GSK-3beta regulates proper mitotic spindle formation in cooperation with a component of the gamma-tubulin ring complex, GCP5

Glycogen synthase kinase-3beta (GSK-3beta) is known to play a role in the regulation of the dynamics of microtubule networks in cells. Here we show the role of GSK-3beta in the proper formation of the mitotic spindles through an interaction with GCP5, a component of the gamma-tubulin ring complex (gammaTuRC). GCP5 bound directly to GSK-3beta in vitro, and their interaction was also observed in intact cells at endogenous levels. Depletion of GCP5 dramatically reduced the GCP2 and gamma-tubulin in the gammaTuRC fraction of sucrose density gradients and disrupted gamma-tubulin localization to the spindle poles in mitotic cells. GCP5 appears to be required for the formation or stability of gammaTuRC and the recruitment of gamma-tubulin to the spindle poles. A GSK-3 inhibitor not only led to the accumulation of gamma-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 gammaTuRC to the centrosome isolated from mitotic cells in vitro. Our findings suggest that GSK-3beta regulates the localization of gammaTuRC, including GCP5, to the spindle poles, thereby controlling the formation of proper mitotic spindles.