CaMKIIβ 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 CaMKII beta, but not CaMKII alpha, localized to prominent F-actin-rich structures at the soma in embryonic cortical neurons. Fluorescence recovery after photo-bleaching analyses of GFP-CaMKII beta binding interactions with F-actin in this CaMKII alpha-free system indicated CaMKII beta binding depended upon a putative F-actin binding domain in the variable region of CaMKII beta. Furthermore, CaMKII alpha decreased CaMKII beta binding to F-actin. We examined the interaction of CaMKII beta with stable and dynamic actin and show that CaMKII beta binding to F-actin was dramatically prolonged when F-actin was stabilized. CaMKII beta 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 CaMKII beta over-expression increased the prevalence of the F-actin-rich structures, disruption of CaMKII beta binding to F-actin reduced them. Taken together, these data suggest that CaMKII beta binding to stable F-actin is important for the in vivo maintenance of polymerized F-actin.