The isolation and characterization of a cDNA encoding phospholipid-specific inositol polyphosphate 5-phosphatase

We report the cDNA cloning and characterization of a novel human inositol p olyphosphate 5-phosphatase (5-phosphatase) that has substrate specificity u nlike previously described members of this large gene family. All previousl y described members hydrolyze water soluble inositol phosphates. This enzym e hydrolyzes only lipid substrates, phosphatidylinositol 3,4,5-trisphosphat e and phosphatidylinositol 4,5-bisphosphate. The cDNA isolated comprises 31 10 base pairs and predicts a protein product of 644 amino acids and M-r 70, 023. We designate this 5-phosphatase as type IV. It is a highly basic prote in (pI = 8.8) and has the greatest affinity toward phosphatidylinositol 3,4 ,5-trisphosphate of known 5-phosphatases. The K-m is 0.65 mu M, 1/10 that o f SHIP (5.95 mu M), another 5-phosphatase that hydrolyzes phosphatidylinosi tol 3,4,5-trisphosphate. The activity of Ei-phosphatase type TV is sensitiv e to the presence of detergents in the in vitro assay. Thus the enzyme hydr olyzes lipid substrates in the absence of detergents or in the presence of n-octyl beta-glucopyranoside or Triton X-100, but not in the presence of ce tyltriethylammonium bromide, the detergent that has been used in other stud ies of the hydrolysis of phosphatidylinositol 4,5-bisphosphate. Remarkably SHIP, a 5-phosphatase previously characterized as hydrolyzing only substrat es with D-3 phosphates, also readily hydrolyzed phosphatidylinositol 4,5-bi sphosphate in the presence of n-octyl P-glucopyranoside but not cetyltrieth ylammonium bromide. We used antibodies prepared against a peptide predicted by the cDNA to identify the B-phosphatase type IV enzyme in human tissues and find that it is highly expressed in the brain as determined by Western blotting. We also performed Western blotting of mouse tissues and found hig h levels of expression in the brain, testes, and heart with lower levels of expression in other tissues. mRNA was detected in many tissues and cell li nes as determined by Northern blotting.