The amino-terminal peptide of HIV-1 gp41 interacts with human serum albumin

Toward Understanding the Molecular Recognition of Albumin by p53-Activating Stapled Peptide ATSP-7041

Reactivation of tumor-suppressing activity of p53 protein by targeting its negative regulator MDM2/MDMX has been pursued as a potential anticancer strategy. A promising dual inhibitor of MDM2/MDMX that has been developed and is currently in clinical trials is the stapled peptide ATSP-7041. The activity of this molecule is reported to be modulated in the presence of serum. Albumin is the most abundant protein in serum and is known to bind reversibly to several molecules. To study this interaction, we develop a protocol combining molecular modeling, docking, and simulations. Exhaustive docking of the peptide with representative simulated structures of human serum albumin led to the identification of probable binding sites on the surface of the protein, including both known canonical and novel binding sites. Sequence differences at putative peptide-binding sites in human and mouse albumin result in differing interaction energies with the peptide and enable us to rationalize the observed differences in vivo. In general, the findings should help in guiding the design of features in such peptides that may affect their distribution and cell permeability, opening a new window in structure-guided design strategies.

Effect of human serum albumin upon the permeabilizing activity of sticholysin II, a pore forming toxin from Stichodactyla heliantus

Sticholysin II (St II) is a haemolytic toxin isolated from the sea anemone Stichodactyla helianthus. The high haemolytic activity of this toxin is strongly dependent on the red cell status and the macromolecule conformation. In the present communication we evaluate the effect of human serum albumin on St II haemolytic activity and its capacity to form pores in the bilayer of synthetic liposomes. St II retains its pore forming capacity in the presence of large concentrations (up to 500 μM) of human serum albumin. This effect is observed both in its capacity to produce red blood cells haemolysis and to generate functional pores in liposomes. In particular, the capacity of the toxin to lyse red blood cells increases in the presence of human serum albumin (HSA). Regarding the rate of the pore forming process, it is moderately decreased in liposomes and in red blood cells, in spite of an almost total coverage of the interface by albumin. All the data obtained in red cells and model membranes show that St II remains lytically active even in the presence of high HSA concentrations. This stubbornness can explain why the toxin is able to exert its haemolytic activity on membranes immersed in complex plasma matrixes such as those present in living organisms.

Biochemistry and biophysics of HIV-1 gp41 - membrane interactions and implications for HIV-1 envelope protein mediated viral-cell fusion and fusion inhibitor design

Human immunodeficiency virus type 1 (HIV-1), the pathogen of acquired immunodeficiency syndrome (AIDS), causes ~2 millions death every year and still defies an effective vaccine. HIV-1 infects host cells through envelope protein - mediated virus-cell fusion. The transmembrane subunit of envelope protein, gp41, is the molecular machinery which facilitates fusion. Its ectodomain contains several distinguishing functional domains, fusion peptide (FP), Nterminal heptad repeat (NHR), C-terminal heptad repeat (CHR) and membrane proximal extracellular region (MPER). During the fusion process, FP inserts into the host cell membrane, and an extended gp41 prehairpin conformation bridges the viral and cell membranes through MPER and FP respectively. Subsequent conformational change of the unstable prehairpin results in a coiled-coil 6-helix bundle (6HB) structure formed between NHR and CHR. The energetics of 6HB formation drives membrane apposition and fusion. Drugs targeting gp41 functional domains to prevent 6HB formation inhibit HIV-1 infection. T20 (enfuvirtide, Fuzeon) was approved by the US FDA in 2003 as the first fusion inhibitor. It is a 36-residue peptide from the gp41 CHR, and it inhibits 6HB formation by targeting NHR and lipids. Development of new fusion inhibitors, especially small molecule drugs, is encouraged to overcome the shortcomings of T20 as a peptide drug. Hydrophobic characteristics and membrane association are critical for gp41 function and mechanism of action. Research in gp41-membrane interactions, using peptides corresponding to specific functional domains, or constructs including several interactive domains, are reviewed here to get a better understanding of gp41 mediated virus-cell fusion that can inform or guide the design of new HIV-1 fusion inhibitors.

