Growth, Differentiation and Sexuality

kDa carboxy-subunit appears only after activation,
and this activation is abrogated when the catalytic
cysteine 297 of YCA1 is mutated. As a consequence
of caspase activation, apoptotic cell death is
induced in yeast. Increased cell death can be
prevented by the caspase-specific inhibitor zVADfmk
(z-Val-Ala-Asp(Ome)-fluoromethyl ketone).
It has been suggested that YCA1p is the
executor for cell death induced by a wide range of
apoptotic stimuli, as disruption of YCA1 abrogates
hydrogen peroxide-, acetic acid-, or age-induced
apoptosis (Madeo et al. 2002).
Two metacaspases have also been found in
Aspergillus fumigatus,andtheiractivityiselevated
when cultures enter the stationary phase (Amin
et al. 2003). Two caspase-like (caspase 3 and
caspase 8) activities have also been identified in
A. nidulans during sporulation. These caspase-like
activities are inhibited by the caspase-3-specific
peptide inhibitor DEVD-fmk (aspartyl-valylalanyl-aspartyl-fmk)
and caspase-8-specific
peptide inhibitor IETD (Ile-Glu-Thr-Asp)-fmk
(Thrane et al. 2004). As in animal cells, the DNA
repair enzyme poly (ADP-ribose) polymerase
(PARP) is cleaved by caspase 3 during apoptosis.
From database searches in the A. nidulans genome,
two genes for metacaspases and one gene for the
PARP have been found. Here the PARP is most
likely a substrate for caspase-like activity in A.
nidulans. The same genes have also been found
in the N. crassa genome. However, no orthologs
of mammalian caspases have been found in these
fungi (Thrane et al. 2004).
F. Other Regulators of Apoptosis
The Bcl-2 family proteins are regulators of apoptosis
in mammalian cells. The proapoptotic Bax
and Bak contain BH1, BH2 and BH3 where BH3
is critical as the death domain (Zha et al. 1996;
Harris and Thompson 2000). The Rad9 protein of
S. pombe (SpRad9) is a DNA-damage checkpoint
protein, but it also contains a group of amino acids
with similarity to the BH-3 death domain. It acts
like Bax, it interacts with Bcl-2 and Bcl-2xL, and it
induces apoptosis in mammalian cells, all of which
require the intact BH-3 domain, as demonstrated
by Komatsu et al. (2000). These authors suggested
that “SpRad9 may represent the first member of the
Bcl-2 protein family in yeast”.
Apoptosis-inducing f actor (AIF) isanapoptogenic
mitochondrial intermembrane protein
that acts beyond or independently of Bcl-2 and
Programmed Cell Death 171
caspase pathways in mammalian cells (Susin
et al. 1999). A yeast homolog, Aif1p, has been
found showing 22% identity and 41% similarity
with human AIF (Wissing et al. 2004). Like the
mammalian AIF, the yeast Aif1p is localized
within the mitochondrial membranes, and it is
translocated to the nucleus upon induction of
apoptosis. It induces apoptosis, and it has a DNase
activity that requires proper cofactors, i.e., Mg ++
and Ca ++ (Wissing et al. 2004). Its apoptogenic
function requires cyclophilin (Cyp1), just as does
AIF of mammalian cells (Cande et al. 2004).
Bax inhibitor-1 (BI-1) is an antiapoptotic protein;
it is highly conserved in mammals, Drosophila,
plants, and has now been found in the budding
yeast. Besides being a Bax-suppressor, yBI-1 can
partially rescue yeast from cell death induced by
oxidative stress and heat shock, and the deletion of
a C-terminal domain of yBI-1 abrogates this death
protection,demonstratingastructure–functionrelation
(Chae et al. 2003).
Inhibitor of apoptosis (IAP) familyproteins
are highly conserved suppressors of apoptosis (reviewed
in Deveraux and Reed 1999). These are characterized
by a ∼70 amino acid Baculoviral IAP repeat
(BIR)domainfirstdiscoveredinthegenomeof
baculoviruses (Birnbaum et al. 1994). IAP-like proteins
containing BIR motifs have also been found
in yeasts. The gene BIR1 is required for efficient
completion of meiosis in S. cerevisiae. Thisgene
contains a putative nuclear localization sequence
(KKKRKFKR 393), and BIR1-GFP is localized to
the nucleus (Uren et al. 1999). The BIR1p is required
for cell division, and it interacts with kinetochore
proteins in controlling chromosome segregation
events (Uren et al. 1999; Yoon and Carbon
1999; Li et al. 2000). It is interesting that a ts-skp1-4
mutant has a pronounced chromosome segregation
defect, and it can be rescued by over-expression of
BIR1p as well as by a 3 ′ -BIR1 fragment (C405),
but not by a 5 ′ -BIR motifs-containing fragment
(Yoon and Carbon 1999). Thus, the BIR motifs
play no part in chromosome segregation fidelity,
and their function is still unclear. So far, BIR motif
has not been linked to apoptosis as is found in
higher eukaryotes, and thus remains to be discovered.
The mammalian HtrA2 (high-temperature
resistance A) protein is a mediator of apoptosis by
its ability to antagonize the IAP protein, such as
human XIAP. The S. cerevisiae HtrA-like protein
has been identified recently in ORF YNL123W,
which codes for a ∼111 kDa protein (Fahrenkrog