Molecular dissection of subunit interfaces in the acetylcholine receptor. Identification of residues that determine agonist selectivity

Partitioning the Hill coefficient into contributions from ligand‐promoted conformational changes and subunit heterogeneity

Heterooligomers that undergo ligand-promoted conformational changes are ubiquitous in nature and involved in many essential processes. Conformational switching often leads to positive cooperativity in ligand binding that is reflected in a Hill coefficient with a value greater than one. The subunits comprising heterooligomers can differ, however, in their affinity for the ligand. Such so-called site heterogeneity results in apparent negative cooperativity that is reflected by a Hill coefficient with a value less than one. Consequently, positive cooperativity due to the ligand-promoted allosteric switch can be masked, in cases of such heterooligomers, by apparent negative cooperativity owing to site heterogeneity. Here, we derived expressions for the Hill coefficient, in the case of a heterodimer, in which the contributions from the ligand-promoted allosteric switch and site heterogeneity are separated. Using these equations and simulations for higher order oligomers, we show under which conditions site heterogeneity can significantly mask the extent of observed positive cooperativity.

Conformational transitions and ligand-binding to a muscle-type nicotinic acetylcholine receptor

Fast synaptic communication requires receptors that respond to the presence of neurotransmitter by opening an ion channel across the post-synaptic membrane. The muscle-type nicotinic acetylcholine receptor from the electric fish, Torpedo, is the prototypic ligand-gated ion channel, yet the structural changes underlying channel activation remain undefined. Here we use cryo-EM to solve apo and agonist-bound structures of the Torpedo nicotinic receptor embedded in a lipid nanodisc. Using both a direct biochemical assay to define the conformational landscape and molecular dynamics simulations to assay flux through the pore, we correlate structures with functional states and elucidate the motions that lead to pore activation of a heteromeric nicotinic receptor. We highlight an underappreciated role for the complementary subunit in channel gating, establish the structural basis for the differential agonist affinities of α/δ versus α /γ sites, and explain why nicotine is less potent at muscle nicotinic receptors compared to neuronal ones.

Mutations of α1F45 residue of GABAA receptor loop G reveal its involvement in agonist binding and channel opening/closing transitions

GABA_A receptors (GABA_ARs) mediate inhibitory neurotransmission in the mammalian brain. Recently, numerous GABA_AR static structures have been published, but the molecular mechanisms of receptor activation remain elusive. Loop G is a rigid β-strand belonging to an extensive β-sheet that spans the regions involved in GABA binding and the interdomain interface which is important in receptor gating. It has been reported that loop G participates in ligand binding and gating of GABA_ARs, however, it remains unclear which specific gating transitions are controlled by this loop. Analysis of macroscopic responses revealed that mutation at the α₁F45 residue (loop G midpoint) resulted in slower macroscopic desensitization and accelerated deactivation. Single-channel analysis revealed that these mutations also affected open and closed times distributions and reduced open probability. Kinetic modeling demonstrated that mutations affected primarily channel opening/closing and ligand binding with a minor effect on preactivation. Thus, α₁F45 residue, in spite of its localization close to binding site, affects late gating transitions. In silico structural analysis suggested an important role of α₁F45 residue in loop G stability and rigidity as well as in general structure of the binding site. We propose that the rigid β-sheet comprising loop G is well suited for long range communication within GABA_AR but this mechanism becomes impaired when α₁F45 is mutated. In conclusion, we demonstrate that loop G is crucial in controlling both binding and gating of GABA_ARs. These data shed new light on GABA_AR activation mechanism and may also be helpful in designing clinically relevant modulators.

Non-adiabatic membrane voltage fluctuations driven by two ligand-gated ion channels

In this paper, we construct a piecewise deterministic Markov process to model the membrane voltage fluctuations driven by two ligand-gated channels. The series-solution method is applied to the third-order ordinary differential equations to get its general solutions. Also, the stationary probability density function (PDF) is just the special solution that satisfies certain boundary conditions. The bifurcation conditions of the PDF at the boundary are obtained analytically. As an application, the PDF is used to calculate the power dissipation of the ionic currents in the (nonequilibrium) steady states.

Conotoxins targeting nicotinic acetylcholine receptors: an overview

Marine snails of the genus Conus are a large family of predatory gastropods with an unparalleled molecular diversity of pharmacologically active compounds in their venom. Cone snail venom comprises of a rich and diverse cocktail of peptide toxins which act on a wide variety of ion channels such as voltage-gated sodium- (Na_V), potassium- (K_V), and calcium- (Ca_V) channels as well as nicotinic acetylcholine receptors (nAChRs) which are classified as ligand-gated ion channels. The mode of action of several conotoxins has been the subject of investigation, while for many others this remains unknown. This review aims to give an overview of the knowledge we have today on the molecular pharmacology of conotoxins specifically interacting with nAChRs along with the structure–function relationship data.

Structure-function elucidation of a new α-conotoxin, Lo1a, from Conus longurionis

α-Conotoxins are peptide toxins found in the venom of marine cone snails and potent antagonists of various subtypes of nicotinic acetylcholine receptors (nAChRs). nAChRs are cholinergic receptors forming ligand-gated ion channels in the plasma membranes of certain neurons and the neuromuscular junction. Because nAChRs have an important role in regulating transmitter release, cell excitability, and neuronal integration, nAChR dysfunctions have been implicated in a variety of severe pathologies such as epilepsy, myasthenic syndromes, schizophrenia, Parkinson disease, and Alzheimer disease. To expand the knowledge concerning cone snail toxins, we examined the venom of Conus longurionis. We isolated an 18-amino acid peptide named α-conotoxin Lo1a, which is active on nAChRs. To the best of our knowledge, this is the first characterization of a conotoxin from this species. The peptide was characterized by electrophysiological screening against several types of cloned nAChRs expressed in Xenopus laevis oocytes. The three-dimensional solution structure of the α-conotoxin Lo1a was determined by NMR spectroscopy. Lo1a, a member of the α4/7 family, blocks the response to acetylcholine in oocytes expressing α7 nAChRs with an IC50 of 3.24 ± 0.7 μM. Furthermore, Lo1a shows a high selectivity for neuronal versus muscle subtype nAChRs. Because Lo1a has an unusual C terminus, we designed two mutants, Lo1a-ΔD and Lo1a-RRR, to investigate the influence of the C-terminal residue. Lo1a-ΔD has a C-terminal Asp deletion, whereas in Lo1a-RRR, a triple-Arg tail replaces the Asp. They blocked the neuronal nAChR α7 with a lower IC50 value, but remarkably, both adopted affinity for the muscle subtype α1β1δε.

Gating of pentameric ligand-gated ion channels: structural insights and ambiguities

Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic communication by converting chemical signals into an electrical response. Recently solved agonist-bound and agonist-free structures greatly extend our understanding of these complex molecular machines. A key challenge to a full description of function, however, is the ability to unequivocally relate determined structures to the canonical resting, open, and desensitized states. Here, we review current understanding of pLGIC structure, with a focus on the conformational changes underlying channel gating. We compare available structural information and review the evidence supporting the assignment of each structure to a particular conformational state. We discuss multiple factors that may complicate the interpretation of crystal structures, highlighting the potential influence of lipids and detergents. We contend that further advances in the structural biology of pLGICs will require deeper insight into the nature of pLGIC-lipid interactions.

