X-ray structure of a pentameric ligand-gated ion channel in an apparently open conformation

Abstract

Pentameric ligand-gated ion channels from the Cys-loop family mediate fast chemo-electrical transduction, but the mechanisms of ion permeation and gating of these membrane proteins remain elusive. Here we present the X-ray structure at 2.9 A resolution of the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel homologue (GLIC) at pH 4.6 in an apparently open conformation. This cationic channel is known to be permanently activated by protons. The structure is arranged as a funnel-shaped transmembrane pore widely open on the outer side and lined by hydrophobic residues. On the inner side, a 5 A constriction matches with rings of hydrophilic residues that are likely to contribute to the ionic selectivity. Structural comparison with ELIC, a bacterial homologue from Erwinia chrysanthemi solved in a presumed closed conformation, shows a wider pore where the narrow hydrophobic constriction found in ELIC is removed. Comparative analysis of GLIC and ELIC reveals, in concert, a rotation of each extracellular beta-sandwich domain as a rigid body, interface rearrangements, and a reorganization of the transmembrane domain, involving a tilt of the M2 and M3 alpha-helices away from the pore axis. These data are consistent with a model of pore opening based on both quaternary twist and tertiary deformation.

Insight in nAChR subtype selectivity from AChBP crystal structures

“…GLIC in an open state [34,35]. These structural data indicate that the nAChR architecture is mostly conserved in both ELIC and GLIC (Figure 13a).…”

“…GLIC hence displays an anti-clockwise rotation of its ECD, accompanied by a clockwise rotation of its transmembrane region relative to the ELIC structure. In the ECD, a rotation of 8° is also observed for the core of the β-sandwich around an axis perpendicular to the inner sheet of the β-sandwich [34].…”

“…Furthermore, the recently solved structures of the integral membrane proteins of the prokaryotic Erwinia chrysanthemi ELIC [33] and Gloeobacter violaceus GLIC [34,35], displaying an architecture very close to that of the Torpedo nAChR also now provide a novel avenue for studying gating mechanisms from a structural standpoint. A c c e p t e d M a n u s c r i p t 8 interactions in the stabilization of the nicotinic ligand cation [36,37].…”

Mutations that stabilize the open state of the Erwinia chrisanthemi ligand-gated ion channel fail to change the conformation of the pore domain in crystals

“…Two main structural models of bacterial nicotinic-type receptors have been inferred from X-ray crystallographic data, thus far: that of the unliganded ELIC channel (PDB ID code: 2VL0), which was deemed to represent the closed-channel conformation (9), and that of the proton-gated GLIC channel in the presence of a desensitizing concentration of protons (PDB ID codes: 3EAM and 3EHZ), which was deemed to represent the open state (5,6). These models have been interpreted to represent the end states of the closed ⇋ open conformational transition (5,6), and thus, form the basis of recently proposed channel-gating mechanisms.…”

“…For example, although GLIC (a proton-gated channel from Gloeobacter violaceus) was crystallized at pH values that strongly favor the desensitized state (pH ∼4.5) (4), the properties of the pore domain inferred from the obtained structural models have been interpreted to suggest that the channel was crystallized, actually, in its open, ion-conductive conformation (5,6; but see ref. 7).…”