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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Figure 16–45 Direct observation of the
dynamic instability of microtubules
in a living cell. Microtubules in a newt
lung epithelial cell were observed after
the cell was injected with a small amount
of rhodamine-labeled tubulin. Notice
the dynamic instability of microtubules
at the edge of the cell. Four individual
microtubules are highlighted for clarity;
each of these shows alternating shrinkage
and growth (Movie 16.7). (Courtesy
of Wendy C. Salmon and Clare
Waterman-Storer.)
the β-monomer produce straight protofilaments that make strong and regular lateral
contacts with one another. But the hydrolysis of GTP to GDP is associated
with a subtle conformational change in the protein, which makes the protofilaments
curved (Figure 16–44B). On a rapidly growing microtubule, the GTP cap
is thought to constrain the curvature of the protofilaments, and the ends appear
straight. But when the terminal subunits have hydrolyzed their nucleotides, this
constraint is removed, and the curved protofilaments spring apart. This cooperative
release of the energy of hydrolysis stored in the microtubule lattice causes
the curled protofilaments to peel off rapidly, and curved oligomers of GDP-containing
tubulin are seen near the ends of depolymerizing microtubules (Figure
MBoC6 m16.17/16.45
16–44C).
Microtubule Functions Are Inhibited by Both Polymer-stabilizing
and Polymer-destabilizing Drugs
Chemical compounds that impair polymerization or depolymerization of microtubules
are powerful tools for investigating the roles of these polymers in cells.
Whereas colchicine and nocodazole interact with tubulin subunits and lead to
microtubule depolymerization, Taxol binds to and stabilizes microtubules, causing
a net increase in tubulin polymerization (see Table 16–1). Drugs like these
have a rapid and profound effect on the organization of the microtubules in living
cells. Both microtubule-depolymerizing drugs (such as nocodazole) and microtubule-polymerizing
drugs (such as Taxol) preferentially kill dividing cells, since
microtubule dynamics are crucial for correct function of the mitotic spindle (discussed
in Chapter 17). Some of these drugs efficiently kill certain types of tumor
cells in a human patient, although not without toxicity to rapidly dividing normal
cells, including those in the bone marrow, intestine, and hair follicles. Taxol in
particular has been widely used to treat cancers of the breast and lung, and it is
frequently successful in treatment of tumors that are resistant to other chemotherapeutic
agents.
A Protein Complex Containing γ-Tubulin Nucleates Microtubules
Because formation of a microtubule requires the interaction of many tubulin heterodimers,
the concentration of tubulin subunits required for spontaneous nucleation
of microtubules is very high. Microtubule nucleation therefore requires
help from other factors. While α- and β-tubulins are the regular building blocks of
microtubules, another type of tubulin, called γ-tubulin, is present in much smaller
amounts than α- and β-tubulin and is involved in the nucleation of microtubule
growth in organisms ranging from yeasts to humans. Microtubules are generally
nucleated from a specific intracellular location known as a microtubule-organizing
center (MTOC) where γ-tubulin is most enriched. Nucleation in many cases
depends on the γ-tubulin ring complex (γ-TuRC). Within this complex, two
accessory proteins bind directly to the γ-tubulin, along with several other proteins
that help create a spiral ring of γ-tubulin molecules, which serves as a template
that creates a microtubule with 13 protofilaments (Figure 16–46).