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PubMed2004

Peptides resulting from the pVP2 C-terminal processing are present in infectious pancreatic necrosis virus particles.

The capsid of birnaviruses contains two proteins, VP2 and VP3, which derive from the processing of a large polyprotein, NH(2)-pVP2-VP4-VP3-COOH. The proteolytic cascade involved in processing the polyprotein, and in the final maturation of pVP2 (the precursor of VP2), has recently been shown to generate VP2 and four structural peptides in infectious bursal disease virus and blotched snakehead virus. The presence of peptides in infectious pancreatic necrosis virus particles was investigated using mass spectrometry and N-terminal sequencing of virus particles. Three peptides deriving from the C terminus of pVP2 (residues 443-486, 487-495 and 496-508 of the polyprotein) and 14 additional peptides produced by further processing of peptides [443-486] and [496-508] were identified. These results indicate that the presence of several virus-encoded peptides in the virions is a hallmark of birnaviruses.

Amino Acid Sequence↗

PubMed1992

Human immunodeficiency virus type 1 protease inhibitors irreversibly block infectivity of purified virions from chronically infected cells.

Synthetic peptide analog inhibitors of human immunodeficiency virus type 1 (HIV-1) protease were used to study the effects of inhibition of polyprotein processing on the assembly, structure, and infectivity of virions released from a T-cell line chronically infected with HIV-1. Inhibition of proteolytic processing of both Pr55gag and Pr160gag-pol was observed in purified virions from infected T cells after treatment. Protease inhibition was evident by the accumulation of precursors and processing intermediates of Pr55gag and by corresponding decreases in mature protein products. Electron microscopy revealed that the majority of the virion particles released from inhibitor-treated cells after a 24-h treatment had an immature or aberrant capsid morphology. This morphological change correlated with the inhibition of polyprotein processing and a loss of infectivity. The infectivity of virion particles purified from these chronically infected cell cultures was assessed following treatment with the inhibitor for 1 to 3 days. Virions purified from cultures treated with inhibitor for 1 or 2 days demonstrated a 95- to 100-fold reduction in virus titers, and treatment for 3 days resulted in complete loss of detectable infectivity. The fact that virions from treated cultures were unable to establish infection over the 7- to 10-day incubation period in the titration experiments strongly suggests that particles produced by inhibitor-treated cells were unable to reactivate to an infectious form when they were purified away from exogenous protease inhibitor. Thus, a block of HIV-1 protease processing of viral polyproteins by specific inhibitors results in a potent antiviral effect characterized by the production of noninfectious virions with altered protein structures and immature morphologies.

Blotting, Western↗

PubMed2000

Comparison of human immunodeficiency virus type 1 Pr55(Gag) and Pr160(Gag-pol) processing intermediates that accumulate in primary and transformed cells treated with peptidic and nonpeptidic protease inhibitors.

Human immunodeficiency virus type 1 (HIV-1) produces two polyproteins, Pr55(Gag) and Pr160(Gag-Pol), that are cleaved into mature functional subunits by the virally encoded protease. Drugs that inhibit this protease are an important part of anti-HIV therapy. We studied the ordered accumulation of Gag and Gag-Pol processing intermediates by variably blocking the protease with HIV-1 protease inhibitors (PIs). Variable protease inhibition caused accumulation of a complex pattern of processing intermediates, which was the same after incubating HIV-1-infected cells with increasing concentrations of either one of the peptidomimetic inhibitors indinavir, saquinavir (SQV), ritonavir (RTV), nelfinavir, and SC-52151 or one of the nonpeptidomimetic inhibitors DMP450, DMP323, PNU-140135, and PNU-109112 for 3 days. The patterns of Gag and Gag-Pol processing intermediate accumulation were nearly identical when the following were compared: cell- versus virion-associated proteins, HIV-1-infected transformed cell lines versus primary human peripheral blood mononuclear cells (PBMCs) and HIV-1(MN) versus HIV-1(IIIB) virus strains. RTV was a more potent inhibitor of p24 production in PBMCs than SQV by approximately 7-fold, whereas SQV was a more potent inhibitor in transformed cells than RTV by approximately 30-fold. Although the antiretroviral potency of HIV-1 PIs may change as a function of cell type, the polyprotein intermediates that accumulate with increasing drug concentrations are the same. These results support sequential processing of Gag and Gag-Pol polyproteins by the HIV-1 protease and may have important implications for understanding common cross-resistance pathways.

