Phosphatidylethanolamine, a key phospholipid of the yeast
Phosphatidylethanolamine (PE), one of the major phospholipids of yeast membranes, is highly important for cellular function and cell proliferation. PE synthesis in the yeast is accomplished by a network of reactions including (i) synthesis of phosphatidylserine (PS) in the endoplasmic reticulum, (ii) decarboxylation of PS by mitochondrial phosphatidylserine decarboxylase 1 (Psd1p) or (iii) Psd2p in a Golgi/vacuolar compartment, (iv) the CDP-ethanolamine pathway (Kennedy pathway) in the endoplasmic reticulum, and (v) the lysophospholipid acylation route catalyzed by Ale1p. To obtain more insight into biosynthesis, membrane assembly, traffic routes and homeostasis of PE single and multiple mutants bearing defects in the respective pathways were used. Recently, these investigations were focused on two subcellular fractions as destinations for PE translocation, namely peroxisomes and the plasma membrane. These two membranes have in common their lack of capacity to synthesize PE. We employed yeast mutants bearing defects in the different pathways of PE synthesis, which are distributed among mitochondria, endoplasmic reticulum and Golgi. These experiments demonstrated that PE formed in all three organelles can be supplied to peroxisomes and the plasma membrane. 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.
Figure 1:Three-dimensional reconstruction of yeast cells grown on oleic acid.
Transmission electron micrographs of ultrathin sections (A, C; bar = 1µm) and corresponding 3D reconstructions of serial sections (B, D) of a chemically fixed wild type cell are shown. Cells shown in A and B were grown on YPD, and cells shown in C and D were grown on oleic acid to induce formation of peroxisomes. CW cell wall; ER endoplasmic reticulum; LP lipid particle; M mitochondrion; N nucleus; V vacuole; PX peroxisomes.
By courtesy of G. Zellnig, KFUG.
To obtain a global view of the role of PE in the cell and to study the effects of an unbalanced PE level we recently subjected a psd1Δ deletion mutant and the corresponding wild type to DNA microarray analysis and examined genome-wide changes in gene expression. Comparison of the gene expression patterns led to the identification of 55 differentially expressed genes. Grouping of these genes into functional categories revealed that PE formation by Psd1p influenced the expression of genes involved in diverse cellular pathways including transport, carbohydrate metabolism and stress response. Moreover, this genome-wide analysis identified a number of gene products with unknown function which will be subjected to detailed molecular analysis addressing PE homeostasis in yeast.