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Vol. 20, Issue 11, 2684-2696, June 1, 2009

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XMAP215–EB1 Interaction Is Required for Proper Spindle Assembly and Chromosome Segregation in Xenopus Egg Extract

Iva Kronja^*, Anamarija Kruljac-Letunic^*, Maïwen Caudron-Herger, Peter Bieling^*, and Eric Karsenti^*

*Department of Cell Biology and Biophysics, European Molecular Biology Laboratory, 69117 Heidelberg, Germany; and Deutsches Krebsforschungszentrum and Bioquant BQ24, 69120 Heidelberg, Germany

Submitted October 22, 2008; Revised March 13, 2009; Accepted April 3, 2009
Monitoring Editor: David G. Drubin

In metaphase Xenopus egg extracts, global microtubule growth is mainly promoted by two unrelated microtubule stabilizers, end-binding protein 1 (EB1) and XMAP215. Here, we explore their role and potential redundancy in the regulation of spindle assembly and function. We find that at physiological expression levels, both proteins are required for proper spindle architecture: Spindles assembled in the absence of EB1 or at decreased XMAP215 levels are short and frequently multipolar. Moreover, the reduced density of microtubules at the equator of EB1 or XMAP215 spindles leads to faulty kinetochore–microtubule attachments. These spindles also display diminished poleward flux rates and, upon anaphase induction, they neither segregate chromosomes nor reorganize into interphasic microtubule arrays. However, EB1 and XMAP215 nonredundantly regulate spindle assembly because an excess of XMAP215 can compensate for the absence of EB1, whereas the overexpression of EB1 cannot substitute for reduced XMAP215 levels. Our data indicate that EB1 could positively regulate XMAP215 by promoting its binding to the microtubules. Finally, we show that disruption of the mitosis-specific XMAP215–EB1 interaction produces a phenotype similar to that of either EB1 or XMAP215 depletion. Therefore, the XMAP215–EB1 interaction is required for proper spindle organization and chromosome segregation in Xenopus egg extracts.

Address correspondence to: Eric Karsenti (karsenti{at}embl.de)

Abbreviations used: APC/C, anaphase-promoting complex/cyclosome; C-EB1, C-terminal fragment of EB1; C-XMAP215, C-terminal fragment of XMAP215; MAP, microtubule-associated protein; MT, microtubule; MAP, microtubule-associated protein.