Properties of a P70 proteolytic factor of murine leukemia viruses

Stephan Oroszlan and the Proteolytic Processing of Retroviral Proteins: Following A Pro

Steve Oroszlan determined the sequences at the ends of virion proteins for a number of different retroviruses. This work led to the insight that the amino-terminal amino acid of the mature viral CA protein is always proline. In this remembrance, we review Steve’s work that led to this insight and show how that insight was a necessary precursor to the work we have done in the subsequent years exploring the cleavage rate determinants of viral protease processing sites and the multiple roles the amino-terminal proline of CA plays after protease cleavage liberates it from its position in a protease processing site.

Across the Hall from Pioneers

I was fortunate to be associated with the lab of Stephen Oroszlan at the US National Cancer Institute from ~1982 until his conversion to Emeritus status in 1995. His lab made groundbreaking discoveries on retroviral proteins during that time, including many features that could not have been inferred or anticipated from straightforward sequence information. Building on the Oroszlan lab results, my colleagues and I demonstrated that the zinc fingers in nucleocapsid proteins play a crucial role in genomic RNA encapsidation; that the N-terminal myristylation of the Gag proteins of many retroviruses is important for their association with the plasma membrane before particle assembly is completed; and that gammaretroviruses initially synthesize their Env protein as an inactive precursor and then truncate the cytoplasmic tail of the transmembrane protein, activating Env fusogenicity, during virus maturation. We also elucidated several aspects of the mechanism of translational suppression in pol gene expression in gammaretroviruses; amazingly, this is a fundamentally different mechanism of suppression from that in most other retroviral genera.

Retroviral proteases and their roles in virion maturation

Proteolytic processing of viral polyproteins is essential for retrovirus infectivity. Retroviral proteases (PR) become activated during or after assembly of the immature, non-infectious virion. They cleave viral polyproteins at specific sites, inducing major structural rearrangements termed maturation. Maturation converts retroviral enzymes into their functional form, transforms the immature shell into a metastable state primed for early replication events, and enhances viral entry competence. Not only cleavage at all PR recognition sites, but also an ordered sequence of cleavages is crucial. Proteolysis is tightly regulated, but the triggering mechanisms and kinetics and pathway of morphological transitions remain enigmatic. Here, we outline PR structures and substrate specificities focusing on HIV PR as a therapeutic target. We discuss design and clinical success of HIV PR inhibitors, as well as resistance development towards these drugs. Finally, we summarize data elucidating the role of proteolysis in maturation and highlight unsolved questions regarding retroviral maturation.

The N-terminus of murine leukaemia virus p12 protein is required for mature core stability

The murine leukaemia virus (MLV) gag gene encodes a small protein called p12 that is essential for the early steps of viral replication. The N- and C-terminal regions of p12 are sequentially acting domains, both required for p12 function. Defects in the C-terminal domain can be overcome by introducing a chromatin binding motif into the protein. However, the function of the N-terminal domain remains unknown. Here, we undertook a detailed analysis of the effects of p12 mutation on incoming viral cores. We found that both reverse transcription complexes and isolated mature cores from N-terminal p12 mutants have altered capsid complexes compared to wild type virions. Electron microscopy revealed that mature N-terminal p12 mutant cores have different morphologies, although immature cores appear normal. Moreover, in immunofluorescent studies, both p12 and capsid proteins were lost rapidly from N-terminal p12 mutant viral cores after entry into target cells. Importantly, we determined that p12 binds directly to the MLV capsid lattice. However, we could not detect binding of an N-terminally altered p12 to capsid. Altogether, our data imply that p12 stabilises the mature MLV core, preventing premature loss of capsid, and that this is mediated by direct binding of p12 to the capsid shell. In this manner, p12 is also retained in the pre-integration complex where it facilitates tethering to mitotic chromosomes. These data also explain our previous observations that modifications to the N-terminus of p12 alter the ability of particles to abrogate restriction by TRIM5alpha and Fv1, factors that recognise viral capsid lattices.

All retroviral genomes contain a gag gene that codes for the Gag polyprotein. Gag is cleaved upon viral maturation to release individual proteins, including matrix, capsid and nucleocapsid, providing the structural components of the virion. In murine leukaemia virus (MLV), Gag cleavage releases an additional protein, named p12, required for both early and late stages of the viral life cycle. The role of p12 during early events is poorly understood, and it is the only MLV protein without a function-associated name. Here, we show that p12 binds to the capsid shell of the viral core and stabilises it. Mutations that give rise to N-terminally altered p12 proteins result in a rapid loss of both p12 and capsid from viral cores, leading to abnormal core morphologies and abolishing the ability of particles to abrogate restriction by cellular factors that target viral capsid lattices. Understanding how the mature retroviral core forms and how it disassembles during infection is important as this determines the infectivity of all retroviruses, including HIV-1. Furthermore, altering core stability has recently become a novel target for HIV-1 therapeutics.

Control of gag-pol gene expression in the Candida albicans retrotransposon Tca2

Background

In the C. albicans retrotransposon Tca2, the gag and pol ORFs are separated by a UGA stop codon, 3' of which is a potential RNA pseudoknot. It is unclear how the Tca2 gag UGA codon is bypassed to allow pol expression. However, in other retroelements, translational readthrough of the gag stop codon can be directed by its flanking sequence, including a 3' pseudoknot.

Results

The hypothesis was tested that in Tca2, gag stop codon flanking sequences direct translational readthrough and synthesis of a gag-pol fusion protein. Sequence from the Tca2 gag-UGA-pol junction (300 nt) was inserted between fused lacZ and luciferase (luc) genes in a Saccharomyces cerevisiae dual reporter construct. Although downstream of UGA, luc was expressed, but its expression was unaffected by inserting additional stop codons at the 3' end of lacZ. Luc expression was instead being driven by a previously unknown minor promoter activity within the gag-pol junction region. Evidence together indicated that junction sequence alone cannot direct UGA readthrough. Using reporter genes in C. albicans, the activities of this gag-pol junction promoter and the Tca2 long terminal repeat (LTR) promoter were compared. Of the two promoters, only the LTR promoter was induced by heat-shock, which also triggers retrotransposition. Tca2 pol protein, epitope-tagged in C. albicans to allow detection, was also heat-shock induced, indicating that pol proteins were expressed from a gag-UGA-pol RNA.

