Contribution of the beta subunit M2 segment to the ion-conducting pathway of the acetylcholine receptor

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

Barbiturates require the N terminus and first transmembrane domain of the delta subunit for enhancement of alpha1beta3delta GABAA receptor currents

GABA(A) receptors are composed predominantly of alphabetagamma receptors, which mediate primarily synaptic inhibition, and alphabetadelta receptors, which mediate primarily extrasynaptic inhibition. At saturating GABA concentrations, the barbiturate pentobarbital substantially increased the amplitude and desensitization of the alpha1beta3delta receptor but not the alpha1beta3gamma2L receptor currents. To explore the structural domains of the delta subunit that are involved in pentobarbital potentiation and increased desensitization of alpha1beta3delta currents, chimeric cDNAs were constructed by progressive replacement of gamma2L subunit sequence with a delta subunit sequence or a delta subunit sequence with a gamma2L subunit sequence, and HEK293T cells were co-transfected with alpha1 and beta3 subunits or alpha1 and beta3 subunits and a gamma2L, delta, or chimeric subunit. Currents evoked by a saturating concentration of GABA or by co-application of GABA and pentobarbital were recorded using the patch clamp technique. By comparing the extent of enhancement and changes in kinetic properties produced by pentobarbital among chimeric and wild type receptors, we concluded that although potentiation of alpha1beta3delta currents by pentobarbital required the delta subunit sequence from the N terminus to proline 241 in the first transmembrane domain (M1), increasing desensitization of alpha1beta3delta currents required a delta subunit sequence from the N terminus to isoleucine 235 in M1. These findings suggest that the delta subunit N terminus and N-terminal portion of the M1 domain are, at least in part, involved in transduction of the allosteric effect of pentobarbital to enhance alpha1beta3delta currents and that this effect involves a distinct but overlapping structural domain from that involved in altering desensitization.

Single-molecule imaging of a fluorescent unnatural amino acid incorporated into nicotinic receptors

We report on the first, to our knowledge, successful detection of a fluorescent unnatural amino acid (fUAA), Lys(BODIPYFL), incorporated into a membrane protein (the muscle nicotinic acetylcholine receptor, nAChR) in a living cell. Xenopus oocytes were injected with a frameshift-suppressor tRNA, amino-acylated with Lys(BODIPYFL) and nAChR (alpha/beta19'GGGU/gamma/delta) mRNAs. We measured fluorescence from oocytes expressing nAChR beta19'Lys(BODIPYFL), using time-resolved total internal reflection fluorescence microscopy. Under conditions of relatively low receptor density (<0.1 receptors/microm(2)), we observed puncta with diffraction-limited profiles that were consistent with the point-spread function of our microscope. Furthermore, diffraction-limited puncta displayed step decreases in fluorescence intensity, consistent with single-molecule photobleaching. The puncta densities agreed with macroscopic ACh-induced current densities, showing that the fUAA was incorporated, and that receptors were functional. Dose-response relations for the nAChR beta19'Lys(BODIPYFL) receptors were similar to those for wild-type receptors. We also studied nAChR beta19'Lys(BODIPYFL) receptors labeled with alpha-bungarotoxin monoconjugated with Alexa488 (alphaBtxAlexa488). The nAChR has two alphaBtx binding sites, and puncta containing the Lys(BODIPYFL) labeled with alphaBtxAlexa488 yielded the expected three discrete photobleaching steps. We also performed positive control experiments with a nAChR containing enhanced green fluorescent protein in the gamma-subunit M3-M4 loop, which confirmed our nAChR beta19'Lys(BODIPYFL) measurements. Thus, we report on the cell-based single-molecule detection of nAChR beta19'Lys(BODIPYFL).

Structural and dynamic studies of the γ-M4 trans-membrane domain of the nicotinic acetylcholine receptor

A structural characterization of a synthetic peptide corresponding to the fourth transmembrane domain (M4-TMD) of the gamma-subunit of the nicotinic acetylcholine receptor from Torpedo californica has been undertaken. Solid-state NMR and CD spectroscopy studies indicate that upon reconstitution into lipid vesicles or magnetically aligned lipid bilayers, the synthetic M4-TMD adopts a linear alpha-helical conformation with the helix aligned within 15 degrees of the membrane normal. Furthermore, analysis of the motional averaging of anisotropic interactions present in the solid-state NMR spectra of the reconstituted peptide, indicate that the dynamics of the peptide within the bilayer are highly sensitive to the phase adopted by the lipid bilayer, providing an insight into how the interaction of lipids with this domain may play a important role in the modulation of this receptor by its lipid environment.

Spontaneous conformational change and toxin binding in alpha7 acetylcholine receptor: insight into channel activation and inhibition

Nicotinic AChRs (nAChRs) represent a paradigm for ligand-gated ion channels. Despite intensive studies over many years, our understanding of the mechanisms of activation and inhibition for nAChRs is still incomplete. Here, we present molecular dynamics (MD) simulations of the alpha7 nAChR ligand-binding domain, both in apo form and in alpha-Cobratoxin-bound form, starting from the respective homology models built on crystal structures of the acetylcholine-binding protein. The toxin-bound form was relatively stable, and its structure was validated by calculating mutational effects on the toxin-binding affinity. However, in the apo form, one subunit spontaneously moved away from the conformation of the other four subunits. This motion resembles what has been proposed for leading to channel opening. At the top, the C loop and the adjacent beta7-beta8 loop swing downward and inward, whereas at the bottom, the F loop and the C terminus of beta10 swing in the opposite direction. These swings appear to tilt the whole subunit clockwise. The resulting changes in solvent accessibility show strong correlation with experimental results by the substituted cysteine accessibility method upon addition of acetylcholine. Our MD simulation results suggest a mechanistic model in which the apo form, although predominantly sampling the "closed" state, can make excursions into the "open" state. The open state has high affinity for agonists, leading to channel activation, whereas the closed state upon distortion has high affinity for antagonists, leading to inhibition.

A stepwise mechanism for acetylcholine receptor channel gating

Muscle contraction is triggered by the opening of acetylcholine receptors at the vertebrate nerve-muscle synapse. The M2 helix of this allosteric membrane protein lines the channel, and contains a 'gate' that regulates the flow of ions through the pore. We used single-molecule kinetic analysis to probe the transition state of the gating conformational change and estimate the relative timing of M2 motions in the alpha-subunit of the murine acetylcholine receptor. This analysis produces a 'Phi-value' for a given residue that reflects its open-like versus closed-like character at the transition state. Here we show that most of the residues throughout the length of M2 have a Phi-value of approximately 0.64 but that some near the middle have lower Phi-values of 0.52 or 0.31, suggesting that alphaM2 moves in three discrete steps. The core of the channel serves both as a gate that regulates ion flow and as a hub that directs the propagation of the gating isomerization through the membrane domain of the acetylcholine receptor.

