Genetic reconstitution of functional acetylcholine receptor channels in mouse fibroblasts

Ligand-gated ion channels as targets of neuroactive insecticides

The Cys-loop superfamily of ligand-gated ion channels (Cys-loop receptors) is one of the most ubiquitous ion channel families in vertebrates and invertebrates. Despite their ubiquity, they are targeted by several classes of pesticides, including neonicotinoids, phenylpyrazols, and macrolides such as ivermectins. The current commercialized compounds have high target site selectivity, which contributes to the safety of insecticide use. Structural analyses have accelerated progress in this field; notably, the X-ray crystal structures of acetylcholine binding protein and glutamate-gated Cl channels revealed the details of the molecular interactions between insecticides and their targets. Recently, the functional expression of the insect nicotinic acetylcholine receptor (nAChR) has been described, and detailed evaluations using the insect nAChR have emerged. This review discusses the basic concepts and the current insights into the molecular mechanisms of neuroactive insecticides targeting the ligand-gated ion channels, particularly Cys-loop receptors, and presents insights into target-based selectivity, resistance, and future drug design.

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

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/.

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.

Importance of the C-terminus of the human 5-HT3A receptor subunit

Amongst the family members of Cys-loop LGICs, the atypical ability of the 5-HT3A subunit to form functional homomeric receptors allowed a direct investigation of the role of the C-terminus. Deletion of the three C-terminal amino acids (DeltaGln453-DeltaTyr454-DeltaAla455) from the h5-HT3A subunit prevented formation of a specific radioligand binding site as well as expression within the cell membrane. Removal of merely the C-terminal residue (DeltaAla455) reduced specific radioligand binding (to 4+/-1% relative to the wild-type; cells grown at 37 degrees C) and also cell membrane expression; these reductions were less evident when the DeltaAla455 expressing cells were grown at 27 degrees C (specific radioligand binding levels 27+/-5% relative to wild-type also grown at 27 degrees C). Mutation of the h5-HT3A C-terminal amino acid, alanine, for either glycine (Ala455Gly), valine (Ala455Val) or leucine (Ala455Leu) reduced specific radioligand binding levels by 24+/-23%, 32+/-12% and 88+/-1%, respectively; the latter mutant also displaying reduced membrane expression. In contrast, mutation to alanine of the two amino acids preceding the C-terminal alanine (Gln453Ala and Tyr454Ala) had no detrimental effects on specific radioligand binding or cell membrane expression levels. The present study demonstrates an important role for the C-terminus in the formation of the functional h5-HT3A receptor. The partial restoration of 5-HT3 receptor binding and cell membrane expression when cells expressing C-terminal mutant 5-HT3A subunits were grown at a lower temperature (27 degrees C) suggests that the C-terminus stabilises the 5-HT3 receptor allowing subunit folding and subsequent maturation.

Assembly and trafficking of nicotinic acetylcholine receptors (Review)

Nicotinic acetylcholine receptors (nAChRs) are members of an extensive super-family of neurotransmitter-gated ion channels. In humans, nAChRs are expressed within the nervous system and at the neuromuscular junction and are important targets for pharmaceutical drug discovery. They are also the site of action for neuroactive pesticides in insects and other invertebrates. Nicotinic receptors are complex pentameric transmembrane proteins which are assembled from a large family of subunits; seventeen nAChR subunits (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon) have been identified in vertebrate species. This review will discuss nAChR subunit diversity and factors influencing receptor assembly and trafficking.

N-linked glycosylation is required for nicotinic receptor assembly but not for subunit associations with calnexin

We investigated how asparagine (N)-linked glycosylation affects assembly of acetylcholine receptors (AChRs) in the endoplasmic reticulum (ER). Block of N-linked glycosylation inhibited AChR assembly whereas block of glucose trimming partially blocked assembly at the late stages. Removal of each of seven glycans had a distinct effect on AChR assembly, ranging from no effect to total loss of assembly. Because the chaperone calnexin (CN) associates with N-linked glycans, we examined CN interactions with AChR subunits. CN rapidly associates with 50% or more of newly synthesized AChR subunits, but not with subunits after maturation. Block of N-linked glycosylation or trimming did not alter CN-AChR subunit associations nor did subunit mutations prevent N-linked glycosylation. Additionally, CN associations with subunits lacking N-linked glycans occurred without subunit aggregation or misfolding. Our data indicate that CN associates with AChR subunits without N-linked glycan interactions. Furthermore, CN-subunit associations only occur early in AChR assembly and have no role in events later that require N-linked glycosylation.

Differential activation of "social" and "solitary" variants of the Caenorhabditis elegans G protein-coupled receptor NPR-1 by its cognate ligand AF9

Natural variations of wild Caenorhabditis elegans isolates having either Phe-215 or Val-215 in NPR-1, a putative orphan neuropeptide Y-like G protein-coupled receptor, result in either "social" or "solitary" feeding behaviors (de Bono, M., and Bargmann, C. I. (1998) Cell 94, 679-689). We identified a nematode peptide, GLGPRPLRF-NH2 (AF9), as a ligand activating the cloned NPR-1 receptor heterologously expressed in mammalian cells. Shifting cell culture temperatures from 37 to 28 degrees C, implemented 24 h after transfections, was essential for detectable functional expression of NPR-1. AF9 treatments linked both cloned receptor variants to activation of Gi/Go proteins and cAMP inhibition, thus allowing for classification of NPR-1 as an inhibitory G protein-coupled receptor. The Val-215 receptor isoform displayed higher binding and functional activity than its Phe-215 counterpart. This finding parallels the in vivo observation of a more potent repression of social feeding by the npr-1 gene encoding the Val-215 form of the receptor, resulting in dispersing (solitary) animals. Since neuropeptide Y shows no sequence homology to AF9 and was functionally inactive at the cloned NPR-1, we propose to rename NPR-1 and refer to it as an AF9 receptor, AF9-R1.

