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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.
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