Image analysis of the heavy form of the acetylcholine receptor from Torpedo marmorata

Orthosteric and allosteric potentiation of heteromeric neuronal nicotinic acetylcholine receptors

Heteromeric nicotinic ACh receptors (nAChRs) were thought to have two orthodox agonist-binding sites at two α/β subunit interfaces. Highly selective ligands are hard to develop by targeting orthodox agonist sites because of high sequence similarity of this binding pocket among different subunits. Recently, unorthodox ACh-binding sites have been discovered at some α/α and β/α subunit interfaces, such as α4/α4, α5/α4 and β3/α4. Targeting unorthodox sites may yield subtype-selective ligands, such as those for (α4β2)₂ α5, (α4β2)₂ β3 and (α6β2)₂ β3 nAChRs. The unorthodox sites have unique pharmacology. Agonist binding at one unorthodox site is not sufficient to activate nAChRs, but it increases activation from the orthodox sites. NS9283, a selective agonist for the unorthodox α4/α4 site, was initially thought to be a positive allosteric modulator (PAM). NS9283 activates nAChRs with three engineered α4/α4 sites. PAMs, on the other hand, act at allosteric sites where ACh cannot bind. Known PAM sites include the ACh-homologous non-canonical site (e.g. morantel at β/α), the C-terminus (e.g. Br-PBTC and 17β-estradiol), a transmembrane domain (e.g. LY2087101) or extracellular and transmembrane domain interfaces (e.g. NS206). Some of these PAMs, such as Br-PBTC and 17β-estradiol, require only one subunit to potentiate activation of nAChRs. In this review, we will discuss differences between activation from orthosteric and allosteric sites, their selective ligands and clinical implications. These studies have advanced understanding of the structure, assembly and pharmacology of heteromeric neuronal nAChRs.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

Allosteric regulation of pentameric ligand-gated ion channels: an emerging mechanistic perspective

Pentameric ligand-gated ion channels (pLGICs) play a central role in intercellular communications in the nervous system by converting the binding of a chemical messenger—a neurotransmitter—into an ion flux through the postsynaptic membrane. They are oligomeric assemblies that provide prototypical examples of allosterically regulated integral membrane proteins. Here, we present an overview of the most recent advances on the signal transduction mechanism based on the X-ray structures of both prokaryotic and invertebrate eukaryotic pLGICs and atomistic Molecular Dynamics simulations. The present results suggest that ion gating involves a large structural reorganization of the molecule mediated by two distinct quaternary transitions, a global twisting and the blooming of the extracellular domain, which can be modulated by ligand binding at the topographically distinct orthosteric and allosteric sites. The emerging model of gating is consistent with a wealth of functional studies and will boost the development of novel pharmacological strategies.

The concept of allosteric interaction and its consequences for the chemistry of the brain

Throughout this Reflections article, I have tried to follow up on the genesis in the 1960s and subsequent evolution of the concept of allosteric interaction and to examine its consequences within the past decades, essentially in the field of the neuroscience. The main conclusion is that allosteric mechanisms built on similar structural principles operate in bacterial regulatory enzymes, gene repressors (and the related nuclear receptors), rhodopsin, G-protein-coupled receptors, neurotransmitter receptors, ion channels, and so on from prokaryotes up to the human brain yet with important features of their own. Thus, future research on these basic cybernetic sensors is expected to develop in two major directions: at the elementary level, toward the atomic structure and molecular dynamics of the conformational changes involved in signal recognition and transduction, but also at a higher level of organization, the contribution of allosteric mechanisms to the modulation of brain functions.

