Two interaction modes of the gp41-derived peptides with gp41 and their correlation with antimembrane fusion activity

Ability of antibodies specific to the HIV-1 envelope glycoprotein to block the fusion inhibitor T20 in a cell-cell fusion assay

The anti-HIV peptide T20 is able to inhibit the syncytia formation between CHO-WT and HeLa CD4(+)cells. We found that several sera of HIV-infected patients have the capacity to block the inhibition of fusion by T20. Suggesting that these sera may contain antibody which can block T20 access and prevent membrane fusion, we studied the ability of a panel of antibodies directed to different regions of HIV-1 envelope glycoprotein to block the inhibition of fusion by T20. We found that the C1 and V3 loop regions of gp120 and the heptad repeat 1, the immunodominant C-C region and the Kennedy epitope of gp41 located in the intracytoplasmic tail were the target for antibodies capable to block the inhibition of syncytia formation by T20. We suggest that these antibodies have the capacity to counteract the anti-fusion effect of T20 by preventing its binding to the interaction sites. Further studies are needed to determine if some of them recognize new T20 interaction sites.

Antibodies purified from sera of HIV-1-infected patients by affinity on the heptad repeat region 1/heptad repeat region 2 complex of gp41 neutralize HIV-1 primary isolates

OBJECTIVE

The objective of this paper was to evaluate the presence and the neutralizing activity of antibodies directed against the complex formed between the two heptad repeat regions (HR1 and HR2) of HIV-1 gp41 in sera of HIV-1-infected patients.

RESEARCH DESIGNS AND METHODS

The HR1 region was represented by the peptide N36 and the maltose-binding protein (MBP)-HR1, the HR2 region by the peptide C34 and MBP44. Antibodies directed to the HR1/HR2 complex were purified from sera by affinity chromatography using MBP-HR1/C34 adsorbed onto a resin.

RESULTS

First, we demonstrated that human monoclonal antibodies, which are directed specifically to the HR1/HR2 complex recognized in enzyme-linked immunosorbent assay the MBP-HR1/C34 and MBP44/N36 mixtures but not the proteins or the peptides individually. We investigated the ability of 50 sera of HIV-1-infected patients to react with the MBP-HR1/C34 and MBP44/N36 complexes. We found that the majority of sera of HIV-1-infected patients recognized the HR1/HR2 complexes but not or to a lower extent the proteins or the peptides individually. Antibodies purified from sera by affinity chromatography using MBP-HR1/C34 adsorbed to a resin neutralized different primary HIV-1 isolates.

CONCLUSION

The presence of antibodies directed to the HR1/HR2 complex in sera of HIV-infected patients highlights the immunogenic character of the complex, whereas the neutralizing activity of these antibodies suggests that immunogens representing HIV-1 HR1/HR2 complexes might be used in anti-HIV vaccine.

Demonstrating the C-terminal boundary of the HIV 1 fusion conformation in a dynamic ongoing fusion process and implication for fusion inhibition

The core complex is a structure involved in the fusion mechanism of many viruses, as well as in intracellular vesicle fusion. A powerful approach for studying the dynamic stages of HIV-1-cell fusion utilizes DP178, a core complex inhibitory peptide derived from the known sequence of the virus. Strikingly, we show that fatty acids can replace the entire C-terminal region of DP178, known to play a crucial role in the activity of the peptide. The inhibitory activity correlated with the length of the fatty acid, with the direction of fatty acid attachment (N- or C-terminus) and, as envisioned by a new triple staining assay, with the concentration of the peptides on cells. Our findings indicate, for the first time, the C-terminal boundary of the endogenous core structure in situ and establish that the C-terminal region of DP178 functions mainly as an anchor to the cell membrane. Apart from the mechanistic implications, such short lipopeptides provide new, promising fusion inhibitors. Because the fusion mechanism of HIV-1 is shared by other pathogen-enveloped viruses and by intracellular vesicle fusion, our results might influence the research and therapeutic efforts in these systems as well.

