Fig 9.

The symmetrically mismatched motor-portal interface contains one unique gp17-gp20 interaction. (A) Fitting of the gp17 pentamer (3EZK) into the cryo-EM density of the packaging machine (32 Å; EMDB ID no. 1572) shows that the HLH motif (cyan surface) interacts with the stalk region of dodecameric portal. The gp17-interacting portal subunits are shown in blue, and the noninteracting subunits are shown in magenta (only the HLH and stalk regions are shown). The HLH interactions are of two types: one unique interaction with 2 portal subunits (star) and four nearly equivalent interactions with 1 portal subunit. (B) Schematic illustrating the 2-1-1-1-1 interaction pattern. The HLH motif (illustrated in red and blue helices) interacts with portal loops (shown as red and blue circles). The active subunit (star) also interacts with the same loops, but those of two adjacent portal subunits rather than those of a single subunit (note the positions of the red and blue helices with respect to the red and blue portal loops).