Growth, Differentiation and Sexuality

176 B.C.K. Lu
Fig. 9.1. Effects of a carbon source on Bax-induced
programmed cell death in WtB1 (wild-type), AtB1 (ANCdefective),
and CtB1 (cytochrome c-deficient) mutants of
yeast: squares glucose (fermentative), triangles mannose
(respiro-fermentative), circles lactate (non-fermentative).
Reproduced, with permission from the authors and the
publisher, from Priault et al. (1999a)
D. Cytochrome c Release and Transmembrane
Potentials in Apoptosis
In multicellular organisms, an early symptom of
apoptosis is the release of cytochrome c from the
mitochondrial membrane space to the cytosol.
This leads to the activation of the caspases, and
a cascade of apoptotic events (see reviews in
Mignotte and Vayssiere 1998). The release of
cytochrome c from mitochondria is also found in
apoptotic cell death induced by acetic acid in S.
cerevisiae (Ludovico et al. 2002), in yeast mutant
asf1/cia1 lacking histone chaperone (Yamaki et al.
2001), and in Bax-expressing yeast cells (Manon
et al. 1997). During acetic acid-induced PCD in the
budding yeast, the mitochondrial transmembrane
potential exhibits a transient hyperpolarization
followed by depolarization (Ludovico et al. 2002).
A decrease of mitochondrial transmembrane
potential (by 50%) is also found in acetic acidinduced
PCD in the food spoilage yeast Z. bailii
(Ludovico et al. 2003), and in yeast lacking the
histone chaperone ASF1/CIA1 (Yamaki et al. 2001).
In one report, however, Bax-expressing yeast
cells exhibit an increase (or hyperpolarization) in
mitochondrial transmembrane potential, ΔΨm,
with no release of cytochrome c (Gross et al. 2000).
The conflicting results remain unresolved. One
possible suggestion could be related to the methodology
of measuring transmembrane potential. The
controversy has been reviewed (Ly et al. 2003).
It is clear, however, that release of cytochrome
c into the cytosol is not essential for Bax-mediated
cell killing in yeast (Gross et al. 2000; Roucou et al.
2000). This is demonstrated by the observation
that, upon the expression of Bax in yeast cells, the
cytochrome c-GFP fusion protein is not released,
while the native cytochrome c is, suggesting that
size is critical for its release through the selective
pore. Nevertheless, cell killing occurs in both cases
(Roucou et al. 2000). The question of mitochondrial
permeability transition pore (PTP) remains
controversial. For Bax-mediated apoptosis in yeast,
the mitochondrial F0F1-ATPase proton pump is
required, and its inhibitor, oligomycin, partially
inhibits Bax-induced cell death (Matsuyama et al.
1998). Other mitochondrial biochemistry (e.g.,
Δatp2, Δatp4, Δcyc1, 7; Δcyp3, Δpor1/2; Š-) may
alsoplayaroleincellkilling,albeitwithvariable
results (evidenced in Gross et al. 2000).
As pointed out above, the release of cytochrome
c from mitochondria is often associated with PCD,
but massive cytochrome c relocalization appears
nottobeabsolutelyrequiredforBax-inducedcell
killing in yeast (Priault et al. 1999a). Indeed, Baxinducedcellkillingisthesameinthecytochrome
c-less mutant as in the wild type, indicating that
cytochrome c is not required for PCD. However,
as demonstrated with the yeast cytochrome c-less
strain, the kinetics of colony-forming efficiency is
identical whether glucose or mannose is used as
the carbon source (CtB1, Fig. 9.1). This is contrary
to what is observed in the wild-type yeast (WtB1,
Fig. 9.1); there is no rapid killing in the mutant
CtB1 when mannose is used as the carbon source.
This would indicate that cytochrome c has a role
in the rapid killing process in yeast, as contended
by Priault et al. (1999a). After all, cytochrome c
is a part of the mitochondrial respiration chain,
and ATP is required for an optimal effect of Bax.