Cell Transformation, Viral↗

PubMed1978

Intracellular precursors to the major glycoprotein of avian oncoviruses in chicken embryo fibroblasts.

A 96,000-dalton glycoprotein, p(96), was present in cell extracts obtained from gs-chf- chicken embryo fibroblasts infected with the avian RNA tumor viruses Rous-associated virus-2 subgroup B (RAV-2) and the Schmidt-Ruppin strain of Rous sarcoma virus subgroup A (SR-RSV-A), as well as from uninfected gsLchf+ (HE) cell extracts. It was not found in cell extracts from uninfected gs-chf- or gs+chf+ (HH) cells, nor from gs-chf- cells infected with envelope-deficient Bryan high-titer Rous sarcoma virus. Immunoprecipitation, kinetic, and biochemical data indicate the this polyprotein contains information that gives rise to the major virion glycoprotein gp85. A second polyprotein of 80,000 daltons, p/80), is also present in the RAV-2- and SR-RSV-A-infected gs-chf- cells. This second polyprotein contains less carbohydrate than p(96), and kinetic and biochemical data indicate that p(80) may be an immature form of p(96).

Animals↗

PubMed1979

Inhibition of Rous sarcoma virus replication by 2-deoxyglucose and tunicamycin: identification of an unglycosylated env gene product.

Two inhibitors of glycosylation, 2-deoxyglucose and tunicamycin, depressed the synthesis of infectious Rous sarcoma virus greater than 100-fold. Under the same conditions only a two- to threefold decrease in the production of virus particles was observed. The noninfectious particles had a lower density (1.145 g/ml) in isopycnic sucrose gradients and lacked the two virion glycoproteins, gp85 and gp37, found on infectious virions. The four internal structural proteins of the virus, p27, p19, p15, and p12, appeared to be assembled normally into the noninfectious virus. Polypeptides related to the Rous sarcoma virus glycoproteins were immunoprecipitated from pulse-labeled Rous sarcoma virus (Prague strain, subgroup B)-transformed cells. pr95gp, the polyprotein precursor to gp85 and gp37, was the major protein precipitated from untreated cells. PR95GP, THE POLYPROTEIN PRECURSOR TO GP85 AND GP37, WAS THE MAJOR PROTEIN PRECIPITATED FROM UNTREATED CELLS. This was absent in both tunicamycin- and 2-deoxyglucose-treatec ells, and a new polypeptide of molecular weight 57,000 to 58,000 was the major species precipitated. In tunicamycin-treated cells this product was unstable and was degraded during a 2-h chase; in 2-deoxyglucose-treated cells, on the other hand, the polypeptide appeared to be more stable and underwent partial glycosylation. The synthesis and processing of pr76, the polyprotein precursor to the internal structural proteins of the virion, occurred normally in both treated and untreated cells. It is concluded that the unglycosylated env gene product is a polypeptide of molecular weight 57,000 to 58,000.

Avian Sarcoma Viruses↗

PubMed1980

Translational products encoded by newly acquired sequences of independently derived feline sarcoma virus isolates are structurally related.

Polyproteins encoded by several independent isolates of feline sarcoma virus (FeSV) were analyzed with respect to molecular weight, extent of phosphorylation, and tryptic peptide composition. As previously reported, cells nonproductively transformed by the Gardner strain of FeSV express a polyprotein which has a molecular weight of approximately 115,000 and contains feline leukemia virus p15, p12, and minor portion of p30. In addition, a major 72,000-dalton possible cleavage product can be identified. Snyder-Theilen FeSV-transformed cells express a major polyprotein of approximately 115,000 daltons and a second highly related 80,000-dalton protein. The p12 structural component of Gardner FeSV P115, but not Snyder-Theilen FeSV 115, corresponds to feline leukemia virus subgroup A with respect to immunological type specificity, a finding consistent with the independent origin of these viruses. Tryptic peptide analysis revealed five methionine-containing peptides specific to the nonstructural portion of Gardner FeSV 115, three of which were also represented in Snyder-Theilen FeSV P115, three of which were also represented in Snyder-Theilen FeSV P115. None of these [35S]methionine-labeled tryptic peptides were present in translational products representative of the complete feline leukemia virus subgroup A genome, including Pr180gag-pol, Pr65gag, and Pr82env. Similarly phosphorylated tryptic peptides within the structural (p12) and nonstructural components of Gardner FeSV P115 and Snyder-Theilen FeSV P115 Are highly related. These findings support the possibility that acquired sequences of two independently derived isolates of FeSV encode structurally related proteins.