The nicotinic receptor ligand binding domain

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

Rearrangement of nicotinic receptor alpha subunits during formation of the ligand binding sites

Muscle nicotinic acetylcholine receptors (AChRs) are pentamers that contain two alpha subunits a beta, gamma (or epsilon), and delta subunit. In this paper, we have characterized subunit processing and folding events leading to formation of the two AChR ligand binding sites. alpha subunit residues, 187-199, which are part of overlapping ACh and alpha-bungarotoxin (Bgt) binding sites on AChRs, were assayed using a monoclonal antibody (mAb) specific for these residues. We found that this region was inaccessible to the mAb early during AChR assembly but became accessible as the first of two Bgt binding sites formed later during assembly, indicating that the region changes conformation as the Bgt binding site appears. Without previous reduction, 20% of the alpha subunits could be alkylated by bromoacetylcholine bromide as the first ACh binding site formed, which further indicated that the disulfide bond between cysteines 192 and 193 does not form until the first ACh binding site appears soon after Bgt binding site formation. When alpha subunits were mutated to add a glycosylation site at residue 187, the number of Bgt binding sites increased threefold, AChRs assembled more efficiently, and 2.5-fold more AChRs reached the cell surface. Our results indicate that binding site formation involves a rate-limiting rearrangement of the alpha subunit that exposes the 187-199 region to the endoplasmic reticulum lumen and determines when cysteines 192 and 193 disulfide bond.

Targeting antigen-specific T cells by genetically engineered antigen presenting cells. A strategy for specific immunotherapy of autoimmune disease

We describe a strategy for specific immunotherapy of autoimmune disease based on targeting the antigen-specific T cells in an experimental model of myasthenia gravis. To address the problem of heterogeneity of the T cell repertoire, we have genetically engineered antigen presenting cells (APCs) to process and present epitopes of the autoantigen, acetylcholine receptor (AChR), to the entire spectrum of AChR-specific syngeneic T cells. APCs derived from BALB/c mice were stably transfected with cDNA for the key immunogenic domain of the AChR alpha-subunit, flanked by sequences of the lysosome-associated membrane protein (LAMP) that direct APCs to process and present the antigen via the MHC Class II pathway. Transfected APCs strongly stimulated AChR-specific T cells from BALB/c mice. Fas ligand, or antibody to Fas, abrogated the T cell response, by inducing apoptosis of the APC-stimulated T cells. The new results of this investigation are (1) that autoreactive T cells can be effectively targeted by autologous APCs that are engineered to present the relevant autoantigen, and (2) that these specifically targeted and activated T cells can be profoundly inhibited by agents that trigger the Fas-mediated apoptosis pathway. The present findings suggest that engineering APCs for simultaneous presentation of the autoantigen and delivery of FasL will provide a powerful strategy for the elimination of autoreactive T cells.

Expression of a neuronal nicotinic acetylcholine receptor in insect and mammalian host cell systems

Different mammalian and insect somatic host cell systems were tested in their ability to express, fold, and assemble alpha7-type neuronal acetylcholine receptor (AChR) both at the transcriptional and translational level. For this purpose we employed clonal cell lines derived from the neural crest, such as PC12 cells from a rat adrenal pheochromocytoma, and GH3 cells isolated from a rat pituitary tumor, as well as non-neuronal cells such as NIH-3T3 fibroblasts from embryonic NIH Swiss mouse and Sf9 cells from ovary tissue of the Spodoptera frugiperda butterfly. Total RNA, isolated from either transfected or non-transfected PC12, GH3 or 3T3 cells, or recombinant AcNPV-infected and mock-infected Sf9 cells was analyzed by Northern blot. PC12 cells, which endogenously express alpha7 AChR, and all its heterologous alpha7-transfectant clones, exhibited variable but generally high amounts of a single transcript. GH3 and NIH-3T3 transfectant clones and recombinant AcNPV-infected Sf9 cells expressed variable levels of alpha7-mRNA, with a single transcript that co-migrated with the 28S rat rRNA. Only the neural crest-derived cell lines appeared to functionally express the alpha7 AChR, as measured by their [125I]alpha-bungarotoxin binding ability. The results suggest that heterologous expression of alpha7 is regulated not at the transcriptional, but at the postranslational level and that not all host cell systems appear to express the cellular factors needed for the correct postranslational modifications leading to mature and functional alpha7 AChR. Furthermore, the results suggest that tightly controlled expression mechanisms have evolved in parallel with this ancient cholinergic sequence.

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

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

Nicotinic receptor assembly requires multiple regions throughout the gamma subunit

Assembly of ionotropic neurotransmitter receptors typified by acetylcholine receptors (AChRs) is thought to be directed by an N-terminal extracellular domain of a subunit. Consistent with this hypothesis, chimeras with the delta subunit N-terminal domain fused to the rest of the gamma subunit can substitute for delta, but not gamma, subunits during AChR assembly. However, chimeras with the gamma subunit N-terminal domain fused to the rest of the delta subunit cannot substitute for gamma or delta subunits during assembly. Furthermore, expression of this chimera with the four wild-type subunits prevents the formation of alpha-bungarotoxin (Bgt) binding sites. Instead of AChR pentamers, complexes are assembled containing only the chimera and either alpha or beta subunits. Based on the results of additional gamma-delta chimeras, there are at least two different regions within the C-terminal half of the chimera required for the dominant-negative effect. Our results indicate that the N-terminal domain of the gamma subunit mediates the initial subunit associations, whereas signals in the C-terminal half of the subunit are required for subsequent subunit interactions.

alpha-bungarotoxin receptors contain alpha7 subunits in two different disulfide-bonded conformations

