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

Murine leukemia virus (MLV) P50 protein induces cell transformation via transcriptional regulatory function

BACKGROUND

The murine leukemia virus (MLV) has been a powerful model of pathogenesis for the discovery of genes involved in cancer. Its splice donor (SD')-associated retroelement (SDARE) is important for infectivity and tumorigenesis, but the mechanism remains poorly characterized. Here, we show for the first time that P50 protein, which is produced from SDARE, acts as an accessory protein that transregulates transcription and induces cell transformation.

RESULTS

By infecting cells with MLV particles containing SDARE transcript alone (lacking genomic RNA), we show that SDARE can spread to neighbouring cells as shown by the presence of P50 in infected cells. Furthermore, a role for P50 in cell transformation was demonstrated by CCK8, TUNEL and anchorage-independent growth assays. We identified the integrase domain of P50 as being responsible for transregulation of the MLV promoter using luciferase assay and RTqPCR with P50 deleted mutants. Transcriptomic analysis furthermore revealed that the expression of hundreds of cellular RNAs involved in cancerogenesis were deregulated in the presence of P50, suggesting that P50 induces carcinogenic processes via its transcriptional regulatory function.

CONCLUSION

We propose a novel SDARE-mediated mode of propagation of the P50 accessory protein in surrounding cells. Moreover, due to its transforming properties, P50 expression could lead to a cellular and tissue microenvironment that is conducive to cancer development.

Ribosome profiling of the retrovirus murine leukemia virus

Background

The retrovirus murine leukemia virus (MuLV) has an 8.3 kb RNA genome with a simple 5′-gag-pol-env-3′ architecture. Translation of the pol gene is dependent upon readthrough of the gag UAG stop codon; whereas the env gene is translated from spliced mRNA transcripts. Here, we report the first high resolution analysis of retrovirus gene expression through tandem ribosome profiling (RiboSeq) and RNA sequencing (RNASeq) of MuLV-infected cells.

Results

Ribosome profiling of MuLV-infected cells was performed, using the translational inhibitors harringtonine and cycloheximide to distinguish initiating and elongating ribosomes, respectively. Meta-analyses of host cell gene expression demonstrated that the RiboSeq datasets specifically captured the footprints of translating ribosomes at high resolution. Direct measurement of ribosomal occupancy of the MuLV genomic RNA indicated that ~ 7% of ribosomes undergo gag stop codon readthrough to access the pol gene. Initiation of translation was found to occur at several additional sites within the 5′ leaders of the gag and env transcripts, upstream of their respective annotated start codons.

Conclusions

These experiments reveal the existence of a number of previously uncharacterised, ribosomally occupied open reading frames within the MuLV genome, with possible regulatory consequences. In addition, we provide the first direct measurements of stop codon readthrough efficiency during cellular infection.

Electronic supplementary material

The online version of this article (10.1186/s12977-018-0394-5) contains supplementary material, which is available to authorized users.

Functional Interplay Between Murine Leukemia Virus Glycogag, Serinc5, and Surface Glycoprotein Governs Virus Entry, with Opposite Effects on Gammaretroviral and Ebolavirus Glycoproteins

Gammaretroviruses, such as murine leukemia viruses (MLVs), encode, in addition to the canonical Gag, Pol, and Env proteins that will form progeny virus particles, a protein called “glycogag” (glycosylated Gag). MLV glycogag contains the entire Gag sequence plus an 88-residue N-terminal extension. It has recently been reported that glycogag, like the Nef protein of HIV-1, counteracts the antiviral effects of the cellular protein Serinc5. We have found, in agreement with prior work, that glycogag strongly enhances the infectivity of MLVs with some Env proteins but not those with others. In contrast, however, glycogag was detrimental to MLVs carrying Ebolavirus glycoprotein. Glycogag could be replaced, with respect to viral infectivity, by the unrelated S2 protein of equine infectious anemia virus. We devised an assay for viral entry in which virus particles deliver the Cre recombinase into cells, leading to the expression of a reporter. Data from this assay showed that both the positive and the negative effects of glycogag and S2 upon MLV infectivity are exerted at the level of virus entry. Moreover, transfection of the virus-producing cells with a Serinc5 expression plasmid reduced the infectivity and entry capability of MLV carrying xenotropic MLV Env, particularly in the absence of glycogag. Conversely, Serinc5 expression abrogated the negative effects of glycogag upon the infectivity and entry capability of MLV carrying Ebolavirus glycoprotein. As Serinc5 may influence cellular phospholipid metabolism, it seems possible that all of these effects on virus entry derive from changes in the lipid composition of viral membranes.

