Bacteriophage T5 Structure Reveals Similarities with HK97 and T4 Suggesting Evolutionary Relationships

Evolutionary relationships between viruses may be obscure by protein sequence but unmasked by structure. Analysis of bacteriophage T5 by cryo-electron microscopy and protein sequence analysis reveals analogies with HK97 and T4 that suggest a mosaic of such connections. The T5 capsid is consistent with the HK97 capsid protein fold but has a different geometry, incorporating three additional hexamers on each icosahedral facet. Similarly to HK97, the T5 major capsid protein has an N-terminal extension, or Δ-domain that is missing in the mature capsid, and by analogy with HK97, may function as an assembly or scaffold domain. This Δ-domain is predicted to be largely coiled-coil, as for that of HK97, but is ∼70% longer correlating with the larger capsid. Thus, capsid architecture appears likely to be specified by the Δ-domain. Unlike HK97, the T5 capsid binds a decoration protein in the center of each hexamer similarly to the “hoc” protein of phage T4, suggesting a common role for these molecules. The tail-tube has unusual trimeric symmetry that may aid in the unique two-stage DNA-ejection process, and joins the tail-tip at a disk where tail fibers attach. This intriguing mix of characteristics embodied by phage T5 offers insights into virus assembly, subunit function, and the evolutionary connections between related viruses.