SPINDLE ASSEMBLY IN XENOPUS EGG EXTRACTS - RESPECTIVE ROLES OF CENTROSOMES AND MICROTUBULE SELF-ORGANIZATION

In Xenopus egg extracts, spindles assembled around sperm nuclei contai n a centrosome at each pole, while those assembled around chromatin be ads do not. Poles can also form in the absence of chromatin, after add ition of a microtubule stabilizing agent to extracts. Using this syste m, we have asked (a) how are spindle poles formed, and (b) how does th e nucleation and organization of microtubules by centrosomes influence spindle assembly? We have found that poles are morphologically simila r regardless of their origin, In all cases, microtubule organization i nto poles requires minus end-directed translocation of microtubules by cytoplasmic dynein, which tethers centrosomes to spindle poles. Howev er, in the absence of pole formation, microtubules are still sorted in to an antiparallel array around mitotic chromatin, Therefore, other ac tivities in addition to dynein must contribute to the polarized orient ation of microtubules in spindles, When centrosomes are present, they provide dominant sites for pole formation. Thus, in Xenopus egg extrac ts, centrosomes are not necessarily required for spindle assembly but can regulate the organization of microtubules into a bipolar array.