A structural dendrogram of the actinobacteriophage major capsid proteins provides important structural insights into the evolution of capsid stability

Many double-stranded DNA viruses, including tailed bacteriophages (phages) and herpesviruses, use the HK97-fold in their major capsid protein to make the capsomers of the icosahedral viral capsid. Following the genome packaging at near-crystalline densities, the capsid is subjected to a major expansion and stabilization step that allows it to withstand environmental stresses and internal high pressure. Several different mechanisms for stabilizing the capsid have been structurally characterized, but how these mechanisms have evolved is still not understood. Using cryo-EM structure determination, structural comparisons, phylogenetic analyses, and Alphafold predictions, we have constructed a detailed structural dendrogram describing the evolution of capsid structural stability within the actinobacteriophages. The cryo-EM reconstructions of ten capsids solved to resolutions between 2.2 and 4 Ångstroms revealed that eight of them exhibit major capsid proteins that are linked by a covalent cross-linking (isopeptide bond) between subunits that was first described in the HK97 phage. Those covalent interactions ultimately lead to the formation of mutually interlinked capsomers that has been compared to the structure of chain mail. However, three of the closely related phages do not exhibit such an isopeptide bond as demonstrated by both our cryo-EM maps and the lack of the required residue. This work raises questions about the importance of previously described capsid stabilization mechanisms.