Growth, Differentiation and Sexuality

These authors concluded that a high ATP/ADP ratio
of yeast is not necessary for Bax-induced growth
arrest, but is required for a rapid Bax-induced cell
death (Priault et al. 1999a,b).
E. Mitochondrial Fission and Apoptosis
Mitochondrial fission and fusion are normal
mitochondrial function (Bossy-Wetzel et al. 2003).
However, mitochondrial fission or fragmentation
leading to damages to mitochondrial integrity is
a key step in apoptosis in mammals and C. elegans
(Bossy-Wetzel et al. 2003; Lee et al. 2004; Jagasla
et al. 2005). Key players are dynamin-like GTPase
family proteins, namely, yeast Dnm-1, its homolog
Drp-1 (dynamin-related protein 1) of mammals
and C. elegans; yeast Mgm-1 (mitochondrial
genome morphology 1), its mammalian ortholog
Opa-1 (optic atrophy 1) and mammalian Mfn-1
(mitofusion 1). These factors have opposing
effects, Dnm-1/Drp-1 for mitochondrial fission
and Mgm1/Opa-1 for fusion (Jones and Fangman
1992; Bossy-Wetzel et al. 2003; Sesaki et al. 2003;
Wong et al. 2003; Sesaki and Jensen 2004; Lee
et al. 2004). The function of Mgm1 in yeast
requires Fzo1 (for Fuzzy onion) and Ugo1 (ugo
is Japanese for fusion), a defect in any one of
which leads to damages to mtDNA and cell death
(Sesaki et al. 2004). Other players are Fis-1 and
Mdv1/Net2 (for mitochondrial division or networks),
the latter being a WD40 motif containing
dynamin-interacting family protein (Cerveny
et al. 2001; Cerveny and Jensen 2003; Lee et al.
2004; Fannjiang et al. 2004). Yeast yFis-1 is an
18-kDa protein that is anchored diffusely to the
mitochondrial outer membrane by a C-terminal
transmembrane domain characteristic of Bcl-2
(Mozdy et al. 2000). It probably mediates normal
mitochondrial fission in yeast, because Δfis-1 cells
exhibit interconnected mitochondrial net-like
morphology, as shown by fluorescent and electron
microscopy (Fannjiang et al. 2004). Interestingly,
yFis1p is antiapoptotic, as is mammalian Bcl-2,
because Δfis-1 cells are more sensitive to acetic
acid-induced cell death than are Δdnm1 cells
(Fannjiang et al. 2004). It is possible that mitochondrial
fission caused by yFis-1 is different
from that caused by Dnm-1 in yeast, the latter
leading to PCD. This above finding is in sharp
contrast to that found in mammalian cells where
hFis-1 is proapoptotic, and down-regulation of
hFis-1 powerfully inhibits cell death (Lee et al.
2004).
Programmed Cell Death 177
In the mammalian system, hFis-1p contains
signal sequence targeting it to the outer mitochondrial
membrane. Bax is normally located
in the cytosol, and only upon death signal is it
recruited to the mitochondrial outer membrane
(Lee et al. 2004). Here hFis-1p is required for
Bax translocation to the outer mitochondrial
membrane, and Bax for complexing with Drp-1
where Drp-1p is required for mitochondrial
fragmentation and cytochrome c release (Lee et al.
2004). The expression of the dominant negative
inhibitor of Drp-1, Drp-1 K38A , in HeLa cells prevents
mitochondrial fragmentation, and inhibits
apoptosis but does not prevent Bax translocation.
Bax, and the fission protein complex, are colocalized
to the constriction sites of mitochondrial
fission in dying HeLa cells, as demonstrated by
silver enhancement of immunogold labeling, and
examined by electron microscopy (Karbowski
et al. 2002).
Since there is no Bax or Bcl-2 in yeast,
a mediator will be needed. This is found in the
Dnm1-interacting factor, WD40 repeat protein
Mdv1/Net2, which is required for mitochondrial
fragmentation (Fannjiang et al. 2004). Interestingly,
there is no mammalian homolog to this
mediator. WD40 repeat protein Mdv1p/Net2p
functions as a molecular adaptor by interacting
with Dnm1p and yFis1p during mitochondrial
fission in yeast cells (Tieu et al. 2002). Mdv1/Net2
contains a novel N-terminal extension region
(NTE) that directly interacts with yFis1p, and
a C-terminal region that contains seven WD
repeats (WD) and directly interacts with Dnm1p.
By using GFP-conjugates, GFP-Mdv1p interacts
and co-localizes with Dnm1p in punctate
structures associated with mitochondria in
a yFis1p-independent manner, and this punctate
structure is essential for mitochondrial fission
function in yeast. Without Dnm1p, GFP-Mdv1p
is uniformly localized to the mitochondrial outer
membrane; without both Dnm1p and yFis1p,
GFP-Mdv1p is diffused in the cytosol (Tieu and
Nunnari 2000; Tieu et al. 2002). Yeast yFis1p
has two important regulatory functions, first,
to regulate assembly of Dnm1p into punctate
structures and target them to mitochondria,
and second, to regulate a rate-limiting, Dnm1pdependent
event during mitochondrial fission
(Tieu et al. 2002). yFis1p inhibits H2O2-induced,
and protease-dependent, cell death in yeast, much
like mammalian Bcl-2 or Bcl-xL (Fannjiang et al.
2004). Yeast yFis1p is not required for mitochon-