Effect of nonpolar substitutions of the conserved Phe11 in the fusion peptide of HIV-1 gp41 on its function, structure, and organization in membranes

A Frame-by-Frame Glance at Membrane Fusion Mechanisms: From Viral Infections to Fertilization

Viral entry and fertilization are distinct biological processes that share a common mechanism: membrane fusion. In viral entry, enveloped viruses attach to the host cell membrane, triggering a series of conformational changes in the viral fusion proteins. This results in the exposure of a hydrophobic fusion peptide, which inserts into the host membrane and brings the viral and host membranes into close proximity. Subsequent structural rearrangements in opposing membranes lead to their fusion. Similarly, membrane fusion occurs when gametes merge during the fertilization process, though the exact mechanism remains unclear. Structural biology has played a pivotal role in elucidating the molecular mechanisms underlying membrane fusion. High-resolution structures of the viral and fertilization fusion-related proteins have provided valuable insights into the conformational changes that occur during this process. Understanding these mechanisms at a molecular level is essential for the development of antiviral therapeutics and tools to influence fertility. In this review, we will highlight the biological importance of membrane fusion and how protein structures have helped visualize both common elements and subtle divergences in the mechanisms behind fusion; in addition, we will examine the new tools that recent advances in structural biology provide researchers interested in a frame-by-frame understanding of membrane fusion.

HIV-1 anchor inhibitors and membrane fusion inhibitors target distinct but overlapping steps in virus entry

HIV-1 entry into cells is mediated by the envelope glycoprotein (Env) and represents an attractive target for therapeutic intervention. Two drugs that inhibit HIV entry are approved for clinical use: the membrane fusion-inhibitor T20 (Fuzeon, enfuvirtide) and the C-C chemokine receptor type 5 (CCR5) blocker maraviroc (Selzentry). Another class of entry inhibitors supposedly target the fusion peptide (FP) and are termed anchor inhibitors. These include the VIRIP peptide and VIRIP derivatives such as VIR165, VIR353, and VIR576. Here, we investigated the mechanism of inhibition by VIR165. We show that substitutions within the FP modulate sensitivity to VIR165, consistent with the FP being the drug target. Our results also revealed that VIR165 acts during an intermediate post-CD4–binding entry step that is overlapping but not identical to the step inhibited by fusion inhibitors such as T20. We found that some but not all resistance mutations to heptad repeat 2 (HR2)-targeting fusion inhibitors can provide cross-resistance to VIR165. In contrast, resistance mutations in the HR1-binding site for the fusion inhibitors did not cause cross-resistance to VIR165. However, Env with mutations located outside this binding site and thought to affect fusion kinetics, exhibited decreased sensitivity to VIR165. Although we found a strong correlation between Env stability and resistance to HR2-based fusion inhibitors, such correlation was not observed for Env stability and VIR165 resistance. We conclude that VIRIP analogs target the FP during an intermediate, post-CD4–binding entry step that overlaps with but is distinct from the step(s) inhibited by HR2-based fusion inhibitors.

Effect of 25-hydroxycholesterol in viral membrane fusion: Insights on HIV inhibition

Recently, it was demonstrated that 25-hydroxycholesterol (25HC), an oxidized cholesterol derivative, inhibits human immunodeficiency virus type 1 (HIV) entry into its target cells. However, the mechanisms involved in this action have not yet been established. The aim of this work was to study the effects of 25HC in biomembrane model systems and at the level of HIV fusion peptide (HIV-FP). Integration of different biophysical approaches was made in the context of HIV fusion process, to clarify the changes at membrane level due to the presence of 25HC that result in the suppressing of viral infection. Lipid vesicles mimicking mammalian and HIV membranes were used on spectroscopy assays and lipid monolayers in surface pressure studies. Peptide-induced lipid mixing assays were performed by Förster resonance energy transfer to calculate fusion efficiency. Liposome fusion is reduced by 50% in the presence of 25HC, comparatively to cholesterol. HIV-FP conformation was assessed by infrared assays and it relies on sterol nature. Anisotropy, surface pressure and dipole potential assays indicate that the conversion of cholesterol in 25HC leads to a loss of the cholesterol modulating effect on the membrane. With different biophysical techniques, we show that 25HC affects the membrane fusion process through the modification of lipid membrane properties, and by direct alterations on HIV-FP structure. The present data support a broad antiviral activity for 25HC.

Structure-Related Roles for the Conservation of the HIV-1 Fusion Peptide Sequence Revealed by Nuclear Magnetic Resonance

Despite extensive characterization of the human immunodeficiency virus type 1 (HIV-1) hydrophobic fusion peptide (FP), the structure-function relationships underlying its extraordinary degree of conservation remain poorly understood. Specifically, the fact that the tandem repeat of the FLGFLG tripeptide is absolutely conserved suggests that high hydrophobicity may not suffice to unleash FP function. Here, we have compared the nuclear magnetic resonance (NMR) structures adopted in nonpolar media by two FP surrogates, wtFP-tag and scrFP-tag, which had equal hydrophobicity but contained wild-type and scrambled core sequences LFLGFLG and FGLLGFL, respectively. In addition, these peptides were tagged at their C-termini with an epitope sequence that folded independently, thereby allowing Western blot detection without interfering with FP structure. We observed similar α-helical FP conformations for both specimens dissolved in the low-polarity medium 25% (v/v) 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), but important differences in contact with micelles of the membrane mimetic dodecylphosphocholine (DPC). Thus, whereas wtFP-tag preserved a helix displaying a Gly-rich ridge, the scrambled sequence lost in great part the helical structure upon being solubilized in DPC. Western blot analyses further revealed the capacity of wtFP-tag to assemble trimers in membranes, whereas membrane oligomers were not observed in the case of the scrFP-tag sequence. We conclude that, beyond hydrophobicity, preserving sequence order is an important feature for defining the secondary structures and oligomeric states adopted by the HIV FP in membranes.

