Introduction to Fungi, Third Edition

SACCHAROMYCES (SACCHAROMYCETACEAE)
271
Fig10.7 The mitotic cell cycle of Saccharomyces cerevisiae.
Actin patches and cables are drawn in white; the septin ring is
black. Secretory vesicles are not drawn, but their distribution
follows essentially that of actin patches. (a) Initiation of the
bud site occurs during the S phase by the assembly of actin
patches around a septin ring. (b) Bud extension during late S
phase.The cap co-ordinates apical bud growth by establishing
actin cables which mediate the transport of secretory vesicles
into the bud, and their fusion in the region of the cap. (c) A
later stage of bud growth; the cap components become
distributed more evenly over the bud membrane surface, and
growth is non-polarized (isodiametric). Meanwhile, the dividing
nucleus is drawn, via cytoplasmic microtubules (not
shown), to an actin/myosin ring superimposed onto the septin
ring.The nucleus divides so that each cell receives one daughter
nucleus. (d) Re-establishment of polarized growth by the
formation of two septin rings, one on either side of the bud
site.Growth leads to closure of the pore between the mother
and daughter cell. Redrawn from Sheu and Snyder (2001), with
kind permission of Springer Science and Business Media.
Useful and comprehensive reviews are those by
Pruyne and Bretscher (2000a,b) and Sheu and
Snyder (2001). The bud site is determined by the
assembly of a protein cap at the inner surface
of the plasma membrane. This cap contains an
essential regulatory protein, Cdc42p, which is
controlled directly by the cell cycle and in turn
determines the sequestration of numerous other
scaffold and regulatory proteins by the cap.
Budding differs between haploid and diploid
cells, the former initiating new buds adjacent
to the previous one, and the latter budding in
a bipolar fashion (Pruyne et al., 2004).
One important group of proteins are the
septins, which form a ring around the bud site
(Fig. 10.7a). Actin filaments are initiated from
the centre of the cap. As the bud extends in a
polarized fashion, the septin ring remains at the
site of bud emergence whereas the cap which
governs bud extension migrates with the bud,
staying at the apex and controlling bud extension
(Fig. 10.7b). Later, the cap components and
actin filament attachment points become distributed
diffusely over the bud surface. This
leads to the fusion of secretory vesicles over
the entire bud surface, and to a change in the
growth pattern from polarized to isodiametric
(Fig. 10.7c).
The septin ring binds numerous proteins,
including important regulatory ones (Versele &
Thorner, 2005). Actin and myosin are attracted
early during bud emergence, and a contractile
actin ring is superimposed on the septin ring.
Later, during nuclear division, cytoplasmic
microtubules are also captured; these, in turn,
are in contact with the microtubules of the
intranuclear mitotic spindle, and thus the dividing
nucleus is drawn towards the ring, with one
daughter nucleus apiece ending up in each of
the two cells (Fig. 10.7c; Kusch et al., 2002).
Cytokinesis is brought about as the actin ring
contracts (Lippincott & Li, 1998). At this point,
septins appear as a double ring, sandwiching
the constricting actin ring. The septin double
ring assembles two caps, and these serve as a
nucleation centre for actin filaments which
direct secretory vesicles to the bud site, closing
the wall between mother and daughter cell
(Fig. 10.7d). The regulatory mechanisms are
immensely complex but are beginning to be
unravelled (Pruyne & Bretscher, 2000a,b;
Pruyne et al., 2004; Versele & Thorner, 2005)
and are likely to be of fundamental significance
because the division of cells by a constricting
actin ring is found also in other fungi and
in animal systems, although apparently not in
plants.