Rat sarcoma virus: further analysis of individual viral isolates and the gene product

Genetic analysis of the rat leukemia virus: influence of viral sequences in transduction of the c-ras proto-oncogene and expression of its transforming activity

The rat leukemia virus (RaLV) is an endogenous retrovirus that is spontaneously released by Sprague-Dawley rat embryo cells. The overall structure of the RaLV genome resembles that of other simple, replication-competent retroviruses, but the sequence of the long terminal repeats (LTR) is unique and unrelated to the known retroviruses. Phylogenetically, the RaLV genome appears to be more closely related to the feline leukemia virus group of retroviruses than to the murine leukemia virus group. A remarkable feature of RaLV is that it is capable of transducing a ras proto-oncogene from rat tumor cells in the form of an acutely transforming virus, designated the Rasheed strain of the rat sarcoma virus (RaSV). With the exception of the c-ras sequence, the genomes of both RaLV and RaSV are collinear. The RaSV-encoded oncogene v-Ra-ras expresses a fusion protein with a molecular mass of 29 kDa, and it exhibits a unique structure that has not been described previously for any known virus. The 5' end of this gene is derived from sequences encoding RaLV matrix followed by 20 bp derived from the U5 region of the RaLV LTR (RS-U5 element) which is joined at its 3' end to sequences derived from all six (coding and noncoding) exons of the c-ras proto-oncogene at the 3' end. This recombinational event represents a novel mechanism among the acutely transforming viruses that have been studied.

Characterization of tumor cell lines from a spontaneous rat sarcoma expressing an endogenous retrovirus

We have characterized various biologic, immunologic and growth properties of several cell lines established from a spontaneous rat sarcoma that was discovered more than 60 yr ago. The tumors consisted of mixed cell types with no detectable host cellular immune response. Cultures derived from single-cell clones of the parental cell line were non-invasive but highly tumorigenic even in adult rats. The cultured cells spontaneously released replication-competent endogenous rat type C virus which did not carry a transforming gene in its genome. Since normal cells from the same rat strain did not produce a retrovirus, it is possible that production of the endogenous retrovirus may have triggered specific cellular changes necessary for the oncogene expression and development of this tumor.

FBR murine osteosarcoma virus. I. Molecular analysis and characterization of a 75,000-Da gag-fos fusion product

The FBR murine osteosarcoma virus complex induces bone tumors with a similar latency and pathology to those induced by the FBJ virus complex. FBR murine sarcoma virus ( FBR -MSV) has been isolated from its helper virus(es) by the establishment of transformed nonproducer cells. These cells were found to express a 75,000-Da protein (P75) which was antigenically related to the p55 oncogene product of the FBJ murine osteosarcoma virus ( FBJ -MSV). P75 also contained antigenic determinants of murine leukemia virus (MLV) gag gene p15, p12, and p30 proteins, and is therefore a gag- fos fusion protein ( P75gag - fos ). P75gag - fos is a phosphoprotein and is found primarily in the nucleus. Only a single species of RNA, of 3.3 kb, was identified in FBR -MSV-transformed nonproducer cells using both fos and MLV probes, which suggested that P75gag - fos was expressed from genome-sized RNA. Chromosomal DNA from one nonproducer cell line was found to contain a single EcoRI restriction fragment of 12 kb pairs (kbp) which encompassed the FBR -MSV provirus. This DNA fragment was molecularly cloned into bacteriophage Charon 30 (lambda FBR -1), and a 7.5-kbp HindIII restriction fragment containing the entire provirus was subsequently subcloned into pBR322 ( pFBR -1). DNA from pFBR -1 was capable of inducing morphological transformation of mouse and rat fibroblasts in tissue culture. In addition, transfected cells expressed the FBR -MSV P75gag - fos protein.

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 the Rasheed rat sarcoma virus oncogene: new mutations

The nucleotide sequence of the oncogene of the Rasheed strain of rat sarcoma virus was determined. The oncogene (Ra-v-ras) encodes a 29,000-dalton (p29) transforming protein. This protein is distinct from the immunologically related 21,000-dalton protein (p21) of the Harvey murine sarcoma virus in its amino terminus and in having additional mutations in its carboxyl terminus. Although the functional significance of these changes is unknown, they appear to occur only in rat sarcoma virus.

