The generation of the two envelope glycoproteins of Rous sarcoma virus from a common precursor polypeptide

Alpharetrovirus envelope-receptor interactions

Infection by all enveloped viruses occurs via the fusion of viral and cellular membranes and delivery of the viral nucleocapsid into the cell cytoplasm, after association of the virus with cognate receptors at the cell surface. This process is mediated by viral fusion proteins anchored in the viral envelope and can be defined based on the requirement for low pH to trigger membrane fusion. In viruses that utilize a pH-dependent entry mechanism, such as influenza virus, viral fusion is triggered by the acidic environment of intracellular organelles after uptake of the virus from the cell surface and trafficking to a low-pH compartment. In contrast, in viruses that utilize a pH-independent entry mechanism, such as most retroviruses, membrane fusion is triggered solely by the interaction of the envelope glycoprotein with cognate receptors, often at the cell surface. However, recent work has indicated that the alpharetrovirus, avian sarcoma and leukosis virus (ASLV), utilizes a novel entry mechanism that combines aspects of both pH-independent and pH-dependent entry. In ASLV infection, the interaction of the envelope glycoprotein (Env) with cognate receptors at the cell surface causes an initial conformational change that primes (activates) Env and renders it sensitive to subsequent low-pH triggering from an intracellular compartment. Thus unlike other pH-dependent viruses, ASLV Env is only sensitive to low-pH triggering following interaction with its cognate receptor. In this manuscript we review current research on ASLV Env-receptor interactions and focus on the specific molecular requirements of both the viral fusion protein and cognate receptors for ASLV entry. In addition, we review data pertaining to the novel two-step entry mechanism of ASLV entry and propose a model by which ASLV Env elicits membrane fusion.

Generation of a water-soluble oligomeric ectodomain of the Rous sarcoma virus envelope glycoprotein

Sequences encoding the transmembrane domain of the Rous sarcoma virus envelope (Env) glycoprotein were deleted and replaced with sequences that signal addition of a glycosyl phosphatidylinositol (GPI) membrane anchor. Stable NIH 3T3 cell lines expressing either the wild-type transmembrane-anchored Env or the Env chimera with a GPI tail were established. The GPI-anchored envelope glycoprotein is expressed, oligomerized, and transported to the cell surface in a manner identical to that of its wild-type transmembrane-anchored counterpart. The GPI-linked protein is quantitatively removed from the cell surface by treatment with phosphatidylinositol phospholipase C. The phosphatidylinositol phospholipase C-released, water-soluble Env glycoprotein ectodomain retains the wild-type oligomeric structure and provides a useful tool for studying the subgroup-specific binding and fusion activities of a prototypic retroviral Env glycoprotein.

The avian retrovirus env gene family: molecular analysis of host range and antigenic variants

The nucleotide sequence of the env gp85-coding domain from two avian sarcoma and leukosis retrovirus isolates was determined to identify host range and antigenic determinants. The predicted amino acid sequence of gp85 from a subgroup D virus isolate of the Schmidt-Ruppin strain of Rous sarcoma virus was compared with the previously reported sequences of subgroup A, B, C, and E avian sarcoma and leukosis retroviruses. Subgroup D viruses are closely related to the subgroup B viruses but have an extended host range that includes the ability to penetrate certain mammalian cells. There are 27 amino acid differences shared between the subgroup D sequence and three subgroup B sequences. At 16 of these sites, the subgroup D sequence is identical to the sequence of one or more of the other subgroup viruses (A, C, and E). The remaining 11 sites are specific to subgroup D and show some clustering in the two large variable regions that are thought to be major determinants of host range. Biological analysis of recombinant viruses containing a dominant selectable marker confirmed the role of the gp85-coding domain in determining the host range of the subgroup D virus in the infection of mammalian cells. We also compared the sequence of the gp85-coding domain from two subgroup A viruses, Rous-associated virus type 1 and a subgroup A virus of the Schmidt-Ruppin strain of Rous sarcoma virus. The comparison revealed 24 nonconservative amino acid changes, of which 6 result in changes in potential glycosylation sites. The positions of 10 amino acid differences are coincident with the positions of 10 differences found between two subgroup B virus env gene sequences. These 10 sites identify seven domains in the sequence which may constitute determinants of type-specific antigenicity. Using a molecular recombinant, we demonstrated that type-specific neutralization of two subgroup A viruses was associated with the gp85-coding domain of the virus.

