Identification of acetylcholine receptor channel-lining residues in the M1 segment of the alpha-subunit

Interrogating permeation and gating of Orai channels using chemical modification of cysteine residues

Chemical modification of ion channels using the substituted cysteine accessibility method has a rich and successful history in elucidating the structural basis of ion channel function. In this approach, cysteine residues are introduced in regions of interest into the protein and their accessibility to water soluble thiol-reactive reagents is determined by monitoring ion channel activity. Because a wide range of these reagents are available with differing size, charge, and membrane solubility, the physio-chemical environment of the introduced cysteine residue and therefore the protein domain of interest can be probed with great precision. The approach has been widely employed for determining the secondary structure of specific ion channel domains, the location and nature of the channel gate, and the conformational rearrangements in the channel pore that underlie the opening/closing of the pore. In this chapter, we describe the use of these and related approaches to probe the functional architecture and gating of store-operated Orai1 channels.

Design, Synthesis and In Vitro Investigation of Novel Basic Celastrol Carboxamides as Bio-Inspired Leishmanicidal Agents Endowed with Inhibitory Activity against Leishmania Hsp90

The natural triterpene celastrol (CE) is here used as lead compound for the design and synthesis of a panel of eleven CE carboxamides that were tested in vitro for their growth inhibitory activity against Leishmania infantum and L.tropica parasites. Among them, in vitro screening identified four basic CE carboxamides endowed with nanomolar leishmanicidal activity, against both the promastigotes and the intramacrophage Leishmania amastigotes forms. These compounds also showed low toxicity toward two human (HMEC-1 and THP-1) and one murine (BMDM) cell lines. Interestingly, the most selective CE analogue (compound 3) was also endowed with the ability to inhibit the ATPase activity of the Leishmania protein chaperone Hsp90 as demonstrated by the in vitro assay conducted on a purified, full-length recombinant protein. Preliminary investigations by comparing it with the naturally occurring Hsp90 active site inhibitor Geldanamycin (GA) in two different in vitro experiments were performed. These promising results set the basis for a future biochemical investigation of the mode of interaction of celastrol and CE-inspired compounds with Leishmania Hsp90.

Mutational Analysis at Intersubunit Interfaces of an Anionic Glutamate Receptor Reveals a Key Interaction Important for Channel Gating by Ivermectin

The broad-spectrum anthelmintic drug ivermectin (IVM) activates and stabilizes an open-channel conformation of invertebrate chloride-selective glutamate receptors (GluClRs), thereby causing a continuous inflow of chloride ions and sustained membrane hyperpolarization. These effects suppress nervous impulses and vital physiological processes in parasitic nematodes. The GluClRs are pentamers. Homopentameric receptors assembled from the Caenorhabditis elegans (C. elegans) GluClα (GLC-1) subunit can inherently respond to IVM but not to glutamate (the neurotransmitter). In contrast, heteromeric GluClα/β (GLC-1/GLC-2) assemblies respond to both ligands, independently of each other. Glutamate and IVM bind at the interface between adjacent subunits, far away from each other; glutamate in the extracellular ligand-binding domain, and IVM in the ion-channel pore periphery. To understand the importance of putative intersubunit contacts located outside the glutamate and IVM binding sites, we introduced mutations at intersubunit interfaces, between these two binding-site types. Then, we determined the effect of these mutations on the activation of the heteromeric mutant receptors by glutamate and IVM. Amongst these mutations, we characterized an α-subunit point mutation located close to the putative IVM-binding pocket, in the extracellular end of the first transmembrane helix (M1). This mutation (αF276A) moderately reduced the sensitivity of the heteromeric GluClαF276A/βWT receptor to glutamate, and slightly decreased the receptor subunits’ cooperativity in response to glutamate. In contrast, the αF276A mutation drastically reduced the sensitivity of the receptor to IVM and significantly increased the receptor subunits’ cooperativity in response to IVM. We suggest that this mutation reduces the efficacy of channel gating, and impairs the integrity of the IVM-binding pocket, likely by disrupting important interactions between the tip of M1 and the M2-M3 loop of an adjacent subunit. We hypothesize that this physical contact between M1 and the M2-M3 loop tunes the relative orientation of the ion-channel transmembrane helices M1, M2 and M3 to optimize pore opening. Interestingly, pre-exposure of the GluClαF276A/βWT mutant receptor to subthreshold IVM concentration recovered the receptor sensitivity to glutamate. We infer that IVM likely retained its positive modulation activity by constraining the transmembrane helices in a preopen orientation sensitive to glutamate, with no need for the aforementioned disrupted interactions between M1 and the M2-M3 loop.

Methanethiosulfonate derivatives as ligands of the STAT3-SH2 domain

With the aim to discover new STAT3 direct inhibitors, potentially useful as anticancer agents, a set of methanethiosulfonate drug hybrids were synthesized. The in vitro tests showed that all the thiosulfonic compounds were able to strongly and selectively bind STAT3-SH2 domain, whereas the parent drugs were completely devoid of this ability. In addition, some of them showed a moderate antiproliferative activity on HCT-116 cancer cell line. These results suggest that methanethiosulfonate moiety can be considered a useful scaffold in the preparation of new direct STAT3 inhibitors. Interestingly, an unusual kind of organo-sulfur derivative, endowed with valuable antiproliferative activity, was occasionally isolated. [Formula: see text].

Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors

General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Excitatory amino acid transporters: roles in glutamatergic neurotransmission

Excitatory amino acid transporters or EAATs are the major transport mechanism for extracellular glutamate in the nervous system. This family of five carriers not only displays an impressive ability to regulate ambient extracellular glu concentrations but also regulate the temporal and spatial profile of glu after vesicular release. This dynamic form of regulation mediates several characteristic of synaptic, perisynaptic, and spillover activation of ionotropic and metabotropic receptors. EAATs function through a secondary active, electrogenic process but also possess a thermodynamically uncoupled ligand gated anion channel activity, both of which have been demonstrated to play a role in regulation of cellular activity. This review will highlight the inception of EAATs as a focus of research, the transport and channel functionality of the carriers, and then describe how these properties are used to regulate glutamatergic neurotransmission.

Direct evidence that scorpion α-toxins (site-3) modulate sodium channel inactivation by hindrance of voltage-sensor movements

The position of the voltage-sensing transmembrane segment, S4, in voltage-gated ion channels as a function of voltage remains incompletely elucidated. Site-3 toxins bind primarily to the extracellular loops connecting transmembrane helical segments S1-S2 and S3-S4 in Domain 4 (D4) and S5-S6 in Domain 1 (D1) and slow fast-inactivation of voltage-gated sodium channels. As S4 of the human skeletal muscle voltage-gated sodium channel, hNav1.4, moves in response to depolarization from the resting to the inactivated state, two D4S4 reporters (R2C and R3C, Arg1451Cys and Arg1454Cys, respectively) move from internal to external positions as deduced by reactivity to internally or externally applied sulfhydryl group reagents, methane thiosulfonates (MTS). The changes in reporter reactivity, when cycling rapidly between hyperpolarized and depolarized voltages, enabled determination of the positions of the D4 voltage-sensor and of its rate of movement. Scorpion α-toxin binding impedes D4S4 segment movement during inactivation since the modification rates of R3C in hNav1.4 with methanethiosulfonate (CH3SO2SCH2CH2R, where R = -N(CH3)3 (+) trimethylammonium, MTSET) and benzophenone-4-carboxamidocysteine methanethiosulfonate (BPMTS) were slowed ~10-fold in toxin-modified channels. Based upon the different size, hydrophobicity and charge of the two reagents it is unlikely that the change in reactivity is due to direct or indirect blockage of access of this site to reagent in the presence of toxin (Tx), but rather is the result of inability of this segment to move outward to the normal extent and at the normal rate in the toxin-modified channel. Measurements of availability of R3C to internally applied reagent show decreased access (slower rates of thiol reaction) providing further evidence for encumbered D4S4 movement in the presence of toxins consistent with the assignment of at least part of the toxin binding site to the region of D4S4 region of the voltage-sensor module.

