FORMATION OF DISULFIDE-LINKED OLIGOMERS OF ACETYLCHOLINE-RECEPTOR IN MEMBRANE FROM TORPEDO ELECTRIC TISSUE

Acetylcholine receptor in electric tissue of Torpedo californica occurs predominantly as a disulfide cross-linked dimer which is converted to monomer by dithiothreitol. Reduced monomers in membrane (but not in solution) are cross-linked to dimers and higher oligomers in the presence of oxidizing agents. The cross-links are disulfide bonds, probably symmetrical, between identical chains on separate monomers. In native dimer, the disulfide cross-links are between δ chains. The oxidizing agent, diamide, however, preferentially cross-links reduced monomers via β chains. Copper phenanthroline cross-links reduced monomers via δ chains and β chains. The sulfhydryl groups involved in cross-linking β chains preexist dithiothreitol reduction; the sulfhydryls involved in cross-linking δ chains are generated by dithiothreitol reduction. The specificity of the oxidative cross-linking reactions leading exclusively to dimers of β chains and/or of δ chains and the kinetics of the loss of receptor monomer and the generation of receptor oligomers with each of the oxidizing agents are consistent with the receptor monomer in membrane containing one β chain and one δ chain as in the proposed chain composition [Reynolds, J. A., & Karlin, A. (1978) Biochemistry 17, 2035] of α 2βγδ. The kinetics of the reactions are also consistent with the random diffusion in two dimensions of the reacting receptor species in the membrane; i.e., it is not necessary to invoke specific association between reduced monomers in the membrane. The functional significance of disulfide cross-linked receptor dimers in torpedo electroplax is not obvious; such dimers are not present in electrophorus electroplax and are apparently not necessary for the translocation of sodium ions by activated receptor in torpedo or electrophorus. © 1979, American Chemical Society. All rights reserved.