round, SUMO is essential but not Nedd8 (Liakopoulos et al. 1998).
The active SCF complex requires at one
side of the post-translational neddylated cullin
scaffold a small RING protein RbxA, which
binds to E2-ubiquitin, and at the other side
the target substrate, which is connected to the
cullin by the general SkpA (suppressor of kinetochore protein mutant A) adaptor. SkpA binds
the specific substrate receptors, the Fbox proteins, which differ considerably to interact with
different substrates, but share a characteristic
N-terminal 40–50 amino acids F-box motif as
SkpA interaction domain. The genome of A.
nidulans encompasses approximately 70 different Fbox proteins and the recruited target proteins are often primed by phosphorylation
prior to ubiquitination (Draht et al. 2007). Fungal adaptation to changing environmental conditions requires the rapid exchange of Fbox
proteins at the SCF complexes in order to
label and degrade different substrates, what
requires the removal of Nedd8 from CulA
(Flick and Kaiser 2013). Several Fbox proteins
are involved in fungal development. Fbox protein GrrA (glucose repression-resistant A) is
needed for the induction of meiosis and thus
for the development of mature ascospores in A.
nidulans (Krappmann et al. 2006). The
corresponding orthologs of the fungal pathogens Cryptococcus neoformans F-box protein 1
(Fbp1) and Gibberella zeae FBP1 are also
involved in sexual reproduction and virulence
(Han et al. 2007; Liu et al. 2011). A. nidulans
Fbx15 is required for asexual and sexual and
Fbx23 for light-dependent development (von
Zeska Kress et al. 2012). The homologous
Fbx15 of the human pathogen A. fumigatus
represses the formation of the secondary
metabolite gliotoxin and provides a general
stress response and virulence to the fungus
(Jo ¨hnk et al. 2016).
The disruption of the UPP leads to an
imbalanced protein degradation of, e.g., secondary metabolite cluster-specific or global
transcription factors. This can be used for the
identification of novel interesting secondary
metabolites (Gerke et al. 2012; Zheng et al.
2017). For instance, the deletion of csnE
(COP9 signalosome E), encoding the catalytically active subunit of the COP9 signalosome,
interferes with the deneddylation of cullins,
which mediate the ubiquitination of proteins
(Beckmann et al. 2015; see Sect. IV.B). In A.
nidulans, this deneddylation defect leads to
the accumulation of 100 metabolites and
resulted in the identification of the antimicrobial 2,4-dihydroxy-3-methyl-6-(2-oxopropyl)
benzaldehyde (DHMBA) (Nahlik et al. 2010;
Gerke et al. 2012).
B. Controlled Removal of Ubiquitin Family
Proteins from Substrates Is Important for
Fungal Growth and Development
The attachment of ubiquitin and UBLs such as
Nedd8 or SUMO to proteins is a reversible
process. Two desumoylating enzymes UlpA
and UlpB were characterized in A. nidulans,
which are required for asexual conidiospore
production and the formation of mature sexual
fruiting bodies (Harting et al. 2013).
The removal of Nedd8 (deneddylation)
from SCF complexes leads to a conformational
change of the cullin protein (Duda et al. 2008)
⁄
ä ⁄
ä
Fig. 8.4 (continued) ways (e.g., the target of rapamycin
Tor kinase) inhibit the formation of the fungal Atg1
autophagy initiation complex for phagophore assembly
site (PAS) in the presence of nutrients. Atg1 is activated
by starvation or during development or interaction
with other organisms and results in coupling of the
ubiquitin-like Atg8 to the lipid phosphatidylethanolamine (PE). Atg8-PE is recruited to PAS, which promotes nucleation and elongation of the phagophore.
The phagophore engulfes proteins, protein aggregates
or organelles like mitochondria or ribosomes resulting
in the double membraned autophagosome. Autophagosomes fuse with the vacuole. The membrane of the
resulting autophagic body is degraded by Atg15 lipase
and the contents by vacuolar hydrolases. Permeases
such as Atg22 can release degraded material back to
the cytoplasm for recycling. The budding of vesicles
from mitochondria is called mitophagy and the budding from peroxisomes is called pexophagy. Aflatoxin
producing secondary metabolite enzymes and their
substrates can be transported by cytoplasm-to-vacuole
transport (Cvt) vesicles and exported from the vacuole
out of the fungal cell by aflatoxisomes
8 Coordination of Fungal Secondary Metabolism and Development
191
The active SCF complex requires at one
side of the post-translational neddylated cullin
scaffold a small RING protein RbxA, which
binds to E2-ubiquitin, and at the other side
the target substrate, which is connected to the
cullin by the general SkpA (suppressor of kinetochore protein mutant A) adaptor. SkpA binds
the specific substrate receptors, the Fbox proteins, which differ considerably to interact with
different substrates, but share a characteristic
N-terminal 40–50 amino acids F-box motif as
SkpA interaction domain. The genome of A.
