Peering Down the Barrel of a Bacteriophage Portal: The Genome Packaging and Release Valve in P22

SummaryThe encapsidated genome in all double-strand DNA bacteriophages is packaged to liquid crystalline density through a unique vertex in the procapsid assembly intermediate, which has a portal protein dodecamer in place of five coat protein subunits. The portal orchestrates DNA packaging and exit, through a series of varying interactions with the scaffolding, terminase, and closure proteins. Here, we report an asymmetric cryoEM reconstruction of the entire P22 virion at 7.8 Å resolution. X-ray crystal structure models of the full-length portal and of the portal lacking 123 residues at the C terminus in complex with gene product 4 (Δ123portal-gp4) obtained by Olia et al. (2011) were fitted into this reconstruction. The interpreted density map revealed that the 150 Å, coiled-coil, barrel portion of the portal entraps the last DNA to be packaged and suggests a mechanism for head-full DNA signaling and transient stabilization of the genome during addition of closure proteins.

► Bacteriophage P22 packages its dsDNA genome in ordered layers at liquid-crystalline density ► DNA remains within the capsid during the addition of tail machine closure proteins ► The 12, C-terminal, 130 residue polypeptides of the portal forms a 200 Å long “barrel” ► The barrel functions as a “Chinese finger trap” to transiently retain the DNA