Bifunctional inositol kinase that acts in concert with the IP6K kinases IP6K1, IP6K2 and IP6K3 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, regulate a variety of cellular processes, including apoptosis, vesicle trafficking, cytoskeletal dynamics, exocytosis, insulin signaling and neutrophil activation. 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. Activated when cells are exposed to hyperosmotic stress. 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 Relocalizes to the plasma membrane upon activation of the PtdIns 3-kinase pathway. The C-terminal acid phosphatase-like domain binds PtdIns(3,4,5)P3 and InsP6. Despite its similarity with the phosphatase domain of histidine acid phosphatases, it has no phosphatase activity. Belongs to the histidine acid phosphatase family. VIP1 subfamily. Intron retention. Extended N-terminus.