Structure of nicotinic acetylcholine receptors

Acetylcholine receptor channel imaged in the open state

The structure of the open-channel form of the acetylcholine receptor has been determined from electron images of Torpedo ray postsynaptic membranes activated by brief (< 5 ms) mixing with droplets containing acetylcholine. Comparison with the closed-channel form shows that acetylcholine initiates small rotations of the subunits in the extracellular domain, which trigger a change in configuration of alpha-helices lining the membrane-spanning pore. The open pore tapers towards the intracellular membrane face, where it is shaped by a 'barrel' of alpha-helices having a pronounced right-handed twist.

Labeling studies of photolabile philanthotoxins with nicotinic acetylcholine receptors: mode of interaction between toxin and receptor

BACKGROUND

The nicotinic acetylcholine receptors (nAChRs) and glutamate receptors are ligand-gated cation channels composed of five separate polypeptide chains. A 43 kDa protein of unknown function is noncovalently associated with the cytoplasmic side of nAChR in vivo. The venoms of many wasps and spiders contain toxins that block the activity of these channels. Philanthotoxin-433 (PhTX-433) is a non-competitive channel blocker found in the venom of the wasp Philanthus. We have used a photolabile derivative to investigate how PhTX-433 interacts with nAChRs.

RESULTS

A radiolabeled PhTX analog, containing a photolabile group substituted on one of its aromatic rings, photocrosslinked to all five subunits (alpha, alpha 1, beta, gamma, delta) of purified nAChR in the absence of the 43 kDa protein. In the presence of the 43 kDa protein, the alpha subunit was preferentially labeled. Proteolysis of the receptor after crosslinking indicated that the hydrophobic end (head) of the PhTx-433 analog bound to the cytoplasmic loop(s) of the alpha-subunit. Binding is inhibited by other non-competitive channel blockers such as the related polyamine-amide toxins from spiders and chlorpromazine.

CONCLUSIONS

These results, coupled with previous structure/activity studies, lead to a putative model of the binding of PhTx and related polyamine toxins to nAChRs in vitro. The 43 kDa protein appears to influence the orientation of toxin binding. Further binding studies are necessary to confirm the model and to define how toxins enter the receptor and how they are oriented within it. A precise understanding of ligand/receptor interaction is crucial for the design of drugs specific for a particular subtype of receptor.

N-glycosylation site tagging suggests a three transmembrane domain topology for the glutamate receptor GluR1

We investigated the transmembrane topology of the glutamate receptor GluR1 by introducing N-glycosylation sites as reporter sites for an extracellular location of the respective site. Our data show that the N-terminus is extracellular, whereas the C-terminus is intracellular. Most importantly, we found only three transmembrane domains (designated TMD A, TMD B, and TMD C), which correspond to the previously proposed TMDs I, III, and IV, respectively. Contrary to earlier models, the putative channel-lining hydrophobic domain TMD II does not span the membrane, but either lies in close proximity to the intracellular face of the plasma membrane or loops into the membrane without transversing it. Furthermore, the region between TMDs III and IV, in previous models believed to be intracellular, is an entirely extracellular domain.

The alpha-conotoxins GI and MI distinguish between the nicotinic acetylcholine receptor agonist sites while SI does not

The alpha-conotoxins are paralytic peptide toxins from Indo-Pacific cone snails. This paper presents a detailed analysis of how alpha-conotoxins inhibit [125I]-alpha-bungarotoxin (125I-BTX) equilibrium binding to the acetylcholine receptor (AChR) from electric organ of Torpedo californica and Torpedo nobiliana. All three alpha-conotoxins studied, SI, GI, and MI, completely inhibited 125I-BTX binding with the same order of potency in both species (MI approximately GI > SI approximately d-tubocurarine). BTX-concentration curves showed that this inhibition is competitive. However, while SI appeared to bind to a homogeneous population of sites, both GI and MI displayed curare-like heterogeneous binding. Studies using partially-blocked AChR demonstrated that both GI and MI display different affinities toward the two agonist sites, much like small curariform antagonists do. The high-affinity site for these two alpha-conotoxins is also the high-affinity d-tubocurarine site, which is believed to be located at the alpha gamma-subunit interface. The high-affinity binding of MI and GI was of the same order of magnitude as that of d-tubocurarine; however, their affinity for the other agonist site was somewhat greater than that of dTC, resulting in less site selectivity. Despite being homologous to GI and MI, SI did not distinguish between the two sites. A possible molecular basis for this difference is presented.

