IDENTIFICATION AND USE OF ANTIVIRAL COMPOUNDS

Abstract: The present invention relates to the identification of host cell proteins that interact with viral proteins required for virus replication, and high throughput assays to identify compounds that interfere with the specific interaction between the viral and host cell protein. Interfering compounds that inhibit viral replication can be used therapeutically to treat viral infection. The invention is based, in part, on the Applicants' discovery of novel interactions between proteins of the influenza virus and human host cell proteins. One of these host cell proteins, referred to herein as NPI-1, interacts with influenza virus protein NP, and may be an accessory protein required for replication of influenza virus. Another of these host cell proteins, referred to herein as NS1I-1, interacts with influenza virus protein NS1. Compounds that interfere with the binding of the host cell and viral proteins, and inhibit viral replication can be useful for treating viral infection in vivo.

Description: IDENTIFICATION AND USE OF ANTIVIRAL COMPOUNDS This application is a continuation-in-part of co- pending application Serial No. 08/246,583, filed May 20, 1994, which is hereby incorporated by reference in its entirety. 1. INTRODUCTION The present invention relates to the identification of new cellular targets for viral intervention, the identification of antiviral compounds that act on the new targets, and the therapeutic use of such antiviral compounds. 2. BACKGROUND OF THE INVENTION Influenza A virus is a negative strand RNA virus belonging to the orthomyxovirus family. The genome of the virus consists of 8 segments and encodes 10 polypeptides. Experimental evidence generated in the laboratory of Scholtissek indicates that the nucleoprotein (NP) is a major determinant of species specificity of influenza viruses (Scholtissek, et al., 1985, Virology 147: 287-294). Phylogenetic analysis divides NP genes into two families: one containing NPs predominantly of avian origin, and one containing those of human origin (Bean, 1984, Virology 133:438- 442; Buckler-White & Murphy, 1986, Virology 155: 345- 355; Gam elin, et al., 1989, Virology 170:71-80; Scholtissek, et al., 1985, supra) . The human virus A/HK/1/68 and viruses having genetically related NPs cannot rescue mutants of the avian virus A/FPV/Rostock/1/34 with ts defects in the NP following double infection of chicken embryo fibroblasts (CEF) at 40 β C (Scholtissek, et al., 1985, supra; Scholtissek, et al., 1978, Virology 91: 79-85). However, the human viruses which failed to rescue the ts mutants on CEF cells were able to do so on Madin- Darby canine kidney(MDCK) cells (Scholtissek, et al., 1978, supra) . Additionally, A/HK/1/68 virus and A/FPV/Rostock/1/34 virus reassortants containing the A/HK/1/68 virus-derived NP replicate in MDBK cells but not in CEFs (Scholtissek, et al., 1978, supra) . The host-specific rescue of FPV ts mutants and the host restriction of A/HK/1/68 virus reassortants suggest that a factor(s) of host origin, which differs between mammalian and avian cells, is responsible for this phenomenon, and that this factor may interact with the influenza A virus NP. However, heretofore, no host protein has been identified. Replication and transcription of influenza virus RNA requires four virus encoded proteins: the NP and the three components of the viral RNA-dependent RNA polymerase, PB1, PB2 and PA (Huang, et al., 1990, J. Virol. 64: 5669-5673). The NP is the major structural component of the virion which interacts with genomic RNA, and is required for antitermination during RNA synthesis (Beaton & Krug, 1986, Proc. Natl. Acad. Sci. USA 83:6282-6286). NP is also required for elongation of RNA chains (Shapiro & Krug, 1988, J. Virol. 62: 2285-2290) but not for initiation (Honda, et al., 1988, J. Bioche . 104: 1021-1026). NS1 is a major non-structural protein expressed by influenza A viruses in infected cells, whose role in infection is not clear. Studies of viruses carrying temperature-sensitive NS1 alleles point to a regulatory role for NS1 in viral gene- expression and/or replication (Wolstenholme, et al., 1980, J. Virol. 35:1-7; Koennecke, et al. , 1981, Virol. 110:16-25; Hatada, et al., 1990, J. Gen. Virol. 71: 1283-1292) , which is also consistent with its preferentially nuclear accumulation (Greenspan, et al., 1988, J. Virol. 62: 3020-3026). Its expression has been shown to interfere with cellular functions in a variety of ways. (Fortes, et al., 1994, EMBO J. 13: 704-712; Qiu & Krug, 1994, J. Virol. 68: 2425-2432; Lu, et al., 1994, Genes Dev. 8: 1817-1828). These effects have been suggested to be mediated through NSl's observed interactions with a variety of RNA's, including single- and double-stranded influenza vRNA (Hatada & Fukuda, 1992, J. Gen. Virol. 73: 3325-3329; Hatada, et al., 1992, J. Gen Virol. 73: 17-25), poly- adenosine RNA (Qiu & Krug, 1994, supra) , and spliceosomal U6 RNA (Lu, et al., 1994, supra) . Despite these studies involving the interaction of NS1 with various RNAs, no host proteins that interact with NS1 during infection have previously been identified or characterized. Little is known about host cell functions which contribute to the intracellular replication of influenza viruses, and cellular factors have not been characterized which directly interact with the viral proteins, much less cellular factor/viral interactions that can be used as targets for therapeutic intervention. 3. SUMMARY OF THE INVENTION The present invention relates to the identification of host cell proteins that interact with viral proteins required for virus replication, and high throughput assays to identify compounds that interfere with the specific interaction between the viral and host cell protein. Interfering compounds that inhibit viral replication can be used therapeutically to treat viral infection. The invention is based, in part, on the Applicants' discovery of a novel interaction between influenza viral proteins, such as NP and NS1, and human host cell proteins referred to herein as NPI-1, NPI-2, NPI-3, NPI-4, NPI-5, NPI-6, and NS1I-1, respectively. The host cell proteins such as NPI-1 and NS1I-1 may be accessory proteins required for replication of influenza virus. Compounds that interfere with the binding of the host cell and viral proteins and inhibit viral replication can be useful for treating viral infection .in vivo. 4. DESCRIPTION OF THE FIGURES FIG. 1A and IB: The interactive trap system, as used in the identification of NP-interacting proteins. FIG. 1A: R100 contains the reporter gene LexAop-LEU2 and a transcriptionally inactive LexA-NP fusion protein (left) . Library proteins are synthesized in R100 transformants in media containing galactose. If the library protein does not interact with the LexA-NP fusion protein, then the LEU2 gene is not transcribed (middle) . If the library protein does interact with the LexA-NP fusion protein, then the LEU2 gene is transcriptionally active, and the cell forms a colony on leu' medium (right) . FIG. IB: The pLexA-NP bait plasmid used in the interactive trap. The coding region of influenza A/PR/8/34 virus nucleoprotein (NP) was subcloned into the EcoRl and Sal I restriction sites of pEG202. This construction encodes a fusion protein which includes 202 amino acids of LexA and the entire coding region of NP (498 amino acids) separated by 3 amino acids encoded by polylinker sequences derived from the cloning process. FIG. 2. Nucleotide sequence of NPI-1 cDNA and deduced protein sequence. The coding sequence starts at nucleotide 1 and ends at nucleotide 1581. The 5' terminus of the library clone is indicated by an asterisk. Regions complementary to nested reverse transcription and 5'RACE primers are underlined. FIG. 3. Comparison of NPI-1 and SRPl. Vertical lines indicate identity; colons and periods indicate conservative changes (Deveraux et al., 1984, Nucl. Acids Res. 12: 387-395). 42 amino acid ARM repeats are aligned vertically according to Peifer et al., 1994, Cell 76: 789-791. For a complete comparison of SRPl to other ARM repeat containing proteins, see Peifer et al., 1994, supra . The ARM consensus sequence is indicated at the bottom; "+" indicates K,R, or H; "-" indicates D or E; M ~" indicates a gap. Since other residues are conserved within the repeats of NPI-1 and SRPl, a consensus sequence derived from only these two proteins is also shown. FIG. 4. GST-NPI-1 binds to NP in vitro. GST (lanes 1,5,6) and GST-NPI-1 (lanes 2,3,7,8) were expressed in bacteria and precipitated from cell lysates on glutathione agarose beads. The complexed beads were then incubated with partially purified influenza virus NP and polymerase preparations (Pol/NP) as indicated. Precipitated proteins were fractionated on a 12.5% SDS polyacrylamide gel, and either stained with Coomassie blue (lanes 1 to 3) , or immunoblotted using the monoclonal antibody HT103 directed against the viral nucleoprotein (lanes 4 to 8). Unprecipitated Pol/NP was separated in lane 4. M, protein molecular weight markers; *, GST-NPI-1 fusion protein; arrows indicate major fusion protein degradation products. FIG 5. Immunoblot of total cellular proteins using polyclonal rabbit sera against NPI-1. Total cell lysates and cytoplasmic cell extracts from HeLa and MDBK cell lines were separated by SDS-PAGE, transferred to nitrocellulose, immunoblotted with anti-NPI-1 sera, and developed by 125 I-protein A. Each lane contains protein from 1 x 10 5 cells. FIG 6. NP is co-iirumunoprecipitated from influenza A virus infected cells by antisera against NPI-1. Infected HeLa cell proteins were labeled with 35 S-methionine and 35 S-cysteine, and total cell lysates were made as described in the text. Complexes of NPI-1 and NP were precipitated using anti-NPI-1 sera. Precipitated proteins were then fractionated by SDS-PAGE and detected by autoradiography. FIGS 7-11. Partial DNA sequences of isolated coding regions of five different proteins that interact with the NP of influenza A, as detected using the interactive trap system in yeast. The proteins whose sequences are provided are as follows: FIG. 7: Partial nucleotide sequence of NPI-2. FIG. 8: Partial nucleotide sequence of NPI-3. FIG. 9: Partial nucleotide sequence of NPI-4. FIG. 10: Partial nucleotide sequence of NPI-5. FIG. 11: Partial nucleotide sequence of NPI-6. Fig. 12. Nucleotide sequence of the NS1I-1 gene and the encoded amino acid sequence of the NS1I-1 protein. The sequence of 2572 bp was determined by dideoxy sequencing of two overlapping clones. The first clone, pK5, was isolated from the yeast library and contains the HeLa cell cDNA comprising nucleotide positions 791 to 2572. The second clone, pRACENSlI-1, resulted from the 5'RACE procedure used to obtain cDNA derived from the 5'-end of NS1I-1 mRNA, and comprises nucleotide positions 1 to 944. Fig. 13. Northern blot analysis of HeLa cell poly(A)-RNA using an NSlI-1-specific probe. Fig. 14. Co-precipitation of NS1 protein from extracts of A/WSN/33-infected MDCK cells by GST-NS1I-1 and glutathione sepharose. Monolayers of MDCK cells were either infected with influenza A/WSN/33 virus at an m.o.i. of 10 or mock-infected, and labeled with 35 S- methionine and cysteine from 5 to 6 hours p.i. Proteins were extracted and used for binding to glutathione sepharose coated with GST-NS1I-1 (lanes 3 and 8) or GST-protein (lane 6) . As controls, extracts were immunoprecipitated with α-NSl (lane 2) , α-Ml (lane 4) , or non-immune serum (lane 5) . Proteins were analyzed by SDS gel electrophoresis and fluorography. Aliquots of the total extracts corresponding to 10% used for the glutathione agarose precipitations are shown (lanes 1 and 7) . The positions of virus proteins and molecular weight markers are indicated to the left. Fig. 15. GST-NS1I-1 co-precipitates NSl proteins of influenza A and B virus strains. Extracts of 35 S-labeled MDCK cells infected with the influenza viruses A/duck/Alberta/76 (Panel A) , A/turkey/Oregon (Panel B) , A/Beijing/32/92 (Panel C) , A/Berkeley/1/68 (Panel D) , and B/Lee/40 (Panel E) were prepared and used in precipitations of viral proteins by glutathione-sepharose coated with GST-NS1I-1 (lanes "GST-K5") or GST-protein (lanes "GST") as described in Fig. 14. In addition, viral proteins were immunoprecipitated using α-NSl-, α-Ml- or non-immune serum (lanes "α-NSl", "α-Ml", "NI", respectively). Analysis was by SDS gel electrophoresis and fluorography. Aliquots of the total extracts corresponding to 10% (Panels C and E) or 6.7% (Panels A, B, and D) , respectively, are also shown (lanes "T") . The positions of viral proteins are indicated to the right. 5. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the identification of host cellular proteins that interact with viral proteins important to viral replication and infection; the identification of compounds that interfere with the specific interaction of the host cell and viral proteins; and the evaluation and use of such compounds as antivirals in the treatment of viral infections in animals, including humans. The invention is described in this section and in the examples, below for the identification and inhibition of interactions between human host cell proteins and influenza viral proteins. For clarity of discussion, particular detail is provided for the isolation of two particular host cell proteins. The first such protein is nucleoprotein interactor 1 (NPI-1) , a human cell protein that interacts with the influenza virus NP protein. The NPI-1 gene and protein, and the protein's interaction with NP protein are described in detail in the example in Section 6, below. Other host cell proteins which interact with the NP protein include, but are not limited to, NPI-2, NPI-3, NPI-4, NPI-5, and NPI-6, and are also described, below. Since the interactions between NP and the NPI-1 through NPI-6 host cell proteins have never before been identified, they provide novel targets for antiviral treatment and serve as excellent models for detailing the aspects of the invention. However, the principles may be analogously applied to the identification of other host cell proteins that interact with any of the four influenza virus proteins (PA, PB1, PB2, in addition to NP) required for viral RNA replication. Particular detail is also provided in the example in Section 7, below, for the identification of nonstructural protein 1 interactor 1 (NS1I-1) . NS1I-1 is a human cell protein that interacts with the influenza virus NS1 protein. This interaction also has never before been described, and, therefore, provides yet another novel target for antiviral treatment. The present invention also contemplates identifying interactions between host cell proteins and other viral proteins (in addition to NS,) required for infection, such as, in the case of influenza virus, NS 2 HA, NA, M,, and M 2 proteins. The principles may also be analogously applied to other RNA viruses, including but not limited to paramyxoviruses, such as parainfluenza viruses, measles viruses, respiratory syncytial virus, bunyviruses, arena viruses, the orthomyxo-like insect virus called Dhori, etc. The host cell proteins so identified may include completely novel proteins, or previously described proteins that have not yet been shown to interact with viral proteins. Any method suitable for detecting protein-protein interactions may be employed for identifying novel viral-host protein interactions, and are considered within the scope of the present invention. For example, some traditional methods are co-immunoprecipitation, crosslinking and copurification through gradients or chromatographic columns. Newer methods result in the simultaneous identification of the genes coding for the protein interacting with a target protein. These methods include probing expression libraries with labeled target protein in a manner similar to antibody probing of λgtll libraries. One such method which detects protein interactions in vivo, the yeast interactive trap system, was successfully used as described herein to identify the host cell proteins NPI-1 through NPI-6, and NS1I-1, described herein, and is described in detail for illustration only and not by way of limitation. The host cell/viral protein interactions identified are considered targets for antiviral intervention. Assays, such as the ones described herein, can be used to identify compounds that interfere with such interactions. The compounds so identified which inhibit virus infection, replication, assembly, or release can be used as an ivirals. In accordance with the invention, a given compound found to inhibit one virus may be tested for antiviral activity against a wide range of different viruses that have analogous dependencies on host cell proteins, including but not limited to paramyxoviruses, such as parainfluenza viruses, measles viruses, respiratory syncytial virus, bunyviruses, arena viruses, the orthomyxo-like insect virus called Dhori, etc. Elucidation of the roles of the interacting proteins will lead to identifying other viruses as targets for intervention. For example, we have found that NPI-1 is important to the import of viral nucleic acid-protein complex into the nucleus of the host cell. Therefore, methods described below that disrupt this process, through interfering with the activity of NPI-1, for example, may be effective in treating viruses with nuclear phases, in addition to those viruses listed above. Such additional viruses include, but are not limited to, human immunodeficiency virus (HIV) , members of the herpes virus family, and adenoviruses. The various aspects of the invention are described in the subsections below with specific reference to host cell proteins that interact with NP (NPI-1 through NPI-6) and NSl (NS1I-1) , with particular emphasis on NPI-1; however, the invention is not limited to NPI-1 and encompasses any viral/host cell protein interactions as targets for therapeutic intervention. 5.1. IDENTIFICATION OF HOST CELL PROTEINS THAT INTERACT WITH VIRAL PROTEINS REQUIRED FOR REPLICATION The previously unidentified gene for the host cell protein NPI-1 was cloned based on its ability to interact with the influenza A virus NP. The NPI-1 is the human homolog of the yeast protein SRPl. Interaction of NPI-1 and NP was demonstrated in yeast by the interactive trap system; in vitro coprecipitation of the NP with a bacterially expressed NPI-1 protein; and in infected cell extracts by coprecipitation of the NP with NPI-1, using anti-NPI-1 sera. The demonstration of this previously unknown interaction is illustrated in the working examples (see Section 6, infra) . The data generated indicate that NPI-1 plays a role in the replication of influenza A viruses. NPI-1 is the first cellular protein characterized which interacts with a protein encoded by influenza viruses. This role, therefore, makes the inhibition of the NP-NPI-1 interaction an excellent target for antiviral therapy. It has not yet been demonstrated at what stage in the replication cycle NPI-1 functions. The NPI-1 could affect any of a number of NP functions which may include: (1) movement of the ribonucleoprotein complex (RNP) to the nucleus during viral entry; (2) vRNA synthesis, including antitermination and elongation; (3) mRNA synthesis, including elongation, polyadenylation, and transport to the cytoplasm; and (4) exit of the RNP from the nucleus during virion assembly. The fact that both NPI-1 and SRPl interact with proteins involved in RNA synthesis implies that there may be fundamental similarities between cellular DNA-dependent transcription and influenza viral RNA-dependent RNA synthesis. Cellular factors, like NPI-1, may be shared by the viral and the cellular RNA synthesis machinery to perform similar functions. In addition, the NPI-1 may tether the viral RNP to areas of the nuclear matrix where splicing and polyadenylation of mRNA occur. It should be noted that although NPI-l was isolated from HeLa cells, this cell line is not productively infected by influenza A virus. However, HeLa cells synthesize influenza viral RNAs and proteins (see Fig. 6, lane 3) , and have previously been used to examine viral RNA synthesis (Beaton & Krug, 1986, supra) . The viral NP exists in two forms in the infected cell. One form is associated with ribonucleoprotein complexes (RNP) , and the other is a free form (Shapiro & Krug, 1988, supra) . Pol/NP preparations used in coprecipitation experiments with NPI-l were purified over cesium chloride/glycerol gradients (Honda et al., 1988, supra) , which dissociate and purify virion proteins away from vRNA. The NP but not the polymerase proteins were detected on Coomassie stained gels in this experiment (Fig 4, lane 3); however, coprecipitation of the viral polymerase proteins was not rigorously tested by immunoblot experiments. Only the NP was coprecipitated from infected HeLa cell extracts (Fig. 6) suggesting that it is free NP which is bound by NPI-l. Only one host factor has been assigned a definitive function in the replication process of a negative strand RNA virus. The cellular casein kinase II has been shown to phosphorylate the phosphoprotein P of the vesicular stomatitis virus (VSV) RNA- dependent RNA polymerase. This is a step which appears to be required in order to activate the viral polymerase (Barik and Banerjee, 1992, Proc. Natl. Acad. Sci. USA 89: 6570-6574; Barik and Banerjee, 1992, J. Virol. 66: 1109-1118). NPI-l and SRPl are 50% identical and 81% conserved at the amino acid level. This is a very high degree of conservation between proteins belonging to organisms as distantly related as humans and yeast, and suggests that the NPI-l/SRPl performs a very basic function in the cell. NPI-l and SRPl hkve eight internal repeats, each of approximately 42-amino acids (Fig. 3) . This repeat, termed the ARM motif, was originally identified in the Drosophila segment polarity gene armadillo (Riggleman, et al., 1989, Genes Dev. 3: 96-113), and it has been identified in a number of other proteins including j8-catenin, plakoglobin, pl20, APC and smGDS (Peifer et al., 1994, supra , and references therein) . Several ARM proteins are associated with cell adhesion structures. Armadillo and its homologues bind to the C-terminal cytoplasmic tail of cadherins, a calcium-dependent class of cell adhesion molecules(CAMs) , linking the CAMs to the underlying cytoskeleton at cell-cell junctions (McCrea, et al., 1991, Science 254: 1359- 1361) . In contrast to the armadillo protein, SRPl and NPI-l appear to be localized to the nucleus. If NPI-l, like SRPl (Yano, et al., 1992, Mol. Cell. Biol. 12: 5640-5651), is associated with the nuclear membrane, it is possible that NPI-l functions to tether viral RNP to the nuclear membranes (Jackson, et al., 1982, Nature 296: 366-368). It should be noted that NPI-l may be related to (or identical with) a nuclear protein that has been found to be involved in V(D)J recombination (Cuomo et al., 1994, Meeting abstract F015, Keystone Symposium on Recombination) . The carboxyl terminal 265 amino acids of the NPI-l, which were sufficient for interaction with the viral NP, contain four and one-half ARM repeats. Individual repeats, in general, are approximately 30 % identical with the ARM consensus sequence. This is consistent with the degree of conservation in ARM repeats of other proteins (Peifer et al., 1994, supra) . Using the same interactive trap system in yeast, five additional DNA sequences were isolated which partially encode proteins that interact with the NP of influenza A virus. Also, using this system, a DNA sequence encoding the NS1I-1 protein was identified based the interaction between NS1I-1 and the NS1 protein of influenza A virus. This protein is the human homolog of porcine 173-estradiol dehydrogenase. Several proteins with a dehydrogenase function have recently been shown to be involved in post- transcriptional events of