Relative locations of the beta and delta chains of the acetylcholine receptor determined by electron microscopy of isolated receptor trimer.

The monomeric unit of the acetylcholine receptor of electric tissue of Torpedo californica has previously been shown to have a subunit composition of alpha 2 beta gamma delta. Receptor in membrane isolated from Torpedo electric tissue occurs as both monomer and dimer. In the dimer which is the predominant form, the monomeric units are cross-linked via a disulfide bond between delta chains. The addition of diamide to receptor-rich membrane causes the formation of trimer and higher oligomers in which the monomeric units are linked by disulfide bonds alternately between pairs of delta chains and between pairs of beta chains. We have isolated receptor trimer and determined the relative locations of the monomeric units by scanning transmission electron microscopy of negatively stained preparations. In face view, the trimer appears as three approximately 90 A disks, each with a central, densely staining pit. From the angles of the triangle formed by the lines connecting the centers of the monomers in the trimer, we infer that the beta-beta disulfide bond is separated from the delta-delta disulfide bond by an angle in the range of 50-80 degrees.