Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
2189151 | Journal of Molecular Biology | 2006 | 10 Pages |
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.