Membrane domains of AChR clusters of cultured rat myotubes revealed by rapid-freeze, deep-etch, rotary-replication.

Acetylcholine receptor clusters of cultured rat myotubes are examined by thin-section EM and by rapid-freeze, deep-etch, rotary-replication technique to observe the cortical cytoskeleton; AChR are localized by binding of fluorescent toxin. Cluster membrane is composed of three types of membrane domain which interdigitate with each other. The AChR domain is rich in the integral membrane receptor, lies further from the substrate than the other two, and is overlain by an extensive irregular meshwork of anastomosing filaments containing actin and a beta isoform of spectrin. Contact domains resemble focal contacts of fibroblasts, in that membrane lying close to the substrate is overlain by bundles of actin filaments running parallel to the membrane; some finer filaments link the parallel filaments to each other and to the membrane. Coated-membrane domains are overlain by polymerized clathrin and, like contact domains, lie close to the substrate and have external connections to it; these domains are associated with coated vesicles, but not with intracellular filaments. The three domains have few if any connections to each other, and occupy mutually exclusive territories in the cluster.