Electron microscopy of complexes of isolated acetylcholine receptor, biotinyl-toxin, and avidin

The principal curarimimetic toxin of Naja naja siamensis derivatized with biotinyl groups binds specifically both to acetylcholine receptor, isolated from Torpedo californica electric tissue, and to avidin. Isolated complexes of receptor monomer or dimer, biotinyl-toxin, and avidin were negatively stained and examined in the scanning transmission electron microscope. We measured the angle made by the radius of each avidin bound at the periphery of a monomeric unit in dimer to the axis connecting the centers of the monomers, starting at the crosslink between the monomers. We infer from the distribution of these angles that one toxin binding site is located in the range of 45 degrees to 85 degrees and another at about 100 degrees further from the crosslink between the monomers. Because it is known that there are two toxin binding sites per monomer, associated with the two alpha chains, the bound avidins presumably point to portions of the alpha chains, indicating their positions relative to that portion of the delta chain located at the crosslink between monomers in dimer.