Exposure of the HIV-1 broadly neutralizing antibody 10E8 MPER epitope on the membrane surface by gp41 transmembrane domain scaffolds

The 10E8 antibody achieves near-pan neutralization of HIV-1 by targeting the remarkably conserved gp41 membrane-proximal external region (MPER) and the connected transmembrane domain (TMD) of the HIV-1 envelope glycoprotein (Env). Thus, recreating the structure that generates 10E8-like antibodies is a major goal of the rational design of anti-HIV vaccines. Unfortunately, high-resolution information of this segment in the native Env is lacking, limiting our understanding of the behavior of the crucial 10E8 epitope residues.In this report, two sequences, namely, MPER-TMD1 (gp41 residues 671–700) and MPER-TMD2 (gp41 residues 671–709) were compared both experimentally and computationally, to assess the TMD as a potential membrane integral scaffold for the 10E8 epitope. These sequences were selected to represent a minimal (MPER-TMD1) or full-length (MPER-TMD2) TMD membrane anchor according to mutagenesis results reported by Yue et al. (2009) J. Virol. 83, 11,588. Immunochemical assays revealed that MPER-TMD1, but not MPER-TMD2, effectively exposed the MPER C-terminal stretch, harboring the 10E8 epitope on the surface of phospholipid bilayers containing a cholesterol concentration equivalent to that of the viral envelope. Molecular dynamics simulations, using the recently resolved TMD trimer structure combined with the MPER in a cholesterol-enriched model membrane confirmed these results and provided an atomistic mechanism of epitope exposure which revealed that TMD truncation at position A700 combined with N-terminal addition of lysine residues positively impacts epitope exposure. Overall, these results provide crucial insights into the design of effective MPER-TMD derived immunogens.     

Structural and immunological reactivity of the principal neutralizing determinant V3 of glycoprotein gp 120 of HIV-1

The variable domain V3 in the outer glycoprotein gp 120 of HIV-1 is a highly important region with respect to immune response during the course of viral infection. Neutralizing antibodies are produced against this domain; in addition, it has been shown to be a functionally active epitope for T helper and cytotoxic T cells. The high degree of amino acid variability in individual HIV-isolates, however, limits the use of the V3-domain in approaches to vaccine development. In order to characterize the residues important for antibody interaction and binding to MHC class I proteins, we constructed a consensus sequence of the V3-domain with broad reactivity [1] and used synthetic peptides derived from this consensus with individual residues altered to alanine. These peptides were used as antigens in ELISA tests to define the amino acids which are important for binding to human and rabbit/anti-peptide immunoglobulins. In addition, we used these alanine-derived peptides in interaction studies with human HLA-A2.1 and mouse H-2D^d by testing their capacity to stabilize the respective MHC class I protein complexes on the surface of mutant cell lines T2 and RMA-S transfected with D^d gene. The experimental tests allowed us to define individual residues involved in antibody and MHC-protein interaction, respectively. In a further approach, we used those results to design interaction models with HLA-A2.1 and H-2D^d. Therefore, a structural model for H-2D^d was built that exhibits an overall similar conformation to the parental crystal structure of HLA-A2.1. The resulting interaction models show V3-peptide bound in an extended β-conformation with a bulge in its centre for both H-2D^d and HLA-A2.1 complexes. The N- and C-termini of V3 peptide reside in conserved pockets within both MHC-proteins. Anchoring residues could be determined that are crucial for the binding of the respective MHC class I haplotype. The cross-reactivity of V3-peptide in enhancing the expression of two different MHC class I molecules (H-2D^d and HLA-A2.1) is shown to be based on similar peptide binding that induces an almost identical peptide conformation.     

