Growth, Differentiation and Sexuality

88 J.P. Latgé and R. Calderone
identified in fungal genome sequences. Molecular
studies have been mostly centred on endoglucanase,
especially since its mode of action (endosplitting
activity efficient on complex polysaccharide
structure) suggested that these glycosylhydrolases
could play a morphogenetic role.
Work in yeast has shown that, like chitinases,
endo β1,3 glucanases are required for cell separation
(Baladron et al. 2002; Martin-Cuadrado et al.
2003). In A. fumigatus, molecular studies have been
exclusively centred on the only endo β1,3 glucanase
identified to date (Eng1p). In spite of its cell wall
localisation, constitutive expression occurs at all
growth stages, which is independent of the presence
of an insoluble and soluble β1,3 glucan substrate in
the culture medium, the lack of phenotype for the
null engl1 mutant suggesting that this endo β1,3
glucanase activity does not play a morphogenetic
role in A. fumigatus (Mouyna et al. 2002). However,
although only one enzyme has been analysed,
the genome of A. fumigatus contains > 20 putative
glucanases which have to be studied before a role
for endo β1,3 glucanase in fungal growth can be
demonstrated.
α1,3 glucanase has been shown to have a role
in cell division in S. pombe. Twoα1,3 glucanase
genes AGS1 and AGS2 have been identified in S.
pombe (Dekker et al. 2004). Deletion of AGS1 results
in clumped cells which remained attached to
each other. It has been proposed that the endo α1,3
glucanase Agn1p acts in concert with the endo β1,3
glucanase Eng1p to achieve efficient cell separation.
AGS2 is essential for endolysis of the ascus wall during
maturation. Similarly, in A. nidulans, anα1,3
glucanase is expressed during sexual development.
Cleistothecia were, however, formed in the mutant
(Wei et al. 2001). This result is not surprising, taking
into account the high number of redundant α1,3
glucanases identified in the genome of Aspergillus
(for example, nine in A. fumigatus).
Expression of glucanases and chitinases is regulated
by a cascade of transcription factors including
Fkh1/2p and Ace2p (Dohrmann et al. 1992; Zhu
et al. 2000). Moreover, recent work (Santos et al.
2003) suggests that Rho4p could be involved in
theregulationofcellwalldegradationinS. pombe.
These data demonstrate that different RHO proteins
regulate different glucan synthases and hydrolases.
B. β1,3 Glucan Branching
and Cross-Linking Enzymes
The chronological steps in the synthesis of the core
structural polysaccharides in the fungal cell wall
are depicted in Fig. 5.10. β1,3 glucan chains produced
by the β1,3 glucan synthase complex remain
unorganised and alkali-soluble until covalent linkages
occur between β1,3 glucans and other cell wall
components. To date, no transglycosidases have
been identified bioinformatically or biochemically
which can achieve the branching of β1,3 glucans
and the subsequent cross-linking of chitin and β1,3
glucan. Two β1,3 glucanosyltransferases have been
identified biochemically to date but neither displays
the branching activity and cross-linking activity
responsible for binding chitin and β1,3 glucans.
The first β1,3 glucanosyltransferase of A. fumigatus
(Bgl2p/Bgt1p) present in both yeasts and
moulds cleaves laminaribiose from the reducing
Fig. 5.10. Temporal events in the biosynthesis
of the structuralβ1,3 glucan-chitin
core. For simplification, only glucan synthase
is represented but chitin synthase
is active at the same time and at the
same location as glucan synthase. Linear
chains of glucan are modified by both
branching and cross-linking enzymes