PYRENESULFONYL AZIDE - MARKER OF ACETYLCHOLINE-RECEPTOR SUBUNITS IN CONTACT WITH MEMBRANE HYDROPHOBIC ENVIRONMENT

A lipophilic photolabel, [3H]pyrenesulfonyl azide, has been synthesized and employed to detect which portions of the acetylcholine receptor (AcChR) molecule may be in contact with the hydrophobic environment of Torpedo cali-fornica electroplax membranes. The probe preferentially partitions into the hydrophobic regions of membrane lipids or Triton X-100 micelles of detergent-solubilized membranes. When irradiated by UV light, the azide generates a nitrene compound which binds covalently, and preferentially, to membrane proteins. In Triton X-100 solubilized AcChR, the 40 000 and 48 000 molecular weight subunits are preferentially labeled, whereas in situ membrane labeling produces incorporation of the radioactive photoproduct in the 48 000 and 55 000 subunits isolated from the membrane. In both solubilized and membrane-bound receptor, the 68 000 molecular weight subunit is poorly labeled. The results suggest that in the membrane environment the 48 000 and 55 000 molecular weight subunits have a pronounced exposure to the membrane lipids, whereas the 68 000 subunit is protected from the label, possibly being partially enveloped by the other polypeptide chains from AcChR. Variations in labeling of receptor subunits in Triton X-100 and native membrane surroundings indicate that in these two environments there is an unequal accessibility of the photolabel probe to various regions of the receptor. © 1979, American Chemical Society. All rights reserved.