Animals↗

PubMed1981

Transformation-defective mutants of Snyder-Theilen feline sarcoma virus lack tyrosine-specific protein kinase activity.

Four phenotypically normal mink cell clones, each containing a transformation-defective provirus of the Snyder-Theilen strain of feline sarcoma virus (ST-FeSV), synthesized an 85,000-dalton viral polyprotein (P85) indistinguishable in size and antigenic complexity from that encoded by wild-type transforming ST-FeSV. An additional transformation-defective, ST-FeSV-containing flat cell clone produced a polyprotein of 88,000 daltons (P88). The viral polyproteins immunoprecipitated from cytoplasmic extracts of these cells lacked the tyrosine-specific protein kinase activity associated with the wild-type ST-FeSV gene product. In addition, the products encoded by representative transformation-defective ST-FeSV genomes were poorly phosphorylated in vivo and lacked detectable phosphotyrosine residues. Whereas proteins of ST-FeSV transformants contained elevated levels of phosphotyrosine, those of mink cells containing transformation-defective ST-FeSV exhibited phosphotyrosine levels no higher than those found in uninfected cells. These findings provide genetic evidence that the tyrosine-specific protein kinase activity associated with ST-FeSV P85 is required for virus-induced transformation.

Animals↗

PubMed1982

Cleavage sites within the poliovirus capsid protein precursors.

Partial amino-terminal sequence analysis was performed on radiolabeled polio-virus capsid proteins VP1, VP2, and VP3. A computer-assisted comparison of the amino acid sequences obtained with that predicted by the nucleotide sequence of the poliovirus genome allows assignment of the amino terminus of each capsid protein to a unique position within the virus polyprotein. Sequence analysis of trypsin-digested VP4, which has a blocked amino terminus, demonstrates that VP4 is encoded at or very near to the amino terminus of the polyprotein. The gene order of the capsid proteins is VP4-VP2-VP3-VP1. Cleavage of VP0 to VP4 and VP2 is shown to occur between asparagine and serine, whereas the cleavages that separate VP2/VP3 and VP3/VP1 occur between glutamine and glycine residues. This finding supports the hypothesis that the cleavage of VP0, which occurs during virion morphogenesis, is distinct from the cleavages that separate functional regions of the polyprotein.

Amino Acid Sequence↗

PubMed1987

Analysis of the complete nucleotide sequence of the picornavirus Theiler's murine encephalomyelitis virus indicates that it is closely related to cardioviruses.

Theiler's murine encephalomyelitis viruses (TMEV) are naturally occurring enteric pathogens of mice which constitute a separate serological group within the picornavirus family. Persistent TMEV infection in mice provides a relevant experimental animal model for the human demyelinating disease multiple sclerosis. To provide information about the TMEV classification, genome organization, and protein processing map, we determined the complete nucleotide sequence of the TMEV genome and deduced the amino acid sequence of the polyprotein coding region. The RNA genome, which is typical of the picornavirus family, is 8,098 nucleotides long. The 5' untranslated region is 1,064 nucleotides long (making it the longest in the picornavirus family after the aphthoviruses) and lacks a poly(C) tract. Computer-generated comparison of the 5' and 3' noncoding regions and polyprotein revealed the highest level of nucleotide and predicted amino acid identity between the TMEV and the cardioviruses encephalomyocarditis virus (EMCV) and Mengo virus. The TMEV polyprotein, which appears to be processed like EMCV since the amino acids flanking the putative proteolytic cleavage sites have been conserved, begins with a short leader peptide followed by 11 other gene products in the standard L-4-3-4 picornavirus arrangement. Because of these similarities, we propose that the TMEV be grouped with the cardioviruses. However, since TMEV and EMCV have different biophysical properties and show no cross-neutralization, they most likely belong in a separate cardiovirus subgroup.

Amino Acid Sequence↗

PubMed1990

Complete nucleotide sequence of infectious Coxsackievirus B3 cDNA: two initial 5' uridine residues are regained during plus-strand RNA synthesis.