Structurally altered peptides reveal an important role for N-terminal heptad repeat binding and stability in the inhibitory action of HIV-1 peptide DP178

Human immunodeficiency virus 1 gp41 folds into a six-helix bundle whereby three C-terminal heptad repeat regions pack in an anti-parallel manner against the coiled-coil formed by three N-terminal heptad repeats (NHR). Peptides that inhibit bundle formation contributed significantly to the understanding of the entry mechanism of the virus. DP178, which partially overlaps C-terminal heptad repeats, prevents bundle formation through an undefined mechanism; additionally it has been suggested to bind other ENV regions and arrest fusion in an unknown manner. We used two structurally altered DP178 peptides; in each, two sequential amino acids were substituted into their d configuration, d-SQ in the hydrophilic N-terminal region and d-LW in the hydrophobic C-terminal. Importantly, we generated an elongated NHR peptide, N54, obtaining the full N-helix docking site for DP178. Interestingly, d-LW retained wild type fusion inhibitory activity, whereas d-SQ exhibited significantly reduced activity. In correlation with the inhibitory data, CD spectroscopy and fluorescence studies revealed that all the DP178 peptides interact with N54, albeit with different stabilities of the bundles. We conclude that strong binding of DP178 N-terminal region to the endogenous NHR, without significant contribution of the C-terminal sequence of DP178 to core formation, is vital for DP178 inhibition. The finding that d-amino acid incorporation in the C terminus did not affect activity or membrane binding as revealed by surface plasmon resonance correlates with an additional membrane binding site, or membrane anchoring role, for the C terminus, which works synergistically with the N terminus to inhibit fusion.

High-throughput screening method of inhibitors that block the interaction between 2 helical regions of HIV-1 gp41

The HIV-1 envelope glycoprotein transmembrane subunit, gp41, mediates the fusion of viral and target cell membranes. The 2 helical regions in the ectodomain of gp41, the N-helix and the C-helix, form a helical bundle complex that has been suggested as a fusion-active conformation. Previously, an enzyme-linked immunosorbent assay (ELISA) method had been established to measure the interaction of 2 helical regions of gp41. In this study, the ELISA method was modified to apply high-throughput screening (HTS) of an organic compound library. A few compounds had been identified to prevent the interaction between 2 helical regions of gp41, and they were further shown to inhibit the gp41-mediated viral infection. In addition, they specifically quenched the fluorescence of tryptophan in the N-helix region, indicating that these compounds bound to the N-helix rather than the C-helix of gp41. These results suggested that this assay method targeting gp41 could be used for HTS of HIV fusion inhibitors.

Different from the HIV fusion inhibitor C34, the anti-HIV drug Fuzeon (T-20) inhibits HIV-1 entry by targeting multiple sites in gp41 and gp120

Fuzeon (also known as T-20 or enfuvirtide), one of the C-peptides derived from the HIV-1 envelope glycoprotein transmembrane subunit gp41 C-terminal heptad repeat (CHR) region, is the first member of a new class of anti-HIV drugs known as HIV fusion inhibitors. It has been widely believed that T-20 shares the same mechanism of action with C34, another C-peptide. The C34 is known to compete with the CHR of gp41 to form a stable 6-helix bundle (6-HB) with the gp41 N-terminal heptad repeat (NHR) and prevent the formation of the fusogenic gp41 core between viral gp41 NHR and CHR, thereby inhibiting fusion between viral and target cell membranes. Here we present data to demonstrate that, contrary to this belief, T-20 cannot form stable 6-HB with N-peptides derived from the NHR region, nor can it inhibit the 6-HB formation of the fusogenic core. Instead, it may interact with N-peptides to form unstable or insoluble complexes. Our data suggest that T-20 has a different mechanism of action from C34. The interaction of T-20 with viral NHR region alone may not prevent the formation of the fusion active gp41 core. We also demonstrate that the T-20-mediated anti-HIV activity can be significantly abrogated by peptides derived from the membrane-spanning domain in gp41 and coreceptor binding site in gp120. These new findings imply that T-20 inhibits HIV-1 entry by targeting multiple sites in gp41 and gp120. Further elucidation of the mechanism of action of T-20 will provide new target(s) for development of novel HIV entry inhibitors.

