-18, 171-UL115-35, 171- UL115-44, and 171-UL115-91 (lanes 9-12) . CHO cell line MCMV-Adhfr (lane 1) was generated by transfection of CHO cells with the mammalian cell expression vector pMCMVAdhfr (Spaete et al. J. Virol. (1990) 64:2922- 2931) and represents a negative control. Cells were labeled with 230 μCl/ml [ 35 S] cysteine for 5 hours in DME lacking cysteine plus 10% dialyzed fetal calf serum. Conditioned media were immunoprecipitated with murine monoclonal antibody 14-4b specific for CMV gH (Urban et al. , J. Virol. (1992) ££:1303-1311) . The protein molecular mass standards are shown on the left of the autoradiogram in kilodaltons, and the mobility of gH and the UL115 precipitated products are indicated on the right. Detailed Description of the Invention The practice of the present invention will employ, unless otherwise indicated, conventional methods of virology, microbiology, molecular biology and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Sambrook, et al. , Molecular Cloning: A Laboratory Manual (2nd Edition, 1989) ; Maniatis et al . , Molecular Cloning: A Laboratory Manual (1982) ; DNA Cloning: A Practical Approach, vol. I & II (D. Glover, ed.) ; Oligonucleotide Synthesis (N. Gait, ed. , 1984) ; Nucleic Acid Hybridization (B. Hames & S. Higgins, eds. , 1985) ; Transcription and Translation (B. Hames & S. Higgins, eds., 1984) ; Animal Cell Culture (R. Freshney, ed. , 1986) ; Perbal, A Practical Guide to Molecular Cloning (1984) . All publications, patents and patent applications cited herein, whether supra or infra, are hereby incorporated by reference in their entirety. I. Definitions In describing the present invention, the following terms will be employed, and are intended to be defined as indicated below. The terms "polypeptide" and "protein" refer to a polymer of amino acid residues and are not limited to a minimum length of the product. Thus, peptides, oligopeptides, dimers, multimers, and the like, are included within the definition. Both full- length proteins and fragments thereof are encompassed by the definition. The terms also include postexpression modifications of the polypeptide, for example, glycosylation, acetylation, phosphorylation and the like. A polypeptide is "immunologically reactive" when it includes one or more epitopes and thus elicits antibodies that neutralize infectivity, and/or mediate antibody-complement or antibody dependent cell cytotoxicity to provide protection to an immunized host. Immunological reactivity may be determined in a standard immunoassay, such as a competition assay, as is known in the art. A "fragment" of a reference polypeptide is any contiguous amino acid sequence found in the reference polypeptide. Such fragments will usually be at least 5 amino acids in length, preferably at least about 10 to 15 amino acids in length. There is no critical upper limit to the length of the fragment which could comprise nearly the full-length of the protein sequence or even a fusion protein. Preferably, the fragment encodes an epitope from the polypeptide, most preferably a neutralizing epitope. A first polypeptide comprises a fragment of another polypeptide even if the homologous domain in the first polypeptide is flanked by amino acid sequences which are not fragments of the other polypeptide. The term "escort" as used herein is defined functionally as any compound capable of associating with a protein coexpressed therewith, thereby facilitating transport of the protein to the cell surface where the protein can be secreted. By way of example, the soluble human FGF receptor and CMV UL115 have been found to be capable of forming complexes with CMV gH, thereby aiding egress of the gH through the host cell. Similarly, HSV gL has been shown to form a complex with HSV gH such that the two proteins can be transported to the cell surface for secretion. Accordingly, these molecules are considered "escorts" in the context of the present invention. It is not necessary that the full-length protein be present in order to serve an escort function. Indeed, the soluble FGF receptor is a truncated molecule having only the extracellular domain (described further below) . Other escorts and proteins with which they interact are described more fully below. "Cytomegalovirus gH or CMV gH polypeptide" refers to a polypeptide comprising a fragment of native human CMV gH. Thus, the term includes both polypeptides comprising the native sequence of gH (full-length and truncated) , as well as analogs thereof. Preferred analogs are those which are substantially homologous to the corresponding native amino acid sequence, and most preferably encode at least one native gH epitope, such as a neutralizing epitope. A particularly preferred class of CMV gH polypeptides are truncated molecules lacking a sufficient portion of the C-terminal transmembrane domain, to promote efficient expression and/or secretion of the CMV gH polypeptide at high levels from the host cell. It is believed that the about 25 C-terminal amino acid residues (residues 718 to 742 of strain Towne, Figure 1) comprise the transmembrane domain, but other regions may also be critical to transmembrane binding. Deletions of all or parts of such domains that eliminate or substantially decrease transmembrane binding, as well as sequences adjacent to these domains are desired. Typically, at least about 5 amino acids will be deleted, preferably at least about 10 residues, and most preferably from at least about 20 residues to 34 residues will be deleted from the C-terminus of the native sequence. Examples of such deletions from one strain are the residues numbered in Figure 1, 732 to 742, 722 to 742, 720 to 742, 718 to 742 and 712 to 742. Of course, other functional deletions can be readily defined by those of ordinary skill by constructing and screening deletions from the same or other domains by expressing the polypeptides in host cells. The only true upper limit to the deletions is the practical limitations of retaining useful epitopes (e.g., neutralizing epitopes) . Typically, however, the deletions will not constitute more than about 100 amino acids of the native gH sequence, particularly the 100 C-terminal residues. It should also be understood that "deletion" of a portion of a transmembrane domain means only that the particular native amino acid sequences do not appear in the polypeptide, and that other amino acids (such as hydrophilic residues) can be substituted for the deleted residues. The term "gL" refers to the gene product of the ULl ORF of HSV. The gene encoding HSV-1 gL has been previously described. McGeoch et al. , J. Gen. Virol . (1988) £9 . :1531-1574. Nucleotide sequence analysis predicts a protein of 224 amino acids with a putative signal sequence and a single site for attachment of N-linked oligosaccharides. The HSV-1 gL protein appears to exist as a 30 kDa precursor form and a 40 kDa mature form. For a further description of HSV-1 gL, see, Hutchinson et al. , J. Virol. (1992) £6:2240-2250, and Hutchinson et al. , Abstract, XVI International Herpesvirus Workshop, July 7-12, 1991. As with gH, the gL for use in the present invention may be a gL polypeptide, either the full-length molecule or an active fragment thereof. Additionally, the term encompasses analogs of the native sequence which are substantially homologous and immunologically reactive. Thus, amino acid substitutions, deletions and additions are contemplated herein that do not destroy the immunoreactivity of the protein. By "UL115" is meant a protein substantially homologous to the gene product of the CMV UL115 ORF, which retains the escort function of the native protein. The strain Towne and AD169 CMV nucleotide sequences for full-length UL115 are depticted in Figure 7. In both strains, the UL115 ORF is 834 bp in length and encodes a primary translation product of 278 amino acids. UL115 appears to be the CMV homologue to HSV-1 gL, which is encoded by the ULl gene of HSV-1. (See, Hutchinson et al . , J. Virol. (1992) £6:2240-2250, and Hutchinson et al. , Abstract, XVI International Herpesvirus Workshop, July 7-12, 1991, for a further discussion of HSV gL.) A "UL115 polypeptide" is a contiguous sequence of amino acids derived from UL115. The polypeptide may consist of the native sequence or may include substitutions, additions or deletions, so long as the molecule retains the ability to form complexes with a protein coexpressed therewith and escort the protein to the cell surface for secretion. The formation of such complexes can be monitored using assays such as immunoprecipitation, described further in the examples. The term "FGF receptor" or "FGF-R" as used herein refers to the human FGF receptor or a fragment thereof which binds FGF in the presence of heparin. The FGF receptor has an amino acid sequence substantially as depicted in Figure 6. The term "rFGF-R" refers to active FGF-R prepared by recombinant means. A preferred form of rFGF-R is sol¬ uble rFGF-R ("sFGF-R" or "EC-FGF") , which is a truncated form obtained by expressing only the extracellular domain. It is surprisingly found that the truncated form is able to act as an escort for viral glycoproteins. The preferred sFGF-R of the invention is a 58 kDa glycoprotein which binds bFGF with a K d of 2-5 nM. As with UL115, an "FGF receptor polypeptide" is a contiguous sequence of amino acids derived from the FGF receptor which may consist of the native sequence or may include substitutions, additions or deletions, so long as the molecule retains the ability to form complexes with a protein coexpressed therewith and escort the protein to the cell surface for secretion. The formation of such complexes can be monitored using assays such as immunoprecipitation, described further in the examples. By "HSV gH" or "HSV gH polypeptide" is meant the art-recognized gH protein from herpes simplex virus, as well as active fragments and analogs thereof. Thus, as above, the term captures amino acid substitutions, deletions and additions which do not destroy the immunoreactivity of the protein. HSV-1 gH has an apparent molecular weight of approximately 110,000 and the gene encoding this protein has been sequenced. (Gompels and Minson, Virology (1986) 111:230-247; McGeoch and Davison, Nucleic Acids Res. (1986) 14:4281-4292.) The gene has been expressed in mammalian cells. Gompels and Minson, J. Virol. (1989) 63:4744-4755. The term "complex" as used herein denotes any association of an escort with a compatible protein, regardless of the nature of the association. Accordingly, the term captures molecules which are covalently bound to one another as well as those associated with each other via electrostatic and other forces. Complexes can be detected using assays such as immunoprecipitation, described in the examples. "Operably linked" refers to a juxtaposition wherein the components so described are in a relationship permitting them to function in their intended manner. A regulatory element "operably linked" to a structural sequence is ligated in such a way that expression of the structural sequence is achieved under conditions compatible with the regulatory elements. "Recombinant" as used herein to describe a polynucleotide means a polynucleotide of genomic, cDNA, semisynthetic, or synthetic origin which, by virtue of its origin or manipulation: (1) is not as¬ sociated with all or a portion of the polynucleotide with which it is associated in nature; and/or (2) is linked to a polynucleotide other than that to which it is linked in nature. The term "recombinant" as used with respect to a protein or polypeptide means a polypeptide produced by expression of a recombinant polynucleotide. "Recombinant host cells, " "host cells," "cells," "cell lines," "cell cultures," and other such terms denoting prokaryotic microorganisms or eukaryotic cell lines cultured as unicellular enti¬ ties, are used interchangeably, and refer to cells which can be, or have been, used as recipients for recombinant vectors or other transfer DNA, and include the progeny of the original cell which has been transfected. It is understood that the progeny of a single parental cell may not necessarily be completely identical in morphology or in genomic or total DNA complement to the original parent, due to accidental or deliberate mutation. Progeny of the parental cell which are sufficiently similar to the parent to be characterized by the relevant property, such as the presence of a nucleotide sequence encoding a desired peptide, are included in the progeny intended by this definition, and are covered by the above terms. A "regulatory element" refers to a polynucleotide sequence which effects the expression of a coding sequence to which it is linked. The term includes promoters, terminators, and when appropriate, leader sequences and enhancers. A "replicon" is any genetic element, e.g., plasmid, cosmid, chromosome, virus, or phage, that behaves as an autonomous unit of polynucleotide replication within a cell. A sequence which is "substantially homologous" to a reference sequence shares at least about 50% sequence homology, preferably at least about 75%, more preferably at least about 85%, and most preferably at least about 90% to 95%. The term "substantially homologous" as used herein also includes proteins showing sequence identity to the reference sequence. "Transformation, " as used herein, refers to the insertion of an exogenous polynucleotide into a host cell, irrespective of the method used for insertion: for example, direct uptake, transduction, or f-mating. The exogenous polynucleotide may be maintained as a nonintegrated vector, for example, a plasmid, or alternatively, may be integrated into the host genome. A "vector" is a replicon in which a heterologous polynucleotide segment is attached, so as to bring about the replication and/or expression of the attached segment, such as a plasmid, transposon, phage, etc. "Coexpression" as used herein refers to the expression of two or more different proteins in a host cell. The polynucleotides encoding for the proteins can be harbored in a