Molecular basis of calmodulin tethering and Ca2+-dependent inactivation ofL-type Ca2+ channels

Ca2+-dependent inactivation (CDI) of L-type Ca2+ channels plays a critical role in controlling Ca2+ entry and downstream signal transduction in excita ble cells. Ca2+-insensitive forms of calmodulin (CaM) act as dominant negat ives to prevent CDI, suggesting that CaM acts as a resident Ca2+ sensor. Ho wever, it is not known how the Ca2+ sensor is constitutively tethered. We h ave found that the tethering of Ca2+-insensitive CaM was localized to the C -terminal tail of arc, close to the CDI effector motif, and that it depende d on nanomolar Ca2+ concentrations, likely attained in quiescent cells. Two stretches of amino acids were found to support the tethering and to contai n putative CaM-binding sequences close to or overlapping residues previousl y shown to affect CDI and Ca2+-independent inactivation. Synthetic peptides containing these sequences displayed differences in CaM-binding properties , both in affinity and Ca2+ dependence, leading us to propose a novel mecha nism for CDI. In contrast to a traditional disinhibitory scenario, we sugge st that apoCaM is tethered at two sites and signals actively to slow inacti vation. When the C-terminal lobe of CaM binds to the nearby CaM effector se quence (IQ motif), the braking effect is relieved, and CDI is accelerated.