The location of a closed channel gate in the GABAA receptor channel

Considerable controversy surrounds the location of the closed channel gate in members of the Cys-loop receptor family of neurotransmitter-gated ion channels that includes the GABA_A, glycine, acetylcholine, and 5-HT₃ receptors. Cysteine-accessibility studies concluded that the gate is near the cytoplasmic end of the channel in acetylcholine and GABA_A receptors but in the middle of the 5-HT_3A receptor channel. Zn²⁺ accessibility studies in a chimeric 5-HT₃-ACh receptor suggested the gate is near the channel's cytoplasmic end. In the 4-Å resolution structure of the acetylcholine receptor closed state determined by cryoelectron microscopy, the narrowest region, inferred to be the gate, is in the channel's midsection from 9' to 14' but the M1–M2 loop residues at the channel's cytoplasmic end were not resolved in that structure. We used blocker trapping experiments with picrotoxin, a GABA_A receptor open channel blocker, to determine whether a gate exists at a position more extracellular than the picrotoxin binding site, which is in the vicinity of α₁Val257 (2') near the channel's cytoplasmic end. We show that picrotoxin can be trapped in the channel after removal of GABA. By using the state-dependent accessibility of engineered cysteines as reporters for the channel's structural state we infer that after GABA washout, with picrotoxin trapped in the channel, the channel appears to be in the closed state. We infer that a gate exists between the picrotoxin binding site and the channel's extracellular end, consistent with a closed channel gate in the middle of the channel. Given the homology with acetylcholine and 5-HT₃ receptors there is probably a similar gate in those channels as well. This does not preclude the existence of an additional gate at a more cytoplasmic location.

Modulation of proton-induced current fluctuations in the human nicotinic acetylcholine receptor channel

The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel that switches upon activation from a closed state to a full conducting state. We found that the mutation delta S268K, located at 12' position of the second transmembrane domain of the delta subunit of the human nAChR generates a long-lived intermediate conducting state, from which openings to a wild-type like conductance level occur on a submillisecond time scale. Aiming to understand the interplay between structural changes near the 12' position and channel gating, we investigated the influence of various parameters: different ligands (acetylcholine, choline and epibatidine), ligand concentrations, transmembrane voltages and both fetal and adult nAChRs. Since sojourns in the high conductance state are not fully resolved in time, spectral noise analysis was used as a complement to dwell time analysis to determine the gating rate constants. Open channel current fluctuations are described by a two-state Markov model. The characteristic time of the process is markedly influenced by the ligand and the receptor type, whereas the frequency of openings to the high conductance state increases with membrane hyperpolarization. Conductance changes are discussed with regard to reversible transfer reaction of single protons at the lysine 12' side chain.

Functional and Structural Analysis of the GABAA Receptor α1 Subunit during Channel Gating and Alcohol Modulation

The substituted cysteine accessibility method has proven useful for investigating structural changes of the gamma-aminobutyric acid type A (GABA(A)) receptor during channel gating and allosteric modulation. In the present study, the surface accessibility and reaction rate of propyl- and hexyl-methanethiosulfonate to cysteine residues introduced into the third transmembrane segment of the GABA(A) receptor alpha(1) subunit were examined. GABA-induced currents in Xenopus oocytes expressing wild type and cysteine mutant GABA(A) receptors were recorded before and after application of methanethiosulfonate (MTS) reagents in the resting, GABA- or alcohol-bound (ethanol or hexanol) states. Our results indicate that a water-filled cavity exists around the Ala(291) and Tyr(294) residues of the third transmembrane segment, in agreement with previous results. Furthermore, our data indicate that a conformational change produced by alcohols (200 mM ethanol or 0.5 mM hexanol) exposure induces the water cavity around the A291C and Y294C residues to extend deeper, causing the A295C and F296C residues to become accessible to the MTS reagents. In addition, exposure of the A291C, Y294C, F296C, and V297C mutants to MTS reagents in the presence of GABA had significant effects on their GABA-induced currents, indicating that the water cavity around A291C and Y294C residues expanded to F296C and V297C by a structural movement caused by GABA binding. Our data show that GABA(A) receptor is a dynamic protein during alcohol modulation and channel gating.

Channel Gating of the Glycine Receptor Changes Accessibility to Residues Implicated in Receptor Potentiation by Alcohols and Anesthetics

The glycine receptor is a target for both alcohols and anesthetics, and certain amino acids in the alpha1 subunit transmembrane segments (TM) are critical for drug effects. Introducing larger amino acids at these positions increases the potency of glycine, suggesting that introducing larger residues, or drug molecules, into the drug-binding cavity facilitates channel opening. A possible mechanism for these actions is that the volume of the cavity expands and contracts during channel opening and closing. To investigate this hypothesis, mutations for amino acids in TM1 (I229C) and TM2 (G256C, T259C, V260C, M263C, T264C, S267C, S270C) and TM3 (A288C) were individually expressed in Xenopus laevis oocytes. The ability of sulfhydryl-specific alkyl methanethiosulfonate (MTS) compounds of different lengths to covalently react with introduced cysteines in both the closed and open states of the receptor was determined. S267C was accessible to short chain (C3-C8) MTS in both open and closed states, but was only accessible to longer chain (C10-C16) MTS compounds in the open state. Reaction with S267C was faster in the open state. I229C and A288C showed state-dependent reaction with MTS only in the presence of agonist. M263C and S270C were also accessible to MTS labeling. Mutated residues more intracellular than M263C did not react, indicating a floor of the cavity. These data demonstrate that the conformational changes accompanying channel gating increase accessibility to amino acids critical for drug action in TM1, TM2, and TM3, which may provide a mechanism by which alcohols and anesthetics can act on glycine (and likely other) receptors.

A fluorophore attached to nicotinic acetylcholine receptor beta M2 detects productive binding of agonist to the alpha delta site

To study conformational transitions at the muscle nicotinic acetylcholine (ACh) receptor (nAChR), a rhodamine fluorophore was tethered to a Cys side chain introduced at the beta 19' position in the M2 region of the nAChR expressed in Xenopus oocytes. This procedure led to only minor changes in receptor function. During agonist application, fluorescence increased by (Delta F/F) approximately 10%, and the emission peak shifted to lower wavelengths, indicating a more hydrophobic environment for the fluorophore. The dose-response relations for Delta F agreed well with those for epibatidine-induced currents, but were shifted approximately 100-fold to the left of those for ACh-induced currents. Because (i) epibatidine binds more tightly to the alpha gamma-binding site than to the alpha delta site and (ii) ACh binds with reverse-site selectivity, these data suggest that Delta F monitors an event linked to binding specifically at the alpha delta-subunit interface. In experiments with flash-applied agonists, the earliest detectable Delta F occurs within milliseconds, i.e., during activation. At low [ACh] (< or = 10 microM), a phase of Delta F occurs with the same time constant as desensitization, presumably monitoring an increased population of agonist-bound receptors. However, recovery from Delta F is complete before the slowest phase of recovery from desensitization (time constant approximately 250 s), showing that one or more desensitized states have fluorescence like that of the resting channel. That conformational transitions at the alpha delta-binding site are not tightly coupled to channel activation suggests that sequential rather than fully concerted transitions occur during receptor gating. Thus, time-resolved fluorescence changes provide a powerful probe of nAChR conformational changes.

