Non-viral cellular substrates for human immunodeficiency virus type 1 protease

Focus on Translation Initiation of the HIV-1 mRNAs

To replicate and disseminate, viruses need to manipulate and modify the cellular machinery for their own benefit. We are interested in translation, which is one of the key steps of gene expression and viruses that have developed several strategies to hijack the ribosomal complex. The type 1 human immunodeficiency virus is a good paradigm to understand the great diversity of translational control. Indeed, scanning, leaky scanning, internal ribosome entry sites, and adenosine methylation are used by ribosomes to translate spliced and unspliced HIV-1 mRNAs, and some require specific cellular factors, such as the DDX3 helicase, that mediate mRNA export and translation. In addition, some viral and cellular proteins, including the HIV-1 Tat protein, also regulate protein synthesis through targeting the protein kinase PKR, which once activated, is able to phosphorylate the eukaryotic translation initiation factor eIF2α, which results in the inhibition of cellular mRNAs translation. Finally, the infection alters the integrity of several cellular proteins, including initiation factors, that directly or indirectly regulates translation events. In this review, we will provide a global overview of the current situation of how the HIV-1 mRNAs interact with the host cellular environment to produce viral proteins.

Related Endogenous Retrovirus-K Elements Harbor Distinct Protease Active Site Motifs

Background: Endogenous retrovirus-K is a group of related genomic elements descending from retroviral infections in human ancestors. HML2 is the clade of these viruses which contains the most intact provirus copies. These elements can be transcribed and translated in healthy and diseased tissues, and some of them produce active retroviral enzymes, such as protease. Retroviral gene products, including protease, contribute to illness in exogenous retroviral infections. There are ongoing efforts to test anti-retroviral regimens against endogenous retroviruses. Herein, we examine the potential activity and diversity of human endogenous retrovirus-K proteases, and their potential for impact on immunity and human disease.

Results: Sequences similar to the endogenous retrovirus-K HML2 protease and reverse transcriptase were identified in the human genome, classified by phylogenetic inference and compared to Repbase reference sequences. The topologies of trees inferred from protease and reverse transcriptase sequences were similar and agreed with the classification using reference sequences. Surprisingly, only 62/480 protease sequences identified by BLAST were classified as HML2; the remainder were classified as other HML groups, with the majority (216) classified as HML3. Variation in functionally significant protease motifs was explored, and two major active site variants were identified – the DTGAD variant is common in all groups, but the DTGVD motif appears limited to HML3, HML5, and HML6. Furthermore, distinct RNA expression patterns of protease variants are seen in disease states, such as amyotrophic lateral sclerosis, breast cancer, and prostate cancer.

Conclusion: Transcribed ERVK proteases exhibit a diversity which could impact immunity and inhibitor-based treatments, and these facets should be considered when designing therapeutic regimens.

Translation initiation is driven by different mechanisms on the HIV-1 and HIV-2 genomic RNAs

The human immunodeficiency virus (HIV) unspliced full length genomic RNA possesses features of an eukaryotic cellular mRNA as it is capped at its 5' end and polyadenylated at its 3' extremity. This genomic RNA is used both for the production of the viral structural and enzymatic proteins (Gag and Pol, respectively) and as genome for encapsidation in the newly formed viral particle. Although both of these processes are critical for viral replication, they should be controlled in a timely manner for a coherent progression into the viral cycle. Some of this regulation is exerted at the level of translational control and takes place on the viral 5' untranslated region and the beginning of the gag coding region. In this review, we have focused on the different initiation mechanisms (cap- and internal ribosome entry site (IRES)-dependent) that are used by the HIV-1 and HIV-2 genomic RNAs and the cellular and viral factors that can modulate their expression. Interestingly, although HIV-1 and HIV-2 share many similarities in the overall clinical syndrome they produce, in some aspects of their replication cycle, and in the structure of their respective genome, they exhibit some differences in the way that ribosomes are recruited on the gag mRNA to initiate translation and produce the viral proteins; this will be discussed in the light of the literature.

