Hyd ubiquitinates the NF-κB co-factor Akirin to operate an effective immune response in Drosophila

Abstract

The Immune Deficiency (IMD) pathway in Drosophila melanogaster is activated in response to microbial challenges, particularly by Gram-negative bacteria, to initiate the innate immune response. To elucidate the nuclear factor κB (NF-κB) signaling pathway, we conducted an in vitro RNA interference (RNAi) screen targeting E3 ubiquitin ligases and identified the HECT-type E3 ubiquitin ligase Hyperplastic discs (Hyd) as a novel component of the IMD pathway. We discovered that Hyd mediates Lys63 (K63)-linked polyubiquitination of the NF-κB cofactor Akirin, which is crucial for Akirin’s efficient binding to the NF-κB transcription factor Relish. This Hyd-dependent interaction is essential for the transcription of immunity-related genes activated by both Relish and Akirin but is not required for genes dependent solely on Relish. Consequently, Hyd plays a critical role in NF-κB transcriptional selectivity downstream of the IMD pathway. Drosophila mutants lacking Akirin or Hyd fail to express the full range of immune-induced antimicrobial peptides and exhibit heightened susceptibility to immune challenges. Furthermore, we found that UBR5, the mammalian homolog of Hyd, is also necessary for NF-κB-mediated activation of Interleukin 6 (IL6) transcription in response to LPS or IL-1β in human cell cultures. Our findings establish a direct link between an E3 ubiquitin ligase and the activation of immune effector genes, advancing our understanding of ubiquitination in inflammation and highlighting a potential target for controlling inflammatory diseases.