(A) A full-length molecule of IX ordered is shown in the corresponding EM density. The closeup views of different regions of the IX molecule in the EM density are shown below. (B) Ribbon diagram showing the secondary structure of the IX polypeptide with a 5-turn linker α helix in the middle and an 11-turn long α helix at the C terminus. (C) Continuous hexagonal network of IX molecules shown in the background of yellow icosahedron. A closeup view of the protein IX network is shown on the right. The four structurally distinct IX polypeptides (P, Q, R, and S) are shown in different colors (dark blue, light blue, cyan, and purple, respectively), and the same-colored copies are related by the icosahedral symmetry. Numbers 1 to 4 identify the locations of the unique hexons. The full-length molecule (R) ordered is shown in cyan, and the IX molecule (P) that contributes an antiparallel helix is shown in blue. Missing connections between the triskelions and coiled coils in P, Q, and S copies are identified by gray sticks. The triskelions at the icosahedral threefold axes are formed by purple IX molecules (S), whereas those at the local threefold axes are formed by three differently colored IX molecules (P, Q, and R). The four-helical bundle (4-HLXB) structures are formed by the CT helices of four structurally distinct IX-molecules that come from four different triskelions. The CT helices of the three IX molecules (Q, R, and S) are arranged in parallel and come from the same facet, whereas the fourth one (P), which joins in an antiparallel orientation, comes from the neighboring facet; this arrangement is similar to that of HAdV-C5. Asterisks (*) identify some of the 4-HLXBs. (D) Schematic diagram showing the hexagonal network of IX molecules shown as lines/sticks that interlace the hexons, which are represented as hexagons, whereas the PBs are shown as pentagons. The subunits (for example, A, B, and C) that constitute the individual hexons are labeled along with the locations of the V1 and V2 barrels of the individual hexon subunits. The centers of IX triskelions interact with the V2 domains of hexon subunits, whereas the 4-HLXB is located at the interface formed by V1 and V2 domains from two different subunits (K and L) of the hexon-4 capsomer on one side and their local twofold-related counterparts from D and E subunits of hexon-2 that belongs to the adjacent facet. The black triangle represents the boundary of the icosahedral facet. (E) Simplified schematic showing the path of the hexagonal network formed by protein IX molecules within a facet. The labels T and 4-HLXB identify the locations of the triskelions and coiled coils, respectively. The 4-HLXBs are encircled by red ovals. (F) Schematic identifying the locations of the triskelions and 4-HLXB with respect to the GON substructure, whose boundary is identified by the black lines overlaid on top of the protein IX network shown in (E). The 4-HLXBs are identified by red ovals.