A Legionella toxin mimics tRNA and glycosylates the translation machinery to trigger a ribosome-associated stress response

ABSTRACT: Pathogenic microorganisms often secrete proteins or nucleic acids that mimic the structure and/or function of molecules expressed within their host counterparts. This molecular mimicry empowers pathogens to target and subvert critical host processes and helps establish their infectious paradigm. 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 enzyme. The 3.1 Å cryo-EM structure of SidI reveals an N-terminal domain that exhibits a characteristic ‘inverted L-shape’ and charge conservation 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 to and enzymatically modify 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 that is highly similar to one activated by elongation inhibitors that target the ribosome and induce ribosome collisions. SidI-mediated perturbations to the ribosome activate 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.

INSTRUMENT(S): Orbitrap Exploris 480, Q Exactive

ORGANISM(S): Homo Sapiens (human)

SUBMITTER: Danielle Swaney  

LAB HEAD: Danielle Swaney

PROVIDER: PXD034240 | Pride | 2023-07-12

REPOSITORIES: Pride