Neuronal nicotinic alpha7 subunits assemble into cell-surface complexes that neither function nor bind alpha-bungarotoxin when expressed in tsA201 cells. Functional alpha-bungarotoxin receptors are expressed if the membrane-spanning and cytoplasmic domains of the alpha7 subunit are replaced by the homologous regions of the serotonin-3 receptor subunit. Bgt-binding surface receptors assembled from chimeric alpha7/serotonin-3 subunits contain subunits in two different conformations as shown by differences in redox state and other features of the subunits. In contrast, alpha7 subunit complexes in the same cell line contain subunits in a single conformation. The appearance of a second alpha7/serotonin-3 subunit conformation coincides with the formation of alpha-bungarotoxin-binding sites and intrasubunit disulfide bonding, apparently within the alpha7 domain of the alpha7/serotonin-3 chimera. In cell lines of neuronal origin that produce functional alpha7 receptors, alpha7 subunits undergo a conformational change similar to alpha7/serotonin-3 subunits. alpha7 subunits, thus, can fold and assemble by two different pathways. Subunits in a single conformation assemble into nonfunctional receptors, or subunits expressed in specialized cells undergo additional processing to produce functional, alpha-bungarotoxin-binding receptors with two alpha7 conformations. Our results suggest that alpha7 subunit diversity can be achieved postranslationally and is required for functional homomeric receptors.

Cells defective in sphingolipids biosynthesis express low amounts of muscle nicotinic acetylcholine receptor

The properties of the nicotinic acetylcholine receptor (AChR) are modulated by its lipid microenvironment. Studies of such modulation are hampered by the cell's homeostatic mechanisms that impede sustained modification of membrane lipid composition. We have devised a novel strategy to circumvent this problem and study the effect of changes in plasma membrane lipid composition on the functional properties of AChR. This approach is based on the stable transfection of AChR subunit cDNAs into cells defective in a specific lipid metabolic pathway. In the present work we illustrate this new strategy with the successful transfection of a temperature-sensitive Chinese hamster ovary (CHO) cell line, SPB-1, with the genes corresponding to the four adult mouse AChR subunits. The new clone, SPB-1/SPH, carries a mutation of the gene coding for serine palmitoyl transferase, the enzyme that catalyses the first step in sphingomyelin (Sph) biosynthesis. This defect causes a decrease of Sph de novo synthesis at non-permissive temperatures. The IC50 for inhibition of alpha-BTX binding with the agonist carbamoylcholine exhibited values of 3.6 and 2.7 microm in the wild-type and Sph-deficient cell lines, respectively. The corresponding IC50 values for the competitive antagonist D-tubocurarine (D-TC) were 2.8 and 3.4 microm, respectively. No differences in single-channel properties were observed between wild-type and mutant cell lines grown at the non-permissive, lipid defect-expressing temperature using the patch-clamp technique. Both cells exhibited two open times with mean values of 0.35 +/- 0.05 and 1.78 +/- 0.2 ms at 12 degrees C. Taken together, these results suggest that the AChR is expressed as the complete heteroligomer. However, only 10-20% of the total AChR synthesized reached the surface membrane in the mutant cell line and exhibited a higher metabolic turnover, with a half-life about 50% shorter than the wild-type cells. When control CHO-K1/A5 cells were treated with fumonisin B1, an inhibitor of sphingosine (sphinganine) N-acetyltransferase (ceramide synthase), a 45.5% decrease in cell surface AChR expression was observed. The results suggest that sphingomyelin deficiency conditions AChR targeting to the plasma membrane.

Transient expression of heteromeric ion channels

Transient transfection is an excellent method for the expression and study of cell-surface, heteromeric ion channels. The cell type, the total amount of DNA, the combination of subunits and the ratio of subunit DNA are all important parameters to consider when attempting to optimize expression. A serious drawback of this method is that the efficiency of subunit assembly is very low in comparison to the efficiency of assembly for stably expressed heteromeric ion channels. The low efficiency of assembly prevents use of transient expression methods for detailed studies of heteromeric AChR assembly, and caution should be taken in the use of these methods for the study of intracellular heteromeric ion channel subunits. After the transient expression of heteromeric AChR subunits, virtually all of the expressing cells contained all four AChR subunits. However, the subunits were heterogeneously distributed among the cells, and the low efficiency of AChR assembly appears to be due to cell-to-cell variations in the ratio of the four subunits.

Formation of the nicotinic acetylcholine receptor binding sites

Nicotinic acetylcholine receptors (AChRs) are activated by ACh binding to two sites located on different alpha subunits. The two alpha subunits, alpha gamma and alpha delta, are distinguished by their interface with gamma and delta subunits. We have characterized the formation of the ACh binding sites and found, contrary to the current model, that the sites form at different times and in a set order. The first site forms on alpha gamma subunits during the process of subunit assembly. Our data are consistent with the appearance of this site on alpha beta gamma delta subunit tetramers soon after the site for the competitive antagonist alpha-bungarotoxin has formed and delta subunits have assembled with alpha beta gamma trimers. The second site is located on alpha delta subunits and forms after AChR subunits have assembled into alpha2 beta gamma delta pentamers. By determining the order in which the ACh binding sites form, we have also identified the sites in which the delta and second alpha subunits associate during subunit assembly.

Neuronal alpha-bungarotoxin receptors differ structurally from other nicotinic acetylcholine receptors

We have characterized the alpha-bungarotoxin receptors (BgtRs) found on the cell surface of undifferentiated pheochromocytoma (PC12) cells. The PC12 cells express a homogeneous population of alpha7-containing receptors that bind alpha-Bgt with high affinity (Kd = 94 pM). The BgtRs mediate most of the response elicited by nicotine, because the BgtR-specific antagonists methyllycaconitine and alpha-Bgt block approximately 90% of the whole-cell current. The binding of nicotinic agonists to cell-surface BgtRs was highly cooperative with four different agonists showing Hill coefficients in the range of 2.3-2.4. A similar agonist binding cooperativity was observed for BgtR homomers formed from chimeric alpha7/5HT3 subunits expressed in tsA 201 cells. Two classes of agonist binding sites, in the ratio of 4:1 for PC12 cell BgtRs and 3:1 for alpha7/5HT3 BgtRs, were revealed by bromoacetylcholine alkylation of the reduced sites on both PC12 BgtRs and alpha7/5HT3 BgtRs. We conclude from this data that PC12 BgtRs and alpha7/5HT3 homomers contain at least three distinguishable agonist binding sites and thus are different from other nicotinic receptors.

