Architecture and assembly mechanism of native glycine receptors

Glycine receptors (GlyRs) are pentameric, ‘Cys-loop’ receptors that form chloride-permeable channels and mediate fast inhibitory signalling throughout the central nervous system 1 , 2 . In the spinal cord and brainstem, GlyRs regulate locomotion and cause movement disorders when mutated 2 , 3 . However, the stoichiometry of native GlyRs and the mechanism by which they are assembled remain unclear, despite extensive investigation 4 – 8 . Here we report cryo-electron microscopy structures of native GlyRs from pig spinal cord and brainstem, revealing structural insights into heteromeric receptors and their predominant subunit stoichiometry of 4α:1β. Within the heteromeric pentamer, the β(+)–α(−) interface adopts a structure that is distinct from the α(+)–α(−) and α(+)–β(−) interfaces. Furthermore, the β-subunit contains a unique phenylalanine residue that resides within the pore and disrupts the canonical picrotoxin site. These results explain why inclusion of the β-subunit breaks receptor symmetry and alters ion channel pharmacology. We also find incomplete receptor complexes and, by elucidating their structures, reveal the architectures of partially assembled α-trimers and α-tetramers. Cryo-electron microscopy structures of pig glycine receptors indicate that they are predominantly assembled with 4α:1β stoichiometry via α-homotrimer and homotetramer intermediates.