EMC holdase:CaV1.2/CaVβ3 complex and CaV1.2 channel structures reveal CaV assembly and drug binding mechanisms

Abstract

Voltage-gated ion channels (VGICs) comprise multiple structural units whose assembly is required for function1,2. There is scant structural understanding of how VGIC subunits assemble and whether chaperone proteins are required. High-voltage activated calcium channels (CaVs)3,4 are paradigmatic multi-subunit VGICs from electrically excitable tissues whose function and trafficking is powerfully shaped by interactions between pore-forming CaV1 or CaV2 CaV13 and auxiliary CaV{beta}5, and CaV2{delta} subunits6,7. Here, we present cryo-EM structures of human brain and cardiac CaV1.2 bound with CaV{beta}3 to a chaperone, the endoplasmic reticulum membrane protein complex (EMC)8,9, and of the isolated CaV1.2/CaV{beta}3/CaV2{delta}-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 CaV2{delta} interaction site. The structures further identify the CaV2{delta} binding site for gabapentinoid anti-pain and anti-anxiety drugs6, show that EMC and CaV2{delta} channel interactions are mutually exclusive, and indicate that EMC to CaV2{delta} handoff involves a Ca2+-dependent step and ordering of multiple CaV1.2 elements. Together, the structures unveil a CaV assembly intermediate and previously unknown EMC client binding sites that have broad implications for biogenesis of VGICs and other membrane proteins.