A novel context for the `MutT' module, a guardian of cell integrity, in a diphosphoinositol polyphosphate phosphohydrolase

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Abstract
Diphosphoinositol pentakisphosphate (PP‐InsP5 or ‘InsP7’) and bisdiphosphoinositol tetrakisphosphate ([PP]2‐InsP4 or ‘InsP8’) are the most highly phosphorylated members of the inositol‐based cell signaling family. We have purified a rat hepatic diphosphoinositol polyphosphate phosphohydrolase (DIPP) that cleaves a β‐phosphate from the diphosphate groups in PP‐InsP5 (Km = 340 nM) and [PP]2‐InsP4 (Km = 34 nM). Inositol hexakisphophate (InsP6) was not a substrate, but it inhibited metabolism of both [PP]2‐InsP4 and PP‐InsP5 (IC50 = 0.2 and 3 μM, respectively). Microsequencing of DIPP revealed a ‘MutT’ domain, which in other contexts guards cellular integrity by dephosphorylating 8‐oxo‐dGTP, which causes AT to CG transversion mutations. The MutT domain also metabolizes some nucleoside phosphates that may play roles in signal transduction. The rat DIPP MutT domain is conserved in a novel recombinant human uterine DIPP. The nucleotide sequence of the human DIPP cDNA was aligned to chromosome 6; the candidate gene contains at least four exons. The dependence of DIPP's catalytic activity upon its MutT domain was confirmed by mutagenesis of a conserved glutamate residue. DIPP's low molecular size, Mg2+ dependency and catalytic preference for phosphoanhydride bonds are also features of other MutT‐type proteins. Because overlapping substrate specificity is a feature of this class of proteins, our data provide new directions for future studies of higher inositol phosphates.