single plasmid, either under the control of the same regulatory elements or under the control of separate elements. Thus, the production of a fusion protein including active portions of the two or more protein sequences would be considered "coexpressed" for purposes of the present definition as would the expression of two genes as a dicistronic construct employing an internal ribosome entry site. Similarly, proteins expressed from the same vector but driven by separate regulatory elements, would also be considered "coexpressed." The term also refers to the expression of two or more proteins from separate constructs. Thus, the expression of proteins encoded from genes present on separate vectors in a host cell would also be considered "coexpression" for purposes of the present invention. II. Modes of Carrying Out the Invention The present invention is based on the discovery of particular escorts capable of shuttling proteins coexpressed therewith to the cell surface where the expressed products can be secreted. Protein yields are thereby increased due to facilitated egress of the proteins and purification of the expressed product can be easily accomplished. Without being bound by a particular theory, it appears that the escort allows egress from the host cell by associating with the protein, thereby providing for release of the protein from the endoplasmic reticulum. It is possible that the escort hides a retention signal and/or confers a tertiary conformation on the protein which permits such egress. The present invention will find use with a wide variety of proteins. Indeed, almost any desired protein can be produced using a compatible escort. Thus, the invention permits increased yields of such useful products as viral and bacterial antigens for use in vaccines and diagnostics, peptide hormones and drugs for use in pharmaceuticals, and marker proteins and the like with a wide variety of uses. Antibodies raised against these proteins are also useful as diagnostics. The present invention is particularly useful in the production of viral glycoproteins, especially those glycoproteins which are expressed intracellularly in recombinant systems. For example, the present invention will find use for the expression of a wide variety of proteins from the herpesvirus family, including proteins derived from herpes simplex virus (HSV) , varicella zoster virus (VZV) , Epstein- Barr virus (EBV) , cytomegalovirus (CMV) and other human herpesviruses such as HHV6 and HHV7. Proteins from other viruses, such as but not limited to, proteins from the hepatitis family of viruses, including hepatitis A virus (HAV) , hepatitis B virus (HBV) , hepatitis C virus (HCV) , the delta hepatitis virus (HDV) and hepatitis E virus (HEV) , as well as retrovirus proteins such as from HTLV-I and HTLV-II and proteins from the human immunodeficiency viruses (HIVs) , such as HIV-1 and HIV-2, can also be conveniently expressed using the present system. (See, e.g. Chee et al . , Cytomegaloviruses (J.K. McDougall, ed. , Springer-Verlag 1990) pp. 125-169, for a review of the protein coding content of cytomegalovirus; McGeoch et al., J. Gen. Virol. (1988) £1:1531-1574, for a discussion of the various HSV-1 encoded proteins; Baer et al. , Nature (1984) 110:207- 211, for the identification of protein coding sequences in an EBV genome, Davison and Scott, J. Gen. Virol. (1986) £7:1759-1816, for a review of VZV; Houghton et al . , Hepatology (1991) 11:381-388, for a discussion of the HCV genome; and Sanchez-Pescador et al., Science (1985) 221:484-492, for an HIV genome.) The above proteins are coexpressed with a compatible escort such that the protein of interest can be efficiently secreted from the host cell. Compatible escorts will be readily identifiable by assaying for the formation of complexes between the putative escort and the protein of interest, using known techniques such as electrophoresis and immunoprecipitation. The detection of such complexes is described further in the examples and in Hutchinson et al., J. Virol. (1992) £6:2240-2250. By way of example, HSV-l gL (encoded by the ULl ORF) has been found to form a complex with HSV-l gH (encoded by the UL22 ORF) (Hutchinson et al. , J __ Virol. (1992) £6:2240-2250) . When gH and gL are coexpressed, they are present at the cell surface, unlike gH and gL expressed in the absence of other viral proteins. Proteins functionally and/or structurally homologous to HSV-l gH and gL are found throughout the herpesvirus family. For instance, the EBV ORF BKRF2 encodes the EBV homologue of HSV-l gL and ORF BXLF2 encodes the homologue for gH (termed gp85) . Baer et al . , Nature (1984) £10:207-211; Heineman et al., J. Virol. (1988) 62:1101-1107. Similarly, gene 60 of VZV is the VZV homologue of HSV ULl and encodes a protein termed VZV37. Davison and Scott, J. Gen. Virol. (1986) £7:1759-1816, McGeoch et al., J. Gen. Virol. (1988) £9:1531-1574; Cranage et al., J. Virol. (1988) £2:1416-1422. Likewise, CMV gH (encoded by UL75) is the homologue to HSV-l gH (Pachl et al., Virology (1989) 169 :418-426; Cranage et al . , J. Virol. (1988) £1:1416-1422; Chee et al. , Cytomegaloviruses (J.K. McDougall, ed. , Springer- Verlag 1990) pp. 125-169) . The HHVs also encode gH (as well as other herpesvirus protein) analogs. Accordingly, glycoprotein homologues, found in other herpesviruses, are also encompassed by the present invention. Other herpesvirus glycoproteins which will be conveniently expressed using escorts include the various HSV glycoproteins, including HSV gB and HSV gD; CMV gB; and the HHV homologues to these proteins. Of particular interest is the present discovery that CMV ULl15 is the homologue of HSV gL. As described further herein, like HSV-l gL and gH, when UL115 and CMV gH are coexpressed, the two proteins form complexes and are secreted from the host cell. As with CMV gL, the FGF receptor has been found to serve an escort function with CMV gH and may function as an escort for other homologous herpesvirus proteins. As explained above, the hepatitis viral products will also be conveniently expressed using the present system. By way of example, the HCV genome encodes several viral proteins, including El (also known as E) and E2 (also known as E2/NSI) . (See, Houghton et al. , Hepatology (1991) 14:381-388, for a discussion of HCV proteins, including El and E2, and EPO publication No. 388,232, for the HCV genomic sequence) . These proteins are membrane associated asialoglycoproteins when expressed in recombinant systems. The HCV virus may gain entry into host cells during infection through either the asialoglycoprotem receptor found on hepatocytes, or through the mannose receptor found on hepatic endothelial cells and macrophages. (See, Exekowitz et al. , J. Ex . Med. (1990) 126:1785-1794; Kurata et al . , J. Biol . Chem. (1990) 2£5:11295-11298; Schuffenecker et al. , Cytogenet. Cell. Genet. (1991) £6:99-102; and Sastry et al., J. Immunol . (1991) 147:692-697 for a discussion of mannose and asialoglycoprotem receptors. ) The mannose receptor has been cloned (Taylor et al . , J. Biol. Chem. (1990) 265:12156- 12162) , as has the asialoglycoprotem receptor (Drickamer et al . , J. Biol. Chem. (1984) 251:770-778; Spiess et al. , Proc. Natl. Acad. Sci. USA (1985) £1:6465-6469; McPhaul and Berg, Proc. Natl. Acad. Sci USA (1986) £3:8863-8867; McPhaul and Berg, Mol . Cell. Biol. (1987) 2:1841-1847) . These receptors will find use as escorts for the recombinant production of HCV proteins such as El and E2. The expression of HIV proteins can also be facilitated using the present methods. The HIV env gene encodes the envelope proteins of the virus, including the glycoproteins gpl60, gpl20 and gp41. As explained above, gpl60 is not properly processed or transported in recombinant systems. (Haffar et al. , J. Virol. (1990) £4:3100-3103.) Furthermore CD4, the cellular receptor for HIV, appears to play a role in protein transport. Specifically, a CD4 mutant, modified to contain a specific retention signal for the endoplasmic reticulum, successfully blocks secretion of gpl20 and surface expression of gpl60. Buonocore and Rose, Nature (1990) 345:625-628. Accordingly, the use of CD4 as an escort for facilitating expression of the HIV envelope proteins is also contemplated herein. Additional examples of proteins and compatible escorts for coexpression therewith will readily occur to those of skill in the art in view of the above discussion. A. Isolating the Desired Gene As explained above, the proteins and escorts for use with the present invention can be recombinantly produced. DNA encoding the protein of interest can be genomic, cDNA or synthetic DNA. Methods for obtaining or synthesizing DNA for subsequent cloning are well known in the art. For example, the desired protein can be purified and the amino acid sequence determined by repetitive cycles of Edman degradation, followed by amino acid analysis by HPLC. Other methods of amino acid sequencing are also known in the art. Once the amino acid sequences are determined, oligonucleotide probes which contain the codons for a portion of the determined amino acid sequences can be prepared and used to screen DNA libraries for genes encoding the subject proteins. The basic strategies for preparing oligonucleotide probes and DNA libraries, as well as their screening by nucleic acid hybridization, are well known to those of ordinary skill in the art. See, e.g., DNA Cloning: Vol. I, supra; Nucleic Acid Hybridization, supra; Oligonucleotide Synthesis, supra; T. Maniatis et al. , supra . First, a DNA library is prepared. Once the library is constructed, oligonucleotides to probe the library are prepared and used to isolate the gene en¬ coding the desired protein. The oligonucleotides are synthesized by any appropriate method. The particular nucleotide sequences selected are chosen so as to cor- respond to the codons encoding a known amino acid sequence from the desired protein. Since the genetic code is degenerate, it will often be necessary to synthesize several oligonucleotides to cover all, or a reasonable number, of the possible nucleotide sequences which encode a particular region of the protein. Thus, it is generally preferred in selecting a region upon which to base the probes, that the region not contain amino acids whose codons are highly degenerate. In certain circumstances, one of skill in the art may find it desirable to prepare probes that are fairly long, and/or encompass regions of the amino acid sequence which would have a high degree of redundancy in corresponding nucleic acid sequences, particularly if this lengthy and/or redundant region is highly characteristic of the protein of interest. It may also be desirable to use two probes (or sets of probes) , each to different regions of the gene, in a single hybridization experiment. Automated oligonucleotide synthesis has made the preparation of large families of probes relatively straightforward. While the exact length of the probe employed is not critical, generally it is recognized in the art that probes from about 14 to about 20 base pairs are usually effective. Longer probes of about 25 to about 60 base pairs are also used. The selected oligonucleotide probes are labeled with a marker, such as a radionucleotide or biotin using standard procedures. The labeled set of probes is then used in the screening step, which consists of allowing the single-stranded probe to hybridize to isolated ssDNA from the library, according to standard techniques. Either stringent or permissive hybridization conditions could be appropriate, depending upon several factors, such as the length of the probe and whether the probe is derived from the same species as the library, or an evolutionarily close or distant species. The selection of the appropriate conditions is within the skill of the art. See, generally, Nucleic Acid hybridization, supra . The basic requirement is that hybridization conditions be of sufficient stringency so that selective hybridization occurs; i.e., hybridization is due to a sufficient degree of nucleic acid homology (e.g., at least about 75%) , as opposed to nonspecific binding. Once a clone from the screened library has been identified by positive hybridization, it can be confirmed by restriction enzyme analysis and DNA sequencing that the particular library insert contains a gene for the desired protein. Alternatively, DNA sequences encoding the proteins of interest can be prepared synthetically rather than cloned. The DNA sequence can be designed with the appropriate codons for the particular amino acid sequence. In general, one will select preferred codons for the intended host if the sequence will be used for expression. The complete sequence is assembled from overlapping oligonucleotides prepared by standard methods and assembled into a complete coding sequence. See, e.g., Edge Nature (1981) 1H:756; Nambair et al . Science (1984) 221:1299; Jay et al. J. Biol. Chem. (1984) 259:6311. B. Expression Systems Once the appropriate coding sequence for the desired protein and escort are isolated or synthesized, they can be coexpressed in a variety of different expression systems; for example those used with mammalian cells, baculoviruses, bacteria, and yeast. For coexpression, two or more plasmids encoding the desired products can be employed. Alternatively, a single construct, encoding both the desired protein and the escort, can be used. The single construct can either consist of a chimeric DNA molecule encoding for a fusion protein including the escort and desired protein, or can include DNAs coding for two separate products. i. Mammalian Systems Mammalian expression systems are known in the art and will find use in the subject invention. Mammalian expression systems will include a mammalian promoter which is any DNA sequence capable of binding mammalian RNA polymerase and initiating the downstream (3') transcription of a coding sequence (e.g., structural gene) into mRNA. A promoter will have a transcription initiating region, which is usually placed proximal to the 5' end of the coding sequence, and a TATA box, usually located 25-30 base pairs (bp) upstream of the transcription initiation site. The TATA box is thought to direct RNA polymerase II to begin RNA synthesis at the correct site. A mammalian promoter will also contain an upstream promoter element, typically located within 100 to 200 bp upstream of the TATA box. An upstream promoter element determines the rate at which transcription is initiated and can act in either orientation (Sambrook et al. , Molecular Cloning: A Laboratory Manual. 