How HIV takes advantage of the cytoskeleton in entry and replication

The host cell cytoskeleton plays a key role in the life cycle of viral pathogens whose propagation depends on mandatory intracellular steps. Accordingly, also the human immunodeficiency virus type 1 (HIV-1) has evolved strategies to exploit and modulate in particular the actin cytoskeleton for its purposes. This review will recapitulate recent findings on how HIV-1 hijacks the cytoskeleton to facilitate entry into, transport within and egress from host cells as well as to commandeer communication of infected with uninfected bystander cells.

Selective killing of human immunodeficiency virus infected cells by non-nucleoside reverse transcriptase inhibitor-induced activation of HIV protease

Background

Current antiretroviral therapy against human immunodeficiency virus (HIV-1) reduces viral load and thereby prevents viral spread, but it cannot eradicate proviral genomes from infected cells. Cells in immunological sanctuaries as well as cells producing low levels of virus apparently contribute to a reservoir that maintains HIV persistence in the presence of highly active antiretroviral therapy. Thus, accelerated elimination of virus producing cells may represent a complementary strategy to control HIV infection. Here we sought to exploit HIV protease (PR) related cytotoxicity in order to develop a strategy for drug induced killing of HIV producing cells. PR processes the viral Gag and Gag-Pol polyproteins during virus maturation, but is also implicated in killing of virus producing cells through off-target cleavage of host proteins. It has been observed previously that micromolar concentrations of certain non-nucleoside reverse transcriptase inhibitors (NNRTIs) can stimulate intracellular PR activity, presumably by enhancing Gag-Pol dimerization.

Results

Using a newly developed cell-based assay we compared the degree of PR activation displayed by various NNRTIs. We identified inhibitors showing higher potency with respect to PR activation than previously described for NNRTIs, with the most potent compounds resulting in ~2-fold increase of the Gag processing signal at 250 nM. The degree of enhancement of intracellular Gag processing correlated with the compound's ability to enhance RT dimerization in a mammalian two-hybrid assay. Compounds were analyzed for their potential to mediate specific killing of chronically infected MT-4 cells. Levels of cytotoxicity on HIV infected cells determined for the different NNRTIs corresponded to the relative degree of drug induced intracellular PR activation, with CC₅₀ values ranging from ~0.3 μM to above the tested concentration range (10 μM). Specific cytotoxicity was reverted by addition of PR inhibitors. Two of the most active compounds, VRX-480773 and GW-678248, were also tested in primary human cells and mediated cytotoxicity on HIV-1 infected peripheral blood mononuclear cells.

Conclusion

These data present proof of concept for targeted drug induced elimination of HIV producing cells. While NNRTIs themselves may not be sufficiently potent for therapeutic application, the results provide a basis for the development of drugs exploiting this mechanism of action.

Altered gene expression in asymptomatic SHIV-infected rhesus macaques (Macacca mulatta)

Simian-Human immunodeficiency virus is a chimeric virus which, in rhesus macaques (Macacca mulatta) closely imitates immunodeficiency virus infection in human (HIV). A relatively new way to study pathogenesis of viral infection is to study alterations in host gene expression induced by the virus. SHIV infection with certain strains does not result in clinical signs. We hypothesized that alterations in gene expression relating to the immune system would be present in SHIV-infected animals despite the lack of clinical signs. Splenic tissue from four adult male Indian-origin Rhesus monkeys serologically positive for non-pathogenic SHIV 89.6 was processed by cDNA microarray analysis. Results were compared with the corresponding outcome using splenic tissues from four unexposed adult male Rhesus monkeys. Subsequent gene analysis confirmed statistically significant variations between control and infected samples. Interestingly, SHIV-infected monkeys exhibited altered expression in genes related to apoptosis, signal transduction, T and B lymphocyte activation and importantly, to immune regulation. Although infected animals appeared asymptomatic, our study demonstrated that SHIV-infected monkeys cannot reliably be used in studies of other infectious agents as their baseline gene expression differs from that of normal Rhesus monkeys. The gene expression differences in SHIV-infected animals relative to uninfected animals offer additional clues to the pathogenesis of altered immune function in response to secondary infection.

