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 cloning and characterization of a gene encoding for membrane-bound PITP, named PITPnm, that is a mammalian homologue of the Drosophila retinal degeneration B (rdgB) gene (Aikawa, Y., Hara, H., and Watanabe, T. (1997) Biochem. Biophys. Res. Commun. 236, 559-564). Here we report the subcellular localization of PITPnm protein and provide evidence for its involvement in phosphatidylinositol 4-phosphate (PtdIns 4-P) synthesis. PITPnm is an integral membrane protein that largely localized in close association with membranes of Golgi vacuoles and the endoplasmic reticulum (ER). The amino terminus region of PITPnm was exposed to cytoplasmic side. Interaction with various phosphoinositides 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 transmembrane 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 possibly in the PtdIns signaling pathway in mammalian cells.