Abstract:
A modified Saccharomyces cerevisiae cell, wherein the cell expresses the Influenza virus ion channel protein M 2 . Also disclosed is a process for detecting modulators of M 2 , which comprises (a) treating such modified Saccharomyces cerevisiae cells with a test substance, and (b) assessing growth in the presence of a test substance, wherein a change in growth signals that the test substance is an M 2  modulator. M 2  inhibitors are useful anti-viral agents.

Description:
This is a continuation of application Ser. No. 08/183,545 filed on Jan. 19, 1994, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to yeast, recombinant DNA techniques, and to processes for detecting inhibitors and/or activators of viral ion channels. 
     BACKGROUND OF THE INVENTION 
     Ion uptake in yeast is accomplished through the concerted actions of transporter proteins with ion selectivity and the electrochemical gradient established by the plasma membrane hydrogen ion ATPase. For example, potassium ions are taken up by transporter proteins encoded by two genes, TRK1 and TRK2, and exchanged stoichiometrically for each hydrogen ion extruded from the cell by the ATPase. Rodriguez-Navarro and Ramos, J. Bacteriol, 159: 940-945 (1984); Gaber, Styles, and Fink, Mol. Cell Biol. 8: 2848-2859 (1988); Ko and Gaber, Genetics 125: 305-312 (1990). Serrano, R.(1991 ) In The Molecular Biology of the Yeast Saccharomyces, Broach, J. R. et al., eds. Cold Spring Harbor, pp 523-585. Perturbations of the electrochemical gradient permit essential ions such as potassium to leak from the cell, resulting in cell death. 
     Several laboratories have used yeast strains defective in potassium ion uptake as hosts for expressing ion channels from other organisms. Certain ion channels from Arabidopsis thaliana restore potassium ion uptake function and permit growth of the strain on media with low levels of potassium ions. Anderson, et al., Proc. Natl. Acad. Sci. U.S.A. 89: 3736-3740 (1992); Sentenac, et al. (1992), Science256: 663-666. 
     The M 2  protein of Influenza A virus is a 97 amino acid polypeptide containing a single membrane-spanning region. In the virus and infected cell membranes, M 2  polypeptides associate into tetramers to form ion channels, thereby providing a function essential for virus replication. Holsinger, L. J. and Lamb, R. A. (1991), Virology 183: 32-43; Sugrue R. L. and Hay, A. J. (1991), Virology 180: 617-624; Pinto, L., et al. (1992), Cell 69: 517-528. The available therapeutic agents for Influenza A virus, amantadine and rimantadine, function by blocking M 2  ion channel activity. Hay, A.(1992), Seminars in Virology 3: 21-30. Inhibition studies with amantadine suggest that M 2  functions at both early and late stages in virus infection by permitting ion flow across various membranes. New inhibitors of the M 2  channel would be effective antiviral agents. A need exists, therefore, for methods of screening for inhibitors and/or activators of the M 2  channel. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention concerns a modified Saccharomyces cerevisiae cell, wherein the cell expresses a nucleic acid sequence for an M 2  protein or a functional derivative or mutant thereof. The yeast cell expresses this protein through its normal transcription and translation mechanisms. 
     The present invention further concerns a process for detecting modulators of the M 2  channel, which comprises: 
     (a) treating Saccharomyces cerevisiae cells with a test substance, wherein the Saccharomyces cerevisiae cells express the M 2  protein or a functional derivative or mutant thereof; and 
     (b) assessing growth in the presence of a test compound; 
     wherein a change in growth in the presence of the test compound signals that the test compound is a modulator of the M 2  channel. This process is thus useful for detecting inhibitors of M 2  channel function, which are useful as anti-influenza agents. 
     Functional expression of ion channels in yeast provides a general means for detecting channel modulators. Compounds that modulate channel function am detected by observing alterations in growth, which is dependent on channel function: Expression of ion channels in yeast is not restricted to restoration of uptake functions and, depending on the particular channel, may result in a range of effects on the host. 
