with only partially moderate growth defects
(Amerik et al. 2000). The ubiquitin-specific
protease Doa4 (degradation of Alpha 4) is associated to the 26S proteasome and recycles ubiquitin chains from proteins that were targeted
for degradation to the 26S proteasome or from
membrane proteins that are targeted to the
vacuole (Swaminathan et al. 1999). The ubiquitin-specific protease Ubp14 hydrolyzes rather
free polyubiquitin chains, which are not
degraded by the 26S proteasome as its unfolding would require too much energy (Amerik
et al. 2000). The C. neoformans ubiquitinspecific protease Ubp5 is required for virulence
and controls melanin and capsule formation
(Fang et al. 2012).
V. Protein Degradation Pathways
Clearance of misfolded, damaged, or no longer
needed proteins and organelles directs time controlled developmental transitions, multicellular
development, pathogenicity, and secondary
metabolism in filamentous fungi and leads to
recycling of building bricks for the synthesis of
new proteins and organelles. Protein degradation is mediated through different pathways
(Fig. 8.4) including the nuclear and cytoplasmic
ubiquitin-26S proteasome pathway (UPP) and
autophagy pathways, which target defective cellular compartments and 26S proteasomes for
degradation in the fungal vacuole. Misfolded
proteins of the endoplasmic reticulum (ER) activate the unfolded protein response and the ER
associated degradation (ERAD) pathway, which
finally results in degradation of these proteins in
26S proteasomes of the cytoplasm. The plant
pathogenic fungus Ustilago maydis requires the
unfolded protein response of the ER for pathogenicity (Heimel et al. 2013; Hampel et al. 2016).
A. Protein Degradation by the 26S Proteasome
The conserved 2.5 MDa 26S proteasome is one
of the major degradation machineries in eukaryotes and has approximately half the size of a
ribosome. This multi-protease complex consists of the 20S barrel-like core particle and on
one or both sides associated 19S regulatory particles consisting of lid and base subcomplexes
(Tomko and Hochstrasser 2013). The lid
receives ubiquitinated substrates and the base,
containing six subunits with ATPase activity,
unfolds substrates by ATP hydrolysis, and translocates them into the barrel of the core particle,
while the JAMM domain metalloprotease DUB
Rpn11 cleaves the ubiquitin chains (Lander et al.
2012; de la Pen ˜a et al. 2018). The core particle
contains four rings. Each ring is built of seven
subunits providing gate and catalytic activity
with different trypsin-, chymotrypsin-, or
caspase-like peptidases. These degrade substrates processively, resulting in recyclable
amino acids (Budenholzer et al. 2017).
26S proteasomal-mediated degradation is
initiated by the recognition of K48polyubiquitinated substrates by Rpn1, Rpn10,
or Rpn13, which possess ubiquitin-binding
domains (Finley 2009). After capturing these
substrates, ubiquitin tags are released by the
intrinsic lid Rpn11 deubiquitinase, which is
embedded in a similar protein subunit architecture as the intrinsic deneddylase of the COP9
signalosome (Meister et al. 2016; see Sect. IV.
A). The eight CSN subunits correspond to eight
Rpn (Rpn3, 5–9, 11, 12) subunits of the lid of
the regulatory particle. The lid contains as
ninth additional subunit the versatile small
multifunctional and intrinsically disordered
Sem1/Dss1 (suppressor of exocyst mutations 1;
deletion of split hand/split foot 1), and a ninth
COP9 signalosome subunit (CSN acidic protein) is also present in metazoa and some plants
and fungi (Barth et al. 2016).
Sem1 stabilizes the assembly of 26S proteasomes by recruiting the receptors Rpn13 and
Rpn10 as well as the tethering factor Ecm29
(extracellular mutant 29) to support complex
formation. The A. nidulans sem1 deletion strain
is delayed in conidiophore development, has
less conidiospores, and produces immature
fruiting bodies without ascospores. Sem1 of A.
nidulans links fungal stress response to development and controls secondary metabolism
similar to CSN or CandA (Kolog Gulko et al.
2018; see Sect. IV.A).
