Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

986 Chapter 17: The Cell Cycle
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.
Microtubule Instability Increases Greatly in Mitosis
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 microtubule
dynamics that occur during mitosis. Many of these changes result from
phosphorylation of specific proteins by M-Cdk and other mitotic protein kinases.
Mitotic Chromosomes Promote Bipolar Spindle Assembly
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 microtubules
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.
Figure 17–27 Activation of the GTPase Ran around mitotic
chromosomes. The Ran protein, like other members of the small
GTPase family (discussed in Chapter 15), can exist in two conformations
depending on whether it is bound to GDP (inactive state) or GTP (active
state). The localization of active Ran in mitosis was determined using
a protein that emits fluorescence at a specific wavelength when it is
activated by Ran-GTP. In the metaphase human cell shown here, Ran
activity (yellow and red) is highest around the chromosomes, between
the poles of the mitotic spindle (indicated by asterisks). (From P. Kaláb
et al., Nature 440:697–701, 2006. With permission from Macmillan
Publishers Ltd.)
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