Introduction to Fungi, Third Edition

272 HEMIASCOMYCETES
Following separation of the daughter cell,
a circular, crater-like bud scar is left as a
permanent mark on the surface of the mother
cell (see Fig. 10.12). The maximum number of
scars that could be accommodated on the surface
of a yeast cell is about 100, suggesting that
individual yeast cells are not capable of unlimited
budding. Individual yeast cells age just like
other organisms, although the timing of death
is determined by complicated genetic factors
and the sum of metabolic energy expended
throughout the life of the yeast cell, rather
than the number of the bud scars per se
(Jazwinski, 2002).
Under certain environmental conditions
(notably nutrient deficiency) diploid and, to a
lesser extent, haploid cells of S. cerevisiae can
change their growth pattern from budding,
which produces heaps of cells only on the agar
surface, to the formation of pseudohyphae which
can grow into the agar. Pseudohyphae may be
of significance in the ecology of S. cerevisiae
because they allow the organism to spread over
and penetrate into substrates, and to assimilate
nutrients more readily (Gimeno et al.,
1992). Formation of pseudohyphae requires an
enhanced adhesion of the cells to each other and
an enhanced polarity of daughter cell growth.
Not surprisingly, the signalling events leading
to pseudohyphal growth are rather complex
(Palecek et al., 2002; Ceccato-Antonini &
Sudbery, 2004).
10.2.5 Membrane cycling in S. cerevisiae
An enormous amount of work has been done
to elucidate the secretory route in S. cerevisiae,
and a sizeable collection of temperature-sensitive
mutants with defects at different points of the
secretory route has been assembled (Schekman,
1992). Further, individual stepwise modifications
to proteins travelling the secretory route can be
identified, especially with respect to their
glycosylation pattern and proteolytic cleavage
of parts of the original polypeptide chain
(Graham & Emr, 1991). The export of proteins
starts with their synthesis in the rough endoplasmic
reticulum and continues with their
processing in a Golgi system. Along this route,
the proteins are modified by the addition of
glycosylation chains, and by the proteolytic
cleavage of signal sequences. Transport has
long been thought to occur by means of vesiclelike
carriers, and the biochemical events leading
to the budding of a vesicle from its source
and its fusion with the destination membrane
(e.g. ER ! Golgi) have been extensively characterized
(Rothman & Orci, 1992). However, it is
still unclear whether discrete vesicular carriers
are an obligate transport system in vegetative
yeast cells. An alternative is the dynamic maturation
model in which sheets of ER become
transformed into Golgi compartments which
gradually dilate and fragment into secretory
vesicles (Rambourg et al., 2001). Whatever their
initial history, secretory vesicles emerge from
the Golgi system (Baba & Osumi, 1987) and
migrate to the growing bud along actin cables
(Finger & Novick, 1998).
The cell membrane shows a high capacity
for endocytosis, i.e. the removal of excess membrane
material and the uptake of specific molecules
by membrane-bound receptors from the
liquid medium of the environment. The occurrence
of endocytosis has been controversial
in filamentous fungi, but it has been obvious
for some time that this must take place in
S. cerevisiae as it is the route through which
mating hormones are internalized and transported
to the vacuole for degradation. While
actin is certainly involved in endocytosis, it is
still unclear whether the actin patches long
known to exist inside the plasma membrane of
S. cerevisiae cells are the scaffold around which
the inward-budding of the plasma membrane is
moulded (Shaw et al., 2001). Endocytosis occurs
when pits are formed at the plasma membrane
and bud inwards to form small vesicles (endocytotic
vesicles) which fuse to form a tubular
early endosome. From there, material is transported
via a late endosome to the vacuole in
which it is degraded (Munn, 2000; Shaw et al.,
2001). The protein ubiquitin plays a vital role as
a tag for endocytosis at the plasma membrane
and for transport of endosomes to the vacuole
(Horák, 2003). The purposes of endocytosis could
include the removal of excess membrane material,
the removal of nutrient uptake systems