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

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

Alpha-bungarotoxin binding to acetylcholine receptor membranes studied by low angle X-ray diffraction

The nicotinic acetylcholine receptor (nAChR) carries two binding sites for snake venom neurotoxins. alpha-Bungarotoxin from the Southeast Asian banded krait, Bungarus multicinctus, is a long neurotoxin which competitively blocks the nAChR at the acetylcholine binding sites in a relatively irreversible manner. Low angle x-ray diffraction was used to generate electron density profile structures at 14-A resolution for Torpedo californica nAChR membranes in the absence and presence of alpha-bungarotoxin. Analysis of the lamellar diffraction data indicated a 452-A lattice spacing between stacked nAChR membrane pairs. In the presence of alpha-bungarotoxin, the quality of the diffraction data and the lamellar lattice spacing were unchanged. In the plane of the membrane, the nAChRs packed together with a nearest neighbor distance of 80 A, and this distance increased to 85 A in the presence of toxin. Electron density profile structures were calculated in the absence and presence of alpha-bungarotoxin, revealing a location for the toxin binding sites. In native, fully-hydrated nAChR membranes, alpha-bungarotoxin binds to the nAChR outer vestibule and contacts the surface of the membrane bilayer.

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

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

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Atomic force microscopy of cloned nicotinic acetylcholine receptor expressed in Xenopus oocytes

The nicotinic acetylcholine receptor (AChR) was expressed in Xenopus oocytes from in vitro transcribed mRNA and was imaged by atomic force microscopy. A characteristic pentameric structure of AChR was readily observed on the extracellular face of the cell membrane, with a central pore surrounded by protruding AChR subunits. These structures were seen only in mRNA-injected oocytes that also gave acetylcholine-induced membrane currents. The size of individual AChR channels, the angles between subunits, and the interchannel spacing were all compatible with the current model of AChR. In addition, localized patches of microscopic AChR clustering were observed, with packing density approaching that at the neuromuscular junction. These findings show the potential of studying cloned membrane proteins in oocytes for both their surface topography and their structure-function relationship in native membrane without the need for crystallization.

A Raman spectroscopic study of acetylcholine receptor-rich membranes from Torpedo marmorata. Interaction of the receptor with carbamylcholine and (+)-tubocurarine

Raman spectroscopy is used to determine structural features of alkali-treated subsynaptic membrane fragments from Torpedo marmorata electric organ, rich in native functional AcChR. Distinct vibrations attributable to the membrane proteins and lipids were identified and studied before and after addition of the agonist carbamylcholine and the competitive antagonist (+)-tubocurarine. The protein secondary structure determined by using amide-I polypeptide vibrational analysis, indicates 47% alpha-helices, 25% beta-sheets, 18% turns and 11% undefined structure. The secondary structure of the AcChR molecule was not subject to large modifications upon addition of carbamylcholine. But, the presence of the (+)-tubocurarine leads to detectable changes in the amide-I region which might be interpreted as reflecting different contributions of alpha-helices and turns in the secondary structure. In addition, Raman spectra provide information about the environment of aromatic amino acids (tyrosine and tryptophan), the (C-C) bonds, the CH2 and CH3 groups of aliphatic side chains, as well as the disulfide (S-S) and cystein (C-S) bonds. The tyrosines seem 'exposed' to the aqueous medium. The Raman spectra of the AcChR-carbamylcholine complex suggest 'exposed' tryptophans, while those of the unliganded membrane-bound AcChR or of the receptor with (+)-tubocurarine are shown 'buried'. The disulfide bridges in the AcChR subunits show identical conformation in the absence and presence of carbamylcholine. On the contrary, considerable changes are found in the AcChR-(+)-tubocurarine complex. Carbamylcholine and especially (+)-tubocurarine decrease lipid fluidity.

The functional architecture of the acetylcholine nicotinic receptor explored by affinity labelling and site-directed mutagenesis

The scientific community will remember Peter Läuger as an exceptional man combining a generous personality and a sharp and skilful mind. He was able to attract by his views the interest of a large spectrum of biologists concerned by the mechanism of ion translocation through membranes. Yet, he was not a man with a single technique or theory. Using an authentically multidisciplinary approach, his ambition was to ‘understand transmembrane transport at the microscopic level, to capture its dynamics in the course of defined physiological processes’ (1987). According to him, ‘new concepts in the molecular physics of proteins’ had to be imagined, and ‘the traditional static picture of proteins has been replaced by the notions that proteins represent dynamic structures, subjected to conformational fluctuations covering a very wide time-range’ (1987).

