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

930 Chapter 16: The Cytoskeleton
microtubule “seam”
γ-tubulins
γ-TuSC
plus end
accessory
proteins
schematic of
seven copies of
γ-TuSC in “lock
washer” spiral
β-tubulin
α-tubulin
γ-tubulin
accessory
proteins in
γ-tubulin ring
complex
γ-TuSC
overlap in
(A) (B) γ-TuSC spiral
(C)
Figure 16–46 Microtubule nucleation by the γ-tubulin ring complex. (A) Two copies of γ-tubulin associate with a pair
of accessory proteins to form the γ-tubulin small complex (γ-TuSC). This image was generated by high-resolution electron
microscopy of individual purified complexes. (B) Seven copies of the γ-TuSC associate to form a spiral structure in which the last
γ-tubulin lies beneath the first, resulting in 13 exposed γ-tubulin subunits in a circular orientation that matches the orientation of
the 13 protofilaments in a microtubule. (C) In many cell types, the γ-TuSC spiral associates with additional accessory proteins
to form the γ-tubulin ring complex (γ-TuRC), which is likely to nucleate the minus end of a microtubule as shown here. Note
the longitudinal discontinuity between two protofilaments, which results from the spiral orientation of the γ-tubulin subunits.
Microtubules often have one such “seam” breaking the otherwise uniform helical packing of the protofilaments. (A and B, from
J.M. Kollman et al., Nature 466:879–883, 2010. With permission from Macmillan Publishers Ltd.)
MBoC6 n16.203/16.46
Microtubules Emanate from the Centrosome in Animal Cells
Many animal cells have a single, well-defined MTOC called the centrosome,
which is located near the nucleus and from which microtubules are nucleated
at their minus ends, so the plus ends point outward and continuously grow and
shrink, probing the entire three-dimensional volume of the cell. A centrosome
typically recruits more than fifty copies of γ-TuRC. In addition, γ-TuRC molecules
are found in the cytoplasm, and centrosomes are not absolutely required for
microtubule nucleation, since destroying them with a laser pulse does not prevent
microtubule nucleation elsewhere in the cell. A variety of proteins have been
identified that anchor γ-TuRC to the centrosome, but mechanisms that activate
microtubule nucleation at MTOCs and at other sites in the cell are poorly understood.
Embedded in the centrosome are the centrioles, a pair of cylindrical structures
arranged at right angles to each other in an L-shaped configuration (Figure
16–47). A centriole consists of a cylindrical array of short, modified microtubules
arranged into a barrel shape with striking ninefold symmetry (Figure 16–48).
Together with a large number of accessory proteins, the centrioles organize the
pericentriolar material, where microtubule nucleation takes place. As described
in Chapter 17, the centrosome duplicates and splits into two parts before mitosis,
each containing a duplicated centriole pair. The two centrosomes move to opposite
sides of the nucleus when mitosis begins, and they form the two poles of the
mitotic spindle (see Panel 17–1).
Microtubule organization varies widely among different species and cell types.
In budding yeast, microtubules are nucleated at an MTOC that is embedded in
the nuclear envelope as a small, multilayered structure called the spindle pole
body, also found in other fungi and diatoms. Higher-plant cells appear to nucleate
microtubules at sites distributed all around the nuclear envelope and at the cell
cortex. Neither fungi nor most plant cells contain centrioles. Despite these differences,
all these cells seem to use γ-tubulin to nucleate their microtubules.
In cultured animal cells, the aster-like configuration of microtubules is robust,
with dynamic plus ends pointing outward toward the cell periphery and stable
minus ends collected near the nucleus. The system of microtubules radiating from