Phosphorylation-dependent Reversible Association of Ca/Calmodulin-dependent Protein Kinase II with the Postsynaptic Densities*

The association of soluble Ca/calmodulin-dependent protein kinase II (CaM kinase II) with postsynaptic densities (PSDs) was determined by activity assay, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and immunoblotting of the enzyme. Soluble CaM kinase II was autophosphorylated with ATP in the presence of Ca and calmodulin, and then it was incubated with PSDs. Autophosphorylated CaM kinase II was precipitated with PSDs by centrifugation. The kinase that was not autophosphorylated did not precipitate with PSDs. These results indicate that the soluble previously autophosphorylated CaM kinase II associates with PSDs and forms PSD-CaM kinase II complex. A maximum of about 60 mg of soluble CaM kinase II bound to 1 mg of PSD protein under the experimental conditions. Ca-independent activity generated by autophosphorylation of the kinase was retained in the PSD-CaM kinase II complex. The CaM kinase II thus associated with PSDs phosphorylated a number of PSD proteins in both the absence and presence of Ca. When the CaM kinase II-PSD complex was incubated at 30 °C, its Ca-independent activity was gradually decreased. This decrease was correlated with dephosphorylation of the kinase and its release from PSD-CaM kinase II complex. These results indicate that CaM kinase II reversibly translocates to PSDs in a phosphorylation-dependent manner.