Many murine leukemia viruses (MLVs) encode a protein called “glycogag.” The function of glycogag is not fully understood, but it can assist HIV-1 replication in the absence of the HIV-1 protein Nef under some circumstances. In turn, Nef counteracts the cellular protein Serinc5. Glycogag enhances the infectivity of MLVs with some but not all MLV Env proteins (which mediate viral entry into the host cell upon binding to cell surface receptors). We now report that glycogag acts by enhancing viral entry and that, like Nef, glycogag antagonizes Serinc5. Surprisingly, the effects of glycogag and Serinc5 upon the entry and infectivity of MLV particles carrying an Ebolavirus glycoprotein are the opposite of those observed with the MLV Env proteins. The unrelated S2 protein of equine infectious anemia virus (EIAV) is functionally analogous to glycogag in our experiments. Thus, three retroviruses (HIV-1, MLV, and EIAV) have independently evolved accessory proteins that counteract Serinc5.

The Nef-like effect of murine leukemia virus glycosylated gag on HIV-1 infectivity is mediated by its cytoplasmic domain and depends on the AP-2 adaptor complex

Human immunodeficiency virus type 1 (HIV-1) Nef enhances the infectivity of progeny virions. However, Nef is dispensable for the production of HIV-1 virions of optimal infectivity if the producer cells are superinfected with certain gammaretroviruses. In the case of the ecotropic Moloney murine leukemia virus (M-MLV), the Nef-like effect is mediated by the glycosylated Gag (glycoGag) protein. We now show that the N-terminal intracellular domain of the type II transmembrane protein glycoGag is responsible for its effect on HIV-1 infectivity. In the context of a fully active minimal M-MLV glycoGag construct, truncations of the cytoplasmic domain led to a near total loss of activity. Furthermore, the cytoplasmic domain of M-MLV glycoGag was fully sufficient to transfer the activity to an unrelated type II transmembrane protein. Although the intracellular region of glycoGag is relatively poorly conserved even among ecotropic and xenotropic MLVs, it was also fully sufficient for the rescue of nef-deficient HIV-1 when derived from a xenotropic virus. A mutagenic analysis showed that only a core region of the intracellular domain that exhibits at least some conservation between murine and feline leukemia viruses is crucial for activity. In particular, a conserved YXXL motif in the center of this core region was critical. In addition, expression of the μ2 subunit of the AP-2 adaptor complex in virus producer cells was essential for activity. We conclude that the ability to enhance HIV-1 infectivity is a conserved property of the MLV glycoGag cytoplasmic domain and involves AP-2-mediated endocytosis.

IMPORTANCE

The Nef protein of HIV-1 and the entirely unrelated glycosylated Gag (glycoGag) protein of a murine leukemia virus (MLV) similarly enhance the infectiousness of HIV-1 particles by an unknown mechanism. MLV glycoGag is an alternative version of the structural viral Gag protein with an extra upstream region that provides a cytosolic domain and a plasma membrane anchor. We now show for the first time that the cytosolic domain of MLV glycoGag contains all the information needed to enhance HIV-1 infectivity and that this function of the cytosolic domain is conserved despite limited sequence conservation. Within the cytosolic domain, a motif that resembles a cellular sorting signal is critical for activity. Furthermore, the enhancement of HIV-1 infectivity depends on an endocytic cellular protein that is known to interact with such sorting signals. Together, our findings implicate the endocytic machinery in the enhancement of HIV-1 infectivity by MLV glycoGag.

