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

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Abstract
AP‐4 is Sa member of the family of heterotetrameric adaptor protein (AP) complexes that mediate the sorting of integral membrane proteins in post‐Golgi compartments. This complex consists of four subunits (ϵ, β4, μ4 and σ4) and localizes to the cytoplasmic face of the trans ‐Golgi network (TGN). Here, we show that the recruitment of endogenous AP‐4 to the TGN in vivo is regulated by the small GTP‐binding protein ARF1. In addition, we demonstrate a direct interaction of the ϵ and μ4 subunits of AP‐4 with ARF1. ϵ binds only to ARF1·GTP and requires residues in the switch I and switch II regions of ARF1. In contrast, μ4 binds equally well to the GTP‐ and GDP‐bound forms of ARF1 and is less dependent on switch I and switch II residues. These observations establish AP‐4 as an ARF1 effector and suggest a novel mode of interaction between ARF1 and an AP complex involving both constitutive and regulated interactions.