Growth, Differentiation and Sexuality

strates recognized by the proteasome are first deubiquitinated,
and then unfolded before they are
translocated into, and degraded by the 20S proteolytic
core (Berndt et al. 2002; Naujokat and Hoffman
2002).
The ubiquitin-proteasome system may be involved
in the regulation of a wide variety of cellular
functions, such as DNA repairs, cell-cycle control,
stress response, and apoptosis. The proteasome
may have proapoptotic or antiapoptotic functions,
depending on which target protein is degraded. If
the target is an inhibitor of apoptosis, such as IAP
or Bcl-2, it is proapoptotic, while if the target is an
inducer of apoptosis, such as Bax or a caspase, it
is antiapoptotic. Studies in mammalian cells have
shownthatinhibitionofproteasomefunctionsby
a proteasome-specific inhibitor can lead to apoptosis.
It can also rescue cells from apoptosis, depending
on the proliferative state of the cell, suggesting
that the ubiquitin-proteasome pathway may be
linked to apoptosis (for reviews, see Drexler 1998;
Orlowski 1999; Naujokat and Hoffmann 2002).
The discovery of apoptosis-like phenotypes
induced by a gene mutation in Cdc48 of the
yeast S. cerevisiae has provided a link between
the ubiquitin-proteasome system and apoptosis.
Cdc48 is a member of the family of AAA-ATPases;
it is a homolog of the mammalian p97 (or VCP)
and of MAC-1 of C. elegans. It is localized in the cytosol,
the nucleus and the endoplasmic reticulum
(ER). In conjunction with different cofactors,
Cdc48/p97 can perform different cellular tasks.
The Cdc48-Ufd1-Npl4 complex functions to move
polyubiquitinated polypeptides that are improperly
assembled or misfolded from the ER into the
cytosol, for their subsequent degradation by the
proteasome (Ye et al. 2003). This retrotranslocationpathwayisaformofqualitycontrolwhereby
misfolded or defective polypeptides are eliminated
(Ellgaard and Helenius 2001). The failure in
this process, as in the case of the yeast mutant
cdc48 S565G , leads to PCD (Madeo et al. 1997).
Cell proliferation and cell death are two opposing
processes that are highly regulated to maintain
the integrity of the organism, and their respective
effector is continuously produced and degraded
depending on the status of the cell. It has been
suggested that in the proliferation pathway, the activator
of apoptosis being produced is identified by
ubiquitination, and the polyubiquitylated proteins
are rapidly degraded by the 26S proteasome. In S.
cerevisiae, one such proteasomal substrate, Stm1,
has been identified to participate in apoptosis-like
Programmed Cell Death 173
cell death. Over-expression of Stm1 in defective
yeast pre1-1 pre4-1 mutant background leads to
cell-cycle arrest and cell death with the phenotypes
of apoptosis. Stm1 is an in vivo substrate of the
proteasome. This is demonstrated by the cycloheximide
chase experiment where Stm1 proteins disappear
in time in the wild-type cells whereas they
arestableinthepre1-1 pre4-1 mutantcells.Itshould
be noted that cells lacking Stm1, as in stm1-1,survive
a low-concentration H2O2 treatment, in contrast
to the wild type. Taken all together, Stm1 is
deemed to be an activator of apoptosis in yeast;
it is normally degraded by the proteasome under
the proliferation pathway. When the proteasome is
suppressed, Stm1 is allowed to accumulate in quantities
that lead to apoptosis (Ligr et al. 2001). How
theproteasomeissuppressedinvivoisunclear.
V. Mitochondria, Oxidative Stress,
and Regulation of Apoptosis
The evolution of eukaryotic cells, by incorporating
the mitochondria from an aerobic prokaryon
into the power-generating system, has an enormous
benefit in terms of energy production, but
it comes with the cost of oxidative stress in the generation
of ROS. Indeed, the mitochondrial respiratory
chain is the main producer of ROS (reviewed
in Green and Reed 1998; Mignotte and Vayssiere
1998). To overcome this oxidative stress, a slate
of antioxidant molecules has also evolved, such as
glutathione, superoxide dismutases, catalases and
peroxidases. Furthermore, to maintain the homeostasis
of a cell population, either in multicellular
metazoans or in unicellular fungi, a system of PCD
has also evolved for which modulation of ROS becomes
the center for regulation of PCD.
A. Role of Mitochondria in Apoptosis
So, what is the role of mitochondria in PCD?
It may be to sequester the apoptosis effector
proteins, such as cytochrome c, AIF (apoptosisinducing
factor) or pro-caspases, etc., within the
mitochondrial membrane space. In the model
systems, such as C. elegans and mammalian cells,
these proteins are normally not permanent to
the outer membrane. In the process of PCD,
these mediators of apoptosis are released to
the cytosol either through the regulation of
outer mitochondrial membrane permeabilization