Functional consequences of mutations in the transmembrane domain and the carboxy-terminus of the murine AE1 anion exchanger

We have characterized mouse AE1-mediated 36Cl- influx and surface AE1 polypeptide expression in Xenopus oocytes injected with cRNA encoding two classes of loss-of-function mutants. The first arose spontaneously. Chimeric mutants constructed with a functional AE1 cDNA localized the site of spontaneous mutation to the transmembrane domain, and DNA sequencing revealed two missense mutations encoding the double-mutant polypeptide V728F/M7301. Each mutation individually produced only partial loss of AE1 transport activity, and coexpression of the individual mutants did not restore full activity. The functional changes produced by the mutations correlated with reduced fractional accumulation of polypeptides at the oocyte surface. The V728F/M7301 polypeptide expressed in mammalian cells displayed complete endoH resistance and rapid degradation. We also examined the effect on AE1 function of engineered removal of its hydrophilic carboxy-terminus. Both delta(c)890 and the internal deletion delta(c)890-917 were functionally inactive in Xenopus oocytes. Lack of transport activity correlated with lack of detectable polypeptide accumulation at the oocyte surface. Coexpression with wt AE1 of some, but not all, of these AE1 mutants partially suppressed wt AE1-mediated 36Cl- uptake. In contrast, coexpression with wt AE1 of soluble N-terminal AE1 fragments was not inhibitory.

The role of the cystine loop in acetylcholine receptor assembly

Nicotinic acetylcholine receptors (AChRs) are composed of alpha, beta, gamma, and delta subunits, assembled into alpha2betagammadelta pentamers. A highly conserved feature of ionotropic neurotransmitter receptors, such as AChRs, is a 15-amino acid cystine "loop." We find that an intact cystine loop is necessary for complete AChR assembly. By preventing formation of the loop with 5 mM dithiothreitol, AChR subunits assemble into alphabetagamma trimers, but the subsequent steps in assembly are blocked. When alpha subunit loop cysteines are mutated to serines, assembly is blocked at the same step as with dithiothreitol. In contrast, when beta subunit loop cysteines are mutated to serines, assembly is blocked at a later step, i.e. after assembly of alphabetagammadelta tetramers and before the addition of the second alpha subunit. After formation of the cystine loop, the alpha subunit undergoes a conformational change, which buries the loop. This conformational change is concurrent with the step in assembly blocked by removal of the disulfide bond of the cystine loop, i.e. after assembly of alphabetagamma trimers and before the addition of the delta subunit. The data indicate that the alpha subunit conformational change involving the cystine loop is key to a series of folding events that allow the addition of unassembled subunits.

Nicotinic acetylcholine receptor channels are influenced by the physical state of their membrane environment

We investigated the effect of the physical state of the cell membrane on the activity of the nicotinic acetylcholine receptor (AChR) in various clonal cell lines transfected with the cDNAs of embryonic or adult AChR by measuring single-channel properties and some membrane physicochemical properties as a function of temperature. Unitary conductance and channel closing rate, alpha, had Q(10) values of 1.2 and 2.2, respectively. Using Eyring's transition state theory, it was calculated that both embryonic and adult-type AChR had relatively low thermal sensitivity of ionic conductance and activation energy (E(a) of 3.0-5.0 kcal-mol(-1) at 20 degrees C), indicating that once the AChR channel opens, ion movement is dominated by diffusional processes. Channel closure exhibited higher energy requirements, with E(a) values of about 13 kcal-mol(-1). This process appears to be more endothermic (higher delta H(a) values) than ion permeation, and it is plausible that the energy acquired by the system can be used in the maintenance of its degree of order, as revealed by the delta S(a) 0 calculated for channel closure. The influence of the membrane environment on AChR function is reinforced by the observation that the conductance of the same, embryonic-type AChR protein, expressed in qualitatively different cellular lipid environments, appeared to have different energetic requirements. A correlation between the electrophysiological and thermodynamic parameters of the AChR and physicochemical properties of the membrane bilayer in which the protein is embedded could be established using measurements of the so-called generalized polarization (GP) of the lipophilic probe laurdan. Both embryonic and adult AChR exhibited a higher GP and a higher sensitivity to temperature-dependent changes in GP when heterologously expressed in stable form in Chinese hamster ovary (CHO)-derived cells than did the native embryonic AChR in BC3H-1 cells, indicating that these two properties are determined by the host membrane and are not inherent properties of the AChR type. In addition, the differences in the macroscopic physical states of the lipids and membrane-associated solvent (water) dipolar relaxation between BC3H-1 and CHO-derived cells indicated by the spectroscopic properties of laurdan suggest that both lipid and associated water may influence the microscopic activity of individual AChR molecules embedded in the lipid bilayer. Finally, the different dependence of AChR channel conductance and mean open time as a function of GP observed between the different AChR subtypes in clonal cell lines suggests the importance of specific lipid-protein interactions in addition to bulk membrane properties.