gene expression (Hentze, 1994, Trends Biochem. Sci. 19: 101-103). This supports an important functional role for the NS1I-1 interaction during the viral life cycle. The various proteins so identified are listed in Table I. TABLE I INTERACTING HOST CELL PROTEINS Host Cell Proteins FIG. Comments NPI-l FIG. 2 New protein sequence, homologous to SRPl of Yeast NPI-2 FIG. 7 Identical to sequences of hnRNP C proteins (Lahiri & Thomas, 1986, Nucl. Acids Res. 14: 4077-4094) New protein sequence New protein sequence New protein sequence New protein sequence New protein sequence, homologous to porcine 17β-estradiol dehydrogenase No * * : **"-o*l laifmuu-l _____--__-_• of fi-wi-h-mlr lave fevnled Ihu wii-ofi n to App-kaπU' ideβificalit-o of NPI-3. NPI- , ■___. NPI-5, ih-ae wcφtm n •n ■ * __.. liln i) try other tra-po ■nd d_ait__u__d Rdil, PC4, •___. BATI, te-φecϋvely. The coding sequence for NPI-2 is identical to sequences coding for the previously identified hnRNP C proteins (Lahiri & Thomas, 1986, supra) . The NPI-3, NPI-4, NPI-5, and NPI-6 coding sequences were unknown prior their identification by Applicants. The NS1I-1 gene is also novel, as explained in detail in the example in Section 7, below. The invention contemplates, in addition to the DNA sequences disclosed herein, 1) any DNA sequence that encodes the same amino acid sequence as encoded by the DNA sequences shown in Figures 2 and 7-12; 2) any DNA sequence that hybridizes to the complement of the coding sequences disclosed herein (see Figs. 2 and 7-12) under highly stringent conditions, e.g.. washing in O.lxSSC/0.1% SDS at 68°C (Ausubel, F.M. et al., eds., 1989, Current Protocols in Molecular Biology, Vol. I, Green Publishing Associates, Inc., and John Wiley & sons. Inc., New York, at p. 2.10.3) and encodes a functionally equivalent gene product; and/or 3) any DNA sequence that hybridizes to the complement of the coding sequences disclosed herein (see Figs. 2 and 7-12) under less stringent conditions, such as moderately stringent conditions, e.g.. washing in 0.2XSSC/0.1% SDS at 42°C (Ausubel, et al. , 1989, supra) , yet which still encodes a functionally equivalent gene product. The invention also encompasses 1) DNA vectors that contain any of the coding sequences disclosed herein (see Figs. 2 and 7-12) , and/or their complements (i.e., antisense) ; 2) DNA expression vectors that contain any of the coding sequences disclosed herein (see Figs. 2 and 7-12) , and/or their complements (i.e., antisense), operatively associated with a regulatory element that directs the expression of the coding and/or antisense sequences; and 3) genetically engineered host cells that contain any of the coding sequences disclosed herein (see Figs. 2 and 7-12), and/or their complements (i.e., antisense), operatively associated with a regulatory element that directs the expression of the coding and/or antisense sequences in the host cell. Regulatory element includes but is not limited to inducible and non- inducible promoters, enhancers, operators and other elements known to those skilled in the art that drive and regulate expression. The invention includes fragments of any of the DNA sequences disclosed herein. Once the host cell proteins are obtained, they can be used to detect interactions with proteins from other viruses, in accordance with the invention. The following description is provided to illustrate this approach and not by way of limitation. Influenza B virus ribonucleoprotein complex was isolated and using a Western immunoblot assay, it was found that the cellular NPI-l was associated with this complex. This result indicates that NPI-l, isolated based on its interaction with influenza A virus NP, also interacts with influenza B virus NP. Thus, compounds that inhibit NP-NPI-1 interactions in influenza A virus and thereby inhibit influenza A viral infection should be similarly effective as antivirals against influenza B virus. Host cell genes that are homologous to those identified herein may be obtained by several methods. In some cases, different host cell proteins that share the property of interacting with the same viral protein, e.g. influenza A virus NP, may also share genetic homology. Thus, the genes identified through the interactive trap selection may be homologous to one another. Once a host cell gene is identified in accordance with the invention, any homologous gene may be obtained using cloning methods well known to those skilled in the art, including but not limited to the use of appropriate probes to detect the homologous genes within an appropriate cDNA or gDNA (genomic DNA) library. (See, for example, Sa brook et al., 1989, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratories, which is incorporated by reference herein its entirety. This method is especially useful for obtaining proteins that may not share the property of binding to the same viral protein, but may nonetheless be genetically homologous. Such homologous proteins may interact with proteins of viruses other than the virus used in the interactive trap. For example, a host cell gene whose product was detected through its interaction with an influenza A viral protein may be homologous to another gene whose product does not interact with influenza A virus, but which does interact with influenza B viral protein. To optimize the detection of such a homologous gene, cDNA libraries may be constructed from cells infected with a virus of interest. Besides influenza B virus, this procedure may be applied analogously to other viruses as well, including but not limited to paramyxoviruses, such as parainfluenza viruses, measles viruses, respiratory syncytial virus, bunyviruses, arena viruses, the orthomyxo-like insect virus called Dhori, as well as human immunodeficiency virus (HIV) , members of the herpes virus family, and adenoviruses. 5.2. SCREENING ASSAYS FOR COMPOUNDS THAT INTERFERE WITH THE INTERACTION OF HOST CELL AND VIRAL PROTEINS REQUIRED FOR VIRAL REPLICATION The host cell protein and the viral protein which interact and bind are sometimes referred to herein as "binding partners". This term also includes peptide fragments, produced as described in the subsections below, comprising the binding domain of each respective protein. Any of a number of assay systems may be utilized to test compounds for their ability to interfere with the interaction of the binding partners. However, rapid high throughput assays for screening large numbers of compounds, including but not limited to ligands (natural or synthetic) , peptides, or small organic molecules are preferred. Compounds that are so identified to interfere with the interaction of the binding partners should be further evaluated for antiviral activity in cell based assays, animal model systems and in patients as described herein. The basic principle of the assay systems used to identify compounds that interfere with the interaction between the viral and host cell proteins involves preparing a reaction mixture containing the viral protein and the host cell protein under conditions and for a time sufficient to allow the two proteins to interact and bind, thus forming a complex. In order to test a compound for inhibitory activity, the reaction is conducted in the presence and absence of the test compound, i.e.. the test compound may be initially included in the reaction mixture, or added at a time subsequent to the addition of the viral and host cell protein; controls are incubated without the test compound or with a placebo. The formation of any complexes between the viral protein and the host cell protein is then detected. The formation of a complex in the control reaction, but not in the reaction mixture containing the test compound indicates that the compound interferes with the interaction of the viral protein and host cell protein. The assay components and various formats that may be utilized are described in the subsections below. 5.2.1. ASSAY COMPONENTS The host cell protein and viral protein binding partners used as components in the assay may be derived from natural sources, e.g.. purified from cells and virus, respectively, using protein separation techniques well known in the art; produced by recombinant DNA technology using techniques known in the art (see e.g.. Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratories Press, Cold Spring Harbor, N.Y.); and/or chemically synthesized in whole or in part using techniques known in the art; e.g.. peptides can be synthesized by solid phase techniques, cleaved from the resin and purified by preparative high performance liquid chromatography (see, e.g.. Creighton, 1983, Proteins: Structures and Molecular Principles, W.H. Freeman & Co., N.Y., pp. 50-60). The composition of the synthetic peptides may be confirmed by amino acid analysis or sequencing; e.g.. using the Edman degradation procedure (see e.g.. Creighton, 1983, supra at pp. 34-49) . The peptide fragments should be produced to correspond to the binding domains of the respective proteins. Any number of methods routinely practiced in the art can be used to identify and isolate the protein's binding site. These methods include but are not limited to mutagenesis of one of the genes encoding the protein and screening for disruption of binding in a co-immunoprecipitation assay, or mutagenesis of the host cell gene and selecting for resistance to viral infection. Compensating mutations in the viral gene can be selected which allow for viral growth in this mutant host. Sequence analysis of the genes encoding the respective proteins will reveal the mutations that correspond to the region of the protein involved in interactive binding. Alternatively, one protein can be anchored to a solid surface using methods described in section 5.2.2. infra , and allowed to interact with and bind to its labeled binding partner, which has been treated with a proteolytic enzyme, such as trypsin. After washing, a short, labeled peptide comprising the binding domain may remain associated with the solid material, which can be isolated and identified by amino acid sequencing. Also, once the gene for the protein is obtained, short gene segments can be engineered to express peptide fragments of the protein, which can then be tested for binding activity and purified or synthesized. Whether produced by molecular cloning methods or by chemical synthetic methods, the amino acid sequence of the binding partners which may be used in the assays of the invention need not be identical to the reported sequence of the genes encoding them. The binding partners may comprise altered sequences in which amino acid residues are deleted, added, or substituted resulting in a functionally equivalent product. For example, functionally equivalent amino acid residues may be substituted for residues within the sequence resulting in a change of sequence. Such substitutes may be selected from other members of the class to which the amino acid belongs; e.g.. the nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan, and methionine; the polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine, and glutamine; the positively charged (basic) amino acids include arginine, lysine, and histidine; the negatively charged (acidic) amino acids include aspartic and glutamic acid. One of the binding partners used in the assay system should be labeled, either directly or indirectly, to facilitate detection of a complex formed between the viral and host cell proteins. Any of a variety of suitable labeling systems may be used including but not limited to radioisotopes such as 125 l; enzyme labelling systems that generate a detectable colorimetric signal or light when exposed to substrate; and fluorescent labels. Where recombinant DNA technology is used to produce the viral and host cell binding partners of the assay it may be advantageous to engineer fusion proteins that can facilitate labeling, immobilization and/or detection. For example, the coding sequence of the viral or host cell protein can be fused to that of a heterologous protein that has enzyme activity or serves as an enzyme substrate in order to facilitate labeling and detection. The fusion constructs should be designed so that the heterologous component of the fusion product does not interfere with binding of the host cell and viral protein. Indirect labeling involves the use of a third protein, such as a labeled antibody, which specifically binds to one of the binding partners, i.e.. either the host cell protein or viral protein used. Such antibodies include but are not limited to polyclonal, monoclonal, chimeric, single chain. Fab fragments and fragments produced by an Fab expression library. For the production of antibodies, various host animals may be immunized by injection with the host cell protein or the viral protein, or a portion thereof. Such host animals may include but are not limited to rabbits, mice, and rats, to name but a few. Various adjuvants may be used to increase the immunological response, depending on the host species, including but not limited to Freund's (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 may be prepared by 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, (Nature, 1975, 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. , 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., 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 No. 4,946,778) can be adapted to produce single chain antibodies specific to one of the binding partners. Antibody fragments which recognize specific epitopes may be generated by known techniques. For example, such fragments include but are not limited to: the 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 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. 