PiUS (Pi uptake stimulator) is an inositol hexakisphosphate kinase

Abstract

A cDNA cloned from its ability to stimulate inorganic phosphate uptake in Xenopus oocytes (phosphate uptake stimulator (PiUS)) shows significant similarity with inositol 1,4,5-trisphosphate 3-kinase. However, the expressed PiUS protein showed no detectable activity against inositol 1,4,5-trisphosphate, nor the 1,3,4,5-or 3,4,5,6-isomers of inositol tetrakisphosphate, whereas it was very active in converting inositol hexakisphosphate (InsP 6 ) to inositol heptakisphosphate (InsP 7 ). PiUS is a member of a family of enzymes found in many eukaryotes and we discuss the implications of this for the functions of InsP 7 and for the evolution of inositol phosphate kinases.z 1999 Federation of European Biochemical Societies.

Protein pyrophosphorylation by inositol pyrophosphates is a posttranslational event

“…Earlier, we noted that IP 7 phosphorylation occurs primarily at stretches of serines flanked by acidic amino acids and concluded that phosphorylation occurs on the serine residues (18) (also see SI Fig. 6).…”

“…In our earlier study, we showed that 5␤[ 32 P]IP 7 phosphorylates a wide range of proteins in both the yeast, S. cerevisiae, and in mammalian tissues (18). We characterized in detail phosphorylation of several of these proteins, such as the yeast proteins NSR1 and SRP40.…”

“…Although yeast extracts alone do not augment IP 7 phosphorylation, yeast extracts plus ATP allow IP 7 phosphorylation of proteins purified from bacteria comparable to phosphorylation of the same proteins purified from yeast. Boiled yeast extracts fail to prime proteins for IP 7 phosphorylation, indicating that the critical factor may be a protein, possibly an enzyme (Fig. 1c).…”

“…6). In Nopp140, deletion of individual serine residues in such a stretch (amino acid 80-100) decreases IP 7 phosphorylation, implying that IP 7 can phosphorylate several of them (Fig. 2a).…”

“…This possibility is further supported by mass spectrometric analysis showing no phosphorylation by IP 7 of the unphosphorylated Nopp140 peptide but addition of one or two phosphates to the prephosphorylated peptides (SI Table 1). The maximum number of phosphates accepted by a peptide from IP 7 equals the number of preexisting phosphate groups on the peptide; single phosphoserine-containing peptides accept one phosphate from IP 7 , whereas the peptide containing two phosphoserine residues accepts one or two phosphates.…”