The chain order of binary unsaturated lipid bilayers modulated by aromatic-residue-containing peptides: an ATR-FTIR spectroscopy study

It highlights the importance of aromatic residues in influencing peptide binding to the membrane, demonstrates that the stability of the membranes depends on the lipid composition and the sequence, structural context, and orientation of the peptides.

Different HIV-1 env frames: gp120 and ASP (antisense protein) biosynthesis, and theirs co-variation tropic amino acid signatures in X4- and R5-viruses

Antisense protein (ASP) is the new actor of viral life of Human Immunodeficiency Virus type 1 (HIV-1) although proposed above 20 years ago. The asp ORF is into complementary strand of the gp120/gp41 junction of env gene. The ASP biological role remains little known. Knowing the Env markers of viral tropism, a dataset of sequences (660 strains) was used to analyze the hypothetical ASP involvement in CCR5 (R5) and/or CXCR4 (X4) co-receptor interaction. Preliminarily, prevalence of ASP and gp120_V3 mutations was performed; following association among mutations were elaborate. The classical V3 tropic-signatures were confirmed, and 36 R5- and 22 X4-tropic ASP mutations were found. Moreover, by analyzing the ASP sequences, 36 out of 179 amino acid positions significantly associated with different co-receptor usage were found. Several statistically significant associations between gp120_V3 and ASP mutations were observed. The dendrogram showed the existence of a cluster associated with R5-usage and a large cluster associated with X4-usage. These results show that gp120_V3 and specific amino acid changes in ASP are associated together with CXCR4 and/or CCR5-usage. These findings implement previous observations on unclear ASP functions. J. Med. Virol. 89:112-122, 2017. © 2016 Wiley Periodicals, Inc.

Functional characteristics of the natural polymorphisms of HIV-1 gp41 in HIV-1 isolates from enfuvirtide-naïve Korean patients

HIV-1 gp41 plays a key role in viral entry. The insertion of Thr at position 4 and Met/Val/Phe substitutions at position 7 are frequently observed in the fusion peptide (FP) motif of gp41 without major enfuvirtide resistance associated with mutation in heptad repeats 1/2 (HR1/2) of HIV-1 isolates from Korean patients. Here, the influence of these mutations on their biological function was evaluated by employing HIV-1 variants with mutant FPs as shown previously and with recombinant HIV-1 using the env genes of 20 HIV-1 isolates from Korean patients. In an infectivity assay, all FP mutants showed lower infectivity than the wild-type NL4-3. In particular, the substitutions at position 7 led to much greater reductions in infectivity than the insertions at position 4. Nevertheless, the replication kinetics of most mutants were similar to those of the wild type, except that the FP mutants with an Ile insertion at position 4 and a Phe substitution at position 7 showed reduced replication. Moreover, most point mutants showed lower IC50 values for enfuvirtide than the wild type, whereas the L7M substitution resulted in a slightly increased IC50 value. The infectivity using the HIV-1 env recombinant viruses decreased in 14 cases but increased slightly in six cases compared with the wild type. Most recombinants were more susceptible to enfuvirtide than the wild type, except for three recombinants that showed slight resistance. Our findings may help to explain the potential mechanisms corresponding to the natural polymorphism of gp41 and to predict the efficiency of enfuvirtide in treatment of HIV-1-infected patients in Korea.

pH-dependent vesicle fusion induced by the ectodomain of the human immunodeficiency virus membrane fusion protein gp41: Two kinetically distinct processes and fully-membrane-associated gp41 with predominant β sheet fusion peptide conformation

The gp41 protein of the Human Immunodeficiency Virus (HIV) catalyzes fusion between HIV and host cell membranes. The ~180-residue ectodomain of gp41 is outside the virion and is the most important gp41 region for membrane fusion. The ectodomain consists of an apolar fusion peptide (FP) region hypothesized to bind to the host cell membrane followed by N-heptad repeat (NHR), loop, and C-heptad repeat (CHR) regions. The present study focuses on the large gp41 ectodomain constructs "Hairpin" (HP) containing NHR+loop+CHR and "FP-Hairpin" (FP-HP) containing FP+NHR+loop+CHR. Both proteins induce rapid and extensive fusion of anionic vesicles at pH4 where the protein is positively-charged but do not induce fusion at pH7 where the protein is negatively charged. This observation, along with lack of fusion of neutral vesicles at either pH supports the significance of attractive protein/membrane electrostatics in fusion. There are two kinetically distinct fusion processes at pH4: (1) a faster ~100 ms⁻¹ process with rate strongly positively correlated with vesicle charge; and (2) a slower ~5 ms⁻¹ process with extent strongly inversely correlated with this charge. The slower process may be more physiologically relevant because HIV/host cell fusion occurs at physiologic pH with gp41 restricted to the narrow region between the two membranes. Previous solid-state NMR (SSNMR) of membrane-associated FP-HP has supported protein oligomers with FP's in an intermolecular antiparallel sheet. There was an additional population of molecules with α helical FPs and the samples likely contained a mixture of membrane-bound and -unbound proteins. For the present study, samples were prepared with fully membrane-bound FP-HP and subsequent SSNMR showed dominant β FP conformation at both low and neutral pH. SSNMR also showed close contact of the FP with the lipid headgroups at both low and neutral pH whereas the NHR+CHR regions had contact at low pH and were more distant at neutral pH, consistent with the protein/membrane electrostatics.

The three lives of viral fusion peptides

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The presence of a fusion peptide (FP) is a hallmark of viral fusion glycoproteins.

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Structure–function relationships underlying FP conservation remain greatly unknown.

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FPs establish interactions satisfying their folding within pre-fusion glycoproteins.

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Upon fusion activation FPs insert into and restructure target membranes.

