Human immunodeficiency virus proteinase dimer as component of the viral polyprotein prevents particle assembly and viral infectivity

Characterization of human immunodeficiency virus type-1 (HIV-1) particles that express protease-reverse transcriptase fusion proteins

We have selectively mutagenized specific residues at the junction between the protease (PR) and reverse transcriptase (RT) genes of human immunodeficiency virus type 1 (HIV-1) to study the effects of PR-RT fusion proteins in the context of a full-length, infectious proviral construct. Mutant viruses derived from COS-7 cells transfected with this construct were analyzed in regard to each of viral replication, maturation, and infectivity. Immunoblot analysis revealed that the mutation prevented cleavage between the PR and RT proteins and that both existed as a PR-RT fusion protein in each of cellular and viral lysates. Interestingly, intracellular PR that existed within the PR-RT fusion protein remained functionally active, whereby HIV-1 precursor proteins were processed efficiently. Furthermore, the RT component of the fusion protein also retained its enzymatic activity as shown in RT assays. Electron microscopy revealed that the mutant viruses containing the PR-RT fusion protein possessed wild-type morphology. These viruses also displayed wild-type sensitivities to inhibitors of each of the HIV-1 PR and RT activities. However, viruses containing the PR-RT fusion protein were 20 times less infectious than wild-type viruses. This defect was further pronounced when mutated Gag-Pol proteins were overexpressed as a consequence of an additional mutation that interfered with frameshifting. Thus, unlike cleavage site mutations at the N terminus of PR, a cleavage site mutation between PR and RT did not affect the enzymatic activities of either PR or RT and viruses containing PR-RT fusion proteins were viable.

A bipartite membrane-binding signal in the human immunodeficiency virus type 1 matrix protein is required for the proteolytic processing of Gag precursors in a cell type-dependent manner

It is unclear whether proteolytic processing of the human immunodeficiency virus type 1 (HIV-1) Gag protein is dependent on virus assembly at the plasma membrane. Mutations that prevent myristylation of HIV-1 Gag proteins have been shown to block virus assembly and release from the plasma membrane of COS cells but do not prevent processing of Gag proteins. In contrast, in HeLa cells similar mutations abolished processing of Gag proteins as well as virus production. We have now addressed this issue with CD4(+) T cells, which are natural target cells of HIV-1. In these cells, myristylation of Gag proteins was required for proteolytic processing of Gag proteins and production of extracellular viral particles. This result was not due to a lack of expression of the viral protease in the form of a Gag-Pol precursor or a lack of interaction between unmyristylated Gag and Gag-Pol precursors. The processing defect of unmyristylated Gag was partially rescued ex vivo by coexpression with wild-type myristylated Gag proteins in HeLa cells. The cell type-dependent processing of HIV-1 Gag precursors was also observed when another part of the plasma membrane binding signal, a polybasic region in the matrix protein, was mutated. The processing of unmyristylated Gag precursors was inhibited in COS cells by HIV-1 protease inhibitors. Altogether, our findings demonstrate that the processing of HIV-1 Gag precursors in CD4(+) T cells occurs normally at the plasma membrane during viral morphogenesis. The intracellular environment of COS cells presumably allows activation of the viral protease and proteolytic processing of HIV-1 Gag proteins in the absence of plasma membrane binding.

Mutations in the primer grip of human immunodeficiency virus type 1 reverse transcriptase impair proviral DNA synthesis and virion maturation

This report describes the effects of mutating highly conserved residues in the primer grip domain of human immunodeficiency virus type 1 reverse transcriptase (RT) on virus formation and infectivity. Among a series of RT mutant viruses, three (M230A, L234D, and W239A) were found to be noninfectious or very poorly infectious. Our data indicate that these mutations in RT caused severe defects in proviral DNA synthesis. Interestingly, assembly and maturation of mutant virus M230A were similar to those of the wild type, while mutants L234D and W239A showed impaired maturation. The immature morphology of RT mutants L234D and W239A is due at least in part to premature cleavage of the gag-pol precursor, prior to virion budding, indicating that intracellular stability of Pr160(gag-pol) is of key importance during virus assembly.

A cellular anti-apoptosis protein is cleaved by the HIV-1 protease

Cleavage of non-viral proteins is rarely observed with the HIV-1 protease (HIV pr). One such cleavage event occurs with bcl-2, an important cytoprotective protein. The loss of bcl-2 has biological consequences, leading to enhanced HIV replication and programmed death of the host cell. A strategy is proposed to suppress HIV with non-cleavable mutants of bcl-2.

Regulation of intracellular human immunodeficiency virus type-1 protease activity

The maturation of HIV-1 virions is accomplished through the proteolytic cleavage of Gag and GagPol precursor polyproteins by the viral-encoded protease (PR). Since virions are assembled from unprocessed polyproteins, the intracellular activation of PR must be limited. An experimental system was established that allows the investigation of the intracellular regulation of PR activity. By expressing Gag in trans with the GagPol precursor, downregulation of the intracellular PR activity associated with GagPol was demonstrated. Inhibition of PR activity was dependent upon the context of PR expression. Sequences capable of mediating this inhibition were localized to capsid. A mechanism through which Gag regulates PR activity is proposed whereby the disproportionate synthesis of Gag inhibits the activation of PR in the cytoplasm. Further elucidation of the mechanism of intracellular inhibition of PR activity may facilitate the development of novel PR inhibitors capable of inhibiting viral replication in vivo.

The HIV protease and therapies for AIDS

New, potent therapies for HIV disease are available, based on synthetic inhibitors of the viral protease, an essential viral enzyme. The results in clinical trials have been impressive with most treated individuals benefiting in terms of reduced quantity of detectable virus, enhanced numbers of CD4 lymphocytes and improvements in quality and duration of life. However, there are some remaining negatives associated with the new drugs, including high cost, side effects and appearance of drug-resistant strains of HIV. Problems and future prospects for use of protease inhibitors and alternate approaches in AIDS are discussed.

