p37(mos) encoded by the HT-1 strain of Moloney murine sarcoma virus has an associated protein kinase activity

The mos gene contains within its sequence the ATP binding site and the phosphate transfer domain conserved among many protein kinases. Earlier studies from our laboratory have provided strong evidence that p37(mos) encoded by the 124 strain of Moloney murine sarcoma virus is a protein-serine kinase. In the present study, we have determined whether env-mos protein encoded by the HT-1 strain of Mo-MuSV (p37(mos)-HT) has protein-serine kinase activity. When assayed under the conditions previously described for the 124 strain of p37(mos) (p37(mos)-124), p37(mos)-HT was phosphorylated poorly in immune complex protein kinase assays. However, inclusion of 2 mM dithiothreitol in both the extraction procedure and during the reaction increased the autophosphorylation significantly. As a comparison, there was only a slight increase in the autophosphorylation of p37(mos)-124. Under these optimal conditions, p37(mos)-HT was found to be about half as efficient as p37(mos)-124 in terms of autophosphorylation per mole. As reported earlier, for the p37(mos)-124, in vitro autophosphorylation of p37(mos)-HT also resulted in the accumulation of a species, termed p43(mos), migrating slower in the SDS polyacrylamide gel. Phosphoamino acid analysis showed that the phosphorylation of both p37 and p43 occurred at serine and threonine residues. The fact that p37(mos)-HT is predicted to have an identical amino sequence in its mos domain to c-mos and that the protein kinase domain of c-mos is conserved among various animal species, strongly suggests that the putative cellular mos gene product will possess protein kinase activity. The assay conditions described here should facilitate studies aimed at detecting the putative protein-serine kinase activity of c-mos.