Conclusion

This is the first demonstration that the LTR promoter directs Tca2 pol protein expression, and that pol proteins are translated from a gag-pol RNA, which thus requires a mechanism for stop codon bypass. However, in contrast to most other retroelement and viral readthrough signals, immediate gag UGA-flanking sequences were insufficient to direct stop readthrough in S. cerevisiae, indicating non-canonical mechanisms direct gag UGA bypass in Tca2.

Mechanical properties of murine leukemia virus particles: effect of maturation

After budding from the host cell, retroviruses undergo a process of internal reorganization called maturation, which is prerequisite to infectivity. Viral maturation is accompanied by dramatic morphological changes, which are poorly understood in physical/mechanistic terms. Here, we study the mechanical properties of live mature and immature murine leukemia virus particles by indentation-type experiments conducted with an atomic force microscope tip. We find that both mature and immature particles have an elastic shell. Strikingly, the virus shell is twofold stiffer in the immature (0.68 N/m) than the mature (0.31 N/m) form. However, finite-element simulation shows that the average Young's modulus of the immature form is more than fourfold lower than that of the mature form. This finding suggests that per length unit, the protein-protein interactions in the mature shell are stronger than those in the immature shell. We also show that the mature virus shell is brittle, since it can be broken by application of large loading forces, by firm attachment to a substrate, or by repeated application of force. Our results are the first analysis of the mechanical properties of an animal virus, and demonstrate a linkage between virus morphology and mechanical properties.

Effects of blocking individual maturation cleavages in murine leukemia virus gag

A single protein, termed Gag, is responsible for retrovirus particle assembly. After the assembled virion is released from the cell, Gag is cleaved at several sites by the viral protease (PR). The cleavages catalyzed by PR bring about a wide variety of physical changes in the particle, collectively termed maturation, and convert the particle into an infectious virion. In murine leukemia virus (MLV) maturation, Gag is cleaved at three sites, resulting in formation of the matrix (MA), p12, capsid (CA), and nucleocapsid (NC) proteins. We introduced mutations into MLV that inhibited cleavage at individual sites in Gag. All mutants had lost the intensely staining ring characteristic of immature particles; thus, no single cleavage event is required for this feature of maturation. Mutant virions in which MA was not cleaved from p12 were still infectious, with a specific infectivity only approximately 10-fold below that of the wild type. Particles in which p12 and CA could not be separated from each other were noninfectious and lacked a well-delineated core despite the presence of dense material in their interiors. In both of these mutants, the dimeric viral RNA had undergone the stabilization normally associated with maturation, suggesting that this change may depend upon the separation of CA from NC. Alteration of the C-terminal end of CA blocked CA-NC cleavage but also reduced the efficiency of particle formation and, in some cases, severely disrupted the ability of Gag to assemble into regular structures. This observation highlights the critical role of this region of Gag in assembly.

Skipper, an LTR retrotransposon of Dictyostelium

The complete sequence of a retrotransposon from Dictyostelium discoideum , named skipper , was obtained from cDNA and genomic clones. The sequence of a nearly full-length skipper cDNA was similar to that of three other partially sequenced cDNAs. The corresponding retrotransposon is represented in approximately 15-20 copies and is abundantly transcribed. Skipper contains three open reading frames (ORFs) with an unusual sequence organization, aspects of which resemble certain mammalian retroviruses. ORFs 1 and 3 correspond to gag and pol genes; the second ORF, pro, corresponding to protease, was separated from gag by a single stop codon followed shortly thereafter by a potential pseudoknot. ORF3 (pol) was separated from pro by a +1 frameshift. ORFs 2 and 3 overlapped by 32 bp. The computed amino acid sequences of the skipper ORFs contain regions resembling retrotransposon polyprotein domains, including a nucleic acid binding protein, aspartyl protease, reverse transcriptase and integrase. Skipper is the first example of a retrotransposon with a separate pro gene. Skipper is also novel in that it appears to use stop codon suppression rather than frameshifting to modulate pro expression. Finally, skipper and its components may provide useful tools for the genetic characterization of Dictyostelium.

HIV-1 protease does not play a critical role in the early stages of HIV-1 infection

We asked whether human immunodeficiency virus type 1 (HIV-1) protease plays a major role in the early stages of infection (i.e. from viral entry to reverse transcription) by using various protease inhibitors (saquinavir, ritonavir, and KNI-272). When assessed in the two-day multinuclear activation of a galactosidase indicator (MAGI) assay, involving a single cycle of HIV-1 replication, all protease inhibitors failed to block infection of HeLa-CD4-LTR-beta-gal cells by HIV-1, while reverse transcriptase (RT) inhibitors (AZT and ddI) completely blocked the infection. Moreover, when HIV-1 proviral DNA synthesis was examined by polymerase chain reaction in HeLa-CD4-LTR-beta-gal cells exposed to HIV-1 and cultured in the presence of protease inhibitors, a significant amount of proviral DNA was detected, while no proviral DNA synthesis was detected when the cells were cultured in the presence of RT inhibitors. Protease inhibitors also failed to block chloramphenicol acetyltransferase (CAT) expression in HLCD4-CAT cells exposed to HIV-1, while RT inhibitors completely suppressed CAT expression. These results strongly suggest, contrary to a previous report by Nagy et al. (1994), that HIV-1 protease does not play a major role in the early stages of infection.