Thioltransferase (glutaredoxin) is detected within HIV-1 and can regulate the activity of glutathionylated HIV-1 protease in vitro

Previous studies have suggested that the two conserved cysteines of the HIV-1 protease may be involved in regulating protease activity. Here, we examined diglutathionylated wild type protease (Cys-67-SSG, Cys-95-SSG) and the monoglutathionylated protease mutants (C67A, Cys-95-SSG and C95A, Cys-67-SSG) as potential substrates for thioltransferase (glutaredoxin). Time-dependent changes in the extent of deglutathionylation of each protein were assayed by reverse phase-high performance liquid chromatography. Glutathione alone was not an effective reductant, whereas thioltransferase displayed differential catalysis toward the Cys-95-SSG and Cys-67-SSG sites. At low thioltransferase concentrations (5 nM), deglutathionylation occurred almost exclusively at Cys-95-SSG. With substantially more thioltransferase (100 nM) Cys-67-SSG was partially deglutathionylated but only at 20% of the rate of Cys-95-SSG reduction. Treatment of the diglutathionylated protease with thioltransferase not only restored protease activity but generated an enzyme preparation that had a 3- to 5-fold greater specific activity relative to the fully reduced form. Immunoblot analysis of HIV-1MN virus with an antibody to thioltransferase detected a band co-migrating with recombinant thioltransferase that persisted following subtilisin treatment, indicating the presence of thioltransferase within HIV-1. Our results implicate thioltransferase in the regulation and/or maintenance of protease activity in HIV-1 infected cells.

Identification of cis-acting repressor activity within human immunodeficiency virus type 1 protease sequences

Human immunodeficiency virus type-1 (HIV-1) Rev overcomes negative elements within viral RNAs to allow expression of gag, pol, and env. The effect of Rev on protein and RNA expression of HIV-1 protease (PR)-containing constructs was investigated utilizing transient transfection of COS cells. Rev, through the Rev response element (RRE), resulted in a large increase in proteolytic activity and cytoplasmic RNA accumulation. Furthermore, Rev increased the level of total RNA produced by a PR-containing construct. The increase in cytoplasmic RNA accumulation in the presence of Rev indicated the presence of cis-acting repressor sequences (CRS) within the RNA produced by this construct. Therefore, components of the construct were analyzed for CRS activity. PR sequences in both sense and antisense orientations exhibited CRS activity. RRE sequences alone conferred a small CRS effect. Additional CRS activity was present within an unspliced RNA containing only nef and LTR sequences. These results indicate a novel form of cis-acting repressor activity within HIV-1 PR; this activity is exerted regardless of the orientation of PR and appears to function at the level of cytoplasmic or nuclear RNA stability.

Human immunodeficiency virus type 1 (HIV-1) protein Vif inhibits the activity of HIV-1 protease in bacteria and in vitro

Human immunodeficiency virus type 1 (HIV-1) Vif is required for productive infection of T lymphocytes and macrophages. Virions produced in the absence of Vif have abnormal core morphology and those produced in primary T cells carry immature core proteins and low levels of mature capsid (M. Simm, M. Shahabuddin, W. Chao, J. S. Allan, and D. J. Volsky, J. Virol. 69:4582-4586, 1995). To investigate whether Vif influences the activity of HIV-1 protease (PR), the viral enzyme which is responsible for processing Gag and Gag-Pol precursor polyproteins into mature virion components, we transformed bacteria to inducibly express truncated Gag-Pol fusion proteins and Vif. We examined the cleavage of polyproteins consisting of matrix to PR (Gag-PR), capsid to PR (CA-PR), and p6Pol to PR (p6Pol-PR) and evaluated HIV-1 protein processing at specific sites by Western blotting using antibodies against matrix, capsid, and PR proteins. We found that Vif modulates HIV-1 PR activity in bacteria mainly by preventing the release of mature MA and CA from Gag-PR, CA from CA-PR, and p6Pol from p6Pol-PR, with other cleavages being less affected. Using subconstructs of Vif, we mapped this activity to the N-terminal half of the molecule, thus identifying a new functional domain of Vif. Kinetic study of p6Pol-PR autocatalysis in the presence or absence of Vif revealed that Vif and N'Vif reduce the rate of PR-mediated proteolysis of this substrate. In an assay of in vitro proteolysis of a synthetic peptide substrate by purified recombinant PR we found that recombinant Vif and the N-terminal half of the molecule specifically inhibit PR activity at a molar ratio of the N-terminal half of Vif to PR of about 1. These results suggest a mechanism and site of action of Vif in HIV-1 replication and demonstrate novel regulation of a lentivirus PR by an autologous viral protein acting in trans.

Molecular analysis of the feline immunodeficiency virus protease: generation of a novel form of the protease by autoproteolysis and construction of cleavage-resistant proteases

The feline immunodeficiency virus (FIV) protease is essential for virion maturation and subsequent viral replication in that it cleaves the Gag and Gag/Pol polyproteins at eight sites to release the respective structural proteins and enzymes. During purification of a recombinant FIV protease (PR), we noted that it underwent autoproteolysis (autolysis) to give discrete cleavage products. These additional PR cleavage sites were defined using N-terminal amino acid sequence analysis and mass spectrometry. Protease breakdown products were also found in FIV virions and were of the same apparent molecular weights as the in vitro autolysis products. Four primary PR autolysis sites were blocked via substitution of either the P1 amino acid with a beta-branched amino acid or the P1' amino acid with lysine. Cleavage-resistant PRs which had Km and k(cat) values similar to those of FIV PR were constructed. An autolysis time course determined that blocking all four primary autolysis sites yielded a cleavage-resistant PR which was enzymatically stable. Concomitant with autolysis is the generation of an N-terminally truncated form of the PR (Thr6/PR) which has enhanced stability with respect to that of FIV PR. A structural basis for the Thr6/PR activity is presented, as are the possible roles of autolysis in the viral replication cycle.

Plasma membrane targeting of chimeric intracisternal A-type particle polyproteins leads to particle release and specific activation of the viral proteinase

Retrovirus morphogenesis involves assembly of structural Gag polyproteins with subsequent budding from the plasma membrane, followed by proteolytic cleavage by the viral proteinase (PR) and extracellular maturation to the infectious virion. Intracisternal A-type particles (IAPs) are defective retroviruses that assemble and bud at the membranes of the endoplasmic reticulum (ER), where they remain as immature particles consisting exclusively of uncleaved polyproteins. To analyze requirements for intracellular polyprotein transport and PR activation, we constructed deletion and substitution mutations in the IAP gag gene, including the putative ER-targeting signal. Mutant polyproteins were transported to various intracellular locations, including the nucleus, the cytoplasm, the ER, and the plasma membrane. Interestingly, assembly of capsid-like particle structures occurred at almost all sites. However, only those polyproteins transported to the plasma membrane were efficiently and specifically cleaved by viral PR, with cleavage occurring predominantly within the virus particle. Thus, at least in the experimental system presented here, retroviral particle assembly can occur at almost any location within the cell, while polyprotein processing and, consequently, virion maturation are confined to a specific cellular site. These results suggest that a factor restricted to the plasma membrane is required to trigger PR activation and maturation of infectious retroviruses.

