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

Abstract

AbstractVoltage-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 CaVα13 and auxiliary CaVβ5, and CaVα2δ subunits6,7. Here, we present cryo-EM structures of human brain and cardiac CaV1.2 bound with CaVβ3 to a chaperone, the endoplasmic reticulum membrane protein complex (EMC)8,9, and of the isolated CaV1.2/CaVβ3/CaVα2δ-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 CaVα2δ interaction site. The structures further identify the CaVα2δ binding site for gabapentinoid anti-pain and anti-anxiety drugs6, show that EMC and CaVα2δ channel interactions are mutually exclusive, and indicate that EMC to CaVα2δ 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.