Identification of two amino acid residues in the epsilon subunit that promote mammalian muscle acetylcholine receptor assembly in COS cells

Pentameric quaternary structure of the intracellular domain of serotonin type 3A receptors

In spite of extensive efforts over decades an experimentally-derived structure of full-length eukaryotic pentameric ligand-gated ion channels (pLGICs) is still lacking. These pharmaceutically highly-relevant channels contain structurally well-conserved and characterized extracellular and transmembrane domains. The intracellular domain (ICD), however, has been orphaned in structural studies based on the consensus assumption of being largely disordered. In the present study, we demonstrate for the first time that the serotonin type 3A (5-HT_3A) ICD assembles into stable pentamers in solution in the absence of the other two domains, thought to be the drivers for oligomerization. Additionally, the soluble 5-HT_3A-ICD construct interacted with the protein RIC-3 (resistance to inhibitors of cholinesterase). The interaction provides evidence that the 5-HT_3A-ICD is not only required but also sufficient for interaction with RIC-3. Our results suggest the ICD constitutes an oligomerization domain. This novel role significantly adds to its known contributions in receptor trafficking, targeting, and functional fine-tuning. The innate diversity of the ICDs with sizes ranging from 50 to 280 amino acids indicates new methodologies need to be developed to determine the structures of these domains. The use of soluble ICD proteins that we report in the present study constitutes a useful approach to address this gap.

Subunit stoichiometry and arrangement in a heteromeric glutamate-gated chloride channel

The invertebrate glutamate-gated chloride-selective receptors (GluClRs) are ion channels serving as targets for ivermectin (IVM), a broad-spectrum anthelmintic drug used to treat human parasitic diseases like river blindness and lymphatic filariasis. The native GluClR is a heteropentamer consisting of α and β subunit types, with yet unknown subunit stoichiometry and arrangement. Based on the recent crystal structure of a homomeric GluClαR, we introduced mutations at the intersubunit interfaces where Glu (the neurotransmitter) binds. By electrophysiological characterization of these mutants, we found heteromeric assemblies with two equivalent Glu-binding sites at β/α intersubunit interfaces, where the GluClβ and GluClα subunits, respectively, contribute the "principal" and "complementary" components of the putative Glu-binding pockets. We identified a mutation in the IVM-binding site (far away from the Glu-binding sites), which significantly increased the sensitivity of the heteromeric mutant receptor to both Glu and IVM, and improved the receptor subunits' cooperativity. We further characterized this heteromeric GluClR mutant as a receptor having a third Glu-binding site at an α/α intersubunit interface. Altogether, our data unveil heteromeric GluClR assemblies having three α and two β subunits arranged in a counterclockwise β-α-β-α-α fashion, as viewed from the extracellular side, with either two or three Glu-binding site interfaces.

Conotoxins targeting nicotinic acetylcholine receptors: an overview

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

Molecular dissection of Cl--selective Cys-loop receptor points to components that are dispensable or essential for channel activity

Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) that bind neurotransmitters to open an intrinsic transmembrane ion channel pore. The recent crystal structure of a prokaryotic pLGIC from the cyanobacterium Gloeobacter violaceus (GLIC) revealed that it naturally lacks an N-terminal extracellular α helix and an intracellular domain that are typical of eukaryotic pLGICs. GLIC does not respond to neurotransmitters acting at eukaryotic pLGICs but is activated by protons. To determine whether the structural differences account for functional differences, we used a eukaryotic chimeric acetylcholine-glutamate pLGIC that was modified to carry deletions corresponding to the sequences missing in the prokaryotic homolog GLIC. Deletions made in the N-terminal extracellular α helix did not prevent the expression of receptor subunits and the appearance of receptor assemblies on the cell surface but abolished the capability of the receptor to bind α-bungarotoxin (a competitive antagonist) and to respond to the neurotransmitter. Other truncated chimeric receptors that lacked the intracellular domain did bind ligands; displayed robust acetylcholine-elicited responses; and shared with the full-length chimeric receptor similar anionic selectivity, effective open pore diameter, and unitary conductance. We suggest that the integrity of the N-terminal α helix is crucial for ligand accommodation because it stabilizes the intersubunit interfaces adjacent to the neurotransmitter-binding pocket(s). We also conclude that the intracellular domain of the chimeric acetylcholine-glutamate receptor does not modulate the ion channel conductance and is not involved in positioning of the pore-lining helices in the conformation necessary for coordinating a Cl- ion within the intracellular vestibule of the ion channel pore.

An intramembrane aromatic network determines pentameric assembly of Cys-loop receptors

Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) that mediate fast synaptic transmission. Here functional pentameric assembly of truncated fragments comprising the ligand-binding N-terminal ectodomains and the first three transmembrane helices, M1-M3, of both the inhibitory glycine receptor (GlyR) alpha1 and the 5HT(3)A receptor subunits was found to be rescued by coexpressing the complementary fourth transmembrane helix, M4. Alanine scanning identified multiple aromatic residues in M1, M3 and M4 as key determinants of GlyR assembly. Homology modeling and molecular dynamics simulations revealed that these residues define an interhelical aromatic network, which we propose determines the geometry of M1-M4 tetrahelical packing such that nascent pLGIC subunits must adopt a closed fivefold symmetry. Because pLGIC ectodomains form random nonstoichiometric oligomers, proper pentameric assembly apparently depends on intersubunit interactions between extracellular domains and intrasubunit interactions between transmembrane segments.

An ER-resident membrane protein complex regulates nicotinic acetylcholine receptor subunit composition at the synapse

Nicotinic acetylcholine receptors (nAChRs) are homo- or heteropentameric ligand-gated ion channels mediating excitatory neurotransmission and muscle activation. Regulation of nAChR subunit assembly and transfer of correctly assembled pentamers to the cell surface is only partially understood. Here, we characterize an ER transmembrane (TM) protein complex that influences nAChR cell-surface expression and functional properties in Caenorhabditis elegans muscle. Loss of either type I TM protein, NRA-2 or NRA-4 (nicotinic receptor associated), affects two different types of muscle nAChRs and causes in vivo resistance to cholinergic agonists. Sensitivity to subtype-specific agonists of these nAChRs is altered differently, as demonstrated by whole-cell voltage-clamp of dissected adult muscle, when applying exogenous agonists or after photo-evoked, channelrhodopsin-2 (ChR2) mediated acetylcholine (ACh) release, as well as in single-channel recordings in cultured embryonic muscle. These data suggest that nAChRs desensitize faster in nra-2 mutants. Cell-surface expression of different subunits of the 'levamisole-sensitive' nAChR (L-AChR) is differentially affected in the absence of NRA-2 or NRA-4, suggesting that they control nAChR subunit composition or allow only certain receptor assemblies to leave the ER.

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.