Conformational mapping of the N-terminal peptide of HIV-1 gp41 in membrane environments using (13)C-enhanced Fourier transform infrared spectroscopy

The N-terminal domain of HIV-1 glycoprotein 41000 (FP; residues 1--23; AVGIGALFLGFLGAAGSTMGARSCONH(2)) participates in fusion processes underlying virus--cell infection. Here, we use physical techniques to study the secondary conformation of synthetic FP in aqueous, structure-promoting, lipid and biomembrane environments. Circular dichroism and conventional, (12)C-Fourier transform infrared (FTIR) spectroscopy indicated the following alpha-helical levels for FP in 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG) liposomes-hexafluoroisopropanol (HFIP)>trifluoroethanol (TFE)>phosphate-buffered saline (PBS). (12)C-FTIR spectra also showed disordered FP structures in these environments, along with substantial beta-structures for FP in TFE or PBS. In further experiments designed to map secondary conformations to specific residues, isotope-enhanced FTIR spectroscopy was performed using a suite of FP peptides labeled with (13)C-carbonyl at multiple sites. Combining these (13)C-enhanced FTIR results with molecular simulations indicated the following model for FP in HFIP: alpha-helix (residues 3-16) and random and beta-structures (residues 1-2 and residues 17-23). Additional (13)C-FTIR analysis indicated a similar conformation for FP in POPG at low peptide loading, except that the alpha-helix extends over residues 1-16. At low peptide loading in either human erythrocyte ghosts or lipid extracts from ghosts, (13)C-FTIR spectroscopy showed alpha-helical conformations for the central core of FP (residues 5-15); on the other hand, at high peptide loading in ghosts or lipid extracts, the central core of FP assumed an antiparallel beta-structure. FP at low loading in ghosts probably inserts deeply as an alpha-helix into the hydrophobic membrane bilayer, while at higher loading FP primarily associates with ghosts as an aqueous-accessible, beta-sheet. In future studies, (13)C-FTIR spectroscopy may yield residue-specific conformations for other membrane-bound proteins or peptides, which have been difficult to analyze with more standard methodologies.     

Intranasal HIV-1-gp160-DNA/gp41 peptide prime-boost immunization regimen in mice results in long-term HIV-1 neutralizing humoral mucosal and systemic immunity

An intranasal DNA vaccine prime followed by a gp41 peptide booster immunization was compared with gp41 peptide and control immunizations. Serum HIV-1-specific IgG and IgA as well as IgA in feces and vaginal and lung secretions were detected after immunizations. Long-term humoral immunity was studied for up to 12 mo after the booster immunization by testing the presence of HIV-1 gp41- and CCR5-specific Abs and IgG/IgA-secreting B lymphocytes in spleen and regional lymph nodes in immunized mice. A long-term IgA-specific response in the intestines, vagina, and lungs was obtained in addition to a systemic immune response. Mice immunized only with gp41 peptides and L3 adjuvant developed a long-term gp41-specific serum IgG response systemically, although over a shorter period (1-9 mo), and long-term mucosal gp41-specific IgA immunity. HIV-1-neutralizing serum Abs were induced that were still present 12 mo after booster immunization. HIV-1 SF2-neutralizing fecal and lung IgA was detectable only in the DNA-primed mouse groups. Intranasal DNA prime followed by one peptide/L3 adjuvant booster immunization, but not a peptide prime followed by a DNA booster, was able to induce B cell memory and HIV-1-neutralizing Abs for at least half of a mouse's life span.     

Induction of high level of specific antibody response to the neutralizing epitope ELDKWA on HIV-1 gp41 by peptide-vaccine

The monoclonal antibody 2F5 recognizing the neutralizing epitope ELDKWA on the C-domain could neutralize 90% of the investigated HIV-1 isolates. Low levels of ELDKWA-epitope-specific antibodies were observed in HIV-1-infected individuals. To induce high levels of antibodies to ELDKW-epitope, C-domain peptide (P2) was conjugated with a carrier peptide (KGGG)(7)-K (K/G). P2-K/G-conjugate induced high level of antibodies in mice by titer 1:25,600 to ELDKWA-epitope. P2-K/G-BSA-conjugate induced antibody response to ELDKWA-epitope (1:320-6400) in mice. The ELDKWA-epitope-specific antibodies of 19.8 and 34.6 microg/per milliliter serum were isolated from two rabbit antiserums (1:25,600). The levels of ELDKWA-epitope-specific antibodies induced in rabbits were greater than 1 microg/ml, a level considered to confer long-term protection. These results demonstrate the potential role of the C-domain peptide of gp41 to develop an effective ELDKWA-based epitope/peptide-vaccine against HIV-1.     

