Molecular Imprinting as a Signal-Activation Mechanism of the Viral RNA Sensor RIG-I

Abstract

SUMMARY RIG-I activates interferon signaling pathways by promoting filament formation of the adaptor molecule, MAVS. Assembly of the MAVS filament is mediated by its CARD domain (CARDMAVS) and requires its interaction with the tandem CARDs of RIG-I (2CARDRIG-I). However, the precise nature of the interaction between 2CARDRIG-I and CARDMAVS, and how this interaction leads to CARDMAVS filament assembly has been unclear. Here we report a 3.6 Å electron microscopy structure of the CARDMAVS filament and a 3.4 Å crystal structure of the 2CARDRIG-I:CARDMAVS complex, representing 2CARDRIG-I “caught in the act” of nucleating the CARDMAVS filament. These structures, together with functional analyses, show that 2CARDRIG-I acts as a template for the CARDMAVS filament assembly, by forming a helical tetrameric structure and recruiting CARDMAVS along its helical trajectory. Our work thus reveals that signal activation by RIG-I occurs by imprinting its helical assembly architecture on MAVS, a previously uncharacterized mechanism of signal transmission.

Structure determination of helical filaments by solid-state NMR spectroscopy

“…S3B. The ensemble of MAVS CARD structures is very well converged (backbone RMSD of 0.5 ± 0.1 Å over 21 protomers) and is very similar to the model of Wu et al (6) when superimposing eight consecutive protomers (Fig. S3E).…”

“…maltose binding protein is available (22), and we have previously presented the sequence-specific secondary structure of this domain in its filamentous form (23). Two competing structural models of MAVS CARD filaments derived from cryo-EM reconstructions were published last year (5,6).…”

“…We further tested the grid-search methodology on a different helical assembly solved by ssNMR: the type III secretion system needle of Salmonella typhimurium (15). The initial atomic coordinates of the PrgL protomer structure and 162 interprotomer distance restraints were taken from PDB ID code 2LPZ (6). Helical symmetry was modeled using 30 copies of the central protomer (15 in both directions of the helical axis), and both left-and righthanded helices were tested.…”

“…However, thus far, only few cryo-EM structures could be obtained with sufficient resolution to decipher fine details of the interprotomer contacts (2,3). In addition, ambiguities in the determination of the helical symmetries have occasionally given rise to contradictory structural models (4)(5)(6)(7)(8). In the last decade, magic angle spinning solid-state NMR (ssNMR) has become a powerful technique to determine atomic resolution structures of protein filaments, in particular of amyloid fibrils (9)(10)(11)(12)(13)(14).…”

The struggle with image glut

“…Edward Egelman, a structural biologist at the University of Virginia in Charlottesville, co-authored a study 2 of the structure of an aggregated, filament-like protein called MAVS -which was at odds with another, earlier model of the protein 3 . Egelman proved the earlier structure was incorrect by downloading and reprocessing the raw data set 4 .…”