Assembling an ion channel: ORF 3a from SARS-CoV

Protein 3a is a 274 amino acid polytopic channel protein with three putative transmembrane domains (TMDs) encoded by severe acute respiratory syndrome corona virus (SARS-CoV). Synthetic peptides corresponding to each of its three individual transmembrane domains (TMDs) are reconstituted into artificial lipid bilayers. Only TMD2 and TMD3 induce channel activity. Reconstitution of the peptides as TMD1 + TMD3 as well as TMD2 + TMD3 in a 1 : 1 mixture induces membrane activity for both mixtures. In a 1 : 1 : 1 mixture, channel like behavior is almost restored. Expression of full length 3a and reconstitution into artificial lipid bilayers reveal a weak cation selective (PK ≈ 2 PCl ) rectifying channel. In the presence of nonphysiological concentration of Ca-ions the channel develops channel activity.

Thiol–Thiosulfonate Chemistry in Polymer Science: Simple Functionalization of Polymers via Disulfide Linkages

Herein we highlight the reaction of thiols with thiosulfonates yielding asymmetric disulfides. The chapter begins with an overview of the synthesis and reactivity of functional thiosulfonates and is followed by a review of polymeric thiosulfonates. We then emphasize the novel use of thiosulfonates as trapping/functionalization agents for macromolecular thiols obtained from parent (co)polymers prepared by reversible addition‐fragmentation chain transfer (RAFT) radical polymerization. We also note how such facile disulfide‐forming chemistries can be readily employed simultaneously with other highly efficient coupling chemistries with an emphasis on the concurrent reaction of activated esters with amines in the presence of thiosulfonates. Finally, we discuss the use of methyl disulfide (SSMe) functional/end‐modified (co)polymers as reagents for the formation of polymeric self‐assembled monolayers (polymer brushes) on metal surfaces such as nanoparticles and quantum dots.

Gating-induced conformational rearrangement of the γ-aminobutyric acid type A receptor β-α subunit interface in the membrane-spanning domain

GABA(A) receptors mediate fast inhibitory synaptic transmission. The transmembrane ion channel is lined by a ring of five α helices, M2 segments, one from each subunit. An outer ring of helices comprising the alternating M1, M3, and M4 segments from each subunit surrounds the inner ring and forms the interface with the lipid bilayer. The structural rearrangements that follow agonist binding and culminate in opening of the ion pore remain incompletely characterized. Propofol and other intravenous general anesthetics bind at the βM3-αM1 subunit interface. We sought to determine whether this region undergoes conformational changes during GABA activation. We measured the reaction rate of p-chloromercuribenzenesulfonate (pCMBS) with cysteines substituted in the GABA(A) receptor α1M1 and β2M3 segments. In the presence of GABA, the pCMBS reaction rate increased significantly in a cluster of residues in the extracellular third of the α1M1 segment facing the β2M3 segment. Mutation of the β2M2 segment 19' position, R269Q, altered the pCMBS reaction rate with several α1M1 Cys, some only in the resting state and others only in the GABA-activated state. Thus, β2R269 is charged in both states. GABA activation induced disulfide bond formation between β2R269C and α1I228C. The experiments demonstrate that α1M1 moves in relationship to β2M2R269 during gating. Thus, channel gating does not involve rigid body movements of the entire transmembrane domain. Channel gating causes changes in the relative position of transmembrane segments both within a single subunit and relative to the neighboring subunits.

Bupropion binds to two sites in the Torpedo nicotinic acetylcholine receptor transmembrane domain: a photoaffinity labeling study with the bupropion analogue [(125)I]-SADU-3-72

Bupropion, a clinically used antidepressant and smoking-cessation drug, acts as a noncompetitive antagonist of nicotinic acetylcholine receptors (nAChRs). To identify its binding site(s) in nAChRs, we developed a photoreactive bupropion analogue, (±)-2-(N-tert-butylamino)-3'-[(125)I]-iodo-4'-azidopropiophenone (SADU-3-72). Based on inhibition of [(125)I]SADU-3-72 binding, SADU-3-72 binds with high affinity (IC(50) = 0.8 μM) to the Torpedo nAChR in the resting (closed channel) state and in the agonist-induced desensitized state, and bupropion binds to that site with 3-fold higher affinity in the desensitized (IC(50) = 1.2 μM) than in the resting state. Photolabeling of Torpedo nAChRs with [(125)I]SADU-3-72 followed by limited in-gel digestion of nAChR subunits with endoproteinase Glu-C established the presence of [(125)I]SADU-3-72 photoincorporation within nAChR subunit fragments containing M1-M2-M3 helices (αV8-20K, βV8-22/23K, and γV8-24K) or M1-M2 helices (δV8-14). Photolabeling within βV8-22/23K, γV8-24K, and δV8-14 was reduced in the desensitized state and inhibited by ion channel blockers selective for the resting (tetracaine) or desensitized (thienycyclohexylpiperidine (TCP)) state, and this pharmacologically specific photolabeling was localized to the M2-9 leucine ring (δLeu(265), βLeu(257)) within the ion channel. In contrast, photolabeling within the αV8-20K was enhanced in the desensitized state and not inhibited by TCP but was inhibited by bupropion. This agonist-enhanced photolabeling was localized to αTyr(213) in αM1. These results establish the presence of two distinct bupropion binding sites within the Torpedo nAChR transmembrane domain: a high affinity site at the middle (M2-9) of the ion channel and a second site near the extracellular end of αM1 within a previously described halothane (general anesthetic) binding pocket.

Fourier transform coupled tryptophan scanning mutagenesis identifies a bending point on the lipid-exposed δM3 transmembrane domain of the Torpedo californica nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (nAChR) is a member of a family of ligand-gated ion channels that mediate diverse physiological functions, including fast synaptic transmission along the peripheral and central nervous systems. Several studies have made significant advances toward determining the structure and dynamics of the lipid-exposed domains of the nAChR. However, a high-resolution atomic structure of the nAChR still remains elusive. In this study, we extended the Fourier transform coupled tryptophan scanning mutagenesis (FT-TrpScanM) approach to gain insight into the secondary structure of the δM3 transmembrane domain of the Torpedo californica nAChR, to monitor conformational changes experienced by this domain during channel gating, and to identify which lipid-exposed positions are linked to the regulation of ion channel kinetics. The perturbations produced by periodic tryptophan substitutions along the δM3 transmembrane domain were characterized by two-electrode voltage clamp and (125)I-labeled α-bungarotoxin binding assays. The periodicity profiles and Fourier transform spectra of this domain revealed similar helical structures for the closed- and open-channel states. However, changes in the oscillation patterns observed between positions Val-299 and Val-304 during transition between the closed- and open-channel states can be explained by the structural effects caused by the presence of a bending point introduced by a Thr-Gly motif at positions 300-301. The changes in periodicity and localization of residues between the closed-and open-channel states could indicate a structural transition between helix types in this segment of the domain. Overall, the data further demonstrate a functional link between the lipid-exposed transmembrane domain and the nAChR gating machinery.

