Lipid transport between intracellular organelles is mediated by vesicular and nonvesicular transport mechanisms and is critical for maintaining the identities of different cellular membranes. Nonvesicular lipid transport between the endoplasmic reticulum (ER) and the Golgi complex has been proposed to affect the lipid composition of the Golgi membranes. Here, we show that the integral ER-membrane proteins VAP-A and VAP-B affect the structural and functional integrity of the Golgi complex. Depletion of VAPs by RNA interference reduces the levels of phosphatidylinositol-4-phosphate (PI4P), diacylglycerol, and sphingomyelin in the Golgi membranes, and it leads to substantial inhibition of Golgimediated transport events. These effects are coordinately mediated by the lipid-transfer/binding proteins Nir2, oxysterolbinding protein (OSBP), and ceramide-transfer protein (CERT), which interact with VAPs via their FFAT motif. The effect of VAPs on PI4P levels is mediated by the phosphatidylinositol/phosphatidylcholine transfer protein Nir2, which is required for Golgi targeting of OSBP and CERT and the subsequent production of diacylglycerol and sphingomyelin. We propose that Nir2, OSBP, and CERT function coordinately at the ER-Golgi membrane contact sites, thereby affecting the lipid composition of the Golgi membranes and consequently their structural and functional identities. INTRODUCTIONThe unique lipid compositions of the secretory and endocytic organelles are critical for maintaining their distinct structural and functional identities (van Meer, 2000). Their identities are maintained despite extensive interconnecting vesicular-trafficking pathways, and they require tight regulation of lipid sorting, metabolism, and transport (van Meer, 1993;Pomorski et al., 2001;van Meer and Sprong, 2004). Increasing lines of evidence suggest that lipid-transfer proteins (LTPs) play a major role in regulating the lipid composition of membranous organelles (Sprong et al., 2001;De Matteis et al., 2007). These proteins facilitate lipid transfer between a donor and acceptor membrane (Holthuis and Levine, 2005), and they usually contain dual-targeting determinants for different membrane compartments. LTPs have been proposed to efficiently transfer lipids at membrane contact sites (MCSs) (Holthuis and Levine, 2005;Levine and Loewen, 2006).MCSs or zones of close apposition between the ER membranes and other cellular membranes, including the plasma membrane (PM), the membranes of the vacuoles, mitochondria, peroxisomes, lipid droplets, late endosomes, lysosomes, and the Golgi apparatus, have been identified in all eukaryotes by morphological and biochemical studies (Shore and Tata, 1977;Levine, 2004;Levine and Loewen, 2006). More recently, electron tomography studies have identified close contacts (10 -20 nm) between the ER and the outer mitochondrial membrane (Perkins et al., 1997) and between a specialized trans-ER and the three trans-most cisternae of the Golgi complex (Ladinsky et al., 1999;Marsh et al., 2001Marsh et al., , 2004. This ...

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The level of diacylglycerol (DAG) in the Golgi apparatus is crucial for protein transport to the plasma membrane. Studies in budding yeast indicate that Sec14p, a phosphatidylinositol (PI)-transfer protein, is involved in regulating DAG homeostasis in the Golgi complex. Here, we show that Nir2, a peripheral Golgi protein containing a PI-transfer domain, is essential for maintaining the structural and functional integrity of the Golgi apparatus in mammalian cells. Depletion of Nir2 by RNAi leads to substantial inhibition of protein transport from the trans-Golgi network to the plasma membrane, and causes a reduction in the DAG level in the Golgi apparatus. Remarkably, inactivation of cytidine [corrected] 5'-diphosphate (CDP)-choline pathway for phosphatidylcholine biosynthesis restores both effects. These results indicate that Nir2 is involved in maintaining a critical DAG pool in the Golgi apparatus by regulating its consumption via the CDP-choline pathway, demonstrating the interface between secretion from the Golgi and lipid homeostasis.

The rearrangement of the Golgi apparatus during mitosis is regulated by several protein kinases, including Cdk1 and Plk1. Several peripheral Golgi proteins that dissociate from the Golgi during mitosis are implicated in regulation of cytokinesis or chromosome segregation, thereby coordinating mitotic and cytokinetic events to Golgi rearrangement. Here we show that, at the onset of mitosis, Cdk1 phosphorylates the peripheral Golgi protein Nir2 at multiple sites; of these, S382 is the most prominent. Phosphorylation of Nir2 by Cdk1 facilitates its dissociation from the Golgi apparatus, and phospho-Nir2(pS382) is localized in the cleavage furrow and midbody during cytokinesis. Mitotic phosphorylation of Nir2 is required for docking of the phospho-Ser/Thr binding module, the Polo box domain of Plk1, and overexpression of a Nir2 mutant, which fails to interact with Plk1, affects the completion of cytokinesis. These results demonstrate a mechanism for coordinating mitotic and cytokinetic events with Golgi rearrangement during cell division.