FIG. 2.

Models of monomeric (A to C) and trimeric (D) gp120 cores in complex with CD4-γ2 and CD4-IgG2. (A) CD4-γ2 with monomeric gp120; (B and C) top and side views, respectively, of CD4-IgG2 with monomeric gp120; (D) CD4-IgG2 with trimeric gp120. For clarity, only two of the four D1D2 domains of CD4-IgG2 are shown in panel D. Color codes: blue, Ig heavy chain; red, IgG2 hinge region disulfide bonds; green, light-chain Cκ domain; yellow, D1D2 domains of CD4-IgG2; orange, gp120 core. The pink structure in panel D represents the D1D2 domains docked in the nearest CD4 binding site not occupied by CD4-IgG2 and is included for orientation purposes. Note that the left-hand CD4 arm (yellow) of CD4-IgG2 is unable to extend and maneuver into the requisite docking position (pink CD4) of the left-hand gp120 subunit.