Charged amino acid motifs flanking each extreme of the alphaM4 transmembrane domain are involved in assembly and cell-surface targeting of the muscle nicotinic acetylcholine receptor

The alphaM4 transmembrane domain of the nicotinic acetylcholine receptor (AChR) is flanked by two basic amino acids (His(408) and Arg(429)) located at its cytoplasmic- and extracellular-facing extremes, respectively, at the level of the phospholipid polar head regions of the postsynaptic membrane. A series of single and double alphaM4 mutants (His(408)Ala, Arg(429)Ala, Arg(429)Glu, His(408)Ala/Arg(429)Ala, and His(408)Ala/Arg(429)Glu) of the adult muscle-type AChR were produced and coexpressed with wild-type beta, delta, and epsilon subunits as stable clones in a mammalian heterologous expression system (CHO-K1 cells). The mutants were studied by alpha-bungarotoxin ([(125)I]alpha-BTX) binding, fluorescence microscopy, and equilibrium sucrose gradient centrifugation. Cell-surface [(125)I]alpha-BTX binding diminished approximately 40% in His(408)Ala and as much as 95% in the Arg(429)Ala mutant. Reversing the amino acid charge (e.g., Arg(429)Glu) abolished cell-surface expression of AChR. Fluorescence microscopy disclosed that AChR was retained at the endoplasmic reticulum, with an enhanced occurrence of unassembled AChR species in the mutant clones. Centrifugation analysis confirmed the lack of fully assembled AChR pentamers in all mutants with the exception of His(408)Ala. We conclude that His(408) and Arg(429) in alphaM4 are involved in assembly and cell-surface targeting of muscle AChR. Arg(429) plays a more decisive role in these two processes, suggesting an asymmetric weight of the charged motifs at each extreme of the alpha subunit M4 transmembrane segment. (c) 2006 Wiley-Liss, Inc.

Stoichiometry of the alpha9alpha10 nicotinic cholinergic receptor

The alpha9 and alpha10 nicotinic cholinergic subunits assemble to form the receptor that mediates synaptic transmission between efferent olivocochlear fibers and hair cells of the cochlea. They are the latest vertebrate nicotinic cholinergic receptor (nAChR) subunits that have been cloned, and their identification has established a distant early divergent branch within the nAChR gene family. The alpha10 subunit serves as a "structural" component leading to heteromeric alpha9alpha10 nAChRs with distinct properties. We now have probed the stoichiometry of recombinant alpha9alpha10 nAChRs expressed in Xenopus oocytes. We have made use of the analysis of the population of receptors assembled from a wild-type subunit and its partner alpha9 or alpha10 subunit bearing a reporter mutation of a valine to threonine at position 13' of the second transmembrane domain (TM2). Because the mutation increased the sensitivity of the receptor for acetylcholine (ACh) but mutations at different subunits were not equivalent, the number of alpha9 and alpha10 subunits could be inferred from the number of components in compound concentration-response curves to ACh. The results were confirmed via the analysis of the effects of a mutation to threonine at position 17' of TM2. Because at this position the mutations at different subunits were equivalent, the stoichiometry was inferred directly from the shifts in the ACh EC50 values. We conclude that the recombinant alpha9alpha10 receptor is a pentamer with a (alpha9)2(alpha10)3 stoichiometry.

Nicotine enhances intracellular nicotinic receptor maturation: A novel mechanism of neural plasticity?

Nicotine addiction, the primary cause of tobacco consumption, is mediated through nicotine binding to brain nicotinic acetylcholine receptor (nAChRs). Upon chronic exposure, nicotine elicits a cascade of events, starting with nAChR activation and desensitization, followed by a long term up-regulation that corresponds to an increase in the number of the high affinity nAChRs, a paradoxical process that occurs in the brain of smokers. Recent investigation of the maturation and trafficking of the major brain alpha4beta2 nAChR demonstrates that up-regulation is initiated in the endoplasmic reticulum soon after protein translation. The data thus far accumulated provide evidence that nicotine elicits up-regulation by promoting maturation of nAChR precursors that would otherwise be degraded. This "maturational enhancer" action of nicotine probably contributes to the long term effect of chronic nicotine, and suggests a novel mechanism of neuronal plasticity through an yet unknown endogenous substance which would modulate the receptor expression under physiological conditions.

Identification of amino acid residues important for assembly of GABA receptor alpha1 and gamma2 subunits

Comparative models of GABA(A) receptors composed of alpha1 beta3 gamma2 subunits were generated using the acetylcholine-binding protein (AChBP) as a template and were used for predicting putative engineered cross-link sites between the alpha1 and the gamma2 subunit. The respective amino acid residues were substituted by cysteines and disulfide bond formation between subunits was investigated on co-transfection into human embryonic kidney (HEK) cells. Although disulfide bond formation between subunits could not be observed, results indicated that mutations studied influenced assembly of GABA(A) receptors. Whereas residue alpha1A108 was important for the formation of assembly intermediates with beta3 and gamma2 subunits consistent with its proposed location at the alpha1(+) side of GABA(A) receptors, residues gamma2T125 and gamma2P127 were important for assembly with beta3 subunits. Mutation of each of these residues also caused an impaired expression of receptors at the cell surface. In contrast, mutated residues alpha1F99C, alpha1S106C or gamma2T126C only impaired the formation of receptors at the cell surface when co-expressed with subunits in which their predicted interaction partner was also mutated. These data are consistent with the prediction that the mutated residue pairs are located close to each other.

Regulation of connexin biosynthesis, assembly, gap junction formation, and removal

Gap junctions (GJs) are the only known cellular structures that allow a direct transfer of signaling molecules from cell-to-cell by forming hydrophilic channels that bridge the opposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for coordination of development, tissue function, and cell homeostasis is now well documented. In addition, recent findings have fueled the novel concepts that connexins, although redundant, have unique and specific functions, that GJIC may play a significant role in unstable, transient cell-cell contacts, and that GJ hemi-channels by themselves may function in intra-/extracellular signaling. Assembly of these channels is a complicated, highly regulated process that includes biosynthesis of the connexin subunit proteins on endoplasmic reticulum membranes, oligomerization of compatible subunits into hexameric hemi-channels (connexons), delivery of the connexons to the plasma membrane, head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic, spatially and temporally organized GJ channel aggregates (so-called plaques), and coordinated removal of channels into the cytoplasm followed by their degradation. Here we review the current knowledge of the processes that lead to GJ biosynthesis and degradation, draw comparisons to other membrane proteins, highlight novel findings, point out contradictory observations, and provide some provocative suggestive solutions.

An Extracellular Protein Microdomain Controls Up-regulation of Neuronal Nicotinic Acetylcholine Receptors by Nicotine

In smoker's brain, rodent brain, and in cultured cells expressing nicotinic receptors, chronic nicotine treatment induces an increase in the total number of high affinity receptors for acetylcholine and nicotine, a process referred to as up-regulation. Up-regulation induced by 1 mm nicotine reaches 6-fold for alpha3beta2 nicotinic receptors transiently expressed in HEK 293 cells, whereas it is much smaller for alpha3beta4 receptors, offering a rationale to investigate the molecular mechanism underlying up-regulation. In this expression system binding sites are mainly intracellular, as shown by [(3)H]epibatidine binding experiments and competition with the impermeant ligand carbamylcholine. Systematic analysis of beta2/beta4 chimeras demonstrates the following. (i) The extracellular domain critically contributes to up-regulation. (ii) Only residues belonging to two beta2 segments, 74-89 and 106-115, confer up-regulation to beta4, mainly by decreasing the amount of binding sites in the absence of nicotine; on an atomic three-dimensional model of the alpha3beta2 receptor these amino acids form a compact microdomain that mainly contributes to the subunit interface and also faces the acetylcholine binding site. (iii) The beta4 microdomain is sufficient to confer to beta2 a beta4-like up-regulation. (iv) This microdomain makes an equivalent contribution to the up-regulation differences between alpha4beta2 and alpha4beta4. We propose that nicotine, by binding to immature oligomers, elicits a conformational reorganization of the microdomain, strengthening the interaction between adjacent subunits and, thus, facilitating maturation processes toward high affinity receptors. This mechanism may be central to nicotine addiction, since alpha4beta2 is the subtype exhibiting the highest degree of up-regulation in the brain.

