Different channel properties of Torpedo acetylcholine receptor monomers and dimers reconstituted in planar membranes.

Abstract

The monomeric and dimeric structures of the nicotinic acetylcholine receptor of T. californica electric tissue, reconstituted in planar lipid bilayers, are functionally different. The native dimer D of MW 500,000 (heavy-form) exhibits a single channel conductance about twice as large as that of the monomer M of MW 250,000 (light form). Under conditions where monomers aggregate, the conductance changes from the level of the monomer M to that of dimers M2. The dimer conductances (D and M2) seem to result from synchronous opening and closing of the two channels in the dimer, giving the impression of single channel activity. This channel cooperativity is apparently mediated by noncovalent interactions between the two monomers, since it requires no disulfide linkage between monomers. Both the monomers M and the dimers D and M2 show at least 1 substate of lower conductivity. The relative population of the 2 conductance levels depends on the ion type (Na+ and K+), indicating ion-specific channel state. Since the channel conductance of isolated dimers resembles those obtained form unextracted microsacs, the dimer with 2 synchronized channels appears to be the in vivo predominant gating unit. In the linear association of dimers, observed in the native membrane, channel synchronization may extend to > 2 channels as suggested by oligomeric channel cooperativity in associations of monomers and dimers.