In most somatic animal cells, each spindle pole is focused at a protein organelle called the centrosome (see Figures 16–47 and 16–48). Each centrosome consists of a cloud of amorphous material (called the pericentriolar matrix) that surrounds a pair of centrioles (Figure 17–24). The pericentriolar matrix nucleates a radial array of microtubules, with their fast-growing plus ends projecting outward and their minus ends associated with the centrosome. The matrix contains a variety of proteins, including microtubule-dependent motor proteins, coiled-coil proteins that link the motors to the centrosome, structural proteins, and components of the cell-cycle control system. Most important, it contains γ-tubulin ring complexes, which are the components mainly responsible for nucleating microtubules (see Figure 16–46).
Some cells—notably the cells of higher plants and the oocytes of many vertebrates—do not have centrosomes, and microtubule-dependent motor proteins and other proteins associate with microtubule minus ends to organize and focus the spindle poles.
Figure 17–23 The metaphase mitotic spindle in an animal cell. The plus ends of the microtubules project away from the spindle pole, while the minus ends are anchored at the spindle poles, which in this example are organized by centrosomes. kinetochore microtubules connect the spindle poles with the kinetochores of sister chromatids, while interpolar microtubules from the two poles interdigitate at the spindle equator. astral microtubules radiate out from the poles into the cytoplasm.
The function of the mitotic spindle depends on numerous microtubule-dependent motor proteins. As discussed in Chapter 16, these proteins belong to two families—the kinesin-related proteins, which usually move toward the plus end of microtubules, and dyneins, which move toward the minus end. In the mitotic spindle, these motor proteins generally operate at or near the ends of the micro-tubules. Four major types of motor proteins—kinesin-5, kinesin-14, kinesins-4/10, and dynein—are particularly important in spindle assembly and function (Figure 17–25).
Kinesin-5 proteins contain two motor domains that interact with the plus ends of antiparallel microtubules in the spindle midzone. Because the two motor domains move toward the plus ends of the microtubules, they slide the two anti-parallel microtubules past each other toward the spindle poles, pushing the poles apart. Kinesin-14 proteins, by contrast, are minus-end directed motors with a single motor domain and other domains that can interact with a neighboring microtubule. They can cross-link antiparallel interpolar microtubules at the spindle midzone and tend to pull the poles together. Kinesin-4 and kinesin-10