Pharmacological Properties of α9α10 Nicotinic Acetylcholine Receptors Revealed by Heterologous Expression of Subunit Chimeras

The nicotinic acetylcholine receptor (nAChR) alpha9 and alpha10 subunits are expressed primarily within hair cells of the inner ear and have been implicated in auditory processing. Although functional recombinant nAChRs generated by the coexpression of alpha9 and alpha10 in Xenopus laevis oocytes have been described previously, there have been no reports of the successful heterologous expression of alpha9alpha10 nAChRs in cultured cell lines. In this study, subunit chimeras (alpha9chi and alpha10chi) have been constructed that contain the extracellular, ligand binding domain of the alpha9 or alpha10 subunits fused to the C-terminal domain of the 5-hydroxytryptamine type 3A (5HT3A) subunit. Specific high-affinity binding of the nicotinic radioligand [3H]methyllycaconitine was detected in membrane preparations of mammalian cells transfected with alpha9chi or alpha10chi alone, but significantly higher levels of binding were detected when alpha9chi and alpha10chi were cotransfected, providing evidence of a requirement for coassembly of alpha9 and alpha10 for the efficient formation of a nicotinic binding site. The pharmacological profile of alpha9chialpha10chi receptors, determined by equilibrium radioligand binding studies, is broadly similar to that determined previously by electrophysiological studies conducted with native and recombinant alpha9alpha10 nAChRs. In agreement with evidence that alpha9alpha10 nAChRs exhibit an atypical pharmacological profile, we have identified specific high-affinity binding of several non-nicotinic ligands including strychnine (a glycine receptor antagonist), bicuculline (a GABAA receptor antagonist), and atropine (a muscarinic acetylcholine receptor antagonist). Results have also been compared with radioligand binding data conducted with a previously described alpha7/5HT3A (alpha7chi) subunit chimera.

Regulation of nicotinic acetylcholine receptor assembly

The four muscle-type nicotinic acetylcholine receptor (AChR) subunits, alpha, beta, gamma, and delta, assemble into functional alpha(2)betagammadelta pentamers in the endoplasmic reticulum (ER) through a series of interdependent folding and oligomerization events. The first stable assembly intermediate is a trimer composed of alpha, beta, and gamma subunits. The formation of alphabetagamma trimers initiates a series of subunit folding and processing events that allow addition of delta subunits to form alphabetagammadelta tetramers. Subunit folding and processing continue with formation of the ligand-binding sites on the alpha subunit of alphabetagammadelta tetramers and the second alpha subunit added to assemble alpha(2)betagammadelta pentamers. AChR assembly is inefficient. Only 20-30% of synthesized subunits assemble into mature receptors in the ER, while the remaining unassembled subunits are degraded. However, the efficiency of subunit assembly can be regulated under certain conditions leading to higher AChR expression. Increased intracellular cAMP levels cause a 2- to 3-fold increase in AChR assembly efficiency and a comparable increase in surface expression. Additionally, block of ubiquitin-proteasome degradation appears to enhance AChR assembly and expression. Thus, the regulation of AChR assembly through posttranslational mechanisms is a potential therapeutic target for increasing AChR expression in diseases in which expression is compromised.

Recall immune memory: a new tool for generating late onset autoimmune myasthenia gravis

Most patients with autoimmune myasthenia gravis (MG) produce autoantibodies against their muscle acetylcholine receptors (AChR), causing debilitating muscle weakness. Approximately 60% of MG patients first exhibit myasthenic symptoms after the age of 40. Yet, in the C57BL/6 mouse model of MG, older mice are resistant to induction of myasthenia gravis. To understand the immunological basis for this resistance, the effects of age on the B-cell responses to AChR from Torpedo californica, the inducing antigen, were addressed. As expected, the primary B-cell response was lower in 20-month-old mice than in 2-month-old mice; the isotype profile was not altered by age. When mice were re-immunized, the anti-T-AChR titers increased in both young and old animals, suggesting that a memory response was elicited. Importantly, memory B-cells activated in young animals were largely resistant to the age-associated loss of immune function and the recall memory response was vigorous. Furthermore, the antibodies produced in re-stimulated older mice were functional, as evidenced by the appearance of MG symptoms in some of these animals. Thus, by eliciting a recall memory response, the first examples of late onset MG in mice have been generated. By analogy, late onset MG in humans may be due to re-activation of B-cell responses initiated at a younger age.

Rapsyn-mediated clustering of acetylcholine receptor subunits requires the major cytoplasmic loop of the receptor subunits

During synaptogenesis at the neuromuscular junction, nicotinic acetylcholine receptors (AChRs) are organized into high-density postsynaptic clusters that are critical for efficient synaptic transmission. Rapsyn, an AChR associated cytoplasmic protein, is essential for the aggregation and immobilization of AChRs at the neuromuscular junction. Previous studies have shown that when expressed in nonmuscle cells, both assembled and unassembled AChR subunits are clustered by rapsyn, and the clustering of the alpha subunit is dependent on its major cytoplasmic loop. In the present study, we investigated the mechanism of rapsyn-induced clustering of the AChR beta, gamma, and delta subunits by testing mutant subunits for the ability to cocluster with rapsyn in transfected QT6 cells. For each subunit, deletion of the major cytoplasmic loop, between the third and fourth transmembrane domains, dramatically reduced coclustering with rapsyn. Furthermore, each major cytoplasmic loop was sufficient to mediate clustering of an unrelated transmembrane protein. The AChR subunit mutants lacking the major cytoplasmic loops could assemble into alphadelta dimers, but these were poorly clustered by rapsyn unless at least one mutant was replaced with its wild-type counterpart. These results demonstrate that the major cytoplasmic loop of each AChR subunit is both necessary and sufficient for mediating efficient clustering by rapsyn, and that only one such domain is required for rapsyn-mediated clustering of an assembly intermediate, the alphadelta dimer.

Identification of critical residues in loop E in the 5-HT3ASR binding site

BACKGROUND

The serotonin type 3 receptor (5-HT3R) is a member of a superfamily of ligand gated ion channels. All members of this family share a large degree of sequence homology and presumably significant structural similarity. A large number of studies have explored the structure-function relationships of members of this family, particularly the nicotinic and GABA receptors. This information can be utilized to gain additional insights into specific structural and functional features of other receptors in this family.

