Growth, Differentiation and Sexuality

272 R. Fischer and U. Kües
and conidiophore number. Similarly to the case
for the MAP-kinases, it is not yet clear how the
COP9 signalling module interacts with regulators
of asexual development.
d) cAMP Signalling
Cyclic AMP is an important secondary messenger
in pro- and eukaryotic cells, and its level is tightly
controlled. Our knowledge of the role of cAMP in
fungi is most advanced in S. cerevisiae, Magnaporthe
grisea and U. maydis, inwhichcAMPsignalling
triggers development together with a MAPkinase
signalling module (see Chap. 18, this volume).
The central enzyme which catalyses the formation
of cAMP is adenylate cyclase, whose enzymatic
activity is regulated by an upstream Gprotein
and which regulates, through the cAMP
level, the activity of downstream protein kinase A
(PkaA). cAMP binding to the regulatory subunit of
PKA causes its dissociation from the catalytic subunit
(PKAc). Active PKAc controls protein activities
via phosphorylation of conserved Ser and Thr
residues. Initial work on asexual development in
A. nidulans already suggested that the cAMP level
could play a role (Clutterbuck 1975). However, it
was unclear whether the observed drop in cAMP
concentration at the onset of development was the
cause of conidiation, or whether this was due to
an independent phenomenon linked to metabolic
alterations. The adenylate cyclase gene, cyaA, has
recently been characterized, and deletion of the
gene caused severe germination phenotypes (Fillinger
et al. 2002). Because of the severe impact of
cyaA deletion on hyphal growth and colony development,
a specific involvement of cAMP in the
regulation of asexual development was not clearly
established in the study of Fillinger et al. (2002).
Better evidence for such an involvement came from
the analysis of the catalytic subunit of protein kinase
A, by deletion and overexpression of the gene
(Shimizu and Keller 2001). Interestingly, PkaA itself
appears to control the cAMP level (Fillinger
et al. 2002). Whereas the lack of the PkaA catalytic
subunit led to a reduced growth rate and a hypersporulation
phenotype, overexpression caused an
increase of aerial, fluffy mycelium (Shimizu and
Keller 2001). These phenotypes resemble to some
extent those of a fadA deletion or the overexpressionofadominantactivealleleofFadArespectively
(see above). These results suggest genetic interaction
between the FlbA-FadA signalling and
the PkaA signalling pathways.
e) Small G-Proteins
This large superfamily of proteins has received
some attention over the past years, because they
are known as important regulators in higher and
lower eukaryotes. The superfamily is divided into
several classes, three of which are the Rho-, Rasand
Rac-type proteins. Whereas Rho and Ras proteins
have been extensively studied in S. cerevisiae
and Schizosaccharomyces pombe, Rac proteins are
not found in these two yeasts, but rather in mycelial
fungi (Boyce et al. 2003). Small G-proteins are able
to bind GTP or GDP, and the GTP-bound form is
theactivespecies.Theproteincontainsanintrinsic
GTPase activity which converts triphosphate into
diphosphate. The balance between the GTP and
GDP form is crucial for signalling, and controlled
by a number of different proteins. In A. nidulans,
a Ras protein was identified by cross-hybridisation
to the S. cerevisiae homologue (Som and Kolaparthi
1994). Whereas deletion of the gene was
lethal, overexpression did not cause any obvious
phenotype. However, the role of the protein can be
studied by generating constitutively active or dominant
negative alleles. The first type of mutation can
be achieved by abolishing the endogenous GTPase
activity, and thus the protein will be locked in the
GTP-bound form. The second variety is blocked in
the GDP-bound form. Overexpression of the different
varieties led to the conclusion that active
Ras is required at different developmental stages,
and that the thresholds for active Ras are different
(Som and Kolaparthi 1994; Fillinger et al. 2002).
In Aspergillus fumigatus, theroleofRasissimilar
to that in A. nidulans, although in the pathogen
two ras genes were identified. Dominant negative
RasB caused conidiation in submersed culture, and
dominant active RasA led to a reduction in conidiation
(Fortwendel et al. 2004). Taken together,
phenotypes of ras mutants are pleiotropic, and the
exact interactions of Ras with the conidiation programme
are not known yet. Similarly, the Rhotype
small G-protein, Cdc42, appears to play several
roles during the life cycle of A. nidulans, and is
especially important for cell polarity (Boyce et al.
2003; Harris and Momany 2004; see Chap. 1, this
volume).
Although not all components of the signalling
cascades have yet been identified in A. nidulans,individual
components known to date show evidence
for the involvement of these pathways. The prime
interest will now be the investigation of the signals
feeding into the pathways, and the cellular reac-