IDENTIFICATION OF A LATENT CA2+ CALMODULIN-DEPENDENT PROTEIN KINASE-II PHOSPHORYLATION SITE IN VASCULAR CALPAIN-II

The Ca2+-dependent protease, calpain II, isolated from vascular smooth muscle was found to be a substrate for Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) in vitro. Phosphorylation was dependent upon prior autolysis of the regulatory subunit of calpain II. The stoichiometry of phosphorylation of native, unautolyzed calpain II was 0.02 +/- 0.01 mol PO4/mol enzyme while for autolyzed calpain, the stoichiometry was 1.04 +/- 0.15 mol PO4/mol enzyme. All phosphate was incorporated into the 76 kDa catalytic subunit of calpain II. A single serine residue in domain III of the catalytic subunit was identified as the phosphate acceptor: RGS*TAGGCR. Phosphorylation doubled enzyme activity measured both as proteolysis of an exogenous substrate (alpha-casein) as well as by intermolecular catalytic subunit autolysis. The effects of phosphorylation could be reversed by dephosphorylation using a type IIA phosphoprotein phosphatase. These results demonstrate that calpain II possesses a latent CaM kinase II phosphorylation site that is unmasked by autolysis.