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FPs can finally combine with transmembrane domains to form integral membrane bundles.

Fusion peptides comprise conserved hydrophobic domains absolutely required for the fusogenic activity of glycoproteins from divergent virus families. After 30 years of intensive research efforts, the structures and functions underlying their high degree of sequence conservation are not fully elucidated. The long-hydrophobic viral fusion peptide (VFP) sequences are structurally constrained to access three successive states after biogenesis. Firstly, the VFP sequence must fulfill the set of native interactions required for (meta) stable folding within the globular ectodomains of glycoprotein complexes. Secondly, at the onset of the fusion process, they get transferred into the target cell membrane and adopt specific conformations therein. According to commonly accepted mechanistic models, membrane-bound states of the VFP might promote the lipid bilayer remodeling required for virus-cell membrane merger. Finally, at least in some instances, several VFPs co-assemble with transmembrane anchors into membrane integral helical bundles, following a locking movement hypothetically coupled to fusion-pore expansion. Here we review different aspects of the three major states of the VFPs, including the functional assistance by other membrane-transferring glycoprotein regions, and discuss briefly their potential as targets for clinical intervention.

Characterization of Gp41 polymorphisms in the fusion peptide domain and T-20 (Enfuvirtide) resistance-associated regions in Korean HIV-1 isolates

HIV-1 gp41 is an envelope protein that plays an essential role in virus entry. The mutation of gp41 affects HIV-1 entry and susceptibility to the fusion inhibitor T-20. Therefore, we analyzed the natural polymorphism of gp41 of 163 HIV-1 isolates from T-20-naïve Koreans infected with HIV-1. This study of gp41 polymorphisms showed that insertions in the fourth threonine (74.8%) and L7M substitutions (85.3%) were more frequent in the fusion peptide motif in Korean HIV-1 isolates compared with those from other countries. Minor T-20 resistance mutations such as L45M (1.2%), N126K (1.2%), and E137K (6.7%) were detected, but the critical T-20 resistance mutations were not detected in the gp41 HR1 and HR2 regions. In addition, the N42S mutation (12.9%) associated with T-20 hypersusceptibility was detected at a high frequency. These results may serve as useful data for studies considering T-20 for use in the development of a more effective anti-retroviral treatment in Korea.

ATR-FTIR studies in pore forming and membrane induced fusion peptides

Infrared (IR) spectroscopy has been shown to be very reliable for the characterization, identification and quantification of structural data. Particularly, the Attenuated Total Reflectance (ATR) technique which became one of the best choices to study the structure and organization of membrane proteins and membrane-bound peptides in biologically relevant membranes. An important advantage of IR spectroscopy is its ability to analyze material under a very wide range of conditions including solids, liquids and gases. This method allows elucidation of component secondary structure elements of a peptide or protein in a global manner, and by using site specific isotope labeling allows determination of specific regions. A few advantages in using ATR-FTIR spectroscopy include; a relatively simple technique, allow the determination of peptide orientation in the membrane, allow the determination of secondary structures of very small peptides, and importantly, the method is sensitive to isotopic labeling on the scale of single amino acids. Many studies were reported on the use of ATR-FTIR spectroscopy in order to study the structure and orientation of membrane bound hydrophobic peptides and proteins. The list includes native and de-novo designed peptides, as well as those derived from trans-membrane domains of various receptors (TMDs). The present review will focus on several examples that demonstrate the potential and the simplicity in using the ATR-FTIR approach to determine secondary structures of proteins and peptides when bound, inserted, and oligomerized within membranes. The list includes (i) a channel forming protein/peptide: the Ca(2+) channel phospholamban, (ii) a cell penetrating peptide, (iii) changes in the structure of a transmembrane domain located within ordered and non-ordered domains, and (iv) isotope edited FTIR to directly assign structure to the membrane associated fusion peptide in context of a Key gp41 Structural Motif. Importantly, a unique advantage of infrared spectroscopy is that it allows a simultaneous study of the structure of lipids and proteins in intact biological membranes without an introduction of foreign perturbing probes. Because of the long IR wavelength, light scattering problems are virtually non-existent. This allows the investigation of highly aggregated materials or large membrane fragments. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

Intramolecular interactions within the human immunodeficiency virus-1 gp41 loop region and their involvement in lipid merging

The human immunodeficiency virus utilizes its gp41 fusion protein to mediate virus-cell membrane fusion. The conserved disulfide loop region in the gp41 hairpin conformation reverses the protein chain, such that the N-terminal heptad repeat and the C-terminal heptad repeat regions interact to form the six-helix bundle. Hence, it is conceivable that the sequential folded N- and C-terminal parts of the loop region also interact. We show that the N- and C-terminal parts of the loop preferably form disulfide-bonded heterodimers with slow oxidation kinetics. Furthermore, when the two parts were linked to a single polypeptide to form the full-length loop, only an intramolecular disulfide-bonded loop was formed. Fluorescence studies revealed that an interaction takes place between the N- and C-terminal parts of the loop in solution, which was sustained in membranes. Functionally, only a combination of the N- and C-loop parts induced lipid mixing of model liposomes, the level of which increased 8-fold when they were connected to a single polypeptide chain. In both cases, the activity was independent of the oxidation state of the cysteines. Overall, the data (i) provide evidence of a specific interaction between the N- and C-terminal parts of the loop, which can further stabilize gp41 hairpin conformation, and (ii) suggest that the interaction between the N- and C-terminal parts of the loop is sufficient to induce lipid merging without forming a disulfide bond.

