Solubilization of aster-forming proteins from yeast: Possible constituents of spindle pole body and reconstitution of asters in vitro.

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Abstract
Spindle pole bodies (SPBs) attached to nuclei were isolated from Saccharomyces cerevisiae; they nucleated microtubules to form asters in vitro. This aster-forming activity was stable in 30% dimethylsulfoxide or 10% 2-propanol and could be solubilized with KCl. The KCl extract contained many protein components, which aggregated upon dialysis against a low concentration salt solution. When incubated with tubulin, the aggregates formed asters. Measurements of the elongation rates of the astral microtubules indicated that the microtubules were nucleated from the SPBs or from the aggregates reconstructed from the KCl extract by dialysis. The plus end was distal to the astral center, as in the case of the microtubule organizing center (MTOC) of mammalian cells. We suggest that the proteins extracted with KCl are responsible for microtubule nucleation in SPBs and that this SPB seems to have the same mechanism for microtubule nucleation as the MTOC in higher eukaryotes.