Ubiquitin binding and conjugation regulate the recruitment of Rabex-5 to early endosomes

Abstract

Rab GTPases and ubiquitination are critical regulators of transmembrane cargo sorting in endocytic and lysosomal targeting pathways. The endosomal protein Rabex‐5 intersects these two layers of regulation by being both a guanine nucleotide exchange factor (GEF) for Rab5 and a substrate for ubiquitin (Ub) binding and conjugation. The ability of trafficking machinery components to bind ubiquitinated proteins is known to have a function in cargo sorting. Here, we demonstrate that Ub binding is essential for the recruitment of Rabex‐5 from the cytosol to endosomes, independently of its GEF activity and of Rab5. We also show that monoubiquitinated Rabex‐5 is enriched in the cytosol. These observations are consistent with a model whereby a cycle of Ub binding and monoubiquitination regulates the association of Rabex‐5 with endosomes.