Cryo-EM analysis of a viral portal protein in situ reveals a switch in the DNA tunnel

The portal protein is a key component of many double-stranded DNA viruses, governing capsidassembly and genome packaging. Twelve subunits of the portal protein form a ring with a central tunnel, through which DNA is translocated into the capsid. It is unknown how the portal protein functions as a gatekeeper, preventing DNA slippage, whilst allowing its passage into the capsidthrough its central tunnel, and how these processes can be controlled by capsidand motor proteins. A cryo-EM structure of a portal protein, determined in situ for immature capsidsof thermostable bacteriophage P23-45, suggests how domain adjustments can be coupled with a switching of properties of the DNA tunnel. Of particular note is an inversion of the conformation of portal loops which define the tunnel9s constriction, accompanied by a switching of surface properties from hydrophobic to hydrophilic. These observations indicate how translocation of DNA into the viral capsidcan be modulated by changes in the properties and size of the central tunnel and how the changing pattern of protein- capsidinteractions across a symmetry-mismatched interface can facilitate these dynamic processes.