Conditional human immunodeficiency virus type 1 protease mutants show no role for the viral protease early in virus replication

The human immunodeficiency virus type 1 protease plays a critical role in the proteolytic processing of precursor polyproteins during virion maturation. Contradictory evidence has been obtained for a possible role for the protease early after infection, i.e., during DNA synthesis and/or integration. We have reexamined this question by using conditional mutants of the protease. In one set of experiments, protease mutants that confer a temperature-sensitive phenotype for processing were used to assess the need for protease activity early after infection. No significant difference from results with wild-type virus was seen when infections were carried out at either 35 or 40 degrees C. In a separate set of experiments, infections were carried out in the presence of a protease inhibitor. In this case, both wild-type virus and a drug-resistant variant were used, the latter as a control to ensure a specific effect of the inhibitor. Infection with either virus was not inhibited at drug concentrations that were up to 10-fold higher than those needed to inhibit intracellular processing by the viral protease. The results obtained by both of these experimental protocols provide evidence that the human immunodeficiency virus type 1 protease does not play a role early after infection.

Assay methods for retroviral proteases

A variety of assay methods for retroviral proteases have been developed in response to different experimental requirements, such as initial identification of a protease, subsequent enzymatic characterization, and high-capacity automated screening of potential inhibitors. This chapter has reviewed a number of these methods above; most have been closely tailored to match specific experimental requirements, and some of them are described in greater detail in other chapters in this volume. They include analysis of polyprotein cleavage using SDS-PAGE, analysis of the determinants of substrate cleavage using either chromogenic peptides or reversed-phase HPLC for product separation after cleavage of unmodified peptides, and the design and utilization of quenched fluoregenic peptides for use in continuous assay.

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.

Isolation, biochemical characterization and crystallization of the p15gag proteinase of myeloblastosis associated virus expressed in E. coli

1. The p15gag proteinase responsible for the processing of the polyprotein precursor of the myeloblastosis associated virus was obtained by a recombinant technique in an E. coli expression system. The massive expression of the intentionally truncated precursor (Pr25lac-delta gag) was accompanied by its structurally correct processing. 2. Three procedures for the purification of the recombinant proteinase from both the cytoplasmic fraction and the inclusion bodies were developed. 3. The purified proteinase was compared with the authentic proteinase isolated from MAV virions by N-terminal sequence analysis and amino acid analysis, molecular weight determination, reverse-phase HPLC and FPLC elution profiles, electrophoretic mobility and isoelectric point determination, and activity assays with proteins and synthetic substrates. The identity of both enzymes was shown. 3. Contrary to reported data, the amino acid sequence of the p15gag proteinase differs from the sequence of the homologous Rous sarcoma virus proteinase in one residue only, as follows from cDNA sequencing. 4. Crystallization of the proteinase from a citrate-phosphate buffer at pH 5.6 afforded hexagonal crystals which diffracted well as 2.3 A without deterioration.

The budding of defective human immunodeficiency virus type 1 (HIV-1) particles from cell clones persistently infected with HIV-1

Three cell clones producing large numbers of infectious or noninfectious particles of human immunodeficiency virus type 1 (HIV-1), designated M 10/LAV-2, M 16/LAV-3, and MT/LAV-17, were isolated from persistently HIV-1-infected MT-4 cells. In M 10/LAV-2, the HIV-1 proteins were defective in the cleavage of gag precursor protein, and the particles were doughnut-shaped with a double-ring structure. These particles were produced by budding at the cell surface from crescentic structures followed by the formation of double-ring structures. The viral proteins in M 16/LAV-3 were defective in the cleavage of env precursor protein. The morphology of the virus particles was intact, and an electron dense bar-shaped core was seen inside a single-ring enveloped structure. The intact particles were released from the cell surface by a budding process in which crescent shape structures first appeared at the cell membrane, then subsequently just before release matured to a complete structure with an electron dense core. In MT/LAV-17, the synthesis of HIV-1 proteins was normal, and the particles were teardrop-shaped with an intact core structure. These particles were produced by budding with an electron dense core at the cell surface. Thus, it was suggested that the morphological maturation of HIV-1 particles was completed just before release from the cell surface in several cell clones producing HIV-1 particles of different morphology.

Inhibition of bacterially expressed HIV protease activity determined by an in vitro cleavage assay with MuLV Pr65gag

HIV protease is a virally coded enzyme that cleaves gag as well as gag-pol precursor polyproteins into functional products needed for virus assembly. A pUC plasmid containing an HIV insert starting at the 5' end of the pol gene and ending just inside the intergrase coding sequence was expressed in E. coli. It provided an 11 kD gene product (protease) that specifically cleaved the Gazdar MuLV Pr65gag precursor into Pr40gag (p30 + p10) and Pr27gag (p15 + p12) intermediates, as well as lower molecular weight gag-encoded products. These were detected by immunoblotting with either MuLV anti-p30 or p12 sera. Using cleavage of MuLV Pr65gag as an assay system, pepstatin A, fusidic acid, and cerulenin were observed to inhibit HIV protease cleavage by greater than 50% at concentrations of 0.1, 0.2-0.5, and 0.5 mM, respectively.

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

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

Ty3, a yeast retrotransposon associated with tRNA genes, has homology to animal retroviruses

Ty3, a retrotransposon of Saccharomyces cerevisiae, is found within 20 base pairs (bp) of the 5' ends of different tRNA genes. Determination of the complete nucleotide sequence of one Ty3 retrotransposon (Ty3-2) shows that the element is composed of an internal domain 4,748 bp long flanked by long terminal repeats of the 340-bp sigma element. Three open reading frames (ORFs) longer than 100 codons are present in the sense strand. The first ORF, TYA3, encodes a protein with a motif found in the nucleic acid-binding protein of retroviruses. The second ORF, TYB3, has homology to retroviral pol genes. The deduced amino acid sequence of the reverse transcriptase domain shows the greatest similarity to Drosophila retrotransposon 17.6, with 43% identical residues. The inferred order of functional domains within TYB3--protease, reverse transcriptase, and endonuclease--resembles the order in Drosophila element 17.6 and in animal retroviruses but is different from that found in yeast elements Ty1 and Ty2. A second Ty3 element (Ty3-1) from a standard laboratory strain was overexpressed and shown to transpose.

