Synaptic vesicles can be coated in vitro in a reaction that is ARF-, A
TP-, and temperature-dependent and requires synaptic vesicle membrane
proteins. The coat is largely made up of the heterotetrameric complex,
adaptor protein 3, recently implicated in Golgi-to-vacuole traffic in
yeast. Depletion of AP3 from brain cytosol inhibits small vesicle for
mation from PC12 endosomes in vitro. Budding from washed membranes can
be reconstituted with purified AP3 and recombinant ARF1. We conclude
that AP3 coating is involved in at least one pathway of small vesicle
formation from endosomes.