title: Loop C and the mechanism of acetylcholine receptor-channel gating.
abstract: Agonist molecules at the two neuromuscular acetylcholine (ACh) receptor (AChR) transmitter-binding sites increase the probability of channel opening. In one hypothesis for AChR activation ("priming"), the capping of loop C at each binding site transfers energy independently to the distant gate over a discrete structural pathway. We used single-channel analyses to examine the experimental support for this proposal with regard to brief unliganded openings, the effects of loop-C modifications, the effects of mutations to residues either on or off the putative pathway, and state models for describing currents at low [ACh]. The results show that (a) diliganded and brief unliganded openings are generated by the same essential, global transition; (b) the radical manipulation of loop C does not prevent channel opening but impairs agonist binding; (c) both on- and off-pathway mutations alter gating by changing the relative stability of the open-channel conformation by local interactions rather than by perturbing a specific site-gate communication link; and (d) it is possible to estimate directly the rate constants for agonist dissociation from and association to both the low and high affinity forms of the AChR-binding site by using a cyclic kinetic model. We conclude that the mechanism of energy transfer between the binding sites and the gate remains an open question.
authors.affiliation: Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214, USA.
authors.forename: Prasad
authors.initials: P
authors.lastname: Purohit
authors.forename: Anthony
authors.initials: A
authors.lastname: Auerbach
mesh_terms: D000495:Allosteric Site; D000595:Amino Acid Sequence; D057809:HEK293 Cells; D006801:Humans; D015640:Ion Channel Gating; D007700:Kinetics; D056004:Molecular Dynamics Simulation; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D018722:Nicotinic Agonists / Q000378:metabolism; D017434:Protein Structure, Tertiary; D011978:Receptors, Nicotinic / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D018722:Nicotinic Agonists; D011978:Receptors, Nicotinic