Murine leukemia virus gag polyproteins: the peptide chain unique to Pr80 is located at the amino terminus

An RNA structural switch regulates diploid genome packaging by Moloney murine leukemia virus

Retroviruses selectively package two copies of their RNA genomes via mechanisms that have yet to be fully deciphered. Recent studies with small fragments of the Moloney murine leukemia virus (MoMuLV) genome suggested that selection may be mediated by an RNA switch mechanism, in which conserved UCUG elements that are sequestered by base-pairing in the monomeric RNA become exposed upon dimerization to allow binding to the cognate nucleocapsid (NC) domains of the viral Gag proteins. Here we show that a large fragment of the MoMuLV 5' untranslated region that contains all residues necessary for efficient RNA packaging (Psi(WT); residues 147-623) also exhibits a dimerization-dependent affinity for NC, with the native dimer ([Psi(WT)](2)) binding 12+/-2 NC molecules with high affinity (K(d)=17+/-7 nM) and with the monomer, stabilized by substitution of dimer-promoting loop residues with hairpin-stabilizing sequences (Psi(M)), binding 1-2 NC molecules. Identical dimer-inhibiting mutations in MoMuLV-based vectors significantly inhibit genome packaging in vivo (approximately 100-fold decrease), whereas a large deletion of nearly 200 nucleotides just upstream of the gag start codon has minimal effects. Our findings support the proposed RNA switch mechanism and further suggest that virus assembly may be initiated by a complex comprising as few as 12 Gag molecules bound to a dimeric packaging signal.

N-terminal cleavage fragment of glycosylated Gag is incorporated into murine oncornavirus particles

Glycosylated Gag (Glycogag) is a transmembrane protein encoded by murine and feline oncornaviruses. While the protein is dispensible for virus replication, Glycogag-null mutants of a neurovirulent murine oncornavirus are slow to spread in vivo and exhibit a loss of pathogenicity. The function of this protein in the virus life cycle, however, is not understood. Glycogag is expressed at the plasma membrane of infected cells but has not been detected in virions. In the present study we have reexamined this issue and have found an N-terminal cleavage fragment of Glycogag which was pelleted by high-speed centrifugation and sedimented in sucrose density gradients at the same bouyant density as virus particles. Its association with virions was confirmed by velocity sedimentation through iodixanol, which effectively separated membrane microvesicles from virus particles. Furthermore, the apparent molecular weight of the virion-associated protein was different from that of the protein extracted from the plasma membrane, suggesting some level of specificity or selectivity of incorporation.

Characterization of glycosylated Gag expressed by a neurovirulent murine leukemia virus: identification of differences in processing in vitro and in vivo

The neuroinvasiveness of a chimeric murine retrovirus, CasFrKP (KP), is dependent on the expression of glycosylated Gag (gp85gag). This viral protein is the product of alternate translation initiation 88 codons upstream of and in frame with the initiation codon of pr65gag, the precursor of the viral core proteins. Although expression of glycosylated Gag affects virus spread in the spleen, it appears not to affect virus spread in vitro in fibroblast cell lines (J. L. Portis et al., J. Virol. 68:3879-3887, 1994). The differential effects of this protein in vitro and in vivo have not been explained, and its function is unknown. We have here compared the in vitro processing of this molecule with that expressed in spleens of infected mice. In vitro, gp85gag was cleaved near the middle of the molecule, releasing the C-terminal half (containing capsid and nucleocapsid domains of pr65gag) as a secreted glycoprotein. The N-terminal half of the protein was associated with the plasma membrane as a approximately 55-kDa glycoprotein bearing the matrix domain of pr65gag as well as the N-terminal 88 residue L domain. This processing scheme was also observed in vivo, although two differences were seen. There were differences in N-linked glycosylation of the secreted form of the protein expressed in the spleen. In addition, whereas the membrane-associated species assumed the orientation of a type II integral membrane protein (N(cyto) C(exo)) in fibroblasts in vitro, a subpopulation of spleen cells was detected in which the N terminus of the protein was exposed at the cell surface. These results suggest that the differential effects of glycosylated Gag expression in vivo and in vitro may be related to differences in posttranslational processing of the protein.