2nd ed. , supra . Mammalian viral genes are often highly expressed and have a broad host range. Therefore, sequences encoding mammalian viral genes provide particularly useful promoter sequences. Examples include the SV40 early promoter, mouse mammary tumor virus LTR promoter, adenovirus major late promoter (Ad MLP) , and herpes simplex virus promoter. In addition, sequences derived from nonviral genes, such as the murine metallothionein gene, also provide useful promoter sequences. Expression may be either constitutive or regulated (inducible) , depending on the promoter and can be induced with glucocorticoid in hormone-responsive cells. Enhancer elements can also be used. The presence of an enhancer element (enhancer) , combined with the promoter elements described above, will typically increase expression levels. An enhancer is a regulatory DNA sequence that can stimulate transcription up to 1000-fold when linked to homologous or heterologous promoters, with synthesis beginning at the normal RNA start site. Enhancers are also active when they are placed upstream or downstream from the transcription initiation site, in either normal or flipped orientation, or at a distance of more than 1000 nucleotides from the promoter (Maniatis et al., Science (1987) 216:1237; Alberts et al. (1989) Molecular Biology of the Cell, 2nd ed.) . Enhancer elements derived from viruses may be particularly useful because they typically have a broader host range. Examples include the SV40 early gene enhancer (Dijkema et al . , EMBO J. (1985) 4 . :761) and the enhancer/promoters derived from the long terminal repeat (LTR) of the Rous Sarcoma Virus (Gorman et al. , Proc. Natl. Acad. Sci. USA (1982b) 21:6777) and from human cytomegalovirus (Boshart et al., Cell (1985) 41:521) . Additionally, some enhancers are regulatable and become active only in the presence of an inducer, such as a hormone or metal ion (Sassone-Corsi and Borelli, Trends Genet. (1986) 1:215; Maniatis et al. , Science (1987) 236:1237) . The desired proteins and escorts can also be expressed as fusion proteins comprised of a leader sequence fragment that provides for secretion of the foreign protein in mammalian cells. Preferably, there are processing sites encoded between the leader fragment and the foreign gene that can be cleaved either in vivo or in vi tro . The leader sequence fragment typically encodes a signal peptide comprised of hydrophobic amino acids which direct the secretion of the protein from the cell. The adenovirus tripartite leader is an example of a leader sequence that provides for secretion of a foreign protein in mammalian cells. Typically, transcription termination and polyadenylation sequences recognized by mammalian cells are regulatory regions located 3' to the trans- lation stop codon and thus, together with the promoter elements, flank the coding sequence. The 3' terminus of the mature mRNA is formed by site-specific posttranscriptional cleavage and polyadenylation (Birnstiel et al. , Cell (1985) 41:349; Proudfoot and Whitelaw (1988) "Termination and 3' end processing of eukaryotic RNA. In Transcription and Splicing (ed. B.D. Hames and D.M. Glover) ; Proudfoot, Trends Biochem. Sci. (1989) 14:105) . These sequences direct the transcription of an mRNA which can be translated into the polypeptide encoded by the DNA. Examples of transcription terminator/polyadenylation signals include those derived from SV40 (Sambrook et al. (1989) "Expression of cloned genes in cultured mammalian cells" in Molecular Cloning: A Laboratory Manual) . Some genes may be expressed more efficiently when introns (also called intervening sequences) are present. Several cDNAs, however, have been effi¬ ciently expressed from vectors that lack splicing signals (also called splice donor and acceptor sites) (see, e.g., Gothing and Sambrook, Nature (1981) 293 : 620) . Introns are intervening noncoding sequences within a coding sequence that contain splice donor and acceptor sites. They are removed by a process called "splicing, " following polyadenylation of the primary transcript (Nevins, Ann. Rev. Biochem. (1983) £2:441; Green, Ann. Rev. Genet. (1986) 10:671; Padgett et al . , Ann. Rev. Biochem. (1986) £5:1119; Krainer and Maniatis, (1988) "RNA splicing" in Transcription and splicing (ed. B.D. Hames and D.M. Glover) ) . Typically, the above described components, comprising a promoter, polyadenylation signal, and transcription termination sequence are put together into expression constructs. Enhancers, introns with functional splice donor and acceptor sites, and leader sequences may also be included in an expression construct, if desired. Expression constructs are often maintained in a replicon, such as an extrachro- mosomal element (e.g., plasmids) capable of stable maintenance in a host, such as mammalian cells or bacteria. Mammalian replication systems include those derived from animal viruses, which require trans- acting factors to replicate. For example, plasmids containing the replication systems of papovaviruses, such as SV40 (Gluzman, Cell (1981) H:175) or polyomavirus, replicate to extremely high copy number in the presence of the appropriate viral T antigen. Additional examples of mammalian replicons include those derived from bovine papillomavirus and Epstein- Barr virus. Additionally, the replicon may have two replicaton systems, thus allowing it to be maintained, for example, in mammalian cells for expression and in a procaryotic host for cloning and amplification. Examples of such mammalian-bacteria shuttle vectors include pMT2 (Kaufman et al. , Mol. Cell. Biol. (1989) 1:946) and pHEBO (Shimizu et al. , Mol. Cell. Biol. (1986) £:1074) . The transformation procedure used depends upon the host to be transformed. Methods for introduction of heterologous polynucleotides into mammalian cells are known in the art and include dextran-mediated transfection, calcium phosphate precipitation, polybrene mediated transfection, protoplast fusion, electroporation, encapsulation of the polynucleotide (s) in liposomes, and direct microinjection of the DNA into nuclei. Mammalian cell lines available as hosts for expression are known in the art and include many immortalized cell lines available from the American Type Culture Collection (ATCC) , including but not limited to, Chinese hamster ovary (CHO) cells, HeLa cells, baby hamster kidney (BHK) cells, monkey kidney cells (COS) , human hepatocellular carcinoma cells (e.g., Hep G2) , and a number of other cell lines. ii. Baculovirus Systems The polynucleotides encoding the desired protein and escort can also be inserted into a suitable insect expression vector, where they are operably linked to the control elements within that vector. Vector construction employs techniques which are known in the art. Generally, the components of the expression system include a transfer vector, usually a bacterial plasmid, which contains both a fragment of the baculovirus genome, and a convenient restriction site for insertion of the heterologous gene or genes to be expressed; a wild type baculovirus with a sequence homologous to the baculovirus-specific fragment in the transfer vector (this allows for the homologous recombination of the heterologous gene into the baculovirus genome) ; and appropriate insect host cells and growth media. After inserting the DNA sequences encoding the desired protein and escort into one or more transfer vectors, the vector(s) and the wild type viral genome are transfected into an insect host cell where the vector(s) and viral genome are allowed to recombine. The packaged recombinant virus is expressed and recombinant plaques are identified and purified. Materials and methods for baculovirus/insect cell expression systems are commercially available in kit form from, inter alia , Invitrogen, San Diego CA ("MaxBac" kit) . These techniques are generally known to those skilled in the art and fully described in Summers and Smith, Texas Agricultural Experiment Station Bulletin No. 1555 (1987) (hereinafter "Summers and Smith") . Prior to inserting the DNA sequences encoding the protein and escort into the baculovirus genome, the above described components, comprising a promoter, leader (if desired) , coding sequence of interest, and transcription termination sequence, are typically assembled into an intermediate transplacement construct (transfer vector) . This construct may contain a single gene and operably linked regulatory elements; multiple genes, each with its own set of operably linked regulatory elements; or multiple genes, regulated by the same set of regulatory elements . Intermediate transplacement constructs are often maintained in a replicon, such as an extrachromosomal element (e.g., plasmids) capable of stable maintenance in a host, such as a bacterium. The replicon will have a replication system, thus allowing it to be maintained in a suitable host for cloning and amplification. Currently, the most commonly used transfer vector for introducing foreign genes into AcNPV is pAc373. Many other vectors, known to those of skill in the art, have also been designed. These include, for example, pVL985 (which alters the polyhedrin start codon from ATG to ATT, and which introduces a BamHl cloning site 32 basepairs downstream from the ATT; see Luckow and Summers, Virology (1989) 17:31) . The plasmid usually also contains the polyhedrin polyadenylation signal (Miller et al . Ann. Rev. Microbiol. (1988) 41:177) and a procaryotic ampicillin-resistance (amp) gene and origin of replication for selection and propagation in E . coli . Baculovirus transfer vectors usually contain a baculovirus promoter. A baculovirus promoter is any DNA sequence capable of binding a baculovirus RNA polymerase and initiating the downstream (5' to 3') transcription of a coding sequence (e.g., structural gene) into mRNA. A promoter will have a transcription initiation region which is usually placed proximal to the 5' end of the coding sequence. This transcription initiation region typically includes an RΝA polymerase binding site and a transcription initiation site. A baculovirus transfer vector may also have a second domain called an enhancer, which, if present, is usually distal to the structural gene. Expression may be either regulated or constitutive. Structural genes, abundantly transcribed at late times in a viral infection cycle, provide particularly useful promoter sequences. Examples include sequences derived from the gene encoding the viral polyhedron protein, Friesen et al. , (1986) "The Regulation of Baculovirus Gene Expression" in: The Molecular Biology of Baculoviruses (ed. Walter Doerfler) ; EPO Publication Νos. 127,839 and 155,476; and the gene encoding the plO protein Vlak et al. , J. Gen. Virol. (1988) £1:765. DΝA encoding suitable signal sequences can be derived from genes for secreted insect or baculovirus proteins, such as the baculovirus polyhedrin gene (Carbonell et al . Gene (1988) 73 :409) . Alternatively, since the signals for mammalian cell posttranslational modifications (such as signal peptide cleavage, proteolytic cleavage, and phosphorylation) appear to be recognized by insect cells, and the signals required for secretion and nuclear accumulation also appear to be conserved between the invertebrate cells and vertebrate cells, leaders of noninsect origin, such as those derived from genes encoding human α-interferon, Maeda et al . , Nature (1985) 315 :592 ; human gastrin-releasing peptide, Lebacq-Verheyden et al. , Molec. Cell. Biol. (1988) £:3129; human IL-2, Smith et al . , Proc. Nat'l Acad. Sci. USA (1985) £1:8404; mouse IL-3, (Miyajima et al., Gene (1987) £8:273; and human glucocerebrosidase, Martin et al. , DNA (1988) 2 99, can also be used to provide for secretion in insects. A recombinant polypeptide or polyprotein when expressed intracellularly will generally include suitable translation initiation signals preceding an ATG start signal. If desired, methionine at the N- terminus may be cleaved from the mature protein by in vi tro incubation with cyanogen bromide. After insertion of the desired DNA sequences, an insect cell host is cotransformed with the heterologous DNA of the transfer vector(s) and the genomic DNA of wild type baculovirus- -usually by cotransfection. The promoter and transcription termination sequence of the construct will typically comprise a 2-5kb section of the baculovirus genome. Methods for introducing heterologous DNA into the desired site in the baculovirus virus are known in the art. (See Summers and Smith, supra; Ju et al. (1987); Smith et al. , Mol. Cell. Biol. (1983) 1:2156; and Luckow and Summers (1989)) . For example, the insertion can be into a gene such as the polyhedrin gene, by homologous double crossover recombination; insertion can also be into a restriction enzyme site engineered into the desired baculovirus gene. Miller et al., Bioessays (1989) 4 . :91. The DNA sequence, when cloned in place of the polyhedrin gene in the expression vector, is flanked both 5' and 3' by polyhedrin-specific sequences and is positioned downstream of the polyhedrin promoter. The newly formed baculovirus expression vector is subsequently packaged into an infectious recombinant baculovirus. Homologous recombination occurs at low frequency (between about 1% and about 5%) ; thus, the majority of the virus produced after cotransfection is still wild-type virus. Thus, the expression system provides a visual screen allowing recombinant viruses to be distinguished from wild-type virus. The polyhedrin protein, which is