5.2.2. ASSAY FORMATS The assay can be conducted in a heterogeneous or homogeneous format. Heterogeneous assays involve anchoring one of the binding partners onto a solid phase and detecting complexes anchored on the solid phase at the end of the reaction. In homogeneous assays, the entire reaction is carried out in a liquid phase. In either approach, the order of addition of reactants can be varied to obtain different information about the compounds being tested. For example, test compounds that interfere with the interaction between the binding partners, e.g.. by competition, can be identified by conducting the reaction in the presence of the test substance; i.e.. by adding the test substance to the reaction mixture prior to or simultaneously with the viral protein and host cell protein. On the other hand, test compounds that disrupt preformed complexes, e.g. compounds with higher binding constants that displace one of the binding partners from the complex, can be tested by adding the test compound to the reaction mixture after complexes have been formed. The various formats are described briefly below. In a heterogeneous assay system, one binding partner, e.g.. either the viral protein or the host cell protein, is anchored onto a solid surface, and its binding partner, which is not anchored, is labeled, either directly or indirectly. In practice, microtiter plates are conveniently utilized. The anchored species may be immobilized by non-covalent or covalent attachments. Non-covalent attachment may be accomplished simply by coating the solid surface with a solution of the protein and drying. Alternatively, an immobilized antibody specific for the protein may be used to anchor the protein to the solid surface. The surfaces may be prepared in advance and stored. In order to conduct the assay, the binding partner of the immobilized species is added to the coated surface with or without the test compound. After the reaction is complete, unreacted components are removed (e.g.. by washing) and any complexes formed will remain immobilized on the solid surface. The detection of complexes anchored on the solid surface can be accomplished in a number of ways. Where the binding partner was pre-labeled, the detection of label immobilized on the surface indicates that complexes were formed. Where the binding partner is not pre-labeled, an indirect label can be used to detect complexes anchored on the surface; e.g.. using a labeled antibody specific for the binding partner (the antibody, in turn, may be directly labeled or indirectly labeled with a labeled anti-Ig antibody) . Depending upon the order of addition of reaction components, test compounds which inhibit complex formation or which disrupt preformed complexes can be detected. Alternatively, the reaction can be conducted in a liquid phase in the presence or absence of the test compound, the reaction products separated from unreacted components, and complexes detected; e.g.. using an immobilized antibody specific for one binding partner to anchor any complexes formed in solution, and a labeled antibody specific for the other binding partner to detect anchored complexes. Again, depending upon the order of addition of reactants to the liquid phase, test compounds which inhibit complex or which disrupt preformed complexes can be identified. In an alternate embodiment of the invention, a homogeneous assay can be used. In this approach, a preformed complex of the host cell and viral protein is prepared in which one of the binding partners is labeled, but the signal generated by the label is quenched due to complex formation (see, e.g.. U.S. Patent No. 4,109,496 by Rubenstein which utilizes this approach for immunoassays) . The addition of a test substance that competes with and displaces one of the binding partners from the preformed complex will result in the generation of a signal above background. In this way, test substances which disrupt the viral protein-host cell protein interaction can be identified. For example, in a particular embodiment for NPI-l, NPI-l can be prepared for immobilization using recombinant DNA techniques described in section 5.2.1., supra . Its coding region can be fused to the glutathione-S-transferase (GST) gene using the fusion vector pGEX-5X-l, in such a manner that its binding activity is maintained in the resulting fusion protein. NP can be purified and used to raise a monoclonal antibody, specific for NP, using methods routinely practiced in the art and described above. This antibody can be labeled with the radioactive isotope 15 I, for example, by methods routinely practiced in the art. In a heterogeneous assay, e.g.. the GST-NPI-1 fusion protein can be anchored to glutathione-agarose beads. NP can then be added in the presence or absence of the test compound in a manner that allows NP to interact with and bind to the NPI-l portion of the fusion protein. After the test compound is added, unbound material can be washed away, and the NP-specific labeled monoclonal antibody can be added to the system and allowed to bind to the complexed binding partners. The interaction between NP and NPI-l can be detected by measuring the amount of radioactivity that remains associated with the glutathione-agarose beads. A successful inhibition of the interaction by the test compound will result in a decrease in measured radioactivity. Alternatively, the GST-NPI-1 fusion protein and NP can be mixed together in liquid in the absence of the solid glutathione-agarose beads. The test compound can be added either during or after the binding partners are allowed to interact. This mixture can then be added to the glutathione-agarose beads and unbound material is washed away. Again the extent of inhibition of the binding partner interaction can be detected by measuring the radioactivity associated with the beads. In another embodiment of the invention, these same techniques can be employed using peptide fragments that correspond to the binding domains of NP and NPI-l, respectively, in place of one or both of the full length proteins. These binding domains can be identified, as described in section 5.2.1., supra . For example, and not by way of limitation, NPI-l can be anchored to a solid material as described above in this section by making a GST-NPI-1 fusion protein and allowing it to bind to glutathione agarose beads. NP can be labeled with a radioactive isotope, such as 35 S, and cleaved with a proteolytic enzyme such as trypsin. Cleavage products can then be added to the anchored GST-NPI-1 fusion protein and allowed to bind. After washing away unbound peptides, labeled bound material, representing the NP binding domain, can be eluted, purified, and analyzed for amino acid sequence by methods described in section 5.2.1., supra . Peptides so identified can be produced synthetically or fused to appropriate facilitative proteins using recombinant DNA technology, as described in section 5.2.1., supra . In accordance with the invention, a given compound found to inhibit one virus may be tested for general antiviral activity against a wide range of different viruses that have analogous dependencies on host cell proteins. For example, and not by way of limitation, a compound which inhibits the interaction of influenza virus NP with NPI-l by binding to the NP binding site can be tested, according to the assays described in section 5.3. infra , against other viruses, particularly those which have similar proteins, e.g.. parainfluenza viruses. 5.3. ASSAYS FOR ANTIVIRAL ACTIVITY Any of the inhibitory compounds which are identified in the foregoing assay systems may be tested for antiviral activity. 5.3.1. VIRAL GROWTH ASSAYS The ability of an inhibitor identified in the foregoing assay systems to prevent viral growth can be assayed by plaque formation or by other indices of viral growth, such as the TCID 50 or growth in the allantois of the chick embryo. In these assays, an appropriate cell line or embryonated eggs are infected with wild-type influenza virus, and the test compound is added to the tissue culture medium either at or after the time of infection. The effect of the test compound is scored by quantitation of viral particle formation as indicated by hemagglutinin (HA) titers measured in the supernatants of infected cells or in the allantoic fluids of infected embryonated eggs; by the presence of viral plaques; or, in cases where a plaque phenotype is not present, by an index such as the TCID 50 or growth in the allantois of the chick embryo, or with a hemagglutination assay. An inhibitor can be scored by the ability of a test compound to depress the HA titer or plaque formation, or to reduce the cytopathic effect in virus-infected cells or the allantois of the chick embryo, or by its ability to reduce viral particle formation as measured in a hemagglutination assay. ■ 5.3.2 ANIMAL MODEL ASSAYS The ability of an inhibitor to prevent replication of influenza virus can be assayed in animal models that are natural or adapted hosts for influenza. Such animals may include mammals such as pigs, ferrets, mice, monkeys, horses, and primates, or birds. As described in detail in Section 5.5 infra , such animal models can be used to determine the LD 50 and the ED 50 in animal subjects, and such data can be used to derive the therapeutic index for the inhibitor of the viral/host cell protein interaction. 