Figure 2.

Model of gp120 engagement of CD4 and CXCR4. Recent structural studies have enhanced our understanding of the molecular interactions between gp120 (cyan) and its receptors. Here, CD4 (green) and CXCR4 (purple), shown as monomers for clarity, are shown simultaneously binding to gp120. (A) Lateral view. (B) Top view. However, the number of CD4 and coreceptor molecules required to interact with Env to mediate productive fusion remains unknown. (C) Gp120 has two key interactions with coreceptor. (1) The base of the V3 loop binds to the amino-terminal domain of the coreceptor, whereas the tip of the V3 loop binds to the second extracellular loop (ECL2). Although both interactions are important, viral strains differ on their dependency of each interaction. (Structural model generated by Wu et al. 2010.)