Growth, Differentiation and Sexuality

80 J.P. Latgé and R. Calderone
A schematic view summarizing the localisation of
the proteins in the yeast and mould cell wall is
showninFig.5.5.
III. Polysaccharide Biosynthesis
A. Chitin
1. Chitin Synthases
Asmentionedabove,chitinisanessentialandthe
most ancestral structural polysaccharide in the cell
wall. A family of integral membrane proteins with
Mr of 100–130 kDa, called chitin synthases, are responsible
for the synthesis of linear chains of β1,4
N-acetylglucosamine from the substrate UDP-Nacetylglucosamine
(Valdivieso et al. 1997; Cabib
et al. 2001; Munro and Gow 2001; Roncero 2002).
The unique and essential Leloir pathway leads
to the synthesis of UDP-GlcNAc. All enzymes
of this pathway are essential in yeast. The first
enzyme in this pathway is the glucosamine-6phosphate
synthase (Gfa1p) which condenses
fructose-6-phosphate and glutamine to produce
glucosamine-6-phosphate and glutamate. It is considered
to be a limiting factor in chitin synthesis in
yeast (Lagorce et al. 2002). This enzyme has been
found in all fungi sequenced to date. Alternatively,
D-glucosamine-6-P and N-acetylglucosamine 6-P
Fig. 5.5. Putative organisation
of fungal cell wall proteins.
GPI (glycosylphosphatidyl
inositol) proteins
anchored to polysaccharides
or membranes are
extracted by hydrofluoric
acid (HF). GPI proteins on
thesurfaceareinvolvedin
cell–cell adhesion. Proteins
extracted by dilute alkali
are bound to the cell wall
by unknown linkages.
SDS-soluble proteins are
secreted proteins in transit
to the cell wall. Proteins
extracted by reducing
agents are linked by S–S
bridges
can be produced directly from D-glucosamine and
N-acetylglucosamine. When added extracellularly,
these aminosugars stimulate chitin synthesis
(Bulik et al. 2003). It has never been investigated
if the presence of > 15 chitinase and chitosanase
genes in the genome of A. fumigatus and other
moulds allow them to provide an intracellular
source of glucosamine and N-acetylglucosamine
for the synthesis of their own chitin (Latgé et al.
2005).
Six families of chitin synthases have been identified
among fungi based on amino acid sequence
analysis, among which three are specific for filamentous
fungi (classes III, V and VI; Bowen et al.
1992). The significance of each of these six classes is
not well understood, since mutations in members
of a common family do not always result in a similar
phenotype. Two groups of mutants can, however, be
identified: the first has reduced chitin content but
normal chitin synthase activity in vitro whereas the
second group is affected in enzyme activity but has
regular cell wall chitin content. The various CHS
genes are usually not redundant but instead perform
distinct and specific functions, even though
they have very homologous sequences. A motif QR-
RRWpresentinallCHSgenesfromdifferentfungi
has been proposed as a catalytic domain, since mutation
in this domain results in a loss of chitin
synthase activity (Nagahashi et al. 1995; Cos et al.
1998). Analysis of the common domains among the