Autophosphorylation of Calmodulin‐Kinase II in Synaptic Junctions Modulates Endogenous Kinase Activity

Abstract

Previous studies have purified from brain a Ca2+/calmodulin-dependent protein kinase II (designated CaM-kinase II) that phosphorylates synapsin I, a synaptic vesicle-associated phosphoprotein. CaM-kinase II is composed of a major Mr 50K [kilodalton] polypeptide and a minor Mr 60K polypeptide; both bind calmodulin and are phosphorylated in a Ca2+/calmodulin-dependent manner. Recent studies have demonstrated that the 50K component of CaM-kinase II and the major postsynaptic density protein (mPSDp) in brain synaptic junctions (SJs) are virturally identical and that the CaM-kinase II and SJ 60K polypeptides are highly related. The photoaffinity analog [.alpha.-32P]8-azido-ATP was used to demonstrate that the 60K and 50K polypeptides of [rat forebrain] SJ-associated CaM-kinase II each bind ATP in the presence of Ca2+ plus calmodulin. This result is consistent with the observation that these proteins are phosphorylated in a Ca2+/calmodulin-dependent manner. Experiments using 32P-labeled peptides obtained by limited proteolysis of 60K and 50K polypeptides from SJs demonstrated that within each kinase polypeptide the same peptide regions contain both autophosphorylation and 125I-calmodulin binding sites. The autophosphorylation of CaM-kinase II could regulate its capacity to bind calmodulin and, thus, its capacity to phosphorylate substrate proteins. 125I-calmodulin overlay techniques and sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that phosphorylated 50K and 60K CaM-kinase II polypeptides bound more calmodulin (50-70%) than did unphosphorylated kinase polypeptides. Levels of in vitro CaM-kinase II activity in SJs were measured by phosphorylation of exogenous synapsin I. SJs containing highly phosphorylated CaM-kinase II displayed greater activity in phosphorylating synapsin I (300% at 15 nM calmodulin) relative to control SJs that contained unphosphorylated CaM-kinase II. The CaM-kinase II activity in phosphorylated SJs was indistinguishable from control SJs at saturating calmodulin concentrations (300-1000 nM). The degree of autophosphorylation of CaM-kinase II in brain SJs modulates its in vitro activity at low physiological calmodulin concentrations: such a process may represent a mechanism of regulating this kinases activity at CNS synapses in situ.