Tubulins

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Tubuline AB heterodimer, Bos taurus.

Tubulins are proteins found in the cells of eukaryotes . They are about 55  kDa and 450 amino acids long. In humans alone, over one hundred paralogous isoforms are known, which are divided into six main groups (α, β, γ, δ, ε, ζ). The most important forms of tubulin, the α- and β-tubulins, are the main components of microtubules .

Occurrence

While eukaryotes, whether they are only protozoa, fungi, plants or animals, have the same tubulin (genetic similarity: 90-95%), prokaryotes have considerably more different tubulin-like proteins ( BtubA and BtubB . There are also TubZ , RepX , FtsZ ) which sometimes only match 40% in their gene sequence. Nevertheless, the microtubules of the bacteria produced from these homologues seem to fulfill essentially the same functions as those of the eukaryotes.

to form

α- and β-tubulin

α-tubulin (red) in human endothelial cells

α- and β-tubulins are globular proteins, which with the aid of a non-covalent bond to a hetero- dimer can connect. These heterodimers, which then aggregate to form the so-called protofilaments, are then able to aggregate into helical hollow bodies, the so-called microtubules . The “microtubule organizing center” ( MTOC ) is the origin of these approximately 25 nanometers thick microtubules . It takes 13 protofilaments to turn the helix .

At the ends of a microtubule there are respectively α-tubulin and β-tubulin. The α-tubulin end is called the minus end, the β-tubulin plus end. α- and β-tubulin are able to bind GTP . In the case of α-tubulin of the protofilaments, the GTP is enclosed at the dimer interface by a loop of the β-tubulin and thus protected from hydrolysis . The GTP of the β-tubulin is, however, hydrolyzed to GDP shortly after it has been attached to another protofilament . The stability at the plus end is higher thanks to the GTP cap. Microtubules depolymerize and polymerize more easily at the plus end. Both processes ( polymerization and depolymerization) take place simultaneously in the cell and thus establish the dynamics of the microtubule network. This property of the microtubules is called dynamic instability .

γ-tubulin

Gamma tubulin is not a component of microtubules. Rather, it is involved in the germination of new microtubules on the centrosome and basal bodies . Gamma tubulin forms a large ring-shaped protein complex with other proteins. This so-called "gamma tubulin ring complex" (gammaTuRC) can cause the polymerization of alpha- and beta-tubulin to form microtubules.

δ- and ε-tubulin

δ- and ε-tubulin are found in the centrioles of animal cells. It is believed that they play a role in the formation of the mitotic spindle .

ζ-tubulin

ζ-tubulin is only known in parasites . A partial sequence was found in clawed frogs Xenopus .

literature

Individual evidence

  1. INTERPRO entry on tubulins in general
  2. Satyajit Sahu, Subrata Ghosh, Daisuke Fujita, Anirban Bandyopadhyay: Live visualizations of single isolated tubulin protein self-assembly via tunneling current: effect of electromagnetic pumping during spontaneous growth of microtubule . In: Scientific Reports . tape 4 , Article 7303 (2014), December 3, 2014, doi : 10.1038 / srep07303 (English).
  3. KK Busiek, W. Margolin: Bacterial actin and tubulin homologs in cell growth and division. In: Current Biology . Volume 25, number 6, March 2015, pp. R243 – R254, doi : 10.1016 / j.cub.2015.01.030 , PMID 25784047 , PMC 5519336 (free full text) (review).
  4. Martin Pilhofer, Mark S. Ladinsky, Alasdair W. McDowall, Giulio Petroni, Grant J. Jensen: Microtubules in Bacteria: Ancient Tubulins Build a Five-Protofilament Homolog of the Eukaryotic Cytoskeleton. In: Journal: PLOS Biology, Published: December 6, 2011, doi : 10.1371 / journal.pbio.1001213
  5. ^ Contzen Pereira: Cytoskeleton and Consciousness: An Evolutionary Based Review. In: Journal: NeuroQuantology, June 2015, Volume 13, Issue 2, Page 232-239
  6. ^ A b William J. Lennarz, M. Daniel Lane (Eds.): Encyclopedia of Biological Chemistry , Vol. 1, p. 372, ISBN 0-12-443710-9 , 2004
  7. INTERPRO: ζ-tubulin