High-resolution Cryo-EM Analysis of the Therapeutic Pseudomonas Phage Pa223.

Cryogenic electron microscopy (cryo-EM) analysis of bacteriophages is a valuable method for deciphering virus composition and conformational plasticity. In this study, we present a high-resolution structural atlas of the Pseudomonas virus Pa223, a phage from the Bruynoghevirus genus that has recently been used in clinical cocktails for treating cystic fibrosis and non-cystic fibrosis bronchiectasis, as well as for compassionate care. By combining bioinformatics, proteomics, cryo-EM single particle analysis, and localized reconstruction, we annotated and built atomic models for eight structural polypeptide chains that form the icosahedral capsid and noncontractile tail. We discovered that the Pa223 capsid is decorated by a spike protein with a unique triple-β helix fold that has no structural homologs in the database. The Pa223 tail features six trimeric tail fibers extending upward, similar to but shorter than those found in phage T7. Unlike T7, the Pa223 tail is extended by two head-to-tail adaptors and sealed by a trimeric tail needle, similar to P22-like phages. We identified a protein bound around the outer perimeter of the portal protein, positioned similarly to the ejection protein gp72, which was identified in the Pseudomonas phage DEV, a Litunavirus phage, and a member of the reclassified Schitoviridae family. This structural clue led us to identify the Pa223 ejection proteins gp53, gp54, and gp56, which bioinformatically resemble those of phage T7 more closely than Schitoviridae. Thus, Pa223 contains various structural elements similar to those in P22-like, T7-like, and Litunavirus phages, providing a foundation for understanding the evolution of ejection proteins in Bruynogheviruses.