In vitro protein kinase C phosphorylation sites of placental lipocortin

Human placental lipocortin is a high-affinity substrate for rat brain protein kinase C in vitro with phosphorylation occurring on serine and threonine residues in a ratio of approximately 2 to 1. Comparison of the ability of various N-terminal-truncated derivatives of lipocortin to serve as phosphorylation substrates, and direct analysis of the N-terminal peptides cleaved from 32P-labeled lipocortin, indicated that threonine-24, serine-27, and serine-28 were the phosphorylation sites. The possibility is discussed that a lysine residue near the carboxy side of the phosphorylation site was involved in lipocortin interaction with the catalytic site of protein kinase C.