Microtubules (MT, ) are small tubes built of tubulin dimmer, composed of two subunits: a- and в-tubulin, both with molecular weight of 55 kDa. Tubulins are organized in a so-called protofilaments that further form microtubule (Fig. 2.17). Each microtubule is typically composed of 13 protofilaments arranged around the circumference with diameter of about 24 nm and length in the range of 200 nm to 25 |tm. A microtubule extends by the addition of tubulin proteins to one of its ends.
Microtubules are found in the cytoplasm of all eukaryotic cells where they are often observed to spread out radially from a microtubule-organizing center (MTOC) located near the nucleus (Fig. 2.17). The microtubules provide a strong scaffold that supports the cell and determines its shape. Most studies have concluded that microtubules play a positive role by regulating
actin polymerization, transporting membrane vesicles or other organelles inside the cell, and/or facilitating the turnover of adhesion plaques. They can also form specialized structures such as centrioles, cilia, and flagella. Both cilia and flagella are cellular appendages, consisting of a core of microtubules enclosed in an extension of the plasma membrane, playing an important role in cellular locomotion.
Figure 2.17 The organization of microtubules in a fibroblast. Microtubules were stained with secondary mouse monoclonal antibody labeled with Alexa Fluor 555 (MTOC—the microtubuleorganizing center, unpublished data of the author).