Interaction between the cytoplasmic domain of ICAM-1 and Pr55Gag leads to acquisition of host ICAM-1 by human immunodeficiency virus type 1

We have examined the molecular basis for the selective incorporation of the adhesion molecule ICAM-1 within human immunodeficiency virus type 1 (HIV-1). The process of ICAM-1 incorporation was investigated by using different ICAM-1 constructs in combination with virus capture and immunoprecipitation studies, Western blot and confocal microscopy analyses, and infectivity assays. Experiments conducted with viruses bearing a truncated version of ICAM-1 revealed that the cytoplasmic domain of ICAM-1 governs insertion of this adhesion molecule into HIV-1. Further experiments suggested that there is an association between ICAM-1 and the virus-encoded Pr55(Gag) polyprotein. This study represents the first demonstration that structural Gag polyproteins play a key role in the uptake of a host-derived cell surface by the virus entity. Taken together, our results indicate that interactions between viral and cellular proteins are responsible for the selective uptake of host ICAM-1 by HIV-1. This observation describes a new strategy by which HIV-1 can modulate its replicative cycle, considering that insertion of ICAM-1 within nascent virions has been shown to increase virus infectivity.

Identification of HIV-1 protease cleavage site in human C1-inhibitor

We have investigated the ability of HIV-1 protease to cleave human complement proteins of the classical complement pathway: C1q, C2 and C4 as well as the regulatory protein, C1-inhibitor. Purified complement proteins were incubated with recombinant HIV-1 protease in vitro and analyzed by SDS-PAGE and immunoblotting assay. The only cleavage site was found in N-terminal region of C1-inhibitor, and it was located between residues Leu-32 and Phe-33 as determined by amino acid sequence analysis of the 85 kDa proteolytic fragment after 12 Edman degradation cycles. The HIV-1 protease cleavage sites were not found in C1q, C2 and C4 protein. HIV-1 protease-susceptible site in N-terminal region of C1-inhibitor is very close to the cleavage sites of some other proteases that are able to induce N-terminal proteolysis of the protein.

Cholesterol depletion of human immunodeficiency virus type 1 and simian immunodeficiency virus with beta-cyclodextrin inactivates and permeabilizes the virions: evidence for virion-associated lipid rafts

Recent evidence suggests that human immunodeficiency virus type 1 (HIV-1) particles assemble and bud selectively through areas in the plasma membrane of cells that are highly enriched with glycosylphosphatidylinositol-anchored proteins and cholesterol, called lipid rafts. Since cholesterol is required to maintain lipid raft structure and function, we proposed that virion-associated cholesterol removal with the compound 2-hydroxy-propyl-beta-cyclodextrin (beta-CD) might be disruptive to HIV-1 and simian immunodeficiency virus (SIV). We examined the effect of beta-CD on the structure and infectivity of cell-free virions. We found that beta-CD inactivated HIV-1 and SIV in a dose-dependent manner and permeabilized the viral membranes, resulting in the loss of mature Gag proteins (capsid, matrix, nucleocapsid, p1, and p6) without loss of the envelope glycoproteins. SIV also lost reverse transcriptase (RT), integrase (IN), and viral RNA. IN appeared to be only slightly diminished in HIV-1, and viral RNA, RT, matrix, and nucleocapsid proteins were retained in HIV-1 but to a much lesser degree. Host proteins located internally in the virus (actin, moesin, and ezrin) and membrane-associated host proteins (major histocompatibility complex classes I and II) remained associated with the treated virions. Electron microscopy revealed that under conditions that permeabilized the viruses, holes were present in the viral membranes and the viral core structure was perturbed. These data provide evidence that an intact viral membrane is required to maintain mature virion core integrity. Since the viruses were not fixed before beta-CD treatment and intact virion particles were recovered, the data suggest that virions may possess a protein scaffold that can maintain overall structure despite disruptions in membrane integrity.

