Binds phospholipid vesicles containing phosphatidylinositol 3-phosphate and participates in early endosomal trafficking. Interacts with BMX. May interact with SSB. Interacts with RAB4 and RAB5 that have been activated by GTP-binding. Broadly expressed, with highest levels in lung, testis, kidney and brain. 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. Phosphorylation on Tyr-389 and/or Tyr-400 is required for interaction with BMX and endosomal targeting.