Retrovirus protease characterized as a dimeric aspartic proteinase

Retroviruses and retroviruslike elements have a protease for specific cleavage of their polyprotein precursors. On the basis of amino acid sequences conserved among species and the sensitivity to protease inhibitors, it was proposed that the retrovirus protease could be classified as an aspartic proteinase. Since the virus protease molecule is comparable to a single domain of aspartic proteinases having two symmetrical domains, we hypothesized and examined the dimer formation of the protease. The results of biochemical molecular mass determination and cross-linking experiments demonstrated that the virus protease molecules self-assemble into dimers. An inhibitory effect of fragmented protease molecules suggests the possibility that the intermolecular association is required for their activity. Other experiments of chemical inactivation suggest a close resemblance of the catalytic features of retrovirus and aspartic proteinases. Characterizations of these bovine and avian virus proteases would provide basic knowledge for the design of retrovirus protease-specific inhibitors, which is one of the possible strategies against human immunodeficiency virus infection.

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

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

Ty3, a yeast retrotransposon associated with tRNA genes, has homology to animal retroviruses

Ty3, a retrotransposon of Saccharomyces cerevisiae, is found within 20 base pairs (bp) of the 5' ends of different tRNA genes. Determination of the complete nucleotide sequence of one Ty3 retrotransposon (Ty3-2) shows that the element is composed of an internal domain 4,748 bp long flanked by long terminal repeats of the 340-bp sigma element. Three open reading frames (ORFs) longer than 100 codons are present in the sense strand. The first ORF, TYA3, encodes a protein with a motif found in the nucleic acid-binding protein of retroviruses. The second ORF, TYB3, has homology to retroviral pol genes. The deduced amino acid sequence of the reverse transcriptase domain shows the greatest similarity to Drosophila retrotransposon 17.6, with 43% identical residues. The inferred order of functional domains within TYB3--protease, reverse transcriptase, and endonuclease--resembles the order in Drosophila element 17.6 and in animal retroviruses but is different from that found in yeast elements Ty1 and Ty2. A second Ty3 element (Ty3-1) from a standard laboratory strain was overexpressed and shown to transpose.

Analysis of mutant Moloney murine leukemia viruses containing linker insertion mutations in the 3' region of pol

Twelve linker insertion mutations have been constructed in the 3' part of the pol gene of Moloney murine leukemia virus. This region of the Moloney murine leukemia virus genome encodes IN or p46pol, which is required for integration of the retroviral DNA into the host cell chromosome. Viral proteins synthesized by these mutants were used to pseudotype a neo-containing retroviral vector. Ten of twelve linker insertion mutant pseudotypes were unable to generate stable proviruses in infected mouse cells, as measured by the formation of G418-resistant colonies. Two mutants mapping at the 3' terminus of the IN-encoding region were competent for the formation of stable vector proviruses (hundreds of G418-resistant colonies per mutant pseudotype-infected plate). Representative linker insertion mutants were also tested for the ability to synthesize viral unintegrated DNA in newly infected cells. All assayed mutants were capable of synthesizing all normal forms of viral unintegrated DNA. The structure of integrated vector proviruses generated by defective and nondefective linker insertion mutants was also analyzed. All replication-competent mutants generated normal proviruses, while the few obtainable proviruses generated by replication-defective mutants were sometimes aberrant in structure. These results argue strongly (and confirm previous data) that the IN-encoding region of pol does not play a significant role in DNA synthesis, but is absolutely required for the formation of normal proviral DNA.

Gene product of Moloney murine leukemia virus required for proviral integration is a DNA-binding protein

The 3'-terminal portion of the retroviral pol gene encodes a function required for the formation of the integrated provirus soon after infection of sensitive cells. To permit the isolation of large quantities of the gene product, we expressed various portions of the pol gene of Moloney murine leukemia virus (M-MuLV) as trpE fusion proteins in Escherichia coli. The proteins were found to exhibit strong DNA-binding activity after extraction and renaturation by two different procedures. In the first method, proteins separated by polyacrylamide gel electrophoresis were blotted to nitrocellulose and assayed when bound to the support. The second procedure involved the isolation of proteins in an insoluble fraction, solubilization with guanidine, and renaturation. The characteristics of the binding activity are described and compared with those of authentic viral protein.

Human immunodeficiency virus has an aspartic-type protease that can be inhibited by pepstatin A

The protease encoded by the human immunodeficiency virus (HIV) processes the viral gag and gag-pol protein precursor by posttranslational cleavage. In this study we have demonstrated by site-specific mutagenesis (Asp----Thr) and by pepstatin A inhibition that the recombinant HIV protease is an aspartic-type protease. Furthermore, incubation of HIV-infected H9 cells with pepstatin A inhibited part of the intracellular processing of the HIV gag protein yet had no apparent toxicity on HIV-infected cells during 48 hr of incubation.

Synthetic peptides as substrates and inhibitors of a retroviral protease

Processing of the gag and pol gene precursor proteins of retroviruses is essential for infectivity and is directed by a viral protease that is itself included in one of these precursors. We demonstrate here that small synthetic peptides can be used as both model substrates and inhibitors to investigate the specificity and molecular parameters of the reaction. The results indicate that a peptide that extends five amino acids but not three amino acids in both directions from a known cleavage site is accurately hydrolyzed by the protease of avian sarcoma-leukosis virus. Substitutions of the amino acids to either side of the peptide bond to be cleaved affect the ability of the peptide (as well as a larger precursor protein) to serve as a substrate. The specificity is more stringent for the amino acid that will become the carboxyl end after cleavage. Some substitutions produced peptides that were not cleaved but could act as inhibitors of cleavage of a susceptible peptide. Thus, small model substrates may be used to explore both the binding and catalytic properties of these important proteases.

