Aromaticity at the water-hydrocarbon core interface of the membrane: consequences on the nicotinic acetylcholine receptor.

Almost all lipid-exposed transmembrane domains of integral proteins contain aromatic residues flanking the hydrophobic segment of the domains. These residues generally reside close to the carbonyl region of the membrane, and several structural and functional roles have been associated to these residues. Although the roles and physicochemical reasons for aromatic preference have been extensively studied using model systems, few studies have been done in a native membrane system. To gain insight about the mechanistic implication for this aromatic preference, we selected position alpha F426 of the muscle-type nicotinic acetylcholine receptor (nAChR). alpha F426 is a lipid-exposed residue at the extracellular segment of the alpha M4 transmembrane domain and is highly conserved among different nAChR subunits and species. We used site-directed mutagenesis, alpha-Bungarotoxin-binding assay, and two-electrodes voltage clamp in Xenopus laevis oocytes to characterize mutations at position alpha F426, which impart different physicochemical properties like volume, polarity, hydrogen bonds, aromaticity and net electrical charge. All mutations except the aromatic residues resulted in a significant reduction of the nAChR cell-surface levels and the macroscopic currents to acetylcholine. These results suggest that position alpha F426 contributes to structural stability and open-close transitions of the nAChR. Finally, the present study also provides information about how intermolecular interactions at position alpha 426 modulate open-close transitions of the nAChR.