Structural and functional properties of peptides based on the N-terminus of HIV-1 gp41 and the C-terminus of the amyloid-beta protein

Given their high alanine and glycine levels, plaque formation, alpha-helix to beta-sheet interconversion and fusogenicity, FP (i.e., the N-terminal fusion peptide of HIV-1 gp41; 23 residues) and amyloids were proposed as belonging to the same protein superfamily. Here, we further test whether FP may exhibit 'amyloid-like' characteristics, by contrasting its structural and functional properties with those of Abeta(26-42), a 17-residue peptide from the C-terminus of the amyloid-beta protein responsible for Alzheimer's. FTIR spectroscopy, electron microscopy, light scattering and predicted amyloid structure aggregation (PASTA) indicated that aqueous FP and Abeta(26-42) formed similar networked beta-sheet fibrils, although the FP fibril interactions were weaker. FP and Abeta(26-42) both lysed and aggregated human erythrocytes, with the hemolysis-onsets correlated with the conversion of alpha-helix to beta-sheet for each peptide in liposomes. Congo red (CR), a marker of amyloid plaques in situ, similarly inhibited either FP- or Abeta(26-42)-induced hemolysis, and surface plasmon resonance indicated that this may be due to direct CR-peptide binding. These findings suggest that membrane-bound beta-sheets of FP may contribute to the cytopathicity of HIV in vivo through an amyloid-type mechanism, and support the classification of HIV-1 FP as an 'amyloid homolog' (or 'amylog').

Membrane perturbing actions of HIV type 1 glycoprotein 41 domains are inhibited by helical C-peptides

To study the membrane actions of various domains of HIV-1 glycoprotein 41,000 (gp41), synthetic peptides were prepared corresponding to the N-terminal fusion region (FP; gp41 residues 519-541), the nearby N-leucine zipper domain (N-peptides; DP-107; gp41 residues 560-597), the C-leucine zipper domain (C-peptides; DP-178; gp41 residues 645-680), and the viral envelope adjacent domain that partially overlaps DP-178 (Pre-TM; gp41 residues 671-690). With erythrocytes, FP, DP-107, and Pre-TM induced hemolysis and cell aggregation; the order for hemolytic activity was Pre-TM > FP > DP-107, but each was equally effective in aggregating cells at the highest peptide concentrations tested. DP-178 produced neither hemolysis nor aggregation, but efficiently reduced FP-, DP-107-, and Pre-TM-induced membrane actions. Fourier transform infrared spectroscopy indicated that the membrane perturbations of Pre-TM, as well as the ability of DP-178 to block membrane activities of other gp41 domains, are dependent on Pre-TM and DP-178 each maintaining helical conformations and tryptophans at residues 673, 677, and 679. These results suggest that the corresponding N-terminal fusion, N-leucine zipper, and viral membrane-adjacent regions of HIV-1 gp41 may similarly promote key membrane perturbations underlying the merging of the viral envelope with the cell surface. Further, the antiviral mechanism of exogenous DP-178 (clinically approved enfuvirtide) may be partially explained by its coordinate inhibition of the fusogenic actions of the FP, DP-107, and Pre-TM regions of gp41.

Analysis of albumin-associated peptides and proteins from ovarian cancer patients

BACKGROUND

Albumin binds low-molecular-weight molecules, including proteins and peptides, which then acquire its longer half-life, thereby protecting the bound species from kidney clearance. We developed an experimental method to isolate albumin in its native state and to then identify [mass spectrometry (MS) sequencing] the corresponding bound low-molecular-weight molecules. We used this method to analyze pooled sera from a human disease study set (high-risk persons without cancer, n = 40; stage I ovarian cancer, n = 30; stage III ovarian cancer, n = 40) to demonstrate the feasibility of this approach as a discovery method.

METHODS

Albumin was isolated by solid-phase affinity capture under native binding and washing conditions. Captured albumin-associated proteins and peptides were separated by gel electrophoresis and subjected to iterative MS sequencing by microcapillary reversed-phase tandem MS. Selected albumin-bound protein fragments were confirmed in human sera by Western blotting and immunocompetition.