Molecular mechanism of cyclic-nucleotide-gated channel activation

Studies on the activation of ligand- and voltage-gated ion channels have identified regions involved in both ligand binding and voltage sensing, but relatively little is known about how such domains are coupled to channel opening. Here we investigate the structural basis for the activation of cyclic-nucleotide-gated channels, which are directly opened by cytoplasmic cyclic nucleotides but are structurally homologous to voltage-gated channels. By constructing chimaeras between cyclic-nucleotide-gated channels cloned from bovine retinal photoreceptors and catfish olfactory neurons, we identify two distinct domains that are important for ligand binding and channel gating. A putative alpha-helix in the carboxy-terminal binding domain determines the selectivity of the channel for activation by cGMP relative to cAMP. A domain in the amino-terminal region determines the ease with which channels open and thus influences agonist efficacy. We propose that channel opening is coupled to an allosteric conformational change in the binding site which enhances agonist binding. Thus, cyclic nucleotides activate the channel by binding tightly to the open state and stabilizing it.

Functional expression of nicotinic acetylcholine receptors containing rat alpha 7 subunits in human SH-SY5Y neuroblastoma cells

Neuronal nicotinic acetylcholine receptors (nAChR) are made from different combinations of subunits encoded by a diverse family of genes. However, the recently cloned alpha 7 gene codes for subunits that can form homooligomeric nAChR complexes when expressed in Xenopus oocytes. Electrophysiological studies reveal that these alpha 7-nAChR function as alpha-bungarotoxin (Bgt)-sensitive, quickly activating/inactivating ion channels with a unique pharmacological profile and an unusually high permeability to calcium ions. Although similar observations have been made in studies of Bgt-sensitive, functional nAChR subtypes that are naturally expressed in neuronal cells, all attempts until now to reconstitute functional alpha 7-nAChR in cell lines have failed. Here we report the successful use of SH-SY5Y human neuroblastoma cells, which naturally express low levels of endogenous alpha 7 transcripts, to stably overexpress heterologous rat nAChR alpha 7 transgenes. These transgenes are expressed as the appropriately-sized alpha 7 messages and protein, and stably transfected SH-SY5Y cells have over 30-times higher levels of specific Bgt binding sites than do wild-type cells. Whole cell current recordings confirm that transfected cells express functional nAChR that are sensitive to blockade by Bgt and display the typical physiological and pharmacological profiles of alpha 7-nAChR. We conclude that stable, functional expression of alpha 7 transgenes in a mammalian cell line has been achieved for the first time.

The histrionicotoxin-sensitive ethidium binding site is located outside of the transmembrane domain of the nicotinic acetylcholine receptor: a fluorescence study

A novel, relatively photostable, long-wavelength fluorescent membrane probe, N-(Texas Red sulfonyl)-5(and 6)-dodecanoylamine (C12-Texas Red), was synthesized and used as an electronic energy acceptor for Förster fluorescence resonance energy transfer (FRET) between ethidium bound to a histrionicotoxin-sensitive binding site on the Torpedo nicotinic acetylcholine receptor (AChR) and the lipid membrane surface. FRET from membrane-partitioned 5-(N-dodecanoylamino)fluorescein (C12-fluorescein) to the membrane-partitioned C12-Texas Red was also determined with a parallel set of cuvettes to (1) compare FRET results with a donor in a known position in the membrane and (2) assess the surface density of the membrane-partitioned C12-Texas Red. Stern-Volmer analysis of the FRET results showed that C12-Texas Red quenched membrane-partitioned C12-fluorescein fluorescence 2.9 times more effectively than it quenched the receptor-bound ethidium fluorescence even though the Förster critical distances for the two donor-acceptor pairs were very similar (49.9 and 54.3 A, respectively). Analysis of the ethidium to C12-Texas Red FRET as a function of acceptor surface density with the assumptions that the donor is attached along the major axis of symmetry of a cylindrical protein embedded perpendicularly into the membrane (On-Axis FRET model) suggested that the distance of closest approach between the receptor-bound ethidium and the membrane surface was approximately 52 A. Because the minimum distance between the surface of the lipid-membrane domain and the major symmetry axis of the AChR is approximately 28 A, the FRET results strongly suggest that the ethidium binding site is not located near the entrance of the luminal transmembrane domain is generally assumed.(ABSTRACT TRUNCATED AT 250 WORDS)