A full-length reverse-transcribed, infectious cDNA copy of coxsackievirus B3 (CVB3) was used to determine the nucleotide sequence of this cardiotropic enterovirus. Comparison of the nucleotide sequence and the deduced amino acid sequence of the viral precursor polyprotein with the sequences of other group B coxsackieviruses (CVB1 and CVB4) demonstrates a high degree of genetic identity. They share about 80% homology at the nucleotide level and about 90% when the amino acid sequences of the polyproteins are compared. The potential processing sites of the coxsackievirus polyproteins, as deduced from alignment with the poliovirus sequence, are conserved among these enteroviruses with the exception of the cleavage sites between VP1 and 2Apro and between polypeptides 2B and 2C. Comparison of the 5' termini of the enteroviral genomes reveals a high degree of identity, including the initial 5' consensus UUAAAACAGC, suggesting essential functions in virus replication. An important finding concerning the molecular basis of infectivity was that both recombinant CVB3 cDNA and in vitro-synthesized CVB3 RNA transcripts are infectious, although two initial 5' uridine residues found on the authentic CVB3 RNA were missing. Here, we report that cDNA-generated CVB3, as well as CVB3 generated by in vitro-synthesized RNA transcripts, regains the authentic initial 5' uridine residues during replication in transfected cells, indicating that the picornaviral primer molecule VPg-pUpU may be uridylylated in a template-independent fashion. The generation of virus or virus mutants with infectious recombinant CVB3 cDNA and in vitro-synthesized infectious CVB3 transcripts should provide a valuable means for studying the molecular basis of the pathogenicity of this cardiotropic enterovirus.

Amino Acid Sequence↗

PubMed1991

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.

Amino Acid Sequence↗

PubMed1991

Structure and organization of the hepatitis C virus genome isolated from human carriers.

Hepatitis C virus (HCV) is a major causative agent of posttransfusion non-A, non-B hepatitis, which often develops into malignant chronic diseases, including liver cirrhosis and hepatocellular carcinoma. We have cloned from human carriers overlapping cDNAs (9,416 bp) covering the entire coding region of the HCV genome. The latter encodes a 3,010-amino-acid polyprotein. In addition, there are 332 and 54 bases of 5' and 3' noncoding sequences, respectively. Our HCV strain has a 77% nucleic acid identity to the HCV strain cloned by workers at Chiron Corporation. The hydrophobicity profile of the putative polyprotein is similar to those of flaviviruses, but it has limited amino acid homology to polyproteins of flaviviruses and other viruses, indicating that HCV is at most distantly related to flaviviruses.

Amino Acid Sequence↗

PubMed1991

Expression of active human immunodeficiency virus type 1 protease by noninfectious chimeric virus particles.

To generate nonpathogenic viral particles which express active human immunodeficiency virus type 1 (HIV-1) protease (PR), plasmids containing sequences from the genomes of HIV-1 and Moloney murine leukemia virus (M-MuLV) were constructed. Either the PR coding region alone; the gag, PR, and reverse transcriptase protein-coding regions; or the complete gag and pol protein-coding regions from HIV-1 were substituted for the corresponding regions of a full-length M-MuLV clone to yield the chimeric plasmids pMoHIV-I, pMoHIV-III, and pMoHIV-IV, respectively. Cell lines which express the viral gag polyprotein were isolated for hybrids pMoHIV-I and pMoHIV-III. These cells produced viral particles which contained processed core proteins. Cleavage of the gag polyprotein in the viral particles was inhibited by the HIV-1 PR inhibitor L-687908, indicating that the viral PR is responsible for the observed processing. The hybrid virions were not infectious; analyses indicated that the viral particles contained little or no reverse transcriptase activity. In addition, particles produced by pMoHIV-III transfectants failed to package the viral genomic RNA. The cell line which expresses and processes the HIV-1 gag polyprotein is a safe and effective reagent for the in vivo evaluation of potential inhibitors of the HIV-1 PR.

Animals↗

PubMed1991

Alternative translation initiation site in the DA strain of Theiler's murine encephalomyelitis virus.

Polyprotein processing studies of Theiler's murine encephalomyelitis virus (TMEV), a group of mouse picornaviruses, demonstrated synthesis of a protein we have called l during in vitro translations from the RNA of DA, a demyelinating strain of TMEV, but not GDVII, an acute neurovirulent strain. We have proposed that l is synthesized from an alternative initiation site in the DA leader (L) coding area out of phase with the polyprotein reading frame (R. P. Roos, W.-P. Kong, B. L. Semler, J. Virol. 63:5344-5353, 1989). We now provide support for this proposal from experiments involving in vitro translation of three separate mutations of an infectious DA cDNA clone: DA"l"-1, which contains a base mismatch at the putative initiation codon of l, DAL-1, which contains a base mismatch at the presumed authentic initiation site of L at the beginning of the polyprotein; and DAL:NheI, which contains nucleotides coding for a four-amino-acid insertion in the L coding area with a termination codon in the l reading frame. Our results demonstrate that the DA strain uses an alternative initiation site and reading frame to in vitro synthesize l. l may have a role in the biological activity of the virus.

