Polyproteins (gag-fes) encoded by the Snyder-Theilen (ST) and the Gardner-Arnstein (GA) strains of feline sarcoma virus (FeSV) were previously shown to be associated with mink or rat cells that were nonproductively transformed in vitro. In the present study we demonstrated that the same gag-fes proteins were found in cat cells transformed in vitro. Of greater importance, these transformation-related proteins were also in cells taken from fresh biopsies of FeSV-induced tumors. Cells from fibrosarcomas induced with ST-FeSV had gag-fes proteins that were characteristic of this strain. Fibrosarcomas and melanomas were induced with GA-FeSV and both types of tumors contained the protein that is characteristic of cells transformed in vitro with this virus. Expression of these proteins in cultured tumor cells appeared to be independent of the passage level. Based on two-dimensional tryptic peptide analysis, the gag-fes proteins of cat tumor cells appeared to be indistinguishable from those found in cells transformed in vitro. The polyproteins of the cat tumor cells have a closely associated protein kinase activity, as demonstrated in the in vitro assay, and phosphorylated tyrosine residues. Gag-fes proteins of either the ST or GA class were not present in cell cultures initiated from five spontaneous cat tumors.
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