LB6
The Rac activator Tiam1 regulates centrosome movement to allow efficient chromosome congression in mitosis
Helen Rushton, Simon Woodcock, Eduardo Castaeda-Saucedo, Gavin White, Angeliki Malliri
Paterson Institute for Cancer Research, University of Manchester, UK
Background
Centrosome separation in vertebrates is critical for bipolar spindle formation
and subsequent chromosome segregation during mitosis. The microtubule motor
Kinesin-5 (Eg5) is essential to push centrosomes apart during bipolar spindle
assembly; its suppression results in monopolar spindles and mitotic arrest.
However, the forces that antagonise Eg5 during centrosome separation in
prophase (before NEBD) are currently unknown. Tiam1, a specific activator of
the GTPase Rac, is required for optimal proliferation of cells in vitro
[1,2] and of tumours in vivo [2,3], but its function during the
cell cycle has not previously been elucidated. Here, we find a novel function
for Tiam1 in the generation of an inward force which antagonises Eg5 in the
forming mitotic spindle, and is required for efficient chromosome congression
and mitotic progression.
Results
We show, using an inducible RNAi system, that depletion of Tiam1 in MDCK
cells leads to an increased mitotic index, which is caused by a pro-longed
mitosis due to inefficiencies in chromosome alignment during pro-metaphase.
Furthermore, we find that Tiam1 and Rac localise at centrosomes during prophase
and pro-metaphase, and that Tiam1-depleted cells have increased
inter-centrosomal distance at these stages, apparently due to an increased rate
of centrosome separation in prophase. Moreover, cells lacking Tiam1 more
readily escape the monopolar spindle mitotic arrest induced by the partial
inhibition of Eg5. Significantly, reduction of Eg5 activity in Tiam1-depleted
cells rectifies the balance of forces operating during bipolar spindle
assembly, rescuing not only their increased centrosomal separation, but also
the chromosome congression errors and mitotic delay.
Conclusion
Our data identify Tiam1/Rac signalling as the first antagonist of
centrosome separation during prophase, demonstrate its requirement in the
balance of forces during bipolar spindle assembly, and show that proper
centrosome separation in prophase is necessary for the efficient congression of
chromosomes in mitosis.
References
[1] Malliri, A., van Es, S., Huveneers, S. & Collard, J.G. The Rac
exchange factor Tiam1 is required for the establishment and maintenance of
cadherin-based adhesions. J Biol Chem 279, 30092-30098 (2004).
[2] Malliri, A. et al. The Rac activator Tiam1 is a Wnt-responsive gene that
modifies intestinal tumour development. J Biol Chem 281, 543-548 (2006).
[3] Malliri, A. et al. Mice deficient in the Rac activator Tiam1 are resistant
to Ras-induced skin tumours. Nature 417, 867-871 (2002).
