Voltage dependence of mouse acetylcholine receptor gating: Different charge movements in di-, mono- and unliganded receptors

1. The voltage dependence of binding and gating in wild-type and mutant recombinant mouse nicotinic acetylcholine receptors (AChRs) was examined at the single-channel level. 2. The closing rate constant of diliganded receptors decreased e-fold with ~66 mV hyperpolarization in both wild-type (adult and embryonic) and mutant receptors. The opening rate constant of a mutant receptor (αY93F) was not voltage dependent. 3. The voltage dependence of closing in monoliganded receptors was examined in several receptors having a mutation in the binding site (αG153S) or pore region (αL251C and εT264P). The closing rate constant of these monoliganded receptors decreased e-fold with ~124 mV hyperpolarization. 4. The voltage dependence of closing and opening in unliganded receptors was examined in two receptors having a mutation in the pore region (αL251C and εT264P). Neither the closing nor the opening rate constants of unliganded receptors were voltage dependent. 5. If z is the amount of charge that moves during channel closure and S is the distance (as a fraction of the electric field) that the charge moves, we conclude that zδ = 0.4 in diliganded receptors, 0.2 in monoliganded receptors, and 0.0 in unliganded receptors. It is likely that charges on the protein, rather than the agonist molecule, move zδ = 0.2 after each ACh molecule has bound. 6. The results suggest that unliganded openings arise from a local, concerted change in the structure of the pore (channel opening) that does not involve the net movement of charged residues. We speculate that as a consequence of agonist binding, charged moieties in the protein change their disposition so that they move with respect to the electric field when the channel gates. The results are consistent with the idea that there is semi-independent movement of distinct domains during AChR gating.