of cytochrome c but also a decrease in the amount of cytochrome c oxidase. They
have shown that the decrease of cytochrome c oxidase is due to the activation of the
mitochondrial AAA protease Ymelp, of which the cytochrome c oxidase subunit 2
(Cox2p) is a substrate. The absence of Ymelp slightly delays Bax-induced cell death,
suggesting a role of this protease in yeast cell death and probably the orthologous
function of its mammalian homolog.
Another mutation resulting in yeast apoptotic cell death is the deletion of ASF1/
CIA1, coding for a histone chaperone. The disruptant arrests preferentially at the G 2 /
M transition, and the cell dies showing markers of apoptosis. Moreover, reduction
of the mitochondrial membrane-potential, dysfunction of the mitochondrial proton
pump, and release of cytochrome c to cytoplasm occur (Yamaki et al., 2001). The
human homolog of Asf1p/Cia1p, CIA, interacts with the largest subunit of TFIID,
CCG1, which is involved in the regulation of apoptosis (Sekiguchi et al., 1995).
Schizosaccharomyces pombe rad9 is a checkpoint gene preventing cells containing
damaged or incompletely replicated DNA from entering mitosis. Rad9 protein
(SpRad9) contains a stretch of amino acids with similarity to the Bcl-2 homology 3
death domain, which is required for SpRad9 interaction with human Bcl-2.
Overexpression of bcl-2 in S. pombe inhibits cell growth independently of rad9, but
enhances the resistance of rad9-null cells to methyl methanesulfonate, and ultraviolet
and ionizing radiation. The authors suggest that SpRad9 may be the first member of
the Bcl-2 protein family to be identified in yeast (Komatsu et al., 2000).
Ceramide is another potential bona fide apoptosis regulator shared by mammals
and yeast. Some mammalian growth modulators, including tumor necrosis factor a,
induce apoptosis or cell-cycle arrest via ceramide, which activates a specific
phosphatase (Kishikawa et al., 1999). Ceramide-induced G1 arrest of S. cerevisiae is
also mediated via activation of a protein phosphatase (Nickels and Broach, 1996).
4.
ROS are central regulators of yeast apoptosisROS (reactive oxygen species) are well established as inducers of apoptosis. Treatment
with low doses of H 2 O 2 induces the apoptotic cascade in mammalian cell cultures.
In addition, in neural cells deprived of nerve growth factor or potassium, ROS
produced by the cell act as a late signal of the apoptotic pathway, downstream of the
action of Bax and caspases (Schulz et al., 1996). In S.cerevisiae, exposure to low doses
of H 2 O 2 or oxidative stress by glutathione depletion induces apoptosis. Inhibition of
translation by cycloheximide prevents development of apoptotic markers in response
to H 2 O 2 , indicating an active role of the cell in the death process.
Furthermore, increased formation of intracellular ROS occurs during yeast
apoptosis, even in the absence of external oxidative stress. Yeast expressing bax and
cdc48S565G mutants is strongly stained after treatment with dihydrorhodamine 123,
indicating an accumulation of ROS (Figure 1). The radicals appear to be necessary
to induce the apoptotic phenotype, as anaerobic growth conditions and radical traps
prevent both cell death and development of apoptotic markers (Madeo et al., 1999).
144 GENETICS OF APOPTOSIS