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© The Rockefeller University Press, 0021-9525/1998//283 $5.00
The Journal of Cell Biology, Volume 143, Number 2, , 1998 283-295

Regular Articles

Spindle Checkpoint Protein Xmad1 Recruits Xmad2 to Unattached Kinetochores



Rey-Huei Chen*, Andrej Shevchenko§, Matthias Mann§, and Andrew W. Murray{ddagger}

* Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, New York 14853; {ddagger} Department of Physiology, University of California, San Francisco, California 94143; and § Protein and Peptide Group, European Molecular Biology Laboratory, Heidelberg, Germany

The spindle checkpoint prevents the metaphase to anaphase transition in cells containing defects in the mitotic spindle or in chromosome attachment to the spindle. When the checkpoint protein Xmad2 is depleted from Xenopus egg extracts, adding Xmad2 to its endogenous concentration fails to restore the checkpoint, suggesting that other checkpoint component(s) were depleted from the extract through their association with Xmad2. Mass spectrometry provided peptide sequences from an 85-kD protein that coimmunoprecipitates with Xmad2 from egg extracts. This information was used to clone XMAD1, which encodes a homologue of the budding yeast (Saccharomyces cerevisiae) checkpoint protein Mad1. Xmad1 is essential for establishing and maintaining the spindle checkpoint in egg extracts. Like Xmad2, Xmad1 localizes to the nuclear envelope and the nucleus during interphase, and to those kinetochores that are not bound to spindle microtubules during mitosis. Adding an anti-Xmad1 antibody to egg extracts inactivates the checkpoint and prevents Xmad2 from localizing to unbound kinetochores. In the presence of excess Xmad2, neither chromosomes nor Xmad1 are required to activate the spindle checkpoint, suggesting that the physiological role of Xmad1 is to recruit Xmad2 to kinetochores that have not bound microtubules.

Key Words: mitosis • checkpoint • kinetochore • Xenopus • Xmad1

Abbreviations used in this paper: APC, anaphase-promoting complex; BUB, budding unihibited by benzimidazole; CSF, cytostatic factor; MAD, mitotic arrest-deficient.

Address all correspondence to Rey-Huei Chen, Section of Biochemistry, Molecular and Cell Biology, Biotechnology Building, Cornell University, Ithaca, NY 14853. Tel.: (607) 255-6542. Fax: (607) 255-2428. E-mail: rc70{at}cornell.edu

M. Mann's present address is Center for Experimental BioInformatics (CEBI), Odense University, Odense, Denmark.


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