G-proteins for spindle geometry

On page 623, Yu et al. demonstrate that unequal cell division in fly embryos results from two apical complexes that somehow affect microtubule dynamics.Unequally sized daughter cells are created in fly neuroblasts by the formation of a longer half spindle in the apical side of the cell. Yu et al. demonstrate that the production of this asymmetric spindle is regulated through the distinct activities of two subunits of the heterotrimeric G-protein complex.

By isolating mutant embryos lacking the Gαi subunit, they show that this subunit is part of one of two redundant apical protein complexes—one including Gαi and Pins, and the other including Par6 and atypical protein kinase C. Loss of both apical pathways results in a symmetric spindle and thus equally sized daughter cells. In wild-type neuroblasts, astral microtubule formation seems to be asymmetrically associated with the apical but not basal centrosome. But if both apical pathways are abolished, astral microtubules form over both centrosomes, suggesting that some normally apically localized molecule can stabilize the association of astral microtubules with the cell cortex.

Loss of a different G-protein subunit, Gβ13F, disrupts the localization of components from both apical pathways. Thus, although Gβ13F is found both apically and basally, it seems to control the localization of the apical components. ▪