Fig. 3.

The solved structure (12) of a 1968 strain of HA, consisting of two chains 329 and 175 residues long, shown here in its monomer form. The commonly used (15) definitions of the five antibody-combining regions are shown in red (A), orange (B), green (C), light blue (D), and dark blue (E). Residues 271-Asp, 220-Arg, 112-Val, 31-Asp, 5-Gly, 3-Leu, and 2-Asp are shown in yellow. These seven residues have not previously been characterized as positively selected but nevertheless show the same genomic pattern of codon variation as many epitopic residues (Fig. 2). The residues in yellow may represent sites that, in the current HA, are directly involved in antibody combination, comutate with epitopic residues, or determine the conformation of epitopes.