Growth, Differentiation and Sexuality

Whereas the structural branched β1,3 glucan and
chitin have been stable since the origin of fungi,
the composition of other polysaccharides such
as mannan has evolved continuously over time.
For example, if short mannan chains were part
of the protein N-glycan in ancient fungi, these
mannan chains evolved over time to become
a non-covalent coat on the yeast cell surface, and
then a constitutive component of the cell wall
in filamentous Ascomycetes (Chaffin et al. 1998;
Fontaine et al. 2000; Masuoka 2004).
If qualitative differences are seen between
species, quantitative differences have been also
noticed. For example, both yeasts and moulds have
chitin in their cell wall, but the amount of chitin in
the mould mycelial cell wall is much higher than
in yeast. This result is in agreement with the shape
of the two fungi: more beams are necessary to
support the structure of a tube such as a mycelium
than that of a balloon such as yeast. Accordingly,
since chitin is thought to be responsible for holding
together the cell wall structure, higher amounts of
chitin are expected in mycelial fungi. In dimorphic
fungi, such as Candida albicans, hyphaecontain
more chitin than is the case for yeast (Chaffin et al.
1998). The number of components of yeast cell
walls seems less than that in filamentous fungi: in
Saccharomyces cerevisiae and the Hemiascomycetous
yeasts, no α1,3 glucans are present whereas
in Schizosaccharomyces pombe no chitin is found.
Variations in composition are also seen in different
fungal stages of one and the same species, such as
spore and vegetative mycelium, suggesting a tight
regulation of expression during the cell cycle. For
example, in the Mucorales, glucan is present in
the spore stage and absent from the mycelium
(Bartnicki-Garcia 1968). Chitin is present in
ascospores and absent from the yeast cell wall of
S. pombe (Perez and Ribas 2004). Chitosan is the
hallmark molecule of the ascospore cell wall of S.
cerevisiae and is absent in yeast cells (Coluccio
et al. 2004). Melanin covers the outer layer of most
conidia of the Ascomycetes but hyphae are hyaline
(Latgé et al. 1988, 2005).
C. Structural Organisation of the Cell Wall
Although significant variations occur in the composition
of the cell wall of different species, a general
scheme can be established, at least for the Ascomycetes
and Basidiomycetes, which represents
the vast majority of all fungi on earth. The fibril-
Fungal Cell Wall 77
lar skeleton of the cell wall is considered to be the
alkali-insoluble fraction, whereas the material in
which the fibrils are embedded is alkali-soluble. It
should be stressed that the linkages disturbed by
the alkali treatment have not been identified yet.
Figure 5.4 is an example of the polysaccharide composition
of Aspergillus and Saccharomyces, which
couldbeusedtorepresenttheputativeschematic
organisation of the cell wall of yeasts and moulds.
The central core of the cell wall is a branched
β1,3/1,6 glucan which is linked to chitin via a β1,4
linkage; 3 and 4% β1,6 glucosidic interchain
linkages have been described in S. cerevisiae
and A. fumigatus respectively (Manners et al.
1973a,b; Fleet 1985; Fontaine et al. 2000). This
core is present in most fungi and at least in
all Ascomycetes and Basidiomycetes, but is
differently decorated depending on the fungal
species. In A. fumigatus, itiscovalentlyboundto
a linear β1,3/1,4 glucan with a [3Glcβ1-4Glcβ1]
repeating unit, and a branched galactomannan
composed of a linear αmannan with a repeating
mannose oligosaccharide unit [6Manα1-2Manα1-
2Manα1-2Manα] and short chains of β1,5
galactofuranose residues (Fontaine et al. 2000). In
S. cerevisiae, the structure of the alkali-insoluble
fraction has not been totally elucidated, but
the data available suggest that in addition to
chitin, β1,6 glucan is bound to the branched
βglucans.
It has been known since the pioneering studies
of Sietsma and Wessels (1979, 1981) that the
cross-linking between β1,3 glucan and chitin is
essential for the formation of a resistant fibrillar
skeletal component in most Ascomycetes and Basidiomycetes.
Identification of the linkage between
β1,3 glucan and chitin was done later by the group
of E. Cabib in S. cerevisiae, andconfirmedinAspergillus
fumigatus (Kollar et al. 1995; Fontaine
et al. 2000). The terminal reducing end of the chitin
chain is attached to the non-reducing end of a β1,3
glucan chain by a β1,4 linkage. Older studies in
Candida albicans have suggested that chitin and
β1,3 glucan can be also linked through a glycosidic
linkage at position 6 of GlcNAc (Surarit et al.
1988). The presence of chitin is, however, not always
required for an organised cell wall, as seen in
S. pombe. In the latter species, the alkali-insoluble
fraction is composed of linear β1,3 glucan bound
toahighlybranchedβ1,6 glucan with β1,3 linked
glucosyl branches at almost every glucose residue
and α1,3 glucan chains which are too long to be
solubilized by alkali (Sugawara et al. 2004).