Neuromuscular activity of Micrurus laticollaris (Squamata: Elapidae) venom in vitro

In this work, we have examined the neuromuscular activity of Micrurus laticollaris (Mexican coral snake) venom (MLV) in vertebrate isolated nerve-muscle preparations. In chick biventer cervicis preparations, the MLV induced an irreversible concentration- and time-dependent (1–30 µg/mL) neuromuscular blockade, with 50% blockade occurring between 8 and 30 min. Muscle contractures evoked by exogenous acetylcholine were completely abolished by MLV, whereas those of KCl were also significantly altered (86% ± 11%, 53% ± 11%, 89% ± 5% and 89% ± 7% for one, three, 10 and 30 µg of venom/mL, respectively; n = 4; p < 0.05). In mouse phrenic nerve-diaphragm preparations, MLV (1–10 µg/mL) promoted a slight increase in the amplitude of twitch-tension (3 µg/mL), followed by neuromuscular blockade (n = 4); the highest concentration caused complete inhibition of the twitches (time for 50% blockade = 26 ± 3 min), without exhibiting a previous neuromuscular facilitation. The venom (3 µg/mL) induced a biphasic modulation in the frequency of miniature end-plate potentials (MEPPs)/min, causing a significant increase after 15 min, followed by a decrease after 60 min (from 17 ± 1.4 (basal) to 28 ± 2.5 (t₁₅) and 12 ± 2 (t₆₀)). The membrane resting potential of mouse diaphragm preparations pre-exposed or not to d-tubocurarine (5 µg/mL) was also significantly less negative with MLV (10 µg/mL). Together, these results indicate that M. laticollaris venom induces neuromuscular blockade by a combination of pre- and post-synaptic activities.

Physostigmine and galanthamine bind in the presence of agonist at the canonical and noncanonical subunit interfaces of a nicotinic acetylcholine receptor

Galanthamine and physostigmine are clinically used cholinomimetics that both inhibit acetylcholinesterase and also interact directly with and potentiate nAChRs. As with most nAChR-positive allosteric modulators, the location and number of their binding site(s) within nAChRs are unknown. In this study, we use the intrinsic photoreactivities of [(3)H]physostigmine and [(3)H]galanthamine upon irradiation at 312 nm to directly identify amino acids contributing to their binding sites in the Torpedo californica nAChR. Protein sequencing of fragments isolated from proteolytic digests of [(3)H]physostigmine- or [(3)H]galanthamine-photolabeled nAChR establish that, in the presence of agonist (carbamylcholine), both drugs photolabeled amino acids on the complementary (non-α) surface of the transmitter binding sites (γTyr-111/γTyr-117/δTyr172). They also photolabeled δTyr-212 at the δ-β subunit interface and γTyr-105 in the vestibule of the ion channel, with photolabeling of both residues enhanced in the presence of agonist. Furthermore, [(3)H]physostigmine photolabeling of γTyr-111, γTyr-117, δTyr-212, and γTyr-105 was inhibited in the presence of nonradioactive galanthamine. The locations of the photolabeled amino acids in the nAChR structure and the results of computational docking studies provide evidence that, in the presence of agonist, physostigmine and galanthamine bind to at least three distinct sites in the nAChR extracellular domain: at the α-γ interface (1) in the entry to the transmitter binding site and (2) in the vestibule of the ion channel near the level of the transmitter binding site, and at the δ-β interface (3) in a location equivalent to the benzodiazepine binding site in GABA(A) receptors.

End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease

The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.

Allosteric activation mechanism of the cys-loop receptors

Binding of a neurotransmitter to its ionotropic receptor opens a distantly located ion channel, a process termed allosteric activation. Here we review recent advances in the molecular mechanism by which the cys-loop receptors are activated with emphasis on the best studied nicotinic acetylcholine receptors (nAChRs). With a combination of affinity labeling, mutagenesis, electrophysiology, kinetic modeling, electron microscopy (EM), and crystal structure analysis, the allosteric activation mechanism is emerging. Specifically, the binding domain and gating domain are interconnected by an allosteric activation network. Agonist binding induces conformational changes, resulting in the rotation of a beta sheet of amino-terminal domain and outward movement of loop 2, loop F, and cys-loop, which are coupled to the M2-M3 linker to pull the channel to open. However, there are still some controversies about the movement of the channel-lining domain M2. Nine angstrom resolution EM structure of a nAChR imaged in the open state suggests that channel opening is the result of rotation of the M2 domain. In contrast, recent crystal structures of bacterial homologues of the cys-loop receptor family in apparently open state have implied an M2 tilting model with pore dilation and quaternary twist of the whole pentameric receptor. An elegant study of the nAChR using protonation scanning of M2 domain supports a similar pore dilation activation mechanism with minimal rotation of M2. This remains to be validated with other approaches including high resolution structure determination of the mammalian cys-loop receptors in the open state.

Functional equivalence of the nicotinic acetylcholine receptor transmitter binding sites in the open state

The subunits of the muscle-type nicotinic acetylcholine receptor (AChR) are not uniformly oriented in the resting closed conformation: the two alpha subunits are rotated relative to its non-alpha subunits. In contrast, all the subunits overlay well with one another when agonist is bound to the AChR, suggesting that they are uniformly oriented in the open receptor. This gating-dependent increase in orientational uniformity due to rotation of the alpha subunits might affect the relative affinities of the two transmitter binding sites, making the two affinities dissimilar (functionally non-equivalent) in the initial ligand-bound closed state but similar (functionally equivalent) in the open state. To test this hypothesis, we measured single-channel activity of the alphaG153S gain-of-function mutant receptor evoked by choline, and estimated the resting closed-state and open-state affinities of the two transmitter binding sites. Both model-independent analyses and maximum-likelihood estimation of microscopic rate constants indicate that channel opening makes the binding sites' affinities more similar to each other. These results support the hypothesis that open-state affinities to the transmitter binding sites are primarily determined by the alpha subunits.

Dynamics of heteropentameric nicotinic acetylcholine receptor: implications of the gating mechanism

The dynamics characteristics of the currently available structure of Torpedo nicotinic acetylcholine receptor (nAChR), including the extracellular, transmembrane, and intracellular domains (ICDs), were analyzed using the Gaussian Network Model (GNM) and Anisotropic Network Model (ANM). We found that a symmetric quaternary twist motion, reported previously in the literature in a homopentameric receptor (Cheng et al. J Mol Biol 2006;355:310-324; Taly et al. Biophys J 2005;88:3954-3965), occurred also in the heteropentameric Torpedo nAChR. We believe, however, that the symmetric twist alone is not sufficient to explain a large body of experimental data indicating asymmetry and subunit nonequivalence during gating. Here we report our results supporting the hypothesis that a combination of symmetric and asymmetric motions opens the gate. We show that the asymmetric motion involves tilting of the TM2 helices. Furthermore, our study reveals three additional aspects of channel dynamics: (1) loop A serves as an allosteric mediator between the ligand binding loops and those at the domain interface, particularly the linker between TM2 and TM3; (2) the ICD can modulate the pore dynamics and thus should not be neglected in gating studies; and (3) the F loops, which are peculiarly longer and poorly-conserved in non-alpha-subunits, have important dynamical implications.

