Assembly and regulation of γ-tubulin complexes

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.

Keywords: CM1 proteins; centrosome; gamma-tubulin; gamma-tubulin complex proteins; microtubule nucleation; microtubule-organizing centres.

Figure 1.

Assembly and recruitment of γ-tubulin complexes. (a) GCP2 and GCP3 interact laterally, and bind longitudinally each to one molecule of γ-tubulin, to form the γTuSC. Assembly of helical complexes from γTuSCs is driven by oligomerization of proteins with a CM1 domain, such as Spc110 in S. cerevisiae. The CM1 domain binds to the amino-terminal region of GCP3, together with a small oligomerization-promoting protein, MOZART1. (b) Soluble γTuRCs are fully assembled in the cytoplasm, and are recruited to the centrosome by NEDD1 and by CM1 proteins, such as Cdk5rap2 in mammals. The inset depicts schematically sequence similarities between GCPs 2, 3, 4, 5 and 6. Conserved secondary structures are found in the amino-terminal grip1 domain, and in the carboxy-terminal grip2 domain (highlighted in green). GCPs 5 and 6 contain unique sequence extensions at their extreme amino-termini and between the grip1 and grip2 domains that are not shared with any other GCPs.

Figure 2.

Augmin complexes recruit γTuRCs to the surface of spindle microtubules, to initiate nucleation of ‘secondary microtubules'. Augmin-dependent recruitment occurs in the presence of NEDD1 and the spindle assembly factor TPX2. TPX2 interacts with the γTuRC via a composite binding sequence that bears resemblance to the SPM and CM1 motifs of yeast Spc110.

Figure 3.

Efficient formation of microtubules from γTuRCs requires additional proteins that interact with early intermediates of nucleation. At early stages of nucleation, single tubulin dimers bind to the γTuRC, some of which are lacking lateral interactions. These early nucleation intermediates are stabilized by TPX2 until a closed tube is formed, independent of the γTuRC-binding property of TPX2. In the next step, tubulin polymerization is supported by the microtubule-associated protein chTOG (= XMAP215 in Xenopus laevis).