BIP associates with newly synthesized subunits of the mouse muscle nicotinic receptor

A slow conformational change in newly synthesized acetylcholine receptor subunits is thought to be a requisite step in the biogenesis of this multi-subunit transmembrane glycoprotein. Previously, we demonstrated that this early conformational change within the alpha-subunit was inefficient and dependent upon disulfide bond formation (Blount, P. and J.P. Merlie. 1990. J. Cell Biol. 111:2613-2622). Here we show that newly synthesized acetylcholine receptor subunits and subunit complexes in the muscle-like cell line, BC3H-1, are associated with Bip, a ubiquitous binding protein of the endoplasmic reticulum. Characterization of the Bip/alpha-subunit complex in stably transfected fibroblasts revealed that Bip associates with newly synthesized unassembled alpha-subunit and some alpha gamma and alpha delta subunit complexes. Significantly, Bip does not associate well with the more mature form of the alpha-subunit containing an intramolecular disulfide bridge. Hence, Bip may play an important role in the conformational maturation and/or editing of unassembled AChR subunits and subunit complexes in vivo.

Mutational analysis of muscle nicotinic acetylcholine receptor subunit assembly

The structural elements required for normal maturation and assembly of the nicotinic acetylcholine receptor alpha subunit were investigated by expression of mutated subunits in transfected fibroblasts. Normally, the wild-type alpha subunit acquires high affinity alpha bungarotoxin binding in a time-dependent manner; however, mutation of the 128 and/or 142 cysteines to either serine or alanine, as well as deletion of the entire 14 amino acids in this region abolished all detectable high affinity binding. Nonglycosylated subunits that had a serine to glycine mutation in the consensus sequence also did not efficiently attain high affinity binding to toxin. In contrast, mutation of the proline at position 136 to glycine or alanine, or a double mutation of the cysteines at position 192 and 193 to serines had no effect on the acquisition of high affinity toxin binding. These data suggest that a disulfide bridge between cysteines 128 and 142 and oligosaccharide addition at asparagine 141 are required for the normal maturation of alpha subunit as assayed by high affinity toxin binding. The unassembled wild-type alpha subunit expressed in fibroblasts is normally degraded with a t1/2 of 2 h; upon assembly with the delta subunit, the degradation rate slows significantly (t1/2 greater than 13 h). All mutated alpha subunits retained the capacity to assemble with a delta subunit coexpressed in fibroblasts; however, mutated alpha subunits that were not glycosylated or did not acquire high affinity toxin binding were rapidly degraded (t1/2 = 20 min to 2 h) regardless of whether or not they assembled with the delta subunit. Assembly and rapid degradation of nonglycosylated acetylcholine receptor (AChR) subunits and subunit complexes were also observed in tunicamycin-treated BC3H-1 cells, a mouse musclelike cell line that normally expresses functional AChR. Hence, rapid degradation may be one form of regulation assuring that only correctly processed and assembled subunits accumulate, and ultimately make functional receptors in AChR-expressing cells.

Assembly intermediates of the mouse muscle nicotinic acetylcholine receptor in stably transfected fibroblasts

We have used fibroblast clones expressing muscle nicotinic acetylcholine receptor alpha and gamma, and alpha and delta subunits to measure the kinetics of subunit assembly, and to study the properties of the partially assembled products that are formed. We demonstrate by coimmunoprecipitation that assembly intermediates in fibroblasts coexpressing alpha and delta subunits are formed in a time-dependent manner. The alpha and gamma- and the alpha and delta-producing transfected cells form complexes that, when labeled with 125I-alpha-bungarotoxin, migrate in sucrose gradients at 6.3S, a value consistent with a hetero-dimer structure. An additional peak at 8.5S is formed from the alpha and gamma subunits expressed in fibroblasts suggesting that gamma may have more than one binding site for alpha subunit. The stability and specificity of formation of these partially assembled complexes suggests that they are normal intermediates in the assembly of acetylcholine receptor. Comparison of the binding of 125I-alpha-bungarotoxin to intact and detergent-extracted fibroblasts indicate that essentially all of the binding sites are retained in an intracellular pool. The fibroblast delta subunit has the electrophoretic mobility in SDS-PAGE of a precursor that does not contain complex carbohydrates. In addition, alpha gamma and alpha delta complexes had lectin binding properties expected of subunits lacking complex oligosaccharides. Therefore, fibroblasts coexpressing alpha and gamma or alpha and delta subunits produce discrete assembly intermediates that are retained in an intracellular compartment and are not processed by Golgi enzymes.