Inhibition of retroviral protease activity by an aspartyl proteinase inhibitor

Retrovirus protease is an enzyme that cleaves gag and gag-pol precursor polyproteins into the functional proteins of mature virus particles. The correct processing of precursor polyproteins is necessary for the infectivity of virus particles: in vitro mutagenesis which introduces deletions into the murine leukaemia virus genome produces a protease-defective virus of immature core form and lacking infectivity. A therapeutic drug effective against disease caused by retrovirus proliferation could likewise interfere with virus maturation. The primary structure has so far been determined for the protease of avian myeloblastosis virus, and of murine, feline and bovine leukaemia viruses. Amino acid sequencing of the retrovirus proteases, either after their purification or from prediction from the nucleotide sequence, shows that they possess the Asp-Thr-Gly sequence characteristic of the aspartyl proteinases. In this report we show that retrovirus proteases belong to the aspartyl proteinase group and demonstrate an inhibition by the aspartyl proteinase-specific inhibitor, pepstatin A, on the activity of bovine leukaemia, Moloney murine leukaemia and human T-cell leukaemia virus proteases.

Mutants of murine leukemia viruses and retroviral replication

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

The functions and relationships of Ty-VLP proteins in yeast reflect those of mammalian retroviral proteins

We have identified the major structural core proteins of Ty virus-like particles (Ty-VLPs) and shown that they are generated by proteolytic cleavage of the primary translation product of TYA, p1. This precursor protein is therefore functionally similar to the gag precursor of retroviruses. Cleavage is mediated by a Ty-encoded protease located at the 5' region of TYB and is accompanied by a change in particle morphology. p1 contains sufficient information for the assembly of a pre-Ty-VLP complex, which does not require the presence of either Ty protease or reverse transcriptase. The results indicate that the requirements and pathway of Ty-VLP formation reflect the initial stages of mammalian retroviral assembly and further support the idea of a common origin for Ty elements and retroviruses.

Characterization of a virus-specific proteolytic activity processing the gag precursor of the simian sarcoma-associated virus

The proteolytic processing of the gag precursor polypeptide pr65gag of simian sarcoma-associated virus (SSAV) has been studied in vivo and in vitro. In SSAV-infected cells (i.e., in vivo) proteins of 52 and 38 kDa and the viral protein p30 could be immunoprecipitated with anti-p30 serum. This cleavage pattern is only in part imitated by in vitro cleavage of the isolated pr65gag with avian myeloblastosis virus (AMV) protease p15. However, in vitro incubation of isolated pr65gag with detergent-disrupted SSAV particles generated products identical in size to those found in vivo, i.e., proteins of 52 and 38 kDa and p30. The extent of cleavage is dependent on the concentration of the disrupted virions added to the incubation mixture. Studies with protease inhibitors suggest that the SSAV enzyme is a serine-type protease like that of other mammalian retroviruses and unlike the protease of avian viruses. The SSAV protease activity eluted from a molecular sieve column in a range of about 10-15 kDa reflecting the molecular weight of the murine leukemia virus (MuLV) protease (Mr = 13.5K). Thus, it appears that there is a close similarity between the proteolytic enzymes present in different mammalian retroviruses such as MuLV and SSAV.

A distinct glucocorticoid hormone response regulates phosphoprotein maturation in rat hepatoma cells

Glucocorticoid hormone-dependent maturation of the mouse mammary tumor virus (MMTV) phosphorylated polyprotein (Pr74) allows experimental access to certain posttranslational regulatory circuits under steroid control in M1.54 cells, an MMTV-infected rat hepatoma cell line. Pulse-chase experiments revealed that [35S]methionine-labeled Pr74 synthesized in uninduced cells could be converted posttranslationally into p24, a stable phosphorylated maturation product, only after 4 h of exposure to 1 microM dexamethasone, a synthetic glucocorticoid. This regulated processing could be prevented by prior exposure, during the chase period, to inhibitors of RNA (actinomycin D) or protein (cycloheximide or puromycin) synthesis. Moreover, half-maximal production of p24 occurred at 10 nM dexamethasone, a concentration that approximated half-maximal receptor binding and stimulation of MMTV transcript synthesis. Kinetic, hormonal, and genetic evidence suggest that p24 expression did not require or result from the overall glucocorticoid-dependent increase in polyprotein concentration. First, 20 h after dexamethasone withdrawal, Pr74 maturation was completely deinduced, whereas the absolute level of this MMTV precursor remained 10-fold over its basal level. Second, progesterone, which competes with dexamethasone for receptor binding, facilitated the regulated production of p24 but prevented the steroid-mediated accumulation of functional MMTV mRNA. Lastly, certain glucocorticoid-responsive variants, derived from M1.54 cells by resistance to complement cytolysis, expressed p24 in the presence or absence of glucocorticoid-induced levels of Pr74. Taken together, our results suggest that the glucocorticoid-regulated maturation of MMTV phosphopolyproteins resulted from an independent hormone response that required normal receptor function and de novo RNA and protein synthesis.

Inhibition of cleavage of Moloney murine leukemia virus gag and env coded precursor polyproteins by cerulenin

