Structural homology of Torpedo californica acetylcholine receptor subunits

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Protein-lipid interactions and Torpedo californica nicotinic acetylcholine receptor function. 2. Membrane fluidity and ligand-mediated alteration in the accessibility of gamma subunit cysteine residues to cholesterol

Fluorescence-quenching and energy-transfer measurements were carried out to further characterize lipid-protein interactions involving the nicotinic acetylcholine receptor (AChR) from Torpedo californica in reconstituted membranes. To assess the fluidity of the receptor microenvironment, cis- and trans-parinaric acids were used to take advantage of the preferential partitioning behavior of the trans isomer for the gel phase. A relatively higher extent of energy transfer from the intrinsic tryptophan fluorescence of AChR in dielaidoylphosphatidylcholine bilayers to cis-parinaric acid in both the gel and the fluid phase suggests that the AChR is surrounded by a relatively fluid annulus of lipids. The ability of AChR to accommodate and interact with specific lipids such as cholesterol and fatty acids in the vicinity of pyrene-labeled cysteine residues in the membranous domain and/or the membrane-water interface region of the gamma subunit was assessed. Pyrene-labeled AChR prepared in (6,7-dibromostearoyl)phosphatidylcholine showed a 25% decrease in fluorescence as sites accessible to phospholipids were occupied; subsequent addition of dibromocholesterol hemisuccinate (DiBrCHS) caused further quenching by about 25%. This result is consistent with the presence of sites accessible to cholesterol, but not accessible to phospholipids, in the vicinity of the cysteine-bound pyrene in the membranous domain of the AChR. Quenching by DiBrCHS was sensitive to the presence of an AChR activator (carbamylcholine) but not a competitive antagonist (alpha-bungarotoxin). The Stern-Volmer quenching constant was 0.123 in the absence of added ligands and 0.167 and 0.134 in the presence of carbamylcholine and alpha-bungarotoxin, respectively, corresponding to accessibilities of 65%, 90%, and 70%.

Protein-lipid interactions and Torpedo californica nicotinic acetylcholine receptor function. 1. Spatial disposition of cysteine residues in the gamma subunit analyzed by fluorescence-quenching and energy-transfer measurements

The nicotinic acetylcholine receptor from Torpedo californica was labeled with a fluorescent, lipophilic probe, N-(1-pyrenyl)maleimide, specific for sulfhydryls in a hydrophobic environment, and was found to alkylate Cys 416, Cys 420 and Cys 451 in the gamma subunit [Li, L., Schuchard, M., Palma, A., Pradier, L., & McNamee, M.G. (1990) Biochemistry 29, 5428-5436]. The spatial disposition of the acetylcholine receptor-bound pyrene with respect to the membrane bilayer was assessed by a combination of fluorescence-quenching and resonance energy transfer measurements, under conditions of selective labeling of the gamma subunit. Quenching of pyrene fluorescence by spin-labeled fatty acids with the doxyl group at positions C-5 and C-12 revealed that the former was more effective, with a Stern-Volmer quenching constant of 0.187 compared to 0.072 for the latter, suggesting that the fluorophore(s) are located closer to the membrane-water interface rather than the hydrophobic interior. Energy transfer was found to occur from tryptophan in the acetylcholine receptor to cysteine-bound pyrene with a distance of separation of approximately 18 A. However, there was no energy transfer when pyrene-labeled AChR was reconstituted into membranes containing brominated phospholipids and cholesterol, suggesting that the fluorophore(s) responsible for energy transfer are located in the membrane domain. Thus, the N-(1-pyrenyl)maleimide can be used to monitor lipid-protein interactions of the AChR.

Detection of alpha-subunit isoforms in human muscle acetylcholine receptor by specific T cells from a myasthenia gravis patient

The nicotinic acetylcholine receptor (AChR) is both the best-characterized transmitter receptor-ion channel and the target for the pathogenic antibodies in the human autoimmune disease myasthenia gravis (MG). In cloning and sequencing its components in man, we found that the alpha-subunit was transcribed in two isoforms, with (P3A+) or without (P3A-) a 75 base pair exon that had not been described in other species. While studying the human T lymphocyte response to recombinant AChR, we found that part of this P3A insert was recognized by one T cell line (from an MG patient), whereas another line only recognized the uninterrupted insertion site. To establish whether this exon is also translated in normal human muscle, we initially raised anti-peptide antibodies to the relevant amino acid sequences, but these failed to bind native AChR (affinity-purified from muscle on alpha-neurotoxin columns). We therefore exploited the great sensitivity and specificity of these T cells to detect the two isoforms after unfolding by antigen-presenting cells, and have been able to show that both are expressed in affinity-purified human muscle AChR.

