Growth, Differentiation and Sexuality

308 R. Debuchy and B.G. Turgeon
interacting with SMTA-1 in S. macrospora (Nolting
and Pöggeler 2005). Interactions of MAT1-1-1 with
other MAT proteins have been investigated, based
on the assumption that a heterodimer involving
MAT proteins from opposite idiomorphs could
trigger developmental events inside the fruiting
body after fertilization. A similar situation has
been observed in diploid cells of S. cerevisiae,
in which a1 and α2 encodedbythemating-type
alleles form a heterodimer required for further
sexual development after mating (reviewed in
Souza et al. 2003). In N. crassa, two-hybrid assays
established the ability of MAT A-1 (MAT1-1-1) to
interact with MAT a-1 (MAT1-2-1; Badgett and
Staben 1999). Mutations that interfere with this
interaction eliminate vegetative incompatibility
(reviewed in Glass and Kuldau 1992), but not
mating, suggesting that this interaction is not
essential for the sexual cycle. In S. macrospora,
an interaction between homologs of the above
proteins has been found, but its role during the
sexual cycle has not been investigated (Jacobsen
and Pöggeler 2001). No interaction between FMR1
(MAT1-1-1) and FPR1 (MAT1-2-1) has been
detected in P. anserina. Instead,aninteraction
between FMR1 and SMR2 was identified (Arnaise
et al. 1995). Mutations that disrupt this interaction
also affect the sexual cycle (Coppin and Debuchy,
unpublished data). Therefore, no conclusive data
support the idea that an interaction between
proteins encoded by opposite idiomorphs is
essential to the sexual cycle. Nevertheless, it must
be emphasized that all of these experiments rely
on the yeast two-hybrid system, which failed to
detect any interaction between the yeast a1 and
α2, although a1/α2 heterodimer formation is well
established.
Examination of all published MAT genes of
Cochliobolus spp. and relatives (e.g., Pleospora, Alternaria
and Phaeosphaeria) reveals that MAT1-1-1
and MAT1-2-1 proteins share at least two conserved
motifs; Motif 1 is located in the HMG box in MAT1-
2-1 and is also found 3 ′ of the α1 boxinMAT1-
1-1; motif 2 is located at the C terminus of both
MAT proteins (Lu and Turgeon, unpublished data;
Fig. 15.5). Motif 2 shows significant similarity to
an α1 box signature motif. Amino acid alignment
to known HMG box domains suggests that motif
1isaHMGboxdomain.Thus,eachMAT gene
of Cochliobolus spp. appears to encode a protein
with both HMG and α1 activities. The shorter idiomorphs
and the single MAT1-1-1 proteins of the
Loculoascomycetes may have evolved to include all
Fig. 15.5. Diagrammatic representation of the MAT1-1-1
and MAT1-2-1 proteins of taxa representing several Loculoascomycete
genera including Cochliobolus (Bipolaris),
Pleospora (Stemphylium), Phaeosphaeria (Stagonospora),
Leptosphaeria maculans and asexual Alternaria alternata.
Alignment of amino-acid sequences of the two “unlike”
MAT proteins reveals shared motifs. A signature motif
within the HMG box (shaded vertical lines) of MAT1-2-1
is also found in the MAT1-1-1 protein (motif 1, hatched
box). A motif resembling a signature α1box(lightly shaded
stippled) motif is found at the C-terminal end of both MAT
proteins (motif 2, striped box). The α1 boxandtheHMG
box overlap slightly in MAT1-1-1. A third common stretch
is RK rich (dotted box). Thus, although the MAT1-1-1 and
MAT1-2-1 genes encode different transcription factors,
these proteins may have dual function and may share
a common evolutionary history. Introns are represented by
inverted triangles (Lu and Turgeon, unpublished data)
activities provided by the three MAT1-1 proteins of
the Pyrenomycetes.
Comparison of MAT1-1-1 gene structure
reveals the conserved position of an intron in
all MAT1-1-1 genes, except in E. nidulans MATB,
which has no intron. The intron is localized after
the first nucleotide of the triplet encoding the
non-conserved residue in the peptide fvgfRXyy
(Fig. 15.4).
2. MAT1-1-2
The MAT1-1-2 proteins contain a conserved region
called the HPG domain, with three invariant
residues, histidine, proline and glycine (Fig. 15.6).
Surprisingly,mutationoftheseresidues,inthe
SMR1 gene of P. anserina, to alanine does not affect
the sexual cycle, whereas a mutation of tryptophan
193 to alanine results in a complete arrest
of the fruiting-body development at an early stage,
thus confirming that this domain is essential for
the sexual cycle (Coppin et al. 2005a). Based on the
high isoelectric point of the conserved domain of
SMR1, Debuchy et al. (1993) have proposed that
it defines a new DNA-binding domain. However,
subsequent investigations of SMR1 do not support
this hypothesis. Subcellular localization of SMR1
by GFP tagging indicates that it has a cytosolic localization
(Coppin et al. 2005a). In agreement with
this observation, analysis of SMR1 with a program
for identification of subcellular localization of pro-