Loss of tumorigenicity correlates with a reduction in pp60 src kinase activity in a revertant subclone of avian sarcoma virus-infected field vole cells

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We have recently isolated an interesting revertant subclone (revertant 866-4) of ESV-infected field vole cells that is indistinguishable from uninfected vole cells with respect to its lack of transformed cell properties. These revertants are not only normal morphologically, but they do not grow in soft agar and are nontumorigenic in athymic nude mice. Despite this lack of transformed cell properties, we have found that this cell line still contains pp60 src at concentrations (0.30 μg/mg cell protein) similar to those (0.13–0.42 μg/mg cell protein) found in transformed and morphologically reverted, but tumorigenic vole cells (partial revertants). However, the most interesting aspect of this newly isolated subclone is the marked reduction in its pp60 src kinase activity (2–3%) when compared with the specific activity of pp60 src immunoprecipitated from transformed and partially revertant vole cell lines. Since the reduction in pp60 src kinase activity strongly correlates with the loss of tumorigenicity in this particular revertant cell line, these data support the contention that this enzymatic activity is a crucial factor in the tumorigenic conversion of cells by avian sarcoma virus. Proteolytic peptide analysis of the structure of pp60 src from revertant 866–4 indicates that it is similar to pp60 src obtained from avian sarcoma virus-transformed chick embryo fibroblasts. Moreover, the reduction in kinase activity does not appear to be due to a lack of phosphorylation of the tyrosine residue in pp60 src. Thus neither an obvious structural alteration nor a reduction in phosphorylation of pp60 src appears responsible for the reduced kinase activity observed, suggesting that some as of yet undetermined feature of pp60 src can influence the pp60 src phosphorylating event.