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{{STRUCTURE_2bg9| PDB=2bg9 | }} '''Refined structure of the nicotinic acetylcholine receptor at 4A resolution..''' ==Background== We present a refined model of the membrane-associated Torpedo acetylcholine (ACh) receptor at 4A resolution. An improved experimental density map was obtained from 342 electron images of helical tubes, and the refined structure was derived to an R-factor of 36.7% (R(free) 37.9%) by standard crystallographic methods, after placing the densities corresponding to a single molecule into an artificial unit cell. The agreement between experimental and calculated phases along the helical layer-lines was used to monitor progress in the refinement and to give an independent measure of the accuracy. The atomic model allowed a detailed description of the whole receptor in the closed-channel form, including the ligand-binding and intracellular domains, which have not previously been interpreted at a chemical level. We confirm that the two ligand-binding alpha subunits have a different extended conformation from the three other subunits in the closed channel, and identify several interactions on both pairs of subunit interfaces, and within the alpha subunits, which may be responsible for their "distorted" structures. The ACh-coordinating amino acid side-chains of the alpha subunits are far apart in the closed channel, indicating that a localised rearrangement, involving closure of loops B and C around the bound ACh molecule, occurs upon activation. A comparison of the structure of the alpha subunit with that of AChBP having ligand present, suggests how the localised rearrangement overcomes the distortions and initiates the rotational movements associated with opening of the channel. Both vestibules of the channel are strongly electronegative, providing a cation-stabilising environment at either entrance of the membrane pore. Access to the pore on the intracellular side is further influenced by narrow lateral windows, which would be expected to screen out electrostatically ions of the wrong charge and size. {{Clear}} <applet load='2bg9.pdb' size='500' frame='true' align='right'/> ==Results== The X-ray structure of the E2020-''Tc''AChE complex shows that E2020 has a <scene name='Sandbox_uc_17/123123/1'>TextToBeDisplayed</scene> along the active-site gorge, extending from the anionic subsite (<scene name='1eve/E2020_close_up_with_84lbld/7'>W84</scene>) of the active site, at the bottom, to the peripheral anionic site (<scene name='1eve/E2020_close_up_with_84_279lbld/5'>near W279</scene>), at the top, via aromatic stacking interactions with conserved aromatic acid residues. E2020 does not, however, interact directly with either the catalytic triad or the 'oxyanion hole' but only <scene name='1eve/E20_interactionshown/8'>indirectly via solvent molecules</scene>. ==Conclusions== This analysis extends earlier electron microscopic analyses of the ACh receptor in Torpedo postsynaptic membranes, imaged either in the absence of ACh, or following brief exposure to ACh to trap the open-channel form. The results together suggest that the channel has the following properties that are fundamental to the way it works: The main ligand-binding a subunits, in the closed channel, are in a “distorted” state, which is stabilised by inter and intra-subunit interactions. In the conformational change to open the channel, the main ligand-binding subunits are the principal mediators, leading to a concerted rearrangement in the membrane involving all the helices lining the pore. The bound ACh opens the channel by causing a localised rearrangement that stabilises. Distribution of charged residues on MA helices shaping the intracellular vestibules of transmitter-gated ion channels: (a) as found in this study for a cation-selective channel; (b) homology model of MA helices of a related anionselective channel (human glycine receptor a1 subunits; amino acid residues 369–394). The panels show C a traces, with bars denoting the C a –C b bonds of residues having negative (red) or positive charge (blue); the view is from the synaptic cleft. In (a) the inside of the vestibule and the windows for the ions are lined predominantly by negatively charged groups, whereas in (b) the opposite distribution applies. Also shown on one of the helices in (b) are the locations in the (cation-selective) 5-HT3A receptor of three arginine residues (green bars); the single-channel conductance increases dramatically when these residues are mutated to neutral or negatively charged residues. Refined Structure of Nicotinic Acetylcholine Receptoralternative “relaxed” conformation of the ligandbinding subunit. The transition to the open state involves rotational movements in the ligand-binding subunits, which unlock their interactions with the porelining helices keeping the channel closed. The gating movements are quite small, being restricted energetically by the need to preserve the conserved hydrophobic cores of the subunits. The ionic surfaces of the vestibules play an important role in facilitating the selective transport of cations through the channel. The high level of amino acid sequence conservation, and the functional specificities able to be achieved with chimeric channels, imply that all channels of the Cys-loop superfamily are constructed around the same three-dimensional framework and function according to the same global principles. These principles applying to the ACh receptor are therefore likely to apply, with minor variations, to other members of the superfamily. ==About this Structure== 1EVE is a 1 chain structure with sequence from [http://en.wikipedia.org/wiki/Torpedo_californica Torpedo californica]. The June 2004 RCSB PDB [http://pdb.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/index.html Molecule of the Month] feature on ''Acetylcholinesterase'' by David S. Goodsell is [http://dx.doi.org/10.2210/rcsb_pdb/mom_2004_6 10.2210/rcsb_pdb/mom_2004_6]. Full crystallographic information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=1EVE OCA]. ==Additional Resources== For additional information, see: [[Alzheimer's Disease]] <br /> ==Reference== Structure of acetylcholinesterase complexed with E2020 (Aricept): implications for the design of new anti-Alzheimer drugs., Kryger G, Silman I, Sussman JL, Structure. 1999 Mar 15;7(3):297-307. PMID:[http://ispc.weizmann.ac.il//pmbin/getpm?pmid=10368299 10368299]

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