Cerulenin, an inhibitor of de novo fatty acid (and cholesterol) biosynthesis, has been shown to significantly decrease (greater than 75%) the amount of Moloney murine leukemia virus (MMuLV) released into the culture medium of chronically infected mouse fibroblasts (I. Katoh, Y. Yoshinaka, and R.B. Luftig, 1986, Virus Res., in press). In order to clarify the mechanism by which this decrease in virus production occurs, we analyzed the kinetics of gag and env coded protein synthesis in M-MuLV infected, cerulenin-treated cells by immunoprecipitation with monospecific antisera to p30, p12, p10, gp70, and p15(E). We found that in pulse (15 min-2 hr)-chase (0-4 hr) experiments the cleavage of not only Pr65gag to p30 and other gag coded proteins but Pr80env to gp70 and Pr15(E) as well, was greatly reduced by cerulenin treatment. Further, since the total amount of label in the Pr65gag and Pr80env bands remained about the same or was slightly decreased in 2-hr pulsed, cerulenin-treated cells, this suggests that cerulenin decreases virus production, in part, by inhibiting the cleavage of both precursor gag and env coded polyproteins during virus assembly and budding at the cell membrane. We also observed that at longer chase periods (4 hr), the effect of cerulenin could be partially overriden in that minor amounts of cleaved gag and env coded polyproteins were produced and assembled into virion particles. However, these particles contained abnormally large amounts of the uncleaved precursor Pr65gag, suggesting that maturation was incomplete. The above results suggest two independent, but not exclusive, possible mechanisms of cerulenin action to block M-MuLV production, viz. cerulenin decreases the pool of fatty acids, thereby inhibiting fatty acid acylation of Pr65gag, as well as Pr80env, and thus preventing the interaction between gag (the p15 antigenic determinant on Pr65gag) and env [the p15(E) antigenic determinant of Pr15(E)] coded gene products at the cell membrane needed for efficient virus assembly (M. Satake and R. B. Luftig, 1983, Virology 124, 259-273), and cerulenin inhibits one or more proteolytic enzymes responsible for the cleavage of Pr65gag and Pr80env.

Analysis of retroviral pol gene products with antisera raised against fusion proteins produced in Escherichia coli

Portions of the pol gene of Moloney murine leukemia virus (MuLV) were expressed as fusion proteins in Escherichia coli, and the purified proteins were used to elicit antibodies in Escherichia coli, and the purified proteins were used to elicit antibodies in rabbits. The sera were used to examine the mature pol gene products contained in virion particles and identified the reverse transcriptase and a second protein, P46pol, encoded by the 3' portion of the gene. The P46 protein was not phosphorylated and was present at the same molar abundance as the reverse transcriptase. The sera were also used to detect the Pr200gag-pol intracellular precursor protein and to analyze its processing to the mature forms. The proteins formed by several Moloney MuLV mutants were analyzed. Further tests revealed cross-reactivity with Friend MuLV and feline leukemia virus proteins, but not with avian retrovirus proteins.

Bovine leukemia virus protease: purification, chemical analysis, and in vitro processing of gag precursor polyproteins

Bovine leukemia virus protease was purified to homogeneity and assayed by using murine leukemia virus Pr65gag, a polyprotein precursor of the viral core structural proteins, as the substrate. A chemical analysis of the protease, including an amino acid composition and NH2- and COOH-terminal amino acid sequence analysis, revealed that it has an Mr of 14,000 and is encoded by a segment of the viral RNA located between the gag gene and the putative reverse transcriptase gene. As expected from the nucleotide sequence data (Rice et al., Virology 142:357-377, 1985), the reading frame for the protease is different from both the gag and reverse transcriptase reading frames. The 5' end of the protease open reading frame extends 38 codons upstream from the codon for the NH2-terminal residue of the mature viral protease and overlaps the gag open reading frame by 7 codons. The 3' end of the protease open reading frame extends 26 codons beyond the codon for the COOH-terminal residue of the mature protease and overlaps 8 codons of the reverse transcriptase open reading frame. Several lines of evidence, such as protein mapping of the gag polyprotein precursor, the characteristic structure of the mRNA, and promotion of the synthesis of a gag polyprotein precursor by lysine tRNA in vitro, suggest that the protease could be translated by frameshift suppression of the gag termination codon. In vitro synthesized bovine leukemia virus gag-related polyproteins were cleaved by the protease into fragments which were the same size as the known components of bovine leukemia virus, suggesting that the specificity of cleavage catalyzed in vitro by the purified protease is the same as the specificity of cleavage found in the virus.

A distinct glucocorticoid hormone response regulates phosphoprotein maturation in rat hepatoma cells

Glucocorticoid hormone-dependent maturation of the mouse mammary tumor virus (MMTV) phosphorylated polyprotein (Pr74) allows experimental access to certain posttranslational regulatory circuits under steroid control in M1.54 cells, an MMTV-infected rat hepatoma cell line. Pulse-chase experiments revealed that [35S]methionine-labeled Pr74 synthesized in uninduced cells could be converted posttranslationally into p24, a stable phosphorylated maturation product, only after 4 h of exposure to 1 microM dexamethasone, a synthetic glucocorticoid. This regulated processing could be prevented by prior exposure, during the chase period, to inhibitors of RNA (actinomycin D) or protein (cycloheximide or puromycin) synthesis. Moreover, half-maximal production of p24 occurred at 10 nM dexamethasone, a concentration that approximated half-maximal receptor binding and stimulation of MMTV transcript synthesis. Kinetic, hormonal, and genetic evidence suggest that p24 expression did not require or result from the overall glucocorticoid-dependent increase in polyprotein concentration. First, 20 h after dexamethasone withdrawal, Pr74 maturation was completely deinduced, whereas the absolute level of this MMTV precursor remained 10-fold over its basal level. Second, progesterone, which competes with dexamethasone for receptor binding, facilitated the regulated production of p24 but prevented the steroid-mediated accumulation of functional MMTV mRNA. Lastly, certain glucocorticoid-responsive variants, derived from M1.54 cells by resistance to complement cytolysis, expressed p24 in the presence or absence of glucocorticoid-induced levels of Pr74. Taken together, our results suggest that the glucocorticoid-regulated maturation of MMTV phosphopolyproteins resulted from an independent hormone response that required normal receptor function and de novo RNA and protein synthesis.