RESULTS

In total, 1208 individual protein sequences were predicted from all 3 pools. The predicted sequences were largely fragments derived from proteins with diverse biological functions. More than one third of these fragments were identified by multiple peptide sequences, and more than one half of the identified species were in vivo cleavage products of parent proteins. An estimated 700 serum peptides or proteins were predicted that had not been reported in previous serum databases. Several proteolytic fragments of larger molecules that may be cancer-related were confirmed immunologically in blood by Western blotting and peptide immunocompetition. BRCA2, a 390-kDa low-abundance nuclear protein linked to cancer susceptibility, was represented in sera as a series of specific fragments bound to albumin.

CONCLUSION

Carrier-protein harvesting provides a rich source of candidate peptides and proteins with potential diverse tissue and cellular origins that may reflect important disease-related information.

Novel role for albumin in innate immunity: serum albumin inhibits the growth of Blastomyces dermatitidis yeast form in vitro

In this study we found that serum inhibitory activity against Blastomyces dermatitidis was principally mediated by albumin. This was confirmed in experiments using albumin from several mammalian species. Analbuminemic rat serum did not inhibit B. dermatitidis growth in vivo; however, the addition of albumin restored inhibitory activity. Inhibitory activity does not require albumin domain III and appears to involve binding of a low-molecular-weight yeast-derived growth factor.

Interaction of albumin with the endothelial cell surface

Endothelial cells (EC) are covered with cell-borne proteoglycans and glycoproteins. Blood plasma proteins (e.g., albumin) adsorb to this glycocalyx forming a complex endothelial surface layer (ESL). We determined the molecular mobility of albumin by electron spin resonance (ESR) in the presence and absence of ECs to analyze interactions with the ESL. Albumin was spin labeled with 5- or 12-4,4-dimethyloxazolidine-N-oxyl (DOXYL)-stearic acid yielding information on the mobility of the molecular surface (5-DOXYL) or the entire protein (12-DOXYL). EC cultures grown on glass coverslips were immersed in labeled albumin and placed in the temperature-regulated cavity of an ESR spectrometer. Alternatively, ECs were labeled and then exposed to native albumin. At 37 degrees C, rotational correlation times determined by modified saturation transfer ESR (ST-ESR) were 26 and 48 ns for 5-DOXYL- and 12-DOXYL-labeled albumin, respectively. Presence of ECs increased rotational correlation time values for 5-DOXYL-stearic acid to 37 ns but not for 12-DOXYL-stearic acid. Albumin was able to completely take up the label from labeled EC within 2 min. The present study shows that modified ST-ESR can be used to determine the mobility of biological macromolecules interacting with cellular surfaces. Reduction in albumin surface mobility in the presence of EC at unchanged mobility of protein proper and fast removal of labeled fatty acids from EC membranes indicate rapid transient interactions between albumin surface and ESL but no rigid incorporation of albumin into a macromolecular network that would interfere with its transport function for poorly water-soluble substances.

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.

The metabolic fate of the Anti-HIV active drug carrier succinylated human serum albumin after intravenous administration in rats

The pharmacokinetics and metabolic fate of the intrinsically active (anti-HIV) drug carrier succinylated human serum albumin (Suc-HSA) was studied in rats. Suc-HSA was prepared by derivatizing HSA with 1,4-[14C]-succinic anhydride, a modification by which all available epsilon NH2-groups in HSA were converted into carboxylic groups. After i.v. injections of 0.3, 1.0, 3.0 and 10.0 mg/kg in freely moving rats, Suc-HSA showed a dose dependent elimination pattern, indicating a saturable elimination pathway. The Michaelis-Menten parameters Vmax and Km were 98.7 micrograms.min-1.kg-1 and 8.5 micrograms.ml-1 respectively. The kinetics of Suc-HSA was influenced by anaesthesia. In anaesthetised animals, Vmax and Km were found to be 26.9 micrograms.min-1.kg-1 and 0.26 microgram.ml-1, respectively. This implies an intrinsic clearance of 100 ml.min-1.kg-1, which is about 10-fold higher as compared to 12 ml.min-1.kg-1 in freely moving animals. Intravenous administration of a sub-saturable dose of 3.0 mg.kg-1 1,4-[14C]-Suc-HSA to freely moving rats resulted in a biphasic elimination with an initial t 1/2 of 20 min and a terminal t 1/2 of 40 hrs. Excretion of metabolites in urine and faeces lasted for at least 48 hours. About 70% of the radioactive dose was excreted in urine, whereas maximally 2% was detected in faeces. Suc-HSA was degraded to its individual amino acids including succinylated lysine (the only radioactive product formed). Succinylated lysine was not further metabolised and mainly excreted via the urine. Immunohistochemical staining showed that even after 48 hrs Suc-HSA could be detected in livers. Together with the urinary excretion patterns, this points to a gradual degradation of Suc-HSA.

