Involved in the trans-Golgi network (TGN)-endosome transport. Important for the correct localization of the adapter protein complex AP-1. Interacts with the clathrin-associated adapter complex AP-1. Colocalizes with AP-1 adapter complex, with clathrin-coated vesicles to the late-Golgi apparatus and with ribosomes. Localization depends on ADP-ribosylation factor (ARF). Deletion, when combined with a conditional mutation in clathrin heavy chain or deletion of GGA genes, accentuates growth defects and increases disruption of clathrin-dependent alpha-factor maturation and transport of carboxypeptidase Y to the vacuole. Causes mislocalization of AP-1, especially in cells at high density (postdiauxic shift), but doesn't affect GGA protein distribution. Sensitive to diltiazem-HCl and hypersensitive to chlorpromazine. Present with 300 molecules/cell in log phase SD medium.