Growth, Differentiation and Sexuality

112 J. Wendland and A. Walther
in a screen for cell division cycle (CDC) mutants
(Hartwell 1971). Inactivation of any one of the
septins, e.g., by placing temperature-sensitive alleles
at the restrictive temperature, causes a failure to
form a septin ring and lethality, whereas deletion
of SHS1 causes only mild defects (Carroll et al.
1998). A recent study analyzed the protein–protein
interactions among septins, also with respect
to the potential to form septin filaments. Based
on these very elegant studies, a model has been
proposed in which a heteropentameric complex of
septins is polymerized into the septin ring (Versele
et al. 2004).
Such a septin ring may serve multiple functions:
first, as a rigid backbone that could stabilize
the neck region; second, as a scaffold that could
direct the assembly and/or attachment of other
proteins required for septation; third, by providing
a diffusion barrier that aids in localized morphogenesis
of only the daughter cell, whilst the
mother cell does not grow in a budding cycle; and
fourth, as a positional cue for proper spindle positioning
(Barral et al. 2000; Takizawa et al. 2000;
Faty et al. 2002; Kusch et al. 2002; Longtine and Bi
2003). In S. cerevisiae, the PAK-kinase Cla4p, which
is a downstream effector protein of Cdc42p, plays
an essential role in septin ring assembly (Schmidt
et al. 2003). This demonstrates a signaling cascade
from Cdc42p via Cla4p to septins in which Cla4p
directly interacts with, and also phosphorylates
septins. Furthermore, besides Cla4p, another effector
of Cdc42p, the formin Bni1, is involved in the
assembly of the septin ring during the early phase
of bud emergence. Other studies also demonstrated
the involvement of GTP–GDP cycles of Cdc42p, and
of the actin cytoskeleton in this process (Gladfelter
et al. 2002; Kadota et al. 2004; Versele et al. 2004).
B. The Acto-Myosin Ring
The assembly and dynamic behavior (see next section)
of the acto-myosin ring is a highly conserved
feature in fungal and animal cytokinesis (Noguchi
et al. 2001). The Cla4p kinase plays a key role in
actin ring formation at septal sites, since a deletion
of CLA4 in A. gossypii resulted in defects in
actin ring formation (Ayad-Durieux et al. 2000).
TheroleofCla4pmaybeindirectandcoupledto
septin function (Fig. 6.3B). Other proteins important
for acto-myosin ring formation in filamentous
fungi may be involved in specific transport processestoseptalsites,aswellasinactinfilament
assembly. In yeast, a cell cycle-dependent reorganization
of the actin cytoskeleton directs growth either
to the bud tip or to the septum. In filamentous
fungi, polarization of the actin cytoskeleton can be
found simultaneously at the tip and at septal sites
(Wendland 2001). The A. gossypii Wiskott-Aldrich
Syndrome protein (WASP)-homolog Wal1p is required
for actin ring formation, and also for apical
positioning of cortical actin patches (Walther
and Wendland 2004). The requirement of Wal1p for
actinringformationmaythusbedueeithertoafailure
in directed vesicle transport (as a secondary
effect of correct positioning of actin patches) or to
the requirement of WASP in the assembly of actin
filaments needed for actin ring formation.
Two other conserved protein families are essential
for actin ring formation. These are members of
the formin and IQGAP protein families (Bi 2001).
Formins are homologs of the limb deformity gene
in the mouse, and share formin homology (FH)domains.
IQGAP-family proteins contain domains
with repeated IQ-amino acid motifs – in fungi
containing a consensus motif of ‘QxxxRGxxxR’ in
which x is any amino acid (see Wendland and
Philippsen 2002) – and a Ras-GTPase activating
protein domain.
The S. cerevisiae genome contains two formins,
encoded by the BNI1 and BNR1 genes. They interact
via N-terminal G-protein binding domains
with Rho-type GTPases, and are thereby activated
(Dong et al. 2003; Evangelista et al. 2003). In S.
cerevisiae, the formin-dependent pathway of actin
ring formation is regulated via the Rho1-GTPase,
and not via Cdc42p (Tolliday et al. 2002). In Bni1p,
the central FH3- and Spa2-binding domains that
may help in the localization of the protein are followed
by a proline-rich FH1, a FH2-dimerization,
and a Bud6-binding domain (Petersen et al. 1998;
Ozaki-Kuroda et al. 2001). Spa2p, Bud6p, and Bni1p
form a protein complex termed the polarisome in
S. cerevisiae, which is required for polarized morphogenesis
(Evangelista et al. 1997; Fujiwara et al.
1998; Sheu et al. 1998). Formin mutants in S. pombe
(cdc12) orDrosophila melanogaster (dia) failto
form an actin ring at cleavage sites that is, however,
formed in S. cerevisiae bni1 mutants (Chang
et al. 1997; Afshar et al. 2000; Vallen et al. 2000).
A formin mutant in a filamentous fungus has been
described so far only in A. nidulans. The A. nidulans
formin SepA was shown to be required for
actin ring formation (Sharpless and Harris 2002).
Temperature-sensitive sepA mutants are aseptate
at the restrictive temperature, but upon shift to the