A new retroelement constituted by a natural alternatively spliced RNA of murine replication-competent retroviruses

Replication of simple retroviruses depends on the recruitment of a single large primary transcript toward splicing, transport/packaging and translation regulations. In this respect, we studied the novel SD' 4.4 kb RNA of murine leukemia retroviruses (MLV) which results from alternative splicing of the primary transcript. We showed that SD' RNA was required for optimal replication since expression of a pre-spliced SD' RNA trans-complemented the impaired infectivity of a SD'-defective mutant. We monitored the fate of this novel transcript throughout early and late events of the viral life cycle. SD' RNA was specifically incorporated into virions demonstrating that the unspliced RNA was not the unique viral RNA present in virions. Furthermore, SD' RNA was reverse transcribed and its DNA copy integrated into the host genome, thus constituting a new splice donor-associated retroelement (SDARE) in infected cells. Finally, we showed that SD' mRNA encoded a 50 kDa polyprotein, and to a lower extent an additional 60 kDa polyprotein, which harbored Gag and integrase domains.

An internal ribosomal entry mechanism promotes translation of murine leukemia virus gag polyprotein precursors

The genomic retroviral RNA is the messenger for the translation of the gag and pol genes encoding the precursors to the major structural proteins and enzymes, respectively, of the virion core. The long 5' untranslated region, the leader, is formed of independent well-structured domains involved in key steps of the viral life cycle such as the initiation of proviral DNA synthesis, genomic RNA dimerization and packaging, and the initiation of gag translation. These functional features and the presence of stable secondary structures between the cap and the gag initiation codon suggested that translation initiation of gag might proceed through a mechanism different from the canonical ribosome scanning process. Interestingly enough, murine leukemia viruses code also for a glycosylated gag precursor, named glyco-gag, initiated at a CUG codon upstream and in the same open reading frame as the AUGgag. We have investigated the translation initiation of gag and glyco-gag precursors of Friend murine leukemia virus (F-MLV) in the rabbit reticulocyte lysate system and in murine cells. Through site-directed mutagenesis of gag and glyco-gag initiation codons, we show that initiation of gag and glyco-gag synthesis does not utilize the classical ribosome scanning. When poliovirus protease 2A is coexpressed in murine cells, expression of MLV-lacZ RNA is not modified, indicating that translation initiation of MLV gag precursors is a cap-independent mechanism. In addition, the F-MLV leader was inserted between two genes in a dicistronic neo-MLV-lacZ mRNA, and its ability to promote expression was examined in vitro and in vivo. Results obtained demonstrate that an internal ribosome entry mechanism promotes translation of F-MLV gag precursors. This finding led us to construct a new dicistronic retroviral vector in which the F-MLV leader can promote both packaging of recombinant genomic RNA and expression of the 3' gene.

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

A nonstructural gag-encoded glycoprotein precursor is necessary for efficient spreading and pathogenesis of murine leukemia viruses

In addition to the Gag-Pol and Env precursors whose translation initiates at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag), glycoGag translation is initiated at CUG codons located upstream of the Gag AUG initiation codon. In contrast to Gag, glycoGag is translocated into the endoplasmic reticulum and is absent from virions. Since glycoGag has been described to be dispensable ex vivo, we investigated the in vivo effects of a glycoGag- mutation in the Friend murine leukemia virus (F-MuLV). F-MuLV induces severe early hemolytic anemia and subsequent erythroleukemia within 2 months after inoculation of newborn mice. We obtained a glycoGag- F-MuLV, strain H5, by inserting an octanucleotide linker downstream of the CUG codon leading to the reading of a stop codon in all reading frames upstream of the Gag AUG. F-MuLV H5 did not induce severe early hemolytic anemia, and latency of erythroleukemia was significantly increased most likely because of an approximately 1-week delay in the in vivo spreading. Accordingly, induction of recombinant polytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-MuLV H5. Western blot (immunoblot) performed after inoculation of newborn mice with this glycoGag- virus indicated the emergence of new glycoGag+ viruses. PCR analyses with F-MuLV-specific primers demonstrated in vivo pseudoreversions restoring the glycoGag reading frame. Our results demonstrated that glycoGag expression is positively selected and essential for full spreading and pathogenic abilities.

