Microtubule

A microtubule is a hollow tube about 25 nanometers wide, built from the protein tubulin, that forms part of the cytoskeleton of eukaryotic cells. Microtubules provide tracks for transport, form the spindle that separates chromosomes and make up the core of cilia and flagella.

Dimers of alpha- and beta-tubulin line up end to end into protofilaments, and thirteen of these form the wall of the tube. This gives the microtubule polarity, with a fast-growing plus end and a slow-growing minus end. Growth requires GTP bound to tubulin, and the cap of GTP-bound subunits keeps the end stable. Most microtubules start at the centrosome.

Tim Mitchison and Marc Kirschner showed in 1984 that microtubules switch between growing and shrinking, a behavior called dynamic instability. If the GTP cap is lost, the end rapidly falls apart, which lets the cell reorganize its network quickly, for example when it enters mitosis. The drug paclitaxel, first isolated from the Pacific yew, freezes microtubules in place, and colchicine prevents assembly; both stop cell division and have been used in medicine.

Motor proteins, kinesin and dynein, walk along them carrying cargo. During division, spindle microtubules attach to the centromere of each chromosome and pull the copies apart. In cilia and flagella, nine doublets of microtubules surround a central pair and are bent by dynein. They work with actin filaments to give the cell its shape.

• Kinesin — a motor that walks along it
• Centrosome — where many microtubules start
• Mitosis — the division that depends on spindle microtubules