Species selectivity of a nicotinic acetylcholine receptor agonist is conferred by two adjacent extracellular beta4 amino acids that are implicated in the coupling of binding to channel gating

5-(Trifluoromethyl)-6-(1-methyl-azepan-4-yl)methyl-1H-quinolin-2-one (TMAQ) is a novel nicotinic acetylcholine receptor (nAChR) agonist with strong selectivity for beta4-containing receptors. TMAQ also exhibits remarkable species selectivity, being a potent agonist of nAChRs containing the human beta4 subunit but having no detectable agonist activity on nAChRs containing the rat beta4 subunit. With the aim of identifying subunit domains and individual amino acids, which contribute to the species selectivity of TMAQ, a series of chimeric and mutated beta4 subunits has been constructed. Recombinant receptors containing wild-type, chimeric, or mutated beta4 subunits have been examined by radioligand binding, intracellular calcium assays, and electrophysiological recording. Two adjacent amino acids located within the extracellular loop D domain of the beta4 subunit (amino acids 55 and 56) have been identified as playing a critical role in determining the agonist potency of TMAQ. Mutagenesis of these two residues within the rat beta4 subunit to the corresponding amino acids in the human beta4 subunit (S55N and I56V mutations) confers sensitivity to TMAQ. The converse mutations in the human beta4 subunit (N55S and V56I) largely abolish sensitivity to TMAQ. In contrast, these mutations have little or no effect on sensitivity to the nonselective nicotinic agonist epibatidine. Despite acting as a potent agonist of human beta4-containing nAChRs, TMAQ acts as an antagonist of rat beta4-containing receptors. Our experimental data, together with homology models of the rat and human alpha3beta4 nAChRs, suggest that amino acids 55 and 56 may be involved in the coupling of agonist binding and channel gating.

Electrostatic steering at acetylcholine binding sites

The electrostatic environments near the acetylcholine binding sites on the nicotinic acetylcholine receptor (nAChR) and acetylcholinesterase were measured by diffusion-enhanced fluorescence energy transfer (DEFET) to determine the influence of long-range electrostatic interactions on ligand binding kinetics and net binding energy. Changes in DEFET from variously charged Tb3+ -chelates revealed net potentials of -20 mV at the nAChR agonist sites and -14 mV at the entrance to the AChE active site, in physiological ionic strength conditions. The potential at the alphadelta-binding site of the nAChR was determined independently in the presence of d-tubocurarine to be -14 mV; the calculated potential at the alphagamma-site was approximately threefold stronger than at the alphadelta-site. By determining the local potential in increasing ionic strength, Debye-Hückel theory predicted that the potentials near the nAChR agonist binding sites are constituted by one to three charges in close proximity to the binding site. Examination of the binding kinetics of the fluorescent acetylcholine analog dansyl-C6-choline at ionic strengths from 12.5 to 400 mM revealed a twofold decrease in association rate. Debye-Hückel analysis of the kinetics revealed a similar charge distribution as seen by changes in the potentials. To determine whether the experimentally determined potentials are reflected by continuum electrostatics calculations, solutions to the nonlinear Poisson-Boltzmann equation were used to compute the potentials expected from DEFET measurements from high-resolution models of the nAChR and AChE. These calculations are in good agreement with the DEFET measurements for AChE and for the alphagamma-site of the nAChR. We conclude that long-range electrostatic interactions contribute -0.3 and -1 kcal/mol to the binding energy at the nAChR alphadelta- and alphagamma-sites due to an increase in association rates.

Site specificity of agonist-induced opening and desensitization of the Torpedo californica nicotinic acetylcholine receptor

Agonist-binding kinetics to the nicotinic acetylcholine receptor (AChR) from Torpedo californica were measured using sequential-mixing stopped-flow fluorescence methods to determine the contribution of each individual site to agonist-induced opening and desensitization. Timed dansyl-C6-choline (DC6C) binding followed by its dissociation upon mixing with high, competing agonist concentrations revealed four kinetic components: an initial, fast fluorescence decay, followed by a transient increase, and then two characteristic decays that reflect dissociation from the desensitized agonist sites. The transient increase resulted from DC6C binding to the open-channel based on its prevention by proadifen, a noncompetitive antagonist. Further characterization of DC6C channel binding by the inhibition of [3H]phencyclidine binding and by equilibrium measurements of DC6C fluorescence yielded KD values of 2-4 microM for the desensitized AChR and approximately 600 microM for the closed state. At this site, DC6C displayed a strongly blue-shifted emission spectrum, higher intrinsic fluorescence, and weaker energy transfer from tryptophans than when bound to either agonist site. The initial, fast fluorescence decay was assigned to DC6C dissociation from the alphadelta site of the AChR in its closed conformation, on the basis of inhibition with the site-selective antagonists d-tubocurarine and alpha-conotoxin MI. Fast decay amplitude data indicated an apparent affinity of 0.9 microM for the closed-state alphadelta site; the closed-state alphagamma-site affinity is inferred to be near 100 microM. These values and the known affinities for the desensitized conformation show that the alphagamma site drives AChR desensitization to a approximately 40-fold greater extent than the alphadelta site, undergoes energetically larger conformational changes, and is the primary determinant of agonist potency.

Structural basis for epibatidine selectivity at desensitized nicotinic receptors

The agonist binding sites of the fetal muscle nicotinic acetylcholine receptor are formed at the interfaces of alpha-subunits and neighboring gamma- and delta-subunits. When the receptor is in the nonconducting desensitized state, the alpha-gamma site binds the agonist epibatidine 200-fold more tightly than does the alpha-delta site. To determine the structural basis for this selectivity, we constructed gamma/delta-subunit chimeras, coexpressed them with complementary wild-type subunits in HEK 293 cells, and determined epibatidine affinity of the resulting complexes. The results reveal three determinants of epibatidine selectivity: gamma104-117/delta106-delta119, gamma164-171/delta166-177, and gammaPro190/deltaAla196. Point mutations reveal that three sequence differences within the gamma104-117/delta106-delta119 region are determinants of epibatidine selectivity: gammaLys104/deltaTyr106, gammaSer111/deltaTyr113, and gammaTyr117/deltaTyr119. In the delta-subunit, simultaneous mutation of these residues to their gamma equivalent produces high affinity, gamma-like epibatidine binding. However, converting gamma to delta affinity requires replacement of the gamma104-117 segment with delta sequence, suggesting interplay of residues in this region. The structural basis for epibatidine selectivity is explained by computational docking of epibatidine to a homology model of the alpha-gamma binding site.

Coupling of agonist binding to channel gating in an ACh-binding protein linked to an ion channel

Neurotransmitter receptors from the Cys-loop superfamily couple the binding of agonist to the opening of an intrinsic ion pore in the final step in rapid synaptic transmission. Although atomic resolution structural data have recently emerged for individual binding and pore domains, how they are linked into a functional unit remains unknown. Here we identify structural requirements for functionally coupling the two domains by combining acetylcholine (ACh)-binding protein, whose structure was determined at atomic resolution, with the pore domain from the serotonin type-3A (5-HT3A) receptor. Only when amino-acid sequences of three loops in ACh-binding protein are changed to their 5-HT3A counterparts does ACh bind with low affinity characteristic of activatable receptors, and trigger opening of the ion pore. Thus functional coupling requires structural compatibility at the interface of the binding and pore domains. Structural modelling reveals a network of interacting loops between binding and pore domains that mediates this allosteric coupling process.

Nicotine and carbamylcholine binding to nicotinic acetylcholine receptors as studied in AChBP crystal structures

Nicotinic acetylcholine receptors are prototypes for the pharmaceutically important family of pentameric ligand-gated ion channels. Here we present atomic resolution structures of nicotine and carbamylcholine binding to AChBP, a water-soluble homolog of the ligand binding domain of nicotinic receptors and their family members, GABAA, GABAC, 5HT3 serotonin, and glycine receptors. Ligand binding is driven by enthalpy and is accompanied by conformational changes in the ligand binding site. Residues in the binding site contract around the ligand, with the largest movement in the C loop. As expected, the binding is characterized by substantial aromatic and hydrophobic contributions, but additionally there are close contacts between protein oxygens and positively charged groups in the ligands. The higher affinity of nicotine is due to a main chain hydrogen bond with the B loop and a closer packing of the aromatic groups. These structures will be useful tools for the development of new drugs involving nicotinic acetylcholine receptor-associated diseases.

