Tubulin is one of several members of a small family of globular
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s. The most common members of the tubulin family are α-tubulin and β-tubulin, the proteins that make up
microtubuleMicrotubules are one of the components of the cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers. Microtubules serve as structural components within cells and are involved in many cellular processes including mitosis, cytokinesis, and vesicular...
s. Each has a molecular weight of approximately 55 kiloDaltons. Microtubules are assembled from
dimerA dimer is a chemical or biological entity consisting of two structurally similar subunits called monomers, which are joined by bonds, which can be strong or weak.- Organic chemistry :...
s of α- and β-tubulin. These subunits are slightly acidic with an
isoelectric pointThe isoelectric point , sometimes abbreviated to IEP, is the pH at which a particular molecule or surface carries no net electrical charge....
between 5.2 and 5.8.
Tubulin was long thought to be specific to
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
s.
Tubulin is one of several members of a small family of globular
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s. The most common members of the tubulin family are α-tubulin and β-tubulin, the proteins that make up
microtubuleMicrotubules are one of the components of the cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers. Microtubules serve as structural components within cells and are involved in many cellular processes including mitosis, cytokinesis, and vesicular...
s. Each has a molecular weight of approximately 55 kiloDaltons. Microtubules are assembled from
dimerA dimer is a chemical or biological entity consisting of two structurally similar subunits called monomers, which are joined by bonds, which can be strong or weak.- Organic chemistry :...
s of α- and β-tubulin. These subunits are slightly acidic with an
isoelectric pointThe isoelectric point , sometimes abbreviated to IEP, is the pH at which a particular molecule or surface carries no net electrical charge....
between 5.2 and 5.8.
Tubulin was long thought to be specific to
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
s. Recently, however, the prokaryotic cell division protein
FtsZFtsZ is a protein encoded by the ftsZ gene that assembles into a ring at the future site of the septum of bacterial cell division. FtsZ, named after "Filamenting temperature-sensitive...
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
GTPGuanosine-5'-triphosphate is a purine nucleotide. It can act as a substrate for the synthesis of RNA during transcription process...
and assemble onto the (+) ends of microtubules while in the GTP-bound state. After the dimer is incorporated into the microtubule, the molecule of GTP bound to the β-tubulin subunit eventually
hydrolysesHydrolysis is a chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions, which may go on to participate in further reactions. It is the type of reaction that is used to break down certain polymers, especially those made by step-growth polymerization...
into GDP through inter-dimer contacts along the microtubule protofilament.. 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
neuronA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
s, and is a popular identifier specific for neurons in nervous tissue.
KataninKatanin is a microtubule-severing AAA protein. It is named after the Japanese sword, katana. Katanin is a heterodimeric protein first discovered in sea urchins. It contains a 60 kDa ATPase subunit, which functions to sever microtubules. This subunit requires ATP and the presence of microtubules...
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 α- and β-tubulin subtypes include:
- α-tubulin
- TUBA1A
Tubulin alpha-1A chain is a protein that in humans is encoded by the TUBA1A gene.-Interactions:TUBA1A has been shown to interact with PAFAH1B1.-Further reading:...
- TUBA1B
Tubulin alpha-1B chain is a protein that in humans is encoded by the TUBA1B gene.-Further reading:...
- TUBA1C
Tubulin alpha-1C chain is a protein that in humans is encoded by the TUBA1C gene.-Further reading:...
- TUBA3C
- TUBA3D
- TUBA3E
- TUBA4A
Tubulin alpha-4A chain is a protein that in humans is encoded by the TUBA4A gene.-Interactions:TUBA4A has been shown to interact with NCOA6 and APC.-Further reading:...
- TUBA8
Tubulin alpha-8 chain is a protein that in humans is encoded by the TUBA8 gene.-Further reading:...
- β-tubulin
- TUBB
Tubulin beta chain is a protein that in humans is encoded by the TUBB gene.-Interactions:TUBB has been shown to interact with NCOA6 and SYT9.-Further reading:...
- TUBB1
Tubulin beta-1 chain is a protein that in humans is encoded by the TUBB1 gene.-Further reading:...
- TUBB2A
Tubulin beta-2A chain is a protein that in humans is encoded by the TUBB2A gene.-Further reading:...
- TUBB2B
- TUBB2C
Tubulin beta-2C chain is a protein that in humans is encoded by the TUBB2C gene.-Further reading:...
- TUBB3
Tubulin beta-3 chain is a protein that in humans is encoded by the TUBB3 gene.-Further reading:...
