Acetylcholine Receptor Gating at Extracellular Transmembrane Domain Interface: the Cys-Loop and M2-M3 Linker

摘要

Acetylcholine receptor channel gating is a propagated conformational cascade that links changes in structure and function at the transmitter binding sites in the extracellular domain (ECD) with those at a "gate" in the transmembrane domain (TMD). We used Φ-value analysis to probe the relative timing of the gating motions of α-subunit residues located near the ECD-TMD interface. Mutation of four of the seven amino acids in the M2-M3 linker (which connects the pore-lining M2 helix with the M3 helix), including three of the four residues in the core of the linker, changed the diliganded gating equilibrium constant (K_(eq)) by up to 10,000-fold (P272 > 1274 > A270 > G275). The average Φ-value for the whole linker was ～0.64. One interpretation of this result is that the gating motions of the M2-M3 linker are approximately synchronous with those of much of M2 (～0.64), but occur after those of the transmitter binding site region (～0.93) and loops 2 and 7 (～0.77). We also examined mutants of six cys-loop residues (V132, T133, H134, F135, P136, and F137). Mutation of V132, H134, and F135 changed K_(cq) by 2800-, 10-, and 18-fold, respectively, and with an average Φ-value of 0.74, similar to those of other cys-loop residues. Even though V132 and I274 are close, the energetic coupling between I and V mutants of these positions was small (≤0.51 kcal mol~(-1)). The M2-M3 linker appears to be the key moving part that couples gating motions at the base of the ECD with those in TMD. These interactions are distributed along an ～16-A border and involve about a dozen residues.