Human calcium-sensing receptor can be suppressed by antisense sequences

We have evaluated the ability of an antisense cDNA sequence, directed to the amino-terminus of the human calcium-sensing receptor (CaR), to reduce the expression and function of an EGFP-tagged CaR (CaR-EGFP) in HEK293 cells. Confocal microscopy and Western blot analysis showed a significant and selective reduction of the expression of CaR-EGFP by the antisense construct. Measurements of changes in intracellular calcium induced by CaR agonists showed that CaR-EGFP function was significantly reduced by the antisense sequence, as was agonist-evoked phosphorylation of extracellular signal-regulated protein kinases (ERK1,2). A sense construct directed to the same region of the receptor had no effect, confirming the specificity of the antisense construct. Our results indicate that a CaR antisense cDNA reduces both the expression and function of the receptor. In the absence of strong, specific pharmacological inhibitors of CaR, the antisense approach will be helpful to elucidate contributions of the CaR to cell physiology.

Acetylcholine binding protein (AChBP): a secreted glial protein that provides a high-resolution model for the extracellular domain of pentameric ligand-gated ion channels

Acetylcholine binding protein (AChBP) has recently been identified from molluskan glial cells. Glial cells secrete it into cholinergic synapses, where it plays a role in modulating synaptic transmission. This novel mechanism resembles glia-dependent modulation of glutamate synapses, with several key differences. AChBP is a homolog of the ligand binding domain of the pentameric ligand-gated ion-channels. The crystal structure of AChBP provides the first high-resolution structure for this family of Cys-loop receptors. Nicotinic acetylcholine receptors and related ion-channels such as GABAA, serotonin 5HT3, and glycine can be interpreted in the light of the 2.7 A AChBP structure. The structural template provides critical details of the binding site and helps create models for toxin binding, mutational effects, and molecular gating.

Properties of the human muscle nicotinic receptor, and of the slow-channel myasthenic syndrome mutant epsilonL221F, inferred from maximum likelihood fits

The mechanisms that underlie activation of nicotinic receptors are investigated using human recombinant receptors, both wild type and receptors that contain the slow channel myasthenic syndrome mutation, epsilonL221F. The method uses the program HJCFIT, which fits the rate constants in a specified mechanism directly to a sequence of observed open and shut times by maximising the likelihood of the sequence with exact correction for missed events. A mechanism with two different binding sites was used. The rate constants that apply to the diliganded receptor (opening, shutting and total dissociation rates) were estimated robustly, being insensitive to the exact assumptions made during fitting, as expected from simulation studies. They are sufficient to predict the main physiological properties of the receptors. The epsilonL221F mutation causes an approximately 4-fold reduction in dissociation rate from diliganded receptors, and a smaller increase in opening rate and mean open time. These are sufficient to explain the approximately 6-fold slowing of decay of miniature synaptic currents seen in patients. The distinction between the two binding sites was less robust, the estimates of rate constants being dependent to some extent on assumptions, e.g. whether an extra short-lived shut state was included or whether the EC50 was constrained. The results suggest that the two binding sites differ by roughly 10-fold in the affinity of the shut receptor for ACh in the wild type, and that in the epsilonL221F mutation the lower affinity is increased so the sites become more similar.

The nicotinic receptor ligand binding domain

The ligand binding domain (LBD) of the nicotinic acetylcholine receptor has served as a prototype for understanding molecular recognition in the family of neurotransmitter-gated ion channels. During the past fifty years, studies progressed from fundamental electrophysiological analyses of ACh-evoked ion flow, to biochemical purification of the receptor protein, pharmacological measurements of ligand binding, molecular cloning of receptor subunits, site-directed mutagenesis combined with functional analysis and recently, atomic structural determination. The emerging picture of the nicotinic receptor LBD is a specialized pocket of aromatic and hydrophobic residues formed at interfaces between protein subunits that changes conformation to convert agonist binding into gating of an intrinsic ion channel.

Contributions of the non-alpha subunit residues (loop D) to agonist binding and channel gating in the muscle nicotinic acetylcholine receptor

The agonist binding site of the nicotinic acetylcholine receptor has a loop-based structure, and is formed by residues located remotely to each other in terms of primary structure. Amino acid residues in sites delta57 and delta59, and the equivalent residues in the epsilon; subunit, have been identified as part of the agonist binding site and designated as loop D. The effects of point mutations in sites delta57, delta59, epsilon;55 and epsilon;57 on agonist binding and channel gating were studied. The mutated receptors were expressed transiently in HEK 293 cells and the currents were recorded using the cell-attached single-channel patch clamp technique. The results demonstrate that the mutations mainly affect channel gating with the major portion of the effect due to a reduction in the channel opening rate constant. For both the delta57/epsilon;55 and the delta59/epsilon;57 site, a mutation in the epsilon; subunit had a greater effect on channel gating than a mutation in the delta subunit. In all instances, agonist binding was affected to a lesser degree than channel gating. Previous data have placed the delta57 and delta59 residues in or near the agonist binding pocket. The data presented here suggest that these two residues (and the homologous sites in the epsilon; subunit) are not involved in specific interactions with the nicotinic agonist and that they affect the activation of the nicotinic receptor by shaping the overall structure of the agonist binding site.

Site-specific fluorescence reveals distinct structural changes with GABA receptor activation and antagonism

Neurotransmitter-operated ion channels, such as the GABA (gamma-aminobutyric acid) receptor, are important in fast synaptic transmission between neurons. Using site-specific fluorescent labeling and simultaneous electrophysiological analysis in Xenopus laevis oocytes expressing recombinant rho1 GABA receptors, we identified agonist-mediated molecular rearrangements at three positions within and near the agonist-binding pocket that were highly correlated with receptor activation. We also show that competitive antagonists induced distinct rearrangements on their own that stabilized the receptor in a closed state. Finally, the allosteric antagonist picrotoxin induced a global conformational change that was sensed in the subunit-subunit interface of the amino (N)-terminal domain, distant from its presumed site of action within the transmembrane domains. This first detection in real time of molecular rearrangements of a ligand-activated receptor provides insights into the structural correlates of activation, antagonism and allosteric modulation.

Mechanism of tacrine block at adult human muscle nicotinic acetylcholine receptors

We used single-channel kinetic analysis to study the inhibitory effects of tacrine on human adult nicotinic receptors (nAChRs) transiently expressed in HEK 293 cells. Single channel recording from cell-attached patches revealed concentration- and voltage-dependent decreases in mean channel open probability produced by tacrine (IC(50) 4.6 microM at -70 mV, 1.6 microM at -150 mV). Two main effects of tacrine were apparent in the open- and closed-time distributions. First, the mean channel open time decreased with increasing tacrine concentration in a voltage-dependent manner, strongly suggesting that tacrine acts as an open-channel blocker. Second, tacrine produced a new class of closings whose duration increased with increasing tacrine concentration. Concentration dependence of closed-times is not predicted by sequential models of channel block, suggesting that tacrine blocks the nAChR by an unusual mechanism. To probe tacrine's mechanism of action we fitted a series of kinetic models to our data using maximum likelihood techniques. Models incorporating two tacrine binding sites in the open receptor channel gave dramatically improved fits to our data compared with the classic sequential model, which contains one site. Improved fits relative to the sequential model were also obtained with schemes incorporating a binding site in the closed channel, but only if it is assumed that the channel cannot gate with tacrine bound. Overall, the best description of our data was obtained with a model that combined two binding sites in the open channel with a single site in the closed state of the receptor.

