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Oligomeric structure of alpha-calmodulin-dependent protein kinase II.

The subunit stoichiometry and symmetry of the neuronal alpha-calmodulin-dependent protein kinase II (alphaCaMKII) is investigated in this report to understand the structural basis of its regulation and mechanism at the molecular level. Two preparations are studied, alphaCaMKII obtained by overexpression in baculovirus-transfected insect cells and CaMKII isolated from rat forebrain. The structures, are studied by electron microscopy and image analysis. Single-particle analysis of individual molecular images reveals a molecule with a circular outline and pronounced 6-fold rotational symmetry of the central part. The central part has an outer radius of approximately 6 nm and is composed of six lobes grouped around a hollow centre. The outer ring extends to approximately 15 nm and consists of 12 apparent domains. These data are interpreted in terms of a three-dimensional model of the alphaCaMKII complex consisting of 12 subunits, each corresponding to a single alphaCaMKII polypeptide chain. The inner ring corresponding to approximately one-third of the molecular mass of the complex is made up of the C-terminal association domains. The 12 association domains are arranged in two concentric hexagonal rings at different axial levels and in rotational register. The outer ring corresponding to the remaining molecular mass of the complex is made up of the 12 N-terminal catalytic domains located at an axial level halfway between the two levels of the association domains. The 6-fold symmetry of stacked association domains may derive from subunit arrangements corresponding to either the C6 or the D6 point group symmetries. The symmetry and the resulting subunit arrangement define the pattern and extent of regulatory autophosphorylation within the alphaCaMKII complex.