microtubule

Microtubules are hollow, rigid cytoskeletal polymers about 25 nanometers in diameter. They are built from tubulin dimers, each consisting of alpha- and beta-tubulin, which assemble into linear protofilaments that laterally associate to form the tube. Tubulin dimers bind GTP; when incorporated into microtubules, GTP is hydrolyzed to GDP, and the balance between growth and shrinkage at the plus end drives dynamic instability.

Dynamic instability and organization of microtubules enable rapid remodeling of the cytoskeleton. The plus ends are generally more dynamic, while the minus ends are stabilized by connections to organizing centers and microtubule-associated proteins. In many animal cells, microtubule nucleation occurs at the centrosome, where gamma-tubulin ring complexes initiate growth. Some cells and structures use alternative organizers, and microtubules also form stable arrays in cilia and flagella.

Functions of microtubules include providing structural support, determining cell shape, and serving as tracks for motor proteins such as kinesin and dynein. These motors transport organelles, vesicles, and chromosomes during cell division, and they power movements of cilia and flagella. Microtubules also contribute to spindle formation in mitosis and meiosis, helping to segregate chromosomes.

Microtubule behavior is modulated by microtubule-associated proteins (MAPs) and post-translational modifications, such as acetylation or detyrosination, which influence stability and motor interactions. Tau, a neuronal MAP, stabilizes microtubules; its abnormal modification is linked to neurodegenerative diseases.

Clinically, microtubules are targets for chemotherapy. Agents like taxanes stabilize microtubules, while vinca alkaloids destabilize them, disrupting cell division.