Staff Members of the Institute of Biochemistry, TU - Institut für ...

Previous investigations in our laboratory were aimed at the molecular biological
identification of novel components involved in PE homeostasis of the yeast Saccharomyces
cerevisiae. For this purpose, a number of genetic screenings were performed. 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 psd1∆ deletion mutant and the corresponding wild type to DNA
microarray analysis and examined genome-wide changes in gene expression caused by a
PSD1 deletion. Comparison of the gene expression pattern of the psd1∆ mutant with the
wild-type 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. Most of the proteins identified are present in the cytosol
followed by the nucleus, cellular membranes, the cell wall and mitochondria. Moreover, this
genome-wide analysis identified a number of gene products with unknown function which
are currently subjected to detailed molecular analysis addressing PE homeostasis in the yeast.
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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 understand cellular PE homeostasis in some more detail, we performed experiments
defining traffic routes of PE within the yeast cell. In recent studies, we investigated
peroxisomes and the plasma membrane as destinations for PE distribution. 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 and demonstrated that PE
formed though all four pathways 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 PE 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.