Growth, Differentiation and Sexuality

and the formin Bni1p, and polarisome localisation
to growth sites occurs in a cell cycle-dependent
manner (Evangelista et al. 1997; Fujiwara et al.
1998). C. albicans SPA2 homologue mutants exhibit
wide, elongated hyphae with nuclear positioning
defects and reduced actin localisation, suggesting
that the polarisome is essential for polarity maintenance
in mycelial fungi (Zheng et al. 2003). The
second mechanism of Cdc42-dependent maintenance
of polarised growth is via the activation
of the PAKs Ste20p and Cla4p, which regulate
myosins, WASP proteins and Arp2/3-dependent
actin nucleation to form actin patches. Intrinsic to
these processes is the role of S. cerevisiae Cdc42 in
endocytosis and exocytosis (reviewed in Johnson
1999). The Cdc42p protein is also required for
polarity maintenance in mycelial fungi, although
the details of how this is achieved have yet to be
uncovered. Expression of a dominant negative
or a constitutively active allele of the Cdc42p
homologue in P. marneffei, cflA, results in swollen,
aberrantly shaped hyphae with a depolarised actin
cytoskeleton (Boyce et al. 2001). Homologues of
proteins which are present in the polarisome of S.
cerevisiae are also required for the maintenance of
polarised growth in mycelial fungi, suggesting that
the Cdc42p/polarisome actin regulation pathway
is conserved. The homologue of the Bni1p formin
in A. nidulans, sepA, is required for actin-mediated
polarised growth and septation of both hyphae
and conidiophores (acting as a scaffold), and is
recruited to the hyphal tip by the transmembrane
protein MesA (Harris et al. 1997; Pearson et al.
2004). Similarly, the Spa2p homologue in A.
gossypii determinesthesiteofgrowthathyphal
tips (Knechtle et al. 2003).
Similarly to S. cerevisiae, Cdc42-dependent
maintenance of polarised growth in mycelial
fungi also leads to activation of WASP proteins,
possibly via the activation of the PAK Cla4p,
which regulates myosins, WASP proteins and
Arp2/3-dependent actin nucleation to form actin
patches in S. cerevisiae. InS. cerevisiae, theWASP
protein Bee1p (which is also a polarisome protein)
interacts with Arp2/3p to nucleate actin during
budding and cytokinesis (Li 1997). In both C.
albicans and A. gossypii, the WASP homologue
(Wal1p) regulates the maintenance of polarised
growth by regulating actin polymerisation and
trafficking of endosomes and vacuoles (Walther
and Wendland 2004a,b).
Despite some of the core components regulating
the maintenance of polarised growth
Generating Fungal Cell Types 13
being conserved between budding yeast and
mycelial fungi, the requirement for additional
cell shapes and developmental programs in
mycelial fungi makes the situation more complex.
In higher eukaryotes, a second Cdc42-like Rho
GTPase, named Rac, also exists. Rac orthologues
are not found in less morphologically complex
eukaryotes such as S. cerevisiae and A. gossypii,
suggesting that RAC genes may have evolved
with increasing cellular complexity. The RAC
orthologue in the dimorphic fungus P. marneffei,
cflB, is also required during polarised growth
maintenance (Fig. 1.8). Deletion of cflB results in
depolarised, swollen hyphae with a delocalised
actin cytoskeleton, and CflB colocalises with actin
at the tips of vegetative hyphal cells (Boyce et al.
2003). A P. marneffei double mutant with the cflB
deletion and the dominant activated cflA allele
has a depolarised phenotype more severe than
either of the single mutants alone, suggesting that
CflA and CflB may have overlapping roles during
actin-mediated polarised growth maintenance
(Boyce et al. 2005). Despite some overlapping
Fig. 1.8. Small GTPases differentially regulate different cell
types in P. marneffei. Representation of the various cell
types of P. marneffei; conidia (centre, left) cangerminate
to produce hyphal cells (centre) which consist of actively
growing apical cells, branch cells and subapical cells, which
in turn can undergo the asexual development program at
25 ◦ C to produce foot, stalk, sterigmata and conidial cell
types (bottom), or the arthroconidiation program at 37 ◦ C
to produce the yeast cell type (top). The various Ras- and
Rho-type GTPases which control morphogenesis of these
cell types are shown, as is their proposed action