Fig. 4: C2 inner plug of the apical cap.

A Ac54 monomers in an ‘echelon’ arrangement on the dsDNA. Two strands of dsDNA are delineated very well in the structure. Inset (G) is marked here. B N- and C-terminal domain organization of Ac54 in the sequence (top) and monomeric and dimeric structural organization (bottom). C Left: Four protomers of two Ac54 dimers interact with dsDNA, each with unique interactions. Only one asymmetric unit containing four protomers is shown for clarity. The C2 counterpart is not shown here. Insets (D–F, H and I) are marked here. They are arranged in the order of seeing the assembly from top to bottom Right: Electrostatic surface of the four Ac54 protomers exhibiting positive charged surface poised to bind to dsDNA. D–I Ac54-dsDNA and Ac54-Ac54 interactions. Residues involved in interactions are shown in ball-and-stick representation. As the dsDNA is a random sequence, potential interacting positions on the dsDNA are highlighted in red. The dsDNA and Ac54 residues of the symmetrical counterpart are marked with a prime (‘). D N-terminal domain of protomer-1 (Ac54-1) interactions with dsDNA. E C-terminal domain of protomer-1 interactions with dsDNA. F Protomer-2 interactions with dsDNA. G Di-sulfide bridge involving Cys13 of protomer-3 of the two symmetry-mates. H Protomer-3 interactions with dsDNA. I Protomer-4 interactions with dsDNA. J Ac54 interactions with pseudo-C8 Ac101/Ac144 subunits. The asymmetric unit has 4 Ac101/Ac144, marked as C2-1 to C2-4. K Same as (J) but with space-filled model for the pseudo-C8 ring to clarify the intricate arrangement allowing only the protomer-2 and 3 to engage in interactions with the pseudo-C8 Ac101/Ac144 subunits. Insets (L) and (M) are marked here. L Protomer-2 interactions with C2-2 and C2-3 units. M Protomer-3 interactions with C2-1, C2-2 and C2-3 units. C2-4 unit of the pseudo-C8 ring is not involved in any interactions with Ac54 protomers. In (L and M), potential interacting residues on Ac54 are highlighted in navy blue and those on Ac101/Ac144 subunits are highlighted in cornflower blue.