title: Mapping the lipid-exposed regions in the Torpedo californica nicotinic acetylcholine receptor.
abstract: To identify regions of the Torpedo nicotinic acetylcholine receptor (AchR) interacting with membrane lipid, we have used 1-azidopyrene (1-AP) as a fluorescent, photoactivatable hydrophobic probe. For AchR-rich membranes equilibrated with 1-AP, irradiation at 365 nm resulted in covalent incorporation in all four AchR subunits with each of the subunits incorporating approximately equal amounts of label. To identify the regions of the AchR subunits that incorporated 1-AP, subunits were digested with Staphylococcus aureus V8 protease and trypsin, and the resulting fragments were separated by SDS-PAGE followed by reverse-phase high-performance liquid chromatography. N-terminal sequence analysis identified the hydrophobic segments M1, M3, and M4 within each subunit as containing the sites of labeling. The labeling pattern of 1-AP in the alpha-subunit was compared with that of another hydrophobic photoactivatable probe, 3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine ([125I]TID). The nonspecific component of [125I]TID labeling [White, B., Howard, S., Cohen, S. G., & Cohen, J.B. (1991) J. Biol. Chem. 266, 21595-21607] was restricted to the same regions as those labeled by 1-AP. The [125I]TID residues labeled in the hydrophobic segment M4 were identified as Cys-412, Met-415, Cys-418, Thr-422, and Val-425. The periodicity and distribution of labeled residues establish that the M4 region is alpha-helical in nature and indicate that M4 presents a broad face to membrane lipid.
authors.affiliation: Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: M P
authors.initials: MP
authors.lastname: Blanton
authors.forename: J B
authors.initials: JB
authors.lastname: Cohen
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001389:Azirines / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D003432:Cross-Linking Reagents; D004591:Electrophoresis, Polyacrylamide Gel; D005456:Fluorescent Dyes; D006868:Hydrolysis; D007457:Iodine Radioisotopes; D050356:Lipid Metabolism; D008969:Molecular Sequence Data; D011721:Pyrenes / Q000737:chemistry; D011978:Receptors, Nicotinic / Q000378:metabolism*; D012697:Serine Endopeptidases / Q000378:metabolism; D014101:Torpedo; D014357:Trypsin / Q000378:metabolism
chemical_list: D001389:Azirines; D003432:Cross-Linking Reagents; D005456:Fluorescent Dyes; D007457:Iodine Radioisotopes; D011721:Pyrenes; D011978:Receptors, Nicotinic; C028591:azidopyrene; C033174:3-(trifluoromethyl)-3-(3-iodophenyl)diazirine; D012697:Serine Endopeptidases; C028344:glutamyl endopeptidase; D014357:Trypsin