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Publications (2)11.75 Total impact

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    ABSTRACT: Sphingolipids are crucial components of membranes, and sphingolipid metabolites serve as signaling molecules. Yeast Orm1 and Orm2 belong to a conserved family of ER membrane proteins that regulate serine palmitoyltransferase, which catalyzes the first and rate-limiting step in sphingolipid synthesis. We now show that sphingolipid synthesis through Orm1 is a target of TOR signaling, which regulates cell growth in response to nutritional signals. Orm1 phosphorylation is dependent on the Tap42-phosphatase complex, which acts downstream of TOR protein kinase complex 1. In temperature-sensitive tap42-11 cells, impaired Orm1 phosphorylation occurs concomitantly with reduced sphingolipid synthesis. A second mechanism for regulating sphingolipid synthesis is through control of Orm2 protein level. The Orm2 protein level responds to ER stress conditions, increasing when cells are treated with tunicamycin or DTT, agents that induce the unfolded protein response (UPR). The sphingolipid intermediates (long chain base and ceramide) are decreased when ORM2 is overexpressed, suggesting that sphingolipid synthesis is repressed under ER stress conditions. Finally, in the absence of the Orms, the UPR is constitutively activated. Lipid dysregulation in the absence of the Orms might signal to the ER from the plasma membrane because UPR activation is dependent on a cell surface sensor and the mitogen-activated protein kinase (MAPK) cell wall integrity pathway. Thus, sphingolipid synthesis and the UPR are coordinately regulated.
    Journal of Cell Science 02/2012; 125(Pt 10):2428-35. · 5.88 Impact Factor
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    ABSTRACT: Sphingolipids are crucial components of membranes, and sphingolipid metabolites serve as signaling molecules. Yeast Orm1 and Orm2 belong to a conserved family of ER membrane proteins that regulate serine palmitoyltransferase, which catalyzes the first and rate-limiting step in sphingolipid synthesis. We now show that sphingolipid synthesis through Orm1 is a target of TOR signaling, which regulates cell growth in response to nutritional signals. Orm1 phosphorylation is dependent on the Tap42–phosphatase complex, which acts downstream of TOR protein kinase complex 1. In temperature-sensitive tap42-11 cells, impaired Orm1 phosphorylation occurs concomitantly with reduced sphingolipid synthesis. A second mechanism for regulating sphingolipid synthesis is through control of Orm2 protein level. The Orm2 protein level responds to ER stress conditions, increasing when cells are treated with tunicamycin or DTT, agents that induce the unfolded protein response (UPR). The sphingolipid intermediates (long chain base and ceramide) are decreased when ORM2 is overexpressed, suggesting that sphingolipid synthesis is repressed under ER stress conditions. Finally, in the absence of the Orms, the UPR is constitutively activated. Lipid dysregulation in the absence of the Orms might signal to the ER from the plasma membrane because UPR activation is dependent on a cell surface sensor and the mitogen-activated protein kinase (MAPK) cell wall integrity pathway. Thus, sphingolipid synthesis and the UPR are coordinately regulated. Introduction Membrane biogenesis during cell growth and proliferation involves coordination of protein and lipid synthesis. In response to nutritional conditions, protein synthesis is modulated by multiple signaling pathways (Smets et al., 2010). One of these is target of rapamycin (TOR), a conserved protein kinase complex that regulates growth in response to nutrients and stresses. The unfolded protein response (UPR) pathway also plays a role in membrane biogenesis by adjusting the capacity of the endoplasmic reticulum (ER) to handle the load of newly synthesized proteins (Cox et al., 1997; Ron and Walter, 2007; Rutkowski and Hegde, 2010). The ER also serves as the initiation site for synthesis of the major lipid components of membranes. Transcriptional regulation serves as a major mechanism for controlling phospholipid and sterol synthesis (Nohturfft and Zhang, 2009). Insight into regulation of sphingolipid synthesis has come from recent discovery of the conserved ORMDL family of ER membrane proteins (Breslow et al., 2010; Han et al., 2010). Sphingolipids are crucial structural components of membranes and contribute to key physical properties (Breslow and Weissman, 2010). Sphingolipid metabolites also have important signaling functions. The ORMDL proteins regulate sphingolipid synthesis by physically associating with serine palmitoyltransferase (SPT), which catalyzes the first and rate-limiting step of sphingolipid synthesis. SPT mediates production of long chain bases from the condensation of serine and palmitoyl CoA (Funato et al., 2002). Ceramide is then generated upon addition of a second long chain fatty acid to a long chain base. Mature sphingolipids are made in the Golgi upon transport of ceramide from the ER. The ORMDL proteins repress SPT activity; in the absence of ORMDL regulation, SPT activity becomes hyperactive and long chain base accumulates (Breslow et al., 2010; Han et al., 2010). In yeast, SPT activity is regulated by Orm1 and Orm2, and phosphorylation of the Orm proteins adjusts SPT activity to maintain sphingolipid homeostasis (Breslow et al., 2010). In cells depleted of Orm1 and Orm2 (orm1D orm2D cells), dysregulation of sphingolipid synthesis results in pleiotropic phenotypes, including impaired growth and constitutive activation of the UPR (Han et al., 2010). As a first step to understanding how sphingolipid synthesis is regulated in response to growth conditions, we have identified the TOR signaling pathway as one regulatory component controlling sphingolipid synthesis through Orm1. In this study, we show that Orm1 phosphorylation and sphingolipid synthesis are dependent on the Sit4–Tap42 complex, a downstream target of the rapamycin-sensitive TOR protein kinase complex, TORC1. Another mechanism for regulating sphingolipid synthesis is by changing the amount of Orm2 protein. Orm2 protein level is increased by agents that cause ER stress, and sphingolipid synthesis is repressed upon ORM2 overexpression.
    Journal of Cell Science 01/2011; 125:2428-2435. · 5.88 Impact Factor