Isolated centrosomes nucleate microtubules when incubated in pure tubulin solutions well below the critical concentration for spontaneous polymer assembly (approximately 15 microM instead of 60 microM). Treatment with urea (2-3 M) does not severely damage the centriole cylinders but inactivates their ability to nucleate microtubules even at high tubulin concentrations. Here we show that centrosomes inactivated by urea are functionally complemented in frog egg extracts. Centrosomes can then be reisolated on sucrose gradients and assayed in different concentrations of pure tubulin to quantify their nucleating activity. We show that the material that complements centrosomes is stored in a soluble form in the egg. Each frog egg contains enough material to complement greater than 6,000 urea-inactivated centrosomes. The material is heat inactivated above 56 degrees C. One can use this in vitro system to study how the microtubule nucleating activity of centrosomes is regulated. Native centrosomes require approximately 15 microM tubulin to begin nucleating microtubules, whereas centrosomes complemented in interphase extracts begin nucleating microtubules around 7-8 microM tubulin. Therefore, the critical tubulin concentrations for polymer assembly off native centrosomes is higher than that observed for the centrosomes first denatured and then complemented in egg extracts. In vivo, the microtubule nucleating activity of centrosomes seems to be regulated by phosphorylation at the onset of mitosis (Centonze, V. E., and G. G. Borisy. 1990. J. Cell Sci. 95:405-411). Since cyclins are major regulators of mitosis, we tested the effect of adding bacterially produced cyclins to interphase egg extracts. Both cyclin A and B activate an H1 kinase in the extracts. Cyclin A-associated kinase causes an increase in the microtubule nucleating activity of centrosomes complemented in the extract but cyclin B does not. The critical tubulin concentration for polymer assembly off centrosomes complemented in cyclin A-treated extracts is similar to that observed for centrosomes complemented in interphase extracts. However, centrosomes complemented in cyclin A treated extracts nucleate much more microtubules at high tubulin concentration. We define this as the "capacity" of centrosomes to nucleate microtubules. It seems that the microtubule nucleating activity of centrosomes can be defined by two distinct parameters: (a) the critical tubulin concentration at which they begin to nucleate microtubules and (b) their capacity to nucleate microtubules at high tubulin concentrations, the latter being modulated by phosphorylation.

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