METHOD FOR DETECTING INHIBITORS OF TAT PROTEIN

Abstract: A composition of matter comprising TAR RNA and tat protein; and a method of evaluating inhibitory effect of a substance on tat protein-TAR RNA binding comprising the steps of preparing a mixture of the test substance and tat protein, adding to the preparation of step 1 a composition of matter containing TAR RNA and observing the preparation of step 2 for evidence of binding between TAR RNA and tat protein.

Description: METHOD FOR DETECTING INHIBITORS OF TAT PROTEIN INTRODUCTION It has been shown that HIV gene expression is regulated by the viral transregulatory protein known as tat. As taught herein, tat protein may now be used in binding assays to screen substances for use as tat inac- tivators. Such inactivators would be useful in treatment of acquired immunodeficiency syndrome (AIDS). BACKGROUND OF THE INVENTION It has been known that HIV-1 gene expression is positively regulated by the viral transregulatory pro¬ tein, tat (Arya, et al., 1985, Sodroski, et al., 1985). Tat protein is an 86 amino acid polypeptide that is translated 'from doubly spliced mRNA containing one 5' non-coding exon and two coding exons. Tat expression is believed to be essential for HIV replication, since mutants lacking tat fail to produce significant levels of viral protein and mRNA. A "tat activation response" element, termed TAR, has been localized within the long terminal repeat (LTR), by deletion analysis, to the region -17 to +80 nucleotides relative to the transcrip¬ tion initiation site (Rosen, et al., dependent and can confer tat inducibility to heterologous promoters, albeit to a reduced level. DESCRIPTION OF THE INVENTION It is the purpose of this invention to provide, an assay which will indicate inhabition of binding of tat protein to TAR. It is a further purpose of this invention to provide a method for evaluating agents which bind tat as a means of identifying useful therapeutic agents. It is a further purpose of this invention to provide reliable methods for producing large amounts of tat protein. Figures; Figure 1 shows the construction of the baculo- virus transfer vector pAcYMl-tat is illustrated. The Ba HI fragment derived from pIBl31-tat, used to construct pAcYMl-tat, was inserted in the E. coli expression vector pREV as in U.S. Patent 4,721,671, which is incorporated herein by reference. Figure 2 shows the extract preparations of both E. coli and baculovirus systems containing tat protein was determined by Western Blot analysis. Figure 3 shows the functional activity of both baculo¬ virus and E. coli preparations were evaluated for func¬ tional activity as determined by induced level of CAT activity. Figure 4 shows a construct which substituted the upstream LTR sequences with the bacteriophage T7 polymerase promoter is illustrated. Figure 5 shows the assay of direct binding are shown using 32 p labeled 80 nt transcript in RNA gel mobility shift experiments of recombinant tat. Retarded bands represent protein-RNA complexes with E. coli tat and baculovirus tat extracts. Note comparison with activity with baculovirus preparations of pgl60 and rev. Figure 6 shows the specificity of binding reaction is determined with RNA gel mobility shift assay using wild type and mutant transcripts. Figure 7 shows the binding to RNA by excess unlabeled wt TAR RNA is completed as illustrated. Since sequences which are responsive to tat are present immediately downstream of the transcription initiation site, tat was believed to interact directly or indirectly with these sequences at either the DNA or RNA level, or both. It had been predicted that the 5' untranslated region of HIV-l mRNA formed a highly stable stem loop structure (Okomoto and Wong-Staal, 1986, Feng and Holland, 1988). The limits of TAR within HIV-l and HIV-2 RNA include one and three stem loops, respective¬ ly. It had been known that while little overall sequence homology exists between HIV-l and HIV-l TAR, the stem- loop structure, as well as the pentanucleotide repeat sequence within the predicted loop, CTGGG, are SUBST I TUTESHEET maintained. Site directed utagenesis of the penta u- cleotide motif has demonstrated the requirement of the sequence (CTGGG) for tat transactivation. In addition to the requirement for recognition of sequences within the predicted TAR loop, complementarity of the bases forming the stem appeared to be essential. It is believed that this is the reason that the 3' border of the TAR region, as determined by progressive deletions, has been mapped to +44. Deletions extending further 5' could diminish base complementarity beyond a critical level needed for formation of RNA- stem loop structure. (While these findings, taken together support the notion that RNA is a target in the tat transactivation, the possibility remains that sequences within TAR are required for interaction at the DNA level with transregulatory factors through the formation of cruciform structures. ) Example 1 In order to investigate the mechanism of tat activation, recombinant tat was produced in both E. coli and baculovirus expression systems. The construction of the baculovirus transfer vector pAcYMl-tat is illustrated in Figure 1. The same Bam HI fragment derived from pIBl31-tat, used to construct pAcYMl-tat was inserted into the E. coli expression vector pREV (U.S. Patent 4,721,671). In this construction, tat was expressed as a fusion protein containing 36 amino acids of E. coli βglucuronidase at the a ino terminus followed by the entire coding sequence of tat. These additional sequences have been shown to confer stability at the protein as well as at the RNA level to various eukaryotic gene products expressed in E. coli (see referenced patent) . Example 2 Extract preparations from both E. coli and baculovirus systems containing tat protein was determined by SDS polyacryiamide gel electrophoresis (not shown) as well as by Western Blot analysis (Figure 2). The baculo- SUBSTITUTE SHEET virus tat migrated as a doublet of 14.5 and 15.5 kDa. The E. coli produced fusion protein as slightly larger, migrating at 17 kDa in a minor band at 15.5 kD . The doublet observed in the baculovirus tat preparation may represent limited proteolysis or, alternatively, may represent two forms of the tat protein at the conforma- tional level, or may result from covalent modification (eg. phosphorylation) . While this may apply to the 15.5 kDa polypeptide in the E. coli preparation, this faster migrating species may correspond to internal initiation at the authentic tat AUG codon. The purity of the baculovirus tat and E. coli tat preparations are esti¬ mated at 5 and 50 percent, respectively, as determined by visualization of stained polyacrylamide gels (data not shown) . Example 3 To test ability of the recombinant tat proteins to transactivate HIV-l LTR directed gene expression, the E. coli and baculovirus protein preparations were intro- duced by "scrape-loading" procedure into He a cells, stably integrated with the HIV-l LTR linked to the bacterial chloramphenicol acetyl transferase gene (Hela/LTR-CAT) . Both baculovirus and E. coli tat prepa¬ rations were functionally active as determined by the induced level of CAT activity (Figure 3). As controls, baculovirus gpl60 infected cell extracts prepared the same way as the baculovirus tat preparation and mock treated with Hela/LTR-CAT cells showed no significant increase in LTR directed gene expression. Example 4 In an effort to study the potential association of tat with the TAR sequence present at the 5' region of HIV m NAs, a construct was designed which substituted the upstream LTR sequences with the bacteriophage T7 poly- merase promoter (Figure 4). The plasmid T7-TAR-CAT was digested with Hind III. This plasmid directs the synthesis of an 80 nucleotide transcript in vitro with T7 SUBSTITOTE SHEET polymerase and nucleoside triphosphates. The T7 enzyme initiates faithfully at the HIV-l cap site, as determined by primer extension of in vitro synthesized RNA (data not shown) . Example 5 32 p labeled 80nt transcript in RNA gel mobility shift experiments were used to assay the direct binding of recombinant tat. Retarded band representing protein- RNA complexes were detected using the E. coli tat and baculovirus tat extracts, but not with baculovirus gplβO or baculovirus rev preparations (Figure 5). The RNA complex formed using the E. coli tat migrated at exactly the same position as one of the complexes formed with the baculovirus tat. An additional complex was seen with baculovirus tat migrating an intermediate . distance between the probe and upper band. (The nature of the two complexes is not clear, but could correspond to the two bands observed for baculovirus tat in the Western Blot. Alternatively, the additional band may represent a vari- ation in the RNA-protein complex, or a monomer or dimer formation. ) No differences were observed in the binding experiments using capped and uncapped RNAs (data not shown) . Example 6 Specificity of binding reaction was determined by the RNA gel mobility shift assay using wild type and mutant transcripts (Figure 6). A four base pair deletion was generated at the SSTI site, +35 to +38 nucleotides relative to the transcription initiation site, in the T7- TAR-CAT plasmid. The resulting plasmids contained a deletion overlapping the loop and stem. This mutation has been shown to abolish transactivation in the HIV-l LTR-CAT background. Unlike the wild type transcript, the deleted (ΔS) transcript failed to form a stable complex with recombinant tat protein in the gel mobility shift assay. The binding to RNA was completed by excess unlabeled wt TAR RNA (Figure 7) . SUBSTITUTE SHEET Discussion: The sequence requirements for binding correlate with the sequences requirements for transactivation, suggesting a correlation between binding of tat protein and TAR. Since the binding of the tat protein to TAR effects transactivation, comparative testing of compounds for the effect on tat protein-TAR RNA binding can be used as a screen to evaluate compositions for activity against the HIV-l gene expression regulated by the tat. The method for evaluating inhibitory effect of the substance would, therefore, consist of the following steps: 1) preparing the mixture of the test substance and tat protein; 2) adding to the preparation which is the product of step (1) a composition of matter containing TAR RNA; and 3) observing the product prepared in step (2) (for evidence of binding between TAR RNA and tat protein. While the use of 32 p labelled 80nt transcript is used in an RNA gel mobility shift experiment to assay direct binding, the invention should not be construed as limited to that embodiment. For example, either the protein or the TAR RNA could be labelled with labels known to those of ordinary skill in the art. The conditions under which the assay to test for binding is run will be adjusted depending on the substance being tested or effect on tat protein TAR RNA binding. As indicated in the samples, the rate at which tat or any complex of that protein migrate on a gel may depend on the vector system in which the components were produced. The data clearly indicated specific and stable binding of tat to TAR RNA. A mutation within TAR which abolished transactivation also produced stable complex formation, suggesting that binding of tat to RNA is a functional step in the transactivation pathway. The assay system can be used as a tool in dissecting the SUBSTITUTESHEET sequence requirements for the tat response as well as a convenient system for screening of compounds which would block tat activation of the HIV gene expression. The specificity of tat interaction with RNA may be further increased in the natural setting by inter¬ action with the transcription complex, other cellular factors, or RNA polymerase II. SUBST I TUTESHEET

Claims: WHAT IS CLAIMED IS: 1. A composition of matter comprising TAR RNA and tat protein. 2. A composition of claim 1 in a gel. 3. A method of evaluating inhibitory effect of a substance on tat protein-TAR RNA binding comprising the steps of: 1) preparing a mixture of the test sub¬ stance and tat protein; 2) adding to the preparation of step 1 a composition of matter containing TAR RNA; and 3) observing the preparation of step 2 for evidence of binding between TAR RNA and tat protein. SUBSTITUTE SHEET

Inventors: RAPPAPORT JAY, RUSCHE JAMES R, WONG-STAAL FLOSSIE, RAPPAPORT, JAY, RUSCHE, JAMES, R., WONG-STAAL, FLOSSIE
Applicants: REPLIGEN CORP, US HEALTH, REPLIGEN CORPORATION, THE UNITED STATES OF AMERICA, REPRESENTED BY THE S
Application year: 1990
Main classification: G01N33
IPC codes: A61K38, A61K49, C07K14, C12N15, C12P21, C12Q1, C12R1