Reversible oxidative modification as a mechanism for regulating retroviral protease dimerization and activation

Human immunodeficiency virus protease activity can be regulated by reversible oxidation of a sulfur-containing amino acid at the dimer interface. We show here that oxidation of this amino acid in human immunodeficiency virus type 1 protease prevents dimer formation. Moreover, we show that human T-cell leukemia virus type 1 protease can be similarly regulated through reversible glutathionylation of its two conserved cysteine residues. Based on the known three-dimensional structures and multiple sequence alignments of retroviral proteases, it is predicted that the majority of retroviral proteases have sulfur-containing amino acids at the dimer interface. The regulation of protease activity by the modification of a sulfur-containing amino acid at the dimer interface may be a conserved mechanism among the majority of retroviruses.

Amino-terminal polypeptides of vimentin are responsible for the changes in nuclear architecture associated with human immunodeficiency virus type 1 protease activity in tissue culture cells

Electron microscopy of human skin fibroblasts syringe-loaded with human immunodeficiency virus type 1 protease (HIV-1 PR) revealed several effects on nuclear architecture. The most dramatic is a change from a spherical nuclear morphology to one with multiple lobes or deep invaginations. The nuclear matrix collapses or remains only as a peripheral rudiment, with individual elements thicker than in control cells. Chromatin organization and distribution is also perturbed. Attempts to identify a major nuclear protein whose cleavage by the protease might be responsible for these alterations were unsuccessful. Similar changes were observed in SW 13 T3 M [vimentin(+)] cells, whereas no changes were observed in SW 13 [vimentin(-)] cells after microinjection of protease. Treatment of SW 13 [vimentin(-)] cells, preinjected with vimentin to establish an intermediate filament network, with HIV-1 PR resulted in alterations in chromatin staining and distribution, but not in nuclear shape. These same changes were produced in SW 13 [vimentin(-)] cells after the injection of a mixture of vimentin peptides, produced by the cleavage of vimentin to completion by HIV-1 PR in vitro. Similar experiments with 16 purified peptides derived from wild-type or mutant vimentin proteins and five synthetic peptides demonstrated that exclusively N-terminal peptides were capable of altering chromatin distribution. Furthermore, two separate regions of the N-terminal head domain are primarily responsible for perturbing nuclear architecture. The ability of HIV-1 to affect nuclear organization via the liberation of vimentin peptides may play an important role in HIV-1-associated cytopathogenesis and carcinogenesis.

Cleavage of vimentin by different retroviral proteases

Proteases (PRs) of retroviruses cleave viral polyproteins into their mature structural proteins and replication enzymes. Besides this essential role in the replication cycle of retroviruses, PRs also cleave a variety of host cell proteins. We have analyzed the in vitro cleavage of mouse vimentin by proteases of human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2), bovine leukemia virus (BLV), Mason-Pfizer monkey virus (M-PMV), myeloblastosis-associated virus (MAV), and two active-site mutants of MAV PR. Retroviral proteases display significant differences in specificity requirements. Here, we show a comparison of substrate specificities of several retroviral proteases on vimentin as a substrate. Vimentin was cleaved by all the proteases at different sites and with different rates. The results show that the physiologically important cellular protein vimentin can be degraded by different retroviral proteases.

