The protein deficient in Lowe syndrome is a phosphatidylinositol-4,5-bisphosphate 5-phosphatase.
Lowe syndrome, also known as oculocerebrorenal syndrome, is caused by mutations in the X chromosome-encoded OCRL gene. The OCRL protein is 51% identical to inositol polyphosphate 5-phosphatase II (5-phosphatase II) from human platelets over a span of 744 aa, suggesting that OCRL may be a similar enzyme. We engineered a construct of the OCRL cDNA that encodes amino acids homologous to the platelet 5-phosphatase for expression in baculovirus-infected Sf9 insect cells. This cDNA encodes aa 264-968 of the OCRL protein. The recombinant protein was found to catalyze the reactions also carried out by platelet 5-phosphatase II. Thus OCRL converts inositol 1,4,5-trisphosphate to inositol 1,4-bisphosphate, and it converts inositol 1,3,4,5-tetrakisphosphate to inositol 1,3,4-trisphosphate. Most important, the enzyme converts phosphatidylinositol 4,5-bisphosphate to phosphatidylinositol 4-phosphate. The relative ability of OCRL to catalyze the three reactions is different from that of 5-phosphatase II and from that of another 5-phosphatase isoenzyme from platelets, 5-phosphatase I. The recombinant OCRL protein hydrolyzes the phospholipid substrate 10- to 30-fold better than 5-phosphatase II, and 5-phosphatase I does not cleave the lipid at all. We also show that OCRL functions as a phosphatidylinositol 4,5-bisphosphate 5-phosphatase in OCRL-expressing Sf9 cells. These results suggest that OCRL is mainly a lipid phosphatase that may control cellular levels of a critical metabolite, phosphatidylinositol 4,5-bisphosphate. Deficiency of this enzyme apparently causes the protean manifestations of Lowe syndrome.
Zhang
X
X
Division of Hematology, Washington University School of Medicine, St. Louis, MO 63110, USA.
Jefferson
A B
AB
Auethavekiat
V
V
Majerus
P W
PW
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001792:Blood Platelets / Q000201:enzymology; D002851:Chromatography, High Pressure Liquid; D017931:DNA Primers; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D009800:Oculocerebrorenal Syndrome / Q000201:enzymology* / Q000235:genetics*; D010744:Phosphoric Monoester Hydrolases / Q000172:deficiency* / Q000235:genetics* / Q000378:metabolism; D016133:Polymerase Chain Reaction; D014176:Protein Biosynthesis; D011506:Proteins / Q000235:genetics* / Q000378:metabolism; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000378:metabolism; D018411:Spodoptera; D014162:Transfection; D014960:X Chromosome; D000099182:Phosphoinositide 5-Phosphatases
D017931:DNA Primers; D007527:Isoenzymes; D010744:Phosphoric Monoester Hydrolases; D011506:Proteins; D011994:Recombinant Proteins; D000099182:Phosphoinositide 5-Phosphatases; C075695:OCRL protein, human