Molecular cloning, genomic analysis, and biological properties of rat leukemia virus and the onc sequences of Rasheed rat sarcoma virus

Rasheed rat sarcoma virus (RaSV) has been shown to code for a protein of 29,000 Mr not present in replication-competent rat type C helper virus (RaLV)-infected cells. This protein is a fused gene product consisting of a portion of the RaLV p15 gag protein and the transformation-specific 21,000 Mr (p21) ras protein, which is also found in Harvey murine sarcoma virus. We now report the molecular cloning of both the SD-1 (Sprague-Dawley) strain of RaLV and the transforming ras sequences of RaSV. Heteroduplex analysis of these cloned DNAs demonstrated that the RaSV ras gene (v-Ra-ras) was inserted into the rat type C viral genome with a small deletion of RaLV genetic information in the 5' region of the gag gene and that the v-Ra-ras gene (0.72 kilobase pair) is homologous to and colinear with the p21 ras gene of Harvey murine sarcoma virus (v-Ha-ras). Restriction enzyme mapping confirmed the homology demonstrated by heteroduplex mapping, showing strong site conservation of restriction endonucleases known to cleave v-Ha-ras. Cloned v-Ra-ras DNA transformed NIH 3T3 cells, inducing the synthesis of the p29 RaSVgag-ras protein.

Monoclonal antibodies to the p21 products of the transforming gene of Harvey murine sarcoma virus and of the cellular ras gene family

We have isolated eight rat lymphocyte-myeloma hybrid cell lines producing monoclonal antibodies that react with the 21,000-dalton transforming protein (p21) encoded by the v-ras gene of Harvey murine sarcoma virus (Ha-MuSV). These antibodies specifically immunoprecipitate both phosphorylated and non-phosphorylated forms of p21 from lysates of cells transformed by Ha-MuSV. All eight react with the products of closely related ras genes expressed in cells transformed by two additional sarcoma viruses (rat sarcoma virus and BALB sarcoma virus) or by a cellular Harvey-ras gene placed under the control of a viral promoter. Three of the antibodies also react strongly with the p21 encoded by the v-ras gene of Kirsten MuSV. These same three antibodies immunoprecipitate the predominant p21 species synthesized normally in a variety of rodent cell lines, including the p21 produced at high levels in 416B murine hemopoietic cells. This suggests that an endogenous gene closely related to Kirsten-ras is expressed in these cells. The monoclonal antibodies have been used to confirm two properties associated with p21; localization at the inner surface of the membrane of Ha-MuSV-transformed cells, assayed by immunofluorescence microscopy, and binding of guanine nucleotides.

Tumorigenic transformation of mammalian cells induced by a normal human gene homologous to the oncogene of Harvey murine sarcoma virus

A normal human gene homologous to the p21 ras oncogene of Harvey murine sarcoma virus induced oncogenic transformation and high p21 ras levels in murine fibroblasts when this gene was ligated to a control element (the long terminal repeat) from a murine or feline retrovirus. These results indicate that high levels of a gene product encoded by a normal human oncogene can induce tumorigenic transformation.

Generation of BALB-MuSV and Ha-MuSC by type C virus transduction of homologous transforming genes from different species

The nature of the cell-derived (bas) sequences of BALB-MuSV, a spontaneous mouse sarcoma virus isolate, was determined. Molecularly cloned bas sequences demonstrated no detectable homology with the onc genes of other mouse transforming viruses, but exhibited a high degree of sequence homology with the ras gene of the rat-derived Harvey murine sarcoma virus (Ha-MuSV) genome. The Ha-MuSV cell-derived sequence (ras) shared a colinear 750 bp region of homology with bas. Moreover, BALB-MuSV transformation was associated with the expression of high levels of a 21,000 dalton protein, immunologically related to the ras gene products, p21. Thus bas and ras represent retroviral transforming gene homologs that were independently transduced by mouse type C viruses from the genomes of different species.

The p21 src genes of Harvey and Kirsten sarcoma viruses originate from divergent members of a family of normal vertebrate genes

The Harvey and Kirsten strains of murine sarcoma virus encode enzymatically and serologically related p21 src proteins which are required for virally mediated cellular transformation. The genes in each virus encoding p21 show such extensive divergence from each other that cloned probes from these genes detect distinct sets of cellular genes in the DNA from several vertebrate species. These data suggest that cellular p21 sarc genes constitute a divergent family of vertebrate genes that can regulate the growth of cells.