Fig. 1.

Unliganded structures of HIV-1 gp120 core. Crystal structures are displayed as Cα-ribbon diagrams with outer domains in gray and inner domains in magenta, blue or light blue, cyan, and green for HIV-1 clades B, C, and E, and SIV, respectively, and the region that in the CD4-bound conformation makes up the bridging sheet in red. The evolutionary relationships of these Env glycoproteins are represented by a dendrogram where the length of connections is proportional to evolutionary distance. [The four HIV-1 structures were determined here; the SIV structure was determined previously by Chen et al. (17).]