RESULTS

Thirteen amino acids in the mouse 5-HT3ASR that correspond to the putative E binding loop of the nicotinic alpha7 receptor were chosen for mutagenesis. Due to the presence of a highly conserved glycine in this region, it has been suggested that this binding loop is comprised of a hairpin turn and may form a portion of the ligand-binding site in this ion channel family. Mutation of the conserved glycine (G147) to alanine eliminated binding of the 5-HT3R antagonist [3H]granisetron. Three tyrosine residues (Y140, Y142 and Y152) also significantly altered the binding of 5-HT3R ligands. Mutations in neighboring residues had little or no effect on binding of these ligands to the 5-HT3ASR.

CONCLUSION

Our data supports a role for the putative E-loop region of the 5-HT3R in the binding of 5-HT, mCPBG, d-tc and lerisetron. 5-HT and mCPBG interact with Y142, d-tc with Y140 and lerisetron with both Y142 and Y152. Our data also provides support for the hypothesis that this region of the receptor is present in a loop structure.

Identification of residues at the alpha and epsilon subunit interfaces mediating species selectivity of Waglerin-1 for nicotinic acetylcholine receptors

Waglerin-1 (Wtx-1) is a 22-amino acid peptide that is a competitive antagonist of the muscle nicotinic receptor (nAChR). We find that Wtx-1 binds 2100-fold more tightly to the alpha-epsilon than to the alpha-delta binding site interface of the mouse nAChR. Moreover, Wtx-1 binds 100-fold more tightly to the alpha-epsilon interface from mouse nAChR than that from rat or human sources. Site-directed mutagenesis of residues differing in the extracellular domains of rat and mouse epsilon subunits indicates that residues 59 and 115 mediate the species difference in Wtx-1 affinity. Mutation of residues 59 (Asp in mouse, Glu in rat epsilon) and 115 (Tyr in mouse, Ser in rat epsilon) converts Wtx-1 affinity for the alpha-epsilon interface of one species to that of the other species. Studies of different mutations at position 59 indicate both steric and electrostatic contributions to Wtx-1 affinity, whereas at position 115, both aromatic and polar groups contribute to affinity. The human nAChR also has lower affinity for Wtx-1 than mouse nAChR, but unlike rat nAChR, residues in both alpha and epsilon subunits mediate the affinity difference. In human nAChR, polar residues (Ser-187 and Thr-189) confer low affinity, whereas in mouse nAChR aromatic residues (Trp-187 and Phe-189) confer high affinity. The overall results show that non-conserved residues at the nAChR binding site, although not crucial for activation by ACh, govern the potency of neuromuscular toxins.

GABA(C) receptors: a molecular view

In the central nervous system inhibitory neurotransmission is primarily achieved through activation of receptors for gamma-aminobutyric acid (GABA). Three types of GABA receptors have been identified on the basis of their pharmacological and electrophysiological properties. The predominant type, termed GABA(A), and a recently identified GABA(C) type, form ligand-gated chloride channels, whereas GABA(B) receptors activate separate cation channels via G proteins. Based on their homology to nicotinic acetylcholine receptors, GABA(C) receptors are believed to be oligomeric protein complexes composed of five subunits in a pentameric arrangement. To date up to five different GABA(C) receptors subunits have been identified in various species. Recent studies have shed new light on the biological characteristics of GABA(C) receptors, including the chromosomal localization of its subunit genes and resulting links to deseases, the cloning of new splice variants, the identification of GABA(C) receptor-associated proteins, the identification of domains involved in subunit assembly, and finally structure/function studies examining functional consequences of introduced mutations. This review summarizes recent data in view of the molecular structure of GABA(C) receptors and presents new insights into the biological function of this protein in the retina.

Effects of purified recombinant neural and muscle agrin on skeletal muscle fibers in vivo

Aggregation of acetylcholine receptors (AChRs) in muscle fibers by nerve-derived agrin plays a key role in the formation of neuromuscular junctions. So far, the effects of agrin on muscle fibers have been studied in culture systems, transgenic animals, and in animals injected with agrin--cDNA constructs. We have applied purified recombinant chick neural and muscle agrin to rat soleus muscle in vivo and obtained the following results. Both neural and muscle agrin bind uniformly to the surface of innervated and denervated muscle fibers along their entire length. Neural agrin causes a dose-dependent appearance of AChR aggregates, which persist > or = 7 wk after a single application. Muscle agrin does not cluster AChRs and at 10 times the concentration of neural agrin does not reduce binding or AChR-aggregating activity of neural agrin. Electrical muscle activity affects the stability of agrin binding and the number, size, and spatial distribution of the neural agrin--induced AChR aggregates. Injected agrin is recovered from the muscles together with laminin and both proteins coimmunoprecipitate, indicating that agrin binds to laminin in vivo. Thus, the present approach provides a novel, simple, and efficient method for studying the effects of agrin on muscle under controlled conditions in vivo.

Structure and function of GABA(C) receptors: a comparison of native versus recombinant receptors

In less than a decade our knowledge of the GABA(C) receptor, a new type of Cl(-)-permeable ionotropic GABA receptor, has greatly increased based on studies of both native and recombinant receptors. Careful comparison of properties of native and recombinant receptors has provided compelling evidence that GABA receptor rho-subunits are the major molecular components of GABA(C) receptors. Three distinct rho-subunits from various species have been cloned and the pattern of their expression in the retina, as well as in various brain regions, has been established. The pharmacological profile of GABA(C) receptors has been refined and more specific drugs have been developed. Molecular determinants that underlie functional properties of the receptors have been assigned to specific amino acid residues in rho-subunits. This information has helped determine the subunit composition of native receptors, as well as the molecular basis underlying subtle variations among GABA(C) receptors in different species. Finally, GABA(C) receptors play a unique functional role in retinal signal processing via three mechanisms: (1) slow activation; (2) segregation from other inhibitory receptors; and (3) contribution to multi-neuronal pathways.

Stoichiometry of human recombinant neuronal nicotinic receptors containing the b3 subunit expressed in Xenopus oocytes

The neuronal nicotinic subunit beta3 forms functional receptors when co-expressed with both an alpha and a beta subunit, such as alpha3 and beta4. We examined the subunit stoichiometry of these 'triplet' alpha3beta4beta3 receptors by expression in Xenopus oocytes of the alpha3, beta4 and beta3 subunits, either in wild-type form or after insertion of a reporter mutation. The mutation chosen was the substitution of a conserved hydrophobic residue in the second transmembrane domain of the subunits (leucine or valine 9THORN ) with a hydrophilic threonine. In other ion channels within the nicotinic superfamily, this mutation type consistently increases the potency of agonists. In muscle-type nicotinic receptors, the magnitude of this effect is approximately constant for each mutant subunit incorporated. In alpha3beta4beta3 receptors, the ACh EC50 was decreased by approximately 17-fold when this mutation was in alpha3 alone and only by fourfold when beta3 alone was mutated. Mutating beta4 was equivalent to mutating alpha3, suggesting that the 'triplet' receptor contains one copy of beta3 and two copies each of alpha3 and beta4. Mutating beta3 and alpha3 or beta3 and beta4 reduced the ACh EC50 further, to values two- to threefold lower than those seen when only alpha3 or beta4 carried the mutation. In 'pair' alpha3beta4 receptors (known to contain two alpha and three beta subunits), mutating beta4 had a greater effect on the ACh EC50 than mutating alpha3, in agreement with an alpha:beta ratio of 2:3 and a constant and independent effect of each copy of the mutation. Our results suggest that alpha3beta4beta3 neuronal nicotinic receptors contain one copy of beta3 and two copies each of alpha3 and beta4 and confirm that in pair alpha3beta4 receptors the alpha/beta subunits are present in a 2:3 ratio.