- TUBB4
Tubulin beta-4 chain is a protein that in humans is encoded by the TUBB4 gene.-Further reading:...
- TUBB4Q
- TUBB6
γ-tubulin
γ-tubulin, another member of the tubulin family, is important in the
nucleationMicrotubules are filaments of the cytoskeleton. They typically form through the polymerization of α- and β-tubulin dimers elongating existing microtubules...
and polar orientation of microtubules. It is found primarily in centrosomes and
spindle pole bodiesThe spindle pole body is the microtubule organizing center in yeast cells, functionally equivalent to the centrosome. Unlike the centrosome the SPB does not contain centrioles. The SPB organises the microtubule cytoskeleton which plays many roles in the cell...
, 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
dimerA dimer is a chemical or biological entity consisting of two structurally similar subunits called monomers, which are joined by bonds, which can be strong or weak.- Organic chemistry :...
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
mutationIn biology, a mutation is a randomly derived change to the nucleotide sequence of the genetic material of an organism.Mutations can be caused by copying errors in the genetic material during cell division, or by exposure to mutagens , or can be induced by the organism itself, by cellular processes...
and
RNAiRNAI is a non-coding RNA that is an antisense repressor of the replication of some E. coli plasmids, including ColE1. Plasmid replication is usually initiated by RNAII, which acts as a primer by binding to its template DNA. The complementary RNAI binds RNAII prohibiting it from its initiation role...
studies that have inhibited its correct expression.
Human γ-tubulin subtypes include:
- TUBG1
Microtubules are filaments of the cytoskeleton. They typically form through the polymerization of α- and β-tubulin dimers elongating existing microtubules...
- TUBG2
Tubulin gamma-2 chain is a protein that in humans is encoded by the TUBG2 gene.-Further reading:...
- TUBGCP2
Gamma-tubulin complex component 2 is a protein that in humans is encoded by the TUBGCP2 gene.-Further reading:...
- TUBGCP3
Gamma-tubulin complex component 3 is a protein that in humans is encoded by the TUBGCP3 gene.-Further reading:...
- TUBGCP4
- TUBGCP5
Gamma-tubulin complex component 5 is a protein that in humans is encoded by the TUBGCP5 gene.-Further reading:...
- TUBGCP6
Gamma-tubulin complex component 6 is a protein that in humans is encoded by the TUBGCP6 gene.-Further reading:...
δ and ε tubulin
Delta (δ) and epsilon (ε) tubulin have been found to localize at
centrioleA centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...
s and may play a role in forming the
mitotic spindleIn cell biology, the spindle apparatus is the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...
during
mitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
, though neither is as well-studied as the α- and β- forms.
Human δ- and ε-tubulin subtypes include:
Pharmacology
Tubulins are targets for anticancer
drugsA pharmaceutical drug, also referred to as medicine, medication or medicament, can be loosely defined as any chemical substance intended for use in the medical diagnosis, cure, treatment, or prevention of disease.- Classification :...
like Taxol and the "Vinca alkaloid" drugs such as
vinblastineVinblastine is an antimicrotubule drug used to treat certain kinds of cancer, including Hodgkin's lymphoma, non-small cell lung cancer, breast cancer, head and neck cancer, and testicular cancer.-History:...
and
vincristineVincristine , also known as leurocristine, is a vinca alkaloid from the Catharanthus roseus , formerly Vinca rosea and hence its name. It is a mitotic inhibitor, and is used in cancer chemotherapy....
. The anti-
goutGout is a disease hallmarked by elevated levels of uric acid in the bloodstream. In this condition, crystals of monosodium urate or uric acid are deposited on the articular cartilage of joints, tendons, and surrounding tissues...
agent
colchicineColchicine is a toxic natural product and secondary metabolite, originally extracted from plants of the genus Colchicum . It was used originally to treat rheumatic complaints, especially gout, and still finds use for these purposes today. It was also prescribed for its cathartic and emetic effects...
binds to tubulin and inhibits microtubule formation, arresting
neutrophilThis article is about neutrophils, cells of the immune system. For organisms that grow in neutral pH environments see: Neutrophile.Neutrophil granulocytes, generally referred to as neutrophils, are the most abundant type of white blood cells in mammals and form an essential part of the innate...
motility and decreasing
inflammationInflammation is the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli as well as initiate the healing process for the tissue. Inflammation is not a...
. The anti-fungal drug
GriseofulvinGriseofulvin is an antifungal drug. It is used both in animals and in humans, to treat fungal infections of the skin and nails...
targets mictotubule formation and has applications in cancer treatment.