Fig. 1.

Diagrams of pentamer packing and C-terminal arm interchange in polyoma and papillomaviruses. In the full-size capsids of viruses from both groups, 72 pentamers of the major capsid protein (called VP1 in the polyomaviruses and L1 in the papillomaviruses) are arrayed at the vertices of a T = 7 d icosahedral lattice. Twelve pentamers (white) are centered on 5-fold symmetry axes, whereas the 60 others (6 of which are shown, colored) are at 6-coordinated positions. In a 12-pentamer (T = 1) icosahedral particle, obtained with N-terminally truncated L1 from HPV16, all 12 pentamers are on icosahedral 5-fold symmetry axes. Left: In the polyomaviruses, the VP1 C-terminal arm “invades” a neighboring pentamer, in the pattern shown, and “inserts” into its structure. Right: In the papillomaviruses, the L1 C-terminal arm invades a neighboring pentamer but turns around and inserts back into its pentamer of origin. The T = 1 shell has arm–arm interactions distinct from those in the virion.