Structural characterization of the T7 bacteriophage portal protein

Abstract

The Escherichia coli infecting T7 bacteriophage shares a common dsDNA packaging mechanism with other bacteriophages of the Caudovirales order, Herpesviruses and Adenoviruses. The packaging machinery comprises the portal protein and the terminase complex. The portal protein is a channel located at a unique portal vertex that provides a conduit for DNA translocation, while the terminase complex recognizes a long concatemer of DNA, performs the nuclease catalytic activity and hydrolyses ATP. Available structural information about portal proteins describes them as oligomeric rings with an axial channel. High quality samples suitable for structural characterization of the portal protein of T7 bacteriophage were obtained and characterized. Both X-ray crystallography and cryo-electron microscopy (cryo-EM) data were collected, and an initial model built on the 5.8Å cryo-EM map was used to phase the crystallographic data, which allowed the building of a model of a tridecameric particle at 2.8Å resolution. The T7 portal particle is 170Å tall and 110Å wide toroidal protein with a central channel that ranges from 23Å to 95Å in diameter. Four domains have been identified in the structure: the wing, the stem, the clip and the crown. The a10-tunnel loop valve is proposed to play an important functional role. During packaging, it may adapt while DNA is translocated and rotated, and once the genome has been packed the side chain of tunnel loop residue Arg368 may be able to seal the channel and stabilize the DNA inside the capsid before tail assembly. Interestingly, these mechanisms would not only imply the flexibility of a loop region, but also the kink of the longer helix of the portal structure, a10.