GSK-3β Regulates Proper Mitotic Spindle Formation in Cooperation with a Component of the γ-Tubulin Ring Complex, GCP5.

Glycogen synthase kinase-3β (GSK-3β) is known to play a role in the regulation of the dynamics of microtubule networks in cells. Here we show the role of GSK-3β in the proper formation of the mitotic spindles through an interaction with GCP5, a component of the γ-tubulin ring complex (γTuRC). GCP5 bound directly to GSK-3β in vitro, and their interaction was also observed in intact cells at endogenous levels. Depletion of GCP5 dramatically reduced the GCP2 and γ-tubulin in the γTuRC fraction of sucrose density gradients and disrupted γ-tubulin localization to the spindle poles in mitotic cells. GCP5 appears to be required for the formation or stability of γTuRC and the recruitment of β-tubulin to the spindle poles. A GSK-3 inhibitor not only led to the accumulation of γ-tubulin and GCP5 at the spindle poles but also enhanced microtubule nucleation activity at the spindle poles. Depletion of GCP5 rescued this disrupted organization of spindle poles observed in cells treated with the GSK-3 inhibitor. Furthermore, the inhibition of GSK-3 enhanced the binding of γTuRC to the centrosome isolated from mitotic cells in vitro. Our findings suggest that GSK-3β regulates the localization of βTuRC, including GCP5, to the spindle poles, thereby controlling the formation of proper mitotic spindles. [ABSTRACT FROM AUTHOR]