5.4. INHIBITORY COMPOUNDS Inhibitory compounds identified in the foregoing screening assays which may be used in accordance with the invention may include but are not limited to small organic molecules, peptides and antibodies. For example, peptides having an amino acid sequence corresponding to the domain of the host cell protein that binds to the viral protein may be used to compete with the native viral protein and, therefore, may be useful as inhibitors in accordance with the invention. Similarly, peptides having an amino acid sequence corresponding to the domain of the viral protein that binds to the host cell protein may be used. Such peptides may be synthesized chemically or produced via recombinant DNA technology using methods well known in the art (e.g.. see Creighton, 1983, supra ; and Sambrook et al., 1989, supra) . Lipofectin or liposomes may be used to deliver the peptides to cells. Alternatively, antibodies that are both specific for the binding domains of either the host cell or viral proteins and interfere with their interaction may be used. Such antibodies may be generated using standard techniques described in Section 5.2.1., supra, against the proteins themselves or against peptides corresponding to the binding domains of the proteins. Such antibodies include but are not limited to polyclonal, monoclonal. Fab fragments, single chain antibodies, chimeric antibodies, etc. Where whole antibodies are used, internalizing antibodies are preferred. However, lipofectin may be used to deliver the antibody or a fragment of the Fab region which binds to the viral or host cell protein epitope into cells. Where fragments of the antibody are used, the smallest inhibitory fragment which binds to the target protein's binding domain is preferred. 5.5. PHARMACEUTICAL PREPARATIONS AND METHODS OF ADMINISTRATION The identified compounds that inhibit viral replication can be administered to a patient at therapeutically effective doses to treat viral infection. A therapeutically effective dose refers to that amount of the compound sufficient to result in amelioration of symptoms of viral infection. Toxicity and therapeutic efficacy of such compounds can be determined by standard pharmaceutical procedures in cell cultures or experimental animals, e.g.. for determining the LD J Q (the dose lethal to 50% of the population) and the ED J0 (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 LDso/ED^. Compounds which exhibit large therapeutic indices are preferred. While compounds that exhibit toxic side effects may be used, care should be taken to design a delivery system that targets such compounds to the site of infection in order to minimize damage to uninfected cells and reduce side effects. The data obtained from the cell culture assays and animal studies can be used in formulating a range of dosage for use in humans. The dosage of such compounds lies preferably within a range of circulating concentrations that include the ED50 with little or no toxicity. The dosage may vary within this range depending upon the dosage form employed and the route of administration utilized. For any compound used in the method of the invention, the therapeutically effective dose can be estimated initially from cell culture assays. A dose may be formulated in animal models to achieve a circulating plasma concentration range that includes the IC50 (i.e.. the concentration of the test compound which achieves a half-maximal infection, or a half-maximal inhibition) as determined in cell culture. Such information can be used to more accurately determine useful doses in humans. Levels in plasma may be measured, for example, by high performance liquid chromatography. Pharmaceutical compositions for use in accordance with the present invention may be formulated in conventional manner using one or more physiologically acceptable carriers or excipients. Thus, the compounds and their physiologically acceptable salts and solvates may be formulated for administration by inhalation or insufflation (either through the mouth or the nose) or oral, buccal, parenteral or rectal administration. For administration by inhalation, the compounds for use according to the present invention are conveniently delivered in the form of an aerosol spray presentation from pressurized packs or a nebuliser, with the use of a suitable propellant, e.g.. dichlorodifluoro ethane, 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 compound and a suitable powder base such as lactose or starch. For oral administration, the pharmaceutical compositions may take the form of, for example, tablets or capsules prepared by conventional means with pharmaceutically acceptable excipients such as binding agents (e.g.. pregelatinised maize starch, polyvinylpyrrolidone or hydroxypropyl methylcellulose) ; fillers (e.g.. lactose, microcrystalline cellulose or calcium hydrogen phosphate) ; lubricants (e.g.. magnesium stearate, talc or silica) ; disintegrants (e.g.. potato starch or sodium starch glycollate) ; or wetting agents (e.g.. sodium lauryl sulphate) . The tablets may be coated by methods well known in the art. Liquid preparations for oral administration may take the form of, for example, solutions, syrups or suspensions, or they may be presented as a dry product for constitution with water or other suitable vehicle before use. Such liquid preparations may be prepared by conventional means with pharmaceutically acceptable additives such as suspending agents (e.g.. sorbitol syrup, cellulose derivatives or hydrogenated edible fats) ; emulsifying agents (e.g.. lecithin or acacia) ; non-aqueous vehicles (e.g.. almond oil, oily esters, ethyl alcohol or fractionated vegetable oils) ; and preservatives (e.g.. methyl or propyl-p-hydroxybenzoates or sorbic acid) . The preparations may also contain buffer salts, flavoring, coloring and sweetening agents as appropriate. Preparations for oral administration may be suitably formulated to give controlled release of the active compound. For buccal administration the compositions may take the form of tablets or lozenges formulated in conventional manner. The 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. Alternatively, the active ingredient may be in powder form for constitution with a suitable vehicle, e.g.. sterile pyrogen-free water, before use. The 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 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 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. 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. 6. EXAMPLE: THE IDENTIFICATION OF NPI-l AND ITS INTERACTION WITH INFLUENZA NUCLEOPROTEIN The yeast interactive trap system was used to identify a cellular protein which interacts with the nucleoprotein of influenza A viruses. This protein, nucleoprotein interactor 1 (NPI-l) is the human homologue of the yeast protein SRPl. SRPl was previously identified as a suppressor of temperature- sensitive RNA polymerase I mutations (Yano, et. al., 1992, Mol. Cell. Biol. 12:5640-5651). A full length cDNA clone of NPI-l was generated from HeLa cell poly A+ RNA. The viral NP, which had been partially purified from influenza A/PR/8/34 virus-infected embryonated eggs, could be coprecipitated from solution by glutathione agarose beads complexed with a bacterially expressed glutathione-S-transferase (GST) - NPI-l fusion protein, confirming the results of the yeast genetic system. Antisera raised against NPI-l identified a 65 kDa polypeptide from total cellular extracts of both HeLa and MDBK cells. In addition, the viral nucleoprotein was co- immunoprecipitated from influenza A/WSN/33 virus-infected HeLa cells by antisera directed against NPI-l, demonstrating an interaction of these two proteins in infected cells, and suggesting that NPI-l plays a role during influenza virus replication. 6.1. MATERIALS AND METHODS 6.1.1. YEAST. BACTERIA AND PLASMIDS Yeast strain EGY48 (Mata trpl ura3 his3 LEU2 : :pLEXAop6-LEU2) (Zervos et al., 1993, Cell 72: 222-232) and plasmids pEG202, pSH18-34, and pRFHMl and the HeLa cell cDNA library constructed in pJG4-5 (Gyuris et al., 1993, Cell 75: 791-803) were previously described. Similar versions of these plasmids and this yeast host strain are available commercially from Clontech as part of a two fusion protein system. pLexA-NP was constructed by subcloning the cDNA of influenza A/PR/8/34 NP as a LexA translational fusion gene into pEG202 (Fig. 1) . Yeast strains constructed as part of these studies are described in Table 2. Escherichia coli MH3 (trpC araD lacX hsdR galϋ galK) and W31005 were previously described (Hall et al., 1984, Celi 36: 1057-1065). 6.1.2. SELECTION OF NP INTERACTORS An interactive trap selection was performed essentially as has been previously described (Gyuris, et al., 1993, supra ; Zervos, et al., 1993, supra) . Strain R100 was transformed by the HeLa cDNA library using the lithium acetate method (Ito, et al., 1983, J. Bacteriol. 153: 163-168). 2 x 10 6 primary yeast transformants were selected on twelve 25 x 25 cm 2 his" trp-glucose plates, pooled and stored at -70°C. Library transformants were selected for leu+ phenotype on hisleu-galactose plates; the efficiency of plating was approximately 10" 4 leu+ colonies per galactose+ colony. Plasmid DNA was isolated from leu-t- library transformants as described by Hoffman and Winston (Hoffman & Winston, 1987, Gene 57: 267-272) and introduced into MH3 cells by electroporation. Library plasmids were selected by plating the transformation mix on lxA+amp+glucose plates (Miller, 1972, Experiments in Molecular Genetics, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY) . cDNAs were analyzed by checking the specificity of interaction with the NP. Each isolated plasmid was introduced into strains R101 and R102. These strains harbor pSH18-34, a reporter plasmid encoding 3-galactosidase with a GAL1 promoter transcriptionally controlled from upstream LexA binding sites. Strain R102 was used as a negative control for NP-specificity of cloned cDNAs. It contains pRFHMl, which encodes LexA fused to a transcriptionally inert fragment of the Drosophila melanogaster bicoid protein. 0-Galactosidase activity was assayed on nitrocellulose replicas of the colonies by freeze fracturing the cells and incubating in buffer containing 5-bromo-4- chloro-3-indolyl-3-D-galactoside (X-gal) (Miller, 1972, supra) . Plasmids which conferred both a leu+ and )8-gal+ phenotypes in the presence of pLexA-NP but not in the presence of pRFHMl were saved for further study. 6.1.3. CLONING OF THE 5' TERMINUS OF NPI-l The 5' terminus of NPI-l was cloned by rapid amplification of cDNA ends ("RACE") by the method of Frohman (Frohman, 1990, in PCR Protocols: A Guide to Methods and Applications, Innis et. al., eds., Academic Press Inc., San Diego, p. 28-38; Frohman, et al., 1988, Proc. Natl. Acad. Sci. USA 85: 8998- 9002) . Reverse transcription of 1 μg of poly A+ HeLa cell RNA was performed using the NPI-l specific oligonucleotide 5'GCAAAGCAGGAGAAACCAC3' . First strand cDNA was tailed with dCTP by terminal transferase. PCR amplification of the reverse transcription product was performed with the nested NPI-l primer 5•GGGTCCATCTGATAGATATGAGAG3' and the 5' RACE anchor primer 5'CUACUACUACUAGGCCACGCGTCGACTACTACGGGIIGGGIIGGGIIG3' (Gibco/BRL) . The PCR product was subcloned into pGEM-T (Promega) and was sequenced by standard protocols. 5'RACE products from three independent experiments were cloned and sequenced in order to avoid errors introduced by PCR. 6.1.4. BACTERIAL EXPRESSION AND PURIFICATION OF GST-NPI-1 The NPI-l cDNA derived from a HeLa cDNA library was subcloned into the EcoRl and Xhol restriction endonuclease sites of the glutathione-S-transferase fusion vector pGEX-5X-l (Pharmacia) to generate the plasmid pGST-NPI-1. Protein was induced from bacterial expression plasmids in W31005 cells with isopropyl-jS-D-galactopyranoside according to standard protocols (Smith & Johnson, 1988, Gene 67: 31-40). Bacteria were pelleted 4 h after induction, washed in ice cold phosphate buffered saline (PBS) , and resuspended in one-tenth culture volume PBS+1% Triton X-100. Bacteria were lysed on ice with four 15 s pulses in a Raytheon sonicator at an output setting of 1 amp. Insoluble material was pelleted at 50,000xg for 30 min in a Beckman TL-100.3 rotor. GST-NPI-1 and GST were purified from bacterial lysates on glutathione-agarose beads(Sigma Chemical Corp.). Beads were swelled according to the manufacturer's instructions and equilibrated in PBS. Typical binding reactions were done in 500 μl of PBS/0.1% Triton X-100, and included 50 μl bacterial lysate and 10 μl of a 50% slurry of glutathione- agarose beads. Binding reactions were incubated for 5 min at room temperature on a rotating wheel. Beads were collected by centrifugation for 5 s in a microfuge, and were washed three times in PBS. 6.1.5. NP BINDING ASSAY To assay binding of NP to GST-NPI-1/bead complexes typical reactions were performed in 500 μl of ice cold PBS+0.05% Nonidet P-40 and contained washed GST-NPl-1/bead complexes and 10 μg partially purified influenza virus polymerase and nucleoprotein preparations (Pol/NP) . Virus was prepared from embryonated eggs infected by influenza A/PR/8/34 virus and POL/NP preparations were purified as previously described (Enami, et al., 1990, Proc. Natl. Acad. Sci. USA 87: 3802-3805; Parvin, et al., 1989, J. Virol. 