Bifunctional inositol kinase that acts in concert with the IP6K kinases to synthesize the diphosphate group-containing inositol pyrophosphates diphosphoinositol pentakisphosphate, PP-InsP5, and bis-diphosphoinositol tetrakisphosphate, (PP)2-InsP4. PP-InsP5 and (PP)2-InsP4, also respectively called InsP7 and InsP8, may regulate a variety of cellular processes, including apoptosis, vesicle trafficking, cytoskeletal dynamics, and exocytosis. Phosphorylates inositol hexakisphosphate (InsP6) at position 1 to produce PP-InsP5 which is in turn phosphorylated by IP6Ks to produce (PP)2-InsP4. Alternatively, phosphorylates PP-InsP5 at position 1, produced by IP6Ks from InsP6, to produce (PP)2-InsP4 (PubMed:25231822). Probably involved in vitamin E homeostasis via the regulation of gamma-tocopherol biosynthesis (By similarity). 1D-myo-inositol hexakisphosphate + ATP = 1-diphospho-myo-inositol 2,3,4,5,6-pentakisphosphate + ADP 5-diphospho-1D-myo-inositol 1,2,3,4,6-pentakisphosphate + ATP + H(+) = 1,5-bis(diphospho)-1D-myo-inositol 2,3,4,6-tetrakisphosphate + ADP Mostly expressed at low levels in roots and reproductive tissues (e.g. flowers and siliques), and, to a lower extent, in vegetative tissues (e.g. roots, shoots, leaves and stems) (PubMed:25231822). Also present in mature pollen (PubMed:25231822, PubMed:25901085). Belongs to the histidine acid phosphatase family. VIP1 subfamily.