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

• Published in: Biochemistry (Easton) 1993-11, Vol.32 (46), p.12413-12419
• Main Authors: Narayanaswami, Vasanthy, Kim, Jeongah, McNamee, Mark G
• Format: Article
• Language: English
• Subjects: Animals; Cysteine - chemistry; Energy Transfer; Freshwater; In Vitro Techniques; Maleimides - chemistry; Membrane Lipids - chemistry; Models, Molecular; Mutagenesis, Site-Directed; Protein Structure, Tertiary; Receptors, Nicotinic - chemistry; Spin Labels; Structure-Activity Relationship; Torpedo; Torpedo californica; Tryptophan - chemistry
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00097a020

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 A. 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.