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

Diphosphoinositol pentakisphosphate (PP-InsP(5) or 'InsP(7)') and bisdiphos phoinositol tetrakisphosphate ([PP](2)-InsP(4) or 'InsP(8)') are the most h ighly phosphorylated members of the inositol-based cell signaling family. W e have purified a rat hepatic diphosphoinositol polyphosphate phosphohydrol ase (DIPP) that cleaves a beta-phosphate from the diphosphate groups in PP- InsP(5) (K-m = 340 nM) and [PP](2)-InsP(4) (K-m = 34 nM). Inositol hexakisp hophate (InsP(6)) was not a substrate, but it inhibited metabolism of both [PP](2)-InsP(4) and PP-InsP(5) (IC50 = 0.2 and 3 mu M, respectively). Micro sequencing of DIPP revealed a 'MutT' domain, which in other contests guards cellular integrity by dephosphorylating 8-oxo-dGTP, which causes AT to CG transversion mutations. The MutT domain also metabolizes some nucleoside ph osphates that may play roles in signal transduction. The rat DIPP MutT doma in is conserved in a novel recombinant human uterine DIPP. The nucleotide s equence of the human DIPP cDNA was aligned to chromosome 6; the candidate g ene contains at least four exons. The dependence of DIPP's catalytic activi ty upon its MutT domain was confirmed by mutagenesis of a conserved glutama te residue. DIPP's low molecular size, Mg2+ dependency and catalytic prefer ence for phosphoanhydride bonds are also features of other MutT-type protei ns, Because overlapping substrate specificity is a feature of this class of proteins, our data provide new directions for future studies of higher ino sitol phosphates.