Growth, Differentiation and Sexuality

106 J. Wendland and A. Walther
Therefore, the relevant data of fungal homologs
of genes involved in the septation process will be
analyzed. In the last section, open questions and
new research directions will be discussed that may
help to define mechanistic differences between
septation and cytokinesis.
II. Selecting Sites of Septation
Fungal cells employ at least three different mechanisms
for the selection of septal sites, differing in
the timing and mechanism of site selection. In the
budding yeasts, for example, in S. cerevisiae, the
septal site is determined with the choice of bud site
selection. In the fission yeast S. pombe, positioning
of the division plane, and thus the septal site
resembles that of animal cells. Filamentous fungi,
such as A. nidulans, position septal sites in different
ways depending on the developmental state of
cells, e.g., germ cells, hyphal cells, or reproductive
cells involved, for example, in conidiation.
A. Bud Site Selection
in Saccharomyces cerevisiae
In S. cerevisiae, the septal site is chosen at the beginning
of a new cell cycle prior to DNA replication
and spindle formation. This defines the position
at which a bud will be formed, and requires
the correct alignment of the mitotic spindle with
the mother bud axis later in the cell cycle. S. cerevisiae
displays a cell type-specific pattern of bud
site selection (Freifelder 1960). Axial budding of
haploid cells (a or α cell types) restricts selection
of bud sites to proximal cell poles with which
daughter cells were connected to their mother cells
(Fig. 6.1A). Bipolar budding of diploid cells (a/α cell
type) allows buds to be formed at both cell poles
(Chant and Pringle 1991, 1995). The choice of bud
site is governed by landmark proteins that are localized
to the cell cortex. The BUD3, BUD4, AXL1,
and BUD10/AXL2 genes encode specific marker
proteins for the axial budding pattern. These proteins
are localized in a transient manner and drive
the bud site selection of the next cell cycle (Chant
and Herskowitz 1991; Halme et al. 1996; Roemer
et al. 1996; Sanders and Herskowitz 1996). Strikingly,
upon re-feeding after a period of starvation,
haploid cells can produce new buds at nonaxial/random
positions, which may allow these cells
toproperlyrespondtonewnutrientsources(Chant
and Pringle 1995). The AXL1 gene was identified as
a key determinant for the axial budding pattern.
Its expression is limited to haploid cell types, and
ectopic expression of AXL1 in diploid cells imposes
the axial budding pattern on these cells (Fujita et al.
1994; Lord et al. 2002). Interestingly, single deletions
of genes required for the axial budding pattern
converts bud site selection to the bipolar budding
pattern in haploid cell types, whereas no phenotype
in diploid cells was observed. In contrast to
haploid landmark proteins, diploid landmarks are
positioned in a persistent manner (Madden and
Snyder 1998; Ni and Snyder 2001). Relevant proteins
for the bipolar budding pattern are Bud8,
Bud9, and Rax1. BUD8 encodes a protein that is
localized to the distal cell pole, whereas Bud9 protein
is localized to the proximal cell pole. Thus,
two distinct proteins are used for marking both
cell poles in diploids (Zahner et al. 1996; Taheri
et al. 2000; Harkins et al. 2001). Deletion of either
BUD8 or BUD9 leads to budding either at the
proximal pole (in bud8/bud8 mutants) or at the
distal pole (in bud9/bud9 mutants). The Bud8 and
Bud9 proteins are highly similar (as a result of gene
duplication and subsequent divergence), and promoter
analyses indicated that the localization of the
resulting proteins is dependent on the cell cycleregulated
expression of the corresponding genes
(Schenkman et al. 2002). Rax1p is necessary for
the establishment of the bipolar budding pattern
and is required for the localization of Bud8 (Fujita
et al. 2004). Rax1p and Rax2p interact with each
other and with Bud8p and Bud9, suggesting a common
function in the establishment of a cortical
landmark used for bipolar bud site selection (Kang
et al. 2004b). Signaling of the cell type-specific landmark
proteins converges on a GTPase module consisting
of the Bud1p/Rsr1p GTPase, its guaninenucleotide
exchange factor (GEF) Bud5p, and its
GTPase-activating protein (GAP) Bud2p. Deletion
of either one of these genes results in random budding
in all cell types, indicating that this module
is required to correctly position the cell polarity
establishment machinery (Chant and Herskowitz
1991; Chant et al. 1991; Park et al. 1993; Kozminski
et al. 2003). To convey positional information, the
landmark proteins need to be able to specifically
activate the Bud1p/Rsr1p module to implement
the correct budding pattern. Since Bud5p activates
Bud1p/Rsr1p by loading the protein with GTP, interactions
of landmark proteins with Bud5p seem
to be crucial. Such interactions have been demonstrated
for Bud10p/Axl2p and Bud5p as well as for