Growth, Differentiation and Sexuality

Fig. 5.9. Elongation of the mannan chain in S. cerevisiae
(adapted from Dean 1999; Stolz and Munro 2002). N, n
indicate x mannose (protein names are in capital italics)
Fungal Cell Wall 87
Mannancanbelinkedtootherpolysaccharides
such as side chains of galactose in A. fumigatus or S.
pombe (Latgé et al. 2005). No enzymes responsible
for the synthesis of these heteropolymers have been
identified in fungi.
IV. Polysaccharide Remodelling
A. Polysaccharide Hydrolysis
In S. cerevisiae, only two chitinase genes (CTS1 and
CTS2) have been identified (King and Butler 1998).
These hydrolases are not essential, but disruption
of the encoding genes leads to defects in cell separation
(Kuranda and Robbins 1991). In C. albicans,
four chitinase genes have been identified among
which only three are expressed. The role of each
individual chitinase has not been totally explored,
since only cht2, cht3 mutants and the double cht2/3
mutant have been obtained to date. Up-regulation
of chitinase activity is seen during yeast-hypha
morphogenesis, a result reinforcing the view that
chitinases have an active role during the growth of
this fungus (Selvaggini et al. 2004). In A. fumigatus,
the situation is more complex, since its genome
contains 18 chitinase-encoding genes. Disruption
of a plant-type chitinase in Aspergillus nidulans
resulted in reduced growth and reduced germination
(Takaya et al. 1998), whereas disruption of
a bacterial-type chitinase in A. fumigatus has no
phenotype (Jacques et al. 2003). This suggests that
the plant-type chitinases have a role during fungal
growth and morphogenesis. An interesting feature
in A. fumigatus is that some of the plant-type chitinases
are GPI-anchored, data compatible with its
putative role in cell wall morphogenesis.
Inhibition of chitin hydrolysis could represent
an anti-fungal drug target, since inhibition of chitin
hydrolysis by allosamidin, a specific chitinase inhibitor,
blocks fungal growth (Papanikolau et al.
2003; Vaaje Kolstad et al. 2004). Cyclopeptide inhibitors
of chitinases have been recently identified
but their anti-fungal efficacy has not been investigated
yet (Houston et al. 2002; Rao et al. 2005).
β1,3 glucan-hydrolysing enzymes have been
also investigated, since β1,3 glucan is the most
abundant cell wall polysaccharide and the formation
of numerous free reducing and non-reducing
ends is necessary for the activity of β1,3 glucanosyltransferases.
In contrast to chitin-binding modules,
noβ1,3 glucan-binding motifs have been identified,
although a high number of glucanases have been