     The present inventors have designed a method to detect inhibitors of the M 2  protein of Influenza A. The aforementioned principle of this screen, growth rescue, is based on the observation by the inventors that expression of high levels of M 2  protein in Saccharomyces cerevisiae impairs growth. The inventors validated this observation by rescuing the growth impairment with amantadine, a known blocker of M 2  function. The inventors obtained genetic evidence to support this observation by converting an amantadine-resistant M 2  allele to amantadine-sensitivity. They subsequently observed amantadine-dependent growth rescue in a strain having the amantadine-sensitive M 2  allele. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a Western blot indicating the levels of M 2  protein produced in yeast. Protein extracts were prepared by lysing log phase cells in a vortex with glass beads. The concentration of total protein was determined using the Bradford reagent. Equivalent amounts of extract (100 μg) from each strain were separated by electrophoresis on 12.5% sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE). After electrophoresis, gels were transferred to nitrocellulose for Coomassie Blue staining or reaction with anti-M 2  antisera (gift of Paul Rota, CDC). FIG. 1 shows a Western blot of M 2  protein using rabbit polyclonal antisera against the M 2  protein. Strains contained either the pGAL vector (designated V), a single-copy plasmid expressing M 2  (designated M 2  single) or a multi-copy plasmid expressing M 2  (designated M 2  multi). Extracts prepared from influenza virus-infected and mock-infected Martin Darby canine kidney cells (designated MDCK) are included as controls. These data indicate that yeast cells containing the multi-copy plasmid produce high levels of M 2  protein when grown on inducing media (galactose). 
     FIG. 2 shows a modified genomic sequence for the M 2  gene (SEQ. ID. NO.: 1) and the resulting amino acid sequence (SEQ. ID. NO.: 2). The sequence was obtained from a cDNA clone constructed at Bristol-Myers Squibb and contains three differences from the sequence published in Markushins et al. (1988), Virus Research 10: 263-272. Amino acid residues 26 to 43, which define the proposed transmembrane region of the protein, are underlined in FIG. 2. The asparagine (Asn) to serine (Ser) change at amino acid 31, which converts the protein to amantadine-sensitivity, is indicated. 
     FIG. 3 shows the growth of cells expressing M 2 . Cultures of strains containing the vector, single-copy M 2  plasmid or multi-copy M 2  plasmid were grown in inducing media at 30° C. with vigorous aeration. A strain expressing the yeast high affinity potassium ion transporter, TRK1, was included as an additional control. Samples were removed at various intervals and the optical density at 600 nm was determined. Absorbance (A 600 ) is plotted versus time (hours). 
     FIG. 4 illustrates growth rescue by addition of amantadine to a culture of cells expressing the Udorn M 2  (gift of Robert Lamb, Northwestern University). Cultures of strains containing the amantadine-resistant M 2  plasmid (WSN) or the amantadine-sensitive M 2  plasmid (Udorn) were grown in inducing media at 30° C. with vigorous aeration. Samples were removed at various intervals and the optical density at 600 nm was determined. Absorbance (A 600 ) is plotted versus time (hours). Amantadine concentration was 100 μM. 
     FIG. 5 shows the growth of strains expressing an allele of M 2  converted from amantadine-resistance (WSN) to amantadine sensitivity. Cultures of strains containing amantadine-resistant M 2  plasmid (WSN) or amantadine-sensitive M 2  plasmid (WSN, N31S) were grown in inducing media at 30° C. with vigorous aeration. Samples were removed at various intervals and the optical density at 600 nm was determined. Absorbance (A 600 ) is plotted versus time (hours). Amantadine was included at concentrations of 10 and 100 μM. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following definitions apply to the terms as used throughout this specification, unless otherwise limited in specific instances. 
     The term &#34;modified&#34; as used with respect to a cell refers to a cell in which the wild-type genome has been altered by addition of one or more heterologous genes, a deficiency in one or more wild-type genes, or a combination thereof. Such modifications may be carried out by transformation and homologous recombination through techniques well understood by those having ordinary skill in the art. 
     The term &#34;an M 2  protein&#34; refers to a protein having at least the smallest portion of the full-length wild-type M 2  protein that can result in growth impairment in Saccharomyces cerevisiae. Proteins having residues 26 to 43 of FIG. 2 are preferred for the M 2  protein. 
     The term &#34;a functional derivative or mutant thereof&#34; as used with respect to a protein refers to a protein differing from the subject protein by one or more amino acid residues but still having the biochemical function of the protein and greater than about 90% sequence homology. In the case of the M 2  protein, a &#34;functional derivative or mutant thereof&#34; refers to such proteins that have ion transport activity but do not have the identical amino acid sequence shown in FIG. 2. Such derivatives and mutants may include proteins that differ from the wild-type protein by amino acid substitutions, deletions, disruptions, and the like. Such differences may be accomplished by genetic means, using such techniques as site-directed mutagenesis or mutagenic polymerase chain reaction (PCR) prior to translation, or by chemical means, using proteases and/or ligases after translation. Functional ion transport activity may be determined by the growth and growth rescue assays described herein (i.e., as in FIGS. 3 and 4). 
