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 factors^2,3,4. 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.

细菌编码无数针对感染噬菌体基因组的防御，包括限制性修饰和 CRISPR-Cas 系统¹。作为回应，大型噬菌体家族使用类核区室通过排除宿主防御因子来保护其复制基因组^2,3,4。然而，这个隔室的主要组成和结构仍然未知。在这里，我们发现噬菌体核壳主要由一种蛋白质组装而成，我们将其命名为 chimallin (ChmA)。结合噬菌体感染细胞核壳的冷冻电子断层扫描和体外最小 chimallin 隔室的冷冻电子显微镜，我们发现 chimallin 作为柔性片材自组装成封闭的微米级隔室。chimallin 壳的结构和组装动力学表明其成核和生长的机制，及其作为噬菌体编码因子介导大分子运输、细胞骨架相互作用和病毒成熟的支架的作用。