Structural Insights into Activation of the Retinal L-type Ca2+ Channel (Cav1.4) by Ca2+-binding Protein 4 (CaBP4)

CaBP4 modulates Ca2+- dependent activity of L-type voltage-gated Ca2+ channels (Cav1.4) in retinal photoreceptor cells. Mg2+ binds to the first and third EF-hands (EF1 and EF3), and Ca2+ binds to EF1, EF3, and EF4 of CaBP4. Here we present NMR structures of CaBP4 in both Mg2+-bound and Ca2+-bound states and model the CaBP4 structural interaction with Cav1.4. CaBP4 contains an unstructured N-terminal region (residues 1-99) and four EF-hands in two separate lobes. The N-lobe consists of EF1 and EF2 in a closed conformation with either Mg2+ or Ca2+ bound at EF1. The C-lobe binds Ca2+ at EF3 and EF4 and exhibits a Ca2+-induced closed-to-open transition like that of calmodulin. Exposed residues in Ca2+-bound CaBP4 (Phe(137), Glu(168), Leu(207), Phe(214), Met(251), Phe(264), and Leu(268)) make contacts with the IQ motif in Cav1.4, and the Cav1.4 mutant Y1595E strongly impairs binding to CaBP4. We conclude that CaBP4 forms a collapsed structure around the IQ motif in Cav1.4 that we suggest may promote channel activation by disrupting an interaction between IQ and the inhibitor of Ca2+-dependent inactivation domain.