Yno1p/Aim14p, a NADPH-oxidase ortholog, controls extramitochondrial reactive oxygen species generation, apoptosis, and actin cable formation in yeast.

The large protein superfamily of NADPH oxidases (NOX enzymes) is found in members of all eukaryotic kingdoms: animals, plants, fungi, and protists. The physiological functions of these NOX enzymes range from defense to specialized oxidative biosynthesis and to signaling. In filamentous fungi, NOX enzymes are involved in signaling cell differentiation, in particular in the formation of fruiting bodies. On the basis of bioinformatics analysis, until now it was believed that the genomes of unicellular fungi like Saccharomyces cerevisiae and Schizosaccharomyces pombe do not harbor genes coding for NOX enzymes. Nevertheless, the genome of S. cerevisiae contains nine ORFs showing sequence similarity to the catalytic subunits of mammalian NOX enzymes, only some of which have been functionally assigned as ferric reductases involved in iron ion transport. Here we show that one of the nine ORFs (YGL160W, AIM14) encodes a genuine NADPH oxidase, which is located in the endoplasmic reticulum (ER) and produces superoxide in a NADPH-dependent fashion. We renamed this ORF YNO1 (yeast NADPH oxidase 1). Overexpression of YNO1 causes YCA1-dependent apoptosis, whereas deletion of the gene makes cells less sensitive to apoptotic stimuli. Several independent lines of evidence point to regulation of the actin cytoskeleton by reactive oxygen species (ROS) produced by Yno1p.

The role of mitochondria in the aging processes of yeast.

This chapter reviews the role of mitochondria and of mitochondrial metabolism in the aging processes of yeast and the existing evidence for the "mitochondrial theory of aging mitochondrial theory of aging ". Mitochondria are the major source of ATP in the eukaryotic cell but are also a major source of reactive oxygen species reactive oxygen species (ROS) and play an important role in the process of apoptosis and aging. We are discussing the mitochondrial theory of aging mitochondrial theory of aging (TOA), its origin, similarity with other TOAs, and its ramifications which developed in recent decades. The emphasis is on mother cell-specific aging mother cell-specific aging and the RLS (replicative lifespan) with only a short treatment of CLS (chronological lifespan). Both of these aging processes may be relevant to understand also the aging of higher organisms, but they are biochemically very different, as shown by the fact the replicative aging occurs on rich media and is a defect in the replicative capacity of mother cells, while chronological aging occurs in postmitotic cells that are under starvation conditions in stationary phase leading to loss of viability, as discussed elsewhere in this book. In so doing we also give an overview of the similarities and dissimilarities of the various aging processes of the most often used model organisms for aging research with respect to the mitochondrial theory of aging mitochondrial theory of aging.

Quantitation of (a)symmetric inheritance of functional and of oxidatively damaged mitochondrial aconitase in the cell division of old yeast mother cells.

Asymmetric segregation of oxidatively damaged proteins is discussed in the literature as a mechanism in cell division cycles which at the same time causes rejuvenation of the daughter cell and aging of the mother cell. This process must be viewed as cooperating with the cellular degradation processes like autophagy, proteasomal degradation and others. Together, these two mechanisms guarantee survival of the species and prevent clonal senescence of unicellular organisms, like yeast. It is widely believed that oxidative damage to proteins is primarily caused by oxygen radicals and their follow-up products produced in the mitochondria. As we have shown previously, old yeast mother cells in contrast to young cells contain reactive oxygen species and undergo programmed cell death. Here we show that aconitase of the mitochondrial matrix is readily inactivated by oxidative stress, but even in its inactive form is relatively long-lived and retains fluorescence in the Aco1p-eGFP form. The fluorescent protein is distributed between old mothers and their daughters approximately corresponding to the different sizes of mother and daughter cells. However, the remaining active enzyme is primarily inherited by the daughter cells. This indicates that asymmetric distribution of the still active enzyme takes place and a mechanism for discrimination between active and inactive enzyme must exist. As the aconitase remains mitochondrial during aging and cell division, our findings could indicate discrimination between active and no longer active mitochondria during the process.