Quaternary structure independent folding of voltage-gated ion channel pore domain subunits

Cristina Arrigoni, Marco Lolicato, David Shaya, Ahmed Rohaim, Felix Findeisen, Lam-Kiu Fong, Claire M Colleran, Pawel Dominik, Sangwoo S Kim, Jonathan P Schuermann, William F DeGrado, Michael Grabe, Anthony A Kossiakoff, Daniel L Minor

Nat Struct Mol Biol. 2022 Jun;29(6):537-548.

PMID: 35655098 DOI: 10.1038/s41594-022-00775-x

Every voltage-gated ion channel (VGIC) has a pore domain (PD) made from four subunits, each comprising an antiparallel transmembrane helix pair bridged by a loop. The extent to which PD subunit structure requires quaternary interactions is unclear. Here, we present crystal structures of a set of bacterial voltage-gated sodium channel (BacNaV) ‘pore only’ proteins that reveal a surprising collection of non-canonical quaternary arrangements in which the PD tertiary structure is maintained. This context-independent structural robustness, supported by molecular dynamics simulations, indicates that VGIC-PD tertiary structure is independent of quaternary interactions. This fold occurs throughout the VGIC superfamily and in diverse transmembrane and soluble proteins. Strikingly, characterization of PD subunit-binding Fabs indicates that non-canonical quaternary PD conformations can occur in full-length VGICs. Together, our data demonstrate that the VGIC-PD is an autonomously folded unit. This property has implications for VGIC biogenesis, understanding functional states, de novo channel design, and VGIC structural origins.