Active site mutagenesis of the AIDS virus protease and its alleviation by trans complementation

trans-acting viral protease is necessary and sufficient for activation of avian leukosis virus reverse transcriptase

The structural and enzymatic components of retroviral cores are formed by proteolytic cleavage of precursor polypeptides, mediated by the viral protease (PR). We described previously the construction of PR-defective avian leukosis viruses. These mutant viruses are noninfectious, and their major internal components are the uncleaved gag and gag-pol polyproteins (Pr76gag and Pr180gag-pol). The reverse transcriptase (RT) activity associated with the PR-defective virions is approximately 500-fold reduced relative to that of wild-type virions, suggesting that specific cleavages activate RT activity. To gain a better understanding of the role that PR plays in the processing and activation of RT, we performed complementation experiments wherein wild-type or PR mutant gag precursors were separately coexpressed with frame-corrected wild-type or PR mutant gag-pol precursors. The results demonstrate that, as in other retrovirus systems, gag-pol precursors can be assembled into virions only when they are rescued by a gag precursor. If the gag precursor is wild type, then the rescued Pr180gag-pol is completely and properly matured, irrespective of whether its embedded PR domain is wild type or mutant. In both cases, the virions produced are fully and equally infectious. This indicates that an active-site mutation in the PR domain of the gag-pol precursor has no effect on avian leukosis virus infectivity when particles are assembled from wild-type gag precursors. In contrast, if the gag precursor has an active-site mutation in PR or is deleted for PR, then the virions are noninfectious and the gag and gag-pol precursors remain unprocessed, even if the embedded PR domain of Pr180gag-pol is wild type. Thus, in this system, virion-associated Pr180gag-pol displays no detectable cis- or trans-acting PR activity. As assayed with an exogenous template, virions with processed gag-pol polyprotein display high levels of RT activity while those with unprocessed Pr180gag-pol display greatly reduced RT activity. These results demonstrate that during virion assembly, the PR supplied by a gag precursor is both necessary and sufficient for trans-activation of RT through proteolytic maturation of copackaged gag-pol polyprotein.

Role of the avian retroviral protease in the activation of reverse transcriptase during virion assembly

The retroviruses of the avian sarcoma-leukosis virus group synthesize their viral protease (PR) in two precursor forms--as a carboxy-terminal domain of the Gag precursor and as an embedded domain within the Gag-Pol precursor. We have shown previously that the Gag-derived PR is fully capable of processing the Gag precursor in the absence of the embedded PR (R.P. Bennett, S. Rhee, R.C. Craven, E. Hunter, and J.W. Wills, J. Virol. 65:272-280, 1991). In this study, we examined the question of whether or not the PR domain of Gag-Pol has an essential role in the maturation of the Pol proteins. The Gag-Pol precursor was expressed in the absence of Gag by use of a simian virus 40-based vector in which the gag and pol reading frames were fused. The fusion protein accumulated to high levels in transfected cells without being released into the medium but could be rescued into particles by coexpression of the Gag protein from a second vector. The resulting particles contained mature Gag and Pol proteins and active reverse transcriptase (RT). Using this complementation system, the effects of PR defects in the Gag and/or Gag-Pol proteins on the activation of RT were examined. The results showed that the presence of a functional PR on the Gag precursor, but not on Gag-Pol, was required for full activation of RT. The embedded PR of Gag-Pol was unable to carry out any detectable processing of the Gag precursor and was able to activate RT to only a low level in the absence of a functional Gag PR domain. Finally, some point mutations in the Gag-Pol PR domain inhibited activation of RT in trans by a wild-type PR, suggesting that the correct conformation of the PR domain in Gag-Pol is prerequisite for activation of RT.

