Identification of the Major Postsynaptic Density Protein as Homologous with the Major Calmodulin‐Binding Subunit of a Calmodulin‐Dependent Protein Kinase

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Abstract
The major postsynaptic density protein (mPSDp), comprising > 50% of postsynaptic density (PSD) protein, is an endogenous substrate for calmodulin-dependent phosphorylation as well as a calmodulin-binding protein in PSD preparations. mPSD seems highly homologous with major calmodulin-binding subunit (.rho.) of [rat brain] tubulin-associated calmodulin-dependent kinase (TACK), and PSD fractions also contain a protein homologous with the .sigma.-subunit of TACK. Homologies between mPSDp and a 63,000 dalton PSD protein and the .rho.- and .sigma.-subunits of TACK were established by the following criteria: identical apparent MW; identical calmodulin-binding properties; manifestation of Ca2+-calmodulin-stimulated autophosphorylation; identical isoelectric points; identical calmodulin binding and autophosphorylation patterns on 2-dimensional gels; homologous 2-dimensional tryptic peptide maps; and similar phosphoamino acid-specific phosphorylation of tubulin. Evidently, mPSDp is a calmodulin-binding protein involved in modulating protein kinase activity in the postsynaptic density and a tubulin kinase system homologous with TACK exists in a membrane-bound form in the PSD.