More barrels from the viral tree of life

Abstract

There is now unmistakable evidence for a revolution in our comprehension of the viral world. Despite their importance for disease and the insights obtained by studying their fascinating intricacy, viral relationships have long been unclear. The article by Khayat et al. (1) in a recent issue of PNAS marked a milestone in progress. Viral origins have been masked by their diversity and complexity, the way they were studied, and their small size. Viruses evolved cunning solutions to the problems of packing information into genomes of limited size and to entering and exploiting their host. Until fairly recently, each family was studied separately by its own group of scientific admirers. Structurally, viruses could only be imaged by low-resolution electron microscopy (EM), with most too large to tackle with crystallography. Advances in imaging techniques, changes in the way that viruses are studied, and increasing structural evidence all are contributing to major change. There is now excellent evidence to support the idea that viruses fall into lineages identifiable by features in common with ancient precursors (2). The best-supported lineage has “double-barrel trimer” coat proteins and icosahedral virions. Its members infect Gram-positive and Gram-negative bacteria, as well as animals, and include very large viruses infecting insects, algae, and amoebae. Structures are at hand for coat proteins of viruses infecting animals, bacteria, and algae, but that for an archaeal virus was lacking. This crucial missing link is found in the thermophilic archaeal Sulfolobus turreted icosahedral virus (STIV) (1). We now have solid structural evidence that at least one lineage originated billions of years ago, before life separated into its three domains: Bacteria, Archaea, and Eukarya. To appreciate this achievement, one must recognize the dramatic progress in describing viral architecture since the first crystal structure of a complete viral particle, or virion, appeared (3). This plant …