Irregular structure of the HIV fusion peptide in membranes demonstrated by solid-state NMR and MD simulations

To better understand peptide-induced membrane fusion at a molecular level, we set out to determine the structure of the fusogenic peptide FP23 from the HIV-1 protein gp41 when bound to a lipid bilayer. An established solid-state (19)F nuclear magnetic resonance (NMR) approach was used to collect local orientational constraints from a series of CF(3)-phenylglycine-labeled peptide analogues in macroscopically aligned membranes. Fusion assays showed that these (19)F-labels did not significantly affect peptide function. The NMR spectra were characteristic of well-behaved samples, without any signs of heterogeneity or peptide aggregation at 1:300 in 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC). We can conclude from these NMR data that FP23 has a well-defined (time-averaged) conformation and undergoes lateral diffusion in the bilayer plane, presumably as a monomer or small oligomer. Attempts to evaluate its conformation in terms of various secondary structures, however, showed that FP23 does not form any type of regular helix or β-strand. Therefore, all-atom molecular dynamics (MD) simulations were carried out using the orientational NMR constraints as pseudo-forces to drive the peptide into a stable alignment and structure. The resulting picture suggests that FP23 can adopt multiple β-turns and insert obliquely into the membrane. Such irregular conformation explains why the structure of the fusion peptide could not be reliably determined by any biophysical method so far.

Comparative analysis of membrane-associated fusion peptide secondary structure and lipid mixing function of HIV gp41 constructs that model the early pre-hairpin intermediate and final hairpin conformations

Fusion between viral and host cell membranes is the initial step of human immunodeficiency virus infection and is mediated by the gp41 protein, which is embedded in the viral membrane. The approximately 20-residue N-terminal fusion peptide (FP) region of gp41 binds to the host cell membrane and plays a critical role in fusion catalysis. Key gp41 fusion conformations include an early pre-hairpin intermediate (PHI) characterized by extended coiled-coil structure in the region C-terminal of the FP and a final hairpin state with compact six-helix bundle structure. The large "N70" (gp41 1-70) and "FP-Hairpin" constructs of the present study contained the FP and respectively modeled the PHI and hairpin conformations. Comparison was also made to the shorter "FP34" (gp41 1-34) fragment. Studies were done in membranes with physiologically relevant cholesterol content and in membranes without cholesterol. In either membrane type, there were large differences in fusion function among the constructs with little fusion induced by FP-Hairpin, moderate fusion for FP34, and very rapid fusion for N70. Overall, our findings support acceleration of gp41-induced membrane fusion by early PHI conformation and fusion arrest after folding to the final six-helix bundle structure. FP secondary structure at Leu7 of the membrane-associated constructs was probed by solid-state nuclear magnetic resonance and showed populations of molecules with either beta-sheet or helical structure with greater beta-sheet population observed for FP34 than for N70 or FP-Hairpin. The large differences in fusion function among the constructs were not obviously correlated with FP secondary structure. Observation of cholesterol-dependent FP structure for fusogenic FP34 and N70 and cholesterol-independent structure for non-fusogenic FP-Hairpin was consistent with membrane insertion of the FP for FP34 and N70 and with lack of insertion for FP-Hairpin. Membrane insertion of the FP may therefore be associated with the early PHI conformation and FP withdrawal with the final hairpin conformation.

Current peptide HIV type-1 fusion inhibitors

There are now 26 antiretroviral drugs and 6 fixed-dose combinations, including reverse transcriptase inhibitors, protease inhibitors, integrase inhibitors and fusion (or entry) inhibitors, approved by the US Food and Drug Administration for clinical use. Although they are clinically effective when used in combination, none of the existing drugs are considered ideal because of toxic side effects and the ascendance of inducing drug-resistant mutants. Development of new antiviral agents is essential. In the past decades, there has been great progress in understanding the structure of HIV type-1 (HIV-1) gp41 and the mechanism of HIV-1 entry into host cells. This opened up a promising avenue for rationally designed agents to interfere with this process. A number of fusion inhibitors have been developed to block HIV-1 replication. Enfuvirtide (T20) was one of those approved for clinical use. This signalled a new era in AIDS therapeutics. It is a synthetic polypeptide with potent inhibitory activity against HIV-1 infection. However, it is sensitive to proteolytic digestion and resistant virus strains are easily induced with multiple clinical use. One of the directions in designing new fusion inhibitors is to overcome these shortages. In the past years, large numbers of promising fusion inhibitory peptides have emerged. The antiviral activities are more potent or they can act differently from that of T20. Some of these new compounds have great potential to be further developed as therapeutic agents. This article reviewed some recent developments of these peptides and the possible role in anti-HIV-1 therapy.

HIV fusion peptide and its cross-linked oligomers: efficient syntheses, significance of the trimer in fusion activity, correlation of beta strand conformation with membrane cholesterol, and proximity to lipid headgroups

For enveloped viruses such as HIV, an approximately 20-residue N-terminal fusion peptide domain in the envelope protein binds to target cell membranes and plays a key role in fusion between the viral and cellular membranes during infection. The chemically synthesized HIV fusion peptide (HFP) catalyzes fusion between membrane vesicles and is a useful model system for understanding some aspects of HIV fusion. Previous studies have shown a common trimeric state for the envelope protein from several different viruses, including HIV, and in this study, practical high-yield syntheses are reported for HFP monomer (HFPmn) and chemically cross-linked HFP dimer (HFPdm), trimer (HFPtr), and tetramer (HFPte). The vesicle fusion rates per strand were ordered as follows: HFPmn < HFPdm < HFPtr approximately HFPte. This suggested that HFPtr is the smallest catalytically efficient oligomer. Solid-state NMR measurements of (13)CO chemical shifts were carried out in constructs labeled at either Ala-6 or Ala-15. For all constructs associated with cholesterol-containing membranes, the chemical shifts of both residues correlated with beta strand conformation while association with membranes without cholesterol resulted in a mixture of helical and beta strand conformations. The dependence of fusion rate on oligomer size is independent of membrane cholesterol content, so one interpretation of the data is fusion activity of both helical and beta strand conformations. Membrane location may be a determinant of fusion activity, and for all constructs in both conformations, a large fraction of the Ala-15 (13)CO groups were 5-6 A from the (31)P atoms in the lipid headgroups, while the Ala-6 (13)CO groups were more distant.