Mutations within the proteolytic cleavage site of the Rous sarcoma virus glycoprotein that block processing to gp85 and gp37

We have investigated the specificity of the proteolytic cleavage of the Rous sarcoma virus glycoprotein precursor by introducing two mutations into the putative cleavage region (Arg-Arg-Lys-Arg). We show that neither a deletion of the cleavage sequence nor a glutamic acid for lysine substitution altered intracellular transport or surface expression of the env gene products. However, both the four-amino-acid deletion and the glutamic acid substitution block processing of the env precursor. Susceptibility of the glutamic acid-substituted env precursor to proteases indicated that tertiary protein structure was unaffected. While inhibitor experiments suggested that more than one endopeptidase might be capable of mediating the proteolytic cleavage, the results presented here point to the presence in the Golgi apparatus of a novel endopeptidase, required for retroviral glycoprotein cleavage, that has a high specificity for lysine-containing peptides.

env genes of avian retroviruses: nucleotide sequence and molecular recombinants define host range determinants

The env gene of avian sarcoma and leukosis retroviruses is allelic in the virus population permitting the virus to use different host cell receptors. This polymorphism has allowed the classification of these viruses into different subgroups. In order to understand further the role of viral sequences involved in determining this host range phenomenon, we constructed molecular recombinants between subgroup A, B, and E viruses and showed that the host range determinant defining subgroup specificity was located within a 1.1-kb region of the genome that included most of the coding region for the env gene product gp85. We also determined the nucleotide sequence of the region of the env gene encoding gp85 for virus isolates representing subgroup A and B viruses. We compared the predicted amino acid sequences of gp85 to themselves and to the previously published sequences of subgroup B, C, and E env genes. Based on these comparisons, we draw the following conclusions: Within the gp85 coding domain, there are four variable regions (VR-1 to VR-4) ranging in size from 9 to 52 amino acids. The variable regions are located in the same relative positions for each of the env gene alleles compared. The variable regions range in homology from 42% (A compared to B) to 57% (C compared to E) in pairwise comparisons; the flanking conserved domains are on average 95% homologous. The sequences of three different subgroup B virus isolates are highly homologous in both the conserved and variable regions. Secondary structure predictions suggest that gp85 is composed mostly of beta sheet topology. Hydrophilic loops within the variable regions may define sites of receptor interaction and binding sites for subgroup specific neutralizing antibodies.

Studies of the invasiveness of the chemically induced mouse sarcoma FS9. I. Monoclonal antibodies to a 37,000 dalton membrane glycoprotein inhibit invasion of fibroblasts in vitro

Monoclonal antibodies against the mouse sarcoma FS9 were prepared. Antibodies recognizing a 37,000 dalton glycoprotein on FS9 tumor cells inhibit invasion by FS9 sarcoma cells of chicken heart fibroblasts in vitro as assessed by Abercrombie's confronted explant assay. Antibodies to other membrane proteins of FS9 tumor cells failed to inhibit invasiveness of FS9 sarcoma cells. The 37,000 dalton glycoprotein, which is neither a histocompatibility antigen nor a gp37 glycoprotein of Rous sarcoma virus nor the MEP described by Gottesman, is present on the surface and in the cytoplasm of FS9 sarcoma cells. The plasma membrane of the non-invasive mouse cell line L929 contains only low concentrations of the 37,000 dalton antigen. Hence, a relationship apparently exists between the increased concentration of this protein on the surface of FS9 cells and their invasiveness.

Avian sarcoma virus gag and env gene structural protein precursors contain a common amino-terminal sequence

