v-mos protein produced by in vitro translation has protein kinase activity

The v-mos protein, termed p37^v-mos, has a closely associated serine/threonine protein kinase activity. To provide further information about its protein kinase activity, we tested the activity of p37^v-mos produced in a cell-free translation system from transcripts generated from a cloned v-mos gene. Anti-mos(37-55) immunoprecipitates of in vitro-produced p37^v-mos were found to possess serine/threonine protein kinase activity, whereas those obtained with anti-mos(260-271), known to block v-mos autophosphorylation, lacked kinase activity. The phosphorylated products were identical in size to p37^v-mos and p43^v-mos produced in protein kinase assays from Moloney murine sarcoma virus-infected cells expressing authentic p37^v-mos. These results provide further proof that the protein kinase activity associated with p37^v-mos is an intrinsic property of the v-mos gene product. This translation system also provides a useful experimental model to study the activation of the mos protein kinase. Thus, protein kinase assays performed on [³⁵S]methionine-labeled p37^v-mos produced p43^v-mos at the expense of p37^v-mos. Phosphatase treatment removed the p43^v-mos species, resulting in an increase of the p37^v-mos-sized protein, confirming our previous interpretation that p43^v-mos is a hyperphosphorylated form of p37^v-mos.