Autoprocessing of the HIV-1 protease using purified wild-type and mutated fusion proteins expressed at high levels in Escherichia coli

Various constructs of the human immunodeficiency virus, type 1 (HIV-1) protease containing flanking Pol region sequences were expressed as fusion proteins with the maltose-binding protein of the malE gene of Escherichia coli. The full-length fusion proteins did not exhibit self-processing in E. coli, thereby allowing rapid purification by affinity chromatography on cross-linked amylose columns. Denaturation of the fusion protein in 5 M urea, followed by renaturation, resulted in efficient site-specific autoprocessing to release the 11-kDa protease. Rapid purification involving two column steps gave an HIV-1 protease preparations of greater than 95% purity (specific activity approximately 8500 pmol.min-1.micrograms protease-1) with an overall yield of about 1 mg/l culture. Incubation of an inactive mutant protease fusion protein with the purified wild-type protease resulted in specific trans cleavage and release of the mutant protease. Analysis of products of the HIV-1 fusion proteins containing mutations at either the N- or the C-terminal protease cleavage sites indicated that blocking one of the cleavage sites influences the cleavage at the non-mutated site. Such mutated full-length and truncated protease fusion proteins possess very low levels of proteolytic activity (approximately 5 pmol.min-1.micrograms protein-1).

Expression and frameshifting but extremely inefficient proteolytic processing of the HIV-1 gag and pol gene products in stably transfected rodent cell lines

Expression, ribosomal frameshifting, and proteolytic processing of HIV-1 GAG and POL proteins were investigated in heterologous mammalian cells in order to elucidate the influence of the cellular background on these events. DNA fragments encoded by the gag and pol region were expressed in two rodent cell lines, LTK- and BHK. Both stably transfected cell lines continuously produce recombinant proteins which react with HIV-specific antisera. The GAG precursor and a 39-kDa proteolytic fragment thereof were the major recombinant proteins detected. Expression of the gag-pol region leads to the production of the GAG-POL precursor. Ribosomal frameshifting at the HIV-1 shifty sequence to a typical extent could be positively demonstrated by an enzyme assay. Despite the presence of the viral protease within the GAG-POL precursors, proteolytic processing of the HIV-derived polyproteins was extremely inefficient. The efficiency could not be enhanced by overexpression of the HIV-1 protease encoding region.

Identification and characterization of human immunodeficiency virus type 1 gag-pol fusion protein in transfected mammalian cells

Three human immunodeficiency virus type 1 (HIV-1) mutants were constructed with mutations in their protease genes: AH2-pSVL, with an in-phase deletion; BH27-pSVL, with an out-of-phase deletion creating a stop codon immediately after the deletion site; and CA-pSVL, with a point mutation creating an Asp-to-Ala substitution at the putative protease active site. The wild-type, HXB2-pSVL, and the mutated viral genomes were used to transfect COS-M6 cells and to produce virions. Immunoblotting assays with a monoclonal antibody (MAb) specific for p24 showed that all three mutant contained a gag precursor, Pr56gag, with AH2 and CA expressing an extra band of about 160 kDa. Similar assays with a MAb specific for HIV-1 reverse transcriptase (RT) also revealed a 160-kDa protein from AH2 and CA virions and two mature p66 and p51 RT subunits from HXB2 virions. In addition, HXB2, AH2, and CA but not BH27 virions exhibited RT activity. The same protein in the 160-kDa band seemed to possess both p24 and RT components, since the MAb against p24 was able to immunoadsorb RT antigen and enzymatic activity. These results indicate that the HIV-1 gag-pol fusion protein produced in mammalian cells expressed significant RT activity.

Expression of an autoprocessing CAT-HIV-1 proteinase fusion protein: purification to homogeneity of the release 99 residue proteinase

The 99 residue human immunodeficiency virus type 1 proteinase has been expressed in Escherichia coli as part of an autocleaving fusion protein. Expression of the fusion protein is toxic to the host cells, however yields of the released proteinase have been improved by optimising induction nad harvest times to increase culture biomass, and decrease degradation of the proteinase. Soluble proteinase was extracted from these cells by a simple and highly efficient three step process. N-terminal sequence analysis confirms that the enzyme preparation is highly pure and correctly autoprocessed. The proteinase cleaves peptide substrate IGCTLNFPISPIETV between F and P at pH 6.0 with a Km of 310 microM and a Kcat of 14s-1. The enzyme is sensitive to its ionic environment, showing stimulation of activity at high salt concentrations, and shows a pH optimising 5.5.

