Architecture of a pentameric assembly of the tail tube protein in SPR phages

The majority of phages, viruses that infect prokaryotes, deliver their genomic material into host cells via a tail structure. Previous research has established that the central tube of these phages, composed of tail tube protein (TTP), is structurally conserved, typically forming either hexameric or trimeric rings. In this study, we revealed a novel pentameric assembly of TTP and present two cryo-EM structures at resolutions of 3.5 angstrom and 3.7 angstrom, respectively. Our detailed structural analysis demonstrates that the inner surface of the pentameric tube is highly negatively charged. Critical residues located on the loop between β3 and β4 play pivotal roles in the formation of the pentameric rings, and mismatches of the interactions between the stacked layers can induce the curvature of the tube. The cryo-EM structure of the TTP polymer at the tube's end uncovered that the β-strands that span amino acids 27-65 move towards the central tunnel, potentially blocking the tunnel through which the phage genome is released. Our study provides new structural insights into a novel assembly of TTP, which will extend the understanding of phage assembly.