EXPRESSION CLONING PROCESSES FOR THE DISCOVERY, CHARACTERIZATION AND ISOLATION OF GENES ENCODING POLYPEPTIDES WITH A PREDETERMINED PROPERTY

Abstract: The present invention is directed to a method for detection, characterization and isolation of nucleic acids encoding proteins of a desired property, such as a particular cellular localization. The invention further provides for rapid expression of such proteins or glycoproteins in mammalian cells and for facilitated purification of the novel secreted proteins or glycoproteins. Further, the invention provides for radioactive labelling of the novel proteins or glycoproteins, for rapid identification of sites of binding including animals and for rapid production of infective viral vectors for use in gene transfer.

Description: EXPRESSION CLONING PROCESSES FOR THE DISCOVERY, CHARACTERIZATION AND ISOLATION OF GENES ENCODING POLYPEPTIDES WITH A PREDETERMINED PROPERTY I. FIELD OF THE INVENTION The present in invention relates, generally, to the field of expressed gene technology and expression cloning. More specifically, the present invention relates to the identification, characterization, and isolation of transcribed nucleic acid sequences encoding polypeptides having a predetermined property, e.g., cellular localization, structure, enzymatic function, or affinity to other molecules, and the production of the corresponding polypeptides. II. BACKGROUND OF THE INVENTION General Background. Proteins are the most prominent biomolecules in living organisms; in addition to their role as structural components and catalysts, they play a crucial role in regulatory processes. Both regulation of cell proliferation and metabolic functions are largely controlled and effected by the cooperation of numerous cellular and extracellular proteins. Lehninger, A.L., 1975, "Biochemistry", Worth Publishers Inc., New York, New York. For example, signal transduction pathways of many kinds that affect critical physiological responses operate through proteins by way of their intermolecular interactions. Metzler, D. E., 1977, "Biochemistry", Academic Press Inc., London. Furthermore, the transcription of genes and the regulation of such transcription is dependent upon and controlled by the interdependence of numerous protein factors. Wainwright, S. D., "Control Mechanisms and Protein Synthesis", Columbia University Press, New York and London. Proper functioning of a multicellular organism does not only depend on the interaction of biomolecules within the cell, but individual cells must also communicate appropriately. Such intercellular communication, and interaction of cells with the environment is often realized by the actions of receptors on the extracellular surface and associated intracellular signal transduction mechanisms. Poste, G., Nicholson, G. L., 1976, "The Cell Surface", Elseviere, Amsterdam. The information is communicated through the cell environment to regulate gene expression or protein activities in the cell. Secreted proteins in the extracellular environment thereby exert potent regulatory effects on certain cellular functions. In view of the above outlined, very simplified paradigm of cell function, particular properties of a protein, including cellular localization, structure, affinity to a binding partner, or enzymatic activity under physiological conditions appear to be highly indicative of its type of function. With respect to a particular cellular localization, secreted proteins, for example, are likely to function as intercellular communicators of signals, while membrane associated receptors having an extracellular and intracellular domain most likely transmit an extracellular signal into the cell. Cytoplasmic proteins may function as intracellular signal transmitters and coordinators. Jeter, J. R., Cameron, I. L., Padilla, I. L., Padilla, G. M., Zimmerman, A. M., 1978, "Cell Cycle Regulation". London. Nuclear proteins are likely to be involved in certain aspects of gene regulation. Zawel et al, 1995, Annu. Rev. Biochem. 64:533-561. Mature proteins found in the Golgi or the ER may have regulatory roles in the post-translational processing of protein precursors, e.g., cleavage or addition of carbohydrates. Hirschberg, 1987, C. Annu. Rev. Biochem. 56:63-87. Membrane-Associated Proteins. For many years, the paradigm of cell function has motivated numerous drug discovery programs to focus on identifying membrane-associated proteins, in particular new receptors, and their respective functions. Porter, R. and O'Connor, M., 1970, "Molecular Properties of Drug Receptors, Ciba Foundation Symposium". J&A Churchill, London. Many examples in fact compel the conclusion that improper function of membrane receptors is a significant source of the development of serious metabolic and proliferative diseases such as cancer. For example, a certain form of Diabetes mellitus, i.e. , the non-insulin-dependent diabetes (NIDDM) may be caused by mutations in the insulin receptor. Ullrich et al. , 1985, Nature 3 3 :756-761 ; Taira et al. , 1989, Science 245:63-66. Furthermore, 30% of all mammary carcinomas are associated with amplification of the receptor tyrosine kinase HER2. Bargman et al., 1986, Cell 45:649-657; Slamon et al., 1989, Science 244:707-712. In addition to traditional drug discovery programs targeting receptors, an ambitiously pursued objective has become to identify membrane-associated receptors as possible gene therapy targets using comparative genomics, which allows determination of changes in gene expression under, e.g., pathological conditions. Wels, et al., 1995, Gene 159(1 ):73-80. Secreted Proteins. While receptors have mostly been considered as important potential therapeutic targets, secreted proteins are of particular interest as potential therapeutic agents. They often have a signalling or hormone function, and hence have a high and specific biological activity. Schoen, F. J., 1994, "Robbins Pathologic Basis of Disease", W.B. Saunders Company, Philadelphia. For example, secreted proteins control physiological reactions such as differentiation and proliferation, blood clotting and thrombolysis, somatic growth and cell death, and immune response. Schoen, F. J., 1994, "Robbins Pathologic Basis of Disease", W.B. Saunders Company, Philadelphia. Significant resources and research efforts have been expended for the discovery and investigation of new secreted proteins controlling biological functions. Many of such secreted proteins, including cytokines and peptide hormones, are manufactured and used as therapeutic agents. Zavyalov et al. , 1997, APMIS 105(3): 161-186. However, of the several thousand expected secreted proteins, only a few are currently used as therapeutic compounds. It can be expected that many of the so far undiscovered secreted proteins of the human organism are effective in correcting physiological disorders and are thus promising candidates for new drugs. In the past, novel cytokines and hormone proteins were identified by assaying a certain cell type for its response to protein fractions or purified proteins. Lauffenburger et al. , 1996, Biotechnology and Bioengineering 52( 1 ):61 -80. Other investigators have used sequence similarities on DNA level to clone novel interferons and interleukins. Nabori et al, \992, Analyt. Biochem. 205(l):42-46. In again another approach, differential display techniques were used to compare the expression patterns of stimulated versus unstimulated cells. Nagata et α/., 1980, Nature 287:401-408. All these methods may yield identification and isolation of certain secreted polypeptides. Recently, a screening method for the identification of cDNA encoding novel secreted mammalian proteins in yeast using the invertase gene as a selection marker has been described. See, United States Patent No. 5,536,637 (the "'637 Patent"). The disclosed technology relies on the concept that leader sequences of mammalian cDNAs are effective in exporting the invertase protein depleted of its leader sequence. This approach yields partial cDNAs which in turn can be used to screen a full-length cDNA library. The novel protein of interest can then be manufactured by standard, but laborious, techniques, including subcloning, transforming a recombinant host, expression, development and implementation of a purification process. Furthermore, since the assays described in the '637 Patent are performed in yeast, the glycosylation pattern of the isolated products will differ significantly from the natural product produced in mammalian cells. This difference is a major impediment in view of the fact that an extremely important feature of secreted proteins (as it is true for the extracellular domain of receptors) is their glycosylation pattern and carbohydrate composition. Rademacher et al. , 1988, Annu. Rev. Biochem. 57:785-838. Nuclear Proteins. In the nucleus, both replication of DNA and transcription of genes is actually implemented. Many nuclear proteins are directly involved in these processes as transcription factors, as cell cycle regulators, or both. Some nuclear proteins are responsible for turning on expression of certain metabolic proteins in response to environmental changes. Zawel et α/., 1995, Annu. Rev. Biochem. 64:533-561. Many others are directly involved in the regulation of cell proliferation. Jeter, J. R., Cameron, I. L., Padilla, G. M., Zimmerman, A. M., 1978, "Cell Cycle Regulation", London. Proteins in this latter class fall into two general categories: (1) dominant transforming genes, including oncogenes; and (2) recessive cell proliferation genes, including tumor suppressor genes and genes encoding products involved in programmed cell death ("apoptosis"). Oncogenes generally encode proteins that are associated with the promotion of cell growth. Because cell division is a crucial part of normal tissue development and continues to play an important role in tissue regeneration, properly regulated oncogene activity is essential for the survival of the organism. However, inappropriate expression or improperly controlled activation of oncogenes may drive uncontrolled cell proliferation and result in the development of severe diseases, such as cancer. Weinberg, 1994, CA Cancer J. Clin. 44:160-170. Tumor suppressor genes, on the other hand, normally act as "brakes" on cell proliferation, thus opposing the activity of oncogenes. Accordingly, inactivation of tumor suppressor genes, e.g. , through mutations or the removal of their growth inhibitory effects may result in the loss of growth control, and cell proliferative diseases such as cancer may develop. Weinberg, 1994, CA Cancer J. Clin. 44:160-170. Related to tumor suppressor genes are genes whose product is involved in the control of apoptosis; rather than regulating proliferation of cells, they influence the survival 5 of cells in the body. In normal cells, surveillance systems are believed to ensure that the growth regulatory mechanisms are intact; if abnormalities are detected, the surveillance system switches on a suicide program that culminates in apoptosis. Several genes that are involved in the process of apoptosis have been described. See, for example, Collins and Lopez Rivas, 1993, TIBS 18:307-308; Martin et al, 1994, 10 TIBS J_9:26-30. Cells that are resistant to apoptosis have an advantage over normal cells, and tend to outgrow their normal counterparts and dominate the tissue. As a consequence, inactivation of genes involved in apoptosis may result in the progression of tumors, and, in fact, is an important step in tumorigenesis. Accordingly, the identification of nuclear proteins addresses two areas of interest. 15 First, oncogenes are prone to be valuable targets for the development of highly specific drugs for the treatment of cancer. Secondly, tumor suppressors and apoptosis inducing proteins can be useful as agents for the treatment of cancer. Other Cellular Localizations. Also many of the remaining cellular localizations are associated with particular functions. For example, most metabolic enzymes are located in 20 the mitochondria. Lehninger, A. L., 1975, "Biochemistry", Worth Publishers Inc., New York. Thus, mitochondrial proteins could reveal targets for the treatment of metabolic diseases. The Golgi apparatus and the ER are associated with post-translational processing of proteins; such processes are valuable targets for the treatment of diseases related to protein folding and glycosylation. Rademacher et al, 1988, Ann. Rev. Biochem. 57:785- 25 838. Enzymatic Activities. A particular enzymatic activity can be indicative of a protein's function. For example, kinases are frequently involved in signal transduction processes. Structures. Protein is also indicative of protein function. Indeed, proteins with 30 similar structures frequently share certain functional properties. Thus, identifying proteins having a structure related to that of a known protein having a particular function of interest can reveal additional proteins having such function. Generally, a method would be desirable which allows one to pre-sort proteins according to a property of interest, e.g., localization in the cell, affinity to binding partners, enzymatic activity, structure and the like. Such a method would allow one to generate libraries of, e.g., secreted proteins, such as cytokines, membrane associated proteins, such as receptors, nuclear proteins, such as transcription factors, mitochondrial proteins, such as respiratory proteins, and so on. Further, it would be desirable to perform screening, isolation and production of any product in mammalian cells in order to achieve the proper glycosylation pattern. Finally, the most preferred method would additionally allow one to identify and isolate proteins of interest per se rather than a partial DNA sequence. The present invention addresses this need. The present invention provides methods and expression systems for the generation of expression libraries encoding polypeptides of a predetermined property, including but not limited to cellular localization, structure, enzymatic function, or affinity to other molecules. The methods and expression systems of the present invention allow one to identify and isolate nucleic acids encoding novel proteins of interest. The methods and expression systems furthermore provide a powerful system for the identification of thus far unelucidated receptor/ligand relationships. Since the methods provided can employed in a wide variety of host cell systems, including mammalian systems, they provide for expression products having an appropriate carbohydrate composition. III. SUMMARY OF THE INVENTION The present invention relates, generally, to the field of expressed gene technology and expression cloning. More specifically, the present invention relates to the identification, characterization, and isolation of transcribed nucleic acid sequences encoding polypeptides having a predetermined property, including, but not limited to, cellular localization, structure, enzymatic function, or affinity to other molecules and the production of the corresponding polypeptides. More specifically, the invention is directed to a method for identifying nucleic acids encoding proteins with a predetermined property of interest. Such a property may include a particular cellular localization, structure, enzymatic function, or affinity to other molecules. In one embodiment of the invention, a plurality of eukaryotic host cells is provided, wherein each host cell has an expression system comprising a different member, each member comprising a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element. In a second step, the eukaryotic host cells are cultured under conditions where the exogenous protein is expressed while expression of endogenous proteins of the eukaryotic host cell is suppressed. In this time window, the exogenous protein may optionally be labelled, or may be treated in a way that allows discrimination from the untreated exogenous proteins. Finally, the member or members of the expression system that encode the exogenous protein or proteins having the property of interest are identified. In another aspect of the invention, a method for identifying a recombinant nucleic acid encoding an exogenous protein having a property of interest is achieved by providing a plurality of eukaryotic host cells, wherein each host cell has an expression system comprising a different member, and each member comprises a recombinant virus having a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element. The eukaryotic host cells are cultured to express the exogenous proteins, and expression systems expressing recombinant nucleic acid encoding an exogenous protein having the property of interest are identified. Optionally, the expression systems are capable of expressing exogenous proteins while endogenous protein production of the eukaryotic host cell is suppressed. Although any kind of recombinant eukaryotic virus can be used, particularly advantageous viruses are alpha viruses. In addition, the exogenous proteins can be preferentially labelled or distinguished from endogenous proteins. The recombinant virus may be capable of directing the generation of viral particles and replicating, or the recombinant virus may lack functions required for propagation. Also encompassed within the invention are methods for generating genetic expression libraries encoding proteins having a predetermined property of interest. In one aspect, such methods entail providing a plurality of eukaryotic host cells, wherein each host cell has an expression system comprising a different member, each member having a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element, culturing the eukaryotic host cells under conditions where said exogenous protein is expressed while expression of endogenous proteins of said eukaryotic host cell is suppressed, and identifying the members that express recombinant nucleic acids encoding exogenous proteins having the property of interest. In another aspect, the methods entail providing a plurality of eukaryotic host cells, wherein each host cell has an expression system comprising a different member, each member being a recombinant virus having a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element, culturing the eukaryotic host cells to express the exogenous proteins, and identifying the members that express recombinant nucleic acids encoding exogenous proteins having the property of interest. The invention further includes libraries of proteins identified using such methods. Additionally, the invention encompasses nucleic acid libraries having a population of eukaryotic expression systems with a plurality of members, each member having a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element for expression in eukaryotic host cells. In one embodiment, the control element directs the expression of the exogenous proteins while expression of endogenous proteins in the eukaryotic host cells are suppressed. In yet another embodiment, the control element is derived from a eukaryotic virus. IV. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 depicts an experiment in which expression systems containing either a nucleic acid encoding a secreted protein or a nucleic acid encoding an intracellular protein from a mixture of nucleic acids were identified using compartment screening. FIGURE 1A is an autoradiogram of labelled proteins precipitated from the supernatants of the screened cell cultures, as described in more detail in Example 1. Lanes are as follows: lane 1- non- infected BHK 21 cells; lane 2- cells transfected with an expression system encoding an intracellular protein, pSinRep5 lacZ; lane 6- cells transfected with an expression system encoding a secreted protein (EPO), pSinRep 5 EPO; lanes 3 to 5 contain mixtures of the 2 expression systems in the ratios 90:10, 50:50, 10:90 (sinRep5 lacZ:SinRep5 EPO) showing increasing amounts of EPO. Protein mass standard is shown on the left side; the molecular weight of EPO is indicated by the arrow. FIGURE IB is an autoradiogram of the labelled proteins in the corresponding cell pellets from lanes 1 and 2 of FIGURE 1A, showing the accumulation of lac Z protein in the cell pellet, and the shutdown of endogenous protein production, in cells infected with pSinRep5 lacZ. FIGURE 2 depicts separation of labelled viral particles from secreted protein, as described in more detail below in Example 2. Shut-down of endogenous protein synthesis is apparent in lanes 4 to 6 (infection with TE 5'2J CAT) and in lanes 7 to 9 (infection with TE 5'2J EPO) as compared to lanes 1 to 3 (non-infected BHK 21 cells). Removal of viral particles is demonstrated by the absence of the characteristic pattern of Sindbis structural proteins (capsid, El and E2). Lane 9 shows diffusion of a protein of the size of EPO through the agarose. In lanes 4 to 9 a soluble protein of viral origin can be seen. It is assumed that this protein is released by proteolytic cleavage. Labelled protein was collected at different time points: 2h (lanes 1,4,7), 4h (lanes 2,5,8) and 8h (lanes 3,6,9). Protein mass standard is shown on the left side, the size of Sindbis viral glycoproteins is indicated by the upper arrow, the molecular weight of EPO is indicated by the lower arrow. FIGURE 3 demonstrates that release of viral soluble proteins is reduced by protease inhibitors. As described below in Example 3, 10 6 BHK 21 cells in a 35mm dish were infected with TE 5'2J CAT. Varying concentrations of the Protease inhibitor cocktail (100, 20, 10, 5, 1 μl per ml, lanes 1 to 5) were applied in the 4 mm agarose overlay. The molecular mass standard is shown on the left; the arrow indicates the expected mass of Sindbis glycoprotein El. FIGURE 4 shows identification of an expression system containing a nucleic acid encoding a protein with a predetermined enzymatic activity in a semi-solid medium screening. Confluent 35mm dishes of BHK 21 cells were infected with 200 pfs SinRep lacZ (Blot 1 on the left) or a mixture of 200 pfs SinRep lacZ / 20 pfu of SinRep 5 SEAP (Blot 2 on the right). Enzymatic activity was detected on the filters with AP staining. Blot l (containing only a lacZ expression system) is negative in SEAP activity whereas blot 2 shows 2 distinct areas with SEAP activity (indicated by arrows). FIGURE 5 is a graph of the pSIN vectors used to illustrate the invention. The source and construction of these vectors is described in Table I. FIGURE 6 is a schematic representation of the pTE vectors used to illustrate the invention. Vectors shown are pTE5'2J (upper left); pTE5'2J CAT (upper right); pTE5'2J SEAP (lower left); and pTE5'2J EPO (lower right). The source and construction of these vectors is described in Table I. FIGURES 7A and 7B depict approximately 20 pfu of pSinRep5 SEAP and 780 pfu pSinRep5 LacZ were mixed in 1 ml Turbodoma HP-1 and a 60 mm dish with BHK 21 cells was infected for 2 hours. An agarose blot assay was performed as described in Example 10. FIGURE 7A shows the AP stained nitrocellulose membrane, blotted SEAP protein is represented by the violet spots of the developed X-ray film (FIGURE 7B, the AP stained membrane exposed to the X-ray film) where the black spots represent labeled secreted protein. Coordinates (indicated by arrows xl,x2,x3 in the AP stained membrane and the corresponding arrows y 1 , y2, y3 in the developed X-ray film) of the spots with SEAP activity can be superimposed with the spots of labeled secreted proteins (compare xl with yl, x2 with y2, x3 with y3). FIGURES 8A, 8B and 8C depict pSinRep 5 SEAP mRNA and pDHEB ts mutant mRNAs were co-electroporated into BHK 21 cells. The cell supernatant of the electroporated cells was analyzed 20 hours post-electroporation by spotting 4 μl supernatant on a nitrocellulose strip and AP staining was done as described before. All the electroporations were positive for SEAP secretion (violet spots on nitrocellulose filter) as shown in FIGURE 8 A. The first passage of the ts mutant viruses was tested for infectivity at 37°C and at 30°C. The supernatants of the infected cells were tested by AP staining for the secreted product SEAP, the upper row in FIGURE 8B represents the supernatant of cells incubated at 37°C, the lower row of FIGURE 8B represents the supernatant of cells incubated at 30°C. The double mutant pSinRep 5 SEAP / pDHEB ts2,20 produced 20 hours post-infection a low amount of SEAP (see FIGURE 8B) but virus particles were amplified and a high amount of product was detected after 48 hours post-infection, as shown in FIGURE 8C). FIGURES 9A, 9B, 9C and 9D depict pSinRep 5 WL13 R alpha-infected BHK 21 cells (at an moi of approximately 0.1) were analyzed by Immunofluorescence. Expressed hIL13 R alpha was detected with IL13-flag, M2 antibody and antiMouse-FITC. FIGURE 9A: BHK 21 infected at an moi of 0.1 with pSinRep 5 hIL13 R alpha, analyzed and sorted with FACS; FIGURE 9B: the same cells as in FIGURE 9A with Immunfluorescence Microscopy; FIGURE 9C: pSinRep 5 LacZ infected cells as negative control; FIGURE 9D:1 :100 diluted pSinRep 5 hill 3 R alpha virus in pSinRep 5 LacZ virus analyzed and sorted with FACS. FIGURE 10 depicts approximately 20 pfu of pSinRep 5 Epo / DHEB were mixed with 200 pfu of pSinRep 5 LacZ. The virus supernatant was incubated for 2 hours on 90% confluent BHK 21 cells in a 60 mm dish, before the supernatant was replaced with 3 ml of 0.8% 41°C warm agarose in lx HP-1 medium. Two days later, a nitrocellulose membrane was applied on the agarose and diffusion blotting was proceeded for 14 hours. Blotted EPO was detected by immuno-detection with anti-EPO antibody (rabbit) and AP-conjugated anti-rabbit antibody. The violet spots dl,d2,d3 (and the other dark spots) represent the blotted EPO derived from the underlying virus plaque representing pSinRep5 EPO virus. FIGURES 11A-11J depict the polynucleotide sequences of pSinRep 5, pSinRep 5 EPO, pSinRep 5 hIL13Ralpha, and pDH-EB. FIGURES 12A-12D depict the polynucleotide sequence of pTE5'2J. FIGURES 13A-13C depict the polynucleotide sequence of 987 BB neo. FIGURE 14 depicts the polynucleotide sequence of CAT. FIGURE 15 depicts the polynucleotide sequence of the Xbal/Apal fragment of synthetic erythropoietin. V. DEFINITIONS Terms used herein are in general as typically used in the art. The following terms are intended to have the following general meanings as they are used