produced by the native virus, is produced at very high levels in the nuclei of infected cells at late times after viral infection. Accumulated polyhedrin protein forms occlusion bodies which also contain embedded particles. These occlusion bodies, up to 15 μm in size, are highly refractile, giving them a bright shiny appearance that is readily visualized under the light microscope. Cells infected with recombinant viruses lack occlusion bodies. To distinguish recombinant virus from wild-type virus, the transfection supernatant is plaqued onto a monolayer of insect cells by techniques known to those skilled in the art. Namely, the plaques are screened under the light microscope for the presence (indicative of wild-type virus) or absence (indicative of recombinant virus) of occlusion bodies. "Current Protocols in Microbiology" Vol. 2 (Ausubel et al. , eds) at 16.8 (Supp. 10, 1990) ; Summers and Smith, supra; Miller et al. (1989). Recombinant baculovirus expression vectors have been developed for infection into several insect cells. For example, recombinant baculoviruses have been developed for, inter alia : Aedes aegypti , Autographa calif ornica, Bombyx mori , Drosophila melanogaster , Spodoptera frugiperda, and ' Trichoplusia ni (PCT Publication No. WO 89/046699; Carbonell et al., J. Virol. (1985) £6:153; Wright, Nature (1986) 321:718: Smith et al. , Mol. Cell. Biol. (1983) 1:2156; and see generally, Fraser, et al. , In Vi tro Cell. Dev. Biol. (1989) 2£:225) . Cells and cell culture media are commercially available for both direct and fusion expression of heterologous polypeptides in a baculovirus/expression system; cell culture technology is generally known to those skilled in the art. See, e.g., Summers and Smith supra . The infected insect cells may then be grown in an appropriate nutrient medium, which allows for expression of the recombinant product. The product may be purified by such techniques as chromatography, e.g., HPLC, affinity chromatography, ion exchange chromatography, etc.; electrophoresis; density gradient centrifugation; solvent extraction, or the like. As appropriate, the product may be further purified, as required, so as to remove substantially any insect proteins which are also secreted in the medium or result from lysis of insect cells, so as to provide a product which is at least substantially free of host debris, e.g., proteins, lipids and polysaccharides . In order to obtain expression, infected cells are incubated under conditions which allow expression of the recombinant coding sequences. These conditions will vary, depending upon the host cell selected. However, the conditions are readily ascertainable to those of ordinary skill in the art, based upon what is known in the art. iii. Bacterial Systems Bacterial expression techniques are known in the art. Bacterial expression systems will include a bacterial promoter which is any DNA sequence capable of binding bacterial RNA polymerase and initiating the downstream (3") transcription of a coding sequence (e.g., structural gene) into mRNA. A promoter will have a transcription initiation region which is usually placed proximal to the 5' end of the coding sequence. This transcription initiation region typically includes an RNA polymerase binding site and a transcription initiation site. A bacterial promoter may also have a second domain called an operator, that may overlap an adjacent RNA polymerase binding site at which RΝA synthesis begins. The operator permits negative regulated (inducible) transcription, as a gene repressor protein may bind the operator and thereby inhibit transcription of a specific gene. Constitutive expression may occur in the absence of negative regulatory elements, such as the operator. In addition, positive regulation may be achieved by a gene activator protein binding sequence which, if present, is usually proximal (5') to the RNA polymerase binding sequence. An example of a gene activator protein is the catabolite activator protein (CAP) , which helps initiate transcription of the lac operon in Escherichia coli (E. coli ) (Raibaud et al. , Annu. Rev. Genet. (1984) 1£:173) . Regulated expression may therefore be either positive or negative, thereby either enhancing or reducing transcription. Sequences encoding metabolic pathway enzymes provide particularly useful promoter sequences. Examples include promoter sequences derived from sugar metabolizing enzymes, such as galactose, lactose (lac) (Chang et al. , Nature (1977) H£:1056) , and maltose. Additional examples include promoter sequences derived from biosynthetic enzymes such as tryptophan (trp) (Goeddel et al., Nuc. Acids Res. (1980) £:4057; Yelverton et al. , Nucl. Acids Res. (1981) 9 . :731; U.S. Patent No. 4,738,921; EPO Publication Nos. 036,776 and 121,775) . The b-lactamase (bla) promoter system (Weissmann (1981) "The cloning of interferon and other mistakes" in Interferon 3 (ed. I. Gresser) ) , bacteriophage lambda PL (Shimatake et al . , Nature (1981) 211:128) and T5 (U.S. Patent No. 4,689,406) promoter systems also provide useful promoter sequences. In addition, synthetic promoters which do not occur in nature also function as bacterial promoters. For example, transcription activation sequences of one bacterial or bacteriophage promoter may be joined with the operon sequences of another bacterial or bacteriophage promoter, creating a synthetic hybrid promoter (U.S. Patent No. 4,551,433) . For example, the tac promoter is a hybrid trp-lac promoter comprised of both trp promoter and lac operon sequences that is regulated by the lac repressor (Mann et al . , Gene (1983) 2£:167; de Boer et al . , Proc. Natl. Acad. Sci. (1983) £0:21) . Furthermore, a bacterial promoter can include naturally occurring promoters of nonbacterial origin that have the ability to bind bacterial RNA polymerase and initiate transcription. A naturally occurring promoter of nonbacterial origin can also be coupled with a compatible RΝA polymerase to produce high levels of expression of some genes in prokaryotes. The bacteriophase T7 RΝA polymerase/promoter system is an example of a coupled promoter system (Studier et al . , J. Mol. Biol. (1986) 189:113; Tabor et al. , Proc Νatl. Acad. Sci. (1985) £1:1074) . In addition, a hybrid promoter can also be comprised of a bacteriophage promoter and an E. coli operator region (EPO Publication o. 267,851) . In addition to a functioning promoter sequence, an efficient ribosome binding site is also useful for the expression of foreign genes in prokaryotes. In E. coli , the ribosome binding site is called the Shine-Dalgarno (SD) sequence and includes an initiation codon (ATG) and a sequence 3-9 nucleotides in length located 3-11 nucleotides upstream of the initiation codon (Shine et al. , Nature (1975) 2£4:34) . A DNA molecule may be expressed intracellu- larly. A promoter sequence may be directly linked with the DNA molecule, in which case the first amino acid at the N-terminus will always be a methionine, which is encoded by the ATG start codon. If desired, methionine at the N-terminus may be cleaved from the protein by in vi tro incubation with cyanogen bromide or by either in vivo or in vi tro incubation with a bacterial methionine N-terminal peptidase (EPO Publication No. 219,237) . Fusion proteins provide an alternative to direct expression. Typically, a DNA sequence encoding the N-terminal portion of an endogenous bacterial protein, or other stable protein, is fused to the 5' end of heterologous coding sequences . Upon expression, this construct will provide a fusion of the two amino acid sequences. For example, a bacteriophage lambda gene can be linked at the 5' terminus of a foreign gene and expressed in bacteria. The resulting fusion protein preferably retains a site for a processing enzyme (factor Xa) to cleave the bacteriophage protein from the foreign gene (Nagai et al. (1984) Nature 309 : 810) . Fusion proteins can also be made with sequences from the lacZ (Jia et al. , Gene (1987) £0:197) , trpE (Allen et al. , J. Biotechnol . (1987) £:93; Makoff et al . , J. Gen. Microbiol. (1989) 135:11) , and Chey (EPO Publication No. 324,647) genes. The DNA sequence at the junction of the two amino acid sequences may or may not encode a cleavable site. Another example is a ubiquitin fusion protein. Such a fusion protein is made with the ubiquitin region that preferably retains a site for a processing enzyme (e.g., ubiquitin specific processing-protease) to cleave the ubiquitin from the foreign protein. Through this method, native foreign proteins can be isolated (Miller et al. , Bio/Technology (1989) 2:698) . Alternatively, foreign proteins can also be secreted from the cell by creating chimeric DNA molecules that encode a fusion protein comprised of a signal peptide sequence fragment that provides for secretion of the foreign protein in bacteria (U.S. Patent No. 4,336,336) . The signal sequence fragment typically encodes a signal peptide comprised of hydrophobic amino acids which direct the secretion of the protein from the cell. The protein is either secreted into the growth media (gram-positive bacteria) or into the periplasmic space, located between the inner and outer membrane of the cell (gram-negative bacteria) . Preferably there are processing sites, which can be cleaved either in vivo or in vitro encoded between the signal peptide fragment and the foreign gene. DNA encoding suitable signal sequences can be derived from genes for secreted bacterial proteins, such as the E. coli outer membrane protein gene (ompA) (Masui et al. , (1983), in: Experimental Manipulation of Gene Expression; Ghrayeb et al. , EMBO J. (1984) 1:2437) and the E. coli alkaline phosphatase signal sequence (phoA) (Oka et al. , Proc. Natl. Acad. Sci. (1985) £1:7212). As an additional example, the signal sequence of the alpha-amylase gene from various Bacillus strains can be used to secrete heterologous proteins from B . subtilis (Palva et al. , Proc. Natl. Acad. Sci. USA (1982) 21:5582; EPO Publication No. 244,042) . Typically, transcription termination sequences recognized by bacteria are regulatory regions located 3' to the translation stop codon, and thus together with the promoter flank the coding sequence. These sequences direct the transcription of an mRNA which can be translated into the polypeptide encoded by the DNA. Transcription termination sequences frequently include DNA sequences of about 50 nucleotides capable of forming stem loop structures that aid in terminating transcription. Examples include transcription termination sequences derived from genes with strong promoters, such as the trp gene in E. coli as well as other biosynthetic genes. Typically, the above described components, comprising a promoter, signal sequence (if desired) , coding sequence of interest, and transcription termination sequence, are put together into expression constructs. Expression constructs are often main¬ tained in a replicon, such as an extrachromosomal element (e.g., plasmids) capable of stable maintenance in a host, such as bacteria. The replicon will have a replication system, thus allowing it to be maintained in a procaryotic host either for expression or for cloning and amplification. In addition, a replicon may be either a high or low copy number plasmid. A high copy number plasmid will generally have a copy number ranging from about 5 to about 200, and typically about 10 to about 150. A host containing a high copy number plasmid will preferably contain at least about 10, and more preferably at least about 20 plasmids. Either a high or low copy number vector may be selected, depending upon the effect of the vector and the foreign protein on the host. Alternatively, the expression constructs can be integrated into the bacterial genome with an integrating vector. Integrating vectors typically contain at least one sequence homologous to the bacterial chromosome that allows the vector to integrate. Integrations appear to result from recombinations between homologous DNA in the vector and the bacterial chromosome. For example, inte¬ grating vectors constructed with DNA from various Bacillus strains integrate into the Bacillus chromosome (EPO Publication No. 127,328) . Integrating vectors may also be comprised of bacteriophage or transposon sequences . Typically, extrachromosomal and integrating expression constructs may contain selectable markers to allow for the selection of bacterial strains that have been transformed. Selectable markers can be expressed in the bacterial host and may include genes which render bacteria resistant to drugs such as ampicillin, chloramphenicol, erythromycin, kanamycin (neomycin) , and tetracycline (Davies et al . , Ann. Rev. Microbiol . (1987) £2:469) • Selectable markers may also include biosynthetic genes, such as those in the histidine, tryptophan, and leucine biosynthetic pathways . Alternatively, some of the above described components can be put together in transformation vectors. Transformation vectors are typically comprised of a selectable marker that is either maintained in a replicon or developed into an integrating vector, as described above. Expression and transformation vectors, either extrachromosomal replicons or integrating vectors, have been developed for transformation into many bacteria. For example, expression vectors have been developed for, inter alia, the following bacteria: Bacillus subtilis (Palva et al. , Proc. Natl. Acad. Sci. USA (1982) 2 :5582; EPO Publication Nos. 036,259 and 063,953; PCT Publication No. WO 84/04541] , E. coli (Shimatake et al . , Nature (1981) 292 . :128; Amann et al . , Gene (1985) 4£:183; Studier et al. , J. Mol. Biol. (1986) 1££:113; EPO Publication Nos. 036,776, 136,829 and 136,907) , Streptococcus cremoris (Powell et al. , Appl . Environ. Microbiol. (1988) £4:655); Streptococcus lividans (Powell et al. , Appl. Environ. Microbiol. (1988) £4:655) , Streptomyces lividans (U.S. Patent No. 4,745,056) . Methods for introducing exogenous DNA into bacterial hosts are well known in the art, and typically include either the transformation of