Superimposed crystal structures of NS1A ED showing “snapshots” of the conformational heterogeneity of the helix:helix interface. All dimer structures were oriented to optimally-superimpose the indole ring atoms of Trp187 (shown in grey). (A) View showing the range of movements of Trp187′ in the second protomer as well as the long helix (residues 171 to 188) in both protomers (α1 and α1′). Crystallographic dimers are colored based on the relative angle between Trp187 and Trp187′ indole rings, from smallest (yellow) to largest (blue). (B) Close-up view of the environment that would be sensed by a fluorine atom at the C5 position of Trp187 (magenta sphere). Residues in the second protomer that are consistently within 5 Å of the fluorine position in Trp187 of the first protomer are labeled. Structure coordinates correspond to selected crystallographic dimer units from the NS1A ED literature coordinates for the following influenza A strains and crystal forms: A/Puerto Rico/8/1934 (H1N1), PDB ID, 2GX9 (Bornholdt and Prasad, 2006); A/Udorn/307/1972 (H3N2), PDB IDs, 3EE8 and 3EE9 (Xia et al., 2009); A/California/07/2009 (H1N1), PDB ID, 3M5R; A/Reassortant/IVR108 (Sydney/5/1995 × Puerto Rico/8/1934) (H3N2), PDB ID, 3O9U (Kerry et al., 2011). Structures were rendered using PyMOL.