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R547
Cytokinesis: Sid signals septation
Kenneth E. Sawin
Components of a novel signal transduction pathway
which initiates septation in fission yeast are located
primarily at the spindle pole body. New studies further
characterizing members of this pathway suggest a
plausible mechanism for how and why this works.
Address: Wellcome Trust Centre for Cell Biology, Institute of Cell and
Molecular Biology, University of Edinburgh, Swann Building, Mayfield
Road, Edinburgh EH9 3JR, UK.
E-mail: [email protected]
Current Biology 2000, 10:R547–R550
0960-9822/00/$ – see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Like DNA replication or chromosome segregation,
cytokinesis is a once-and-only-once event in the life of a
cell. What controls where, when and how a cell divides?
The fission yeast Schizosaccharomyces pombe has recently
become a focus for genetic studies of eukaryotic cytokinesis, and this work has revealed, among other things, a novel
signal transduction pathway controlling septation [1–3].
What is remarkable about this pathway is that nearly all of
its components reside in the nuclear-localized spindle pole
body, the yeast equivalent of the centrosome, which Boveri
[4] described one hundred years ago as “the true divisionorgan of the cell”. Papers published earlier this year from
the groups of Gould [5] and McCollum [6,7] have given a
further indication of how the spindle pole body becomes
the headquarters for this pathway, and also how the signals
generated there finally reach the division site. Interestingly, an analogous pathway also exists in the budding
yeast Saccharomyces cerevisiae, although in that organism the
same genetic hardware appears to control not septation
per se, but rather a more general exit from mitosis [8–10].
Cytokinesis in fission yeast requires the accomplishment of
three basic tasks, which are separable genetically — that is,
one can find mutants that are specifically defective in one
task and not the others [1–3,11]. First, a prospective division site at the cell cortex must be identified, based on
nuclear position, in the middle of the cell. Mutations in
genes important for this process lead to all subsequent
events occurring in the wrong place. Second, a ring of
actomyosin and associated proteins forms at the cell
middle. This occurs during mitosis itself and depends on a
large number of genes, mostly related to actin organization.
The third step of cell division occurs later, after the completion of anaphase. Patches of actin appear in the neighborhood of the medial ring, which contracts, concomitant
with the centripetal deposition of a primary septum. Division is later completed by degradation of the primary
septum; this is replaced by secondary septa on either side,
which become the new ends of the two daughter cells.
Even before any detailed molecular characterization, it was
recognized that the genes specifically involved in septum
formation are likely to function together as a single module
[12]. More recently, however, with the identification of
new components, the cloning of nearly all of the genes
involved and the elucidation of their dependency relationships, a clear pathway controlling septation has emerged.
Collectively, the genes involved in septation are referred
to as sid genes, from their ‘septum initiation defective’
mutant phenotype. Loss-of-function mutation of a sid
gene leads to a failure in deposition of septal material and
a continuation of cell growth and the nuclear cell cycle,
resulting in long, multinucleate cells [11,13]. Although all
sid mutants have essentially the same phenotype, it has
been possible to determine the order of sid genes in a
common pathway, primarily by two methods. First, overexpression or mutation of certain pathway components
leads to aberrant septation, but this can be blocked by
mutations in more ‘downstream’ genes. Second, most of
the sid gene products localize to one or both of the spindle
pole bodies in interphase and/or mitosis (see below), and
thus one can ask whether a given sid gene product is correctly localized in a given sid mutant. Together, these
types of experiment have provided a compelling picture
of how the septation signal is transduced from the spindle
pole body to the division site (see Figure 1).
At the heart of the Sid pathway is the protein kinase Cdc7p,
shown by Fankhauser and Simanis [14] to regulate septation in a dosage-dependent manner. During interphase,
Cdc7p shows no discrete localization, but in early mitosis it
becomes localized to both spindle pole bodies, at opposite
ends of the mitotic spindle, and later asymmetrically to a
single spindle pole body [15]. This asymmetric localization,
which may help to prevent the cell from forming more than
one septum, is driven by association with the GTP-bound
form of the GTPase switch protein Spg1p [15,16]. The
GTPase activity of Spg1p is itself positively regulated by
Cdc16p and Byr4p, acting as a two-component GTPaseactivating protein [17,18], and it may also be negatively regulated by the product of the cdc11+ gene acting as a
guanine-nucleotide exchange factor [16]. The most recent
work has involved the molecular cloning and characterization of additional members of the Sid pathway, and in so
doing has not only filled in the gaps but also focussed attention on the precise relationships between components, and
specifically on how the formal nature of a signalling scheme
corresponds to physical protein complexes within cells.
