FADD is recruited to activated STING oligomers to initiate caspase-mediated NF-κB activation in Drosophila melanogaster.

STING is an evolutionarily conserved key regulator of innate immunity. In the model organism Drosophila melanogaster, STING activates the NF-κB-like transcription factor Relish, initially characterized for its role in the antibacterial IMD pathway. The versatile FADD/Caspase-8 axis is widely used in various immune signaling pathways throughout the animal kingdom, including the IMD pathway. Here, we show that it functions downstream of STING in Drosophila to mediate Relish activation by the Caspase-8 homolog DREDD. We present a detailed structural model illustrating how the adapter protein FADD interacts with two separate STING dimers in the activated oligomerized form of STING, thus providing a molecular explanation for the activation-dependent recruitment of FADD. We further show that FADD interacts with IMD in a structurally distinct but functionally related manner, highlighting how the STING and IMD pathways differentially utilize the adapter protein FADD. Our results illustrate how an ancestral module is incorporated into different innate immune pathways, providing insights into the evolution of host-pathogen interactions.