bacteria treated with CaCl_ or other agents, such as divalent cations and DMSO. DNA can also be introduced into bacterial cells by electroporation. Transformation procedures usually vary with the bacterial species to be transformed. See, e.g., (Masson et al. , FEMS Microbiol. Lett. (1989) 60:273: Palva et al. , Proc. Natl. Acad. Sci. USA (1982) 21:5582; EPO Publication Nos. 036,259 and 063,953; PCT Publication No. WO 84/04541, Bacillus) , (Miller et al., Proc. Natl. Acad. Sci. (1988) £5:856; Wang et al., J. Bacteriol. (1990) 172:949 , Campy1obacter) , (Cohen et al . , Proc. Natl. Acad. Sci. USA (1973) £9:2110; Dower et al . , Nucleic Acids Res. (1988) 16:6127; Kushner (1978) "An improved method for transformation of Escherichia coli with ColEl-derived plasmids" in Genetic Engineering: Proceedings of the International Symposium on Genetic Engineering (eds. H.W. Boyer and S. Nicosia) ; Mandel et al . , J. Mol . Biol. (1970) £1:159; Taketo Biochim. Biophys. Acta (1988) 949:318: Escherichia) , (Chassy et al. , FEMS Microbiol. Lett. (1987) 44:173 Lactobacillus) ; (Fiedler et al . , Anal . Biochem (1988) 170:38 , Pseudomonas) ; (Augustin et al . , FEMS Microbiol. Lett. (1990) £6:203, Staphylococcus) , (Barany et al . , J. Bacteriol. (1980) 144:698: Harlander (1987) "Transformation of Streptococcus lactis by electroporation, " in: Streptococcal Genetics (ed. J. Ferretti and R. Curtiss III); Perry et al. , Infec. Immun. (1981) 11:1295; Powell et al. , Appl. Environ. Microbiol. (1988) £4:655; Somkuti et al . , Proc. 4th Eyr. Cong. Biotechnology (1987) 1:412, Streptococcus) . iv. Yeast Expression Yeast expression systems are also known to one of ordinary skill in the art. A yeast promoter is used in such systems and is any DNA sequence capable of binding yeast RNA polymerase and initiating the downstream (3') transcription of a coding sequence (e.g., structural gene) into mRNA. A promoter will have a transcription initiation region which is usually placed proximal to the 5' end of the coding sequence. This transcription initiation region typically includes an RNA polymerase binding site (the "TATA Box") and a transcription initiation site. A yeast promoter may also have a second domain called an upstream activator sequence (UAS) , which, if present, is usually distal to the structural gene. The UAS permits regulated (inducible) expression. Constitu- tive expression occurs in the absence of an UAS. Regulated expression may be either positive or negative, thereby either enhancing or reducing transcription. Yeast is a fermenting organism with an active metabolic pathway, therefore sequences encoding enzymes in the metabolic pathway provide particularly useful promoter sequences. Examples include alcohol dehydrogenase (ADH) (EPO Publication No. 284,044) , enolase, glucokinase, glucose-6-phosphate isomerase, glyceraldehyde-3-phosphate-dehydrogenase (GAP or GAPDH) , hexokinase, phosphofructokinase, 3- phosphoglycerate mutase, and pyruvate kinase (PyK) (EPO Publication No. 329,203) . The yeast PH05 gene, encoding acid phosphatase, also provides useful promoter sequences (Myanohara et al . , Proc. Natl. Acad. Sci. USA (1983) 80:1) . In addition, synthetic promoters which do not occur in nature also function as yeast promoters. For example, UAS sequences of one yeast promoter may be joined with the transcription activation region of another yeast promoter, creating a synthetic hybrid promoter. Examples of such hybrid promoters include the ADH regulatory sequence linked to the GAP transcription activation region (U.S. Patent Nos. 4,876,197 and 4,880,734) . Other examples of hybrid promoters include promoters which consist of the regulatory sequences of either the ADH2, GAL4, GAL10. OR PH05 genes, combined with the transcriptional activation region of a glycolytic enzyme gene such as GAP or PyK (EPO Publication No. 164,556) . Furthermore, a yeast promoter can include naturally occurring promoters of nonyeast origin that have the ability to bind yeast RNA polymerase and initiate transcription. Examples of such promoters are described in, inter alia, Cohen et al. , Proc. Natl. Acad. Sci. USA (1980) 22:1078; Henikoff et al. , Nature (1981) 283 :835; Hollenberg et al . , Curr. Topics Microbiol. Immunol. (1981) 1£:119; Hollenberg et al . , (1979) "The Expression of Bacterial Antibiotic Resistance Genes in the Yeast Saccharomyces cerevisiae" in: Plasmids of Medical, Environmental and Commercial Importance (eds. K.N. Timmis and A. Puhler) ; Mercerau-Puigalon et al., Gene (1980) 11:163; Panthier et al . , Curr. Genet. (1980) 1:109. A DNA molecule may be expressed intracellu- larly in yeast. A promoter sequence may be directly linked with the DNA molecule, in which case the first amino acid at the N-terminus of the recombinant protein will always be a methionine, which is encoded by the ATG start codon. If desired, methionine at the N-terminus may be cleaved from the protein by in vi tro incubation with cyanogen bromide. Fusion proteins provide an alternative for yeast expression systems, as well as in mammalian, baculovirus, and bacterial expression systems. Typically, a DNA sequence encoding the N-terminal portion of an endogenous yeast protein, or other stable protein, is fused to the 5' end of heterologous coding sequences. Upon expression, this construct will provide a fusion of the two amino acid sequences. For example, the yeast or human superoxide dismutase (SOD) gene, can be linked at the 5' terminus of a foreign gene and expressed in yeast. The DNA sequence at the junction of the two amino acid sequences may or may not encode a cleavable site. See, e.g., EPO Publication No. 196,056. Another example is a ubiquitin fusion protein. Such a fusion protein is made with the ubiquitin region that preferably retains a site for a processing enzyme (e.g., ubiquitin- specific processing protease) to cleave the ubiquitin from the foreign protein. Through this method, therefore, native foreign protein can be isolated (see, e.g., PCT Publication No. WO 88/024066) . Alternatively, foreign proteins can also be secreted from the cell into the growth media by creating chimeric DNA molecules that encode a fusion protein comprised of a leader sequence fragment that provide for secretion in yeast of the foreign protein. Preferably, there are processing sites encoded between the leader fragment and the foreign gene that can be cleaved either in vivo or in vi tro . The leader sequence fragment typically encodes a signal peptide comprised of hydrophobic amino acids which direct the secretion of the protein from the cell. DNA encoding suitable signal sequences can be derived from genes for secreted yeast proteins, such as the yeast invertase gene (EPO Publication No. 012,873; JPO Publication No. 62,096,086) and the A- factor gene (U.S. Patent No. 4,588,684) . Alternatively, leaders of nonyeast origin, such as an interferon leader, exist that also provide for secretion in yeast (EPO Publication No. 060,057) . A preferred class of secretion leaders are those that employ a fragment of the yeast alpha-factor gene, which contains both a "pre" signal sequence, and a "pro" region. The types of alpha-factor fragments that can be employed include the full-length pre-pro alpha factor leader (about 83 amino acid residues) as well as truncated alpha-factor leaders (typically about 25 to about 50 amino acid residues) (U.S. Patent Nos. 4,546,083 and 4,870,008; EPO Publication No. 324,274) . Additional leaders employing an alpha- factor leader fragment that provides for secretion include hybrid alpha-factor leaders made with a presequence of a first yeast, but a pro-region from a second yeast alphafactor. (See, e.g., PCT Publication No. WO 89/02463.) Typically, transcription termination sequences recognized by yeast are regulatory regions located 3' to the translation stop codon, and thus together with the promoter flank the coding sequence. These sequences direct the transcription of an mRNA which can be translated into the polypeptide encoded by the DNA. Examples of such sequences include those found in association with sequences coding for glycolytic enzymes. Typically, the above described components, comprising a promoter, leader (if desired) , coding sequence of interest, and transcription termination sequence, are put together into expression constructs. Expression constructs are often maintained in a replicon, such as an extrachromosomal element (e.g., plasmids) capable of stable maintenance in a host, such as yeast or bacteria. The replicon may have two replication systems, thus allowing it to be maintained, for example, in yeast for expression and in a procaryotic host for cloning and amplification. Examples of such yeast-bacteria shuttle vectors include YEp24 (Botstein et al . , Gene (1979) £:17-24) , pCl/1 (Brake et al . , Proc. Natl. Acad. Sci USA (1984) £1:4642-4646) , and YRpl7 (Stinchcomb et al. , J. Mol . Biol . (1982) 158:157) . In addition, a replicon may be either a high or low copy number plasmid. A high copy number plasmid will generally have a copy number ranging from about 5 to about 200, and typically about 10 to about 150. A host containing a high copy number plasmid will preferably have at least about 10, and more preferably at least about 20. Either a high or low copy number vector may be selected, depending upon the effect of the vector and the foreign protein on the host. See, e.g., Brake et al. , supra . Alternatively, the expression constructs can be integrated into the yeast genome with an integrating vector. Integrating vectors typically contain at least one sequence homologous to a yeast chromosome that allows the vector to integrate, and preferably contain two homologous sequences flanking the expression construct. Integrations appear to result from recombinations between homologous DNA in the vector and the yeast chromosome (Orr-Weaver et al., Methods in Enzymol. (1983) 101:228-245) . An integrating vector may be directed to a specific locus in yeast by selecting the appropriate homologous sequence for inclusion in the vector. See Orr-Weaver et al. , supra . One or more expression constructs may integrate, possibly affecting levels of recombinant protein produced (Rine et al . , Proc. Natl. Acad. Sci. USA (1983) ££:6750) . The chromosomal sequences included in the vector can occur either as a single segment in the vector, which results in the integra¬ tion of the entire vector, or two segments homologous to adjacent segments in the chromosome and flanking the expression construct in the vector, which can result in the stable integration of only the expression construct. Typically, extrachromosomal and integrating expression constructs may contain selectable markers to allow for the selection of yeast strains that have been transformed. Selectable markers may include biosynthetic genes that can be expressed in the yeast host, such as ADE2 , HIS4. LEU2. TR 1, and ALG7. and the G418 resistance gene, which confer resistance in yeast cells to tunicamycin and G418, respectively. In addition, a suitable selectable marker may also provide yeast with the ability to grow in the presence of toxic compounds, such as metals. For example, the presence of CUPl allows yeast to grow in the presence of copper ions (Butt et al. Microbiol, Rev. (1987) £1:351) . Alternatively, some of the above described components can be put together into transformation vectors. Transformation vectors are typically comprised of a selectable marker that is either maintained in a replicon or developed into an integrating vector, as described above. Expression and transformation vectors, either extrachromosomal replicons or integrating vectors, have been developed for transformation into many yeasts. For example, expression vectors have been developed for, inter alia, the following yeasts: Candida albicans (Kurtz et al . , Mol. Cell. Biol. (1986) £:142) , Candida maltosa (Kunze et al. , J. Basic Microbiol. (1985) 1£:141) , Hansenula polymorpha (Gleeson et al., J. Gen. Microbiol. (1986) 112:3459; Roggenkamp et al., Mol . Gen. Genet. (1986) 2£1:302), Kluyveromyces fragilis (Das et al. , J. Bacteriol . (1984) 158 : 1165) , Kluyveromyces lactis (De Louvencourt et al., J. Bacteriol. (1983) 154:737; Van den Berg et al., Bio/Technology (1990) £:135), Pichia guillerimondii (Kunze et al. , J. Basic Microbiol. (1985) 2£:141) , Pichia pastoris (Cregg et al . , Mol. Cell. Biol. (1985) £:3376; U.S. Patent Nos. 4,837,148 and 4,929,555) , Saccharomyces cerevisiae (Hinnen et al., Proc. Natl. Acad. Sci. USA (1978) 2£:1929; Ito et al., J. Bacteriol. (1983) 153 :163) . Schizosaccharomyces pombe (Beach and Nurse, Nature (1981) 300 : 706) , and Yarrowia lipolytica (Davi ow et al., Curr. Genet. (1985) 10:380471; Gaillardin et al . , Curr. Genet. (1985) 1£:49) . Methods of introducing exogenous DNA into yeast hosts are well known in the art, and typically include either the transformation of spheroplasts or of intact yeast cells treated with alkali cations. Transformation procedures usually vary with the yeast species to be transformed. See, e.g., Kurtz et al. , Mol. Cell. Biol. (1986) £:142; Kunze et al. , J. Basic Microbiol. (1985) 2£:141; Candida) ; (Gleeson et al. , J. Gen. Microbiol. (1986) 111:3459; Roggenkamp et al. , Mol. Gen. Genet. (1986) 202:302; Hansenula) ; (Das et al., J. Bacteriol. (1984) 158:1165 : De Louvencourt et al., J. Bacteriol. 