Actin associates with the nucleocapsid domain of the human immunodeficiency virus Gag polyprotein

Recently, it was shown that actin molecules are present in human immunodeficiency virus type 1 (HIV-1) particles. We have examined the basis for incorporation and the location of actin molecules within HIV-1 and murine retrovirus particles. Our results show that the retroviral Gag polyprotein is sufficient for actin uptake. Immunolabeling studies demonstrate that actin molecules localize to a specific radial position within the immature particle, clearly displaced from the matrix domain underneath the viral membrane but in proximity to the nucleocapsid (NC) domain of the Gag polyprotein. When virus or subviral Gag particles were disrupted with nonionic detergent, actin molecules remained associated with the disrupted particles. Actin molecules remained in a stable complex with the NC cleavage product (or an NC-RNA complex) after treatment of the disrupted HIV-1 particles with recombinant HIV-1 protease. In contrast, matrix and capsid molecules were released. The same result was obtained when mature HIV-1 particles were disrupted with detergent. Taken together, these results indicate that actin molecules are associated with the NC domain of the viral polyprotein.

Mechanisms of enveloped virus entry into animal cells

The ability of viruses to transfer macromolecules between cells makes them attractive starting points for the design of biological delivery vehicles. Virus-based vectors and sub-viral systems are already finding biotechnological and medical applications for gene, peptide, vaccine and drug delivery. Progress has been made in understanding the cellular and molecular mechanisms underlying virus entry, particularly in identifying virus receptors. However, receptor binding is only a first step and we now have to understand how these molecules facilitate entry, how enveloped viruses fuse with cells or non-enveloped viruses penetrate the cell membrane, and what happens following penetration. Only through these detailed analyses will the full potential of viruses as vectors and delivery vehicles be realised. Here we discuss aspects of the entry mechanisms for several well-characterised viral systems. We do not attempt to provide a fully comprehensive review of virus entry but focus primarily on enveloped viruses.

Viral manipulations of the actin cytoskeleton

Viruses succeed as intracellular parasites because of their ability to invade cells and appropriate the cellular machinery required during their life cycle. The actin cytoskeleton of the host cell does not escape viral infection unscathed, but is often co-opted by the virus at many different stages of its life cycle to facilitate the infection process.

SFV infection in CHO cells: cell-type specific restrictions to productive virus entry at the cell surface

Alphaviruses, such as Semliki Forest virus, normally enter cells by penetration from acidic organelles of the endocytic pathway. The virions are internalised intact from the cell surface before undergoing acid-induced fusion in endosomes. To investigate the possibility that endocytosis might play a role in delivering virions to specific sites for replication, we compared SFV infection of baby hamster kidney (BHK) cells and Chinese hamster ovary (CHO) cells following either normal virus fusion in endosomes or experimentally-induced fusion at the cell surface. Whereas baby hamster kidney cells were infected efficiently following fusion in endosomes or at the plasma membrane, Chinese hamster ovary cells were only infected following fusion from endocytic organelles. Virions fused at the plasma membrane of CHO cells failed to initiate viral RNA and protein synthesis. Similar results were observed when CHO cells were challenged with a rhabdovirus, vesicular stomatitis virus. These data suggest that in certain cell types a barrier, other than the plasma membrane, can prevent infection by alpha- and rhabdoviruses fused at the cell surface. Moreover, they suggest the endocytic pathway provides a mechanism for bringing viral particles to a site, or sites, in the cell where replication can proceed.

Frequency of development of acute global left ventricular dysfunction in human immunodeficiency virus infection

OBJECTIVES

This study evaluated prospectively the frequency, clinical outcome and pathologic findings of acute global left ventricular dysfunction in human immunodeficiency virus (HIV) infection during the various stages of the disease.

BACKGROUND

Acute global left ventricular dysfunction in the course of HIV infection is still a poorly defined clinical entity, and little is known about the outcome after the acute onset.

METHODS

Between January 1988 and June 1992, 136 HIV-positive (HIV+) patients without clinical, electrocardiographic or echocardiographic evidence of cardiovascular dysfunction on admission were prospectively studied with serial echocardiograms. Patients were assigned to three groups: 1) anti-HIV+ asymptomatic (17 patients, 12.5%); 2) acquired immunodeficiency syndrome (AIDS)-related complex (26 patients, 19.1%); 3) AIDS (93 patients, 68.4%).

