Figure 3

Model of canonical helical faces. (A) C_α atoms that form a helical face are shown in space filling. Residues 3 and 4, 10 and 11 and 17 and 18 complement the anchoring residues 0, 7, 14 and 21, respectively. (B) Helical wheels illustrating the seven canonical helical faces centered at each of the seven positions of the heptad repeat. The central helical wheel contains numbers that label the helical faces 0 through 6. These numbers correspond to heptad positions a through f around which the helical surfaces are centered. The corresponding helical face is depicted as an outer wheel located next to that label. The outer helical wheels depict residues forming each of the seven helical faces, e.g., face 0 is formed by residues at heptad positions a-d-e, face 1: b-e-f, face 2: c-f-g, face 3: d-g-a, face 4: e-a-b, face 5: f-b-c, and face 6: g-c-d.