Constraints on the assembly of spherical virus particles.

Examination of protein structure shows that it is not possible to deform protein domains to the extent required by the Caspar-Klug quasi-symmetry surface lattices for the description of viral capsids (D. L. D. Caspar and A. Klug (1962). Cold Spring Harbor Symp. Quant. Biol. 27, 1-24). However, flexibility in proteins can be achieved by a number of ligand-induced events. One type of alteration is that of quaternary structural changes in oligomers. This strategy has been used by southern bean mosaic virus and tomato bushy stunt virus where dimers attain two different states in the assembled capsid. Alterations of subunit interactions can be induced by association with RNA, cations, or other oligomeric units and, hence, successful assembly results from a stepwise aggregation. The nature of the oligomers (dimers, trimers, or pentamers) must be the underlying reason for the occurrence of the Caspar-Klug lattices and the organization into icosahedra. An analysis of the surface lattices shows which types of oligomers will be necessary for assembly.