Loose protein packing around the extracellular half of the GABA(A) receptor beta1 subunit M2 channel-lining segment

GABA(A) receptors are ligand-gated ion channels formed by the pseudosymmetrical assembly of five homologous subunits around the central channel axis. The five M2 membrane-spanning segments largely line the channel. In the present work we probed the water surface accessibility of the beta(1) subunit M2 segment using the substituted cysteine accessibility method. We assayed the reaction of the negatively charged sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS(-)), by its effect on subsequent currents elicited by EC(50) and saturating GABA concentrations. pCMBS(-), applied with GABA, reacted with 14 of the 19 residues tested. At the M2 cytoplasmic end from 2' to 6' only beta(1)A252C (2') and beta(1)T256C (6') were pCMBS(-)-reactive in the presence of GABA. We infer that the M2 segments are tightly packed in this region. Toward the extracellular half of M2 all residues from beta(1)T262C (12') through beta(1)E270C (20') reacted with pCMBS(-) applied with GABA. We infer that this region is highly mobile and loosely packed against the rest of the protein. Based on differences in pCMBS(-) reaction rates two domains can be distinguished on the putative channel-lining side of M2. A faster reacting domain includes the 2', 9', 12', 13', and 16' residues. The slower reacting face contains the 6', 10', and 14' residues. We hypothesize that these may represent the channel-lining faces in the closed and open states and that gating involves an 80-100 degrees rotation of the M2 segments. These results are consistent with the loose packing of the M2 segments inferred from the structure of the homologous Torpedo nicotinic acetylcholine receptor.

Nicotinic acetylcholine receptors: from structure to brain function

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system where they are involved in fast synaptic transmission. nAChRs are pentameric structures that are made up of combinations of individual subunits. Twelve neuronal nAChR subunits have been described, alpha2-alpha10 and beta2-beta4; these are differentially expressed throughout the nervous system and combine to form nAChRs with a wide range of physiological and pharmacological profiles. The nAChR has been proposed as a model of an allosteric protein in which effects arising from the binding of a ligand to a site on the protein can lead to changes in another part of the molecule. A great deal is known about the structure of the pentameric receptor. The extracellular domain contains binding sites for numerous ligands, which alter receptor behavior through allosteric mechanisms. Functional studies have revealed that nAChRs contribute to the control of resting membrane potential, modulation of synaptic transmission and mediation of fast excitatory transmission. To date, ten genes have been identified in the human genome coding for the nAChRs. nAChRs have been demonstrated to be involved in cognitive processes such as learning and memory and control of movement in normal subjects. Recent data from knockout animals has extended the understanding of nAChR function. Dysfunction of nAChR has been linked to a number of human diseases such as schizophrenia, Alzheimer's and Parkinson's diseases. nAChRs also play a significant role in nicotine addiction, which is a major public health concern. A genetically transmissible epilepsy, ADNFLE, has been associated with specific mutations in the gene coding for the alpha4 or beta2 subunits, which leads to altered receptor properties.

Structure and gating mechanism of the acetylcholine receptor pore

The nicotinic acetylcholine receptor controls electrical signalling between nerve and muscle cells by opening and closing a gated, membrane-spanning pore. Here we present an atomic model of the closed pore, obtained by electron microscopy of crystalline postsynaptic membranes. The pore is shaped by an inner ring of 5 alpha-helices, which curve radially to create a tapering path for the ions, and an outer ring of 15 alpha-helices, which coil around each other and shield the inner ring from the lipids. The gate is a constricting hydrophobic girdle at the middle of the lipid bilayer, formed by weak interactions between neighbouring inner helices. When acetylcholine enters the ligand-binding domain, it triggers rotations of the protein chains on opposite sides of the entrance to the pore. These rotations are communicated through the inner helices, and open the pore by breaking the girdle apart.

Structural effects of quinacrine binding in the open channel of the acetylcholine receptor

Noncompetitive inhibitors of the nicotinic acetylcholine (ACh) receptors suppress cation flux directly by binding in and blocking the open channel or indirectly by stabilizing closed states of the receptor. The lidocaine derivative QX-314 and the acridine derivative quinacrine act directly as open channel blockers, but can act indirectly as well. The binding site for quinacrine in the open channel of mouse-muscle ACh receptor was mapped in cysteine-substituted mutants of the alpha subunit expressed with wild-type beta, gamma, and delta subunits. In the open state, substituted cysteines in the inner half of the second membrane-spanning segment (M2), but not in the outer half, were protected by quinacrine from reaction with 2-aminoethyl methanethiosulfonate. In addition, an alkylating derivative, quinacrine mustard, affinity labeled a subset of the substituted cysteines in M2, but only in the open state. These results, mapped onto a model of the open channel surrounded by five alpha-helical M2s, imply that quinacrine binds midway down M2 in the same site previously mapped for QX-314. A cysteine substituted for a residue in the outer third of alphaM1, which reacted with 2-aminoethyl methanethiosulfonate only in the presence of ACh, reacted faster in the additional presence of quinacrine or QX-314. It is proposed that channel opening involves both the opening of the resting gate at the inner end of M2 and the removal of an obstruction formed by the outer end of M1 that retards diffusion of blockers into the closed channel. Blocker binding in the open channel causes a further change in structure.

A photochemical approach to the lipid accessibility of engineered cysteinyl residues

Ordinary electrophilic reagents react too slowly in a nonpolar environment to be useful for the determination of the accessibility to lipid of continuous stretches of residues mutated to cysteine. By contrast, photoactivated 5-iodonaphthyl-1-azide (INA) reacted readily with 2-mercaptoethanol and dodecanethiol in nonpolar solvents and in liposomes. Continuous stretches of residues in the amphipathic N-terminal helix and first transmembrane helix of the bacterial potassium channel KcsA were replaced with cysteine, and the mutants were expressed in Escherichia coli and isolated in inner membranes. These membranes were dissolved in detergent and reconstituted into asolectin liposomes incorporating INA. The extent of light-induced reaction of INA with each cysteine was assayed by subsequent reaction with the gel-shifting, SH-specific methoxy-polyethylene glycol-2-pyridine disulfide. The pattern of apparent second-order rate constants for the photoreactions of eight substituted cysteines in the N-terminal helix conformed to other measures of lipid exposure. The pattern of the rate constants for the photoreactions of 15 cysteines in the first transmembrane helix had peaks every third residue, which partly conformed to other measures of lipid exposure.

Comparative surface accessibility of a pore-lining threonine residue (T6') in the glycine and GABA(A) receptors

The substituted cysteine accessibility method was used to probe the surface exposure of a pore-lining threonine residue (T6') common to both the glycine receptor (GlyR) and gamma-aminobutyric acid, type A receptor (GABA(A)R) chloride channels. This residue lies close to the channel activation gate, the ionic selectivity filter, and the main pore blocker binding site. Despite their high amino acid sequence homologies and common role in conducting chloride ions, recent studies have suggested that the GlyRs and GABA(A)Rs have divergent open state pore structures at the 6' position. When both the human alpha1(T6'C) homomeric GlyR and the rat alpha1(T6'C)beta1(T6'C) heteromeric GABA(A)R were expressed in human embryonic kidney 293 cells, their 6' residue surface accessibilities differed significantly in the closed state. However, when a soluble cysteine-modifying compound was applied in the presence of saturating agonist concentrations, both receptors were locked into the open state. This action was not induced by oxidizing agents in either receptor. These results provide evidence for a conserved pore opening mechanism in anion-selective members of the ligand-gated ion channel family. The results also indicate that the GABA(A)R pore structure at the 6' level may vary between different expression systems.

