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118 Structural Flexibility and Functional Valency of CD4-IgG2 (PRO 542) and CD4-gamma2: Potential for Crosslinking HIV-1 Envelope Spikes
K. H. Roux*1, Z. Ping1, and W. C. Olson2
1Florida State Univ., Tallahassee, USA and 2Progenics Pharma., Inc., Tarrytown, NY, USA
Background: Homodimeric CD4-gamma2 and hetero-tetrameric CD4-IgG2 (PRO 542) are fusion proteins in which human IgG2 VH domains in the former and both VH and VL domains in the latter have been substituted with the D1D2 domains of human CD4. Compared with monovalent CD4-based proteins, CD4-gamma2 and CD4-IgG2 possess improved antiviral and pharmacokinetic properties, with the CD4-IgG2 construct being particularly promising.
Methods: We have analyzed both constructs in complex with rsgp120 by negative stain electron microscopy and molecular modeling to document their structures and functional valencies.
Results: Individual CD4-gamma2 molecules were shown to adopt a simple Y-shape and to bind to 2 soluble gp120 monomers as expected. For CD4-IgG2, each of the 4 CD4 arms radiated from a central Fc fragment and could bind soluble gp120. This configuration suggests minimal steric interaction between the adjacent H and L chain-associated CD4 arms and a functional valency of 4. Modeling of the constructs in complex with soluble gp120 was consistent with the electron micrographs. Modeling of the CD4-based constructs in complex with gp120 trimers as arrayed on the viral surface revealed that neither construct appears capable of binding more than 1 gp120 subunit of a single gp120/gp41 trimer spike. However, CD4-IgG2, but not CD4-gamma2, has significant potential to crosslink 2 to 3 adjacent trimer spikes.
Conclusions: The unexpected structural flexibility and dimensions of CD4-IgG2 likely contribute to its markedly enhanced antiviral activity as compared with CD4-gamma2.
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