154:1165 ; Van den Berg et. al. (1990) Bio/Technology (1983) £:135; Kluyveromyces) ; (Cregg et al., Mol. Cell. Biol. (1985) £:3376; Kunze et al., J. Basic Microbiol. (1985) 2£:141; U.S. Patent Nos. 4,837,148 and 4,929,555; Pichia) ; (Hinnen et al. , Proc. Natl. Acad. Sci. USA (1978) 2£;1929; Ito et al. , J. Bacteriol. (1983) 153:163) ; Saccharomyces) ; (Beach and Nurse, Nature (1981) 300:706; Schizosaccharomyces) ; (Davidow et al. , Curr. Genet. (1985) 1£:39; Gaillardin et al. , Curr. Genet. (1985) 1£:49; Yarrowia) . C. Purification of the Desired Polypeptide Once expressed and secreted, the polypeptide of interest can be purified from collected media using any of several techniques known in the art. Convenient techniques include affinity chromatography and immunoprecipitation, (see, e.g., Weir and Moss, (1985) ) . For example, CMV gH can be easily purified using Murine monoclonal antibody 1G6 (Rasmussen et al., PNAS (1984) £1:876880) , either by immunoprecipitation or affinity column chromatography. For affinity chromatography, the ligand may be covalently coupled to solid supports such as cellulose, polystyrene, polyacrylamide, crosslinked dextran, beaded agarose or controlled pore glass using bifunctional coupling agents that react with functional groups on the support and functional groups (i.e., reactive amino acid side chains) on the ligand molecule. See Scientific Foundations of Clinical Biochemistry, vol. 1, pp. 202 et seq. (1978) . The resulting ligand-bearing solid phase is contacted with disruptates of cells transformed with the gene encoding the protein of interest or conditioned media from the same, using reducing conditions, pH, ionic strength, temperature (typically physiological) , and residence times that permit the desired polypeptide to bind to the immobilized ligand. The cells may be disrupted by sonication, lysing or other methods. The solid phase is separated from the disruptate after the incubation and washed with buffer to remove residual unbound disruptate. The protein is eluted from the solid phase by passing an elutant that dissociates hydrogen bonds through the bed. Bases that lower the pH to below about 3 or NaCl solutions above about 2M are commonly used elutants. If monoclonal antibodies are used, these can be prepared by selection of an antibody raised against the protein of interest. Monoclonal antibodies to the protein may be made by the somatic cell hybridization techniques described initially by Kohler and Milstein, Nature (1975) 256:495-497. The procedure involves immunizing a host animal (typically a mouse because of the availability of murine myelomas) with the protein of interest. Antibody-producing cells (e.g., peripheral blood lymphocytes, and splenocytes) are taken from the immunized host and mixed with a suitable tumor fusion partner in a liquid growth medium containing a fusogen such as polyethylene glycol of molecular weight 2000 to 5000. After the fusion, the cells are washed to remove residual fusion medium and incubated in a selective growth medium (i.e., a growth medium containing additives to which the parent tumor line is sensitive) such as HAT medium. Only hybrid cells that possess the parent noncancerous cells' ability to survive culture in the selective medium and the parent tumor cells' immortality survive culture in the selective medium. Surviving hybrids may be expanded and their culture media screened for the presence of appropriate antibodies by radioimmunoassay (RIA) , a microneutralization assay that detects inhibition of viral cytopathic effect (CPE) in cell cultures, or other assays that detect antiviral activity (e.g., plaque reduction) . Positive cultures may be screened for their ability to recognize and bind to the desired protein by immunoprecipitating labeled infected cell extracts with the positive cultures and analyzing the precipitate by SDS-PAGE for the presence of a labeled protein component. Hybrids that produce antibody that binds specifically to the protein may be subcloned and grown in vitro or in vivo by known procedures. ' The antibody may be isolated from the resulting culture medium or body fluid, as the case may be, by conventional procedures for isolating immunoglobulins. Below are examples of specific embodiments for carrying out the present invention. The examples are offered for illustrative purposes only, and are not intended to limit the scope of the present invention in any way. III. Examples Example 1 Nucleotide and Amino Acid Sequencing of CMV gH The two main laboratory isolates of human CMV are AD169 (Rasmussen, L. , et al . , Proc. Natl. Acad. Sci. USA (1984) £1:876-880) , and strain Towne (Pachl, C, et al., Virology (1989) 169:418-426) . Both strains encode a gH, and these glycoproteins share substantial sequence similarity and are immunologically cross-reactive. Other strains of CMV can readily be used for the source of gH sequences. The identification and isolation of the 3910 bp Hindlll to PstI fragment of the strain Towne CMV genome, which contains a 2226 bp gH open reading frame is described in Pachl, C, et al. , Virology (1989) l£l:418-426, and shown in Figures 1 & 2. The gH sequence is 742 amino acids (84.3 kD) and has the characteristics of a membrane glycoprotein. Immunologically equivalent fragments of human CMV gH, for example, from AD169 or strain Towne, may be identified by making analogs of the polynucleotide sequence encoding the protein that are truncated at the 3' and/or 5' ends of the sequence and/or have one or more internal deletions, expressing the analog polynucleotide sequences, and determining whether the resulting fragments immunologically react with a CMV antibody or induce the production of such antibodies in vivo, particularly neutralizing antibodies. For example, deletion within or encompassing a 13 amino acid hydrophobic peptide (residues Met., _ to Ala_ _) may facilitate secretion of gH. Similarly, deletions from within or encompassing the 34 residues (709-742) from the C- terminal transmembrane and internal regions, facilitate secretion. Deletions of the last 22 and 23 C-terminal residues are described in the examples, below. These fragments retain either the first two or three amino acids, respectively, of the putative gH transmembrane domain. Example 2 Cloning Truncated Human CMV gH in a Mammalian system For this example, human CMV virus was grown in cultures of human foreskin fibroblast (HF) cells with Dulbecco's modified Eagle (DME) medium (Gibco Laboratories, Grand Island, N.Y.) , as described by Spaete and Mocarski, J. Virol. (1985) 5£:135-143, but supplemented with 10% fetal calf serum (FCS) . COS 7 cells were grown in the same medium described for HF cells. Plasmid pRL108a (LaFemina and Hayward (1980) in Animal Virus Genetics (B.N. fields, R. Jaenisch, and C.F. Fox, Eds.) Vol. 28, pp. 39-55, Academic Press, New York) , encodes the Hindlll H fragment of CMV (Towne) cloned into plasmid pBR322. The gH gene was subcloned from the CMV (Towne) Hindlll H fragment (pRL108a) as a 5.15 kb EcoRI to Ncol fragment and inserted into plasmid pGEM2 (Promega, Madison, WI) , generating plasmid pXgH5. Expression plasmid pSVgH2 was constructed by isolating a 2577 bp Smal to Tthllll (partial) fragment, encoding the gH gene, from plasmid pXgH5. This fragment was treated with Klenow fragment to blunt the ends and ligated into the Smal site of the SV40-based expression plasmid pSV7d (Truett et al., DNA (1988) 4:333-349, generating plasmid pSVgH2. Expression plasmids encoding truncated gH were constructed which contain a gH gene in which most of the transmembrane region of gH was deleted by removal of the last 22 amino acids from the C-terminus of gH. These constructs retain the first three amino acids of the gH transmembrane domain, (Leu 718 -Leu 7ις - Met 720 ) . Two of these constructs, pSVgH6a and pSVgHδb, differed from each other in the polylinker sequences at the 3' end of gH. For pSVgH6a, an additional ten amino acids were encoded by the polylinker region, 3' to Met 720 (Gly 721 -Ser 7:2 -Arg 723 -Gly 724 -Ser 725 -Val 726 -Asp 727 -Leu 72 g- Asp 729 -Lys 730 ) and for pSVgH6b, an additional six amino acids were encoded by the polylinker (Leu 721 -Glu 722 -Asp 723 - Pro 724 -Ser 725 -Thr 726 ) . Plasmids pSVgH6a and pSVgH6b were generated by isolating a 2262 bp EcoRI plus BsphI (partial) fragment from pSVgH2. This fragment was then ligated to EcoRI plus Smal cut pSV7d (for pSVgH6a) or to EcoRI plus Xbal cut pSV7d (for pSVgH6b) . A third plasmid, pCMAdgH6 (Figure 2, ATCC Accession No. 69035) , bearing a truncated gH gene as above, was generated by cloning a 2775 bp EcoRI to Sail gH fragment from plasmid pSVgH6b into the Sail site of plasmid pCMVAdhfr (Spaete et al. J. Virol. (1990) £4:2922-2931) . This gH fragment encodes a 22 amino acid C-terminally truncated gH lacking the transmembrane and cytoplasmic domains (including the six additional amino acids encoded by the polylinker of pSVgH6b as described above) , and includes the selectable dhfr gene. Plasmid pCMAdgH6 was used to transfeet Chinese hamster ovary (CHO) cell line 171 as described previously. (Spaete et al. , Progress in Cytomegalovirus Research (M.P. Landini, ed. , 1991) pp. 133-136) . Briefly, CHO cell line 171 is a 0.1 μM methotrexate amplified derivative of parent cell line CR-7, transfected with pCMAdgH6. Media from transfected cells was analyzed by a standard ELISA to screen for positive clones. RIP analysis of medium conditioned by growth of cell line 171 was not positive for truncated gH. Example 3 Cloning Full-length Human CMV gH in a Baculovirus System A baculovirus-gH transfer vector encoding full-length gH (gH2) was prepared as follows. Baculovirus vector pAc373 (Smith et al . , Proc. Natl. Acad. Sci. (1985) £1:8404-8408) was cut with BamHl and the 2495 bp NotI to Xbal gH fragment of plasmid pSVgH2 (described in example 2) was filled and ligated into this BamHl site. The resulting plasmid, designated pACgH2 (see Figure 3) , encodes a gH construct where transcription is driven by the baculovirus polyhedrin gene promoter. This DNA plasmid was mixed with wildtype baculovirus viral DNA, the mixture was transfected into cells derived from Spodoptera frugiperda (Sf9 cells) , and recombinant plaques were isolated and plaque purified. Several recombinant virus clones were used to infect cells, and at four to six days after infection, cell lysates and conditioned media were analyzed by ELISA and Western blot. No extracellular expression of full-length gH (pACgH2) was detected. Extracellular expression of gH in Sf9 cells infected with a recombinant baculovirus containing pACgH2 was also analyzed by radioimmunoprecipitation (RIP) , using monoclonal IG6 or human sera. No gH- specific bands were detected from RIPs of baculovirus- pACgH2 (full-length gH) media, indicating that extracellular expression was not achieved. To test whether full-length gH was expressed intracellularly, Sf9 cells were infected at a MOI of 5 with gH2 (full-length gH) . Five days post infection, the cells were stained with a monoclonal antibody against gH, as the primary antibody. Cells were incubated with goat F(ab') 2 anti-mouse IgG-FITC conjugate, treated with paraformaldehyde and analyzed for cell surface gH expression on a Coulter EPICS-C flow cytometer. As a positive control, cells were infected with truncated gH (described below) , fixed with paraformaldehyde and permeabilized with detergent before primary antibody treatment to visualize intracellular gH expression. Intracellular staining was observed with both full-length and truncated gH. Thus, full-length gH and truncated gH are expressed intracellularly. Example 4 Cloning Truncated Human CMV gH in a baculovirus system A baculovirus-gH transfer vector encoding a fragment of gH, lacking a C-terminal domain, was prepared. Baculovirus vector pAc373 (Smith et al. , Proc. Natl. Acad. Sci. (1985) £2:8404-8408) was cut with BamHl and the 2178 bp NotI to Sail fragment from pCM6-H6 was filled and ligated into this BamHl site. pCM6-H6 was constructed by ligation of an EcoRI/Sall gH fragment from pSVgH6b (as described in Example 2) to EcoRI/Sall digested mammalian cell expression vector pCMV6c (Chapman et al. , Nuc. Acids Res. (1991) 11:3979-3986) . The resulting plasmid, designated pACgH6 (ATCC Accession No. 68373, see Figure 4) encodes a gH construct where transcription is driven by the baculovirus polyhedrin gene promoter. In this truncated gH segment most of the transmembrane region of gH was deleted by removal of the last 23 amino acids from the C-terminus of gH. The fragment retains only the first two amino acids of the gH transmembrane domain (Leu 71Q -Leu 7ιg , Figure 1) . The plasmid was mixed with wild-type baculovirus viral DNA, used to cotransfect Spodoptera frugiperda cells and recombinant plaques were isolated and plaque purified. Several recombinant virus clones were used to infect cells, and at four to six days after infection, cell lysates and conditioned media were analyzed by ELISA and Western blot. For all the pACgH6 containing clones, ELISA analysis showed gH reactivity for the culture media, indicating that truncated gH was expressed extracellularly in this system. While ELISA analysis of cell lysates was unable to detect the intracellular presence of truncated gH, RIP analysis was positive for truncated gH, indicating a low but detectable intracellular presence of truncated gH. Example 5 Cloning and characterization of the FGF Receptor A. Oligonucleotide Synthesis Oligonucleotide adapters, probes and sequencing primers were synthesized by the phosphoramidite method using Applied Biosystems (Foster City, CA) model 380A and 380B synthesizers, purified by polyacrylamide gel electrophoresis and desalted on SEP-PAK C 18 cartridges (Waters, Milford, MA) . The oligonucleotide probes used for screening the cDNA library were complementary to nucleotides 1- 30 (5 ' -ATAACGGACCTTGTAGCCTCCAATTCTGTG-3 ' ) and nucleotides 1840-1869 (5 ' -GCGGCGTTTGAGT- CCGCCATTGGCAAGCTG-3 ' ) of the published fig nucleic acid sequence (Ruta et al . , Oncogene (1988) £:9-15) . The two PCR primers used to amplify the extracellular region of the FGF receptor ( flg5 ) cDNA consisted of a sense primer, P4 (5'- CCAACCTCTAGAGGATCCACTGGGATGTGGAGCTGGAAGTGC-3 ' ) con- taining the ribosome binding site plus amino acids 1-6 of flgS and an antisense primer, P3 (5 ' -GTAAGCGGCCGCG- GATCCTTACTACTCCAGGTACAGGGGCGA-3 ' ) containing amino acids 369-374 of flgS and directly followed by a termination codon. Both primers contain BamHl sites to facilitate cloning into pAc373. Two additional PCR primers were used to identify two and three immunoglobulinlike domain FGF receptors in various tissues. They were a sense primer, PI (5'-CCATTTG- GATCCGTCACAGCCACACTCTGCACCGCT-3 ' ) encoding