Evidence that the major postsynaptic density protein is a component of a Ca2+/calmodulin-dependent protein kinase.

Polypeptides of Mr 50,000 and 60,000 in isolated synaptic junctions have been compared to polypeptides of corresponding molecular weight in Ca2+/calmodulin-dependent protein kinase II. The polypeptides of corresponding molecular weight from the two preparations were shown by several criteria to be indistinguishable. These criteria included 125I-labeled tryptic/chymotryptic peptide patterns, 32P-labeled proteolytic peptide maps, and crossreactivity on immunoblots using polyclonal and monoclonal antibodies. Furthermore, studies examining the phosphorylation of substrate proteins, by the endogenous synaptic junction kinase and by Ca2+/calmodulin-dependent protein kinase II, indicated that the two enzymes have similar substrate specificities. Since the Mr 50,000 polypeptide present in synaptic junctions is known to be the major postsynaptic density protein, the present results indicate that the major postsynaptic density protein is a component of Ca2+/calmodulin-dependent protein kinase II.