Growth, Differentiation and Sexuality

86 J.P. Latgé and R. Calderone
resulting in cell lysis and the loss of cell polarity
(Hochstenbach et al. 1998).
Three homologous AGS genes involved in α1,3
glucan synthesis have been identified in A. fumigatus
(Beauvais et al. 2004). Their sequences are very
homologous to the S. pombe α1,3 glucan synthase
genes but, in contrast to this yeast, all genes are expressed
during vegetative growth and none of the
AGS genes is essential in A. fumigatus. Inspiteof
the high homology between the three genes, a partial
reduction in α1,3 glucan was seen only in the
cell wall of the AGS1 mutant. Expression study suggested
that all three genes are involved in the synthesis
of α1,3 glucan of A. fumigatus, and each is
able to compensate for the lack of the other genes.
AGS1 seems to be the main responsible for synthesis
at the cell wall level, as confirmed by its cell wall
localisation.
Alterations in the α1,3 glucan content of other
human fungal pathogens affect their pathogenicity.
In C. neoformans, only AGS1 was found. The
ags1 mutant grows to a very limited extent at 37 ◦ C
and does not encapsulate, because the capsule is
anchored to the α1,3 glucan (Reese and Doering
2003). The lack of virulence in spontaneous α1,3
glucan mutants of Histoplasma capsulatum, Blastomyces
dermatitidis and Paracoccidioides brasiliensis
could be due to a higher sensitivity to phagocytes
due to the total lack of α1,3 glucan (San-Blas
et al. 1977; Hogan and Klein 1994; Klimpel and
Goldman 1988). By contrast, in A. fumigatus,where
the three genes are active and able to complement
themselves, a limited modification of the α1,3 glucan
content of the cell wall of the AGS mutants
of A. fumigatus was not associated with a reduction
in the virulence of this fungus (Beauvais et al.
2004).
α1,3 glucan synthesis is also under the regulationoftheRho2p-GTPasesbutitisadifferentRho
than that for β1,3 glucan synthesis. In S. pombe,
Rho2-GTP acts as a positive regulator of α1,3 glucan
synthase (Hirata et al. 1998; Calonge et al. 2000).
The proteins activating Rho2p are not known.
b) β1,6 Glucan Synthesis
β1,6 glucan constitutes approximately 10% of the
yeast cell wall. Many genes involved in β1,6 glucan
synthesis have been identified based on resistance
of mutants to the K1 killer toxin, which kills yeast
following binding to β1,6 glucan (Shahinian and
Bussey 2000). However, most of the genes identified
in these screens encode proteins located along the
secretory pathway from ER to plasma membrane
(Shahinian et al. 1998). None of these genes have
been associated with an enzymatic activity directly
responsible for β1,6 glucan synthesis. It seems indeed
that many mutations result in β1,6 glucan
synthesis alterations (Dijkgraaf et al. 2002; Machi
et al. 2004), but the role of the mutated genes in β1,6
glucan biosynthesis is only indirect (Abeijon and
Chen 1998; Breinig et al. 2004). KRE6, oneofthe
genes putatively involved in β1,6 glucan synthesis,
has been recently found in A. fumigatus, indicating
that its role is also indirect, since β1,6 glucan
does not exist in A. fumigatus. Onlyrecently,an
immunoassay for β1,6 glucan synthesis in vivo has
been developed. This assay shows that β1,6 glucan
synthesis is produced when UDP-glucose and
GTP are provided to a membrane preparation, and
that β1,6 glucan synthesis is under the regulation of
Rho1p, like β1,3 glucan synthesis (Vink et al. 2004).
C. Mannan Synthesis
High-MW mannan biosynthesis will be discussed
sinceonlythistypeiscellwallassociated(Fig.5.9).
The N-mannan chains are usually considered as
a “coating” component of the yeast cell wall. This
is true in the case of the peptidomannans of yeasts,
butnotsoinA. fumigatus and other moulds where
mannan (as galactomannan) is an essential component
of the cell wall and is bound covalently to
the other polysaccharides. Although the mannan
composition of the A. fumigatus cellwallisverydifferent
to that of yeast mannans, a comparative genomic
study has indicated that orthologues of most
yeast mannosyltransferase genes can be found in
the genomes of moulds. OCH1, the gene initiating
the synthesis of the long N-mannan chains in yeasts
(Dean 1999; Stolz and Munro 2002), is present in
the genome of A. fumigatus as a family of three
proteins, whereas it is unique in S. cerevisiae.Other
genes coding for mannosyltransferases responsible
for the synthesis of linear α1,6 and α1,2 mannan
in yeast have unique orthologues in A. fumigatus,
although the end products of this mannosyltransferase
complex will be structurally different in the
two fungal species. This result is in agreement with
the lack of specificity of the different gene products
shown in vitro (Bussey, personal communication)
and indicates a timely regulation, different
in the two species. Regulators or specific inhibitors
of high-MW mannan synthesis have not been reported
yet.