CryoEM structure and assembly mechanism of a bacterial virus genome gatekeeper

Numerous viruses use a portal system for dsDNA entry and exit from their capsid. Here the authors report the atomic structure of phage SPP1 portal DNA gatekeeper and its mechanism of assembly. They also identify evolution breakpoints between different tailed bacteriophages morphotypes and herpesviruses. Numerous viruses package their dsDNA genome into preformed capsids through a portal gatekeeper that is subsequently closed. We report the structure of the DNA gatekeeper complex of bacteriophage SPP1 (gp6(12)gp15(12)gp16(6)) in the post-DNA packaging state at 2.7 angstrom resolution obtained by single particle cryo-electron microscopy. Comparison of the native SPP1 complex with assembly-naive structures of individual components uncovered the complex program of conformational changes leading to its assembly. After DNA packaging, gp15 binds via its C-terminus to the gp6 oligomer positioning gp15 subunits for oligomerization. Gp15 refolds its inner loops creating an intersubunit beta-barrel that establishes different types of contacts with six gp16 subunits. Gp16 binding and oligomerization is accompanied by folding of helices that close the portal channel to keep the viral genome inside the capsid. This mechanism of assembly has broad functional and evolutionary implications for viruses of the prokaryotic tailed viruses-herpesviruses lineage.

Automated summary

The authors report the atomic structure and assembly mechanism of the phage SPP1 portal DNA gatekeeper, as well as identify evolutionary breakpoints between different tailed bacteriophages morphotypes and herpesviruses.