Tubulin



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 which makes up microtubules. Each has a molecular weight of approximately 55 kiloDaltons. Microtubules are assembled from dimers of &alpha;- and &beta;-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 into GDP.

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:
 * α-tubulin
 * TUBA1A
 * TUBA1B
 * TUBA1C
 * TUBA3C
 * TUBA3D
 * TUBA3E
 * TUBA4A
 * TUBA8
 * β-tubulin
 * TUBB
 * TUBB1
 * TUBB2A
 * TUBB2B
 * TUBB2C
 * TUBB3
 * TUBB4
 * TUBB4Q
 * TUBB6

γ-tubulin
&#947;-tubulin, another member of the tubulin family, is important in the nucleation and polar orientation of microtubules.

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 during mitosis, though neither is as well-studied as the α- and β- forms

Pharmacology
Tubulins are targets for anticancer drugs like Taxol® 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.