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A Legionella toxin exhibits tRNA mimicry and glycosyl transferase activity to target the translation machinery and trigger a ribotoxic stress response.

ABSTRACT: A widespread strategy employed by pathogens to establish infection is to inhibit host cell protein synthesis. Legionella pneumophila, an intracellular bacterial pathogen and the causative organism of Legionnaires disease, secretes a subset of protein effectors into host cells that inhibit translation elongation. Mechanistic insights into how the bacterium targets translation elongation remain poorly defined. We report here that the Legionella effector SidI functions in an unprecedented way as a transfer RNA (tRNA) mimic that directly binds to and glycosylates the ribosome. The 3.1 A cryo-EM structure of SidI reveals an N-terminal domain with an inverted-L shape and surface charge distribution characteristic of tRNA mimicry and a C-terminal domain that adopts a glycosyl transferase fold that licenses SidI to utilize GDP-mannose as a sugar precursor. This coupling of tRNA mimicry and enzymatic action endows SidI with the ability to block protein synthesis with a potency comparable to ricin, one of the most powerful toxins known. In Legionella infected cells, the translational pausing activated by SidI elicits a stress response signature mimicking the ribotoxic stress response, which is activated by elongation inhibitors that induce ribosome collisions. SidI-mediated effects on the ribosome activate the stress kinases ZAKalpha and p38, which in turn drive an accumulation of the protein activating transcription factor 3 (ATF3). Intriguingly, ATF3 escapes the translation block imposed by SidI, translocates to the nucleus, and orchestrates the transcription of stress-inducible genes that promote cell death, revealing a major role for ATF3 in the response to collided ribosome stress. Taken together, our findings elucidate a novel mechanism by which a pathogenic bacterium employs tRNA mimicry to hijack a ribosome-to-nuclear signaling pathway that regulates cell fate.

INSTRUMENT(S): Orbitrap Fusion Lumos

ORGANISM(S): Canavalia Brasiliensis (ncbitaxon:61861) Bovidae (ncbitaxon:9895) Homo Sapiens (ncbitaxon:9606)

SUBMITTER: Peter Walter Shaeri Mukherjee

PROVIDER: MSV000091992 | MassIVE | Fri May 19 10:23:00 BST 2023

REPOSITORIES: MassIVE

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Project description: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.

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A Legionella toxin exhibits tRNA mimicry and glycosyl transferase activity to target the translation machinery and trigger a ribotoxic stress response.

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