Cyclic AMP-regulated AChR assembly is independent of AChR subunit phosphorylation by PKA

Forskolin treatment of cells expressing Torpedo acetylcholine receptors leads to enhanced assembly efficiency of subunits, which correlates with increased phosphorylation of the gamma subunit. To determine the role of the two potential protein kinase A sites of the gamma subunit in receptor assembly, cell lines expressing different mutant receptors were established. Mouse fibroblast cell lines stably expressing wild-type Torpedo acetylcholine receptor alpha, beta, delta subunits plus one of three gamma subunit mutations (S353A, S354A, or S353,354A) were established to identify the protein kinase A phosphorylation sites of gamma in vivo, and to determine if increased phosphorylation of the gamma subunit leads to enhanced expression of receptors. We found that both serines (353, 354) in gamma are phosphorylated in vivo by protein kinase A, however, phosphorylation of either or both of these sites does not lead to increased assembly efficiency. We established a cell line expressing alpha, beta, and gamma(S353,354A) subunits only (no delta), and found that the presence of delta (or its phosphorylation) is also not necessary for the observed stimulation by forskolin. alpha beta gamma, alpha gamma, and beta gamma associations were stimulated by forskolin but alpha beta and alpha delta interactions were not. These data imply that the presence of gamma is necessary for forskolin action. We postulate that forskolin may stimulate acetylcholine receptor expression through a cellular protein that is involved in the folding and/or assembly of protein complexes, and that forskolin may regulate the action of such a protein through phosphorylation.

Stable transfection of the NR1 subunit in Chinese hamster ovary cells fails to produce a functional N-methyl-D-aspartate receptor

The NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor cDNA was stably transfected into Chinese hamster ovary (CHO) cells. Northern analysis revealed 3 clonal cell lines expressing high levels of NMDAR1 mRNA (BA1, BA2 and BA3). NMDAR1 protein was readily detected by Western analysis in only one of the clonal populations, BA1. Whole-cell patch clamp analysis revealed a lack of functional NMDA receptors in all of the cell lines, including BA1. NMDA-mediated responses were obtained only after a second subunit, NMDAR2A, was transiently expressed in BA1 cells. Therefore, the NR1 subunit alone may not be sufficient to form functional NMDA receptors in CHO cells.

Acetylcholine receptor assembly: subunit folding and oligomerization occur sequentially

The temperature sensitivity of nicotinic acetylcholine receptors (AChRs) from T. californica was used to identify steps in AChR subunit folding and oligomerization. Assembly intermediates were isolated by lowering to an assembly-permissive temperature. The earliest identifiable assembly intermediates, alpha beta gamma trimers, form minutes after subunit synthesis. alpha beta gamma delta tetramers are formed slowly by the addition of delta subunits to trimers, and finally a second alpha subunit is added to form alpha 2 beta gamma delta pentamers. Between these oligomerization steps, subunits fold as monitored by alpha-bungarotoxin-binding site formation, appearance of antigenic epitopes, changes in apparent molecular weight, and changes in detergent solubility. Subunit folding requires specific combinations of subunits and correlates in time with subunit additions, suggesting that these subunit folding events contribute to subunit recognition site formation during assembly.

Heterologous expression of the membrane proteins that control cellular excitability

Versatile and potent expression systems are needed to decipher the structure and functions of the many excitability proteins that have been identified through molecular cloning. This article reviews the use of recombinant vaccinia viruses (VV), which have been recently explored for the heterologous expression of eukaryotic proteins. Vaccinia viruses feature a series of favourable properties, most of all a broad host range and high efficiency of infection, that make them uniquely suited as flexible expression vectors. In one type of experiment, the recombinant virus simply harbors the cDNA for the foreign protein; in a second type the virus harbors the cDNA for the specific and efficient RNA polymerase of bacteriophage T7, which in turn generates RNA from a separate introduced plasmid or virus. Both variations have been successfully applied to the expression and analysis of voltage-dependent ion channels, neurotransmitter receptors and other excitability proteins in many cell lines and postmitotic cells in culture. VV vectors promise to be particularly useful to study membrane proteins that require posttranslational processing, association with cell-specific subunits or coupling to endogenous second messengers pathways.

Use of Torpedo-mouse hybrid acetylcholine receptors reveals immunodominance of the alpha subunit in myasthenia gravis antisera

The nicotinic acetylcholine receptor (AChR), a pentameric complex of alpha 2 beta gamma delta subunits, is the autoantigen in the human autoimmune disease myasthenia gravis (MG). Anti-AChR antibodies are found in approximately 90% of MG patients and using indirect methods (competitive binding to solubilized AChR), peptides, or synthetic peptides, the majority of these antibodies have been shown to bind to the AChR alpha subunit. In order to determine directly the AChR subunit specificities of MG antibodies, we employed as antigens a novel set of hybrid AChR composed of species cross-reacting and non-cross-reacting subunits stably expressed in fibroblasts. Sequence similarities of homologous subunits among species can vary widely, with mammalian subunits having 87%-96% identity and Torpedo-mammalian subunits having 54%-80% identity. These findings are reflected in antigenic specificities, with human anti-AChR antisera frequently recognizing mouse AChR but rarely recognizing Torpedo. By establishing separate cell lines stably expressing all-Torpedo, all-mouse, and different combinations of Torpedo and mouse subunits, we were able to provide the first direct evidence of a predominant anti-alpha subunit specificity in MG antisera. Functional hybrid AChR stably expressed in an intact cell membrane provide us with a system that best mimics the in vivo environment of the MG antibody in a binding assay. Such a system allows us to investigate a perplexing observation in the field: a poor correlation between the patient's clinical status and antibody titer. Those antibodies which can interfere with AChR function, such as ones with the ability to cross-link AChR and induce their accelerated internalization and degradation (antigenic modulation) might represent a subpopulation of MG antibodies important in disease induction or maintenance. In this report, we demonstrate that wild-type and hybrid AChR expressed in fibroblasts can be antigenically modulated by intermolecular cross-linking antibodies as AChR are in native muscle cells. Because we can monitor dynamic interactions between AChR and MG antibodies, this system may allow us to define crucial pathogenic epitopes in MG by expressing hybrid, chimeric, and mutant AChR.