GABA(A) receptor M2-M3 loop secondary structure and changes in accessibility during channel gating

The gamma-aminobutyric acid type A (GABA(A)) receptor M2-M3 loop structure and its role in gating were investigated using the substituted cysteine accessibility method. Residues from alpha(1)Arg-273 to alpha(1)Ile-289 were mutated to cysteine, one at a time. MTSET(+) or MTSES(-) reacted with all mutants from alpha(1)R273C to alpha(1)Y281C, except alpha(1)P277C, in the absence and presence of GABA. The MTSET(+) closed-state reaction rate was >1000 liters/mol-s at alpha(1)N274C, alpha(1)S275C, alpha(1)K278C, and alpha(1)Y281C and was <300 liters/mol-s at alpha(1)R273C, alpha(1)L276C, alpha(1)V279C, alpha(1)A280C, and alpha(1)A284C. These two groups of residues lie on opposite sides of an alpha-helix. The fast reacting group lies on a continuation of the M2 segment channel-lining helix face. This suggests that the M2 segment alpha-helix extends about two helical turns beyond alpha(1)N274 (20'), aligned with the extracellular ring of charge. At alpha(1)S275C, alpha(1)V279C, alpha(1)A280C, and alpha(1)A284C the reaction rate was faster in the presence of GABA. The reagents had no functional effect on the mutants from alpha(1)A282C to alpha(1)I289C, except alpha(1)A284C. Access may be sterically hindered possibly by close interaction with the extracellular domain. We suggest that the M2 segment alpha-helix extends beyond the predicted extracellular end of the M2 segment and that gating induces a conformational change in and/or around the N-terminal half of the M2-M3 loop. Implications for coupling ligand-evoked conformational changes in the extracellular domain to channel gating in the membrane-spanning domain are discussed.

First and second transmembrane segments of alpha3, alpha4, beta2, and beta4 nicotinic acetylcholine receptor subunits influence the efficacy and potency of nicotine

The first three transmembrane segments (M1-M3) of human nicotinic acetylcholine receptors (nAChRs) have been implicated in determining the efficacy of nicotine by studies of alpha3/alpha4 subunit chimeras. Nicotine has full efficacy on the alpha4beta2 nAChR and partial efficacy on the alpha3beta2 nAChR. Now, we have exchanged individually three amino acids between the alpha4 and the alpha3 subunits at positions 226(M1), 258(M2), and 262(M2). Also, similar exchanges were made in the beta2 and beta4 subunits at positions 224(M1), 226(M1), and 254(M2) (using alpha subunit numbering). Expression of these mutated nAChRs in Xenopus laevis oocytes showed that the mutated M1 amino acids were important in influencing the potency of ACh and nicotine. It is hypothesized that these M1 amino acids affect the stability between the resting and activated states of the nAChR. M2 amino acids altered the efficacy of nicotine, usually without altering its potency. When the residue located at position 258 in the M2 region of the alpha subunit was valine (as in the alpha3 subunit), the resulting nAChR exhibited partial efficacy for nicotine that was voltage-dependent. Therefore, we believe that these M2 amino acids contribute to the formation of a binding site for nicotine in the alpha3beta2 nAChR channel, which results in a low-affinity channel block, causing the lower efficacy of nicotine on this nAChR.

Evidence for a centrally located gate in the pore of a serotonin-gated ion channel

Serotonin-gated ion channels (5-HT3) are members of the ligand-gated channel family, which includes channels that are opened directly by the neurotransmitter acetylcholine, GABA, glycine, or glutamate. Although there is general agreement that the second transmembrane domain (M2) lines the pore, the position of the gate in the M2 is less certain. Here, we used substituted cysteine accessibility method (SCAM) to provide new evidence for a centrally located gate that moves during channel activation. In the closed state, three cysteine substitutions, located on the extracellular side of M2, were modified by methanethiosulfonate (MTS) reagents. In contrast, 13 cysteine substitutions were modified in the open state with MTS reagents. The pattern of inhibition (every three to four substitutions) was consistent with an alpha helical structure for the middle and cytoplasmic segments of the M2 transmembrane domain. Unexpectedly, open-state modification of two amino acids in the center of M2 with three different MTS reagents prevented channels from fully closing in the absence of neurotransmitter. Our results are consistent with a model in which the central region of the M2 transmembrane domain is inaccessible in the closed state and moves during channel activation.

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.

Emerging structure of the nicotinic acetylcholine receptors

The conversion of acetylcholine binding into ion conduction across the membrane is becoming more clearly understood in terms of the structure of the receptor and its transitions. A high-resolution structure of a protein that is homologous to the extracellular domain of the receptor has revealed the binding sites and subunit interfaces in great detail. Although the structures of the membrane and cytoplasmic domains are less well determined, the channel lining and the determinants of selectivity have been mapped. The location and structure of the gates, and the coupling between binding sites and gates, remain to be established.

Structural and electrostatic properties of the 5-HT3 receptor pore revealed by substituted cysteine accessibility mutagenesis

5-HT(3) receptors are members of the Cys loop family of ligand-gated ion channels. We used the substituted cysteine accessibility method to identify amino acid residues in the channel forming domain, M2 that face the water-accessible surface and to locate their position in the ion conduction pathway. Cysteine was substituted for each residue, one at a time, in the M2 segment (Asp(274)-Asp(298)). 5-Hydroxytryptamine EC(50) values for functional mutants did not vary from wild type (1.4 +/- 0.2 microm) by more than 10-fold, and five mutants were nonfunctional. Covalent modification of the mutant receptors with sulfydryl reagents revealed 11 residues to be water-accessible, with a pattern consistent with an alpha-helix except at Leu(285) and Leu(293). The data suggest that charge selectivity begins at a more cytoplasmic level than Val(291). Modification at some positions (Val(291), Leu(293), Ile(294), Leu(287), and Ser(280)) resulted in channels that were locked open. Reaction rates with accessible cysteines were voltage-dependent at some residues, suggesting that access occurs via the ion channel. Overall the data observed are similar but not identical to that reported for other members of the family and confirms the high degree of structural and functional homology between receptors in the Cys loop receptor family.

Structure of ligand-gated ion channels: critical assessment of biochemical data supports novel topology

Rapid signaling across the synaptic junction is partially mediated by the ligand-gated ion channel superfamily (LGICS), which includes inhibitory glycine and GABA receptors and excitatory acetylcholine and serotonin receptors. The glycine receptor (GlyR) can assemble as homopentamers of alpha subunits, and baculovirus expression systems are capable of overexpressing large quantities of active receptors. Limited proteolysis coupled to mass spectrometry on reconstituted alpha1 GlyR homopentamers identified proteolytic cleavages within proposed transmembrane domains postulated to fold as bilayer-spanning alpha helices in the "classical" model and identified unexpected membrane-associated regions in the N-terminal domain (J. F. Leite et al., 2000, J. Biol. Chem. 275, 13683-13689). In this review, optimized sequence alignments were used to integrate these proteolysis data with biochemical information determined in studies of all the LGICS members in order to construct a novel topological model.