nidulans encompasses approximately 70 different Fbox proteins and the recruited target proteins are often primed by phosphorylation
prior to ubiquitination (Draht et al. 2007). Fungal adaptation to changing environmental conditions requires the rapid exchange of Fbox
proteins at the SCF complexes in order to
label and degrade different substrates, what
requires the removal of Nedd8 from CulA
(Flick and Kaiser 2013). Several Fbox proteins
are involved in fungal development. Fbox protein GrrA (glucose repression-resistant A) is
needed for the induction of meiosis and thus
for the development of mature ascospores in A.
nidulans (Krappmann et al. 2006). The
corresponding orthologs of the fungal pathogens Cryptococcus neoformans F-box protein 1
(Fbp1) and Gibberella zeae FBP1 are also
involved in sexual reproduction and virulence
(Han et al. 2007; Liu et al. 2011). A. nidulans
Fbx15 is required for asexual and sexual and
Fbx23 for light-dependent development (von
Zeska Kress et al. 2012). The homologous
Fbx15 of the human pathogen A. fumigatus
represses the formation of the secondary
metabolite gliotoxin and provides a general
stress response and virulence to the fungus
(Jo ¨hnk et al. 2016).
The disruption of the UPP leads to an
imbalanced protein degradation of, e.g., secondary metabolite cluster-specific or global
transcription factors. This can be used for the
identification of novel interesting secondary
metabolites (Gerke et al. 2012; Zheng et al.
2017). For instance, the deletion of csnE
(COP9 signalosome E), encoding the catalytically active subunit of the COP9 signalosome,
interferes with the deneddylation of cullins,
which mediate the ubiquitination of proteins
(Beckmann et al. 2015; see Sect. IV.B). In A.
nidulans, this deneddylation defect leads to
the accumulation of 100 metabolites and
resulted in the identification of the antimicrobial 2,4-dihydroxy-3-methyl-6-(2-oxopropyl)
benzaldehyde (DHMBA) (Nahlik et al. 2010;
Gerke et al. 2012).
B. Controlled Removal of Ubiquitin Family
Proteins from Substrates Is Important for
Fungal Growth and Development
The attachment of ubiquitin and UBLs such as
Nedd8 or SUMO to proteins is a reversible
process. Two desumoylating enzymes UlpA
and UlpB were characterized in A. nidulans,
which are required for asexual conidiospore
production and the formation of mature sexual
fruiting bodies (Harting et al. 2013).
The removal of Nedd8 (deneddylation)
from SCF complexes leads to a conformational
change of the cullin protein (Duda et al. 2008)
⁄
ä ⁄
ä
Fig. 8.4 (continued) ways (e.g., the target of rapamycin
Tor kinase) inhibit the formation of the fungal Atg1
autophagy initiation complex for phagophore assembly
site (PAS) in the presence of nutrients. Atg1 is activated
by starvation or during development or interaction
with other organisms and results in coupling of the
ubiquitin-like Atg8 to the lipid phosphatidylethanolamine (PE). Atg8-PE is recruited to PAS, which promotes nucleation and elongation of the phagophore.
The phagophore engulfes proteins, protein aggregates
or organelles like mitochondria or ribosomes resulting
in the double membraned autophagosome. Autophagosomes fuse with the vacuole. The membrane of the
resulting autophagic body is degraded by Atg15 lipase
and the contents by vacuolar hydrolases. Permeases
such as Atg22 can release degraded material back to
the cytoplasm for recycling. The budding of vesicles
from mitochondria is called mitophagy and the budding from peroxisomes is called pexophagy. Aflatoxin
producing secondary metabolite enzymes and their
substrates can be transported by cytoplasm-to-vacuole
transport (Cvt) vesicles and exported from the vacuole
out of the fungal cell by aflatoxisomes
8 Coordination of Fungal Secondary Metabolism and Development
191