herein: The term "cellular localization " refers to a defined localization in the cellular context, including the extracellular space and any defined intracellular compartment, including, but not limited to, the nucleus, mitochondria, lysosomes, endoplasmatic reticulum (ER), Golgi, cytoplasm, cell membrane, endocytotic or exocytotic vesicles, cytoskeleton, peroxisomes. In microbes or plant cells, further cellular localizations may include the cell wall and chloroplast. The term "control element" refers to a nucleic acid component capable of directing expression of an operatively associated nucleic acid encoding a polypeptide. Generally, a "control element" comprises regulatory sequences required for transcription and/or translation of genetic information. The control element further comprises a component which promotes, under certain conditions or circumstances, selective expression of the operatively linked nucleic acid, while expression of endogenous host proteins is suppressed. Optionally, the "control element" may further comprise components facilitating packaging of the operatively associated nucleic acid. The term "exogenous protein " refers to a protein which is encoded by a recombinant nucleic acid. If the "exogenous protein" is expressed in a cell, it is encoded by a recombinant nucleic acid which has been introduced into the cell or its progenitors. In the expression systems of the invention, the nucleic acid encoding the exogenous protein is operatively linked to a control element. In some circumstances, the exogenous protein can be the same protein as one endogenously produced by the cell. The term "virus " refers to a bio-entity capable of introducing genetic information into a cell. The genetic information may be introduced in the form of RNA, DNA, or derivatives thereof. The cell may be eukaryotic or prokaryotic. VI. DETAILED DESCRIPTION OF THE INVENTION A. General Overview Of The Invention The present invention is directed to methods and expression systems allowing rapid identification, characterization and isolation of transcribed nucleic acid sequences encoding polypeptides having a predetermined property, including, but not limited to, cellular localization, structure, enzymatic function, or affinity to other molecules. The invention is based, in part, on the inventors' surprising discovery of materials and compositions that allow the discrimination between a unique exogenous protein of interest and all other "common" proteins present in the system, including endogenous cellular proteins and recombinant proteins which are perpetual part of the expression system. Generally, the methods of the invention employ eukaryotic host cell systems, into which the expression systems are introduced for any further characterization according to the invention. Preferably, the employed expression systems have an inherent feature which allows introduction of only one expression system per eukaroytic cell. Such systems have the advantage that they allow rapid one step cloning of individual expressed sequences. The system which is described in more detail below offers a number of advantages. First, the system of this invention is an expression cloning system which allows the direct cloning of full-length cDNAs in one step. Second, it facilitates rapid expression of the protein of interest without further laborious handling such as subcloning and establishment of a production cell line. Third, as the products may be expressed in mammalian cells, the system may yield correctly folded, glycosylated and active material. Fourth, the system allows rapid purification of the material without explicit knowledge of the nature of the protein. Fifth, it allows labelling of the material facilitating rapid identification of sites of binding in any context including animals. Finally, the system directly provides vectors useful for gene transfer into cultured cells, tissues, or animals. In one aspect, the invention discloses methods which allow the identification and isolation of novel and known proteins having a predetermined property, for example cellular localization. More specifically, within a plurality of expression systems encoding unique exogenous proteins, which are expressed in suitable host cell systems, the disclosed methods allow one to identify and isolate those expression systems which encode exogenous proteins located in a cellular compartment of interest. The method is based, in one of its aspects, on a unique feature of the employed expression system which allows, during a certain time window, high expression of the exogenous proteins while the synthesis of endogenous proteins is suppressed. If during this time window the expressed exogenous proteins are treated in a certain detectable way, for example by radioactive labelling, they may be identified easily within a chosen cellular fraction. Alternatively, all proteins expressed prior to the time window in which synthesis of endogenous protein is suppressed may be treated in an identifiable way; in this scenario the exogenous proteins which are expressed after such treatment, while expression of endogenous proteins is inhibited may be identified by not exhibiting the feature caused by the treatment. In one aspect, the methods of the invention may be used to identify and isolate individual expression systems encoding secreted proteins. During a time window in which expression of the endogenous proteins of the host cell system is suppressed, the exogenous proteins are labelled, for example by incubation with radioactive amino acids. Subsequently, all secreted proteins are separated from the host cells comprising the individual expression systems. This separation is performed in a manner which allows one to correlate the secreted protein fractions with the host cell/expression system from which it derived. For example, the host cells comprising the individual expression systems may be grown in semisolid medium, such as soft agar at a density which allows physical separation of individual clones or colonies. If the secreted proteins are separated from the cells by a vertical diffusion process on, for example, a nitrocellulose filter, which may be achieved by placing the filter on top of the semisolid medium, the secreted proteins are bound on the filter in a pattern which exactly mirrors the physical distribution pattern of the corresponding host cells in the semisolid medium. Those host cells/expression systems which comprise exogenous secreted proteins can thus easily be identified by presence of the label in the fraction of secreted proteins. Likewise, the methods may be applied to identify and isolate membrane associated proteins, nuclear proteins, mitochondrial proteins, lysosomal proteins, Golgi apparatus proteins, ER proteins, etc., as described in more detail within the body of this application. In short, the above principle may, with modifications, be applied to identify and isolate exogenous proteins localized in any other predetermined cellular localization of interest. Furthermore, the methods and expression systems of the invention may be applied to identify and isolate nucleic acids encoding exogenous proteins having a particular enzymatic activity, binding affinity to a binding partner, or structure of interest. In a second general aspect, the invention provides methods which allow expression cloning of nucleic acids encoding exogenous proteins for which a predetermined binding molecule and substrate (ligand) is available. For example, to identify an unknown secreted ligand for a known receptor, receptor protein may be immobilized on a filter, e.g., a nitrocellulose filter, in a manner that the filter is saturated with protein, i.e., does not bind any further protein molecules. A plurality of expression systems is provided that has been prepared using expressed nucleic acids from a source known to express the receptor's ligand, which can be determined using standard methods. The expression systems are again introduced and expressed in suitable host cells in a manner that host cells comprising individual expression systems are physically separable in an identifiable way, e.g., by growing the cells in semisolid medium. Similarly, as described for the identification of expression systems comprising secreted proteins, the exogenous proteins are labelled within the time window in which endogenous protein expression is suppressed. Subsequently, the secreted proteins are transferred on the nitrocellulose filter which is saturated with the receptor protein. Since all binding positions on the filter are saturated with protein, see, supra, only a ligand binding to the receptor may adhere to the filter. Exogenous protein corresponding to the receptor's ligand may be identified due to its label, and correlated to the expression system it is derived from. Accordingly, if a receptor for a predetermined ligand is to be cloned, and binding of said labelled ligand to the host cells is detected, labelled ligand is subjected to host cells comprising a plurality of expression systems and detecting binding of said labelled ligand to the host cells. Individual expression systems giving rise to ligand binding may be isolated. 0 One advantage provided by the expression systems provided by the present invention is that they may be introduced and expressed in a wide variety of host cells. The expression cloning of receptors and ligands can therefore be performed in a host cell system of the same or related species as the receptor and ligand are derived. This is a valuable feature in particular in view of the significance of the glycosylation pattern which is known 5 to be species dependent and might influence receptor ligand binding. B. The Process For Identification And Isolation Of Proteins Having A Predetermined Property 1. The Expression System Q One important component for the methods provided by the present invention is an expression system having a number of features. To practice the methods of the invention, one or a plurality of unique expression systems is generated and introduced in eukaroytic host cells leading to a population of host cells, with generally each cell containing one unique type of expression system. It will be understood, however that the 5 population of host cells can contain some cells having the same expression system. Generally, the expression system provided by the present invention is a nucleic acid molecule comprising two critical elements. First, it comprises a control element; secondly. it comprises a heterologous nucleic acid encoding a recombinant protein, referred to as "exogenous protein", which is operatively linked to the control element. Q The Control Element. In its most basic version, the control element comprises components which facilitate transcription and translation of the exogenous protein. Preferably, the control element also is or contains a component which promotes, under certain circumstances, selective expression of the exogenous protein while expression of endogenous proteins in a host is suppressed. The control element may further comprise additional components, as detailed below. A control element useful for the practice of the invention may be derived from a number of sources. Most typically, the control element is derived from a virus. A variety of viral control elements have been described that are capable of directing expression in eukaryotic cells. Particularly preferred control elements are further capable of promoting, under certain circumstances, expression of nucleic acids in eukaryotic cells while expression of endogenous proteins is inhibited. Such viral control elements both already described and yet to be discovered are within the scope of the invention. Viruses of the group of RNA viruses are preferred vectors for the present invention. Most preferably viruses of the group of alpha virus are used as viral vectors, because they themselves have the ability to block host protein synthesis. Examples of suitable alpha viruses are Sindbis virus, Semliki forest virus or Venezuelan equine encephalitis virus. A variety of alpha virus derived control elements has been described. See, among other places, Liljestrόm and Garoff, 1991, Biotechnology 9:356-361; Hahn et al, 1992, Proc. Natl. Acad. Sci. USA 89:2679-2683; Bredenbeek, 1993, J. Virol. 67:6439-6446. Alpha Viruses are positive-strand RNA viruses of the family of Togaviridae. Strauss et al. , 1994, Microbiological Reviews pp. 491-562. Alpha viruses can function in a broad range of host cells, including mammalian avian, amphibian, reptilian and insect cells. The genome of alpha viruses comprises elements capable of directing the expression of proteins encoded by nucleic acids of said viral genome in large amounts. The expression of proteins encoded by the alpha viral genome is independent of expression of proteins encoded by the genome of the host cell. Transcription of cellular genes may be arrested while the expression of virally encoded nucleic acids is not noticeably affected. Moreover, the expression of the viral genome of alpha viruses in a cell was found to result in the inhibition of translation of cellular messenger RNA molecules. Frolov et α/., 1994, J Virol. 68:1721-1727. This feature of alpha virus mediated expression of proteins in cells is particularly useful for the practice of the invention. A number of, e.g., the double subgenomic vectors of the alpha virus group are provided, in which a recombinant nucleic acid may be inserted at a site in the genome downstream of a second subgenomic promoter or another genetic element leading to expression of the exogenous nucleic acid. Hahn et al., PNAS, 89:2679-2683. It is well known that for example internal ribosome entry sites are functional in a Sindbis virus background. An RNA molecule containing a viral internal ribosome entry site upstream of a resistance gene, e.g., a neomycine resistance gene, in the cytoplasma is sufficient to survive the selection with G418. Therefore, the viral internal ribosome entry site is sufficient for translation. Also, many sites within the genome are functional for expression of the exogenous nucleic acid. Sindbis virus vectors have been engineered which contain the second subgenomic promoter/expression cassette upstream or downstream of the structural genes. Frolov et α/., 1996. PNAS, 93: 11371-11377. Additionally, for increased cloning capacity and increased safety of the vectors, the structural proteins can be supplied by helper functions provided either by a packaging cell line or a helper virus particle. Bredenbeek et al., 1993, J. Virol, pp. 6433-6446. Removing the structural proteins from the replicon increases the cloning capacity of the vector used, which is advantageous for cloning of large secreted proteins and glycoproteins. See, infra. As pointed out with these examples many modifications of the genetic arrangements are possible within the scope of the invention. Although alpha viral control elements are most preferred, a number of other viral control elements may be employed which fulfill the criterion that synthesis of proteins under viral regulation can occur when protein synthesis of the host cell is arrested. According to the present invention, a variety of methods can be applied to uncouple viral protein synthesis from host cell protein synthesis. For example, all eukaroytic genes which encode proteins are transcribed by RNA polymerase II. Thus, if a control element is used which does not use RNA polymerase II for transcription, the host protein synthesis may selectively be inhibited using an RNA polymerase II inhibitor; inhibition of transcription leads consequently also to inhibition of translation. A variety of other RNA polymerase II specific inhibitors have been described and are useful for the practice of this embodiment of the invention. The skilled artisan would readily know which RNA polymerase II specific inhibitor to use. For example, antibiotics have been demonstrated as useful for such inhibition. Such antibiotics are, for example, Actinomycin D, Aflatoxin Bl, Amatoxin, which are useful for the practice of this embodiment of the invention. In another embodiment of the invention, the control element may be derived from a prokaryotic cell. In that case, the expression system would also include the gene encoding bacterial RNA polymerase. Again, host transcription could then be selectively inhibited using a RNA polymerase II inhibitor, such as Actinomycin D, Aflatoxin B 1 , Amatoxin. The skilled artisan would readily know which control element to use in the practice of the invention. In still another embodiment of the invention, control elements derived from eukaroytic systems may be used in the expression systems of the invention. For example, one may use a promoter or enhancer element specific for RNA polymerase I or III as control element. This would render the expression of the nucleic acid encoding the exogenous protein dependent upon RNA polymerase I or III, but not on RNA polymerase II. As in eukaryotic host cells genes encoding proteins have been demonstrated to be transcribed by RNA polymerase II, and it is believed in the art that RNA polymerase II is solely responsible for such transcription. Therefore, in the practice of this embodiment, one may, for example, inhibit RNA polymerase II dependent transcription by adding an RNA polymerase II specific inhibitor, thus inhibiting the expression of endogenous proteins without such an effect on the expression of the exogenous protein. The Nucleic Acid Encoding An Exogenous Protein. As set forth above, the expression system of the invention comprises in addition to a control element a recombinant nucleic acid encoding an exogenous protein which is operatively linked to the control element. The recombinant nucleic acid may be derived from any source, i.e., any organism, tissue or cell type, disease state, etc. In one embodiment of the invention, a plurality of different nucleic acids is inserted into a plurality of expression systems, so that a plurality of expression systems is generated each encoding a unique exogenous protein. Alternatively, one known or unknown nucleic acid of interest encoding one particular exogenous protein to be characterized may be inserted into the expression system. I one embodiment of the invention, the nucleic acid component encoding an exogenous protein may be derived from a nucleic acid library. This embodiment is particularly preferred if the objective is to identify and isolate nucleic acids encoding exogenous proteins with a predetermined property of interest. The library may be obtained from a tissue or cell type of interest. This library may be a cDNA library, a genomic library, an RNA library, a heterologous RNA library, or any other kind of library comprising transcribed nucleic acid from any kind of organism, tissue, or cell type known to the skilled artisan. In preferred embodiments, the library is derived from a mammalian source, in most preferred embodiments form a human source; however, it may be also derived from reptilian, amphibian, avian, insect, plant, fungi, bacterial cells, etc. In some instances, the recombinant nucleic acid will be derived from a subtractive library, for example a library which comprises cDNAs differentially expressed in a disease state when compared to the corresponding healthy tissue. A nucleic acid library typically comprises a number of different nucleic acid species, each species having a distinct nucleic acid sequence when compared to other species in the library. The number of nucleic acid species, or complexity, of a library may vary widely, depending on a number of parameters. For instance, in case of a cDNA library, the complexity of the library depends on the complexity of the RNA pool used to generate the cDNA library. Suitable nucleic acid libraries may be generated using standard methods, see, among other places, Sambrook et al. Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor (1989). Alternatively, a suitable library may be obtained from numerous commercial sources, including, but not limited to, Clontech, Palo Alto, California; and Stratagene, La Jolla, California. Optionally, the genetic library may be propagated in E. coli, phages, yeast or the like. Alternatively, the nucleic acid encoding the exogenous protein may be derived from mRNA isolated from a tissue or cell type of interest. In this case, the mRNA is reverse transcribed into cDNA. Starting with cellular RNA or, preferably, mRNA, many methods are known for the preparation of cDNA. Most preferably directed cDNA is produced which can be inserted into the viral vector such that the clones carry the inserts in sense orientation. Stratagene, e.g., offers several kits for the generation of directed cDNA (lambda ZAP cDNA kit). Further, standard protocols for the generation of cDNA are available, and can be found, among other places, in Sambrook et al. , Molecular Cloning: A Laboratory Manual. 2nd Ed., Cold Spring Harbor (1989). In another embodiment of the invention, the nucleic acid encoding an exogenous protein is a particular and specific nucleic acid of interest. The so generated expression system is particularly useful for characterization of a particular nucleic acid or protein of interest as it conveniently allows to determine where a protein encoded by a nucleic acid molecule is located in the cell, thereby revealing important information to the skilled artisan as to the function of that protein. The nucleic acid of interest may be DNA or RNA. Whether the nucleic acid is available as DNA or RNA, it can easily be used with a control element as part of the expression system of the invention. Once the artisan has chosen a control element useful for the method of the invention, it will require only routine experimentation to use the nucleic acid of interest with that control element. For example, if the nucleic acid of interest is available in the form of RNA and the control element of choice is available in the form of DNA, the nucleic acid of interest can be converted into DNA using routine reverse transcriptase assays. Also, the control element could be converted into RNA by using routine transcription assays. Following either conversion, the resulting two DNA or RNA molecules can be ligated together using routine DNA ligase assays or RNA ligase assays. Finally, the resulting expression system comprising the nucleic acid of interest and the control element combined, can then be used in the invention. Similar to the above example, if the nucleic acid of interest is available in the form of DNA and the control element in the form of RNA, either routine conversion method can be applied followed by the appropriate routine ligase method. The skilled artisan would readily know which procedure for transcription, reverse transcription, or ligation to follow in order to practice the invention. Alternatively, the nucleic acid of interest may be DNA of any kind. This includes, but is not limited to, complementary DNA ("cDNA"), genomic DNA, cDNA that was manipulated to include additional nucleic acids, or any other kind of DNA. Any kind of DNA can be used in the method of the invention. Any kind of DNA can be linked to the control element molecule by using routine ligation assays. The expression of genomic DNA is well within the scope of the invention as the use of eukaryotic cell types facilitates the necessary processing of heterogenous RNA molecules that are created through transcription of genomic DNA, to yield messenger RNA useful for cellular protein synthesis. The nucleic acid encoding the exogenous protein, if derived from a cDNA library or from RNA or from any other source may be integrated into the expression system using standard procedure. Such integration may be facilitated by, for example, DNA ligation or RNA ligation procedures to connect a nucleic acid molecule of the library with a control element molecule. Additionally, different methods of assembly of the cDNA or RNA and the viral vector are available. Standard ligation of the nucleic acids as DNA are the preferred way to carry out the assembly step. Other methods are of course feasible, e.g., RNA molecules can be ligated using the enzyme T4 RNA ligase. Numerous standard methods are available and described, for example, in Sambrook et al. , Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor (1989). Viral Propagation. When using a virally derived control element in the practice of the invention, one may want to propagate the virus in order to have the expression system available for future experimentation. The propagation of a virus requires the availability of certain molecular components to facilitate the assembly of viral particles. Some molecular components, in the case of proteins, typically have to be encoded by nucleic acids of the viral genome, depending on the particular virus. However, one cannot add unlimited numbers of nucleic acids to the genome of a virus while maintaining the ability of the resulting derivative of the viral genome to become packaged into viral particles. Therefore, when trying to maximize the size of a nucleic acid of interest that can be incorporated into a viral genome for characterization in the method of the invention, it is necessary to delete nucleic acids found in the wild type viral genome from that genome. However, such deletion may render the resulting expression system incapable of propagation of the virus the control element is derived from. This problem may, however, be overcome by using a helper function that provides for the necessary components for viral propagation. In one embodiment of the invention, such a helper function may be provided by use of a helper nucleic acid. A helper nucleic acid may be, for example, a defective helper RNA, as used for alpha virus based expression systems, to facilitate the propagation of the alpha virus. In another embodiment of the invention, such a helper function may be provided through a packaging cell line. A packaging cell line encodes in its genome the components necessary for viral propagation usually encoded by the viral genome, but deleted for purposes of cloning utility. These components will therefore be expressed in the cell line used in the method of the invention and propagation of the virus will be facilitated. The generation of such a packaging cell line is well within the knowledge of the skilled artisan by using routine experimentation. For example, depending on the virus chosen for the practice of the invention, one may take the exact same nucleotide sequence deleted from the viral genome, link it to a control element known to facilitate high level expression of nucleic acids in the chosen cell line, and transfect the resulting nucleic acid into the cell line. An example of such a vector is provided below. Specifically, the vector 987 BBneo (SEQ ID NO:2) can be transfected into BHK 21 cells with the Ca-phosphate method and subsequently selected with 200 mg/l G418 according to known methods. This can be further facilitated by, for example, using standard screening procedures for the establishment of cell lines with new properties, like, for example, the screen for the presence of a marker providing resistance to a certain toxin. Such a marker, like, for example, the neomycin resistance gene which renders cells resistant to neomycin selection, may be included in the expression system comprising the nucleic acids encoding components necessary for virus propagation and the control element specific for the chosen cell line. 2. Introduction Of The Expression System In Host Cells The expression system, or typically a plurality of unique expression systems comprising library inserts encoding exogenous proteins, are introduced into suitable eukaryotic host cells. In this manner, a plurality of eukaryotic host cells can be provided, wherein each host cell has an expression system comprising a different member. Of course, in this population there may be multiple host cells that contain the same member. The host cells can be derived from any kind of organism, depending on the compatibility of the expression system. Preferably, the host cell is identical with or related to the cellular source of the recombinant nucleic acids encoding the exogenous protein, which, in preferred embodiments is mammal. Introduction of the expression systems into eukaryotic host cells can be carried out using a number of different well known procedures. Transfection with CaPO 4 , polyethylenimine, lipofection, electroporation, are only a number of available techniques to introduce nucleic acids into animal cells. An RNA molecule containing a viral internal ribosome entry site upstream of a resistance gene, e.g., a neomycine resistance gene, in the cytoplasma is sufficient to survive the selection with G418. Therefore, the viral internal ribosome entry site is sufficient for translation. An advantage of the alpha viruses is their 0 broad host range; infection of mammalian, insect, avian, amphibian, and reptilian cells from various different tissues and different degree of transformation have been reported. Cells containing exogenous viral nucleic acids can be used to produce viral particles which can be collected from the supernatant or by cell lysis. Alternatively the cells containing the exogenous nucleic acids can be used to induce plaque formation in a feeder culture. 