γ-Tubulin complexes in microtubule nucleation and beyond

Abstract

Tremendous progress has been made in understanding the functions of γ-tubulin and, in particular, its role in microtubule nucleation since the publication of its discovery in 1989. The structure of γ-tubulin has been determined, and the components of γ-tubulin complexes have been identified. Significant progress in understanding the structure of the γ-tubulin ring complex and its components has led to a persuasive model for how these complexes nucleate microtubule assembly. At the same time, data have accumulated that γ-tubulin has important but less well understood functions that are not simply a consequence of its function in microtubule nucleation. These include roles in the regulation of plus-end microtubule dynamics, gene regulation, and mitotic and cell cycle regulation. Finally, evidence is emerging that γ-tubulin mutations or alterations of γ-tubulin expression play an important role in certain types of cancer and in other diseases.

FIGURE 1:

γ-Tubulin or γ-tubulin complexes in microtubule nucleation and beyond. (A) Nucleation of microtubule assembly by a γTuRC. The γTuRC consists of GCPs that bind to γ-tubulin and to each other, forming a complex in which a ring of γ-tubulin molecules effectively mimics the plus end of a microtubule. It forms a pre-existing nucleus for microtubule assembly. At physiological tubulin concentrations, spontaneous assembly of tubulin into microtubules is rare. Instead, almost all microtubule growth occurs from preformed nucleating structures, principally the γTuRC. (B) In S. cerevisiae only two GCPs exist, Spc97 and Spc98. They assemble with γ-tubulin to form γTuSCs. (C) The finding that GCP2–6 all bind to γ-tubulin raises the possibility that GCPs and γ-tubulin may assemble into alternative γTuSC-like structures (Kollman et al., 2011) that assemble along with γTuSCs to form the γTuRC. (D) Designations for GCPs in various organisms in which they have been studied extensively. (E) Functions of γ-tubulin. γ-Tubulin complexes nucleate microtubules from the centrosome and other MTOCs in both interphase and mitosis or from microtubule surfaces in mitosis. In interphase and mitosis, γ-tubulin has been shown to regulate microtubule plus-end dynamics. In mitosis, it plays a role in the spindle assembly checkpoint (SAC) and the control of mitotic exit. In interphase, γ-tubulin plays an important role in inhibiting APC/C^Cdh1, thereby promoting the transition from the G₁ to the S phase. It also appears to regulate E2F1-mediated gene expression and complexes with Rad51, suggesting a role in the Rad51-mediated DNA damage response.