Nonequivalence of alpha-bungarotoxin binding sites in the native nicotinic receptor molecule

In the native, membrane-bound form of the nicotinic acetylcholine receptor (M-AcChR) the two sites for the cholinergic antagonist alpha-bungarotoxin (alpha-BGT) have different binding properties. One site has high affinity, and the M-AcChR/alpha-BGT complexes thus formed dissociate very slowly, similar to the complexes formed with detergent-solubilized AcChR (S-AcChR). The second site has much lower affinity (KD approximately 59 +/- 35 nM) and forms quickly reversible complexes. The nondenaturing detergent Triton X-100 is known to solubilize the AcChR in a form unable, upon binding of cholinergic ligands, to open the ion channel and to become desensitized. Solubilization of the AcChR in Triton X-100 affects the binding properties of this second site and converts it to a high-affinity, slowly reversible site. Prolonged incubation of M-AcChR at 4 degrees C converts the low-affinity site to a high-affinity site similar to those observed in the presence of Triton X-100. Although the two sites have similar properties when the AcChR is solubilized in Triton X-100, their nonequivalence can be demonstrated by the effect on alpha-BGT binding of concanavalin A, which strongly reduces the association rate of one site only. The Bmax of alpha-BGT to either Triton-solubilized AcChR or M-AcChR is not affected by the presence of concanavalin A. Occupancy of the high-affinity, slowly reversible site in M-AcChR inhibits the Triton X-100 induced conversion to irreversibility of the second site. At difference with alpha-BGT, the long alpha-neurotoxin from Naja naja siamensis venom (alpha-NTX) binds with high affinity and in a very slowly reversible fashion to two sites in the M-AcChR (Conti-Tronconi & Raftery, 1986). We confirm here that Triton-solubilized AcChR or M-AcChR binds in a very slowly reversible fashion the same amount of alpha-NTX.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular basis of the two nonequivalent ligand binding sites of the muscle nicotinic acetylcholine receptor

We have stably expressed in fibroblasts different pairs of alpha and non-alpha subunits of the mouse muscle nicotinic acetylcholine receptor (AChR). The gamma and delta, but not the beta, subunits associated efficiently with the alpha subunit, and they extensively modified its binding characteristics. The alpha gamma and alpha delta complexes formed distinctly different high affinity binding sites for the competitive antagonist d-tubocurarine that, together, completely accounted for the two nonequivalent antagonist binding sites in native AChR. The alpha delta complex and native AChR had similar affinities for the agonist carbamylcholine. In contrast, although the alpha gamma complex contains the higher affinity competitive antagonist binding site, it had an affinity for carbamylcholine that was an order of magnitude less than that of the alpha delta complex or the AChR. The comparatively low agonist affinity of the alpha gamma complex may represent an allosterically regulated binding site in the native AChR. These data support a model of two nonequivalent binding sites within the AChR and imply that the basis for this nonequivalence is the association of the alpha subunit with the gamma or delta subunit.

Three-dimensional structure of the nicotinic acetylcholine receptor and location of the major associated 43-kD cytoskeletal protein, determined at 22 A by low dose electron microscopy and x-ray diffraction to 12.5 A

