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

990 Chapter 17: The Cell Cycle
sister chromatid
inner
kinetochore
(A) LOW TENSION
Aurora-B kinase
P
P
P
P microtubule
plus end
outer
kinetochore
BI-ORIENTATION
(B) HIGH TENSION
Ndc80 complex
POLEWARD
FORCE
and the kinetochore sends an inhibitory signal that loosens the grip of its microtubule
attachment site, allowing detachment to occur. When bi-orientation occurs,
the high tension at the kinetochore shuts off the inhibitory signal, strengthening
microtubule attachment. In animal cells, tension not only increases the affinity of
MBoC6 n17.202/17.34
the attachment site but also leads to the attachment of additional microtubules
to the kinetochore. This results in the formation of a thick kinetochore fiber composed
of multiple microtubules.
The tension-sensing mechanism depends on the protein kinase Aurora-B,
which is associated with the kinetochore and is thought to generate the inhibitory
signal that reduces the strength of microtubule attachment in the absence
of tension. It phosphorylates several components of the microtubule attachment
site, including the Ndc80 complex, decreasing the site’s affinity for a microtubule
plus end. When bi-orientation occurs, the resulting tension somehow reduces
phosphorylation by Aurora-B, thereby increasing the affinity of the attachment
site (Figure 17–34).
Following their attachment to the two spindle poles, the chromosomes are
tugged back and forth, eventually assuming a position equidistant between the
two poles, a position called the metaphase plate. In vertebrate cells, the chromosomes
then oscillate gently at the metaphase plate, awaiting the signal for the
sister chromatids to separate. The signal is produced, with a predictable lag time,
after the bi-oriented attachment of the last of the chromosomes.
Figure 17–34 How tension might
increase microtubule attachment to the
kinetochore. These diagrams illustrate
one speculative mechanism by which
bi-orientation might increase microtubule
attachment to the kinetochore. A single
kinetochore is shown for clarity; the spindle
pole is on the right. (A) When a sisterchromatid
pair is unattached to the spindle
or attached to just one spindle pole, there
is little tension between the outer and inner
kinetochores. The protein kinase Aurora-B
is tethered to the inner kinetochore and
phosphorylates the microtubule attachment
sites, including the Ndc80 complex
(blue), in the outer kinetochore as shown,
thereby reducing the affinity of microtubule
binding. Microtubules therefore associate
and dissociate rapidly, and attachment
is unstable. (B) When bi-orientation is
achieved, the forces pulling the kinetochore
toward the spindle pole are resisted by
forces pulling the other sister kinetochore
toward the opposite pole, and the resulting
tension pulls the outer kinetochore away
from the inner kinetochore. As a result,
Aurora-B is unable to reach the outer
kinetochore, and microtubule attachment
sites are not phosphorylated. Microtubule
binding affinity is therefore increased,
resulting in the stable attachment of
multiple microtubules to both kinetochores.
The dephosphorylation of outer kinetochore
proteins depends on a phosphatase that is
not shown here.
Multiple Forces Act on Chromosomes in the Spindle
Multiple mechanisms generate the forces that move chromosomes back and forth
after they are attached to the spindle, and produce the tension that is so important
for the stabilization of correct attachments. In anaphase, similar forces pull the
separated chromatids to opposite ends of the spindle. Three major spindle forces
are particularly critical, although their strength and importance vary at different
stages of mitosis.
The first major force pulls the kinetochore and its associated chromatid along
the kinetochore microtubule toward the spindle pole. It is produced by proteins at
the kinetochore itself. By an uncertain mechanism, depolymerization at the plus
end of the microtubule generates a force that pulls the kinetochore poleward. This
force pulls on chromosomes during prometaphase and metaphase but is particularly
important for moving sister chromatids toward the poles after they separate
in anaphase. Interestingly, this kinetochore-generated poleward force does not
require ATP or motor proteins. This might seem implausible at first, but it has
been shown that purified kinetochores in a test tube, with no ATP present, can
remain attached to depolymerizing microtubules and thereby move. The energy
that drives the movement is stored in the microtubule and is released when the
microtubule depolymerizes; it ultimately comes from the hydrolysis of GTP that
occurs after a tubulin subunit adds to the end of a microtubule (discussed in
Chapter 16).