Regulates both the synthesis and turnover of phosphatidylinositol 3,5-bisphosphate (1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3,5-bisphosphate) or PtdIns(3,5)P2) (PubMed:16837550). Catalyzes the phosphorylation of phosphatidylinositol 3-phosphate (1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3-phosphate)) on the fifth hydroxyl of the myo-inositol ring, to form PtdIns(3,5)P2 (PubMed:16837550). Required for endocytic-vacuolar pathway and nuclear migration. Has a role at a late stage in endosome-related membrane trafficking, at a point when signal termination has occurred. Is not required for receptor silencing. a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3-phosphate) + ATP = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3,5-bisphosphate) + ADP + H(+) Mainly associated with membranes of the late endocytic pathway. Flies contain undetectable phosphatidylinositol 3,5-bisphosphate levels, show profound increases in cell and organ size, and die at the pupal stage. Mutant larvae contain highly enlarged multivesicular bodies and late endosomes that are inefficiently acidified. Even though endocytic receptor trafficking is impaired in fab1 mutants, Notch, Wingless, and Dpp signaling is unaffected.