Structural Basis for the Activation of Innate Immune Pattern-Recognition Receptor RIG-I by Viral RNA

Abstract

RIG-I is a key innate immune pattern-recognition receptor that triggers interferon expression upon detection of intracellular 5'triphosphate double-stranded RNA (5'ppp-dsRNA) of viral origin. RIG-I comprises N-terminal caspase activation and recruitment domains (CARDs), a DECH helicase, and a C-terminal domain (CTD). We present crystal structures of the ligand-free, autorepressed, and RNA-bound, activated states of RIG-I. Inactive RIG-I has an open conformation with the CARDs sequestered by a helical domain inserted between the two helicase moieties. ATP and dsRNA binding induce a major rearrangement to a closed conformation in which the helicase and CTD bind the blunt end 5'ppp-dsRNA with perfect complementarity but incompatibly with continued CARD binding. We propose that after initial binding of 5'ppp-dsRNA to the flexibly linked CTD, co-operative tight binding of ATP and RNA to the helicase domain liberates the CARDs for downstream signaling. These findings significantly advance our molecular understanding of the activation of innate immune signaling helicases.

Higher antiviral response of RIG-I through enhancing RIG-I/MAVS-mediated signaling by its long insertion variant in zebrafish

“…The activity of RIG-I is auto-repressed through intramolecular interaction between the CARD and the RD, and upon viral dsRNA binding, conformational change of RIG-I results in the release of CARDs from RD repression, leading to the association with the adaptor protein MAVS for the initiation of antiviral signaling cascade [11,33,35,36]. It has been shown that the intact tandem CARDs are essential and sufficient for signaling [9,36,37], and mammalian RIG-I mutants with a CARD deletion or CARD point mutations (T55I or S183I) all function as a dominant inhibitor in its signaling [9,11,38].…”

“…It has been shown that the intact tandem CARDs are essential and sufficient for signaling [9,36,37], and mammalian RIG-I mutants with a CARD deletion or CARD point mutations (T55I or S183I) all function as a dominant inhibitor in its signaling [9,11,38]. It has also been revealed in mammals that the RIG-I downstream signaling via MAVS requires the recruitment of TRIM25 and other ubiquitination enzymes to synthesize unanchored K63-linked polyubiquitin chains binding to the CARD domains [36,37,39,40]. In the present study, RIG-Ia, the insertion variant of RIG-I with additional 38-aa residues in the CARD2 domain had no direct role in the activation of type I IFN promoter or antiviral defense, although the expression level of RIG-Ia could be induced under E. tarda or SVCV infections.…”

Rig-I

“…Les structures tridimensionnelles de la protéine RIG-I entière de canard [4] et de formes tronquées comportant les domaines CARD-hélicase, hélicase, et hélicase-CTD, complexées ou non avec de l'ARNdb, viennent d'être simultanément résolues à l'automne 2011 par quatre équipes, dont la nôtre, travaillant sur le RIG-I de canard, humain ou de la souris [4][5][6][7]. Les informations obtenues sont complémentaires.…”

“…Les informations obtenues sont complémentaires. Elles convergent vers un modèle cohérent des changements de conformation responsables de la commutation d'un état autoréprimé inactif (ou dormant) vers un état activé [4][5][6][7]. La première hélice  remonte vers Hel1 qui est prise en sandwich par la seconde hélice .…”

“…Au sein de la protéine RIG-I en mode autoréprimé (sans ARNdb), le lien Hel2-CTD est flexible et le CTD est « libre », car la structure de CTD n'est pas « vue » pour cause de mobilité dans le cristal de RIG-I examiné par diffraction aux rayons X [4]. La comparaison de la structure de RIG-I avec des domaines CARD mais sans ligand et de la structure de RIG-I complexée à une molécule d'ARNdb et à un analogue non hydrolysable de l'ATP révèle un changement de conformation majeur (Figure 1 Compétition des domaines CARD pour la liaison du domaine hélicase à l'ARNdb La résolution de la structure tridimensionnelle de RIG-I entière et d'une forme tronquée du CTD complexé à l'ARNdb [4] nous a permis pour la première fois de voir une structure de deux domaines CARD contigus et leur orientation dans la molécule. Les domaines CARD1 et CARD2 comprennent respectivement 7 et 6 hélices .…”