The three-dimensional structure of the nicotinic acetylcholine receptor (AChR) from Torpedo californica, crystallized both before and after removal of associated proteins, most notably the main 43-kD cytoskeletal protein that interacts both with AChR and actin, is determined to a resolution of 22 A. This is the first structural analysis where the 43-kD protein has been removed from the sample before crystallization. Thus, it provides the most reliable assessment of what constitutes the structure of the minimal five subunit AChR complex, and, by comparison with the native membrane, of the location of the 43-kD cytoskeletal protein. Image reconstruction of two-dimensional crystals includes information from electron images of up to +/- 52 degrees tilted specimens of latticed AChR. Hybrid density maps that include x-ray diffraction perpendicular to the membrane to 12.5 A resolution were used and eliminate some of the distortions introduced in maps based only on electron microscopic analyses. Comparison of the difference Fourier density maps between AChR with its normal complement of associated proteins, and without them shows that the main density, assigned to the actin-binding 43-kD component is closely associated with the lipid bilayer as well as with the cytoplasmic domain of the AChR. It binds beside the AChR, not beneath it as suggested by others (C. Toyoshima and N. Unwin 1988. Nature [Lond.]. 336:237-240). There is good agreement between the volumes of density for structural components and expected volumes based on their molecular weight. Acetylcholine receptors aggregate in the absence of any cytoskeletal proteins, suggesting that the AChR alone is sufficient to encode and stabilize clustering, and perhaps to do so during synaptogenesis. The main 43-kD component may play a role in location and rate of association of AChR. We show that the disulfide bond that cross-links delta-delta chains of adjacent pentamers in about 80% of AChR, is not required to stabilize the lattice of AChR. Latticed tube structures are stable indefinitely. The lattices described here have 20% less volume of lipid than those originally obtained and characterized by J. Kistler and R. M. Stroud (1981. Proc. Natl. Acad. Sci. USA. 78:3678-3682), or those subsequently characterized by A. Brisson and P. N. T. Unwin (1984. J. Cell Biol. 99:1202-1211) and A. Brisson and P. N. T. Unwin (1985. Nature (Lond.). 315:474-477).

The lipid environment of the nicotinic acetylcholine receptor in native and reconstituted membranes

Detailed knowledge of the membrane framework surrounding the nicotinic acetylcholine receptor (AChR) is key to an understanding of its structure, dynamics, and function. Recent theoretical models discuss the structural relationship between the AChR and the lipid bilayer. Independent experimental data on the composition, metabolism, and dynamics of the AChR lipid environment are analyzed in the first part of the review. The composition of the lipids in which the transmembrane AChR chains are inserted bears considerable resemblance among species, perhaps providing this evolutionarily conserved protein with an adequate milieu for its optimal functioning. The effects of lipids on the latter are discussed in the second part of the review. The third part focuses on the information gained on the dynamics of AChR and lipids in the membrane, a section that also covers the physical properties and interactions between the protein, its immediate annulus, and the bulk lipid bilayer.

Identification of synaptophysin as a hexameric channel protein of the synaptic vesicle membrane

The quaternary structure and functional properties of synaptophysin, a major integral membrane protein of small presynaptic vesicles, were investigated. Cross-linking and sedimentation studies indicate that synaptophysin is a hexameric homo-oligomer, which in electron micrographs exhibits structural features common to channel-forming proteins. On reconstitution into planar lipid bilayers, purified synaptophysin displays voltage-sensitive channel activity with an average conductance of about 150 picosiemens. Because specific channels and fusion pores have been implicated in vesicular uptake and release of secretory compounds, synaptophysin may have a role in these processes.

Electron microscopic observations of reconstituted proteoliposomes with the purified major intrinsic membrane protein of eye lens fibers

The purified major intrinsic protein of the lens fiber plasma membrane (MP26) reconstituted into liposomes favored membrane-to-membrane close contacts as visualized by freeze fracture and immunoelectron microscopy. Reconstituted apposed unilamellar vesicles formed pentalaminar profiles, and multilamellar liposomes showed regions of stacked bilayers. Immunogold labeling, using antibody directed against MP26, demonstrated that this polypeptide is present in regions of membrane-to-membrane close interaction. Fracture faces displayed both randomly distributed clusters of 8-nm polygonal intramembrane particles and membrane domains where a bidimensional lattice of repeating subunits was present. The structural pleomorphism which characterized the MP26-reconstituted proteoliposomes seems quite comparable to that visualized in natural fiber plasma membrane domains.

