Genome organization of retroviruses IX. Analysis of the genomes of Friend spleen focus-forming (F-SFFV) and helper murine leukemia viruses by heteroduplex-formation

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.

Role of a membrane glycoprotein in Friend virus-induced erythroleukemia: studies of mutant and revertant viruses

We previously reported the isolation and characterization of spontaneous, transmissible mutants of Friend spleen focus-forming virus (SFFV) that are nonpathogenic in adult NIH/Swiss mice and that contain abnormalities in nonoverlapping regions of their envelope glycoprotein (env) genes (M. Ruta, R. Bestwick, C. Machida, and D. Kabat, 1983, Proc. Natl. Acad. Sci. USA 80, 4704-4708). In newborn NIH/Swiss mice, these mutant SFFVs form revertants that are pathogenic in mice of all ages. At least two of three studied revertants contain second site env mutations which affect the sizes and proteolytic fragmentation patterns of their encoded glycoproteins. A variety of structural and genetic evidence suggests that the xenotropic- and ecotropic-related regions of the SFFV glycoprotein fold into separate globular domains that are connected by a flexible proline-rich joint. A glutamyl peptide bond within this joint is exceptionally susceptible to cleavage with Staphylococcus aureus V8 protease. Moreover, disulfide bonds occur within the xenotropic-related domain, but not between the globular domains. These results provide strong additional evidence that the env gene is required for SFFV pathogenesis, and they provide a new system for identifying the features of glycoprotein structure and localization which are essential for its leukemogenic activity.

Tissue-specific expression of the newly acquired ecotropic Emv-18 provirus in Fv-2 congenic mice

Expression of endogenous retroviral sequences in Fv-2 congenic mouse strains was examined by Northern blot analysis. Endogenous ecotropic virus transcripts were observed in total spleen RNA of B6.S (Fv-2ss) mice. Endogenous ecotropic transcripts were not detected in spleen RNA of C57BL/6, the Fv-2rr congenic partner of B6.S, nor in spleens of the C57BL/10 (Fv-2rr) and B10.C (Fv-2ss) congenic strains. Mendelian segregation analysis revealed that only backcross mice segregating the newly acquired Fv-2-linked endogenous ecotropic provirus had endogenous ecotropic transcripts in spleen RNA. Examination of different tissues of B6.S mice showed that Emv-18 transcription was highest in spleen and bone marrow, tissues in which Fv-2 has been shown to function. These results support the conclusion that chromosomal location is an important factor controlling Emv-18 expression in B6.S mice. We also report the presence in the spleen of a novel xenotropic virus transcript detectable only in B6.S mice.

A weakly pathogenic Rauscher spleen focus-forming virus mutant that lacks the carboxyl-terminal membrane anchor of its envelope glycoprotein

A mutant Rauscher spleen focus-forming virus (mutant 4-3) that causes mild splenic erythroblastosis in mice has a 44-base-pair deletion in the 3' region of its envelope glycoprotein (env) gene. The encoded glycoprotein terminates prematurely, lacks a hydrophobic membrane anchor, and has a shortened intracellular lifespan. An active site for causing erythroblast proliferation may occur in the undamaged amino-terminal domain of the env glycoprotein.

Sequence comparisons of the anemia- and polycythemia-inducing strains of Friend spleen focus-forming virus

A nucleotide sequence analysis carried out on the envelope gene of the anemia-inducing strain of the Friend spleen focus-forming virus (F-SFFVA) reveals that its product has some unique features in common with previously described polycythemia-inducing strains of F-SFFV (F-SFFVP). (i) It contains an amino terminus that is highly related to the gp70 of mink cell focus-inducing viruses, (ii) it is a fusion protein containing the amino terminus of gp70 and the carboxy terminus of p15E, and (iii) it lacks the R-peptide normally found at the carboxy end of the p15E region. Although the envelope genes of F-SFFVA and F-SFFVP are quite similar overall, they do show sequence variation, particularly at the 3' end in the p15E-related region. These variations may contribute to previously observed differences in the response of F-SFFVP- and F-SFFVA-infected erythroid cells to regulatory hormone or to differences in the way the envelope glycoproteins are processed. The long terminal repeat regions of F-SFFVA and the Lilly-Steeves strain of F-SFFVP were also sequenced and compared with each other and with a previously published sequence of another F-SFFVP long terminal repeat. The sequences were found to be reasonably similar to each other but different from their ecotropic parent, Friend murine leukemia virus, as a result of a deletion of one copy of the direct tandem repeat in the enhancer regions. The observation that all SFFVS have this common change in the long terminal repeat enhancer region raises the possibility that it is required for pathogenicity.