Electron microscopic evidence for the assembly of soluble pentameric extracellular domains of the nicotinic acetylcholine receptor

Exploitation of soluble extracellular domains (ECDs) of the nicotinic acetylcholine receptor may provide a route to crystallographic studies aimed at exploring the structure and function of the intact receptor. The first step towards this goal is to manufacture and isolate soluble fragments that fold and assemble to form a functionally relevant complex. The baculovirus insect cell expression system was used to co-express soluble ECDs of all four muscle-type nicotinic acetylcholine receptor subunits (alpha, beta, gamma & delta-ECD) from Torpedo. Protein complexes were purified using either the conformationally sensitive monoclonal antibody mAb35, specific for a folded alpha subunit, or a NiNTA affinity resin, specific for a polyhistidine tag engineered on the delta-ECD. Western blotting with subunit specific antibodies confirmed the co-expression of each ECD and furthermore, indicated that the alpha, beta and gamma-ECDs were being co-purified with the polyhistidine-tagged delta-ECD. Chemical cross-linking was used to show that these co-purified proteins had indeed interacted specifically to form soluble oligomeric complexes. A low-resolution, three-dimensional image of these purified complexes, composed only of ECDs, was obtained by electron microscopy. They were shown to resemble the extracellular vestibule of the native receptor, having the same pseudo-pentameric symmetry, size and shape. Expression of incomplete sets of the four nicotinic acetylcholine receptor ECDs did not yield detectable complexes.

Identification of residues within GABA(A) receptor alpha subunits that mediate specific assembly with receptor beta subunits

GABA(A) receptors can be constructed from a range of differing subunit isoforms: alpha, beta, gamma, delta, and epsilon. Expression studies have revealed that production of GABA-gated channels is achieved after coexpression of alpha and beta subunits. The expression of a gamma subunit isoform is essential to confer benzodiazepine sensitivity on the expressed receptor. However, how the specificity of subunit interactions is controlled during receptor assembly remains unknown. Here we demonstrate that residues 58-67 within alpha subunit isoforms are important in the assembly of receptors comprised of alphabeta and alphabetagamma subunits. Deletion of these residues from the alpha1 or alpha6 subunits results in retention of either alpha subunit isoform in the endoplasmic reticulum on coexpression with the beta3, or beta3 and gamma2 subunits. Immunoprecipitation revealed that residues 58-67 mediated oligomerization of the alpha1 and beta3 subunits, but were without affect on the production of alpha/gamma complexes. Within this domain, glutamine 67 was of central importance in mediating the production of functional alpha1beta3 receptors. Mutation of this residue resulted in a drastic decrease in the cell surface expression of alpha1beta3 receptors and the resulting expression of beta3 homomers. Sucrose density gradient centrifugation revealed that this residue was important for the production of a 9S alpha1beta3 complex representing functional GABA(A) receptors. Therefore, our studies detail residues that specify GABA(A) receptor alphabeta subunit interactions. This domain, which is conserved in all alpha subunit isoforms, will therefore play a critical role in the assembly of GABA(A) receptors composed of alphabeta and alphabetagamma subunits.

Mutation causing congenital myasthenia reveals acetylcholine receptor beta/delta subunit interaction essential for assembly

We describe a severe postsynaptic congenital myasthenic syndrome with marked endplate acetylcholine receptor (AChR) deficiency caused by 2 heteroallelic mutations in the beta subunit gene. One mutation causes skipping of exon 8, truncating the beta subunit before its M1 transmembrane domain, and abolishing surface expression of pentameric AChR. The other mutation, a 3-codon deletion (beta426delEQE) in the long cytoplasmic loop between the M3 and M4 domains, curtails but does not abolish expression. By coexpressing beta426delEQE with combinations of wild-type subunits in 293 HEK cells, we demonstrate that beta426delEQE impairs AChR assembly by disrupting a specific interaction between beta and delta subunits. Studies with related deletion and missense mutants indicate that secondary structure in this region of the beta subunit is crucial for interaction with the delta subunit. The findings imply that the mutated residues are positioned at the interface between beta and delta subunits and demonstrate contribution of this local region of the long cytoplasmic loop to AChR assembly.

Identification of 70 amino acids important for GABA(C) receptor rho1 subunit assembly

gamma-Aminobutyric acid (GABA) is the most important inhibitory neurotransmitter in the mammalian central nervous system and gates at least three subclasses of receptors, termed GABA(A), GABA(B) and GABA(C). Accumulating evidence indicates that GABA(C) receptors are composed exclusively of rho subunits. The N-terminal half of the rho subunits has been shown to mediate formation of homo- and heterooligomeric GABA(C) receptors. In this study, we searched for specific sequences within the N-terminus of the rho1 subunit involved in the assembly process. Assembly sequences were localized to a 128-amino acid region by deletion of progressively larger regions of a chimeric rho1beta1 subunit previously shown to disrupt rho1 and rho2 assembly. To confirm this observation, a series of GABA(A) receptor beta subunit chimeras containing different regions of the rho1 N-terminus were tested for interference with rho1 and rho2 subunit assembly into functional GABA receptors. Transfer of 70 residues within the 128 amino acid region to the beta1 subunit created a chimera that disrupted rho1, but not rho2, assembly into functional receptors. These observations refine the location of signals involved in rho1 subunit assembly, and suggest that different signals exist for the formation of rho1 homooligomeric and rho1/rho2 heterooligomeric GABA(C) receptors.

Identification of amino acid residues within GABA(A) receptor beta subunits that mediate both homomeric and heteromeric receptor expression

GABA(A) receptors are believed to be heteropentamers that can be constructed from six subunit classes: alpha(1-6), beta(1-4), gamma(1-3), delta, epsilon, and pi. Given that individual neurons often express multiple receptor subunits, it is important to understand how these receptors assemble. To determine which domains of receptor subunits control assembly, we have exploited the differing capabilities of the beta2 and beta3 subunits to form functional cell surface homomeric receptors. Using a chimeric approach, we have identified four amino acids in the N-terminal domain of the beta3 subunit that mediate functional cell surface expression of this subunit compared with beta2, which is retained within the endoplasmic reticulum. Substitution of these four amino acids-glycine 171, lysine 173, glutamate 179, and arginine 180-into the beta2 subunit was sufficient to enable the beta2 subunit to homo-oligomerize. The effect of this putative "assembly signal" on the production of heteromeric receptors composed of alphabeta and betagamma subunits was also analyzed. This signal was not critical for the formation of receptors composed of either alpha1beta2 or alpha1beta3 subunits, suggesting that mutation of these residues did not disrupt subunit folding. However, this signal was important in the formation of betagamma2 receptors. These residues did not seem to affect the initial association of beta2 and gamma2 subunits but appeared to be important for the subsequent production of functional receptors. Our studies identify, for the first time, key residues within the N-terminal domains of receptor beta subunits that mediate the selective assembly of GABA(A) receptors.

