Growth, Differentiation and Sexuality

eight chitin synthases of A. fumigatus has shown
twoclustersofthreeandfourgenesandasingleton
which are associated with chitin synthesis and
chitin synthase activities respectively (Latgé et al.
2005). A bioinformatic analysis of these two chitin
synthase families in A. fumigatus showed that most
of the motifs are found in both families but their
localisation in each family is different, suggesting
that the binding domain of the UDP GlcNac will
be the same and that the overall organisation of
the domains inside each protein would lead to different
outcomes (Latgé et al. 2005). This result fits
also with the differential sensitivity of the three
yeast CHS genes to nikkomycin X and Z and polyoxin
D, suggesting that the active-site domains of
the different CHS proteins differ appreciably (Cabib
1991).
Chitin biosynthesis is understood best in the
model yeast S. cerevisiae. Three chitin synthases
are responsible for the synthesis of S. cerevisiae
chitin (Fig. 5.6). Chs1p acts as a repair enzyme
during cell separation. It is a fairly stable protein
(Choi et al. 1994), and its levels do not change significantly
during the cell cycle (Ziman et al. 1996).
Fig. 5.6. Chitin synthesis in S. cerevisiae and the function of
the different synthase genes (adapted from Cabib et al. 2001)
Chitin is indicated by shading. A Chitin synthase 3 (CSIII)
catalyses the synthesis of chitin in the entire cell wall and
at the chitin ring. B Chitin synthase 2 (CSII) catalysesthe
synthesis of the primary septum chitin. C, D Completion of
the septum and separation of the daughter cell by a chitinase
(Chit) with extra chitin provided by chitin synthase 1 (CSI).
BS Bud scar
Fungal Cell Wall 81
Chs2p is responsible for septal chitin biosynthesis.
Like Chs1p, Chs2p activity was originally described
as zymogenic, but again there is no direct evidence
for this type of regulation in vivo. Chs2p activity
peaks just before cytokinesis (Choi et al. 1994). Expression
of this gene is strongly reduced during
mating and sporulation, two conditions in which
no primary septum is formed (Choi et al. 1994).
Chs2p is thought to be transported to the septum
site by the general secretory pathway, where it acts
in the formation of the primary septum (Shaw et al.
1991). Its function depends directly on the formation
of the actomyosin ring (Schmidt et al. 2002).
The non-zymogenic Chs3p is involved in the synthesis
of bulk chitin of the cell wall of the mother
cell and of the septum, and for the synthesis of
chitin as a response to cell wall stress. Chs3p levels
remain fairly constant inside the cell, mainly because
of a very extended protein half-life (Ziman
et al. 1996; Roncero 2002; Fig. 5.6).
In S. cerevisiae, noneoftheCHSgenesareessential,
although the cell wall of the triple mutant is
very thick, and its survival results from the acquisition
of suppressors (Schmidt 2004). This is in contrast
with C. albicans where the CHS family comprises
four genes, CHS1 (class II), CHS2 (class I),
CHS3 (class IV) and CHS8 (class I), while the class
II CHS1 is essential for cell viability (Munro and
Gow 2001; Munro et al. 2001).
The largest gene families of chitin synthases
are found in filamentous fungi, with up to 11 genes
in Aspergillus oryzae. The presence of the highest
number of genes in filamentous Ascomycetes is correlated
with a higher chitin content in the mould
cell wall. In A. fumigatus, eightgeneshavebeen
identified, among which six were inactivated. Mutants
with the most altered phenotype result only
from inactivation of CHSE and G genes belonging
to classes III and V (Mellado et al. 1996; Aufauvre-
Brown et al. 1997). The phenotypes of these mutants
include a reduction in hyphal growth, periodic
swellings along the length of hyphae, and
a block of conidiation which is partially restored
by growth in the presence of an osmotic stabilizer.
A double CHSE/CHSG disruptant has been
obtained, and the phenotype of the double mutant
is only additive (Mellado et al. 2003): the cell
wall still contains chitin (half of the concentration
of the parental strain) and the mutant still displays
zymogenic and non-zymogenic chitin synthase activities.
Although the understanding of the chitin
synthases in filamentous fungi remains very incomplete,
cellular specialization for the different