Recovery of Glycosylated gag Virus from Mice Infected with a Glycosylated gag-Negative Mutant of Moloney Murine Leukemia Virus

Two independent pathways for gag gene expression exist in Moloney murine leukemia virus (M-MuLV). One begins with Pr65(gag) that is processed and cleaved into the internal structural proteins of the virion. The other pathway begins with the glycosylated gag polyprotein, gPr80(gag). gPr80(gag) consists of Pr65(gag) plus additional N-terminal residues and it is glycosylated. A glycosylated-gag-negative mutant of M-MuLV (Ab-X-MLV) was previously constructed and shown to replicate in tissue culture. To test for the importance of glycosylated gag in vivo, the Ab-X-MLV mutant was inoculated intraperitoneally into newborn NIH Swiss mice. Mutant-infected mice developed typical lymphoblastic lymphomas at rates comparable to wild-type M-MuLV at either high (2 x 10(4) XC pfu/animal) or low (2 x 10(2) XC pfu/animal) doses. However, when viral protein expression was examined in the resultant tumors, six out of six mice showed evidence of virus that had recovered gPr80(gag) expression. These results suggest that glycosylated gag is important for M-MuLV propagation or leukemogenesis in vivo. Copyright 1994 S. Karger AG, Basel

Multiple antibiotic resistance in Pseudomonas aeruginosa: evidence for involvement of an efflux operon

An outer membrane protein of 50 kDa (OprK) was overproduced in a siderophore-deficient mutant of Pseudomonas aeruginosa capable of growth on iron-deficient minimal medium containing 2,2'-dipyridyl (0.5 mM). The expression of OprK in the mutant (strain K385) was associated with enhanced resistance to a number of antimicrobial agents, including ciprofloxacin, nalidixic acid, tetracycline, chloramphenicol, and streptonigrin. OprK was inducible in the parent strain by growth under severe iron limitation, as provided, for example, by the addition of dipyridyl or ZnSO4 to the growth medium. The gene encoding OprK (previously identified as ORFC) forms part of an operon composed of three genes (ORFABC) implicated in the secretion of the siderophore pyoverdine. Mutants defective in ORFA, ORFB, or ORFC exhibited enhanced susceptibility to tetracycline, chloramphenicol, ciprofloxacin, streptonigrin, and dipyridyl, consistent with a role for the ORFABC operon in multiple antibiotic resistance in P. aeruginosa. Sequence analysis of ORFC (oprK) revealed that its product is homologous to a class of outer membrane proteins involved in export. Similarly, the products of ORFA and ORFB exhibit homology to previously described bacterial export proteins located in the cytoplasmic membrane. These data suggest that ORFA-ORFB-oprK (ORFC)-dependent drug efflux contributes to multiple antibiotic resistance in P. aeruginosa. We propose, therefore, the designation mexAB (multiple efflux) for ORFAB.

Characterization of monoclonal antibodies against the human immunodeficiency virus matrix protein, p17gag: identification of epitopes exposed at the surfaces of infected cells

Eight monoclonal antibodies reactive with the matrix protein of human immunodeficiency virus type 1 (HIV-1), p17gag, were isolated from rats which had been immunized with solubilized HIV-1 lysate. The epitope specificities of these antibodies were determined with a series of synthetic peptides representing overlapping regions of p17. Six of the antibodies were mapped to three distinct regions of p17, while two antibodies (G11g1 and G11h3) reacted only with intact recombinant p17, suggesting that they were directed against conformational or discontinuous epitopes. All the antibodies bound to HIV-infected cells which had been permeabilized with acetone, but only G11g1 and G11h3 reacted with live HIV-infected cells. Specificity studies with diverse virus strains demonstrated that these two antibodies recognized distinct epitopes, one which was group specific for HIV-1, and one which was shared with HIV type 2 and simian immunodeficiency virus. Binding competition studies indicated that these epitopes were proximal in native p17. Despite their reactivity with intact cells, these two antibodies did not possess appreciable virus-neutralizing activity. These results indicate that a form of p17 is expressed on the surfaces of live HIV-infected cells which is accessible to some, but not all, antibodies against p17. These cell surface molecules may play a role in the generation of antibodies against p17gag that are characteristic of early stages of HIV infection, and they may act as natural targets for the immune system and as potential targets for immunotherapy of HIV-infected cells.

