CELEBRATING OUR TENTH YEAR HISTORICAL NEWS AND VIEWS gp41: HIV’s shy protein Michael B Zwick, Erica O Saphire & Dennis R Burton The first X-ray crystal structures of gp41, the protein that mediates fusion of HIV-1 to target cells, were solved in the mid-1990s. The structures provide a foundation for understanding viral entry and the mechanism of action of compounds that block fusion. The first fusion inhibitor has recently entered the clinic, and the hope is that more potent and broadly active compounds, based on molecular design, will follow. The structure of gp41 was a long-sought and a bc Merged viral and elusive goal for HIV biologists. The difficulties Viral membrane Viral membrane host cell membrane of working with a membrane protein, the strong tendency of gp41 to aggregate, and other http://www.nature.com/naturemedicine hurdles stood in the way.When Chan et al.1 and Weissenhorn et al.2 finally produced the first structures in 1997, followed shortly by Tan et gp41 N al.3, they began to give molecular form to the apparatus of viral fusion, whose outline had CHR emerged earlier in the decade. The structures are the focus of much work to design new CHR inhibitors of the fusion process, and serve as an Deborah Maizels C outstanding example of how findings in basic N N N science can stimulate exploration of new thera- Figure 1 The conformations of HIV-1 gp41 before, NHR NHR during and after the fusion of the viral and host-cell C peutics. T20 C This is what fusion looks like: envelope spikes membranes. The HIV-1 envelope spike is a putative composed of gp41 and gp120 decorate the sur- trimer of gp120-gp41 heterodimers. (a) Before N fusion. In the native, untriggered conformation, the © 2004 Nature Publishing Group face of HIV-1. The surface glycoprotein of the ectodomain (extraviral portion) of gp41 is largely cloaked by gp120 (transparent here). The structural spike, gp120, engages CD4 and CCR5 or details of this native configuration are not well elucidated. (b) Fusion intermediate(s). Upon binding CD4 CXCR4 coreceptors, primarily on macrophages and coreceptor, gp120 undergoes conformational changes that expose gp41 and activate the fusion and CD4+ T cells. gp120 undergoes conforma- machinery, notably the N-HR regions and the fusion peptide (FP). gp41 is probably in a somewhat extended tional changes to activate gp41, a transmem- arrangement in which the N-HR region of three gp41 molecules form an α-helical bundle, positioning the brane glycoprotein that uses conserved fusion peptides for insertion into the host-cell membrane. The C-HR region of gp41 is drawn here without defined secondary structure, but some models show it in an α-helical conformation. gp41-targeting fusion structural elements to mediate the fusion of the inhibitors such as T20 act on the fusion-intermediate conformation(s) of gp41 to inhibit six-helix bundle host and viral membranes. Genetic information formation. (c) After fusion. Fusion seems to coincide with chain reversal, or ‘jackknifing’, such that the can then flow from the virus to the target cell. antiparallel N-HR and C-HR regions form a six-helix bundle, represented here by the structure of The structure of the spikes is dictated by the Weissenhorn et al.2. virus’ need to bind and enter target cells while avoiding a host neutralizing antibody response. much as possible. This serves two purposes: it closely opposing membranes. The fusion In such a response, host antibodies bind to the restricts the ability of conserved epitopes to process itself takes only minutes, which signifi- virus and prevent it from entering the target cell elicit antibodies (low immunogenicity), and it cantly limits access to the transiently exposed To manage this problem, HIV-1 has evolved to helps prevent antibodies that are elicited from epitopes on gp41. In contrast, the postfusion keep the vulnerable conserved regions of the binding to conserved epitopes (low antigenic- conformation of gp41 is a stable repeating unit envelope spike—such as those on gp41 that ity). Neutralizing antibodies against conserved that is highly immunogenic. This postfusion comprise the fusion machinery—hidden as epitopes are a particular threat to HIV-1, as conformation appears to be so radically differ- escape via mutation is more difficult than for ent from the fusion-intermediate conforma- variable regions of the envelope spikes. tions of gp41 that elicited antibodies are unable Michael B. Zwick, Erica O. Saphire and Dennis R. In the mature, ‘untriggered’ spike, much of to bind to the intermediate conformations and Burton are in the Departments of Immunology and the surface of gp41 is masked by gp120. After therefore are unable to inhibit fusion. Molecular Biology, The Scripps Research Institute, receptor engagement, during which gp41 is This description was given a molecular La