Phosphorylation of p36 in vitro by protein kinase C

The 36kDa subunit of protein I (p36) is a major substrate of several tyrosine protein kinases. Here we demonstrate that protein kinase C catalyzes the incorporation of 1.7 moles of phosphate per mole of protein I. Phosphorylation is absolutely dependent on the presence of both calcium and phospholipid, and is specific for serine and threonine residues. Phosphorylation of protein I by the c-AMP dependent protein kinase, phosphorylase kinase, casein kinase I, and casein kinase II was not observed. The in vivo significance of protein kinase C dependent phosphorylation of p36 is discussed.