Key Concepts of Section 18.1

• Tubulin is the major structural component of microtubules (see Figure 18-3). Microtubule-associated proteins (MAPs) associate with tubulin and help mediate the assembly, dynamics, and function of microtubules.

• Free tubulin exists as a dimer in which the α-subunit contains a trapped and nonhydrolyzable GTP and the β-subunit binds an exchangeable and hydrolyzable GTP.

• αβ-Tubulin assembles into microtubules, each of which is made up of 13 laterally associated protofilaments, with an α-subunit exposed at the (−) end and a β-subunit at the (+) end of each protofilament.

• In cilia and flagella, as well as in centrioles and basal bodies, doublet or triplet microtubules exist in which the additional microtubules have 10 protofilaments (see Figure 18-4).

• All microtubules are nucleated from microtubule-organizing centers (MTOCs), and many remain anchored with their (−) ends there. Thus the end away from the MTOC is always the (+) end.

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  The centrosome is the MTOC that nucleates the radial array of microtubules in interphase animal cells. Two centrosomes, or spindle poles, are the MTOCs that nucleate the microtubules of the mitotic spindle in animal cells. Basal bodies are the MTOCs that assemble the microtubules of cilia and flagella (see Figure 18-5).

• Centrosomes consist of two centrioles and the surrounding pericentriolar material, which contains the γ-TuRC microtubule-nucleating complex (see Figures 18-6 and 18-7).