Structural Basis for Bacteriophage P22 Assembly and Infection Initiation

Salmonella phage P22 deploys a highly coordinated tail machine to recognize hosts and initiate infection. Here, we present a cryo-EM structure of wild-type P22 that defines how the tail apparatus assembles onto the capsid and how they interface. Flexible loop residues on both the portal protein gp1 and the capsid protein gp5 undergo pronounced positional shifts and engage multiple partners to accommodate the C12-C5 symmetry mismatch at the portal-capsid interface. The portal protein gp1 forms a distinctive ∼15-nm barrel that projects deep into the capsid interior. Comparison with a mutant lacking the three internal E (ejection) proteins indicates that these proteins reside within the portal-tail lumen in a poorly ordered state, yet are essential for stabilizing the extended portal barrel. We further show how the hub protein gp10 orchestrates the assembly of four distinct particle isomers through its coordinated interactions with portal gp1, adaptor gp4, tailspike gp9, and needle gp26. Finally, cryo-electron tomography reveals that the gp10 hub acts as a structural foundation for the assembly of one E protein into an extracellular channel that breaches the cell surface, with other E proteins forming a genome-translocating trans-envelope conduit.