EMC chaperone-Ca_V structure reveals an ion channel assembly intermediate

Voltage-gated ion channels (VGICs) comprise multiple structural units, the assembly of which is required for function^1,2. Structural understanding of how VGIC subunits assemble and whether chaperone proteins are required is lacking. High-voltage-activated calcium channels (Ca_Vs)^3,4 are paradigmatic multisubunit VGICs whose function and trafficking are powerfully shaped by interactions between pore-forming Ca_V1 or Ca_V2 Ca_Vα₁ (ref. ³), and the auxiliary Ca_Vβ⁵ and Ca_Vα₂δ subunits^6,7. Here we present cryo-electron microscopy structures of human brain and cardiac Ca_V1.2 bound with Ca_Vβ₃ to a chaperone-the endoplasmic reticulum membrane protein complex (EMC)^8,9-and of the assembled Ca_V1.2-Ca_Vβ₃-Ca_Vα₂δ-1 channel. These structures provide a view of an EMC-client complex and define EMC sites-the transmembrane (TM) and cytoplasmic (Cyto) docks; interaction between these sites and the client channel causes partial extraction of a pore subunit and splays open the Ca_Vα₂δ-interaction site. The structures identify the Ca_Vα₂δ-binding site for gabapentinoid anti-pain and anti-anxiety drugs⁶, show that EMC and Ca_Vα₂δ interactions with the channel are mutually exclusive, and indicate that EMC-to-Ca_Vα₂δ hand-off involves a divalent ion-dependent step and Ca_V1.2 element ordering. Disruption of the EMC-Ca_V complex compromises Ca_V function, suggesting that the EMC functions as a channel holdase that facilitates channel assembly. Together, the structures reveal a Ca_V assembly intermediate and EMC client-binding sites that could have wide-ranging implications for the biogenesis of VGICs and other membrane proteins.