CaMKIIbeta binding to stable F-actin in vivo regulates F-actin filament stability.

Ca2(+)/calmodulin-dependent protein kinase II (CaMKII) is a serine/threonine kinase that is best known for its role in synaptic plasticity and memory. Multiple roles of CaMKII have been identified in the hippocampus, yet its role in developing neurons is less well understood. We show here that endogenous CaMKIIbeta, but not CaMKIIalpha, localized to prominent F-actin-rich structures at the soma in embryonic cortical neurons. Fluorescence recovery after photobleaching analyses of GFP-CaMKIIbeta binding interactions with F-actin in this CaMKIIalpha-free system indicated CaMKIIbeta binding depended upon a putative F-actin binding domain in the variable region of CaMKIIbeta. Furthermore, CaMKIIalpha decreased CaMKIIbeta binding to F-actin. We examined the interaction of CaMKIIbeta with stable and dynamic actin and show that CaMKIIbeta binding to F-actin was dramatically prolonged when F-actin was stabilized. CaMKIIbeta binding to stable F-actin was disrupted when it was bound by Ca2(+)/calmodulin or when it was highly phosphorylated, but not by kinase inactivity. Whereas CaMKIIbeta over-expression increased the prevalence of the F-actin-rich structures, disruption of CaMKIIbeta binding to F-actin reduced them. Taken together, these data suggest that CaMKIIbeta binding to stable F-actin is important for the in vivo maintenance of polymerized F-actin.