Membrane-perturbing domains of HIV type 1 glycoprotein 41

Structural and functional studies were performed to assess the membrane actions of peptides based on HIV-1 glycoprotein 41,000 (gp41). Previous site-directed mutagenesis of gp41 has shown that amino acid changes in either the N-terminal fusion or N-leucine zipper region depressed viral infection and syncytium formation, while modifications in the C-leucine zipper domain both increased and decreased HIV fusion. Here, synthetic peptides were prepared corresponding to the N-terminal fusion region (FP-I; gp41 residues 519-541), the nearby N-leucine zipper domain (DP-107; gp41 residues 560-597), and the C-leucine zipper domain (DP-178; gp41 residues 645-680). With erythrocytes, FP-I or DP-107 induced dose-dependent hemolysis and promoted cell aggregation; FP-I was more hemolytic than DP-107, but each was equally effective in aggregating cells. DP-178 produced neither hemolysis nor aggregation, but blocked either FP-I- or DP-107-induced hemolysis and aggregation. Combined with previous nuclear magnetic resonance and Fourier transform infrared spectroscopic results, circular dichroism (CD) spectroscopy showed that the alpha-helicity for these peptides in solution decreased in the order: DP-107 >> DP-178 > FP-I. CD analysis also indicated binding of DP-178 to either DP-107 or FP-I. Consequently, DP-178 may inhibit the membrane actions mediated by either FP-I or DP-107 through direct peptide interactions in solution. These peptide results suggest that the corresponding N-terminal fusion and N-leucine zipper regions participate in HIV infection, by promoting membrane perturbations underlying the merging of the viral envelope with the cell surface. Further, the C-leucine zipper domain in "prefusion" HIV may inhibit these membrane activities by interacting with the N-terminal fusion and N-leucine zipper domains in unactivated gp41. Last, exogenous DP-178 may bind to the N-terminal and N-leucine zipper domains of gp41 that become exposed on HIV stimulation, thereby preventing the fusogenic actions of these gp41 regions leading to infection.

Membrane interactions of the synthetic N-terminal peptide of HIV-1 gp41 and its structural analogs

Structural and functional studies assessed the membrane actions of the N terminus of HIV-1 glycoprotein 41000 (gp41). Earlier site-directed mutagenesis has shown that key amino acid changes in this gp41 domain inhibit viral infection and syncytia formation. Here, a synthetic peptide corresponding to the N terminus of gp41 (FP; 23 residues, 519-541), and also FP analogs (FP520V/E with Val-->Glu at residue 520; FP527L/R with Leu-->Arg at 527; FP529F/Y with Phe-->Tyr at 529; and FPCLP1 with FP truncated at 525) incorporating these modifications were prepared. When added to human erythrocytes at physiologic pH, the lytic and aggregating activities of the FP analogs were much reduced over those with the wild-type FP. With resealed human erythrocyte ghosts, the lipid-mixing activities of the FP analogs were also substantially depressed over that with the wild-type FP. Combined with results from earlier studies, theoretical calculations using hydrophobic moment plot analysis and physical experiments using circular dichroism and Fourier transform infrared spectroscopy indicate that the diminished lysis and fusion noted for FP analogs may be due to altered peptide-membrane lipid interactions. These data confirm that the N-terminal gp41 domain plays critical roles in the cytolysis and fusion underlying HIV-cell infection.

