The function of RBSN (rabenosyn, RAB effector, ENSG00000131381) is as follows. 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). {ECO:0000269|PubMed:11062261, ECO:0000269|PubMed:11788822, ECO:0000269|PubMed:15020713, ECO:0000269|PubMed:22308388}.