(a) Topology of the P22 portal protomer, colored by domains. Ribbon diagram of the portal protein monomers from the bacteriophages P22 (b), SPP1 (c) and phi29 (d) , colored based on similarity to the P22 portal protein in (a). The three proteins have MW of ~83 kDa, ~55 kDa, and ~33 kDa, respectively. Despite the overall similarity in fold, the sequence similarity is below 20%. (e,f) Monomer organization in the portal dodecamer. Ribbon diagram of the full length portal protein, in (e) top and (f) side view, with only one protomer colored by domain as in and , and the other eleven shown in gray. Each monomer of portal is nearly vertical in the hip domain (blue), but has an apparent tilt of ~30° relative to the 12-fold axis in both the leg domain (red) and the barrel (yellow).