The Membrane Proximal External Region of the HIV-1 Envelope Glycoprotein gp41 Contributes to the Stabilization of the Six-Helix Bundle Formed with a Matching N′ Peptide

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Abstract
The HIV-1 envelope glycoprotein gp41 undergoes a sequence of extensive conformational changes while participating in the fusion of the virus with the host cell. Since the discovery of its postfusion conformation, the structure and function of the protease-resistant six-helix bundle (6-HB) have been the subject of extensive investigation. In this work, we describe additional determinants (S528−Q540 and W666−N677) in the fusion peptide proximal region (FP-PR) and the membrane proximal external region (MPER) that stabilize the six-helix bundle and are involved in the interaction of T-20 (FUZEON, an anti-HIV-1 fusion inhibitor drug) with the gp41 FP-PR. Circular dichroism and sedimentation equilibrium measurements indicate that the 1:1 mixture of N′ and C′ peptides comprising residues A541−T569 and I635−K665 from the gp41 first and second helical repeats, HR1 and HR2, respectively, fail to form a stable six-helix bundle. Triglutamic acid and triarginine tags were added to these N′ and C′ peptides, respectively, at the termini distant from the FP-PR and the MPER to alter their pI and increase their solubility at pH 3.5. The tagged HR1 and HR2 peptides were elongated by addition of residues S528−Q540 from the FP-PR and residues W666−N677 from the MPER, respectively. A 1:1 complex of the elongated peptides formed a stable six-helix bundle which melted at 60 °C. These results underscore the importance of a detailed high-resolution characterization of MPER interactions, the results of which may improve our understanding of the structure−function relationship of gp41 and its role in HIV-1 fusion.