Phosphorylation at Ser26in the ATP-binding site of Ca 2+/calmodulin-dependent kinase II as a mechanism for switching off the kinase activity

Abstract

CaMKII (Ca2+/calmodulin-dependent kinase II) is a serine/threonine phosphotransferase that is capable of long-term 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 Ser26, a residue located within the ATP-binding site, terminates the sustained activity of the enzyme. To test the physiological importance of phosphorylation at Ser26, we generated a phosphospecific Ser26 antibody and demonstrated an increase in Ser26 phosphorylation upon depolarization and contraction of blood vessels. To determine if the phosphorylation of Ser26 affects the kinase activity, we mutated Ser26 to alanine or aspartic acid. The S26D mutation mimicking the phosphorylated state of CaMKII causes a dramatic decrease in Thr287 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 Ser26 of CaMKII γ inhibits the catalytic activity of the enzyme towards its autophosphorylation site at Thr287 most probably by blocking ATP binding. We propose that Ser26 phosphorylation constitutes an important mechanism for switching off CaMKII activity. © 2013 The Author(s).