A protein tyrosine phosphatase-like inositol polyphosphatase from Selenomonas ruminantium subsp. lactilytica has specificity for the 5-phosphate of myo-inositol hexakisphosphate

Although it is becoming well known that myo-inositol polyphosphates and the enzymes involved in their metabolism play a critical role in eukaryotic systems, little is understood of their significance in prokaryotic systems. A novel protein tyrosine phosphatase (PTP)-like inositol polyphosphatase (IPPase) gene has been cloned from Selenomonas ruminantium subsp. lactilytica (phyAsrl). The deduced amino acid sequence of PhyAsrl is most similar to a PTP-like IPPase from the anaerobic bacterium S. ruminantium (35% identity), but also shows similarity (19–30% identity) to various other putative prokaryotic PTPs. Recombinant PhyAsrl could dephosphorylate myo-inositol hexakisphosphate (Ins P6) in vitro, and maximal activity was displayed at an ionic strength of 200 mM, a pH of 4.5, and a temperature of 55 °C. In order to elucidate its substrate specificity and pathway of Ins P6 dephosphorylation, a combination of kinetic and high-performance ion-pair chromatography studies were conducted. The data indicated that PhyAsrl has a general specificity for polyphosphorylated myo-inositol substrates, but can also dephosphorylate molecules containing high energy pyrophosphate bonds in vitro. PhyAsrl is unique from other microbial IPPases in that it preferentially cleaves the 5-phosphate position of Ins P6. Furthermore, it can produce Ins(2)P via a highly unique and ordered pathway of sequential dephosphorylation: Ins P6, Ins(1,2,3,4,6)P5, D-Ins(1,2,3,6)P4, Ins(1,2,3)P3, and D/L-Ins(1,2)P2. Finally, reverse transcription PCR was used to determine that phyAsrl is constitutively expressed, and together with bioinformatic analysis, was used to gain an understanding of its physiological significance.