Regulation of sphingolipid synthesis via Orm1 and Orm2 in yeast

Abstract

Sphingolipids are critical components of membranes and sphingolipid metabolites also serve as signaling molecules. Yeast Orm1 and Orm2 belong to a conserved family of ER membrane proteins that regulate serine palmitoyltransferase, catalyzing the first and rate-limiting step in sphingolipid synthesis. We now show that sphingolipid synthesis via 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 regulating sphingolipid synthesis is via controlling Orm2 protein level. 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 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 may signal to the ER from the plasma membrane as UPR activation is dependent on a cell surface sensor and the MAPK cell wall integrity pathway. Thus, sphingolipid synthesis and the UPR are coordinately regulated.