Coordination of cleavage of gag and env gene products of murine leukemia virus: implications regarding the mechanism of processing

Mouse 3T6 cells infected with Murine Leukemia Virus (MuLV) were cloned to yield several sublines producing viruses distinct from one another with respect to the ratio of uncleaved to cleaved gag gene-coded polyprotein, Pr65gag. The virus produced by the cloned sublines also differed in the ratio of the env gene-coded protein, p15E, to its product, p12E. The two ratios, Pr65gag/p30 and p15E/p12E, were found to be highly correlated among the cloned cell lines. Velocity gradient separation of the virions produced by individual sublines, followed by polypeptide analysis, demonstrated that the particles were inhomogeneous with respect to extent of cleavage both of PR65gag and of p15E. The two cleavages were again highly correlated. These data indicate that the gag and env gene product cleavages are not independent events but are tightly coupled.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Translational readthrough of an amber termination codon during synthesis of feline leukemia virus protease

Feline leukemia virus contains a protease which apparently has the same specificity as murine leukemia virus protease. It cleaves in vitro the Pr65gag of Gazdar-mouse sarcoma virus into the constituent p15, p12, p30, and p10 proteins. We purified the protease and determined its NH2-terminal amino acid sequence (the first 15 residues). Alignment of this amino acid sequence with the nucleotide sequence (I. Laprevotte, A. Hampe, C. H. Sherr, and F. Galibert, J. Virol. 50:884-894, 1984) reveals that the protease is a viral-coded enzyme and is located at the 5' end of the pol gene. As previously found for murine leukemia virus (Y. Yoshinaka, I. Katoh, T. D. Copeland, and S. Oroszlan, Proc. Natl. Acad. Sci. U.S.A. 82:1618-1622, 1985), feline leukemia virus protease is synthesized through in-frame suppression of the gag amber termination codon by insertion of a glutamine in the fifth position, and the first four amino acids are derived from the gag gene.

Simian sarcoma virus-encoded gag-related protein: in vitro cleavage by Friend leukemia virus-associated proteolytic activity

The simian sarcoma virus (SSV) encodes a gag-related 65,000-Da protein (SSV p65) which is not processed in SSV nonproducer cells (SSV-NP cells) (H.-J. Thiel, T. J. Matthews, E. M. Broughton, K. J. Weinhold, D. P. Bolognesi, T. Graf, and H. Beug (1981a), Virology 114, 124-131). In order to cleave SSV p65, retroviral particles containing this antigen were incubated with extracts from the heterologous helper virus Friend leukemia virus (FLV). Superinfection of SSV-NP cells by FLV has been previously shown to result in processing of SSV p65 in vivo (H.-J. Thiel, F. Weiland, R. Hafenrichter, T. J. Matthews, and K. J. Weinhold (1982), Virology 123, 229-234). In vitro cleavage was most efficient in the presence of a nonionic detergent (greater than 0.1% Nonidet-P40) and a reducing agent (greater than 5 mM dithiothreitol) at a pH of 7.0. The products, termed SSV p55 (p15, p12, p30), SSV p30, SSV p25 (p15, p12), and SSV p10, were characterized by (1) molecular weight, (2) kinetics experiments, (3) incorporation of different radiolabeled amino acids, and (4) comparison with SSAV structural proteins. Kinetics experiments with two amino acids ([3H]leucine, [35S]cysteine) revealed that initial processing of SSV p65 produced SSV p55 and SSV p10, with subsequent processing of SSV p55 occurring thereafter. In contrast to the Moloney system, the major intermediate p40 (p30, p10) could not be clearly demonstrated. A direct comparison of SSAV p10 and the cleavage product SSV p10 by SDS-PAGE suggests that SSAV pr65gag and SSV p65 differ slightly by molecular weight.

Murine leukemia virus protease is encoded by the gag-pol gene and is synthesized through suppression of an amber termination codon

We have purified from Moloney murine leukemia virus (Mo-MuLV) a protease that has the capacity of accurately cleaving the polyprotein precursor Pr65gag into the mature viral structural proteins. Both the NH2- and COOH-terminal amino acid sequences have been determined and aligned with the amino acid sequence deduced from the DNA sequence of Mo-MuLV by other workers. The results show that: (i) the protease is located at the 5' end of the pol gene, and the first four amino acids are overlapped with the 3' end of the gag gene; (ii) the fifth amino acid residue is glutamine, which is inserted by suppression of the UAG termination codon at the gag-pol junction; and (iii) the protease is composed of 125 amino acids with calculated Mr = 13,315, and the COOH terminus of the protease is adjacent to the NH2 terminus of reverse transcriptase. The map order of the gag-pol gene is proposed to be 5'-p15-p12-p30-p10-protease-reverse transcriptase-endonuclease-3'.

A deletion mutation in the 5' part of the pol gene of Moloney murine leukemia virus blocks proteolytic processing of the gag and pol polyproteins

Deletion mutations in the 5' part of the pol gene of Moloney murine leukemia virus were generated by restriction enzyme site-directed mutagenesis of cloned proviral DNA. DNA sequence analysis indicated that one such deletion was localized entirely within the 5' part of the pol gene, did not affect the region encoding reverse transcriptase, and preserved the translational reading frame downstream of the mutation. The major viral precursor polyproteins (Pr65gag, Pr200gag-pol, and gPr80env) were synthesized at wild-type levels in cell lines carrying the mutant genome. These cell lines assembled and released wild-type levels of virion particles into the medium. Cleavage of both Pr65gag and Pr200gag-pol precursors to the mature proteins was completely blocked in the mutant virions. Surprisingly, these virions contained high levels of active reverse transcriptase; examination of the endogenous reverse transcription products synthesized by the mutant virions revealed normal amounts of minus-strand strong-stop DNA, indicating that the RNA genome was packaged and that reverse transcription in detergent-permeabilized virions was not significantly impaired. Processing of gPr80env to gP70env and P15E was not affected by the mutation, but cleavage of P15E to P12E was not observed. The mutant particles were poorly infectious; analysis indicated that infection was blocked at an early stage. The data are consistent with the idea that the 5' part of the pol gene encodes a protease directly responsible for processing Pr65gag, and possibly Pr200gag-pol, to the structural virion proteins. It appears that cleavage of the gag gene product is not required for budding and release of virions and that complete processing of the pol gene product to the mature form of reverse transcriptase is not required for its functional activation.

