Figure 1.

Top, a rhombic triacontahedron depicting the T = 3 quasi-symmetric quaternary structure of Tomato Bushy Stunt Virus and Southern Bean Mosaic Virus, the first two icosahedral virus structures deposited in the PDB. The triangle enclosing the “commas” is the icosahedral asymmetric unit and a “comma” represents a single subunit with the folds shown in Figure 2. They are related by an approximate q3 fold axis at the center of the triangle. Bottom, a side view of the icosahedral asymmetric unit with the particle center at the bottom of the figure where the boundary icosahedral symmetry elements (5-3-3-5) intersect. Quasi-symmetry elements are defined by vectors representing q2 and q3. Note, unlike the icosahedral symmetry elements, they do not intersect at the center.