Modeling anesthetic binding sites within the glycine alpha one receptor based on prokaryotic ion channel templates: the problem with TM4

Ligand-gated ion channels (LGICs) significantly modulate anesthetic effects. Their exact molecular structure remains unknown. This has led to ambiguity regarding the proper amino acid alignment within their 3D structure and, in turn, the location of any anesthetic binding sites. Current controversies suggest that such a site could be located in either an intra- or intersubunit locale within the transmembrane domain of the protein. Here, we built a model of the glycine alpha one receptor (GlyRa1) based on the open-state structures of two new high-resolution ion channel templates from the prokaryote, Gloebacter violaceus (GLIC). Sequence scoring suggests reasonable homology between GlyRa1 and GLIC. Three of the residues notable for modulating anesthetic action are on transmembrane segments 1-3 (TM1-3): (ILE229, SER 267, and ALA 288). They line an intersubunit interface, in contrast to previous models. However, residues from the fourth transmembrane domain (TM4) that are known to modulate a variety of anesthetic effects are quite distant from this putative anesthetic binding site. While this model can account for a large proportion of the physicochemical data regarding such proteins, it cannot readily account for the alterations on anesthetic effects that are due to mutations within TM4.

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.

Cysteine accessibility analysis of the human alpha7 nicotinic acetylcholine receptor ligand-binding domain identifies L119 as a gatekeeper

A large number of structurally diverse ligands have been produced to selectively target α7 nicotinic acetylcholine receptors (nAChRs). We applied the method of scanning cysteine accessibility mutations (SCAM) to the ligand-binding domain of the α7 nAChR to identify subdomains of particular importance to the binding and subsequent activation by select agonists. We evaluated the activity of four structurally distinct α7 agonists on wild-type human α7 and 44 targeted mutants expressed in Xenopus oocytes. Responses were measured prior and subsequent to the application of the sulfhydryl reagent methanethiosulfonate ethylammonium (MTSEA). One mutant (C116S) served as a Cys-null control, and the additional mutants were made in the C116S background. In many cases, the insertion of free cysteines into the agonist-binding site had a negative effect on function, with 12 of 44 mutants showing no detectable responses to ACh, and with only 19 of the 44 mutants showing sufficiently large responses to permit further study. Several of the cysteine mutations, including W55C, showed selectively reduced responses to the largest agonist tested, 2-methoxy,4-hydroxy-benzylidene anabaseine. Interestingly, although homology models suggest that most of the introduced cysteine mutations should have had good solvent accessibility, application of MTSEA had no effect or produced only modest changes in the agonist response profile of most mutants. Consistent with previous studies implicating W55 to play important roles in agonist activation, MTSEA treatment further decreased the functional responses of W55C to all the test agonists. While the cysteine mutation at L119 itself had relatively little effect on receptor function, treatment of L119C receptors with MTSEA or alternative cationic sulfhydryl reagents profoundly decreased activation by all agonists tested, suggesting a general block of gating. The homologous mutation in heteromeric nAChRs produced similar results, provided that the mutation was placed in the beta subunit complementary surface of the ligand-binding domain. Structural models locate the L119 residue directly across the subunit interface from the C-loop of the primary face of the binding domain. Our data suggest that a covalent modification of L119C by MTSEA or other cationic reagents might block the binding of even small agonists such as TMA through electrostatic interactions. Reaction of L119C with small non-polar reagents increases activation by small agonists but can block the access of large ligands such as benzylidene anabaseines to the ligand-binding domain.

The Molecular Mechanisms of Anesthetic Action: Updates and Cutting Edge Developments from the Field of Molecular Modeling

For over 160 years, general anesthetics have been given for the relief of pain and suffering. While many theories of anesthetic action have been purported, it has become increasingly apparent that a significant molecular focus of anesthetic action lies within the family of ligand-gated ion channels (LGIC’s). These protein channels have a transmembrane region that is composed of a pentamer of four helix bundles, symmetrically arranged around a central pore for ion passage. While initial and some current models suggest a possible cavity for binding within this four helix bundle, newer calculations postulate that the actual cavity for anesthetic binding may exist between four helix bundles. In either scenario, these cavities have a transmembrane mode of access and may be partially bordered by lipid moieties. Their physicochemical nature is amphiphilic. Anesthetic binding may alter the overall motion of a ligand-gated ion channel by a “foot-in-door” motif, resulting in the higher likelihood of and greater time spent in a specific channel state. The overall gating motion of these channels is consistent with that shown in normal mode analyses carried out both in vacuo as well as in explicitly hydrated lipid bilayer models. Molecular docking and large scale molecular dynamics calculations may now begin to show a more exact mode by which anesthetic molecules actually localize themselves and bind to specific protein sites within LGIC’s, making the design of future improvements to anesthetic ligands a more realizable possibility.

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.

Mechanistic contributions of residues in the M1 transmembrane domain of the nicotinic receptor to channel gating

The nicotinic receptor (AChR) is a pentamer of homologous subunits with an alpha(2)betaepsilondelta composition in adult muscle. Each subunit contains four transmembrane domains (M1-M4). Position 15' of the M1 domain is phenylalanine in alpha subunits while it is isoleucine in non-alpha subunits. Given this peculiar conservation pattern, we studied its contribution to muscle AChR activation by combining mutagenesis with single-channel kinetic analysis. AChRs containing the mutant alpha subunit (alphaF15'I) as well as those containing the reverse mutations in the non-alpha subunits (betaI15'F, deltaI15'F, and epsilonI15'F) show prolonged lifetimes of the diliganded open channel resulting from a slower closing rate with respect to wild-type AChRs. The kinetic changes are not equivalent among subunits, the beta subunit, being the one that produces the most significant stabilization of the open state. Kinetic analysis of betaI15'F of AChR channels activated by the low-efficacious agonist choline revealed a 10-fold decrease in the closing rate, a 2.5-fold increase in the opening rate, a 28-fold increase in the gating equilibrium constant in the diliganded receptor, and a significant increase opening in the absence of agonist. Mutations at betaI15' showed that the structural bases of its contribution to gating is complex. Rate-equilibrium linear free-energy relationships suggest an approximately 70% closed-state-like environment for the beta15' position at the transition state of gating. The overall results identify position 15' as a subunit-selective determinant of channel gating and add new experimental evidence that gives support to the involvement of the M1 domain in the operation of the channel gating apparatus.

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.

