Ocrl1, a Ptdlns(4.5)P-2 5-phosphatase, is localized to the trans-Golgi network of fibroblasts and epithelial cells

Ptdlns(4,5)P-2 and Ptdlns(4,5)P-2 5-phosphatases play important roles in di verse aspects of cell metabolism, including protein trafficking. However, t he relative importance of the Ptdlns(4,5)P-2 5-phosphatases in regulating P tdlns(4,5)P-2 levels for specific cell processes is not well understood. Oc rl1 is a Ptdlns(4,5)P-2 5-phosphatase that is deficient in the oculocerebro renal syndrome of Lowe, a disorder characterized by defects in kidney and l ens epithelial cells and mental retardation. Ocrl1 was originally localized to the Golgi in fibroblasts, but a subsequent report suggested a lysosomal localization in a kidney epithelial cell line. In this study we defined th e localization of ocrl1 in fibroblasts and in two kidney epithelial cell li nes by three methods: immunofluorescence, subcellular fractionation, and a dynamic perturbation assay with brefeldin A. We found that ocrl1 was a Golg i-localized protein in all three cell types and further identified it as a protein of the trans-Golgi network (TGN). The TGN is a major sorting site a nd has the specialized function in epithelial cells of directing proteins t o the apical or basolateral domains. The epithelial cell phenotype in Lowe syndrome and the localization of ocrl1 to the TGN imply that this Ptdlns(4, 5)P-2 5-phosphatase plays a role in trafficking.