Autophosphorylation of calmodulin-kinase II in synaptic junctions modulates endogenous kinase activity.
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 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 virtually identical and that the CaM-kinase II and SJ 60K polypeptides are highly related. In the present study the photoaffinity analog [alpha-32P]8-azido-ATP was used to demonstrate that the 60K and 50K polypeptides of 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. These results suggested that the autophosphorylation of CaM-kinase II could regulate its capacity to bind calmodulin and, thus, its capacity to phosphorylate substrate proteins. By using 125I-calmodulin overlay techniques and sodium dodecyl sulfate-polyacrylamide gel electrophoresis we found 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 (300-1,000 nM). These findings show that the degree of autophosphorylation of CaM-kinase II in brain SJs modulates its in vitro activity at low and possibly physiological calmodulin concentrations; such a process may represent a mechanism of regulating this kinase's activity at CNS synapses in situ.
D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000378:metabolism; D000345:Affinity Labels / Q000378:metabolism; D000818:Animals; D001386:Azides / Q000378:metabolism; D001921:Brain / Q000201:enzymology*; D002118:Calcium / Q000494:pharmacology; D002147:Calmodulin / Q000378:metabolism / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D010777:Photochemistry; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D013569:Synapses / Q000201:enzymology*
D000345:Affinity Labels; D001386:Azides; D002147:Calmodulin; D002352:Carrier Proteins; D010750:Phosphoproteins; C017040:8-azidoadenosine 5'-triphosphate; D000255:Adenosine Triphosphate; D011494:Protein Kinases; D002118:Calcium