63: 5142-5152) . NP was bound for 1 h at 4°C on a rotating wheel. Beads were collected by centrifugation for 5 s in a microfuge, and were washed three times in PBS+0.05% NP-40. Washed beads were resuspended in 50 μl SDS sample buffer (Sambrook, et al., 1989, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratories Press, Cold Spring Harbor, NY) , boiled for 5 min, and pelleted in a microfuge. 10 μl of each supernatant was separated by electrophoresis on a 12.5% SDS-polyacrylamide gel. Gels were either stained with Coomassie blue or processed for immunoblot analysis. NP was detected by immunoblotting with the monoclonal antibody HT103. 6.1.6. ANTISERA AND IMMUNOBLOTTING Polyclonal rabbit antisera against NPI-l was generated by immunization of a female NZY Rabbit (Buckshire Farms) with 200 μg of purified GST-NPI-1 in complete Freund's adjuvant, followed by two boosts of 100 μg in incomplete Freund's adjuvant at three week intervals. The specificity of antisera was demonstrated by immunoblot analysis of GST-NPI-1 in bacterial lysates. Immunoblots were performed by standard methods (Harlow and Lane, 1998, Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratories Press, Cold Spring Harbor, NY) . Sera were used at a dilution of 1:1000. 6.1.7. VIRUSES AND CELLS Total cell lysates from HeLa and MDBK cells were generated by direct lysing of cells in SDS-sample buffer, followed by shearing of chromosomal DNA by passage through a 21 ga. syringe. Cytoplasmic extracts were generated by lysing cells in ice cold NP-40 lysis buffer (10 mM Tris-Cl, pH 8.0; 100 mM NaCl; 1 mM EDTA; 1 mM DTT; 1% Nonidet P-40; 1 mM 4-(2- aminoethyl)benzenesulfonylfluoride-hydrochloride (Pefabloc) ) . After 10 min on ice nuclei were removed by centrifugation. Proteins were separated by SDS- PAGE, transferred to nitrocellulose and visualized by immunoblotting. To generate infected cell lysates containing metabolically labeled proteins 4xl0 6 HeLa cells were infected with influenza A/WSN/33 virus at a multiplicity of 10 for 45 min at 37°C. Infection was allowed to proceed in DMEM + 0.1% BSA for 5 h at which time cells were labeled with 50 μCi 35 S-methionine + 50 μCi 35 S-cystine in MEM-cys-met for 1 h. Extracts were prepared by resuspending infected cells in 650 μl ice cold NP-40 lysis buffer followed by two 15 s pulses in a Raytheon sonicator to disrupt nuclei. Insoluble cell debris was removed by centrifugation at 100,000xg in a TL-100.3 Beckman rotor. 5 μl anti-NPI- 1 sera was incubated on ice for 1 h with 100 μl infected cell lysates. Immune complexes were precipitated from solution by incubation with Sepharose-4B linked protein G beads (Sigma) for 1 h. Beads were collected by centrifugation, washed three times in NP-40 lysis buffer, and resuspended in SDS- sa ple buffer. Precipitated proteins were separated by SDS-PAGE and visualized by autoradiography. 6.2. RESULTS 6.2.1. ISOLATION OF NPI-l The interactive trap was used to identify proteins which specifically interact with the influenza A virus nucleoprotein (NP) . The interactive trap is one of several genetic systems recently developed which uses the modular nature of transcription activators to detect protein:protein interactions (Chien, et al., 1991, Proc. Natl. Acad. Sci. USA 88: 9578-9582; Dalton & Treisman, 1992, Cell 68: 597-612; Durfee, et al., 1993, Genes Dev. 7: 555- 569; Gyuris, et al., 1993, supra ; Vojtek, et al., 1993, Cell 74: 205-214; Zervos, et al., 1993, supra) . The interactive trap consists of three components: (1) a reporter gene that has no basal transcription; (2) a fusion protein which contains a LexA DNA binding domain that is transcriptionally inert; and (3) proteins encoded by an expression library, which are expressed as fusion proteins containing an activation domain (Fig. 1A) . Interaction of the LexA fusion protein and the fusion protein containing the activation domain will constitute a bimolecular transcriptional activator which, in this case, will confer the ability to grow on media lacking leucine (Gyuris, et al., 1993, supra ; Zervos, et al., 1993, supra) . In the absence of this interaction the leu2 gene is not transcribed. The NP gene of influenza A/PR/8/34 virus was subcloned as a translational fusion gene with the LexA gene into pEG202 to generate pLexA-NP (Fig. IB) . Strain R100 (Table II) , which contains pLexA-NP, was transformed with a HeLa cell cDNA library constructed in pJG4-5. pJG4-5 contains an activation domain under control of a GAL1 promoter (Gyuris, et al., 1993, supra) . TABLE II YEAST STRAINS USED Strains Genotype EGY48 Mata trpl ura3 his3 LEU2 : :pLEXAop6-LEU2 R100 EGY48, pLexA-NP(TRP1) R101 EGY48, pLexA-NP, pSH18-34(HIS3) R102 EGY48, pRFHMl(TRP1) , pSH18-34 Library plasmids were rescued from 100 leu+ colonies. Reproducibility of the interaction of the NP with the encoded library proteins was tested by transforming library plasmids into strain R101. Transformants were screened for galactose-dependent /5-galactosidase activity and growth on media lacking leucine. Specificity for NP was analyzed by checking the ability of library plasmids to confer growth on leu' media and /3-galactosidase activity in connection with a different LexA fusion plasmid, pRFHMl, encoding a fragment of the Drosophila melanogaster bicoid protein. Twenty-three library plasmids were confirmed to encode NP-interactive proteins. Twelve identical 2.1 kbp clones encoded the carboxy terminal fragment of a protein termed nucleoprotein interactor 1 (NPI-l) . Partial DNA sequencing showed that NPI-l is the human homologue of the yeast SRPl gene (infra) . 6.2.2. CLONING AND SEQUENCING OF THE NPI-l CDNA The 2.1 kbp NPI-l cDNA in pJG4-5 was sequenced by standard protocols. The 5' cDNA terminus of the NPI-l gene was cloned by 5' RACE. cDNAs from 3 independently derived NPI-l 5'RACE products were cloned and sequenced. Nucleotide and derived amino acid sequences of NPI-l are shown in Figure 2. The sequence reveals a 2.9 kbp cDNA which encodes a protein of 527 amino acids with a calculated molecular weight of 58,754 Da and a pi = 4.74. The carboxyl terminal 265 amino acids were encoded by the interactive trap library plasmid and interact with the viral NP. Comparison of the deduced amino acid sequences in the GenBank and EMBL data bases using the FASTA and TFASTA programs (Deveraux, et al., 1984, Nucleic Acids Res. 12: 387-395) demonstrated that NPI-l is the human homologue of the Saccharomyces cerevisiae protein SRPl (Yano, et al., 1992, Mol. and Cell. Biol. 12: 5640- 5651) . SRPl was cloned as an allele-specific suppressor of ts mutations in the zinc-binding domain of the A190 subunit of RNA polymerase I. The amino acid sequence is highly conserved between NPI-l and SRPl: 50% identity and 81% similarity at the amino acid level. The amino terminus of NPI-l has a potential nuclear localization signal (Chelsky, et al., 1989, Mol. Cell. Biol. 9:2487-2492); amino acids 25 to 49 are rich in arginine, and contain a stretch of four consecutive arginines at amino acids 28 to 31. NPI-l, like SRPl, contains a series of 8 consecutive ARM motifs, which are 42 amino acid protein subsequences originally identified in the Drosophila armadillo protein (Peifer et al., Cell 76: 789-791, 1994; Yano, et al., 1992, supra) (Fig. 3, infra) . 6.2.3. NPI-l BINDS TO NP IN VITRO In order to demonstrate that the NPI-l binds to the viral NP, the NPI-l cDNA fragment (amino acids 262 to 527) was subcloned into the bacterial expression vector pGEX-5X-l yielding a glutathione S-transferase fusion gene. The expressed fusion protein was purified from bacterial lysates on glutathione agarose beads. NP, which had been partially purified with the viral polymerase from influenza A/PR/8/34 virus was specifically precipitated from solution by glutathione agarose beads complexed with GST-NPI-1 (Fig. 4) . The NP band migrates slightly faster than that of the GST- NPI-1 fusion protein. The identity of this protein was confirmed by immunoblot analysis using the anti-NP monoclonal antibody HT103 (Fig 4, lane 8). 6.2.4. IMMUNODETECTION OF NPI-l IN CELL EXTRACTS Rabbit antisera raised against GST-NPI-1 were used to identify a polypeptide from total cellular extracts of both HeLa and MDBK cells with an apparent molecular weight of 65 kDa (Fig. 5) . The molecular weight predicted from the derived amino acid sequence of the cDNA is slightly smaller (59 kDa) . A lower amount of NPI-l was present in the cytoplasmic fraction generated by lysis of cells in the presence of NP-40 than in the total cellular extract suggesting that most of NPI-l is located in the nucleus (Fig. 5) . This is consistent with results localizing the NPI-l homologue SRPl to the nucleus of yeast cells by immunofluorescence (Yano, et al., 1992, supra) . Localization of NPI-l to a particular intracellular compartment by immunofluorescence experiments has not been posssible due to the high background fluorescence of the antisera preparations used. 6.2.5. NPI-l INTERACTS WITH NP IN INFECTED CELLS Since NP formed a complex with NPI-l in vitro, we examined whether NP and NPI-l form a complex in infected cells. NP was specifically coimmunoprecipitated from extracts of influenza A/WSN virus infected HeLa cells by antisera directed against NPI-l (Fig 6) . This demonstrates an interaction of the viral NP and the cellular NPI-l during influenza A virus infection. 7. Example: THE IDENTIFICATION OF NS1I-1 AND ITS INTERACTION WITH INFLUENZA NUCLEOPROTEIN NS1 In the example described below, the yeast interactive trap system was used to identify a human protein, NS1I-1 (NSl-interactor-1) , from a HeLa cell cDNA library on the basis of its binding to NSl of influenza A virus. NS1I-1 is shown herein to be recognized not only by NSl proteins from five human and avian influenza A strains, but also by NSl of influenza B virus. Surprisingly, NS1I-1 is homologous to a steroid dehydrogenase isolated from pigs (Leenders, et al., 1994, Eur. J. Biochem. 222: 221- 227) . Several proteins with a dehydrogenase function have recently been shown not only to have enzymatic activity but also to be involved in post- transcriptional events of gene-expression (Hentze, 1994, supra) . This strong conservation supports an important functional role of the NS1I-1 interaction during the viral life cycle. 7.1. MATERIALS AND METHODS 7.1.1. YEAST. E.COLI STRAINS. AND PLASMIDS Manipulations of nucleic acids, Escherichia coli and yeast followed essentially standard procedures as described elsewhere (Ausubel, et al. , 1992, Current Protocols in Molecular Biology, Green Publishing Associates, Inc., and John Wiley & sons, Inc. , New York) . The yeast strains EGY40 (Mata trpl ura3 his3 ) and EGY48 (Mata trpl ura3 hiε3 LEU2 : :pLEX- Aop6-LEU2) as well as plasmids pEG202, pRFHMl, and pSH18-34, and the HeLa cell cDNA constructed in pJG4-5 have been described (Gyuris, et al., 1993, supra ; Zervos, et al., 1993, supra) . E. coli strains used for cloning and expression were MH3 (trpc araD lacX hsdR galU galK) , DH5α (FΦ80dlacZΔM15 Δ(lacZY-argF)U169 deoR recAl endAl hsdR17(r κ -m κ +) supE44λ-thi- gyrA96 relAl) , and BL26 (FompT hsdS B (r B ~ m B J gal dcm) . pLexA- NS1 was constructed by subcloning the cDNA of the NS segment of influenza virus A/PR/8/34 downstream of the LexA gene in pEG202. pGEX-NSlI-1 was constructed by subcloning the HeLa cDNA-insert of library plasmid pK5 as an EcoRI/Xbol-fragment into pGEX-5X-l (Pharmacia) . DNA-oligonucleotides used were: GSP-I, 5'- dTCCTGATGTTGCTGTAGACG-3' , GSP-II, 5'-dGCACGACTAGTATGATTTGC-3' , and the 5'RACE anchor primer (BRL) , 5*- dCUACUACUACUAGGCCACGCGTCGACTAGTACGGGIIGGGIIGGGIIG-3' 7.1.2. IDENTIFICATION OF NS1-INTERACTORS The interactive trap selection was performed essentially as described for NPI-l in Section 6.1.2, above. The selection strain was constructed by transforming EGY48 with the bait plasmid pLexA-NSl and the lacZ-reporter plasmid pSH18-34. Expression of lacZ from pSH18-34 is transcriptionally controlled by a GAL1 promoter and LexA-dependent operator sites. A HeLa cell cDNA library was introduced into the selection strain using the lithium acetate method (Ito, et al., 1983, supra) . Primary transformants were selected on trphisura" glucose plates. 