EMC chaperone-CaV 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(V)s)(3,4) are paradigmatic multisubunit VGICs whose function and trafficking are powerfully shaped by interactions between pore-forming Ca(V)1 or Ca(V)2 CaV alpha(1) (ref. 3), and the auxiliary Ca-V beta(5) and Ca-V alpha(2)delta subunits(6,7). Here we present cryo-electron microscopy structures of human brain and cardiac Ca(V)1.2 bound with Ca-V beta(3) to a chaperone-the endoplasmic reticulum membrane protein complex (EMC)(8,9)-and of the assembled Ca(V)1.2-Ca-V beta(3)-Ca-V alpha(2)delta-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-V alpha(2)delta-interaction site. The structures identify the Ca-V alpha(2)delta-binding site for gabapentinoid anti-pain and anti-anxiety drugs(6), show that EMC and Ca-V alpha(2)delta interactions with the channel are mutually exclusive, and indicate that EMC-to-Ca-V alpha(2)delta hand-off 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 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.

Automated summary

This article reveals the assembly mechanism of high-voltage-activated calcium channels and their interactions with chaperone proteins through cryo-electron microscopy structure. The results indicate that chaperone proteins play an important role in the process of channel assembly.