Figure 3: Structure of the 3E1 Fab–CA09 HA complex.

(a) The overall structure of the 3E1 Fab–CA09 HA complex is shown in cartoon representation from the side view and top view. HA1 and HA2 are coloured in pale green and pale cyan, respectively. The heavy chain and light chain of 3E1 Fab are coloured in magenta and yellow, respectively. The glycans are shown with spheres. 3E1 Fab binds to the stem region of HA. (b) The interaction interface between 3E1 Fab and HA looking down at the Fab (paratope). The Fab is shown in surface representation, with heavy chain in violet, and the HA in cartoon. The residues of the heavy chain and light chain at the antigen-binding site coloured in light pink and orange, respectively. The key residues of the epitope are shown with side chains: these of HA1 are denoted with a subscript 1 and these of HA2 with a subscript 2, and these of the fusion peptide are coloured in red, these of the F subdomain in cyan, and these of the β-strand in green. (c) The interaction interface between 3E1 Fab and HA looking down at the HA (epitope). The HA is shown in surface representation and the Fab in cartoon. The key residues of the antigen-binding site are shown with side chains. (d–f) Specific interactions between the fusion peptide (red) and β-strand (green) and the HCDR loops (magenta) (d), between the F subdomain (cyan) and the HCDR3 loop (magenta) (e) and between the F subdomain (cyan) and the LCDR loops (yellow) (f). The key residues are shown with side chains.