Growth, Differentiation and Sexuality

Fig. 7.4. A,B Comparison between mating cell fusion in
Saccharomyces cerevisiae and anastomosis in Neurospora
crassa. A In S. cerevisiae, receptionofamatingsignalvia
interaction of the peptide pheromones with their cognate
receptors results in the dissociation of the heterotrimeric
G-protein (Gα,Gβ,Gγ). Gβγ interacts with the PAK kinase
Ste20, resulting in the activation of the pheromone response
MAP kinase pathway (Ste11, Ste7 and Fus3, bound to
the scaffold protein Ste5) and subsequent transcriptional
activation of genes required for cell fusion via Ste12.
Endocytosis of the receptor–ligand complex is associated
with chemotropism toward an appropriate mating partner.
Polarization of the cytoskeleton to the schmoo tip occurs
via interactions between Gβγ and Cdc24, and subsequent
recruitment and activation of Cdc42. Activated Cdc42
interacts with Fus1, which plays a role in events during cell
fusion. Components of the MAP kinase pathway interact
with polarization components, such as Spa2, Bud6, and
the formin Bni1. Cell wall remodeling, via the Slt2 MAP
7-transmembrane receptors (Galagan et al. 2003;
Borkovich et al. 2004), including some that resemble
cAMP receptors in Dictyostelium discoideum.
D. discoideum shows a chemotactic phenotype in
response to pulses of cAMP (Arkowitz 1999; Kay
2002).
In S. cerevisiae, the binding of a secreted
peptide pheromone (α, ora-factor) to its cognate
GCPR (Ste2 and Ste3, respectively) initiates mating
(Cross 1988; Fig. 7.4). Oligomerization of the
Anastomosis in Filamentous Fungi 129
kinase pathway (Mkk2, Mkk1 and Slt2), is involved in
schmoo formation and mating cell fusion (for review, see
Banuett 1998; Dohlman and Thorner 2001). Components
not mentioned in the text are not included in the figure.
B In N. crassa, mutations in orthologs of STE11 and FUS3
(nrc-1 and mak-2, respectively) result in mutants unable
to undergo germling or hyphal fusion (Pandey et al. 2004).
In addition, mutations in ham-2, encoding a putative
membrane protein, and in so, a putative cytoplasmic
protein, result in strains unable to undergo both hyphal
and germling fusion (Xiang et al. 2002; Fleissner et al.
2005). In F. graminearum, a strain containing a mutation in
the ortholog of SLT2 failstoformaheterokaryon(Houetal.
2002); the N. crassa ortholog of SLT2 is called mak-3. The
nature of the receptor and ligand involved in anastomosis
is unknown. NCU04612.1 is the N. crassa NCU number
(http:/www.broad.mit.edu/annotation/fungi/neurospora/)
for the predicted ortholog of STE7; a mutant containing
alesioninthisgeneispredictedtobeahyphalfusionmutant
Ste2 pheromone receptor is required for efficient
signaling (Overton and Blumer 2000), while
the C-terminal portion of Ste2 is required for
internalization via endocytosis. Endocytosis of the
pheromone receptor–ligand complex is required
for chemotropism toward an appropriate mating
partner (Vallier et al. 2002). These data have led to
the model that endocytosis of the receptor/ligand
complex plays a role in marking the spot for
polarization of the cytoskeleton, resulting in