Abstract
(English)
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Sphingolipids are important components of membranes and help to participate in the establishment of distinct membrane domains. The precursors involved in sphingolipid synthesis are also signalling molecules that control diverse events, including membrane traffic. Our work has focussed on the mechanisms of ceramide synthesis from endogenous and exogenous precursors, the intracellular transport of ceramide, and the roles of sphingoid bases and ceramides in membrane trafficking. We have made substantial progress in all three areas. One of the least characterised enzymes in sphingolipid synthesis is the ceramide synthase. We have identified two genes that are required for ceramide synthesis, LAG1 and LAC1 (Schörling, S et al, 2001). Recently, we have shown that these are in fact subunits of the acyl-CoA dependent ceramide synthase with specificity towards C26 fatty acyl CoA. A mammalian gene related to the yeast genes is also involved in ceramide synthesis and seems also to regulate fatty acyl chain length of ceramides (Venkataraman, K et al 2002). Ceramides can be made from endogenous synthesis of sphingoid bases, but also from exogenously added sphingoid bases. We have shown that exogenously added sphingoid bases require a cycle of phosphorylation and dephosphorylation on the endoplasmic reticulum membrane before their efficient incorporation into ceramides. The finding the phosphorylation event takes place on the ER membrane is surprising and suggests that sphingoid base phosphorylation on the membrane and in the cytoplasm play different roles (Funato, K et al, 2002A). Once synthesised on the ER membrane, ceramide is transported to the Golgi compartment by vesicular and non-vesicular mechanisms. The non-vesicular pathway is ATP independent, cytosolic protein-dependent and requires ER-Golgi membrane contacts to be efficient (Funato, K and Riezman, H., 2001). One of the functions of sphingolipid synthesis is for the transport of glycosylphosphatidylinositol (GPI) -anchored proteins from the ER to the Golgi compartment. We have shown that the critical component of sphingolipid synthesis that is necessary for the transport step is ceramide. The role of ceramide in the transport seems to be to ensure a stable association of GPI-anchored proteins with the ER membrane, because in the absence of ceramide synthesis, GPI-anchored proteins are no longer tightly membrane associated, even though they have hydrophobic GPI-anchors (Watanabe et al, 2002). We have recently written a review on the biosynthesis and trafficking of sphingolipids, concentrating on yeast (Funato et al., 2002B).
Funato, K., and H. Riezman (2001) Vesicular and non-vesicular transport of ceramide from ER to the Golgi apparatus in yeast. J. Cell Biol., 155, 949-959.
Venkataraman, K., C. Riebeling, J. Bodennec, H. Riezman, J.C. Allegood, M.C. Sullards, A.H. Merrill and A.H. Futerman (2002) Upstream of growth and differentiation factor 1 (uog1), a mammalian homolog of the yeast longevity assurance gene 1 (LAG1), regulates N-stearoyl-sphinganine (C18-(dihydro)ceramide) synthesis in a fumonisin B1-independent manner in mammalian cells. J. Biol. Chem. 377, 35642-35649.
Funato, K., B. Vallée and H. Riezman (2002B) Biosynthesis and trafficking of sphingolipids in the yeast Saccharomyces cerevisiae. Biochemistry, 41, 15105-15114.
Watanabe, R., K. Funato, K. Venkataraman, A.H. Futerman and H. Riezman (2002) Sphingolipids are required for the stable membrane association of glycosylphosphatidylinositol-anchored proteins in yeast. J. Biol. Chem., 277, 49538-49544.
Funato, K., R. Lombardi, B. Vallée and H. Riezman (2002A) Lcb4p is a Key Regulator of Ceramide Synthesis from Exogenous Long Chain Sphingoid Base in Saccharomyces cerevisiae. J. Biol. Chem., Dec 18, epub ahead of print.
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