Drosophila proteasome subunit Rpn8 controls IMD pathway activation via PGRP-SC2 degradation

The ubiquitin-proteasome system (UPS) is a highly conserved protein degradation pathway in insects, playing crucial roles in cellular homeostasis, signal transduction, and various biological processes. Here, we demonstrate that the Drosophila melanogaster (fruit fly) regulatory particle non-ATPase 8 (Rpn8), a key subunit of the 26 S proteasomal cap, is indispensable for fly survival upon bacterial infections. Through targeted genetic knockdown, we observed that Rpn8-silenced flies exhibited decreased expressions of several antimicrobial peptides upon bacterial challenges, concomitant with impaired bacterial clearances. Further proteomic and biochemical analyses revealed that Rpn8 controls the proteasome-mediated degradation of peptidoglycan recognition protein SC2 (PGRP-SC2), a typical amidase that antagonizes immune deficiency (IMD) signaling. Silencing of PGRP-SC2 prevented the functional role of Rpn8 in controlling the fly antibacterial immune defense, suggesting that Rpn8 mediates Drosophila innate immunity in a PGRP-SC2-dependent manner. Intriguingly, overexpression of Bombyx mori (silkworm) Rpn8 rescued the immune defects in Rpn8 RNAi flies after bacterial infections. Collectively, our findings not only establish Rpn8 as a novel immune modulator that bridges UPS-mediated protein degradation with innate immune defense in Drosophila but also offer insights into conserved mechanisms of host-pathogen interactions in other insect species.