Residues in the epsilon subunit of the nicotinic acetylcholine receptor interact to confer selectivity of waglerin-1 for the alpha-epsilon subunit interface site

Waglerin-1 (Wtx-1) is a 22-amino acid peptide that competitively antagonizes muscle nicotinic acetylcholine receptors (nAChRs). Previous work demonstrated that Wtx-1 binds to mouse nAChRs with higher affinity than receptors from rats or humans, and distinguished residues in alpha and epsilon subunits that govern the species selectivity. These studies also showed that Wtx-1 binds selectively to the alpha-epsilon binding site with significantly higher affinity than to the alpha-delta binding site. Here we identify residues at equivalent positions in the epsilon, gamma, and delta subunits that govern Wtx-1 selectivity for one of the two binding sites on the nAChR pentamer. Using a series of chimeric and point mutant subunits, we show that residues Gly-57, Asp-59, Tyr-111, Tyr-115, and Asp-173 of the epsilon subunit account predominantly for the 3700-fold higher affinity of the alpha-epsilon site relative to that of the alpha-gamma site. Similarly, we find that residues Lys-34, Gly-57, Asp-59, and Asp-173 account predominantly for the high affinity of the alpha-epsilon site relative to that of the alpha-delta site. Analysis of combinations of point mutations reveals that Asp-173 in the epsilon subunit is required together with the remaining determinants in the epsilon subunit to achieve Wtx-1 selectivity. In particular, Lys-34 interacts with Asp-173 to confer high affinity, resulting in a DeltaDeltaG(INT) of -2.3 kcal/mol in the epsilon subunit and a DeltaDeltaG(INT) of -1.3 kcal/mol in the delta subunit. Asp-173 is part of a nonhomologous insertion not found in the acetylcholine binding protein structure. The key role of this insertion in Wtx-1 selectivity indicates that it is proximal to the ligand binding site. We use the binding and interaction energies for Wtx-1 to generate structural models of the alpha-epsilon, alpha-gamma, and alpha-delta binding sites containing the nonhomologous insertion.

Experimentally based model of a complex between a snake toxin and the alpha 7 nicotinic receptor

To understand how snake neurotoxins interact with nicotinic acetylcholine receptors, we have elaborated an experimentally based model of the alpha-cobratoxin-alpha7 receptor complex. This model was achieved by using (i) a three-dimensional model of the alpha7 extracellular domain derived from the crystallographic structure of the homologous acetylcholine-binding protein, (ii) the previously solved x-ray structure of the toxin, and (iii) nine pairs of residues identified by cycle-mutant experiments to make contacts between the alpha-cobratoxin and alpha7 receptor. Because the receptor loop F occludes entrance of the toxin binding pocket, we submitted this loop to a dynamics simulation and selected a conformation that allowed the toxin to reach its binding site. The three-dimensional structure of the toxin-receptor complex model was validated a posteriori by an additional double-mutant experiment. The model shows that the toxin interacts perpendicularly to the receptor axis, in an equatorial position of the extracellular domain. The tip of the toxin central loop plugs into the receptor between two subunits, just below the functional receptor loop C, the C-terminal tail of the toxin making adjacent additional interactions at the receptor surface. The receptor establishes major contacts with the toxin by its loop C, which is assisted by principal (loops A and B) and complementary (loops D, F, and 1) functional regions. This model explains the antagonistic properties of the toxin toward the neuronal receptor and opens the way to the design of new antagonists.

Mapping the agonist binding site of the nicotinic acetylcholine receptor by cysteine scanning mutagenesis: antagonist footprint and secondary structure prediction

To further define the surface of the Torpedo californica nicotinic acetylcholine receptor (nAChR) contributing to the agonist binding site structure, we used the substituted Cys accessibility method to identify novel residues and determined the "footprint" of residues protected from modification by the reversible competitive antagonist d-tubocurarine (dTC). nAChRs containing single Cys substitutions within regions of the alpha- or gamma-subunit primary structure known to contribute to the agonist binding site were expressed in Xenopus laevis oocytes. Cys substitutions in binding site segments A (alphaTyr-93 and alphaAsn-94), C (alphaTyr-198), and D (gammaGlu-57) had been shown previously to be accessible for modification. We now introduced cysteines from alphaAsp-195 to alphaIle-201 and from gammaAla-106 to gammaAsp-113 and identified positions accessible for modification in segments C (alphaAsp-195, alphaThr-196, alphaPro-197, alphaAsp-200, and alphaIle-201) and E (gammaAsn-107 and gammaLeu-109). dTC protected against alkylation in segments D (gammaGlu-57) and E (gammaLeu-109) but not in segment A (alphaTyr-93 and alphaAsn-94). In segment C, dTC protection experiments revealed a pattern in which every other residue (alpha196, alpha198, and alpha200, but not alpha197 or alpha201) was protected from alkylation. This pattern of protection provides evidence that bound dTC is near amino acids in segments C, D, and E but not in segment A, and identifies a beta-strand surface within segment C contributing to the binding site. These results are discussed in terms of a homology model, based on the molluscan acetylcholine binding protein crystal structure, of the T. californica nAChR agonist binding site.

The binding site of acetylcholine receptor as visualized in the X-Ray structure of a complex between alpha-bungarotoxin and a mimotope peptide

We have determined the crystal structure at 1.8 A resolution of a complex of alpha-bungarotoxin with a high affinity 13-residue peptide that is homologous to the binding region of the alpha subunit of acetylcholine receptor. The peptide fits snugly to the toxin and adopts a beta hairpin conformation. The structures of the bound peptide and the homologous loop of acetylcholine binding protein, a soluble analog of the extracellular domain of acetylcholine receptor, are remarkably similar. Their superposition indicates that the toxin wraps around the receptor binding site loop, and in addition, binds tightly at the interface of two of the receptor subunits where it inserts a finger into the ligand binding site, thus blocking access to the acetylcholine binding site and explaining its strong antagonistic activity.

Acetylcholine receptor delta subunit mutations underlie a fast-channel myasthenic syndrome and arthrogryposis multiplex congenita

Limitation of movement during fetal development may lead to multiple joint contractures in the neonate, termed arthrogryposis multiplex congenita. Neuromuscular disorders are among the many different causes of reduced fetal movement. Many congenital myasthenic syndromes (CMSs) are due to mutations of the adult-specific epsilon subunit of the acetylcholine receptor (AChR), and, thus, functional deficits do not arise until late in gestation. However, an earlier effect on the fetus might be predicted with some defects of other AChR subunits. We studied a child who presented at birth with joint contractures and was subsequently found to have a CMS. Mutational screening revealed heteroallelic mutation within the AChR delta subunit gene, delta 756ins2 and delta E59K. Expression studies demonstrate that delta 756ins2 is a null mutation. By contrast, both fetal and adult AChR containing delta E59K have shorter than normal channel activations that predict fast decay of endplate currents. Thus, delta E59K causes dysfunction of fetal as well as the adult AChR and would explain the presence of joint contractures on the basis of reduced fetal movement. This is the first report of the association of AChR gene mutations with arthrogryposis multiplex congenita. It is probable that mutations that severely disrupt function of fetal AChR will underlie additional cases.