Efficacy of fusion peptide homologs in blocking cell lysis and HIV-induced fusion

Contrary to earlier reports, we have found that tri- and hexapeptides analogous or homologous with segments of the 23-residue N-terminal fusion sequence (FS) of the viral transmembrane glycoprotein gp41 (residues 517-539) did not significantly inhibit HIV-1-induced syncytium formation, using an uninfected cell-infected cell fusion assay. In contrast, we found that the high molecular weight apolipoprotein A-1 and a 23-residue analog of the FS, with the phenylalanine residues at positions 524 and 527 replaced with alanine residues, were effective inhibitors. Although the tripeptides were ineffective as inhibitors of syncytium formation, we found a number of them inhibited red cell lysis induced by the synthetic peptide AVGIGALFLGFLGAAGSTMGARS (based on the HIV-1 gp41 FS). This effect was also seen with apolipoprotein A-1. The Ala524,527 analog of the fusion sequence could not be tested in this system because it was hemolytic. We concluded that the smaller peptides were effective inhibitors of hemolysis because they interfered with pore formation by the fusion sequence peptide, either by disrupting the pores or by preventing the peptide from adopting the alpha-helical conformation found in the pores. On the other hand, membrane fusion, which is a prelude to syncytium formation, has been shown to require the fusion sequence in the beta-strand conformation. We argue that small peptides would be unable to block interaction between such strands, although larger molecules, such as apolipoprotein A-1 and the Ala524,527 analog, would be able to do so and thus inhibit fusion. It seems, therefore, that a successful drug directed against the FS-cell membrane interaction stage of syncytium formation would need to be of relatively high molecular weight and complexity.

Membrane fusion induced by the HIV type 1 fusion peptide: modulation by factors affecting glycoprotein 41 activity and potential anti-HIV compounds

Peptides representing a sequence of 23 amino acid residues at the N terminus of human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp41 bind and subsequently induce fusion of large unilamellar vesicles (LUV), an activity presumably related to gp41 function in viral infection. These in vitro effects can be modulated by several factors that are known to affect HIV-1 infectivity and gp41-mediated virus-cell fusion. Peptide-induced membrane fusion but not peptide binding can be inhibited by two factors known to block gp41 activity: a polar amino acid substitution V --> E in position 2 and the presence of the N-terminal hexapeptide of gp41 in addition to the parent sequence. Whereas inclusion of the alternative gp120 receptor galactosylceramide in membranes has virtually no effect, membrane cholesterol stimulates fusion activity. In view of its putative physiological relevance, we have used the fusion activity of the peptides as a tool to evaluate the inhibitory effect of antivirals that might target this sequence. We describe three dissimilar effects: Amphotericin B inhibits in a cholesterol-independent way peptide-induced fusion but not binding, human serum albumin inhibits binding and consequently fusion, and dextran sulfate (M(r) 5000) does not affect either binding or fusion.

Mechanism of anti-HIV activity of negatively charged albumins: biomolecular interaction with the HIV-1 envelope protein gp120

A novel class of polyanionic proteins with potent anti-human immunodeficiency virus type 1 activity, the negatively charged albumins (NCAs), have been reported previously. In vitro antiviral assays established that these compounds preferentially inhibit virus-cell fusion and syncytium formation and that virus-cell binding is less affected. Here the interaction of the NCAs with synthetic peptides composed of 15-36 amino acids and corresponding to different parts of the gp120 envelope protein is described. Among the gp120 peptides tested, binding of the NCAs was observed only with the s0-called V3 loop (amino acids 296-330) and the C-terminal part of gp120. A higher number of negatively charged residues in the albumins resulted in higher binding affinities. NCAs in which, in addition to negative charges, up to 7 or 14 lactose or mannose groups were introduced, respectively did not exhibit increasing binding affinity. In contrast, mannosylated albumin containing about 14 mannose groups showed an increased binding compared with native albumin. Binding of the NCAs to the V3 and C-terminal oligopeptide was competitively inhibited by sulfated polysaccharide heparin and dextran sulfate. This finding indicates that the binding between the gp120 peptides and the NCAs is likely caused by electrostatic interactions. However, the fact that the dissociation constants of dextran sulfate and heparin are orders of magnitude larger compared with the NCAs indicates that the spatial structure of the proteins and/or hydrophobic interactions between the NCAs and the envelope protein may also be involved.

