Unified mechanisms for self-RNA recognition by RIG-I Singleton-Merten syndrome variants.

Charlotte Lässig,Sebastian Michalski,Olga Fettscher,Karl-Peter Hopfner,Jacob Lucián Gorenflos López,Katja Lammens

doi:10.7554/elife.38958

Charlotte Lässig, Sebastian Michalski + Show 4 more

Open Access

https://doi.org/10.7554/elife.38958

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Abstract

The innate immune sensor retinoic acid-inducible gene I (RIG-I) detects cytosolic viral RNA and requires a conformational change caused by both ATP and RNA binding to induce an active signaling state and to trigger an immune response. Previously, we showed that ATP hydrolysis removes RIG-I from lower-affinity self-RNAs (<xref ref-type="bibr" rid="bib19">Lässig et al., 2015</xref>), revealing how ATP turnover helps RIG-I distinguish viral from self-RNA and explaining why a mutation in a motif that slows down ATP hydrolysis causes the autoimmune disease Singleton-Merten syndrome (SMS). Here we show that a different, mechanistically unexplained SMS variant, C268F, which is localized in the ATP-binding P-loop, can signal independently of ATP but is still dependent on RNA. The structure of RIG-I C268F in complex with double-stranded RNA reveals that C268F helps induce a structural conformation in RIG-I that is similar to that induced by ATP. Our results uncover an unexpected mechanism to explain how a mutation in a P-loop ATPase can induce a gain-of-function ATP state in the absence of ATP.

Highlights

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are cytosolic innate immune sensors that recognize viral double-strandedRNAs
The RIG-I Singleton-Merten syndrome (SMS) variants C268F and E373A are located within SF2 ATP binding and hydrolysis motifs I and II, respectively, (Fairman-Williams et al, 2010) (Figure 1—figure supplement 1) and were previously shown to be constitutively active in reporter cells without any external dsRNA trigger (Jang et al, 2015; Lassig et al, 2015) (Figure 1—figure supplement 2A)
RIG-I E373A’s enhanced immune signaling ability can be explained by proficient ATP binding but reduced ATP hydrolysis, since the motif II glutamate is implicated in positioning and polarizing the attacking water molecule in the hydrolysis reaction

Summary

Introduction

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are cytosolic innate immune sensors that recognize viral double-stranded (ds)RNAs. RLRs (RIG-I, MDA5 and LGP2) are members of the socalled Superfamily II (SF2) helicases/translocases. Simultaneous binding of an RNA ligand and ATP to RIG-I switches the protein into an active state in which the otherwise shielded 2CARD is released (Zheng et al, 2015). The similarity of epitopes of viral RNA recognized by RLRs, in particular dsRNA stems, to some endogenous ribonucleic acids has required the immune system to develop mechanisms besides merely recognizing 5’-di/triphosphate-containing RNA ends to discriminate self from non-self. We show that active site rearrangements of several amino acid side chains in RIG-I C268F mimic an ATP-bound state and activate the protein for signaling upon recognition of RNA ligands in the absence of ATP

Results and discussion

B Singleton-Merten syndrome signal-off state signal-off state

Materials and methods