Figure 238

Diagram of a microtubule in a longitudinal and cross-sectional profile. At the left, the diagram depicts the process of polymerization and depolymerization of tubulin dinners during the process of microtubule assembly. Each tubulin dimer consists of an «-tubulin and a /3-tubulin subunit. On the right, each microtubule contains 13 tubulin dimers within its cross section. The minus (-) end of the microtubule contains a ring of y-tubulin, which is necessary for microtubule nucle-ation. This end is usually embedded within the MTOC and possesses numerous capping proteins. The plus (+) end of the microtubule is the growing end to which tubulin dimers bound to GTP molecules are incorporated. Incorporated tubulin dimers hydrolyze GTP, which releases the phosphate groups to form polymers with GDP-tubulin molecules.

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2.38). The dimers polymerize in an end-to-end fashion, head to tail, with the a molecule of one dimer bound to the ¡3 molecule of the next dimer in a repeating pattern. The polymer thus formed is called a protofilament. Axial periodicity seen along the 5-nm-diameter dimers corresponds to the length of the protein molecules. A small, l-/xm segment of microtubule contains approximately 16,000 tubulin dimers.

Microtubules grow from ytubulin rings within the MTOC that serve as nucleation sites for each microtubule

Microtubule formation can be traced to hundreds of y-tubulin rings that form an integral part of the MTOC (Fig.

2.39). The a- and /3-tubulin dimers are added to a y-tubu-lin ring in an end-to-end fashion (see Fig. 2.38). Polymerization of tubulin dimers requires the presence of guanosine triphosphate (GTP) and Mg2+. Each tubulin molecule binds GTP before it is incorporated into the forming microtubule.

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