Base Sequence↗

PubMed1992

cis-acting lesions targeted to the hydrophobic domain of a poliovirus membrane protein involved in RNA replication.

The structural requirements of the hydrophobic domain contained in poliovirus polypeptide 3AB were studied by using a molecular genetic approach in combination with an in vitro biochemical analysis. We report here the generation and analysis of deletion, insertion, and amino acid replacement mutations aimed at decreasing the hydrophobic character of the domain. Our results indicated that the hydrophobicity of this region of 3AB is necessary to maintain normal viral RNA synthesis. However, in vitro membrane association assays of the mutated proteins did not establish a direct correlation between 3AB membrane association and viral RNA synthesis. Some of the lethal mutations we engineered produced polyproteins with abnormal P2- and P3-processing capabilities due to an alteration in the normal cleavage order of the polyprotein. A detailed analysis of these mutants suggests that P2 is not the major precursor for polypeptides 2A and 2BC and that P2 protein products are derived from P2-P3-containing precursors (most likely P2-P3 or P2-3AB). Such precursors are likely to result from primary polyprotein cleavage events that initiate a proteolytic cascade not previously documented. Our results also indicated that the function provided by the hydrophobic domain of 3AB cannot be provided in trans. We discuss the implications of these results on the formation of limited-diffusion replication complexes as a means of sequestering P2- and P3-region polypeptides required for RNA synthesis and protein processing.

Amino Acid Sequence↗

PubMed1992

Importance of p12 protein in Mason-Pfizer monkey virus assembly and infectivity.

Mason-Pfizer monkey virus (M-PMV) represents the prototype type D retrovirus, characterized by the assembly of intracytoplasmic A-type particles within the infected-cell cytoplasm. These immature particles migrate to the plasma membrane, where they are released by budding. The gag gene of M-PMV encodes a novel protein, p12, just 5' of the major capsid protein (CA) p27 on the polyprotein precursor. The function of p12 is not known, but an equivalent protein is found in mouse mammary tumor virus and is absent from the type C retroviruses. In order to determine whether the p12 protein plays a role in the intracytoplasmic assembly of capsids, a series of in-frame deletion mutations were constructed in the p12 coding domain. The mutant gag genes were expressed by a recombinant vaccinia virus-T7 polymerase-based system in CV-1 cells or in the context of the viral genome in COS-1 cells. In both of these high-level expression systems, mutant Gag precursors were competent to assemble but were not infectious. In contrast, when stable transfectant HeLa cell lines were established, assembly of the mutant precursors into capsids was drastically reduced. Instead, the polyprotein precursors remained predominantly soluble in the cytoplasm. These results show that while p12 is not required for the intracytoplasmic assembly of M-PMV capsids, under the conditions of low-level protein biosynthesis seen in virus-infected cells, it may assist in the stable association of polyprotein precursors for capsid assembly. Moreover, the presence of the p12 coding domain is absolutely required for the infectivity of M-PMV virions.

Amino Acid Sequence↗

PubMed1992

Matrix protein of Akv murine leukemia virus: genetic mapping of regions essential for particle formation.

Type C retroviruses assemble at the plasma membrane of the infected cell. Attachment of myristic acid to the N terminus of the Gag precursor polyprotein has been shown to be essential for membrane localization and virus morphogenesis. Here, we report that the matrix (MA) protein contains regions that in conjunction with myristylation are important for Gag protein stability and the assembly of murine leukemia viruses. We identified these domains by generating a series of Akv murine leukemia virus mutants carrying small in-frame deletions within the coding region of the MA protein encompassing 129 amino acids. Studies show that mutants with deletions within the segment encoding the first 102 amino acids were all replication defective, whereas the C-terminal residues 103 to 124 seem not to have any critical function in virus maturation. Cells expressing the replication-defective genomes did not release any detectable Gag proteins. In one mutant, deletion of 3 amino acids in the N terminus resulted in an inefficiently myristylated, stable Gag polyprotein. The remaining defect genomes encoded unstable Gag proteins, although they were modified with myristic acid. The results suggest that the matrix domain plays an important role in stabilizing the Gag polyprotein.

Amino Acid Sequence↗

PubMed1993

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.

Amino Acid Sequence↗