Incorporation of caged cysteine and caged tyrosine into a transmembrane segment of the nicotinic ACh receptor

The nonsense codon suppression technique was used to incorporate o-nitrobenzyl cysteine or o-nitrobenzyl tyrosine (caged Cys or Tyr) into the 9' position of the M2 transmembrane segment of the gamma-subunit of the muscle nicotinic ACh receptor expressed in Xenopus oocytes. The caged amino acids replaced an endogenous Leu residue that has been implicated in channel gating. ACh-induced current increased substantially after ultraviolet (UV) irradiation to remove the caging group. This represents the first successful incorporation of caged Cys into a protein in vivo and the first incorporation of caged amino acids within a transmembrane segment of a membrane protein. The bulky nitrobenzyl group does not prevent the synthesis, assembly, or trafficking of the ACh receptor. When side chains were decaged using 1-ms UV light flashes, the channels with caged Cys or caged Tyr responded with strikingly different kinetics. The increase in current upon photolysis of caged Cys was too rapid for resolution by the voltage-clamp circuit [time constant (tau) <10 ms], whereas the increase in current upon photolysis of caged Tyr was dominated by a phase with tau approximately 500 ms. Apparently, the presence of a bulky o-nitrobenzyl Tyr residue distorts the receptor into an abnormal conformation. Upon release of the caging group, the receptor relaxes, with tau approximately 500 ms, into a conformation that allows the channel to open. Tyr at the 9' position of the gamma-subunit greatly increases the ability of ACh to block the channel by binding within the channel pore. This is manifested in two ways. 1) A "rebound," or increase in current, occurs upon removal of ACh from the bathing medium; and 2) at ACh concentrations >400 microM, inward currents are decreased through the mutated channel. The ability to incorporate caged amino acids into proteins should have widespread utility.

The surface accessibility of the glycine receptor M2-M3 loop is increased in the channel open state

Mutations in the extracellular M2-M3 loop of the glycine receptor (GlyR) alpha1 subunit have been shown previously to affect channel gating. In this study, the substituted cysteine accessibility method was used to investigate whether a structural rearrangement of the M2-M3 loop accompanies GlyR activation. All residues from R271C to V277C were covalently modified by both positively charged methanethiosulfonate ethyltrimethylammonium (MTSET) and negatively charged methanethiosulfonate ethylsulfonate (MTSES), implying that these residues form an irregular surface loop. The MTSET modification rate of all residues from R271C to K276C was faster in the glycine-bound state than in the unliganded state. MTSES modification of A272C, L274C, and V277C was also faster in the glycine-bound state. These results demonstrate that the surface accessibility of the M2-M3 loop is increased as the channel transitions from the closed to the open state, implying that either the loop itself or an overlying domain moves during channel activation.

Protein mobility and GABA-induced conformational changes in GABA(A) receptor pore-lining M2 segment

Protein movements underlying ligand-gated ion channel activation are poorly understood. Here we used disulfide bond trapping to examine the proximity and mobility of cysteines substituted for aligned GABAA receptor alpha1 and beta1 M2 segment channel-lining residues in resting and activated receptors. With or without GABA, disulfide bonds formed at alpha1N275C/beta1E270C (20') and alpha1S272C/beta1H267C (17'), near the extracellular end, suggesting that this end is more mobile and/or flexible than the rest of the segment. Near the middle of M2, at alpha1T261C/beta1T256C (6'), a disulfide bond formed only in the presence of GABA and locked the channels open. Channel activation must involve an asymmetric rotation of two adjacent subunits toward each other. This would move aligned engineered cysteines on different subunits into proximity and allow disulfide bond formation without blocking conduction. Asymmetric rotation of M2 segments is probably a common gating mechanism in other ligand-gated ion channels.

Chemically charging the pore constriction opens the mechanosensitive channel MscL

MscL is a bacterial mechanosensitive channel that protects the cell from osmotic downshock. We have previously shown that substitution of a residue that resides within the channel pore constriction, MscL's Gly-22, with all other 19 amino acids affects channel gating according to the hydrophobicity of the substitution (). Here, we first make a mild substitution, G22C, and then attach methanethiosulfonate (MTS) reagents to the cysteine under patch clamp. Binding MTS reagents that are positively charged ([2-(trimethylammonium)ethyl] methanethiosulfonate and 2-aminoethyl methanethiosulfonate) or negatively charged (sodium (2-sulfonatoethyl)methanethiosulfonate) causes MscL to gate spontaneously, even when no tension is applied. In contrast, the polar 2-hydroxyethyl methanethiosulfonate halves the threshold, and the hydrophobic methyl methanethiolsulfonate increases the threshold. These observations indicate that residue 22 is in a hydrophobic environment before gating and in a hydrophilic environment during opening to a substate, a finding consistent with our previous study. In addition, we have found that cysteine 22 is accessible to reagents from the cytoplasmic side only when the channel is opened whereas it is accessible from the periplasmic side even in the closed state. These results support the view that exposure of hydrophobic surfaces to a hydrophilic environment during channel opening serves as the barrier to gating.

Acetylcholine receptor channel structure in the resting, open, and desensitized states probed with the substituted-cysteine-accessibility method

The nicotinic acetylcholine (ACh) receptors cycle among classes of nonconducting resting states, conducting open states, and nonconducting desensitized states. We previously probed the structure of the mouse-muscle ACh receptor channel in the resting state obtained in the absence of agonist and in the open states obtained after brief exposure to ACh. We now have probed the structure in the stable desensitized state obtained after many minutes of exposure to ACh. Muscle-type receptor has the subunit composition alpha(2)betagammadelta. Each subunit has four membrane-spanning segments, M1-M4. The channel lumen in the membrane domain is lined largely by M2 and to a lesser extent by M1 from each of the subunits. We determined the rates of reaction of a small, sulfhydryl-specific, charged reagent, 2-aminoethyl methanethiosulfonate with cysteines substituted for residues in alphaM2 and the alphaM1-M2 loop in the desensitized state and compared these rates to rates previously obtained in the resting and open states. The reaction rates of the substituted cysteines are different in the three functional states of the receptor, indicating significant structural differences. By comparing the rates of reaction of extracellularly and intracellularly added 2-aminoethyl methanethiosulfonate, we previously located the closed gate in the resting state between alphaG240 and alphaT244, in the predicted M1-M2 loop at the intracellular end of M2. Now, we have located the closed gate in the stable desensitized state between alphaG240 and alphaL251. The gate in the desensitized state includes the resting state gate and an extension further into M2.

Acetylcholine receptor channel structure in the resting, open, and desensitized states probed with the substituted-cysteine-accessibility method

The nicotinic acetylcholine (ACh) receptors cycle among classes of nonconducting resting states, conducting open states, and nonconducting desensitized states. We previously probed the structure of the mouse-muscle ACh receptor channel in the resting state obtained in the absence of agonist and in the open states obtained after brief exposure to ACh. We now have probed the structure in the stable desensitized state obtained after many minutes of exposure to ACh. Muscle-type receptor has the subunit composition alpha(2)betagammadelta. Each subunit has four membrane-spanning segments, M1-M4. The channel lumen in the membrane domain is lined largely by M2 and to a lesser extent by M1 from each of the subunits. We determined the rates of reaction of a small, sulfhydryl-specific, charged reagent, 2-aminoethyl methanethiosulfonate with cysteines substituted for residues in alphaM2 and the alphaM1-M2 loop in the desensitized state and compared these rates to rates previously obtained in the resting and open states. The reaction rates of the substituted cysteines are different in the three functional states of the receptor, indicating significant structural differences. By comparing the rates of reaction of extracellularly and intracellularly added 2-aminoethyl methanethiosulfonate, we previously located the closed gate in the resting state between alphaG240 and alphaT244, in the predicted M1-M2 loop at the intracellular end of M2. Now, we have located the closed gate in the stable desensitized state between alphaG240 and alphaL251. The gate in the desensitized state includes the resting state gate and an extension further into M2.

