Cytoskeleton-membrane interactions at the postsynaptic density of Xenopus neuromuscular junctions

Acetylcholine receptors of mature muscle fibres are concentrated in the postsynaptic membrane by mechanisms that are not yet understood. As one possibility, receptors might be anchored to cytoskeletal elements in the postsynaptic density that is located beneath the membrane where receptors are concentrated. To address this possibility, we examined the cytoskeleton at the postsynaptic density and determined the organization of cytoskeletal filaments relative to clustered acetylcholine receptors (AChR). Xenopus nerve-muscle co-cultures were sheared to expose the cytoplasmic membrane surface, then quick-frozen, deep-etched, and rotary-replicated. Areas with a high concentration of AChR had aggregates of particles protruding from the cytoplasmic surface of the membrane, with actin microfilaments attached to and cross-linking the aggregates. Microfilaments contacted only a few of the particles in an aggregate. These findings suggest that short-range interactions may bind individual AChR into small aggregates, while microfilaments tie these aggregates together at the nerve-muscle junction.