Eukaryotic The tubulin superfamily contains six families (alpha-(α), beta-(β), gamma-(γ), delta-(δ), epsilon-(ε), and zeta-(ζ) tubulins).
α-Tubulin Human α-tubulin subtypes include: •
TUBA1A •
TUBA1B •
TUBA1C •
TUBA3C • TUBA3D • TUBA3E •
TUBA4A •
TUBA8 β-Tubulin '' sp. All drugs that are known to bind to human tubulin bind to β-tubulin. These include
paclitaxel,
colchicine, and the
vinca alkaloids, each of which have a distinct binding site on β-tubulin.
albendazole almost exclusively binds to the β-tubulin of worms and other lower eukaryotes.
Class III β-tubulin is a microtubule element expressed exclusively in
neurons, and is a popular identifier specific for neurons in nervous tissue. It binds colchicine much more slowly than other
isotypes of β-tubulin.
β1-tubulin, sometimes called class VI β-tubulin, is the most divergent at the amino acid sequence level. It is expressed exclusively in megakaryocytes and platelets in humans and appears to play an important role in the formation of platelets.
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. Human β-tubulins subtypes include: •
TUBB •
TUBB1 •
TUBB2A • TUBB2B •
TUBB2C •
TUBB3 •
TUBB4 • TUBB4Q • TUBB6 • TUBB8
γ-Tubulin 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. Besides forming a γ-TuRC to nucleate and organize microtubules, γ-tubulin can polymerize into filaments that assemble into bundles and meshworks. Human γ-tubulin subtypes include: •
TUBG1 •
TUBG2 Members of the γ-tubulin ring complex: •
TUBGCP2 •
TUBGCP3 •
TUBGCP4 •
TUBGCP5 •
TUBGCP6 δ and ε-Tubulin Delta (δ) and epsilon (ε) tubulin have been found to localize at
centrioles and may play a role in centriole structure and function, though neither is as well-studied as the α- and β- forms. Human δ- and ε-tubulin genes include: • δ-tubulin:
TUBD1 • ε-tubulin:
TUBE1 ζ-Tubulin Zeta-tubulin () is present in many eukaryotes, but missing from others, including placental mammals. It has been shown to be associated with the basal foot structure of centrioles in multiciliated epithelial cells.
FtsZ Many bacterial and
euryarchaeotal cells use
FtsZ to divide via
binary fission. All
chloroplasts and some
mitochondria, both organelles derived from
endosymbiosis of bacteria, also use FtsZ. It was the first prokaryotic
cytoskeletal protein identified.
TubZ TubZ (; pBt156) was identified in
Bacillus thuringiensis as essential for
plasmid maintenance.
CetZ CetZ () is found in the
euryarchaeal clades of
Methanomicrobia and
Halobacteria, where it functions in cell shape differentiation.
"Odinarchaeota" tubulin Asgard archaea tubulin from hydrothermal-living "Odinarchaeota" (OdinTubulin) was identified as a genuine tubulin. OdinTubulin forms protomers and protofilaments most similar to eukaryotic microtubules, yet assembles into ring systems more similar to
FtsZ, indicating that OdinTubulin may represent an evolution intermediate between FtsZ and microtubule-forming tubulins. == Pharmacology ==