Involvement of PITPnm, a mammalian homologue of Drosophila rdgB, in phosphoinositide synthesis on Golgi membranes

Phosphatidylinositol transfer protein (PITP) is involved in phospholipase C -mediated signaling and membrane trafficking. We previously reported clonin g and characterization of a gene encoding for membrane-bound PITP, named PI TPnm, that is a mammalian homologue of the Drosophila retinal degeneration B (rdgB) gene (Aikawa, Y., Hara, H., and Watanabe, T. (1997) Biochem. Bioph ys. Res. Commun, 236, 559-564). Here we report the subcellular localization of PITPnm protein and provide evidence for its involvement in phosphatidyl inositol 4-phosphate (PtdIns 4-P) synthesis. PITPnm is an integral membrane protein that largely localized in close association with membranes of Golg i vacuoles and the endoplasmic reticulum (ER). The amino terminus region of PITPnm was exposed to cytoplasmic side. Interaction with various phosphoin ositides was observed in the amino terminus region spanning from 196 amino acids to 257 amino acids of PITPnm. At the amino terminus regions of 1-372 amino acids, PITPnm formed a complex with type III PtdIns 4-kinase. The tra nsmembrane and carboxyl-terminal portions (residues 418-1242) functioned to retain the PITPnm in the Golgi vacuole. These results suggest that PITPnm plays a role in phosphoinositide synthesis on the Golgi vacuoles and possib ly in the PtdIns signaling pathway in mammalian cells.