Discovery and optimization of a natural HIV-1 entry inhibitor targeting the gp41 fusion peptide

A variety of molecules in human blood have been implicated in the inhibition of HIV-1. However, it remained elusive which circulating natural compounds are most effective in controlling viral replication in vivo. To identify natural HIV-1 inhibitors we screened a comprehensive peptide library generated from human hemofiltrate. The most potent fraction contained a 20-residue peptide, designated VIRUS-INHIBITORY PEPTIDE (VIRIP), corresponding to the C-proximal region of alpha1-antitrypsin, the most abundant circulating serine protease inhibitor. We found that VIRIP inhibits a wide variety of HIV-1 strains including those resistant to current antiretroviral drugs. Further analysis demonstrated that VIRIP blocks HIV-1 entry by interacting with the gp41 fusion peptide and showed that a few amino acid changes increase its antiretroviral potency by two orders of magnitude. Thus, as a highly specific natural inhibitor of the HIV-1 gp41 fusion peptide, VIRIP may lead to the development of another class of antiretroviral drugs.     

A short segment of the HIV-1 gp120 V1/V2 region is a major determinant of resistance to V1/V2 neutralizing antibodies

Antibody PG9 is a prototypical member of a class of V1/V2-directed antibodies that effectively neutralizes diverse strains of HIV-1. We analyzed strain-specific resistance to PG9 using sequence and structural information. For multiply resistant strains, mutations in a short segment of V1/V2 resulted in gain of sensitivity to PG9 and related V1/V2 neutralizing antibodies, suggesting both a common mechanism of HIV-1 resistance to and a common mode of recognition by this class of antibodies.     

The complete chirospectroscopic signature of the peptide 3(10)-helix in aqueous solution

We synthesized by solution methods a water-soluble, terminally blocked heptapeptide based on five markedly helicogenic, C(alpha)-tetrasubstituted alpha-amino acids C(alpha)-methyl-L-norvalines and two strongly hydrophilic 2-amino-3-[1-(1,4,7-triazacyclononane)]-L-propanoic acid residues at positions 2 and 5. A Fourier transform infrared absorption and NMR analysis in deuterated chloroform and aqueous solutions of the heptapeptide and two side-chain protected synthetic precursors confirmed our working hypothesis that all oligomers are folded in the 3(10)-helical conformation. Based on these findings, we exploited this heptapeptide as a chiral reference compound for detailed electronic CD, vibrational CD, and Raman optical activity characterizations of the 3(10)-helix in aqueous solution.     

Neutralization resistance of virological synapse-mediated HIV-1 Infection is regulated by the gp41 cytoplasmic tail

Human immunodeficiency virus type 1 (HIV-1) infection can spread efficiently from infected to uninfected T cells through adhesive contacts called virological synapses (VSs). In this process, cell-surface envelope glycoprotein (Env) initiates adhesion and viral transfer into an uninfected recipient cell. Previous studies have found some HIV-1-neutralizing patient sera to be less effective at blocking VS-mediated infection than infection with cell-free virus. Here we employ sensitive flow cytometry-based infection assays to measure the inhibitory potency of HIV-1-neutralizing monoclonal antibodies (MAb) and HIV-1-neutralizing patient sera against cell-free and VS-mediated infection. To various degrees, anti-Env MAbs exhibited significantly higher 50% inhibitory concentration (IC(50)s) against VS-mediated infection than cell-free infection. Notably, the MAb 17b, which binds a CD4-induced (CD4i) epitope on gp120, displayed a 72-fold reduced efficacy against VS-mediated inocula compared to cell-free inocula. A mutant with truncation mutation in the gp41 cytoplasmic tail (CT) which is unable to modulate Env fusogenicity in response to virus particle maturation but which can still engage in cell-to-cell infection was tested for the ability to resist neutralizing antibodies. The ΔCT mutation increased cell surface staining by neutralizing antibodies, significantly enhanced neutralization of VS-mediated infection, and had reduced or no effect on cell-free infection, depending upon the antibody. Our results suggest that the gp41 CT regulates the exposure of key neutralizing epitopes during cell-to-cell infection and plays an important role in immune evasion. Vaccine strategies should consider immunogens that reflect Env conformations exposed on the infected cell surface to enhance protection against VS-mediated HIV-1 spread.     