amino acids 14 to 21 of flgr5 and an antisense primer P2 (5'- CCATTTGTCGACTTCCATCTTTTCTGGGGATGTCCA-3' ) encoding the complement of amino acids 154 to 161 of flg5 . The primers contain BamHl and Sail . sites to facilitate cloning into M13 sequencing plasmids. B. RNA Isolation and Construction and Screening of the cDNA Library RNA was isolated by the guanidinium thiocyanate method (J.M. Chirgwin et al . , Biochem. (1979) 18:5294-5299) with modifications (G.J. Freeman et al., Proc. Natl. Acad. Sci. USA (1983) 80:4094- 4098) . Poly(A) + RNA was purified by a single fractionation over oligo(dT) cellulose (H. Aviv & P. Leder, Proc. Natl. Acad. Sci. USA (1972) 69:1408- 1412) . The construction and screening of the Hep G2 library in λZAP has been described (Zapf et al. , J. Biol. Chem. (1990) 265:14892-14898) . The probes were labeled with T 4 polynucleotide kinase and [γ- 32 P] -ATP (Sambrook et al. , (1989) Molecular Cloning: A Laboratory Manual 2nd Ed. (Cold Spring Harbor Laboratory, Cold Spring Harbor, NY) to a specific activity of 1-2 x 10 8 cpm/mg. Approximately 600,000 recombinant phages from the Hep G2 cDNA library were screened on duplicate nitrocellulose filters (Millipore, HATF 137) , with two fig oligonucleotide probes. Areas of plaques that hybridized to both probes were further purified. c. Plasmid Isolation, Subcloning and Sequencing Bluescript SK(- ) plasmids containing the putative fig cDNA inserts were released from λZAP by the M13 rescue/excision protocol described by the supplier (Stratagene) . Plasmid DNA was isolated by the alkaline lysis method (Sambrook et al. , (1989) Molecular Cloning: A Laboratory Manual 2nd Ed. (Cold Spring Harbor Laboratory, Cold Spring Harbor, NY) ) . The cDNA inserts containing the putative fig sequence were excised from the Bluescript SK( " ) vector by Bglll or EcoRI digestion and fractionated by agarose gel electrophoresis. Inserts were excised from the gel and passively eluted for 16 h with gentle shaking in 10 mM Tris-hydrochloride, pH 7.5, 1 mM EDTA (TE) , purified on elutip-D columns (Schleicher and Schuell) and subcloned into M13 sequencing vectors (Yanisch- Perron et al . , Gene (1985) 11:103-119) . PCR-amplified DNA was similarly purified. DNA sequencing was performed by the dideoxy chain termination method (Sanger et al. , Proc. Natl. Acad. Sci. USA (1977) 24:5463-5467) using M13 primers as well as specific internal primers. Ambiguous regions were resolved using 7-deaza-2 ' -deoxyguanosine-5 ' -triphosphate (Barr et al., Biotechniques (1986) 4:428-432) and Sequenase (US Biochemicals) . To isolate full-length FGF receptor encoded cDNAs, 600,000 recombinants from a λZAP-human hepatoma cell line (Hep G2) cDNA library were screened with oligonucleotide probes derived from the 5'- and 3'- ends of a partial fig cDNA (Ruta et al . , Oncogene (1988) 1:9-15) . Six clones were identified that hybridized to both probes. Bglll restriction endonuclease digestion of the cDNA inserts and gel analysis suggested that three of the six clones contained the complete coding sequence. Four Bglll fragments of 1.6, 1.1, 0.6, and 0.55 Kb and two ' EcoRI fragments of 2.7 and 1.2 Kb were identified in the longest cDNA clone, flgS (Figure 5) . Bglll and EcoRI sites are also present in the flanking adapters that were used to make the cDNA library. The Bglll and EcoRI fragments of _flgτ5 cDNA were isolated, cloned into M13 mpl9 and sequenced. A detailed sequencing strategy is shown in Figure 5. The flg5 cDNA encodes a protein of 820 amino acids and is flanked by 671 and 753 nucleotides of 5 ' - and 3 ' -untranslated regions, respectively. The encoded protein revealed a structure that included a signal peptide, three extracellular immunoglobulinlike domains, an acidic amino acid-rich region, a transmembrane domain and a split intracellular tyrosine kinase domain. These domains have been identified previously on the chicken (Lee et al . , Science (1989) 245:57-60) , the mouse (Reid et al. , Proc. Natl. Acad. Sci. USA (1990) £2:1596-1600) and most recently, several human FGF receptors deduced from cDNA sequences (Isacchi et al . , Nuc. Acids Res. (1990) 1£:1906; Johnson et al . , Mol. Cell Biol. (1990) 10:4728-4736) . The encoded receptor also contains eight consensus N-linked glycosylation sites in the extracellular region and one in the cytoplasmic tyrosine kinase domain. The amino acid sequence encoded by flgb cDNA is shown in Figure 6 (top row) . For comparison, five other previously identified forms of the human FGF receptors are shown (Isacchi et al. , supra ; Johnson et al. , supra) and are aligned for maximum amino acid sequence identity. The identified structural domains are indicated above the flgS sequence, as are regions corresponding to the PCR primers. The putative signal peptidase cleavage site (von Heijne, Nuc. Acids Res. (1986) 14:4683-4690) after Ala 21 is indicated ) . Differences or deletions of amino acids are boxed. The three most notable differences between the six FGF receptors are: i) a large deletion near the N- terminus in FGF receptors 3-6 (aa 31 . 119 ) that spans the entire first immunoglobulmlike domain; ii) truncation of receptors 5 and 6, which differ from the other FGF receptors in their carboxyl terminal amino acids (aa 221 . 300 and aa 223 . 302 respectively) , with consequent deletion of their transmembrane and cytoplasmic domains; and iii) deletion of amino acids 148 and 149 in FGF recep¬ tors 1, 3 and 5. Other differences in FGF receptor-3 (aa 101 ) and FGF receptor-2 (aa 8]7 ) are also noted. The partial fig sequence is not shown, but has an N-ter¬ minal amino acid corresponding to position 198 of FGF receptor-1. Accordingly, it may be encoded by the cDNAs of FGF receptors 1, 2, 3 or 4. It is important to note however, that the fig sequence displays a difference from FGF receptors 1-4 in the tyrosine kinase domain at aa 670 . 674 , due to three nucleic acid deletions flanking this region that results in a limited frame shift. D. PCR Amplification Amplification reactions were performed according to the supplier of the PCR kit (Perkin Elmer Cetus) . PCR primers and template were at a final con¬ centration of 1 mM and 0.1-0.5 mg/mL, respectively. The cDNA encoding f " 2gr5 was used as a template DNA for the construction of EC-FGF receptor in pAc373. For expression studies, template DNA was reverse transcribed from mRNA as described (Zapf et al. , supra) . 30 cycles of PCR were performed using a Perkin Elmer Cetus DNA thermal cycler. Each cycle consisted of a 94°C, 1 min denaturation step; a 55°C, 2 min annealing step; and a 72°C, 3 min extension step. The extension step in the last cycle was 7 min. E. Construction of Recombinant EC-FGF Receptor Virus The PCR amplified DNA fragment encoding the extracellular domain of the FGF receptor was digested with BamHl, gel purified and ligated to BamHl cut, calf intestinal phosphatase-treated pAc373. Recombinant plasmids were analyzed for EC-FGF receptor cDNAs inserted in the correct orientation by restriction endonuclease digestion and agarose gel electrophoresis. The recombinant plasmid was cotransfected with wild-type AcMNPV viral DNA into Sf9 cells by the calcium phosphate transfection procedure (Summers and Smith, supra) . Recombinant viruses were identified in the first round of plaque screening by hybridization with flgr5 cDNA that was 32 P-labeled by replacement synthesis (Sambrook et al. , supra) . The recombinant viruses were further purified by visual screening for the occlusion negative phenotype in two additional rounds. The recombinant baculovirus that expressed EC-FGF receptor was constructed by ligating PCR- amplified DNA encoding amino acids 1-374 of the flg cDNA into the BamHl site of the baculovirus transfer vector pAc373. The PCR primers contained flanking BamHl sites to facilitate cloning. In addition, the 5 1 sense primer (P4) contained, directly upstream from the initiation codon, the -1 to -5 nucleotides of the flg5 cDNA sequence that are implicated in ribosome binding (Kozak, M. , Nuc. Acids. Res. (1984) 12:857- 87239) . The 3 '-antisense primer (P3) contained two termination codons TAG and TAA directly after amino acid 374. Cotransfection of Sf9 cells with AcMNPV viral DNA and the recombinant construct (pAc373-EC-FGF receptor) by the calcium phosphate method (Summers and Smith, supra) generated recombinant baculovirus that were subsequently purified by plaque hybridization and visual screening. Example 6 FGF Receptor Binding and Activity Assays. A. Radioreceptor assay The effects of the EC-FGF receptor on the binding of radioiodinated basic FGF to its receptor was examined using a radioreceptor assay as described in the art. Briefly, baby hamster kidney cells ' were maintained in Hepes (25 mM) buffered DMEM supplemented with 5% calf serum and antibiotics and were grown to subconfluence in 24-well dishes. The cells were washed twice with phosphate buffered saline and incubated for 3 hours at 4°C with the indicated concentrations of the peptides and 1 ng (100,000 cpm) of labelled basic FGF in 300 μL of DMEM containing 0.1% gelatin. The medium was aspirated and the cells washed twice with 0.5 mL PBS and twice with 0.5 πiL of PBS containing 2 M NaCl. The amount of 125 I-FGF bound to the high affinity receptor was determined by quantitating the amount of radioactivity in the cell lysate obtained with 0.1% Triton ® X-100 in PBS, pH 8.4. B. Mitogenesis assay The effects of the peptides on mitogenesis was determined using Swiss 3T3 fibroblasts as described. Briefly, cells were plated at a concentration of 20,000 cells/well in 96 microwells and grown for two days in Hepes (25 mM) buffered DMEM containing 10% fetal calf serum and antibiotics. On the third day, the cells were washed twice with DMEM with no additives and the cells synchronized by a fur¬ ther incubation for two days in 0.5% fetal calf serum. At the time of assay, the test substances (basic FGF, EC-FGFR or both together) were added directly to the cells in 10 μL of DMEM supplemented with 0.1% BSA. Eighteen hours later, 1 μCi of 3 H-thymidine was added to the cells, and 24 hours after the addition of the peptides, the media was aspirated, the cells washed with PBS and the proteins precipitated with 50% trichloroacetic acid. After three washes, the cells were solubilized overnight with 1 N NaOH and the amount of radioactivity incorporated into DNA was determined by scintillation counting. C. Cell Proliferation Assays The EC-FGF receptor was tested for its ability to inhibit basic FGF stimulated adrenal capillary endothelial (ACE) cell proliferation. Aliquots of receptor preparation were added to ACE cells and four days later, the cell number was estab¬ lished using a Coulter particle counter. For com¬ parison purposes, 2 ng/ml of recombinant human basic FGF increased cell proliferation from 27,500 ± 2,100 cells/well to 133,300 ± 1,800 cells/well. D. Receptor dependent tyrosine phosphorylation Swiss 3T3 cells were treated at 37°C for 5 minutes with no additives or with basic FGF (15 ng/mL) , EC-FGF receptor (10 mg/mL) or basic FGF (15 ng/mL) and EC-FGF (10 mg/mL) added together. The cells were then harvested in a 2.5χ Laemmli's buffer, the proteins separated on 8% polyacrylamide SDS-PAGE gels and the presence of tyrosine phosphorylated proteins examined by Western blotting with a specific anti-phosphotyrosine antibody. The FGF binding properties of EC-FGF recep¬ tor was determined using a soluble binding assay (adapted from the assay described by Robinson et al . , J. Immunol. Meth. (1990) 132:63-71) . EC-FGF receptor, attached to concanavalin A coated plastic wells, was incubated with 125 I-bFGF and increasing concentrations of bFGF. Scatchard analysis of I25 I-FGF binding indicated a K d of less than 5 nM. An completely accurate K d determination was not possible due to the nonspecific binding of 15 I-FGF. Several blocking agents included in the assays, such as BSA, gelatin and heparan sulfate, were ineffective at blocking the nonspecific binding of 12S I-FGF at low concentrations of 125 I-FGF. The biological activity of the EC-FGF receptor was tested in several additional assay systems. First, the addition of EC-FGF receptor to endothelial cells in culture was shown to inhibit the proliferative effect of basic FGF. Because this cell type is known to synthesize basic FGF, it was suspected that the recombinant receptor might inhibit basal endothelial cell growth. As predicted, the expressed EC-FGF receptor can inhibit basal cell pro¬ liferation. Specificity of this effect was studied by incubating various cell types, that do not synthesize basic FGF, with the EC-FGF receptor. No effects were observed on BHK cells, A431 cells or on CHO cells. As expected, however, the addition of EC-FGF receptor to 3T3 cells inhibited the mitogenic response to basic FGF. Furthermore, it was observed that the EC-FGF receptor inhibited the growth of melanoma cells, a cell type previously shown to be dependent on the autocrine production of basic FGF. To establish that the FGF/EC-FGF receptor complex did not recognize the basic FGF receptor, two experiments were performed. First, the addition of the EC-FGF receptor preparation to BHK cells during the radioreceptor assay prevented the binding of ' d i¬ basic FGF to its receptor indicating that it binds basic FGF. The binding of 125 l-basic FGF to its low affinity receptor was also inhibited. Secondly, basic FGF fails to activate the tyrosine phosphorylation of either its cell membrane receptor or the characteristic 90 kDa substrate identified by Coughlin et al., J. Biol. Chem. (1988) 263 :988-993 when incubated in the presence of EC-FGF receptor. Example 7 Cloning of UL115 A. pUL115 Plasmid pUL115 (ATCC Accession No. 69036) is a staging vector which contains the UL115 ORF. This plasmid was constructed as follows. Plasmid pRL103 (LaFemina and Hayward (1980) in Animal Virus Genetics (B.N. fields, R. Jaenisch, and C.F. Fox, Eds.) Vol. 28, pp. 39-55, Academic Press, New York) contains the CMV Towne strain Hindlll C fragment cloned into Hindlll of pBR322. Plasmid