Dimerization and activation of the herpes simplex virus type 1 protease

The quaternary state of the herpes simplex virus type 1 (HSV-1) protease has been analyzed in relation to its catalytic activity. The dependence of specific activity upon enzyme concentration indicated that association of the 27-kDa subunits strongly increased activity. Size-exclusion chromatography identified the association as a monomer-dimer equilibrium. Isolation of monomeric and dimeric species from a size-exclusion column followed by immediate assay identified the dimer as the active form of the enzyme. Activation of the protease by antichaotropic cosolvents correlated with changes in the monomer-dimer equilibrium. Thus, dimerization of the enzyme was enhanced in solvents containing glycerol or the anions citrate or phosphate. These are substances previously identified as activators of HSV-1 protease (Hall, D. L., and Darke, P. L. (1995) J. Biol. Chem. 270, 22697-22700). The relative potencies of these cosolvents as enzyme activators correlated with their efficiency in promoting dimerization. Under all solvent conditions examined, the dependence of specific activity upon enzyme concentration was consistent with a kinetic model in which only the dimer is active. Dissociation constants for the HSV-1 protease dimer determined with this model at 15 degrees C, pH 7.5, were 964 and 225 nM in 20% glycerol with 0.2 and 0.5 M citrate present, respectively. The activation of the HSV-1 protease by antichaotropic cosolvents was hereby shown to be similar in nature to the activation of the other well characterized herpesvirus protease, that from human cytomegalovirus.

Altered Rous sarcoma virus Gag polyprotein processing and its effects on particle formation

Proteolytic processing of the Rous sarcoma virus (RSV) Gag precursor was altered in vivo through the introduction of amino acid substitutions into either the polyprotein cleavage junctions or the PR coding sequence. Single amino acid substitutions (V(P2)S and P(P4)G), which are predicted from in vitro peptide substrate cleavage data to decrease the rate of release of PR from the Gag polyprotein, were placed in the NC portion of the NC-PR junction. These substitutions do not affect the efficiency of release of virus-like particles from COS cells even though recovered particles contain significant amounts of uncleaved Pr76gag in addition to mature viral proteins. Single amino acid substitutions (A(P3)F and S(P1)Y), which increase the rate of PR release from Gag, also do not affect budding of virus-like particles from cells. Substitution of the inefficiently cleaved MA-p2 junction sequence in Gag by eight amino acids from the rapidly cleaved NC-PR sequence resulted in a significant increase in cleavage at the new MA-p2 junction, but again without an effect on budding. However, decreased budding was observed when the A(P3)F or S(P1)Y substitution was included in the NC-PR junction sequence between the MA and p2 proteins. A budding defect was also caused by substitution into Gag of a PR subunit containing three amino acid substitutions (R105P, G106V, and S107N) in the substrate binding pocket that increase the catalytic activity of PR. The defect appears to be the result of premature proteolytic processing that could be rescued by inactivating PR through substitution of a serine for the catalytic aspartic acid residue. This budding defect was also rescued by single amino acid substitutions in the NC-PR cleavage site which decrease the rate of release of PR from Gag. A similar budding defect was caused by replacing the Gag PR with two PR subunits covalently linked by four glycine residues. In contrast to the defect caused by the triply substituted PR, the budding defect observed with the linked PR dimer could not be rescued by NC-PR cleavage site mutations, suggesting that PR dimerization is a limiting step in the maturation process. Overall, these results are consistent with a model in which viral protein maturation occurs after PR subunits are released from the Gag polyprotein.

Activity of tethered human immunodeficiency virus 1 protease containing mutations in the flap region of one subunit

The tethered-dimer protease of human immunodeficiency virus 1 (HIV-1) [Cheng Y.-S. E., Yin, F.H., Foundling, S., Blomstrom, D. & Kettner, C. A. (1990) Proc. Natl Acad. Sci. USA 87, 9660-9664] and its mutants containing amino acid substitutions or deletions or both in only one flap region were expressed in Escherichia coli. These mutant enzymes showed various degrees of self-processing and significantly reduced catalytic activity toward oligopeptide substrates compared with the wild type. Kinetic parameters determined for one of the oligopeptide substrates showed a dramatic increase in K(m) and decrease in Kcat values. Unexpectedly, the substrate cleavage was more efficient in low salt concentration for a mutant containing a shortened hydrophilic flap. Assays with oligopeptides representing naturally occurring cleavage sites or oligopeptides containing single amino acid substitutions at the P2 and P2' substrate positions showed only moderate changes in the substrate specificity of the mutant proteases. Predicted structures for the mutants were constructed by molecular modeling and used to interpret the results of kinetic measurements. In general, the data suggest that the mutated part of the flaps does not have a major role in determining substrate specificity; rather, it provides the hydrophobic environment and hydrogen-bond interactions with the conserved water that are necessary for efficient substrate binding and catalysis.

Analysis of cleavage site mutations between the NC and PR Gag domains of Rous sarcoma virus

In retroviruses, the viral protease (PR) is released as a mature protein by cleavage of Gag, Gag-Pro, or Gag-Pro-Pol precursor polypeptides. In avian sarcoma and leukemia viruses (ASLV), PR forms the C-terminal domain of Gag. Based on the properties of a mutation (cs22) in the cleavage site between the upstream NC domain and the PR domain, the proteolytic liberation of PR previously was inferred to be essential for processing of Gag and Pol proteins. To study this process in more detail, we have analyzed the effects that several mutations at the NC-PR cleavage site have on proteolytic processing in virus-like particles expressed in COS and quail cells. Mutant Gag proteins carrying the same mutations also were synthesized in vitro and tested for processing with purified PR. In both types of studies, N-terminal sequencing of the liberated PR domain was carried out to exactly identify the site of cleavage. Finally, synthetic peptides corresponding to the mutant proteins were assessed for the ability to act as substrates for PR. The results were all consistent and led to the following conclusions. (i) In vivo, if normal processing between NC and PR is prevented by mutations, limited cleavage occurs at a previously unrecognized alternative site three amino acids downstream, i.e., in PR. This N-terminally truncated PR is inactive as an enzyme, as inferred from the global processing defect in cs22 and a similar mutant. (ii) In Gag proteins translated in vitro, purified PR cleaves this alternative site as rapidly as it does the wild-type site. (iii) Contrary to previously accepted rules describing retroviral cleavage sites, an isoleucine residue placed at the P1 position of the NC-PR cleavage site does not hinder normal processing. (iv) A proline residue placed at the P2 position in this cleavage site blocks normal processing.

