Fig. 1.
Image reconstruction of MDA5-dsRNA filaments. (A and B) Negatively stained electron micrographs of MDA5 bound to viral genomic dsRNA with ATP (A) and ATP-γ-S (B). (C) ΔCARD-MDA5 with ATP-γ-S. (D) Power spectrum from 9,154 overlapping segments of the ATP-γ-S filaments in B showing a strong meridional layer line (blue arrow) and a weak near-meridional layer line (red arrow). (E) Improved power spectrum after sorting by twist (1,362 segments). (F and G) Reconstructions from this subset (F) and from ΔCARD-MDA5 filaments with ATP-γ-S (G) revealing an asymmetric unit of four domains (labeled 1–4 in F) arranged as a ring around a central RNA cavity (4-σ contour). (H) At the 1-σ contour level, two lobes of density extend from domain 1 in full-length MDA5 (gray) but not in ΔCARD-MDA5 (blue), identifying the approximate position of the CARDs or the CARD–Hel1 linker (black arrows). A difference map shows density at the same two positions (Fig. S2).