Molecular cloning of biologically active Rauscher spleen focus-forming virus and the sequences of its env gene and long terminal repeat

Rauscher and Friend spleen focus-forming viruses (R- and F-SFFVs) cause similar progressive erythroleukemias dependent upon a virus-encoded membrane glycoprotein. Moreover, these SFFV glycoproteins are immunologically related to each other and to the recombinant-type glycoproteins encoded by the env genes of dual tropic murine leukemia viruses. To better understand these diseases and the viral origins, we isolated a pathogenically active molecular clone of R-SFFV proviral DNA, sequenced its 3'-terminal 2,163-base-pair (bp) region, and compared these sequences with previously determined sequences of F-SFFV. The 516-bp R-SFFV long terminal repeat is highly homologous to those of F-SFFV and Friend murine leukemia virus, although only the latter contains a 65-bp direct repeat in its U3 region. The env gene of R-SFFV encodes a glycoprotein with 408 amino acids that is identical in its basic domain organization to the glycoprotein of F-SFFV. Thus, the junctions between the dual tropic-related and ecotropic sequences occur at the same nucleotide, and both SFFV env genes contain identical 585-bp deletions in their ecotropic domains and single-bp insertions which cause premature terminations at the same amino acid in their ecotropic p15E domains. Consistent with their independent origins, however, the env sequences of R- and F-SFFV are distinctive in both their 5' dual tropic-related and 3' ecotropic-related domains. Furthermore, there are several consistent amino acid differences between the polycythemic F-SFFV sequences and the anemia-inducing R-SFFV sequence. The striking similarities of the independently formed F- and R-SFFV env genes imply that all of the glycoprotein domains arranged in a precise organization may be required for its leukemogenic activity

Heteroduplex analysis of molecular clones of the pathogenic Friend virus complex: Friend murine leukemia virus, Friend mink cell focus-forming virus, and the polycythemia- and anemia-inducing strains of Friend spleen focus-forming virus

The pathogenic Friend virus complex is of considerable interest in that, although members of this group are genetically related, they differ markedly in biochemical and biological properties. Heteroduplex mapping of molecular clones of the Friend virus complex, which includes the replication-competent ecotropic Friend murine leukemia virus (F-MuLV) and mink cell focus-forming virus (F-MCF) and replication-defective polycythemia- and anemia-inducing strains of spleen focus-forming virus (SFFVp and SFFVa, respectively), was employed to provide insight into the molecular basis of their relationships. In heteroduplexes of F-MuLV X F-MCF, a major substitution of 0.89 kilobases in the env gene of F-MCF was discerned. Heteroduplexes of SFFVp X F-MuLV or F-MCF and SFFVa X F-MuLV or F-MCF showed several major deletions in the pol gene region and a single major deletion in the 3' half of the env gene region of SFFVp and SFFVa. A major substitution of 0.89 kilobases was mapped to the 5' end of the env deletion of SFFVp and SFFVa in heteroduplexes with F-MuLV, similar to that seen in F-MuLV X F-MCF heteroduplexes. In contrast, this env gene region was totally homologous in F-MCF X SFFVp or SFFVa and SFFVp X SFFVa heteroduplexes. Our results suggest that (i) both SFFVp and SFFVa lack part of the env gene at its 3' end, corresponding to the p15(E) coding region, (ii) major deletions occur in the pol and env genes which account for the replication defectiveness of SFFVp and SFFVa, (iii) minor substitutions occur in the gag gene region of SFFVa that are not present in SFFVp, F-MuLV, or F-MCF, (iv) a major substitution exists in the gp70 region of the env gene between F-MuLV and F-MCF that probably accounts for the differences in their host range specificities, (v) this substitution in F-MCF is identical to the gp70 part of the gp52 coding region of SFFVp and SFFVa, and (vi) heteroduplexes to F-MCF show unambiguously that no additional large substitutions are present in SFFVp or SFFVa that could account for differences in their leukemogenicity.

