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

CA(2+)-binding protein-1 (CaBP1) and Calmodulin (CaM) are highly related CA(2+)-binding proteins that directly interact with, and yet differentially regulate, voltage-gated CA(2+) channels. Whereas CaM enhances inactivation of CA(2+) currents through CA(v)1.2 (L-type) CA(2+) 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 CA(2+)-independent binding of CaBP1 to the N-terminal domain (NT) of alpha(1)1.2, which was in contrast to CA(2+)-dependent binding of CaM to this region. Deletion of the NT abolished the effects of CaBP1 in prolonging CA(v)1.2 CA(2+) currents, but spared CA(2+)-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.