Alternative splicing of a Drosophila GABA receptor subunit gene identifies determinants of agonist potency

Alternative splicing of the Drosophila melanogaster Rdl gene yields four ionotropic GABA receptor subunits. The two Rdl splice variants cloned to date, RDL(ac) and RDL(bd) (DRC17-1-2), differ in their apparent agonist affinity. Here, we report the cloning of a third splice variant of Rdl, RDL(ad). Two-electrode voltage clamp electrophysiology was used to investigate agonist pharmacology of this expressed subunit following cRNA injection into Xenopus laevis oocytes. The EC(so) values for GABA and its analogues isoguvacine, muscimol, isonipecotic acid and 3-amino sulphonic acid on the RDL(ad) homomeric receptor differed from those previously described for RDL(ac) and DRC17-1-2 receptors. In addition to providing a possible physiological role for the alternative splicing of Rdl, these data delineate a hitherto functionally unassigned region of the N-terminal domain of GABA receptor subunits, which affects agonist potency and aligns closely with known determinants of potency in nicotinic acetylcholine receptors. Thus, using expression in Xenopus oocytes, we have demonstrated differences in agonist potency for the neurotransmitter GABA (and four analogues) between splice variant products of the Drosophila melanogaster Rdl gene encoding homomer-forming GABA receptor subunits.

Crystal structure of an ACh-binding protein reveals the ligand-binding domain of nicotinic receptors

Pentameric ligand gated ion-channels, or Cys-loop receptors, mediate rapid chemical transmission of signals. This superfamily of allosteric transmembrane proteins includes the nicotinic acetylcholine (nAChR), serotonin 5-HT3, gamma-aminobutyric-acid (GABAA and GABAC) and glycine receptors. Biochemical and electrophysiological information on the prototypic nAChRs is abundant but structural data at atomic resolution have been missing. Here we present the crystal structure of molluscan acetylcholine-binding protein (AChBP), a structural and functional homologue of the amino-terminal ligand-binding domain of an nAChR alpha-subunit. In the AChBP homopentamer, the protomers have an immunoglobulin-like topology. Ligand-binding sites are located at each of five subunit interfaces and contain residues contributed by biochemically determined 'loops' A to F. The subunit interfaces are highly variable within the ion-channel family, whereas the conserved residues stabilize the protomer fold. This AChBP structure is relevant for the development of drugs against, for example, Alzheimer's disease and nicotine addiction.

Location of the polyamine binding site in the vestibule of the nicotinic acetylcholine receptor ion channel

To map the structure of a ligand-gated ion channel, we used the photolabile polyamine-containing toxin MR44 as photoaffinity label. MR44 binds with high affinity to the nicotinic acetylcholine receptor in its closed channel conformation. The binding stoichiometry was two molecules of MR44 per receptor monomer. Upon UV irradiation of the receptor-ligand complex, (125)I-MR44 was incorporated into the receptor alpha-subunit. From proteolytic mapping studies, we conclude that the site of (125)I-MR44 cross-linking is contained in the sequence alpha His-186 to alpha Leu-199, which is part of the extracellular domain of the receptor. This sequence partially overlaps in its C-terminal region with one of the three loops that form the agonist-binding site. The agonist carbachol and the competitive antagonist alpha-bungarotoxin had only minor influence on the photocross-linking of (125)I-MR44. The site where the hydrophobic head group of (125)I-MR44 binds must therefore be located outside the zone that is sterically influenced by agonist bound at the nicotinic acetylcholine receptor. In binding and photocross-linking experiments, the luminal noncompetitive inhibitors ethidium and triphenylmethylphosphonium were found to compete with (125)I-MR44. We conclude that the polyamine moiety of (125)I-MR44 interacts with the high affinity noncompetitive inhibitor site deep in the channel of the nicotinic acetylcholine receptor, while the aromatic ring of this compound binds in the upper part of the ion channel (i.e. in the vestibule) to a hydrophobic region on the alpha-subunit that is located in close proximity to the agonist binding site. The region of the alpha-subunit labeled by (125)I-MR44 should therefore be accessible from the luminal side of the vestibule.

Contributions of Torpedo nicotinic acetylcholine receptor gamma Trp-55 and delta Trp-57 to agonist and competitive antagonist function

Results of affinity-labeling studies and mutational analyses provide evidence that the agonist binding sites of the nicotinic acetylcholine receptor (nAChR) are located at the alpha-gamma and alpha-delta subunit interfaces. For Torpedo nAChR, photoaffinity-labeling studies with the competitive antagonist d-[(3)H]tubocurarine (dTC) identified two tryptophans, gammaTrp-55 and deltaTrp-57, as the primary sites of photolabeling in the non-alpha subunits. To characterize the importance of gammaTrp-55 and deltaTrp-57 to the interactions of agonists and antagonists, Torpedo nAChRs were expressed in Xenopus oocytes, and equilibrium binding assays and electrophysiological recordings were used to examine the functional consequences when either or both tryptophans were mutated to leucine. Neither substitution altered the equilibrium binding of dTC. However, the deltaW57L and gammaW55L mutations decreased acetylcholine (ACh) binding affinity by 20- and 7,000-fold respectively. For the wild-type, gammaW55L, and deltaW57L nAChRs, the concentration dependence of channel activation was characterized by Hill coefficients of 1.8, 1.1, and 1.7. For the gammaW55L mutant, dTC binding at the alpha-gamma site acts not as a competitive antagonist but as a coactivator or partial agonist. These results establish that interactions with gamma Trp-55 of the Torpedo nAChR play a crucial role in agonist binding and in the agonist-induced conformational changes that lead to channel opening.

Structure-activity relationships in a peptidic alpha7 nicotinic acetylcholine receptor antagonist

alpha-Conotoxins are small disulfide-constrained peptide toxins which act as antagonists at specific subtypes of nicotinic acetylcholine receptors (nACh receptors). In this study, we analyzed the structures and activities of three mutants of alpha-conotoxin ImI, a 12 amino acid peptide active at alpha7 nACh receptors, in order to gain insight into the primary and tertiary structural requirements of neuronal alpha-conotoxin specificity. NMR solution structures were determined for mutants R11E, R7L, and D5N, resulting in representative ensembles of 20 conformers with average pairwise RMSD values of 0.46, 0.52, and 0.62 A from their mean structures, respectively, for the backbone atoms N, C(alpha), and C' of residues 2-11. The R11E mutant was found to have activity near that of wild-type ImI, while R7L and D5N demonstrated activities reduced by at least two orders of magnitude. Comparison of the structures reveals a common two-loop architecture, with variations observed in backbone and side-chain dihedral angles as well as surface electrostatic potentials upon mutation. Correlation of these structures and activities with those from previously published studies emphasizes that existing hypotheses regarding the molecular determinants of alpha-conotoxin specificity are not adequate for explaining peptide activity, and suggests that more subtle features, visualized here at the atomic level, are important for receptor binding. These data, in conjunction with reported characterizations of the acetylcholine binding site, support a model of toxin activity in which a single solvent-accessible toxin side-chain anchors the complex, with supporting weak interactions determining both the efficacy and the subtype specificity of the inhibitory activity.

Alpha-conotoxins

alpha-Conotoxins (alpha-CgTxs) are a family of Cys-enriched peptides found in several marine snails from the genus Conus. These small peptides behave pharmacologically as competitive antagonists of the nicotinic acetylcholine receptor (AChR). The data indicate that (1) alpha-CgTxs are able to discriminate between muscle- and neuronal-type AChRs and even among distinct AChR subtypes; (2) the binding sites for alpha-CgTxs are located, like other cholinergic ligands, at the interface of alpha and non-alpha subunits (gamma, delta, and epsilon for the muscle-type AChR, and beta for several neuronal-type AChRs); (3) some alpha-CgTxs differentiate the high- from the low-affinity binding site found on either alpha/non-alpha subunit interface; and that (4) specific residues in the cholinergic binding site are energetically coupled with their corresponding pairs in the toxin stabilizing the alpha-CgTx-AChR complex. The alpha-CgTxs have proven to be excellent probes for studying the structure and function of the AChR family.

