Pentameric Assembly Architecture of the Tail Tube Protein in SPR Phages

Most phages—viruses infecting prokaryotes—inject their genomes via a tail structure. The central tail tube, composed of tail tube protein (TTP), typically forms conserved hexameric or trimeric rings. In this paper, we report a novel pentameric TTP assembly, solved by cryo-electron microscopy (cryo-EM) at 3.5 Å and 3.7 Å resolution. Structural analysis reveals a highly negatively charged inner surface of this pentameric tube. Key residues in the loop connecting β3 and β4 strands are crucial for pentameric ring formation. Mismatches in interactions between stacked layers can induce curvature in the tube. The cryo-EM structure of the TTP polymer at the tube’s end shows that β-strands spanning amino acids 27-65 shift toward the central tunnel, potentially obstructing the passage of the phage genome. This study provides new structural insights into a unique TTP assembly, enhancing our understanding of phage assembly processes.