Rab4/Rab5 effector protein acting in early endocytic membrane fusion and membrane trafficking of recycling endosomes. Required for endosome fusion either homotypically or with clathrin coated vesicles. Plays a role in the lysosomal trafficking of CTSD/cathepsin D from the Golgi to lysosomes. Also promotes the recycling of transferrin directly from early endosomes to the plasma membrane. Binds phospholipid vesicles containing phosphatidylinositol 3-phosphate (PtdInsP3) (PubMed:11062261, PubMed:11788822, PubMed:15020713). Plays a role in the recycling of transferrin receptor to the plasma membrane (PubMed:22308388). Interacts with EHD1, RAB4A, RAB5A, RAB14, RAB22A, RAB24 and VPS45 (PubMed:11062261, PubMed:15020713, PubMed:16034420). Binds simultaneously to RAB4A and RAB5A in vitro (PubMed:16034420). Interacts with RAB4A and RAB5A that has been activated by GTP binding (PubMed:11062261, PubMed:16034420, PubMed:20098723, PubMed:11788822). Enriched in endosomes that are in close proximity to clathrin-enriched regions at the cell surface.