Quantitative analysis of the interaction between the envelope protein domains and the core protein of human hepatitis B virus

Interaction between preformed nucleocapsids and viral envelope proteins is critical for the assembly of virus particles in infected cells. The pre-S1 and pre-S2 and cytosolic regions of the human hepatitis B virus envelope protein had been implicated in the interaction with the core protein of nucleocapsids. The binding affinities of specific subdomains of the envelope protein to the core protein were quantitatively measured by both ELISA and BIAcore assay. While a marginal binding was detected with the pre-S1 or pre-S2, the core protein showed high affinities to pre-S with apparent dissociation constants (K(D)(app)) of 7.3+/-0.9 and 8.2+/-0.4microM by ELISA and BIAcore assay, respectively. The circular dichroism analysis suggested that conformational change occurs in pre-S through interaction with core protein. These results substantiate the importance of specific envelope domains in virion assembly, and demonstrate that the interaction between viral proteins can be quantitatively measured in vitro.

Direct evidence that C-peptide inhibitors of human immunodeficiency virus type 1 entry bind to the gp41 N-helical domain in receptor-activated viral envelope

While it has been established that peptides modeling the C-helical region of human immunodeficiency virus type 1 gp41 are potent in vivo inhibitors of virus replication, their mechanism of action has yet to be determined. It has been proposed, but never directly demonstrated, that these peptides block virus entry by interacting with gp41 to disrupt the formation or function of a six-helix bundle structure. Using a six-helix bundle-specific monoclonal antibody with isolate-restricted Env reactivity, we provide the first direct evidence that, in receptor-activated viral Env, C-peptide entry inhibitors bind to the gp41 N-helical coiled-coil to form a peptide/protein hybrid structure and, in doing so, disrupt native six-helix bundle formation.

C-terminal octylation rescues an inactive T20 mutant: implications for the mechanism of HIV/SIMIAN immunodeficiency virus-induced membrane fusion

T20, a synthetic peptide corresponding to a C-terminal segment of the envelope glycoprotein (gp41) of human and simian immunodeficiency viruses, is a potent inhibitor of viral infection. We report here that C-terminal octylation of simian immunodeficiency virus gp41-derived T20 induces a significant increase in its inhibitory potency. Furthermore, when C-terminally octylated, an otherwise inactive mutant in which the C-terminal residues GNWF were replaced by ANAA has potency similar to that of the wild type T20. This effect cannot be explained by a trivial inhibitory effect of the octyl group added to the peptides, because the N-terminally octylated peptides have the same activity as the non-octylated parent peptides. The effects caused by octylation on the oligomerization, secondary structure, and membrane-interaction properties of the peptides were investigated. Our results shed light on the mechanism of inhibition by T20 and provide experimental support for the existence of a pre-hairpin intermediate.

Construction of recombinant targeting immunogens incorporating an HIV-1 neutralizing epitope into sites of differing conformational constraint

