In metazoans, the insulin signalling pathway has a key function in regulating energy metabolism and organismal growth1. Its activation stimulates a highly conserved downstream kinase cascade that includes phosphatidylinositol-3-OH kinase (PI(3)K) and the serine–threonine protein kinase Akt. This study identifies a new component of insulin signalling in Drosophila, the steppke gene (step). step encodes a member of the cytohesin family of guanine nucleotide exchange factors (GEFs), which have been characterized as activators for ADP-ribosylation factor (ARF) GTPases2,3,4. In step mutant animals both cell size and cell number are reduced, resulting in decreased body size and body weight in larvae, pupae and adults. step acts upstream of PI(3)K and is required for the proper regulation of Akt and the transcription factor FOXO. Temporally controlled interference with the GEF activity of the Step protein by feeding the chemical inhibitor SecinH3 causes a block of insulin signalling and a phenocopy of the step mutant growth defect. Step represses its own expression and the synthesis of growth inhibitors such as the translational repressor 4E-BP. Our findings indicate a crucial role of an ARF–GEF in insulin signalling that has implications for understanding insulin-related disorders, such as diabetes and obesity.
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We thank W. Kolanus for valuable discussions on cytohesins; M. Famulok for providing the Step inhibitor SecinH3 and for sharing unpublished results; S. Cohen and M. Günther Behr for reagents; A. Schmitz for help with pAkt quantifications; C. Müller for technical assistance; and the members of the Famulok and Hoch laboratories for helpful comments. This work was supported by grants from the Deutsche Forschungsgemeinschaft, the Sonderforschungsbereiche 645 (to M.H.) and 704 (to M.H. and B.F.). Author Contributions T.B. and I.Z. performed the growth and body size analysis in step mutants. T.B. analysed the insulin signalling activity in step and chico mutants and under starvation using quantitative RT–PCR. B.F. performed the cell cycle analysis, cell size analysis in step mutants, pAkt quantifications, in vitro and in vivo FOXO localization studies, in vitro growth inhibition analysis, step enhancer analysis and ectopic FOXO expression experiments. I.Z. generated the in vivo chemical genetics data and the Dp110-CAAX rescue experiment. I.Z. and B.F. contributed to the step rescue experiment. M.H. supervised the research project, and assisted in the experimental design. All authors discussed the experimental results. B.F. and M.H. wrote the manuscript.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Fuss, B., Becker, T., Zinke, I. et al. The cytohesin Steppke is essential for insulin signalling in Drosophila. Nature 444, 945–948 (2006). https://doi.org/10.1038/nature05412
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