The Epsilon Hinge‐Ear Region Regulates Membrane Localization of the AP‐4 Complex

Adaptor protein (AP) complexes are key factors for the spatial and temporal regulation of intracellular trafficking events. Four complexes (AP‐1, ‐2, ‐3, ‐4) are known, among which AP‐4 is only poorly characterized. Recent work suggests a role for AP‐4 in the intracellular trafficking of the β‐amyloid precursor protein and molecular genetics showed that the loss of functional AP‐4 is associated with congenital neuronal disorders of severe cognitive dysfunction. To unravel the molecular mechanisms controlling AP‐4 functions, we established the intracellular expression of recombinant AP‐4 complex. This approach combined with the analysis of mutant complexes allowed us to discover that the epsilon adaptin hinge‐ear region has a function in membrane recruitment of AP‐4. We further show that this process is phosphorylation dependent and involves PP2A‐like protein phosphatases and a staurosporine‐sensitive kinase. Deletion of the residues 839‐871 in the carboxy‐terminal region of the hinge of epsilon adaptin abrogated the membrane/cytosol recycling of AP‐4. As targets of phosphorylation, we identified three serine residues: S847, S868 and S871. We conclude that the terminal hinge region and the appendage of the AP‐4 epsilon subunit are involved in membrane association in a process that is controlled by phosphorylation and dephosphorylation events.