Phosphoproteins in Postsynaptic Densities

The postsynaptic density (PSD) is a dense submembranous filamentous array located behind and in intimate contact with the postsynaptic membrane. PSDs have been isolated from synaptosomal plasma membranes and synaptosomes using various detergents. Canine cerebral cortex and midbrain PSDs have identical protein patterns with SDS-polyacrylamide gel electrophoresis (SDS PAGE) and contain about 10 major and at least 20 minor proteins. No membranes are seen by electron microscopy and the phospholipid content is 1% or less. No nucleic acid has been found either so that the structure is composed almost exclusively of protein. Enzymatic marker studies and radioactive mixing experiments reveal 0.1–6% contamination of the PSD fraction by mitochondrial, synaptic vesicle, plasma membrane and myelin membrane proteins. Myelin basic protein and intermediate filament protein (have been identified as contaminants. There are no Mg²⁺ or Ca2+-ATPase nor adenylate cyclase activities present in the fraction. A CAMP-dependent protein kinase and the two substrates for this kinase, proteins Ia and Ib are present. The presence of actin has been identified by amino acid analysis, immunology, and by gel mobility.