Ligand-receptor interactions in the nicotinic acetylcholine receptor probed using multiple substitutions at conserved tyrosines on the alpha subunit

Affinity labeling studies have identified several conserved tyrosine residues in the alpha subunit of the nicotinic acetylcholine receptor (alpha Y93, alpha Y190, and alpha Y198) as being in or near the ligand binding site. Mutagenesis studies from several laboratories have shown that substitution of phenylalanine for tyrosine at these positions reduces the apparent affinity for ACh. We have examined this apparent reduction in affinity further through the use of multiple substitutions at each position. Substitution of either phenylalanine, tryptophan, or serine resulted in an apparent decrease in agonist affinity, but the degree of reduction depended on both the position and the nature of the substitution. Analysis of the effects of each substitution suggests that each residue interacts with the quaternary N of ACh, and that each residue may make a different type of interaction with the agonist.

Electrostatic potential of the acetylcholine binding sites in the nicotinic receptor probed by reactions of binding-site cysteines with charged methanethiosulfonates

All of the potent agonists and competitive antagonists of the acetylcholine receptors are positively charged, onium compounds. Among the interactions involved in the binding of these compounds, electrostatic forces undoubtedly make an important contribution. There is evidence that the acetylcholine binding site contains both acidic and aromatic amino acids. The acidic side chains could provide long-range charge-charge interactions with acetylcholine, while the aromatic side chains could provide short-range cation-pi-electron and hydrophobic interactions. To probe the long-range electrostatic interactions in the binding site, the rate constants for the reactions of sulfhydryl-specific reagents with cysteines in the binding site have been determined as a function of ionic strength. The reagents are the positively charged methanethiosulfonate ethylammonium and methanethiosulfonate ethyltrimethylammonium, the negatively charged methanethiosulfonate ethylsulfonate, and the neutral methyl methanethiosulfonate. In addition, the rate constants of the reactions of these methanethiosulfonates with positively charged, negatively charged, and uncharged simple thiol compounds have been similarly determined. An analysis of these rate constants in terms of absolute rate theory and Debye-Hückel theory is consistent with the acetylcholine binding site containing two to three negative charges and an electrostatic potential at zero ionic strength of about -80 mV relative to bulk solution.

Kainate-binding proteins: phylogeny, structures and possible functions

Recent advances have demonstrated that the family of [3H]kainate-binding proteins and kainate receptors comprise a number of related polypeptides. In all the cases so far investigated, the kainate-binding proteins from non-mammalian vertebrates have M(r) values in the range of 40-50 kDa whereas mammalian kainate receptors and kainate-binding proteins have M(r) values in the order of 100 kDa. There have not, as yet, been any reports of 40-50 kDa kainate-binding proteins in mammalian CNS and, despite the cloning of increasing numbers of cDNAs encoding new kainate-binding proteins, the relationships between these two general groups of polypeptides remain unclear. Nonetheless, there is now a wealth of phylogenetic, structural and molecular biological data available about these proteins. In this review, Jeremy Henley outlines the properties and structures of kainate-binding proteins and offers some possibilities as to the roles of these often hugely abundant proteins.

Chimaeric nicotinic-serotonergic receptor combines distinct ligand binding and channel specificities

The neuronal nicotinic alpha 7 (nAChR) and 5-hydroxytryptamine (5HT3) receptors are ligand-gated ion channels with a homologous topological organization and have activation and desensitization reactions in common. Yet these homo-oligomeric receptors differ in the pharmacology of their binding sites for agonists and competitive antagonists, and in their sensitivity to Ca2+ ions. The alpha 7 channel is highly permeable to Ca2+ ions and external Ca2+ ions potentiate, in an allosteric manner, the permeability response to acetylcholine, as shown for other neuronal nAChRs. The 5HT3 channel, in contrast, is not permeable to Ca2+ ions, but blocked by them. To assign these properties to delimited domains of the primary structure, we constructed several recombinant chimaeric alpha 7-5HT3 receptors. We report here that one of the constructs expresses a functional receptor that contains the serotonergic channel still blocked by Ca2+ ions, but is activated by nicotinic ligands and potentiated by external Ca2+ ions.