Flavivirus structure. (a) Diagram of the packing of 180 E subunits in the surface of a virion. The proteins are clustered as dimers. Each is represented by a symbol, colored to correspond to the domain representation in b. (b) The ectodomain of the E dimer, viewed as if looking toward the surface of the virion. Domains I, II, and III are labeled and colored in red, yellow, and blue, respectively. An arrow points to the fusion loop on one subunit. The locations of two histidines at the domain I–domain III interface are shown by orange triangles. His 146 is on domain I; His 323, close to the fusion peptide of the partner subunit, is on domain III. Black triangles mark a potential receptor-binding loop.