GABA-induced intersubunit conformational movement in the GABAA receptor alpha 1M1-beta 2M3 transmembrane subunit interface: experimental basis for homology modeling of an intravenous anesthetic binding site

The molecular basis of general anesthetic interactions with GABA(A) receptors is uncertain. An accurate homology model would facilitate studies of anesthetic action. Construction of a GABA(A) model based on the 4 A resolution acetylcholine receptor structure is complicated by alignment uncertainty between the acetylcholine and GABA(A) receptor M3 and M4 transmembrane segments. Using disulfide crosslinking we previously established the orientation of M2 and M3 within a single GABA(A) subunit. The resultant model predicts that the betaM3 residue beta2M286, implicated in anesthetic binding, faces the adjacent alpha1-M1 segment and not into the beta2 subunit interior as some models have suggested. To assess the proximity of beta2M286 to the alpha1-M1 segment we expressed beta2M286C and gamma2 with 10 consecutive alpha1-M1 cysteine (Cys) mutants, alpha1I223C to alpha1L232C, in and flanking the extracellular end of alpha1-M1. In activated states, beta2M286C formed disulfide bonds with alpha1Y225C and alpha1Q229C based on electrophysiological assays and dimers on Western blots, but not with other alpha1-M1 mutants. beta2F289, one helical turn below beta2M286, formed disulfide bonds with alpha1I228C, alpha1Q229C and alpha1L232C in activated states. The intervening residues, beta2G287C and beta2C288, did not form disulfide bonds with alpha1-M1 Cys mutants. We conclude that the beta2-M3 residues beta2M286 and beta2F289 face the intersubunit interface in close proximity to alpha1-M1 and that channel gating induces a structural rearrangement in the transmembrane subunit interface that reduces the betaM3 to alphaM1 separation by approximately 7 A. This supports the hypothesis that some intravenous anesthetics bind in the betaM3-alphaM1 subunit interface consistent with azi-etomidate photoaffinity labeling.

Probing protein packing surrounding the residues in and flanking the nicotinic acetylcholine receptor M2M3 loop

Nicotinic acetylcholine receptors (nAChR) are cation-selective, ligand-gated ion channels of the cysteine (Cys)-loop gene superfamily. The recent crystal structure of a bacterial homolog from Erwinia chrysanthemi (ELIC) agrees with previous structures of the N-terminal domain of AChBP (acetylcholine-binding protein) and of the electron-microscopy-derived Torpedo nAChR structure. However, the ELIC transmembrane domain is significantly more tightly packed than the corresponding region of the Torpedo nAChR. We investigated the tightness of protein packing surrounding the extracellular end of the M2 transmembrane segment and around the loop connecting the M2 and M3 segments using the substituted cysteine accessibility method. The M2 20' to 27' residues were highly water accessible and the variation in reaction rates were consistent with this region being alpha-helical. At all positions tested, the presence of ACh changed methanethiosulfonate ethylammonium (MTSEA) modification rates by <10-fold. In the presence of ACh, reaction rates for residues in the last extracellular alpha-helical turn of M2 and in the M2M3 loop increased, whereas rates in the penultimate alpha-helical turn of M2 decreased. Only three of eight M2M3 loop residues were accessible to MTSEA in both the presence and absence of ACh. We infer that the protein packing around the M2M3 loop is tight, consistent with its location at the interdomain interface where it is involved in the transduction of ligand binding in the extracellular domain to gating in the transmembrane domain. Our data indicate that the Torpedo nAChR transmembrane domain structure is a better model than the ELIC structure for eukaryotic Cys-loop receptors.

Residues accessible in the binding-site crevice of transmembrane helix 6 of the CB2 cannabinoid receptor

We have used the substituted-cysteine accessibility method (SCAM) to map the residues in the sixth membrane-spanning segment of the CB2 cannabinoid receptor that contribute to the surface of the water-accessible binding-site crevice. Using a background of the mutant C2.59S which is relatively insensitive to the methanethiosulfonate (MTS) reagents, we mutated to cysteine, one at a time, 34 consecutive residues in TMH6 of the CB2 receptor. These mutant receptors were then expressed in HEK293 cells. By incubating HEK293 cells stably transfected with CB2 receptors with the small, charged, hydrophilic, thiol-specific reagent methanethiosulfonate ethylammonium (MTSEA), [(3)H]CP55940 binding was significantly inhibited for six mutant receptors. All six of the mutants that reacted with MTSEA were protected from the reaction when pretreated with the cannabinoid agonist WIN55212-2, suggesting that MTSEA modification occurred within the binding crevice. Therefore, the side chains of the residues at these reactive loci (V6.51, L6.52, L6.54, M6.55, L6.59, and T6.62) are on the water-accessible surface of the binding-site crevice. These residues are extracellular to the TMH6 CWXP hinge motif. The pattern of accessibility is consistent with a alpha-helical conformation for this segment of TMH6. Molecular modeling studies performed in the context of the CB2 model show that V6.51, L6.52, L6.54, M6.55, L6.59, and T6.62 face into the CB2 binding pocket, further confirming our SCAM results. These results are similar to the accessibility patterns determined by SCAM studies of TMH6 in the opioid and dopamine D2 receptors.

Tryptophan scanning of the acetylcholine receptor's betaM4 transmembrane domain: decoding allosteric linkage at the lipid-protein interface with ion-channel gating

The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel protein that mediates fast excitatory synaptic transmission in the peripheral and central nervous systems. Changes in the structure and function of the AChR can lead to serious impairment of physiological processes. In this study, we combined site-directed mutagenesis, radioligand binding assays, electrophysiological recordings and Fourier analyses to characterize the functional role and structural aspects of the betaM4 transmembrane domain of the Torpedo AChR. We performed tryptophan replacements, from residues L438 through F455, along the betaM4 transmembrane domain. Expression levels of mutants F439W-G450W and F452W-I454W produced peak currents similar to or lower than those in wild-type (WT). Tryptophan substitutions at positions L438 and T451 led to a deficiency in either subunit expression or receptor assembly. Mutations L440W, V442W, C447W and S453W produced a gain-of-function response. Mutation F455W produced a loss of ion channel function. The periodicity profile of the normalized expression level (closed state) and EC(50) (open state) revealed a minor conformational change of 0.4 residues/turn of the betaM4 domain. These findings suggest that a minor movement of the betaM4 domain occurs during channel activation.

Functional contribution of alpha3L8' to the neuronal nicotinic alpha3 receptor

The role of position L8', located in transmembrane domain 1 of the neuronal nicotinic alpha3 subunit, was characterized by using two-electrode voltage clamp in Xenopus oocytes. Four amino acids (Ala, Ser, Phe, and Tyr) were inserted at this conserved position, and the mutant subunit was coexpressed with either wild-type beta2 or beta4 subunits. These substitutions led to significant alterations in the pharmacodynamic parameters of cholinergic agents, resulting in loss of function. Ala and Ser substitutions resulted in losses in agonist (ACh, nicotine, and DMPP) potency and intrinsic activity at both alpha3beta2 and alpha3beta4 receptors. Similarly, significant changes in antagonist potency were produced by the Ala and Ser substitutions. Phe and Tyr mutations did not alter the receptor's EC(50) for ACh or nicotine but reduced the EC(50) for DMPP at both receptors. The Phe mutation also reduced the intrinsic activity of all agonists tested at both receptors. The Tyr mutation, though, led to a decrease in intrinsic activity for all agonists at the alpha3beta2 receptor, yet resulted in no changes for DMPP, a decrease for nicotine, and an increase for ACh at the alpha3beta4 receptor. The most dramatic changes in the receptor's functional properties were produced by substitutions that introduced the largest changes in amino acid volume. Additional replacements (Gly, Thr, and Val) suggested an inverse correlation between amino acid volume at position alpha3L8' and EC(50) for alpha3beta4 nAChRs; however, alpha3beta2 nAChRs displayed a nonlinear correlation. These data demonstrate that structural alterations at position alpha3L8' could propagate to the agonist-binding site.

