In pursuit of the high-resolution structure of nicotinic acetylcholine receptors

Abstract

The nicotinic acetylcholine receptor (nAChR) has been studied extensively for well over four decades because of its important physiological roles and medical relevance. A large body of data from biochemical and biophysical studies are now available. The structural information, which is needed to integrate existing data to address the mechanism and function of nAChRs, started to emerge in recent years. Structural studies of acetylcholine binding proteins (AChBPs) have greatly facilitated the study of nAChRs. The recently determined crystal structures of the prokaryotic homologues of nAChRs will probably have similar impact over time. However, a direct structural model of nAChRs at high resolution will be important for mechanistic studies and drug development. Here we will review some of the recent efforts in this area and use the high-resolution structure of the extracellular domains of nAChR alpha1 to illustrate the potential insights one may gain at higher resolution.

Lin Chen (University of Southern California, USA) obtained his PhD degree in Chemistry and Biochemistry in the Department of Chemistry at Harvard University in 1994. He did his postdoctoral training in structural biology in the Department of Molecular and Cellular Biology at Harvard University. His research interests include: (i) mechanisms of eukaryotic gene regulation, including the molecular basis of signal transduction, transcription regulation and epigenetic control of chromosome structure; (ii) structure and function of nicotinic acetylcholine receptors (nAChRs) and other ligand-gated ion channels (LGICs) involved in neuronal signalling.

Figure 1. Mutations that stabilize nAChR α1 ECD

A, the three mutations (boxed and indicated by arrow) are mapped on the surface of nAChR α1 ECD (dark green) and away from the binding site of α-bungarotoxin (orange) and the glycan (magenta). B, the mutation Val8Glu establishes a salt bridge with Lys84. The surrounding structure is well ordered, showing well-defined electron density. C, the mutation Trp149Arg establishes a salt bridge with Asp89. The side chains of both residues show well-defined electron density. D, the mutation Val151Ala removes an exposed hydrophobic residue. The surrounding structure is well ordered.

Figure 2. Receptor-specific structural features and their functional implications

The high-resolution structure of nAChR α1 ECD bound to α-bungarotoxin was put into the electron microscopic model of nAChR. The details of MIR, the Cys-loop and the toxin-binding interactions with receptor and glycan are shown in the indicated boxes.

Figure 3. Potential roles of glycan in nAChR channel gating

A, the crystal structure reveals that the glycan chain provides a physical linker between loop C and the Cys-loop. B, a hypothetical model of the roles of glycan in nAChR channel gating.

Figure 5. A summary model of high-resolution studies of the nAChR α1 ECD and the mechanistic implications in the whole receptor

One subunit of the nAChR pentamer is highlighted in red (depicted in ribbon). Other subunits are coloured in grey (two are visible in this view). Loop C, the carbohydrate chain (in space-filling model), the Cys-loop, and the hydration pocket (the two buried water molecules are shown as blue spheres) function as a group of physically linked mobile elements (boxed).

Figure 4. Specific packing defect that is conserved in nAChR but absent in AChBP

The structure of the nAChR α1 ECD is represented by a ribbon and surface model. Shown is a cross-section of the structure viewed from the stop of the β sandwich fold. Thr52, Ser126, the two buried water molecules and nearby cavities are evident from this view.