Engineering human immunodeficiency virus 1 protease heterodimers as macromolecular inhibitors of viral maturation

Dimerization of human immunodeficiency virus type 1 protease (HIV-1 PR) monomers is an essential prerequisite for viral proteolytic activity and the subsequent generation of infectious virus particles. Disruption of the dimer interface inhibits this activity as does formation of heterodimers between wild-type and defective monomers. A structure-based approach was used to identify amino acid substitutions at the dimer interface of HIV-1 PR that facilitate preferential association of heterodimers and inhibit self-association of the defective monomers. Expression of the designed PR monomers inhibits activity of wild-type HIV-1 PR and viral infectivity when assayed in an ex vivo model system. These results show that it is possible to design PR monomers as macromolecular inhibitors that may provide an alternative to small molecule inhibitors for the treatment of HIV infection.

Lack of integrase can markedly affect human immunodeficiency virus type 1 particle production in the presence of an active viral protease

The Gag-Pol polyprotein of human immunodeficiency virus type 1 is not required for efficient viral particle assembly or release. However, in this report we demonstrate that the synthesis of a truncated Gag-Pol precursor due to a premature termination codon in pol can reduce the ability of a full-length provirus to direct the formation of viral particles. Marked effects on particle production were seen when premature termination codons were introduced into the integrase (IN)-coding region. By contrast, a mutant which lacked both IN and reverse transcriptase (RT) formed particles with normal efficiency. Particle production by IN mutants was restored to wild-type levels when a second premature termination codon was introduced at the 5' end of the RT-coding sequence. Particle formation was similarly restored by a second site mutation in the viral protease (PR) gene which prevented proteolytic processing of the Gag polyprotein. Finally particle formation was restored in the presence of A77003, a specific inhibitor of human immunodeficiency virus type 1 PR. These results suggest that the effects of a lack of IN sequences on particle formation require the synthesis of a Gag-Pol precursor which contains RT sequences and are due to inappropriate PR activity.

Apoptosis mediated by HIV protease is preceded by cleavage of Bcl-2

Expression of the human immunodeficiency virus type 1 (HIV) protease in cultured cells leads to apoptosis, preceded by cleavage of bcl-2, a key negative regulator of cell death. In contrast, a high level of bcl-2 protects cells in vitro and in vivo from the viral protease and prevents cell death following HIV infection of human lymphocytes, while reducing the yields of viral structural proteins, infectivity, and tumor necrosis factor alpha. We present a model for HIV replication in which the viral protease depletes the infected cells of bcl-2, leading to oxidative stress-dependent activation of NF kappa B, a cellular factor required for HIV transcription, and ultimately to cell death. Purified bcl-2 is cleaved by HIV protease between phenylalanine 112 and alanine 113. The results suggest a new option for HIV gene therapy; bcl-2 muteins that have noncleavable alterations surrounding the HIV protease cleavage site.

Inhibitors of human immunodeficiency virus type 1 zinc fingers prevent normal processing of gag precursors and result in the release of noninfectious virus particles

The Cys-Xaa2-Cys-Xaa4-His-Xaa4-Cys zinc fingers of retroviral nucleocapsid proteins are prime antiviral targets because of conservation of the Cys and His chelating residues and the absolute requirement of these fingers in both early and late phases of retroviral replication. We previously reported that certain disulfide-substituted benzamides (DIBAs) chemically modify the Cys residues of the fingers, resulting in inhibition of human immunodeficiency virus type 1 (HIV-1) replication (W. G. Rice, J. G. Supko, L. Malspeis, R. W. Buckheit, Jr., D. Clanton, M. Bu, L. Graham, C. A. Schaeffer, J. A. Turpin, J. Domagala, R. Gogliotti, J. P. Bader, S. M. Halliday, L. Coren, R. C. Sowder II, L. O. Arthur, and L. E. Henderson, Science 270:1194-1197, 1995). We now examine the consequences of the interaction of DIBAs with the zinc fingers of the HIV-1 p7 nucleocapsid protein and its Pr55gag precursor. In HIV-1-infected U1 cells, DIBAs inhibited the release of infectious virions, and even under conditions in which virion particles were produced, the particles were noninfectious. DIBAs caused abnormal processing of Gag precursors, and the inhibitory effect on processing was not due to inhibition of the HIV-1 protease enzyme or Pr55gag myristoylation. Rather, the defect in processing was due to the formation of intermolecular cross-linkages among the zinc fingers of adjacent Gag molecules, rendering the precursors no longer recognizable by HIV-1 protease. Likewise, DIBAs caused intermolecular cross-linkage among recombinant Pr55gag packaged into pseudovirions, thereby generating modified precursors that were resistant to the action of protease. Thus, DIBAs chemically modified the mutationally intolerant retroviral zinc fingers in infected cells, interrupting protease-mediated maturation of virions and leading ultimately to the production of compromised virions.

Generation of lymphocyte cell lines coexpressing CD4 and wild-type or mutant HIV type 1 glycoproteins: implications for HIV type 1 Env-induced cell lysis

To gain more insight into the processes leading to HIV-1 Env-induced cell death, we aim to coexpress stably wild-type and relevant mutant variants of both HIV-1 Env and human CD4 in lymphocyte cell lines. Here we report on the generation and characterization of several cell lines inducibly or constitutively expressing wild-type or cleavage-defective HIV-1 glycoproteins and human CD4 either singly or in combination. Coexpression of CD4 and wild-type Env led to the formation of multinucleated syncytia, to growth arrest and cell death, effects that all could be prevented by cultivation in the presence of monoclonal antibodies that inhibit cell surface membrane fusion. Cell lines coexpressing CD4 and mutated, noncleavable Env, detectable at the cell surface and still retaining CD4-binding capacity, were not retarded in their growth and cytolysis did not occur. These results indicate that cell lysis requires cell surface interaction of CD4 and gp120/41 and cleavage of gp160 to gp120 and gp41.

