Ligand-gated ion channels (LGICs) mediate excitatory and inhibitory transmission in the nervous system. Among them, the pentameric or 'Cys-loop' receptors (pLGICs) compose a family that until recently was found in only eukaryotes. Yet a recent genome search identified putative homologues of these proteins in several bacterial species. Here we report the cloning, expression and functional identification of one of these putative homologues from the cyanobacterium Gloeobacter violaceus. It was expressed as a homo-oligomer in HEK 293 cells and Xenopus oocytes, generating a transmembrane cationic channel that is opened by extracellular protons and shows slow kinetics of activation, no desensitization and a single channel conductance of 8 pS. Electron microscopy and cross-linking experiments of the protein fused to the maltose-binding protein and expressed in Escherichia coli are consistent with a homo-pentameric organization. Sequence comparison shows that it possesses a compact structure, with the absence of the amino-terminal helix, the canonical disulphide bridge and the large cytoplasmic domain found in eukaryotic pLGICs. Therefore it embodies a minimal structure required for signal transduction. These data establish the prokaryotic origin of the family. Because Gloeobacter violaceus carries out photosynthesis and proton transport at the cytoplasmic membrane, this new proton-gated ion channel might contribute to adaptation to pH change.

Chronic exposure to nicotine elicits upregulation of high-affinity nicotinic receptors in the smoker's brain. To address the molecular mechanism of upregulation, we transfected HEK293 cells with human alpha4beta2 receptors and traced the subunits throughout their intracellular biosynthesis, using metabolic labeling and immunoprecipitation techniques. We show that high-mannose glycosylated subunits mature and assemble into pentamers in the endoplasmic reticulum and that only pentameric receptors reach the cell surface following carbohydrate processing. Nicotine is shown to act inside the cell and to increase the amount of beta subunits immunoprecipitated by the conformation-dependent mAb290, indicating that nicotine enhances a critical step in the intracellular maturation of these receptors. This effect, which also takes place at concentrations of nicotine found in the blood of smokers upon expression of alpha4beta2 in SH-SY5Y neuroblastoma cells, may play a crucial role in nicotine addiction and possibly implement a model of neural plasticity.

Benzodiazepines are widely used anxiolytics and anticonvulsants, and their potent sedative properties are routinely used in presurgical anaesthesia. However, they are also known to induce a strong anterograde amnesia in patients. Specific benzodiazepine antagonists have recently been described, some of which have intrinsic pharmacological properties that are opposite to those of benzodiazepines. These have been called inverse agonists and they have been shown to be proconvulsant or convulsant whereas benzodiazepines are anticonvulsants. Inverse agonists are also anxiogenic rather than anxiolytic. Since benzodiazepines induce anterograde amnesia, we have investigated the possibility that inverse agonists might also have an opposite effect for this property and so enhance acquisition (learning) and (or) retention (memory). We report here that, in three different animal models, an inverse agonist of the beta-carboline group, methyl beta-carboline-3-carboxylate (beta-CCM), enhances animal performance in three different tasks used to investigate learning and memory.

