Abstract
Microtubules are major constituents of the cytoskeleton in all eukaryotic cells. They are essential for chromosome segregation during cell division, for directional intracellular transport and for building specialized cellular structures such as cilia or flagella. Their assembly has to be controlled spatially and temporally. For this, the cell uses multiprotein complexes containing γ-tubulin. γ-Tubulin has been found in two different types of complexes, γ-tubulin small complexes and γ-tubulin ring complexes. Binding to adaptors and activator proteins transforms these complexes into structural templates that drive the nucleation of new microtubules in a highly controlled manner. This review discusses recent advances on the mechanisms of assembly, recruitment and activation of γ-tubulin complexes at microtubule-organizing centres.
Highlights
Microtubules are tubular polymers that assemble from heterodimers of a and b-tubulin
Microtubules can be assembled reproducibly in vitro, at tubulin concentrations of approximately 20 mM [1]. This value matches well the concentration of tubulin that was measured in cytoplasmic extracts [2], raising the question why microtubules grow off specific organizing centres in the cell, instead of polymerizing ubiquitously in an uncontrolled manner
The major constituents of g-tubulin complexes comprise g-tubulin, a member of the tubulin family, and ‘g-tubulin complex proteins’ (GCPs). g-Tubulin was originally discovered in the fungus Aspergillus nidulans, as a suppressor of a & 2018 The Authors
Summary
Microtubules are tubular polymers that assemble from heterodimers of a and b-tubulin. Microtubule assembly occurs in multiple steps: initially, a small number of tubulin dimers need to oligomerize, to form a stable nucleus with correct geometry This is considered to be a slow process, because a dynamic equilibrium between dimers and oligomers exists, and detachment of dimers at this stage leads to immediate loss of the nucleus. Multiprotein complexes of g-tubulin are used as templates for the longitudinal association with a/b-tubulin dimers, reducing the duration of the nucleation process [3] These complexes are essential to permit the rapid formation of spindle microtubules at early stages of mitosis. Their absence leads to severe defects in spindle formation, cell cycle arrest and cell death [4,5,6]. As these g-tubulin complexes are only active upon recruitment to specific microtubule-organizing centres (MTOCs) such as the centrosome, the cell possesses spatial and temporal control over the growth of microtubules
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