High-yield purification of HIV-1 proteinase expressed by a synthetic gene in Escherichia coli

A rapid and simple purification procedure for human immunodeficiency virus type 1 (HIV-1) proteinase from a synthetic gene expressed in Escherichia coli has been developed. The synthetic gene was constructed from oligonucleotides containing several restriction enzyme sites in order to allow simple construction of homologous genes. The protein was translated as a precursor which was autocatalytically processed into the mature protein as shown by N-terminal sequence analysis of the purified protein. Immunoblot analysis was used to verify the nature of the expression product and it was found that 2 of 10 anti-peptide antibodies, covering the whole proteinase sequence, were able to react with the enzyme in crude bacterial lysates. These two anti-peptide antibodies represent a continuous sequence partially overlapping the active site. The purification involves two initial precipitation steps followed by cation-exchange and size-exclusion chromatography. A high yield and a high specific activity were achieved.

Human immunodeficiency viral protease is catalytically active as a fusion protein: characterization of the fusion and native enzymes produced in Escherichia coli

Processing of the gag and pol gene precursor proteins of retroviruses is essential for the production of mature infectious virions. The processing is directed by a viral protease that itself is part of these precursors and is presumed to cleave itself autocatalytically. To facilitate study of this process, the protease was produced as a fusion protein in Escherichia coli. In this construct, the 10,793-Da protease was preceeded by two copies of a modified IgG binding domain derived from protein A. The IgG binding domain was linked to the protease by an Asp-Pro peptide bond which could not be cleaved by the viral protease. A dimer of the 25,400-Da fusion protein was catalytically active, specifically cleaving a substrate peptide at the correct Tyr-Pro bond. Thus, the fusion protein could serve as a model of the viral gag-pol polyprotein. The finding that the fusion protein was catalytically active supports the suggestion that a gag-pol dimer can initiate a proteolytic cascade after budding of the immature virus. The fusion protein also provided a source of authentic protease. The protease was released from the fusion construct by incubation with formic acid, cleaving the Asp-Pro linkage which had been inserted between the IgG binding domain and the protease.

Effect of two novel inhibitors of the human immunodeficiency virus protease on the maturation of the HIV gag and gag-pol polyproteins

The ability of two novel synthetic compounds to inhibit the HIV protease-mediated processing of HIV-1 precursor polyproteins was investigated in an in vitro gag-protease mixed lysate assay system and in an assay using recombinant baculoviruses engineered to express the HIV-1 gag and pol genes in cultured insect cells. With the in vitro mixed lysate assay we have shown that both compounds at 1 microM can completely inhibit the HIV-1 and HIV-2 protease-mediated release of p24 from the HIV-1 gag precursor at pH 5.5 and pH 7.0. In the intracellular baculovirus system these compounds were shown to inhibit the protease-mediated maturation of gag and also the excision of the protease moiety from its precursor.

Properties of avian retrovirus particles defective in viral protease

The structural and enzymatic components of retroviral cores are formed by proteolytic cleavage of precursor polypeptides, mediated by the viral protease (PR). We constructed an active-site mutation, D37I, in the PR of avian leukosis virus. The D37I mutation was introduced into an infectious DNA clone, and quail cell lines expressing the mutant virus were established. These cell lines produce normal amounts of virus particles, the major internal protein components of which are the uncleaved gag and gag-pol precursors. As in other retroviral systems, the protease-defective virions are noninfectious and retain the "immature" type A morphology as determined by thin-section transmission electron microscopy. The virion cores are stable at nonionic detergent concentrations that completely disrupt wild-type cores. Digestion of mutant virions with exogenous PR in the presence of detergent leads to complete and correct cleavage of the gag precursor but incomplete cleavage of the gag-pol precursor. The protease-defective virions encapsidate normal amounts of genomic RNA and tRNA(Trp) that is properly annealed to the primer-binding site, but some of the genomic RNA remains monomeric. Results from UV cross-linking experiments show that the gag polyprotein of mutant virions interacts with viral RNA and that this interaction occurs through the nucleocapsid (NC) domain. However, within mutant virions the interaction of the NC domain with RNA differs from that of mature NC with RNA in wild-type virions. Reverse transcriptase (RT) activity associated with mutant virions is diminished but still detectable. Digestion of the virions with PR leads to a fivefold increase in activity, but this PR-mediated activation of RT is incomplete. Since in vitro cleavage of the gag-pol precursor is also incomplete, we hypothesize that amino acid sequences N terminal to the reverse transcriptase domain inhibit RT activity.

