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 (Phe¹³⁷, Glu¹⁶⁸, Leu²⁰⁷, Phe²¹⁴, Met²⁵¹, Phe²⁶⁴, and Leu²⁶⁸) 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.