EMC chaperone-Ca V structure reveals an ion channel assembly intermediate.

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 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 assembled Ca V 1.2/Ca V β 3 /Ca V α 2 δ-1 channel. These provide a view of an EMC:client complex and define EMC sites, the TM and Cyto docks, whose interaction with the client channel causes partial extraction of a pore subunit and splays open the Ca V α 2 δ interaction site. The structures 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 divalent ion-dependent step and Ca V 1.2 element ordering. Disruption of the EMC:Ca V complex compromises Ca V function suggesting that the EMC acts as a channel holdase that facilitates channel assembly. Together, the structures unveil a Ca V assembly intermediate and EMC client binding sites, with potentially wide-reading implications for biogenesis of VGICs and other membrane proteins.