Editing Microtubule (section)

==Microtubule dynamics==

===Dynamic instability===
[[File:MicrotubuleDynamicInstability.ogv|thumb|Animation of the microtubule dynamic instability.  Tubulin dimers bound to GTP (red) bind to the growing end of a microtubule and subsequently hydrolyze GTP into GDP (blue).]]

Dynamic instability refers to the coexistence of assembly and disassembly at the ends of a microtubule. The microtubule can dynamically switch between growing and shrinking phases in this region.<ref>{{Cite book |url=https://archive.org/details/cellmolecularbio04edkarp/page/355 |title=Cell and Molecular Biology: Concepts and Experiments |vauthors=Karp G |publisher=John Wiley & Sons |year=2005 |isbn=978-0-471-46580-5 |location=USA |page=[https://archive.org/details/cellmolecularbio04edkarp/page/355 355]}}</ref> Tubulin dimers can bind two molecules of GTP, one of which can be hydrolyzed subsequent to assembly.  During polymerization, the tubulin dimers are in the [[Guanosine triphosphate|GTP]]-bound state.<ref name="weisenberg" />  The GTP bound to α-tubulin is stable and it plays a structural function in this bound state. However, the GTP bound to β-tubulin may be [[hydrolysis|hydrolyzed]] to [[Guanosine diphosphate|GDP]] shortly after assembly. The assembly properties of GDP-tubulin are different from those of GTP-tubulin, as GDP-tubulin is more prone to depolymerization.<ref>{{Cite journal |vauthors=Weisenberg RC, Deery WJ, Dickinson PJ |date=September 1976 |title=Tubulin-nucleotide interactions during the polymerization and depolymerization of microtubules |journal=Biochemistry |volume=15 |issue=19 |pages=4248–54 |doi=10.1021/bi00664a018 |pmid=963034}}</ref> A GDP-bound tubulin subunit at the tip of a microtubule will tend to fall off, although a GDP-bound tubulin in the middle of a microtubule cannot spontaneously pop out of the polymer.  Since tubulin adds onto the end of the microtubule in the GTP-bound state, a cap of GTP-bound tubulin is proposed to exist at the tip of the microtubule, protecting it from disassembly.  When hydrolysis catches up to the tip of the microtubule, it begins a rapid depolymerization and shrinkage. This switch from growth to shrinking is called a catastrophe.  GTP-bound tubulin can begin adding to the tip of the microtubule again, providing a new cap and protecting the microtubule from shrinking. This is referred to as "rescue".<ref name="pmid6504138">{{Cite journal |vauthors=Mitchison T, Kirschner M |year=1984 |title=Dynamic instability of microtubule growth |journal=Nature |volume=312 |issue=5991 |pages=237–42 |bibcode=1984Natur.312..237M |doi=10.1038/312237a0 |pmid=6504138 |s2cid=30079133}}</ref>

==="Search and capture" model===

In 1986, [[Marc Kirschner]] and [[Tim Mitchison]] proposed that microtubules use their dynamic properties of growth and shrinkage at their plus ends to probe the three dimensional space of the cell. Plus ends that encounter kinetochores or sites of polarity become captured and no longer display growth or shrinkage. In contrast to normal dynamic microtubules, which have a half-life of 5–10 minutes, the captured microtubules can last for hours. This idea is commonly known as the "search and capture" model.<ref>{{Cite journal |vauthors=Kirschner M, Mitchison T |date=May 1986 |title=Beyond self-assembly: from microtubules to morphogenesis |journal=Cell |volume=45 |issue=3 |pages=329–42 |doi=10.1016/0092-8674(86)90318-1 |pmid=3516413 |s2cid=36994346}}</ref> Indeed, work since then has largely validated this idea. At the kinetochore, a variety of complexes have been shown to capture microtubule (+)-ends.<ref name="pmid18097444">{{Cite journal |vauthors=Cheeseman IM, Desai A |date=January 2008 |title=Molecular architecture of the kinetochore-microtubule interface |journal=Nature Reviews. Molecular Cell Biology |volume=9 |issue=1 |pages=33–46 |doi=10.1038/nrm2310 |pmid=18097444 |s2cid=34121605}}</ref> Moreover, a (+)-end capping activity for interphase microtubules has also been described.<ref name="pmid11058078">{{Cite journal |vauthors=Infante AS, Stein MS, Zhai Y, Borisy GG, Gundersen GG |date=November 2000 |title=Detyrosinated (Glu) microtubules are stabilized by an ATP-sensitive plus-end cap |url=http://jcs.biologists.org/cgi/pmidlookup?view=long&pmid=11058078 |url-status=live |journal=Journal of Cell Science |volume=113 |issue=22 |pages=3907–19 |doi=10.1242/jcs.113.22.3907 |pmid=11058078 |archive-url=https://web.archive.org/web/20240221025058/https://journals.biologists.com/jcs/cgi/pmidlookup |archive-date=2024-02-21 |access-date=2014-06-23}}</ref> This later activity is mediated by [[formins]],<ref name="pmid11483957">{{Cite journal |vauthors=Palazzo AF, Cook TA, Alberts AS, Gundersen GG |date=August 2001 |title=mDia mediates Rho-regulated formation and orientation of stable microtubules |journal=Nature Cell Biology |volume=3 |issue=8 |pages=723–9 |doi=10.1038/35087035 |pmid=11483957 |s2cid=7374170}}</ref> the [[adenomatous polyposis coli]] protein, and [[MAPRE1|EB1]],<ref name="pmid15311282">{{Cite journal |vauthors=Wen Y, Eng CH, Schmoranzer J, Cabrera-Poch N, Morris EJ, Chen M, Wallar BJ, Alberts AS, Gundersen GG |date=September 2004 |title=EB1 and APC bind to mDia to stabilize microtubules downstream of Rho and promote cell migration |journal=Nature Cell Biology |volume=6 |issue=9 |pages=820–30 |doi=10.1038/ncb1160 |pmid=15311282 |s2cid=29214110}}</ref> a protein that tracks along the growing plus ends of microtubules.