The initiation site for translation of the avian sarcoma virus glycoprotein precursor, Pr63env, has been determined by analyzing the amino-terminal peptides of Pr63env and the polyprotein precursor Pr76gag encoded by the viral gag gene. The acceptor splice junction used to form the env gene mRNA has also been identified. Hybrid-selected virus-specific mRNAs were translated in vitro in the presence of either L-[35S]methionine to label at every methionine residue or L-[35S]methionine-tRNAMeti to label specifically at the amino-terminal methionine residues. Tryptic peptide maps of Pr63env labeled at every methionine residue contain all of the peptides, plus one additional peptide, present in the map of Pr57env, a nonglycosylated env-encoded polypeptide of molecular weight 57,000 immunoprecipitated from tunicamycin-treated cells. Specific amino-terminal labeling of the in vitro-synthesized polypeptides showed that the peptide missing from Pr57env corresponds to the amino-terminal tryptic peptide of Pr63env, which is removed in vivo as part of the amino-terminal signal peptide. Comparison of the amino-terminal tryptic peptides of Pr63env and Pr76gag showed that they are identical. In contrast, the chymotryptic amino-terminal peptides of Pr76gag and Pr63env are not identical. The location of the acceptor-splice junction in the env mRNA of the Prague A strain of avian sarcoma virus was determined by mung bean nuclease mapping to be at nucleotide 5,078. Fusion of the gag and env gene sequences during splicing results in use of the same AUG codon to initiate synthesis of Pr76gag and Pr63env. This sequence is contained within the 397-nucleotide 5' terminal leader that is spliced to the body of the env mRNA. The possible significance of these results for the regulation of avian sarcoma virus synthesis and translation is discussed.

Complete sequence of the Rous sarcoma virus env gene: identification of structural and functional regions of its product

The amino-terminal amino acid sequences of gp85 and gp37, the envelope glycoproteins of Rous sarcoma virus (RSV), were determined. Alignment of these sequences with the amino acid sequence predicted from the complete nucleotide sequence of the Prague strain of RSV, subgroup C (PR-C), has allowed us to delineate the env gene-coding region of this virus. The coding sequences for gp85 and gp37 have been placed in an open reading frame that extends from nucleotide 5045 to nucleotide 6862 and predict sizes of 341 amino acids (36,962 molecular weight) for gp85 and 198 amino acids (21,566 molecular weight) for gp37. Carbohydrate makes a significant contribution to the observed molecular weights of these polypeptides--the amino acid sequence contains 14 potential glycosylation sites (Asn-X-Ser/Thr) in gp85 and two in gp37. Experiments aimed at estimating the number of carbohydrate side chains yielded results consistent with most or all of these sites being occupied. Although an initiation codon is located early (codon 4) in the open reading frame, it is likely that splicing yields an mRNA on which translation initiates at the same AUG as that of the gag gene to produce a nascent polypeptide in which gp85 is preceded by a 62-amino-acid-long leader peptide. This leader contains the hydrophobic sequence (signal sequence) necessary for translocation across the endoplasmic reticulum and is completely removed from the env gene product during translation. The polyprotein precursor, Pr95env, is cleaved to gp85 and gp37 at the carboxyl side of the basic sequence:-Arg-Arg-Lys-Arg-. gp85 is attached through a disulphide linkage to gp37, and although the positions of the cysteines involved in this linkage are not known, the presence of a 27-amino-acid-long hydrophobic region at the carboxy-terminus of gp37 is consistent with its role as a membrane anchor for the viral glycoprotein complex. The location of host range variable regions with respect to the possible tertiary structure of the complex is discussed.

Glycopeptides of the type-common glycoprotein gD of herpes simplex virus types 1 and 2