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.

Expression of human immunodeficiency virus type 1 (HIV-1) gag antigens on the surface of a cell line persistently infected with HIV-1 that highly expresses HIV-1 antigens

MT-4 cells persistently infected with human immunodeficiency virus type 1 (HIV-1) (MT-4/HIV-1) were recently isolated (K. Ikuta, C. Morita, M. Nakai, N. Yamamoto, and S. Kato, Japan. J. Cancer Res. (Gann), 79, 418-423, 1988). Mouse hybridoma cell clones producing monoclonal antibodies (MoAbs) to HIV-1 gag p24 and p18, and pol reverse transcriptase (RT) were isolated by using this MT-4/HIV-1 cell line for the screening of MoAb production by the immunofluorescence (IF) test. By indirect IF tests of acetone-fixed cells with these MoAbs, the IF intensities in MT-4/HIV-1 cells were found to be higher than those in the other HIV-1 infected cells, such as MOLT-4/HIV-1, HL-60/HIV-1, and U937/HIV-1 cells. Cell surface expression of the HIV-1 gag p24 and p18 antigens examined by IF and radioimmune techniques with these MoAbs revealed the p24 and p18 antigens to be expressed strongly on the cell surface of MT-4/HIV-1 cells and faintly on the cell surface of MOLT-4/HIV-1 cells, respectively. However, monoclonal antibody isolated in the present study failed to detect pol RT antigen on the surface of MT-4/HIV-1 cells. These results indicate that the gag p24 and p18 antigens are expressed, at least in part, on the surface of HIV-1-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.

Nucleotide sequence of a radiation leukemia virus genome

The complete nucleotide sequence of an infectious molecular clone of a radiation murine leukemia proviral DNA RadLV/VL3(T+L+) has been determined. The sequence of the RNA genome is 8318 nucleotides long and contains three large open reading frames encoding the gag, pol, and env gene products. With the exception of a xenotropiclike R peptide and the LTR which bears structural similarities to a xenotropic LTR, displaying typical enhancerlike sequences, the remaining sequences are strikingly similar to the endogenous, ecotropic Akv murine leukemia virus. Therefore, it could be postulated that the leukemogenic properties of RadLV/VL3(T+L+) were generated by a recombination event between a xenotropic virus and an Akv-like ecotropic virus.

Antigenic differences among multiply charged Moloney murine leukemia virus p30 polypeptides found inside infected cells

At least three Moloney murine leukemia virus (M-MuLV) p30 polypeptides (p30's), viz., a major species at pI 6.3 and two minor ones at pI 6.1 and pI 6.6, have previously been identified in purified virions by 2-dimensional gel electrophoresis and chromatofocusing (Katoh, I., Yoshinaka, Y. and Luftig, R.B. (1984) J. Gen. Virol. 65, 733-741). We have observed a similar, but distinctive pI pattern for [35S]methionine-labeled MuLV p30's in lysates from chronically infected (MuLV) cells. The variation in pI pattern of the intracellular MuLV p30's was dependent on the type of p30 reactive antibody used for immunoprecipitation. Specifically: a p30 spot with pI 6.3 was always precipitated as the major spot with three different antibodies, minor spots with pI 6.0 and 6.6 were variably seen dependent on the antibody used, and an intracellular p30 spot at pI 6.1 was only precipitated with a rat p30 monoclonal antibody but not with monospecific mouse or intact MuLV cross-reacting p30 sera. These results indicate that first, there are differences between the pI pattern of virion and intracellular MuLV p30's, and second, the antigenic determinants of intracellular p30's vary dependent on the antibody used for immunoprecipitation.

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.

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.

