Deletion of the V1/V2 Region Does Not Increase the Accessibility of the V3 Region of Recombinant gp125

Previous analyses of HIV-1 surface glycoprotein indicate that both the V1/V2 region and the interaction of gp120 with CD4 influence the accessibility of the V3 region on gp120. In this study we investigated the accessibility of the V3 region of HIV-2 recombinant gp125 proteins using V3-specific mAbs (7C8 and 3C4) and analyzed the binding kinetics of soluble CD4 (sCD4) to recombinant HIV-1 gp120 and HIV-2 gp125 proteins by surface plasmon resonance (SPR) analysis. Our results indicated that 7C8 recognized monomers of gp125 and gp125 Δv1v2, (lacking the V1/V2 region) while 3C4 was sensitive to the conformation of gp125, recognizing only oligomers of gp125 Δv1v2. Furthermore, SPR analysis of 7C8 binding to gp125 demonstrated that the deletion of the V1/V2 region did not increase the accessibility of the V3 region in gp125 Δv1v2. Comparative SPR analyses of sCD4 binding HIV recombinant surface glycoproteins revealed a lower affinity of sCD4 to gp125 as compared to gp120. Moreover, the analyses suggest that conformational changes only occur in HIV-1 gp120 upon interaction with CD4. We hypothesize that the V3 region is accessible in HIV-2 gp125 and thus may not require interaction with CD4 to induce conformational reorientation of the V1/V2 region.