Molecular dissection of subunit interfaces in the acetylcholine receptor: identification of residues that determine curare selectivity

The acetylcholine receptor from vertebrate skeletal muscle is a transmembrane channel that binds nerve-released acetylcholine to elicit rapid transport of small cations. Composed of two alpha subunits and one beta, one gamma, and one delta subunit, the receptor is a cooperative protein containing two sites that bind agonists, curariform antagonists, and snake alpha-toxins. Until recently the two binding sites were thought to reside entirely within each of the two alpha subunits, but affinity labeling and expression studies have demonstrated contributions by the gamma and delta subunits. Affinity labeling and mutagenesis studies have identified residues of the alpha subunit that contribute to the binding site, but the corresponding gamma- and delta-subunit residues remain unknown. By making gamma-delta chimeras and following the nearly 100-fold difference in curare affinity for the two binding sites, the present work identified residues of the gamma and delta subunits likely to be near the binding site. Two sets of binding determinants were identified in homologous positions of the gamma and delta subunits. The determinants lie on either side of a disulfide loop found within the major extracellular domain of the subunits. This loop is common to all acetylcholine, gamma-aminobutyrate, and glycine receptor subunits.

A splice variant of the N-methyl-D-aspartate (NMDAR1) receptor

A splice variant of the NMDA receptor (NMDAR1) was discovered containing a deletion of 37 amino acids near the carboxyl tail and has been designated NMDAR1b. The 111 nucleotides corresponding to the deleted amino acid sequence were found in a separate exon bounded by consensus intron/exon junction sequences in rat genomic DNA. A partial restriction map of genomic DNA bounding this region placed the deleted exon approximately 600 base pairs (bp) downstream of the upstream exon. RT/PCR analysis of RNA from different brain regions showed that the deletion variant is more abundantly expressed in the brain stem and cerebellum while the full-length form is expressed more abundantly in the olfactory bulb, striatum, hippocampus, and cortex. Northern analysis of poly(A)+ RNA from different brain regions with probes specific for the deleted exon (i.e., full-length form) and for the splice junction (deletion form) indicated approximately 4.4 kb transcripts. The probe for the deleted exon hybridized to transcripts in olfactory bulb, cortex, striatum, and hippocampus while the splice junction probe hybridized most strongly to transcripts in cerebellum. The results suggest an interesting rostral to caudal shift in the expression of splice variants of the NMDAR1 which may signify important functional differences in native forms of NMDA receptors.

Reporter epitopes: a novel approach to examine transmembrane topology of integral membrane proteins applied to the alpha 1 subunit of the nicotinic acetylcholine receptor

The development of a novel immunological method called the "reporter epitope" technique to probe the transmembrane topology of integral membrane proteins is described. Using this method, synthetic oligonucleotides encoding epitopes (reporter epitopes) for well characterized monoclonal antibodies (reporter mAbs) were inserted at various locations within the human acetylcholine receptor (AChR) alpha 1 subunit cDNA. The engineered subunits were then expressed along with Torpedo beta 1, gamma, and delta subunits in Xenopus oocytes, and the transmembrane location of the site of insertion was determined by the binding of the 125I-labeled reporter mAbs to whole oocytes. Control reporter epitope insertions at alpha 347 exhibited the expected cytoplasmic location. Reporter epitopes inserted at alpha 429 are located on the extracellular surface. Reporter epitopes that are 16-48 amino acids long do not disrupt assembly or function of hybrid AChRs when inserted near the carboxy terminus (at alpha 429) or in the large cytoplasmic domain (at alpha 347). However, because two reporter epitopes inserted at alpha 157 obliterated subunit assembly and a third reporter epitope when tolerated at this position was inaccessible from the extracellular surface and only marginally accessible after detergent solubilization of the AChRs, a definitive transmembrane location for this region was not possible. Nonetheless, the use of this approach has been successfully demonstrated, and it may be generally applicable to the study of other integral membrane proteins.

FTIR analysis of nicotinic acetylcholine receptor secondary structure in reconstituted membranes

Using Fourier-transform infrared resonance spectroscopy, we examined the structure of the purified Torpedo californica nicotinic acetylcholine receptor in reconstituted dioleoylphosphatidylcholine membranes in H2O and D2O. Using the amide-I band, we calculated the secondary structure of nAChR in H2O to be approx. 19% alpha-helix, 42% beta-structure, 24% turns and 15% unordered. The secondary structure content in D2O was estimated to be 14% alpha-helix, 37% beta-structure, 29% turns and 20% unordered. In the presence of phosphatidic acid the beta-structure content in D2O increased significantly from 37% to 42%. This suggests that an ionic interaction between negatively-charged lipid head groups and positively-charged peptide side chains may stabilize a beta-structure conformation that is necessary for receptor function. The inclusion of cholesterol in the reconstituted membranes significantly increased the alpha-helix content from 14% to 17%. These results support the hypothesis that cholesterol may induce a transmembrane region to undergo a unordered-to-helix transition which is necessary to maintain the integrity of the ion channel. Additionally, we found that nAChR did not undergo major secondary structure changes when subjected to conditions that induce desensitization. This is consistent with the view that the mechanism of desensitization consists of small quaternary rearrangements of the subunits rather than large changes in receptor secondary structure.