Modified oligopeptides designed to interact with the HIV-1 proteinase inhibit viral replication

The human immunodeficiency virus 1 (HIV-1) codes for a proteinase that cuts viral proteins at specific sites. We have tested 13 modified oligopeptides related to these cleavage sites to see if they inhibit viral replication. To indicate whether a decrease in replication could be due to a general inhibition of cell metabolism, we also measured the effect of the peptides on cellular protein synthesis. Three of the peptides tested (Ac-Gln-Asn-Sta-Val-NH2, Ac-Gln-Asn-Sta-Val-Val-NH2, and Ac-Glu-Asn-Sta-Ile-NH2) inhibited HIV-1 replication at concentrations that did not inhibit protein synthesis. Ac-Gln-Asn-Sta-Val-NH2 was the most potent, causing an approximately 40% decrease in viral replication, measured as the synthesis of HIV-1 antigens and the formation of infectious particles.

Rational design of peptide-based HIV proteinase inhibitors

A series of peptide derivatives based on the transition-state mimetic concept has been designed that inhibit the proteinase from the human immunodeficiency virus (HIV). The more active compounds inhibit both HIV-1 and HIV-2 proteinases in the nanomolar range with little effect at 10 micromolar against the structurally related human aspartic proteinases. Proteolytic cleavage of the HIV-1 gag polyprotein (p55) to the viral structural protein p24 was inhibited in chronically infected CEM cells. Antiviral activity was observed in the nanomolar range (with one compound active below 10 nanomolar) in three different cell systems, as assessed by p24 antigen and syncytium formation. Cytotoxicity was not detected at 10 and 5 micromolar in C8166 and JM cells, respectively, indicating a high therapeutic index for this new class of HIV proteinase inhibitors.

High frequency of isolation of defective human immunodeficiency virus type 1 and heterogeneity of viral gene expression in clones of infected U-937 cells

Limiting-dilution techniques were employed to derive single-cell clones from U-937 cells that had been chronically infected with human immunodeficiency virus type 1. All clones thus obtained were positive for the presence of viral antigens; however, not all of the clones produced infectious progeny virus, as detected by the presence of reverse transcriptase (RT) activity in culture fluids. Six of these clones were monitored over time to determine whether their phenotype of human immunodeficiency virus type 1 expression was stable. Three clones maintained production of RT activity at a high level and showed a very high percentage of cells positive for viral p24 antigen, as determined by indirect immunofluorescence. The other three clones showed variations in either their levels of RT activity or the number of cells positive for p24, after which they stabilized. Infectious virus could be recovered from only three clones, as assessed by coculture experiments with different cell types. Two other clones were shown to produce noninfectious viruses. Molecular analyses at the DNA, RNA, and protein levels showed extensive variations between the viral isolates recovered from each clone.

A simple Escherichia coli system for monitoring HIV protease activity: analysis of two temperature-sensitive protease mutants

A simple Escherichia coli system has been developed for the detection of human immunodeficiency virus (HIV) protease activity. In this system, the protease sequence is placed downstream of the HIV gag polypeptide in an operon arrangement. Upon expression of the operon, gag serves as the substrate for the protease; the level of protease activity can be determined by measurement of the cleavage product of gag in cell extracts by Western immunoblotting. This system is useful in both detection of protease mutations generated by mutagenesis and in testing substrate specificity of the protease by mutagenesis of the gag sequence. Using this system, we have observed that modification of the N-terminus of HIV protease renders the enzyme temperature sensitive; the temperature sensitivity is made more pronounced by the conserved change of valine to isoleucine at residue eleven.