Altered glycosylation sites of the delta subunit of the acetylcholine receptor (AChR) reduce alpha delta association and receptor assembly

We have used mutagenesis to investigate the potential N-glycosylation sites in the delta subunit of the mouse muscle acetylcholine receptor (AChR). Of the three sites, Asn76, Asn143, and Asn169, only the first two were glycosylated when the delta subunit was expressed in COS cells. Because the heterologously expressed delta subunit was similar in its properties to that expressed in C2 muscle cells, the sites of glycosylation are likely to be the same in both cases. In COS cells, mutations of the delta subunit that prevented glycosylation at either of the sites did not change its metabolic stability nor its steady-state level. These results are in contrast to those found previously for the alpha subunit, in which glycosylation at a single site metabolically stabilized the polypeptide (Blount, P., and Merlie, J. P. (1990) J. Cell Biol. 111, 2613-2622). Mutations of the delta subunit that prevented glycosylation, however, decreased its ability to form an alpha delta heterodimer when the alpha and delta subunit were expressed together. When all four subunits of the AChR (alpha, beta, delta, and epsilon) were coexpressed, mutation of the delta subunit to prevent glycosylation resulted in a reduced amount of fully assembled AChR and reduced surface AChR levels, consistent with the role of the heterodimer in the assembly reaction. These results suggest that glycosylation of the delta subunit at both Asn76 and Asn143 is needed for its efficient folding and/or its subsequent interaction with the alpha subunit.

GABAC receptor rho subunits are heterogeneously expressed in the human CNS and form homo- and heterooligomers with distinct physical properties

In the central nervous system, receptors for gamma-aminobutyric acid (GABA) are responsible for inhibitory neurotransmission. Anatomical and electrophysiological studies indicate that GABAC receptors are composed of rho subunits. While the rho 1 subunit of various species forms homooligomeric receptors with GABAC-like properties, molecular cloning has identified additional rho subunits whose functional role is unclear. By RT-PCR, we demonstrated that rho 1 expression is primarily restricted to the retina, whereas the rho 2 subunit was present in all brain regions tested. Transfection of HEK-293 cells with rho 2 cDNA resulted in GABA-gated whole-cell currents that differed from those mediated by the rho 1 subunit in two respects: maximal amplitude (rho 1:rho 2 approximately 4:1) and inactivation time course (rho 1:rho 2 approximately 2:1). Cotransfection of rho 1 and rho 2 cDNA in a 1:1 ratio generated whole-cell currents with large amplitudes characteristic of rho 1 but more rapid inactivation typical for rho 2. This observation suggested formation of heterooligomeric GABAC receptors with distinct features. Therefore, we tested the assembly of rho 1 and rho 2 subunits by cotransfecting rho 2 cDNA together with a chimeric rho 1 beta 1 subunit, known to interfere with rho 1 assembly in a dominant-negative fashion. Reduction of rho 2 generated currents correlated with the ratio of chimeric to rho 2 cDNA. Secondly, we determined that the picrotoxinin sensitivity of cells transfected with various ratios of rho 1 and rho 2 cDNA differed from that expected of a pure mixture of homooligomeric receptors. The latter two observations support the idea that rho 1 and rho 2 subunits form heterooligomeric GABAC receptors in mammalian cells. Together, our results indicate that the presence of both rho subunits enables the formation of heterooligomeric receptors with physical properties distinct from homooligomers, thus increasing the diversity of GABAC receptors in the CNS.

Disorders of neuromuscular junction ion channels

Ion channel defects produce a clinically diverse set of disorders that range from cystic fibrosis and some forms of migraine to renal tubular defects and episodic ataxias. This review discusses diseases related to impaired function of the skeletal muscle acetylcholine receptor and calcium channels of the motor nerve terminal. Myasthenia gravis is an autoimmune disease caused by antibodies directed toward the skeletal muscle acetylcholine receptor that compromise neuromuscular transmission. Congenital myasthenias are genetic disorders, a subset of which are caused by mutations of the acetylcholine receptor. Lambert-Eaton myasthenic syndrome is an immune disorder characterized by impaired synaptic vesicle release likely related to a defect of calcium influx. The disorders will illustrate new insights into synaptic transmission and ion channel structure that are relevant for all ion channel disorders.

Inhibition of glucose trimming with castanospermine reduces calnexin association and promotes proteasome degradation of the alpha-subunit of the nicotinic acetylcholine receptor

To identify factors involved in the expression of ligand-gated ion channels, we expressed nicotinic acetylcholine receptors in HEK cells to characterize roles for oligosaccharide trimming, calnexin association, and targeting to the proteasome. The homologous subunits of the acetylcholine receptor traverse the membrane four times, contain at least one oligosaccharide, and are retained in the endoplasmic reticulum until completely assembled into the circular arrangement of subunits of delta-alpha-gamma-alpha-beta to enclose the ion channel. We previously demonstrated that calnexin is associated with unassembled subunits of the receptor, but appears to dissociate when subunits are assembled in various combinations. We used the glucosidase inhibitor castanospermine to block oligosaccharide processing, and thereby inhibit calnexin's interaction with the oligosaccharides in the receptor subunits. Castanospermine treatment reduces the association of calnexin with the alpha-subunit of the receptor, and diminishes the intracellular accumulation of unassembled receptor subunit protein. However, treatment with castanospermine does not appear to alter subunit folding or assembly. In contrast, co-treatment with proteasome inhibitors and castanospermine enhances the accumulation of polyubiquitin-conjugated alpha-subunits, and generally reverses the castanospermine induced loss of alpha-subunit protein. Co-transfection of cDNAs encoding the alpha- and delta-subunits, which leads to the expression of assembled alpha- and delta- subunits, also inhibits the loss of alpha-subunits expressed in the presence of castanospermine. Taken together, these observations indicate that calnexin association reduces the degradation of unassembled receptor subunits in the ubiquitin-proteasome pathway.

Identification of residues in the neuronal alpha7 acetylcholine receptor that confer selectivity for conotoxin ImI

To identify residues in the neuronal alpha7 acetylcholine subunit that confer high affinity for the neuronal-specific toxin conotoxin ImI (CTx ImI), we constructed alpha7-alpha1 chimeras containing segments of the muscle alpha1 subunit inserted into equivalent positions of the neuronal alpha7 subunit. To achieve high expression in 293 human embryonic kidney cells and formation of homo-oligomers, we joined the extracellular domains of each chimera to the M1 junction of the 5-hydroxytryptamine-3 (5HT-3) subunit. Measurements of CTx ImI binding to the chimeric receptors reveal three pairs of residues in equivalent positions of the primary sequence that confer high affinity of CTx ImI for alpha7/5HT-3 over alpha1/5HT-3 homo-oligomers. Two of these pairs, alpha7Trp55/alpha1Arg55 and alpha7Ser59/alpha1Gln59, are within one of the four loops that contribute to the traditional non-alpha subunit face of the muscle receptor binding site. The third pair, alpha7Thr77/alpha1Lys77, is not within previously described loops of either the alpha or non-alpha faces and may represent a new loop or an allosterically coupled loop. Exchanging these residues between alpha1 and alpha7 subunits exchanges the affinities of the binding sites for CTx ImI, suggesting that the alpha7 and alpha1 subunits, despite sequence identity of only 38%, share similar protein scaffolds.