Primary structure of the human muscle acetylcholine receptor. cDNA cloning of the gamma and epsilon subunits

cDNA sequences encompassing the full coding region for the human muscle acetylcholine receptor (AChR) epsilon and gamma subunits have been isolated. The deduced amino-acid sequences indicate that the mature epsilon subunit contains 473 amino acids and is preceded by a 20-amino-acid signal peptide. As predicted from genomic clones, the gamma subunit contains 495 amino acids preceded by a 22-amino-acid signal peptide. In common with the human alpha, beta, gamma and delta subunits the epsilon subunit is highly conserved between mammalian species. The epsilon subunit gene is not closely linked to the gamma and delta subunits on chromosome 2 but rather is located with the beta subunit on chromosome 17. Expression of the alpha-, beta-, gamma-, delta- and epsilon-subunit cRNAs in rabbit-reticulocyte lysates followed by analysis on SDS/PAGE show glycosylated proteins with apparent molecular masses of 44-60 kDa.

Negatively charged amino acid residues in the nicotinic receptor delta subunit that contribute to the binding of acetylcholine

In nicotinic receptors, the binding sites for acetylcholine are likely to contain negatively charged amino acid side chains that interact with the positively charged quaternary ammonium group of acetylcholine and of other potent agonists. We previously found that a 61-residue segment of the delta subunit contains aspartate or glutamate residues within 1 nm of cysteines in the acetylcholine binding site on the alpha subunit. We have now mutated, one at a time, the 12 aspartates and glutamates in this segment of the mouse muscle delta subunit and have expressed the mutant receptors in Xenopus oocytes. Both the concentration of acetylcholine eliciting half-maximal current (Kapp) and the Ki for the inhibition by acetylcholine of alpha-bungarotoxin binding were increased 100-fold by the mutation of delta Asp180 to Asn and 10-fold by the mutation of delta Glu189 to Gln. These two residues, and their homologs in the gamma and epsilon subunits, are likely to contribute to the acetylcholine binding sites.

Structure of nicotinic acetylcholine receptors

Nicotinic acetylcholine (ACh) receptors convert the binding of ACh into the opening of a cation-conducting channel. New information about the regions of the receptor most immediately involved in its function, namely the ACh-binding sites, the gate and the channel, has come from two approaches. One is the identification by labelling and by mutagenesis of residues contributing to these regions. Another is the determination of the three-dimensional structure of the receptor by electron microscopy. Although the identification of functionally relevant residues is incomplete and residues cannot yet be resolved in the three-dimensional structure, the two approaches are converging. There is still room in the gap for speculation.

B cell autoimmunity to acetylcholine receptor and its subunits in Lewis rats over the course of experimental autoimmune myasthenia gravis

Experimental autoimmune myasthenia gravis (EAMG) is induced by a single injection of acetylcholine receptor (AChR) with complete Freund's adjuvant and represents a useful animal model for studying the mechanisms by which autoimmune responses to AChR and its subunits are coupled to the development of human myasthenia gravis. Using an immunospot assay, we enumerated cells secreting IgG antibodies against Torpedo AChR and the alpha-, beta-, gamma- and delta-subunits of Torpedo AChR in lymph nodes, spleen and thymus from Lewis rats over the course of EAMG. Cells secreting IgG antibodies to AChR and to all four subunits were detected at higher numbers in the three immune organs in EAMG compared to controls. Numbers were highest in lymph nodes followed by spleen and thymus. Cells secreting IgG antibodies against native AChR were always higher than those against individual subunits. The immunogenicity between the four subunits did not differ, with the exception that the alpha-subunit induced a slightly higher B cell response in thymus and lymph nodes. The patterns of B cell responses were similar when analyzed over the course of EAMG from week 2 to week 5, and there was no restriction of the B cell repertoire early in EAMG. Anti-AChR and anti-subunit antibody-secreting cells were also detected in control animals immunized with adjuvant only, but at numbers which were much lower, and which were within the same level as numbers of cells secreting IgG antibodies to the control antigen myelin basic protein, probably reflecting naturally occurring autoimmune B cells.

T-cell immunity to acetylcholine receptor and its subunits in Lewis rats over the course of experimental autoimmune myasthenia gravis

Lymph nodes, spleen and thymus obtained from Lewis rats were examined over the course of experimental autoimmune myasthenia gravis (EAMG) for the distribution and the number of antigen-reactive CD4+ T helper cells which, upon recognition of Torpedo acetylcholine receptor (AChR) or the alpha, beta, gamma or delta subunits of Torpedo AChR, responded by secretion of interferon-gamma (IFN-gamma). T cells with these specificities were detected in these three immune organs. Numbers were highest in lymph nodes. In spleen and thymus, numbers of antigen-reactive T cells did not differ. T cells reacting against the intact AChR were more frequent than T cells recognizing any of the subunits. The immunogenicity between the four subunits did not differ, with the exception that the alpha subunit induced a slightly higher T-cell response. No restriction of the T-cell repertoire to the four subunits was detected during early compared to late phases of EAMG. The AChR and subunit-reactive T cells could--via secretion of effector molecules including IFN-gamma--play an important role in the initiation and perpetuation of EAMG, and consequently also of human myasthenia gravis. T cells with the same specificities were also detected in control animals injected with adjuvant only, but at much lower numbers which were within the range of T cells recognizing the control antigen myelin basic protein. They could represent naturally occurring autoimmune T cells.