High-level synthesis of recombinant HIV-1 protease and the recovery of active enzyme from inclusion bodies

A complete chemical synthesis and assembly of genes for the production of human immunodeficiency virus type-I protease (HIV-PR) and its precursors are described. The T7 expression system was used to produce high levels of active HIV-PR and its precursors in Escherichia coli inclusion bodies. The gene encoding the open reading frames of HIV-PR was expressed in E. coli as a 10-kDa protein, while the genes encoding HIV-PR precursors were expressed as larger proteins, which were partially processed in E. coli to the 10-kDa form. These processing events are autoproteolytic, since a single-base mutation, changing the active-site aspartic acid to glycine, completely abolished the conversion. HIV-PR can be released with 8 M urea from washed cellular inclusion bodies, resulting in a preparation with few bacterial host proteins. After refolding, this preparation contains no nonspecific protease or peptidase activities. The recombinant HIV-PR isolated from inclusion bodies cleaves HIV-PR substrates specifically with a specific activity comparable to column-purified HIV-PR.

Rapid purification of homodimer and heterodimer HIV-1 reverse transcriptase by metal chelate affinity chromatography

We have modified an Escherichia coli vector expressing 66-kDa HIV-1 reverse transcriptase (p66) so that it simultaneously expresses this and the pol-coded protease. The twin expression cassette yields high quantities of both reverse transcriptase and protease; however, under these conditions, 50% of the over-expressed p66 reverse transcriptase is processed, resulting in accumulation of large quantities of p66/p51 enzyme. Furthermore, addition of a poly(histidine) affinity label at the amino terminus of the reverse-transcriptase-coding sequence (His-p66) permits a simple, rapid purification of milligram quantities of either p66 or p66/p51 enzyme from a crude lysate by metal chelate affinity chromatography. Purified His-p66 and His-p66/His-p51 reverse transcriptase exhibit both reverse transcriptase and RNase H activity. Purification by metal chelate chromatography of a p66/p51 enzyme wherein only the p66 component is labelled strengthens the argument for the existence of a heterodimer.

Substrate analogue inhibition and active site titration of purified recombinant HIV-1 protease

The aspartyl protease of human immunodeficiency virus 1 (HIV-1) has been expressed in Escherichia coli at high levels, resulting in the formation of inclusion bodies which contain denatured insoluble aggregates of the protease. After solubilization of these inclusion bodies in guanidinium chloride, the protease was purified to apparent homogeneity by a single-step reverse-phase HPLC procedure. The purified, but inactive, protein was denatured in 8 M urea and refolded to produce the active protease. Enzyme activity was demonstrated against the substrate H-Val-Ser-Gln-Asn-Tyr-Pro-Ile-Val-OH, modeled after the cleavage region between residues 128 and 135 in the HIV gag polyprotein. With this substrate, a Vmax of 1.3 +/- 0.2 mumol/(min.mg) and KM of 2.0 +/- 0.3 mM were determined at pH 5.5. Pepstatin (Iva-Val-Val-Sta-Ala-Sta-OH) and substrate analogues with the Tyr-Pro residues substituted by Sta, by Phe psi [CH2N]Pro, and by Leu psi [CH(OH)CH2]Val inhibited the protease with KI values of 360 nM, 3690 nM, 3520 nM, and less than 10 nM, respectively. All were competitive inhibitors, and the tightest binding compound provided an active site titrant for the quantitative determination of enzymatically active HIV-1 protease.

