Growth, Differentiation and Sexuality

GRISEA is an orthologue of the yeast transcription
factor MAC1 which controls cellular copper
homeostasis in yeast via the control of the expression
of different target genes. Among these, CTR1
encodes a yeast high-affinity copper transporter
(Dancis et al. 1994). The GRISEA protein is able to
rescue a mac1-deficiency mutant back to respiratory
competence (Borghouts and Osiewacz 1998).
Like MAC1, GRISEA activity is controlled by cellular
copper, most likely via copper binding and
conformational changes of the copper-loaded protein
(Graden and Winge 1997; Zhu et al. 1998; Mc-
Daniels et al. 1999).
Upon copper depletion, GRISEA activates
the transcription of different target genes including
PaGrg1, PaSod2 and PaCtr3 (Borghouts
and Osiewacz 1998; Kimpel and Osiewacz 1999;
Borghouts et al. 2002a,b). PaCtr3 encodes a highaffinity
copper transporter which enables efficient
copper transport across the plasma membrane
even under low copper concentrations in the
environment. In the grisea mutant, high-affinity
copper uptake is impaired, leading to cellular
copper deficiency which results in a pleiotropic
phenotypewithreducedpigmentationofmycelia
and ascospores, a reduced growth rate, reduced
Fig. 10.2. A,B Genetic regulation of copper homeostasis in
the wild type (A) and the long-lived grisea (B) mutantof
P. anserina. Factors and processes indicated in brackets are
missing or reduced in the mutant grisea. A In the wild type,
copper uptake proceeds via the high-affinity transporter
PaCTR3 which is transcriptionally regulated via GRISEA.
The copper is bound to PaSOD1, tyrosinase and PaMt1
(metallothionein), and other proteins. In the nucleus, an
unknown transcription factor induces PaMt1 transcription
under high copper concentration, whereas the expression
of PaSod2 and PaCtr3 is repressed under these con-
Fungal Senescence and Longevity 193
fertility and an increased life span. The pleiotropic
effect of the mutation of Grisea is explained by
the fact that, due to cellular copper depletion,
different copper-dependent cellular activities are
impaired. Among others, these are the activity
of tyrosinase (Fig. 10.2) which is involved in
pigment synthesis. Another affected enzyme is
superoxide dismutase 1 (SOD1). This enzyme is
involved in ROS scavenging in both the cytoplasm
and in the intermembrane space of mitochondria
(Sturtz et al. 2001). In P. anserina, incontrastto
yeast where transcription of the corresponding
gene is copper dependent (Gralla et al. 1991),
Sod1 transcription is constitutive and, due to the
dependency on copper as a cofactor, SOD1 activity
is regulated post-transcriptionally by binding of
the metal (Borghouts et al. 2002b).
In P. anserina, the impact of copper on life span
appears to be largely the result of mitochondrial
impairments. Here, a stabilization of the mtDNA
in mutant grisea, which can be reversed by the addition
of copper to the growth medium, is one of
the factors which play a role. The reduction of ROS
scavenging caused by the mitochondrial SOD2 gene
not being expressed in the grisea mutant should
rather have a negative effect on life span. How-
ditions. In the mitochondrion of the wild type, PaSOD2 is
present (under low copper concentration), mtDNA is instable
and respiration proceeds via COX. B In the mutant
grisea, copper uptake proceeds via an unknown low-affinity
copper transporter (X). The low copper concentration in
the cell leads to PaSOD1 deficiency, and reduced PaMT1
activity in the cytoplasm. Lack of functional GRISEA inhibits
PaSod2 and PaCtr3 expression. Consequently, Pa-
SOD2 is missing in mitochondria of the grisea mutant,
the mtDNA is stabilised and AOX is used as terminal oxidase