Activation of nicotinic acetylcholine receptors on cultured Drosophila and other insect neurones

1. Using whole-cell and single channel recordings, we have examined the properties of acetylcholine (ACh)-activated currents in neurones from larval and pupal Drosophila melanogaster (fruit fly), larval and embryonic Musca domestica (house fly), and nymphal Schistocerca gregaria (locust). 2. In all preparations, single channel recordings revealed two major classes of ACh-activated channels, with average conductances of approximately 32 and 59 pS. 3. At ACh concentrations from 1 to 10 microM, channel activity in Drosophila larval neurones occurs in bursts with an average of 1-2 openings. Open times and burst durations are described by one or two exponentials. Burst durations for the 32 pS channel (approximately 3 ms, slow component) were longer than those for the 59 pS channel (approximately 1.0 ms). The mean open interval duration for the 32 pS channel (slow component) was also longer than that of the 59 pS channel. 4. At high ACh (20-200 microM) concentrations, bursts of the smaller conductance channel occur in clusters separated by long-lived periods without channel activity. Considerable kinetic heterogeneity was observed among clusters. 5. The whole-cell dose-response curve suggests that activation of current by ACh increases up to at least 100 microM and that multiple ligand binding steps are involved. 6. Drosophila and Musca larval neuronal ACh-activated channels show some unique features in their cholinergic pharmacological properties: (a) they are only weakly activated by the potent neuromuscular nicotinic agonist suberyldicholine, (b) hexamethonium and decamethonium are weak, but approximately equi-effective blockers, and (c) alpha- and kappa-bungarotoxin (BTX) both blocked reversibly, though alpha-BTX appears to be the more potent inhibitor.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Acetylcholine receptor/channel molecules of insects

Acetylcholine-gated ion channels of the nicotinic type are abundant in the nervous system of insects. The channels are permeable to Na+, K+ and probably Ca(2+), and unlike most vertebrate neuronal nicotinic acetylcholine receptors the receptor/channel molecule is blocked by alpha-bungarotoxin (alpha-Bgt). Such alpha-Bgt-sensitive receptors are present at synapses and on cell bodies of insect neurones. Single channel recordings have shown the existence of multiple conductances of nAChRs. Studies on several different insect preparations have provided evidence for more than one open state and several closed states of insect nAChRs. Functional insect nAChR channels have now been investigated in situ, following reconstitution of a purified protein in bilayers, and as a result of expressing in Xenopus oocytes messenger RNA encoding receptor subunits.

Profile of the regions on the alpha-chain of human acetylcholine receptor recognized by autoantibodies in myasthenia gravis

Eighteen synthetic overlapping peptides encompassing the entire extracellular part (residues alpha 1-210) of the alpha-chain of human acetylcholine receptor (AChR) and a 19th peptide (residues alpha 262-276) corresponding to an extracellular connection between two transmembrane regions were prepared and used for the measurement, by solid-phase radioimmunoassay, of the binding of autoantibodies in plasma from myasthenia gravis (MG) patients. Autoantibodies were found to recognize only a limited number of the synthetic peptides. The regions recognized resided predominantly within the areas alpha 10-30, alpha 111-145 and alpha 175-198 and, less frequently, region alpha 45-77. Differences in the recognition profile of the peptides from patient to patient indicated that the autoantibody responses were under genetic control. However, by using a mixture of the appropriate peptides, it was possible to determine autoantibodies in all 15 myasthenia sera and to distinguish between these, normal human sera and other neurological or autoimmune diseases. The mapping of the continuous antigenic regions recognized by autoantibodies on the alpha-chain of human AChR has permitted a comparison of the regions recognized by autoantibodies and autoimmune T-cells from the same donor. It also provided a peptide-based direct antibody binding method for diagnosis of MG.

Structure and function of inhibitory glycine receptors

Ion transport by peptide channels has been the major theme in the work of the late P. Läuger. His theoretical and experimental approaches provided the basis for a deeper understanding of pore-mediated ion permeation through biological membranes. This review on a ligand–gated ion channel protein from the mammalian brain is dedicated to the memory of this outstanding scientist.

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

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

Pore size and negative charge as structural determinants of permeability in the Torpedo nicotinic acetylcholine receptor channel

To gain an insight into the molecular basis of ion permeation mechanism through the nicotinic acetylcholine receptor (AChR) channel, we have determined permeability ratios of organic cations relative to Na+ of specifically mutated Torpedo californica AChR channels expressed in Xenopus oocytes. The mutations involved mainly the side chains of the amino acid residues in the intermediate ring, where mutations have been found to exert strong effects on single-channel conductance and ion selectivity among alkali metal cations. The results obtained reveal that both the size and the net charge of the side chains of the intermediate ring are involved in determining the permeability, and provide experimental evidence that the pore size at the intermediate ring is a critical determinant of permeability. Our findings further suggest that changes in net charge exert effects on permeability by affecting the pore size of the channel.

Acetylcholine receptor channel structure probed in cysteine-substitution mutants

In order to understand the structural bases of ion conduction, ion selectivity, and gating in the nicotinic acetylcholine receptor, mutagenesis and covalent modification were combined to identify the amino acid residues that line the channel. The side chains of alternate residues--Ser248, Leu250, Ser252, and Thr254--in M2, a membrane-spanning segment of the alpha subunit, are exposed in the closed channel. Thus alpha 248-254 probably forms a beta strand, and the gate is closer to the cytoplasmic end of the channel than any of these residues. On channel opening, Leu251 is also exposed. These results lead to a revised view of the closed and open channel structures.

