Differential Inactivation of Postsynaptic Density-Associated and Soluble Ca2+/Calmodulin-Dependent Protein Kinase II by Protein Phosphatases 1 and 2A

Abstract: Autophosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) at Thr²⁸⁶ generates Ca²⁺-independent activity. As an initial step toward understanding CaMKII inactivation, protein phosphatase classes (PP1, PP2A, PP2B, or PP2C) responsible for dephosphorylation of Thr²⁸⁶ in rat forebrain subcellular fractions were identified using phosphatase inhibitors/activators, by fractionation using ion exchange chromatography and by immunoblotting. PP2A-like enzymes account for >70% of activity toward exogenous soluble Thr²⁸⁶-autophosphorylated CaMKII in crude cytosol, membrane, and cytoskeletal extracts; PP1 and PP2C account for the remaining activity. CaMKII is present in particulate fractions, specifically associated with postsynaptic densities (PSDs); each protein phosphatase is also present in isolated PSDs, but only PP1 is enriched during PSD isolation. When isolated PSDs dephosphorylated exogenous soluble Thr²⁸⁶-autophosphorylated CaMKII, PP2A again made the major contribution. However, CaMKII endogenous to PSDs (³²P autophosphorylated in the presence of Ca²⁺/calmodulin) was predominantly dephosphorylated by PP1. In addition, dephosphorylation of soluble and PSD-associated CaMKII in whole forebrain extracts was catalyzed predominantly by PP2A and PP1, respectively. Thus, soluble and PSD-associated forms of CaMKII appear to be dephosphorylated by distinct enzymes, suggesting that Ca²⁺-independent activity of CaMKII is differentially regulated by protein phosphatases in distinct subcellular compartments.