5 3. The Screening Process For The Identification And Isolation Of Exogenous Proteins Having A Predetermined Property In order to facilitate screening for individual exogenous proteins, a cell culture is provided which allows physical separation of different viral clones and Q physical separation of viral particles and/or viral proteins from exogenous novel secreted proteins or glycoproteins of interest. According to the present invention, the detection of clones secreting novel radioactive labelled proteins or glycoproteins can be performed in various ways. Physical separation of the viral clones can be achieved for example by plaque formation in semisolid 5 medium consisting of normal growth medium containing agarose or carboxymethylcellulose (similar to phage screening). Alternatively single virus particles (isolated by limited dilution) or plaque purified clones can be inoculated into wells of, e.g., a 96 well plate containing animal cells. Addition of single cells to a feeder layer in 96 well plates will infect the cells with a single clone. Other methods of physical separation of 0 viral clones are possible without leaving the scope of the invention. Blocking Of Residual Host Protein Synthesis. Where selective labelling of virally encoded exogenous proteins is desired, host cellular protein synthesis has to be silent. According to the present invention several methods can be used to suppress cellular protein synthesis. Using viruses of the alpha virus group addresses this problem, since the virus shuts down cellular protein synthesis entirely, see, supra. Alternatively, depending on the control element employed, see, supra, if other viruses are used host cell directed RNA polymerase II transcription and translation can be suppressed specifically by addition of agents such as Actinomycin D, Aflatoxin Bl, Amatoxin. Also in cases where not all cells are subject to viral replication the remaining fraction of cells can be transcriptionally/translationally be arrested with this substance. Condreay et al, 1988, J. Virol, 62:2629-2635. The Labelling Step. In a preferred embodiment of the invention, the exogenous protein expressed by the expression system is rendered distinguishable from the endogenous proteins of the host cell system by specific labelling during a time window in which endogenous protein expression is suppressed while the exogenous protein is expressed. For example, a pulse of radioactively labelled amino acids of suitable length and intensity is applied such that sufficient radioactive protein can be recovered from the clone for detection. The system may be varied in at least three dimensions, i.e., (\) pulse length, (2) pulse intensity, and (3) site of plaques. At a suitable time after infection when sufficient cells undergo viral replication and express virally encoded proteins and when suppression of RNA polymerase II transcription and/or translation are complete, a pulse of radioactive amino acids is added to the culture. Of course, many different combinations of pulse and chase times, at different plaque sizes, choice of isotopes, mode of autoradiography (X-Ray film vs. Phosphoimager) etc. are suitable. Examples of suitable combinations of labelling times, isotopes, concentrations etc. are given in the examples. The application of radioactive amino acids can be carried out as a pulse with a subsequent chase period. Other schemes are suitable, e.g., a continuous incorporation of radioactive amino acids. Monomeric amino acids are not precipitable under certain conditions at which proteins are precipitable and can therefore be subtracted. Suitable isotopes are 35 S, 3 H, 14 C, in any of the twenty (20) natural amino acids. Alternatively endogenous proteins can be treated in a way to allow discrimination with newly synthesized exogenous proteins. Treatments of the present invention are: 1) removal of sialic acid residues from the cell surface by sialidase. New membrane proteins being synthesized from the exogenous expression system display sialic acid residues on the cell surface allowing detection; 2) proteolytic digestion of endogenous proteins (e.g., on the cell surface). Other methods are possible and within the scope at the present invention. Soluble Viral Proteins. When using an expression system with a control element derived from a virus comprising viral proteins, one may encounter the problem of soluble viral proteins that are expressed when the exogenous protein is expressed. This problem may be of varying significance. For example, when using the semisolid medium, e.g., soft- agar-plate, based screening method, such soluble viral proteins could bind the membrane designed to bind the exogenous protein. In one embodiment of the invention, such soluble viral proteins are kept from interfering with the screen for desired exogenous proteins by using a filter that can bind or stop the flow of such soluble viral proteins. In another embodiment, antibodies specific to the soluble viral proteins are attached to such filter papers so to bind the soluble viral proteins. A variety of filter papers useful to bind or stop the flow of soluble viral proteins has been described. The skilled artisan would readily know which filter to use for the practice of the invention. It is also well within the knowledge of the skilled artisan to attach antibodies specific for soluble viral proteins to such filters. In another embodiment of the invention, antibodies are crosslinked to the semisolid medium (e.g., NuFix Agarose, FMC Bioproducts Rockland USA, which prevents interfering of the viral proteins with the screen. In another embodiment of the invention, the generation of soluble viral protein is reduced by using serum containing medium. Any amount of reduction is desirable, preferably to levels that allow detection of secreted proteins of interest in the background of viral proteins. Yet another way of reducing the generation of soluble viral proteins is by using a protease inhibitor or inhibitors. An example of using protease inhibitors to reduce the amounts of soluble viral proteins is illustrated below by way of a working example. Any one or number of protease inhibitors can be used and are well known to those of ordinary skill in the art. In another embodiment of the invention, the generation of soluble viral protein is reduced by using an expression system containing a control element that is based on a mutant virus. A variety of mutant virus strains has been described which are useful for this embodiment of the invention. The skilled artisan would readily know which mutant viral strain to use for this embodiment. For example, mutant viral strains of the genus alpha virus have been described that are useful for the practice of this embodiment of the invention. A mutant Sindbis Virus strain useful for the practice of this embodiment of the invention having the genotypes, for example, ts20, tslO, and ts23. Lindquist et al., 1986, Virology, 151 :10-20; Arias et al, 1983, J Mol Biol, 168:87-102; Carleton and Brown, 1996, J. Virol. , 70:952- 959. Such temperature-sensitive virus mutants are defective in cleavage of PE2 (ts20) and defective in folding of El (tslO; ts23). As a consequence, the viral proteins do not reach the cell surface at the nonpermissive temperature. Leakage of soluble proteins is reduced by use of these mutants. In another embodiment of the invention, the generation of soluble viral proteins is reduced using a cell line deficient in the proteolysis of viral proteins in the method of the invention. A variety of cell lines deficient in the proteolysis of viral proteins has been described. Watson et al, 1991, J. Virol. 65:2332-2339. The skilled artisan would readily know which cell line deficient in proteolysis of viral proteins would be useful for the practice of the invention. A cell line useful for the practice of this embodiment of the invention would be deficient in the proteolytic cleavage of proteins of the virus used in the method of the invention. For example, when using an Alpha Virus in the practice of the invention, cell lines described that are deficient in the cleavage of the viral proteins PE2 to E2 and E3 may be useful. More particular screening processes of the invention are exemplified below. Compartment Screening. In one embodiment of the invention, host cells comprising unique expression systems encoding exogenous proteins may be screened by placing them into separate compartments. For example, individual cells may be picked physically and placed into a separate compartment, for example, a well in a 96 well plate. Of course, if desired, a number of cells may be placed into a compartment or well, to facilitate the screening of cells in groups. Alternatively, viral particles are diluted to 1 pfu per compartment leading to monoclones. Semisolid Medium Screening. In another embodiment of the invention, cells used in the method of the invention may be separated by growing them in soft agar. For example, cells can be transfected with the nucleic acids of the expression system and then spread out onto plates in a semisolid medium, i.e., soft agar. This soft agar, or semisolid medium may consist of, for example, normal growth medium containing agarose or carboxymethylcellulose. Alternatively, a confluent layer of cells is infected with virus, e.g., alphavirus at an MOI that allows plagues to grow to a size of 1 -3mm. Three hours postinfection the virus solution is removed and replaced by semi-solid medium. This semisolid medium inhibits diffusion of virus particles and allows plaque formation. Other methods of maintaining cells in a semi solid medium known to the skilled artisan are also within the scope of the invention. Once plated out in the soft agar, or semisolid, medium, the host cells can then be screened. This allows one to reduce lateral diffusion of substances on the plate and therefore reduces cross contamination of the various cells or viruses that contain expression systems expressing different exogenous proteins. In addition, this experimental setup allows the flow of proteins secreted by the cells, for example, extracellular proteins encoded by the nucleic acid of interest, from the cells to a filter that can bind such proteins. Filters useful for the binding of proteins are, for example, nitrocellulose filters and PVDF or nylon membranes. Once exogenous proteins are bound to a filter, they may be screened by different methods known to the skilled artisan. In one embodiment of the invention, exogenous proteins are selectively labelled, see supra, to determine their location on the filter. For example, by exposing the filter to a film, the film, once developed, will have a dark spot in the place where it was juxtaposed to a spot of the filter that had bound labelled exogenous protein. When using proper markings, one may correlate each spot on the film to a particular area on the soft agar plate. Thus, one may locate the cells that express the exogenous protein which gave rise to a spot on the film. Therefore, one may also locate the particular expression system that contains the nucleic acid encoding the exogenous protein that was secreted and gave rise to the spot on the film. In another embodiment of the invention, one may screen filters with bound exogenous protein by exposing the filters to a ligand. For example, one method of choice may be to expose the filter with the bound exogenous proteins to a labelled lectin, thereby screening for secreted glycoproteins. Or, for example, one may screen the filter with an antibody specific to a particular post-translational modification such as, for example, a sugar moiety. Also, one may use antibodies that are specific to a particular protein of interest. Furthermore, this system facilitates the easy separation of viral particles from the exogenous proteins. Separation can be accomplished by, for example, using a different filter paper, i.e., one that binds or stops the flow of viral particles but not the exogenous protein. This additional filter paper may be placed underneath the filter that binds the exogenous protein. Therefore, the viral particles would not reach the filter paper that binds the exogenous protein. Filters can be rendered more effective by, for example, attaching antibodies against the soluble viral proteins to the filter which bind the soluble viral proteins but not the exogenous protein. Detection And Isolation Of Secreted Proteins. In a preferred embodiment of the invention, samples are prepared for detection of secreted radioactive protein and detecting radioactivity versus background. According to the method of physical separation of the clones chosen, different modes of detection of the secreted protein or glycoprotein can be applied. For example, in cases where single clones were inoculated in microtiterplates, precipitation of the secreted radioactive protein or glycoprotein in the supernatant can be carried out with, e.g., the TCA method. (Ma et al, 1994, Cancer Chemother. Pharmacol. 38(4):391-394). If physical separation of the clones was achieved using semisolid growth medium, then the membrane capturing the secreted radioactive protein or glycoprotein is washed and prepared for autoradiography. Many different commercially available membranes or filters are useful as tracer membranes. Protein captured at the membrane has to be bound such that the clones secreting radioactive protein can be detected. Several different washing procedures can be applied to link precipitable protein to the membrane and remove unincorporated radioactive amino acids. Several methods are suitable for autoradiography including exposure with or without intensifying agents to an X-Ray film and Phosphoimaging. Ligand Selection. In another embodiment of the invention, cells containing expression systems expressing different exogenous proteins may be separated by ligand recognition. For example, when screening for nucleic acids of interest encoding receptor molecules of the cellular membrane, a ligand capable of recognizing such receptors may be used to screen for cells that express such receptors. That screen may be carried out by, for example, attaching the ligand to a solid support and exposing cells expressing the protein of interest to the ligand on the solid support. By binding cells that express membrane receptor molecules of interest, the cells expressing such receptors may be separated from cells not expressing such receptors. In one embodiment of the invention, the ligand may be a protein. In another embodiment, the ligand may be an antibody. In yet another embodiment, the ligand may be a lectin. In a further embodiment, the ligand may be a non-protein. The choice of the ligand is dependent upon the protein that is to be selected for. A large number of ligands capable of binding proteins based on the peptide backbone, a post-translational modification, or other criteria have been described. The skilled artisan would readily know which ligand is useful for the practice of this embodiment of the invention. In another embodiment of the invention, the ligand is attached to a solid support. A variety of solid supports useful for the practice of this embodiment have been described. For example, one may attach the ligand to a plate to which one would add the cells expressing exogenous protein. Once on the plate to which the ligand is bound, cells will bind to the ligand if they express a membrane protein which recognizes the ligand. Cells bound to the ligand will therefore remain on the plate when a washing solution is added for the removal of cells that do not bind to the ligand. This way, cells expressing a membrane receptor which binds the ligand will be removed from cells that do not. Consequently, this is a fast and simple way to identify membrane receptors and the nucleic acids that encode them. As membrane receptors are main targets for drug development, this embodiment of the invention will be highly useful for pharmaceutical research and drug discovery. Membrane Protein Selection. In another embodiment of the invention, one may select for cells expressing an exogenous protein based on the exogenous protein being a membrane protein with an extracellular domain. For example, one may express exogenous proteins in cells while expression of endogenous proteins is inhibited or while using an expression system the operation of which inhibits the expression of endogenous proteins, see, supra. Using this setup, one may, for example, treat the cells with proteases after inhibition of expression of endogenous proteins has set in, but while expression of exogenous protein is still continuing. After the cell surface has been deprived of extracellular proteinaceous protrusions, i.e., cellular membrane receptors and so on, the expression of the exogenous protein may replenish such protrusions. However, such replenishment will only occur if the exogenous protein expressed in a particular cell is a membrane receptor molecule. Therefore, at this step, only cells containing an expression system that encodes a membrane receptor molecule will have extracellular proteinaceous protrusions. Such protrusions may be used to bind the cell to any structure that binds proteins, regardless of whether such binding occurs specifically or nonspecifically with regard to the structure of the protein that is bound. Cells bound to such a protein binding structure, may then, for example be further analyzed as to the sequence of the nucleic acid encoding the exogenous protein. A variety of structures that bind proteins have been described and are well known to the skilled artisan. For example, nitrocellulose, PVPF or nylon, filters may be used in this embodiment as a protein binding structure. Other structures, known to the skilled artisan, that bind proteins are also within the scope of the invention. Membrane Glycoprotein Screening. In one embodiment of the invention, cell membrane glycoproteins are identified using the method of the invention. To facilitate the identification of cell membrane glycoproteins, a variety of method have been described that are useful for the practice of this embodiment of the invention. For example, one may alter the chemical reactivity of cell surface sugar residues and thereby facilitate the specific recognition and identification of such sugar residues. One method useful for such alteration of chemical reactivity is, for example, the introduction of reactive ketone groups into cell surface oligosaccharides. Mahal et al, 1997, Science 276: 1125-1128. In this method one introduces new chemically reactive groups into cell surface glycoproteins. When this method is applied under conditions where the exogenous protein is expressed in the method of the invention while the expression of endogenous proteins is inhibited, the exogenous protein will preferably incorporate the reactive group, thereby facilitating the recognition of the exogenous protein. Other methods known to the skilled artisan useful for the practice of this embodiment of the invention and are also within the scope of the invention. In another embodiment of the invention, membrane glycoproteins are identified using the method of the invention through selective glycosylation of such membrane glycoproteins. A variety of methods are known to selectively glycosylate membrane proteins and are within the scope of the invention. For example, one may first remove all cell surface sialic acid residues by using an enzyme exhibiting sialidase activity. After such removal, one may, using the method of the invention, express the exogenous protein in cells while the expression of endogenous proteins is inhibited. Thereby, the exogenous protein will be preferably transported in sialiated form to the cell surface which facilitates its improved identification. Screening For Proteins Located In Other Cellular Fractions Of Interest. In one embodiment, the present invention provides a general approach with which expression systems comprising a nucleic acid encoding an exogenous protein located in any cellular location of interest may be identified. Host cells harboring expression system, comprising a unique nucleic acid encoding an exogenous protein are provided in a manner that physical separation of each host cell comprising an individual expression system is possible. Preferably, the expression systems comprise viral control elements. Most preferably, the host cell/expression system allows that viral particles comprising the expression system be released from the host cells. For example, host cells may be plated in semisolid medium. At first, filter which is capable of binding viral particles is placed in the semisolid medium under conditions and for a duration which allow to captive viral particles on the filter. The skilled artisan will readily know what filters may be useful for production of this embodiment of the invention. For example, the filter may be a nitrocellose filter. The first filter having the virus particles attended to it is treated in a way which facilitates recovery of infectious virus particles comprising an expression system. Subsequently, the host cells in the semisolid medium are exposed to a composition which allows labelling of the expressed exogenous protein which expression of endogenous proteins is inhibited. For example, the exogenous proteins may be labelled with radioactive amino acids while endogenous protein expression is suppressed. In a next step, host cells in the semisolid medium are and/or conditions which allow recovery of the cellular fraction/compartment of interest. The skilled artisan will know what conditions would be appropriate depending on the cellular compartment/fraction under consideration. In a next step, a second filter having attached to it, e.g., antibodies against certain structures comprised within the cellular fraction of interest is placed on the semisolid medium. It is critical that the filter is saturated, i.e., does not allow for any further molecules to bind to its surface. The second filter is placed under conditions and for a duration which allows the predetermined cellular compartment of interest to adhere. If, for example, the cellular component of interest is in the nucleus, antibodies to nuclear membrane surface proteins would be attached to the filter. For other compartments, corresponding methods would be used. The second filter thus constitutes a "replica" of the first filter, where the first filter has attached to it the recoverable expression system, while the second filter has attached to it proteins derived from the cellular component of interest. Those individual clones of host cells which comprise an expression system comprising a nucleic acid encoding an exogenous protein located within the cellular fraction/component of interest may be identified through presence of the label. Additionally, cellular compartments can be physically separated using classical methods well-known in the art (e.g., differential centrifugation). Identification Of Unknown Ligands Of Known Receptors. In one embodiment, the invention allows for the identification of unknown secreted ligands of known receptor of interest. Host cells comprising expression system comprising unique recombinant nucleic acids encoding exogenous proteins are cultured in a way that they are a physically separable. For example, for this purpose the host cells may be cultured in semisolid medium. The cells are incubated with compositions which allow labelling of expressed exogenous proteins while expression of endogenous proteins is inhibited. A filter coated with the receptor protein of interest whereby all binding sites are saturated (either by the receptor protein or by any other neutral protein) is placed on the semisolid medium under conditions and for a duration which allows all secreted proteins to move to the filter. Since all protein binding sites on the filter are saturated, only secreted proteins binding to the receptor of interest may adhere to the filter. Those exogenous proteins may be identified by means of their label. As the filters represent a replica, i.e., a mirror image of the original plate, colonies harboring the expression system comprising a nucleic acid encoding a ligand of the receptor of interest may be easily identified. Identification of Unknown Receptors Of Known Ligands. In one embodiment, the invention allows for the identification of unknown receptors for a known ligand. Host cells comprising expression system comprising unique recombinant nucleic acids encoding exogenous proteins are cultured in a way that they are physically separable. For example, a confluent layer of cells can be infected with a low MOI of an infectious virus carrying the plurality of expression systems. After infection, the cells are incubated at 34 °C in semisolid medium with a melting point around 37°C (e.g., low melting agarose). After 0 plaques of sufficient size are produced, a filter is placed on top long enough to capture viral particles which serves as a replica of the plate. Subsequently, the agarose is melted by heating the plates to 39 °C. After removal of the agarose and washing, the cells are incubated with the labelled ligand of choice. After washing of the cells, the binding of the ligand can be detected by autoradiography. Viral particles containing the expression 5 system (and nucleic acid) of interest can then be recovered from the replica filter. 