Dimerization of the human cytomegalovirus protease: kinetic and biochemical characterization of the catalytic homodimer

The single-chain 28 kDa human cytomegalovirus (HCMV) protease catalytic domain containing the A143Q mutation has been kinetically and conformationally characterized. The specific activity of the HCMV A143Q protease (HCMVp) increases as the protease concentration increases, suggesting that this protease oligomerizes at high protein concentration to form a more active species. Both cross-linking and light-scattering studies of HCMVp show the existence of a homodimer with an apparent molecular mass of 56 kDa under low ionic strength and high protein concentration. The cosolvent and solute effects of glycerol, trisodium citrate, and NaCl as well as the temperature effects on the HCMVp activity and quaternary structure were investigated. The effects induced by cosolvents and temperature can largely be explained by their influences in the dimerization or oligomerization state of HCMVp. The dissociation constant (Kd) for the HCMVp homodimer was determined to be 8 +/- 1 microM with all activity attributed to the dimeric form. Monomeric HCMVp is inactive. This report demonstrates that in vitro, HCMV A143Q protease exists as an obligate catalytic homodimer. This protease dimerization may have regulatory significance during viral replication.

Sequence-specific resonance assignments of the 1H-NMR spectra and structural characterization in solution of the HIV-1 transframe protein p6

The frameshift protein p6* encoded directly upstream of the protease in the human immunodeficiency virus type 1 (HIV-1) pol reading frame is thought to be a natural inhibitor of protease activation and to play a role in the polyprotein processing of Gag and Gag-Pol precursors. To allow structural characterization of the p6* transframe protein, the p6* coding region was cloned into the vector pGEX-KG and expressed in Escherichia coli as a fusion protein with glutathione S-transferase (GST) under the control of the tac promoter. Thrombin cleavage of the construct resulted in a 70-amino-acid polypeptide which is extended by two additional residues at the N-terminus compared to the natural p6* sequence. The native purification procedure including an affinity and a size-exclusion chromatography step yielded sufficient amounts of highly pure protein suitable for NMR spectroscopy. Fluorescence, circular dichroism and 1H-NMR spectroscopy were applied to characterize the structure of protein. Two-dimensional NMR spectra provided essentially complete sequence-specific resonance assignments at pH 5.9. Although there is evidence for a helix-forming tendency in the N-terminus of the protein, the experiments indicate that p6* has no overall stable secondary or tertiary structure with the single tryptophan exposed in aqueous solution. However, the results reported herein open the way to characterize further the interaction of p6* with the HIV-1 protease in structural and functional in vitro studies.

A transient precursor of the HIV-1 protease. Isolation, characterization, and kinetics of maturation

Recently, the mechanism of autoprocessing of the protease (PR) of the human immunodeficiency virus type 1 from the model polyprotein, MBP-DeltaTF-PR-DeltaPol, which contains the protease linked to short native flanking sequences (DeltaTF and DeltaPol) fused to the maltose binding protein (MBP) of Escherichia coli, was reported (Louis, J. M., Nashed, N. T., Parris, K. D., Kimmel, A. R., and Jerina, D. M. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 7970-7974). According to this mechanism, intramolecular cleavage of the N-terminal strands of the dimeric MBP-DeltaTF-PR-DeltaPol protein leads to the formation of the PR-DeltaPol intermediate, which is subsequently converted to the mature protease by cleavage of the C-terminal strands. We now report the purification and characterization of the PR-DeltaPol intermediate and the kinetics of its processing to the mature protease. Unlike the MBP-DeltaTF-PR-DeltaPol precursor, PR-DeltaPol has proteolytic activity similar to that of the mature enzyme at pH 5.0. The pH rate profile for kcat/Km is similar to that of the mature protease above pH 4.0. Although the PR-DeltaPol is more sensitive than the mature protease toward denaturing reagents, both the enzymatic activity and the intrinsic fluorescence of PR-DeltaPol are linearly dependent on the protein concentration, indicating that the protein is largely in its dimeric form above 10 nM. In contrast to the first-order kinetics observed for the proteolytic reaction at the N terminus of the protease, the proteolytic reaction at the C terminus of the protease is second order in protein concentration. These results are discussed in terms of a mechanism in which the C-terminally located DeltaPol peptide chains are cleaved intermolecularly to release the mature protease.

An active-site mutation in the human immunodeficiency virus type 1 proteinase (PR) causes reduced PR activity and loss of PR-mediated cytotoxicity without apparent effect on virus maturation and infectivity

Infectious retrovirus particles are derived from structural polyproteins which are cleaved by the viral proteinase (PR) during virion morphogenesis. Besides cleaving viral polyproteins, which is essential for infectivity, PR of human immunodeficiency virus (HIV) also cleaves cellular proteins and PR expression causes a pronounced cytotoxic effect. Retroviral PRs are aspartic proteases and contain two copies of the triplet Asp-Thr-Gly in the active center with the threonine adjacent to the catalytic aspartic acid presumed to have an important structural role. We have changed this threonine in HIV type 1 PR to a serine. The purified mutant enzyme had an approximately 5- to 10-fold lower activity against HIV type 1 polyprotein and peptide substrates compared with the wild-type enzyme. It did not induce toxicity on bacterial expression and yielded significantly reduced cleavage of cytoskeletal proteins in vitro. Cleavage of vimentin in mutant-infected T-cell lines was also markedly reduced. Mutant virus did, however, elicit productive infection of several T-cell lines and of primary human lymphocytes with no significant difference in polyprotein cleavage and with similar infection kinetics and titer compared with wild-type virus. The discrepancy between reduced processing in vitro and normal virion maturation can be explained by the observation that reduced activity was due to an increase in Km which may not be relevant at the high substrate concentration in the virus particle. This mutation enables us therefore to dissociate the essential function of PR in viral maturation from its cytotoxic effect.

