Three-Dimensional Structure of the Enveloped Bacteriophage Φ12: An Incomplete T=13 Lattice Is Superposed on an Enclosed T=1 Shell

Background: Bacteriophage phi 12 is a member of the Cystoviridae, a unique group of lipid containing membrane enveloped bacteriophages that infect the bacterial plant pathogen Pseudomonas syringae pv. phaseolicola. The genomes of the virus species contain three double-stranded (dsRNA) segments, and the virus capsid itself is organized in multiple protein shells. The segmented dsRNA genome, the multi-layered arrangement of the capsid and the overall viral replication scheme make the Cystoviridae similar to the Reoviridae. Methodology/Principal Findings: We present structural studies of cystovirus phi 12 obtained using cryo-electron microscopy and image processing techniques. We have collected images of isolated phi 12 virions and generated reconstructions of both the entire particles and the polymerase complex (PC). We find that in the nucleocapsid (NC), the phi 12 P8 protein is organized on an incomplete T = 13 icosahedral lattice where the symmetry axes of the T = 13 layer and the enclosed T = 1 layer of the PC superpose. This is the same general protein-component organization found in phi 6 NC's but the detailed structure of the entire phi 12 P8 layer is distinct from that found in the best classified cystovirus species phi 6. In the reconstruction of the NC, the P8 layer includes protein density surrounding the hexamers of P4 that sit at the 5-fold vertices of the icosahedral lattice. We believe these novel features correspond to dimers of protein P7. Conclusions/Significance: In conclusion, we have determined that the phi 12 NC surface is composed of an incomplete T = 13 P8 layer forming a net-like configuration. The significance of this finding in regard to cystovirus assembly is that vacancies in the lattice could have the potential to accommodate additional viral proteins that are required for RNA packaging and synthesis.