Examination of sera from human immunodeficiency virus type 1 (HIV-1)-infected individuals for antibodies reactive with peptides corresponding to the principal neutralizing determinant of HIV-1 gp120 and for in vitro neutralizing activity.

Sera from human immunodeficiency virus type 1 (HIV-1)-infected individuals from the United States and Tanzania were examined for antibody reactivity to four synthetic peptides which corresponded to the principal neutralizing determinant from the V3 region of HIV-1 gp120. We observed that the majority of sera from both countries contained antibodies reactive with a V3 peptide whose sequence is based on that of the HIV-1 MN isolate. We were unable to establish a relationship between the presence of V3-reactive antibodies, as measured by enzyme-linked immunosorbent assay and neutralization of homologous HIV-1 isolates, in sera from either the United States or Tanzania. We observed that some sera which contained high antibody titers to the V3 peptides failed to neutralize HIV-1, while others with no antibody reactivity to the panel of V3 peptides exhibited in vitro neutralizing activity. These results suggest that neutralizing epitopes exist outside the V3 loop and that the presence of V3-reactive antibodies in sera does not imply in vitro neutralization of the homologous HIV-1 isolate. In addition, it appears that the V3 loop may consist of both neutralizing and nonneutralizing epitopes. The identification of neutralizing as well as nonneutralizing epitopes will be important for the design of potential HIV-1 vaccines.     

Mutations of Gln64 in the HIV-1 gp41 N-terminal heptad repeat render viruses resistant to peptide HIV fusion inhibitors targeting the gp41 pocket

To prove that the peptidic HIV-1 fusion inhibitors containing the pocket-binding domain (PBD) mainly target the hydrophobic pocket in the gp41 N-terminal heptad repeat (NHR), we constructed pseudoviruses by replacement of Q64 in the gp41 pocket region with Ala (Q64A) or Leu (Q64L). These viruses were highly resistant to C34 and CP32M containing the PBD, while they were susceptible to T20 (enfuvirtide) lacking the PBD but containing the GIV-motif-binding domain (GBD) and lipid-binding domain (LBD). They were also sensitive to C52L, which contains the PBD, GBD, and LBD. Those mutations may disrupt the hydrophilic interaction between Q64 in the NHR and N113 in the peptides containing the PBD. This report provides insights into the mechanisms of drug resistance, with implications for the design of novel HIV fusion and entry inhibitors.     

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

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 degrees 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.     

The LLSGIV stretch of the N-terminal region of HIV-1 gp41 is critical for binding to a model peptide,T20

A number of peptides and peptide analogs derived from the membrane proximal region of gp41 ectodomain are found to be effective inhibitors of human immunodeficiency virus type 1 (HIV-1)-mediated fusion events. One of them, T20 (aa 638-673), was found disordered and sparingly soluble in water, but became soluble upon mixing with selected, structured peptides from the amino terminal heptad repeat (HR1) region of gp41 using a simple and sensitive method of reduction in the scattering of T20 suspension. From the results on mapping the locus of interaction with T20 by employing partially overlapping peptides derived from HR1, it was concluded that the LLSGIV segment was a critical docking site for the C-terminal peptide of gp41 in its putative inhibitory action consistent with a previous fluorescence study. It was also found that peptides capable of solubilizing T20 dispersion have a high content of helix, as well as beta-strand, conformation in aqueous solution. Specificity of T20/HR1-derived peptide binding was ascertained by using a scrambled sequence of a T20-active peptide and a plateau in scattering reduction of T20 suspension with variation in the concentration of a T20-active HR1 peptide. Implications on the mechanism of T20 inhibition and the sequence of folding of the gp41 core structure are discussed.     

