Crystal structure of HIV-1 gp41 including both fusion peptide and membrane proximal external regions.

Winfried Weissenhorn,Michael M Kozlov,David Schibli,Ganesh Natrajan,Michael Farzan,Felix Campelo,Victor Buzon

doi:10.1371/journal.ppat.1000880

Abstract

The HIV-1 envelope glycoprotein (Env) composed of the receptor binding domain gp120 and the fusion protein subunit gp41 catalyzes virus entry and is a major target for therapeutic intervention and for neutralizing antibodies. Env interactions with cellular receptors trigger refolding of gp41, which induces close apposition of viral and cellular membranes leading to membrane fusion. The energy released during refolding is used to overcome the kinetic barrier and drives the fusion reaction. Here, we report the crystal structure at 2 Å resolution of the complete extracellular domain of gp41 lacking the fusion peptide and the cystein-linked loop. Both the fusion peptide proximal region (FPPR) and the membrane proximal external region (MPER) form helical extensions from the gp41 six-helical bundle core structure. The lack of regular coiled-coil interactions within FPPR and MPER splay this end of the structure apart while positioning the fusion peptide towards the outside of the six-helical bundle and exposing conserved hydrophobic MPER residues. Unexpectedly, the section of the MPER, which is juxtaposed to the transmembrane region (TMR), bends in a 90°-angle sideward positioning three aromatic side chains per monomer for membrane insertion. We calculate that this structural motif might facilitate the generation of membrane curvature on the viral membrane. The presence of FPPR and MPER increases the melting temperature of gp41 significantly in comparison to the core structure of gp41. Thus, our data indicate that the ordered assembly of FPPR and MPER beyond the core contributes energy to the membrane fusion reaction. Furthermore, we provide the first structural evidence that part of MPER will be membrane inserted within trimeric gp41. We propose that this framework has important implications for membrane bending on the viral membrane, which is required for fusion and could provide a platform for epitope and lipid bilayer recognition for broadly neutralizing gp41 antibodies.

Highlights

HIV-1 employs its trimeric env glycoprotein, composed of the receptor binding domain gp120 and the membrane anchored fusion protein subunit gp41 to enter host cells
Mutagenesis analyses indicate that both linkers to the membrane anchors, fusion peptide proximal region (FPPR) and membrane proximal external region (MPER), are implicated in fusion [23,24] and the transmembrane region (TMR) play an important role in fusion pore enlargement [22,25,26]
HIV-1 employs its envelope glycoprotein complex (Env) composed of gp120 and gp41 to catalyze cell entry. Both Env subunits undergo conformational changes triggered by the gp120-mediated interactions with cellular receptors

Summary

Introduction

HIV-1 employs its trimeric env glycoprotein, composed of the receptor binding domain gp120 and the membrane anchored fusion protein subunit gp to enter host cells. Gp41 catalyses membrane fusion and current models suggest that receptor binding leads to the exposure of the gp fusion peptide (FP), which interacts with the target cell membrane producing an intermediate, pre-hairpin state bridging two membranes. This pre-hairpin has a relatively long half-life [6] and constitutes the target for inhibitory peptides [7,8,9] and neutralizing antibodies directed against HR1 [10][11] and MPER [12,13]. Mutagenesis analyses indicate that both linkers to the membrane anchors, FPPR and MPER, are implicated in fusion [23,24] and the TMRs play an important role in fusion pore enlargement [22,25,26]

Methods

Results

Discussion

Conclusion