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performed. Three components identified through these screenings were recently characterized in more detail, namely Oxa1p, Mrpl22p and Vps4p. Oxa1p is a component of the inner mitochondrial membrane and involved in the assembly of proteins into this compartment (Figure 1). Deletion of OXA1 causes a defect in the import of Psd1p, PtdSer decarboxylase 1, into mitochondria. Since Psd1p is the major enzyme of PtdEtn formation in the yeast, the oxa1 mutation results in a marked decrease of PtdEtn compared to wild type. Psd1p Targeting IM Sorting β-Subunit LGST -Subunit ++++ + TOM OMM TIM22 TIM23 Oxa1p Mba1p Psd1p IMM Psd1p Figure 1: The Oxa1p route of Psd1p import into mitochondria Mrpl22p is a component of mitochondrial ribosomes. Deletion of the respective gene causes defects comparable to psd1, namely decrease of cellular and mitochondrial PtdEtn and defects in mitochondrial respiration. The molecular reason for these findings is not yet known. Vps4p is a component of the machinery governing protein traffic to yeast vacuoles. In strains deleted of VPS4 an alternative pathway of protein transport to vacuoles, the so-called autophagy/cytosol to vacuole targeting (Cvt) pathway, is induced. The latter route requires PtdEtn binding to Atg8p, a component involved in autophagy. Thus, shortage of PtdEtn in respective mutants compromises the autophagic process and causes a growth defect. To understand PtdEtn homeostasis in some more detail, we performed experiments defining traffic routes of PtdEtn within the yeast cell. For these investigations we have chosen two subcellular fractions as destinations for PtdEtn translocation, namely peroxisomes and the plasma membrane. These two membranes have in common their lack of capacity to synthesize PtdEtn themselves. We employed yeast mutants bearing defects in the three pathways of PtdEtn synthesis, which are distributed among mitochondria, endoplasmic reticulum and Golgi. These experiments 16
demonstrated that PtdEtn formed in all three organelles can be supplied to peroxisomes and to the plasma membrane. The fatty acid composition of peroxisomal and plasma membrane phospholipids was mostly influenced by the available pool of total species synthesized, although a certain balancing effect was observed regarding the assembly of PtdEtn species into the two target compartments. We assume that the phospholipid composition of peroxisomes and the plasma membrane is mainly affected by the synthesis of the respective components and subject to equilibrium, and to a lesser extent affected by specific transport and assembly processes. Organelle association and membrane contact between organelles may be a relevant mechanism for lipid transport between organelles, but components governing this process and related events have to be studied in future investigations. Neutral lipid storage in lipid particles and mobilization Yeast cells have the ability to store neutral lipids TAG (triacylglycerols) and SE (steryl esters) in subcellular structures named lipid particles/droplets. Upon requirement, TAG and SE can be mobilized and serve as building blocks for membrane biosynthesis. In an ongoing project in our laboratory, we investigate and characterize enzymatic steps which lead to the storage of TAG and SE and at the same time to the biogenesis of lipid particles. Moreover, we study processes involved in the mobilization of TAG and SE depots. In the yeast, the SE synthases Are1p and Are2p, and the TAG synthases Dga1p and Lro1p contribute to the formation of lipid depots (Figure 2). We have to distinguish between de novo formation of lipid particles on one hand and incorporation of neutral lipids into preformed lipid particles on the other hand. Using a dga1 lro1 are1 are2 quadruple mutant and the related set of triple mutants we performed cell biological and Figure 2: Pathways of triacylglycerol (TAG) and steryl ester (SE) metabolism in the yeast. DAG: diacylglycerol; FFA: free fatty acids; PL: phospholipid. 17
Page 1 and 2: Staff Members of the Institute of B
Page 3 and 4: 1973 Dr. F. PALTAUF, docent of the
Page 5 and 6: The PhD program “Molecular Enzymo
Page 7 and 8: methyl group of the substrate (S)-r
Page 9 and 10: eceives further support by addition
Page 11 and 12: The absence of vitamin B 6 biosynth
Page 13 and 14: FWF-Doktoratskolleg “Molecular En
Page 15: Cell Biology Group Group leader: G
Page 19 and 20: iosynthesis in a type of feedback r
Page 21 and 22: T. Hoefken, Institute of Biochemist
Page 23 and 24: Molecular Biology Group Karin Athen
Page 25 and 26: proteins. The “standard” proteo
Page 27 and 28: specific action of sphingolipid med
Page 29 and 30: in gel-digest, the isolated peptide
Page 31 and 32: 3) Wehinger A, Tancevski I, Schgoer

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