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. 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 The N-terminal kinase domain produces inositol polyphosphates. The C-terminal acid phosphatase-like domain binds inositol polyphosphates and negatively regulates their accumulation. The C-terminal domain reduces the amount of inositol pyrophosphates in a dose-dependent manner in vitro. Belongs to the histidine acid phosphatase family. VIP1 subfamily. Although related to histidine acid phosphatases, it lacks the conserved active sites, suggesting that it has no phosphatase activity.