Aromaticity at the water-hydrocarbon core interface of the membrane: consequences on the nicotinic acetylcholine receptor

Almost all lipid-exposed transmembrane domains of integral proteins contain aromatic residues flanking the hydrophobic segment of the domains. These residues generally reside close to the carbonyl region of the membrane, and several structural and functional roles have been associated to these residues. Although the roles and physicochemical reasons for aromatic preference have been extensively studied using model systems, few studies have been done in a native membrane system. To gain insight about the mechanistic implication for this aromatic preference, we selected position alpha F426 of the muscle-type nicotinic acetylcholine receptor (nAChR). alpha F426 is a lipid-exposed residue at the extracellular segment of the alpha M4 transmembrane domain and is highly conserved among different nAChR subunits and species. We used site-directed mutagenesis, alpha-Bungarotoxin-binding assay, and two-electrodes voltage clamp in Xenopus laevis oocytes to characterize mutations at position alpha F426, which impart different physicochemical properties like volume, polarity, hydrogen bonds, aromaticity and net electrical charge. All mutations except the aromatic residues resulted in a significant reduction of the nAChR cell-surface levels and the macroscopic currents to acetylcholine. These results suggest that position alpha F426 contributes to structural stability and open-close transitions of the nAChR. Finally, the present study also provides information about how intermolecular interactions at position alpha 426 modulate open-close transitions of the nAChR.

Cross-linking of sites involved with alcohol action between transmembrane segments 1 and 3 of the glycine receptor following activation

The glycine receptor is a member of the Cys-loop, ligand-gated ion channel family and is responsible for inhibition in the CNS. We examined the orientation of amino acids I229 in transmembrane 1 (TM1) and A288 in TM3, which are both critical for alcohol and volatile anesthetic action. We mutated these two amino acids to cysteines either singly or in double mutants and expressed the receptors in Xenopus laevis oocytes. We tested whether disulfide bonds could form between A288C in TM3 paired with M227C, Y228C, I229C, or S231C in TM1. Application of cross-linking (mercuric chloride) or oxidizing (iodine) agents had no significant effect on the glycine response of wild-type receptors or the single mutants. In contrast, the glycine response of the I229C/A288C double mutant was diminished after application of either mercuric chloride or iodine only in the presence of glycine, indicating that channel gating causes I229C and A288C to fluctuate to be within 6 A apart and form a disulfide bond. Molecular modeling was used to thread the glycine receptor sequence onto a nicotinic acetylcholine receptor template, further demonstrating that I229 and A288 are near-neighbors that can cross-link and providing evidence that these residues contribute to a single binding cavity.

The Polarity of Lipid-Exposed Residues Contributes to the Functional Differences between Torpedo and Muscle-Type Nicotinic Receptors

A comparison between the Torpedo and muscle-type acetylcholine receptors (AChRs) reveals differences in several lipid-exposed amino acids, particularly in the polarity of those residues. The goal of this study was to characterize the role of eight lipid-exposed residues in the functional differences between the Torpedo and muscle-type AChRs. To this end, residues alphaS287, alphaC412, betaY441, gammaM299, gammaS460, deltaM293, deltaS297 and deltaN305 in the Torpedo AChR were replaced with those found in the muscle-type receptor. Mutant receptor expression was measured in Xenopus oocytes using [(125)I]-alpha-bungarotoxin, and AChR ion channel function was evaluated using the two-electrode voltage clamp. Eight mutant combinations resulted in an increase (1.5- to 5.2-fold) in AChR expression. Four mutant combinations produced a significant 46% decrease in the ACh 50% inhibitory concentration (EC(50)), while three mutant combinations resulted in 1.7- to 2-fold increases in ACh EC(50). Finally, seven mutant combinations resulted in a decrease in normalized, ACh-induced currents. Our results suggest that these residues, although remote from the ion channel pore, (1) contribute to ion channel gating, (2) may affect trafficking of AChR into specialized membrane domains and (3) account for the functional differences between Torpedo and muscle-type AChR. These findings emphasize the importance of the lipid-protein interface in the functional differences between the Torpedo and muscle-type AChRs.

Naturally occurring variations in the human 5-HT3A gene profoundly impact 5-HT3 receptor function and expression

BACKGROUND

The serotonin [5-hydroxytryptamine (5-HT)]-gated ion channel 5-HT3 is involved in the mediation of postoperative and radiotherapy/chemotherapy-induced nausea/emesis and in irritable bowel syndrome. It has also been suggested to play a role in various psychiatric diseases. Five naturally occurring single nucleotide polymorphisms leading to amino acid changes have been identified in the human 5-HT3A gene.

METHODS AND RESULTS

We investigated the functional effects of these polymorphisms on the 5-HT3A receptor using fluorescence-based cellular assays. Notably, variants A33T, S253N, and M257I displayed 5-HT-induced maximal responses of 3-64% of the wild-type response, whereas R344H and P391R exhibited wild-type-like function. All variants displayed wild-type-like potencies of 5-HT and three 5-HT3 antagonists. Furthermore, all variants displayed Kd values similar to that of the wild-type receptor in a [H]GR65630-binding assay. The surface expression of A33T, M257I, and R344H was reduced 2-4-fold compared with the wild-type, despite similar total expression levels. Finally, coexpression of wild-type 5-HT3A or 5-HT3B subunits with 5-HT3A variants A33T, S253N, or M257I resulted in mixed or heteromeric receptors, characterized by significantly reduced maximal responses to 5-HT compared with the wild-type receptors.

CONCLUSIONS

Three polymorphisms of the 5-HT3A gene gave rise to functionally impaired receptors whose function could not be rescued by either wild-type 5-HT3A or 5-HT3B. Three of the variant receptors were surface-expressed at reduced levels in spite of total expression levels similar to wild-type, indicating that these variants affect receptor biogenesis and/or trafficking. These severe single nucleotide polymorphism effects hold promise for identification of new 5-HT3A gene-disease causalities.

Structure of the first transmembrane domain of the neuronal acetylcholine receptor beta2 subunit

The recent cryoelectron microscopy structure of the Torpedo nicotinic acetylcholine receptor (nAChR) at 4-A resolution shows long helices for all transmembrane (TM) domains. This is in disagreement with several previous reports that the first TM domain of nAChR and other Cys-loop receptors are not entirely helical. In this study, we determined the structure and backbone dynamics of an extended segment encompassing the first TM domain (TM1e) of nAChR beta(2) subunit in dodecylphosphocholine micelles, using solution-state NMR and circular dichroism (CD) spectroscopy. Both CD and NMR results show less helicity in TM1e than in Torpedo nAChR structure (Protein Data Bank: 2BG9). The helical ending residues at the C-terminus are the same in the TM1e NMR structure and the Torpedo nAChR structure, but the helical starting residue (I-217) in TM1e is seven residues closer to the C-terminus. Interestingly, the helical starting residue is two residues before the highly conserved P-219, in accordance with the hypothesis that proline causes helical distortions at three residues preceding it. The NMR relaxation measurements show a dynamics pattern consistent with TM1e structure. The substantial nonhelical content adds greater flexibilities to TM1e, thereby implicating a different molecular basis for nAChR function compared to a longer and more rigid helical TM1.

