The GCN2-ATF4 Signaling Pathway Induces 4E-BP to Bias Translation and Boost Antimicrobial Peptide Synthesis in Response to Bacterial Infection

Abstract

Bacterial infection often leads to suppression of mRNA translation, but hosts are nonetheless able to express immune response genes through as yet unknown mechanisms. Here, we use a Drosophila model to demonstrate that antimicrobial peptide (AMP) production during infection is paradoxically stimulated by the inhibitor of cap-dependent translation, 4E-BP (eIF4E-binding protein; encoded by the Thor gene). We found that 4E-BP is induced upon infection with pathogenic bacteria by the stress-response transcription factor ATF4 and its upstream kinase, GCN2. Loss of gcn2, atf4, or 4e-bp compromised immunity. While AMP transcription is unaffected in 4e-bp mutants, AMP protein levels are substantially reduced. The 5′ UTRs of AMPs score positive in cap-independent translation assays, and this cap-independent activity is enhanced by 4E-BP. These results are corroborated in vivo using transgenic 5′ UTR reporters. These observations indicate that ATF4-induced 4e-bp contributes to innate immunity by biasing mRNA translation toward cap-independent mechanisms, thus enhancing AMP synthesis. The cap-dependent translation inhibitor 4E-BP is induced during bacterial infection in Drosophila, and animals lacking 4E-BP are immunocompromised. How an effective innate immune response is mounted in a translation-suppressive environment and why 4E-BP is required in this process were unknown. Vasudevan et al. demonstrate that antimicrobial peptides, which are key components of the Drosophila innate immune response, are translated cap-independently via IRES-like elements in their 5′ UTRs and that their translation is enhanced by 4E-BP-mediated suppression of cap-dependent translation.