Analysis of v-mos encoded proteins in cells transformed by several related murine sarcoma viruses

Transformation of a mammary epithelial cell line by the v-raf and v-mos oncogenes

Defective retroviruses were used to investigate the effects of the oncogenes v-raf and v-mos on the behaviour of mammary epithelial cells. Cultures of the mammary epithelial cell lines #43 and NMuMG infected with either virus showed obvious areas of overgrowing cells, which were not seen when cells were infected with control viruses not containing an oncogene. Several clonal cell lines were derived from the areas of overgrowth on #43 cells. Most of these were tumorigenic on subcutaneous inoculation. In culture, several of the cloned lines showed marked morphological abnormalities, had altered rates of growth and saturation densities, and were able to clone in soft agar. Unexpectedly, one raf-containing and one mos-containing line grew more slowly in culture than did control or uninfected cells. Other lines showed only some of these changes. In the mos-containing lines the level of viral transcript corresponded approximately to the extent of transformation, whereas the raf-containing lines showed no such correlation.

Use of the mouse mammary tumor virus long terminal repeat to promote steroid-inducible expression of v-mos

We used the mouse mammary tumor virus long terminal repeat to promote dexamethasone-regulated expression of the Moloney murine sarcoma virus (M-MSV) transforming gene, v-mos. A recombinant DNA vector containing the mouse mammary tumor virus long terminal repeat fused to the M-MSV 124 v-mos gene was cotransfected with a plasmid containing the herpes simplex virus thymidine kinase gene (tk) into 3T3TK- cells. Individual clones of cells which grew in hypoxanthine-aminopterin-thymidine medium were tested for dexamethasone-regulated expression of p37mos as well as several transformation-specific phenotypic parameters. In the absence of dexamethasone, the v-mos transfectants appeared morphologically similar to the control cells despite low basal levels of p37mos expression. Upon hormone treatment, the levels of p37mos increased 5- to 10-fold, coincident with morphological changes typical of M-MSV transformation of 3T3 cells. The ability to form foci in monolayers also correlated with p37mos induction. The extent of morphological changes varied in individual clones of cells with similar levels of induced p37mos. Although the induced levels of p37mos were comparable to those seen in stable M-MSV 124 virus-transformed NIH 3T3 cells, the transfectants were unable to grow in soft agar under conditions which support growth of the virus-transformed cells. Acute infection of the transfectants with M-MSV 124 virus, a situation which resulted in elevated levels of p37mos, allowed these cells to grow in soft agar. The results described in this paper suggest that different threshold levels of p37mos may be necessary for the expression of various parameters of the transformed phenotype and also that continued expression of p37mos is necessary for maintenance of the transformed state. However, it also appears that the sensitivity to given levels of p37mos varies among clonal cell lines.

The transforming protein of Moloney murine sarcoma virus is a soluble cytoplasmic protein

The transforming gene, v-mos, of Moloney murine sarcoma virus (M-MuSV) encodes a 37,000-dalton phosphoprotein, p37mos. Since the biochemical function of this protein is unknown, we have determined the subcellular location of p37mos in M-MuSV 124-transformed cells. Using two different methods of cell lysis and fractionation, we found that newly synthesized as well as mature p37mos is a soluble cytoplasmic protein. In agreement with these results, immunofluorescent staining of cells acutely infected with M-MuSV 124, using an antiserum directed against a synthetic v-mos peptide, produced a diffuse cytoplasmic pattern. Gel filtration experiments and glycerol gradient sedimentation analysis suggest that the bulk of p37mos exists as a monomer and is not involved in a specific association with other cellular proteins. These properties of p37mos are different from those of other characterized retroviral transforming proteins.

Detection of an 85,000-dalton phosphoprotein in ts110 murine sarcoma virus-infected cells with antiserum against a v-mos peptide

We have used an antiserum directed against a synthetic v-mos peptide (anti-C3 serum) to screen ts110 murine sarcoma virus (MuSV)-infected cells for the presence of v-mos-encoded proteins. Anti-C3 serum specifically recognized an 85,000-dalton protein doublet (P85) from [35S]methionine-labeled ts110 MuSV-infected producer cells grown at 32 degrees C, the permissive temperature for transformation. The P85 doublet was also recognized by an antiserum directed against the viral gag protein p15. P85 was present but at 2- to 10-fold-lower levels in ts110 MuSV-infected producer cells grown at 39 degrees C, the restrictive temperature for transformation. The P85gag-mos fusion product was the only v-mos protein reproducibly detected in this ts110 MuSV-transformed cell line. Immunoprecipitation of 32P-labeled cells with anti-C3 serum revealed that the upper band of the P85 doublet is phosphorylated, containing mostly phosphoserine and some phosphothreonine. Cells acutely infected with ts110 MuSV contained slightly higher levels of P85 than did the ts110 MuSV-infected producer cell line. Anti-C3 serum specifically recognized a 33,000-dalton protein (p33) in the acutely infected cells labeled with [35S]methionine. p33 was present in trace amounts and may represent a previously unidentified ts110 MuSV-encoded v-mos protein.