Location of subunits within the acetylcholine receptor by electron image analysis of tubular crystals from Torpedo marmorata

The binding sites on the nicotinic acetylcholine receptor of labels specific for the alpha-, beta-, and delta-subunits were determined by electron image analysis, using tubular crystals of receptors grown from the postsynaptic membranes of Torpedo marmorata electric organ. The labels were alpha-bungarotoxin (which attaches to the acetylcholine binding sites on the pair of alpha-subunits), Fab35 (a monoclonal antibody Fab fragment directed against the main immunogenic region of the alpha-subunit), Fab111 (a monoclonal antibody Fab fragment directed against a cytoplasmic site on the beta-subunit), and wheat germ agglutinin (which binds to N-acetylglucosamine residues on the delta-subunit). These labels, bound to receptors in the crystals, were located by comparing labeled with native structures, averaged in each case over more than 5,000 molecules. From the assignments made, we find that the clockwise arrangement of subunits around the receptor, viewed from the synaptic face, is: alpha, beta, alpha, gamma, and delta; that the main immunogenic region is at (or close to) the side of the alpha-subunit; and that the two acetylcholine binding sites are at the synaptic end of the alpha-subunits, 27-28 A from the central axis and approximately 53 A apart. In the crystal lattice, neighboring molecules are paired so that their delta- and alpha-subunits are juxtaposed, an organization that appears to relate closely to the grouping of receptors in vivo.

Location of antigenic determinants on primary sequences of subunits of nicotinic acetylcholine receptor by peptide mapping

The binding domains of 28 monoclonal antibodies (mAbs) against the alpha, beta, and delta subunits of the Torpedo acetylcholine receptor were mapped on the primary sequences of these subunits. Small peptide fragments (2000-20,000 daltons) of the purified subunits were obtained by digestion with staphylococcal V8 protease and papain, separated on a discontinuous polyacrylamide gel electrophoretic system, and electroblotted onto diaminophenyl thioether paper. The blots were probed with the various monoclonal antibodies and also with antibodies against carboxy-terminal decapeptides of the alpha, beta, and delta subunits to identify the carboxy-terminal fragments. From inspection of the binding patterns of the various antibodies to the subunits fragments and the molecular weights of these fragments, and by using the carboxy termini of the subunits as reference points, it was possible to deduce the regions on the primary sequence of each subunit in which the antibodies bound and in some cases to order the binding sites within these sequences. mAb 148, which inhibits receptor function by cross-linking receptor molecules on the cytoplasmic side, was mapped to the sequence beta 368-406. The main immunogenic region of the native receptor, which is of pathological importance in the autoimmune disease myasthenia gravis, was mapped by using mAb 210 to within 80 amino acid residues (alpha 46-127). The overall antigenic structure of alpha subunits was examined. Synthetic peptides have been used to locate determinants responsible for 83% of the antibodies in antisera to denatured alpha subunits and 46% of the antibodies to denatured alpha subunits in antisera to intact receptor. Theoretical models of the transmembrane orientation of the subunit polypeptide chains were tested by determining whether mapped monoclonal antibodies bound to the extracellular or intracellular surface of receptor-rich membranes. Our results confirm previous reports that the carboxy termini of the subunits are exposed on the intracellular surface, as is part of the region between a putative channel-forming domain (M5) and a putative membrane-spanning region (M3). However, contrary to current theoretical models, the region between M5 and the putative membrane-spanning sequence M4 also appears to be on the intracellular surface, implying that M4 and M5 are not membrane-spanning domains.(ABSTRACT TRUNCATED AT 400 WORDS)

Transmembrane topography of nicotinic acetylcholine receptor: immunochemical tests contradict theoretical predictions based on hydrophobicity profiles

