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© The Rockefeller University Press, 0021-9525/1997//615 $5.00
The Journal of Cell Biology, Volume 138, Number 3, , 1997 615-628

Article

Spindle Assembly in Xenopus Egg Extracts: Respective Roles of Centrosomes and Microtubule Self-Organization



Rebecca Heald, Régis Tournebize, Anja Habermann, Eric Karsenti, and Anthony Hyman

Cell Biology Program, European Molecular Biology Laboratory, 69117 Heidelberg, Germany

In Xenopus egg extracts, spindles assembled around sperm nuclei contain a centrosome at each pole, while those assembled around chromatin beads do not. Poles can also form in the absence of chromatin, after addition of a microtubule stabilizing agent to extracts. Using this system, we have asked (a) how are spindle poles formed, and (b) how does the nucleation and organization of microtubules by centrosomes influence spindle assembly? We have found that poles are morphologically similar regardless of their origin. In all cases, microtubule organization into poles requires minus end–directed translocation of microtubules by cytoplasmic dynein, which tethers centrosomes to spindle poles. However, in the absence of pole formation, microtubules are still sorted into an antiparallel array around mitotic chromatin. Therefore, other activities in addition to dynein must contribute to the polarized orientation of microtubules in spindles. When centrosomes are present, they provide dominant sites for pole formation. Thus, in Xenopus egg extracts, centrosomes are not necessarily required for spindle assembly but can regulate the organization of microtubules into a bipolar array.

1. Abbreviation used in this paper: NuMA, nuclear protein that associates with mitotic apparatus.

Please address all correspondence to Dr. Rebecca Heald, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany. Tel.: 49-6221-387324. Fax: 49-6221-387306. E-mail:Heald{at}EMBL-Heidelberg.de

As of October 1997, Dr. Heald's address will be Department of Molecular and Cell Biology, University of California, Berkeley, 311 Life Sciences Addition, Berkeley, CA 94720.


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