Antigen/antibody content of circulating immune complexes in HIV-infected patients

Dual infection with HIV and hepatitis B virus (HBV) is not an uncommon feature. Immunity impairment due to HIV infection can be the cause of a higher rate of HBV replication with less intensive liver damage and less effective immune response to HBV. Many HIV-infected patients have an elevated level of circulating immune complexes (CIC) in serum, throughout all stages of illness evolution. The aim of our study was to estimate p24 and HBsAg content of CIC in dually infected patients, and the prevalence of major classes of complexed antibodies (IgM and IgG). We examined 146 samples of sera from 105 HIV positive patients of the Institute for Infectious and Tropical Diseases during 1992 and 1993. On those sera we performed p24Ag and HbsAg detection, with and without prior dissociation of CIC, we determined serum level of CIC and immunoglobulin classes IgM and IgG level in sera and in polyethilenglycol (PEG) precipitates of sera. Acid dissociation of immune complexes revealed a high proportion of HIV antigen positive sera in all stages of HIV disease progression. HbsAg in serum of HIV positive patients was also found coupled in immune complexes much more frequently than in the HIV negative control group. In many instances both antigens were simultaneously found coupled in CIC. Immune complexes detected have been shown to contain both IgM and IgG immunoglobulins, while IgM antibodies were associated to immune complexes in higher proportion than IgG, compared to total serum immunoglobulins.

The amino-terminal peptide of HIV-1 glycoprotein 41 fuses human erythrocytes

The ability of synthetic peptides based on the amino-terminus of HIV-1 glycoprotein 41,000 (gp41) to fuse human erythrocytes was investigated. Previous site-directed mutagenesis studies have shown an important role for the N-terminal gp41 domain in HIV-fusion, in which replacement of hydrophobic amino acids with polar residues inhibits viral infection and syncytia formation. Here, a synthetic peptide (FP; 23 amino acid residues 519-541) corresponding to the N-terminus of HIV-1 gp41, and also a FP analog (FP526L/R) with Arg replacing Leu-526, were prepared with solid phase techniques. The lipid mixing and leakage of resealed ghosts triggered by these peptides were examined with fluorescence quenching techniques. Peptide-induced aggregation of human erythrocytes was studied using Coulter counter sizing and scanning electron microscopy (SEM). Using resealed erythrocyte ghosts at physiologic pH, FP induces rapid lipid mixing between red cell membranes at doses previously shown to hemolyze intact cells. FP also causes leakage from resealed ghosts, and promotes the formation of multicelled aggregates with whole erythrocytes. Contrarily, similar FP526L/R concentrations did not induce red cell lysis, lipid mixing, leakage or aggregation. Since the fusogenic potency of FP and FP526L/R parallels earlier gp41 mutagenesis studies showing that substitution of Arg for Leu-526 blocks fusion activity, these data suggest that the N-terminal gp41 domain in intact HIV participates in fusion.

Antivirals that target the amino-terminal domain of HIV type 1 glycoprotein 41

Functional and structural studies were made to assess whether a class of antiviral agents targets the N-terminal domain of the glycoprotein 41,000 (gp41) of human immunodeficiency virus type 1 (HIV-1). Previous experiments have shown that the amino-terminal peptide (FP-I; 23 amino acids, residues 519-541) of HIV-1 gp41 is cytolytic to both human erythrocytes (non-CD4+ cells) and Hut-78 cells (CD4+ lymphocytes). Accordingly, FP-I-induced hemolysis may be used as a surrogate assay for evaluating the role of the N-terminal gp41 domain in HIV-cell interactions. Here, we studied the blocking of FP-I-induced lysis of erythrocytes by the following anti-HIV agents: (1) IgG [i.e., anti-(518-541) IgG] raised to an immunoconjugate of Arg-FP-I, (2) apolipoprotein A-1 (apo A-1) and a peptide based on apo A-1, (3) dextran sulfate, (4) gp41 peptide (residues 637-666), and (5) anionic human serum albumins. Dose-response curves indicated that their relative potency in inhibiting FP-I-induced hemolysis was approximately correlated with their previously reported anti-HIV activity. Electron spin resonance (ESR) studies showed that FP-I spin labeled at the N-terminal alanine binds to anti-(518-541) IgG, dextran sulfate, and anionic albumins. The high in vitro antiviral activity and low cytotoxicity of these agents suggest that blocking membrane-FP-I interactions offers a novel approach for AIDS therapy or prophylaxis.