HIV-1 protease: mutagenesis of asparagine 88 indicates a domain required for dimer formation

Considerable interest exists in the HIV-1 protease for biochemical studies as a potential therapeutic target of acquired immunodeficiency syndrome. We have produced the retroviral enzyme in E. coli from a synthetic gene encoding the protease that was constructed by assembling six overlapping and complementary oligonucleotides into the vector pKK223-3. When expressed in E. coli, the recombinant protease was able to correctly process the HIV-1 core protein p24 from a beta-galactosidase-gag fusion protein and to use a heptapeptide as a substrate for proteolytic cleavage. A single base pair mutation was identified in a recombinant that resulted in the substitution of lysine for asparagine at position 88 and a significant loss of enzyme activity. Through site-directed mutagenesis, the Asn88 was changed to five other residues representative of all classes of amino acids. The correlation between enzyme activity and amino acid substitution suggests that the protease domain surrounding position 88 affects the protein's potential for forming an active homodimeric protein and hence, indicates a biochemical interaction that could be inhibited by novel antiviral compounds.

Biosynthesis of hepatitis B virus e antigen: directed mutagenesis of the putative aspartyl protease site

The C gene products of all mammalian hepadnaviruses contain a region with sequence similarities to the catalytic center of the aspartyl proteases. This region could have the capacity to cleave precore proteins, leading to the synthesis of e antigen. By site-directed mutagenesis on a plasmid containing the hepatitis B virus C gene, we have replaced either the Asp residue of the putative aspartyl protease catalytic center or an Asp residue located 3 amino acids upstream. Transient expression of the mutated hepatitis B virus C gene in human and mouse cells showed that none of these mutations prevented the secretion of an accurately processed HBe antigen. Thus, we demonstrated that the aspartyl protease responsible for e antigen precursor processing is not C gene encoded but is more likely to be a cellular enzyme. From these results, we suggest a model for the mechanism of e antigen synthesis.

A fluorescent oligopeptide energy transfer assay with broad applications for neutral proteases

A fluorescent peptide substrate to explore the protease specificity for the amino acid regions C- and N-terminal to the cleavage site has been designed. Intramolecular quenching of indole fluorescence by an N-terminal dansyl group separated by six amino acid residues forms the basis of this assay. For a particular enzyme, specificity can be designed into the peptide sequence by means of the number of residues that separate the two chromophores. In the present instance, the heptapeptide Dns-Gly-Lys-Tyr-Ala-Pro-Trp-Val is used to assay angiotensin converting enzyme (ACE), Astacus protease, carboxypeptidase A, alpha-chymotrypsin, and trypsin, all of which cleave the peptide in accord with their known specificity: Trypsin and Astacus protease hydrolyze only the Lys-Tyr and Tyr-Ala bonds, respectively. alpha-Chymotrypsin primarily cleaves the Tyr-Ala bond while ACE makes three successive dipeptidyl cleavages from the C-terminus. Carboxypeptidase rapidly hydrolyzes first the Trp-Val and then the Pro-Trp bond. For all of the enzymes, catalytic activity (kcat/Km) is in the range from 10(5) to 10(6) M-1 s-1. Hydrolysis causes a fluorescence increase in the 310 to 410 nm region of 8.6- to 13.6-fold depending on the enzyme that is assayed. Assays can be designed based on the increase in tryptophan fluorescence or by individual product analyses using thin-layer or high-performance liquid chromatography. The specificity and sensitivity of such internally quenched fluorescent oligopeptides would seem to be ideal for the assay of specific endoproteases.

Recombinant HIV-1 reverse transcriptase: purification, primary structure, and polymerase/ribonuclease H activities

