IMP Research Report 2002

Figure 2: Centralspindlin localizes to the spindle midzone in
anaphase. A mammalian cell in anaphase has been stained for
MKLP-1 (the mammalian ZEN-4 ortholog; green), tubulin (red) and
DNA (blue).
Figure 3: A schematic model of the centralspindlin complex. The
interacting domains of CYK-4 (blue) and ZEN-4 (red) have been
defined and the nature of the complex studied with biochemical
techniques. Based on these data, we have proposed this working
model of centralspindlin bound to a microtubule (green).
Central spindle assembly begins at the metaphase to
anaphase transition, when chromosomes move
polewards on the shrinking kinetochore microtubules.
At this time, spindle microtubules become bundled to
form the central spindle (Figure 2). We have found an
evolutionarily conserved protein complex, the
centralspindlin complex, consisting of a Rho family GAP,
CYK-4, and a kinesin like protein, ZEN-4, that is directly
involved in central spindle assembly (Figure 3).
Embryos deficient for CYK-4 or ZEN-4 are defective in
member of this kinase complex, CSC-1, which forms a
complex with ICP-1 and the survivin-like protein BIR-1.
We have reconstituted this tetrameric kinase complex
with recombinant proteins and are investigating its
regulation. ICP-1 and AIR-2 promote the stable
localization of ZEN-4 to the central spindle. We are
investigating whether ZEN-4 or CYK-4 is a direct
substrate of the AIR-2 kinase and if so, how
phosphorylation affects the activity of the centralspindlin
complex.
both central spindle assembly and cytokinesis. CYK-4
contains a GAP domain that stimulates GTP hydrolysis
by Rho-family GTPases. It is likely that CYK-4 promotes
the completion of cytokinesis by virtue of its ability to
promote GTP hydrolysis by Rho. We have reconstituted
centralspindlin-mediated microtubule bundling in vitro
and are using this system to understand how this
structurally unique kinesin-like protein functions at the
molecular level and how it is regulated in both space
and time.
The analysis of cytokinesis in C. elegans embryos
lacking a central spindle reveals that this structure is
not required at the early stages of cytokinesis. However,
the central spindle does appear to have a critical role
in furrow formation in other systems. We have recently
gained insight into this apparent discrepancy. We have
found that the central spindle is required for furrow
initiation even in C. elegans embryos, but only under
certain conditions. We are further studying the
properties of the mitotic spindle that underlie its ability
A second protein that localizes to the central spindle is
the Aurora-B like kinase AIR-2. In previous years we
demonstrated that Aurora-B binds to ICP-1 (the
nematode Incenp) and that ICP-1 is required for
localization of this kinase. Embryos lacking AIR-2 and
to induce a single, precisely positioned, cleavage furrow
and are beginning to dissect the molecular basis of this
process.
Contact: mglotzer@imp.univie.ac.at
ICP-1 are defective in both chromosome segregation
and cytokinesis. We have recently identified another
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