Subunit structures have been determined by crystallography. The L subunit, B and C are covalently linked, their interface stabilized by hydrophobic interactions. The S subunit , A domain, helix interacts with a helix in the B domain of the large subunit. In the diagram the large subunit is coloured in two shades of green, the small subunit is blue. The quaternary structure is roughly spherical with icosahedral symmetry.
Many plant viruses are also filamentous.
It the discs are stacked, then a tube is created with room for the nucleic acid genome in the middle. A closer examination of these virus structures shows that the coat proteins are not arranged cylindrically but helically. This is because of the propensity for nucleic acids to adopt helical structures. By arranging the protein subunits helically then equivalent bondings between the proteins and nucleic acid can be made - except for the two end subunits. All known filamentous viruses are helical.