A transmembrane site determines sensitivity of neuronal nicotinic acetylcholine receptors to general anesthetics

Neuronal nicotinic acetylcholine receptors (nAChRs) are potential targets for a wide variety of general anesthetics. We recently showed that alpha(4)beta(2) nAChRs are more sensitive than alpha(4)beta(4) receptors to the gaseous anesthetics nitrous oxide and xenon. The present study examines chimeric and point mutant rat nAChRs expressed in Xenopus oocytes and identifies a single amino acid residue (beta(2)-Val(253) or beta(4)-Phe(255)) near the middle of the second transmembrane segment (TM2) that determines gaseous anesthetic sensitivity. Mutations of this residue in beta subunits and the homologous residue of alpha(4) subunits (alpha(4)-Val(254)) showed that this position also determines sensitivities of nAChRs to acetylcholine, isoflurane, pentobarbital, and hexanol. In contrast, these mutations did not affect actions of ketamine. The positively charged sulfhydryl-specific reagent methanethiosulfonate ethylammonium reacted with a cysteine introduced at alpha(4)-Val(254) or beta(2)-Val(253), and irreversibly reduced anesthetic sensitivities of nAChRs. Propyl methanethiosulfonate is an anesthetic analog that covalently binds to a TM2 site of gamma-aminobutyric acid(A) and glycine receptors and irreversibly enhances receptor function. However, propyl methanethiosulfonate reversibly inhibited cysteine-substitution mutants at alpha(4)-Val(254) or beta(2)-Val(253) of nAChRs, and did not affect anesthetic sensitivity. Thus, residues alpha(4)-Val(254) and beta(2)-Val(253) alter channel gating and determine anesthetic sensitivity of nAChRs, but are not likely to be anesthetic-binding sites.

Identification of sites of incorporation in the nicotinic acetylcholine receptor of a photoactivatible general anesthetic

Most general anesthetics including long chain aliphatic alcohols act as noncompetitive antagonists of the nicotinic acetylcholine receptor (nAChR). To locate the sites of interaction of a long chain alcohol with the Torpedo nAChR, we have used the photoactivatible alcohol 3-[(3)H]azioctanol, which inhibits the nAChR and photoincorporates into nAChR subunits. At 1 and 275 microm, 3-[(3)H]azioctanol photoincorporated into nAChR subunits with increased incorporation in the alpha-subunit in the desensitized state. The incorporation into the alpha-subunit was mapped to two large proteolytic fragments. One fragment of approximately 20 kDa (alpha V8-20), containing the M1, M2, and M3 transmembrane segments, showed enhanced incorporation in the presence of agonist whereas the other of approximately 10 kDa (alpha V8-10), containing the M4 transmembrane segment, did not show agonist-induced incorporation of label. Within alpha V8-20, the primary site of incorporation was alpha Glu-262 at the C-terminal end of alpha M2, labeled preferentially in the desensitized state. The incorporation at alpha Glu-262 approached saturation between 1 microm, with approximately 6% labeled, and 275 microm, with approximately 30% labeled. Low level incorporation was seen in residues at the agonist binding site and the protein-lipid interface at approximately 1% of the levels in alpha Glu-262. Therefore, the primary binding site of 3-azioctanol is within the ion channel with additional lower affinity interactions within the agonist binding site and at the protein-lipid interface.

Asymmetric and independent contribution of the second transmembrane segment 12' residues to diliganded gating of acetylcholine receptor channels: a single-channel study with choline as the agonist

Mutagenesis studies have suggested that the second transmembrane segment (M2) plays a critical role during acetylcholine receptor liganded gating. An adequate description of the relationship between gating and structure of the M2 domain, however, has been hampered by the fact that many M2 mutations increase the opening rate constant to levels that, in the presence of acetylcholine, are unresolvably fast. Here, we show that the use of saturating concentrations of choline, a low-efficacy agonist, is a convenient tool to circumvent this problem. In the presence of 20 mM choline: (a) single-channel currents occur in clusters; (b) fast blockade by choline itself reduces the single-channel conductance by approximately 50%, yet the excess open-channel noise is only moderate; (c) the kinetics of gating are fitted best by a single-step, C <--> O model; and (d) opening and closing rate constants are within a well resolvable range. Application of this method to a series of recombinant adult mouse muscle M2 12' mutants revealed that: (a) the five homologous M2 12' positions make independent and asymmetric contributions to diliganded gating, the delta subunit being the most sensitive to mutation; (b) mutations at delta12' increase the diliganded gating equilibrium constant in a manner that is consistent with the sensitivity of the transition state to mutation being approximately 30% like that of the open state and approximately 70% like that of the closed state; (c) the relationship between delta12' amino acid residue volume, hydrophobicity or alpha-helical tendency, and the gating equilibrium constant of the corresponding mutants is not straightforward; however, (d) rate and equilibrium constants for the mutant series are linearly correlated (on log-log plots), which suggests that the conformational rearrangements upon mutation are mostly local and that the position of the transition state along the gating reaction coordinate is unaffected by these mutations.

Kinetic, mechanistic, and structural aspects of unliganded gating of acetylcholine receptor channels: a single-channel study of second transmembrane segment 12' mutants

The spontaneous activity of adult mouse muscle acetylcholine receptor channels, transiently expressed in HEK-293 cells, was studied with the patch-clamp technique. To increase the frequency of unliganded openings, mutations at the 12' position of the second transmembrane segment were engineered. Our results indicate that: (a) in both wild type and mutants, a C <--> O kinetic scheme provides a good description of spontaneous gating. In the case of some mutant constructs, however, additional states were needed to improve the fit to the data. Similar additional states were also needed in one of six patches containing wild-type acetylcholine receptor channels; (b) the delta12' residue makes a more pronounced contribution to unliganded gating than the homologous residues of the alpha, beta, and straightepsilon subunits; (c) combinations of second transmembrane segment 12' mutations in the four different subunits appear to have cumulative effects; (d) the volume of the side chain at delta12' is relevant because residues larger than the wild-type Ser increase spontaneous gating; (e) the voltage dependence of the unliganded gating equilibrium constant is the same as that of diliganded gating, but the voltage dependences of the opening and closing rate constants are opposite (this indicates that the reaction pathway connecting the closed and open states of the receptor changes upon ligation); (f) engineering binding-site mutations that decrease diliganded gating (alphaY93F, alphaY190W, and alphaD200N) reduces spontaneous activity as well (this suggests that even in the absence of ligand the opening of the channel is accompanied by a conformational change at the binding sites); and (g) the diliganded gating equilibrium constant is also increased by the 12' mutations. Such increase is independent of the particular ligand used as the agonist, which suggests that these mutations affect mostly the isomerization step, having little, if any, effect on the ligand-affinity ratio.

