Printed in U.S.A. Changes in Conformation upon Agonist Binding, and Nonequivalent Labeling, of the Membrane-spanning Regions of the Nicotinic

All four subunits of the acetylcholine receptor (AChR) are labeled by the lipid-soluble photolabel 3-(trifluoromethyl)-3-(m-[12"I]iodophenyl)diazirine (['261]TID) with different stoichiometries and levels of saturable modification sites, dependent on the confor-mational state of the AChR. This probe is specific for hydrophobic targets such as the membrane-spanning regions of intrinsic proteins. In the resting state, the y subunit is labeled 4.5 times greater and the B and 6 subunits 1.65-1.69 greater than the a subunit. Car-bamylcholine-induced desensitization of the AChR lowers the level and alters the stoichiometry of TID incorporation into each subunit. This effect is shown to be specific in two ways. First, it is eliminated by prior equilibration with excess a-bungarotoxin