Ocrl1, a PtdIns(4,5)P25-Phosphatase, Is Localized to theTrans-Golgi Network of Fibroblasts and Epithelial Cells

Abstract

PtdIns(4,5)P2and PtdIns(4,5)P25-phosphatases play important roles in diverse aspects of cell metabolism, including protein trafficking. However, the relative importance of the PtdIns(4,5)P25-phosphatases in regulating PtdIns(4,5)P2levels for specific cell processes is not well understood. Ocrl1 is a PtdIns(4,5)P25-phosphatase that is deficient in the oculocerebrorenal syndrome of Lowe, a disorder characterized by defects in kidney and lens 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 the localization of ocrl1 in fibroblasts and in two kidney epithelial cell lines by three methods: immunofluorescence, subcellular fractionation, and a dynamic perturbation assay with brefeldin A. We found that ocrl1 was a Golgi-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 and has the specialized function in epithelial cells of directing proteins to the apical or basolateral domains. The epithelial cell phenotype in Lowe syndrome and the localization of ocrl1 to the TGN imply that this PtdIns(4,5)P25-phosphatase plays a role in trafficking.