PiUS (Pi uptake stimulator) is an inositol hexakisphosphate kinase

A cDNA cloned from its ability to stimulate inorganic phosphate uptake in X enopus oocytes (phosphate uptake stimulator (PiUS)) shows significant simil arity with inositol 1,4,5-trisphosphate 3-kinase, However, the expressed Pi US protein showed no detectable activity against inositol 1,4,5-trisphospha te, nor the 1,3,4,5- or 3,4,5,6-isomers of inositol tetrakisphosphate, wher eas it was very active in converting inositol hexakisphosphate (InsP(6)) to inositol heptakisphosphate (InsP(7)). PiUS is a member of a family of enzy mes found in many eukaryotes and we discuss the implications of this for th e functions of InsP7 and for the evolution of inositol phosphate kinases. ( C) 1999 Federation of European Biochemical Societies.