Fatty acids can substitute the HIV fusion peptide in lipid merging and fusion: an analogy between viral and palmitoylated eukaryotic fusion proteins

Various fusion proteins from eukaryotes and viruses share structural similarities such as a coiled coil motif. However, compared with eukaryotic proteins, a viral fusion protein contains a fusion peptide (FP), which is an N-terminal hydrophobic fragment that is primarily involved in directing fusion via anchoring the protein to the target cell membrane. In various eukaryotic fusion proteins the membrane targeting domain is cysteine-rich and must undergo palmitoylation prior to the fusion process. Here we examined whether fatty acids can replace the FP of human immunodeficiency virus type 1 (HIV-1), thereby discerning between the contributions of the sequence versus hydrophobicity of the FP in the lipid-merging process. For that purpose, we structurally and functionally characterized peptides derived from the N terminus of HIV fusion protein - gp41 in which the FP is lacking or replaced by fatty acids. We found that fatty acid conjugation dramatically enhanced the capability of the peptides to induce lipid mixing and aggregation of zwitterionic phospholipids composing the outer leaflet of eukaryotic cell membranes. The enhanced effect of the acylated peptides on membranes was further supported by real-time atomic force microscopy (AFM) showing nanoscale holes in zwitterionic membranes. Membrane-binding experiments revealed that fatty acid conjugation did not increase the affinity of the peptides to the membrane significantly. Furthermore, all free and acylated peptides exhibited similar alpha-helical structures in solution and in zwitterionic membranes. Interestingly, the fusogenic active conformation of N36 in negatively charged membranes composing the inner leaflet of eukaryotic cells is beta-sheet. Apparently, N-terminal heptad repeat (NHR) can change its conformation as a response to a change in the charge of the membrane head group. Overall, the data suggest an analogy between the eukaryotic cysteine-rich domains and the viral fusion peptide, and mark the hydrophobic nature of FP as an important characteristic for its role in lipid merging.

Chemical shift assignment and structural plasticity of a HIV fusion peptide derivative in dodecylphosphocholine micelles

A "HFPK3" peptide containing the 23 residues of the human immunodeficiency virus (HIV) fusion peptide (HFP) plus three non-native C-terminal lysines was studied in dodecylphosphocholine (DPC) micelles with 2D 1H NMR spectroscopy. The HFP is at the N-terminus of the gp41 fusion protein and plays an important role in fusing viral and target cell membranes which is a critical step in viral infection. Unlike HFP, HFPK3 is monomeric in detergent-free buffered aqueous solution which may be a useful property for functional and structural studies. H alpha chemical shifts indicated that DPC-associated HFPK3 was predominantly helical from I4 to L12. In addition to the highest-intensity crosspeaks used for the first chemical shift assignment (denoted I), there were additional crosspeaks whose intensities were approximately 10% of those used for assignment I. A second assignment (II) for residues G5 to L12 as well as a few other residues was derived from these lower-intensity crosspeaks. Relative to the I shifts, the II shifts were different by 0.01-0.23 ppm with the largest differences observed for HN. Comparison of the shifts of DPC-associated HFPK3 with those of detergent-associated HFP and HFP derivatives provided information about peptide structures and locations in micelles.

Solid-state nuclear magnetic resonance measurements of HIV fusion peptide to lipid distances reveal the intimate contact of beta strand peptide with membranes and the proximity of the Ala-14-Gly-16 region with lipid headgroups

Human immunodeficiency virus (HIV) infection begins with fusion between viral and host cell membranes and is catalyzed by the HIV gp41 fusion protein. The approximately 20 N-terminal apolar residues of gp41 are called the HIV fusion peptide (HFP), interact with the host cell membrane, and play a key role in fusion. In this study, the membrane location of peptides which contained the HFP sequence (AVGIGALFLGFLGAAGSTMGARS) was probed in samples containing either only phospholipids or phospholipids and cholesterol. Four HFPs were examined which each contained 13CO labeling at three sequential residues between G5 and G16. The 13CO chemical shifts indicated that HFP had predominant beta strand conformation over the labeled residues in the samples. The internuclear distances between the HFP 13CO groups and the lipid 31P atoms were measured using solid-state nuclear magnetic resonance rotational-echo double-resonance experiments. The shortest 13CO-31P distances of 5-6 A were observed for HFP labeled between A14 and G16 and correlated with intimate association of beta strand HFP and membranes. These results were confirmed with measurements using HFPs singly labeled with 13CO at A6 or A14. To our knowledge, these data are the first measurements of distances between HIV fusion peptide nuclei and lipid P, and qualitative models of the membrane location of oligomeric beta strand HFP which are consistent with the experimental data are presented. Observation of intimate contact between beta strand HFP and membranes provides a rationale for further investigation of the relationship between structure and fusion activity for this conformation.

A critical evaluation of the conformational requirements of fusogenic peptides in membranes

It is generally assumed that fusogenic peptides would require a certain conformation, which triggers or participates in the rate-determining step of membrane fusion. Previous structure analyses of the viral fusion peptide from gp41 of HIV-1 have yielded contradictory results, showing either an alpha-helical or a beta-stranded conformation under different conditions. To find out whether either of these conformations is relevant in the actual fusion process, we have placed sterically demanding substitutions into the fusion peptide FP23 to prevent or partially inhibit folding and self-assembly. A single substitution of either D- or L-CF(3)-phenylglycine was introduced in different positions of the sequence, and the capability of these peptide analogues to fuse large unilamellar vesicles was monitored by lipid mixing and dynamic light scattering. If fusion proceeds via a beta-stranded oligomer, then the D- and L-epimers are expected to differ systematically in their activity, since the D-epimers should be unable to form beta-structures due to sterical hindrance. If an alpha-helical conformation is relevant for fusion, then the D-epimers would be slightly disfavoured compared to the L-forms, hence a small systematic difference in fusion activity should be observed. Interestingly, we find that (1) all D- and L-epimers are fusogenically active, though to different extents compared to the wild type, and--most importantly--(ii) there is no systematic preference for either the D- or L-forms. We therefore suggest that a well-structured alpha-helical peptide conformation or a beta-stranded oligomeric assembly can be excluded as the rate-determining state. Instead, fusion appears to involve conformationally disordered peptides with a pronounced structural plasticity.

