SUMOylation of Dorsal attenuates Toll/NF-κB signalling

In Drosophila, Toll/NF-{kappa}B signalling plays key roles in both animal development and in host defence. The activation, intensity and kinetics of Toll signalling is regulated by post-translational modifications such as phosphorylation, SUMOylation or ubiquitination that target multiple proteins in the Toll/NF-{kappa}B cascade. Here, we have generated a CRISPR-Cas9 edited Dorsal (DL) variant that is SUMO conjugation resistant (SCR). Intriguingly, embryos laid by dlSCR mothers overcome dl haploinsufficiency and complete the developmental program. This ability appears to be a result of higher transcriptional activation by DLSCR. In contrast, SUMOylation dampens DL transcriptional activation, ultimately conferring robustness to the dorso-ventral program. In the larval immune response, dlSCR animals show increase in crystal cell numbers, stronger activation of humoral defence genes, high cactus levels and cytoplasmic stabilization of DL:Cactus complexes. A mathematical model that evaluates the contribution of the small fraction of SUMOylated DL (<5%) suggests that it acts to block transcriptional activation, driven primarily by DL that is not SUMO conjugated. Our findings define SUMO conjugation as an important regulator of the Toll signalling cascade, in both development and in host defense. Our results broadly indicate that SUMO attenuates DL at the level of transcriptional activation. Further, we hypothesize that SUMO conjugation of DL may be part of a Ubc9 dependant feedback circuit that restrains Toll/NF-{kappa}B signalling.