A Cysteine-scanning Mutagenesis Study of Transmembrane Domain 8 of the Electrogenic Sodium/Bicarbonate Cotransporter NBCe1

Na/HCO(3) cotransporters (NBCs) such as NBCe1 are members of a superfamily of bicarbonate transporters that includes anion exchangers. Residues within putative transmembrane domain 8 (TMD8) of anion exchanger 1 are involved in ion translocation (Tang, X. B., Kovacs, M., Sterling, D., and Casey, J. R. (1999) J. Biol. Chem. 274, 3557-3564), and the corresponding domain in NBCe1 variants is highly homologous. We performed cysteine-scanning mutagenesis to examine the role of TMD8 residues in ion translocation by rat NBCe1-A. We accessed function and/or sulfhydryl sensitivity and p-chloromercuribenzene sulfonate (pCMBS) accessibility of 21 cysteine-substituted NBC mutants expressed in Xenopus oocytes using the two-electrode, voltage clamp technique. Five NBC mutants displayed <10% wild-type activity: P743C, A744C, L746C, D754C, and T758C. For the remaining 16 mutants, we compared transporter-mediated inward currents elicited by removing external Na(+) before and after exposing oocytes to either 2-aminoethylmethane thiosulfonate (MTSEA) or pCMBS. MTSEA inhibited NBC mutants T748C, I749C, I751C, F752C, M753C, and Q756C by 9-19% and stimulated mutants A739C, A741C, L745C, V747C, Q755C, and I757C by 11-21%. pCMBS mildly inhibited mutants A739C, A740, V747C, and Q756C by 5 or 8%, and stimulated I749C by 10%. However, both sulfhydryl reagents strongly inhibited the L750C mutant by > or =85%. Using the substituted cysteine accessibility method, we examined the accessibility of the NBC mutant L750C under different transporter conditions. pCMBS accessibility is (i) reduced when the transporter is active in the presence of both Na(+) and HCO(3)(-), likely due to substrate competition with pCMBS; (ii) reduced in the presence of a stilbene inhibitor; and (iii) stimulated at more positive membrane potentials. In summary, TMD8 residues of NBCe1, particularly L750, are involved in ion translocation, and accessibility is influenced by the state of transporter activity.

Novel beta subunit mutation causes a slow-channel syndrome by enhancing activation and decreasing the rate of agonist dissociation

We traced the cause of a slow-channel syndrome (SCS) in a patient with progressive muscle weakness, repetitive compound muscle action potential and prolonged low amplitude synaptic currents to a V --> F substitution in the M1 domain of the beta subunit (betaV229F) of the muscle acetylcholine receptor (AChR). In vitro expression studies in Xenopus oocytes indicated that the novel mutation betaV229F expressed normal amounts of AChRs and decreased the ACh EC50 by 10-fold compared to wild type. Kinetic analysis indicated that the mutation displayed prolonged mean open duration and repeated openings during activation. Prolonged openings caused by the betaV229F mutation were due to a reduction in the channel closing rate and an increase in the effective channel opening rate. Repeated openings of the channel during activation were caused by a significant reduction in the agonist dissociation constant. In addition, the betaV229F mutation produced an increase in calcium permeability. The kinetic and permeation studies presented in this work are sufficient to explain the consequences of the betaV229F mutation on the miniature endplate currents and thus are direct evidence that the betaV229F mutation is responsible for compromising the safety margin of neuromuscular transmission in the patient.

Dynamic Structural Investigations on the Torpedo Nicotinic Acetylcholine Receptor by Time-Resolved Photoaffinity Labeling

An increasing number of high-resolution structures of membrane-embedded ion channels (or soluble homologues) have emerged during the last couple of years. The most pressing need now is to understand the complex mechanism underlying ion-channel function. Time-resolved photoaffinity labeling is a suitable tool for investigating the molecular function of membrane proteins, especially when high-resolution structures of related proteins are available. However until now this methodology has only been used on the Torpedo nicotinic acetylcholine receptor (nAChR). nAChRs are allosteric cation-selective receptor channels that are activated by the neurotransmitter acetylcholine (ACh) and implicated in numerous physiological and pathological processes. Time-resolved photoaffinity labeling has already enabled local motions of nAChR subdomains (i.e. agonist binding sites, ion channel, subunit interface) to be understood at the molecular level, and has helped to explain how small molecules can exert their physiological effect, an important step toward the development of drug design. Recent analytical and technical improvements should allow the application of this powerful methodology to other membrane proteins in the near future.

Structural determinants of benzodiazepine allosteric regulation of GABA(A) receptor currents

Benzodiazepine enhancement of GABA(A) receptor current requires a gamma subunit, and replacement of the gamma subunit by the delta subunit abolishes benzodiazepine enhancement. Although it has been demonstrated that benzodiazepines bind to GABA(A) receptors at the junction between alpha and gamma subunits, the structural basis for the coupling of benzodiazepine binding to allosteric enhancement of the GABA(A) receptor current is unclear. To determine the structural basis for this coupling, the present study used a chimera strategy, using gamma2L-delta GABA(A) receptor subunit chimeras coexpressed with alpha1 and beta3 subunits in human embryonic kidney 293T cells. Different domains of the gamma2L subunit were replaced by delta subunit sequence, and diazepam sensitivity was determined. Chimeric subunits revealed two areas of interest: domain 1 in transmembrane domain 1 (M1) and domain 2 in the C-terminal portion of transmembrane domain 2 (M2) and the M2-M3 extracellular loop. In those domains, site-directed mutagenesis demonstrated that the following two groups of residues were involved in benzodiazepine transduction of current enhancement: residues Y235, F236, T237 in M1; and S280, T281, I282 in M2 as well as the entire M2-M3 loop. These results suggest that a pocket of residues may transduce benzodiazepine binding to increased gating. Benzodiazepine transduction involves a group of residues that connects the N terminus and M1, and another group of residues that may facilitate an interaction between the N terminus and the M2 and M2-M3 loop domains.

Sortases and the art of anchoring proteins to the envelopes of gram-positive bacteria

The cell wall envelopes of gram-positive bacteria represent a surface organelle that not only functions as a cytoskeletal element but also promotes interactions between bacteria and their environment. Cell wall peptidoglycan is covalently and noncovalently decorated with teichoic acids, polysaccharides, and proteins. The sum of these molecular decorations provides bacterial envelopes with species- and strain-specific properties that are ultimately responsible for bacterial virulence, interactions with host immune systems, and the development of disease symptoms or successful outcomes of infections. Surface proteins typically carry two topogenic sequences, i.e., N-terminal signal peptides and C-terminal sorting signals. Sortases catalyze a transpeptidation reaction by first cleaving a surface protein substrate at the cell wall sorting signal. The resulting acyl enzyme intermediates between sortases and their substrates are then resolved by the nucleophilic attack of amino groups, typically provided by the cell wall cross bridges of peptidoglycan precursors. The surface protein linked to peptidoglycan is then incorporated into the envelope and displayed on the microbial surface. This review focuses on the mechanisms of surface protein anchoring to the cell wall envelope by sortases and the role that these enzymes play in bacterial physiology and pathogenesis.