Conformational flexibility and strand arrangements of the membrane-associated HIV fusion peptide trimer probed by solid-state NMR spectroscopy

The human immunodeficiency virus (HIV) fusion peptide (HFP) is the N-terminal apolar region of the HIV gp41 fusion protein and interacts with target cell membranes and promotes membrane fusion. The free peptide catalyzes vesicle fusion at least to the lipid mixing stage and serves as a useful model fusion system. For gp41 constructs which lack the HFP, high-resolution structures show trimeric protein and suggest that at least three HFPs interact with the membrane with their C-termini in close proximity. In addition, previous studies have demonstrated that HFPs which are cross-linked at their C-termini to form trimers (HFPtr) catalyze fusion at a rate which is 15-40 times greater than that of non-cross-linked HFP. In the present study, the structure of membrane-associated HFPtr was probed with solid-state nuclear magnetic resonance (NMR) methods. Chemical shift and intramolecular (13)CO-(15)N distance measurements show that the conformation of the Leu-7 to Phe-11 region of HFPtr has predominant helical conformation in membranes without cholesterol and beta strand conformation in membranes containing approximately 30 mol % cholesterol. Interstrand (13)CO-(13)CO and (13)CO-(15)N distance measurements were not consistent with an in-register parallel strand arrangement but were consistent with either (1) parallel arrangement with adjacent strands two residues out-of-register or (2) antiparallel arrangement with adjacent strand crossing between Phe-8 and Leu-9. Arrangement 1 could support the rapid fusion rate of HFPtr because of placement of the apolar N-terminal regions of all strands on the same side of the oligomer while arrangement 2 could support the assembly of multiple fusion protein trimers.

The HIV-neutralizing monoclonal antibody 4E10 recognizes N-terminal sequences on the native antigen

Characterization of the epitope recognized by the broadly neutralizing anti-HIV Ab 4E10 has, heretofore, focused on a linear sequence from the gp41 pretransmembrane region (PTMR). Attempts to generate neutralizing Abs based on this linear epitope sequence have been unsuccessful. We have characterized the antigenic determinants on recombinant glycosylated full-length Ags, and nonglycosylated and truncated Ags recognized by 4E10 using epitope extraction and excision assays in conjunction with MALDI mass spectrometry. The mAb recognized the peptides (34)LWVTVYYGVPVWK(46) and (512)AVGIGAVFLGFLGAAGSTMGAASMTLTVQAR(542) located at the N-terminal region of gp120 and gp41, respectively. Immunoassays verified AV(L/M)FLGFLGAA as the gp41 epitope core. Recognition of the peptide from the gp41 PTMR was detected only in constructs in which the N termini of the mature envelope proteins were missing. In this region, the epitope core is located in the sequence (672)WFDITNWLWY(681). We hypothesize that the hydrophobic surface of the paratope functions as a "trap" for the viral sequences, which are responsible for insertion into the host cell membrane. As the N-terminal region of gp120, the fusogenic peptide of gp41, and the PTMR of gp41 show high sequence homology among various HIV strains, this model is consistent with the broadly neutralizing capabilities of 4E10.

Membrane-perturbing properties of three peptides corresponding to the ectodomain of hepatitis C virus E2 envelope protein

Based on the predicted capacity to interact with membranes at the interface, we have found three regions in the ectodomain of the hepatitis C virus envelope glycoprotein E2 (430-449, 543-560 and 603-624) with the ability to destabilize membranes. Three peptides corresponding to the sequence of these regions have been synthesized and their interaction with liposomes have been characterized. The three peptides were able to insert deeply into the hydrophobic core of negatively charged phospholipids as stated by fluorescence depolarization of the probe 1,6-diphenyl-1,3,5-hexatriene. Peptides E2(430-449) and E2(603-624) were able to induce aggregation of phosphatidylglycerol vesicles in a concentration-dependent manner both at neutral and acidic pH while peptide E2(543-560) did not induce any increase of optical density at 360 nm in the concentration range studied. The three peptides induced lipid mixing and the release of the internal contents in a dose-dependent manner when acidic phospholipids were used. Fourier transformed infrared spectroscopy indicated that the peptides adopted mainly a beta-sheet conformation which is not modified by the presence of acidic phospholipids. Taken together, our results point out to the involvement of these three regions in the fusion mechanism of HCV at the plasma membrane level.

The N-terminal 12 Residue Long Peptide of HIV gp41 is the Minimal Peptide Sufficient to Induce Significant T-cell-like Membrane Destabilization in Vitro

Here, we predicted the minimal N-terminal fragment of gp41 required to induce significant membrane destabilization using IMPALA. This algorithm is dedicated to predict peptide interaction with a membrane. We based our prediction of the minimal fusion peptide on the tilted peptide theory. This theory proposes that some protein fragments having a peculiar distribution of hydrophobicity adopt a tilted orientation at a hydrophobic/hydrophilic interface. As a result of this orientation, tilted peptides should disrupt the interface. We analysed in silico the membrane-interacting properties of gp41 N-terminal peptides of different length derived from the isolate BRU and from an alignment of 710 HIV strains available on the Los Alamos National Laboratory. Molecular modelling results indicated that the 12 residue long peptide should be the minimal fusion peptide. We then assayed lipid-mixing and leakage of T-cell-like liposomes with N-terminal peptides of different length as first challenge of our predictions. Experimental results confirmed that the 12 residue long peptide is necessary and sufficient to induce membrane destabilization to the same extent as the 23 residue long fusion peptide. In silico analysis of some fusion-incompetent mutants presented in the literature further revealed that they cannot insert into a modelled membrane correctly tilted. According to this work, the tilted peptide model appears to explain at least partly the membrane destabilization properties of HIV fusion peptide.

