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

The v-mos protein, termed p37(v-mos), has a closely asociated 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) immuno-precipitates of in vitro-produced p37(v-mos) were found to posses serine/threonine protein kase 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)