pRL103 was cut with restriction enzymes BamHl and BstEII and the 1053 bp fragment was isolated and purified from a 1% agarose gel. This 1053 bp fragment was cut with Narl and the 957 bp BamHI/Narl fragment containing the UL115 ORF was isolated. pBluescript II KS +/- vector (Stratagene) was digested with BamHl and AccI and the 2915 bp vector ligated to the 957 bp BamHI/Narl fragment, creating pUL115. This ligation destroyed the AccI and Narl sites and retained the BamHl site. B. pVLUL115 Plasmid pVLUL115 is a baculovirus expression vector incorporating the UL115 ORF and was constructed as follows. The pUL115 staging vector was digested with BamHl and Kpnl and the 981 bp fragment was gel isolated and purified. Plasmid pVL1392 (Webb and Summers, J. Meth. Cell Mol. Biol. (1990) 1:173-188) was cut with BamHl and PstI to obtain a 9249 bp vector fragment. The 981 bp and 9249 bp fragments were then ligated in the presence of Klenow to produce pVLUL115. The resulting ligation restored the BamHl site and destroyed the PstI and Kpnl sites to produce a 10,230 bp plasmid. C. pMCUL115 Plasmid pMCUL115 is a mammalian cell expression vector with the MCMV immediate early promoter used to drive transcription of the UL115 gene. This plasmid was constructed as follows. pUL115 was digested with Asp718 and Xbal to remove UL115 from the bluescript vector. Then the pmcsr vector was digested with Asp718 and Xbal. The Asp718- Xbal UL115 fragment was then ligated directly into the pmcsr Asp7l8-Xbal sites restoring both sites. The resulting plasmid was termed pMCUL115. D. pMCUL115neo Plasmid pMCUL115neo is a PSV2neo-based mammalian expression vector which contains the UL115 ORF whose expression is promoted by the MCMV immediate early promoter, along with the neomycin selectable marker. The plasmid was constructed as follows. pMCUL115 was digested with Sfil and Xbal, and a 2.3 kb fragment was isolated, gel purified, and Genecleaned. Vector pSV2neo (Southern and Berg, J. Molec. and App. Gen. (1982) 1:327-341) was digested with EcoRI and BamHl and a 4.5 kb fragment was gel purified, isolated and Genecleaned. Both the 4.5 kb vector and the 2.3 kb MCUL115 fragments were treated with Klenow to prepare for blunt end ligation. The vector was phosphatased, phenol chloroform extracted and ethanol precipitated. Blunt end ligation of the 4.5 kb vector and the 2.3 kb MCMV-UL115 fragment resulted in pMCUL115neo. Example 8 Coexpression of CMV gH and UL115 in a Recombinant Baculovirus Vector Recombinant baculovirus vectors expressing full length (gH2) and C-terminally truncated gH (gH6) , were generated as described in Examples 3 and 4, respectively. A recombinant baculovirus (rBV) vector expressing the gene product of the UL115 ORF was generated by cotransfection of wildtype baculovirus DNA with transfer vector pVLUL115 (Example 7B) as described in Examples 3 and 4. Recombinant baculovirus vector rBVgH6 expressing truncated gH was used in coinfection experiments with rBVUL115 in SF9 cells. Cells were infected or coinfected at a MOI of 1, labeled with [ 35 S] cysteine, and lysates and supernatants were examined for expression by radioimmunoprecipitation using murine monoclonal antibody (MAb) 14-4b (Urban et al. , J. Virol. (1992) £6:1303-1311) which is a conformational dependent neutralizing antibody specific for CMV gH. The results showed that rBVUL115 and rBVgH6 expressed maximally at two days post infection (dpi) , and that a prominant protein migrating at 32 kDa was precipitated or coprecipitated with gH by the monoclonal antibody. Western blot analysis of lysates or supernatants using rabbit antiserum raised against a peptide from UL115 reacted specifically with UL115 gene product. Accordingly, this evidenced the presence of a CMV gH/UL115 complex. Example 9 Coexpression of CMV gH and UL115 in Mammalian Cells Plasmid pMCUL115neo described in Example 7D was transfected into stable gH secreting cell line 171 (Example 2) , or GRP2gH previously transfected with gH expression plasmid pMCGRP2gH, in order to generate new gH secreting cell lines. Plasmid pMCGRP2gH was generated by ligating a 2166 bp Notl/Xbal fragment encoding gH taken from pCMgH6 into Notl/Xbal digested mammalian cell expression vector pMCGRP2SR, a derivative of pMCMVAdhrf (Spaete et al . J. Virol. (1990) £4:2922-2931) . Plasmid vector pSV2neo was transfected into cell lines 171 and GRP2gH as a control. Clones were selected for resistance to neomycin and 96 clones from each transfection series were picked and assayed by ELISA. Clones secreting the highest amounts of gH were expanded to 6-well plates and reassayed by ELISA. Selected cell lines were further assayed by radioimmunoprecipitation using MAb 14-4b (Urban et al . , J. Virol. (1992) 66:1303- 1311) . As shown in Figure 8, cell lines secreting gH at levels detectable by RIP are also those which express the gene product of the UL115 ORF. Additional characterization of the coprecipiated UL115/gH complex under nonreducing conditions revealed that UL115 and gH are linked by disulfide bonds. Example 10 Coexpression of truncated CMV gH with the Soluble FGF Receptor CHO cell line 171 described in Example 2 was transfected with plasmid pFGFrtpaneo#3 designed to express a soluble form of the FGF r . Plasmid pFGFrtpaneo#3 was generated by ligating a 3 kbp SFil/Hindlll fragment from pFGF r tpa, encoding the human CMV immediate early promoter, tpa leader and C- terminally truncated FGF r into EcoRI/BamHI digested pSV2neo (Southern and Berg, J. Molec. and App. Gen. (1982) 1:327-341) . The fragments were treated with Klenow fragment to blunt the ends prior to ligation. Plasmid pFGF r tpa was constructed by ligating a 1 kbp CR generated Nhol/Hindlll fragment encoding the C- terminally truncated FGF r into an Nhol/Hindlll digested mammalian cell expression vector pCMV6al20 (Chapman et al., Nuc. Acids Res. (1991) 11:3979-3986) . The fragment encoding the truncated FGF r was generated with the following primer pairs: 5' GGA TCC GCT AGC AGG CCG TCC CCG ACC TTG 3' 5' GGA TCC AAG CTT TTA CTC CAG GTA CAG GGG CGA 3' . Plasmid PB5 flgS was used .as the template. Transfected cells were selected for resistance to neomycin and resistant clones were picked and assayed for levels of gH secretion by ELISA. Cell line 171-3-16 is representative of one such selected cell line (Figure 8, lane 2) . Radioimmunoprecipitation of selected gH secreting cell lines revealed gH specific bands on autoradiographs after a five day exposure. However, growth of the cell lines at 30°C prior to RIP resulted in increased secretion of gH with the consequence of being able to see a detectable band on autoradiographs in 24 hours after labeling the cells 4 hours in 250 μCi/ml [ 35 S]methionine. A coprecipitated 60 kDa molecule was shown to be C-terminally truncated FGF r by radioimmunoprecipitation using anti-FGF r antibodies and an FGF r competion assay, as described in Example 6A. Thus, secretion of gH is facilitated by coexpression of truncated FGF r . Example 11 Specificity of Receptor/Ligand Coexpression As a test of the generality of receptor/ligand coexpression strategies, cell lines transfected with an expression plasmid encoding heparan sulfate proteoglycan were assayed for the effect on gH secretion. 159 clones selected to express heparan sulfate proteoglycan were compared by ELISA for levels of gH secretion with 60 vector control lines. There was no difference in levels of gH secretion from either population of selected clones, suggesting that there is specificity involved in successful coexpression strategies. Thus, methods for increasing cell surface expression of proteins using escorts are described. Although preferred embodiments of the subject invention have been described in some detail, it is understood that obvious variations can be made without departing from the spirit and the scope of the invention as defined by the appended claims. Deposits of Strains Useful in Practicing the Invention A deposit of biologically pure cultures of the following strains was made with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Maryland. The accession number indicated was assigned after successful viability testing, and the requisite fees were paid. Access to said cultures will be available during pendency of the patent application to one determined by the Commissioner to be entitled thereto under 37 CFR 1.14 and 35 USC 122. All restriction on availability of said cultures to the public will be irrevocably removed upon the granting of a patent based upon the application. Moreover, the designated deposits will be maintained for a period of thirty (30) years from the date of deposit, or for five (5) years after the last request for the deposit; or for the enforceable life of the U.S. patent, whichever is longer. Should a culture become nonviable or be inadvertently destroyed, or, in the case of plasmid-containing strains, lose its plasmid, it will be replaced with a viable culture (s) of the same taxonomic description. These deposits are provided merely as convenience to those of skill in the art, and are not an admission that a deposit is required under 35 USC §112. The nucleic acid sequences of these plasmids, as well as the amino acid sequences of the polypeptides encoded thereby, are incorporated herein by reference and are controlling in the event of any conflict with the description herein. A license may be required to make, use, or sell the deposited materials, and no such license is hereby granted. Strain Deposit Date ATCC No. pACgH6 (in E. coli HB101) 7/27/90 68373 pUL115 (in E. coli HB101) 7/23/92 69036 pCMAdgH6 (in E. coli HB101) 7/23/92 69035

Claims: CLAIMS 1. A method for recombinantly producing a protein comprising coexpressing in a host cell a first gene encoding said protein with a second gene encoding an escort, under conditions whereby said protein is secreted from said host cell. 2. The method of claim 1 wherein said protein is a viral protein. 3. The method of claim 2 wherein said viral protein comprises an immunologically reactive herpesvirus polypeptide. 4. The method of claim 3 wherein said herpesvirus polypeptide is an immunologically reactive cytomegalovirus glycoprotein H polypeptide. 5. The method of claim 4 wherein said cytomegalovirus glycoprotein H polypeptide is a truncated human cytomegalovirus glycoprotein H lacking all or a portion of a transmembrane binding domain which is present in native human cytomegalovirus glycoprotein H. 6. The method of claim 5 wherein about 22 amino acids are deleted from the C-terminus of the amino acid sequence of cytomegalovirus glycoprotein H. 7. The method of claim 5 wherein about 23 amino acids are deleted from the C-terminus of the amino acid sequence of cytomegalovirus glycoprotein H. 8. The method of claim 5 wherein said escort is a fibroblast growth factor receptor polypeptide. 9. The method of claim 8 wherein said fibroblast growth factor receptor polypeptide is soluble fibroblast growth factor receptor. 10. The method of claim 5 wherein said escort is a UL115 polypeptide. 11. A method for recombinantly producing an immunologically reactive truncated cytomegalovirus glycoprotein H wherein said truncated glycoprotein H lacks all or a portion of a transmembrane binding domain which is present in native human cytomegalovirus glycoprotein H, said method comprising coexpressing in a host cell a first gene encoding said truncated glycoprotein H with a second gene encoding soluble fibroblast growth factor receptor, under conditions whereby said truncated glycoprotein H ' is secreted from said host cell. 12. A method for recombinantly producing an immunologically reactive truncated cytomegalovirus glycoprotein H wherein said truncated glycoprotein H lacks all or a portion of a transmembrane binding domain which is present in native human cytomegalovirus glycoprotein H, said method comprising coexpressing in a host cell a first gene encoding said truncated glycoprotein H with a second gene encoding a UL115 polypeptide, under conditions whereby said truncated glycoprotein H is secreted by said host cell. 13. A complex comprising a recombinant, immunologically reactive herpesvirus polypeptide and an escort. 14. The complex of claim 13 wherein said herpesvirus polypeptide is an immunologically reactive cytomegalovirus glycoprotein H polypeptide and said escort is a fibroblast growth factor receptor polypeptide. 15. The complex of claim 14 wherein said glycoprotein H polypeptide is a truncated human cytomegalovirus glycoprotein H lacking all or a portion of a transmembrane binding domain which is present in native human cytomegalovirus glycoprotein H and said fibroblast growth factor receptor polypeptide is soluble fibroblast growth factor receptor. 16. The complex of claim 15 wherein about 22 amino acids are deleted from the C-terminus of the amino acid sequence of cytomegalovirus glycoprotein H. 17. The complex of claim 15 wherein about 23 amino acids are deleted from the C-terminus of the amino acid sequence of cytomegalovirus glycoprotein H. 18. The complex of claim 13 wherein said herpesvirus polypeptide is an immunologically reactive cytomegalovirus glycoprotein H polypeptide and said escort is a UL115 polypeptide. 19. The complex of claim 18 wherein said glycoprotein H polypeptide is full-length glycoprotein H. 20. The complex of claim 18 wherein said glycoprotein H polypeptide is a truncated human cytomegalovirus glycoprotein H lacking all or a portion of a transmembrane binding domain which is present in native human cytomegalovirus glycoprotein H. 21. The complex of claim 20 wherein about 22 amino acids are deleted from the C-terminus of the amino acid sequence of cytomegalovirus glycoprotein H. 22. The complex of claim 20 wherein about 23 amino acids are deleted from the C-terminus of the amino acid sequence of cytomegalovirus glycoprotein H.

Inventors: SPAETE RICHARD L, SPAETE, RICHARD, L.
Applicants: CHIRON CORP, CHIRON CORPORATION
Application year: 1993
Main classification: C12N15
IPC codes: A61K39, A61P31, C07K14, C07K19, C12N15, C12P21, C12R1