A growing number of jumbo bacteriophages, with genomes exceeding 200 kb, have been found to establish a Phage Nucleus—a micron-scale, proteinaceous structure encompassing the replicating phage DNA. Bacteriophage and host proteins associated with replication and transcription are concentrated inside the Phage Nucleus while nucleotide synthesis, translation, and numerous other host and exogenous proteins are effectively excluded, including CRISPR-Cas and restriction endonuclease host defense systems. Here, we show that fragments of the Phage Nucleus isolated from ΦPA3 infected
Pseudomonas aeruginosa cells form a square lattice and demonstrate that the recombinantly purified primary Phage Nuclear Enclosure (PhuN) protein spontaneously assembles into sheets also constructed from a square lattice which we resolve to 3.8 Å by cryo-EM. Our structure reveals that the flexible termini and large loops mediate adaptable inter-tetramer contacts that drive shell assembly into a C2-symmetric lattice. While the interfaces between subunits are mostly well packed, two of the interfaces are open, forming clear channels that likely have important functional implications for the transport of proteins, mRNA and small molecules.