Mutations in IgY-selected A/California/7/2004 HA. (A) Amino acid residues found to be mutated in HA of IgY-selected viruses are indicated on the structure of the HA monomer viewed from the above (top), from the interior of the HA trimer (back), or from the front. The receptor binding site (RBS, magenta) and previously described human epitopes A (navy), B (yellow), C (pink), D (orange) and E (red) are shown . Mutated residues observed in this study (Table 2) are highlighted (olive) and indicated by olive labels, except for E62K (dark red), falling in the previously described human epitope E. Note that we have not to date detected mutations in the A or B epitopes, most frequently associated with changes in HA in humans. (B) Observed mutations are shown in the context of the HA trimer. Individual monomers are colored salmon (HA1) and grey (HA2), green (HA1) and cyan (HA2), and purple (HA1; HA2 not visible). Mutations are shown in red, with red labels. Note that two mutations (Y94H and E62K) occur on the solvent-exposed surface near the trimer interface, although most (N31S, P214Q, P221S, Y308H, and C1372Y) are deeply buried within the interface and have the potential to alter intersubunit interactions. The figure was prepared using MacPyMol (http://www.pymol.org, DeLano Scientific LLC), using the crystal structure of A/X-31 (H3N2) HA (, PDB Accession ID: 2VIU) as a model.