Functional and physical interactions of the adaptor protein complex AP-4 with ADP-ribosylation factors (ARFs)

AP-4 is a member of the family of heterotetrameric adaptor protein (AP) com plexes that mediate the sorting of integral membrane proteins in post-Golgi compartments. This complex consists of four subunits (epsilon, beta4, mu4 and sigma4) and localizes to the cytoplasmic face of the trans-Golgi networ k (TGN). Here, we show that the recruitment of endogenous AP-4 to the TGN i n vivo is regulated by the small GTP-binding protein ARF1. In addition, we demonstrate a direct interaction of the epsilon and mu4 subunits of AP-4 wi th ARF1. epsilon binds only to ARF1.GTP and requires residues in the switch I and switch II regions of ARF1. In contrast, mu4 binds equally well to th e GTP- and GDP-bound forms of ARF1 and is less dependent on switch I and sw itch II residues. These observations establish AP-4 as an ARF1 effector and suggest a novel mode of interaction between ARF1 and an AP complex involvi ng both constitutive and regulated interactions.