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Figure 1
Sid4p
Sid4p SPB scaffold?
Symmetric at SPB
Cdc16p, Byr4p
Spg1p GTPase
Total protein symmetric at SPB
GTP-bound form asymmetric at SPB
Spg1p ( GTP)
Spg1p ( GDP)
Cdc11p??
Cdc7p protein kinase
Asymmetric at SPB
Cdc7p
Sid1p protein kinase
Asymmetric at SPB
Cell-cycle timing
Sid2p protein kinase
Symmetric at SPB
Movement of signal from
'active' SPB to division site?
Sid1p
Sid2p
Cdc14p
Mob1p
Current Biology
Proposed order of sid gene products in the pathway controlling
septation in the fission yeast S. pombe. At left is shown the molecular
function and important aspects of localization of sid gene products to
the spindle pole body (SPB) during mitosis; some details have been left
out. The network at right shows how localization of different sid gene
products to the spindle pole body depends on other sid genes. An
arrow from ‘A’ to ‘B’ indicates that localization of ‘B’ to the spindle pole
body depends on ‘A’, either directly or indirectly. Note the absence of
arrows to Sid2p or Mob1p from Spg1p, Sid1p, and Cdc14p. Mob1p
localization depends only partially on Cdc7p and Cdc11p (dashed
arrows); Sid2p shows a stronger dependence on Cdc7 and Cdc11p,
presumably because of its additional dependence on Mob1p.
The work of Chang and Gould [5] has clearly put sid4+ at
the top of the pathway. Sid4p is a protein which is of
unknown function, but which has been found to be
localized to the spindle pole body throughout the cell
cycle, independently of all other sid genes. By contrast,
the localization of all Sid proteins tested to date depends
on Sid4p function. An amino-terminal region of Sid4p is
sufficient for its localization, although this is not very
efficient, suggesting that normal localization of Sid2p to
the spindle pole body may depend on multimerization of
the protein into a higher-order structure, which is
mediated through more carboxy-terminal regions.
but are also transiently associated with the division site
around the time of septation. The two proteins are also
physically associated in vivo, and while Mob1p is required
for Sid2p localization to the spindle pole body (but not
vice-versa), Sid2p is required for Mob1p to leave the
spindle pole body and relocate to the division site [7].
At the other end are Sid2p [19] and Mob1p, identified by
Hou et al. [7]. Sid2p was identified in the original sid
screen [13] and is, like Cdc7p and Sid1p (see below), a
protein kinase. However, Sid2p kinase activity peaks at
the time of septation and requires all other sid genes,
clearly putting it at the bottom of the pathway. Mob1p was
identified from database searches on the basis of its similarity to a protein in budding yeast which binds to a
budding yeast Sid2p homolog [20]. Both Sid2p and Mob1p
localize to the spindle pole body throughout the cell cycle
What is most exciting about Sid2p and Mob1p is that they
provide the missing link between the spindle pole body
and the division site. What they do there is still anyone’s
guess, but the most salient observation in this light is that
Sid2p/Mob1p localization to the division site depends on
Cdc15p, a component of the medial actomyosin ring
[7,19]. Interestingly, among medial ring mutants, cdc15
mutants are unique in that they fail to deposit any septal
material during their defective cytokinesis. So the finger is
clearly pointed at Cdc15p as a target for Sid2p/Mob1p.
