- Laughlin, Thomas G;
- Deep, Amar;
- Prichard, Amy M;
- Seitz, Christian;
- Gu, Yajie;
- Enustun, Eray;
- Suslov, Sergey;
- Khanna, Kanika;
- Birkholz, Erica A;
- Armbruster, Emily;
- McCammon, J Andrew;
- Amaro, Rommie E;
- Pogliano, Joe;
- Corbett, Kevin D;
- Villa, Elizabeth
Bacteria encode myriad defences that target the genomes of infecting bacteriophage, including restriction-modification and CRISPR-Cas systems1. In response, one family of large bacteriophages uses a nucleus-like compartment to protect its replicating genomes by excluding host defence factors2-4. However, the principal composition and structure of this compartment remain unknown. Here we find that the bacteriophage nuclear shell assembles primarily from one protein, which we name chimallin (ChmA). Combining cryo-electron tomography of nuclear shells in bacteriophage-infected cells and cryo-electron microscopy of a minimal chimallin compartment in vitro, we show that chimallin self-assembles as a flexible sheet into closed micrometre-scale compartments. The architecture and assembly dynamics of the chimallin shell suggest mechanisms for its nucleation and growth, and its role as a scaffold for phage-encoded factors mediating macromolecular transport, cytoskeletal interactions, and viral maturation.