Growth, Differentiation and Sexuality

permissive temperature, multiple septa form with
a wild type-like spacing, suggesting that the correct
assembly of the protein complexes at septal
sites was not inhibited (Trinci and Morris 1979).
The mutation in this sepA1 mutant was found to
be a single-point mutation resulting in an amino
acid exchange from leucine to serine at position
1369 within the FH2-domain of the SepA protein.
This, therefore, does not result in a lack of SepA
protein, but may yield a protein in which essential
interactions are blocked at elevated temperatures.
Interestingly, SepA localizes not only to septal sites
but also to hyphal tips (Sharpless and Harris 2002).
IQGAP-family members are conserved proteins
that share an N-terminal calponin homology
(CH)-domain required for actin bundling, central
‘IQ’-repeats, and a C-terminal GTPase activating
domain which is required for ring constriction
(Shannon and Li 1999). The S. cerevisiae IQGAP,
IQG1/CYK1, is an essential gene and Iqg1p/Cyk1p
is required for actin ring assembly (Epp and
Chant 1997; Lippincott and Li 1998). Iqg1p/Cyk1p
does not require formins for its localization to
the septal site – rather, it is localized via the
interaction of its IQ-repeats with a myosin light
chain (Mlc1p) that appears to be recruited by the
septin ring and localizes to the bud neck prior to,
and independently of Iqg1p/Cyk1p (Boyne et al.
2000; Shannon and Li 2000; Tolliday et al. 2002).
These data suggest independent roles for formins
and IQGAP-proteins in actin ring formation, but
the mechanistic interactions are currently unclear.
In filamentous fungi, septation results in the
compartmentalization of hyphae and, therefore,
may not be essential. An A. gossypii cyk1 mutant
strain was found to be viable, and grew with wildtype
extension rates but turned out to be aseptate.
Agcyk1 mutants failed to generate an actin ring at
presumptive septal sites (Wendland and Philippsen
2002). Further analyses demonstrated that the Ag-
Bud3p homolog plays a role in the localization of
AgCyk1p (see Sect. IV.).
In S. cerevisiae, the myosin light chain Mlc1p
interacts with Iqg1p/Cyk1p, with the conventional
type II myosin encoded by the MYO1 gene, and
also with the type V myosin, encoded in yeast by
the MYO2/MYO4/genes (Stevens and Davis 1998;
Boyne et al. 2000). Myo1p forms a ring at the presumptive
bud site in S. cerevisiae prior to bud emergence.
This ring formation requires septin ring assembly,
and loss of MYO1 leads to the inability to
form an acto-myosin ring (Bi et al. 1998). The actomyosin
ring in S. cerevisiae is formed even in the
Septation in Fungi 113
absence of one of the formins (Bni1p or Bnr1p),
or in strains lacking HOF1/CYK2 (which is related
to Schizosaccharomyces pombe cdc15 –hence,its
name ‘homolog of fifteen’). An interesting model
was postulated in which Bni1p and Myo1p form
a pathway involved in acto-myosin ring constriction,
and Bnr1p and Hof1p/Cyk2p are involved in
linking acto-myosin ring constriction with septum
formation (Kamei et al. 1998; Lippincott and Li
1998; Vallen et al. 2000). Once the assembly of protein
complexes is completed, a signal needs to be
generated to initiate acto-myosin ring constriction
and chitin synthesis at the septum (see Sect. V.).
At this point, we can ask which set of proteins
of a fungus is localized to a future site of
septation. This question has been addressed in S.
cerevisiae. In a global study including about 75%
of the whole proteome, the subcellular localization
of GFP-tagged proteins was analyzed (Ross-
Macdonald et al. 1999; Kumar et al. 2002; Huh et al.
2003). These efforts yielded close to 100 proteins
that localized either at the tip of the bud or at
the septum. Different classes of proteins were distinguished,
belonging to bud site-selection genes,
proteins involved in bud emergence, septins, proteins
required for actin ring assembly, septum formation,
secretion, and exit of mitosis. Comparison
of this gene set with other sequenced genomes of
filamentous fungi, e.g., A. gossypii and N. crassa,revealed
a large number of conserved genes, further
demonstrating a generally conserved mechanism
of septation/cytokinesis (Table 6.1).
IV. Dynamic Contraction of the Acto-
Myosin Ring and Chitin Synthesis
Lead to Septum Formation
The basic conserved feature between fungal and animal
cells is the contraction of the acto-myosin ring
during cytokinesis (Satterwhite and Pollard 1992;
Fishkind and Wang 1995). In fungal cells, this is
connected with chitin synthesis and depositioning
at the septal sites (Schmidt et al. 2002). The actomyosin
ring is not essential in S. cerevisiae and A.
gossypii, in contrast to animal cells and S. pombe
(Bi et al. 1998; Lippincott and Li 1998; Wendland
and Philippsen 2002). Finally, there are differences
in the timing of actin ring formation in different
cell systems (Vallen et al. 2000).
Infilamentous ascomycetes,septationresults in
the generation of similarly sized hyphal compart-