TrV proteins VP1, VP2 and VP3 and relation to the symmetry axes of the capsid. (a) Ribbon diagrams colour-ramped from the N-terminus to the C-terminus: blue to red through yellow. All three proteins adopt the standard jelly-roll topology and have their N-termini lying in the particle interior. The symmetry axes of the particle are represented as thin black lines, with a pentagon (fivefold), ellipse (twofold) and triangle (threefold) denoting the respective icoshahedral axes of the particle. The proposed proteolytic residues (1223–1225 and 3254–3256; see text) are also depicted. (b, c) Virion assembly. The TrV capsid proteins and their interactions about the fivefold axes. (b) Simplified lateral view. VP1 (blue) and only part of VP3 (red) is shown for clarity. The N-tails of VP3 intertwine around the fivefold axis to form a β-cylinder, while two loops and a β-sheet join VP1 in the formation of the projections. (c) Top view of a penton (a pentamer of protomers), showing the presence of a channel with a minimum aperture of 6 Å and the extended conformation of the VP3 N-termini surrounding the fivefold axes (VP1, blue; VP2, red, VP3, green).