Growth, Differentiation and Sexuality

6 Septation and Cytokinesis in Fungi
J. Wendland 1 ,A.Walther 1
CONTENTS
I. Introduction ......................... 105
II. Selecting Sites of Septation .............. 106
A. Bud Site Selection
in Saccharomyces cerevisiae ........... 106
B. Placement of the Site of Cell Division
in Schizosaccharomyces pombe ......... 108
C. SeptationinFilamentousFungi........ 109
III. Protein Complexes at Septal Sites ......... 110
A. TheSeptinRing .................... 111
B. TheActo-MyosinRing............... 112
IV. Dynamic Contraction of the Acto-Myosin
Ring and Chitin Synthesis Lead to Septum
Formation ........................... 113
V. Coordination of the End of Mitosis
with Cytokinesis ...................... 116
VI. Conclusions .......................... 116
References ........................... 117
I. Introduction
Fungi display either of two growth modes:
yeast-like or filamentous. Yeast-like growth, as
exemplified by the unicellular baker’s yeast Saccharomyces
cerevisiae, is characterized by cytokinesis,
aprocessthatresultsinthepartialdegradation
of the chitin-rich septum to allow separation of
daughter cells from their mother cells. Polarized
growth in filamentous fungi produces hyphae that
are compartmentalized by septation. Septation in
filamentous fungi is carried out by proteins whose
homologs act during cytokinesis in yeast cells.
However, septa do not separate the cytoplasms of
adjacent compartments, due to the presence of
a septal pore. Chitin hydroloysis is absent at septal
sites, which allows the formation of a multicellular
mycelium. Cytokinesis and septation mark the
ultimate step of a cell cycle and, therefore, need to
1 Junior Research Group, Growth Control of Fungal Pathogens,
Leibniz Institute for Natural Products Research and Infection
Biology, Hans-Knöll Institute and Department of Microbiology,
Friedrich-Schiller University Jena, Beutenbergstr. 11a, 07745 Jena,
Germany
be tightly controlled with other processes such as
daughter cell growth/tip growth, DNA replication
and mitosis. Although the concept of cell division
is a fundamental problem of all cells, differing
solutions have been developed in bacteria, fungi,
plants, and animals. Cell division has to deal with
the following tasks:
1. a site has to be chosen as division plane,
2. protein complexes assemble at this chosen site,
3. in fungal, as well as in animal cells, a contractile
acto-myosin ring that has been formed earlier
becomes dynamic at the end of mitosis, which
4. is accompanied by chitin deposition.
Very elaborate control mechanisms, called either
mitotic exit network (MEN) or septation initiation
network (SIN), have evolved to link mitosis with
cytokinesis in yeast-like organisms, although in filamentous
ascomycetes that generate multinucleate
cellular compartments, a strict coupling of mitotic
exit and septation seems questionable. Synthesis of
a chitin-rich septum occurs both in yeast-like and
filamentous fungi. In yeast, daughter cells actively
separate from their mother cells by dissolving the
chitin ring structure, using specific chitinases. In
filamentous fungi, however, the septum remains
intact, and the cytoplasm of apical and subapical
compartments is connected via septal pores.
These differences between septation and
cytokinesis, as well as similarities between fungi
and animal cells have provided the basis for the
use of tractable fungal model systems to elucidate
the underlying mechanisms. In this chapter,
we will discuss the key events leading to septation/cytokinesis.
We start with the yeast model
systems S. cerevisiae and Schizosaccharomyces
pombe, and then compare knowledge acquired
in these organisms with results from filamentous
fungi, particularly Neurospora crassa, Aspergillus
nidulans, and Ashbya gossypii. Analysis of conserved
features is facilitated by the availability of
genome sequences for these fungi (see Table 6.1).
The Mycota I
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© Springer-Verlag Berlin Heidelberg 2006