CaBP1 regulates voltage-dependent inactivation and activation of Ca(V)1.2 (L-type) calcium channels.

CaBP1 is a Ca(2+)-binding protein that regulates the gating of voltage-gated (Ca(V)) Ca(2+) channels. In the Ca(V)1.2 channel α(1)-subunit (α(1C)), CaBP1 interacts with cytosolic N- and C-terminal domains and blunts Ca(2+)-dependent inactivation. To clarify the role of the α(1C) N-terminal domain in CaBP1 regulation, we compared the effects of CaBP1 on two alternatively spliced variants of α(1C) containing a long or short N-terminal domain. In both isoforms, CaBP1 inhibited Ca(2+)-dependent inactivation but also caused a depolarizing shift in voltage-dependent activation and enhanced voltage-dependent inactivation (VDI). In binding assays, CaBP1 interacted with the distal third of the N-terminal domain in a Ca(2+)-independent manner. This segment is distinct from the previously identified calmodulin-binding site in the N terminus. However, deletion of a segment in the proximal N-terminal domain of both α(1C) isoforms, which spared the CaBP1-binding site, inhibited the effect of CaBP1 on VDI. This result suggests a modular organization of the α(1C) N-terminal domain, with separate determinants for CaBP1 binding and transduction of the effect on VDI. Our findings expand the diversity and mechanisms of Ca(V) channel regulation by CaBP1 and define a novel modulatory function for the initial segment of the N terminus of α(1C).

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