We have carried out detailed structural studies of the glycopeptides of glycoprotein gD of herpes simplex virus types 1 and 2. We first examined and compared the number of N-asparagine-linked oligosaccharides present in each glycoprotein. We found that treatment of either pgD-1 or pgD-2 with endo-beta-N-acetylglucosaminidase H (Endo H) generated three polypeptides which migrated more rapidly than pgD on gradient sodium dodecyl sulfate-polyacrylamide gels. Two of the faster-migrating polypeptides were labeled with [(3)H]mannose, suggesting that both pgD-1 and pgD-2 contained three N-asparagine-linked oligosaccharides. Second, we characterized the [(3)H]mannose-labeled tryptic peptides of pgD-1 and pgD-2. We found that both glycoproteins contained three tryptic glycopeptides, termed glycopeptides 1, 2, and 3. Gel filtration studies indicated that the molecular weights of these three peptides were approximately 10,000, 3,900, and 1,800, respectively, for both pgD-1 and pgD-2. Three methods were employed to determine the size of the attached oligosaccharides. First, the [(3)H]mannose-labeled glycopeptides were treated with Endo H, and the released oligosaccharide was chromatographed on Bio-Gel P6. The size of this molecule was estimated to be approximately 1,200 daltons. Second, Endo H treatment of [(35)S]methionine-labeled glycopeptide 2 reduced the molecular size of this peptide from approximately 3,900 to approximately 2,400 daltons. Third, glycopeptide 2 isolated from the gD-like molecule formed in the presence of tunicamycin was approximately 2,200 daltons. From these experiments, the size of each N-asparagine-linked oligosaccharide was estimated to be approximately 1,400 to 1,600 daltons. Our experiments indicated that glycopeptides 2 and 3 each contained one N-asparagine-linked oligosaccharide chain. Although glycopeptide 1 was large enough to accommodate more than one oligosaccharide chain, the experiments with Endo H treatment of the glycoprotein indicated that there were only three N-asparagine-linked oligosaccharides present in pgD-1 and pgD-2. Further studies of the tryptic glycopeptides by reverse-phase high-performance liquid chromatography indicated that all of the glycopeptides were hydrophobic in nature. In the case of glycopeptide 2, we observed that when the carbohydrate was not present, the hydrophobicity of the peptide increased. The properties of the tryptic glycopeptides of pgD-1 were compared with the properties predicted from the deduced amino acid sequence of gD-1. The size and amino acid composition compared favorably for glycopeptides 1 and 2. Glycopeptide 3 appeared to be somewhat smaller than would be predicted from the deduced sequence of gD-1. It appears that all three potential glycosylation sites predicted by the amino acid sequence are utilized in gD-1 and that a similar number of glycosylation sites are present in gD-2.

Alterations in the transport and processing of Rous sarcoma virus envelope glycoproteins mutated in the signal and anchor regions

The env gene of Rous sarcoma virus codes for two glycoproteins which are located on the surface of infectious virions. Subcloning of these coding sequences in the place of the late region of SV40 DNA has allowed the expression of a normally glycosylated, functionally active glycoprotein complex on the surface of monkey cells. Through the use of site-directed mutagenesis, the role of specific amino acids in the signal peptide, signal peptidase cleavage site, and membrane anchor region have been investigated. Amino-terminal mutations have shown that deletion of the signal peptidase cleavage site along with one or two amino acids of the hydrophobic signal peptide results in the synthesis of an unglycosylated, uncleaved, and presumably cytoplasmically located precursor. Nevertheless, changing the signal peptidase cleavage site from ala/asp to ala/asn does not block the translocation of the glycoprotein across the membrane or the action of the peptidase. At the other end of the molecule, carboxy-terminal mutations have shown that the deletion of the hydrophobic membrane anchor region is not sufficient for the secretion of the truncated glycoprotein. Interpretations of these results based on recent models for protein transport and secretion are discussed.

The structure of the Rous sarcoma virus glycoprotein complex: in vitro phenotypic mixing of rous sarcoma virus

Gp 85 the major envelope glycoprotein of Rous sarcoma virus can be released from intact by virus treatment with 2-mercaptoethanol. Attempts were made to reassociate released gp 85 to 2-mercaptoethanol treated virus. In order to study the biological activity and to analyse the reaction product with biochemical methods gp 85 from a subgroup different from the recipient virus was used. Successful in vitro phenotypic mixing could be shown. The biochemical analysis, however, revealed that no reassociation of gp 85 and gp 35 could be obtained. The biological activity was due to a strong association of whole gp 85-gp 35 molecules derived from the gp 85 preparation to the recipient virus. These molecules were present as a minor contamination in the gp 85 preparation.

Evidence for a Gp85-related glycoprotein in cells transformed by the Bryan high titer strain of Rous sarcoma virus

The Bryan High Titer strain of Rous Sarcoma Virus (BH-RSV) is a deletion mutant in the env-gene coding for the viral envelope glycoproteins gp35 and gp85. In this report experimental evidence is described that cells, transformed by BH-RSV, express a glycoprotein immunologically related to gp85. Animals bearing BH-RSV induced tumors produce antibodies reacting with gp85 of nondefective RSV. Lysates of a BH-RSV transformed quail cell line, R(-):Q, inhibit the immunoprecipitation of gp85 by antibodies against the group-specific determinant of gp85. In R(-):Q cell lysates and in the culture supernatant a glycoprotein of an apparent molecular weight 40,000 (gp40) is found that reacts with monospecific antisera against gp85 of nondefective RSV. In newly synthesized BH-RSV a gp40 associated with the virion is detectable but is easily lost during purification of the virus. Further, a 95k glycoprotein and a 95k phosphoprotein are specifically precipitated from R(-):Q cells by an antiserum against gp85. From these results we conclude that the deletion of the env-gene is incomplete such that part of gp85, bearing group-specific antigenic determinants, is expressed in BH-RSV transformed cells. Analysis of BH-RSV particles freshly harvested from R(-):Q cells reveals that they contain almost exclusively the gag-precursor pr76 and little or no processed gag-proteins. Therefore the R(-):Q cell line seems to be suitable for the study of virus maturation occurring after the budding process.