4. Recovery Of Viral Particles Comprising Exogenous Proteins Of Interest In a next step, those viral particles from the samples or area where increased radioactivity was observed are identified and recovered. The identity of the well or the position of the plaques where exogenous protein of interest was detected can be traced back using the developed film and viral particles can be recovered. At this point, recombinant virus particles containing a nucleic acid sequence giving rise to expression of the protein of interest are collected. According to the invention the particles can be applied g for, e.g. , the one of the procedures described below. C. Applications Of The Identified Expression Systems Encoding Proteins Of Predetermined Properties The pools of expression systems selected with the above screening methods having a predetermined property, e.g. , cellular localization, structure, enzymatic function, 0 or affinity to other molecules, may be used for further screening procedures. Alternatively, individual expression systems may be randomly picked and subjected to characterization and functional analysis. See, infra. Additional Screening Rounds To Avoid Contamination. Individual clones may be picked and the procedure of steps 3 and 4 can be repeated to plaque purify the clones and to avoid contaminations. Passaging The Viral Particles To Increase Their Number. The viral particles isolated and optionally amplified by passaging can be applied to one of the following procedures. Many useful application for viral particles containing exogenous nucleic acids encoding secreted proteins or glycoproteins are apparent, of which some are describe below. Therefore the present invention is not limited to the applications described above. Infection Of A Cell Culture To Induce Production Of Novel Secreted Protein Or Glycoprotein, In Either Labelled Or Unlabelled Form. Novel secreted proteins or glycoproteins can be produced at larger scale by infecting a substrate cell line growing in vessels such as T-flasks, spinner flask, roller bottles, bioreactors, perfusion reactors and the like with the isolated viral particles. Depending on the nature of the genetic construct chosen above, production will occur under slightly different conditions. When one way vectors are packaged in a packaging cell line the particles will induce only one round of infection (accordingly the MOI should be higher than one). Also a broad spectrum of host cell lines can be used for production of the novel secreted protein or glycoprotein. Labelling of the novel secreted protein or glycoprotein can be achieved by addition of radioactive amino acids as described above. This offers the advantage that product formation can easily be monitored by SDS polyacrylamide gel electrophoresis and subsequent autoradiography. In this way an unknown product can be easily detected for optimization of production parameters. Application Of The So Produced Supernatant To Protein Purification Or Isolation Methods And Detecting Novel Secreted Protein Or Glycoprotein By Its Radioactive Label. The novel secreted protein or glycoprotein can easily be purified from the supernatant of the above step using standard protein purification techniques. The unique possibility to label the secreted protein during production, see supra, simplifies purification processes significantly. Eluent from chromatography columns or the like can automatically be collected using a sample collector. The samples containing the novel secreted protein can then easily be identified by detection of radioactivity. Enriched and purified material can be obtained in this way. Unlabelled material can be obtained by application of identical chromatography conditions to unlabelled cell culture supernatant. D. Characterization And Functional Analysis Of Individual Identified Proteins Several procedures known in the art may be employed for further characterization and functional analysis of individual identified exogenous proteins. 1. Sequencing of The Nucleic Acid Encoding The Exogenous Protein In one embodiment, the identity and the sequence of the exogenous nucleic acid in the recovered viral particles can be investigated by sequencing according to standard methods. Detailed description of suitable protocols can be found, for example, in Sambrook et al. , Molecular Cloning: A Laboratory Manual, 2nd Ed.. Cold Spring Harbor (1989). Sequence comparisons with known polynucleotide sequences in data bases may reveal indications about the function of an identified and isolated exogenous protein. 2. Analysis Of The Expression Pattern Of The Identified Exogenous Protein In one embodiment, the cDNA of the identified exogenous protein or fragments thereof are used as a probe to detect the expression of its mRNA. For example, sections of tissue samples may be prepared and examined by in situ hybridization with a suitable, labelled probe. Alternately, mRNA extracts may be prepared and analyzed in Northern blot analysis. Alternatively, synthetic oligonucleotides designed according to the identified protein's cDNA sequence may be generated and used as hybridization probes. Detailed description of suitable protocols can be found, for example, in Sambrook et al. , Molecular Cloning: A Laboratory Manual, 2nd Ed.. Cold Spring Harbor (1989). In one embodiment, the level of the gene's expression is assayed by detecting and measuring its transcription. For example, RNA from a cell type or tissue known, or suspected to over- or under- express the gene, such as cancerous tissue, is isolated and tested utilizing hybridization or PCR techniques such as are described herein. The isolated cells can be derived from cell culture, from a patient, or from test animals. Such analyses may reveal both quantitative and qualitative aspects of the expression pattern of the gene encoding the identified exogenous protein, including activation or inactivation of its gene expression. Hybridization probes for Northern blot, Southern blot, and in situ hybridization may be labelled by a variety of reporter groups, including radionuclides such as 32 P, 35 S, and 3 H (in the case of in situ hybridization), or enzymatic labels, such as alkaline phosphatase, coupled to the probe via avidin biotin coupling systems, and the like. The labelled hybridization probes may be prepared by any method known in the art for the synthesis of DNA and RNA molecules. See, Section VI. K, infra. An additional use for nucleic acid hybridization probes involves their use as primers for polymerase chain reaction (PCR). PCR is described in detail in U.S. Patents 4,965,188, 4,683,195, and 4,800,195. DNA may be used in hybridization or amplification assays of biological samples to detect abnormalities involving gene structure, including point mutations, insertions, deletions and chromosomal rearrangements. Such assays include, but are not limited to, Southern analyses, single stranded conformational polymorphism analyses (SSCP), and PCR analyses. Diagnostic methods for the detection of specific mutations of the gene encoding the identified exogenous protein can involve for example, contacting and incubating nucleic acids including recombinant DNA molecules, cloned genes or degenerate variants thereof, obtained from a sample, e.g., derived from a patient sample or other appropriate cellular source, with one or more labelled nucleic acid reagents including recombinant DNA molecules, cloned genes or degenerate variants thereof, under conditions favorable for the specific annealing of these reagents to their complementary sequences within the gene. Preferably, the lengths of these nucleic acid reagents are at least 15 to 30 nucleotides. After incubation, all non-annealed nucleic acids are removed from the nucleic acid molecule hybrid. The presence of nucleic acids which have hybridized, if any, is then detected. Using such a detection system, the nucleic acid from the cell type or tissue of interest can be immobilized, for example, to a solid support such as a membrane, or a plastic surface such as that on a microtiter plate or polystyrene beads. In this case, after incubation, non- annealed, labelled nucleic acid reagents are easily removed. Detection of the remaining, annealed, labelled gene's nucleic acid reagents is accomplished using standard techniques well-known to those in the art. The sequences to which the nucleic acid reagents have annealed is compared to the annealing pattern expected from a normal gene sequence in order to determine whether a gene mutation is present. Alternative methods for the detection of the gene's specific nucleic acid molecules, in patient samples or other appropriate cell sources, may involve their amplification, e.g., by PCR (the experimental embodiment set forth in Mullis, K.B., 1987, U.S. Patent No. 4,683,202, see, supra), followed by the detection of the amplified molecules using techniques well known to those of skill in the art. If mutations are intended to be determined, the resulting amplified sequences can be compared to those which would be expected if the nucleic acid being amplified contained only normal copies of the gene in order to determine whether a gene mutation exists. 3. Detection Of The Identified Exogenous Protein Using Antibodies Antibodies directed against the identified exogenous protein of interest or conserved variants or peptide fragments thereof, may also be used to gain more insight of its expression pattern in vivo. Antibodies may also be used to detect abnormalities in the level of the gene's expression, or abnormalities in the structure and/or temporal, tissue, cellular, or subcellular location of its gene product, and may be performed in vivo or in vitro, such as, for example, on biopsy tissue. The analysis may be performed on any tissue or cell type, or, with labelled antibodies, even in vivo in a test animal. Alternatively, the tissue or cell type to be analyzed may include those which are known, or suspected, to aberrantly express the identified exogenous protein of interest, such as, for example, cancerous tissue. The protein isolation methods employed herein may, for example, be such as those described in Harlow and Lane (Harlow, E. and Lane, D., 1988, "Antibodies: A Laboratory Manual". Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York). The isolated cells can be derived from cell culture, a laboratory animal, or a patient. For example, antibodies, or fragments of antibodies useful in the present invention may be used to quantitatively or qualitatively detect the presence of the identified exogenous protein of interest or conserved variants or peptide fragments thereof. This can be accomplished, for example, by immunofluorescence techniques employing a fluorescently labelled antibody (see, this Section, infra) coupled with light microscopic, flow cytometric, or fluorimetric detection. The antibodies (or fragments thereof) or fusion or conjugated proteins useful in the present invention may, additionally, be employed histologically, as in immunofluorescence, immunoelectron microscopy or non-immuno assays, for in situ detection of the identified exogenous protein of interest or conserved variants or peptide fragments thereof, or for catalytic subunit binding (in the case of labelled catalytic subunit fusion protein). In situ detection may be accomplished by removing a histological specimen from a patient, and applying thereto a labelled antibody or fusion protein of the present invention. The antibody (or fragment) or fusion protein is preferably applied by overlaying the labelled antibody (or fragment) onto a biological sample. Through the use of such a procedure, it is possible to determine not only the presence of the identified exogenous protein of interest, or conserved variants or peptide fragments, but also its distribution in the examined tissue. Using the present invention, those of ordinary skill will readily perceive that any of a wide variety of histological methods (such as staining procedures) can be modified in order to achieve such in situ detection. Immunoassays and non-immunoassays for identified exogenous protein of interest or conserved variants or peptide fragments thereof will typically comprise incubating a sample, such as a biological fluid, a tissue extract, freshly harvested cells, or lysates of cells which have been incubated in cell culture, in the presence of a detectably labelled antibody capable of identifying the identified exogenous protein of interest or conserved variants or peptide fragments thereof, and detecting the bound antibody by any of a number of techniques well-known in the art. The biological sample may be brought in contact with and immobilized onto a solid phase support or carrier such as nitrocellulose, or other solid support which is capable of immobilizing cells, cell particles or soluble proteins. The support may then be washed with suitable buffers followed by treatment with the detectably labelled antibody or fusion protein. The solid phase support may then be washed with the buffer a second time to remove unbound antibody or fusion protein. The amount of bound label on solid support is then detected by conventional means. By "solid phase support or carrier" is intended any support capable of binding an antigen or an antibody. Well-known supports or carriers include glass, polystyrene, polypropylene, polyethylene, dextran, nylon, amylases, natural and modified celluloses, polyacrylamides, gabbros, and magnetite. The nature of the carrier can be either soluble to some extent or insoluble for the purposes of the present invention. The support material may have virtually any possible structural configuration so long as the coupled molecule is capable of binding to an antigen or antibody. Thus, the support configuration may be spherical, as in a bead, or cylindrical, as in the inside surface of a test tube, or the external surface of a rod. Alternatively, the surface may be flat such as a sheet, test strip, etc. Preferred supports include polystyrene beads. Those skilled in the art will know many other suitable carriers for binding antibody or antigen, or will be able to readily ascertain the same. The binding activity of a given lot of antibody or fusion protein is determined according to well known methods. Those skilled in the art will be able to readily determine operative and optimal assay conditions. With respect to antibodies, one of the ways in which the antibody can be detectably labelled is by linking the same to an enzyme and use in an enzyme immunoassay (EIA) (Voller, 1978, Diagnostic Horizons 2: 1 -7, Microbiological Associates Quarterly Publication, Walkersville, MD); Voller et al, 1978, J. Clin. Pathol 31:507-520; Butler, 1981, Meth. Enzymol 73:482-523; Maggio, E. (ed.), 1980, Enzyme Immunoassay. CRC Press, Boca Raton, FL,; Ishikawa et al, (eds.), 1981. Enzyme Immunoassay. Kgaku Shoin, Tokyo). The enzyme which is bound to the antibody will react with an appropriate substrate, preferably a chromogenic substrate, in such a manner as to produce a chemical moiety which can be detected, for example, by spectrophotometric, fluorimetric or by visual means. Enzymes which can be used to detectably label the antibody include, but are not limited to, malate dehydrogenase, staphylococcal nuclease, delta-5-steroid isomerase, yeast alcohol dehydrogenase, alpha-glycerophosphate, dehydrogenase, triose phosphate isomerase, horseradish peroxidase, alkaline phosphatase, asparaginase, glucose oxidase, beta-galactosidase, ribonuclease, urease, catalase, glucose-6-phosphate dehydrogenase, glucoamylase and acetylcholinesterase. The detection can be accomplished by colorimetric methods which employ a chromogenic substrate for the enzyme. Detection may also be accomplished by visual comparison of the extent of enzymatic reaction of a substrate in comparison with similarly prepared standards. Detection may also be accomplished using any of a variety of other immunoassays. For example, by radioactively labelling the antibodies or antibody fragments, it is possible to detect the identified exogenous protein of interest through the use of a radioimmunoassay (RIA) (see, for example, Weintraub, B., Principles of Radioimmunoassays. Seventh Training Course on Radioligand Assay Techniques, The Endocrine Society, March, 1986). The radioactive isotope can be detected by such means as the use of a gamma counter or a scintillation counter or by autoradiography. It is also possible to label the antibody with a fluorescent compound. When the fluorescently labelled antibody is exposed to light of the proper wave length, its presence can then be detected due to fluorescence. Among the most commonly used fluorescent labelling compounds are fluorescein isothiocyanate, rhodamine, phycoerythrin, phycocyanin, allophycocyanin, o-phthaldehyde and fluorescamine. The antibody can also be detectably labelled using fluorescence emitting metals such as 152 Eu, or others of the lanthanide series. These metals can be attached to the antibody using such metal chelating groups as diethylenetriaminepentacetic acid (DTP A) or ethylenediaminetetraacetic acid (EDTA). The antibody also can be detectably labelled by coupling it to a chemiluminescent compound. The presence of the chemiluminescent-tagged antibody is then determined by detecting the presence of luminescence that arises during the course of a chemical reaction. Examples of particularly useful chemiluminescent labelling compounds are luminol, isoluminol, theromatic acridinium ester, imidazole, acridinium salt and oxalate ester. Likewise, a bioluminescent compound may be used to label the antibody of the present invention. Bioluminescence is a type of chemiluminescence found in biological systems in, which a catalytic protein increases the efficiency of the chemiluminescent reaction. The presence of a bioluminescent protein is determined by detecting the presence of luminescence. Important bioluminescent compounds for purposes of labelling are luciferin, luciferase and aequorin. Infection Of Animals Tissues Or Cells. The viral particles obtained by the procedure described above can be used as gene transfer vectors to deliver the gene of interest to a tissue or animal. This approach is useful for the investigation of the nature of the protein of interest or could be used, e.g., for the production of antibodies against the novel protein as a result of its expression at the host. E. Uses Of Proteins Having A Predetermined Property The nucleic acids which are identified, characterized and isolated using the methods and expression systems of this invention, and, more importantly, proteins encoded by same, may be useful for a number of applications. First, in many instances they will be useful for research applications and laboratory use, for example the discovery and isolation of new growth factors, cytokines, or hormones may facilitate the growth of cells in culture which could not be cultured before. The discovery and isolation of membrane receptors, cytoplasmic, and nuclear proteins will be useful to gain more insight in important cellular signal transduction and control processes. The discovery and isolation of organelle proteins may provide more insight into metabolic, anabolic, and processing functions. However, some of the genes and gene products identified and isolated by the present invention may directly be used as therapeutic agents or, alternatively, as therapeutic targets. 1. Use As Therapeutic Agents Proteins, in particular the secreted proteins, identified and isolated with the expression systems of the present invention may be useful as therapeutic agents. In fact, a number of the already known secreted proteins, including cytokines and peptide hormones, are manufactured and used as therapeutic agents. Many severe diseases are caused by lack or insufficient amounts of certain secreted proteins which serve as intercellular communicators, for example in response to environmental changes or other physiological needs. A well known example is diabetes mellitus, which most frequently is caused by deficiencies in the production of the peptide hormone insulin. Many other examples are known. In light of the fact that only a small percentage of all secreted proteins have been identified, isolated and characterized thus far, it can be expected that many of the novel secreted proteins uncovered by the present invention will be useful as therapeutic compounds for the treatment of diseases, including, but not limited to, diseases relating to aberrant cell proliferation, metabolic signals, or certain other incapabilities of cells to communicate properly. In addition, proteins which are derived from other cellular localizations may be useful as therapeutic agents. For example, a number of proteins expressed in the nucleus, including apoptosis-inducing proteins or tumor suppressors, are prone to be used as agents to treat, e.g., cancer. The skilled artisan will be able to determine which proteins identified using the methods and expression systems of the invention may be useful as therapeutic targets. For example, in vivo assays using inhibitory antibodies or antisense strategies may be employed to elucidate the function of a protein. Further, purified forms of the protein may be used for cellular assays and tests in animal disease models to determine their value as therapeutic compound. 2. Use As Therapeutic Targets Another important aspect of the invention is to provide proteins which serve as specific therapeutic targets for the treatment pathological disorders. While many diseases relating to inappropriate function or abundance of secreted proteins may be treated by administration of the respective protein (or an equivalent thereof), the situation is often different for proteins located within the cell. Deficiencies in the production of hormones or cytokines may frequently be remedied by their administration to reinstate a physiological response. In contrast, many other cellular fractions, in particular the cell membrane and the nucleus, are known to include numerous proteins which, if they function improperly, may be the cause of severe diseases, including cancer and other proliferative disorders, and a variety of metabolic diseases. For example, breast cancer is, in 30% of all occurrences, thought to be caused by overexpression of a cell surface receptor, / ' . e. , the receptor tyrosine kinase HER2. Slamon, 1989, supra. It is believed that this type of breast cancer may be treated by administration of HER2 inhibitors to the appropriate target site. As an example of a metabolic disease, type II diabetes mellitus (Non-Insulin-Dependent- Diabetes Mellitus, NIDDM) may be caused by aberrant expression or defective insulin receptors. Taira, 1989, supra. Type II diabetes could be treated with a therapeutic compound which surpasses the receptor and activates a molecule downstream of the insulin receptor's signal transduction pathway's. Alternatively, type II diabetes could be treated with compounds which are able to activate the defective insulin receptor. In the case of many diseases, both diseases related to inappropriate cell proliferation and diseases relating to metabolic defects, the biological cause is not yet identified, but is thought to be due to inappropriate function or abundance of cellular proteins. Proteins identified by the methods of the present invention will be useful for the development of targeted therapeutic approaches, including drug development and gene therapy, or as 0 diagnostics/reagents. If a protein identified by the methods of the invention is used as therapeutic target, it may be subjected to suitable assays for the identification and isolation of compounds modulating its activity. 