The CP_C NTA forms an extensive network at the capsid interior that includes a β-annulus motif.

A solvent-excluded surface view of the MrNV capsid is shown, coloured by radius and clipped to reveal the capsid interior. The CPC NTAs are coloured mauve to highlight the extended network formed (a). The white frame highlights the view presented in (b), in which the cryoEM map is shown as a transparent isosurface with a single CPC molecule presented as a ribbon diagram with rainbow colouring. Black frames highlight the views presented in subsequent figures (c–f). (c) A wall-eyed stereo-pair view of a CC dimer from the capsid interior shows the NTA emerging from the S-domain β-jelly roll to cross the icosahedral 2-fold symmetry axis (pink diamond). (d) Moving from C- to N-termini, the NTA runs between the symmetry-related CPC (pink) and an adjacent CPB (yellow), donating a strand to a β-sheet within the CPB. This sheet also includes a β-strand donated by a symmetry-related CPC NTA (pink). (e) The NTA then forms a β-annulus at the icosahedral 3-fold axis (green triangle) and donates a strand to a β-sheet within a second CPB (yellow), which similarly also includes a β-strand from a symmetry related CPC (pink). Thus, CPC NTAs are interdigitated between other CPC NTAs and CPB S domains. (f) The N-terminal 29 residues of CPC are not resolved in our cryoEM map; however, the last visible CPC NTA density terminates beneath a symmetry-related icosahedral 2-fold symmetry axis (pink diamond), suggesting that the RNA binding site is proximal to this region. CP, capsid protein; cryoEM, cryogenic electron microscopy; MrNV, M. rosenbergii nodavirus; NTA, N-terminal arm; P domain, protruding domain; S domain, shell domain.