Regulation of protein kinase C mu by basic peptides and heparin - Putative role of an acidic domain in the activation of the kinase

Protein kinase C mu is a novel member of the protein kinase C (PKC) family that differs from the other isoenzymes in structural and enzymatic properties. No substrate proteins of PKC mu have been identified as yet. Moreover, the regulation of PKC mu activity remains obscure, since a structural region corresponding to the pseudosubstrate domains of other PKC isoenzymes has not been found for PKC mu. Here we show that aldolase is phosphorylated by PKC mu in vitro. Phosphorylation of aldolase and of two substrate peptides by PKC mu is inhibited by various proteins and peptides, including typical PKC substrates such as histone H1, myelin basic protein, and p53. This inhibitory activity seems to depend on clusters of basic amino acids in the protein/peptide structures. Moreover, in contrast to other PRC isoenzymes PKC mu is activated by heparin and dextran sulfate. Maximal activation by heparin is about twice and that by dextran sulfate four times as effective as maximal activation by phosphatidylserine plus 12-O-tetradecanoylphorbol-13-acetate, the conventional activators of c-and nPKC isoforms. We postulate that PKC mu contains an acidic domain, which is involved in the formation and stabilization of an active state and which, in the inactive enzyme, is blocked by an intramolecular interaction with a basic domain. This intramolecular block is thought to be released by heparin and possibly also by 12-P-tetradecanoylphorbol-13-acetate/phosphatidylserine, whereas basic peptides and proteins inhibit PKC mu activity by binding to the acidic domain of the active enzyme.