Detection of nicotinic receptor ligands with a light addressable potentiometric sensor

The nicotinic acetylcholine receptor, purified from Torpedo electric organ, was coupled to a light addressable potentiometric sensor (LAPS) to form a LAPS-receptor biosensor. Receptor-ligand complexes containing biotin and urease were captured on a biotinylated nitrocellulose membrane via a streptavidin bridge and detected with a silicon-based sensor. Competition between biotinylated alpha-bungarotoxin and nonbiotinylated ligands formed the basis of this assay. This biosensor detected both agonists (acetylcholine, carbamylcholine, succinylcholine, suberyldicholine, and nicotine) and competitive antagonists (d-tubocurarine, alpha-bungarotoxin, and alpha-Naja toxin) of the receptor with affinities comparable to those obtained using radioactive ligand binding assays. Consistent with agonist-induced desensitization of the receptor, the LAPS-receptor biosensor reported a time-dependent increase in affinity for the agonist carbamylcholine as expected, but not for the antagonists.

Stable expression of heterologous multisubunit protein complexes established by calcium phosphate- or lipid-mediated cotransfection

For most of the studies conducted in our laboratory, we were interested in expressing the AChR in a nonmuscle cell background, in part, to distinguish inherent AChR properties from those contributed by its environment. The fibroblast host cells we used do not express endogenous AChRs, and although there can be considerable daily variability, for most of the studies, approximately 80% of the cells did not express any type of endogenous channel (S. Sine, personal observations, 1988). Thus, characterization of Torpedo AChRs in fibroblasts was simplified by not having endogenous channel expression. We have shown that the pharmacological and electrophysiological properties of AChRs appear fully correct, only alpha 2 beta gamma delta pentamers are expressed on the cell surface, AChRs expressed in fibroblasts can be regulated as they are in muscle cells by agents that increase intracellular levels of cAMP and the AChRs cluster in fibroblasts as they do in muscle cells in response to extracellularly added clustering factors. We have stably expressed Torpedo AChRs in NIH 3T3 and L fibroblasts as well as rat muscle L6 cells. Different transfectants express different levels of AChRs, with the numbers varying between about 2.5 x 10(4) and 1.5 x 10(6) surface AChRs per cell. In choosing a host cell for expressing a protein of interest, it is always prudent to characterize the line prior to transfection for expression of the homolog of the protein being introduced. When transfecting channel proteins, one might also wish to characterize the host cell for endogenous channel expression. Other considerations for selecting a host cell depend on the types of experiments that will be performed. For example, if fluorescent microscopic experiments will be performed, the ability of the cells to adhere to glass coverslips can be a very important consideration.

Molecular analysis of neuronal physiology by gene transfer into neurons with herpes simplex virus vectors

A genetic analysis of mammalian neuronal physiology might now be possible due to the development of defective herpes simplex virus vectors, which allow gene transfer directly into mature neurons, in culture or in the adult brain. Genetically altered proteins that play critical roles in neuronal physiology, including those responsible for the generation of action potentials, synthesis and release of neurotransmitters, and signal transduction enzymes, can now be stably expressed in neurons. The effect of such altered proteins on neuronal physiology can therefore be examined, using the tools of modern neuroscience. Genetic manipulation is biochemically specific and stable, and can be targeted both to a particular cell type and to a particular subregion of the cell to yield insights into the molecular basis for specific brain functions.

Extracellular synaptic factors induce clustering of acetylcholine receptors stably expressed in fibroblasts

The clustering of nicotinic acetylcholine receptors (AChRs) is one of the first events observed during formation of the neuromuscular junction. To determine the mechanism involved in AChR clustering, we established a nonmuscle cell line (mouse fibroblast L cells) that stably expresses just one muscle-specific gene product, the AChR. We have shown that when Torpedo californica AChRs are expressed in fibroblasts, their immunological, biochemical, and electrophysiological properties all indicate that fully functional cell surface AChRs are produced. In the present study, the cell surface distribution and stability of Torpedo AChRs expressed in fibroblasts (AChR-fibroblasts) were analyzed and shown to be similar to nonclustered AChRs expressed in muscle cells. AChR-fibroblasts incubated with antibodies directed against the AChR induced the formation of small AChR microclusters (less than 0.5 micron 2) and caused an increase in the internalization rate and degradation of surface AChRs (antigenic modulation) in a manner similar to that observed in muscle cells. Two disparate sources of AChR clustering factors, extracellular matrix isolated from Torpedo electric organ and conditioned media from a rodent neuroblastoma-glioma hybrid cell line, each induced large (1-3 microns 2), stable AChR clusters with no change in the level of surface AChR expression. By exploiting the temperature-sensitive nature of Torpedo AChR assembly, we were able to demonstrate that factor-induced clusters were produced by mobilization of preexisting surface AChRs, not by directed insertion of newly synthesized AChRs. AChR clusters were never observed in the absence of extracellular synaptic factors. Our results suggest that these factors can interact directly with the AChR.

Acetylcholine receptor assembly is stimulated by phosphorylation of its gamma subunit

Different combinations of Torpedo acetylcholine receptor (AChR) subunits stably expressed in mouse fibroblasts were used to establish a role for phosphorylation in AChR biogenesis. When cell lines expressing fully functional AChR complexes (alpha 2 beta gamma delta) were labeled with 32P, only gamma and delta subunits were phosphorylated. Forskolin, which causes a 2- to 3-fold increase in AChR expression by stimulating subunit assembly, increased unassembled gamma phosphorylation, but had little effect on unassembled delta. The forskolin effect on subunit phosphorylation was rapid, significantly preceding its effect on expression. The pivotal role of the gamma subunit was established by treating alpha beta gamma and alpha beta delta cell lines with forskolin and observing increased expression of only alpha beta gamma complexes. This effect was also observed in alpha gamma, but not alpha delta cells. We conclude that the cAMP-induced increase in expression of cell surface AChRs is due to phosphorylation of unassembled gamma subunits, which leads to increased efficiency of assembly of all four subunits.

