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/calmodulin‐dependent protein kinase, becomes phosphorylated when a postsynaptic density preparation from rat cerebral cortex is incubated in medium containing calcium and calmodulin. Upon longer incubation, however, the level of phosphorylation declines, suggesting the presence of a phosphatase 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 achieved. Under these conditions, the maximal phosphorylation of major postsynaptic density protein is accompanied by a nearly complete shift in its electrophoretic mobility from 50 kDa to 54 kDa, similar to that described for the a subunit of the soluble calcium/calmodulin‐dependent protein kinase II. Of the four major groups of serine/threonine protein phosphatases, the enzyme responsible for the dephosphorylation of major postsynaptic density protein is neither type 2C, which is insensitive to Microcystin‐LR, nor type 2B, which is calcium‐dependent. As Microcystin‐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 phosphorylation of the major postsynaptic density protein observed in preparations containing postsynaptic densities is not due to a difference between the cytoplasmic and postsynaptic density‐associated calcium/calmodulin‐dependent kinases as previously proposed, but to a phosphatase activity, presumably belonging to the type 1 group.