Growth, Differentiation and Sexuality

172 B.C.K. Lu
et al. 2004). It is localized in the nucleus and
is named Nma111p (for nuclear mediator of
apoptosis). Over-expression of Nma111 in yeast
brings about cell death with all the hallmarks of
apoptosis, while nma111 and yca1 null mutant
control cells are TUNEL negative and exhibit no
accumulation of ROS (Fahrenkrog et al. 2004).
Nma111p belongs to the HtrA family of serine
proteases, and its proapoptogenic property depends
on its serine protease activity. No cell death
occurswhenserine235ismutatedtocysteineby
site-directed mutagenesis (Fahrenkrog et al. 2004).
Since Nma111p and BIR1p are both localized in
the nucleus, it remains to be discovered whether
the two interact in any way.
III. Heterokaryon Incompatibility
The first case of programmed cell death in fungi
extensively investigated is known as heterokaryon
incompatibility, also termed vegetative incompatibility.
N. crassa and P. anserina are the main model
systems used (reviewed in Glass et al. 2000; Glass
and Kaneko 2003). The genes that regulate this nonself
recognition are named het (for heterokaryon
incompatibility) and vic (for vegetative incompatibility)
loci (for reviews, see Glass et al. 2000; Saupe
et al. 2000; Glass and Kaneko 2003).
To understand the mechanism of incompatibility
reaction and the genes involved, a het-R het-V
self-incompatible strain of P. anserina has been
used as a model system. This strain includes two incompatible
genes (het-R and het-V) in the haploid
nucleus, and it behaves like a temperature-sensitive
mutant. At the permissive temperature (32 ◦ C), it
grows normally like the wild type whereas at the
restrictive temperature (26 ◦ C) incompatibility reaction
is triggered in all cells, making it possible
for unequivocal recognition (Labarère 1973). With
this system, genes that are highly induced during
incompatibility reaction have been identified
and isolated, and these are named idi (induced
during incompatibility) genes (idi-1, idi-2, idi-3,
idi-4, idi-6 and idi-7). These genes are also induced
two- to tenfold by carbon or nitrogen starvation,
indicative of their involvement in autophagy, and
by rapamycin, which inhibits the TOR (target of
rapamycin) kinase pathway (Bourges et al. 1998;
Dementhon et al. 2003). The idi-6 and idi-7 are
both directly involved in incompatibility cell death
as well as in autophagy, and they may provide
a link to type II programmed cell death by autophagy
(Paoletti et al. 2001; Pinan-Lucarré et al.
2003). Interestingly, idi-4 turns out to be a basic
leucine zipper (bZIP) transcription factor that carries
a DNA binding site (Dementhon et al. 2004;
Dementhon and Saupe 2005). Its expression turns
on idi-2, idi-7 and idi-4, and causes autophagic cell
death (Dementhon et al. 2004).
For hyphal compartmentalization and death
(HCD), vegetative incompatibility may recruit the
autophagic pathway in which the formation and
accumulation of autophagic vesicles in the vacuole
occur, and in which final degradation of these autophagic
bodies takes place by vacuolar proteases.
The strong evidence that links the idi-6/pspA (initially
identified as P. anserina serine protease A)
gene to autophagic cell death came from observations
that inactivation of the PSPA vacuolar protease
prevents degradation of autophagic bodies in
the vacuole (Dementhon et al. 2003; Pinan-Lucarré
et al. 2003). Also, both pspA and idi-7 null mutants
can survive or delay cell death (reviewed by Pinan-
Lucarré et al. 2003). On the other hand, vegetative
incompatibility may also recruit the apoptotic
pathway to achieve cell killing, as recently demonstrated
by TUNEL assay in N. crassa showing DNA
fragmentation associated with nuclear pyknosis as
well as cytoplasmic vacuolation (Marek et al. 2003).
IV. Ubiquitin-Proteasome System
and Apoptosis
During cell-cycle progression, many intracellular
proteins are synthesized and turned over by a cascade
of events in the cytosol. This is achieved by
theubiquitin-proteasomepathway,andnotbythe
lysosomal pathway.Proteinsthataretobedegraded
are first tagged by an addition of multiple ubiquitin
molecules, and the polyubiquitinated proteins are
then transferred to the 26S proteasome for degradation
(Schreader et al. 2003; Yen et al. 2003; see
also Finley and Chau 1991 for review).
The 26S proteasome of S. cerevisiae is an ATPdependent
multicatalytic-multifunctional protease
complex (Glickman et al. 1998; Naujokat and Hoffmann
2002 for review). It is made up of two subunits:
a cylindrical 20S catalytic core particle (CP),
and two 19S regulatory particles (RP), one on each
side of the CP. Several CP proteolytic subunits (Pre1
to Pre4) have been identified (Heinemeyer et al.
1991; Gerlinger et al. 1997). Polyubiquitinated sub-