Binds phospholipid vesicles containing phosphatidylinositol 3-phosphate and participates in endosomal trafficking. Homodimer. Binds STX6. Binds RAB5A, RAB5B, RAB5C and RAB22A that have been activated by GTP-binding. Interacts with ERBB2 (By similarity). Interacts with RAB31. Interacts with SAMD9 AND SAMD9L (By similarity). May interact with PLEKHF2 (By similarity). The FYVE-type zinc finger domain mediates interactions with phosphatidylinositol 3-phosphate in membranes of early endosomes and penetrates bilayers. The FYVE domain insertion into PtdIns(3)P-enriched membranes is substantially increased in acidic conditions (By similarity). Truncated C-terminus.