CUG initiation codon used for the synthesis of a cell surface antigen coded by the murine leukemia virus

Murine leukemia virus (MuLV) codes for two precursors of the group-specific antigens, Pr65gag and Pr75gag, in vivo. While Pr65gag is the precursor to the virion structural proteins, Pr75gag undergoes glycosylation and is found on the surface of the infected cell as gp85gag, and it is thought to play a role in virus maturation and spread. Pr65gag synthesis starts at an AUG codon within a favourable initiation context (AAUAUGG at positions 618 to 624). The gp85gag start codon is upstream but its precise location is not known. To map the initiation codon of gp85gag, we used deletion and site-directed mutagenesis of the leader sequence of MuLV RNA and in vitro translation of the RNAs. Synthesis of the MuLV gp85gag protein appears to be initiated at a CUG codon located within a favourable context (ACCCUGG at positions 354 to 359 for Moloney-MuLV). The possible function of gp85gag was investigated by expressing Moloney-MuLV and Friend-MuLV proviral DNA and mutants deficient for gp85gag synthesis in mouse and rat cells. The results indicate that the gp85gag protein probably facilitates the spread of virus infection in tissue culture.

Evidence that the packaging signal of Moloney murine leukemia virus extends into the gag region

Replication-competent retroviruses can be modified to carry nonviral genes. Such gene transfer vectors help define regions of the retroviral genome that are required in cis for retroviral replication. Moloney murine leukemia virus has been used extensively in vector construction, and all of the internal protein-encoding regions can be removed and replaced with other genes while still allowing production of virions containing and transmitting the altered retroviral genome. However, inclusion of a portion of the gag region from Moloney murine leukemia virus markedly increases the titer of virus derived from these vectors. We determined that this effect was due to more efficient packaging of the vector RNA into particles and did not depend on protein synthesis from the gag region. We conclude that the retrovirus packaging signal extends into the gag region. We have found that retroviral vectors containing the complete packaging signal allow more efficient gene transfer into a variety of cell types. In addition, these results may help explain why many oncogenic retroviruses have retained gag sequences and often express transforming proteins that are gag-onc hybrids.

Monoclonal antibody to the amino-terminal L sequence of murine leukemia virus glycosylated gag polyproteins demonstrates their unusual orientation in the cell membrane

To analyze cell surface murine leukemia virus gag protein expression, we have prepared monoclonal antibodies against the spontaneous AKR T lymphoma KKT-2. One of these antibodies, 43-13, detects an AKR-specific viral p12 determinant. A second monoclonal antibody, 43-17, detects a novel murine leukemia virus-related antigen found on glycosylated gag polyproteins (gp95gag, gp85gag, and gp55gag) on the surface of cells infected with and producing ecotropic endogenous viruses, but does not detect antigens within these virions. The 43-17 antibody immunoprecipitates the precursor of the cell surface gag protein whether in its glycosylated or unglycosylated state, but does not detect the cytoplasmic precursor of the virion gag proteins (Pr65gag). Based on these findings, we have localized the 43-17 determinant to the unique amino-terminal part of the glycosylated gag polyprotein (the L domain). We have determined that gp95gag contains L-p15-p12-p30-p10 determinants, whereas gp85gag lacks the carboxyterminal p10 determinant, and gp55gag lacks both p30 and p10 carboxy terminal determinants. Analysis of cell surface gag expression with the 43-17 antibody leads us to propose that the L domain plays a crucial role in (i) the insertion and orientation of murine leukemia virus gag polyproteins in the cell membrane and (ii) the relative abundance of expression of AKR leukemia virus versus Moloney murine leukemia virus glycosylated gag polyproteins in infected cells.

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.