A link between the 26S proteasome of filamentous fungi and development and control of
8 Coordination of Fungal Secondary Metabolism and Development
193
(Amerik et al. 2000). The ubiquitin-specific
protease Doa4 (degradation of Alpha 4) is associated to the 26S proteasome and recycles ubiquitin chains from proteins that were targeted
for degradation to the 26S proteasome or from
membrane proteins that are targeted to the
vacuole (Swaminathan et al. 1999). The ubiquitin-specific protease Ubp14 hydrolyzes rather
free polyubiquitin chains, which are not
degraded by the 26S proteasome as its unfolding would require too much energy (Amerik
et al. 2000). The C. neoformans ubiquitinspecific protease Ubp5 is required for virulence
and controls melanin and capsule formation
(Fang et al. 2012).
V. Protein Degradation Pathways
Clearance of misfolded, damaged, or no longer
needed proteins and organelles directs time controlled developmental transitions, multicellular
development, pathogenicity, and secondary
metabolism in filamentous fungi and leads to
recycling of building bricks for the synthesis of
new proteins and organelles. Protein degradation is mediated through different pathways
(Fig. 8.4) including the nuclear and cytoplasmic
ubiquitin-26S proteasome pathway (UPP) and
autophagy pathways, which target defective cellular compartments and 26S proteasomes for
degradation in the fungal vacuole. Misfolded
proteins of the endoplasmic reticulum (ER) activate the unfolded protein response and the ER
associated degradation (ERAD) pathway, which
finally results in degradation of these proteins in
26S proteasomes of the cytoplasm. The plant
pathogenic fungus Ustilago maydis requires the
unfolded protein response of the ER for pathogenicity (Heimel et al. 2013; Hampel et al. 2016).
A. Protein Degradation by the 26S Proteasome
The conserved 2.5 MDa 26S proteasome is one
of the major degradation machineries in eukaryotes and has approximately half the size of a
ribosome. This multi-protease complex consists of the 20S barrel-like core particle and on
one or both sides associated 19S regulatory particles consisting of lid and base subcomplexes
(Tomko and Hochstrasser 2013). The lid
receives ubiquitinated substrates and the base,
containing six subunits with ATPase activity,
unfolds substrates by ATP hydrolysis, and translocates them into the barrel of the core particle,
while the JAMM domain metalloprotease DUB
Rpn11 cleaves the ubiquitin chains (Lander et al.
2012; de la Pen ˜a et al. 2018). The core particle
contains four rings. Each ring is built of seven
subunits providing gate and catalytic activity
with different trypsin-, chymotrypsin-, or
caspase-like peptidases. These degrade substrates processively, resulting in recyclable
amino acids (Budenholzer et al. 2017).
26S proteasomal-mediated degradation is
initiated by the recognition of K48polyubiquitinated substrates by Rpn1, Rpn10,
or Rpn13, which possess ubiquitin-binding
domains (Finley 2009). After capturing these
substrates, ubiquitin tags are released by the
intrinsic lid Rpn11 deubiquitinase, which is
embedded in a similar protein subunit architecture as the intrinsic deneddylase of the COP9
signalosome (Meister et al. 2016; see Sect. IV.
A). The eight CSN subunits correspond to eight
Rpn (Rpn3, 5–9, 11, 12) subunits of the lid of
the regulatory particle. The lid contains as
ninth additional subunit the versatile small
multifunctional and intrinsically disordered
Sem1/Dss1 (suppressor of exocyst mutations 1;
deletion of split hand/split foot 1), and a ninth
COP9 signalosome subunit (CSN acidic protein) is also present in metazoa and some plants
and fungi (Barth et al. 2016).
Sem1 stabilizes the assembly of 26S proteasomes by recruiting the receptors Rpn13 and
Rpn10 as well as the tethering factor Ecm29
(extracellular mutant 29) to support complex
formation. The A. nidulans sem1 deletion strain
is delayed in conidiophore development, has
less conidiospores, and produces immature
fruiting bodies without ascospores. Sem1 of A.
nidulans links fungal stress response to development and controls secondary metabolism
similar to CSN or CandA (Kolog Gulko et al.
2018; see Sect. IV.A).
A link between the 26S proteasome of filamentous fungi and development and control of
8 Coordination of Fungal Secondary Metabolism and Development
193