     The present invention may employ the amantadine-sensitive strain SGY1444, which contains pGAL::M 2  (N31S) construction in pYES2, ATCC No. 74266. The ATCC designation reflects a deposit under the Budapest Treaty with the Amedcan Type Culture Collection, 12301 Parklawn Drive, Rockville, Md. 20852-1776. 
     These strains contain plasmids that require constant selection on minimal media lacking uracil to ensure their maintenance. The preferred strain background is MATα, ade2-1, can1-100, his3-11.15, leu2-3, 112, trp1-1, ura3-1. 
     The yeast cell of the present invention begins with a wild type yeast strain with regard to ion channel activity. A variety of viral ion channels may be introduced into this strain to assess whether these channels impair growth. This analysis may reveal ion channels that have been cloned but not yet described physiologically. Each application results in a strain expressing a foreign ion channel, useful in a screen for modulators of the channel. 
     A yeast strain expressing an ion channel can be adapted to natural products screening. A simple screen design involving growth inhibition or ion uptake in agar plates or in liquid culture may detect compounds that modulate channel function. For screening of activators of an ion channel, the screen may include such modifications such as pH adjustment. 
     In the course of developing this system, the inventors introduced plasmids that contain the M 2  gene under the transcriptional control of an inducible promoter (GAL 1) into a yeast strain defective in potassium uptake. To analyze M 2  expression in the transporter mutant, the inventors compared three different transformed strains: (1) a transformant containing the parent vector lacking an insert, (2) a transformant containing the M 2  (WSN) gene expressed on a single-copy plasmid, and (3) a transformant containing the M 2  (WSN) gene expressed on a multi-copy plasmid (note that the WSN allele of M 2  is amantadine resistant). When these strains were grown on inducing media, expression of M 2  protein to levels comparable to those in virus-infected canine kidney cells (MDCK) was detected only in the strain containing the multi-copy plasmids (FIG. 1 ). Growth of this strain was significantly impaired relative to control strains (FIG. 3). Thus, levels of M 2  expression and growth impairment correlate directly. This observation is consistent with studies of M 2  expression in baculovirus-infected insect cells and in Xenopus oocytes. Black, R. et al., (1993), J. Gen Virol. 74: 143-146. This result allowed use of wild type yeast incorporating the M 2  gene rather than only strains defective in potassium uptake. 
     To confirm this interpretation, the inventors examined growth of yeast transformants expressing the Udorn allele of M 2 , which is amantadine-sensitive. In the presence of amantadine, these transformants grow at near wild-type rates, whereas growth of the transformants expressing the amantadine-resistant WSN allele is not remediated with amantadine (FIG. 4). Additional genetic evidence was obtained from an experiment in which the amantadine-resistant allele of M 2  (WSN) was converted back to amantadine-sensitivity by changing amino acid 31 from asparagine to serine (WSN, N31S). Both WSN alleles are toxic when overexpressed in yeast; however, only the toxicity of the WSN(N31S) allele is blocked by addition of amantadine to the media (FIG. 5). 
     While not intending to be constrained by theory, the present inventors suggest that M 2  exerts its effect on yeast cells by functioning as an unregulated pore in a manner similar to killer toxin, which disrupts membrane integrity and causes ion leakage. Killer toxin, a low molecular weight polypeptide product of the yeast viral double-stranded RNA (dsRNA), is secreted by most yeast strains. It kills sensitive cells by binding initially to a 1,6-β-D-glucan cell wall receptor (reviewed in Bussey, H. et al., (1990), Experientia 46: 193-200). Exposure to toxin results in a rapid inhibition of net proton pumping, which reduces the proton gradient across the plasma membrane. Concomitantly, toxin-treated cells show an inhibition of potassium ion and amino acid uptake, accompanied by acidification of the cytoplasm and potassium ion efflux. Killer toxin has since been shown to form ion channels in liposomes and in bilayers. Martinac, B. et al., (1990), PNAS 87: 6228-6232; Kagan, B., (1983), Nature 302: 709-711. The toxicity of M 2  in yeast may be the consequence of a similar membrane effect. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Construction of Expression Plasmids 
     pGAL::M 2 . The following oligonucleotides were used to amplify sequences encoding the WSN M 2  protein from a full length cDNA clone of segment 7 of WSN virus: ##STR1## 
     The PCR-amplified product was digested with BamHI and XbaI, gel-purified and cidned into an identically digested pYES2 vector (Invitrogen). The plasmid was verified by standard restriction digests and the insert was analyzed by DNA sequencing. 
