Lipid domains of acetylcholine receptor clusters detected with saponin and filipin.

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Abstract
The acetylcholine receptor (AChR) clusters of cultured rat myotubes contain 2 distinct, interdigitating, membrane domains, one rich in AChR, the other poor in AChR but associated with sites of myotube-substrate contact (Bloch et Geiger, 1980). Two cholesterol-specific cytochemical probes, saponin and filipin, were used to investigate the lipid nature of these membrane domains. When studied with freeze-fracture EM or fluorescence microscopy, these reagents reacted moderately and preferentially with the AChR-rich domains of AChR clusters. Little or no reaction with the membrane in contact domains was seen. Membrane regions surrounding the AChR clusters reacted extensively with filipin. In rat myotubes, the composition or the state of the lipids differs between the 2 membrane domains of AChR clusters, and between clusters and surrounding membrane. In chick myotubes, AChR clusters do not appear to react with filipin or saponin, although surrounding membrane reacts extensively with these reagents.

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