1 x 10 6 cells representing 3.3 x 10 5 independent transformants were plated on 150 mm trphisuraleu'-galactose plates to select for clones expressing NSl-interacting proteins. Viable cells were replica-transferred to a nitrocellulose filter and assayed for 3-galactosidase activity using 5-bromo-4-chloro-3-indolyl 3-D- galactoside (X-gal) as described (Ausubel et al., 1992, supra) . Positive clones were tested in a second round of selection by replica plating onto X-gal trp " his'ura" galactose plates. Plasmid DNA was isolated from yeast clones expressing 3-galactosidase activity only on galactose plates and library plasmids were recovered by transformation into E.coli MH3 as described in Section 6.1.2, above. The specificity of the isolated plasmids was tested by co-transformation with pLexA-NSl or pRFHMl into EGY40 harboring pSH18- 34. pRFHMl expresses an unrelated LexA-bicoid fusion protein. The resulting strains were assayed for β- galactosidase activity on X-gal trphisura " plates containing glucose or galactose. Plasmids that induced 3-galactosidase only in the presence of galactose and only in conjunction with pLexA-NSl were considered to encode true interacting proteins. 7.1.3. CLONING OF NS1I-1 5'-END CDNA Cloning of cDNA derived from the 5•-end of NS1I-1 mRNA followed a RACE-procedure (rapid amplification of cDNA ends) (Frohmanm, et al., 1988, supra) using a 5'RACE-kit (BRL) . First strand cDNA was synthesized from 1 μg of HeLa cell poly(A)-RNA hybridized to 2.5 pmol NSlI-1-specific oligonucleotide GSP-I using reverse transcriptase. The cDNA was tailed at the 5'-end with dC by terminal transferase. The product was used as a template for the amplification of a 5'RACE-product by PCR using a nested oligonucleotide GSP-II and an anchor primer provided by the kit. The resulting fragment was subcloned in pGEM-T (Promega) to form pRACENSlI-1, and sequenced by the standard dideoxy method. The NCBI- search was conducted using Fasta, Tfasta. Sequence comparison was conducted using Bestfit. 7.1.4. NORTHERN BLOT ANALYSIS 1 μg of HeLa cell poly(A)-RNA was separated on a 1% agarose-formaldehyde gel, transferred to a nylon membrane (Nytran, Amersham) , and UV-crosslinked. The RNA was hybridized to a 32 P-labled, NSlI-1-specific probe derived form a fragment (corresponding to positions +791 to +1745) of the original pK5 library isolate as described (Ausubel, et al., 1992, supra) . 7.1.5. VIRUSES. CELLS. AND EXTRACTS Influenza strains A/WSN/33 (H1N1) , A/Berkeley/1/68 (H2N2) , A/Beijing/32/92 (H3N2) , A/duck/Alberta/76 (N12N5) , A/turkey/Oregon/71 (H7N5) , and B/Lee/40 were grown in the allantoic cavity of 10 days old embryonated chicken eggs. Confluent monolayers of Madin Darby canine kidney-(MDCK)-cells were infected with influenza viruses at an m.o.i. of 10 for one hour in 35 mm dishes. Infection was continued at 37°C (influenza A viruses) or 35°C (influenza B/Lee/40) for 5 hours in MEM-medium containing 0.1% bovine serum albumin. Cells were labeled with 100 μCi of 35 S-methionine and 35 S-cysteine (ICN) per dish for one hour in MEM-metcys-medium. Cells were washed and scraped in ice-cold phosphate buffered saline (PBS) . Cells from one dish were lysed with 500 μl NET-N buffer (lOmM Tris/HCL pH 8.0, 1 mM EDTA, 150 mM Nacl, 0.05% Nonidet P 40) and two 30 second pulses in a Raytheon sonicator at a setting of 1A. Lysates were centrifuged for 10 minutes at 20,000 rpm in a TL100.3 rotor. The supernatants were used for precipitation of proteins. 7.1.6. EXPRESSION OF GST-NS1I-1 FUSION PROTEIN IN E. COLI AND PRECIPITATION OF VIRAL PROTEINS FROM CELL EXTRACTS NS1I-1 was expressed in E. coli BL26 from pGEX-NSlI-1 as a GST (glutathione-S-transferase)-NS1I- 1 fusion protein with a predicted molecular weight of 77 kDa. Production of GST-NS1I-1 was induced using isopropyl-/3-D-galactopyranoside essentially as described (Smith, et al., 1988, supra) . GST-NS1I-1 was adsorbed from bacterial lysates to glutathione sepharose beads (Pharmacia) as recommended by the manufacturer. Beads were washed three times with PBS to remove contaminating proteins. 10 μl of glutathione sepharose coated with GST-NS1I-1 fusion protein was rotated with 100 μl extract of virus- infected MDCK-cells (see above) in 750 μl NET-100 buffer (20 mM Hepes,pH 8.0, lOOmM NaCl, 0.5 mM DTT) for 90 minutes at 4°C. The beads were washed three times with PBS/0.05% NP-40 and precipitated proteins were analyzed by SDS-gel electrophoresis and autoradiography. In parallel reactions, viral proteins were immunoprecipitated from 50 μl of infected cell extracts using 5 μl of anti-NSl or anti- Mi antiserum and protein A agarose as described (Harlow & Lane, 1988, supra) . As a negative control, GST protein was expressed in BL26 from pGEX-5X-l and used the same way in the co-precipitation assay. 7.2. RESULTS 7.2.1. ISOLATION OF NS1-INTERACTING FACTORS The yeast interaction trap system (Gyuris, et al., 1993, supra ; Zervos, et al., 1993, supra) was used to identify cellular proteins that interact with the non-structural protein NSl of influenza A virus. A LexA-NSl fusion protein was used as bait to screen library in which HeLa cell cDNAs were expressed as fusions with an acidic transcription activation domain (Gyuris, 1993 #159) . Colonies were selected, in which either of two reporter genes, LEU2 and lacZ, were activated by the cDNA-encoded proteins. This double selection scheme was used to increase the stringency, because in an initial screen a high proportion of candidates scored negative in subsequent genetic tests. The library plasmids were isolated from the selected clones. The binding specificity of the encoded fusion proteins was tested by assaying the activation of a lacZ-reporter gene encoded on pSH18-34. Expression of |8-galactosidase from this plasmid is transcriptionally controlled by LexA-specific operator sites. The isolated library plasmids were co- transformed with pLexA-NSl or pRFHMl into EGY40 harboring pSH18-34. pRFHMl expresses a LexA-bicoid fusion protein and was used as a non-specific operator-binding control. The resulting strains were assayed for j8-galactosidase activity specifically on X-gal plates containing galactose, but not glucose. From 3.3 x 10 5 independent library transformants, three plasmids were isolated that induced galactose-specific activation of the lacZ reporter gene only in combination with pLexA-NSl. Sequence analysis indicated that the three plasmids were each derived from different cellular cDNAs. 7.2.2. CLONING AND SEQUENCE ANALYSIS OF NS1I-1 One of the isolated plasmids, pK5, was analyzed further. It carried a cDNA-insert of 1781 bp with an open reading frame of 1413 nucleotides followed by 368 nucleotides of a potentially untranslated region (Fig. 12) . The cDNA terminated with an oligo(A)-tract and had a consensus poly(A)- site at positions 2526-2531. Northern blot analysis of HeLa cell poly(A)-RNA using a NSlI-1-specific probe detected one single transcript of about 3.0 kb suggesting that the pK5 insert represented an incomplete cDNA (Fig. 13) . The remaining NS1I-1 cDNA was cloned by a 5'RACE procedure (Frohman, et al., 1988, supra) . Four independent clones were sequenced that differed only in length at the very 5'-end. The longest 5'RACE product, contained in pRACENSlI-1, extended the NS1I-1 sequence for 893 nucleotides upstream totalling in a cDNA of 2674 bp (Fig. 12) . The sequence has one long open reading frame encoding a protein of 735 amino acids with a predicted molecular mass of 79.5 kDa and a pi of 9.06. The putative ATG-start codon is located 103 nucleotides downstream of the 5•-end and is in the context of a sequence consistent with its being a translational start (Kozak, 1989, J. Cell Biol. 108: 229-241). Sequence comparisons through the EMBL- and Genbank databases using the FASTA- and TFASTA-analysis programs revealed that NS1I-1 is highly homologous to porcine 17 / 8-estradiol dehydrogenase (Leenders, et al., 1994, supra) . The two cDNAs are 86% identical on the nucleic acid level. The encoded proteins are 84% identical and are 92% similar when allowing for conserved amino acid substitutions. NS1I-1 cDNA also shows strong homology to ten human cDNA fragments that have been isolated as expressed sequence tags, as revealed by a BLAST-analysis of the NCBI-database (fragments are between 134 to 556 bp in length) . These cDNAs were derived from different tissues including liver, spleen, brain, adipose tissue, and adrenals tissue indicating a broad expression of NS1I- 1 in the body. The encoded NS1I-1 protein features two conserved sequence motifs of the short-chain alcohol dehydrogenase family (Persson, et al., 1991, Eur. J. Biochem. 200: 537-543) . Specifically, amino acids 15 to 22 (TGAGAGCG) are similar to the potential co- factor binding site, and residues 163 to 167 (YSAAK) correspond to a short stretch that has been suggested to participate in catalysis (Chen, et al., 1993, Biochemistry 32: 3342-3346). The presence of the tri- peptide AKL at the carboxy-terminus was also noted. Similar tri-peptide motifs have been found to serve as targeting signals for import into microbodies (for a review, see de Hoop & Ab, 1992, Biochem. J. 286: 657- 669) . However, the presence of this signal does not automatically direct a protein to these organelles (de Hoop & Ab, 1992, supra) . 7.2.3. NS1I-1 BINDS NSl PROTEIN FROM EXTRACTS OF INFLUENZA VIRUS INFECTED CELLS In order to confirm a physical interaction between NS1I-1 protein and NSl expressed in influenza virus infected cells, a co-precipitation assay was performed as similarly described in Section 6.2.3, above, for NPI-l. A glutathione-S-transferase(GST)- NS1I-1 fusion gene was constructed and expressed in E. coli . GST-NS1I-1 fusion protein from bacterial lysate was absorbed to the affinity matrix glutathione agarose and purified from contaminating bacterial proteins. The immobilized fusion protein was used to bind and precipitate 35 S-labeled proteins from extracts of MDCK cells infected with human influenza A/WSN/33 viruses (Fig. 14) . The NSl protein of this strain is 98% identical to its counterpart from A/PR/8/34 used in the yeast interaction screen. Aliquots of the same extract were used to in parallel reactions to immunoprecipitate influenza virus proteins NSl and Ml. The precipitated proteins were analyzed by SDS-gel electrophoresis and visualized by fluorography. Fig. 14 shows, that GST-NS1I-1 efficiently precipitated a protein band co-migrating with immunoprecipitated NSl protein from infected cell extract (compare lanes 2 and 3) . This interaction was specific for NS1I-1 since no proteins were detected in precipitates using GST only (lane 6) . In addition, no proteins were precipitated by GST-NS1I-1 from mock-infected cells (lane 8) , showing that a virus induced protein was recognized by NS1I-1. This experiment confirmed, that NS1I-1 interacts specifically with the NSl protein of influenza A virus. If this interaction is important for the viral life-cycle one would expect it to be conserved. Consequently, the binding of NS1I-1 to NSl proteins from other influenza A strains should be detectable despite of