The kinase activity of the v-fms encoded protein has a low pH optimum.

The protein encoded by v-fms, the oncogene of the Susan McDonough strain of feline sarcoma virus, is a member of the protein tyrosine kinase family. The kinase activity of the v-fms encoded protein has been reported to be low compared to other members of this enzyme family. We found that the optimal pH in vitro for the autophosphorylation of the immunoprecipitated v-fms encoded protein kinase activity was about pH 5.0; the activity at this pH was 15-fold higher than at the pH (7.4) used in standard kinase assays. The low pH optimum of the kinase activity of the v-fms encoded protein was observed when this protein was immunoprecipitated with each of four independent polyclonal antisera. v-fms proteins from transfected rat, mink or hamster cells all showed the same pH optimum for the kinase activity, as did the protein encoded by the feline c-fms gene. Autophosphorylation of v-fms in vitro at pH 5.0 occurred exclusively on tyrosine residues. Enolase was a substrate for the v-fms encoded protein kinase, and the pH profile for phosphorylation of this substrate in vitro paralleled that seen for the autophosphorylation of v-fms encoded proteins. The discovery of the low pH optimum of the kinase activity exhibited by v-fms proteins may be useful for further characterization of this activity in vitro, as well as for phenotypic classification of other members of the protein tyrosine kinase family.