Research ArticleImmunology

Genome-wide RNAi screening implicates the E3 ubiquitin ligase Sherpa in mediating innate immune signaling by Toll in Drosophila adults

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Sci. Signal.  27 Oct 2015:
Vol. 8, Issue 400, pp. ra107
DOI: 10.1126/scisignal.2005971

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Sherpa guides Toll signaling

Mammalian Toll-like receptors (TLRs) recognize pathogen-derived molecular patterns and stimulate the innate immune response. These proteins are named for their counterparts in the fruit fly Drosophila in which the first of these, Toll, was identified. Kanoh et al. showed that the fly Toll innate immune signaling pathway required an E3 ubiquitin ligase called Sherpa. Flies expressing a ligase-defective mutant Sherpa had compromised immune responses to bacterial infection. The adaptor protein dMyd88, which interacts with Toll, relied on both Sherpa and the Drosophila SUMO homolog for proper subcellular localization. Thus, this study reveals additional similarities between the fly and mammalian innate immune systems.

Abstract

The Drosophila Toll pathway plays important roles in innate immune responses against Gram-positive bacteria and fungi. To identify previously uncharacterized components of this pathway, we performed comparative, ex vivo, genome-wide RNA interference screening. In four screens, we overexpressed the Toll adaptor protein dMyd88, the downstream kinase Pelle, or the nuclear factor κB (NF-κB) homolog Dif, or we knocked down Cactus, the Drosophila homolog of mammalian inhibitor of NF-κB. On the basis of these screens, we identified the E3 ubiquitin ligase Sherpa as being necessary for the activation of Toll signaling. A loss-of-function sherpa mutant fly exhibited compromised production of antimicrobial peptides and enhanced susceptibility to infection by Gram-positive bacteria. In cultured cells, Sherpa mediated ubiquitylation of dMyd88 and Sherpa itself, and Sherpa and Drosophila SUMO (small ubiquitin-like modifier) were required for the proper membrane localization of an adaptor complex containing dMyd88. These findings highlight a role for Sherpa in Drosophila host defense and suggest the SUMOylation-mediated regulation of dMyd88 functions in Toll innate immune signaling.

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