5 F. Generation And Use Of Antibodies Directed Against Proteins Identified By The Methods Of The Invention Various procedures known in the art may be used for the production of antibodies to epitopes of the recombinantly produced proteins identified and isolated employing the methods of the present invention. Such antibodies include but are not ø limited to polyclonal, monoclonal, chimeric, single chain, Fab fragments and fragments produced by an Fab expression library. In one embodiment of the invention, such antibodies are used as tools for the investigation of expression pattern, abundance, function and other characteristics of the proteins identified by the methods of the invention. For example, cells or tissue sections 5 may be exposed to antibodies, allowing to characterize expression specificity and abundance of the identified proteins of the invention. Various types of antibodies, including monoclonal, polyclonal and single chain antibodies may be used for such experiments determining cell and tissue specific expression pattern of the identified protein. Numerous techniques are well-established which allow detection of antibody binding to the expressed proteins in the cell or tissue section. Furthermore, labelled antibodies, e.g., radiolabelled antibodies may be used to determine the expression patterns and abundance of the identified protein in vivo. See, supra. For example, the labelled antibody may be injected into a test subject, and its binding sites monitored in order to determine in which cell and/or tissue types the identified protein is located. To the extent that the originally identified and isolated protein of interest is the product of a species different from the test animal, the use of polyclonal antibodies is preferred. Thus, for example, if the identified protein used to generate the antibody is human, and the test subject is not human, polyclonal antibodies, with exceptions, will be preferred. Furthermore, antibodies may be employed to analyze the physiological function of the protein identified with the methods and expression systems of the invention. For example, inhibitory antibodies, i.e., antibodies binding to certain functional epitopes of the identified protein of interest may identified, rationally or empirically, and injected into test cells or laboratory animals. Effects on the cells, certain tissues or physiological functions may be identified, allowing to elucidate the biological role of the identified protein and its potential involvement in aberrant or pathological conditions. Once the biological function and role in disease development is identified, development and design of targeted treatment strategies may be initiated. One of such strategies is the identification and isolation of compounds favorably modulating the function of a target protein, which is discussed in more detail below. In other embodiments, antibodies are useful, e.g., as diagnostic or therapeutic agents. As therapeutic agents, neutralizing antibodies, i.e., those which compete for binding with a ligand, substrate or adapter molecule, or interfering with the proteins of the invention, where the proteins are used as therapeutic targets, are of especially preferred interest. For use as diagnostic agents, monoclonal antibodies that bind to the identified protein are radioactively labelled allowing detection of their location and distribution in the body after injection. Radioactivity tagged antibodies may be used as a non- invasive diagnostic tool for imaging in vivo the presence of a tumors and metastases associated with the expression of the identified protein of the invention. Immunotoxins may also be designed which target cytotoxic agents to specific sites in the body. For example, high affinity monoclonal antibodies may be covalently complexed to bacterial or plant toxins, such as diphtheria toxin, abrin, or ricin. A general method of preparation of antibody/hybrid molecules may involve use of thiol-crosslinking reagents such as SPDP, which attack the primary amino groups on the antibody and by disulfide exchange, attach the toxin to the antibody. The hybrid antibodies may be used to specifically eliminate cells expressing the protein identified by the methods of the invention. For the production of antibodies, various host animals are immunized by injection with the identified protein of interest including, but not limited to, rabbits, mice, rats, etc. Various adjuvants may be used to increase the immunological response, depending on the host species, including but not limited to Freund's adjuvance (complete and incomplete), mineral gels such as aluminum hydroxide, surface active substances such as lysolecithin, pluronic polyols, polyanions, peptides, oil emulsions, keyhole limpet hemocyanin, dinitrophenol, and potentially useful human adjuvants such as BCG (bacille Calmette- Guerin) and Corynebacterium parvum. Monoclonal antibodies to the proteins of the invention may be prepared using any technique which provides for the production of antibody molecules by continuous cell lines in culture. These include, but are not limited to, the hybridoma technique originally described by Kohler and Milstein, 1975, Nature 256:495-497, the human B-cell hybridoma technique (Kosbor et al, 1983, Immunology Today 4:72; Cote et al, 1983, Proc. Natl. Acad. Sci. U.S.A. 80:2026-2030) and the EBV-hybridoma technique (Cole et al, 1985, Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96). In addition, techniques developed for the production of "chimeric antibodies" (Morrison et al. , 1984, Proc. Natl. Acad. Sci. U.S.A. 81:6851-6855; Neuberger et al, 1984, Nature 312:604-608: Takeda et al, 1985, Nature 314:452-454) by splicing the genes from a mouse antibody molecule of appropriate antigen specificity together with genes from a human antibody molecule of appropriate biological activity can be used. Alternatively, techniques described for the production of single chain antibodies (U.S. Patent 4,946,778) can be adapted to produce single chain antibodies specific for the proteins of the invention. Antibody fragments which contain specific binding sites of the cell proliferation gene may be generated by known techniques. For example, such fragments include, but are not limited to, F(ab') 2 fragments which can be produced by pepsin digestion of the antibody molecule and the Fab fragments which can be generated by reducing the disulfide bridges 5 of the F(ab') 2 fragments. Alternatively, Fab expression libraries may be constructed (Huse et al, 1989, Science 246:1275-1281) to allow rapid and easy identification of monoclonal Fab fragments with the desired specificity to the protein of the invention. G. Use Of The Identified Nucleic Acids Encoding A Target Protein Of j ø Interest For Development Of Antisense Approaches And Ribozymes The development and the use of oligonucleotide or oligoribonucleotide sequences comprising antisense DNA or RNA molecules or ribozymes that function to inhibit the translation of the cell proliferation gene mRNA may fulfill either of two purposes. First, such approaches may be used to investigate the function of novel proteins j 5 identified with the methods of the present invention. Second, these approaches may serve as actual treatment methods once the function of a protein and its involvement in pathological conditions is established. For example, antisense DNA or RNA molecules act to directly block the translation of targeted gene by binding to the targeted mRNA and thus preventing protein translation. 2ø Ribozymes are enzymatic RNA molecules capable of catalyzing the specific cleavage of RNA. The mechanism of ribozyme action appears to involve site specific hybridization of the ribozyme molecule to complementary sequences of the target RNA, followed by endonucleolytic cleavage. In one embodiment of the invention, ribozyme molecules are engineered that specifically catalyze endonucleolytic cleavage of mRNA of 5 the genes identified with the methods of the invention. Suitable target sites for ribozyme activity are identified by first scanning the target molecule for potential ribozyme cleavage motifs, second by evaluating the structural features of the about 15 to 25 amino acids corresponding to the region of the target molecule containing the identified cleavage recognition site. Further, the suitability of the ø candidate targets may also be evaluated by testing their accessibility to hybridization with complementary oligonucleotides, using ribonuclease protection assays. Bordonaro et al. , 1994, Biotechniques 16:428-430. The labelled hybridization probes antisense DNA and RNA oligonucleotides and ribozymes of the subject invention are prepared by any method known in the art for the synthesis of DNA and RNA molecules. For example, oligonucleotides may be synthesized chemically using commercially available DNA or RNA synthesizers like machines sold by Applied Biosystems. Alternatively, RNA molecules may be generated by in vitro and in vivo transcription of DNA sequences encoding the antisense RNA molecule. Such DNA sequences may be incorporated into a wide variety of vectors which comprise suitable RNA polymerase promoters such as the T3, T7, or the SP6 polymerase promoters. Alternatively, antisense cDNA constructs that synthesize antisense RNA constitutively or inducibly, may be introduced stably into cell lines. Various modifications to the DNA and RNA molecules may be introduced as a means of increasing the intracellular stability and half-life. For example, flanking sequences of ribo- or deoxyribo-nucleotides may be added to the 5' and/or 3' ends of the molecule, or phosphorothioate or 2' O-methyl rather than phosphodiester linkages may be used within the oligonucleotide backbone. Xu et al. , 1996, Nucleic Acid Res. 24: 1602- 1607. H. Generation Of Compounds Targeting The Identified, Isolated And Characterized Proteins 1. Identification Of Compounds For the identification and isolation of compounds modifying, inhibiting or enhancing the function of an identified and characterized protein according to the invention, suitable cellular systems expressing the identified protein may be employed. Alternatively, the proteins identified by the process of the invention may be isolated and used for in vitro or in vivo assays for the identification and isolation of compounds specifically interfering with their activity. Generally, the type of assay employed will largely depend on the nature and functional characteristics of the identified protein used as target, in the following referred to as "target protein". For example, if the target protein is a growth factor receptor, a cellular assay may be employed wherein the influence of compounds on proliferation is measured. If the target protein is a secreted protein, assays may be employed involving the effects of isolated secreted protein on suitable systems, e.g., receptor systems which allow one to measure the function of the secreted protein. If the target protein is a transcription factor, cellular systems may be employed which allow assaying the impact of compounds on expression of genes which are controlled by the transcription factor. If the target protein is an apoptosis regulator, cellular systems may be employed in which cell death (or survival) is driven by the respective target protein, and the impact of compounds may be assayed. More specifically, cells in an appropriate assay system expressing the target protein may be exposed to chemical compounds or compound libraries to identify compounds having the desired modulating effects. Alternatively, the target protein may be expressed in suitable expression systems, designed to allow for high-throughput testing of compounds from any source, optionally isolated, to identify molecules binding to or having measurable inhibitory effects on the target protein. Nucleotide sequences encoding the target protein identified and isolated using the methods of the invention may be used to produce the corresponding purified protein using well-known methods of recombinant DNA technology. Among the many publications that teach methods for the expression of genes after they have been isolated is Gene Expression Technology. Methods and Enzymology. Vol.: 185. edited by Goeddel, Academic Press, San Diego, California (1990). The protein of the invention chosen as target protein may be expressed in a variety of host cells, either prokaryotic or eukaryotic. In many cases, the host cells would be eukaryotic, more preferably host cells would be mammalian. Host cells may be from species either the same or different than the species from which the nucleic acid sequences encoding the protein identified with the methods of the invention are naturally present, i.e. , endogenous. Advantages of producing the chosen target protein by recombinant DNA technology in cellular expression systems other than the expression systems primarily used see supra, for the original identification and isolation of the proteins according to their cellular localization include the development of optimized assay systems for the identification of modulating compounds. Generally, the expression systems of the invention have the advantage that they readily provide a system for the production of large amounts of recombinant proteins. However, under certain circumstances which the skilled artisan will appreciate, alternative expression systems may, in some instances, also prove advantageous for obtaining highly enriched sources of the proteins for purification and the availability of simplified purification procedures. Methods for recombinant production of 5 proteins are generally very well established in the art, and can be found, among other places in Sambrock et al. , supra. In an embodiment of the invention, cells transformed with expression vectors encoding the identified protein of the invention are cultured under conditions favoring expression of its gene sequence and the recovery of the recombinantly-produced protein 10 from the cell culture. A target protein of interest produced by a recombinant cell may be secreted or may be contained intracellularly, depending on the nature of the gene and the particular genetic construction used. In general, it is more convenient to prepare recombinant proteins in secreted form. Purification steps will depend on the nature of the production and the particular protein produced. Purification methodologies are well 15 established in the art; the skilled artisan will know how to optimize the purification conditions. General protocols of how to optimize the purification conditions for a particular protein can be found, among other places, in Scopes in: Protein Purification: Principles and Practice, 1982, Springer- Verlag New York, Heidelberg, Berlin. In addition to recombinant production, peptide fragments may be produced by direct 20 peptide synthesis using solid-phase techniques. See, Stewart et al. , Solid-Phase Peptide Synthesis (1969), W. H. Freeman Co., San Francisco; and Merrifield, 1963, J. Am. Chem. Soc. 85:2149-2154. In vitro polypeptide synthesis may be performed using manual techniques or by automation. Automated synthesis may be achieved, for example, using Applied Biosystems 25 431 A Peptide Synthesizer (Foster City, California) following the instructions provided in the instruction manual supplied by the manufacturer. In an embodiment of the invention, the target protein and/or cell lines expressing the target protein are used to screen for antibodies, peptides, organic molecules or other ligands that act as agonist or antagonists of the cell proliferation gene activity. For example, 30 antibodies capable of interfering with the activity, e.g., enzymatic activity of the target protein, or with its interaction with a ligand, adapter molecule, or substrate are used to inhibit the target protein's function. In cases where amplification of the target protein's function is desired, antibodies which mimic, e.g., a ligand, an adapter molecule or substrate of the corresponding the signal transduction pathway may be developed. Obviously, if desired, antibodies may be generated which modify the activity, function, or specificity of the target protein. Alternatively, screening of peptide libraries or organic compounds with recombinantly expressed target protein or cell lines expressing the target protein may be useful for identification of therapeutic molecules that function by inhibiting, enhancing, or modifying its biological activity. Synthetic compounds, natural products, and other sources of potentially biologically active materials can be screened in a number of ways. The ability of a test compound to inhibit, enhance or modulate the function of the target protein may be determined with suitable assays measuring the target protein's function. For example, responses such as its activity, e.g., enzymatic activity, or the target protein's ability to bind its ligand, adapter molecule or substrate may be determined in in vitro assays. Cellular assays can be developed to monitor a modulation of second messenger production, changes in cellular metabolism, or effects on cell proliferation. These assays may be performed using conventional techniques developed for these purposes. Finally, the ability of a test compound to inhibit, enhance or modulate the function of the target protein will be measured in suitable animal models in vivo. For example, mouse models will be used to monitor the ability of a compound to inhibit the development of solid tumors, or effect reduction of the solid tumor size. In an embodiment of the invention, random peptide libraries consisting of all possible combinations of amino acids attached to a solid phase support are used to identify peptides that are able to interfere with the function of the target protein. For example, peptides may be identified binding to a ligand-, adapter molecule- or substrate binding site of a given target protein or other functional domains of the target protein, such as an enzymatic domain. Accordingly, the screening of peptide libraries may result in compounds having therapeutic value as they interfere with its activity. Identification of molecules that are able to bind to the target protein may be accomplished by screening a peptide library with recombinant soluble target protein. Methods for expression and purification of the selected target protein and may be used to express recombinant full length target protein or fragments thereof, depending on the functional domains of interest. In order to identify and isolate the peptide/solid phase support that interacts and forms a complex with the target protein, it is necessary to label or "tag" the target protein molecule or fragment thereof. For example, the target protein may be conjugated to enzymes such as alkaline phosphatase or horseradish peroxidase or to other reagents such as fluorescent labels which may include fluorescein isothyiocynate (FITC), phycoerythrin (PE) or rhodamine. Conjugation of any given label to the target protein may be performed using techniques that are routine in the art. In addition to using soluble target protein molecules or fragments thereof, in another embodiment, peptides that bind to the target protein may be identified using intact cells. The use of intact cells is preferred in instances where the target protein which comprises cell surface receptors, which require the lipid domain of the cell membrane to be functional. Methods for generating cell lines expressing the target protein identified with the methods and expression systems of the invention. The cells used in this technique may be either live or fixed cells. The cells are incubated with the random peptide library and will bind to certain peptides in the library. The so formed complex between the target cells and the relevant solid phase support/peptide may be isolated by standard methods known in the art, including differential centrifugation. In the case the target protein is a membrane bound receptor or a receptor that requires the lipid domain of the cell membrane to be functional, an alternative to whole cell assays is to reconstitute the receptor molecules into liposomes where a label or "tag" can be attached. In another embodiment, cell lines that express the chosen target protein or, alternatively isolated target protein or fragments thereof, are used to screen for molecules that inhibit, enhance, or modulate the target protein's activity or, where applicable, signal transduction. Such molecules may include small organic or inorganic compounds, or other molecules that effect the target protein activity or that promote or prevent the complex formation with its ligand, adapter molecules, or substrates. Synthetic compounds, natural products, and other sources of potentially biologically active materials can be screened in a number of ways, which are generally known by the skilled artisan. For example, the ability of a test molecule to interfere with the chosen target protein's function may be measured using standard biochemical techniques. Alternatively, cellular responses such as activation or suppression of a catalytic activity, phosphorylation, dephosphorylation, or other modification of other proteins, activation or modulation of second messenger production, changes in cellular ion levels, association, dissociation or translocation of signalling molecules, or transcription or translation of specific genes may also be monitored. These assays may be performed using conventional techniques developed for these purposes in the course of screening. Further, effects on the target protein's function may, via signal transduction pathways, affect a variety of cellular processes. Cellular processes under the control of the its signalling pathway may include, but are not limited to, normal cellular functions, proliferation, differentiation, maintenance of cell shape, and adhesion, in addition to abnormal or potentially deleterious processes such as unregulated cell proliferation, loss of contact inhibition and, blocking of differentiation or cell death. The qualitative or quantitative observation and measurement of any of the described cellular processes by techniques known in the art may be advantageously used as a means of scoring for signal transduction in the course of screening. Various technologies may be employed for the screening, identification, and evaluation of compounds which interact with the chosen target protein of the invention, which compounds may affect various cellular processes under the control of said target protein. For example, the target protein or a functional derivative thereof, in pure or semi- pure form, in a membrane preparation, or in a whole live or fixed cell is incubated with the compound. Subsequently, under suitable conditions, the effect of the compound on the target protein's function is scrutinized, e.g., by measuring its activity, or its signal transduction, and comparing the activity to that of the target protein, incubated under same conditions, without the compound, thereby determining whether the compound stimulates or inhibits the target protein's activity. In addition to the use of whole cells expressing the chosen target protein for the screening of compounds, the invention also includes methods using soluble or immobilized target protein. For example, molecules capable of binding to the target protein may be identified within a biological or chemical preparation. For example, the target protein, or functional fragments thereof, e.g., fragments containing a specific domain of interest, is immobilized to a solid phase matrix, subsequently a chemical or biological preparation is contacted with the immobilized target protein for an interval sufficient to allow the compound to bind. Any unbound material is then washed away from the solid phase matrix, and the presence of the compound bound to the solid phase is detected, whereby the compound is identified. Suitable means are then employed to elute the binding compound. 2. Source Of Candidate Test Compounds The test compounds employed for assays for the identification of modulators of a target protein's activity are obtained from any commercial source, including Aldrich (1001 West St. Paul Ave., Milwaukee, WI 53233), Sigma Chemical (P.O. Box 14508, St. Louis, MO 63178), Fluka Chemie AG (Industriestrasse 25, CH-9471 Buchs, Switzerland (Fluka Chemical Corp. 980 South 2nd Street, Ronkonkoma, NY 11779)), Eastman Chemical Company, Fine Chemicals (P.O Box 431, Kingsport, TN 37662), Boehringer Mannheim GmbH (Sandhofer Strasse 116, D-68298 Mannheim), Takasago (4 Volvo Drive, Rockleigh, NJ 07647), SST Corporation (635 Brighton Road, Clifton, NJ 07012), Ferro (111 West Irene Road, Zachary, LA 70791), Riedel-deHaen Aktiengesellschaft (P.O. Box D-30918, Seelze, Germany), PPG Industries Inc., Fine Chemicals (One PPG Place, 34th Floor, Pittsburgh, PA 15272); further any kind of natural products may be screened using the assay cascade of the invention, including microbial, fungal or plant extracts. 