Nicotinic acetylcholine receptors (nAChR) are pentameric ligandgated ion channels composed of subunits that consist of an extracellular domain that carries the ligand-binding site and a distinct ion-pore domain. Signal transduction results from the allosteric coupling between the two domains: the distance from the binding site to the gate of the pore domain is 50 Å. Normal mode analysis with a C␣ Gaussian network of a new structural model of the neuronal ␣7 nAChR showed that the lowest mode involves a global quaternary twist motion that opens the ion pore. A molecular probe analysis, in which the network is modified at each individual amino acid residue, demonstrated that the major effect is to change the frequency, but not the form, of the twist mode. The largest effects were observed for the ligand-binding site and the Cys-loop. Most (24͞27) of spontaneous mutations known to cause congenital myasthenia and autosomal dominant nocturnal frontal lobe epilepsy are located either at the interface between subunits or, within a given subunit, at the interface between rigid blocks. These interfaces are modified significantly by the twist mode. The present analysis, thus, supports the quaternary twist model of the nAChR allosteric transition and provides a qualitative interpretation of the effect of the mutations responsible for several receptor pathologies.allosteric transition ͉ nicotinic receptor ͉ pathological mutations ͉ normal mode perturbation scanning N icotinic acetylcholine receptors (nAChRs) play a central role in intercellular communications in the brain and at the neuromuscular junction. They are involved in nicotine addiction as well as in cognitive processes such as attention, access to consciousness, learning, and memory, and their pathologies include autism, schizophrenia, Parkinson's disease, and Alzheimer's disease (references in ref. 1). Understanding the functional organization of the nAChR at the atomic level thus is of considerable interest in itself and is a source of insights for the development of new drug therapies.nAChRs are members of the Cys-loop superfamily of ligandgated ion channels. They are hetero-or homopentameric integral membrane proteins with a fivefold axis of pseudosymmetry perpendicular to the membrane. Each subunit can be subdivided into two principal domains: extracellular and transmembrane. The extracellular domain (ECD) carries the acetylcholine (ACh) binding site at the boundary between subunits, and the transmembrane ion-pore domain (IPD) delineates an axial cation-specific channel (2, 3). These topologically distinct domains are coupled allosterically to each other. Therefore, nAChRs possess the structural elements necessary to convert a chemical signal, typically a local increase of extracellular ACh concentration, into an electrical signal generated by the opening of the ion channel.Electrophysiological analysis of nAChRs has shown that rapid delivery of ACh promotes fast opening of the channel, and that a prolonged application of ACh leads to a slow decrease of the re...

Neurotransmitters such as acetylcholine (ACh) and glycine mediate fast synaptic neurotransmission by activating pentameric ligandgated ion channels (LGICs). These receptors are allosteric transmembrane proteins that rapidly convert chemical messages into electrical signals. Neurotransmitters activate LGICs by interacting with an extracellular agonist-binding domain (ECD), triggering a tertiary͞quaternary conformational change in the protein that results in the fast opening of an ion pore domain (IPD). However, the molecular mechanism that determines the fast opening of LGICs remains elusive. Here, we show by combining whole-cell and single-channel recordings of recombinant chimeras between the ECD of ␣7 nicotinic receptor (nAChR) and the IPD of the glycine receptor (GlyR) that only two GlyR amino acid residues of loop 7 (Cys-loop) from the ECD and at most five ␣7 nAChR amino acid residues of the M2-M3 loop (2-3L) from the IPD control the fast activation rates of the ␣7͞Gly chimera and WT GlyR. Mutual interactions of these residues at a critical pivot point between the agonist-binding site and the ion channel fine-tune the intrinsic opening and closing rates of the receptor through stabilization of the transition state of activation. These data provide a structural basis for the fast opening of pentameric LGICs.allosteric proteins ͉ chimeric receptor ͉ Cys-loop receptor ͉ transition state P entameric ligand-gated ion channels (LGICs), such as the cationic nicotinic acetylcholine receptor (nAChR) and the anionic glycine receptor (GlyR), mediate fast excitatory or inhibitory chemical neurotransmission between neurons (1-6). A unique feature of these receptors is that they activate the ion channel, a process known as gating, in less than a ms. For nicotinic receptors, a detailed single-channel analysis has recently established a speed limit for the opening of the ion channel in the s time range (7). Perturbations of this rapid transmission pathway by natural mutants lead to severe diseases such as congenital myasthenic syndromes (8), hereditary hyperekplexia (9), or epileptic disorders (10).Pentameric LGICs, or Cys-loop receptors, are composed of five homologous subunits, sharing a common structural organization, arranged (pseudo)symmetrically around the central ion pore (1, 2). All subunits are made of two distinct topological domains: the extracellular (ECD) and the ion pore domains (IPD). First, the ECD is folded into a twisted ␤-sandwich core, as revealed by x-ray crystallographic studies of the mollusk acetylcholine-binding protein (AChBP), a soluble pentameric protein homologous to the extracellular domain of LGICs (11-14). Second, electron microscopy images of Torpedo nAChR at 4-Å resolution revealed that the four transmembrane segments (M1 to M4) of the IPD are folded into ␣-helices joined by linking loops of variable lengths (15). By combining these structural data, we built a 3D model of the full ␣7 nAChR (16). In this model, the coupling zone located at the interface between the two domains is framed by f...

