Essential Cell Biology 5th edition

582 CHAPTER 17 Cytoskeleton
nucleating sites
(γ-tubulin ring complexes)
centrosome
matrix
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γ-tubulin ring complex
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(A)
pair of
centrioles
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(B)
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microtubules grow at their
plus ends from γ-tubulin ring
complexes of the centrosome
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(C)
0.5 µm
Figure 17–13 Tubulin polymerizes from
nucleation sites on a centrosome.
(A) Schematic drawing showing that an
animal cell centrosome consists of an
amorphous matrix of various proteins,
including the γ-tubulin rings (red ) that
nucleate microtubule growth, surrounding
a pair of centrioles, oriented at right angles
to each other. Each member of the centriole
pair is made up of a cylindrical array of short
microtubules. (B) Diagram of a centrosome
with attached microtubules. The minus end
of each microtubule is embedded in the
centrosome, having grown from a γ-tubulin
ring complex, whereas the plus end of each
microtubule extends into the cytoplasm.
(C) An image of a centrosome,
reconstructed from serial sections of a
Caenorhabditis elegans cell, showing
a dense thicket of microtubules (green)
emanating from γ-tubulin ring complexes
(red ). A pair of centrioles, themselves
made of short microtubules (blue), can be
seen at the center. (C, from E.T. O’Toole
et al., J. Cell Biol. 163:451–456, 2003. With
permission from The Rockefeller University
Press.)
Figure 17–14 Each microtubule grows and
shrinks independently of its neighbors.
The array of microtubules anchored in a
centrosome is continually changing, as
some microtubules grow (red arrows) and
ECB5 E17.13/17.14
others shrink (blue arrows).
to the cell nucleus when the cell is not in mitosis—organizes an array of
microtubules that radiates outward through the cytoplasm (see Figure
17–11B). The centrosome consists of a pair of centrioles, surrounded by
a matrix of proteins. The centrosome matrix includes hundreds of ringshaped
structures formed from a special type of tubulin called γ-tubulin,
and each γ-tubulin ring complex serves as the starting point, or nucleation
site, for the growth of one microtubule (Figure 17–13A). The αβ-tubulin
dimers add to each γ-tubulin ring complex in a specific orientation, with
the ECB5 result e17.12/17.13 that the minus end of each microtubule is embedded in the
centrosome, and growth occurs only at the plus end that extends into the
cytoplasm (Figure 17–13B and C).
The paired centrioles at the center of an animal cell centrosome are
curious structures. Each centriole, sitting perpendicular to its partner, is
made of a cylindrical array of short microtubules (see Figure 17–13C).
Yet centrioles have no role in the nucleation of microtubules from the
centrosome: the γ-tubulin ring complex alone is sufficient. Thus, their
function remains something of a mystery, especially as most plant cells
lack them. Centrioles do, however, act as the organizing centers for the
microtubules in cilia and flagella, where they are called basal bodies (see
Figure 17–11D), as we discuss later.
Why do microtubules need nucleating sites such as those provided by the
γ-tubulin rings in the centrosome? The answer is that it is much harder
to start a new microtubule from scratch, by first assembling a ring of
αβ-tubulin dimers, than it is to add such dimers to a preexisting γ-tubulin
ring complex. Although purified αβ-tubulin dimers at a high concentration
can polymerize into microtubules spontaneously in vitro, the concentration
of free αβ-tubulin in a living cell is too low to drive the difficult first
step of assembling the initial ring of a new microtubule. By providing
organizing centers at specific sites, and keeping the concentration of free
αβ-tubulin dimers low, cells can control more precisely where microtubules
form.
Microtubules Display Dynamic Instability
Once a microtubule has been nucleated, it typically grows outward from
the organizing center for many minutes by the addition of αβ-tubulin
dimers to its free plus end. Then, without warning, the microtubule can
suddenly undergo a transition that causes it to shrink rapidly by losing
tubulin dimers from its plus end (Movie 17.2). The microtubule may
shrink partially and then, no less suddenly, start growing again, or it may
disappear completely, to be replaced by a new microtubule that grows
from the same γ-tubulin ring complex (Figure 17–14).