Growth, Differentiation and Sexuality

Fig. 6.1. Selection of a cell division site. A In Saccharomyces
cerevisiae, the mother cell is bigger than the daughter cell.
The mother cell bears a prominent bud scar at the septal site,
while the daughter cell has a fainter birth scar (due to Cts1p
chitinase activity –see below).Cell poles can be defined such
thattheproximalpoleistheonewithwhichthedaughtercell
was connected to the mother cell. Hence, the opposite pole
is defined as the distal pole. Each budding event leads to the
formation of a permanent mark – the chitin-rich bud scar.
Two cell type-specific budding patterns determine bud site
selection (and thus the position of the cleavage plane), and
give rise to a distinctive distribution of bud scars. The axial
budding pattern of haploid a orα cells leads to the formation
of bud scar rows on the proximal cell pole, whereas diploid
(a/α) cells use both poles for budding. A new bud scar in
haploid cells is positioned adjacent to the bud scar of the
preceding cell cycle, based on the expression of haploidspecific
landmark proteins. Relevant genes for each budding
pattern are listed. Lack of either BUD1, BUD2,orBUD5 leads
to loss of positional information and random budding. B In
Schizosaccharomyces pombe, selection of the cleavage plane
is dependent on the positioning of Mid1p. Mid1p localizes to
the nucleus and, upon its phosphorylation, exits the nucleus
to form a band in the vicinity of the nucleus, which serves
as a landmark to assemble protein complexes for septation,
e.g., the acto-myosin ring
sitioning mutants were observed to affect the localization
of Mid1p (Chang 2001). The localization
of Mid1p may be facilitated by the pleckstrin homology
(PH)-domain of the protein that could, for
example, establish interactions with lipids in the
cell membrane. Based on its sequence similarity
to Mid1p, another protein, Mid2p, was identified
(Berlin et al. 2003). However, both proteins have
divergent functions (see below; Tasto et al. 2003).
Homologs of Mid1p, termed anillin, were found
in Drosophila and human cells in which cell cycle
localization patterns were similar to that in S.
pombe, indicating that the mechanisms of cleavage
plane selection in S. pombe and mammalian
Septation in Fungi 109
cells are based on conserved molecular machineries
(Oegema et al. 2000).
C. Septation in Filamentous Fungi
In filamentous ascomycetes such as N. crassa, A.
nidulans and A. gossypii, the search for homologs
either of the S. cerevisiae Budproteinsorofthe
S. pombe Mid1p revealed a number of proteins
with high similarity to the yeast Bud proteins but
no clear homolog to Mid1p (Wendland 2003).
However, selection of septation sites in filamentous
fungi depends on different cell types, or the developmental
stage of particular cells. In A. nidulans
and A. gossypii, germination of spores produces
round-shaped germ cells (Fig. 6.2A). From these
germ cells, hyphae are branched off in a characteristic
bipolar germination pattern similar to bipolar
Fig. 6.2. Septation pattern in filamentous fungi. A Spore
germination in Ashbya gossypii leads to the formation of
a round-shaped germ cell. A bipolar germination pattern
results in the successive protrusion of two hyphae. Septa
are positioned at the neck between germ cell and hypha.
Hyphae are compartmentalized by septation, which occurs
in regular intervals. Mature mycelia of A. gossypii display
a characteristic dichotomous tip branching pattern. This
is remarkable, since both branches simultaneously form
septa at their bases. B Hyphal induction in Candida albicans
results in germ tube formation out of a yeast cell.
The position of the first septum is peculiar, since it is not
formed at the bud neck but rather 10–15 μm within the hyphal
tube (reason unknown). C Aspergillus nidulans forms
specialized cell types during condiogenesis. The metulae,
phialides and conidiospores are uninucleate, and metulae
and phialides show a unipolar budding pattern