Ca2+/Calmodulin-dependent Protein Kinase Kinase β Is Regulated by Multisite Phosphorylation

Ca2+/calmodulin-dependent protein kinase kinase beta (CaMKK beta) is a serine/threonine-directed kinase that is activated following increases in intracellular Ca2+. CaMKK beta activates Ca2+/calmodulin-dependent protein kinase I, Ca2+/calmodulin-dependent protein kinase IV, and the AMP-dependent protein kinase in a number of physiological pathways, including learning and memory formation, neuronal differentiation, and regulation of energy balance. Here, we report the novel regulation of CaMKK beta activity by multisite phosphorylation. We identify three phosphorylation sites in the N terminus of CaMKK beta, which regulate its Ca2+/calmodulin-independent autonomous activity. We then identify the kinases responsible for these phosphorylations as cyclin-dependent kinase 5 (CDK5) and glycogen synthase kinase 3 (GSK3). In addition to regulation of autonomous activity, we find that phosphorylation of CaMKK beta regulates its half-life. We find that cellular levels of CaMKK beta correlate with CDK5 activity and are regulated developmentally in neurons. Finally, we demonstrate that appropriate phosphorylation of CaMKK beta is critical for its role in neurite development. These results reveal a novel regulatory mechanism for CaMKK beta-dependent signaling cascades.