Murine leukemia virus maturation: protease region required for conversion from "immature" to "mature" core form and for virus infectivity

Murine leukemia virus (MuLV) genome encodes a protease (Y. Yoshinaka, I. Katoh, T.D. Copeland, and S. Oroszlan (1985), Proc. Natl. Acad. Sci. USA 82, 1618-1622), which has been shown to cause maturation, specified as morphological conversion from "immature" to "mature" form of virus cores. To examine whether "immature" particles have infectivity or not, we constructed mutant DNAs with deletions in the protease region. The NIH/3T3 cells transfected with mutant DNAs produced "immature" particles, having immature morphology and containing Pr65gag, a polyprotein precursor of core proteins. The specific infectivity of the extracellularly released and purified particles was shown to be greatly reduced based on reverse transcriptase activity and protein content as compared with the "mature" particles obtained from wild-type DNA-transfected cells. The mutant genomes encoded functionally normal surface glycoprotein, gp70. These results strongly suggest that maturation of MuLV from "immature" to "mature" form of virus particles is indispensable to virus infectivity. The importance of processing of gag and pol, as well as transmembrane protein precursors by the viral protease is discussed.

Myristylation of gag-onc fusion proteins in mammalian transforming retroviruses

Four cell lines producing transforming proteins encoded by three mammalian oncogenes (fes, abl, and ras) were investigated for incorporation of [3H]myristate into gag-onc fusion proteins. Using 5-min pulse-labelings, fusion proteins of Abelson murine leukemia virus, Gardner-Arnstein strain of feline sarcoma virus (FeSV), and Snyder-Theilen strain of FeSV were shown to be myristylated. In a 4-hr pulse, p29gag-ras of rat sarcoma virus (RaSV) was also shown to incorporate radiolabel. The fatty acid was recovered from this labeled protein by acid hydrolysis, and identified by reverse-phase thin-layer chromatography to be [3H]myristic acid. The results indicate that substitution of viral gag sequences by cellular oncogene sequences does not abolish their ability to become post-translationally modified by this long chain fatty acid (A. Schultz and S. Oroszlan, J. Virol. 46, 355-361). It is assumed that in the fusion proteins the myristyl moiety is linked through an amide linkage to the amino-terminal glycine as previously found for several retroviral gag precursor polyproteins (L. E. Henderson, H. C. Krutzsch, and S. Oroszlan, Proc. Natl. Acad. Sci. USA 80, 339-343). The possible role of myristylation of transforming proteins is discussed.

Nucleotide sequence of AKV murine leukemia virus

AKV is an endogenous, ecotropic murine leukemia virus that serves as one of the parents of the recombinant; oncogenic mink cell focus-forming viruses that arise in preleukemic AKR mice. I report the 8,374-nucleotide-long sequence of AKV, as determined from the infectious molecular clone AKR-623. The 5'-leader sequence of AKV extends to nucleotide 639, after which lies a long open reading frame encoding the gag and pol gene products. The reading frame is interrupted by a single amber codon separating the gag and pol genes. The pol gene overlaps the env gene within the 3' region of the AKV genome. The nucleotide sequence of the 5' region of AKV reveals the following features. (i) The 5'-leader sequence lacks any AUG codon to initiate translation of gPr80gag, suggesting that gPr80gag is not required for the replication of AKV. (ii) A short portion of the leader region diverges in sequence from the closely related Moloney murine leukemia virus and appears to be related to a sequence highly repeated in eucaryotic genomes. (iii) As in Moloney murine leukemia virus, there is a potential RNA secondary structure flanking the amber codon that separates the gag and pol genes. This structure might function as a regulatory protein binding site that controls the relative levels of synthesis of the gag and pol precursors. The nucleotide sequence of the 3' region of AKV is compared with sequences reported previously from both infectious and noninfectious molecular clones of AKV.

