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

MICROTUBULES
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its activity and shifts the balance of its competition with catastrophe factors. This
shift results in a tenfold increase in the dynamic instability of microtubules during
mitosis, a transition that is critical for the efficient construction of the mitotic
spindle (discussed in Chapter 17).
In many cells, the minus ends of microtubules are stabilized by association
with a capping protein or the centrosome, or else they serve as microtubule depolymerization
sites. The plus ends, in contrast, efficiently explore and probe the
entire cell space. Microtubule-associated proteins called plus-end tracking proteins
(+TIPs) accumulate at these active ends and appear to rocket around the cell
as passengers at the ends of rapidly growing microtubules, dissociating from the
ends when the microtubules begin to shrink (Figure 16–53).
The kinesin-related catastrophe factors and XMAP215 mentioned above
behave as +TIPs and act to modulate the growth and MBoC6 shrinkage m16.45/16.53 of the microtubule
end to which they are attached. Other +TIPs control microtubule positioning by
helping to capture and stabilize the growing microtubule end at specific cellular
targets, such as the cell cortex or the kinetochore of a mitotic chromosome.
EB1 and its relatives, small dimeric proteins that are highly conserved in animals,
plants, and fungi, are key players in this process. EB1 proteins do not actively
move toward plus ends, but rather recognize a structural feature of the growing
plus end (see Figure 16–53). Several of the +TIPs depend on EB1 proteins for their
plus-end accumulation and also interact with each other and with the microtubule
lattice. By attaching to the plus end, these factors allow the cell to harness the
energy of microtubule polymerization to generate pushing forces that can be used
for positioning the spindle, chromosomes, or organelles.
Tubulin-Sequestering and Microtubule-Severing Proteins
Destabilize Microtubules
As it does with actin monomers, the cell sequesters unpolymerized tubulin subunits
to maintain a pool of active subunits at a level near the critical concentration.
One molecule of the small protein stathmin (also called Op18) binds to two
tubulin heterodimers and prevents their addition to the ends of microtubules
(Figure 16–54). Stathmin thus decreases the effective concentration of tubulin
subunits that are available for polymerization (an action analogous to that of the
drug colchicine), and enhances the likelihood that a growing microtubule will
switch to the shrinking state. Phosphorylation of stathmin inhibits its binding to
tubulin, and signals that cause stathmin phosphorylation can increase the rate
of microtubule elongation and suppress dynamic instability. Stathmin has been
implicated in the regulation of both cell proliferation and cell death. Interestingly,
mice lacking stathmin develop normally but are less fearful than wild-type mice,
reflecting a role for stathmin in neurons of the amygdala, where it is normally
expressed at high levels.
Severing is another mechanism employed by the cell to destabilize microtubules.
To sever a microtubule, thirteen longitudinal bonds must be broken, one
for each protofilament. The protein katanin, named after the Japanese word for
Figure 16–53 +TIP proteins found at the
growing plus ends of microtubules.
(A) Frames from a fluorescence time-lapse
movie of the edge of a cell expressing
fluorescently labeled tubulin that incorporates
into microtubules (red) as well as the +TIP
protein EB1 tagged with a different color
(green). The same microtubule is marked
(asterisk) in successive movie frames. When
the microtubule is growing (frames 1, 2),
EB1 is associated with the tip. When the
microtubule undergoes a catastrophe and
begins shrinking, EB1 is lost (frames 3, 4).
The labeled EB1 is regained when growth
of the microtubule is rescued (frame 5).
See Movie 16.8. (B) In the fission yeast
Schizosaccharomyces pombe, the plus ends
of the microtubules (green) are associated
with the homolog of EB1 (red) at the two
poles of the rod-shaped cells. (A, courtesy
of Anna Akhmanova and Ilya Grigoriev;
B, courtesy of Takeshi Toda.)
stathmin
tubulin dimer
2.5 nm
Figure 16–54 Sequestration of tubulin by
stathmin. Structural studies with electron
microscopy and crystallography suggest
that the elongated stathmin protein binds
along the side of two tubulin heterodimers.
(Adapted from M.O. Steinmetz et al.,
EMBO J. 19:572–580, 2000. With
permission MBoC6 from n16.204/16.54
John Wiley and Sons.)