Cargo-sorting signals promote polymerization of adaptor protein-1 in an Arf-1.GTP-independent manner.

Adaptor protein-1 (AP-1) is recruited onto the trans-Golgi network via binding to Arf-1.GTP, cargo-sorting signals and phosphoinositides, where it orchestrates the assembly of clathrin-coated vesicular carriers that transport cargo molecules to endosomes. Here we show that cytosolic AP-1 polymerizes when recruited onto enriched Golgi membranes and liposomes containing covalently attached cargo-sorting signal peptides. Incubation of cytosolic or purified AP-1 with soluble sorting signal peptides also resulted in AP-1 polymerization, showing that Arf-1.GTP and membranes are not required for this process. We propose that cargo-induced polymerization of AP-1 contributes to stabilization of the coat complex in the formation of clathrin-coated buds.