Analysis of early events in acetylcholine receptor assembly

Mammalian cell lines expressing nicotinic acetylcholine receptor (AChR) subunit cDNAs from Torpedo californica were used to study early events in AChR assembly. To test the hypothesis that individual subunits form homooligomeric intermediates before assembling into alpha 2 beta gamma delta pentamers, we analyzed the sedimentation on sucrose density gradients of each subunit expressed separately in cell lines. We have shown previously that the acute temperature sensitivity of Torpedo AChR subunit assembly is due, in part, to misfolding of the polypeptide chains (Paulson, H.L., and T. Claudio. 1990. J. Cell Biol. 110:1705-1717). We use this phenomenon to further analyze putative assembly-competent intermediates. In nonionic detergent at an assembly-permissive temperature, the majority of alpha, beta, gamma, and delta subunits sediment neither as 3-4S monomers nor as 9S complexes, but rather as 6S species whether synthesized in fibroblasts, myoblasts, or differentiated myosyncytia. Several results indicate that the 6S species are complexes comprised predominantly of incorrectly folded subunit polypeptides. The complexes represent homoaggregates which form rapidly within the cell, are stable to mild SDS treatment and, in the case of alpha, contain some disulfide-linked subunits. The coprecipitation of alpha subunit with BiP or GRP78, a resident protein of the ER, further indicates that at least some of these internally sequestered subunits also associated with an endogenous protein implicated in protein folding. The majority of subunits expressed in these cell lines appear to be aggregates of subunits which are not assembly intermediates and are not assembly-competent. The portion which migrates as monomer, in contrast, appears to be the fraction which is assembly competent. This fraction increases at temperatures more permissive for assembly, further indicating the importance of the monomer as the precursor to assembly of alpha 2 beta gamma delta pentamers.

Efficiency of acetylcholine receptor subunit assembly and its regulation by cAMP

Assembly of nicotinic acetylcholine receptor (AChR) subunits was investigated using mouse fibroblast cell lines stably expressing either Torpedo (All-11) or mouse (AM-4) alpha, beta, gamma, and delta AChR subunits. Both cell lines produce fully functional cell surface AChRs. We find that two independent treatments, lower temperature and increased intracellular cAMP can increase AChR expression by increasing the efficiency of subunit assembly. Previously, we showed that the rate of degradation of individual subunits was decreased as the temperature was lowered and that Torpedo AChR expression was acutely temperature sensitive, requiring temperatures lower than 37 degrees C. We find that Torpedo AChR assembly efficiency increases 56-fold as the temperature is decreased from 37 to 20 degrees C. To determine how much of this is a temperature effect on degradation, mouse AChR assembly efficiencies were determined and found to be only approximately fourfold more efficient at 20 than at 37 degrees C. With reduced temperatures, we can achieve assembly efficiencies of Torpedo AChR in fibroblasts of 20-35%. Mouse AChR in muscle cells is also approximately 30% and we obtain approximately 30% assembly efficiency of mouse AChR in fibroblasts (with reduced temperatures, this value approaches 100%). Forskolin, an agent which increases intracellular cAMP levels, increased subunit assembly efficiencies twofold with a corresponding increase in cell surface AChR. Pulse-chase experiments and immunofluorescence microscopy indicate that oligomer assembly occurs in the ER and that AChR oligomers remain in the ER until released to the cell surface. Once released, AChRs move rapidly through the Golgi membrane to the plasma membrane. Forskolin does not alter the intracellular distribution of AChR. Our results indicate that cell surface expression of AChR can be regulated at the level of subunit assembly and suggest a mechanism for the cAMP-induced increase in AChR expression.

Identification of cultured cells expressing ligand-gated cationic channels

We have identified cultured cells that express ligand-gated cation channels using a simple method which may also be applied to the screening of chemical agents for their use as agonists or antagonists. This assay is based upon the observation that many ligand-gated cation channels are permeable to lithium and agonists induce the flux of lithium into the cells which contain them. Since the accumulation of intracellular lithium can alter the cell cycle, the measurement of [3H]thymidine ([3H]thy) incorporation should reflect this occurrence. This expectation was realized using the PC12 cell line which expresses neuronal-like nicotinic acetylcholine receptor (nAChR). When cholinergic agonists are applied to PC12 cells in the presence of lithium-containing buffer and cells are subsequently pulsed with [3H]thy, the radiolabel incorporation into these cells relative to controls is reduced. If cholinergic antagonists are included or if the concentration of agonist either rapidly desensitizes receptors or is insufficient to induce channel opening, the reduction in [3H]thy incorporation is not observed. This method also provides a rapid way to screen cultured cell lines for those that express ligand-gated cation channels. This assay offers the potential to be automated for the low cost screening of drugs which act upon ligand-gated ion channels.

cAMP stimulation of acetylcholine receptor expression is mediated through posttranslational mechanisms

When the four Torpedo acetylcholine receptor (AcChoR) subunit cDNAs are stably integrated into the genome of mouse fibroblast cells, alpha 2 beta gamma delta pentamers with proper pharmacological and electrophysiological properties are expressed on the cell surface. Expression of these AcChoRs can be regulated by agents that stimulate intracellular cAMP levels with the result that increased numbers of cell-surface AcChoRs are produced. Theophylline, 8-(4-chlorophenylthio)-adenosine 3':5'-cyclic monophosphate, cholera toxin, and forskolin stimulated AcChoR cell-surface expression 1.2-, 1.6-, 2.2-, and 2.3-fold, respectively. cAMP-stimulated expression is mediated through a posttranslational mechanism, and the observed increase in surface AcChoRs correlates with increased lifetimes of each newly synthesized subunit. Increased subunit lifetimes are not observed in cell lines expressing each subunit individually, indicating that subunit stabilization arises through heterologous subunit-subunit interactions.

