A Legionella toxin mimics tRNA and glycosylates the translation machinery to trigger a ribotoxic stress response.

Pathogens often secrete proteins or nucleic acids that mimic the structure and/or function of molecules expressed within their host counterparts. Such molecular mimicry empowers pathogens to subvert critical host processes and helps establish infection. We report here that the intracellular bacterium Legionella pneumophila (L.p.) secretes a toxin named SidI (substrate of icm/dot transporter I) which possesses a transfer RNA (tRNA) like fold and functions as a mannosyl transferase . The 3.1 Å cryo-EM structure of SidI reveals an N-terminal domain that exhibits a characteristic ‘inverted L-shape’ and charge distribution that is present in two known protein mimics of tRNAs, the bacterial elongation factor EF-G and the mammalian release factor eRF1. In addition, SidI’s C-terminal domain adopts a glycosyl transferase B fold and shares considerable homology to a known mannosyl transferase. This molecular coupling of fold and enzymatic function allows SidI to bind and glycosylate components of the host translational apparatus, including the ribosome, resulting in a robust block of protein synthesis that is comparable to the potency exhibited by the toxin ricin. Additionally, we uncovered that the translational pausing activated by SidI elicits a stress response signature reminiscent to the ribotoxic stress response activated by elongation inhibitors that induce ribosome collisions. SidI mediated perturbations to the ribosome activates the stress kinases ZAKα and p38, that in turn drive the accumulation of the protein activating transcription factor 3 (ATF3). Intriguingly, ATF3 escapes the translation block imposed by SidI, translocates to the nucleus, and subsequently orchestrates the transcription of stress-inducible genes that culminates in the activation of a cell death program. Thus, using Legionella and its effectors as tools, we have unravelled the role of a ribosome to nuclear signaling pathway that regulates cell fate. Expression profiling by RNA-seq of HEK293-FcgR cells infected with either WT or ΔdotA Legionella (as a time course) and HEK293T cells treated with ansiomycin or didemnin-B (for 4 hours) with two bioreps for each condition.