Introduction to Fungi, Third Edition

258 ARCHIASCOMYCETES
Fig 9.6 The cytoskeleton and the cell cycle in
Schizosaccharomyces pombe. A new cell initially grows only at
the old end (a) before bipolar growth is assumed (b). In
growing cells, microtubules (dark lines) are orientated
longitudinally, forming a basket around the nucleus (large
sphere) and projecting into the poles. Actin patches (white
dots) are located in the growing tips, as they are in
filamentous fungi. At mitosis (c), microtubules form the
intranuclear spindle while actin aggregates to form a cortical
ring in the vicinity of the dividing nucleus (d,e).The ring
constricts as the wall of the septum is laid down; upon
completion of nuclear migration into the poles, the nuclear
spindlebreaksdownandthebasketofcytoplasmic
microtubules reforms (f). After cell fission has been
completed (g), actin relocates into the old end (h) at which
growth is resumed. Redrawn from Brunner and Nurse (2000),
Philosophical Transactions ofthe Royal Society,bycopyright
permission of The Royal Society.
9.3.2 Morphogenesis in S. pombe
Many aspects of the cell biology and ultrastructure
of S. pombe have been studied extensively,
and the results pertaining to the cytoskeleton
are of particular relevance to filamentous fungi.
Freshly divided cells of S. pombe grow only
at one end (the ‘old end’ opposite the septum),
i.e. growth is initially monopolar (Fig. 9.6a)
before bipolar growth starts in early G2 phase
(Fig. 9.6b). In growing cells, microtubules are
located in the cytoplasm and are orientated
longitudinally, enclosing the nucleus like a
basket. Mutant and inhibitor studies have
revealed a crucial role for microtubules in
co-ordinating polarized growth, i.e. in focusing
cell wall extension to either or both of the
two opposite poles (Brunner & Nurse, 2000;
Hayles & Nurse, 2001). Microtubules seem to be
involved in transporting the Tea1 protein to the
poles. This protein acts as a termination signal
for microtubule elongation, and by
attracting microtubules it effectively controls
its own transport (Mata & Nurse, 1998; Sawin
& Nurse, 1998). Interactions between Tea1p
and other proteins mark the cell end, i.e. the
site of Tea1p accumulation, thereby fixing the
growth direction in S. pombe (Niccoli et al., 2003;
Sawin & Snaith, 2004). Actin is also located in the
growing poles of S. pombe. We can speculate
that actin filaments and microtubules fulfil
a similar role in S. pombe as in the apices of
filamentous fungi, with microtubules mediating
long-distance transport and actin fine-tuning
the fusion of secretory vesicles with the plasma
membrane.
When mitosis starts, there is a complete
remodelling of the cytoskeleton, with the disappearance
of cytoplasmic microtubules and the