The PI(3,5)P2 regulatory complex regulates both the synthesis and turnover of phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2). Catalyzes the phosphorylation of phosphatidylinositol 3-phosphate on the fifth hydroxyl of the myo-inositol ring, to form phosphatidylinositol 3,5-bisphosphate (By similarity). Plays an important role in maintenance of endomembrane homeostasis including endocytosis, vacuole formation, and vacuolar acidification processes. Required for development of viable pollen. Might mediate recycling of auxin transporters. 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(+) Component of the PI(3,5)P2 regulatory complex at least composed of ATG18, SAC/FIG4, FAB1 and VAC14. Ubiquitous with highest expression levels in the root hair zone, pollen, and stamens. Leaf-curling phenotype. Root growth inhibition, root gravitropic response, and hyposensitivity to exogenous auxin. Fab1a and fab1b double mutant displays an abnormal vacuolar phenotype late in pollen development leading to inviable pollen (PubMed:19846542).