RESULTS

During a mean follow-up period of 415 +/- 220 days, seven patients, all in the AIDS subgroup, developed clinical and echocardiographic findings of acute global left ventricular dysfunction; of these, six (85%) died of congestive heart failure. Mean survival time from symptom onset was 41 +/- 13 days. Necropsy findings in five patients revealed acute lymphocytic myocarditis in three, cryptococcal myocarditis in one and interstitial edema and fibrosis in one. In only one patient was left ventricular dysfunction reversible with treatment.

CONCLUSIONS

Although infrequent, acute global left ventricular dysfunction is not rare in the course of HIV infection. It seems to occur exclusively during the AIDS stage. Acute global left ventricular dysfunction is often fatal but may be reversible and is mainly associated with the pathologic findings of acute myocarditis.

Staphylococcal protein A is a novel heterologous substrate for the HIV-1 protease

Upon in vitro processing of the recombinant HIV-1/gag p24 protein, expressed in Escherichia coli as a fusion protein, by HIV-1 protease, a cleavage site within the staphylococcal protein A fusion partner was found. N-terminal sequencing of the protein A fragments showed that HIV-1 protease cleavage occurred between phenylalanine-235 and tyrosine-236 within the sequence Gln-Asn-Ala-Phe/Tyr-Glu-Ile-Leu (QNAF/YEIL) in the IgG-binding domain C of the protein A encoded by the pRIT2T fusion gene vector (Pharmacia). Results presented here have proven that the protease-sensitive site is viable in vitro on the protein A alone and other chimeric protein, protein A/beta-galactosidase. A possible significance of this phenomenon in biotechnology work is discussed.

Viral interactions with the host-cell cytoskeleton: the role of retroviral proteases

A surprisingly large number of animal viruses interact with cytoskeletal elements inside infected cells at different stages of replication. For example, a viral protease is activated during assembly of murine leukemia viruses at the cell surface, which causes both the morphological conversion of 'immature' to 'mature' particles and a drastic reduction of the actin stress fiber network.

Human immunodeficiency virus type-1 reverse transcriptase and ribonuclease H as substrates of the viral protease

A study has been made of the susceptibility of recombinant constructs of reverse transcriptase (RT) and ribonuclease H (RNase H) from human immunodeficiency virus type 1 (HIV-1) to digestion by the HIV-1 protease. At neutral pH, the protease attacks a single peptide bond, Phe440-Tyr441, in one of the protomers of the folded, active RT/RNase H (p66/p66) homodimer to give a stable, active heterodimer (p66/p51) that is resistant to further hydrolysis (Chattopadhyay, D., et al., 1992, J. Biol. Chem. 267, 14227-14232). The COOH-terminal p15 fragment released in the process, however, is rapidly degraded by the protease by cleavage at Tyr483-Leu484 and Tyr532-Leu533. In marked contrast to this p15 segment, both p66/p51 and a folded RNase H construct are stable to breakdown by the protease at neutral pH. It is only at pH values around 4 that these latter proteins appear to unfold and, under these conditions, the heterodimer undergoes extensive proteolysis. RNase H is also hydrolyzed at low pH, but cleavage takes place primarily at Gly436-Ala437 and at Phe440-Tyr441, and only much more slowly at residues 483, 494, and 532. This observation can be reconciled by inspection of crystallographic models of RNase H, which show that residues 483, 494, and 532 are relatively inaccessible in comparison to Gly436 and Phe440. Our results fit a model in which the p66/p66 homodimer exists in a conformation that mirrors that of the heterodimer, but with a p15 segment on one of the protomers that is structurally disordered to the extent that all of its potential HIV protease cleavage sites are accessible for hydrolysis.