Molecular composition of GABAC receptors

In the central nervous system inhibitory neurotransmission is primarily achieved through activation of receptors for gamma-aminobutyric acid (GABA). Three types of GABA receptors have been identified on the basis of their pharmacology and electrophysiology. The predominant type, termed GABAA and a recently identified type, GABAC, have integral chloride channels, whereas GABAB receptors couple to separate K+ or Ca2+ channels via G-proteins. By analogy to nicotinic acetylcholine receptors, native GABAA receptors are believed to be heterooligomers of five subunits, drawn from five classes (alpha, beta, gamma, delta, epsilon/chi). An additional class, called rho, is often categorized with GABAA receptor subunits due to a high degree of sequence similarity. However, rho subunits are capable of forming functional homooligomeric and heterooligomeric receptors, whereas GABAA receptors only express efficiently as heterooligomers. Intriguingly, the pharmacological properties of receptors formed from rho subunits are very similar to those exhibited by GABAC receptors and rho subunits and GABAC responses have been colocalized to the same retina cells, indicating that rho subunits are the sole components of GABAC receptors. In contrast, the propensity of GABAA receptor and rho subunits to form multimeric structures and their coexistence in retinal cells suggests that GABAC receptors might be heterooligomers of rho and GABAA receptor subunits. This review will summarize our current understanding of the molecular composition of GABAC receptors based upon studies of rho subunit assembly.

Water-soluble nicotinic acetylcholine receptor formed by alpha7 subunit extracellular domains

Water-soluble models of ligand-gated ion channels would be advantageous for structural studies. We investigated the suitability of three versions of the N-terminal extracellular domain (ECD) of the alpha7 subunit of the nicotinic acetylcholine receptor (AChR) family for this purpose by examining their ligand-binding and assembly properties. Two versions included the first transmembrane domain and were solubilized with detergent after expression in Xenopus oocytes. The third was truncated before the first transmembrane domain and was soluble without detergent. For all three, their equilibrium binding affinities for alpha-bungarotoxin, nicotine, and acetylcholine, combined with their velocity sedimentation profiles, were consistent with the formation of native-like AChRs. These characteristics imply that the alpha7 ECD can form a water-soluble AChR that is a model of the ECD of the full-length alpha7 AChR.

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)

The N-terminal domain of human GABA receptor rho1 subunits contains signals for homooligomeric and heterooligomeric interaction

gamma-Aminobutyric acid type C (GABAC) receptors identified in retina appear to be composed of GABA rho subunits. The purpose of this study was to localize signals for homooligomeric assembly of rho1 subunits and to investigate whether the same region contained signals for heterooligomeric interaction with rho2 subunits. In vitro translated human rho1 was shown to be membrane-associated, and proteinase K susceptibility studies indicated that the N terminus was oriented in the lumen of ER-derived microsomal vesicles. This orientation suggested the involvement of the N terminus of rho1 in the initial steps of subunit assembly. To test this hypothesis, mutants were created containing only N-terminal sequences (N-rho1) or C-terminal sequences (C-rho1) of rho1. Co-immunoprecipitation studies revealed that N-rho1, but not C-rho1, interacted with rho1 in vitro. When coexpressed in Xenopus oocytes, N-rho1 interfered with rho1 receptor formation. Together, these data suggested that signals for rho1 homooligomeric assembly reside in the N-terminal half of the subunit. Sequential immunoprecipitations were then performed upon cotranslated rho1 and rho2 subunits which demonstrated that rho1 and rho2 interacted in vitro. Co-immunoprecipitation indicated that N-rho1 specifically associated with rho2. Therefore, the N-terminal regions of rho subunits contain the initial signals for both homooligomeric and heterooligomeric assembly into receptors with GABAC properties.

Predicted structure of the extracellular region of ligand-gated ion-channel receptors shows SH2-like and SH3-like domains forming the ligand-binding site

Fast synaptic neurotransmission is mediated by ligand-gated ion-channel (LGIC) receptors, which include receptors for acetylcholine, serotonin, GABA, glycine, and glutamate. LGICs are pentamers with extracellular ligand-binding domains and form integral membrane ion channels that are selective for cations (acetylcholine and serotonin 5HT3 receptors) or anions (GABAA and glycine receptors and the invertebrate glutamate-binding chloride channel). They form a protein superfamily with no sequence similarity to any protein of known structure. Using a 1D-3D structure mapping approach, we have modeled the extracellular ligand-binding domain based on a significant match with the SH2 and SH3 domains of the biotin repressor structure. Refinement of the model based on knowledge of the large family of SH2 and SH3 structures, sequence alignments, and use of structure templates for loop building, allows the prediction of both monomer and pentamer models. These are consistent with medium-resolution electron microscopy structures and with experimental structure/function data from ligand-binding, antibody-binding, mutagenesis, protein-labeling and subunit-linking studies, and glycosylation sites. Also, the predicted polarity of the channel pore calculated from electrostatic potential maps of pentamer models of superfamily members is consistent with known ion selectivities. Using the glycine receptor alpha 1 subunit, which forms homopentamers, the monomeric and pentameric models define the agonist and antagonist (strychnine) binding sites to a deep crevice formed by an extended loop, which includes the invariant disulfide bridge, between the SH2 and SH3 domains. A detailed binding site for strychnine is reported that is in strong agreement with known structure/function data. A site for interaction of the extracellular ligand-binding domain with the activation of the M2 transmembrane helix is also suggested.

Acetylcholine receptor subunit homomer formation requires compatibility between amino acid residues of the M1 and M2 transmembrane segments

The neuronal nicotinic acetylcholine receptor (nAChR) subunits alpha3 and alpha7 have different assembly behavior when expressed in heterologous expression systems: alpha3 subunits require other subunits to assemble functional nAChRs, whereas alpha7 subunits can produce homomeric nAChRs. A previous analysis of alpha7/alpha3 chimeric constructs identified a domain comprising the first putative membrane-spanning segment, M1, as essential to homomeric assembly. The present study dissected further this domain, identifying three amino acid residues, which are located at the most intracellular third of the M1 transmembrane segment, as important in the assembly of homomers. Moreover, formation of homooligomeric complexes seems to require a compatible accommodation between this region and certain residues of the second transmembrane segment, M2. Thus, compatibility between defined domains of the M1 and M2 transmembrane segments appears as a determinant factor governing homomer association of nAChR subunits.

Assembly of the nicotinic acetylcholine receptor. The first transmembrane domains of truncated alpha and delta subunits are required for heterodimer formation in vivo

To investigate the mechanism of assembly of the mouse muscle acetylcholine receptor, we have expressed truncated N-terminal fragments of the alpha and delta subunits in COS cells and have examined their ability to fold, to associate into heterodimers, and to form a ligand-binding site. Truncated fragments of the alpha subunit that include all, part, or none of the first transmembrane domain (M1) folded to acquire alpha-bungarotoxin binding activity. Neither the full-length alpha subunit nor any of the fragments were expressed on the cell surface, although the shortest folded fragment lacking a transmembrane domain was secreted into the medium. When coexpressed with the delta subunit, the alpha subunit fragment possessing M1 formed a heterodimer containing a ligand-binding site, but shorter fragments, which lack transmembrane segments, did not associate with the delta subunit. N-terminal delta subunit fragments gave similar results. An N-terminal delta subunit fragment that contains M1 associated with the alpha subunit to form a heterodimer, while a fragment lacking M1 did not. These results show that a complete M1 domain is necessary for association of truncated N-terminal alpha and delta subunits into a heterodimer with high affinity ligand binding activity.

