Phosphorylation at Ser 26 in the ATP-binding site of Ca 2 + /calmodulin-dependent kinase II as a mechanism for switching off the kinase activity

Synopsis CaMKII (Ca 2 + /calmodulin-dependent kinase II) is a serine/threonine phosphotransferase that is capable of longterm retention of activity due to autophosphorylation at a specific threonine residue within each subunit of its oligomeric structure. The γ isoform of CaMKII is a significant regulator of vascular contractility. Here, we show that phosphorylation of CaMKII γ at Ser 26 , a residue located within the ATP-binding site, terminates the sustained activity of the enzyme. To test the physiological importance of phosphorylation at Ser 26 , we generated a phosphospecific Ser 26 antibody and demonstrated an increase in Ser 26 phosphorylation upon depolarization and contraction of blood vessels. To determine if the phosphorylation of Ser 26 affects the kinase activity, we mutated Ser 26 to alanine or aspartic acid. The S26D mutation mimicking the phosphorylated state of CaMKII causes a dramatic decrease in Thr 287 autophosphorylation levels and greatly reduces the catalytic activity towards an exogenous substrate (autocamtide-3), whereas the S26A mutation has no effect. These data combined with molecular modelling indicate that a negative charge at Ser 26 of CaMKII γ inhibits the catalytic activity of the enzyme towards its autophosphorylation site at Thr 287 most probably by blocking ATP binding. We propose that Ser 26 phosphorylation constitutes an important mechanism for switching off CaMKII activity.