The stoichiometry of trimeric SIV glycoprotein interaction with CD4 differs from that of anti-envelope antibody Fab fragments

Human and simian immunodeficiency viruses infect host lymphoid cells by bin ding CD4 molecules via their gp160 envelope glycoproteins. Biochemical stud ies on recombinant SIVmac32H (pJ5) envelope ectodomain gp140 precursor prot ein show that the envelope is a trimer. Using size exclusion chromatography , quantitative amino acid analysis, analytical ultracentrifugation, and CD4 -based competition assay, we demonstrate that the stoichiometry of CD4 rece ptor-oligomeric envelope interaction is 1:1. By contrast, Fab fragments of both neutralizing and non-neutralizing monoclonal antibodies bind at a 3:1 ratio. Thus, despite displaying equivalent CD4 binding sites on each of the three gp140 protomers within an uncleaved trimer, only one site binds the soluble 4-domain human CD4 extracellular segment. The anti-cooperativity an d the faster k(off) of gp140 trimer:CD4 versus gp120 monomer:CD4 interactio n suggest that CD4-induced conformational change is impeded in the intact e nvelope. The implications of these findings for immunity against human immu nodeficiency virus and simian immunodeficiency virus are discussed.