Polypeptides of Mason-Pfizer monkey virus. I. Synthesis and processing of the gag-gene products

Mason-Pfizer monkey virus (M-PMV), the prototype D-type retrovirus, differs from the mammalian C-type retroviruses by preassembling core structures in the cytoplasm of infected cells during morphogenesis. Studies that define the protein composition of M-PMV virions and identify two gag-related polyprotein precursors in M-PMV infected cells are reported. The polyprotein precursor to the internal structural (gag) proteins of M-PMV was identified by immunoprecipitation from lysates of pulse-labeled, virus-infected cells with an antiserum to the major structural protein, p27. Tryptic peptide-mapping experiments have shown that this precursor (Pr78) is cleaved to yield five virion structural polypeptides--p27, pp16, p14, p12, and p10. The pp16 polypeptide represents an additional gag-gene encoded polypeptide, not described previously; it is a phosphoprotein and present in virions in a number of forms. A second gag-related polyprotein precursor, P95, is also present in infected cells although in smaller amounts. This nonglycosylated polypeptide contains all of the leucine-containing tryptic peptides of Pr78 plus three others. Studies of the rate of synthesis and half-life of this protein argue against it being the major gag-gene precursor polypeptide. The possibility that it represents a precursor to the viral protease is discussed.

Murine leukemia virus mutant with a frameshift in the reverse transcriptase coding region: implications for pol gene structure

The molecular defect in the nonconditional B-tropic MuLV pol mutant, clone 23 (Gerwin et al., J. Virol. 31:741-751, 1979), has been characterized by recombinant DNA technology. The entire mutant genome was cloned from an EcoRI digest of integrated cellular DNA into bacteriophage lambda Charon 4A and then subcloned at the EcoRI site of pBR322. NIH-3T3 cells transfected with the plasmid clone, termed pRTM (RTM, reverse transcriptase mutant), reproduced the properties of clone 23 virus-infected cells. In vivo ligation experiments involving cotransfection of subclones of pRTM and wild-type murine leukemia virus localized the defect in the clone 23 genome to an approximately 400-base-pair region in the pol gene between the SalI and XhoI sites. Sequence analysis of this region in the wild-type and mutant genomes revealed that the mutant has one additional C residue located 231 bases downstream of the last base of the SalI recognition site. This 1-base insertion brings three TGA termination codons into phase. Thus, the mutation in clone 23 leads to premature termination of translation, explaining the presence in clone 23 virions of a truncated polymerase with low levels of enzymatic activity. It was previously shown that the gag precursor is cleaved normally in clone 23-infected cells; therefore, if a virus-coded protease is involved in this cleavage, it must be encoded by sequences upstream of the reverse transcriptase region of the pol gene. This consideration, coupled with the observed molecular weight of the mutant polymerase and our precise determination of its C terminus, have led to a proposal for the genetic organization of the murine leukemia virus pol gene.

Impaired cleavage of the joint gag-pol polyprotein precursor and virion assembly in a temperature-sensitive mutant of Rous sarcoma virus

A temperature-sensitive coordinate mutant tsLA83 of Prague (PR-B) strain of Rous sarcoma virus at the nonpermissive temperature (41 degrees) produces noninfectious virus particles (NI-LA83) which contained only 3% of the reverse-transcriptase activity present in infectious virions. Analyses of [35S]methionine-labeled NI-LA83 showed the presence of all of the viral proteins except reverse transcriptase. Pulse-chase analyses of the virus-specified proteins in cells infected with LA83 or PR-B showed that the gag and glycoprotein precursors, Pr76gag and gPr95env, respectively, were processed at both 35 and 41 degrees. The reverse-transcriptase precursor, Pr180gag-pol, however, was not processed in LA83-infected cells at 41 degrees. In contrast, cells infected with LA83 or PR-B at 35 degrees as well as with PR-B at 41 degrees showed normal cleavage of Pr180gag-pol. A shiftdown of LA83-infected cells at 41 degrees to the permissive temperature 35 degrees resulted in the normal processing of Pr180gag-pol and production of infectious virus containing reverse transcriptase. Electron microscopic analysis showed that at 41 degrees cells infected with LA83 showed a large number of budding structures but fewer released particles. A shiftdown from 41 to 35 degrees resulted in an increase of virus particles with a concomitant decrease in budding structures suggesting that the processing of reverse-transcriptase precursor is related to virion assembly.

Comparative immunofluorescence of murine leukemia virus-derived membrane-associated antigens

We recently reported that the distribution and location of Moloney murine leukemia virus-derived membrane-associated gp70 and p30 antigens on the surface of 3.7% formaldehyde-fixed, chronically infected mouse fibroblasts were completely distinct, as judged by immunofluorescent light microscopy (M. Satake, P. N. McMillan, and R. B. Luftig (1981), Proc. Nat. Acad. Sci. USA 78, 6266). gp70, one of the two env gene products, exhibited a multiple-dot fluorescent pattern on the external surface of infected cells, while p30, one of the gag gene products, exhibited a diffuse fluorescence pattern which was apparently derived from Pr65gag molecules associated with the cytoplasmic face of the cell membrane. We have now examined the membrane fluorescence patterns of p15E, the other env gene product, as well as p15, p12, and p10, the other gag gene products. In these studies, both multivalent and monoclonal antibodies as well as fluorescein- and rhodamine-conjugated probes were used. We found that: (i) each of the env gene products, gp70 and p15E, exhibited characteristic and distinctive multiple-dot staining patterns. Further, each protein was labeled on intact cells with 125I-protein A plus homologous antiserum, confirming that both gp70 and p15E had externally exposed antigenic determinants. (ii) Among the gag gene products, p15 exhibited a different membrane fluorescence pattern than the diffuse pattern seen with p30, p12, and p10. The p15 pattern had an additional multiple-dot component. (iii) By double immunofluorescence we observed that the p15E and p15 multiple-dot patterns were superimposable at the same loci on infected cells. These three results suggest, first, that the cleavage of gp70 and p15E occurs prior to the arrival of the env polyprotein precursor at the cell surface and, second, there is an association between p15E and p15 antigenic determinants at the cell membrane. This latter association between an env and a gag gene product may be important for viral assembly.