Mechanisms of nicotinic acetylcholine receptor channel gating: The extracellular domain

Nicotinic acetylcholine receptor-channel (AChR) gating is an organized sequence of molecular motions that couples a change in the affinity for ligands at the transmitter binding sites with a change in the ionic conductance of the pore. The transmitter binding site (TBS) and the 'gate' of the channel are separated by at least 45 A. Using patch clamp recording and model based kinetic analyses of single-channel currents, we studied the effect of side-directed mutagenesis, in different regions of the AChR, on the basic parameters of channel function: agonist binding, channel gating and desensitization.;Loop 5 (L5) links the beta4 and beta5 strands. Most of the mutations in this region increase gating (up to 168-fold) but have little or no effect on ligand binding or desensitization. L5 moves early in the gating reaction (phi = 0.93), approximately in synchrony with the movement of the TBS.;Loop 2 (L2) links the beta1 and beta2 strands and Loop 7 (L7) links the beta6 and beta7 strands. Mutations in L2 and L7 effect channel gating without altering agonist affinity. L2 (phi = 0.80) and L7 (phi = 0.77) follow L5 and the TBS in the gating reaction sequence.;The extracellular domain of the AChR is composed entirely of beta-strands and connecting loops. These strands fold into a hydrophobic beta-sandwich core. Over 40 mutations (at 22 residues) in this region showed minimal effects on agonist affinity or channel gating. Only one residue showed a moderate effect on channel gating (M144). This residues moves along with L2 and L7 in the gating reaction sequence (phi = 0.84).;We conclude that during channel opening, the beta-sandwich core does not undergo a significant change in its local structure or environment. The loop regions in the agonist-binding subunit interface undergo substantial structural changes (as evidenced by the changes in the gating equilibrium constant) and thereby may serve to relay the conformational changes that propagate between the binding site and the membrane domain. We have identified at least three domains in the extracellular region of the receptor that move at relatively different time scales during the gating reaction, TBS/L5, L2/L7 and the M2--M3 linker.