Structural analysis of the spleen focus-forming virus envelope gene product

The structure of the envelope gene product (gp52) of the spleen focus-forming virus was analyzed and compared to that of envelope proteins (gp70 and p15E) of another pathogenic Friend virus recombinant, Friend mink cell focus-inducing virus (F-MCF). This has enabled us to confirm and extend previous nucleotide sequence data regarding the make up of specific domains of the SFFV glycoprotein. Amino-terminal 23,000-Da V8 protease fragments from gp52 of the Lilly-Steeves strain of SFFV and from gp70 of a pathogenic F-MCF isolate produced tryptic peptide fingerprints in which the mobilities of the trypsin-generated peptides were identical. The carboxyl-terminal, 21,000 Da, V8 protease fragment of gp52, however, has a unique fingerprint that contained a single highly charged trypsin-generated peptide. This peptide migrated to the same position as a peptide in F-MCF p15E, thus indicating that the p15E-related nucleotide sequences, that follow the large envelope deletion, are translated in the same reading frame as those in the standard p15E. Although R peptide determinants can be detected in F-MCF Pr15E, they could not be detected in gp52. The amino-terminal 23K domain of gp52, like that of MCF gp70, contains two oligosaccharide attachment sites. The other two attachment sites are located within the 21K carboxyl-terminal domain.

Retrovirus transduction: segregation of the viral transforming function and the herpes simplex virus tk gene in infectious Friend spleen focus-forming virus thymidine kinase vectors

A series of deletions and insertions utilizing the herpesvirus thymidine kinase gene (tk) were constructed in the murine retrovirus Friend spleen focus-forming virus (SFFV). In all cases, the coding region for the SFFV-specific glycoprotein (gp55), which is implicated in erythroleukemic transformation, was left intact. These SFFV-TK and SFFV deletion vectors were analyzed for expression of tk and gp55 after DNA-mediated gene transfer. In addition, virus rescued by cotransfection of these vectors with Moloney murine leukemia virus was analyzed for infectious TK-transducing virus, gp55 expression, and erythroleukemia-inducing ability. The experiments demonstrated that deletions or insertions within the intron for the gp55 env gene can interfere with expression of gp55 after both DNA-mediated gene transfer and virus infection. In contrast, the gene transfer efficiency of the tk gene was unaffected in the SFFV-TK vectors, and high-titer infectious TK virus could be recovered. Revertant viruses capable of inducing erythroleukemia and expressing gp55 were generated after cotransfection of the SFFV-TK vectors with murine leukemia virus. The revertant viruses lost both tk sequences and the ability to transduce TK- fibroblasts to a TK+ phenotype. These experiments demonstrate that segregation of the TK and erythroleukemia functions can occur in retrovirus vectors which initially carry both markers.

A follow-up study of the development of Rauscher erythroleukemia

Expression of cell membrane antigens recognized by antiserum raised against purified Friend murine leukemia virus (Fr-MuLV) envelope glycoprotein (gp71) and by xenotropic MuLV-coded cell surface antigen (XenCSA) specific antibodies was studied in the course of the development of Rauscher erythroleukemia in the spleen of Balb/c mice. DNA content vs immunofluorescence or light scattering of cells were simultaneously analyzed. At early stages of the disease (4-5 days after infection) the gp71 and XenCSA-related antigen expression is enhanced mainly on S-G2/M-phase cells as compared to the majority of G1-phase cells or to the endogenous background of uninfected cells. Later (around 10 days after infection) an approximately ten-fold increased gp71-related antigen density is reached in every phase of the cell cycle. These data show that the virus-induced transition from resting to proliferating state is coupled to enhanced expression of both helper and defective viral env-gene products in the cell membrane of mitotic and G1-phase cells as well.