Nicotinic receptors at the amino acid level

nAChRs are pentameric transmembrane proteins into the superfamily of ligand-gated ion channels that includes the 5HT3, glycine, GABAA, and GABAC receptors. Electron microscopy, affinity labeling, and mutagenesis experiments, together with secondary structure predictions and measurements, suggest an all-beta folding of the N-terminal extracellular domain, with the connecting loops contributing to the ACh binding pocket and to the subunit interfaces that mediate the allosteric transitions between conformational states. The ion channel consists of two distinct elements symmetrically organized along the fivefold axis of the molecule: a barrel of five M2 helices, and on the cytoplasmic side five loops contributing to the selectivity filter. The allosteric transitions of the protein underlying the physiological ACh-evoked activation and desensitization possibly involve rigid body motion of the extracellular domain of each subunit, linked to a global reorganization of the transmembrane domain responsible for channel gating.

Localization of agonist and competitive antagonist binding sites on nicotinic acetylcholine receptors

Identification of all residues involved in the recognition and binding of cholinergic ligands (e.g. agonists, competitive antagonists, and noncompetitive agonists) is a primary objective to understand which structural components are related to the physiological function of the nicotinic acetylcholine receptor (AChR). The picture for the localization of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are located mainly on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are identical, the observed high and low affinity for different ligands on the receptor is conditioned by the interaction of the alpha subunit with other non-alpha subunits. This molecular interaction takes place at the interface formed by the different subunits. For example, the high-affinity acetylcholine (ACh) binding site of the muscle-type AChR is located on the alphadelta subunit interface, whereas the low-affinity ACh binding site is located on the alphagamma subunit interface. Regarding homomeric AChRs (e.g. alpha7, alpha8, and alpha9), up to five binding sites may be located on the alphaalpha subunit interfaces. From the point of view of subunit arrangement, the gamma subunit is in between both alpha subunits and the delta subunit follows the alpha aligned in a clockwise manner from the gamma. Although some competitive antagonists such as lophotoxin and alpha-bungarotoxin bind to the same high- and low-affinity sites as ACh, other cholinergic drugs may bind with opposite specificity. For instance, the location of the high- and the low-affinity binding site for curare-related drugs as well as for agonists such as the alkaloid nicotine and the potent analgesic epibatidine (only when the AChR is in the desensitized state) is determined by the alphagamma and the alphadelta subunit interface, respectively. The case of alpha-conotoxins (alpha-CoTxs) is unique since each alpha-CoTx from different species is recognized by a specific AChR type. In addition, the specificity of alpha-CoTxs for each subunit interface is species-dependent. In general terms we may state that both alpha subunits carry the principal component for the agonist/competitive antagonist binding sites, whereas the non-alpha subunits bear the complementary component. Concerning homomeric AChRs, both the principal and the complementary component exist on the alpha subunit. The principal component on the muscle-type AChR involves three loops-forming binding domains (loops A-C). Loop A (from mouse sequence) is mainly formed by residue Y(93), loop B is molded by amino acids W(149), Y(152), and probably G(153), while loop C is shaped by residues Y(190), C(192), C(193), and Y(198). The complementary component corresponding to each non-alpha subunit probably contributes with at least four loops. More specifically, the loops at the gamma subunit are: loop D which is formed by residue K(34), loop E that is designed by W(55) and E(57), loop F which is built by a stretch of amino acids comprising L(109), S(111), C(115), I(116), and Y(117), and finally loop G that is shaped by F(172) and by the negatively-charged amino acids D(174) and E(183). The complementary component on the delta subunit, which corresponds to the high-affinity ACh binding site, is formed by homologous loops. Regarding alpha-neurotoxins, several snake and alpha-CoTxs bear specific residues that are energetically coupled with their corresponding pairs on the AChR binding site. The principal component for snake alpha-neurotoxins is located on the residue sequence alpha1W(184)-D(200), which includes loop C. In addition, amino acid sequence 55-74 from the alpha1 subunit (which includes loop E), and residues gammaL(119) (close to loop F) and gammaE(176) (close to loop G) at the low-affinity binding site, or deltaL(121) (close to the homologous region of loop G) at the high-affinity binding site, are i

Electrostatic interactions regulate desensitization of the nicotinic acetylcholine receptor

To determine the importance of electrostatic interactions for agonist binding to the nicotinic acetylcholine receptor (AChR), we examined the affinity of the fluorescent agonist dansyl-C6-choline for the AChR. Increasing ionic strength decreased the binding affinity in a noncompetitive manner and increased the Hill coefficient of binding. Small cations did not compete directly for dansyl-C6-choline binding. The sensitivity to ionic strength was reduced in the presence of proadifen, a noncompetitive antagonist that desensitizes the receptor. Moreover, at low ionic strength, the dansyl-C6-choline affinities were similar in the absence or presence of proadifen, a result consistent with the receptor being desensitized at low ionic strength. Similar ionic strength effects were observed for the binding of the noncompetitive antagonist [(3)H]ethidium when examined in the presence and absence of agonist to desensitize the AChR. Therefore, ionic strength modulates binding affinity through at least two mechanisms: by influencing the conformation of the AChR and by electrostatic effects at the binding sites. The results show that charge-charge interactions regulate the desensitization of the receptor. Analysis of dansyl-C6-choline binding to the desensitized conformation using the Debye-Hückel equation was consistent with the presence of five to nine negative charges within 20 A of the acetylcholine binding sites.

Pairwise electrostatic interactions between alpha-neurotoxins and gamma, delta, and epsilon subunits of the nicotinic acetylcholine receptor

alpha-Neurotoxins bind with high affinity to alpha-gamma and alpha-delta subunit interfaces of the nicotinic acetylcholine receptor. Since this high affinity complex likely involves a van der Waals surface area of approximately 1200 A(2) and 25-35 residues on the receptor surface, analysis of side chains should delineate major interactions and the orientation of bound alpha-neurotoxin. Three distinct regions on the gamma subunit, defined by Trp(55), Leu(119), Asp(174), and Glu(176), contribute to alpha-toxin affinity. Of six charge reversal mutations on the three loops of Naja mossambica mossambica alpha-toxin, Lys(27) --> Glu, Arg(33) --> Glu, and Arg(36) --> Glu in loop II reduce binding energy substantially, while mutations in loops I and III have little effect. Paired residues were analyzed by thermodynamic mutant cycles to delineate electrostatic linkages between the six alpha-toxin charge reversal mutations and three key residues on the gamma subunit. Large coupling energies were found between Arg(33) at the tip of loop II and gammaLeu(119) (-5.7 kcal/mol) and between Lys(27) and gammaGlu(176) (-5.9 kcal/mol). gammaTrp(55) couples strongly to both Arg(33) and Lys(27), whereas gammaAsp(174) couples minimally to charged alpha-toxin residues. Arg(36), despite strong energetic contributions, does not partner with any gamma subunit residues, perhaps indicating its proximity to the alpha subunit. By analyzing cationic, neutral and anionic residues in the mutant cycles, interactions at gamma176 and gamma119 can be distinguished from those at gamma55.

