STRUCTURAL ORGANIZATION OF DEVELOPING ACETYLCHOLINE-RECEPTOR AGGREGATES

We report the first quantitative ultrastructural analysis of newly for med acetylcholine receptor aggregates. Aggregates were induced in Xeno pus muscle cell cultures with agrin, labeled with gold particles, and detected using high resolution scanning electron microscopy. Aggregate s are readily discernible at the ultrastructural level within 2 h of s timulation by agrin. The size and density profiles of the developing a ggregates show that receptors reach maximal density very quickly in sm all ''nano-aggregates'' and that the aggregation process is not limite d by the diffusion rate of the receptor. Quantitative analysis of labe l locations indicates that the receptor distribution within aggregates is nonrandom. Instead, the newly aggregated receptors appear to be bo und to a localized scaffold conforming to a hexagonal (close-packed) g eometry with a spacing of approximately 9.9 nm. (C) 1997 John Wiley & Sons, Inc.