On allosteric mechanisms and acetylcholine receptors

Introduction to membrane proteins

Membrane proteins (MPs) mediate important physiological processes for the cell via extracellular and intracellular interactions. To better understand the biochemical and structural bases of these interactions, well-characterized preparations of purified MPs are required. This introduction reviews common problems encountered in MP preparation.

From the stochasticity of molecular processes to the variability of synaptic transmission

The variability of the postsynaptic response following a single action potential arises from two sources: the neurotransmitter release is probabilistic, and the postsynaptic response to neurotransmitter release has variable timing and amplitude. At individual synapses, the number of molecules of a given type that are involved in these processes is small enough that the stochastic (random) properties of molecular events cannot be neglected. How the stochasticity of molecular processes contributes to the variability of synaptic transmission, its sensitivity and its robustness to molecular fluctuations has important implications for our understanding of the mechanistic basis of synaptic transmission and of synaptic plasticity.

Acetylcholine and epibatidine binding to muscle acetylcholine receptors distinguish between concerted and uncoupled models

The muscle acetylcholine receptor (AChR) has served as a prototype for understanding allosteric mechanisms of neurotransmitter-gated ion channels. The phenomenon of cooperative agonist binding is described by the model of Monod et al. (Monod, J., Wyman, J., and Changeux, J. P. (1965) J. Mol. Biol. 12, 88-118; MWC model), which requires concerted switching of the two binding sites between low and high affinity states. The present study examines binding of acetylcholine (ACh) and epibatidine, agonists with opposite selectivity for the two binding sites of mouse muscle AChRs. We expressed either fetal or adult AChRs in 293 HEK cells and measured agonist binding by competition against the initial rate of 125I-alpha-bungarotoxin binding. We fit predictions of the MWC model to epibatidine and ACh binding data simultaneously, taking as constants previously determined parameters for agonist binding and channel gating steps, and varying the agonist-independent parameters. We find that the MWC model describes the apparent dissociation constants for both agonists but predicts Hill coefficients that are far too steep. An Uncoupled model, which relaxes the requirement of concerted state transitions, accurately describes binding of both ACh and epibatidine and provides parameters for agonist-independent steps consistent with known aspects of AChR function.

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 approximately 66 mV hyperpolarization in both wild-type (adult and embryonic) and mutant receptors. The opening rate constant of a mutant receptor (alpha Y93F) 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 (alpha G153S) or pore region (alpha L251C and epsilon T264P). The closing rate constant of these monoliganded receptors decreased e-fold with approximately 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 (alpha L251C and epsilon T264P). Neither the closing nor the opening rate constants of unliganded receptors were voltage dependent. 5. If z if the amount of charge that moves during channel closure and delta is the distance (as a fraction of the electric field) that the charge moves, we conclude that z delta = 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 z delta = 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.

Selection of distinct conformational states of the 5-HT3 receptor by full and partial agonists

1. 5-Hydroxytryptamine 5-HT3 receptor-mediated ion currents evoked by 5-HT, quaternary 5-HT (5-HTQ), meta-chlorophenylbiguanide (mCPBG), dopamine and tryptamine in N1E-115 mouse neuroblastoma cells have been investigated in whole-cell voltage clamp and single channel patch clamp experiments. 2. The concentration-dependent activation and desensitization of the ion currents evoked by the agonists yield the potency order: mCPBG > 5-HTQ approximately 5-HT >> tryptamine > dopamine, and the efficacy order: 5-HT approximately mCPBG approximately 5-HTQ >> dopamine approximately tryptamine. Thus, 5-HT, 5-HTQ and mCPBG are full agonists, whereas dopamine and tryptamine are partial agonists at the 5-HT3 receptor. 3. Full and partial agonists cause complete cross-desensitization and activate single channels with similar conductances and open lifetimes. This shows that full and partial agonists act on the same population of 5-HT3 receptors. 4. The time course of recovery from desensitization depends on the agonist used. Recovery from partial agonist-induced desensitization is single exponential, whereas the desensitization induced by full agonists recovers with sigmoid kinetics, suggesting at least 3 steps between 4 states. 5. During the process of recovery from cross-desensitization, the full agonists activate a larger fraction of the 5-HT3 receptors than the partial agonists, irrespective of the agonist used to induce desensitization. 6. It is concluded that full and partial agonists induce distinct desensitized states and, during recovery from desensitization, recognize distinct conformations of unoccupied 5-HT3 receptors. This conformational selection is likely to account for the different efficacies of full and partial 5-HT3, receptor agonists.