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Ca(2+)-Calmodulin-dependent protein kinase II (CaMKII) is an abundant synaptic protein that was recently shown to regulate the organization of actin filaments leading to structural modifications of synapses. CaMKII is a dodecameric complex with a special architecture that provides it with unique potential for organizing the actin cytoskeleton. We report using biochemical assays that the beta isoform of CaMKII binds to and bundles actin filaments, and the disposition of betaCaMKII within the actin bundles was revealed by cryoelectron tomography. In addition, betaCaMKII was found to inhibit actin polymerization, suggesting that it either serves as a capping protein or binds monomeric actin, reducing the amount of freely available monomers to nucleate polymer assembly. By means of fluorescent cross-correlation spectroscopy, we determined that betaCaMKII does indeed bind to monomeric actin, reaching saturation at a stoichiometry of 12:1 actin monomers per betaCaMKII holoenzyme with a binding constant of 2.4 x 10(5) m(-1). In cells, betaCaMKII has a dual functional role; it can sequester monomeric actin to reduce actin polymerization and can also bundle actin filaments. Together, these effects would impact both the dynamics of actin filament assembly and enhance the rigidity of the filaments once formed, significantly impacting the structure of synapses.
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