Structure and orientation study of fusion peptide FP23 of gp41 from HIV-1 alone or inserted into various lipid membrane models (mono-, bi- and multibi-layers) by FT-IR spectroscopies and Brewster angle microscopy

In the present work, we study the structure and the orientation of the 23 N-terminal peptide of the HIV-1 gp 41 protein (AVGIGALFLGFLGAAGSTMGARS) called FP23. The behaviour of FP23 was investigated alone at the air/water interface and inserted into various lipid model systems: in monolayer or multibilayers of a DOPC/cholesterol/DOPE/DOPG (6/5/3/2) and in a DMPC bilayer. PMIRRAS and polarized ATR spectroscopy coupled with Brewster angle microscopy and spectral simulations were used to precisely determine the structure and the orientation of the peptide in its environment as well as the lipid perturbations induced by the FP23 insertion. The infra-red results show the structural polymorphism of the FP23 and its ability to transit quasi irreversibly from an alpha-helix to antiparallel beta-sheets. At the air/water interface, the transition is induced by compression of the peptide alone and is modulated by compression and lipid to peptide ratio (Ri) when FP23 is inserted into a lipid monolayer. In multibilayers and in a single bilayer, there is coexistence in quasi equal proportions of alpha-helix and antiparallel beta-sheets of FP23 at low peptide content (Ri=100, 200) while antiparallel beta-sheets are predominant at high FP23 concentration (Ri=50). In (multi)bilayer systems, evaluation of dichroic ratios and sprectral simulations show that both the alpha-helix and the antiparallel beta-sheets are tilted at diluted FP23 concentrations (tilt angle of alpha-helix with respect to the normal of the interface=36.5+/-3.0 degrees for FP23 in multibilayers of DOPC/Chol/DOPE/DOPG at Ri=200 and 39.0+/-5.0 degrees in a single bilayer of DMPC at Ri=100 and tilt angle of the beta-sheets=36.0+/-2.0 degrees for the beta-sheets in multibilayers and 30.0+/-2.0 degrees in the lipid bilayer). In parallel, the FP23 induces an increase of the lipid chain disorder which shows both by an increase of the methylene stretching frequencies and an increase of the average C-C-C angle of the acyl chains. At high FP23 content (Ri=50), the antiparallel beta-sheets induce a complete disorganization of the lipid chains in (multi)bilayers.

The HIV Fusion Peptide Adopts Intermolecular Parallel β-Sheet Structure in Membranes when Stabilized by the Adjacent N-Terminal Heptad Repeat: A 13C FTIR Study

The HIV gp41 protein mediates fusion with target host cells. The region primarily involved in directing fusion, the fusion peptide (FP), is poorly understood at the level of structure and function due to its toxic effect in expression systems. To overcome this, we used a synthetic approach to generate the N70 construct, whereby the FP is stabilized in context of the adjacent auto oligomerization domain. The amide I profile of unlabeled N70 in membranes reveals prominent alpha-helical contribution, along with significant beta-structure. By truncating the N terminus (FP region) of N70, beta-structure is eliminated, suggesting that the FP adopts a beta-structure in membranes. To assess this directly, (13)C Fourier-transformed infra-red analysis was carried out to map secondary structure of the 16 N-terminal hydrophobic residues of the fusion peptide (FP16). The (13)C isotope shifted absorbance of the FP was filtered from the global secondary structure of the 70 residue construct (N70). On the basis of the peak shift induced by the (13)C-labeled residues of FP16, we directly assign beta-sheet structure in ordered membranes. A differential labeling scheme in FP16 allows us to distinguish the type of beta-sheet structure as parallel. Dilution of each FP16-labeled N70 peptide, by mixing with unlabeled N70, shows directly that the FP16 beta-strand region self-assembles. We discuss our structural findings in the context of the prevailing gp41 fusion paradigm. Specifically, we address the role of the FP region in organizing supramolecular gp41 assembly, and we also discuss the mechanism by which exogenous, free FP constructs inhibit gp41-induced fusion.

Identification and characterization of the putative fusion peptide of the severe acute respiratory syndrome-associated coronavirus spike protein

Severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is a newly identified member of the family Coronaviridae and poses a serious public health threat. Recent studies indicated that the SARS-CoV viral spike glycoprotein is a class I viral fusion protein. A fusion peptide present at the N-terminal region of class I viral fusion proteins is believed to initiate viral and cell membrane interactions and subsequent fusion. Although the SARS-CoV fusion protein heptad repeats have been well characterized, the fusion peptide has yet to be identified. Based on the conserved features of known viral fusion peptides and using Wimley and White interfacial hydrophobicity plots, we have identified two putative fusion peptides (SARS(WW-I) and SARS(WW-II)) at the N terminus of the SARS-CoV S2 subunit. Both peptides are hydrophobic and rich in alanine, glycine, and/or phenylalanine residues and contain a canonical fusion tripeptide along with a central proline residue. Only the SARS(WW-I) peptide strongly partitioned into the membranes of large unilamellar vesicles (LUV), adopting a beta-sheet structure. Likewise, only SARS(WW-I) induced the fusion of LUV and caused membrane leakage of vesicle contents at peptide/lipid ratios of 1:50 and 1:100, respectively. The activity of this synthetic peptide appeared to be dependent on its amino acid (aa) sequence, as scrambling the peptide rendered it unable to partition into LUV, assume a defined secondary structure, or induce both fusion and leakage of LUV. Based on the activity of SARS(WW-I), we propose that the hydrophobic stretch of 19 aa corresponding to residues 770 to 788 is a fusion peptide of the SARS-CoV S2 subunit.