Chimeric analysis of a neuronal nicotinic acetylcholine receptor reveals amino acids conferring sensitivity to alpha-bungarotoxin

We have investigated the molecular determinants responsible for alpha-bungarotoxin (alphaBgtx) binding to nicotinic acetylcholine receptors through chimeric analysis of two homologous alpha subunits, one highly sensitive to alphaBgtx block (alpha1) and the other, alphaBgtx-insensitive (alpha3). By replacing rat alpha3 residues 184-191 with the corresponding region from the Torpedo alpha1 subunit, we introduced a cluster of five alpha1 residues (Trp-184, Trp-187, Val-188, Tyr-189, and Thr-191) into the alpha3 subunit. Functional activity and alphaBgtx sensitivity were assessed following co-expression in Xenopus oocytes of the chimeric alpha3 subunit (alpha3/alpha1[5]) with either rat beta2 or beta4 subunits. Agonist-evoked responses of alpha3/alpha1[5]-containing receptors were blocked by alphaBgtx with nanomolar affinity (IC(50) values: 41 nM for alpha3/alpha1[5]beta2 and 19 nM for alpha3/alpha1[5]beta4). Furthermore, receptors containing the single point mutation alpha3K189Y acquire significant sensitivity to alphaBgtx block (IC(50) values: 186 nM for alpha3K189Ybeta2 and 179 nM for alpha3K189Ybeta4). Another alpha3 chimeric subunit, alpha3/alpha7[6], similar to alpha3/alpha1[5] but incorporating the corresponding residues from the alphaBgtx-sensitive alpha7 subunit, also conferred potent alphaBgtx sensitivity to chimeric receptors when co-expressed with the beta4 subunit (IC(50) value = 31 nM). Our findings demonstrate that the residues between positions 184 and 191 of the alphaBgtx-sensitive subunits alpha1 and alpha7 play a critical functional role in the interaction of alphaBgtx with nicotinic acetylcholine receptors sensitive to this toxin.

Molecular recognition in nicotinic acetylcholine receptors: the importance of pi-cation interactions

We explore the significance of pi-cation interactions in the binding of ligands to nicotinic acetylcholine receptors. Specifically, the Austin method of semiempirical molecular orbital theory is utilized to estimate the interaction of aromatic amino acid side chains with the cation-containing heterocyclic ring fragments of nicotinic ligands. Variational interaction energies (E(i)) of side chain-ligand fragment pairs are shown to be distance-dependent and follow a Morse-like potential function. The tryptophan side chain shows the most pronounced interaction with the cation fragments, followed by tyrosine and phenylalanine. For a given side chain, cationic fragments exhibit characteristically different E(i) profiles, with the azabicyclo[2.2.1]heptane fragment of the potent nicotinic ligand epibatidine eliciting the greatest interaction energy of the study set. Most significantly, the minimum energy values calculated for numerous fragments correlate with the binding affinity of the parent ligands- we show this to be the case for heteropentameric (alpha4beta2) and homopentameric (alpha7) nicotinic acetylcholine receptor subtypes. Furthermore, intermolecular distances corresponding to the Morse-like potential minimum also correlate with high-affinity binding. A number of parallel calculations were conducted at the Hartree-Fock 6-31G ab initio level of theory in an effort to substantiate these findings.

Molecular investigations on the nicotinic acetylcholine receptor: conformational mapping and dynamic exploration using photoaffinity labeling

The nicotinic acetylcholine receptor (nAChR) is a well-understood member of the ligand-gated ion channels superfamily. The members of this signaling proteins group, including 5HT3, GABA(A), glycine, and ionotropic glutamate receptors, are thought to share common secondary, tertiary, and quaternary structures on the basis of a very high degree of sequence similarity. Despite the absence of X-ray crystallographic data, considerable progress on structural analysis of nAChR was achieved from biochemical, mutational, and electron microscopy data allowing the emergence of a three-dimensional image. Photoaffinity labeling and site-directed mutagenesis gave information on the tertiary structure with respect to the agonist/antagonist binding sites, the ion channel, and its selectivity filter. nAChR is an allosterical protein that undergoes interconversion among several conformational states. Time-resolved photolabeling was used in an attempt to elucidate the structural changes that occur in nAChR on neurotransmitter activation. Tertiary and quaternary rearrangements were found in the cholinergic binding pocket and in the channel lumen, but the structural determinant and the functional link between the binding of agonist and the channel gating remain unknown. Time-resolved photolabeling of the functional activated A state using photosensitive agonists might help in understanding the dynamic process leading to the interconversion of the different states.

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.

Pairwise interactions between neuronal alpha7 acetylcholine receptors and alpha-conotoxin ImI

The present work uses alpha-conotoxin ImI (CTx ImI) to probe the neurotransmitter binding site of neuronal alpha7 acetylcholine receptors. We identify key residues in alpha7 that contribute to CTx ImI affinity, and use mutant cycles analysis to identify pairs of residues that stabilize the receptor-conotoxin complex. We first mutated key residues in the seven known loops of alpha7 that converge at the subunit interface to form the ligand binding site. The mutant subunits were expressed in 293 HEK cells, and CTx ImI binding was measured by competition against the initial rate of 125I-alpha-bungarotoxin binding. The results reveal a predominant contribution by Tyr-195 in alpha7, accompanied by smaller contributions by Thr-77, Tyr-93, Asn-111, Gln-117, and Trp-149. Based upon our previous identification of bioactive residues in CTx ImI, we measured binding of receptor and toxin mutations and analyzed the results using thermodynamic mutant cycles. The results reveal a single dominant interaction between Arg-7 of CTx ImI and Tyr-195 of alpha7 that anchors the toxin to the binding site. We also find multiple weak interactions between Asp-5 of CTx ImI and Trp-149, Tyr-151, and Gly-153 of alpha7, and between Trp-10 of CTx ImI and Thr-77 and Asn-111 of alpha7. The overall results establish the orientation of CTx ImI as it bridges the subunit interface and demonstrate close approach of residues on opposing faces of the alpha7 binding site.

Structure of the agonist-binding sites of the Torpedo nicotinic acetylcholine receptor: affinity-labeling and mutational analyses identify gamma Tyr-111/delta Arg-113 as antagonist affinity determinants

Photoaffinity labeling of the Torpedo nicotinic acetylcholine receptor (nAChR) with [3H]d-tubocurarine (dTC) has identified a residue within the gamma-subunit which, along with the analogous residue in delta-subunit, confers selectivity in binding affinities between the two agonist sites for dTC and alpha-conotoxin (alpha Ctx) MI. nAChR gamma-subunit, isolated from nAChR-rich membranes photolabeled with [3H]dTC, was digested with Staphylococcus aureus V8 protease, and a 3H-labeled fragment was purified by reversed-phase high-performance liquid chromatography. Amino-terminal sequence analysis of this fragment identified 3H incorporation in gamma Tyr-111 and gamma Tyr-117 at about 5% and 1% of the efficiency of [3H]dTC photoincorporation at gamma Trp-55, the primary site of [3H]dTC photoincorporation within gamma-subunit [Chiara, D. C., and Cohen, J. B. (1997) J. Biol. Chem 272, 32940-32950]. The Torpedo nAChR delta-subunit residue corresponding to gamma Tyr-111 (delta Arg-113) contains a positive charge which could confer the lower binding affinity seen for some competitive antagonists at the alpha-delta agonist site. To test this hypothesis, we examined by voltage-clamp analysis and/or by [125I]alpha-bungarotoxin competition binding assays the interactions of acetylcholine (ACh), dTC, and alpha Ctx MI with nAChRs containing gamma Y111R or delta R113Y mutant subunits expressed in Xenopus oocytes. While these mutations affected neither ACh equilibrium binding affinity nor the concentration dependence of channel activation, the gamma Y111R mutation decreased by 10-fold dTC affinity and inhibition potency. Additionally, each mutation conferred a 1000-fold change in the equilibrium binding of alpha Ctx MI, with delta R113Y enhancing and gamma Y111R weakening affinity. Comparison of these results with previous results for mouse nAChR reveals that, while the same regions of gamma- (or delta-) subunit primary structure contribute to the agonist-binding sites, the particular amino acids that serve as antagonist affinity determinants are species-dependent.