Conserved and divergent features of neuronal CaMKII holoenzyme structure, function, and high-order assembly

Neuronal CaMKII holoenzymes (alpha and beta isoforms) enable molecular signal computation underlying learning and memory but also mediate excitotoxic neuronal death, Here, we provide a comparative analysis of these signaling devices, using single-particle electron microscopy (EM) in combination with biochemical and livecell imaging studies. In the basal state, both isoforms assemble mainly as 12-mers (but also 14-mers and even 16-mers for the beta isoform). CaMKII alpha and beta isoforms adopt an ensemble of extended activatable states (with average radius of 12.6 versus 16.8 nm, respectively), characterized by multiple transient intra- and interholoenzyme interactions associated with distinct functional properties. The extended state of CaMKII beta allows direct resolution of intra-holoenzyme kinase domain dimers. These dimers could enable cooperative activation by calmodulin, which is observed for both isoforms. High-order CaMKII clustering mediated by inter-holoenzyme kinase domain dimerization is reduced for the isoform for both basal and excitotoxicity-induced clusters, both in vitro and in neurons.