Discovery of molecular and catalytic diversity among human diphosphoinositol-polyphosphate phosphohydrolases - An expanding Nudt family

The turnover of the "high energy" diphosphoinositol polyphosphates by Ca2+- and cyclic nucleotide-modulated enzymes is considered a regulatory, molecular switching activity. Target processes may include intracellular trafficking. Following our earlier identification of a prototype human (d) under bar iphospho (i) under bar nositol-(p) under bar olyphosphate phosphohydrolase (hDIPP1), we now describe new 21-kDa human isoforms, hDIPP2 alpha and hDIPP2 beta, distinguished from each other solely by hDIPP2 beta possessing one additional amino acid (Gln(86)). Candidate DIPP2 alpha and DIPP2 beta homologues in rat and mouse were also identified. The rank order for catalytic activity is hDIPP1 > hDIPP2 alpha > hDIPP2 beta. Differential expression of hDIPP isoforms may provide flexibility in response times of the molecular switches. The 76% identity between hDIPP1 and the hDIPP2s includes conservation of an emerging signature sequence, namely, a Nudt (MutT) motif with a GX(2)GX(6)G carboxy extension. Northern and Western analyses indicate expression of hDIPP2s is broad but atypically controlled; these proteins are translated from multiple mRNAs that differ in the length of the 3'-untranslated region because of utilization of an array of alternative (canonical and noncanonical) polyadenylation signals. Thus, cells can recruit sophisticated molecular processes to regulate diphosphoinositol polyphosphate turnover.