Editing Microtubule (section)

== Structure ==
[[File:Tubulin dimer 1JFF.png|thumb|Cartoon representation of the structure of α(yellow)/β(red)-tubulin heterodimer, GTP and GDP.<ref>{{Cite journal |vauthors=Löwe J, Li H, Downing KH, Nogales E |date=November 2001 |title=Refined structure of alpha beta-tubulin at 3.5 A resolution |url=https://zenodo.org/record/1229896 |url-status=live |journal=Journal of Molecular Biology |volume=313 |issue=5 |pages=1045–57 |doi=10.1006/jmbi.2001.5077 |pmid=11700061 |archive-url=https://web.archive.org/web/20210122161041/https://zenodo.org/record/1229896 |archive-date=2021-01-22 |access-date=2019-09-09}}</ref>]]

In [[eukaryote]]s, microtubules are long, hollow cylinders made up of polymerized [[Tubulin#Eukaryotic|α- and β-tubulin]] [[protein dimer|dimers]].<ref name="weisenberg">{{Cite journal |vauthors=Weisenberg RC |date=September 1972 |title=Microtubule formation in vitro in solutions containing low calcium concentrations |journal=Science |volume=177 |issue=4054 |pages=1104–5 |bibcode=1972Sci...177.1104W |doi=10.1126/science.177.4054.1104 |pmid=4626639 |s2cid=34875893}}</ref> The inner space of the hollow microtubule cylinders is referred to as the lumen. The α and β-tubulin subunits are ~50% identical at the amino acid level, and both have a molecular weight of approximately 50 kDa.<ref name="desai">{{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><ref>{{Cite journal |last1=Desai |first1=A. |last2=Mitchison |first2=T. J. |date=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 |issn=1081-0706 |pmid=9442869}}</ref>

These α/β-tubulin [[protein dimer|dimers]] [[polymerize]] end-to-end into linear '''protofilaments''' that associate laterally to form a single microtubule, which can then be extended by the addition of more α/β-tubulin dimers.  Typically, microtubules are formed by the parallel association of thirteen protofilaments, although microtubules composed of fewer or more protofilaments have been observed in various species&nbsp;<ref>{{Cite journal |vauthors=Chaaban S, Brouhard GJ |date=2017 |title=A microtubule bestiary: structural diversity in tubulin polymers |journal=Molecular Biology of the Cell |volume=28 |issue=22 |pages=2924–31 |doi=10.1091/mbc.E16-05-0271 |pmc=5662251 |pmid=29084910}}</ref> as well as ''in vitro''.<ref>{{Cite journal |vauthors=Chrétien D, Metoz F, Verde F, Karsenti E, Wade RH |date=June 1992 |title=Lattice defects in microtubules: protofilament numbers vary within individual microtubules |journal=Journal of Cell Biology |volume=117 |issue=5 |pages=1031–40 |doi=10.1083/jcb.117.5.1031 |pmc=2289483 |pmid=1577866}}</ref>

Microtubules have a distinct polarity that is critical for their biological function. Tubulin polymerizes end to end, with the β-subunits of one tubulin dimer contacting the α-subunits of the next dimer. Therefore, in a protofilament, one end will have the α-subunits exposed while the other end will have the β-subunits exposed. These ends are designated the (−) and (+) ends, respectively. The protofilaments bundle parallel to one another with the same polarity, so, in a microtubule, there is one end, the (+) end, with only β-subunits exposed, while the other end, the (−) end, has only α-subunits exposed. While microtubule elongation can occur at both the (+) and (−) ends, it is significantly more  rapid at the (+) end.<ref>{{Cite journal |vauthors=Walker RA, O'Brien ET, Pryer NK, Soboeiro MF, Voter WA, Erickson HP, Salmon ED |date=October 1988 |title=Dynamic instability of individual microtubules analyzed by video light microscopy: rate constants and transition frequencies |journal=The Journal of Cell Biology |volume=107 |issue=4 |pages=1437–48 |citeseerx=10.1.1.525.507 |doi=10.1083/jcb.107.4.1437 |pmc=2115242 |pmid=3170635}}</ref>

