Microtubules

Microtubules serve a range of cellular functions including separation of chromosomes during mitosis, maintenance of cell shape, beating of cilia and flagella, and transport of organelles and cargo within a cell. Structurally, microtubules are hollow cylinders composed of alpha- and beta-Tubulin heterodimers that join end-to-end to form protofilaments. Protofilaments associate laterally to give rise to 25 nm microtubules. Microtubules undergo rapid assembly and disassembly which is critical to its role in active processes such as ciliary beating. Microtubules are assisted by two main classes of microtubule associated proteins (MAPs) including motor proteins and non-motor proteins. Motor proteins such as Kinesin and Dynein are critical for axonal transport, chromosome movement during mitosis, and movement of cilia and flagella. Some non-motor MAPs such as Tau stabilize microtubules and promote the assembly of microtubules while others such as Katanin sever microtubules, a function necessary for segregation of sister chromatids during anaphase. A more recently described class of MAPs are the plus-end-tracking proteins (+-TIPS) which bind to the plus-end of microtubules where they interact with cell targets and regulate microtubule dynamics.

Distinct from alpha- and beta-Tubulin is gamma-Tubulin which is contained within the microtubule organizing center (MTOC). Through associations with other proteins, gamma-Tubulin acts as a scaffold for alpha/beta Tubulin dimers to begin polymerization. Other Tubulin proteins such as delta and epsilon-Tubulin are not as well characterized.

The critical role that microtubules serve is emphasized by the large number of cancer chemotherapies that target their assembly and disassembly. Additionally, defects in microtubule assembly/disassembly are associated with neurodegenerative diseases, such as Alzheimer's disease.