INHIBITION OF ENDOGENOUS PHOSPHATASE IN A POSTSYNAPTIC DENSITY FRACTION ALLOWS EXTENSIVE PHOSPHORYLATION OF THE MAJOR POSTSYNAPTIC DENSITY PROTEIN

The major postsynaptic density protein, proposed to be a calcium/calmo dulin-dependent protein kinase, becomes phosphorylated when a postsyna ptic density preparation from rat cerebral cortex is incubated in medi um containing calcium and calmodulin. Upon longer incubation, however, the level of phosphorylation declines, suggesting the presence of a p hosphatase activity. When Microcystin-LR, a phosphatase inhibitor, is included in the phosphorylation medium, the decline in phosphorylation is prevented and a higher maximal level of phosphorylation can be ach ieved. Under these conditions, the maximal phosphorylation of major po stsynaptic density protein is accompanied by a nearly complete shift i n its electrophoretic mobility from 50 kDa to 54 kDa, similar to that described for the a subunit of the soluble calcium/calmodulin-dependen t protein kinase II. Of the four major groups of serine/threonine prot ein phosphatases, the enzyme responsible for the dephosphorylation of major postsynaptic density protein is neither type 2C, which is insens itive to Microcystin-LR, nor type 2B, which is calcium-dependent. As M icrocystin-LR is much more potent than okadaic acid in inhibiting the dephosphorylation of major postsynaptic density protein, it is likely that the postsynaptic density-associated phosphatase is a type 1. The above results indicate that the relatively low level of phosphorylatio n of the major postsynaptic density protein observed in preparations c ontaining postsynaptic densities is not due to a difference between th e cytoplasmic and postsynaptic density-associated calcium/calmodulin-d ependent kinases as previously proposed, but to a phosphatase activity , presumably belonging to the type 1 group.