     pGAL::TRK1. The following oligonucleotides were used to amplify sequences encoding the yeast Trk1 protein from a plasmid containing the TRK1 gene (this plasmid was isolated from a yeast genomic library by hybridization with 5&#39; and 3&#39; DNA fragments unique to the TRK1 gene): ##STR2## 
     The PCR-amplified product was digested with BamHI and XhoI, gel-purified and cloned into identically digested pYEUra3 vector (Clontech). The plasmid was verified by restriction digests and complements a trk1 deletion mutation when expressed in vivo. 
     Assay for growth 
     Cultures of strains containing either pYES2 vector, pGAL::TRK1 or pGAL::M 2 , were grown in inducing media (2% galactose) at 30° C. with vigorous aeration. Optical density was determined at various intervals by measuring absorbance at 600 nm. Amantadine was prepared fresh and added to cultures at the time of induction at 10 and 100 μM final concentrations. 
     
         __________________________________________________________________________SEQUENCE LISTING(1) GENERAL INFORMATION:(iii) NUMBER OF SEQUENCES: 6(2) INFORMATION FOR SEQ ID NO:1:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 294 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: DNA (genomic)(ix) FEATURE:(A) NAME/KEY: CDS(B) LOCATION: 1..291(xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:ATGAGTCTTCTAACCGAGGTTGAAACGCCAATCAGAAACGAATGGGGG48MetSerLeuLeuThrGluValGluThrProIleArgAsnGluTrpGly151015TGCAGATGCAACGATTCAAGTGATCCTCTCGTCATTGCAGCAAATATC96CysArgCysAsnAspSerSerAspProLeuValIleAlaAlaAsnIle202530ATTGGAATCTTGCACTTGATATTGTGGATTCTTGATCGTCTTTTTTTC144IleGlyIleLeuHisLeuIleLeuTrpIleLeuAspArgLeuPhePhe354045AAATGCATTTATCGTCGCTTTAAATACGGTTTGAAAAGAGGGCCTTCT192LysCysIleTyrArgArgPheLysTyrGlyLeuLysArgGlyProSer505560ACGGAAGGAGTGCCAGAGTCTATGAGGGAAGAATATCGAAAGGAACAG240ThrGluGlyValProGluSerMetArgGluGluTyrArgLysGluGln65707580CAGAATGCTGTGGATGTTGACGATGGTCATTTTGTCAACATAGAGCTG288GlnAsnAlaValAspValAspAspGlyHisPheValAsnIleGluLeu859095GAGTAA294Glu(2) INFORMATION FOR SEQ ID NO:2:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 97 amino acids(B) TYPE: amino acid(D) TOPOLOGY: linear(ii) MOLECULE TYPE: protein(xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:MetSerLeuLeuThrGluValGluThrProIleArgAsnGluTrpGly151015CysArgCysAsnAspSerSerAspProLeuValIleAlaAlaAsnIle202530IleGlyIleLeuHisLeuIleLeuTrpIleLeuAspArgLeuPhePhe354045LysCysIleTyrArgArgPheLysTyrGlyLeuLysArgGlyProSer505560ThrGluGlyValProGluSerMetArgGluGluTyrArgLysGluGln65707580GlnAsnAlaValAspValAspAspGlyHisPheValAsnIleGluLeu859095Glu(2) INFORMATION FOR SEQ ID NO:3:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 57 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: cDNA(xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:GGATCCGGATCCAGCAAAAGCAGGTAGATATTGAAAGATGAGTCTTCTAACCGAGGT57(2) INFORMATION FOR SEQ ID NO:4:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 38 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: cDNA(xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:TCTAGATCTAGATTACTCCAGCTCTATGCTGACAAATG38(2) INFORMATION FOR SEQ ID NO:5:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 36 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: cDNA(xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:CGGGATCCAAAAAATGCATTTTAGAAGAACGATGAG36(2) INFORMATION FOR SEQ ID NO:6:(i) SEQUENCE CHARACTERISTICS:(A) LENGTH: 30 base pairs(B) TYPE: nucleic acid(C) STRANDEDNESS: single(D) TOPOLOGY: linear(ii) MOLECULE TYPE: cDNA(xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:CCCGCTCGAGCGATGAGTGGGGATTTTGTC30__________________________________________________________________________