Introduction to Fungi, Third Edition

270 HEMIASCOMYCETES
template is unaffected during that process
(for details, see Haber, 1998). Hence, yeast cells
can repeatedly switch their mating type. There is
an element of selectivity in the mating type
switch because, for example, a-cells choose the
silent a-locus 85 90% of the time (Weiler &
Broach, 1992). In the case of a-cells, preference
for the switch to the a mating type is brought
about by the a2 protein (for details, see Haber,
1998).
10.2.3 The cell wall of S. cerevisiae
The wall of S. cerevisiae represents a considerable
biochemical investment, making up 15 30% of
the dry weight of vegetative cells. Up to three
wall layers can be distinguished by electron
microscopy. They differ in their chemical composition,
and the relatively simple architecture of
the wall of S. cerevisiae is considered a model for
other fungi (Molina et al., 2000; de Nobel et al.,
2001). The middle layer is electron-translucent
and consists of the main structural scaffold
of branched b-(1,3)-glucan molecules which bind
b-(1,6)-glucans and chitin. The latter, however, is
present only in low quantities (1 2% of the total
wall material) and it is unevenly distributed,
being concentrated in a ring around the region
where the bud emerges. The outer wall layer of
S. cerevisiae is electron-dense because it consists
mainly of proteins. These determine the
cell surface properties, including the porosity
(pore size) of the cell wall (Zlotnik et al., 1984)
and adhesiveness to other cells (flocculation; see
p. 274). The outer wall proteins may be highly
glycosylated in S. cerevisiae by the addition of
large mannose chains. In pathogenic yeasts such
as Candida albicans, this outer layer is also
important because of its involvement in attachment
of the fungus to its host, and because it
conveys antigenic properties. There are two main
groups of outer cell wall proteins (CWPs) in
S. cerevisiae. The members of one are modified
by a glycosylphosphatidylinositol (GPI) chain
which is indirectly linked to the b-(1,3)-glucans
of the central wall layer via the b-(1,6)-glucans.
These proteins are called GPI CWPs. The second
type of outer cell wall protein is called Pir CWP
(Pir ¼ protein with internal repeats) and is
linked directly to the b-(1,3)-glucan component
(Kapteyn et al., 1999; de Nobel et al., 2001). Both
GPI CWPs and Pir CWPs are structural
proteins. The innermost layer (periplasmic
space) is also electron-dense and consists of
proteins, but these are mostly enzymes which
are too large to pass through the central layer.
They are therefore restrained by the glucan layer
(de Nobel & Barnett, 1991).
The polarity of wall synthesis in S. cerevisiae
is controlled by the localization of the plasma
membrane-bound enzymes (glucan synthetases,
chitin synthetases) which produce the elements
of the middle layer, and by the secretion of the
structural outer wall proteins as well as enzymes
which cross-link the various elements of the
cell wall. Cell wall synthesis is thus regulated
spatially by the polarity of the yeast cell, and
temporally by the cell cycle; the transcription
of many genes involved in cell wall synthesis
is cell cycle-dependent (Molina et al., 2000;
Rodríguez-Peña et al., 2000).
10.2.4 Morphogenesis and the cell cycle
of S. cerevisiae
One oddity about S. cerevisiae is that it does
not require microtubules for the maintenance
of its cellular polarity, as shown by mutant
and inhibitor studies. In other eukaryotes,
including filamentous fungi and also the fission
yeast Schizosaccharomyces pombe, microtubules
are employed for long-distance transport processes.
It is possible that they are dispensible in
S. cerevisiae simply because of the small distance
between the mother cell and the growing
bud. Of course, microtubules are required in
S. cerevisiae as in all other eukaryotes for nuclear
division. Actin, in contrast, is crucial for cell
polarity and cell viability in S. cerevisiae (Pruyne
& Bretscher, 2000b; Pruyne et al., 2004).
The cell cycle of S. cerevisiae is similar to that
of Schizosaccharomyces pombe (see p. 256) and other
eukaryotes in its regulatory mechanisms (Lewin,
2000; Alberts et al., 2002), except that the G2
phase is lacking, and that cell division (cytokinesis)
is initiated early in the cycle, the bud being
already present during the S phase. A summary
of the budding process is given in Fig. 10.7.