Models of the extracellular domain of the nicotinic receptors and of agonist- and Ca ²⁺ -binding sites

Abstract

We constructed a three-dimensional model of the amino-terminal extracellular domain of three major types of nicotinic acetylcholine receptor, (␣7)5, (␣4)2(␤2)3, and (␣1)2␤1␥␦, on the basis of the recent x-ray structure determination of the molluscan acetylcholine-binding protein. Comparative analysis of the three models reveals that the agonist-binding pocket is much more conserved than the overall structure. Differences exist, however, in the side chains of several residues. In particular, a phenylalanine residue, present in ␤2 but not in ␣7, is proposed to contribute to the high affinity for agonists in receptors containing the ␤2 subunit. The semiautomatic docking of agonists in the ligand-binding pocket of (␣7)5 led to positions consistent with labeling and mutagenesis experiments. Accordingly, the quaternary ammonium head group of nicotine makes a -cation interaction with W148 (␣7 numbering), whereas the pyridine ring is close to both the cysteine pair 189 -190 and the complementary component of the binding site. The intrinsic affinities inferred from docking give a rank order epibatidine > nicotine > acetylcholine, in agreement with experimental values. Finally, our models offer a structural basis for potentiation by external Ca 2؉ .

Insight in nAChR subtype selectivity from AChBP crystal structures

“…Hence, CLR modeling strategies have heavily relied on the X-ray structures of AChBPs and their complexes together with the structure of the Torpedo nAChR [24][25][26][27]. The availability of the recently solved X-ray structure of the monomeric mouse α 1 nAChR subunit in complex with α-Bungarotoxin [28] indeed illustrates that AChBP constitutes a very good model for studying nAChRs, especially when considering the high degree of structural similarity between the α 1 and AChBP structures; mouse α 1 nAChR subunit superposes to carbamylcholine-bound LsAChBP with an r.m.s.…”

An Extracellular Protein Microdomain Controls Up-regulation of Neuronal Nicotinic Acetylcholine Receptors by Nicotine

“…Visualization of the ␤2 residues conferring up-regulation to ␤4 on an atomic model of the ␣3␤2 receptor extracellular domain derived from AChBP (30,34) (Fig. 5) revealed that these amino acids form a compact microdomain.…”

“…These residues form a compact microdomain located at the interface between ␣ (light gray) and ␤2 (dark blue) and contact the upper part of the nicotinic site. Epibatidine is shown in pink, according to previous docking calculations (30).…”

“…Cell disruption was performed through sonication (3 times for 10 s) as previously described (17), and binding was performed in the usual conditions. Molecular Modeling-Molecular models were constructed with Modeler (28) and Autodock (29) as described elsewhere (30). Handling of models and generation of pictures were performed with Deep-View (31), Rasmol (32), and ViewerLite (Accelrys).…”

Nicotinic Acetylcholine Receptors

“…The sequence determinants for this effect have been investigated for chick recombinant α7/5HT3 chimaeric receptors by Galzi et al (1996) who have identified residues α7 161-172 as particularly important: in the AChBP these residues are on the minus face, at the end of loop 9, which is near the extracellular end of the pore. Le Novère et al (2002) proposed, on the basis of their modelling of the α7 subunit on the AchBP, that the binding site for calcium is formed at the subunit interface by residues belonging to different subunits. These residues include some identified by Galzi et al (1996), such as E44 and E172, but also D43 and D41.…”