CA2-INDEPENDENT AUTOPHOSPHORYLATION OF POSTSYNAPTIC DENSITY-ASSOCIATED CA2+()CALMODULIN-DEPENDENT PROTEIN-KINASE/

A major protein in the postsynaptic density fraction is alpha-CAM kina se II, the alpha-subunit of the Ca2+/calmodulin-dependent protein kina se. Autophosphorylation of the postsynaptic density-associated CaM kin ase II is likely to be a crucial event in the induction of activity-de pendent synaptic modification. This study focuses on the regulation an d consequences of Ca2+-independent autophosphorylation of the enzyme. In isolated postsynaptic densities, a sub-stochiometric level of autop hosphorylation in the presence of Ca2+ is sufficient to trigger maxima l Ca2+-independent autophosphorylation of alpha-CaM Kinase II. A major fraction of the sites phosphorylated in the absence of Ca2+ can be de phosphorylated by the endogenous phosphatase activity in the preparati on. Ca2+-independent autophosphorylation is correlated with a drastic decrease in calmodulin binding to postsynaptic densities. This may rep resent a physiological mechanism that lowers the calmodulin trapping c apacity of the organelle, thus increasing the availability of calmodul in to other elements within a spine.