Editing Microtubule (section)

==Proteins that interact with microtubules==

===Microtubule-associated proteins (MAPs)===

{{Main|Microtubule-associated protein}}

MAPs have been shown to play a crucial role in the regulation of microtubule dynamics ''in-vivo''. The rates of microtubule polymerization, depolymerization, and catastrophe vary depending on which [[microtubule-associated protein]]s (MAPs) are present. The originally identified MAPs from brain tissue can be classified into two groups based on their molecular weight. This first class comprises MAPs with a molecular weight below 55-62 kDa, and are called [[tau proteins|τ (tau) proteins]]. ''In-vitro'', tau proteins have been shown to directly bind microtubules, promote nucleation and prevent disassembly, and to induce the formation of parallel arrays.<ref>{{Cite journal |vauthors=Mandelkow E, Mandelkow EM |date=February 1995 |title=Microtubules and microtubule-associated proteins |journal=Current Opinion in Cell Biology |volume=7 |issue=1 |pages=72–81 |doi=10.1016/0955-0674(95)80047-6 |pmid=7755992}}</ref>  Additionally, tau proteins have also been shown to stabilize microtubules in axons and have been implicated in Alzheimer's disease.<ref>{{Cite journal |vauthors=Bramblett GT, Goedert M, Jakes R, Merrick SE, Trojanowski JQ, Lee VM |date=June 1993 |title=Abnormal tau phosphorylation at Ser396 in Alzheimer's disease recapitulates development and contributes to reduced microtubule binding |journal=Neuron |volume=10 |issue=6 |pages=1089–99 |doi=10.1016/0896-6273(93)90057-X |pmid=8318230 |s2cid=23180847}}</ref>  The second class is composed of MAPs with a molecular weight of 200-1000 kDa, of which there are four known types: MAP-1, [[MAP2|MAP-2]], MAP-3 and [[MAP4|MAP-4]]. MAP-1 proteins consists of a set of three different proteins: [[MAP1A|A]], [[MAP1A|B]] and C. The C protein plays an important role in the retrograde transport of vesicles and is also known as [[Dynein#Cytoplasmic dynein|cytoplasmic dynein]].  MAP-2 proteins are located in the dendrites and in the body of neurons, where they bind with other cytoskeletal filaments.  The MAP-4 proteins are found in the majority of cells and stabilize microtubules.  In addition to MAPs that have a stabilizing effect on microtubule structure, other MAPs can have a destabilizing effect either by cleaving or by inducing depolymerization of microtubules.  Three proteins called [[katanin]], [[spastin]], and fidgetin have been observed to regulate the number and length of microtubules via their destabilizing activities. Furthermore, [[CRACD-like protein]] is predicted to be localized to the microtubules.<ref>{{Cite web |title=The Human Protein Atlas |url=http://www.proteinatlas.org |url-status=live |archive-url=https://web.archive.org/web/20170501045727/http://www.proteinatlas.org/ |archive-date=2017-05-01 |access-date=2017-04-27 |website=www.proteinatlas.org}}</ref>

MAPs are determinants of different cytoskeletal forms of [[axon]]s and [[dendrite]]s, with microtubules being farther apart in the [[dendrite]]s <ref>{{Cite journal |last=Hirokawa, N |year=1994 |title=The neuronal cytoskeleton: roles in neuronal morphogenesis and organelle transport |journal=Molecular Neurobiology: Mechanisms Common to Brain, Skin and Immune System. Series: Progress in Clinical and Biological Research. Willey-Liss, Inc. |volume=390 |pages=117–143 |pmid=7536943}}</ref>

===Plus-end tracking proteins (+TIPs)===

{{Main|Microtubule plus-end tracking protein}}

Plus end tracking proteins are MAP proteins which bind to the tips of growing microtubules and play an important role in regulating microtubule dynamics.  For example, +TIPs have been observed to participate in the interactions of microtubules with chromosomes during mitosis.  The first MAP to be identified as a +TIP was [[CLIP1]]70 (cytoplasmic linker protein), which has been shown to play a role in microtubule depolymerization rescue events. Additional examples of +TIPs include [[MAPRE1|EB1]], [[MAPRE2|EB2]], [[MAPRE3|EB3]], [[DCTN1|p150Glued]], [[DCTN2|Dynamitin]], [[Lis1]], [[CLIP2|CLIP115]], [[CLASP1]], and [[CLASP2]].{{citation needed|date=May 2015}}

===Motor proteins===
[[File:CytoplasmicDyneinOnMT noLabels.png|thumb|A cytoplasmic dynein motor bound to a microtubule.]]
[[File:Kinesin cartoon.png|thumb|A kinesin molecule bound to a microtubule.]]
Microtubules can act as substrates for motor proteins that are involved in important cellular functions such as vesicle trafficking and cell division. Unlike other microtubule-associated proteins, motor proteins utilize the energy from ATP hydrolysis to generate mechanical work that moves the protein along the substrate. The major motor proteins that interact with microtubules are [[kinesin]], which usually moves toward the (+) end of the microtubule, and [[dynein]], which moves toward the (−) end.
* [[Dynein]] is composed of two identical heavy chains, which make up two large globular head domains, and a variable number of intermediate and light chains. Dynein-mediated transport takes place from the (+) end towards the (-) end of the microtubule.  [[Adenosine triphosphate|ATP]] hydrolysis occurs in the globular head domains, which share similarities with the AAA+ (ATPase associated with various cellular activities) protein family. ATP hydrolysis in these domains is coupled to movement along the microtubule via the microtubule-binding domains.  Dynein transports vesicles and organelles throughout the cytoplasm.  In order to do this, dynein molecules bind organelle membranes via a protein complex that contains a number of elements including [[dynactin]].
* [[Kinesin]] has a similar structure to dynein.  Kinesin is involved in the transport of a variety of intracellular cargoes, including vesicles, organelles, protein complexes, and mRNAs toward the microtubule's (+) end.<ref>{{Cite journal |vauthors=Hirokawa N, Noda Y, Tanaka Y, Niwa S |date=October 2009 |title=Kinesin superfamily motor proteins and intracellular transport |journal=Nature Reviews. Molecular Cell Biology |volume=10 |issue=10 |pages=682–96 |doi=10.1038/nrm2774 |pmid=19773780 |s2cid=18129292}}</ref>

Some viruses (including [[retroviruses]], [[herpesviridae|herpesviruses]], [[parvoviruses]], and [[adenoviruses]]) that require access to the nucleus to replicate their genomes attach to [[#Motor proteins|motor proteins]].