Induction of mucosal and systemic neutralizing antibodies against human immunodeficiency virus type 1 (HIV-1) by oral immunization with bovine Papillomavirus-HIV-1 gp41 chimeric virus-like particles

Human immunodeficiency virus type 1 (HIV-1) envelope-specific neutralizing antibodies are generated late after initial infection, and the neutralizing antibody response is weak in the infected individuals. Administration of neutralizing antibodies such as 2F5 to HIV-1-infected individuals resulted in reductions in viral loads. Because HIV-1 is transmitted mainly via mucosa and because HIV-specific neutralizing antibodies reduce HIV-1 in infected individuals, a vaccine that can induce both mucosal and systemic HIV-1-specific neutralizing antibodies may be used to prevent and to treat HIV-1 infection. In this study, we made a bovine papillomavirus (BPV) L1-HIV-1 gp41 fusion protein in which ELDKWA of gp41 was inserted into the N terminus of BPV L1 (amino acids 130 to 136). Expression of the fusion protein in insect cells led to the assembly of chimeric virus-like particles (CVLPs). The CVLPs had sizes similar to those of BPV particles and were able to bind to the cell surface and penetrate the cell membrane. Oral immunization of mice with CVLPs induced gp41-specific serum immunoglobulin G (IgG) and intestinal secretory IgA. However, intramuscular immunization with the CVLPs resulted in similar amounts of gp41-specific IgG but low levels of secretory IgA. The antibodies specifically recognized the fixed HIV-1 gp41 on the cell surface. Importantly, the sera and fecal extracts from mice orally immunized with the CVLPs neutralized HIV-1(MN) in vitro. Thus, BPV-HIV-1 gp41 CVLPs may be used to prevent and to treat HIV-1 infection.     

An infectious chimeric human immunodeficiency virus type 2 (HIV-2) expressing the HIV-1 principal neutralizing determinant.

The human immunodeficiency virus type 1 strain MN (HIV-1MN) principal neutralizing determinant (PND, V3 loop) was introduced into infectious molecular clones HIV-2KR and simian immunodeficiency virus mm239 (SIVmm239) by hybridization PCR, replacing the corresponding HIV-2 or SIV envelope cysteine loops with the HIV-1 coding sequence. The HIV-2 chimera (HIV-2KR-MNV3) was found to be capable of infecting a number of T-cell lymphoblastic cell lines as well as primary peripheral blood mononuclear cells. In contrast, the SIV chimera (SIV239MNV3) was not replication competent. Envelope produced by HIV-2KR-MNV3 but not the parental HIV-2KR was recognized by V3-specific and HIV-1-specific polyclonal antisera in radioimmunoprecipitation assays. HIV-2-specific antisera recognized both the chimeric and parental virus but not HIV-1MN. The chimeric HIV-2KR-MNV3 virus proved to be exquisitely susceptible to neutralization by HIV-1-specific and V3-specific antisera, suggesting the potential for use in animal models designed to test HIV-1 vaccine candidates which target the PND.     

Cross-reactive potential of rabbit antibodies raised against a cyclic peptide representing a chimeric V3 loop of HIV-1 gp120 studied by biosensor technique and ELISA

Abstract Rabbit antibodies were induced against a free cyclic peptide representing the chimeric sequence of a consensus V3 loop of HIV-1 gp120. The reactivity of these antibodies was tested in a biosensor system (BIAcore^™, Pharmacia AB, Uppsala, Sweden) and in ELISA with the peptide immunogen in its cyclic and linear forms, as well as with peptides corresponding to the V3 region of different HIV-1 variants. The antibodies reacted with all the peptides tested both in ELISA and in biosensor assays and recognized the cyclic form of the chimeric peptide better than the linear form. Although antibodies raised against the V3 region of particular HIV-1 variants cross-react with other HIV-1 strains, it seems that the use of a chimeric peptide as immunogen improved the cross-reactivity spectrum of recognition of the antibodies. The anti-V3 antibodies were also tested for their ability to neutralize in vitro four HIV-1 laboratory strains. Only the HIV_MN variant was found to be neutralized. Compared to conventional solid phase immunoassays, the BIAcore presents several advantages for measuring the differential reactivity of peptide analogues. In view of their broadly cross-reactive potential, antibodies raised against a consensus sequence should be useful in immunodiagnosis of viral antigenic variants.     

