Mechanism of NACHO-mediated assembly of pentameric ligand-gated ion channels

Pentameric ligand-gated ion channels (pLGICs) such as γ-aminobutyric acid receptors (GABA _A Rs) and nicotinic acetylcholine receptors (nAChRs) are cell surface proteins of crucial importance for animal physiology. The molecular mechanisms that control their assembly at the endoplasmic reticulum (ER) remain unknown. Here, we identified and determined the cryo-EM structure of an assembly intermediate containing two α1 subunits of GABA _A R each bound to an ER-resident membrane protein NACHO. The structure showed how NACHO shields the principal (+) transmembrane interface of α1 subunits containing an immature extracellular conformation. Crosslinking and structure-prediction revealed an adjacent surface on NACHO for β2 subunit interactions to guide stepwise oligimerisation. Mutations of either subunit-interacting surface on NACHO also impaired the formation of homopentameric α7 nAChRs, pointing to a generic framework for pLGIC assembly.