Recombinant HIV-1 reverse transcriptase (RT) was stably overproduced as a soluble protein in Escherichia coli using a double-plasmid expression system in which an RT precursor protein was expressed and processed in vivo by HIV-1 protease produced in trans. The RT thus produced consisted of an equimolar mixture of two polypeptides, p66 and p51, which were copurified to greater than 90% homogeneity and were found to share a common NH2 terminus as judged by sequence analysis of the polypeptide mixture. The observed sequence confirmed correct in vivo cleavage by protease at the protease-RT polyprotein junction to yield an NH2 terminus identical to that of genuine viral RT (M. M. Lightfoote et al. (1986) J. Virol. 60, 771-775; F. diMarzo Veronese et al. (1986) Science 231, 1289-1291). The bacterially expressed RT had a specific activity similar to that of viral RT and inhibition studies with phosphonoformate confirmed that it was indistinguishable from the viral enzyme with respect to sensitivity to this inhibitor. Polymerase activated gel analysis of the mixture indicated that p66 was associated with a higher level of RT activity than p51. RNase H activated gel analysis suggested that the purified preparation of recombinant RT was free of endogenous E. coli RNase H, and that the RNase H activity of RT was exclusively associated with the p66 polypeptide, supporting the hypothesis that the RNase H domain is located in the COOH-terminal region of the molecule.

Conserved folding in retroviral proteases: crystal structure of a synthetic HIV-1 protease

The rational design of drugs that can inhibit the action of viral proteases depends on obtaining accurate structures of these enzymes. The crystal structure of chemically synthesized HIV-1 protease has been determined at 2.8 angstrom resolution (R factor of 0.184) with the use of a model based on the Rous sarcoma virus protease structure. In this enzymatically active protein, the cysteines were replaced by alpha-amino-n-butyric acid, a nongenetically coded amino acid. This structure, in which all 99 amino acids were located, differs in several important details from that reported previously by others. The interface between the identical subunits forming the active protease dimer is composed of four well-ordered beta strands from both the amino and carboxyl termini and residues 86 to 94 have a helical conformation. The observed arrangement of the dimer interface suggests possible designs for dimerization inhibitors.

Members of the RTVL-H family of human endogenous retrovirus-like elements are expressed in placenta

A cDNA clone homologous to the RTVL-H family of human retrovirus-like elements was isolated from a human placenta cDNA library. The nucleotide sequence of the 1084-bp cDNA revealed an open reading frame (ORF) that may encode a 146 amino acid protein with significant homology to retroviral proteases. Downstream from the putative protease ORF a 3' long terminal repeat (LTR) containing U3 and R regions was found. The cDNA sequence ends in a poly(A) tail appropriately positioned downstream from a polyadenylation signal in the LTR. Northern-blot analysis showed that several distinct RTVL-H homologous transcripts are present in human placenta. We also show that repetitive RTVL-H homologous sequences are present in the genomes of both gorilla and African green monkey.

The proteinase inhibitor pepstatin A inhibits formation of reverse transcriptase in H9 cells infected with human immunodeficiency virus 1

Retroviruses depend on a virus-encoded proteinase. As this enzyme is an interesting target for antiviral therapy, we examined the effect of various low-molecular-weight proteinase inhibitors, as well as a few oligopeptides related to the proteolytic cleavage sites, on the replication of HIV-1 in H9 cells. The increase in reverse transcriptase activity during incubation was assumed to reflect viral replication. Cellular DNA synthesis was measured to quantitate the adverse effects of the inhibitors on the cells. Only one of the substances tested, pepstatin A, had an appreciable selective effect on viral replication. Substances that decreased DNA synthesis generally caused an equally large decrease in reverse transcriptase activity.

The protease and gag gene products of the human immunodeficiency virus: authentic cleavage and post-translational modification in an insect cell expression system

Three recombinant baculoviruses which are capable of expressing human immunodeficiency virus (HIV) protease, p55gag, and both products simultaneously in insect cell culture have been constructed. Upon co-infection of cells with the protease and p55gag-expressing viruses, authentic processing of the gag precursor is observed to take place. This processing could be reproduced in vitro using mixtures of cellular lysates containing the expressed proteins. When expressed alone, uncleaved p55gag precursor appears to form retroviral core-like particles within the cytoplasm of infected cells. Metabolic labeling studies of the baculovirus-expressed gag products have demonstrated that p17 is myristylated at its amino terminus, and that p24 is phosphorylated. In these respects, the insect cell system is evidently capable of carrying out post-translational processing resembling that which occurs in authentic HIV-1 replication.

