Nicotinic acetylcholine receptor studied by molecular dynamics simulation: Structural insights and gating mechanism

Abstract

The pentameric nicotinic acetylcholine receptor (nAChR) is essential for neurotransmission. The binding of acetylcholine to the binding site in the extracellular domain (ECD) triggers the opening of the cation channel in the transmembrane domain (TMD). Although there are diverse studies about the nAChR, the relationship between subunits and propagation from ECD to TMD, leading to the gating is still ambiguous. An experimental study identified 3 residues (Lys185, Asp187 and Ile188) of α6 subunit as the determinant of the selectivity on the α-conotoxin BuIA. However, the atomic detail of the structure-function relationship is still elucidated. In this research, we performed molecular dynamic simulations with two toxin-bound α4β2 nAChR systems: wild type α4β2 nAChR and mutant type that we replaced 3 residues of α4 subunit by 3 corresponding residues of α6 subunit (Tyr185Lys, Thr187Asp and Arg188Ile). The result showed that after mutation, α4β2 systems lost salt-bridge between Asp199-Arg188, and that the hydrogen bond pair was replaced by a new one between Lys185-Asp187. Thus, loop C of mutant α4β2 lost rigid form and became more flexible than the wild type. In the mutant simulation, we also recognized the spatial reducing between toxin and binding interface that was constructed by the interface between two adjacent subunits: principal and complementary subunits. In another work, we performed molecular dynamics simulations of the systems relating to wild type 42 nAChR and mutant type 42 nAChR in either presence or absence of acetylcholine (ach) binding. The mutant systems were prepared by replacing two residues of 4 subunit by corresponding 6 residues (I41A and D42N). In fact, the mutant 42 nAChR function was reduced dramatically through electrophysiological experiments. The results of the study have shown that there was an open conformation of the receptor which corresponds to a less tilted arrangement from ECD to TMD through the coupling region, combined with the tilting inward of transmembrane helix (TM) TM1 and outward of TM3 helices to the channel center. Subsequently, the TM2 helices tilted outward to the channel center and made the pore wider. In contrast, the opposite conformation was observed in closed channel systems. Furthermore, we observed that the mutation caused the disorder in the ECD arrangement in ach bound mutant type systems, and disturbed the arrangement of the coupling region. Hence, that channel remained in its closed conformation.