Construction and characterization of Moloney murine leukemia virus mutants unable to synthesize glycosylated gag polyprotein

Murine leukemia virus (MuLV) encodes two independent pathways for expression of the gag gene. One pathway results in processing and cleavage of the precursor Pr65gag to yield the internal capsid proteins of the virion and is analogous to gag polyprotein precursors for all classes of retroviruses. The other pathway, which is not encoded by several other classes of retroviruses, begins with a glycosylated polyprotein gPr80gag . gPr80gag is synthesized independently of Pr65gag; it contains Pr65gag peptides and additional amino-terminal protein. It is modified by further addition of carbohydrate, exported to the cell surface, and released from the cell but does not appear in virus particles. To investigate the role of glycosylated gag in MuLV infection, two mutants of Moloney MuLV (M-MuLV) deficient for synthesis of gPr80gag but able to synthesize Pr65gag were constructed. The mutants were obtained by substitution into a molecular clone of M-MuLV DNA by DNA from two acutely transforming viruses, Ableson MuLV (Ab-MuLV) and Moloney murine sarcoma virus (M-MSV). Both Ab-MuLV and M-MSV are derived from M-MuLV and they express M-MuLV gag sequences, but some strains do not synthesize glycosylated gag protein. For Ab-MuLV, a 177-base-pair Pst I fragment from the P90 strain containing the initiation codon for Pr65gag was substituted for the equivalent fragment in M-MuLV DNA. For M-MSV, 1.5 kilobases at the 5' end of the genome was substituted. Transfection of the recombined DNAs onto NIH-3T3 cells produced infectious M-MuLV, although the infected cells did not produce gPr80gag. Therefore glycosylated gag is not absolutely required for MuLV replication. Deletion of the glycosylated gag pathway did not significantly reduce the level of virus production, although a minor difference in XC plaque morphology was observed.

Provirus of M7 baboon endogenous virus: nucleotide sequence of the gag-pol region

A 3,023-base nucleotide sequence of the M7 baboon endogenous virus genome, spanning the 5' noncoding region as well as the entire gag gene and part of the pol gene, is reported. Within the 562-base 5' noncoding region, a 21-base sequence complementary to the OH terminus of tRNApro is located immediately downstream from the long terminal repeat. Amino acid sequences were deduced from the 1,596 nucleotides comprising the gag gene, and the four structural gag polypeptides, p12, p15, p30, and p10, appeared to be coded contiguously. Only one termination codon interrupted the M7 gag and pol genes. The data suggest that 55 additional amino acids may be attached to the NH2 terminus of the gag precursor protein. However, such a sequence was not detected in virions or in virus-infected cells. With the exception of the p15 region, nucleotide and amino acid sequences of the gag and pol regions of M7 virus exhibited strong homologies to those of Moloney leukemia virus.

Structure of glycosylated and unglycosylated gag and gag-pol precursor proteins of Moloney murine leukemia virus

Precursor polyproteins containing translational products of the gag gene of Moloney murine leukemia virus were purified by gel electrophoresis and cleaved into large fragments by hydroxylamine, mild acid hydrolysis, or cyanogen bromide. The hydroxylamine cleavage method (specific for asparagine-glycine bonds) was adapted especially for this study. The electrophoretic mobility and antigenicity of the fragments and, in some cases, the presence or absence of [35S]methionine revealed detailed information on the structure of Pr65gag, gPr80gag, and Pr75gag (the unglycosylated variant of gPr80gag formed in vivo in the presence of tunicamycin or in vitro in a reticulocyte cell-free system). When compared with Pr65gag, gPr80gag contains 7,000 daltons of additional amino acids, presumably as, or as part of, a leader sequence at or very close to its N terminus. We present evidence that this leader may have replaced part of the p15 sequence. Furthermore, gPr80gag contains three separate carbohydrate groups. One is attached to the presumed leader sequence or to the p15 domain, and two are attached to the p30 domain. Each of the Moloney murine leukemia virus gag precursor proteins Pr65gag, gPr80gag, and Pr75gag corresponds with a read-through product into the pol gene. We designated these products Pr180gag-pol, gPr200gag-pol, and Pr190gag-pol (the unglycosylated variant of gPr200gag-pol), respectively. gPr200gag-pol contains all of the extra amino acids and carbohydrate groups present in gPr80gag and at least one carbohydrate group in its pol sequences.

