Lipid-protein interactions and nicotinic acetylcholine receptor function

The effect of bilayer composition, specifically the presence of cholesterol, upon the function of the reconstituted nicotinic acetylcholine receptor (nAcChoR) was investigated using stopped-flow fluorescence. The nAcChoR was purified and reconstituted from the electroplaques of Torpedo nobiliana, using affinity column chromatography, into bilayers of defined composition and the function of each sample assessed and compared with those of the native receptor. Investigation of the effect of bilayer composition upon the kinetics of agonist binding to the nAcChoR, using the fluorescent acetylcholine analogue, Dns-C6-Cho, established that the receptor pre-existed in equilibrium between the resting and two desensitised states. However, Dns-C6-Cho inhibited channel gating at high concentrations and another fluorescent probe was sought. The kinetics of carbachol induced nAcChoR conformational changes, reported by ethidium bromide (a non-competitive inhibitor) fluorescence, in native membranes were characterised and an assay developed to investigate whether cholesterol mediated rapid conformational changes in reconstituted samples. It was found that ethidium bromide reported on the carbachol-induced development of the fast desensitised state from the open channel state, and that this conformational change was sensitive to changes in bilayer composition. The onset of fast desensitisation from the open channel state was not observed when the receptor was reconstituted into DOPC or DOPC-DOPA bilayers. However, increasing the cholesterol content in these bilayers increased the amplitude of the component reporting this conformational change, while the observed rate at which it occurred was independent of bilayer cholesterol content. This result agrees with the suggestion that cholesterol facilitates channel opening and the onset of fast desensitisation by binding to specific sites on the nAcChoR and that these must be occupied for a functionally viable receptor (Jones and McNamee, 1988).