Structural analysis and assembly of the HIV-1 Gp41 amino-terminal fusion peptide and the pretransmembrane amphipathic-at-interface sequence.

The amino-terminal region within the HIV-1 gp41 aromatic-rich pretransmembrane domain is an amphipathic-at-interface sequence (AIS). AIS is highly conserved between different viral strains and isolates and recognized by the broadly neutralizing 2F5 antibody. The atomic structure of the native Fab2F5-bound AIS appears to involve a nonhelical extended region and a beta-turn structure. We previously described how an immunogenic complex forms, based on the stereospecific interactions between AIS and the gp41 amino-terminal fusion peptide (FP). Here, we have analyzed the structure generated by these interactions using synthetic hybrids containing AIS and FP sequences connected through flexible tethers. The monoclonal 2F5 antibody recognized FP-AIS hybrid sequences with an apparently higher affinity than the linear AIS. Indeed, these hybrids exhibited a weaker capacity to destabilize membranes than FP alone. A combined structural analysis, including circular dichroism, infrared spectroscopy, and two-dimensional infrared correlation spectroscopy, revealed the existence of specific conformations in FP-AIS hybrids, predominantly involving beta-turns. Thermal denaturation studies indicated that FP stabilizes the nonhelical folded AIS structure. We propose that the assembly of the FP-AIS complex may act as a kinetic trap in halting the capacity of FP to promote fusion.