Asymmetric division of Drosophila neuroblasts (NBs) and the Caenorhabditis elegans zygote uses polarity cues provided by the Par proteins, as well as heterotrimeric G-protein-signalling that is activated by a receptor-independent mechanism mediated by GoLoco/GPR motif proteins1,2. Another key component of this non-canonical G-protein activation mechanism is a non-receptor guanine nucleotide-exchange factor (GEF) for Gα, RIC-8, which has recently been characterized in C. elegans and in mammals3,4,5,6. We show here that the Drosophila Ric-8 homologue is required for asymmetric division of both NBs and pI cells. Ric-8 is necessary for membrane targeting of Gαi, Pins and Gβ13F, presumably by regulating multiple Gα subunit(s). Ric-8 forms an in vivo complex with Gαi and interacts preferentially with GDP–Gαi, which is consistent with Ric-8 acting as a GEF for Gαi. Comparisons of the phenotypes of Gαi, Ric-8, Gβ13F single and Ric-8;Gβ13F double loss-of-function mutants indicate that, in NBs, Ric-8 positively regulates Gαi activity. In addition, Gβγ acts to restrict Gαi (and GoLoco proteins) to the apical cortex, where Gαi (and Pins) can mediate asymmetric spindle geometry.
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We thank H. Bellen, C.Q. Doe, D. Glover, Y.N. Jan, D. St Johnston, T. Kaufman, C. Klambt, J.A. Knoblich, E. Knust, F. Matsuzaki, S. Roth, F. Schweisguth, X. Yang, A. Wodarz, Developmental Studies Hybridoma Bank (University of Iowa) and the Bloomington Stock Center for generously providing antibodies and fly stocks. We thank F. S. Willard for discussion and support, and W. Chia thanks Y. Cai for stimulating discussions. This work was supported by Temasek Life Sciences Laboratory, Singapore Millennium Foundation (F. Y.) and Wellcome Trust (W. C.).
The authors declare no competing financial interests.
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