2F5 is one of the very few monoclonal antibodies with the capacity to neutralize a wide spectrum of type 1 human immunodeficiency virus (HIV-1) strains and primary isolates. Constructing an immunogen that contains a conformational mimic of the epitope recognized by 2F5 could provide the means to induce a broadly neutralizing anti-HIV-1 antibody response. Thus, in an effort to create a targeted, adjuvant-independent immunogen able to induce a 2F5-like antibody response, the gp41 sequence recognized by 2F5 (ELDKWAS) was genetically incorporated into different regions of an antibody specific for a framework determinant on human leukocyte antigen (HLA)-DR. All constructs were expressed, secreted from Sf9 insect cells, and found to retain the anti-HLA-DR specificity of the parental antibody. Three of the four constructs in which the ELDKWAS sequence was incorporated into a beta-turn (BT)-like conformational site were recognized by the 2F5 antibody. In contrast, none of the five constructs with the same sequence incorporated into surface-exposed regions of helical turn had any detectable 2F5 reactivity. In addition to demonstrating the significant plasticity of several regions in the antibody molecule in terms of accepting foreign sequences without loss of expression or binding specificity, these results also suggest that the native epitope recognized by the 2F5 antibody may be more beta-turn-like than helical in conformation. Importantly, with respect to vaccine development, the 2F5-reactive antibody constructs represent candidate immunogens for the adjuvant-independent induction of an HIV-1, neutralizing 2F5-like antibody response in humans.

Virus-cell fusion inhibitory activity for the polysaccharides from various Korean edible clams

In order to find potent virus-cell fusion inhibitory components from Korean edible clams, thirteen prepared polysaccharides were introduced to syncytia formation inhibition assay, which is based on the interaction between the HIV-1 envelope protein gp120/41 and the cellular membrane protein CD4 of T lymphocytes. Among them, Meretrix petechialis showed a potent virus-cell fusion inhibitory activity. Fusion index (FI) and percent (%) fusion inhibition of the polysaccharide of this clam were 0.21 +/- 0.02, and 67.52 +/- 4.09 at 100 microg/ml, respectively. It exhibited almost equivalent virus-cell fusion inhibitory activity to that of dextran sulfate which was used as a standard control.

Cooperative helix stabilization by complex Arg-Glu salt bridges

Among the interactions that stabilize the native state of proteins, the role of electrostatic interactions has been difficult to quantify precisely. Surface salt bridges or ion pairs between acidic and basic side chains have only a modest stabilizing effect on the stability of helical peptides or proteins: estimates are roughly 0.5 kcal/mol or less. On the other hand, theoretical arguments and the occurrence of salt bridge networks in thermophilic proteins suggest that multiple salt bridges may exert a stronger stabilizing effect. We show here that triads of charged side chains, Arg(+)-Glu(-)-Arg(+) spaced at i,i+4 or i,i+3 intervals in a helical peptide stabilize alpha helix by more than the additive contribution of two single salt bridges. The free energy of the triad is more than 1 kcal/mol in excess of the sum of the individual pairs, measured in low salt concentration (10 mM). The effect of spacing the three groups is severe; placing the charges at i,i+4 or i,i+3 sites has a strong effect on stability relative to single bridges; other combinations are weaker. A conservative calculation suggests that interactions of this kind between salt bridges can account for much of the stabilization of certain thermophilic proteins.

Design of a peptide inhibitor that blocks the cell fusion mediated by glycoprotein 41 of human immunodeficiency virus type 1

Fusion between the envelope of HIV and the plasma membrane of target cells is mediated by gp41, the envelope glycoprotein of HIV. Peptides derived from the membrane-proximal helical motif of the extracellular domain of gp41 effectively inhibit the infection of HIV, and their inhibitory activities are known to be correlated with the helical propensity of the peptides. We have designed small peptides that can form a stable alpha helix and thereby inhibit gp120/41-mediated cell fusion. A 19-mer peptide from the membrane-proximal helical motif of gp41 had no secondary structure in solution, and failed to block gp41-mediated cell fusion. When amino acids with low helical propensity were substituted, and helix-capping sequences were introduced at both ends of the peptides, the modified peptides formed a stable helical structure. They also bound to the coiled-coil motif of gp41 presented at the C terminus of thioredoxin and blocked the cell fusion mediated by gp120/41. These results implied that such modification was enough to change a short peptide derived from gp41 into a potent inhibitor against the infection of HIV.