Abstract:
HIV protease necessary for the natural synthesis of HIV has been identified and sequenced. Rabbit antiserum against the C-terminal portion of HIV protease has been produced and used to isolate natural HIV protease and characterize its activity. In addition, a method for producing synthetic HIV protease having the correct stereospecific conformation and specific HIV proteolytic activity has been achieved.

Description:
BACKGROUND OF THE INVENTION 
     We conducted research which identified and structurally, biochemically and enzymologically characterized human immunodeficiency virus (HIV) protease, as well as its natural polyprotein substrates, in order to develop drugs that inhibit protease activity. It was known from our work on protease deficient MuLV mutants that when precursor polyproteins are not cleaved mature infectious virus can not be produced. Instead, non-infectious particles are made that, however, remain immunogenic because they carry complete envelopes. The idea underlying this research was to ultimately prepare chemical inhibitors that penetrate the infected cell, become incorporated into the budding virus, bind with high affinity to the viral protease or precursor polyproteins, prevent cleavage and lead to the production of non-infectious but still immunogenic viral progeny. The use of these chemical inhibitors would block the spread of HIV infection while allowing for antigenic stimulation of host immunity. 
     SUMMARY OF THE INVENTION 
     We have identified the amino acid sequence that constitutes human immunodeficiency virus (HIV) protease, a proteolytic enzyme specific for the virus known as HIV, LAV and HTLV-III. This enzyme is necessary in the natural synthesis of HIV in the cells of subjects infected by the virus. In the course of natural synthesis it is necessary for protein of the virus to be lysed from precursor proteins. It is this function for which the proteolytic enzyme we have identified and synthesized is specific. Without HIV protease the active virus cannot be reproduced in infected cells and the natural synthesis process will be stopped short of completion. As a result, with our discovery of the structure of this specific protease and our synthesis of the active enzyme, a protease inhibitor specific for this enzyme can be designed. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 provides the amino acid sequence of the HIV protease. 
     FIG. 2 illustrates the regions of sequence conservation between a number of different retroviral proteases. 
     FIG. 3 provides a portion of the DNA sequence of HIV containing the coding region for the HIV protease. 
     FIG. 4 is a graphic representation of the chromatographic separation of various substrates before and after cleavage with the synthetic HIV protease. FIGS. 4a and 4b illustrate cleavage of MuLV synthetic peptides before and after addition of the HIV protease. FIGS. 4c and 4d illustrate cleavage of the MuLV substrate using cathepsin D and renin respectively. FIGS. 4e and 4f illustrate cleavage of an HIV peptide substrate before and after incubation with the HIV protease. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The DNA sequence coding for HIV was known. As we had previously identified proteases specific for human retroviruses, and we had identified homologies in the sequences coding for proteases necessary in the natural synthesis of these other retroviruses, we were able to identify sections in the DNA sequence coding for HIV proteins that should be within the sequence coding for a protease specific for HIV and necessary for its natural synthesis. (Copeland and Oroszlan, &#34;A Synthetic Dodecapeptide Substrate For Type C RNA Tumor Virus Associated Proteolytic Enzyme,&#34; PEPTIDES: Synthesis-Structure-Function, Pierce Chemical Company, 1981, and Yoshinaka et al, &#34;Murine Leukemia Virus Protease Is Encoded By The gag-pol Gene and Is Synthesized Through Suppression of an Amber Termination Codon&#34;, Proc. Natl. Acad. Sci., USA, Vol. 82, pages 1618-1622, March 1985, included herein in their entirety by reference). Our previous work indicated to us that certain sequences were conserved in retroviral proteases; these are set forth in FIG. 2. 
     HIV protease is a 99 amino acid peptide, which has a molecular weight of 11K-11.5K daltons measured by SDS PAGE. The amino acid sequence is given in FIG. 1, as marked, beginning with ProGlnIle . . . . FIG. 3 indicates the position in the 99 amino acid peptide in the HIV sequence. The peptide sequence is shown in the third line in each grouping. The first line in each grouping is the DNA sequence coding for the HIV protease. The second line in each grouping is an alternative amino acid sequence, which is correct for the expression of certain portions of HIV protein. However, due to a reading frame shift prior to the protease, the sequence we have identified in the third line and repeated in FIG. 1, is the actual amino acid sequence of the protease. After identifying the amino acid sequence for HIV protease, we synthesized the C-terminal peptide consisting of 15 amino acids (IleGlyArgAsnLeuLeuThr GlnIleGlyCysThrLeuAsPhe). With this 15 amino acid synthetic peptide we generated a rabbit antiserum. The rabbit antiserum was then used to isolate the natural protease from HIV. The activity of the natural protease was identified and confirmed by cleaving natural proteins in natural virus substrates and in synthetic substrates characteristic of all retroviruses and, therefore, characteristic of cleavage sites in HIV itself. This assay was conducted according to the method discussed in Copeland and Oroszlan (supra), essentially by incubating the substrate with the suspected protease, and then separating using PAGE and HPLC procedures. 
     Having confirmed that the sequence we identified was for HIV protease, we proceeded by solid phase synthesis using the Merrifield method to synthesize the 99 amino acid peptide (Merrifield R. B. (1963), &#34;Solid Phase Peptide Synthesis I&#34;. J. Amer. Chem. Soc. 85, 2149-2154, included herein by reference). The synthesis was done using the semiautomatic synthesis procedures with an Applied Biosystems program and an Applied Biosystems 430 Peptide synthesizer (Foster City, Calif.). 
     After synthesis, the peptide was removed from the resin on which it was constructed and blocking groups were eliminated from the synthetic peptide by conventional procedures. The synthetic protease was then recovered and purified. 
     As synthesized, the peptide is linear and demonstrates no activity as a proteolytic enzyme. In addition to removing blocking groups, therefore, it was necessary to convert the protease to its natural stereospecific conformation in order to exhibit proteolytic activity. As produced, the synthetic HIV protease was extracted in a strong acid. It was necessary, thereafter, to submit it to treatment and purification using specific buffer systems and dialysis. Thereafter, we proceeded by trial and error to effect renaturation and to refold the peptide into its natural stereospecific confirmation. This was done through recovery in guanidine hydrochloride solution, concentration, recovery of all solutes and recovery in an aqueous solution. The proper folding was accomplished through a series of trial and error steps. The correct folding of the peptide is the result of intra and intermolecular forces and bonding, and of the characteristics of the media to which we subjected the synthetic peptide. 
     We were able to obtain a synthetic protein that showed specific proteolytic activity for HIV. Assays to confirm that we had produced the active peptide were conducted by incubating the synthetic protease with natural viral substrates and with synthetic viral substrates. The results demonstrated that our synthetic HIV protease was specific for the characteristic cleavage sites. The assay methods are described in Oroszlan and Copeland, UCLA Symposium on Molecular and Cellular Biology (included herein by reference). This activity is illustrated in FIG. 4. FIG. 4a illustrates MuLV synthetic peptides containing the known proteolytic cleavage site. FIG. 4b shows the same peptide after being incubated with our synthetic HIV protease. The two peaks to the left of the main peptide indicate products of cleavage of the MuLV substrate. FIGS. 4c and 4d illustrate cleavage of the same substrate using two other proteases, cathepsin D and renin, respectively. As can be seen from the products to the right of the substrate peak, those enzymes cleave the substrate at different cleavage site. FIG. 4e shows a synthetic HIV peptide substrate. FIG. 4f illustrates the results after incubation after our synthetic HIV protease. The products indicated by the peaks to the left of the substrate peak demonstrate proteolysis activity.