tive function of which has not been resolved,
yet (Iracane et al. 2018). Interestingly, although
heterogeneous in size, the secondary structure
of unconventional introns and the consensus
cleavage site (CNG’CNGN) (Fig. 3.2) are conserved among eukaryotes, although endogenous Ire1 and foreign Hac1-encoding mRNAs
display limited interspecies compatibility
(Zhang et al. 2016b). In filamentous fungi,
translation of the unspliced Hac1 mRNA is
controlled by an extended 5’ UTR (untranslated
region) of the mRNA, which is shortened under
ER stress conditions and translationally derepressed (Joubert et al. 2011; Mulder et al. 2004;
Saloheimo et al. 2003).
3. Ire1 Functions Independent of Hac1 mRNA
Splicing
While in most fungi the Hac1 mRNA represents
the sole Ire1 target, in higher eukaryotes, Ire1
activity is as well connected to the degradation
of various ER-bound mRNAs, a process
referred to as regulated Ire1-dependent decay
(RIDD) (Hollien et al. 2009). Splicing of Hac1/
XBP1 mRNA and RIDD are mediated by distinct enzymatic activities (Li et al. 2018; Tam
et al. 2014). Interestingly and in contrast to
most other fungi, the ER stress response pathway in Schizosaccharomyces pombe and Candida glabrata is mediated independent of
HAC1 mRNA splicing suggesting that RIDD is
not exclusively found in higher eukaryotes but
an evolutionary ancient trait (Kimmig et al.
2012; Miyazaki et al. 2013). In S. pombe, the
induction of ER stress results in cleavage and
subsequent degradation of ER-associated
mRNAs. Cleavage of bip1 mRNA results in the
loss of its polyA tail but also in increased stability and translation (Kimmig et al. 2012). S.
pombe lacks a potential Hac1 homolog, and
Hac1 is not involved in ER stress resistance in
C. glabrata. Remarkably, expression of C. glabrata Hac1 in S. cerevisiae DHAC1 strains
restores ER stress resistance (Miyazaki et al.
2013). The observation that in most fungi,
Dire1 and Dhac1 strains display distinct phenotypes suggests additional roles for Ire1, independent of unconventional splicing of HAC1
mRNA (Heimel 2015).
4. Hac1-Like Transcription Factors in Fungi
Hac1 represents the central transcriptional regulator that orchestrates the UPR by adapting
gene expression to the demands imposed on the
ER. Hac1 is a bZIP transcription factor that
binds as a homodimer to UPREs (unfolded
protein response elements), in the promoters
of regulated genes (Fig. 3.1), inducing or elevating their transcription. Cellular responses to ER
stress are manifested by an increase of the ER
folding capacity, restructuring of ER-associated
transport processes, enlargement of the ER, and
the targeted degradation of terminally misfolded proteins by the ERAD pathway (ERassociated degradation) (Amara et al. 1989;
Chen et al. 1988; Ng et al. 2000; Travers et al.
2000). Genome-wide expression studies in different fungi identified a core set of UPRregulated genes encoding ER chaperones and
foldases, proteins involved in the synthesis of
fatty acids and phospholipids, protein glycosylation, protein translocation, and protein degradation (Guillemette et al. 2007; Travers et al.
2000; Feng et al. 2011; Pinter et al. 2019). While
several genes encoding the ER chaperones Bip1,
Lhs1 (Lumenal Hsp Seventy 1), and the foldases
and associated proteins Pdi1 (Protein Disulfide
Isomerase 1), Mpd1 (Multicopy suppressor of
PDI1 Deletion 1), Ero1 (ER Oxidation 1), and
Cne1 (Calnexin 1) are universal UPR markers, a
substantial amount of UPR regulated genes
appears to be unique for certain fungal species.