ATP-gated P2X receptors are trimeric ion channels, as recently confirmed by X-ray crystallography. However, the structure was solved without ATP and even though extracellular intersubunit cavities surrounded by conserved amino acid residues previously shown to be important for ATP function were proposed to house ATP, the localization of the ATP sites remains elusive. Here we localize the ATP-binding sites by creating, through a proximity-dependent "tethering" reaction, covalent bonds between a synthesized ATPderived thiol-reactive P2X2 agonist (NCS-ATP) and single cysteine mutants engineered in the putative binding cavities of the P2X2 receptor. By combining whole-cell and single-channel recordings, we report that NCS-ATP covalently and specifically labels two previously unidentified positions N140 and L186 from two adjacent subunits separated by about 18 Å in a P2X2 closed state homology model, suggesting the existence of at least two binding modes. Tethering reaction at both positions primes subsequent agonist binding, yet with distinct functional consequences. Labeling of one position impedes subsequent ATP function, which results in inefficient gating, whereas tethering of the other position, although failing to produce gating by itself, enhances subsequent ATP function. Our results thus define a large and dynamic intersubunit ATPbinding pocket and suggest that receptors trapped in covalently agonist-bound states differ in their ability to gate the ion channel.affinity labeling | chemical modification | purinergic receptor P 2X receptors are oligomeric ATP-gated ion channels selective to cations (1) and are involved in physiological processes as diverse as synaptic transmission, the response to inflammation, and pain perception (2). Upon ATP binding, structural rearrangements of the subunit interface (3-5) lead to the opening of the ion channel (6-8), but the entire molecular sequence of events that couple ATP binding to channel opening remains unknown. The recent X-ray structure of the P2X4 receptor in a closed resting state represents in this regard a decisive step (9). It confirms the trimeric stoichiometry of the ion channel, in agreement with the fact that there are three activatable ATP-binding sites (10), and provides a structural context to interpret functional data (11).Early studies based on mutagenesis data have identified highly conserved extracellular residues important for ATP function and have proposed that ATP binding occurs through the extracellular domain (12-19), presumably at the subunit interface (15,17). When mapped on the crystal structure, most of these residues are observed to line a large and deep intersubunit cavity shaped like an open "jaw" and located approximately 45 Å away from the ion channel domain (9). This observation thus suggests that these residues participate in ATP binding; however, the crystal structure was solved in the absence of ATP, and therefore no direct evidence support this hypothesis to date.We used the proximity-dependent "tethering" approach (20) to localiz...

ion channel ͉ membrane protein ͉ structure ͉ acetylcholine

Ionotropic glycine receptors (GlyRs) are present in the central nervous system well before the establishment of synaptic contacts. Immature nerve cells are known, at least in the spinal cord, to express α2 homomeric GlyRs, the properties of which are relatively unknown compared to those of the adult synaptic form of the GlyR (mainly α1/β heteromeres). Here, the kinetics properties of GlyRs at the single‐channel level have been recorded in real‐time by means of the patch‐clamp technique in the outside‐out configuration coupled with an ultra‐fast flow application system (< 100 µs). Recordings were performed on chinese hamster ovary (CHO) cells stably transfected with the α2 GlyR subunit. We show that the onset, the relaxation and the desensitisation of α2 homomeric GlyR‐mediated currents are slower by one or two orders of magnitude compared to synaptic mature GlyRs and to other ligand‐gated ionotropic channels involved in fast synaptic transmission. First latency analysis performed on single GlyR channels revealed that their slow activation time course was due to delayed openings. When synaptic release of glycine was mimicked (1 mm glycine; 1 ms pulse duration), the opening probability of α2 homomeric GlyRs was low (Po≈ 0.1) when compared to mature synaptic GlyRs (Po= 0.9). This low Po is likely to be a direct consequence of the relatively slow activation kinetics of α2 homomeric GlyRs when compared to the activation kinetics of mature α1/β GlyRs. Such slow kinetics suggest that embryonic α2 homomeric GlyRs cannot be activated by fast neurotransmitter release at mature synapses but rather could be suited for a non‐synaptic paracrine‐like release of agonist, which is known to occur in the embryo.