Asymmetry of the rat acetylcholine receptor subunits in the narrow region of the pore

The acetylcholine receptor (AChR) channel is a pentameric protein in which every subunit contributes to the conducting parts of the pore. Recent studies of rat nicotinic AChR channels mutated in the alpha-subunit revealed that a threonine residue (alpha T264) in the transmembrane segment M2 forms part of the narrow region of the channel. We have mutated the residues at homologous positions in the beta-, gamma-, and delta-subunits and measured the resulting change in channel conductance. For all subunits the conductance is inversely related to the volume of the amino acid residue, suggesting that they form part of the channel narrow region. Exchanges of residues between subunits do not alter the conductance, suggesting a ring-like structure formed by homologous amino acids. To investigate the relative contribution of amino acid residues at these positions in determining the channel conductance, receptors carrying the same amino acid in each subunit in the narrow region were constructed. They form functional channels in which the conductance is inversely related to the volume of the amino acids in the narrow region. Channels in which the narrow region is formed by four serines and one valine have the same conductance if the valine is located in the alpha-, beta-, or gamma-subunits, but it is smaller if the valine is located in the delta-subunit. The results suggest a structural asymmetry of the AChR channel in its narrow region formed by the hydroxylated amino acids of alpha-, gamma- and delta-subunits, where the delta-subunit serine is a main determinant of the channel conductance.

The roles of serine and threonine sidechains in ion channels: a modelling study

The ion channel of the nicotinic acetylcholine receptor (nAChR) is believed to be lined by transmembrane M2 helices. A "4-8-12" sequence motif, comprising serine (S) or threonine (T) residues at positions 4, 8 and 12 of M2, is conserved between different members, anion and cation selective, of the nAChR superfamily. Parallel bundles of 4-8-12 motif-containing helices are considered as simplified models of ion channels. The relationship between S and T sidechain conformations and channel-ion interactions is explored via evaluation of interaction energies of K+ and of Cl- ions with channel models. Energy calculations are used to determine optimal chi 2 (C alpha-C beta-O gamma-H gamma) values in the presence of K+ or Cl- ions. 4-8-12 motif-containing bundles may form favourable interactions with either cations or anions, dependent upon the chi 2 values adopted. Parallel-helix and tilted-helix bundles are considered, as are heteromeric models designed to mimic the Torpedo nAChR. The main conclusion of the study is that conformational flexibility at chi 2 enables both S and T residues to form favourable interactions with anions or cations. Consequently, there is apparently no difference between S and T residues in their interactions with permeant ions, which suggests that the presence of T vs. S residues within the 4-8-12 motif is not a major mechanism whereby anion/cation selectivity may be generated. The implications of these studies with respect to more elaborate models of nAChR and related receptors are considered.

Possible relationship between coding recognition amino acid sequence motif or residue(s) and post-translational chemical modification of proteins

1. The "code-sequence" of N-glycosylation site(s), the amino acids located around O-glycosylation site(s), the sequence motifs of several kinases, the sequence motifs of--sulfation, amidation, isoprenylation, myristoylation, palmitoylation and N-acetylation, Aspartic and Asparagine hydroxylation-site, gamma-carboxyglutamate domain, phosphopantetheine attachment site etc. are extensively listed, compared to those reported by "PROSITE" Computer Screen Center and discussed. 2. The structural aspects of protein-DNA recognition are quoted as discussion and conclusion.

Modulation of anti-acetylcholine receptor antibody specificities and of experimental autoimmune myasthenia gravis by synthetic peptides

Synthetic peptides corresponding to selected sequences from the nicotinic acetylcholine receptor (AChR) were employed to identify possible antigenic determinants within the receptor which can modulate the anti-AChR response and experimental autoimmune myasthenia gravis (EAMG). Immunization of rabbits with peptides T alpha 73-89, T alpha 351-368, T delta 354-367 and H alpha 351-368, prior to AChR inoculation, affected the course of EAMG in six out of eight rabbits. These six protected rabbits survived three inoculations of AChR and survived for at least five months after the third injection with AChR, whereas control rabbits died following one or two injections of AChR. The survival of peptide-preimmunized rabbits injected with AChR seemed to correlate with the antibody specificities in immunoblots. Following AChR inoculation there was a shift in reactivity, from a subunit-restricted response, to reactivity with all subunits of the receptor. This shift was delayed in protected rabbits. This may indicate that the reactivity with the entire Torpedo receptor molecule represents a loss of tolerance to AChR which culminates in the autoimmune disease, EAMG.

