Presence of calmodulin in postsynaptic densities isolated from canine cerebral cortex

The postsynaptic density fraction isolated from canine cerebral cortex has been found to contain some 10 major and at least 20 minor proteins. This structure is remarkably resistant to various extraction conditions; however, a major 18,000 molecular weight component has been extracted by EGTA treatment and purified by DEAE-Sephadex A-25 column chromatography. This protein has been identified as calmodulin by the following criteria: the 18,000 molecular weight protein from the postsynaptic density fraction is similar to both purified canine and porcine brain calmodulin in stimulating a partially purified cyclic nucleotide phosphodiesterase in a calcium-dependent manner; all three proteins co-migrate as a single band on sodium dodecyl sulfate polyacrylamide gels; the amino acid composition of the 18,000 molecular weight protein is similar to that of brain calmodulin, and contains ε-N-trimethyllysine; the protein from the postsynaptic density fraction, canine brain, and porcine brain calmodulin all exhibit an enhanced migration rate on sodium dodecyl sulfate polyacrylamide gels in the presence of calcium, but not of magnesium. In addition, purified calmodulin from porcine brain, canine brain, and postsynaptic density fraction could all be reconstituted in the presence of calcium, but not of magnesium, into a postsynaptic density preparation rendered deficient in calmodulin.