gp140^v-fms Molecules Expressed at the Surface of Cells Transformed by the Mcdonough Strain of Feline Sarcoma Virus Are Phosphorylated in Tyrosine and Serine

Abstract

Cells transformed by the McDonough strain of feline sarcoma virus express at their surface a v-fms-specific transmembrane glycoprotein designated gp140^v-fms. By labeling with 32Pi, gp140^v-fms was shown to be phosphorylated 30-fold more in serine residues than were the cytosolic v-fms polypeptides gp180^gag-fms and gp120^v-fms. By using the phosphotyrosine phosphatase-specific inhibitor sodium orthovanadate, an additional tyrosine phosphorylation was observed in vivo, again involving predominantly gp140^v-fms. In vitro studies showed that the v-fms proteins were phosphorylated by protein kinase C in a calcium- and phosphatidylserine-dependent manner.