Coated vesicles are present in the myoplasm of embryonic chick myotubes grown in vitro. They are most numerous beneath regions of the surface membrane that contain a high density of acetylcholine receptors (AChR). Prolonged exposure of myotubes to saline extract of chick brain increases the number of intracellular AChR and the number of coated vesicles. This suggests that coated vesicles contain AChR, and this hypothesis was tested with horseradish peroxidase-alpha-bungarotoxin (HRP-alpha BTX) conjugates. The conjugates enter saponin-permeabilized cells and, as judged by the inhibition of [125I] alpha BTX binding, they label the entire intracellular AChR pool. Approximately 50% of the coated vesicles contained HRP-alpha BTX reaction product. In addition, reaction product was detected in Golgi cisternae and along membranes that bound a subsurface tubulovesicular network. The majority of labeled vesicles are probably involved in exocytosis rather than endocytosis of AChR because very few coated vesicles were labeled when HRP-alpha BTX was added to the medium bathing intact cells. Moreover, inhibition of protein synthesis with puromycin resulted in a large decrease in the number of labeled vesicles. These results suggest that a subpopulation of coated vesicles ferry newly synthesized AChR to the cell surface.

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