Trans-dominant inhibitory human immunodeficiency virus type 1 protease monomers prevent protease activation and virion maturation

Production of infectious human immunodeficiency virus (HIV) requires proper polyprotein processing by the dimeric viral protease. The trans-dominant inhibitory activity of a defective protease monomer with the active site Asp-25 changed to Asn was measured by transient transfection. A proviral plasmid that included the drug-selectable Escherichia coli gpt gene was used to deliver the wild-type (wt) or mutant proteases to cultured cells. Coexpression of the wt proviral DNA (HIV-gpt) with increasing amounts of the mutant proviral DNA (HIV-gpt D25N) results in a concomitant decrease in proteolytic activity monitored by in vivo viral polyprotein processing. The viral particles resulting from inactivation of the protease were mostly immature, consisting predominantly of unprocessed p55gag and p160gag-pol polyproteins. In the presence of HIV-1 gp160 env, the number of secreted noninfectious particles correlated with the presence of increasing amounts of the defective protease. Greater than 97% reduction in infectivity was observed at a 1:6 ratio of wt to defective protease DNA. This provides an estimate of the level of inhibition required for effectively preventing virion processing. Stable expression of the defective protease in monkey cells reduced the yield of infectious particles from these cells by 90% upon transfection with the wt proviral DNA. These results show that defective subunits of the viral protease exert a trans-dominant inhibitory effect resulting from the formation of catalytically compromised heterodimers in vivo, ultimately yielding noninfectious viral particles.

Potent inhibition of human immunodeficiency virus type 1 (HIV-1) replication by inducible expression of HIV-1 PR multimers

Constructs were generated in which expression of human immunodeficiency virus type 1 (HIV-1) protease (PR) was placed under control of the HIV-1 long terminal repeat, thus requiring the HIV-1 Tat protein for expression of PR. The activity of PR was assessed by cotransfection with a construct producing a Gag substrate. Expression of PR as an intramolecular multimer resulted in a large increase in PR activity in comparison with the level obtained with the expression of PR as a monomer. A cytotoxic effect of PR expression was also exhibited by the constructs expressing PR multimers. CD4+ T-cell lines were generated with a construct producing PR as a linked tetramer and screened for PR activity and inducibility. The replication of HIV-1 in these cell lines was several orders of magnitude reduced in comparison with that in cell lines not expressing PR. Infection in these cell lines could be detected early after infection but disappeared over time. Infection of the PR-expressing cell lines could be increased several orders of magnitude by the addition of a specific inhibitor of PR, U75875 (Upjohn), after infection of the cells, demonstrating that the potent inhibition of HIV-1 replication in these cells was directly due to the expression of PR.

Cloning, bacterial expression, and characterization of the Mason-Pfizer monkey virus proteinase

We have cloned and expressed the 3' region of the Mason-Pfizer monkey virus pro gene in Escherichia coli. The recombinant 26-kDa precursor undergoes rapid self-processing both in E. coli and in vitro at the NH2 terminus, yielding a proteolytically active 17-kDa protein, p17. This initial cleavage is followed in vitro by a much slower self-processing that leads to emergence of proteolytically active p12 and a COOH-terminal cleavage product p5. We have found the NH2-terminal processing site of both the p17 and p12 to be identical and similar to the amino terminus of the mouse mammary tumor virus proteinase. We have also identified the COOH-terminal processing site of the p12 form. Using purified recombinant proteins and synthetic oligopeptide substrates based on naturally occurring retroviral processing sites, we have determined the enzymatic activity and specificity of the Mason-Pfizer monkey virus proteinase to be more closely related to that of myeloblastosis-associated virus proteinase rather than that of the Human immunodeficiency virus type 1 proteinase. Inhibition studies using peptide inhibitors support these results.

Intrinsic activity of human immunodeficiency virus type 1 protease heterologous fusion proteins in mammalian cells

We have generated various mammalian expression constructs that produce fusion proteins of human immunodeficiency virus type 1 (HIV-1) protease (PR) with the HIV-1 Nef protein. The expression of these proteins is inducible by the HIV-1 Tat protein. High-level expression of proteolytically active PR was produced from PR imbedded into Nef coding sequences, flanked by PR cleavage sites. The fusion protein was cleaved nearly to completion and did not exhibit the regulated processing that is seen with the virally encoded PR. No cytotoxic effect of PR expression was detected. The self-cleavage of PR could be inhibited by a specific inhibitor of HIV-1 PR (U75875). Elimination of the aminoterminal PR cleavage site did not have a measurable effect on cleavage of the precursor fusion protein. The cleaved fusion proteins appeared to be extremely unstable in the transfected cells. These findings demonstrate the intrinsic activity of HIV-1 PR in mammalian cells, in the context of a heterologous fusion protein.

An assembly domain of the Rous sarcoma virus Gag protein required late in budding

The Gag protein of Rous sarcoma virus has the ability to direct particle assembly at the plasma membrane in the absence of all the other virus-encoded components. An extensive deletion analysis has revealed that very large regions of this protein can be deleted without impairing budding and has suggested that the essential functions map to three discrete regions. In the studies reported here, we establish the location of assembly domain 2 (AD2) within the proline-rich p2b sequence of this Gag protein. AD2 mutants lacking the p2b sequence were completely defective for particle release even though their Gag proteins were tightly associated with the membrane fraction and exhibited high levels of protease activity. Mutations that inactivate the viral protease did not restore budding to wild-type levels for these mutants, indicating that the defect is not due simply to a loss of protease regulation. AD2 mutants could be rescued into dense particles in genetic complementation assays, indicating that their defect is not due to a gross alteration of the overall conformation of the protein and that the assembly function is not needed on every Gag molecule in the population. Several mutants with amino acid substitutions in the p2b sequence were found to have an intermediate capacity for budding. Inactivation of the protease of these mutants stabilized the Gag polyprotein within the cells and allowed an increase in particle release; however, the rate of budding remained slow. We favor the idea that AD2 is a dynamic region of movement, perhaps serving as a molecular hinge to allow the particle to emerge from the surface of the cell during budding.

