Extended Data Fig. 5: Superposition of the nsP1 complex in the FHV tomographic reconstruction of spherule necks.

a, Cytoplasmic view of the nsP1 complex (coloured by protomer) superposed into the tomography map reconstructed from FHV growing spherules (light grey surface)⁴. The volume is shown at two contour levels, 1.2 (left) and 0.9 (right), using ChimeraX. The superposition shows the 12-fold symmetry of both structures and that they are of a similar shape and size. The FHV central crown appears to be slightly larger. The panel on the left at lower contour level allows for visualization of the membrane around the crown. b, Lateral view of the superposition. The nsP1 fits into the lower part of the FHV crown in contact with the membrane. In the right panel, it is possible to appreciate the membrane bending carried out by the complex together with 12 external detached volumes of unidentified origin. From this view, an extension of the crown is visible at the top of the ring. This region probably corresponds to the polymerase domain of FHV protein A, which is in the same polypeptide chain as the capping enzyme that is homologous to alphavirus nsP1. c, Bottom view of the superposition. In this orientation, the inner shape of the viral spherule can be seen. The FHV volume was provided by P. Ahlquist, who has since published the same structure at higher resolution³⁹.