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When challenged with a pathogen, the host tries to maintain cellular integrity by mounting an immune response to restrict pathogen growth while repairing any cellular damage. A new paper in Cell Host & Microbe reveals that the insect pathogen Pseudomonas entomophila prevents both of these processes by arresting protein synthesis in the gut of its Drosophila melanogaster host.

P. entomophila infection induces global suppression of translation in the D. melanogaster gut.

Although oral infection with P. entomophila stimulates local and systemic expression of antimicrobial peptide genes, the infection is lethal, suggesting that there is a block in the immune response to infection and that this block occurs downstream of transcription. In addition, P. entomophila infection damages the gut epithelium, and there is a lack of epithelial renewal, indicating that the cellular repair pathways in the gut might also be inhibited. Bruno Lemaitre, Nicolas Buchon and colleagues began investigating these phenomena using transgenic D. melanogaster expressing a Diptericin–lacZ reporter gene. Although this reporter gene was transcribed in P. entomophila-infected D. melanogaster, there was a block in LacZ protein expression, whereas this block was not detected in flies infected with the non-lethal pathogen Erwinia carotovora subsp. carotovora str. 15 (Ecc15). The uncoupling of transcription and translation was not specific for genes encoding antimicrobial peptides, as a methionine analogue incorporation assay (which measures total protein synthesis) revealed that P. entomophila infection induces global suppression of translation in the D. melanogaster gut.

How is this block in translation achieved? In eukaryotes, cap-dependent protein synthesis is typically regulated through eukaryotic translation initiation factor 2α (eIF2α) phosphorylation, which suppresses translation. Initial western blot analysis showed that eIF2α in the D. melanogaster gut was phosphorylated following infection with P. entomophila but not Ecc15. Phosphorylation of eIF2α can be achieved by various stress-responsive kinases, including GCN2. The authors again used the Diptericin–lacZ reporter construct to analyse translational activity and found that RNAi-mediated inactivation of GCN2 in the D. melanogaster gut removed the translational block. Another key component of protein synthesis in eukaryotes is the translational repressor eIF4E-binding protein 1 (4E-BP1), which is targeted by the kinase TOR (target of rapamycin); when TOR is inactive, 4E-BP1 is hypophosphorylated, and cap-dependent translation is inhibited. The authors found that at 16 hours after infection P. entomophila causes a reduction in 4E-BP1 phosphorylation and that this involves inhibition of TOR kinase activity. So, the translational arrest induced by P. entomophila infection involves activation of the kinase GCN2 and inhibition of the TOR pathway. Examination of epithelial turnover in D. melan ogaster in which GCN2 had been inactivated by RNAi also revealed that this translational arrest does inhibit epithelial renewal.

Translation inhibition is connected to oxidative stress, and P. entomophila infection is known to induce a strong oxidative burst in the host. In D. melanogaster fed P. entomophila in conjunction with antioxidants, the inhibition of translation was alleviated; conversely, in D. melanogaster fed Ecc15 plus paraquat (a potent inducer of reactive oxygen species), translation was inhibited. Thus, the translational arrest that follows P. entomophila infection is dependent on the host-generated oxidative burst. Finally, the experiments also revealed a role for monalysin, a P. entomophila pore-forming toxin that belongs to the aerolysin family, although further work is required to elucidate the precise mechanisms involved.

Inhibiting host translation is a strategy that is most often associated with viruses. However, over the past 12 months, the protozoan parasite Leishmania major and bacterial pathogens including Legionella pneumophila and now P. entomophila have also been shown to target translation. The authors conclude with the suggestion that “inhibition of protein synthesis could play a central role in host-pathogen interactions”.