3. Indications For The Use Of Compounds Modulating The Activity Of Target Proteins Of The Invention According To Its Predetermined Property Depending on the nature and the function of the target protein, the compounds identified by the above exemplified assays and methods may be modulators of cell proliferation activity or modulators of certain metabolic functions. As such, the compounds produced by the processes and assays of the invention are useful for the treatment of disease related to aberrant, uncontrolled or inappropriate cell proliferation, including excess or diminished proliferation. Alternatively, the compounds may be useful for the treatment of diseases relating to aberrant metabolic functions. A large number of disease states involve excess or diminished cell proliferation. Generally, many of these diseases may be treated with DNA sequences, proteins, or small molecules that influence cell proliferation. In some instances the goal is to stimulate proliferation; in others, to prevent or inhibit proliferation of cells. The list of diseases directly involving cell growth includes, but is not limited to, cancer, psoriasis, inflammatory diseases, such as rheumatoid arthritis, restenosis, immunological activation or suppression, including tissue rejection, neurodegeneration or expansion of neuronal cells and viral infection. Numerous diseases involve aberrant metabolic functions, including, but not limited to, diabetes mellitus, fibrosis, cystic fibrosis. Further, pathological conditions relate both to aberrant cell proliferation and metabolic defects. Accordingly, pharmaceutical compositions comprising a therapeutically effective amount of a compound identified by the methods described above will be useful for the treatment of diseases driven by unregulated or inappropriate cell proliferation, including cancer, such as glioma, melanoma, Kaposi's sarcoma, psoriasis, hemangioma and ovarian, breast, lung, pancreatic, prostate, colon and epidermoid cancer, rheumatoid arthritis, restenosis, immunological activation or suppression, including tissue rejection, neurodegeneration or expansion of neuronal cells; and diseases relating to metabolic dysfunction, including, but not limited to diabetes mellitus, fibrosis, cystic fibrosis. I. Formulations/Route Of Administration The present invention provides two forms of therapeutic compounds. First, the invention provides therapeutics which resemble a naturally occurring functional polypeptide or peptide, or functional fragment or derivative thereof. Such therapeutics are, for example, proteins which are secreted in their natural form; such proteins may be identified directly with the expression systems of the invention. This type of therapeutics includes, but are not limited to, cytokines, hormones, growth factors, etc. Second, the invention provides therapeutic compounds which modulate the function of target proteins identified using the methods and expression systems of the invention. Typically, these types of compounds are small organic molecules, but they also include peptide compounds and antibodies. The skilled artisan will appreciate that the following methods of administration, formulation and treatment methods will need to be adjected depending on the type of compound chosen, and the type of disease to be treated. The identified therapeutic compound can be administered to a human patient alone or in pharmaceutical compositions where they are mixed with suitable carriers or excipient(s) at therapeutically effective doses to treat or ameliorate a variety of disorders. A therapeutically effective dose further refers to that amount of the compound sufficient to result in amelioration of symptoms as determined, for example, in a decrease or increase of cell proliferation, or in a restitution of metabolic functions. Techniques for formulation and administration of the compounds of the instant application may be found in "Remington's Pharmaceutical Sciences," Mack Publishing Co., Easton, PA, latest edition. 1. Routes Of Administration. Suitable routes of administration may, for example, include oral, rectal, transmucosal, or intestinal administration; parenteral delivery, including intramuscular, subcutaneous, intramedullary injections, as well as intrathecal, direct intraventricular, intravenous, intraperitoneal, intranasal, or intraocular injections. Alternately, one may administer a compound of the invention in a local rather than systemic manner, for example, via injection of the compound directly into a solid tumor, often in a depot, or in a sustained release formulation. Furthermore, one may administer the drug via a targeted drug delivery system, for example, in a liposome coated with tumor-specific antibody. The liposomes will be targeted to and taken up selectively by the tumor. 2. Composition/Formulation The pharmaceutical compositions of the present invention may be manufactured by means of conventional mixing, dissolving, granulating, dragee-making, levigating, emulsifying, encapsulating, entrapping or lyophilizing processes. Pharmaceutical compositions for use in accordance with the present invention thus may be formulated in a conventional manner using one or more physiologically acceptable carriers comprising excipients and auxiliaries which facilitate processing of the active therapeutic compounds into preparations which can be used pharmaceutically. Proper formulation is dependent upon the route of administration chosen. For injection, the agents of the invention may be formulated in aqueous solutions, preferably in physiologically compatible buffers such as Hank's solution, Ringer's solution, or physiological saline buffer. For transmucosal administration, penetrants appropriate to the barrier to be permeated are used in the formulation. Such penetrants are generally known in the art. For oral administration, the therapeutic compounds can be formulated readily by combining the active therapeutic compounds with pharmaceutically acceptable carriers well known in the art. Such carriers enable the therapeutic compounds of the invention to be formulated as tablets, pills, dragees, capsules, liquids, gels, syrups, slurries, suspensions and the like, for oral ingestion by a patient to be treated. Pharmaceutical preparations for oral use can be obtained as a solid excipient, optionally grinding a resulting mixture, and processing the mixture of granules, after adding suitable auxiliaries, if desired, to obtain tablets or dragee cores. Suitable excipients include fillers such as sugars, including lactose, sucrose, mannitol, or sorbitol; cellulose preparations such as, for example, maize starch, wheat starch, rice starch, potato starch, gelatin, gum tragacanth, methyl cellulose, hydroxypropylmethyl-cellulose, sodium carboxymethylcellulose, and/or polyvinylpyrrolidone (PVP). If desired, disintegrating agents may be added, such as the cross-linked polyvinyl pyrrolidone, agar, or alginic acid or a salt thereof such as sodium alginate. Dragee cores are provided with suitable coatings. For this purpose, concentrated sugar solutions may be used, which may optionally contain gum arabic, talc, polyvinyl pyrrolidone, carbopol gel, polyethylene glycol, and/or titanium dioxide, lacquer solutions, and suitable organic solvents or solvent mixtures. Dyestuffs or pigments may be added to the tablets or dragee coatings for identification or to characterize different combinations of active therapeutic compound doses. Pharmaceutical preparations which can be used orally include push-fit capsules made of gelatin, as well as soft, sealed capsules made of gelatin and a plasticizer, such as glycerol or sorbitol. The push-fit capsules can contain the active ingredients in admixture with fillers such as lactose, binders such as starches, and/or lubricants such as talc or magnesium stearate and, optionally, stabilizers. In soft capsules, the active therapeutic compounds may be dissolved or suspended in suitable liquids, such as fatty oils, liquid paraffin, or liquid polyethylene glycols. In addition, stabilizers may be added. All formulations for oral administration should be in dosages suitable for such administration. For buccal administration,the compositions may take the form of tablets or lozenges formulated in conventional manner. For administration by inhalation, the therapeutic compounds for use according to the present invention are conveniently delivered in the form of an aerosol spray presentation from pressurized packs or a nebulizer, with the use of a suitable propellant, e.g., dichlorodifluoromethane, trichlorofluoromethane, dichlorotetrafluoroethane, carbon dioxide or other suitable gas. In the case of a pressurized aerosol the dosage unit may be determined by providing a valve to deliver a metered amount. Capsules and cartridges of, e.g., gelatin, for use in an inhaler or insufflator, may be formulated containing a powder mix of the therapeutic compound and a suitable powder base such as lactose or starch. The therapeutic compounds may be formulated for parenteral administration by injection, e.g., by bolus injection or continuous infusion. Formulations for injection may be presented in unit dosage form, e.g., in ampoules or in multi-dose containers, with an added preservative. The compositions may take such forms as suspensions, solutions or emulsions in oily or aqueous vehicles, and may contain formulatory agents such as suspending, stabilizing and/or dispersing agents. Pharmaceutical formulations for parenteral administration include aqueous solutions of the active therapeutic compounds in water-soluble form. Additionally, suspensions of the active compounds may be prepared as appropriate oily injection suspensions. Suitable lipophilic solvents or vehicles include fatty oils such as sesame oil, or synthetic fatty acid esters, such as ethyl oleate or triglycerides, or liposomes. Aqueous injection suspensions may contain substances which increase the viscosity of the suspension, such as sodium carboxymethyl cellulose, sorbitol, or dextran. Optionally, the suspension may also contain suitable stabilizers or agents which increase the solubility of the therapeutic compounds to allow for the preparation of highly concentrated solutions. Alternatively, the active ingredient may be in powder form for constitution with a suitable vehicle, such as sterile pyrogen-free water, before use. The therapeutic compounds may also be formulated in rectal compositions such as suppositories or retention enemas, e.g., containing conventional suppository bases such as cocoa butter or other glycerides. In addition to the formulations described previously, the therapeutic compounds may also be formulated as a depot preparation. Such long acting formulations may be administered by implantation (for example subcutaneously or intramuscularly) or by intramuscular injection. Thus, for example, the therapeutic compounds may be formulated with suitable polymeric or hydrophobic materials (for example as an emulsion in an acceptable oil) or ion exchange resins, or as sparingly soluble derivatives, for example, as a sparingly soluble salt. A pharmaceutical carrier for the hydrophobic therapeutic compounds of the invention is a cosolvent system comprising benzyl alcohol, a nonpolar surfactant, a water- miscible organic polymer, and an aqueous phase. The cosolvent system may be the VPD co-solvent system. VPD is a solution of 3% w/v benzyl alcohol, 8% w/v of the nonpolar surfactant polysorbate 80, and 65% w/v polyethylene glycol 300, made up to volume in absolute ethanol. The VPD co-solvent system (VPD:5W) consists of VPD diluted 1 :1 with a 5% dextrose in water solution. This co-solvent system dissolves hydrophobic therapeutic compounds well, and itself produces low toxicity upon systemic administration. Naturally, the proportions of a co-solvent system may be varied considerably without destroying its solubility and toxicity characteristics. Furthermore, the identity of the co-solvent components may be varied: for example, other low-toxicity nonpolar surfactants may be used instead of polysorbate 80; the fraction size of polyethylene glycol may be varied; other biocompatible polymers may replace polyethylene glycol, e.g., polyvinyl pyrrolidone; and other sugars or polysaccharides may be substituted for dextrose. Alternatively, other delivery systems for hydrophobic pharmaceutical therapeutic compounds may be employed. Liposomes and emulsions are well known examples of delivery vehicles or carriers for hydrophobic drugs. Certain organic solvents such as dimethylsulfoxide also may be employed, although usually with a greater toxicity. Additionally, the therapeutic compounds may be delivered using a sustained-release system, such as semipermeable matrices of solid hydrophobic polymers containing the therapeutic agent. Various sustained-release materials have been established and are well known by those skilled in the art. Sustained-release capsules may, depending on their chemical nature, release the therapeutic compounds for a few weeks up to over 100 days. Depending on the chemical nature and the biological stability of the therapeutic reagent, additional strategies for protein stabilization may be employed. The pharmaceutical compositions also may comprise suitable solid or gel phase carriers or excipients. Examples of such carriers or excipients include but are not limited to calcium carbonate, calcium phosphate, various sugars, starches, cellulose derivatives, gelatin, and polymers such as polyethylene glycols. Many of the therapeutic compounds of the invention may be provided as salts with pharmaceutically compatible counterions. Pharmaceutically compatible salts may be formed with many acids, including but not limited to hydrochloric, sulfuric, acetic, lactic, tartaric, malic, succinic, etc. Salts tend to be more soluble in aqueous or other protonic solvents that are the corresponding free base forms. 3. Effective Dosage. Pharmaceutical compositions suitable for use in the present invention include compositions wherein the active ingredients are contained in an effective amount to achieve its intended purpose. More specifically, a therapeutically effective amount means an amount effective to prevent development of or to alleviate the existing symptoms of the subject being treated. Determination of the effective amounts is well within the capability of those skilled in the art, especially in light of the detailed disclosure provided herein. For any therapeutic compound used in the method of the invention, the therapeutically effective dose can be estimated initially from cell culture assays. For example, a dose can be formulated in animal models to achieve a circulating concentration range that includes the IC 50 as determined in cell culture (i.e., the concentration of the test therapeutic compound which achieves a half-maximal inhibition or activation of the cell proliferation activity, or restitution of a metabolic function). Such information can be used to more accurately determine useful doses in humans. A therapeutically effective dose refers to that amount of the therapeutic compound that results in amelioration of symptoms or a prolongation of survival in a patient. Toxicity and therapeutic efficacy of such therapeutic compounds can be determined by standard pharmaceutical procedures in cell cultures or experimental animals, e.g. , for determining the LD 50 (the dose lethal to 50% of the population) and the ED 50 (the dose therapeutically effective in 50% of the population). The dose ratio between toxic and therapeutic effects is the therapeutic index and it can be expressed as the ratio between LD 50 and ED 50 . Therapeutic compounds which exhibit high therapeutic indices are preferred. The data obtained from these cell culture assays and animal studies can be used in formulating a range of dosage for use in human. The dosage of such therapeutic compounds lies preferably within a range of circulating concentrations that include the ED 50 with little or no toxicity. The dosage may vary within this range depending upon the dosage form employed and the route of administration utilized. The exact formulation, route of administration and dosage can be chosen by the individual physician in view of the patient's condition. (See, e.g., Fingl et al, 1975, in "The Pharmacological Basis of Therapeutics", Ch. 1 pi). Dosage amount and interval may be adjusted individually to provide plasma levels of the active moiety which are sufficient to maintain the kinase modulating effects, or minimal effective concentration (MEC). The MEC will vary for each therapeutic compound but can be estimated from in vitro data; e.g., the concentration necessary to achieve 50-90% inhibition of the kinase using the assays described herein. Dosages necessary to achieve the MEC will depend on individual characteristics and route of administration. However, HPLC assays or bioassays can be used to determine plasma concentrations. Dosage intervals can also be determined using MEC value. Therapeutic compounds should be administered using a regimen which maintains plasma levels above the MEC for 10-90% of the time, preferably between 30-90% and most preferably between 50-90%. In cases of local administration or selective uptake, the effective local concentration of the drug may not be related to plasma concentration. The amount of composition administered will, of course, be dependent on the subject being treated, on the subject's weight, the severity of the affliction, the manner of administration and the judgment of the prescribing physician. 4. Packaging. The compositions may, if desired, be presented in a pack or dispenser device which may contain one or more unit dosage forms containing the active ingredient. The pack may for example comprise metal or plastic foil, such as a blister pack. The pack or dispenser device may be accompanied by instructions for administration. Compositions comprising a therapeutic compound of the invention formulated in a compatible pharmaceutical carrier may also be prepared, placed in an appropriate container, and labelled for treatment of an indicated condition. Suitable conditions indicated on the label may include inhibition or activation of cell proliferation, restitution of a metabolic function, treatment of a tumor, treatment of arthritis, and the like. The following examples for the generation and use of the selection systems of the invention are given to enable those skilled in the art to more clearly understand and to practice the present invention. The present invention, however, is not limited in scope by the exemplified embodiments, which are intended as illustrations of single aspects of the invention only, and methods which are functionally equivalent are within the scope of the invention. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims. VII. EXAMPLES A. Materials And Methods The following are experimental procedures and materials used for the Examples set forth below. Construction OfcDNA Libraries Into The SinRep5 Vector. Total mRNA of any tissue or cell line is isolated by using the QuickPrep (Micro mRNA purification kit (Pharmacia, Pharmacia Biotech Europe GmbH, Duebendorf, Switzerland) according to supplier's recommendation. The first strand synthesis is carried out in the presence of 5- methyl-cytosine to protect internal StuI restriction sites, using the Stratagene (Stratagene AG, Basel, Switzerland) ZAP cDNA synthesis kit according to supplier's instructions. After brief digestion with RNaseH, the second strand is synthesized using DNA polymerase I to produce 3' overhangs which can be recessed by T4 DNA polymerase (3' exonuclease activity). The double stranded DNA is finally ligated into the pSinRep5 vector (Invitrogen, NV Leek, Netherl; see FIGURE 5 and Bredenbeek et al, 1993, J. Virol. 11:6439-6446). Subsequent digestion of the resulting DNA with StuI linearizes the plasmids for in vitro transcription. The construction of cDNA libraries in the TE vectors (See FIGURE 6 and Frolov et al, 1996, PNAS 93:11371-11377; the complete sequence of pTE5'2J is given in SEQ ID NO:l) is carried out essentially as described for the pSinRep5. The only exception is that during the second strand synthesis 5-methyl-adenosine is used to protect internal Xbal restriction sites to clone the resulting DNA fragment into the pTE plasmid pTE SEAP via Xbal/StuI or Apal. The plasmids used in this study are described in detail in Table I. TABLE I Description of the DNA construct: pSinRep5 construct: P SinRep5: from Invitrogen Sindbis expression system European Headquarters: Invitrogen, NV Leek, Netherland Bredenbeek et α/., Nov. 1993, J. Virology 67: 6439- 6446. SP6 promotor: 9933-9951 Non-structural genes: 60-7598 Subgenomic promotor: 7580-7603 Transcriptional start: 7598 Multiple cloning site: 7647-7689 polyA tail: 7997-8033 Amp resistance gene: 8227-9085 ColEl origin: 9232-9861 pSinRep5 EPO: pSinRep 5 digested with Xbal/SphI was ligated with a synthetic EPO sequence (Xbal/SphI) sequence of EPO see appendix: sequence 1 pSinRep5 SEAP: pSinRep5 digested with Xbal/StuI and ligated with Nhel/Clal fragment of pSEAP 2 Basic, Clontech Laboratories, Inc. , Palo Alto, USA. pSinRep5 lacZ: from Invitrogen Sindbis expression system. pTE constructs: pTE 5'2J: A general description of the related vectors pTE 3'2J is given in: Hahn et al, 1992, Proc. Natl. Acad. Sci. USA 89,:2679-2683. Frolov et al, 1996, Proc. Natl. Acad. Sci. USA 93:11371-11377. The detailed sequence of TE 5'2J is given as SEQ ID NO: 1. pTE 5'2J EPO: pTE was digested with Xbal/Apal was ligated with the Xbal/Apal EPO fragment derived from pSinRep5. pTE 5'2J SEAP: pTE was digested with Apal, the ends were treated with Klenow enzyme before digestion with Xbal. The vector was ligated with the Nhel/Clal fragment from pSEAP 2 Basic (Clontech Laboratories, Inc. , Palo Alto USA). pTE 5'2J CAT: pTE 5'2J was digested with Xbal, the ends were filled according to standard procedures with Klenow enzyme CAT PCR fragment as shown in appendix: sequence 3, was ligated blunt into vector TE 5'2J. Helper constructs: DHEB: Bredenbeek et α/., 1993, J. Virology 67 :6439-6446. 987 BB neo: (SEQ ID NO: 2). In Vitro Transcription Of Sindbis cDNA Libraries. The linearized vector DNA (pSinRep5 or pTE, linearized by NotI digestion; helper DNA DH-EB (Bredenbeek et al. , 1993, J Virol, jj,: 6439-6446) linearized by EcoRI digestion) were made RNase-free by purification over QiaQuick PCR purification columns (QIAGEN AG, Basel, Switzerland) and elution with DEPC-H2O. Subsequent in vitro transcription was carried out using an SP6 in vitro transcription kit (Invitroscript CAP Kit, Invitrogen BV, NV Leek, The Netherlands) according to the manufacturer's recommendation. The resulting 5'-capped mRNA was analyzed on reducing and non-reducing agarose gels. Generation Of Sindbis Virus Particles. Two (2) to five (5) μg of in vitro transcribed mRNA was electroporated (for TE constructs) or co-electroporated (for PSinRep 5 and helper DH-EB) into BHK 21 cells (ATCC No. CCL10) according to the Invitrogen 's manual (Invitroscript CAP Kit, Invitrogen BV, NV Leek, The Netherlands). The 5'-capped mRNA of pSinRep5 encodes the viral non-structural proteins, which induce the viral replication steps. The co-electroporated helper mRNA (DH-EB) delivers the viral structural proteins. After incubation for eighteen (18) hours at 37°C, 5% C0 2 in Turbodoma HP-1 medium containing 1% FCS, the cell supernatant was harvested and the amount of released infectious virus particles was determined by plaque assays. Plaque Assay. Dilution series of the harvested virus particles, see, supra, were carried out in 1 ml Turbodoma HP- 1 (1 :10 4 , 1 : 5x 104, 1 :10 5 , 1 : 5x 10 5 , 1 : 10 6 , 1 : 5x 10 6 ) on 90%) confluent BHK 21 cells in 6-well-plates. After two (2) hours incubation at 37°C (in the case of the temperature-sensitive mutants from the complementation group C,D and E, the plates were incubated at the permissive temperature of 30°C (Lindquist et al, 1986, Virology 111:10-20; Arias et al, J. Mol. Biol. 168:87-102; Carleton and Brown, 1996, J. Virol. 70:952-959) the medium was replaced with 2 ml 41°C warm 0.8% agarose (Carl Roth GmbH, Karlsruhe, Germany) in Turbodoma HP-1 medium (Dr. F. Messi, Cell Culture Technologies, Zurich, CH). Plaques of 1 to 4 mm diameter had been formed after two (2) days of incubation at the permissive temperature. The plaques were counted and the corresponding numbers of plaque forming units ("pfu") per ml were calculated. Agarose Blot Assay (ABA). 90% confluent BHK 21 cell layers in 60 mm dishes were incubated at the permissive temperature for two (2) hours with HP-1 medium containing approximately 800 to 1000 plaque forming units. The supernatant was replaced by overlaying the cells with 1.5 mm layer of 41°C warm 0.8% agarose in lx HP-1 and the cells were incubated for two (2) days at the permissive temperature. Secreted proteins were detected by 35 S Met/Cys labeling of the infected cells. For this purpose, the medium was replaced for 30 minutes with 2 ml of RPMI 1640 (Met/Cys deficient medium) before 20 μCi 35 S Met/Cys (Hartmann Analytic GmbH, Braunschweig, Germany) was added. Release of viral proteins was inhibited by adding a protease inhibitor cocktail and/or crosslinked antibodies against the Sindbis Virus glycoproteins as described in the examples. The temperature-sensitive Sindbis mutants were shifted to 40°C for two (2) hours pre-labeling to inhibit virus particle release. The 20 μCi 35 S Met/Cys were applied for 2 hours at 37°C, before the cells and the agarose layer were washed twice with 1 ml of RPMI 1640 (Met/Cys deficient medium). Afterwards pre-wetted nitrocellulose membranes were placed on top of the agarose for 4 to 20 hours. The membrane were removed and washed with buffers containing between 0.1 % Triton XI 00 as described in the examples (Sigma, St. Louis, USA) and were exposed to X-ray films (Hyperfilm bmax, Amersham, Sweden) after drying. Black spots on the developed X-ray film indicated Sindbis Virus particles containing the cDNA for a secreted protein. Amplification Of Selected Virus Particles. Positive plaques were identified by superposition of the X-ray film with the agarose overlay. The corresponding plaques were picked with a 10 μl tip and eluted in 200 μl PBS overnight at 4°C. After pelleting the agarose, 60% confluent BHK 21 cells in a 12-well plate were infected at the permissive temperature with the eluted virus for two (2) hours in 0.5 ml Turbodoma HP-1 before the medium was replaced with 1 ml Turbodoma HP-1 medium. After incubated for 20 hours at the permissive temperature the cell supernatant was harvested and a dilution of 1 :20 in 2 ml Turbodoma HP-1 was incubated for two (2) hours in a T25 flask with 60% confluent BHK 21 cells, before the medium was replaced with 10 ml HP-1 medium. After 20 hours incubation the resulting supernatant contained about 104 to 106 pfu per ml, which corresponds to a total of about 10 5 to 10 7 pfu per T25 supernatant. Sequencing Of The Selected Sindbis Virus Particles. The virus mRNA was isolated using a viral RNA isolation kit (High Pure Viral RNA Kit, Boehringer Mannheim, Mannheim, Germany) and RT-PCR was done with Superscript one-step RT-PCR according to Gibco (Gibco/BRL, Life Technologies AG, Basel, Switzerland) according to the recommedation of the manufactorer. The 3' primer for the RT-PCR was either the oligo GCGCGGGCCCT 20 (specific for the poly A tail) or the oligo GCGCGGGCCCCGCTG CGTGGCATTATGCACC specific for the 3 * CIS sequence of the Sindbis Virus; the 5' primer was the oligo GCGCGGGCCCCGCTGCGTGGCATTATGCACC. The RT-PCR product was digested with the restriction enzymes Bspl20I and EcoRI, gel-purified and ligated in pBluescript (digested with EcoRI and Bspl20I) and finally sequenced using the oligos "-40 forward and reverse primer" of the multiple cloning site. Immunofluorescence Microscopy of Sindbis Virus Infected Cells. 60% confluent BHK 21 cells in όwell plates were infected for 2 hours at the permissive temperature at a moi of 0.1 in 1 ml HP-1 medium with Sindbis virus particles containing a heterologous cDNA encoding a membrane protein (receptor). 