In contrast to Ire1, Hac1-like proteins are
only conserved in their bZIP domains
(Fig. 3.3a) and display wide variations with
respect to protein size. While HAC1 in S. cerevisiae encodes a protein of 238 amino acids
(aa), the molecular mass of Hac1-like proteins
in filamentous fungi is increased and in U.
maydis more than doubled (574 aa). The
increased size of Hac1-like proteins provides
platforms for interaction with other proteins
and thus the incorporation and connection of
other signaling pathway to the UPR. Experi3 Genetics of the Unfolded Protein Response in Fungi
53
yet (Iracane et al. 2018). Interestingly, although
heterogeneous in size, the secondary structure
of unconventional introns and the consensus
cleavage site (CNG’CNGN) (Fig. 3.2) are conserved among eukaryotes, although endogenous Ire1 and foreign Hac1-encoding mRNAs
display limited interspecies compatibility
(Zhang et al. 2016b). In filamentous fungi,
translation of the unspliced Hac1 mRNA is
controlled by an extended 5’ UTR (untranslated
region) of the mRNA, which is shortened under
ER stress conditions and translationally derepressed (Joubert et al. 2011; Mulder et al. 2004;
Saloheimo et al. 2003).
3. Ire1 Functions Independent of Hac1 mRNA
Splicing
While in most fungi the Hac1 mRNA represents
the sole Ire1 target, in higher eukaryotes, Ire1
activity is as well connected to the degradation
of various ER-bound mRNAs, a process
referred to as regulated Ire1-dependent decay
(RIDD) (Hollien et al. 2009). Splicing of Hac1/
XBP1 mRNA and RIDD are mediated by distinct enzymatic activities (Li et al. 2018; Tam
et al. 2014). Interestingly and in contrast to
most other fungi, the ER stress response pathway in Schizosaccharomyces pombe and Candida glabrata is mediated independent of
HAC1 mRNA splicing suggesting that RIDD is
not exclusively found in higher eukaryotes but
an evolutionary ancient trait (Kimmig et al.
2012; Miyazaki et al. 2013). In S. pombe, the
induction of ER stress results in cleavage and
subsequent degradation of ER-associated
mRNAs. Cleavage of bip1 mRNA results in the
loss of its polyA tail but also in increased stability and translation (Kimmig et al. 2012). S.
pombe lacks a potential Hac1 homolog, and
Hac1 is not involved in ER stress resistance in
C. glabrata. Remarkably, expression of C. glabrata Hac1 in S. cerevisiae DHAC1 strains
restores ER stress resistance (Miyazaki et al.
2013). The observation that in most fungi,
Dire1 and Dhac1 strains display distinct phenotypes suggests additional roles for Ire1, independent of unconventional splicing of HAC1
mRNA (Heimel 2015).
4. Hac1-Like Transcription Factors in Fungi
Hac1 represents the central transcriptional regulator that orchestrates the UPR by adapting
gene expression to the demands imposed on the
ER. Hac1 is a bZIP transcription factor that
binds as a homodimer to UPREs (unfolded
protein response elements), in the promoters
of regulated genes (Fig. 3.1), inducing or elevating their transcription. Cellular responses to ER
stress are manifested by an increase of the ER
folding capacity, restructuring of ER-associated
transport processes, enlargement of the ER, and
the targeted degradation of terminally misfolded proteins by the ERAD pathway (ERassociated degradation) (Amara et al. 1989;
Chen et al. 1988; Ng et al. 2000; Travers et al.
2000). Genome-wide expression studies in different fungi identified a core set of UPRregulated genes encoding ER chaperones and
foldases, proteins involved in the synthesis of
fatty acids and phospholipids, protein glycosylation, protein translocation, and protein degradation (Guillemette et al. 2007; Travers et al.
2000; Feng et al. 2011; Pinter et al. 2019). While
several genes encoding the ER chaperones Bip1,
Lhs1 (Lumenal Hsp Seventy 1), and the foldases
and associated proteins Pdi1 (Protein Disulfide
Isomerase 1), Mpd1 (Multicopy suppressor of
PDI1 Deletion 1), Ero1 (ER Oxidation 1), and
Cne1 (Calnexin 1) are universal UPR markers, a
substantial amount of UPR regulated genes
appears to be unique for certain fungal species.
In contrast to Ire1, Hac1-like proteins are
only conserved in their bZIP domains
(Fig. 3.3a) and display wide variations with
respect to protein size. While HAC1 in S. cerevisiae encodes a protein of 238 amino acids
(aa), the molecular mass of Hac1-like proteins
in filamentous fungi is increased and in U.
maydis more than doubled (574 aa). The
increased size of Hac1-like proteins provides
platforms for interaction with other proteins
and thus the incorporation and connection of
other signaling pathway to the UPR. Experi3 Genetics of the Unfolded Protein Response in Fungi
53