Signaling at the neuromuscular synapse requires C(losed-channel)?O(pen-channel) 'gating'of acetylcholine receptors (AChRs). The thermodynamic foundation of this allosteric transition is well understood: transmitter molecules released from the nerve terminal bind to AChRs with higher affinity to O vs. C to increase the probability that the channel is Open. The microscopic events within C?O are less certain. We will use single-channel kinetics and phi-value analysis to probe the interior of AChR gating and illuminate the ultra-fast protein rearrangements within this reaction. So far, results show that AChRs change from C-to-O in 4 steps. The first amino acids to move are not at the transmitter binding sites but in a distant membrane domain linker that joins the M2 and M3 helices of the subunit. Further, the unlocking of a double-gate in M2 (all subunits) occurs in the final 2 gating activation steps. Most side chain gating movements are 'resettling'events that have only local energetic consequences. We will investigate two new hypotheses for AChR gating: 1) communication between the binding sites and the gate is not by a structural-mechanical process but, rather, by the vibrational entropy of the entire backbone, and 2) allosteric communication commences with low-affinity binding of the agonist to the resting receptor.
Chemical synaptic transmission requires the activation of membrane receptors by neurotransmitters. We will use the nerve-muscle synaptic receptor as a model system for understanding the mechanism of this activation. What we will learn increases our basic knowledge of molecular neuroscience, especially with regard to receptor physiology, pharmacology and diseases.