20 hours post-infection, the cells were dissociated with cell dissociation solution (Sigma-Aldrich, Steinheim, Germany) and were washed twice at 4°C with 1% BSA in HBSS. The resuspended cells were then incubated with ligand-flag fusion protein at concentrations between 100 ng/ml and 1 μg and with 5 μg/ml monoclonal antibody M2 for 15 min on ice. The cells were washed twice before being incubated for 30 min with 10 μg/ml FITC-conjugated secondary antibody in 1% BSA in HBSS at 4°C. After two (2) washing steps with 1% BSA in HBSS, the cells were analyzed with an immunfluorescence microscope (Leica DMIL, Leica, Heerburg, Switzerland) or the cells were analyzed and sorted with a FACS sorter. Cloning Ofts2, ts20 And The Double Mutation ts2,20 In DH-EB. The mutations ts2, ts20 and the combination of these two mutations were inserted into DH-EB (Bredenbeek et al, 1993, J Virology p. 6439-6446) by PCR using the following oligonucleotides: pimers: oligo Aat II GCACTTAAGTTGGAGGCCGAC oligo BssH II GGCACTCACGGCGCGCTTTACAGGC oligo Hpa 1.1 GCAATGTtAAcaGGTCTGTATCTAATTGG oligo Hpa 1.2 CAGACCtgTTaACATTGCTCACCACCAGG oligo Pvu 1.1 GACAGCGGTCGatCGATCATGGATAACTC oligo Pvu 1.2 GAGTTATCCATGATCGatCGACCGCTGTC The restriction sites used for analysis are underlined, mutated nucleotides are indicated in small letters and bold. PCR reactions were performed using the following combinations: PCR1 : oligo Aat II - oligo Hpa 1.1 (for ts20) PCR2: oligo BssH II - oligo Hpa 1.2 (for ts20) PCR3: oligo Aat II - oligo Pvu 1.1 (for ts2) PCR4: oligo BssH II - oligo Pvu 1.2 (for ts2) 100 pmol of each oligo and 5 ng of the template DNA were used in the 100 μl reaction mixture, containing 4 units of Taq or Pwo polymerase, 0.1 μM dNTPs and 1.5 mM MgSO 4 . The polymerase was added directly before starting the PCR reaction (starting point 95°C). The temperature cycles were as following: 95°C for 1.5 min, followed by 5 cycles of 95°C (30 seconds), 54°C (30 seconds), 72°C (120 seconds) and followed by 25 cycles of 95°C (30 seconds), 64°C (30 seconds), 72°C (120 seconds). The four PCR (PCR 1 to 4) fragments were purified using Qia spin PCR kit (QIAGEN, Inc., Chatsworth, CA 91311, USA) and finally digested in the appropriate buffer using 10 units of Dralll and Hpal, Hpal and BssHII, Dralll and Pvul, or BssHII and Pvul respectively. The digestion was performed for 6 hours at 37°C, before the DNA fragments were gel-purified (gene-clean; Bio 101 Inc., Vista, CA 92083, USA). The purified PCR fragments 1 and 2 and the framents 3 and 4 were finally ligated into Dralll / BssHII digested and gel-purified pDH-EB (Bredenbeek et al, J. Virology ' Nov. 1993, p. 6439-6446). The correct sequence of the obtained vectors was checked by restriction enzyme digestion and was verified by DNA sequencing using the following primers: oligo seq ts2: TTCCAAGCCATCAGAGGGG oligo seq ts20: AGATTAGCACCTCAGGACCG The correct vectors DH-EB ts2 and DH-EB ts20 were digested for 4 hours at 37°C with 10 units of Bsp68I and EcoRI in the appropriate buffer. The 4.5 kb DNA fragment of pDH-EB ts 2 and the 3.4 kb fragment of pDH-EB ts20 were gel-purified (as described above) and ligated to create DH-EB ts2,20. The correct sequence of the obtained vector was checked by restriction enzyme digestion and was verified by DNA sequencing using the primers oligo seq ts2 and oligo seq ts20 (described above). Production Of Milligram Quantities Of Secreted Proteins. A batch of BHK 21 cells is infected with the virus supernatant containing the protein of interest. Subsequent purification will yield several milligrams of the pure protein. B. Example 1: Compartment Screening and Identification of Nucleic Acids Encoding Proteins with a Predetermined Localization Confluent cultures of BHK cells were infected with viral particles containing two expression systems with two different nucleic acids (pSinRep5 EPO and pSinRep5 lacZ), both packaged with the helper construct DHBB. One culture was not infected as a negative control (Vial 1, lane 1 in FIGURE 1A). Twenty-four hours postinfection, the cells were starved in starvation medium lacking Methionine and Cysteine for 30 minutes. A pulse of lOμCi 35S Met/Cys was added and replaced by fresh medium after 30 minutes. After an incubation of two hours, the cells and the supernatant were collected. The cells were resuspended in Laemmli buffer and boiled for 10 minutes. Five hundred μl supernatant was added to 600 μl methanol and 200 μl chloroform. The upper phase was discarded and precipitated protein was pelleted by adding 800μl of methanol. Both the precipitated supernatant proteins and cell pellets were resuspended in 25 μl Laemmli buffer, and the proteins separated on a 15%) SDS polyacrylamide gel. After drying, the gel was exposed overnight to an X-ray film. The labelled proteins found in the supernatant were run on the gel presented in FIGURE 1 A; FIGURE IB depicts labelled proteins found in the cell pellet corresponding to the supernatants in lanes 1 and 2 of FIGURE 1A. Identification of a nucleic acid encoding a secreted protein was possible by detection of a band on the X-ray film (FIGURE 1 A), which is the result of a unique labelled protein in the supernatant encoded by the exogenous expression system. FIGURE 1 A, lane 1 shows that uninfected cells (containing no exogenous expression system) secrete several unidentified proteins. The corresponding cell pellet is shown in FIGURE IB where many proteins were labelled and a multitude of bands is visible. The blank lane 2 in FIGURE 1A indicates the presence of an expression system containing a nucleic acid encoding an intracellular protein in vial 2. Accordingly, lane 2 in FIGURE IB confirms this finding by the presence of a unique band which stems from a single labelled protein from the cell pellet. Expression of all other endogenous proteins was suppressed (compare with lane 1) allowing unique labelling and a clear identification of the nucleic acid of vial 2 as a nucleic acid coding for an intracellular protein (which confirms the published finding that lac Z is in fact an intracellular protein). A strong band resulting from a single radioactive protein in the supernatant in FIGURE 1A, lane 6 indicates the presence of an expression system containing a nucleic acid encoding a secreted protein. Vial 6 (lane 6 in FIGURES 1 A and IB) was infected with the pSinRepS EPO construct. This finding confirms that EPO is a secreted protein. Endogenous protein expression was again suppressed allowing unique labelling and a clear identification of the nucleic acid of vial 6 as a nucleic acid coding for a secreted protein. Lanes 3-5 show labelled secreted proteins from mixtures of the two expression systems. Increasing proportions of the EPO expression system with 10%, 50% and 90% give rise to a more uniquely labelled protein in the supernatant. These results demonstrate that compartment screening can identify nucleic acids which encode proteins of a predetermined cellular localization. C. Example 2: Separation of Labelled Viral Particles from Secreted Protein for Semisolid Medium Screening. For optimal identification of expression systems encoding secreted proteins in semisolid medium, it is preferred that a) endogenous protein synthesis is suppressed; and 5 b) labelled viral particles and other labelled soluble viral protein are prevented from interfering with the screen. Confluent layers of BHK 21 cells in 35mm dishes were infected with pTEC5'2JCAT or pTE5'2JEPO double subgenomic infective viral particles at an MOI of 5. As a negative control, uninfected BHK 21 cells were used. Fourteen hours post infection, the medium was removed and the cells were overlaid with 4mm of 0.8% j ø agarose in RPMI 1640 medium containing lOμCi 35 SMet/Cys per dish. After gelling, the cultures were overlaid with 1 ml of Cys/Met free RPMI, and the medium was collected after 2 hours, 4 hours and 8 hours. The protein was precipitated and separated on a 15% SDS gel. The gel was exposed to an X-ray film overnight. It could be demonstrated that endogenous protein synthesis was inhibited again and that labelling was specific (FIGURE ••5 2: compare lanes 1-3 with lanes 4-9). Moreover, diffusion of viral particles was also inhibited since the characteristic pattern of the three structural viral proteins (capsid, El and E2) could not be detected (lanes 4-9). In both supernatants of the virally infected cultures, a labelled protein with a size of about 60 kD was present (lanes 4-9). It was speculated that this protein is a viral protein 2ø and that it might result from proteolytic cleavage of one of the glycoproteins of Sindbis virus. In the case of pTE5'2JEPO after 8 hours, an additional protein with the size of EPO was detected (lane 9) demonstrating the principal feasibility of semi-solid medium screening. This experiment demonstrates that labelled viral particles can be separated from secreted protein by limited diffusion in 0.8% agarose. However, without further measures, 5 the release of soluble viral protein occurs. Such a release is undesirable because it may interfere with the phenotypic screen. Example 3 describes a method that inhibits the release of unwanted viral proteins. D. Example 3: Inhibition of Release of Viral Soluble Proteins ø Confluent layers of BHK cells were infected with double subgenomic pTE5'2JCAT viral particles at an MOI of 5. Fourteen hours post infection, the medium was removed and the cells were overlaid with 4 mm of 0.8% agarose in RPMI 1640 medium containing lOμCi 35 SMet/Cys per dish and varying amounts of Mini Protean protease inhibitor cocktail solution (seven times concentrated as described in the manual, Boehringer Mannheim, Rotkreuz, Switzerland). One hundred μl, 20μl, lOμl, 5μl and lμl were used (see lanes 1-5 in FIGURE 3). After gelling, the cultures were overlaid with 1 ml of Cys/Met free RPMI, and the medium was collected after 4 hours. The protein was precipitated and separated on a 15% SDS gel. The gel was exposed to an X-ray film overnight. The results (depicted in FIGURE 3) demonstrated that release of viral protein was inhibited by addition of the protease inhibitor. At concentrations above 20μl/ml (lanes 1 and 2), the release of soluble protein was inhibited showing that neither viral particles nor viral protein diffuses through the agarose layer, and that no endogenous or viral protein interfered with the screen. E. Example 4: Identification of a Nucleic Acid Encoding a Protein with a Predetermined Enzymatic Activity in a Mixture of Nucleic Acids in Semi-Solid Medium Screening An expression system containing a nucleic acid encoding a protein with phosphatase activity was identified in a mixture of two expression systems. pSinRep5 lacZ and pSinRep5 SEAP were packaged individually with the helper DHEB, yielding infective particles. The two expression systems were mixed in a ratio of 10:1 (pSinRep5 lacZ: pSinRep5 SEAP). Confluent layers of BHK 21 cells in 35 mm dishes were infected with this mixture of expression systems and alternatively only with the pSinRep5 lacZ expression system. Two hours postinfection, the medium was removed and the cells were overlaid with 1.5 ml 0.8% agarose in lx HP-1 medium Two days later, a nitrocellulose filter was placed on top of the agarose for 4 hours. The filter was removed, washed in PBS and placed in a solution of 100 mM TrisHCl, 100 mM NaCI, 370 mg/nitroblue tetrazolium and 250 mg/l 5-bromo-4-chloro-3- indolylphosphate (all Sigma) to detect alkaline phosphatase activity. In the sample containing 10% of the pSinRep5 SEAP expression system, two spatially distinct areas with alkaline phosphatase activity could be detected (FIGURE 5). The two corresponding plaques that gave rise to the enzymatic conversion of the substrate and a negative control from an area that stained negative on the screen were isolated. The cell culture was then fixed in 0.5% glutaraldehyde in PBS and lac Z activity was detected in the cells by X-Gal staining as described in the instruction manual "Sindbis expression system" Invitrogen, San Diego, USA (not shown). About 20 to 30 plaques stained blue in this cell culture reflect the original distribution of expression systems. Viruses from the positive alkaline phosphatase plaques and the negative control were eluted in PBS overnight by shaking at 4°C. The PBS was added to a fresh culture of confluent BHK cells in a 35mm dish and SEAP activity was confirmed 2 days post infection by the following assay. Five hundred μl 2x SEAP buffer (2mM L-homoarginine, 0.2 M diethanolamine, 0.1 mM MgCl 2 in H 2 O) plus lOOμl substrate solution (120mM nitrophenylphosphate in H 2 O) were mixed with 400 μl of heat inactivated (10 minutes 60 °C) cell culture supernatant. SEAP activity was revealed by a color change from purple to yellow in the sample from the positive areas of the culture, whereas the negative controls did not give rise to a color change. This example demonstrates that an expression system containing a nucleic acid encoding a protein with a predetermined enzymatic activity can be isolated from a mixture of expression systems in semisolid medium screening. F. Example 5: Stable Amplification of Expression Systems The positive viral particles (containing the nucleic acid coding for SEAP) from Example 4 were amplified over three passages. One percent of the supernatant from the 35 mm dish of Example 4 was used to infect an 80% confluent T25 (25 cm 2 ) cell culture flash containing BHK 21. Twenty-four hours post infection, the supernatant was removed and assayed for SEAP activity. SEAP activity in this supernatant was proven by a fast color change in this sample. The particles were passaged in this way for a total of four times, thereby amplifying the expression system by a factor of 10 8 within 5 days. This result demonstrates that the expression system can be amplified rapidly and efficiently to yield large amounts of a protein of interest. Moreover, this process yields substantial amounts of the expression system interest for further investigation of the protein's function and usefulness in any other mammalian system. G. Example 6: Identification Of A Nucleic Acid Encoding A Secreted Protein In A Mixture Of Nucleic Acids In Semi-Solid Medium Screening An expression system containing a nucleic acid encoding a secreted protein with phosphatase activity was identified in a mixture of two expression systems. pSinRep5'LacZ and pSinRep5' SEAP were packaged individually with the helper DHEB, yielding infective particles. 90% confluent BHK 21 cell layers in 60 mm dishes (Easy Grip, Falcon 3004, Becton Dickinson and Company, England) were incubated at 37(C for 2 hours with 1 ml of lx HP-1 medium containing approximately 780 plaque forming units ("pfu") of pSinRep5 LacZ and 20 pfu of pSinRep 5 SEAP (secreted alkaline phosphatase). After removal of the medium, the cells were overlaid with 3 ml 41°C warm 0.8% agarose (Carl Roth GmbH, Karlsruhe, Germany) in HP-1 medium and were then incubated for 2 days at 37°C in 5% CO 2 . The agarose was overlaid with 1 ml lx RPMI 1640 MetVCys " for 10 min before being replaced by 1 ml of fresh lx RPMI 1640 MetVCys " . After 30 min starvation, the medium was replaced with 0.8 ml lx RPMI 1640 MetVCys " plus 20 μCi of 35 S Met/Cys. After labeling for 4 hour, the agarose overlay was washed 3 times for 10 min with 1 ml lx HP-1, before the 54 mm pre-wetted nitrocellulose filter (0.45 μm membrane, BA85, Schleicher & Schuell, Germany) was applied for 16 to 18 hours. Diffusion blotting was proceeded during 18 hours, before SEAP activity was detected by AP staining (10 ml 100 mM TrisHCl, 100 mM NaCI, 370 mg nitroblue-tetrazolium and 250 mg/l 5-bromo-4chloro-3indolylphopshate (all Sigma)) resulting in violet spots (FIGURE 7). The AP-developing was stopped by removal of the AP staining solution and with subsequent washing steps (3 times with TBS containing 0.1 % Triton X-100). The nitrocellulose was then dried and exposed for at least 24 hours to an X-ray-film (Hyperfilm βmax, Amersham, Sweden) before being developed with an AGFA Curix 60 (Schenk, Winterthur, Switzerland) machine (FIGURE 7). The coordinates of the radioactive spots were determined and the equivalent regions were picked from the agarose layer with a 10 μl tip. The eluted virus (as described above) was passaged in a 60%> confluent BHK 21 12-well plate. Presence of a gene encoding SEAP in the insert of the eluted virus was determined by spotting 4 μl supernatant on a nitrocellulose membrane and by AP staining as described above. The correspondance of the spots on the Secreted Alkaline Phosphatase stained filters with the spots on the X-ray film clearly demonstrated that nucleic acids encoding a secreted protein can be identified in a mixture of nucleic acids using semi-solid medium screening. H. Example 7: Identification Of Nucleic Acids Encoding Secreted Proteins In A Sindbis Virus Library Representing A cDNA Library In Semi- Solid Medium Screening A directed cDNA library from ECV 304 cells (ATCC CRL-1998, human endothelial cell line) containing a Not I site at the 3' end and a 5' blunt end was ligated into Stu I, Bsp 120 L digested pSinRep 5 vector and 10 7 primary clones were obtained after transformation of E.coli DHlOb. After preparation of plasmid DNA and linearization with Not I, the library was transcribed in vitro as described before and packaged with the helper DHEB, yielding infective particles. 90% confluent BHK 21 cell layers in 60 mm dishes (Easy Grip, Falcon 3004, Becton Dickinson and Company, England) were incubated at 37°C for 2 hours with 1 ml of lx HP-1 medium containing approximately 800 pfu of pSinRep 5 ECV 304 library. After removal of the medium, the cells were overlaid with 3 ml 41°C warm 0.8%> agarose (Carl Roth GmbH, Karlsruhe, Germany) in HP-1 medium and were then incubated for 2 days at 37°C in 5% CO2. The agarose was overlaid with 1 ml lx RPMI 1640 MetVCys ' for 10 min which was replaced by 1 ml of fresh lx RPMI 1640 MetVCys " . After 30 min starvation, the medium was replaced with 0.8 ml lx RPMI 1640 MetVCys " containing 20 μCi of 35 S Met Cys. After 4 hour labeling, the agarose overlay was washed 3 times for 10 min with 1 ml lx HP-1, before the 54 mm pre-wetted nitrocellulose filter (0.45 μm membrane, BA85, Schleicher & Schuell, Gremany) was applied. Diffusion blotting was proceeded during 18 hours at 37°C, then the nitrocellulose was washed three (3) times 10 minutes with TBS containing 0.1 % Triton X-100 before the membrane was dried and exposed for at least 24 hours to an X-ray film (Hyperfilm βmax, Amersham, Sweden), which was subsequently developed with an AGFA Curix 60 (Schenk, Winterthur, Switzerland) machine. The coordinates of the radioactive spots were determined and the equivalent regions were picked from the agarose layer with a 10 μl tip. The virus was eluted as decribed before and was passaged on 60%> confluent BHK 21 cells in 12-well plates. After reamplification of the virus particles on 60% confluent BHK 21 cells in 6-well plates, the viral RNA was isolated using the "high pure viral RNA kit" (Boehringer Mannheim, Mannheim, Germany). RT-PCR was carried out with the Superscript one-step RT-PCR kit according to the manufacturer (Gibco/BRL, Life Technologies AG, Basel, Switzerland) and the primers described before. The RT-PCR product was digested with the restriction enzymes Bspl20I and EcoRI, gel-purified and ligated in pBluescript (digested with EcoRI and Bspl20I) and finally sequenced using the oligos "-40 forward and reverse primer" of the multiple cloning site. Example 8: Identification Of A Nucleic Acid Encoding A Secreted Protein In A Mixture Of Nucleic Acids In Semi-Solid Medium Screening Using Sindbis TS Mutants Recombinant SEAP Sindbis Virus Stocks. EcoRI-linearized DH-EB helper constructs (DH-EB, DH-EB ts2, DH-EB ts20, DH-EB ts2,20) and Notl-linearized pSinRep 5 SEAP and pSinRep5 LacZ DNA were made RNase-free by purification over QiaQuick PCR purification columns (QIAGEN AG, Basel, Switzerland) and elution with DEPC-H 2 O. SP6 in vitro transcription was carried out according to the manufacturer Invitrogen (Invitroscript CAP Kit, Invitrogen BV, NV Leek, The Netherlands). 5 μg of SinRep 5' SEAP and 5 μg of the helper transcript were co-electroporated into BHK 21 cells according to Invitrogen (Invitroscript CAP Kit, Invitrogen BV, NV Leek, The Netherlands). The supernatants were harvested 30 hours post-electroporation (incubated at 30°C, 5% CO 2 ) and were assayed for SEAP activity. 4 μl of supernatant was spotted on nitrocellulose membrane (Schleicher & Schuell) and SEAP activity was detected by AP staining method as described above (FIGURE 8). First Passage Ofts Mutants. The recombinant virus stocks were passaged in a 60% confluent 6-well plate with BHK 21 cells. 100 μl of each stock plus 900 μl of HP-1 were added for two (2) hours to the BHK cells at 30°C, then the virus supernatant was replaced with 2 ml HP-1. The cells were incubated for 24 hours either at the permissive temperature at 30°C or at the nonpermissive temperature 37°C before the supernatant was assayed for SEAP activity (as described before) (FIGURE 8). Plaque Assay For The ts Mutants. Dilution series of the harvested virus particles, see, supra, were carried out in 1 ml Turbodoma HP-1 (1 :10 4 , 1 : 5x 10 4 , 1 :10 5 , 1 : 5x 10 5 , 5 1 : 10 6 , 1 : 5x 10 6 ) on 90% confluent BHK 21 cells in 60 mm tissue culture dishes (Easy Grip, Falcon 3004, Becton Dickinson and Company, England). The cells were infected at 30°C for 2 hours with the diluted virus at Dilution series of the harvested virus particles, see, supra, were carried out in 1 ml Turbodoma HP- 1 (1 :10 4 , 1 : 5x 10 4 , 1 :10 s , 1 : 5x 10 5 , 1 : 10 6 , 1 : 5x 10 6 ). The supernatant was then replaced with 0.8% agarose in HP-1 and incubated for 10 2 days at 37°C or 30°C in 5% CO 2 . The plaques were then counted. Agarose Blot Assay With A Mixture OfpSinRep SEAP And pSinRep LacZ An expression system containing a nucleic acid encoding a secreted protein was identified in a mixture of two expression systems. pSinRep5'LacZ and pSinRep5' SEAP were packaged individually with the ts mutant helpers DHEB (see TABLE I), yielding infective particles. 15 90% confluent BHK 21 cell layers in 60 mm dishes (Easy Grip, Falcon 3004, Becton Dickinson and Company, England) were incubated at 30°C for 2 hours with 1 ml of lx HP-1 medium containing approximately 780 plaque forming units ("pfu") of pSinRep 5 LacZ / DHEB ts and 20 pfu of pSinRep 5 SEAP (secreted alkaline phosphatase)/ DHEB ts. After removal of the medium, the cells were overlaid with 3 ml 41°C 0.8% agarose (Carl 0 Roth GmbH, Karlsruhe, Germany) in HP-1 medium and were then incubated for 2 days at 30°C in 5% CO 2 . Two (2) hours pre-starvation, the cells were shifted up to 40°C in 5% CO 2 . The agarose was overlaid with 1 ml lx RPMI 1640 MetVCys " for 10 min before the 1 ml RPMI medium was replaced with 1 ml of fresh lx RPMI 1640 MetVCys " . After 30 min starvation 5 at 40°C, the medium was replaced with 0.8 ml lx RPMI 1640 MetVCys " containing 20 μCi of 35 S Met/Cys. After 4 hour labeling at 40°C, the agarose overlay was washed 3 times 10 min with 1 ml lx HP-1, before a 54 mm pre-wetted nitrocellulose filter (0.45 μm membrane, BA85, Schleicher & Schuell, Gremany) was applied. Diffusion blotting was proceeded at 40°C during 18 hours, before SEAP activity 0 W as detected by AP staining (10 ml 100 mM Tris-HCI, 100 mM NaCI, 370 mg nitroblue-tetrazolium and 250 mg/l 5-bromo-4chloro-3indolylphopshate (all Sigma)) resulting in violet spots (FIGURE 8). The AP staining was stopped by removing the AP solution and by susequent washing steps (3 times TBS containing 0.1% Triton XlOO). The nitrocellulose was dried and exposed for at least 24 hours to an X-ray film (Hyperfilm bmax, Amersham, Sweden) before being developed with a AGFA Curix 60 (Schenk, Winterthur, Switzerland) machine. The coordinates of the radioactive spots were determined and the equivalent regions were picked from the agarose layer with a 10 μl pipette tip. J. Example 9: Identification Of A Nucleic Acid Encoding A Secreted Protein In A Mixture Of Nucleic Acids In Semi-Solid Medium Library Screening Using The TS Mutants pSinRep5' cDNA library ECV 304 was packaged with the mutant helpers DHEB, yielding infective particles. 