Capsid structure and its stability at the late stages of bacteriophage SPP1 assembly

The structure of bacteriophage SPP1 capsid was determined at sub-nanometer resolution by cryo-electron microscopy and single-particle analysis. The icosahedral capsid is composed of the major capsid protein gp13 and the auxiliary protein gp12 that are organized in a T=7 lattice. DNA is arranged in layers with a distance of ∼24.5 Å. Gp12 forms spikes that are anchored at the centre of gp13 hexamers. In a gp12-deficient mutant the centres of hexamers are closed by loops of gp13 coming together to protect the SPP1 genome from the outside environment. The HK97-like fold was used to build a pseudo-atomic model of gp13. Its structural organization remains unchanged upon tail binding and following DNA release. Gp13 exhibits enhanced thermostability in the DNA-filled capsid. A remarkable convergence between thermostability of the capsid and of the other virion components was found, revealing that the overall architecture of the SPP1 infectious particle co-evolved towards high robustness