Mapping the agonist binding site of the nicotinic acetylcholine receptor. Orientation requirements for activation by covalent agonist

To characterize the structural requirements for ligand orientation compatible with activation of the Torpedo nicotinic acetylcholine receptor (nAChR), we used Cys mutagenesis in conjunction with sulfhydryl-reactive reagents to tether primary or quaternary amines at defined positions within the agonist binding site of nAChRs containing mutant alpha- or gamma-subunits expressed in Xenopus oocytes. 4-(N-Maleimido)benzyltrimethylammonium and 2-aminoethylmethanethiosulfonate acted as irreversible antagonists when tethered at alphaY93C, alphaY198C, or gammaE57C, as well as at alphaN94C (2-aminoethylmethanethiosulfonate only). [2-(Trimethylammonium)-ethyl]-methanethiosulfonate (MTSET), which attaches thiocholine to binding site Cys, also acted as an irreversible antagonist when tethered at alphaY93C, alphaN94C, or gammaE57C. However, MTSET modification of alphaY198C resulted in prolonged activation of the nAChR not reversible by washing but inhibitable by subsequent exposure to non-competitive antagonists. Modification of alphaY198C (or any of the other positions tested) by [(trimethylammonium)methyl]methanethiosulfonate resulted only in irreversible inhibition, while modification of alphaY198C by [3-(trimethylammonium)propyl]methanethiosulfonate resulted in irreversible activation of nAChR, but at lower efficacy than by MTSET. Thus changing the length of the tethering arm by less than 1 A in either direction markedly effects the ability of the covalent trimethylammonium to activate the nAChR, and agonist activation depends on a very selective orientation of the quaternary ammonium within the agonist binding site.

Acetylcholine receptors and myasthenia

Much progress has been made in the 26 years since initial studies of the first purified acetylcholine receptors (AChRs) led to the discovery that an antibody-mediated autoimmune response to AChRs causes the muscular weakness and fatigability characteristic of myasthenia gravis (MG) and its animal model, experimental autoimmune myasthenia gravis (EAMG). Now, the structure of muscle AChRs is much better known. Monoclonal antibodies to muscle AChRs, developed as model autoantibodies for studies of EAMG, were used for initial purifications of neuronal AChRs, and now many homologous subunits of neuronal nicotinic AChRs have been cloned. There is a basic understanding of the pathological mechanisms by which autoantibodies to AChRs impair neuromuscular transmission. Immunodiagnostic assays for MG are used routinely. Nonspecific approaches to immunosuppressive therapy have been refined. However, fundamental mysteries remain regarding what initiates and sustains the autoimmune response to muscle AChRs and how to specifically suppress this autoimmune response using a practical therapy. Many rare congenital myasthenic syndromes have been elegantly shown to result from mutations in muscle AChRs. These studies have provided insights into AChR structure and function as well as into the pathological mechanisms of these diseases. Evidence has been found for autoimmune responses even to some central nervous system neurotransmitter receptors, but only one neuronal AChR has so far been implicated in an autoimmune disease. Thus far, only two neuronal AChR mutations have been found to be associated with a rare form of epilepsy, but many more neuronal AChR mutations will probably be found to be associated with disease in the years ahead.

The intrinsic electrostatic potential and the intermediate ring of charge in the acetylcholine receptor channel

A ring of aligned glutamate residues named the intermediate ring of charge surrounds the intracellular end of the acetylcholine receptor channel and dominates cation conduction (Imoto et al. 1988). Four of the five subunits in mouse-muscle acetylcholine receptor contribute a glutamate to the ring. These glutamates were mutated to glutamine or lysine, and combinations of mutant and native subunits, yielding net ring charges of -1 to -4, were expressed in Xenopus laevis oocytes. In all complexes, the alpha subunit contained a Cys substituted for alphaThr244, three residues away from the ring glutamate alphaGlu241. The rate constants for the reactions of alphaThr244Cys with the neutral 2-hydroxyethyl-methanethiosulfonate, the positively charged 2-ammonioethyl-methanethiosulfonate, and the doubly positively charged 2-ammonioethyl-2'-ammonioethanethiosulfonate were determined from the rates of irreversible inhibition of the responses to acetylcholine. The reagents were added in the presence and absence of acetylcholine and at various transmembrane potentials, and the rate constants were extrapolated to zero transmembrane potential. The intrinsic electrostatic potential in the channel in the vicinity of the ring of charge was estimated from the ratios of the rate constants of differently charged reagents. In the acetylcholine-induced open state, this potential was -230 mV with four glutamates in the ring and increased linearly towards 0 mV by +57 mV for each negative charge removed from the ring. Thus, the intrinsic electrostatic potential in the narrow, intracellular end of the open channel is almost entirely due to the intermediate ring of charge and is strongly correlated with alkali-metal-ion conductance through the channel. The intrinsic electrostatic potential in the closed state of the channel was more positive than in the open state at all values of the ring charge. These electrostatic properties were simulated by theoretical calculations based on a simplified model of the channel.

Gamma-aminobutyric acid increases the water accessibility of M3 membrane-spanning segment residues in gamma-aminobutyric acid type A receptors

Gamma-aminobutyric acid type A (GABA(A)) receptors are members of the ligand-gated ion channel gene superfamily. Using the substituted cysteine accessibility method, we investigated whether residues in the alpha(1)M3 membrane-spanning segment are water-accessible. Cysteine was substituted, one at a time, for each M3 residue from alpha(1)Ala(291) to alpha(1)Val(307). The ability of these mutants to react with the water-soluble, sulfhydryl-specific reagent pCMBS(-) was assayed electrophysiologically. Cysteines substituted for alpha(1)Ala(291) and alpha(1)Tyr(294) reacted with pCMBS(-) applied both in the presence and in the absence of GABA. Cysteines substituted for alpha(1)Phe(298), alpha(1)Ala(300), alpha(1)Leu(301), and alpha(1)Glu(303) only reacted with pCMBS(-) applied in the presence of GABA. We infer that the pCMBS(-) reactive residues are on the water-accessible surface of the protein and that GABA induces a conformational change that increases the water accessibility of the four M3 residues, possibly by inducing the formation of water-filled crevices that extend into the interior of the protein. Others have shown that mutations of alpha(1)Ala(291), a water-accessible residue, alter volatile anesthetic and ethanol potentiation of GABA-induced currents. Water-filled crevices penetrating into the interior of the membrane-spanning domain may allow anesthetics and alcohol to reach their binding sites and thus may have implications for the mechanisms of action of these agents.

Contribution of individual subunits to the multimeric P2X(2) receptor: estimates based on methanethiosulfonate block at T336C

P2X receptors are membrane proteins that incorporate a cation-selective ion channel that can be opened by the binding of extracellular ATP. They associate as hetero- and homo-multimers of currently unknown stoichiometry. In this study, we have used Xenopus laevis oocytes to express rat P2X(2) receptor subunits, which carry a cysteine mutation at position 336. ATP-induced currents at this mutant receptor subunit were blocked by more than 90% when exposed to [2-(trimethylammonium) ethyl] methanethiosulfonate (MTSET), whereas currents from wild-type subunits were not affected. To compare mutant and wild-type channel expression, we introduced an epitope in their extracellular domains and found for both channels a similar linear relationship between antibody binding and currents induced by ATP. To study the contribution of the individual subunits to the block by MTSET, we coinjected different mixtures of wild-type and mutant-encoding mRNAs. We found that the inhibition by MTSET depended linearly on the proportion of mutant subunits, which was clearly contrary to the hypothesis that a single mutant subunit could act in a dominant fashion. Subsequent concatenation of wild-type and mutant-encoding cDNAs resulted in an inhibition by MTSET that also depended linearly on the number of mutant subunits and was independent of the position of the mutant subunit, as long as only two or three P2X(2) subunits were joined. With four or six subunits joined, however, the inhibition by MTSET became strongly position-dependent. The present results show that a "per-subunit" channel block causes the blocking effects of MTSET and they suggest that not four but maximally three subunits actively participate in the channel formation.

