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Published 3 January 2006. doi:10.1083/jcb.200508180
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 172, Number 1, 19-25
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Xorbit/CLASP links dynamic microtubules to chromosomes in the Xenopus meiotic spindle



Eva Hannak and Rebecca Heald

Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720

Correspondence to Rebecca Heald: heald{at}socrates.berkeley.edu


Abstract

A family of microtubule (MT)-binding proteins, Orbit/multiple asters/cytoplasmic linker protein–associated protein, has emerged as an important player during mitosis, but their functional mechanisms are poorly understood. In this study, we used meiotic egg extracts to gain insight into the role of the Xenopus laevis homologue Xorbit in spindle assembly and function. Xorbit immunodepletion or its inhibition by a dominant-negative fragment resulted in chromosome alignment defects and aberrant MT structures, including monopolar and small spindles. Xorbit-depleted extracts failed to nucleate MTs around chromatin-coated beads, indicating its essential requirement for spindle assembly in the absence of centrosomes and kinetochores. Xorbit's MT stabilizing effect was most apparent during anaphase, when spindle MTs depolymerized rapidly upon Xorbit inhibition. Biochemical interaction between a COOH-terminal Xorbit fragment and the kinetochore-associated kinesin centromeric protein E may contribute to Xorbit's role in chromosome congression. We propose that Xorbit tethers dynamic MT plus ends to kinetochores and chromatin, providing a stabilizing activity that is crucial for spindle assembly and chromosome segregation.

Abbreviations used in this paper: APC, adenomatous polyposis coli; CENP-E, centromeric protein E; CLASP, CLIP-associated protein; CLIP, cytoplasmic linker protein; CSF, cytostatic factor arrested; CT, COOH terminal; MT, microtubule.


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