Characterization of gp41-derived proteins. Panel A. Different gp41-derived proteins used in this study are depicted. The different regions of gp41 are depicted in blue (fusion peptide), red (helicoidal region 1, HR1), brown (disulfide loop), green (HR2), yellow (membrane proximal external region, MPER) and purple (Transmembrane region, TM). The GFP fused to the C-terminal sequence is also depicted in light green. Panel B. Flow cytometry analysis of MPER exposure on the surface of transfected cells. 293T cells transiently transfected with the constructions shown in panel A were analyzed for cell surface MPER exposure. Plots of GFP expression and binding of control, 4E10 and 2F5 antibodies are shown. Panel C. 293T cells stably expressing the MIN (left panels) or STAPLE (middle panels) constructions were selected and the binding profile of different antibodies was compared with a 293T cell line stably expressing a full-length HIV-1 envelope construct (right panels). Antibodies tested were the anti-MPER mAb 2F5, the anti-gp120 glycan shield mAb 2G12 and plasma samples from HIV-1 infected or uninfected individuals.