In vivo modification of retroviral gag gene-encoded polyproteins by myristic acid

It has recently been shown by mass spectral analysis (Henderson et al., Proc. Natl. Acad. Sci. U.S.A. 80:339-343, 1983) that the p15gag protein of murine leukemia viruses contains a novel post-translational modification, an amino-terminal myristyl (tetradecanoyl) amide. In this report we show that p15gag is the only structural protein to contain this fatty acid. In addition, the gag precursor polyproteins of type B, C, and D retroviruses have been examined for the presence of myristic acid by metabolic labeling and immunoprecipitation studies. In a panel of mammalian type C retroviruses we found that the precursor polyprotein Pr65gag homologs, but not the glycosylated forms (gPr80gag homologs), were specifically labeled after a 5-min incubation of infected cells with [3H]myristic acid. The gag precursor polyprotein was also labeled in mouse mammary tumor virus and in Mason-Pfizer monkey virus, but Pr76gag of Rous sarcoma virus failed to incorporate [3H]myristate. Under similar conditions, [3H]palmitate was not found to be incorporated into any viral gag proteins. Thus, myristylation appears to be a common feature of mammalian type B, C, and D retroviruses but not of avian retroviruses.

Biological, chemical, and immunological studies of Rauscher ecotropic and mink cell focus-forming viruses from JLS-V9 cells

Two murine leukemia viruses were isolated from JLS-V9 cells which had been infected with Rauscher plasma virus. One virus was XC positive and failed to grow on mink or cat cells and thus was an ecotropic virus. The other virus formed cytopathic foci on mink cells, was XC negative, and fell into the mink cell focus-forming (MCF) viral interference group and was thus an MCF virus. The glycoproteins of the two viruses could be distinguished immunologically, by peptide mapping, and by size in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The MCF virus produced gp69, and the ecotropic virus produced gp71, explaining the origin of the heterogeneous glycoprotein (gp69 and gp71) of Rauscher leukemia virus. Amino-terminal sequences of gp69 and gp71 were determined. The MCF sequence was distinct from the ecotropic sequence, but retained partial homology to it. The data show that the glycoproteins are encoded by related yet distinct genes. The protein structural data support the proposal that MCF virus gp70 molecules have nonecotropic sequences at the amino terminus, with ecotropic sequences occurring at the 3' end of the gene. The Rauscher MCF virus glycoprotein lacks a glycosylation site found at position 12 of the ecotropic sequence.

Further studies on the glycosylated gag gene products of Rauscher murine leukemia virus: identification of an N-terminal 45,000-dalton cleavage product

A glycosylated 45,000-Mr protein containing Rauscher murine leukemia virus p15 and p12 antigenic sites and tryptic peptides was identified in Rauscher murine leukemia virus-infected cells. This glycoprotein, termed gP45gag, was also shown to contain a single tryptic peptide also present in gPr80gag and its unglycosylated apoprotein precursor Pr75gag, but lacking in Pr65gag or Pr40gag. The presence of this peptide only in viral precursor proteins containing the so-called leader (L) sequence strongly suggests that gPr45gag is an N-terminal fragment of larger glycosylated gag polyproteins, composed of L sequences in addition to p15 and p12. The kinetics of appearance of radiolabeled gPr45gag and its disappearance during chase-incubation is suggestive of a precursor-like role for this intermediate gene product. An observed 27,000-Mr glycosylated polypeptide, termed gP27gag and containing p15 but not p12, p30, or p10 antigenic determinants, is a candidate cleavage product derived from gPr45gag. These observations suggest that gPr45gag and its putative cleavage product gP27gag represent an authentic pathway for intracellular processing of glycosylated core proteins.