Envelope assembly mutant of rous sarcoma virus

The properties of a novel nonconditional envelope mutant of Rous sarcoma virus, rdPN3/2-SR-D, defective in the assembly of viral glycoproteins into mature virions, are described.

Synthesis and processing of viral glycoproteins in two nonconditional mutants of Rous sarcoma virus

We have studied the pattern of glycoprotein synthesis in two nonconditional mutants of Rous sarcoma virus. One mutant, SE33, produces no viral particles but synthesizes Pr92env, which is cleaved intracellularly to mature glycoproteins. The second mutant, SE521, encodes a gPr92env which is not cleaved to gp85 or gp37 and therefore produces virions with the phenotype of Bryan RSV(-) or NY8. Neither of these mutants have detectable genomic deletions. The study of these mutants has led to the following conclusions. (i) In the absence of particle production or p15 synthesis, gPr92env can be cleaved to the mature glycoprotein which is found on the cell surface. (ii) Noncleaved gPr92env is not packaged into virions but is found on the cell surface. (iii) gPr92env alone can account for subgroup specific viral interference. (iv) gPr92env is probably transported to the cell surface before additional glycosylation or cleavage to mature virion glycoprotein. The nonprocessed precursor of SE521 appears to be glycosylated normally, and thus far we have been unable to determine the basis for the defect in this mutant.

Novel p19-related protein in Rous-associated virus type 61: implications for avian gag gene order

Virions of Rous-associated virus type 61 contain a previously unrecognized p19-related protein, called p19f, which comigrates with gag protein p12 during electrophoresis in sodium dodecyl sulfate-polyacrylamide gels but can be separated by gel filtration chromatography in 6 M guanidine hydrochloride. It is shown that the existence of p19f accounts for the earlier inability to order p27 and p12 by the pactamycin mapping procedure. Remapping with pactamycin by using methods which take this new protein into account yielded a gag gene order of NH2-p219-p27-p12-p15-COOH. It also confirmed earlier positions for the env and pol genes and placed unclassified protein p10 near a translational initiation site. The pactamycin-derived mapping position of p12 differs from reports based on tryptic analysis. An analysis of procedural shortcomings emphasizes the need for more definitive determinations of the avian gag gene order.

Murine leukemia virus proteins expressed on the surface of infected cells in culture

Infection of JLS-V9 cells in culture with Rauscher murine leukemia virus induced the appearance on the cell surface of two classes of viral proteins: Rauscher murine leukemia virus gp70, and glycoproteins related to the viral core (gag) proteins with apparent molecular weights in sodium dodecyl sulfate polyacrylamide gels of 80 x 10(3) and 95 x 10(3). The latter proteins were identified by lactoperoxidase-catalyzed iodination of the cell surface and by metabolic labeling with [(3)H]mannose followed by immunoprecipitation with an antiserum directed against the major viral core protein, p30. Tryptic peptide maps of chloramine T-iodinated proteins indicated that 80 x 10(3) - and 95 x 10(3)-molecular-weight proteins were closely related. The 95 x 10(3)-molecular-weight protein from Rauscher murine leukemia virus-infected cells had a tyrosine fingerprint which was identical to that of the 95 x 10(3)-molecular-weight gag surface polyprotein of endogenous virus-producing AKR-A cells, suggesting that expression on the cell surface of glycosylated forms of gag precursor polyproteins may not be an exclusive property of leukemic thymocytes, but a more general phenomenon in murine leukemia virus infection. Tryptic fingerprint analysis of iodinated viral and cell-bound gp70's before and after desialylation indicated a lower level of glycosylation in the cell-bound gp70 population than in virions. Analysis of only surface-iodinated gp70 showed a simple pattern of exposed tryptic peptides which was very similar in Rauscher murine leukemia virus-infected cells and in AKR-A cells.

