A Tubulin is one of several members of a small family of globular proteins. 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 kiloDaltons. 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.
Tubulin was long thought to be specific to eukaryotes. Recently, however, the prokaryotic cell division protein FtsZ was shown to be evolutionarily related to tubulin.
α-tubulin and β-tubulin
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. After being incorporated into the microtubule, the bound molecule of GTP will hydrolyse
Although both subunits bind GTP, only the β-subunit has GTPase activity; that is, β-tubulin can hydrolyse GTP to GDP whereas α-tubulin cannot. 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.
Class III β-tubulin is a microtubule element expressed exclusively in neurons, and is a popular identifier specific for neurons in nervous tissue.
Katanin is a protein complex that severs β-tubulin, and is necessary for rapid microtubule transport in neurons and in higher plants.
Human α- and β-tubulin subtypes include:
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
studies that have inhibited its correct expression.
Human γ-tubulin subtypes include:
δ and ε tubulin
Delta (δ) and epsilon (ε) tubulin have been found to localize at centrioles
and may play a role in forming the mitotic spindle
, though neither is as well-studied as the α- and β- forms.
Human δ- and ε-tubulin subtypes include:
Tubulins are targets for anticancer drugs
and the "Vinca alkaloid" drugs such as vinblastine
. The anti-gout
binds to tubulin and inhibits microtubule formation, arresting neutrophil
motility and decreasing inflammation
. The anti-fungal drug Griseofulvin
targets mictotubule formation and has applications in cancer treatment.