their considerable variation in the primary structure (Baez, et al., 1981, Virology 113: 397-402; Ludwig, et al., 1991, Virology 183: 566-577). Therefore the interaction between NS1I-1 and NSl was examined using the same co-precipitation assay described above, with extracts from cells infected with different influenza A and B virus strains. Mutations accumulate in the NSl gene at a steady rate over time (Buonagurio, et al., 1985, Science 232: 980-982). Thus, the time-span between the isolation of two strains is reflected in the sequence variation of its NSl proteins (Ludwig, et al., 1991, supra ; Buonagurio, et al., 1985, supra) . NS1I-1 binding to NSl proteins from two recently isolated human influenza A strains A/Beijing/32/92 and A/Berkeley/1/68 was examined. As can be seen in Fig. 15, Panels C and D, respectively, NSl proteins from both strains were specifically precipitated (Fig 15, Panels C and D, lanes "GST-K5") . A low immunoprecipitation efficiency of NSl protein from the Beijing-strain (Panel C) was reproducibly observed. The NSl proteins of A/Berkeley/1/68 and A/WSN/33 are 90.8% identical to each other. The NSl sequence of A/Beijing/32/92 is not known. The following analyses were conducted to examine whether GST-NS1I-1 is also recognized by the more divergent NSl proteins of the avian influenza strains A/duck/Alberta/76 and A/turkey/Oregon/71. The NSl proteins of these strains are 66.5% and 63.6% identical, respectively, to A/WSN/33. Significantly, NSl of A/turkey/Oregon/71 is only 124 amino acids in length, lacking most of the carboxy-terminal half of other NSl proteins, which consist of 207 to 237 amino acids (Norton, et al., 1987, Virology 156: 204-213). Nevertheless, precipitation of a protein band co- migrating with NSl from both strains was observed (Fig. 15, Panels A and B, lanes "GST-K5") . The NSl and Ml proteins of A/duck/Alberta/76 could not be separated by the gel system used. Significant amounts of nucleoprotein in the GST-NS1I-1 precipitates of these avian strains were reproducibly detected for undetermined reasons. Finally, the co-precipitation assay was used to test the human influenza B virus B/Lee/40. Surprisingly, GST-NS1I-1 precipitated specifically the influenza B virus NSl protein, although it is only 20.6% identical to NSl from A/WSN/33 (Fig. 15, Panel E, lane "GST-K5") . Taken together, the binding of GST-NS1I-1 to NSl proteins expressed by several influenza A and B virus stains could be demonstrated, despite the great divergence of their primary structures. This result strongly supports an important function of this interaction during the viral life cycle, and indicates that the NS1I-1 interaction is an excellent target for antiviral intervention. The present invention is not to be limited in scope by the specific embodiments described which are intended as single illustrations of individual aspects of the invention, and functionally equivalent methods and components are within the scope of the invention. Indeed, various modifications of the invention, in addition to those shown and 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. International Application No: PCT/ / MICROORGANISMS Optional Sheet in connection with the microorganism referred to on page _, lines of the description ' A. IDENTIFICATION OF DEPOSIT ' Further deposits are identified on an additional sheet Name of depositary institution ' American Type Culture Collection Address of depositary institution (including postal code and country) * 12301 Parklawn Drive Rockville, MD 20852 US Date of deposit " November 30, 1994 Accession Number ' 75950 B. ADDITIONAL INDICATIONS ' (leave blank if not applicable) This information is continued on a separate attached sheet C. DESIGNATED STATES FOR WHICH INDICATIONS ARE MADE ' K*..-..™^^-.*,^.-., D. SEPARATE FURNISHING OF INDICATIONS ' (leave blank if not applicable) The indications listed below will be submitted to the International Bureau later ' (Specify the general nature of the indications e.g , "Accession Number of Deposit") E. D This sheet was received with the International application when filed (to be checked by the receiving Office) (Authorized Officer) D The date of receipt (from the applicant) by the International Bureau ** (Authorized Officer) Form PCT/RO/134 (January 1981 ) International Application No: PCT/ Form CT/RO/134 (cont.) American Type Culture Collection 12301 Parklawn Drive Rockville. MD 20852 US Accession No. Date of Deposit 75951 November 30, 1994

Claims: WHAT IS CLAIMED IS; 1. An assay for identifying a substance that inhibits the specific interaction of a host cell protein, that is not a cell surface receptor protein, with a viral protein required for viral infection, replication, assembly or release, comprising: (a) contacting a protein or peptide containing an amino acid sequence corresponding to the binding site of the host cell protein with a protein or peptide having an amino acid sequence corresponding to the binding site of the viral protein, under conditions and for a time sufficient to permit binding and the formation of a complex, in the presence of a test substance, and (b) detecting the formation of a complex, in which the ability of the test substance to inhibit the interaction between the host cell protein and the viral protein is indicated by a decrease in complex formation as compared to the amount of complex formed in the absence of the test substance. 2. An assay for identifying a substance that inhibits the interaction of influenza virus NP with a host cell protein comprising: (a) contacting a protein or peptide containing an amino acid sequence corresponding to the binding site of influenza virus NP with a protein or peptide containing an amino acid sequence corresponding to the binding site of the host cell protein, under conditions and for a time sufficient to permit binding and formation of a complex, in the presence of a test substance, and (b) detecting the formation of a complex, in which the ability of a test substance to inhibit the interaction between influenza virus NP and the host cell protein is indicated by a decrease in complex formation as compared to the amount of complex formed in the absence of the test substance. 3. The assay of Claim 2 in which the host cell protein is NPI-l. 4. The assay of Claim 3 in which the host cell protein is NPI-2. 5. The assay of Claim 3 in which the host cell protein is NPI-3. 6. The assay of Claim 3 in which the host cell protein is NPI-4. 7. The assay of Claim 3 in which the host cell protein is NPI-5. 8. The assay of Claim 3 in which the host cell protein is NPI-6. 9. An assay for identifying a substance that inhibits the interaction of influenza virus NSl with a host cell protein comprising: (a) contacting a protein or peptide containing an amino acid sequence corresponding to the binding site of influenza virus NSl with a protein or peptide containing an amino acid sequence corresponding to the binding site of the host cell protein, under conditions and for a time sufficient to permit binding and formation of a complex, in the presence of a test substance, and (b) detecting the formation of a complex, in which the ability of a test substance to inhibit the interaction between influenza virus NSl and the host cell protein is indicated by a decrease in complex formation as compared to the amount of complex formed in the absence of the test substance. 10. The assay of Claim 9 in which the host cell protein is NS1I-1. 11. The assay of Claim 1, 2, or 9 in which one protein or peptide of the complex is immobilized, and the other protein or peptide is labeled with a signal- generating compound. 12. The assay of Claim 11 in which an immobilized antibody is used to anchor the immobilized protein or peptide. 13. The assay of Claim 11 in which a labeled antibody is used to label the protein or peptide with a signal-generating compound. 14. The assay of Claim 11 in which the protein or peptide substrate is immobilized prior to the reaction so that the reaction is conducted in a solid- liquid phase. 15. The assay of Claim 1, 2, or 9 in which the proteins or peptides are contacted in a liquid phase to form a complex which is separated from the liquid phase at the end of the reaction. 16. The assay of Claim 15, in which the complex formed is separated from the liquid phase by immobilizing the complex on a solid phase. 17. The assay of Claim 16 in which the complex is captured by an immobilized antibody specific for one of the proteins or peptide binding partners. 18. A method for treating influenza virus infection, comprising administering to an infected individual, a therapeutically effective amount of a substance that inhibits the specific interaction of a host cell protein with a viral protein required for viral infection, replication, assembly or release. 19. The method of Claim 18 in which the viral protein is NP. 20. The method of Claim 18 the host cell protein is NPI-l. 21. The method of Claim 18 the host cell protein is NPI-2. 22. The method of Claim 18 the host cell protein is NPI-3. 23. The method of Claim 18 the host cell protein is NPI-4. 24. The method of Claim 18 the host cell protein is NPI-5. 25. The method of Claim 18 the host cell protein is NPI-6. 26. The method of Claim 18 the viral protein is NSl. 27. The method of Claim 18 the host cell protein is NS1I-1. 28. An isolated DNA sequence which encodes the amino acid sequence of NPI-l, or which selectively hybridizes to the complement of the coding sequence of NPI-l and encodes a functionally equivalent gene product. 29. An isolated DNA sequence which encodes the complement of the DNA sequence of Claim 28. 30. A DNA vector containing the DNA sequence of Claim 28. 31. A DNA vector containing the DNA sequence of Claim 29. 32. An expression vector containing the DNA sequence of Claim 28 operatively associated with a regulatory element that directs the expression of the DNA sequence. 33. A genetically engineered host cell containing the DNA sequence of Claim 28 operatively associated with a regulatory element that directs expression of the DNA sequence in the host cell. 34. A DNA sequence which encodes the amino acid sequence of NPI-2, NPI-3, NPI-4, NPI-5, or NPI-6 or which selectively hybridizes to the complement of the coding sequence of NPI-2, NPI-3, NPI-4, NPI-5, or NPI- 6 and encodes a functionally equivalent gene product. 35. A DNA sequence which encodes the complement of the DNA sequence of Claim 34. 36. A DNA vector containing the DNA sequence of Claim 34. 37. A DNA vector containing the DNA sequence of Claim 35. 38. An expression vector containing the DNA sequence of Claim 34 operatively associated with a regulatory element that directs the expression of the DNA sequence. 39. A genetically engineered host cell containing the DNA sequence of Claim 34 operatively associated with a regulatory element that directs expression of the DNA sequence in the host cell. 40. An isolated DNA sequence which encodes the amino acid sequence of NS1I-1, or which selectively hybridizes to the complement of the coding sequence of NS1I-1 and encodes a functionally equivalent gene product. 41. An isolated DNA sequence which encodes the complement of the DNA sequence of Claim 40. 42. A DNA vector containing the DNA sequence of Claim 40. 43. A DNA vector containing the DNA sequence of Claim 41. 44. An expression vector containing the DNA sequence of Claim 40 operatively associated with a regulatory element that directs the expression of the DNA sequence. 45. A genetically engineered host cell containing the DNA sequence of Claim 40 operatively associated with a regulatory element that directs expression of the DNA sequence in the host cell.

Inventors: O'NEILL ROBERT, PALESE PETER, O'NEILL, ROBERT, PALESE, PETER
Applicants: MOUNT SINAI MEDICAL CENTER, THE MOUNT SINAI MEDICAL CENTER
Application year: 1995
Main classification: C12Q1
IPC codes: A61K38, A61P31, C07K14, C12N1, C12N15, C12P21, C12Q1, C12R1, G01N33