Low chemical specificity of the nicotinic acetylcholine receptor sterol activation site

AbstractThe nicotinic acetylcholine receptor (nAcChoR) has an absolute requirement for cholesterol if agonist-stimulated channel opening is to occur [Biochemistry 25 (1986) 830]. Certain non-polar analogs could replace cholesterol in vectorial vesicle permeability assays. Using a stopped-flow fluorescence assay to avoid the limitations of permeability assays imposed by vesicle morphology, it was shown that polar conjugates of cholesterol could also satisfy the sterol requirement [Biochim. Biophys. Acta 1370 (1998) 299]. Here this assay is used to explore the chemical specificity of sterols. Affinity-purified nAcChoRs from Torpedo were reconstituted into bilayers at mole ratios of 58:12:30 [1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-sn-glycero-3-phosphate (DOPA)/steroid]. When the enantiomer of cholesterol was used, or when the stereochemistry at the 3-hydroxy group was changed from β to α by substituting epicholesterol for cholesterol, activation was still supported. The importance of cholesterol's planar ring structure was tested by comparing planar cholestanol (5α-cholestan-3β-ol) with nonplanar coprostanol (5β-cholestan-3β-ol). Both supported activation. Thus, these steroids support activation independent of structural features known to be important for modulation of lipid bilayer properties. This provides indirect support for a steroid binding site possessing very lax structural requirements