Analysis of HIV type 1 reverse transcriptase expression in a human cell line

The functional analysis of human immunodeficiency virus type-1 (HIV-1) reverse transcriptase (RT) subunits on transient and constitutive expression, in the absence or presence of the HIV-1 protease (PR) expression, in a human cell line is described. HIV-1 RT is a heterodimer composed of a 51-kDa subunit (p51) and a 66-kDa subunit (p66). Cloning and expression of the RT region of the HIV-1 pol gene in the HT-1080 human fibrosarcoma cell line yielded p66 without any detectable p51 and a low level of RT activity could be measured. Transient expression of PR and RT in cis generated p51 and p66, but when RT and PR were expressed in trans only p66 was produced. Attempts to establish a stable cell line expressing the PR-RT region of the pol gene were hampered by an apparent intolerance of HT-1080 cells to the HIV-1 PR expression. Therefore, to generate p51 independent of PR expression, the 51-kDa subunit was cloned separately. p51 lacked detectable RT activity. Coexpression of p51 and p66 resulted in a dramatic increase in RT activity. Stable HT-1080 cells producing both p51 and p66 exhibited on average a 15-fold increase in RT activity compared to the parental cell line. Immunofluorescence revealed a diffuse cytoplasmic localization of p51 and p66. To date, this is the first example of a human cell line that is constitutively expressing HIV-1 RT in the absence of HIV-1 infection.

Kinetics and mechanism of autoprocessing of human immunodeficiency virus type 1 protease from an analog of the Gag-Pol polyprotein

Upon renaturation, the polyprotein MBP-delta TF-Protease-delta Pol, consisting of HIV-1 protease and short native sequences from the trans-frame protein (delta TF) and the polymerase (delta Pol) fused to the maltose-binding protein (MBP) of Escherichia coli, undergoes autoprocessing to produce the mature protease in two steps. The initial step corresponds to cleavage of the N-terminal sequence to release the protein intermediate Protease-delta Pol, which has enzymatic activity comparable to that of the mature enzyme. Subsequently, the mature enzyme is formed by a slower cleavage at the C terminus. The rate of increase in enzymatic activity is identical to that of the appearance of MBP-delta TF and the disappearance of the MBP-delta TF-Protease-delta Pol. Initial rates are linearly dependent on the protein concentration, indicating that the N-terminal cleavage is first-order in protein concentration. The reaction is competitively inhibited by pepstatin A and has a pH rate profile similar to that of the mature enzyme. These results and molecular modeling studies are discussed in terms of a mechanism in which a dimeric full-length fusion protein must form prior to rate-limiting intramolecular cleavage of the N-terminal sequence that leads to an increase in enzymatic activity.

Crystal structure of a tethered dimer of HIV-1 proteinase complexed with an inhibitor

HIV-1 proteinase (HIV PR) is a dimeric enzyme composed of two identical polypeptide chains that associate with twofold symmetry. We have determined to 1.8 A the crystal structure of a covalently tethered dimer of HIV PR. The tethered dimer:inhibitor complex is identical in nearly every respect to the complex of the same inhibitor with the wild type dimeric molecule, except for the linker region. Our results suggest that the tethered dimer may be a useful surrogate enzyme for studying the effects of single site mutations on substrate and inhibitor binding as well as on enzyme asymmetry, and for simulating independent mutational drift of the two domains which has been proposed to have led to the evolution of modern day, single-chain aspartic proteinases.

Expression of virus-encoded proteinases: functional and structural similarities with cellular enzymes

Many viruses express their genome, or part of their genome, initially as a polyprotein precursor that undergoes proteolytic processing. Molecular genetic analyses of viral gene expression have revealed that many of these processing events are mediated by virus-encoded proteinases. Biochemical activity studies and structural analyses of these viral enzymes reveal that they have remarkable similarities to cellular proteinases. However, the viral proteinases have evolved unique features that permit them to function in a cellular environment. In this article, the current status of plant and animal virus proteinases is described along with their role in the viral replication cycle. The reactions catalyzed by viral proteinases are not simple enzyme-substrate interactions; rather, the processing steps are highly regulated, are coordinated with other viral processes, and frequently involve the participation of other factors.

Functional chimeras of the Rous sarcoma virus and human immunodeficiency virus gag proteins

The Gag protein encoded by Rous sarcoma virus (RSV) is the only viral product required for the process of budding whereby virus particles are formed at the plasma membrane. Deletion analysis of this Gag molecule has revealed several regions (assembly domains) that are important for budding. One of these domains is located at the amino terminus and is needed for membrane binding. Another is located within the carboxy-terminal third of the protein. Though there is little sequence homology among the Gag proteins of unrelated retroviruses, it seemed possible that their assembly domains might be functionally conserved, and to explore this idea, numerous Gag chimeras were made. The results indicate that the first 10 amino acids of the human immunodeficiency virus (HIV) Gag protein can suppress the block to budding caused by deletions in the RSV MA sequence, much as described previously for the first 10 residues from the Src oncoprotein (J.W. Wills, R.C. Craven, R. A. Weldon, Jr., T. D. Nelle, and C.R. Erdie, J. Virol. 65:3804-3812, 1991). In addition, the carboxy-terminal half of the HIV Gag protein was fused to a truncated RSV Gag molecule, mutant Bg-Bs, which is unable to direct core assembly. This chimera was able to produce particles at a rate identical to that of RSV and of a density similar to that of authentic virions. Deletion analysis of the carboxy-terminal chimera revealed two small regions within the HIV NC protein that were sufficient for endowing mutant Bg-Bs with these properties. Chimeras lacking both regions produced particles of a low density, suggesting that these sequences may be involved in the tight packing of Gag molecules during assembly. In a related set of experiments, replacement of the RSV protease with that of HIV resulted in premature processing within the RSV sequence and a block to budding. Particle assembly was restored when the HIV PR activity was inactivated by mutagenesis. Collectively, the data presented here illustrate the functional similarities of Gag proteins from unrelated retroviruses.

