A mechanism for acetylcholine receptor gating based on structure, coupling, phi, and flip

Area of Science:

• Biochemistry
• Structural Biology
• Neuroscience

Background:

• Nicotinic acetylcholine receptors (nAChRs) are crucial allosteric proteins mediating neurotransmission.
• Receptor function relies on conformational changes (gating) between resting and active states.
• Understanding the allosteric pathways linking ligand binding to channel gating is essential.

Purpose of the Study:

• To elucidate the molecular mechanisms connecting transmitter-binding sites (TBSs) to the ion channel gate in nAChRs.
• To investigate the coupling between distant residues and domain rearrangements during gating.
• To develop a structure-based model for nAChR allosteric transitions.

Main Methods:

• Mutant cycle analyses to quantify residue-pair coupling.
• Phi value analyses to map domain rearrangements during the gating transition state.
• Microscopic current simulations to analyze gating kinetics and intermediate states.

Main Results:

• Identified significant allosteric coupling between the αM2-M3 linkers and TBSs (~30 Å apart).
• Phi value analysis indicated the M2-M3 linker is the first region to reach the gating transition state.
• Gating involves passage through four intermediate states, with brief channel closures (flips) arising from these states.

Conclusions:

• A novel structure-based model for nAChR gating is proposed, initiated by M2-M3 linker and TBS rearrangements.
• The resting-to-active transition involves extracellular domain twisting, transmembrane helix tilting, and gate dilation.
• Extracellular domain twisting is the most energetically costly step in activation, while bubble collapse is rate-limiting for deactivation.