Mapping the substrate binding site of the prostaglandin transporter PGT by cysteine scanning mutagenesis

We have identified a cDNA, PGT, that encodes a widely expressed transporter for prostaglandin (PG) E(2), PGF(2alpha), PGD(2), 8-iso-PGF(2alpha), and thromboxane B(2). To begin to understand the molecular mechanisms of transporter function, we have initiated a structure-function analysis of PGT to identify its substrate-binding region. We have found that by introducing the small, water-soluble, thiol-reactive anion Na(2-sulfonatoethyl)methanethiosulfonate (MTSES) into the substrate pathway, we were able to cause inhibition of transport that could be reversed with dithiothreitol. Importantly, co-incubation with PGE(2) protected PGT from this inhibition, suggesting that MTSES gains access to the aqueous pore pathway of PGT to form a mixed disulfide near the substrate-binding site. To identify the susceptible cysteine, we mutated, one at a time, all six of the putative transmembrane cysteines to serine. Only the mutation of Cys-530 to serine within putative transmembrane 10 became resistant to inhibition by MTSES. Thus, Cys-530 is the substrate-protectable, MTSES-inhibitable residue. To identify other residues that may be lining the substrate-binding site, we initiated cysteine-scanning mutagenesis of transmembrane 10 using Cys-530 as an entry point. On a C530S, MTSES-resistant background, residues in the N- and C-terminal directions were individually mutated to cysteine (Ala-513 to His-536), one at a time, and then analyzed for MTSES inhibition. Of the 24 cysteine-substituted mutants generated, 6 were MTSES-sensitive and, among these, 4 were substrate-protectable. The pattern of sensitivity to MTSES places these residues on the same face of an alpha-helix. The results of cysteine-scanning mutagenesis and molecular modeling of putative transmembrane 10 indicate that the substrate binding of PGT is formed among its membrane-spanning segments, with 4 residues along the cytoplasmic end of helix 10 contributing to one surface of the binding site.

Mutational analysis of the charge selectivity filter of the alpha7 nicotinic acetylcholine receptor

In the alpha7 nicotinic acetylcholine receptors, we analyze the contribution of mutations E237A and V251T, together with the proline insertion P236', in the conversion of the charge selectivity from cationic to anionic. We show that the triple mutant exhibits spontaneous openings displaying anionic selectivity. Furthermore, at position 251, hydrophilic or even negatively charged residues are compatible with an anionic channel. In contrast, the additional proline yields an anionic channel only when inserted between positions 234 and 237; insertion before 234 yields a cationic channel and after 238 alters the receptor surface expression. The coiled 234-238 loop thus directly contributes to the charge selectivity filter of the alpha7 channel.

NMDAR channel segments forming the extracellular vestibule inferred from the accessibility of substituted cysteines

In NMDA receptor channels, the M2 loop forms the narrow constriction and the cytoplasmic vestibule. The identity of an extracellular vestibule leading toward the constriction remained unresolved. Using the substituted cysteine accessibility method (SCAM), we identified channel-lining residues of the NR1 subunit in the region preceding M1 (preM1), the C-terminal part of M3 (M3C), and the N-terminal part of M4 (M4N). These residues are located on the extracellular side of the constriction and, with one exception, are exposed to the pore independently of channel activation, suggesting that the gate is at the constriction or further cytoplasmic to it. Permeation of Ca2+ ions was decreased by mutations in M3C and M4N, but not by mutations in preM1, suggesting a functionally distinct contribution of the segments to the extracellular vestibule of the NMDA receptor channel.

Delimiting the binding site for quaternary ammonium lidocaine derivatives in the acetylcholine receptor channel

The triethylammonium QX-314 and the trimethylammonium QX-222 are lidocaine derivatives that act as open-channel blockers of the acetylcholine (ACh) receptor. When bound, these blockers should occlude some of the residues lining the channel. Eight residues in the second membrane-spanning segment (M2) of the mouse-muscle alpha subunit were mutated one at a time to cysteine and expressed together with wild-type beta, gamma, and delta subunits in Xenopus oocytes. The rate constant for the reaction of each substituted cysteine with 2-aminoethyl methanethiosulfonate (MTSEA) was determined from the time course of the irreversible effect of MTSEA on the ACh-induced current. The reactions were carried out in the presence and absence of ACh and in the presence and absence of QX-314 and QX-222. These blockers had no effect on the reactions in the absence of ACh. In the presence of ACh, both blockers retarded the reaction of extracellularly applied MTSEA with cysteine substituted for residues from alphaVal255, one third of the distance in from the extracellular end of M2, to alphaGlu241, flanking the intracellular end of M2, but not with cysteine substituted for alphaLeu258 or alphaGlu262, at the extracellular end of M2. The reactions of MTSEA with cysteines substituted for alphaLeu258 and alphaGlu262 were considerably faster in the presence of ACh than in its absence. That QX-314 and QX-222 did not protect alphaL258C and alphaE262C against reaction with MTSEA in the presence of ACh implies that protection of the other residues was due to occlusion of the channel and not to the promotion of a less reactive state from a remote site. Given the 12-A overall length of the blockers and the alpha-helical conformation of M2 in the open state, the binding site for both blockers extends from alphaVal255 down to alphaSer248.

The location of the gate in the acetylcholine receptor channel

The cation-conducting channel of the nicotinic acetylcholine (ACh) receptor is lined by the first (M1) and second (M2) membrane-spanning segments of each of its five subunits. Six consecutive residues, alphaS239 to alphaT244, in the alpha subunit M1-M2 loop and at the intracellular end of M2 were mutated to cysteine. The accessibility of the substituted cysteines were probed with small, cationic, sulfhydryl-specific reagents added extracellularly and intracellularly. In the closed state of the channel, there is a barrier to these reagents added from either side between alphaG240 and alphaT244. ACh induces the removal of this barrier, which acts as an activation gate. The residues alphaG240, alphaE241, alphaK242, and alphaT244 line a narrow part of the channel, in which this gate is located.

State-dependent accessibility and electrostatic potential in the channel of the acetylcholine receptor. Inferences from rates of reaction of thiosulfonates with substituted cysteines in the M2 segment of the alpha subunit

Ion channel function depends on the chemical and physical properties and spatial arrangement of the residues that line the channel lumen and on the electrostatic potential within the lumen. We have used small, sulfhydryl-specific thiosulfonate reagents, both positively charged and neutral, to probe the environment within the acetylcholine (ACh) receptor channel. Rate constants were determined for their reactions with cysteines substituted for nine exposed residues in the second membrane-spanning segment (M2) of the alpha subunit. The largest rate constants, both in the presence and absence of ACh, were for the reactions with the cysteine substituted for alpha Thr244, near the intracellular end of the channel. In the open state of the channel, but not in the closed state, the rate constants for the reactions of the charged reagents with several substituted cysteines depended on the transmembrane electrostatic potential, and the electrical distance of these cysteines increased from the extracellular to the intracellular end of M2. Even at zero transmembrane potential, the ratios of the rate constants for the reactions of three positively charged reagents with alpha T244C, alpha L251C, and alpha L258C to the rate constant for the reaction of an uncharged reagent were much greater in the open than in the closed state. This dependence of the rate constants on reagent charge is consistent with an intrinsic electrostatic potential in the channel that is considerably more negative in the open state than in the closed state. The effects of ACh on the rate constants for the reactions of substituted Cys along the length of alpha M2, on the dependence of the rate constants on the transmembrane potential, and on the intrinsic potential support a location of a gate more intracellular than alpha Thr244.