Structural flexibility and functional valency of CD4-IgG2 (PRO 542): potential for crosslinking HIV-1 envelope spikes

CD4-immunoglobulin G2 (CD4-IgG2) incorporates four copies of the D1D2 domains of CD4 into an antibody-like molecule that potently neutralizes primary human immunodeficiency virus type 1. Here electron microscopy was used to explore the structure and functional valence of CD4-IgG2 in complex with gp120. CD4-g2, a divalent CD4-immunoglobulin fusion protein, was evaluated in parallel. Whereas CD4-g2–gp120 complexes adopted a simple Y-shaped structure, CD4-IgG2–gp120 complexes consisted of four gp120s arrayed about a central CD4-IgG2 molecule, a structure more reminiscent of complement C1q. Molecular modeling corroborated the electron microscopy data and further indicated that CD4-IgG2 but not CD4-g2 has signifi-cant potential to cross-link gp120-gp41 trimers on the virion surface, suggesting a mechanism for the height-ened antiviral activity of CD4-IgG2. CD4-immunoglobulin G2 (CD4-IgG2; PRO 542) is a recom-binant antibody-like fusion protein wherein the heavy- and light-chain variable domains of human IgG2 have been re-placed with the D1D2 domains of human CD4 (2). CD4-g2 is a homodimer comprising D1D2 genetically fused to the hinge and Fc region of the g2 heavy chain (2). Unlike monovalen