env Gene of Rous sarcoma virus: identification of the gene product by cell-free translation

Cell-free translation of polyadenylic acid-selected, denatured virion 70S RNA of the Schmidt-Ruppin strain of Rous sarcoma virus (subgroup A) yields a 64,000-Mr polypeptide which is specifically immunoprecipitated by a group-specific serum raised against envelope glycoprotein gp85. This polypeptide is not synthesized from the virion RNA of the replication-defective mutant rdNY8SR-A, which contains an extensive deletion within the envelope (env) gene. From this genetic evidence we conclude that the 64,000-Mr polypeptide represents the nonglycosylated product of the env gene and propose the designation of P64env. The 64,000-Mr polypeptide is translated from a 26S to 28S polyadenylated RNA species, whereas the p60src product is synthesized from a 20S to 22S RNA, and both Pr76gag and P180gag-pol are synthesized predominately from 34S RNA. The product of the env gene of Rous-associated virus-2 was also identified by cell-free translation.

Effect of p15-associated protease from an avian RNA tumor virus on avian virus-specific polyprotein precursors

A proteolytic activity is associated with structural protein p15 in avian RNA tumor viruses. Its effect on the known intracellular viral polyprotein precursors obtained by immunoprecipitation was investigated. Cleavage of Pr76gag resulted in the sequential appearance of p15, p27, and p19. The intracellular precursor Pr180gag-pol was also cleaved by p15, whereas the intracellular glycoprotein precursors of avian RNA tumor viruses, Pr92env, remained unaffected by p15 under all conditions tested. The specificities of the antibodies used to precipitate the precursors influenced the pattern of intermediates and cleavage products obtained by p15 treatment. If virus harvested from the the Prague strain of Rous sarcoma virus, subgroup C-transformed cells at 15-min intervals was incubated at 37 degrees C for further maturation, RNA-dependent DNA polymerase activity showed an optimum of DNA synthesis with 70S viral RNA or synthetic template-primers after short incubation periods. The presence of additional p15 during incubation resulted in a shift of the enzyme activity peak toward earlier time points. Virus harvested at 3-h intervals contained significant amounts of Pr180gag-pol and Pr76gag. The addition of p15 resulted in the cleavage of Pr180gag-pol and Pr76gag, but only a few distinct low-molecular-weight polypeptides appeared. Treatment of purified RNA-dependent DNA polymerase with p15 in vitro resulted in a disappearance of the beta subunit and an enrichment of the alpha subunit. In addition, a polypeptide of 32 x 10(3) molecular weight was generated. The cleavage pattern observed differed from the one obtained by trypsin treatment.

Origin of the minor glycoproteins of murine leukemia viruses

Polyacrylamide gel electrophoretic analysis and immunoprecipitation were used to study glycoproteins from purified Rauscher murine leukemia virus (R-MuLV) and from AKR thymic lymphoblastoid cell membranes. In addition to gp70, a minor glycoprotein of approximately 52,000 daltons (gp52) was demonstrated in purified R-MuLV preparations, which was antigenically related to gp70. Analysis of R-MuLV glycopeptides obtained after exhaustive Pronase digestion showed that gp70 has at least two different glycopeptide size classes with molecular weights of 5,100 and 2,900, respectively. gp52, however, contained only a single glycopeptide size class of approximately 5,100 daltons, indicating that the two glycoproteins contain distinct carbohydrate components. Trypsin treatment of R-MuLV converted gp70 into a product with a molecular mass of approximately 52,000 daltons as well as a 45,000-dalton minor product, with little effect on virus infectivity. Similarly, trypsin treatment of 125I-labeled glycoproteins derived from AKR mouse lymphoblastoid cell membranes generated fragments antigenically related to gp70 and similar in size to those obtained by trypsin treatment of R-MuLV. In both cases, the appearance of cleavage products was accompanied by a decrease in gp70 during trypsin treatment. The occurrence of glycosylated components antigenically related to gp70 in AKR membrane glycoprotein preparations and in purified R-MuLV preparations which were similar to those generated by trypsin treatment supports the concept that these minor components arise from proteolytic cleavage of gp70.