Characterization of a small (25-kilodalton) derivative of the Rous sarcoma virus Gag protein competent for particle release

Retroviral Gag proteins have the ability to induce budding and particle release from the plasma membrane when expressed in the absence of all of the other virus-encoded components; however, the locations of the functional domains within the Gag protein that are important for this process are poorly understood. It was shown previously that the protease sequence of the Rous sarcoma virus (RSV) Gag protein can be replaced with a foreign polypeptide, iso-1-cytochrome c from a yeast, without disrupting particle assembly (R. A. Weldon, Jr., C. R. Erdie, M. G. Oliver, and J. W. Wills, J. Virol. 64:4169-4179, 1990). An unexpected product of the chimeric gag gene is a small, Gag-related protein named p25C. This product was of interest because of its high efficiency of packaging into particles. The goal of the experiments described here was to determine the mechanism by which p25C is synthesized and packaged into particles. The results demonstrate that it is not the product of proteolytic processing of the Gag-cytochrome precursor but is derived from an unusual spliced mRNA. cDNA clones of the spliced mRNA were obtained, and each expressed a product of approximately 25 kDa, designated p25M1, which was released into the growth medium in membrane-enclosed particles that were much lighter than authentic retrovirions as measured in sucrose density gradients. DNA sequencing revealed that the clones encode the first 180 of the 701 amino acids of the RSV Gag protein and no residues from iso-1-cytochrome c. This suggested that a domain in the carboxy-terminal half of Gag is important for the packaging of Gag proteins into dense arrays within the particles. In support of this hypothesis, particles of the correct density were obtained when a small segment from the carboxy terminus of the RSV Gag protein (residues 417 to 584) was included on the end of p25.

Proteolytic processing of Ty3 proteins is required for transposition

Ty3 is a retroviruslike element found in Saccharomyces cerevisiae. It encodes GAG3 and GAG3-POL3 polyproteins which are processed into mature proteins found in the Ty3 viruslike particle. In this study, the region encoding a protease that is homologous to retroviral aspartyl proteases was identified and shown to be required for production of mature Ty3 proteins and transposition. The Ty3 protease has the Asp-Ser-Gly consensus sequence found in copia, Ty1, and Rous sarcoma virus proteases, rather than the Asp-Thr-Gly found in most retroviral proteases. The Asp-Ser-Gly consensus is flanked by residues similar to those which flank the active sites of cellular aspartyl proteases. Mutations were made in the Ty3 active-site sequence to examine the role of the protease in Ty3 particle maturation and to test the functional significance of the Ser active-site variant in the consensus sequence. Mutation of the active-site Asp blocked processing of Gag3 and Gag3-Pol3 and allowed identification of a GAG3-POL3 polyprotein. This protein was turned over rapidly in cells expressing the mutant Ty3. Changing the active-site Ser to Thr caused only a modest reduction in the levels of certain Ty3 proteins. Five putative cleavage sites of this protease in Ty3 GAG3 and GAG3-POL3 polyproteins were defined by amino-terminal sequence analysis. The existence of an additional protein(s) of unknown function, encoded downstream of the protease-coding region, was deduced from the positions of these amino termini and the sizes of known Ty3 proteins. Although Ty3 protease cleavage sites do not correspond exactly to known retroviral protease cleavage sites, there are similarities. Residues P3 through P2' in the regions encompassing each of the five sites are uncharged, and no P1 position is occupied by an amino acid with a branched beta carbon.

Synthetic "interface" peptides alter dimeric assembly of the HIV 1 and 2 proteases

Retroviral proteases are obligate homodimers and play an essential role in the viral life cycle. Dissociation of dimers or prevention of their assembly may inactivate these enzymes and prevent viral maturation. A salient structural feature of these enzymes is an extended interface composed of interdigitating N- and C-terminal residues of both monomers, which form a four-stranded beta-sheet. Peptides mimicking one beta-strand (residues 95-99), or two beta-strands (residues 1-5 plus 95-99 or 95-99 plus 95-99) from the human immunodeficiency virus 1 (HIV1) interface were shown to inhibit the HIV1 and 2 proteases (PRs) with IC50's in the low micromolar range. These interface peptides show cognate enzyme preference and do not inhibit pepsin, renin, or the Rous sarcoma virus PR, indicating a degree of specificity for the HIV PRs. A tethered HIV1 PR dimer was not inhibited to the same extent as the wild-type enzymes by any of the interface peptides, suggesting that these peptides can only interact effectively with the interface of the two-subunit HIV PR. Measurements of relative dissociation constants by limit dilution of the enzyme show that the one-strand peptide causes a shift in the observed Kd for the HIV1 PR. Both one- and two-strand peptides alter the monomer/dimer equilibrium of both HIV1 and HIV2 PRs. This was shown by the reduced cross-linking of the HIV2 PR by disuccinimidyl suberate in the presence of the interface peptides. Refolding of the HIV1 and HIV2 PRs with the interface peptides shows that only the two-strand peptides prevent the assembly of active PR dimers. Although both one- and two-strand peptides seem to affect dimer dissociation, only the two-strand peptides appear to block assembly. The latter may prove to be more effective backbones for the design of inhibitors directed toward retroviral PR dimerization in vivo.

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.

Assembly and processing of avian retroviral gag polyproteins containing linked protease dimers

Assembly and maturation of retroviral particles requires the aggregation and controlled proteolytic cleavage of polyprotein core precursors by a precursor-encoded protease (PR). Active, mature retroviral PR is a dimer, and the accumulation of precursors at sites of assembly may facilitate subunit interaction and subsequent activation of this enzyme. In addition, it has been suggested that cellular cytoplasmic components act as inhibitors of PR activity, so that processing is delayed until the nascent virions leave this compartment and separate from the surface of host cells. To investigate the mechanisms that control PR activity during virus assembly, we studied the in vivo processing of retroviral gag precursors that contain tandemly linked PR subunits in which dimerization is concentration independent. Sequences encoding four different linked protease dimers were independently joined to the end of the Rous sarcoma virus (RSV) gag gene in a simian virus 40-based plasmid vector which expresses a myristoylated gag precursor upon transfection of COS-1 cells. Three of these plasmids produced gag precursors that were incorporated into viruslike particles and proteolytically cleaved by the dimers to mature core proteins that were indistinguishable from the processed products of wild-type gag. The amount of viral gag protein that was assembled and packaged in these transfections was inversely related to the relative proteolytic activities of the linked PR dimers. The fourth gag precursor, which contained the most active linked PR dimer, underwent rapid intracellular processing and did not form viruslike particles. In the absence of the plasma membrane targeting signal, processing of all four linked PR dimer-containing gag precursors was completed entirely within the cell. From these results, we conclude that the delay in polyprotein core precursor processing that occurs during normal virion assembly does not depend on a cytoplasmic inhibitor of PR activity. We suggest that dimer formation is not only necessary but may be sufficient for the initiation of PR-directed maturation of gag and gag-pol precursors.

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.