Fig. 9: Single-channel currents from the muscle AChR lacking allosteric loops C.
From: Disentangling the mechanistic role of loop-C capping in Cys-loop receptor activation
a, b Bursts of single-channel openings recorded in the cell-attached configuration from the wild-type adult mouse-muscle AChR; as previously reported, the single-channel conductance decreased somewhat as the pH was lowered44. c, d Bursts of single-channel openings recorded in the cell-attached configuration from mutants carrying the indicated mutations in the β1, δ or ε subunit. The reporter mutation (Leu-to-His at position 9ʹ of the M2 α-helix) introduced a pore-lining proton-binding side chain whose protonation lowers the single-channel conductance, and therefore, manifests as a current sublevel whose occupancy depends on pH44,45,46. Because of the lumen-facing orientation of the engineered histidines in the open-channel conformation of the (cation-selective) muscleAChR, their side-chain pKas are bulk-like44, and thus, side-chain protonation–deprotonation events could be clearly observed, even at pH 7.4. The occurrence of these main-level ⇌ sublevel current fluctuations allowed us to detect the presence of the loop-C-deleted subunit in the pentameric assembly. As expected62 from the slower deactivation timecourses recorded upon application of a high concentration of ACh (100 μM; Fig. 7b), bursts of single-channel openings recorded from the mutants at a low concentration of ACh (1 μM) were on average longer than those recorded from the wild-type muscle AChR. For all panels, the indicated pH values correspond to those of the pipette solution; openings are downward deflections; the pipette potential was –100 mV; and—for display purposes—the traces were low-pass filtered at 5 kHz. Black dashed lines denote the zero-current baseline. Source data are provided as a Source Data file.
