Microtubule

Microtubules are the widest fibers of the cytoskeleton, about 25 nanometers across. Each is a hollow cylinder whose wall is assembled from dimers of two closely related proteins, alpha-tubulin and beta-tubulin, arranged in rows called protofilaments, usually 13 of them side by side.

Microtubules are dynamic. They grow and shrink by gaining or losing tubulin at their ends, mostly at the faster-growing plus end, and in animal cells most of them start at the centrosome near the nucleus. This constant rebuilding lets them do several jobs. They resist squashing forces that would deform the cell, and they act as tracks along which motor proteins such as kinesin and dynein carry vesicles and organelles. During division they form the mitotic spindle, attaching to chromosomes at their kinetochores and pulling the copies apart. In cilia and flagella, a ring of nine microtubule pairs around a central pair, the 9 + 2 arrangement, forms the core that bends to make them beat.

Because a dividing cell needs a working spindle, drugs that interfere with microtubules are used against cancer. Some, such as paclitaxel, lock microtubules in place, while others, such as the vinca alkaloids, keep them from forming. Colchicine, a gout medicine, also acts by binding tubulin.