Acetylcholine receptor-channel gating
Cadugan, David J.
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The probability that muscle type acetylcholine receptors become conductive to ions increases in response to binding of two ligand molecules. The mechanism of this gating reaction is not yet fully understood. A wide range of studies reveal the importance of many residues throughout the protein in contributing to the gating reaction. Here, we investigate the contributions of residues in three subdomains of the AChR. We use single channel recording of mutant AChRs and rate-equilibrium relationship analysis, measurements of the energy changes upon mutation of side chains, and double mutant cycle analysis to describe the motion of specific residues during the gating reaction. First, we find that the third transmembrane domain moves as a rigid body late in the channel opening reaction, probably synchronously with the channel gate. Second, we study residues in the signature cys loop and M2-M3 linker, which together may form one link between the extracellular domain and the transmembrane domains. The cys loop moves before the M2-M3 linker, both of which move before M3. Third, a triad of residues I49 (loop 2), A96 (loop 5), and Y127 (â strand 6). A96 and Y127 experience extremely large changes in energy during channel gating. A96 is energetically linked with I49 and with Y127. Rate equilibrium analysis indicates that all three of these residues move approximately synchronously with the cys loop and distinctly after the ligand binding site. These residues form part of a novel pathway for gating, almost directly between the ligand binding site and the channel pore and along the interface of the ligand binding subunits with the adjacent subunits.