Structure of a potentially open state of a proton-activated pentameric ligand-gated ion channel

A lipid-dependent uncoupled conformation of the acetylcholine receptor

Lipids influence the ability of Cys-loop receptors to gate open in response to neurotransmitter binding, but the underlying mechanisms are poorly understood. With the nicotinic acetylcholine receptor (nAChR) from Torpedo, current models suggest that lipids modulate the natural equilibrium between resting and desensitized conformations. We show that the lipid-inactivated nAChR is not desensitized, instead it adopts a novel conformation where the allosteric coupling between its neurotransmitter-binding sites and transmembrane pore is lost. The uncoupling is accompanied by an unmasking of previously buried residues, suggesting weakened association between structurally intact agonist-binding and transmembrane domains. These data combined with the extensive literature on Cys-loop receptor-lipid interactions suggest that the M4 transmembrane helix plays a key role as a lipid-sensor, translating bilayer properties into altered nAChR function.

Ligand-specific conformational changes in the alpha1 glycine receptor ligand-binding domain

Understanding the activation mechanism of Cys loop ion channel receptors is key to understanding their physiological and pharmacological properties under normal and pathological conditions. The ligand-binding domains of these receptors comprise inner and outer beta-sheets and structural studies indicate that channel opening is accompanied by conformational rearrangements in both beta-sheets. In an attempt to resolve ligand-dependent movements in the ligand-binding domain, we employed voltage-clamp fluorometry on alpha1 glycine receptors to compare changes mediated by the agonist, glycine, and by the antagonist, strychnine. Voltage-clamp fluorometry involves labeling introduced cysteines with environmentally sensitive fluorophores and inferring structural rearrangements from ligand-induced fluorescence changes. In the inner beta-sheet, we labeled residues in loop 2 and in binding domain loops D and E. At each position, strychnine and glycine induced distinct maximal fluorescence responses. The pre-M1 domain responded similarly; at each of four labeled positions glycine produced a strong fluorescence signal, whereas strychnine did not. This suggests that glycine induces conformational changes in the inner beta-sheet and pre-M1 domain that may be important for activation, desensitization, or both. In contrast, most labeled residues in loops C and F yielded fluorescence changes identical in magnitude for glycine and strychnine. A notable exception was H201C in loop C. This labeled residue responded differently to glycine and strychnine, thus underlining the importance of loop C in ligand discrimination. These results provide an important step toward mapping the domains crucial for ligand discrimination in the ligand-binding domain of glycine receptors and possibly other Cys loop receptors.

Probing protein packing surrounding the residues in and flanking the nicotinic acetylcholine receptor M2M3 loop

Nicotinic acetylcholine receptors (nAChR) are cation-selective, ligand-gated ion channels of the cysteine (Cys)-loop gene superfamily. The recent crystal structure of a bacterial homolog from Erwinia chrysanthemi (ELIC) agrees with previous structures of the N-terminal domain of AChBP (acetylcholine-binding protein) and of the electron-microscopy-derived Torpedo nAChR structure. However, the ELIC transmembrane domain is significantly more tightly packed than the corresponding region of the Torpedo nAChR. We investigated the tightness of protein packing surrounding the extracellular end of the M2 transmembrane segment and around the loop connecting the M2 and M3 segments using the substituted cysteine accessibility method. The M2 20' to 27' residues were highly water accessible and the variation in reaction rates were consistent with this region being alpha-helical. At all positions tested, the presence of ACh changed methanethiosulfonate ethylammonium (MTSEA) modification rates by <10-fold. In the presence of ACh, reaction rates for residues in the last extracellular alpha-helical turn of M2 and in the M2M3 loop increased, whereas rates in the penultimate alpha-helical turn of M2 decreased. Only three of eight M2M3 loop residues were accessible to MTSEA in both the presence and absence of ACh. We infer that the protein packing around the M2M3 loop is tight, consistent with its location at the interdomain interface where it is involved in the transduction of ligand binding in the extracellular domain to gating in the transmembrane domain. Our data indicate that the Torpedo nAChR transmembrane domain structure is a better model than the ELIC structure for eukaryotic Cys-loop receptors.