Fig. 3.

Interactions between the A₃ and P₁ helices. (A) The helical wheel diagram (Top) shows the interaction between the A₃’s heptad motif “AxYxAx[LV]” and P₁’s “PxxP” motif. The helical net of the interactions between the α- and PPII helices is also shown, where the A₃-repeat heptad motifs are highlighted in blue and the P₁ repeat PxxP motifs are highlighted in orange. Asparagine residues within the A₃ region, intervening the heptads, that are involved in hydrogen bonding with the P₁ PPII helix are highlighted in red. The conserved tyrosines and leucines (highlighted in yellow) of the heptad sequences are nestled between prolines of the P repeat. Additionally, the phenolic oxygen atom of the tyrosine residues participate in water-mediated hydrogen bonding (red-dashed lines). (B) The stereo diagram of the heptad interactions shows that tyrosine side chains nestle between the prolines in a knobs-in-holes interaction, which is highlighted by surface plots for the A and P repeats. (C) The stereo diagram of the region intervening the heptads shows a dominant direct hydrogen bonding between the asparagine side chains of A₃ and the main chain oxygen and nitrogen of the PPII helix. Two prolines (Pro855 and Pro858) break this pattern and face outward.