CLUSTERED ACETYLCHOLINE-RECEPTORS HAVE 2 LEVELS OF ORGANIZATION IN XENOPUS MUSCLE-CELLS

We studied the organization of acetylcholine receptor (AChR) clusters by shearing cultured Xenopus muscle cells with a stream of buffer, and preparing rotary replicas of the exposed cytoplasmic surface of the s arcolemma. AChR clusters contained numerous particles that protruded f rom the sarcolemma and formed an irregular array composed of discrete aggregates. AChR were located within these particle aggregates, as sho wn by comparison of the replicas to labeling by fluorescent alpha-bung arotoxin, and by immunogold cytochemistry with antibodies specific for the receptor. The aggregates were cross-linked by a dense network of 7 nm filaments that replicated with the banded pattern characteristic of actin microfilaments. The organization of receptors into the small aggregates was independent of the organization of these aggregates int o clusters, as alkaline extraction removed the microfilament network a nd disrupted the irregular array of particle aggregates, but did not d isperse individual receptors from the aggregates. We conclude that two levels of interactions organize AChR clusters in Xenopus muscle cells : short-range interactions that assemble individual AChR into small ag gregates, and long-range interactions, perhaps mediated by actin micro filaments, that anchor the aggregates into larger clusters. (C) 1994 W iley-Liss, Inc.