Isolation of a recombinant murine leukemia virus utilizing a new primer tRNA

We have previously described the construction of a mutant of Moloney murine leukemia virus bearing a deletion at the normal site of integration of the viral DNA. We have now recovered a revertant of the virus after abortive infection of mouse cells and have determined the structure of the new virus. The revertant is a recombinant virus containing a 500-base-pair patch of new sequences derived from the mouse genome. The integration site was perfectly restored to the wild-type sequence, although the patch of DNA was overall only 80% homologous to Moloney murine leukemia virus. Surprisingly, the tRNA primer binding site was no longer homologous to the usual proline tRNAs, but was a perfect match for glutamine tRNA. This result suggests that the Moloney murine leukemia virus reverse transcriptase is not specific to one tRNA, but can utilize different tRNAs to prime the synthesis of viral DNA. Comparisons with published reports allowed the identification of sequences that are 94% homologous to the patch sequence, present in one of the endogenous retroviral sequences of the mouse. No replication-competent members of this family, utilizing the glutamine tRNA primer, have been previously isolated.

Characterization of gP85gag as an antigen recognized by Moloney leukemia virus-specific cytolytic T cell clones that function in vivo

The gag membrane protein gP85gag, encoded by Moloney murine leukemia virus (M-MLV), was identified as a target molecule recognized by Moloney murine sarcoma virus--M-MLV (M-MSV--M-MLV)-specific cytolytic T lymphocyte (CTL) clones. Target cells infected with Ab-X-MLV, an M-MLV-derived mutant virus not encoding gP85gag, were not lysed by the CTL clones. The same CTL clones were shown previously to induce the destruction of M-MLV-induced tumor cells in the peritoneal cavity. We have now characterized CTL-resistant antigen-loss tumor cell variants that have lost the surface antigen, but which retain transcriptionally silent M-MLV genomes. A cloned antigen-loss variant that reverted in vitro to the CTL-susceptible phenotype reexpressed M-MLV genomes that had undergone an insertion event in the region of the viral DNA coding for the gag membrane protein. Intravenous injection of virus-specific CTL clones inhibited tumor formation in mice injected subcutaneously with M-MSV--M-MLV.

A membrane-associated, carbohydrate-modified form of the v-abl protein that cannot be phosphorylated in vivo or in vitro

Abelson murine leukemia virus encodes a transforming protein which contains tyrosine kinase activity and is phosphorylated in vivo and in vitro. We found that P160 and P160-derived virus strains expressed an additional, altered v-abl protein which could not be phosphorylated. The altered v-abl protein (L-v-abl) differed from the phosphorylated form (K-v-abl) in that it was glycosylated and localized exclusively to the membrane fraction. Tunicamycin inhibition of N-linked carbohydrate addition did not restore phosphorylation. It did, however, reveal that L-v-abl had additional sequences relative to K-v-abl. The coding sequences required for this region and for the expression of L-v-abl were identified by replacing sequences in the P120 virus genome, which did not express L-v-abl, with sequences from the P160 virus genome. The necessary sequences were localized to the Moloney murine leukemia virus-derived gag gene. Comparison between the in vitro altered P120 and wild-type P120 virus strains indicated that expression of L-v-abl did not increase the efficiency of lymphoid transformation. Although the biological role of L-v-abl is not clear, our analyses have revealed that a specific amino terminal gag sequence can prevent v-abl from acting as a kinase substrate and can alter the cellular localization and modification of v-abl. These properties distinguish L-v-abl from previously reported v-abl proteins.

The nucleotide sequence of the Akv murine leukemia virus genome

The nucleotide sequence of an infectious molecular clone of the Akv murine leukemia virus has been determined by the dideoxy chain termination method after subcloning in bacteriophage M13 vectors. The sequence predicts an RNA genome of 8371 nucleotides containing three large open reading frames corresponding to the gag, pol, and env genes. Signal sequences for transcription, splicing, and translation have been identified. The positions of 95 major RNase T1 resistant oligonucleotides of the Akv RNA genome have been located.