90% confluent BHK 21 cell layers in 60 mm dishes (Easy Grip, Falcon 3004, Becton Dickinson and Company, England) were incubated at 30°C for 2 hours with 1 ml of lx HP-1 medium containing approximately 800 pfu of the pSinRep 5 cDNA library ECV 304. After removal of the medium, the cells were overlaid with 3 ml 0.8% agarose (Carl Roth GmbH, Karlsruhe, Germany) in HP-1 medium and were then incubated for 2 days at 30°C in 5% CO 2 . Two (2) hours pre-starvation, the cells were shifted to 40°C in 5%> C0 2 . The agarose was overlaid with 1 ml lx RPMI 1640 MetVCys " for 10 min before being replaced by 1 ml of fresh lx RPMI 1640 MetVCys " . After 30 min starvation, the medium was replaced with 0.8 ml lx RPMI 1640 MetVCys " plus 20 μCi of 35 S Met/Cys. After 4 hour labeling, the agarose overlay was washed 3 times for 10 min with 1 ml lx HP-1, before the 54 mm pre-wetted nitrocellulose filter (0.45 μm membrane, BA85, Schleicher & Schuell, Germany) was applied for 16 to 18 hours. Diffusion blotting was proceeded during 18 hours, before the nitrocellulose was dried and exposed for at least 24 hours to an X-ray film(Hyperfilm βmax, Amersham, Sweden) and then developed with a AGFA Curix 60 (Schenk, Winterthur, Switzerland) machine. The coordinates of the radioactive spots were determined and the equivalent regions were picked from the agarose layer with a 10 μl tip. K. Example 10: Expression Cloning Using Sindbis Virus: Cloning A Receptor For A Known Ligand By FACS Single Cell Sorting 60% confluent BHK 21 cultures in T 150 flasks were infected at 37°C for 2 hours at an moi of 0.1 in lx HP-1 medium. Different dilutions of pSinRep 5 hIL 13 Ra (human Interleukin 13 Receptor, subunit alpha; hIL13 Ra digested from pDR2-hIL13Ra with Xbal and PvuII, cloned into pSinRep5 digested with Xbal and StuI) /DHEB with pSinRep 5 LacZ/DHEB (1 :0 / 1:100 / 1 :100 * 000) were used with a total moi of 0.1. The medium was then replaced with fresh lx HP-1 medium and the cells were incubated at 37°C in 5% CO 2 for 20 hours. ø The cells were then resuspended with 5 ml cell dissociation solution (Sigma-Aldrich, Steinheim, Germany) followed by two additional washing steps in HBSS containing 1% BSA. 1 ml 1%BSA, 1 μg/ml IL13flag (according to IBR GmbH, Waengi, Switzerland) and 5 μg/ml mAb M2 (according to to IBR GmbH, Waengi, Switzerland) in HBSS were incubated with the cells at 4°C for 15 min. The cells were washed twice with 5 1.5 ml 1 % BSA/HBSS and incubated with 1 ml 1 % BSA and 10 μg/ml FITC conjugated secondary antibody for 30 min at 4°C. The cells were washed twice with 4°C 1% BSA in HBSS and resuspended in HBSS at a concentration of 3x 10 6 / ml. Single cell sorting into 96-well plates was done for cells with FITC fluorescence intensity above background (see FIGURE 9). ø FIGURE 9 shows that the Interleukin 13-Receptor alpha is transiently expressed and that single cells expressing the correct receptor for a chosen ligand can be sorted by FACS analysis. L. Example 11: Expression Cloning Using Sindbis Viruses: Cloning A Ligand For A Known Receptor 5 An infectious Sindbis Virus library is produced in double subgenomic Sindbis Virus vectors (pTE) or in pSinRep5 copackaged with the DH-EB helper as described supra. Confluent BHK 21 cells in 150 mm dishes are incubated for two (2) hours with HP-1 medium containing about 5,000 pfu of the infectious sindbis virus library. After removal of the medium, the cells are overlaid with 2 mm of 0.8% agarose in lx HP-1 0 medium and are incubated for three days at 37 °C. A nitrocellulose filter is placed on top of the agarose and incubated for at least three hours. The membrane is removed and subsequently washed with TBS. After agitating the filters for 15 minutes in TBS, the filter is blocked by incubation in 1%> skim milk for one (1) hour. After three washing steps in TBS, the filters are incubated in TBS containing the radiolabelled solubilized receptor for at least one (1) hour. After several washing steps in TBS, the filters are dried and exposed to X-ray films. Dark spots indicate colonies expressing a binding partner of the chosen receptor of interest. M. Example 12: Cloning A Cell Associated Protein For Which The Ligand ø Is Known An infectious Sindbis Virus library is produced in double subgenomic Sindbis Virus vectors (pTE) or in pSinRep5 copackaged with the DH-EB helper as described in the Materials and Methods section, supra. BHK 21 cells are grown on nitrocellulose filters in 150 mm dishes to confiuency. The cells are incubated for two hours 5 with HP-1 medium containing about 5,000 pfu. After removal of the medium, the cells are overlaid with 2 mm of 0.8% low melting agarose in lx HP-1 medium and are incubated for two days at 37 °C. After this incubation, a nitrocellulose filter is placed on top of the agarose to capture viral particles. After 24 hours of incubation, the nitrocellulose filters are removed and placed in new 150 mm dishes where a new culture of BHK cells is added at a ø cell density of 400,000 cells per cm 2 in medium containing 10%> FCS. After adhesion of the cells, this culture is again overlaid with 2 mm agarose and incubated at 37 °C. This culture serves as a stock of virus particles for later isolation. The initial plate is heated to 40 °C to melt the agarose. After removal of the melted agarose, the cells are fixed to the membrane by known methods (0.5% glutaraldehye in PBS, 3%> paraformaldeyde, -20 °C 5 methanol and the like) according to the expected localization (nucleus, cytoplasmic membrane, etc.) and nature of the protein that is to be cloned. After several washing steps in TBS, the filters are incubated in TBS containing the radioactive labelled ligand. After several washing steps, the filters are dried and exposed to X-ray films. Dark spots indicate colonies expressing a binding partner of the chosen ligand. 0 N. Example 13: Discovery And Expression Of Novel Membrane Proteins A Sindbis Virus library in pSinRep5 as described in Example 6 is coelectroporated with the DHBB helper to produce one way virions described supra. After determination of the virus titer, confluent cultures of 987 BB neo cells (See Table I and SEQ ID NO:2 for a description of the vector by which BHK 21 cells were transfected to yield the 987BBneo cell line after selection in 200μg/ml G418). After removal of the medium, the cells are overlaid with 2 mm of agarose to allow plaque formation. After three days, a coated polyester mat as described in Motobu et al (in E.C. Beuvery et al. (eds.) "Animal Cell Technology; Developments towards the 21th century", pp. 811-815 (1995) Kluwer Academic Publishers) is placed on top of the agarose and BHK 21 cells are added on top at a cell density of 400,000 cells/cm 2 at a concentration of 1,000,000 cells/ml of medium. After adhesion of the cells, the same amount of fresh medium is added and the culture is incubated for an additional 12 hours. The liquid medium is removed and replaced by labelling medium (10 μCi of 35 S methionine/cysteine). After 12 hours of incorporation, the labelling medium is removed and the culture is washed by adding 10 ml of normal medium. This medium is replaced every 10 minutes for a total of two hours. After removal of all liquid medium, the cells are overlaid with a 2 mm layer of medium containing 0.8% agarose and 0.1% trypsin. After gelling of the agarose, a nitrocellulose membrane is placed on top of the agarose and the culture is incubated for an additional one to six (6) hours. Proteolytically released fragments of membrane proteins diffuse to the membrane and are captured on the nitrocellulose filter. Autoradiographing the filter indicates the clones coding for membrane proteins. O. Example 14: Discovery And Expression Of Novel Organelle Specific Proteins An infectious Sindbis Virus library is produced in double subgenomic Sindbis Virus vectors (pTE) or in pSinRep5 copackaged with the DH-EB helper as described in Materials and Methods. BHK 21 cells are grown in 150 mm dishes to confiuency. The cells are incubated for two hours with HP-1 medium containing about 5,000 pfu. After removal of the medium, the cells are overlaid with 2 mm of 0.8% low melting agarose in lx HP-1 medium and are incubated for one to two days at 37 °C. After this incubation, a nitrocellulose filter is placed on top of the agarose to capture viral particles. After at least 12 (twelve) hours of incubation, the nitrocellulose filters are removed and placed in new 150 mm dishes where a new culture of BHK cells is added at a cell density of 400,000 cells per cm 2 in medium containing 10% FCS. After adhesion of the cells, this culture is again overlaid with 2 mm agarose and incubated at 37 °C. This culture serves as a stock of virus particles for later isolation. The initial plate is overlaid with a second layer of 0.8% agarose in RPMI 1640 medium containing 40 μCi of 35 S labelled mix of methionine and cysteine. After two hours of labelling, the plates are heated to 40 °C to melt the agarose. After removal of the melted agarose, the cells are lysed by known methods to yield intact organelles. A blocked 0 nitrocellulose filter with immobilized antibodies against a surface protein of the organelle to be captured is placed on top of the homogenate. After several washing steps, the filters are dried and exposed to X-ray films. Autoradiographing the filter indicates the clones coding for proteins localized in the desired organelle. 5 P. Example 15: Expression Cloning Using Sindbis Virus: Cloning A New Receptor For A Known Ligand By FACS Single Cell Sorting 60%) confluent BHK 21 cells in T 150 flask are infected for 2 hours with pSinRep ECV cDNA library at an moi of 0.1 in 20 ml Turbodoma HP-1 medium. The medium is then replaced with 40 ml fresh lx HP-1 medium and the cells are incubated at ø 37°C in 5% CO 2 for 20 hours. The cells are then resuspended with 5 ml cell dissociation solution (Sigma-Aldrich, Steinheim, Germany) and washed twice in HBSS containing 1% BSA. The cells are incubated in 1 ml HBSS in the presence of 1% BSA, 1 μg/ml ligand-flag fusion protein and 5 μg/ml mAb M2 at 4°C for 15 min and washed twice with 1.5 ml 1%BSA/HBSS before 5 subsequent incubation with 1 ml 1% BSA and 10 μl/ml antiM2 FITC for 30 min at 4°C. The cells are washed twice with 1%> BSA in HBSS before resuspension in HBSS at a concentration of 3x 10 6 / ml. Single cell sorting into 96-well plates is done for cells with FITC activity above background. The supernatant of the 96-well plates are used for a plaque assay in 6-well plates ø with 90%) confluent BHK 21 cells. Plaques are picked with 10 μl pipette tips and the eluted as described before. The eluted virus is passaged in a 60% confluent BHK 21 12-well plate in 1 ml Turbodoma HP-1 at the permissive temperature. The insert of the eluted virus is determined by reamplification of the virus particles in 2 ml Turbodoma HP-1 medium in a 6-well plate containing 60% BHK 21 cell layers. Viral RNA is isolated using a "high pure viral RNA kit" (Boehringer Mannheim, Mannheim, Germany) and RT-PCR is done with the Superscript one-step RT-PCR kit according to the manufacturer (Gibco/BRL, Life Technologies AG, Basel, Switzerland) and the primers as described before. The RT-PCR product ligation into pBluescript and the sequence determination are done as described above. 0 Q: Example 16: Expression Cloning Of A Ligand For A Known Binding Partner pSinRep5' EPO and pSinRep 5 LacZ were packaged with the helper DHEB, yielding infective particles. 90%> confluent BHK 21 cell layers in 60 mm dishes (Easy Grip, Falcon 3004, Becton Dickinson and Company, England) were incubated at 37°C for 5 two (2) hours with 1 ml of lx HP-1 medium containing approximately 25 to 30 pfu of pSinRep EPO / DH-EB and approximately 180 to 200 pfu of pSinRep 5 LacZ / DH-EB. After removal of the medium, the cells were overlaid with 3 ml 41°C warm 0.8% agarose (Carl Roth GmbH, Karlsruhe, Germany) in HP-1 medium and the plates were incubated for two (2) days at 37°C in 5% CO 2 . ø A pre-wetted 54 mm nitrocellulose filter (0.45 μm membrane, BA85, Schleicher & Schuell, Germany) was applied for sixteen (16) hours, before the nitrocellulose was blocked with 1% BSA in TBS for two (2) hours. The first antibody (anti-EPO, polyclonal, Research Diagnostics, Inc., USA) specific for the screened secreted protein was incubated for one (1) hour at the dilution of 1 :3000 in TBS (according to the manufacturer). The 5 membrane was then washed three (3) times with TBS 0.05% Tween before the second antibody (AP conjugated anti-rabbit IgG, Jackson ImmunoResearch Laboratories, Inc., USA) was incubated with the filter at a dilution of 1 :4000 in TBS 1% BSA. After washing the membrane with TBS 0.05% Tween, AP staining was carried out as described above. The plaques corresponding to the AP positive spots (see FIGURE 10) were picked ø from the agarose layer with a 10 μl pipette tip. Elution of the virus was done as described. The virus was passaged in a 60% confluent BHK 21 12-well plate. The insert of the eluted virus was determined by reamplifying of the virus particles in a 60%) BHK 21 6-well plate before a dot blot against EPO was done as described above. FIGURE 10 shows, that a specific secreted ligand for a known binding partner (receptor, antibody) can be cloned successfully and that the system can be efficiently used to screen viruses representing cDNA libraries. All references cited within the body of the instant specification are hereby incorporated by reference in their entirety.

Claims: CLAIMS WHAT IS CLAIMED IS: L A method for identifying a recombinant nucleic acid encoding an exogenous protein having a property of interest comprising: a. providing a composition comprising a plurality of eukaryotic host cells, wherein each host cell has an expression system comprising a different member, each member comprising a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element; b. culturing said eukaryotic host cells under conditions where said exogenous protein is expressed while expression of endogenous proteins of said eukaryotic host cell is suppressed; and c. identifying a member comprising a recombinant nucleic acid encoding an exogenous protein having the property of interest. 2. The method of Claim 1 , wherein the property is a predetermined cellular localization. 3. The method of Claim 2, wherein the predetermined cellular localization is the extracellular space. 4. The method of Claim 2, wherein the predetermined cellular localization is the cell membrane. 5. The method of Claim 2, wherein the predetermined cellular localization is selected from the group consisting of nucleus, lysosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxysomes, endosomes and cytoplasm. 6. The method of Claim 1 , wherein the property is a binding affinity to a binding partner. 7. The method of Claim 1 , wherein the property is an enzymatic activity. 8. The method of Claim 1 , wherein the property is a structural property. 9. The method of Claim 1 , wherein said exogenous protein is labelled while expression of said endogenous proteins is suppressed. 10. The method of Claim 9, wherein said exogenous protein is labelled with a marker selected from the group consisting of radioactive marker and spectroscopically detectable marker. 1 1. The method of claim 10, wherein said radioactive marker is selected from the group consisting of radioactive amino acids and radioactive sugars. 12. The method of Claim 1 , wherein each of said members is expressed in an individual eukaryotic host cell. 13. The method of Claim 9, wherein said control element is derived from a virus. 14. The method of Claim 13, wherein said virus suppresses expression of endogenous proteins. 15. The method of Claim 13, wherein said virus is an alpha virus. 16. The method of Claim 15, wherein said Alpha Virus is selected from the group consisting of Sindbis Virus, Semliki Forest Virus, and Venezuelan Equine Encephalitis Virus. 17. The method of Claim 13, wherein said expression system further directs the generation of viral particles. 18. The method of Claim 17, further comprising the step of separating said viral particles from said exogenous protein. 19. The method of Claim 18, wherein said viral particles are separated from said exogenous protein through filtration, chromatography or precipitation. 20. The method of Claim 14, wherein said control element lacks at least one function required for propagation of said virus. 21. The method of Claim 20, wherein the function required for propagation of said virus is provided through a helper function. 22. The method of Claim 21 , wherein said helper function is provided by a viral packaging cell line or a helper virus particle. 23. The method of Claim 1, wherein said control element is derived from a virus which is capable of directing expression of said nucleic acid encoding an exogenous protein while expression of endogenous proteins is suppressed. 24. The method of Claim 23, wherein expression of said endogenous proteins is suppressed by inhibiting transcription or translation of nucleic acids encoding endogenous proteins. 25. The method of Claim 1, wherein said control element promotes transcription of said recombinant nucleic acid encoding said exogenous protein in the presence of an inhibitor of cellular RNA Polymerase II, and wherein in step b, said eukaryotic host cells are cultured in the presence of an effective amount of an inhibitor of cellular RNA Polymerase II. 26. The method of Claim 25, wherein said inhibitor is selected from the group consisting of Actinomycin D, Aflatoxin Bl, and Amatoxin. 27. The method of Claim 17, wherein said viral particles produce solubilized viral protein, and wherein production of said solubilized viral protein is reduced. 28. The method of Claim 27, wherein generation of solubilized protein of said virus is reduced through use of a protease inhibitor. 29. The method of Claim 27, wherein generation of solubilized protein of said virus is reduced through use of a mutant virus strain. 30. The method of Claim 29, wherein said mutant virus strain is selected from the group consisting of the temperature sensitive Sindis mutants ts20, tslO, and ts23. 31. The method of Claim 27, wherein generation of solubilized protein of said virus is reduced through use of a cell line deficient in cleavage of a protein of said virus. 32. The method of Claim 31 , wherein said cell line is deficient in cleavage of protein PE2 to E2 and E3. 33. The method of Claim 1, wherein said nucleic acid encoding an exogenous protein is derived from a nucleic acid library comprising a plurality of distinct nucleic acids encoding polypeptides. 34. The method of Claim 1, wherein said plurality of members is screened using physical separation of said eukaryotic host cells each carrying one individual member of said plurality of members. 35. The method of Claim 34, wherein said physical separation is achieved by separating said eukaryotic host cells in a semi-solid medium. 36. The method of Claim 34, wherein said physical separation is achieved by placing eukaryotic host cells in separate compartments, wherein each compartment contains host cells carrying identical expression systems of said plurality of expression systems. 37. The method of Claim 1 , wherein said expression system is capable of self- replication. 38. A method for identifying a recombinant nucleic acid encoding an exogenous protein having a property of interest comprising: a. providing a composition comprising a plurality of eukaryotic host cells, wherein each host cell has an expression system comprising a different member, each member comprising a recombinant virus having a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element; b. culturing said eukaryotic host cells under conditions where said exogenous protein is expressed; and c. identifying a member comprising a recombinant nucleic acid encoding an exogenous protein having the property of interest. 39. The method of Claim 38, wherein the property is a predetermined cellular localization. 40. The method of Claim 39, wherein the predetermined cellular localization is the extracellular space. 41. The method of Claim 39, wherein the predetermined cellular localization is the cell membrane. 42. The method of Claim 39, wherein the predetermined cellular localization is selected from the group consisting of nucleus, lysosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxysomes, endosomes and cytoplasm. 43. The method of Claim 38, wherein the property is a binding affinity to a binding partner. 44. The method of Claim 38, wherein the property is an enzymatic activity. 45. The method of Claim 38, wherein the property is a structural property. 46. The method of Claim 38, wherein the eukaryotic host cells are cultured under conditions where said exogenous protein is expressed while expression of endogenous proteins of the eukaryotic host cell is suppressed. 47. The method of Claim 38, wherein said exogenous protein is labelled while expression of said endogenous proteins is suppressed. 48. The method of Claim 47, wherein said exogenous protein is labelled with a marker selected from the group consisting of radioactive marker and spectroscopically detectable marker. 49. The method of claim 48, wherein said radioactive marker is selected from the group consisting of radioactive amino acids and radioactive sugars. 50. The method of Claim 38, wherein each of said members is expressed in an individual eukaryotic host cell. 51. The method of Claim 38, wherein said virus suppresses expression of endogenous proteins. 52. The method of Claim 38, wherein said virus is an alpha virus. 53. The method of Claim 52, wherein said Alpha Virus is selected from the group consisting of Sindbis Virus, Semliki Forest Virus, and Venezuelan Equine Encephalitis Virus. 54. The method of Claim 38, wherein said virus further directs the generation of viral particles. 55. The method of Claim 54, further comprising the step of separating said viral particles from said exogenous protein. 56. The method of Claim 55, wherein said viral particles are separated from said exogenous protein through filtration, chromatography or precipitation. 57. The method of Claim 51 , wherein said virus lacks at least one function required for propagation of said virus. 58. The method of Claim 57, wherein the function required for propagation of said virus is provided through a helper function. 59. The method of Claim 58, wherein said helper function is provided by a viral packaging cell line or a helper virus particle. 60. The method of Claim 38, wherein said virus is capable of directing expression of said nucleic acid encoding an exogenous protein while expression of endogenous proteins is suppressed. 61. The method of Claim 60, wherein expression of said endogenous proteins is suppressed by inhibiting transcription or translation of nucleic acids encoding endogenous proteins. 62. The method of Claim 54, wherein said viral particles produce solubilized viral protein, and wherein production of said solubilized viral protein is reduced. 63. The method of Claim 62, wherein generation of solubilized protein of said virus is reduced through use of a protease inhibitor. 64. The method of Claim 62, wherein generation of solubilized protein of said virus is reduced through use of a mutant virus strain. 65. The method of Claim 64, wherein said mutant virus strain is selected from the group consisting of the temperature sensitive Sindis mutants ts20, tslO, and ts23. 66. The method of Claim 62, wherein generation of solubilized protein of said virus is reduced through use of a cell line deficient in cleavage of a protein of said virus. 67. The method of Claim 66, wherein said cell line is deficient in cleavage of protein PE2 to E2 and E3. 68. The method of Claim 38, wherein said nucleic acid encoding an exogenous protein is derived from a nucleic acid library comprising a plurality of distinct nucleic acids encoding polypeptides. 69. The method of Claim 38, wherein said plurality of members is screened using physical separation of said eukaryotic host cells each carrying one individual member of said plurality of members. 70. The method of Claim 69, wherein said physical separation is achieved by separating said eukaryotic host cells in a semi-solid medium. 71. The method of claim 69, wherein said physical separation is achieved by placing eukaryotic host cells in separate compartments, wherein each compartment contains host cells carrying identical expression systems of said plurality of expression systems. 72. The method of Claim 38, wherein said recombinant virus is capable of self- replication. 73. A method for generating a genetic expression library encoding proteins having a predetermined property of interest comprising: a. providing a composition comprising a plurality of eukaryotic host cells, wherein each host cell has an expression system comprising a different member, each member comprising a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element; b. culturing said eukaryotic host cells under conditions where said exogenous protein is expressed while expression of endogenous proteins of said eukaryotic host cell is suppressed; and c. identifying the members comprising recombinant nucleic acids encoding exogenous proteins having the property of interest. 74. The method of Claim 73, wherein said nucleic acid encoding an exogenous protein is derived from a nucleic acid library comprising a plurality of nucleic acids encoding a distinct polypeptides. 75. A genetic expression library comprising a plurality of expression systems encoding proteins located in a predetermined cellular localization, produced by the method of Claim 73. 76. The expression library of Claim 75, wherein the proteins are secreted proteins. 77. The expression library of Claim 75, wherein the proteins are cell membrane associated receptors. 78. The expression library of Claim 75, wherein the cellular localization is selected from the group consisting of nucleus, lysosome, and mitochondria. 79. A library of exogenous proteins produced using the method of claim 73. 80. A nucleic acid library comprising a population of eukaryotic expression systems having a plurality of different members, each member comprising a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element, wherein the control element directs the expression of the exogenous proteins while expression of endogenous proteins of said eukaryotic host cells are suppressed. 81. A method for generating a genetic expression library encoding proteins having a predetermined property of interest comprising: a. providing a composition comprising a plurality of eukaryotic host cells, wherein each host cell has an expression system comprising a different member, each member comprising a recombinant virus having a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element; b. culturing said eukaryotic host cells under conditions where said exogenous protein is expressed; and c. identifying members comprising recombinant nucleic acids encoding exogenous proteins having the property of interest. 82. The method of Claim 81 , wherein said nucleic acid encoding an exogenous protein is derived from a nucleic acid library comprising a plurality of nucleic acids encoding a distinct polypeptides. 83. A genetic expression library comprising a plurality of expression systems encoding proteins located in a predetermined cellular localization, produced by the method of Claim 81. 84. The expression library of Claim 83, wherein the proteins are secreted proteins. 85. The expression library of Claim 83, wherein the proteins are cell membrane associated receptors. 86. The expression library of Claim 83, wherein the cellular localization is selected from the group consisting of nucleus, lysosome, and mitochondria. 87. A library of exogenous proteins produced using the method of Claim 81. 88. A nucleic acid library comprising a population of eukaryotic expression systems having a plurality of different members, each member comprising a recombinant virus having a recombinant nucleic acid encoding an exogenous protein operatively linked to a control element.

Inventors: BAILEY JAMES E, KOLLER DANIEL, ORBERGER GEORG H, RENNER WOLFGANG A, BAILEY, JAMES, E., KOLLER, DANIEL, ORBERGER, GEORG, H., RENNER, WOLFGANG, A.
Applicants: BAILEY JAMES E, CYTOS BIOTECHNOLOGY GMBH, KOLLER DANIEL, ORBERGER GEORG H, RENNER WOLFGANG A, BAILEY, JAMES, E., KOLLER, DANIEL, ORBERGER, GEORG, H., RENNER, WOLFGANG, A.
Application year: 1998
Main classification: C12Q1
IPC codes: C12N15, C12Q1