Inhibition of HIV replication in cell culture by the specific aspartic protease inhibitor pepstatin A

After incubation of H9 cells infected with human immunodeficiency virus (HIV) with pepstatin A at 10(-4) M for 2, 4, or 11 days, the culture medium contained significantly less HIV core antigen (p24) than controls without pepstatin A and no or only borderline activity of reverse transcriptase was detected. In addition, after pepstatin A treatment no infectious HIV at 2 or 4 days and only minimal amounts at 11 days were detectable in the culture medium.

Possible role of some groups in the structure and function of HIV-1 protease as revealed by molecular modeling studies

Retroviral proteases belong to the class of aspartic proteases. A molecular model of HIV-1 protease has been built on the basis of the consensus template specific for the domains of these enzymes. The template region comprises more than a half of the HIV-1 protease monomer structure, it includes the active site, formed at the junction of the two monomers, binding pockets of the enzyme, and some other molecular segments. These regions can be more conveniently described than other parts of the structure. Some properties of the HIV-1 protease molecule are discussed, as well as of probable inhibitors. The properties of the model structure are in good agreement with the recent results of crystallographic studies of Rous sarcoma virus protease.

Effective blocking of HIV-1 proteinase activity by characteristic inhibitors of aspartic proteinases

Inhibitory constants (Ki) between 5 and 35 nM were derived (under different conditions of pH and ionic strength) for the interaction of HIV-1 proteinase with acetyl-pepstatin and H-261, two characteristic inhibitors of aspartic proteinases. Thus this enzyme, essential for replication of the AIDS virus, may be classified unequivocally as belonging to this proteinase family.

Peptide substrates and inhibitors of the HIV-1 protease

Oligopeptides containing the consensus retroviral protease cleavage sequence Ser/Thr-X-Y-Tyr/Phe-Pro are substrates for purified recombinant HIV-1 protease with Km's in the millimolar range. The minimum sequence containing the consensus pentapeptide which serves as a good substrate is a heptapeptide spanning the P4-P3' residues. Substitution of reduced Phe-Pro or Tyr-Pro dipeptide isosteres or the statine analog 3-hydroxy-4-amino-5-phenylpentanoic acid for the scissile dipeptide afforded inhibitors of HIV-1 protease with Ki values in the micromolar range, three orders of magnitude better in affinity than the corresponding substrates. Inhibitors of HIV-1 protease may provide a novel and potentially useful therapeutic approach to the treatment of acquired immune deficiency syndrome (AIDS).

Recombinant HIV1 protease secreted by Saccharomyces cerevisiae correctly processes myristylated gag polyprotein

The protease of the human immunodeficiency virus type I (HIV1) was expressed both intracellularly and extracellularly in Saccharomyces cerevisiae. Intracellular expression of the protease was achieved by fusing a 179 amino acid precursor form of the protease to human superoxide dismutase (hSOD). Self-processing of the viral enzyme from the hybrid precursor was demonstrated to occur within the yeast host. Secretion of the protease was achieved by fusing the leader sequence of yeast alpha-factor to the precursor form of the protease or to the 99 amino acid mature form of the protease. Authentic and active forms of the retroviral enzyme were detected in yeast supernatants of cells expressing the precursor or the mature form of the protease. A D25E active site variant of the retroviral enzyme exhibited diminished autocatalytic activity when expressed intracellularly or secreted from yeast. The wild-type protease was active in an in vitro assay on the natural substrate, myristylated gag precursor, Pr53gag. Correct processing of Pr53gag at the Tyr 138-Pro 139 junction was confirmed by amino terminal sequence analysis of the resulting capsid protein (CA, p24). The secreted protease was purified to homogeneity from yeast media using preparative isoelectric focusing and reverse-phase HPLC. Amino terminal sequence analysis showed a sequence beginning at amino acid 1 of the mature enzyme (Pro) and another sequence beginning at amino acid 6 (Trp). This shorter sequence may represent a natural autolytic product of the protease.