The Tryptophan-Rich Motif of HIV-1 gp41 Can Interact with the N-Terminal Deep Pocket Site: New Insights into the Structure and Function of gp41 and Its Inhibitors.

Refolding of the HIV-1 gp41 N- and C-terminal heptad repeats (NHR and CHR, respectively) into a six-helix bundle (6-HB) juxtaposes viral and cellular membranes for fusion. The CHR-derived peptide T20 is the only clinically approved viral fusion inhibitor and has potent anti-HIV activity; however, its mechanism of action is not fully understood. In this study, we surprisingly found that T20 disrupted the α-helical conformation of the NHR-derived peptide N54 through its C-terminal tryptophan-rich motif (TRM) and that synthetic short peptides containing the TRM sequence, TRM8 and TRM12, disrupted the N54 helix in a dose-dependent manner. Interestingly, TRM8 efficiently interfered with the secondary structures of three overlapping NHR peptides (N44, N38, and N28) and interacted with N28, which contains mainly the deep NHR pocket-forming sequence, with high affinity, suggesting that TRM targeted the NHR pocket site to mediate the disruption. Unlike TRM8, the short peptide corresponding to the pocket-binding domain (PBD) of the CHR helix had no such disruptive effect, and the CHR peptide C34 could form a stable 6-HB with the NHR helix; however, addition of the TRM to the C terminus of C34 resulted in a peptide (C46) that destroyed the NHR helix. Although the TRM peptides alone had no anti-HIV activity and could not block the formation of 6-HB conformation, substitution of the TRM for the PBD in C34 resulted in a mutant inhibitor (C34TRM) with high binding and inhibitory capacities. Combined, the present data inform a new mode of action of T20 and the structure-function relationship of gp41.IMPORTANCE The HIV-1 Env glycoprotein mediates membrane fusion and is conformationally labile. Despite extensive efforts, the structural property of the native fusion protein gp41 is largely unknown, and the mechanism of action of the gp41-derived fusion inhibitor T20 remains elusive. Here, we report that T20 and its C-terminal tryptophan-rich motif (TRM) can efficiently impair the conformation of the gp41 N-terminal heptad repeat (NHR) coiled coil by interacting with the deep NHR pocket site. The TRM sequence has been verified to possess the ability to replace the pocket-binding domain of C34, a fusion inhibitor peptide with high anti-HIV potency. Therefore, our studies have not only facilitated understanding of the mechanism of action of T20 and developed novel HIV-1 fusion inhibitors but also provided new insights into the structural property of the prefusion state of gp41.