Correlation of polypeptide composition with functional events in acetylcholine receptor-enriched membranes from Torpedo californica.

Membrane vesicles containing partially inactivated acetylcholine receptor (AcChoR) channels may produce a full 22Na+ flux response because an excess of channels may exist above the level needed to completely empty the vesicles of ions. Therefore, attempts to use ion flux amplitudes as indicators of AcChoR function may fail due to the presence of these excess AcChoR channels. Random inactivation of variable fractions of AcChoR channels in vesicles by the irreversible binding of the neurotoxin alpha-bungarotoxin provides a tool for assessing the size of the excess receptor population. Using this approach, we predict that the dependence of the flux response on partial inactivation by alpha-bungarotoxin will drastically change if an essential AcChoR component is substantially removed from the membranes. Membranes from which Mr 43,000, Mr 90,000, and other polypeptides had been substantially removed by base extraction exhibited a flux response after random inactivation that was indistinguishable from that of untreated membranes. Therefore, those components which are substantially removed by base extraction do not appear to be essential for AcChoR-mediated ion flux.