Threonine in the selectivity filter of the acetylcholine receptor channel

The acetylcholine receptor (AChR) is a cation selective channel whose biophysical properties as well as its molecular composition are fairly well characterized. Previous studies on the rat muscle alpha-subunit indicate that a threonine residue located near the cytoplasmic side of the M2 segment is a determinant of ion flow. We have studied the role of this threonine in ionic selectivity by measuring conductance sequences for monovalent alkali cations and bionic reversal potentials of the wild type (alpha beta gamma delta channel) and two mutant channels in which this threonine was replaced by either valine (alpha T264V) or glycine (alpha T264G). For the wild type channel we found the selectivity sequence Rb greater than Cs greater than K greater than Na. The alpha T264V mutant channel had the sequence Rb greater than K greater than Cs greater than Na. The alpha T264G mutant channel on the other hand had the same selectivity sequence as the wild type, but larger permeability ratios Px/PNa for the larger cations. Conductance concentration curves indicate that the effect of both mutations is to change both the maximum conductance as well as the apparent binding constant of the ions to the channel. A difference in Mg2+ sensitivity between wild-type and mutant channels, which is a consequence of the differences in ion binding, was also found. The present results suggest that alpha T264 form part of the selectivity filter of the AChR channel were large ions are selected according to their dehydrated size.

Modulation of nicotinic acetylcholine receptor channel by pH: a difference in pH sensitivity of Torpedo and mouse receptors expressed in Xenopus oocytes

1. In this study, effects of pH on the ion channel function of nicotinic acetylcholine receptor (nAChR) from Torpedo californica electroplax and mouse muscle BC3H-1 cells were investigated using Xenopus laevis oocytes injected with in vitro synthesized RNA transcripts. The acetylcholine (ACh)-induced whole-cell peak current responses and slow desensitization rates were measured by voltage-clamp. 2. The ACh-induced peak currents of Torpedo nAChRs were reversibly diminished when extracellular pH was reduced from 7.4 to 5.0 and increased at pH greater than 7.4. This pH dependence had an apparent pKa of 7.0. In contrast, the peak current of mouse muscle nAChRs were reversibly decreased at both acidic and alkaline pH's. This bell-shaped pH profile with a maximum at pH 7.4 had two apparent pKa values of 5.6 and 9.2. 3. The peak current responses of four mouse-Torpedo nAChR hybrids consisting of three Torpedo subunits and one mouse nAChR subunit displayed similar pH profiles at acidic pH, with a pKa of 6.0 to 6.5, which lay between the pKa 5.6 for mouse and the pKa 7.0 for Torpedo nAChRs. Two of these combinations, alpha M beta T gamma T delta T and alpha T beta T gamma M delta T, also had an alkaline pH dependence of the current response similar to that of mouse receptor, with a pKa of 9.3 to 9.5. 4. The slow desensitization rate of Torpedo nAChRs increased at both acidic and alkaline pH's, with two pKa values of 6.5 and 9.5, whereas that of mouse muscle receptors remained unchanged from pH 6.5 to pH 9.0 and increased at pH less than 6.0, with a pKa of 4.7. 5. Substitution of each subunit of Torpedo nAChR with a mouse counterpart resulted in a pH dependence pattern (pKa 6.0 to 6.4 and pKa 9.1 to 9.3) similar to that of Torpedo nAChR except the substitution with mouse beta subunit (pKa 4.8 and pKa 8.9), which appears to carry the characteristic acidic group determining the pH dependence of mouse nAChR desensitization. 6. The apparent pKa values obtained from the pH dependence studies of nAChR channel activation and desensitization suggest the involvement of different amino acid residues in determining channel functions of Torpedo and mouse nAChRs. The groups with pKa 7.0 and 6.5 on Torpedo nAChR can be tentatively identified as His residues, whereas those with pKa 5.6 and 4.7 on mouse receptor as Glu or Asp residues. The alkaline pKa of 8.9 to 9.5 may be Cys, Tyr, or Lys.

The unc-18 Gene Encodes a Novel Protein Affecting the Kinetics of Acetylcholine Metabolism in the Nematode Caenorhabditis elegans

Genes affecting acetylcholine (ACh) levels without influencing choline acetyltransferase activity have been identified in Caenorhabditis elegans. We have examined one such gene, unc-18. We isolated a transposon-insertion allele for unc-18 and used it to clone a genomic region containing the unc-18 locus. The unc-18 location within this region was determined by rescuing the unc-18 mutant phenotype in a germ-line transformation experiment and identifying transcripts affected by four independent unc-18 mutations. A single-sized poly(A)+ RNA was synthesized from the gene. Expression of the transcript appears to be stage specific: The transcript is found in abundance at the early larval stage but in decreased amounts at the fourth larval and the adult stages. These results show that the unc-18 gene plays a role in development as well as in the kinetics of ACh metabolism.

Epitope mapping of monoclonal antibodies to Torpedo acetylcholine receptor gamma subunits, which specifically recognize the epsilon subunit of mammalian muscle acetylcholine receptor

Epitopes for four monoclonal antibodies (mAbs) to the gamma subunit of Torpedo nicotinic acetylcholine receptor (AChR), and one mAb crossreactive with the gamma and delta subunits of Torpedo AChR were mapped using overlapping synthetic peptides corresponding to the complete amino acid sequence of Torpedo gamma subunit. The epitopes for all mAbs were within a 50 residue sequence region, on the cytoplasmic surface of the AChR. Three mAbs crossreacted with mammalian muscle AChRs. Two of them specifically recognized the epsilon subunit of AChRs at adult neuromuscular junction. The epsilon-specific mAbs were used, in conjunction with mAbs specific for the alpha and beta subunits and anti-peptide antisera specific for the epsilon, gamma and delta subunits, to identify in Western blots the subunit complement of embryonic and adult bovine muscle AChRs.

