Biochemical and immunochemical evidence that the "major postsynaptic density protein" is a subunit of a calmodulin-dependent protein kinase.

Abstract

By 2 biochemical and 1 immunochemical criteria the major postsynaptic density protein (mPSDp) is indistinguishable from the 50-kilodalton (kDa) .alpha. subunit of a brain calmodulin-dependent protein kinase. The 2 proteins comigrate on NaDodSO4/polyacrylamide gels. Iodinated tryptic peptide maps of the 2 are identical. A monoclonal antibody (6G9) that was raised [from murine hybridoma cells] against the protein kinase binds on immunoblots to a single 50 kDa band in crude [rat] brain homogenates and to both the .alpha. subunit of the purified kinase and the mPSDp from postsynaptic density fractions. The purified kinase holoenzyme also contains a 60-kDa subunit termed .beta.. A comparison of the peptide map of .beta. with the maps of 60-kDa proteins from the postsynaptic density fraction suggests that .beta. is present there but is not the only protein present in this MW range. The calmodulin-dependent protein kinase is a major constituent of the postsynaptic density fraction and thus may be a component of type I postsynaptic densities.