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

932 Chapter 16: The Cytoskeleton
centrosome
containing centriole
pair
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cut
here
with
needle
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melanophore cell
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severed
cell
fragment
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WAIT
4 HOURS
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new microtubuleorganizing
center lacking
centrioles
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cell fragment
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reorganized
microtubules
Figure 16–49 A microtubule array can
find the center of a cell. After the arm of a
fish pigment cell is cut off with a needle, the
microtubules in the detached cell fragment
reorganize so that their minus ends end up
near the center of the fragment, buried in a
new microtubule-organizing center.
apical plasma membrane. From this asymmetrical location, a microtubule array
extends along the long axis of the cell, with plus ends directed toward the basal
surface (see Figure 16–4).
Microtubule-Binding Proteins Modulate Filament Dynamics and
Organization
Microtubule polymerization dynamics are very different in cells than in solutions
of pure tubulin. Microtubules in cells
MBoC6
exhibit
m16.33/16.49
a much higher polymerization rate
(typically 10–15 μm/min, relative to about 1.5 μm/min with purified tubulin at
similar concentrations), a greater catastrophe frequency, and extended pauses in
microtubule growth, a dynamic behavior rarely observed in pure tubulin solutions.
These and other differences arise because microtubule dynamics inside the
cell are governed by a variety of proteins that bind tubulin dimers or microtubules,
as summarized in Panel 16–4.
Proteins that bind to microtubules are collectively called microtubule-associated
proteins, or MAPs. Some MAPs can stabilize microtubules against disassembly.
A subset of MAPs can also mediate the interaction of microtubules with other
cell components. This subset is prominent in neurons, where stabilized microtubule
bundles form the core of the axons and dendrites that extend from the
cell body (Figure 16–50). These MAPs have at least one domain that binds to the
microtubule surface and another that projects outward. The length of the projecting
domain can determine how closely MAP-coated microtubules pack together,
as demonstrated in cells engineered to overproduce different MAPs. Cells overexpressing
MAP2, which has a long projecting domain, form bundles of stable
microtubules that are kept widely spaced, while cells overexpressing tau, a MAP
with a much shorter projecting domain, form bundles of more closely packed
microtubules (Figure 16–51). MAPs are the targets of several protein kinases, and
phosphorylation of a MAP can control both its activity and localization inside
cells.
Microtubule Plus-End-Binding Proteins Modulate Microtubule
Dynamics and Attachments
Cells contain numerous proteins that bind the ends of microtubules and thereby
influence microtubule stability and dynamics. These proteins can influence the
Figure 16–50 Localization of MAPs in the axon and dendrites of a
neuron. This immunofluorescence micrograph shows the distribution of
the proteins tau (green) and MAP2 (orange) in a hippocampal neuron in
culture. Whereas tau staining is confined to the axon (long and branched in
this neuron), MAP2 staining is confined to the cell body and its dendrites.
The antibody used here to detect tau binds only to unphosphorylated tau;
phosphorylated tau is also present in dendrites. (Courtesy of James
W. Mandell and Gary A. Banker.) 10 µm