In our preceding paper [Ratnam, M., Sargent, P. B., Sarin, V., Fox, J. L., Le Nguyen, D., Rivier, J., Criado, M., & Lindstrom, J. (1986) Biochemistry (preceding paper in this issue)], we presented results from peptide mapping studies of purified subunits of the Torpedo acetylcholine receptor which suggested that the sequence beta 429-441 is on the cytoplasmic surface of the receptor. Since this finding contradicts earlier theoretical models of the transmembrane structure of the receptor, which placed this sequence of the beta subunit on the extracellular surface, we investigated the location of the corresponding sequence (389-408) and adjacent sequences of the alpha subunit by a more direct approach. We synthesized peptides including the sequences alpha 330-346, alpha 349-364, alpha 360-378, alpha 379-385, and alpha 389-408 and shorter parts of these peptides. These peptides corresponded to a highly immunogenic region, and by using 125I-labeled peptides as antigens, we were able to detect in our library of monoclonal antibodies to alpha subunits between two and six which bound specifically to each of these peptides, except alpha 389-408. We obtained antibodies specific for alpha 389-408 both from antisera against the denatured alpha subunit and from antisera made against the peptide. These antibodies were specific to alpha 389-396. In binding assays, antibodies specific for all of these five peptides bound to receptor-rich membrane vesicles only after permeabilization of the vesicles to permit access of the antibodies to the cytoplasmic surface of the receptors, suggesting that the receptor sequences which bound these antibodies were located on the intracellular side of the membrane. Electron microscopy using colloidal gold to visualize the bound antibodies was used to conclusively demonstrate that all of these sequences are exposed on the cytoplasmic surface of the receptor. These results, along with our previous demonstration that the C-terminal 10 amino acids of each subunit are exposed on the cytoplasmic surface, show that the hydrophobic domain M4 (alpha 409-426), previously predicted from hydropathy profiles to be transmembranous, does not, in fact, cross the membrane. Further, these results show that the putative amphipathic transmembrane domain M5 (alpha 364-399) also does not cross the membrane. Our results thus indicate that the transmembrane topology of a membrane protein cannot be deduced strictly from the hydropathy profile of its primary amino acid sequence. We present a model for the transmembrane orientation of receptor subunit polypeptide chains which is consistent with current data.

Functional domains of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is a multisubunit, membrane-spanning protein that contains a gated, cation-conducting channel. Our approach to the understanding of the function of this receptor in molecular terms has been to locate its functionally significant sites in the sequences of its subunits and in its three-dimensional structure. In addition, we have tried to correlate transitions in the properties of these sites with functional transitions of the receptor. On binding acetylcholine, the nicotinic acetylcholine receptor enters at least two transient states, the open state and the rapid-onset desensitized state, and, in the continued presence of agonist, finally subsides into the slow-onset desensitized state. The transitions of the receptor between these various states are susceptible to regulation by acetylcholine and its congeners acting at one type of site and by a broad class of noncompetitive inhibitors (NCIs), including local anesthetics, acting at other sites. The chain composition of the receptor is alpha 2 beta gamma delta. The two acetylcholine binding sites are on the alpha chains, and two residues contributing to these sites, Cys-192 and Cys-193, have been identified. Furthermore, these adjacent Cys residues are cross-linked by a disulfide bond. In the quaternary structure of the receptor, the chains appear to be arranged in the order alpha gamma alpha beta delta around a central channel. Both the alpha and beta chains contribute to functionally significant NCI binding sites. The addition to receptor-rich membrane from Torpedo electric tissue of agonists (but not competitive antagonists) renders these NCI sites susceptible to photolabeling by the NCI quinacrine azide (QA). Furthermore, this susceptibility is transient, arising in milliseconds and subsiding in hundreds of milliseconds. These transiently susceptible sites are protected by other NCIs against photolabeling by QA. The time-course of the susceptibility and its dependence on agonist-concentration suggest that it might be the transient, rapid-onset desensitized state of the receptor that is most susceptible to photolabeling by QA.

Quaternary structure of the acetylcholine receptor

The five membrane-spanning subunits of the acetylcholine receptor have been resolved in electron microscope images and are shown to lie at pentagonally symmetrical positions around the channel over a large fraction of their length. The channel consists of a wide synaptic portion and a narrow portion extending through the membrane into the interior of the cell.

Acetylcholine receptor: an allosteric protein

The nicotine receptor for the neurotransmitter acetylcholine is an allosteric protein composed of four different subunits assembled in a transmembrane pentamer alpha 2 beta gamma delta. The protein carries two acetylcholine sites at the level of the alpha subunits and contains the ion channel. The complete sequence of the four subunits is known. The membrane-bound protein undergoes conformational transitions that regulate the opening of the ion channel and are affected by various categories of pharmacologically active ligands.