also phosphorylates components of the nuclear lamina, the structural framework beneath the envelope. The phosphorylation of these lamina components and of several inner-nuclear-envelope proteins leads to disassembly of the nuclear lamina and the breakdown of the envelope membranes into small vesicles.
Most animal cells in interphase contain a cytoplasmic array of microtubules radiating out from the single centrosome. As discussed in Chapter 16, the microtubules of this interphase array are in a state of dynamic instability, in which individual microtubules are either growing or shrinking and stochastically switch between the two states. The switch from growth to shrinkage is called a catastrophe, and the switch from shrinkage to growth is called a rescue. New microtubules are continually being created to balance the loss of those that disappear completely by depolymerization.
Entry into mitosis signals an abrupt change in the cell’s microtubules. The interphase array of few, long microtubules radiating from the single centrosome is converted to a larger number of shorter and more dynamic microtubules emanating from both centrosomes. During prophase, and particularly in prometaphase and metaphase (see Panel 17–1), the half-life of microtubules decreases dramatically. This increase in microtubule instability, coupled with the increased ability of centrosomes to nucleate microtubules as mentioned earlier, results in remarkably dense and dynamic arrays of spindle microtubules that are ideally suited for capturing sister chromatids.
Microtubule dynamics are controlled in the cell by a variety of regulatory proteins, including microtubule-associated proteins (MAPs) that promote stability and catastrophe factors that destabilize microtubule plus ends. Changes in the activities of these regulatory proteins are responsible for the changes in micro-tubule dynamics that occur during mitosis. Many of these changes result from phosphorylation of specific proteins by M-Cdk and other mitotic protein kinases.
Chromosomes are not just passive passengers in the process of spindle assembly. By creating a local environment that favors both microtubule nucleation and microtubule stabilization, they play an active part in spindle formation. The influence of the chromosomes can be demonstrated by using a fine glass needle to reposition them after the spindle has formed. For some cells in metaphase, if a single chromosome is tugged out of alignment, a mass of new spindle micro-tubules rapidly appears around the newly positioned chromosome, while the spindle microtubules at the chromosome’s former position depolymerize. This property of the chromosomes seems to depend, at least in part, on a guanine nucleotide exchange factor (GEF) that is bound to chromatin; the GEF stimulates a small GTPase in the cytosol called Ran to bind GTP in place of GDP. The activated Ran-GTP, which is also involved in nuclear transport (discussed in Chapter 12), releases microtubule-stabilizing proteins from protein complexes in the cytosol, thereby stimulating the local nucleation and stabilization of microtubules around chromosomes (Figure 17–27). Local microtubule stabilization is also promoted by the protein kinase Aurora-B, which associates with mitotic chromosomes.