Virus particles are all individuals
Virus particles of the same type had been thought to have identical structures, like a mass-produced toy, but a new visualization technique has revealed otherwise.
Motor driven packaging of a dsDNA genome into a pre-formed protein capsid through a unique portal vertex is essential in the life cycle of a large number of dsDNA viruses. We have used single-particle electron cryo-microscopy to study the multi-layer structure of the portal vertex of the bacteriophage T7 procapsid, the recipient of T7 DNA packaging. Focused asymmetric reconstruction (FAR) method was developed and applied to selectively resolve neighboring pairs of symmetry-mismatched layers of the portal vertex. However, structural features in all layers of the multi-layer portal vertex could not be resolved simultaneously. Our results imply that layers with mismatched symmetries can join together in several different relative orientations, and that orientations at different interfaces assort independently to produce structural isomers, a process that we call combinatorial assembly isomerism. This isomerism explains rotational smearing in previously reported asymmetric reconstructions of the portal vertex of T7 and other bacteriophages. Combinatorial assembly isomerism may represent a new regime of structural biology in which globally varying structures assemble from a common set of components. Our reconstructions collectively validate previously proposed symmetries, compositions and sequential order of T7 portal vertex layers, resolving in tandem the 5-fold gp 10 shell, 12-fold gp8 portal ring, and an internal core stack consisting of 12-fold gp14 adaptor ring, 8-fold bowl-shaped gp15, and 4-fold gp16 tip. We also found a small tilt of the core stack relative to the icosahedral 5-fold axis, and propose that this tilt assists DNA spooling without tangling during packaging.