Theoretical studies of the M2 transmembrane segment of the glycine receptor: models of the open pore structure and current-voltage characteristics

The pentameric glycine receptor (GlyR), a member of the nicotinicoid superfamily of ligand-gated ion channels, is an inhibitory Cl(-) channel that is gated by glycine. Using recently published NMR data of the second transmembrane segment (M2) of the human alpha1 GlyR, structural models of pentameric assemblies embedded in a lipid bilayer were constructed using a combination of experimentally determined constraints coupled with all-atom energy minimization. Based on this structure of the pentameric M2 "pore", Brownian dynamics simulations of ion permeation through this putative conducting open state of the channel were carried out. Simulated I-V curves were in good agreement with published experimental current-voltage curves and the anion/cation permeability ratio, suggesting that our open-state model may be representative of the conducting channel of the full-length receptor. These studies also predicted regions of chloride occupancy and suggested residues critical to anion permeation. Calculations of the conductance of the cation-selective mutant A251E channel are also consistent with experimental data. In addition, both rotation and untilting of the pore helices of our model were found to be broadly consistent with closing of the channel, albeit at distinct regions that may reflect alternate gates of the receptor.

A model of the closed form of the nicotinic acetylcholine receptor m2 channel pore

The nicotinic acetylcholine receptor is a neurotransmitter-gated ion channel in the postsynaptic membrane. It is composed of five homologous subunits, each of which contributes one transmembrane helix--the M2 helix--to create the channel pore. The M2 helix from the delta subunit is capable of forming a channel by itself. Although a model of the receptor was recently proposed based on a low-resolution, cryo-electron microscopy density map, we found that the model does not explain much of the other available experimental data. Here we propose a new model of the M2 channel derived solely from helix packing and symmetry constraints. This model agrees well with experimental results from solid-state NMR, chemical reactivity, and mutagenesis experiments. The model depicts the channel pore, the channel gate, and the residues responsible for cation specificity.

Modeling noncompetitive antagonism of a nicotinic acetylcholine receptor

Models of closed and open channel pores of a muscle-type nicotinic acetylcholine receptor (nAChR) channel comprising M1 and M2 segments are presented. A model of the closed channel is proposed in which hydrophobic residues of the Equatorial Leucine ring screen the oxygen domain formed by the Serine ring, thereby preventing ion flux without completely occluding the pore. This model demonstrates a high similarity with the structure derived from a recent electron microscopy study. We propose that hydrophobic residues of the Equatorial Leucine ring are retracted when the pore is open. Our models provide a possible resolution of the nAChR gate controversy. We have also obtained explanations for the complex mechanisms underlying inhibition of nAChR by philanthotoxins (PhTXs). PhTX-343, containing a spermine moiety with a charge of +3, binds deep in the pore near the Serine ring where classical open channel blockers of nAChR bind. In contrast, PhTX-(12), which has a single charged amino group is unable to reach deeply located rings because of steric restrictions. Both philanthotoxins may bind to a hydrophobic site located close to the external entrance of the pore in a region that includes residues associated with the regulation of desensitization.

Gating-enhanced accessibility of hydrophobic sites within the transmembrane region of the nicotinic acetylcholine receptor's {delta}-subunit. A time-resolved photolabeling study

General anesthetics often interact more strongly with sites on open than on closed states of ligand-gated ion channels. To seek such sites, Torpedo membranes enriched in nicotinic acetylcholine receptors (nAChRs) were preincubated with the hydrophobic probe 3-(trifluoromethyl)-3-(m-iodophenyl) diazirine ([125I]TID) and exposed to agonist for either 0 ms (closed state), 1.5 and 10 ms (activated states), 1 s (fast desensitized state), or > or =1 h (equilibrium or slow desensitized state) and then rapidly frozen (<1 ms) and photolabeled. Within 1.5 ms, the fractional change in photoincorporation relative to the closed state decreased to 0.7 in the beta- and gamma-subunits, whereas in the alpha-subunit, it changed little. The most dramatic change occurred in the delta-subunit, where it increased to 1.6 within 10 ms but fell to 0.7 during fast desensitization. Four residues in the delta-subunit's transmembrane domain accounted for the enhanced photoincorporation induced by a 10-ms agonist exposure both when TID was added simultaneously with agonist and when it was preincubated with membranes. In the published closed state structure, two residues (deltaThr274 and deltaLeu278) are situated toward the extracellular end of helix M2, both contralateral to the ion channel and adjacent to the third residue (deltaPhe232) on M1. The fourth labeled residue (deltaIle288) is toward the end of the M2-M3 loop. Contact with these residues occurs on the time scale of a rapid phase of TID inhibition in Torpedo nAChRs, suggesting the formation of a transient hydrophobic pocket between M1, M2, and M3 in the delta-subunit during gating.

Homology Modeling of a Human Glycine Alpha 1 Receptor Reveals a Plausible Anesthetic Binding Site

The superfamily of ligand-gated ion channels (LGICs) has been implicated in anesthetic and alcohol responses. Mutations within glycine and GABA receptors have demonstrated that possible sites of anesthetic action exist within the transmembrane subunits of these receptors. The exact molecular arrangement of this transmembrane region remains at intermediate resolution with current experimental techniques. Homology modeling methods were therefore combined with experimental data to produce a more exact model of this region. A consensus from multiple bioinformatics techniques predicted the topology within the transmembrane domain of a glycine alpha one receptor (GlyRa1) to be alpha helical. This fold information was combined with sequence information using the SeqFold algorithm to search for modeling templates. Independently, the FoldMiner algorithm was used to search for templates that had structural folds similar to published coordinates of the homologous nAChR (1OED). Both SeqFold and Foldminer identified the same modeling template. The GlyRa1 sequence was aligned with this template using multiple scoring criteria. Refinement of the alignment closed gaps to produce agreement with labeling studies carried out on the homologous receptors of the superfamily. Structural assignment and refinement was achieved using Modeler. The final structure demonstrated a cavity within the core of a four-helix bundle. Residues known to be involved in modulating anesthetic potency converge on and line this cavity. This suggests that the binding sites for volatile anesthetics in the LGICs are the cavities formed within the core of transmembrane four-helix bundles.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

The αM1 segment of the nicotinic acetylcholine receptor exhibits conformational flexibility in a membrane environment

The transmembrane domain of the nicotinic acetylcholine receptor (nAChR) is predominantly alpha-helical, and of the four distinctly different transmembrane M-segments, only the helicity of M1 is ambiguous. In this study, we have investigated the conformation of a membrane-embedded synthetic M1 segment by solid-state nuclear magnetic resonance (NMR) methods. A 35-residue peptide representing the extended alphaM1 domain 206-240 of the Torpedo californica nAChR was synthesized with specific 13C - and 15N-labelled amino acids, and was incorporated in different phosphatidylcholine model membranes. The chemical shift of the isotopic labels was resolved by magic angle spinning (MAS) NMR and could be related to the secondary structure of the alphaM1 analog at the labelled sites. Our results show that the membrane-embedded alphaM1 segment forms an unstable alpha-helix, particularly near residue Leu18 (alphaLeu223 in the entire nAChR). This non-helical tendency was most pronounced when the peptide was incorporated in fully hydrated phospholipid bilayers, with an estimated 40-50% of the peptides having an extended conformation at position Leu18. We propose that the conserved proline residue at position 16 in the alphaM1 analog imparts a conformational flexibility on the M1 segments that could enable membrane-mediated modulation of nAChR activity.