Retrovirus transduction: segregation of the viral transforming function and the herpes simplex virus tk gene in infectious Friend spleen focus-forming virus thymidine kinase vectors

A series of deletions and insertions utilizing the herpesvirus thymidine kinase gene (tk) were constructed in the murine retrovirus Friend spleen focus-forming virus (SFFV). In all cases, the coding region for the SFFV-specific glycoprotein (gp55), which is implicated in erythroleukemic transformation, was left intact. These SFFV-TK and SFFV deletion vectors were analyzed for expression of tk and gp55 after DNA-mediated gene transfer. In addition, virus rescued by cotransfection of these vectors with Moloney murine leukemia virus was analyzed for infectious TK-transducing virus, gp55 expression, and erythroleukemia-inducing ability. The experiments demonstrated that deletions or insertions within the intron for the gp55 env gene can interfere with expression of gp55 after both DNA-mediated gene transfer and virus infection. In contrast, the gene transfer efficiency of the tk gene was unaffected in the SFFV-TK vectors, and high-titer infectious TK virus could be recovered. Revertant viruses capable of inducing erythroleukemia and expressing gp55 were generated after cotransfection of the SFFV-TK vectors with murine leukemia virus. The revertant viruses lost both tk sequences and the ability to transduce TK- fibroblasts to a TK+ phenotype. These experiments demonstrate that segregation of the TK and erythroleukemia functions can occur in retrovirus vectors which initially carry both markers.

Loss of leukemogenicity caused by mutations in the membrane glycoprotein structural gene of Friend spleen focus-forming virus

Friend virus infection of mice causes progressive leukemogenesis--a rapid splenic erythroblastosis that develops weeks later into a disseminating erythroleukemia. Furthermore, the replication-defective Friend spleen focus-forming virus (F-SFFV) encodes a membrane glycoprotein with an apparent Mr of 55,000 (designated gp55), which is structurally and immunologically related to the membrane envelope glycoproteins of dual tropic murine leukemia viruses. We now have isolated three spontaneous F-SFFV mutants that encode abnormally sized gp55-related glycoproteins with apparent Mrs of 40,000, 54,000, and 58,000, respectively. RNA blot and Southern blot analyses indicate that the mutant nucleic acids do not have substantial deletions or insertions in their glycoprotein gene regions. Protein fragmentation patterns indicate that the mutations affect nonoverlapping domains of the glycoprotein. Furthermore, these mutant glycoproteins seem to be defective in their processing to the plasma membranes. Although transmitted efficiently between cultured cells, the mutants have dramatically reduced leukemogenicities compared with the same titers of wild-type F-SFFV. We conclude that the gp55 structural gene is necessary for initiating the erythroblast proliferative phase of Friend disease and that changes in membranes can be primary causes rather than only secondary consequences of tumor progression.

Envelope gene of the Friend spleen focus-forming virus: deletion and insertions in 3' gp70/p15E-encoding region have resulted in unique features in the primary structure of its protein product

A nucleotide sequence was determined for the envelope (env) gene of the polycythemia-inducing strain of the acute leukemia-inducing Friend spleen focus-forming virus (SFFV) and from this the amino acid sequence of its gene product, gp52, was deduced. All major elements of the gene were found to be related to genes of other retroviruses that code for functional glycoproteins. Although the carboxyl terminus of gp52 is encoded by sequences highly related to sequences in its putative parent, ecotropic Friend murine leukemia virus, the majority of the protein (69%), including the amino terminus, is encoded by dualtropic virus-like sequences. Nucleotide sequence comparisons suggest that the nonecotropic region may be more closely related to the 5' substitution in dualtropic mink cell focus-inducing viruses that it is to the 5' end of xenotropic virus env genes. A large deletion and two unique insertions have been located in the env gene of polycythemia-inducing SFFV and may account for some of the unusual structural characteristics, aberrant processing, and pathogenic properties of gp52. As a consequence of the deletion, amino-terminal gp70 and carboxyl-terminal p15E-encoding sequences are juxtaposed and it appears that translation from the p15E region, 3' to the deletion, continues in the standard reading frame used by other retroviruses. Insertions of six base pairs and one base pair at the very 3' end of the gp52-encoding region results in a SFFV-unique amino acid sequence and a premature termination codon.

