Virus evolution: how far does the double β-barrel viral lineage extend?

During the past few years one of the most astonishing findings in the field of virology has been the realization that viruses that infect hosts from all three domains of life are often structurally similar. The recent burst of structural information points to a need to create a new way to organize the virosphere that, in addition to the current classification, would reflect relationships between virus families. Using the vertical β-barrel major capsid proteins and ATPases related to known viral genome-packaging ATPases as examples, we can now re-evaluate the classification of viruses and virus-like genetic elements from a structural standpoint.