EMC holdase:Ca _V 1.2/Ca _V β ₃ complex and Ca _V 1.2 channel structures reveal Ca _V assembly and drug binding mechanisms

Voltage-gated ion channels (VGICs) comprise multiple structural units whose assembly is required for function ^1,2 . There is scant structural understanding of how VGIC subunits assemble and whether chaperone proteins are required. High-voltage activated calcium channels (Ca _V s) ^3,4 are paradigmatic multi-subunit VGICs from electrically excitable tissues whose function and trafficking is powerfully shaped by interactions between pore-forming Ca _V 1 or Ca _V 2 Ca _V α _{1 ³} and auxiliary Ca _V β ₅ , and Ca _V α ₂ δ subunits ^6,7 . Here, we present cryo-EM structures of human brain and cardiac Ca _V 1.2 bound with Ca _V β ₃ to a chaperone, the endoplasmic reticulum membrane protein complex (EMC) ^8,9 , and of the isolated Ca _V 1.2/Ca _V β ₃ /Ca _V α ₂ δ-1 channel. These provide an unprecedented view of an EMC holdase:client complex and define EMC sites, the TM and Cyto docks, whose interaction with the client channel cause partial extraction of a pore subunit and splay open the Ca _V α ₂ δ interaction site. The structures further identify the Ca _V α ₂ δ binding site for gabapentinoid anti-pain and anti-anxiety drugs ⁶ , show that EMC and Ca _V α ₂ δ channel interactions are mutually exclusive, and indicate that EMC to Ca _V α ₂ δ handoff involves a Ca ²⁺ -dependent step and ordering of multiple Ca _V 1.2 elements. Together, the structures unveil a Ca _V assembly intermediate and previously unknown EMC client binding sites that have broad implications for biogenesis of VGICs and other membrane proteins.