Tubulin

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Tubulin (tubul- + -in) is one of several members of a small family of globular proteins. The tubulin superfamily includes five distinct families, the alpha-, beta-, gamma-, delta-, and epsilon-tubulins and a sixth family (zeta-tubulin) which is present only in kinetoplastid protozoa.[1] The most common members of the tubulin family are α-tubulin and β-tubulin, the proteins that make up microtubules. Each has a molecular weight of approximately 55,000 Daltons. Microtubules are assembled from dimers of α- and β-tubulin. These subunits are slightly acidic with an isoelectric point between 5.2 and 5.8.[2]

Tubulin was long thought to be specific to eukaryotes. Recently, however, the prokaryotic cell division protein FtsZ was shown to be related to tubulin.[3]

To form microtubules, the dimers of α- and β-tubulin bind to GTP and assemble onto the (+) ends of microtubules while in the GTP-bound state.[4] The β-tubulin subunit is exposed on the plus end of the microtubule while the α-tubulin subunit is exposed on the minus end. After the dimer is incorporated into the microtubule, the molecule of GTP bound to the β-tubulin subunit eventually hydrolyzes into GDP through inter-dimer contacts along the microtubule protofilament.[5] Whether the β-tubulin member of the tubulin dimer is bound to GTP or GDP influences the stability of the dimer in the microtubule. Dimers bound to GTP tend to assemble into microtubules, while dimers bound to GDP tend to fall apart; thus, this GTP cycle is essential for the dynamic instability of the microtubule.

α-Tubulin[edit]

Human α-tubulin subtypes include:[citation needed]

β-Tubulin[edit]

β-tubulin in Tetrahymena sp.

All drugs that are known to bind to human tubulin bind to β-tubulin.[6] These include paclitaxel, colchicine, and the vinca alkaloids, each of which have a distinct binding site on β-tubulin.[6]

Class III β-tubulin is a microtubule element expressed exclusively in neurons,[7] and is a popular identifier specific for neurons in nervous tissue. It binds colchicine much more slowly than other isotypes of β-tubulin.[8]

β1-tubulin, sometimes called class VI β-tubulin,[9] is the most divergent at the amino acid sequence level.[10] It is expressed exclusively in megakaryocytes and platelets in humans and appears to play an important role in the formation of platelets.[10]

Katanin is a protein complex that severs microtubules at β-tubulin subunits, and is necessary for rapid microtubule transport in neurons and in higher plants.[11]

Human β-tubulins subtypes include:[citation needed]

γ-Tubulin[edit]

γ-Tubulin, another member of the tubulin family, is important in the nucleation and polar orientation of microtubules. It is found primarily in centrosomes and spindle pole bodies, since these are the areas of most abundant microtubule nucleation. In these organelles, several γ-tubulin and other protein molecules are found in complexes known as γ-tubulin ring complexes (γ-TuRCs), which chemically mimic the (+) end of a microtubule and thus allow microtubules to bind. γ-tubulin also has been isolated as a dimer and as a part of a γ-tubulin small complex (γTuSC), intermediate in size between the dimer and the γTuRC. γ-tubulin is the best understood mechanism of microtubule nucleation, but certain studies have indicated that certain cells may be able to adapt to its absence, as indicated by mutation and RNAi studies that have inhibited its correct expression.

Human γ-tubulin subtypes include:

Members of the γ-tubulin ring complex:

δ And ε tubulin[edit]

Delta (δ) and epsilon (ε) tubulin have been found to localize at centrioles and may play a role in forming the mitotic spindle during mitosis, though neither is as well-studied as the α- and β- forms.

Human δ- and ε-tubulin subtypes include:[citation needed]

Pharmacology[edit]

Tubulins are targets for anticancer drugs like Taxol, Tesetaxel and the "Vinca alkaloid" drugs such as vinblastine and vincristine. The anti-gout agent colchicine binds to tubulin and inhibits microtubule formation, arresting neutrophil motility and decreasing inflammation. The anti-fungal drug Griseofulvin targets microtubule formation and has applications in cancer treatment.

Tubulin domains[edit]

Tubulin/FtsZ family, GTPase domain[edit]

Tubulin
PDB 1ia0 EBI.jpg
kif1a head-microtubule complex structure in atp-form
Identifiers
Symbol Tubulin
Pfam PF00091
Pfam clan CL0442
InterPro IPR003008
PROSITE PDOC00201
SCOP 1tub
SUPERFAMILY 1tub

In molecular biology, Tubulin/FtsZ family, GTPase domain is an evolutionary conserved protein domain.

This GTPase protein domain is found in all tubulin chains,[12] as well as the bacterial FtsZ family of proteins.[3][13] These proteins are involved in polymer formation. Tubulin is the major component of microtubules, while FtsZ is the polymer-forming protein of bacterial cell division that forms part of a ring in the middle of the dividing cell that is required for constriction of the cell membrane and cell envelope to yield two daughter cells. FtsZ can polymerise into tubes, sheets, and rings in vitro, and is ubiquitous in bacteria and archaea.

See also[edit]

References[edit]

  1. ^ NCBI CCD cd2186
  2. ^ Williams RC Jr, Shah C, Sackett D (November 1999). "Separation of tubulin isoforms by isoelectric focusing in immobilized pH gradient gels". Anal Biochem 275 (2): 265–7. doi:10.1006/abio.1999.4326. PMID 10552916. 
  3. ^ a b Nogales E, Downing KH, Amos LA, Löwe J (June 1998). "Tubulin and FtsZ form a distinct family of GTPases". Nat Struct Biol 5 (6): 451–8. doi:10.1038/nsb0698-451. PMID 9628483. 
  4. ^ Heald R, Nogales E (January 2002). "Microtubule dynamics". J Cell Sci 115 (Pt 1): 3–4. PMID 11801717. 
  5. ^ Howard J, Hyman AA (April 2003). "Dynamics and mechanics of the microtubule plus end". Nature 422 (6933): 753–8. doi:10.1038/nature01600. PMID 12700769. 
  6. ^ a b Zhou J, Giannakakou P (January 2005). "Targeting Microtubules for Cancer Chemotherapy". Current Medicinal Chemistry - Anti-Cancer Agents 5 (1): 65–71. doi:10.2174/1568011053352569. PMID 15720262. 
  7. ^ http://www.ncbi.nlm.nih.gov/pubmed/23379899
  8. ^ Luduena RF (May 1993). "Are tubulin isotypes functionally significant". Molecular Biology of the Cell 4 (5): 445–457. doi:10.1091/mbc.4.5.445. PMID 8334301. 
  9. ^ http://www.ncbi.nlm.nih.gov/gene?Db=gene&Cmd=ShowDetailView&TermToSearch=81027
  10. ^ a b Lecine P et al. (August 2000). "Hematopoietic-specific beta 1 tubulin participates in a pathway of platelet biogenesis dependent on the transcription factor NF-E2". Blood 96 (4): 1366–1373. PMID 10942379. 
  11. ^ McNally FJ, Vale RD (November 1993). "Identification of katanin, an ATPase that severs and disassembles stable microtubules". Cell 75 (3): 419–29. doi:10.1016/0092-8674(93)90377-3. PMID 8221885. 
  12. ^ Nogales E, Wolf SG, Downing KH (January 1998). "Structure of the alpha beta tubulin dimer by electron crystallography". Nature 391 (6663): 199–203. doi:10.1038/34465. PMID 9428769. 
  13. ^ Löwe J, Amos LA (January 1998). "Crystal structure of the bacterial cell-division protein FtsZ". Nature 391 (6663): 203–6. doi:10.1038/34472. PMID 9428770. 

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