Complete nucleotide sequence of an infectious clone of Friend spleen focus-forming provirus: gp55 is an envelope fusion glycoprotein

The Friend spleen focus-forming provirus is 6,296 base pairs (bp) in length. Compared to Moloney murine leukemia virus, it has undergone five major deletions, three substitutions, and a number of minor alterations. Otherwise, these viruses are about 90% homologous. A 16-bp palindrome is found in the region thought to be involved in packaging and dimerization of the RNA genome. Premature termination of translation of the gag polyprotein is attributed to a 13-bp deletion in the p12 region. A substitution of xenotropic env sequences was identified in the 5' region of the env gene; 150 nucleotides 3' to this substitution, a deletion of 585 bp removes the site where the normal env precursor protein is cleaved to form gp70 and p15(E), resulting in a fusion protein of Mr 44,725. Due to these changes, the env product gp55 is expected to have a substantially different conformation on the cell surface compared to either a xenotropic or ecotropic gp70 protein, and may be responsible for the rapid erythroleukemic potential of spleen focus-forming virus.

Complete nucleotide sequence of the gene for the specific glycoprotein (gp55) of Friend spleen focus-forming virus

The complete nucleotide sequence of the gene for the specific glycoprotein (gp55) of the polycythemic strain of Friend spleen focus-forming virus (SFFV) was derived from the cloned SFFV DNA intermediate. The gp55 gene is present within 1.4 kilobases of the 5' side of the 3'long terminal repeat sequence. The open reading frame predicts the primary translation product has a total of 409 amino acids with a Mr of 44,752. Comparisons of the deduced amino acid sequence of gp55 with those of the envelope (env) gene products of murine leukemia viruses (MuLVs) revealed that gp55 is composed of three distinct regions. The amino-terminal 80% of the molecule has a high degree of sequence homology with the amino-terminal portion of the gp70 of the Moloney mink cell focus-forming virus (BALB/Mo-MCFV). This portion of the BALB/Mo-MCFV gp70 is known to be coded for by the acquired xenotropic env-like sequence. The sequence of the following 66 amino acids of gp55 is highly homologous to that of the middle portion of the p15E of Moloney MuLV (Mo-MuLV). The sequence of the Carboxyl-terminal 12 amino acids is specific to gp55 and a comparison of the nucleotide sequence showed that this specific amino acid sequence is due to the presence of seven extra nucleotides compared with the sequence of the Mo-MuLV.

Genetic structure of Rauscher spleen focus-forming virus

Rauscher spleen focus-forming virus contains functional gag and pol genes and a partially deleted env gene which is structurally related to the env genes of dual tropic murine leukemia viruses.

Glycosylation and intracellular transport of spleen focus-forming virus glycoproteins

We have investigated the pattern of glycosylation of the membrane glycoproteins encoded by a polycythemic strain of spleen focus-forming virus (SFFV). These include a major species designated gp52 and its processed form which is designated gp65. The SFFV glycoproteins were found to be predominantly intracellular, although a portion of gp65 is expressed on the cell surface. gp65 was observed to be highly sialylated and resistant to digestion with endoglycosidase-H (endo-H). In contrast, gp52 was endo-H sensitive and the polyacrylamide gel electrophoresis profile of the endo-H digests suggested the presence of four glycosylation sites. Analysis of tryptic glycopeptides from gp52 by reverse-phase high-performance liquid chromatography also suggested the presence of four glycosylation sites. Glycopeptide analysis of Pronase digests of gp52 revealed two major size classes with molecular weights of 2200 and 1500, which correspond to two of the four oligosaccharide size classes reported previously for MuLV gp70's (M.C. Kemp, N.G. Famulari, P.V. O'Donnell, and R.W. Compans, 1980, J. Virol. 34, 154). Both glycopeptide size classes were sensitive to digestion with endo-H. The glycopeptide profile of gp65 was found to be very heterogeneous and the predominant form was a 2900-dalton size class. In addition a fucosyl glycopeptide of 2500 daltons was observed in gp65, but not in F-MuLV or F-MCF glycoproteins. In the presence of the sodium ionophore monensin, the processing of gp52 to gp65 was inhibited. Instead a smaller protein of about 60,000 daltons was observed, which did not arrive at the cell surface, a situation analogous to the processing and post-translational modification reported for gp52 from anemic isolates of SFFV (S.K. Ruscetti, J.A. Field, and E.M. Scolnick, 1981, Nature (London) 294, 663).