Comparative modeling of a GABAA alpha1 receptor using three crystal structures as templates

We built a model of a GABAA alpha1 receptor (GABAAR) that combines the ligand binding (LBD) and the transmembrane domains (TMD). We used six steps: (1) a four-alpha helical bundle in the crystal structure of bovine cytochrome c oxidase (2OCC) was identified as a template for the TMD of a single subunit. (2) The five pore-forming alpha helices of a bacterial mechanosensitive channel (1MSL) served as a template for the pentameric ion channel. (3) Five copies of the tetrameric template from 2OCC were superimposed on 1MSL to produce a homopentamer containing 20 alpha helices arranged around a funnel-shaped central pore. (4) Five copies of the GABAAR sequence were threaded onto the alpha-helical segments of this template and inter-helical loops were generated to produce the TMD model. (5) A model of the LBD was built by threading the aligned sequence of GABAAR onto the crystal structure of the acetylcholine binding protein (1I9B). (6) The models of the LBD and the TMD were aligned along a common five-fold axis, moved together along that axis until in vdW contact, merged, and then optimized with restrained molecular dynamics. Our model corresponds closely with recently published coordinates of the acetylcholine receptor (1OED) but also explains additional features. Our model reveals structures of loops that were not visible in the cryoelectron micrograph and satisfies most labeling and mutagenesis data. It also suggests mechanisms for ligand binding transduction, ion selectivity, and anesthetic binding.

Tryptophan scanning mutagenesis of the gammaM4 transmembrane domain of the acetylcholine receptor from Torpedo californica

The periodicity of structural and functional effects induced by tryptophan scanning mutagenesis has been successfully used to define function and secondary structure of various transmembrane domains of the acetylcholine receptor of Torpedo californica. We expand the tryptophan scanning of the AchR of T. californica to the gammaM4 transmembrane domain (gammaTM4) by introducing tryptophan, at residues 451-462, along the gammaTM4. Wild type (WT) and mutant AChR were expressed in Xenopus laevis oocytes. Using [(125)I]alpha-bungarotoxin binding assays and voltage clamp, we determined that the nAChR expression, EC(50), and Hill coefficient values for WT are 1.8 +/- 0.4 fmol, 30.3 +/- 1.1 microM, and 1.8 +/- 0.3, respectively. Mutations L456W, F459W, and G462W induce a significant increase in nAChR expression (2.8 +/- 0.5, 3.6 +/- 0.6, and 3.0 +/- 0.5 fmol, respectively) when compared with WT. These data suggest that these residues are important for AChR oligomerization. Mutations A455W, L456W, F459W, and G462W result in a significant decrease in EC(50) (19.5 +/- 1.7, 11.4 +/- 0.7, 16.4 +/- 3.8, and 19.1 +/- 2.6 microM, respectively), thus suggesting a gain in function when compared with WT. In contrast, mutation L458W induced an increase in EC(50) (42.8 +/- 6.8 microM) or loss in function when compared with WT. The Hill coefficient values were the same for WT and all of the mutations studied. The periodicity in function (EC(50) and macroscopic peak current) and nAChR expression reveals an average of 3.3 and 3.0 amino acids respectively, thus suggesting a helical secondary structure for the gammaTM4.

Evidence that the TM1-TM2 Loop Contributes to the ρ1 GABA Receptor Pore

Considerable evidence indicates the second transmembrane domain (TM2) of the gamma-aminobutyric acid (GABA) receptor lines the integral ion pore. To further delineate the structures that constitute the ion pore and selectivity filter of the rho1 GABA receptor, we used the substituted cysteine accessibility method with charged reagents to identify anion- and cation-accessible surfaces. Twenty-one consecutive residues were mutated to cysteine, one at a time, in the presumed intracellular end of the first transmembrane domain (TM1; Ala(271)-Met(276)), the entire linker connecting TM1 to TM2 (Leu(277)-Arg(287)), and the presumed intracellular end of TM2 (Ala(288)-Ala(291)). Positively (MTSEA(+)) and negatively (pCMBS(-)) charged sulfhydryl reagents, as well as Cd(2+), were added extracellularly to test accessibility of the engineered cysteines. Four of the mutants, all at the intracellular end of TM2 (R287C, V289C, P290C, A291C), were accessible to positively charged reagents, whereas seven mutants (A271C, T272C, L277C, W279C, V280C, P290C, A291C) were functionally modified by negatively charged pCMBS(-). These seven modified residues were at the intracellular end of TM2, in the TM1-TM2 linker, and at the intracellular end of TM1. In nearly all cases (excluding P290C), the rate and the degree of modification were state-dependent, with greater accessibility in the presence of agonist. Select cysteine mutants were combined with a point mutation (A291E) that converted the pore from chloride- to non-selective. In this case, positively charged reagents could modify residues in the TM1-TM2 linker (Leu(277) and Val(280)), supporting the notion that the modifying reagents were reaching their target through the pore. Taken together, our results suggest that, up to its intracellular end, the TM2 domain is not charge selective. In addition, we propose that the TM1-TM2 linker and the intracellular end of TM1 are along the pathway of the permeating ion. These findings may lend new insights into the structure of the GABA receptor pore.

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.

Agonist-Induced Transitions of the Acetylcholine Receptor

Anti-acetylcholine receptor (AChR) monoclonal antibody 383C binds to the beta-hairpin loop alpha(187-199) of only one of the two Torpedo AChR alpha subunits. The loop recognized is associated with the alpha subunit corresponding to the high-affinity d-tubocurarine (dTC) binding site. Desensitization of the receptor with carbamylcholine completely blocks the binding of 383C. Mild reduction of AChR alpha subunit cys 192-193 disulfide with DTT and subsequent reaction with 5-iodoacetamidofluorescein label only the high-affinity dTC alpha subunit. Rhodamine-labeled alpha-bungarotoxin (R-Btx) binds to the unlabeled AChR alpha subunit as monitored by fluorescence resonance energy transfer between the fluorescein and rhodamine dyes. A 10-A contraction of the distance between the dyes is observed following the addition of carbamylcholine. In a small angle X-ray diffraction experiment exploiting anomalous X-ray scattering from Tb(III) ions titrated into AChR Ca(II) binding sites, we find evidence for a change in the Tb(III) ion distribution in the region of the ion channel following addition of carbamylcholine to the AChR. The carbamylcholine-induced loss of the 383C epitope, the 10-A contraction of the beta-hairpin loop, and the loss of multivalent cations from the channel likely represent the first molecular transitions leading to AChR channel opening.