Solution conformation of a peptide corresponding to the principal neutralizing determinant of HIV-1IIIB: a two-dimensional NMR study.

The 24 amino acid peptide RP135 corresponds in its amino acid sequence to the principal neutralizing determinant (PND) of the IIIB isolate of HIV-1. Although the sequence of the PND is highly variable, its central part, containing the sequence GPGR, is conserved in most HIV isolates. Using 2D NMR and CD spectroscopy, we have studied the conformation of RP135 and of two shorter versions: one (P547) that includes the GPGR sequence with the N-terminal part of the peptide and the other (P344) that includes GPGR and the C-terminal segment of RP135. In water, the C-terminal part of RP135 was found to exist in several transient turnlike conformations ("nascent helix"). A helical conformation was found to be stabilized by the addition of TFE. A transient turn was observed also in the GPGR sequence, both in water and in aqueous TFE solutions. While no nascent helix conformations could be observed in the N-terminal part of RP135 in water, a helical conformation was partially stabilized by the addition of TFE. The conformations of the two shorter versions of the peptide were similar to those of the corresponding parts of RP135, except that the transient turn in GPGR could not be detected in P547 dissolved in water. The turn in GPGR was previously predicted by Larosa et al. (1990) and was observed by Chandrasekhar et al. (1991) in the PND peptide of HIV-1MN (RP142), which shares only 56% identity with RP135. However, nascent helix conformations were not observed in aqueous solutions of RP142.(ABSTRACT TRUNCATED AT 250 WORDS)     

Molecular dynamics simulation of the stability of a 22-residue α-helix in water and 30% trifluoroethanol

A molecular dynamics (MD) simulation was performed on the α-helix H8-HC5, the C-terminal part of myoglobin (residue 132–153), under periodic boundary conditions in two different solutions, water and water with 30% (v/v) 2,2,2-trifluoroethanol (TFE), at 300 K to investigate the stability of the helix. In both simulations, the initial configuration was a canonical right-handed α-helix. In the course of the MD trajectory in water (200 ps), the helix clearly destabilized and began to unfold after 100 ps. In the TFE solution, two stable parts of helical regions were observed after 70 ps of a 200-ps MD simulation, supporting the notion that TFE acts as a structure-forming solvent. © 1993 John Wiley & Sons, Inc.     

Conformational mapping of the N-terminal peptide of HIV-1 gp41 in lipid detergent and aqueous environments using 13C-enhanced Fourier transform infrared spectroscopy

The N-terminal domain of HIV-1 glycoprotein 41,000 (gp41) participates in viral fusion processes. Here, we use physical and computational methodologies to examine the secondary structure of a peptide based on the N terminus (FP; residues 1-23) in aqueous and detergent environments. (12)C-Fourier transform infrared (FTIR) spectroscopy indicated greater alpha-helix for FP in lipid-detergent sodium dodecyl sulfate (SDS) and aqueous phosphate-buffered saline (PBS) than in only PBS. (12)C-FTIR spectra also showed disordered FP conformations in these two environments, along with substantial beta-structure for FP alone in PBS. In experiments that map conformations to specific residues, isotope-enhanced FTIR spectroscopy was performed using FP peptides labeled with (13)C-carbonyl. (13)C-FTIR results on FP in SDS at low peptide loading indicated alpha-helix (residues 5 to 16) and disordered conformations (residues 1-4). Because earlier (13)C-FTIR analysis of FP in lipid bilayers demonstrated alpha-helix for residues 1-16 at low peptide loading, the FP structure in SDS micelles only approximates that found for FP with membranes. Molecular dynamics simulations of FP in an explicit SDS micelle indicate that the fraying of the first three to four residues may be due to the FP helix moving to one end of the micelle. In PBS alone, however, electron microscopy of FP showed large fibrils, while (13)C-FTIR spectra demonstrated antiparallel beta-sheet for FP (residues 1-12), analogous to that reported for amyloid peptides. Because FP and amyloid peptides each exhibit plaque formation, alpha-helix to beta-sheet interconversion, and membrane fusion activity, amyloid and N-terminal gp41 peptides may belong to the same superfamily of proteins.