Thymopoietin, a thymic polypeptide, potently interacts at muscle and neuronal nicotinic alpha-bungarotoxin receptors

Current studies suggest that several distinct populations of nicotinic acetylcholine (ACh) receptors exist. One of these is the muscle-type nicotinic receptors with which neuromuscular nicotinic receptor ligands and the snake toxin alpha-bungarotoxin interact. alpha-Bungarotoxin potently binds to these nicotinic receptors and blocks their function, two characteristics that have made the alpha-toxin a very useful probe for the characterization of these sites. In neuronal tissues, several populations of nicotinic receptors have been identified which, although they share a nicotinic pharmacology, have unique characteristics. The alpha-bungarotoxin-insensitive neuronal nicotinic receptors, which may be involved in mediating neuronal excitability, bind nicotinic agonists with high affinity but do not interact with alpha-bungarotoxin. Subtypes of these alpha-toxin-insensitive receptors appear to exist, as evidenced by findings that some are inhibited by neuronal bungarotoxin whereas others are not. In addition to the alpha-bungarotoxin-insensitive sites, alpha-bungarotoxin-sensitive neuronal nicotinic receptors are also present in neuronal tissues. These latter receptors bind alpha-bungarotoxin with high affinity and nicotinic agonists with an affinity in the microM range. The function of the nicotinic alpha-bungarotoxin receptors are as yet uncertain. Thymopoietin, a polypeptide linked to immune function, appears to interact specifically with nicotinic receptor populations that bind alpha-bungarotoxin. Thus, in muscle tissue where alpha-bungarotoxin both binds to the receptor and blocks activity, thymopoietin also potently binds to the receptor and inhibits nicotinic receptors-mediated function. In neuronal tissues, thymopoietin interacts only with the nicotinic alpha-bungarotoxin site and not the alpha-bungarotoxin-insensitive neuronal nicotinic receptor population. These observations that thymopoietin potently and specifically interacts with nicotinic alpha-bungarotoxin-sensitive receptors in neuronal and muscle tissue, together with findings that thymopoietin is an endogenously occurring agent, could suggest that this immune-related polypeptide represents a ligand for the alpha-bungarotoxin receptors. The function of thymopoietin at the alpha-bungarotoxin receptor is as yet uncertain; however, a potential trophic, as well as other roles are suggested.

Primary structure and expression of the gamma 2 subunit of the glutamate receptor channel selective for kainate

The presence and primary structure of a novel subunit of the mouse glutamate receptor channel, designated as gamma 2, have been revealed by cloning and sequencing the cDNA. The gamma 2 subunit has structural characteristics common to the neurotransmitter-gated ion channel family and shares a high amino acid sequence identity with the rat KA-1 subunit, thus constituting the gamma subfamily of the glutamate receptor channel. Expression of the gamma 2 subunit together with the beta 2 subunit in Xenopus oocytes yields functional glutamate receptor channels selective for kainate.

Species- and subtype-specific recognition by antibody WF6 of a sequence segment forming an alpha-bungarotoxin binding site on the nicotinic acetylcholine receptor alpha subunit

The monoclonal antibody WF6 competes with acetylcholine and alpha-bungarotoxin (alpha-BGT) for binding to the Torpedo nicotinic acetylcholine receptor (nAChR) alpha 1 subunit. Using synthetic peptides corresponding to the complete Torpedo nAChR alpha 1 subunit, we previously mapped a continuous epitope recognized by WF6, and the prototope for alpha-BGT, to the sequence segment alpha 1(181-200). Single amino acid substitution analogs have been used as an initial approach to determine the critical amino acids for WF6 and alpha-BGT binding. In the present study, we continue our analysis of the structural features of the WF6 epitope by comparing its cross-reactivity with synthetic peptides corresponding to the alpha 1 subunits from the muscle nAChRs of different species, the rat brain alpha 2, alpha 3, alpha 4 and alpha 5 nAChR subtypes, and the chick brain alpha-BGT binding protein subunits, alpha BGTBP alpha 1 and alpha BGTBP alpha 2. Our results indicate that WF6 is able to cross-react with the muscle alpha 1 subunits of different species by virtue of conservation of several critical amino acid residues between positions 190-198 of the alpha 1 subunit. These studies further define the essential structural features of the sequence segment alpha 1(181-200) required to form the epitope for WF6.

The N-terminal domains of acetylcholine receptor subunits contain recognition signals for the initial steps of receptor assembly

Ligand-gated ion channels are oligomeric membrane proteins in which homologous subunits specifically recognize one another and assemble around an aqueous pore. To identify domains responsible for the specificity of subunit association, we used a dominant-negative assay in which truncated subunits of the mouse muscle acetylcholine receptor (AChR) were coexpressed with the four wild-type subunits in transfected COS cells. Fragments of the alpha, delta, and gamma subunits consisting solely of the extracellular N-terminal domain blocked surface expression of the AChR and the formation of alpha delta heterodimers, an early step in the assembly pathway of the AChR. Immunoprecipitation and sucrose gradient sedimentation experiments showed that an N-terminal fragment of the alpha subunit forms a specific complex with the intact delta subunit. Thus the extracellular N-terminal domain of the alpha, delta, and gamma subunits contains the information necessary for specific subunit association.