The lateral association of the protofilaments generates a pseudo-helical structure, with one turn of the helix containing 13 tubulin dimers, each from a different protofilament.  In the most common "13-3" architecture, the 13th tubulin dimer interacts with the next tubulin dimer with a vertical offset of 3 tubulin monomers due to the helicity of the turn. There are other alternative architectures, such as 11-3, 12-3, 14-3, 15-4, or 16-4, that have been detected at a much lower occurrence.<ref>{{Cite journal |vauthors=Sui H, Downing KH |date=August 2010 |title=Structural basis of interprotofilament interaction and lateral deformation of microtubules |journal=Structure |volume=18 |issue=8 |pages=1022–31 |doi=10.1016/j.str.2010.05.010 |pmc=2976607 |pmid=20696402}}</ref> Microtubules can also morph into other forms such as helical filaments, which are observed in [[protist]] organisms like [[foraminifera]].<ref>{{Cite journal |vauthors=Bassen DM, Hou Y, Bowser SS, Banavali NK |date=August 2016 |title=Maintenance of electrostatic stabilization in altered tubulin lateral contacts may facilitate formation of helical filaments in foraminifera |journal=Scientific Reports |volume=6 |page=31723 |bibcode=2016NatSR...631723B |doi=10.1038/srep31723 |pmc=4990898 |pmid=27539392}}</ref> There are two distinct types of interactions that can occur between the subunits of lateral protofilaments within the microtubule called the A-type and B-type lattices.  In the A-type lattice, the lateral associations of protofilaments occur between adjacent α and β-tubulin subunits (i.e. an α-tubulin subunit from one protofilament interacts with a β-tubulin subunit from an adjacent protofilament).  In the B-type lattice, the α and β-tubulin subunits from one protofilament interact with the α and β-tubulin subunits from an adjacent protofilament, respectively.  Experimental studies have shown that the B-type lattice is the primary arrangement within microtubules.  However, in most microtubules there is a seam in which tubulin subunits interact α-β.<ref>{{Cite journal |vauthors=Nogales E |year=2000 |title=Structural insights into microtubule function |journal=Annual Review of Biochemistry |volume=69 |pages=277–302 |doi=10.1146/annurev.biochem.69.1.277 |pmid=10966460}}</ref>

The sequence and exact composition of molecules during microtubule formation can thus be summarised as follows: A β-tubulin connects in the context of a non-existent covalent bond with an α-tubulin, which in connected form are a heterodimer, since they consist of two different polypeptides (β-tubulin and α-tubulin). So after the heterodimers are formed, they join together to form long chains that rise figuratively in one direction (e.g. upwards). These heterodimers, which are connected in a certain direction, form protofilaments. These long chains (protofilaments) now gradually accumulate next to each other so that a tube-like structure is formed, which has a lumen typical of a tube. Accordingly, mostly 13 protofilaments form the outer wall of the microtubules. The heterodimers consist of a positive and negative end, with alpha-tubulin forming the negative end and beta-tubulin the positive end. Due to the fact that the heterodimers are stacked on top of each other, there is always a negative and positive end. Microtubules grow by an addition of heterodimers at the plus end.

Some species of ''[[Prosthecobacter]]'' also contain microtubules. The structure of these bacterial microtubules is similar to that of eukaryotic microtubules, consisting of a hollow tube of protofilaments assembled from heterodimers of bacterial tubulin A (BtubA) and bacterial tubulin B (BtubB).  Both BtubA and BtubB share features of both α- and β-[[tubulin]]. Unlike eukaryotic microtubules, bacterial microtubules do not require chaperones to fold.<ref>{{Cite journal |vauthors=Schlieper D, Oliva MA, Andreu JM, Löwe J |date=June 2005 |title=Structure of bacterial tubulin BtubA/B: evidence for horizontal gene transfer |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=102 |issue=26 |pages=9170–5 |bibcode=2005PNAS..102.9170S |doi=10.1073/pnas.0502859102 |pmc=1166614 |pmid=15967998 |doi-access=free}}</ref> In contrast to the 13 protofilaments of eukaryotic microtubules, bacterial microtubules comprise only five.<ref name="Pilhofer-2011">{{Cite journal |vauthors=Pilhofer M, Ladinsky MS, McDowall AW, Petroni G, Jensen GJ |date=December 2011 |title=Microtubules in bacteria: Ancient tubulins build a five-protofilament homolog of the eukaryotic cytoskeleton |journal=PLOS Biology |volume=9 |issue=12 |pages=e1001213 |doi=10.1371/journal.pbio.1001213 |pmc=3232192 |pmid=22162949 |doi-access=free}}</ref>