Disclosure of cholesterol recognition motifs in transmembrane domains of the human nicotinic acetylcholine receptor

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Abstract
Cholesterol influences ion-channel function, distribution and clustering in the membrane, endocytosis, and exocytic sorting of the nicotinic acetylcholine receptor (AChR). We report the occurrence of a cholesterol recognition motif, here coined “CARC”, in the transmembrane regions of AChR subunits that bear extensive contact with the surrounding lipid, and are thus optimally suited to convey cholesterol-mediated signaling from the latter. Three cholesterol molecules could be docked on the transmembrane segments of each AChR subunit, rendering a total of 15 cholesterol molecules per AChR molecule. The CARC motifs contribute each with an energy of interaction between 35 and 52 kJ.mol−1, adding up to a total of about 200 kJ.mol−1 per receptor molecule, i.e. ∼40% of the lipid solvation free energy/ AChR molecule. The CARC motif is remarkably conserved along the phylogenetic scale, from prokaryotes to human, suggesting that it could be responsible for some of the above structural/functional properties of the AChR.