Recruitment of the endosomal WASH complex is mediated by the extended 'tail' of Fam21 binding to the retromer protein Vps35.

The retromer complex is a conserved endosomal protein sorting complex that sorts membrane proteins into nascent endosomal tubules. The recognition of membrane proteins is mediated by the cargo-selective retromer complex, a stable trimer of the Vps35 (vacuolar protein sorting 35), Vps29 and Vps26 proteins. We have recently reported that the cargo-selective retromer complex associates with the WASH (Wiskott-Aldrich syndrome homologue) complex, a multimeric protein complex that regulates tubule dynamics at endosomes. In the present study, we show that the retromer-WASH complex interaction occurs through the long unstructured 'tail' domain of the WASH complex-Fam21 protein binding to Vps35, an interaction that is necessary and sufficient to target the WASH complex to endosomes. The Fam21-tail also binds to FKBP15 (FK506-binding protein 15), a protein associated with ulcerative colitis, to mediate the membrane association of FKBP15. Elevated Fam21-tail expression inhibits the association of the WASH complex with retromer, resulting in increased cytoplasmic WASH complex. Additionally, overexpression of the Fam21-tail results in cell-spreading defects, implicating the activity of the WASH complex in regulating the mobilization of membrane into the endosome-to-cell surface pathway.