Molecular mechanism for divergent regulation of Cav1.2 Ca2+ channels by calmodulin and Ca2+-binding protein-1

Ca2+-binding protein-1 (CaBP1) and calmodulin (CaM) are highly related Ca2+-binding proteins that directly interact with, and yet differentially regulate, voltagegated Ca2+ channels. Whereas CaM enhances inactivation of Ca2+ currents through Ca(v)1.2 (L-type) Ca2+ channels, CaBP1 completely prevents this process. How CaBP1 and CaM mediate such opposing effects on Ca(v)1.2 inactivation is unknown. Here, we identified molecular determinants in the alpha(1)-subunit of Ca(v)1.2 (alpha(1)1.2) that distinguish the effects of CaBP1 and CaM on inactivation. Although both proteins bind to a well characterized IQ-domain in the cytoplasmic C-terminal domain of alpha(1)1.2, mutations of the IQ-domain that significantly weakened CaM and CaBP1 binding abolished the functional effects of CaM, but not CaBP1. Pulldown binding assays revealed Ca2+-independent binding of CaBP1 to the N-terminal domain (NT) of alpha(1)1.2, which was in contrast to Ca2+-dependent binding of CaM to this region. Deletion of the NT abolished the effects of CaBP1 in prolonging Ca(v)1.2 Ca2+ currents, but spared Ca2+-dependent inactivation due to CaM. We conclude that the NT and IQ-domains of alpha(1)1.2 mediate functionally distinct interactions with CaBP1 and CaM that promote conformational alterations that either stabilize or inhibit inactivation of Ca(v)1.2.