Mutagenesis of essential functional residues in acetylcholinesterase

The cholinesterases are serine hydrolases that show no global similarities in sequence with either the trypsin or the subtilisin family of serine proteases. The cholinesterase superfamily includes several esterases with distinct functions and other proteins devoid of the catalytic serine and known esterase activity. To identify the residues involved in catalysis and conferring specificity on the enzyme, we have expressed wild-type Torpedo acetylcholinesterase (EC 3.1.1.7) and several site-directed mutants in a heterologous system. Mutation of serine-200 to cysteine results in diminished activity, while its mutation to valine abolishes detectable activity. Two conserved histidines can be identified at positions 425 and 440 in the cholinesterase family; glutamine replacement at position 440 eliminates activity whereas the mutation at 425 reduces activity only slightly. The assignment of the catalytic histidine to position 440 defines a rank ordering of catalytic residues in cholinesterases distinct from trypsin and subtilisin and suggests a convergence of a catalytic triad to form a third, distinct family of serine hydrolases. Mutation of glutamate-199 to glutamine yields an enzyme with a higher Km and without the substrate-inhibition behavior characteristic of acetylcholinesterase. Hence, modification of the acidic amino acid adjacent to the serine influences substrate association and the capacity of a second substrate molecule to affect catalysis.

Properties of embryonic and adult muscle acetylcholine receptors transiently expressed in COS cells

We used transient transfection in COS cells to compare the properties of mouse muscle acetylcholine receptors (AChRs) containing alpha, beta, delta, and either gamma or epsilon subunits. gamma- and epsilon-AChRs had identical association rates for binding 125I-alpha-bungarotoxin, and identical curves for inhibition of toxin binding by d-tubocurarine, but epsilon-AChRs had a significantly longer half-time of turnover in the membrane than gamma-AChRs. A myasthenic serum specific for the embryonic form of the AChR reduced toxin binding to gamma-, but not epsilon-AChRs. The gamma-AChRs had channel characteristics of embryonic AChRs, whereas the major class of epsilon-AChR channels had the characteristics of adult AChRs. Two minor channel classes with smaller conductances were also seen with epsilon-AChR. Thus, some, but not all, of the differences between AChRs at adult endplates and those in the extrasynaptic membrane can be explained by the difference in subunit composition of gamma- and epsilon-AChRs.

Cloned GABA receptors are maintained in a stable cell line: allosteric and channel properties

The cloned cDNAs encoding the alpha 1 and beta 1 subunits of the bovine brain GABA(A) receptor have been co-transfected, using a dexamethasone-inducible promoter, into cultured hamster ovary cells, with selection to form a stable cell line. The use, alternatively, of a much stronger constitutive promoter led to cell death consequent upon high receptor density. After induction, the cells contained the alpha 1 and beta 1 mRNAs. The expressed receptors showed the high-affinity binding of [3H]muscimol and of the GABA(A) receptor channel blocker, t-butylphosphorothionate (TBPS), and the characteristic enhancement of the former by a pregnanolone. Their GABA-activated current was potentiated by the barbiturate, pentobarbitone, was reversibly blocked by bicuculline and picrotoxin, but was not enhanced by benzodiazepines. In mouse spinal cord neurons GABA activates channel openings to at least four conductance states (45, 30, 19 and 12 pS) with the 30 pS state being the most frequently observed (main) state. However, the main state of the alpha 1/beta 1 GABA(A) receptor was the 19 pS state. The enhancement of GABA(A) receptor current by barbiturates wa due to prolongation of mean channel lifetime, whereas the reduction of GABA(A) receptor current by picrotoxin was due to reduction of channel opening frequency and mean channel lifetime. Stable cell lines containing subunit combinations of this receptor should provide a powerful tool for the elucidation of its channel features and control mechanisms.

Temperature-sensitive expression of all-Torpedo and Torpedo-rat hybrid AChR in mammalian muscle cells

When the four subunits of the Torpedo californica nicotinic acetylcholine receptor (AChR) are expressed in mammalian fibroblasts, they properly assembly into alpha 2 beta gamma delta pentamers only at temperatures lower than 37 degrees C (Claudio, T., W. N. Green, D. S. Hartman, D. Hayden, H. L. Paulson, F. J. Sigworth, S. M. Sine, and A. Swedlund. 1987. Science (Wash. DC). 238:1688-1694). Experiments here with rat L6 myoblast cell lines indicate that this temperature sensitivity is not specific to fibroblasts, but is intrinsic to Torpedo subunits. A clonal isolate of L6 cells cotransfected with the four Torpedo subunit cDNAs synthesizes the exogenous AChR subunits at 37 degrees and 26 degrees C, but expresses Torpedo AChR complexes only at the lower temperature. When Torpedo alpha alone is expressed in L6 myotubes, hybrid AChRs are formed, again only at temperatures below 37 degrees C. These hybrid AChRs can contain either two Torpedo alpha subunits or one each of rat and Torpedo alpha, proving that the two alpha subunits in an AChR pentamer need not derive from the same polysome. Further analysis of hybrid and all-Torpedo AChR established that there is no internally sequestered pool of AChR at the nonpermissive temperature, and that the AChR, once formed, is thermostable. Two lines of experimentation with alpha subunits expressed in fibroblasts indicate that alpha polypeptides exhibit different conformations at 26 degrees and 37 degrees C, favoring the hypothesis that the temperature-sensitive step occurs before assembly and reflects, at least in part, misfolding of subunits: at 37 degrees C, there is a reduction in the fraction of alpha subunits that (a) bind the AChR antagonist alpha-bungarotoxin with high affinity; and (b) bind a monoclonal antibody that recognizes correctly folded and/or assembled alpha subunit.