Examination of subconductance levels arising from a single ion channel

Single-channel records often show frequent currents at a main conductance level and occasional currents at subconductance levels. In some instances, the conductances occur at regular levels that are multiples of a minimum conductance. It is well-appreciated that multiple conductance levels may arise either from the co-operative gating of more than one pore or from changes that occur in a single pore. In this paper, we used theoretical models of ion permeation to examine subconductances arising in a single-pore channel. In particular, the work focuses on the following question: how can an ion channel that provides only one aqueous pore through the membrane produce regular subconductances and a main conductance that all have the same selectivity and the same ion binding affinity? The three types of ion permeation models used in this study showed that a single-pore channel can have subconductances because of long-lived conformational states, because of alterations in rapid fluctuations between conformational states, or because of slight alterations in the electrostatic properties in the channel's entrance vestibules. Regular subconductances with the same selectivity and binding affinity can arise in a single pore even if the energy profile changes do not meet the constant peak offset condition. The results show that the appearance of regular subconductance levels in a single-channel recording is not sufficient evidence to conclude that identical pores have co-operative gating, as would arise in a channel that is a multi-pore complex.

Cloning of cDNA for the glutamate-binding subunit of an NMDA receptor complex

The amino acids L-glutamic and L-aspartic acids form the most widespread excitatory transmitter network in mammalian brain. The excitation produced by L-glutamic acid is important in the early development of the nervous system, synaptic plasticity and memory formation, seizures and neuronal degeneration. The receptors activated by L-glutamic acid are a target for therapeutic intervention in neurodegenerative diseases, brain ischaemia and epilepsy. There are two types of receptors for the excitatory amino acids, those that lead to the opening of cation-selective channels and those that activate phospholipase C (ref. 11). The receptors activating ion channels are NMDA (N-methyl-D-aspartate) and kainate/AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate)-sensitive receptors. The complementary DNAs for the kainate/AMPA receptor and for the metabotropic receptor have been cloned. We report here on the isolation and characterization of a protein complex of four major proteins that represents an intact complex of the NMDA receptor ion channel and on the cloning of the cDNA for one of the subunits of this receptor complex, the glutamate-binding protein.

Structural determinants within residues 180-199 of the rodent alpha 5 nicotinic acetylcholine receptor subunit involved in alpha-bungarotoxin binding

Synthetic peptides corresponding to sequence segments of the nicotinic acetylcholine receptor (nAChR) alpha subunits have been used to identify regions that contribute to formation of the binding sites for cholinergic ligands. We have previously defined alpha-bungarotoxin (alpha-BTX) binding sequences between residues 180 and 199 of a putative rat neuronal nAChR alpha subunit, designated alpha 5 [McLane, K. E., Wu, X., & Conti-Tronconi, B. M. (1990) J. Biol. Chem. 265, 9816-9824], and between residues 181 and 200 of the chick neuronal alpha 7 and alpha 8 subunits [McLane, K. E., Wu, X., Schoepfer, R., Lindstrom, J., & Conti-Tronconi, B. M. (1991) J. Biol. Chem. (in press)]. These sequences are relatively divergent compared with the Torpedo and muscle nAChR alpha 1 alpha-BTX binding sites, which indicates a serious limitation of predicting functional domains of proteins based on homology in general. Given the highly divergent nature of the alpha 5 sequence, we were interested in determining the critical amino acid residues for alpha-BTX binding. In the present study, the effects of single amino acid substitutions of Gly or Ala for each residue of the rat alpha 5(180-199) sequence were tested, using a competition assay, in which peptides compete for 125I-alpha-BTX binding with native Torpedo nAChR.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and amino-terminal sequence of the bovine cardiac sodium-calcium exchanger: evidence for the presence of a signal sequence

The Na(+)-Ca2+ exchange carrier was purified from bovine cardiac tissue by a new procedure which relies principally upon anion-exchange chromatography. The purified protein exhibited two major bands on sodium dodecyl sulfate gels, at 120 and 160 kDa. The relative intensities of the two bands could be altered by variations in the procedures used for preparing the samples for electrophoresis, suggesting that they represent two different conformational states of the same protein. The NH2-terminal amino acid sequences of the 120- and 160-kDa bands were identical and agreed closely with a region of the deduced amino acid sequence of the recently cloned canine cardiac exchanger. The NH2-terminal sequence was preceded in the deduced sequence by a 32-residue segment that exhibited the characteristics of a signal sequence; the initial amino acid in the NH2-terminal sequence followed immediately after the predicted cleavage site for the signal sequence. The Na(+)-Ca2+ exchanger appears to be unique among membrane transport carriers in encoding a cleaved signal sequence. The characteristics of the sequences flanking the first putative transmembrane segment of the mature exchanger suggest that the signal sequence is necessary to ensure the correct topological orientation of the exchanger in the membrane.