Basolateral maturation of retroviruses in polarized epithelial cells

We have investigated the maturation sites of avian and mammalian C-type retroviruses in polarized epithelial cells. Examination of thin sections of Madin Darby canine kidney cells infected with RD114 or avian reticuloendotheliosis virus revealed that these viruses mature from the basolateral membrane domains. Similar results were obtained with a continuous line of mouse mammary epithelial cells infected with Friend, Moloney, Rauscher, or Kirsten murine leukemia viruses, or Friend virus-related or Moloney virus-related mink cell focus-forming viruses. Immunofluorescence observations indicate that viral glycoproteins are inserted only at the basolateral membranes in these cells. Because of the availability of DNA and protein sequence data, and of molecularly cloned viruses, these virus systems offer advantages for molecular studies on directional transport of plasma membrane glycoproteins.

Structure of glycosylated and unglycosylated gag polyproteins of Rauscher murine leukemia virus: carbohydrate attachment sites

The structural relationships among the gag polyproteins Pr65gag, Pr75gag, and gPr80gag of Rauscher murine leukemia virus were studied by endoglycosidase H digestion and formic acid cleavage. Fragments were identified by precipitation with specific antisera to constituent virion structural proteins followed by one-dimensional mapping. Endoglycosidase H reduced the size of gPr80gag to that of Pr75gag. By comparing fragments of gPr80gag and the apoprotein Pr75gag, the former was shown to contain two mannose-rich oligosaccharide units. By comparing fragments of Pr65gag and Pr75gag, the latter was shown to differ from Pr65gag at the amino terminus by the presence of a leader peptide approximately 7,000 daltons in size. The internal and carboxyl-terminal peptides of the two unglycosylated polyproteins were not detectably different. The location of the two N-linked carbohydrate chains in gPr80gag has been specified. One occurs in the carboxyl-terminal half of the polyprotein at asparagine177 of the p30 sequence and the other is found in a 23,000-dalton fragment located in the amino-terminal region of gPr80gag and containing the additional amino acid sequences not found in Pr65gag plus a substantial portion of p15.

Kinetics of synthesis and processing of precursor polypeptides of murine leukemia virus in frog oocytes following microinjection of viral RNA

Microinjection of Rauscher murine leukemia viral RNA into living oocytes from Xenopus laevis, in contrast to cell-free systems, allowed detailed studies on the processing of newly synthesized viral precursor polypeptides. The viral messenger appeared to be stable for at least 5 days. Maximal rate of translation of 70-S virion RNA was observed 10 h after injection. The predominant translation products after a 1-h labeling period were three precursor polypeptides of Mr 77000, 75000 and 65000. Following longer labeling periods the most stable precursor polypeptide of Mr 65000 was most prominent. In addition, several intermediates of Mr 35000--60000 were observed. After about 24 h, mature viral core proteins appeared. The rate of synthesis of the 75000-Mr and 77000-Mr viral proteins decreased gradually after injection, suggesting that viral core polypeptides somehow regulated processing or synthesis of the group-specific antigen precursors. A heterogeneous group of 90000--95000-Mr polypeptides seemed to be post-translationally modified products of the 75000-Mr and 77000-Mr proteins. However, in this study no envelope-related polypeptides were synthesized, when viral RNA (70-S or 35-S) was injected into the cytoplasm or the nucleus of Xenopus oocytes.

Sequence relationship of glycosylated and unglycosylated gag polyproteins of Moloney murine leukemia virus

Both glycosylated and unglycosylated polyproteins coded by the gag gene are produced in cells infected with Moloney murine leukemia virus. GpP80gag is a glycosylated precursor of a larger gag glycoprotein exported to the cell surface, whereas Pr65gag is an unglycosylated precursor of the virion internal structural proteins. GpP80gag contains not only carbohydrate, but also additional polypeptide sequences not found in Pr65gag. In the experiment reported here, we localized the differences between GpP80gag and Pr65gag with respect to the domains of the individual gag proteins. This was done by comparison of partial proteolytic cleavage fragments from Pr65gag, from GpP80gag, and from the unglycosylated form of GpP80gag (P75gag) which had been immunoprecipitated by antisera specific for gag proteins p30, p15, and p10. We conclude that the additional polypeptide sequences in GpP80gag are located at or very near the amino terminus of the polyprotein. The carbohydrate in GpP80gag is attached to polypeptide sequences held in common between GpP80gag and Pr65gag.