Distinct forebrain and cerebellar isozymes of type II Ca^(2+)/calmodulin-dependent protein kinase associate differently with the postsynaptic density fraction

Abstract

Forebrain and cerebellar Type II Ca2+/calmodulin-dependent protein kinases have different subunit compositions. The forebrain holoenzyme, characterized in our laboratory, is a 650-kDa holoenzyme composed of 50-kDa alpha-subunits and 60-kDa beta-subunits assembled in approximately a 3:1 ratio (Bennett, M. K., Erondu, N. E., and Kennedy, M. B. (1983) J. Biol. Chem. 258, 12735-12744). The cerebellar isozyme is a 500-kDa holoenzyme composed of alpha-subunits and beta-subunits assembled in almost the converse ratio, approximately four beta-subunits for each alpha-subunit. When compared by tryptic peptide mapping and by immunochemical techniques, the beta-subunits from the two brain regions are indistinguishable and the alpha-subunits appear closely related. The specific activities, substrate specificities, and catalytic constants of the cerebellar and forebrain isozymes are similar, suggesting that the alpha- and beta-subunits contain similar catalytic sites. However, two differences in the properties of the isozymes may result in functional differences between them in vivo. First, the apparent affinity of the cerebellar kinase for Ca2+/calmodulin is 2-fold higher than that of the forebrain kinase. Second, the two isozymes appear to associate differently with subcellular structures. Approximately 85% of the cerebellar kinase and 50% of the forebrain kinase remain in the particulate fraction after homogenization under standard conditions. However, they are present in different amounts in postsynaptic density fractions. Postsynaptic densities prepared from forebrain contain the forebrain isozyme. Immunochemical measurements show that it comprises approximately 16% of their total protein. In contrast, postsynaptic densities prepared from cerebellum contain the cerebellar isozyme, but it comprises only approximately 1-2% of their total protein. Thus, the alpha-subunit may play a role in anchoring Type II Ca2+/calmodulin-dependent protein kinase to postsynaptic densities.

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