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). Plays a role in the recycling of transferrin receptor to the plasma membrane (By similarity). Interacts with EHD1, RAB4A, RAB5A, RAB22A, RAB24 and VPS45. Binds simultaneously to RAB4A and RAB5A in vitro. Interacts with RAB4A and RAB5A that has been activated by GTP binding (By similarity). Enriched in endosomes that are in close proximity to clathrin-enriched regions at the cell surface (By similarity).