PROTEIN-LIPID INTERACTIONS AND TORPEDO-CALIFORNICA NICOTINIC ACETYLCHOLINE-RECEPTOR FUNCTION .1. SPATIAL DISPOSITION OF CYSTEINE RESIDUES IN THE GAMMA-SUBUNIT ANALYZED BY FLUORESCENCE-QUENCHING AND ENERGY-TRANSFER MEASUREMENTS

The nicotinic acetylcholine receptor from Torpedo californica was labeled with a fluorescent, lipophilic probe, N-(1-pyrenyl)maleimide, specific for sulfhydryls in a hydrophobic environment, and was found to alkylate Cys 416, Cys 420 and Cys 451 in the gamma subunit [Li, L., Schuchard, M., Palma, A., Pradier, L., & McNamee, M. G. (1990) Biochemistry 29, 5428-5436]. The spatial disposition of the acetylcholine receptor-bound pyrene with respect to the membrane bilayer was assessed by a combination of fluorescence-quenching and resonance energy transfer measurements, under conditions of selective labeling of the gamma subunit. Quenching of pyrene fluorescence by spin-labeled fatty acids with the doxyl group at positions C-5 and C-12 revealed that the former was more effective, with a Stern-Volmer quenching constant of 0.187 compared to 0.072 for the latter, suggesting that the fluorophore(s) are located closer to the membrane-water interface rather than the hydrophobic interior. Energy transfer was found to occur from tryptophan in the acetylcholine receptor to cysteine-bound pyrene with a distance of separation of approximately 18 angstrom. However, there was no energy transfer when pyrene-labeled AChR was reconstituted into membranes containing brominated phospholipids and cholesterol, suggesting that the fluorophore(s) responsible for energy transfer are located in the membrane domain. Thus, the N-(1-pyrenyl)maleimide can be used to monitor lipid-protein interactions of the AChR.