title: Isolation and characterization of two 3-phosphatases that hydrolyze both phosphatidylinositol 3-phosphate and inositol 1,3-bisphosphate.
abstract: Inositol-polyphosphate 3-phosphatase catalyzes the hydrolysis of the 3-position phosphate bond of inositol 1,3-bisphosphate (Ins(1,3)P2) to form inositol 1-monophosphate and inorganic phosphate (Bansal, V.S., Inhorn, R.C., and Majerus, P.W. (1987) J. Biol. Chem. 262, 9444-9447). Phosphatidylinositol 3-phosphatase catalyzes the analogous reaction utilizing phosphatidylinositol 3-phosphate (PtdIns(3)P) as substrate to form phosphatidylinositol and inorganic phosphate (Lips, D.L., and Majerus, P.W. (1989) J. Biol. Chem. 264, 19911-19915). We now demonstrate that these enzyme activities are identical. Two forms of the enzyme, designated Type I and II 3-phosphatases, were isolated from rat brain. The Type I 3-phosphatase consisted of a protein doublet that migrated at a relative Mr of 65,000 upon sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The Mr of this isoform upon size-exclusion chromatography was 110,000, suggesting that the native enzyme is a dimer. The Type II enzyme consisted of equal amounts of an Mr = 65,000 doublet and an Mr = 78,000 band upon SDS-polyacrylamide gel electrophoresis. This isoform displayed an Mr upon size-exclusion chromatography of 147,000, indicating that it is a heterodimer. The Type II 3-phosphatase catalyzed the hydrolysis of Ins(1,3)P2 with a catalytic efficiency of one-nineteenth of that measured for the Type I enzyme, whereas PtdIns(3)P was hydrolyzed by the Type II 3-phosphatase at three times the rate measured for the Type I 3-phosphatase. The Mr = 65,000 subunits of the two forms of 3-phosphatase appear to be the same based on co-migration on SDS-polyacrylamide gels and peptide maps generated with Staphylococcus aureus protease V8 and trypsin. The peptide map of the Mr = 78,000 subunit was different from that of the Mr = 65,000 subunits. Thus, we propose that the differing relative specificities of the Type I and II 3-phosphatases for Ins(1,3)P2 and PtdIns(3)P are due to the presence of the Mr = 78,000 subunit of the Type II enzyme.
authors.affiliation: Division of Hematology-Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: K K
authors.initials: KK
authors.lastname: Caldwell
authors.forename: D L
authors.initials: DL
authors.lastname: Lips
authors.forename: V S
authors.initials: VS
authors.lastname: Bansal
authors.forename: P W
authors.initials: PW
authors.lastname: Majerus
mesh_terms: D000818:Animals; D001921:Brain / Q000201:enzymology; D002384:Catalysis; D002460:Cell Line; D002850:Chromatography, Gel; D004591:Electrophoresis, Polyacrylamide Gel; D006358:Hot Temperature; D006868:Hydrolysis; D007295:Inositol Phosphates / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism*; D051379:Mice; D010449:Peptide Mapping; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000037:antagonists & inhibitors / Q000302:isolation & purification* / Q000378:metabolism; D051381:Rats
chemical_list: D007295:Inositol Phosphates; D007527:Isoenzymes; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; C055525:phosphatidylinositol 3-phosphate; C051853:inositol 1,3-bisphosphate; D010744:Phosphoric Monoester Hydrolases; C061816:phosphatidylinositol-3-phosphatase