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. 2020 Aug;10(8):a038778. doi: 10.1101/cshperspect.a038778

Figure 1.

Figure 1.

Hemagglutinin (HA) structures and receptor-binding site (RBS). (A) Structure of trimeric HA. The location of the RBS is shown in lime on the HA structure. HA1 is shown in gray and HA2 in salmon. The globular head domain sits on top of the stem domain. (B) The major structural elements, namely 130-loop, 150-loop, 190-helix, and 220-loop, of the HA RBS are shown. Highly conserved residues W153, H183, L194, and Y195 are shown in stick representation. The alpha carbons (Cαs) of HA1 residues 190, 225, 226, and 228, which are involved in the H1, H2, and H3 receptor-specificity switch, are shown in sphere representations. (C) Avian-type receptor (α2,3-linked sialic acid) binds to HA with an extended configuration (Xu et al. 2012). (D) Human-type receptor (α2,6-linked sialic acid) binds to HA with a folded-back configuration (Xu et al. 2012).