Growth, Differentiation and Sexuality

Ste20 homolog Smu1 (Smith et al. 2004). cla4 mutants
display a phenotype similar to that of rac1
mutants, implying that Cla4 might act as effector
for Rac1 (Leveleki et al. 2004). Deletion of smu1
resulted in a delayed mating response in a mating
type-specific manner, and also in a substantial reduction
of disease (Smith et al. 2004). The genome
of U. maydis contains two additional genes coding
for proteins with a CRIB domain. These are homologues
of the Skm1 protein kinase from yeast,
andaproteinrelatedtothehumanN-WASPprotein
which has been identified in mammalian systems as
Wilskott-Aldrich syndrome protein involved in organizing
the actin cytoskeleton (Stradal et al. 2004).
Testing isolated CRIB domains of all these proteins
in the yeast two-hybrid system revealed that the
CRIB domain of Cla4, Smu1 and Skm1 are recognized
by both Cdc42 and Rac1, whereas the CRIB
domain of the U. maydis WASP protein is specific
for Cdc42 (Leveleki et al. 2004). Since cdc42 and
rac1 deletion mutants are viable in U. maydis,this
organism provides a unique opportunity to study
biological functions of individual players in this
important regulatory network.
V. Cytoskeletal Networks
for Morphology
In U. maydis, the transition from a budding
yeast form to the filamentous conjugation hyphae
marks the beginning of the pathogenic cycle
(Banuett 1995; Kahmann et al. 1999). Conjugation
hyphae grow in a directed and polarized fashion
towards each other, fuse their cytoplasm and form
the dikaryotic hypha, which consists of a single
tip-growing cell which leaves empty cell sections
behind while scanning the plant surface for a site
of invasion (see above). Polarized growth is the
key factor in these morphogenetic transitions, and
is therefore essential for successful infection of the
host plant.
Tip growth of polarized fungal hyphae requires
anterograde membrane traffic to the apical cell
pole (Gow 1995; Geitmann and Emons 2000). It
is thought that intracellular vesicle and organelle
transport is mediated by filaments of the cytoskeleton
which provide the tracks along which mechanoenzymes,
the so-called molecular motors, transport
their cargo for delivery to the hyphal tip (Steinberg
2000; Xiang and Plamann 2003). In fungi, indications
exist that F-actin as well as microtubules
Regulatory and Structural Networks in Ustilago maydis 385
participate in membrane transport (Heath 1995),
though the importance for each system is weighed
differently in each fungus. Numerous lines of evidence
indicate that F-actin has fundamental roles
in polarized growth of fungi (Harold 1990; Heath
1995). Transport along the actin cytoskeleton allows
the delivery of wall vesicles to the growing
apex where exocytosis takes place (Gow 1995). Factinisalsoinvolvedinfungalendocytosis(Kübler
and Riezman 1993; Kaksonen et al. 2003; Huckaba
et al. 2004).
U. maydis cells contain long actin cables
(Banuett and Herskowitz 2002; Weber et al. 2003)
which are generally thought to support the tipward
traffic of wall compounds needed for proper
hyphal growth (Harold 1990; Heath 1995; Ayscough
et al. 1997). Disruption of F-actin by latrunculin
A led to severe defects in morphogenesis in
yeast-like sporidia, and affected the formation and
directed growth of conjugation hyphae as well as
the development of dikaryotic hyphae. Moreover,
it was found that F-actin is essential for cell fusion
during mating of compatible cells (Fuchs et al.
2005). In addition, class V myosin Myo5, which
most likely transports membranous vesicles along
F-actin filaments, was found to be required for
hyphal growth as well as pathogenic development
(Weber et al. 2003). In higher eukaryotes as well as
in the yeasts S. cerevisiae and S. pombe, myosinV
is thought to participate in vesicle transport (Johnston
et al. 1991; Win et al. 2001), suggesting that
Myo5 might deliver vesicles containing cell wall
componentstothegrowingtip.InS. cerevisiae,
myosin V is responsible for transport of a chitin
synthase (Santos and Snyder 1997), and enzymes
of this class might also be required for tip growth
of U. maydis hyphae. Indeed, it was recently
demonstrated that five of the eight chitin synthases
present in U. maydis localize to the growth region
of yeast-like and hyphal cells, and their localization
depends on an intact F-actin cytoskeleton (Weber
et al. 2005). These data reinforce the essential role
for F-actin-based transport in polarized growth of
U. maydis.
In contrast to the well-established role of Factin
in fungal morphogenesis, the role of microtubules
in fungi is less clear (Heath 1995). Microtubules
have no obvious function in Candida albicans
(Yokoyama et al. 1990) and S. cerevisiae (Huffaker
et al. 1988), whereas microtubule integrity is
necessary for fast tip growth in Aspergillus nidulans
(Horio and Oakley 2005). Moreover, disrupting
microtubules induces multiple growth sites in