Anionic residue in the alpha-subunit of the nicotinic acetylcholine receptor contributing to subunit assembly and ligand binding

To ascertain the anionic sites on the nicotinic receptor to which acetylcholine and other quaternary ammonium ligands bind, we have examined the role of an aspartyl residue (Asp-152) in the alpha-subunit. Prior photolytic labeling with agonist analogues of the neighboring residues Trp-149 and Tyr-151 suggests that their side chains reside on the binding face (also termed the (+)- or counterclockwise face) of the alpha-subunit. Asp-152 presents an anionic charge in the vicinity of these aromatic residues. Modification of the aspartate to asparagine (D152N) creates a glycosylation signal (Asn-152-Gly-Ser), and we find, on the basis of altered electrophoretic migration, that glycosylation occurs at this position upon cotransfection of the mutant alpha-subunit with beta-, gamma-, and delta-subunits. Glycosylation results in a reduction in the capacity of the receptor to assemble; this reduction is manifest in the initial step of dimer formation between the alphagamma- and alphadelta-subunits. The alpha-subunit mutant receptor reaching the assembled pentamer exhibits an altered selectivity for certain ligands. Little reduction in alpha-bungarotoxin binding is observed, whereas affinities for agonists and competitive alkaloid antagonists are reduced substantially. Separation of the contributions of charge removal and glycosylation addition shows that both factors affect agonist affinity, with the charge influence being far more predominant. These findings raise the possibility that a component of the coulombic attraction stabilizing the binding of agonists comes from the aspartyl residue at position 152 in the alpha-subunit.

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

Agonists and antagonists select between the alphagamma and ns31744adelta binding sites of the fetal muscle acetylcholine receptor owing to different contributions by the gamma and delta subunits. To identify determinants of selectivity for agonists, we constructed a panel of gamma-delta subunit chimeras, co-expressed them with the alpha subunit in 293 HEK cells, and measured carbamylcholine binding affinity of intracellular complexes. Wild-type alphadelta complexes bind carbamylcholine about 30-fold more tightly than alphagamma complexes. This degree of selectivity is similar to that of the resting state of the receptor determined by kinetic analysis of single-channel events. We identify a primary set of determinants of selectivity, Lysgamma34/Serdelta36 and Phegamma172/Iledelta178, and a secondary set, Glugamma57/Aspdelta59 and Cysgamma115/Tyrdelta117. The contributions of all four determinants are subunit-dependent and are modified by interaction with one another. Coexpression of point mutant subunits with complementary wild-type subunits to form cell surface pentamers shows that Lysgamma34/Serdelta36 and Phegamma172/Iledelta178 contribute in a manner consistent with affecting selectivity of the resting state of the receptor, while Glugamma57 appears to contribute to the affinity of the desensitized state. The four determinants either coincide with or are close to residues known to contribute to the acetylcholine binding site. These results suggest that a minimum of four loops in the gamma and delta subunits contribute to the agonist binding site.

Involvement of the chaperone protein calnexin and the acetylcholine receptor beta-subunit in the assembly and cell surface expression of the receptor

The nicotinic acetylcholine receptor at the neuromuscular junction is a ligand-gated ion channel assembled in the endoplasmic reticulum from four distinct glycoprotein subunits into the pentameric configuration of alpha2betagammadelta. The individual homologous subunits form specific contacts at interfaces with neighboring subunits to achieve the appropriate orientation and order of each subunit in surrounding the ion channel. Assembly is thought to proceed through the formation of intermediates composed of dimers of the alphadelta and alphagamma subunits which are eventually joined by the beta-subunit to achieve a circular structure enclosing the gated ion channel. In this study, we transfect cDNAs encoding receptor subunits in various combinations into HEK-293 cells to identify intracellular factors that influence the assembly and cell surface expression of the receptor. Our data derived from brefeldin A-treated cells indicate that intracellular association of the receptor subunits with the beta-subunit increases the pool of fully assembled receptors available for transport to the cell surface, presumably by protection from degradation. In addition, we determined that the chaperone protein calnexin is associated with the isolated alpha-, beta-, and delta-subunits of the receptor, but calnexin is not detected in association with assembled alphadelta subunit dimers. Calnexin is also detected in association with maturely folded, unassembled alpha-subunits, as observed by the recognition of this complex by the monoclonal antibody mAb 35, believed to be specific for correctly folded alpha-subunits. Thus, calnexin appears to associate with the individual nascent subunits, thereby facilitating their assembly into the mature pentameric receptor.

Serotonergic modulation of muscle acetylcholine receptors of different subunit composition

Modulation of muscle acetylcholine (AcCho) receptors (AcChoRs) by serotonin [5-hydroxytryptamine (5HT)] and other serotonergic compounds was studied in Xenopus laevis oocytes. Various combinations of alpha, beta, gamma, and delta subunit RNAs were injected into oocytes, and membrane currents elicited by AcCho were recorded under voltage clamp. Judging by the amplitudes of AcCho currents generated, the levels of functional receptor expression were: alpha beta gamma delta > alpha beta delta > alpha beta gamma > alpha gamma delta. The alpha beta gamma delta and alpha beta delta AcChoR Subtypes were strongly blocked by 5HT, whereas the alpha beta gamma receptor was blocked only slightly. The order of blocking potency of AcChoRs by 5HT was: alpha beta delta > alpha beta gamma delta > alpha beta gamma. 5HT receptor antagonists, such as methysergide and spiperone, were even more potent blockers of AcChoRs than 5HT but did not show much subunit selectivity. Blockage of alpha beta gamma delta and alpha beta delta receptors by 5HT was voltage-dependent, and the voltage dependence was abolished when the delta subunit was omitted. These findings may need to be taken into consideration when trying to elucidate the mode of action of many clinically important serotonergic compounds.

Intersubunit contacts governing assembly of the mammalian nicotinic acetylcholine receptor

Through specific intersubunit contacts, the four subunits of the nicotinic acetylcholine receptor assemble into an alpha 2 beta gamma delta pentamer. The specificity of subunit association leads to formation of proper ligand binding sites and to transport of assembled pentamers to the cell surface. To identify determinants of subunit association, we constructed chimeric subunits, transfected them into HEK 293 cells, and studied their association with wild-type subunits. We used beta gamma chimeras to determine sequences that associate with the alpha subunit to form a ligand binding site and found residues 21-131 of the gamma subunit sufficient to form the site. Residues 51-131 of the beta subunit do not form a binding site, but do promote surface expression of pentamers; of these residues, R117 is key for surface expression. We studied formation of tetramers by alpha and gamma subunits and dimers by alpha and delta subunits, and used gamma delta chimeras to identify sequences that result in either dimers or tetramers. The conserved residues I145 and T150 of the gamma subunit promote alpha gamma alpha gamma tetramer formation, whereas the corresponding residues in the delta subunit, K145 and K150, allow only alpha delta dimer formation.