Figure 7. Three-dimensional comparison of the F10 epitope in group 1 and group 2 hemagglutinins.

Stereo overlay of crystal structures of the five known hemagglutinin subtypes in the region of the F10 epitope, showing conservation and differences between the two phylogenetic groups. H1, H5 and H9 (group 1) are in shades of red and yellow (PDB 1RU7, 2IBX and 1JSD); H3 and H7 (group 2) are in shades of blue (PDB 1MQL and 1TI8). R.m.s. differences for pairwise overlays are 0.56 ± 0.11 Å (observed range, group 1), 0.75 Å (group 2) and 1.21 ± 0.12 Å between groups. Consistent differences between phylogenetic groups include the orientation of Trp21₂ and alternative side chain directions at 18₁ and 38₁, which are linked to the packing of buried His111₂ (the putative pH trigger in group 1; absent in group 2), and the burial of the larger tyrosine (group 1) versus histidine (the putative pH trigger in group 2) at 17₁. Of particular note, Asn38₁ is glycosylated in four members of the group 2 clusters. Other epitope residues are indicated by numbered light blue circles.