Editing Microtubule (section)

===Nucleation===
{{main|Microtubule nucleation}}

Nucleation is the event that initiates the formation of microtubules from the tubulin dimer.  Microtubules are typically [[nucleate]]d and organized by organelles called [[microtubule-organizing center]]s (MTOCs). Contained within the MTOC is another type of tubulin, γ-tubulin, which is distinct from the α- and β-subunits of the microtubules themselves. The γ-tubulin combines with several other associated proteins to form a lock washer-like structure known as the "γ-tubulin ring complex" (γ-TuRC).  This complex acts as a template for α/β-tubulin dimers to begin polymerization; it acts as a cap of the (−) end while microtubule growth continues away from the MTOC in the (+) direction.<ref>{{Cite journal |vauthors=Desai A, Mitchison TJ |year=1997 |title=Microtubule polymerization dynamics |journal=Annual Review of Cell and Developmental Biology |volume=13 |pages=83–117 |doi=10.1146/annurev.cellbio.13.1.83 |pmid=9442869}}</ref>

The [[centrosome]] is the primary MTOC of most cell types. However, microtubules can be nucleated from other sites as well. For example, [[cilia]] and [[flagella]] have MTOCs at their base termed [[basal bodies]]. In addition, work from the Kaverina group at Vanderbilt, as well as others, suggests that the [[Golgi apparatus]] can serve as an important platform for the nucleation of microtubules.<ref>{{Cite journal |vauthors=Vinogradova T, Miller PM, Kaverina I |date=July 2009 |title=Microtubule network asymmetry in motile cells: role of Golgi-derived array |journal=Cell Cycle |volume=8 |issue=14 |pages=2168–74 |doi=10.4161/cc.8.14.9074 |pmc=3163838 |pmid=19556895}}</ref> Because nucleation from the centrosome is inherently symmetrical, Golgi-associated microtubule nucleation may allow the cell to establish asymmetry in the microtubule network. In recent studies, the Vale group at UCSF identified the protein complex augmin as a critical factor for centrosome-dependent, spindle-based microtubule generation. It that has been shown to interact with γ-TuRC and increase microtubule density around the mitotic spindle origin.<ref>{{Cite journal |vauthors=Uehara R, Nozawa RS, Tomioka A, Petry S, Vale RD, Obuse C, Goshima G |date=April 2009 |title=The augmin complex plays a critical role in spindle microtubule generation for mitotic progression and cytokinesis in human cells |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=106 |issue=17 |pages=6998–7003 |bibcode=2009PNAS..106.6998U |doi=10.1073/pnas.0901587106 |pmc=2668966 |pmid=19369198 |doi-access=free}}</ref>

Some cell types, such as plant cells, do not contain well defined MTOCs. In these cells, microtubules are nucleated from discrete sites in the cytoplasm. Other cell types, such as [[trypanosomatid]] parasites, have a MTOC but it is permanently found at the base of a flagellum. Here, nucleation of microtubules for structural roles and for generation of the mitotic spindle is not from a canonical centriole-like MTOC.