Subcellular localization of the env-related glycoproteins in Friend erythroleukemia cells

A scheme was developed for the subcellular fractionation of murine erythroleukemia cells transformed by Friend leukemia virus. The subcellular localization of the env-related glycoproteins was determined by immune precipitation with antiserum against gp70, the envelope glycoprotein of the helper virus, followed by gel electrophoresis. In cells labeled for 2 h with [35S]methionine, the glycoprotein encoded by the defective spleen focus-forming virus, gp55SFFV, was found primarily in the nuclear fraction and in fractions containing dense cytoplasmic membranes such as endoplasmic reticulum. A similar distribution was noted for gp85env, the precursor to gp70. The concentration of viral glycoproteins in the nuclear fraction could not be accounted for by contamination with endoplasmic reticulum. In pulse-chase experiments, neither glycoprotein underwent major redistribution. However, labeled gp85env disappeared from intracellular membranes with a half-time of 30 min to 1 h, whereas labeled gp55SFFV was stable during a 2-h chase. In plasma membrane preparations with very low levels of contamination with endoplasmic reticulum, gp70 was the major viral env-related glycoprotein detected; a minor amount of gp55SFFV and no gp85env could be detected. The unexpected result of these experiments is the amount of viral glycoproteins found in the nuclear fraction. Presence of viral proteins in the nucleus could be relevant to the mechanism of viral leukemogenesis.

Avian reticuloendotheliosis virus: characterization of genome structure by heteroduplex mapping

The genome structure of defective, oncogenic avian reticuloendotheliosis virus (REV) was studied by heteroduplex mapping between the full-length complementary DNA of the helper virus REV-T1 and the 30S REV RNA. The REV genome (5.5 kilobases) had a deletion of 3.69 kilobases in the gag-pol region, confirming the genetic defectiveness of REV. In addition, REV lacked the sequences corresponding to the env gene but contained, instead, a contiguous stretch (1.6 to 1.9 kilobases) of the specific sequences presumably related to viral oncogenicity. Unlike those of other avian acute leukemia viruses, the transformation-specific sequences of REV were not contiguous with the gag-pol deletion. Thus, REV has a genome structure similar to that of a defective mink cell focus-inducing virus or a defective murine sarcoma virus. An additional class of heteroduplex molecules containing the gag-pol deletion and two other smaller deletion loops was observed. These molecules probably represented recombinants between the oncogenic REV and its helper virus.

Plasma membrane glycoproteins encoded by cloned Rauscher and Friend spleen focus-forming viruses

Rauscher spleen focus-forming virus (SFFV) was cloned free of its helper virus into normal rat kidney and mouse fibroblasts, and the resulting nonproducer fibroblast clones were analyzed. Our results suggested that Rauscher SFFV encodes a glycoprotein with an apparent Mr of 54,000 (gp54) that reacts with antisera made to the envelope glycoprotein (gp70) of ecotropic murine leukemia viruses, as well as with a rat antiserum that reacts with the gp70's of dual-tropic mink cell focus-inducing and HIX viruses but not with the gp70's of ecotropic viruses. In these respects and in its tryptic peptide map, Rauscher SFFV-encoded gp54 is nearly identical to the gp55 glycoprotein which we previously reported to be encoded by Friend SFFV (Dresler et al., J. Virol. 30:564--575, 1979). However, gp54 is slightly smaller, and it lacks one methionine-containing tryptic peptide that occurs in gp55. Studies with cytotoxic antiserum in the presence of complement and with a rosetting technique which employed sheep erythrocytes coupled to protein A suggested that the gp54 and gp55 glycoproteins are weakly expressed on the surface membranes of SFFV-infected cells. In addition, the Rauscher SFFV genome also encodes gag polyproteins which appear to be identical to the gag polyproteins encoded by helper Rauscher murine leukemia virus, but differ from the antigenically related polyproteins encoded by some but not all clones of Friend SFFV. Furthermore, the glycosylated gag polyproteins encoded by Rauscher SFFV and by some Friend SFFVs also appear to be expressed on the surface membranes of infected cells. These results suggest that similar env gene recombination and partial deletion events were involved in the independent origins of two different strains of acute erythroleukemia virus.