Analysis of the Sites in P56lck Whose Phosphorylation is Induced by Tetradecanoyl Phorbol Acetate

p56lck is a member of the src family of tyrosine kinases that is expressed almost exclusively in lymphocytes. Previous studies have shown that treatment of T cells with activators of protein kinase C induces serine phosphorylation of p56lck. We show here that treatment of Jurkat cells with 12-O-tetradecanoyl phorbol-13-acetate also induces threonine phosphorylation of p56lck. Chymotryptic mapping shows that at least three sites in p56lck undergo phosphorylation in TPA-treated Jurkat cells. Cyanogen bromide cleavage analysis demonstrates that these new phosphorylation sites are located in an amino-terminal 32 kDa fragment containing amino acid residues 14-261.