Allosteric signal initiation and communication in neuromuscular acetylcholine receptors

Acetylcholine receptors (AChRs) are integral to nerve-muscle communication, transitioning between a closed (C) and open (O) state. Upon acetylcholine (ACh) binding, the channel gate opens, facilitating ion flow. However, the mechanism behind how ACh communicates allosteric signals to the channel gate remains unclear.

In the absence of ACh, both wild-type and mutant AChRs, including those causing congenital myasthenia syndrome, exhibit continuous gating. The debate persists on whether the activation pathways in the presence and absence of ACh are similar. To address this, researchers employed kinetic modelling, phi- and activation energy estimation from over 60 residues derived from single-channel current recordings, and molecular dynamics simulations.

The study reveals two distinct gating pathways in unliganded AChRs: a major and minor pathway. Crucially, the activation pathways differ when ACh is present, indicating non-identical activation in both scenarios. Kinetic analysis of the minor pathway, coupled with the correlation between state residence probabilities and C-loop conformations, suggests two key findings.

Firstly, the C-loop capping initiates allosteric communication independently of the ACh presence. Secondly, the minor gating pathway is a pre-existing route that becomes more favorable in the presence of ACh.

Moreover, the researchers identified a continuous, live-wire-like allosteric network connecting the neurotransmitter binding sites (TBS) and the gate in the liganded receptor. This network comprises residues that lower the activation energy barrier by more than -4 kcal/mol. Conversely, in unliganded receptors, the major gating initiates from hub residues within the allosteric network.

The findings provide unprecedented insights into the mechanism of allosteric signal initiation and communication in AChRs, shedding light on the complex processes underlying neuromuscular function.