The other interesting point about Sid2p and Mob1p is
that, although they are at the bottom of a formal signalling
pathway, they are found at the spindle pole body throughout interphase, and at both spindle pole bodies during
mitosis, whereas some ‘upstream’ components, such as
Dispatch
Sid1p, Cdc14p and Cdc7p, are associated with the spindle
pole body only during mitosis, and in late mitosis with a
single spindle pole body rather than both spindle poles
[7,19]. Moreover, while the asymmetric spindle pole body
localization of, for example, Sid1p and Cdc14p depends
on all upstream components of the pathway [6], the
spindle pole body localization of Sid2p and Mob1p is
independent of Spg1p, Sid1p and Cdc14p, and Mob1p
localization may also be somewhat independent of Cdc7p
and the still uncloned cdc11+ (see Figure 1).
So what looks like a linear pathway with respect to
signalling begins to seem a little more complex from a structural point of view and not, for example, simply a pathway
of successive recruitment of proteins to the spindle pole
body. How might this shape our thinking of how the signalling complexes are actually arrayed at the spindle pole
body, and how they function? Chang and Gould [5] suggest
that Sid4p may serve as a kind of scaffolding platform for
the entire Sid module. Sid4p could recruit Spg1 — and
thereby Cdc7p, and thereby Sid1p/Cdc14p, as these three
proteins depend on all upstream components for localization — and at the same time recruit a Mob1p/Sid2p
complex more or less independently of upstream components (although this may be modulated by soluble, nonspindle-pole-body-localized Cdc7p).
By contrast, the work of Guertin et al. [6] contributes
significantly to a picture of temporal control. Sid1p, a
protein kinase, and its binding partner, Cdc14p, lie
downstream of Cdc7p in the pathway, and upstream of
Sid2p. Like Spg1p–GTP and Cdc7p, Sid1p and Cdc14p
are localized to the spindle pole body only in mitosis,
and primarily to a single spindle pole body in late
mitosis. However, the temporal appearance of Sid1p and
Cdc14p at the spindle pole body is later than that of
Cdc7, and also, unlike Cdc7p, the localization of Sid1p
and Cdc14p to the spindle pole body seems to depend
on inactivation of the mitotic, cyclin-dependent kinase
composed of Cdc2p and Cdc13p, providing a compelling
link between the completion of mitosis and the onset of
septation. This link is strengthened by the fact that at
least some of the elements involved in controlling
mitotic progression in fission yeast (and budding yeast)
are enriched at the spindle pole body [10].
Thus, not only do these new papers fill out the pathway
by ordering nearly all of the known genes involved, but
also they animate it into a series of spatially and temporally controlled events to which we can relate even as nongeneticists. The model of Guertin et al. [6], with the
additional data on Mob1p and Sid4p, might be summarized as follows: Sid4p provides a landing site for Spg1p
and also Sid2p/Mob1p. In mitosis, the GTP-bound form
of Spg1p, by virtue of Cdc16p, Byr4p and Cdc11p, spatially restricts signalling to a single spindle pole body.
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Signalling is handed off to Cdc7, which is recruited as an
effector molecule of Spg1p. But Cdc7p, although active,
cannot propagate the signal to Sid1p and Cdc14p until
they are recruited to the spindle pole body, which
depends on a signal of a successful mitosis from the cellcycle machinery. The timing of firing of septation is then
provided by the kinase activity of Sid1p/Cdc14p, which
promotes activation and release of Mob1p/Sid2p from the
spindle pole body, allowing it to find the division site.
To determine whether or not this is all moonshine will
require more detailed biochemical analysis, as fractionation of complexes and the identification of the physiological kinase substrates in the Sid pathway will help to link
all of the components together. But the basic elements of
the model now provide a framework for further experiments to address the question of how the cell makes one,
and only one, septum per cell cycle. For example, what is
ultimately responsible for the asymmetry of signalling
components at the spindle pole body? Is it a chance event,
or driven by inherent asymmetries in spindle pole body
reproduction [18]? What is the precise role of
Sid2p/Mob1p at the division site, and how does it get
there? And what is the nature of the relationship between
the Sid pathway and the cell-cycle machinery, and with
the polo kinase Plo1p, which has several functions in both
mitosis and cytokinesis, and is also found at the spindle
pole body [21,22]? Answers to these and other interesting
questions should soon be forthcoming.
Acknowledgements
I thank Irina Stancheva and Hiro Ohkura for discussion and comments. In
several places for the sake of brevity I have cited recent reviews rather than
the primary literature.
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