Jolla, California 92037, USA. perhaps most vulnerable, access to the fusion- underpinning in 1997, with the X-ray crystal e-mail: [email protected] intermediate state of gp41 is restricted by the structures of the putative postfusion form of NATURE MEDICINE VOLUME 10 | NUMBER 2 | FEBRUARY 2004 133 HISTORICAL NEWS AND VIEWS HIV-1 gp41 (refs. 1–3). Before then, it was prone to aggregation, making them much less In another approach, a small molecule was known that the N- and C-terminal portions of potent than T20; potency is increased substan- designed to mimic a hydrophobic surface along the external domain of gp41 contain heptad tially with soluble, trimeric N-HR con- one face of a heptad repeat α-helix; the pro- repeat sequences. Peptides corresponding to the structs12,13. teomimetic was able to partially disrupt the six- N-terminal (N-HR) and C-terminal (C-HR) In principle, antibodies that could bind to helix bundle and inhibit HIV-1 mediated heptad repeat regions of gp41, notably T21 the N-HR inner helical core after receptor acti- fusion20. (DP107) and T20 (DP178), respectively, could vation should also block entry of HIV-1, similar As the hidden secrets of gp41 continue to be inhibit HIV-1 entry into cells4–6.T20, rebranded to the fusion inhibitors. In terms of vaccine revealed, new leads for HIV-1 therapies and as enfuvirtide, was approved by the US Food design, however, eliciting neutralizing antibod- vaccines will emerge. Interesting new targets for and Drug Administration for patients last fall. ies to this target may be difficult because of spa- intervention may be uncovered by obtaining To overcome the technical hurdles of crystal- tial and temporal limitations during the fusion detailed structural information on the native lizing gp41, it was necessary to truncate the process. The broadly neutralizing antibodies and fusion-intermediate conformations of protein to make it stable and soluble. Each of defined to date seem to recognize gp41 epitopes gp41. It will also be useful to elucidate the influ- the three laboratories crystallized a proteolyti- that are close to the viral membrane but acces- ence on gp41 structure and function of the seg- cally stable core of the extraviral domain of sible during fusion14,15.Vaccinologists await ments that were omitted in the gp41 crystal gp41; (devoid of the N-terminal hydrophobic structures for the fusion intermediates. structures. fusion peptide, a disulfide-loop region linking The gp41 structures have stimulated Small-molecule drug design has been the N-HR and C-HR regions, and the mem- progress in the design and selection of fusion extremely effective in inhibiting the HIV-1 pro- brane-proximal region). inhibitors. In one study, an engineered trimer tease and reverse transcriptase, but it remains to Although each of the groups crystallized frag- of the inner N-HR helical peptides was used to be seen how well it will work against the enve- ments of different sizes, the resulting structures select peptides from a random phage display lope complex. Since drug resistance is so preva- were nearly identical. gp41 was found to have a library. These peptides, selected as proteolyti- lent for HIV-1 and has already been trimeric core consisting of parallel α-helices cally stable mirror-image or D-peptides, documented for T20, drug cocktails may be the http://www.nature.com/naturemedicine corresponding to the N-HR region of gp41. potently inhibit HIV-1 entry into cells13.In best solution. Indeed, the range of potential tar- Packed around this trimeric core were three, another study, investigators constructed a five- gets on gp41 and a growing arsenal of other slightly oblique C-HR α-helices, arranged helix bundle that lacks one of the outer C-HR entry inhibitors21 make synergy with existing antiparallel to the inner core to create a trimer of peptides. The five-helix bundle potently inhib- and future drug combinations an exciting pos- heterodimers, or a six-helix bundle (Fig. 1). ited HIV-1 entry, probably by binding to a sin- sibility. Notably, this arrangement had also been gle C-HR region on HIV-1 gp41 and 16 This material is part of Nature Medicine's 10 year observed in the pH-induced conformation of preventing six-helix bundle formation .In a anniversary series. For more content related to these the influenza hemagglutinin molecule7 and the particularly inspired demonstration of struc- special focus issues, please see http://www.nature.com/ transmembrane subunit of Moloney murine ture-based design, the solvent-exposed residues nm/special_focus/anniversary/index.html 8 α leukemia virus .The structural motif has since on one face of the -helix of a gp41 C-HR pep- 1. Chan, D.C., Fass, D., Berger, J.M. & Kim, P.S. Cell 89, been found in Ebola virus, Newcastle disease tide were substituted onto the solvent-exposed 263–273 (1997).