Growth, Differentiation and Sexuality

378 M. Feldbrügge et al.
Fig. 18.3. Signalling network during mating. Components
of the MAP kinase module (hexagons) communicate with
a conserved cAMP signalling pathway (indicated on the left)
as well as with putative cell cycle components (indicated on
the right). The same symbols are used as those described
in the caption of Fig. 18.2. The cAMP signalling pathway
consists of a heterotrimeric G protein (Gpa3, Bpp1, and
a γ subunit), adenylate cylase (Uac1) and protein kinase
A (Ubc1 and Adr1). Phosphorylation of Prf1 through PKA
and MAPK signalling (encircled P) is used to differentiate
between a and b gene expression (see text for details). The
novel MAPK Crk1 as well as the HMG box transcription
factors Rop1 and Prf1 regulate prf1 expression transcriptionally
maydis, the input of environmental signals might
be particularly important to achieve temporal
and local control of pheromone signalling to
allow mating on the plant surface. One of these
communicating signalling pathways required
for pheromone response and further pathogenic
development is an evolutionarily conserved
cAMP signalling pathway which consists of a heterotrimeric
G protein, containing Gpa3 and Bpp1
as α and β subunit respectively (Regenfelder et al.
1997; Müller et al. 2004). G protein-mediated stimulation
of adenylate cyclase Uac1 (Gold et al. 1994)
leads to the production of cAMP. This secondary
messenger regulates the cAMP-dependent protein
kinase A (PKA), consisting of the regulatory
subunit Ubc1 and the catalytic subunit Adr1 (Gold
et al. 1997; Dürrenberger et al. 1998; Lee et al.
2003; Fig. 18.3). The connection to pheromone
signalling has been revealed by finding that the
mating defect of compatible gpa3Δ strains can be
rescued by addition of external cAMP (Krüger
et al. 1998). How this crosstalk operates was
revealed by investigating mfa1 gene expression.
Initially, it was observed that strains carrying
deletions in genes encoding components of the
cAMP pathway, such as gpa3Δ or uac1Δ, exhibit
strongly reduced mfa1 expression. Conversely,
strains expressing constitutively active versions
of Gpa3 or protein kinase A (gpa3 Q206L or ubc1Δ,
respectively) showed elevated mfa1 expression.
This indicated that high internal cAMP levels
induce mfa1 gene expression (Krüger et al. 1998).
In U. maydis, a second Ras protein, Ras1, has
been described whose deletion has no significant
effect on cellular morphology. A constitutive
active version of Ras1 is likely to activate mfa1
expression via the cAMP cascade, and it has
been hypothesized that this might occur via
an RA domain in Uac1 (Müller et al. 2003a;
Fig. 18.3).
TheobservationsthatPrf1bindingsitesare
necessary and sufficient for cAMP-mediated induction
of mfa1, and the finding that PKA phosphorylation
sites in Prf1 are essential for mfa1 expression
provided compelling evidence that PKA phosphorylation
of Prf1 is the key signalling node mediating
crosstalk between cAMP and MAP kinase pathways
(Kaffarnik et al. 2003; Fig. 18.3). Although
phosphorylation of Prf1 through Adr1 appears sufficient
for the induction of the genes in the a locus,
transcriptional activation of the b genes requires
Prf1 to be phosphorylated by Adr1 as well as Kpp2
(Kaffarnik et al. 2003; Fig. 18.3).
In addition to its regulation by phosphorylation,
Prf1 activity is also intensively regulated
on the transcriptional level (Fig. 18.3). As mentioned
above, the pheromone-responsive MAPK
module mediates pheromone-induced prf1 expression.
This is likely to operate by positive autoregulation
via two PREs which are present in the prf1
promoter and are recognized by Prf1 in vitro (Hartmann
et al. 1999; Brefort et al. 2005). However,
recent results indicate a more complex transcriptional
regulation of prf1, involving at least two
additional proteins. One of these is the sequencespecific
HMG box protein Rop1 (regulator of prf1)
which is essential for prf1 expression during fungal
growth in axenic culture. This regulation is mediated
by direct binding of Rop1 to three distinct
response elements present in the prf1 promoter
(Fig. 18.3). In the absence of rop1, prf1 transcrip-