Pentameric Assembly Architecture of the Tail Tube Protein in SPR Phages

Abstract 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 β 3 and β 4 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.