NAChR binding domains

The nAChRs contain multiple binding domains that can accommodate different classes of endogenous and exogenous ligands. The nAChR ligand binding domain consists of seven loops (A-G) spaced on the protein chains of the a and... 

The refined 4 A resolution electron microscopy structure of the hetero-pentameric musde-type, (al)2jSyd uAChR has elegantly illustrated considerable structural similarity of L-AChBP with the nAChR ligand-binding domain. Therefore, L-AChBP is now considered a structural and functional surrogate of the nAChRs. 

Figure 2.1 Diagram of nicotinic acetylcholine receptor (nAChR) structure. A top view of (A) an a7 nAChR and (B) a p2 nAChR shows that homomeric and heteromeric classes of nAChRs are both pentameric in structure. Each subunit is made up of four transmembrane domains with the M2 domain making up the ion pore. (C) A side view of the four transmembrane regions shows the N terminus, C terminus, and large M3-M4 intracellular loop that make up each nAChR subunit. The extracellular loops are available for binding to ligands and the intracellular loop is available for regulation of the nAChR by intracellular signaling proteins.

Fig. 4 Current model for nicotine upregulation of a4p2 nAChRs. a Schematic of a ceU indicating major steps in the lifecycle of a nAChR. Nicotine accumulates within the cell. Within the endoplasmic reticulum, nicotine binds to nAChR subunits to facilitate assembly, or binds at the interface of an aP subunit pair to enhance maturation of a pentameric nAChR (Sallette et al. 2004, 2005). The strong influence of nicotine on maturation of the P2 subunit might also favour a change in nAChR stoichiometry, from (a4)3(P2)2 to (a4)2(P2)3 (Moroni et al. 2006). These actions could result in an increase in the membrane insertion of competent nAChRs. The possibflity of an additional action of nicotine to impede nAChR turnover or degradation is indicated by the dotted line, b Binding of nicotine to the extracellular domain of unassembled nAChR subunits facilitates assembly, c Binding of nicotine at an aP interface facilitates maturation of a pentameric nAChR. Items b and c adapted from Nashmi and Lester (2007), with permission from Elsevier...

The adult nicotinic acetylcholine receptor (nAChR) is an intrinsic membrane protein with five distinct subunits (a2B T). A Cartoon of the one of five subunits of the AChR in the end plate surface of adult mammalian muscle. Each subunit contains four helical domains labeled Ml to M4. The 2 domains line the channel pore. B Cartoon of the full nAChR. The N termini of two subunits cooperate to form two distinct binding pockets for acetylcholine (ACh). These pockets occur at the (X-B and the 3-Ct subunit interfaces. 

Structurally, nAChRs are comprised of homologous or heterologous combinations of five polypeptide subunits. Each subunit is composed of four transmembrane (TM 1 ) domains arranged around a central, water-filled pore. The extracellular N-terminal domain of each of the five subunits makes up the agonist binding site [35]. The pentameric nAChR, formed by various combinations of a and p subunits, is widely distributed in mammalian brain [36, 37]. Eight forms of the a subunit (ot2-otlO) and three forms of the p subunit (p2-p4) have been identified. The mechanism for the neuroprotective/trophic effects of nicotine is not known but seems to be multifaceted, as the reader will appreciate throughout this chapter. 

Three cholesterol molecules could be docked on each AQiR subunit, rendering a total of 15 cholesterol molecules per nAChR molecule. In full agreement with the results of photoaffinity labeling studies, Baier et al. found specific cholesterol-binding sites in transmembrane domains TMl, TM3, and TM4. Interestingly, once boimd to TMl, cholesterol could not interact with TM2 due to steric hindrance. Molecular models of nAchR in complex with 15 cholesterol molecules are shown in Fig. 7.2. 

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