Asymmetric cell divisions, which help to generate cell diversity during development, often involve eccentric positioning of the mitotic spindle. On p. , Afshar and colleagues report that the Gα proteins GOA-1 and GPA-16 activate the same spindle positioning pathway in one-cell C. elegans embryos but are regulated differently. The researchers show, for example, that both proteins localize to the cortex of the one-cell embryo, and that both bind RIC-8 and GPR-1/2 – all four proteins help to generate the pulling force needed to position the mitotic spindle. However, whereas GPR-1/2 acts as a guanine nucleotide-dissociation inhibitor for both GPA-1 and GOA-1, RIC-8 is a guanine nucleotide-exchange factor for GOA-1 only. Conversely, RIC-8 is required for the cortical localization of GPA-16 but not GOA-1. These and other observations provide new insights into the mechanisms of asymmetric spindle positioning.