Model peptides to study the effects of P2 and P3 substitutions in statine-containing HIV proteinase inhibitors

Through a series of synthetic model peptides, we have examined the structural requirements of the P2 and P3 residues in statine-based HIV protease (PR) inhibitors. Results agree with the general observations that, the more bulky the P3 aromatic hydrophobic side chain, the more potent is the inhibitor. At P2, an isopropyl side chain is critical in maintaining potency. Three-dimensional modeling demonstrates that the steric bulk of a leucyl residue or the unfavorable energy transfer, from water to enzyme, for a basic amino acid residue at P2 markedly compromises activity. A naphthylalaninyl-valyl P3-P2 substituted analogue inhibits PR with an IC50 value of 6 nM, and was also effective as an antiviral agent.

Potential role of the viral protease in human immunodeficiency virus type 1 associated pathogenesis

Infection with the human immunodeficiency virus type 1 (HIV-1) results in a variety of pathological changes culminating in the acquired immune deficiency syndrome (AIDS). While most of these changes can readily be accounted for either by direct effects of HIV-1 on the immune system or by indirect effects of secondary infectious agents as a result of faulty immune surveillance, the direct cause for a number of disease states, including some neuropathies, myopathies, nephropathy, thrombocytopenia, wasting syndromes and increased incidence of cancers (primarily lymphoma) has remained an enigma. We have recently shown that the HIV-1 protease, a viral encoded enzyme necessary for virus maturation and infectivity, can cleave a variety of host cell cytoskeletal proteins in vitro. Potential substrates for the HIV-1 protease are found in all of the cell types affected in these unexplained diseases. Recent proposals suggest that elements of the cytoskeleton may play an important role in the regulation of large scale genetic regulation. We propose that some of the degenerative changes associated with infection by HIV-1 are a direct consequence of cleavage of host cell cytoskeletal proteins, which in turn may be responsible for the increased incidence of cancer in HIV-1 infected individuals as a result of the perturbation of the regulation of gene expression by cytoskeletal components.

Specific inhibitor of human immunodeficiency virus proteinase prevents the cytotoxic effects of a single-chain proteinase dimer and restores particle formation

The active form of the retroviral proteinase (PR) is a homodimer of monomeric subunits expressed as integral parts of the viral gag-pol precursor polyproteins, and dimerization of polyproteins is presumed to be important for regulation of PR activity. Expression of a single-chain dimer of the human immunodeficiency virus (HIV) type 1 PR as a component of the viral polyprotein has been shown to prevent particle assembly and viral infectivity (H.-G. Kräusslich, Proc. Natl. Acad. Sci. USA 88:3213-3217, 1991). Ro31-8959, a specific inhibitor of HIV PR, blocked proteolysis of polyproteins containing either wild-type or single-chain dimer PR at the same inhibitor concentration. Different inhibitor concentrations gave three phenotypic effects for the linked PR: at a concentration of 10 nM, cytotoxicity was prevented yet viral polyproteins were almost completely processed and no particles were released. The majority of HIV capsid proteins was found in the soluble cytoplasmic fraction, whereas at a concentration of 1 microM inhibitor most HIV gag proteins were associated with an insoluble fraction. Release of particles consisting of partially processed polyproteins was observed at 100 nM Ro31-8959, and polyprotein processing was blocked at 10 microM. Particles derived from the dimer-containing provirus were noninfectious independently of the inhibitor concentration. Production of infectious HIV after transfection of wild-type provirus was abolished at 100 nM and markedly reduced at 10 nM Ro31-8959.

Fibronectin is a non-viral substrate for the HIV proteinase

The retrovirus encoded proteinase (PR) is required for the proper maturation of viral particles into infectious virus. The PR had been considered highly substrate specific, cleaving exclusively the viral gag and gag-pol protein precursor. It has recently been reported, however, that cytoskeleton and other cellular filament proteins can be cleaved by the HIV-1 PR. Here we have evidence that a cell-associated protein, the fibronectin (A-chain), is also cleaved in vitro specifically by this PR. The possibility of a cytotoxic role of the PR is conceivable.