Two types of brain calmodulin-dependent protein kinase II: morphological, biochemical and immunochemical properties

Two forms of the soluble calmodulin-dependent protein kinase type II can be isolated from rat brain: one oligomeric enzyme complex contains the α and β subunits of the enzyme, whereas the other oligomeric species is comprised of a constant ratio of the subunits of the kinase and tubulin in the presence of several other minor polypeptides. The unassociated enzyme oligomer does not detectably exchange with the tubulin-containing form, and both forms rechromatograph by ion-exchange to their respective positions. In the molecular complex of proteins eluting at high ionic strength, the ratio of kinase subunits to tubulin remains constant throughout sedimentation velocity centrifugation and gel permeation chromatography. Furthermore, a similar complex of proteins is coprecipitated by the anti-kinase monoclonal antibody. Hydrodynamic parameters demonstrate that the tubulin-associated enzyme is larger than the unassociated enzyme, and displays heterodisperse behavior as well. Electron microscopic examination of negatively stained enzyme preparations reveals that the free enzyme constitutes uniform 10-20 nm diameter oligomers in contrast to the tubulin-associated kinase which forms elongated structures with varying morphology. Interestingly, enzyme purified through the calmodulin-Sepharose step can also form 'polymers' featuring ultrastructural similarities to postsynaptic densities and brain microsomal cytokeletal preparations. We discuss the relevance of these observations to the ability of the type II calmodulin-dependent protein kinase to interact with other polypeptides and to form cytoskeletal structures such as the postsynaptic density.