A. Dosemeci et Ts. Reese, INHIBITION OF ENDOGENOUS PHOSPHATASE IN A POSTSYNAPTIC DENSITY FRACTION ALLOWS EXTENSIVE PHOSPHORYLATION OF THE MAJOR POSTSYNAPTIC DENSITY PROTEIN, Journal of neurochemistry, 61(2), 1993, pp. 550-555
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.