The P22 tail machine at subnanometer resolution reveals the architecture of an infection conduit.

The portal channel is a key component in the life cycle of bacteriophages and herpesviruses. The bacteriophage P22 portal is a 1 megadalton dodecameric oligomer of gp1 that plays key roles in capsid assembly, DNA packaging, assembly of the infection machinery, and DNA ejection. The portal is the nucleation site for the assembly of 39 additional subunits generated from multiple copies of four gene products (gp4, gp10, gp9, and gp26), which together form the multifunctional tail machine. These components are organized with a combination of 12-fold (gp1, gp4), 6-fold (gp10, trimers of gp9), and 3-fold (gp26, gp9) symmetry. Here we present the 3-dimensional structures of the P22 assembly-naive portal formed from expressed subunits (gp1) and the intact tail machine purified from infectious virions. The assembly-naive portal structure exhibits a striking structural similarity to the structures of the portal proteins of SPP1 and phi29 derived from X-ray crystallography.