Fusion Peptide−Phospholipid Noncovalent Interactions As Observed by Nanoelectrospray FTICR−MS

Viral infections are propagated by the fusing of the viral membrane with a host cell membrane. Initiation of the fusion process occurs upon perturbation of the membrane of the cell under attack by a subunit of the viral protein known as a fusion peptide. Fusion peptides must insert into the lipid-rich host cell membrane to initiate rupture and merging of the two entities, but much remains unknown about the details of the fusion process. We present detailed electrospray mass spectrometry studies of binding specificities of model fusion peptides P294 and P326 with cell membrane phospholipids, i.e., phosphatidylcholines (PCs, such as 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)) and phosphatidylglycerols (PGs, such as 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG)). The fusion peptides clearly bind more strongly to negatively charged DMPG than to zwitterionic DMPC. Detected binding between P294/P326 and PC/PG in 100% aqueous solution was disrupted by addition of methanol, which is known to weaken hydrophobic interactions; a higher percentage of methanol was needed to destroy a stronger initial binding. Further increases in the methanol volume fraction generally resulted in a reappearance of peptide-lipid binding, with binding strength quotients of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC)/1,2-dilauroyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DLPG)-peptide complexes rising more steeply than those of DMPC/DMPG-peptide complexes. Compared to fusion peptides P294 and P326, a hydrophilic peptide, fibrinopeptide B, showed much weaker affinity for zwitterionic DMPC, but had moderate binding affinity to negatively charged DMPG in 100% aqueous solutions. However, upon progressive addition of methanol, this hydrophilic peptide showed only a minor initial decrease in binding to DMPG before the detected binding eventually increased. These results contrast with those obtained for the hydrophobic peptides, and offer corroborative evidence that hydrophobic interactions play a key role in the mass spectrometrically observed binding between fusion peptides and phospholipids. Because the rate of viral infection has been found to be pH-dependent, the effect of initial solution pH on peptide-lipid binding was also studied. As the pH was lowered, P326-DMPC binding had a steep and immediate weakening, whereas the P294-DMPC binding was slightly strengthened at pH 3.7 and then gradually weakened with a further decrease in pH. Both P326 and P294 exhibited affinities toward unsaturated lipids; (18:1)PC bound slightly more strongly to P294 than (18:3)PC. These experiments offer further evidence of the ability of electrospray mass spectrometry to provide binding information concerning noncovalent interactions that were established principally by the hydrophobic effect in solution.

Atypical fusion peptide of Nelson Bay virus fusion-associated small transmembrane protein

A 10-kDa nonstructural transmembrane protein (p10) encoded by a reovirus, Nelson Bay virus, has been shown to induce syncytium formation (34). Sequence analysis and structural studies identified p10 as a type I membrane protein with a central transmembrane domain, a cytoplasmic basic region, and an N-terminal hydrophobic domain (HD) that was hypothesized to function as a fusion peptide. We performed mutational analysis on this slightly hydrophobic motif to identify possible structural requirements for fusion activity. Bulky aliphatic residues were found to be essential for optimal fusion, and an aromatic or highly hydrophobic side chain was found to be required at position 12. The requirement for hydrophilic residues within the HD was also examined: substitution of 10-Ser or 14-Ser with hydrophobic residues was found to reduce cell surface expression of p10 and delayed the onset of syncytium formation. Nonconservative substitutions of charged residues in the HD did not have an effect on fusion activity. Taken together, our results suggest that the HD is involved in both syncytium formation and in determining p10 transport and surface expression.

The interactions of the HIV gp41 fusion peptides with zwitterionic membrane mimics determined by NMR spectroscopy

The wild-type (wt) N-terminal 23-residue fusion peptide (FP) of the human immunodeficiency virus (HIV) fusion protein gp41 and its V2E mutant have been studied by nuclear magnetic resonance (NMR) spectroscopy in dodecylphosphocholine (DPC) micelles as membrane mimics. A number of NMR techniques have been used. Pulsed field-gradient diffusion measurements in DPC and in 4:1 DPC/sodium dodecylsulfate mixed micelles showed that there is no major difference between the partition coefficients of the fusogenic wt peptide and the V2E mutant in these micelles, indicating that there is no correlation between the activity of the fusion peptides and their membrane affinities. The nuclear Overhauser enhancement (NOE) patterns and the chemical shift index for these two peptides indicated that both FP are in an alpha helical conformation between the Ile4 to Leu12 or to Ala15 region. Simulated annealing showed that the helical region extends from Ile4 to Met19. The two FPs share similar conformational characteristics, indicating that the conformation of the FP is not an important factor determining its activity. The spin-label studies, utilizing spin labels 5- and 16-doxystearic acids in the DPC micelles, provided clear indication that the wt FP inserts its N-terminus into the micelles while the V2E mutant does not insert into the micelles. The conclusion from the spin-label results is corroborated by deuterium amide proton exchange experiments. The correlation between the oblique insertion of the FP and its fusogenic activity is in excellent agreement with results from our molecular dynamics simulation and from other previous studies.

Solid-state nuclear magnetic resonance studies of HIV and influenza fusion peptide orientations in membrane bilayers using stacked glass plate samples

The human immunodeficiency virus (HIV) and influenza virus fusion peptides are approximately 20-residue sequences which catalyze the fusion of viral and host cell membranes. The orientations of these peptides in lipid bilayers have been probed with 15N solid-state nuclear magnetic resonance (NMR) spectroscopy of samples containing membranes oriented between stacked glass plates. Each of the peptides adopts at least two distinct conformations in membranes (predominantly helical or beta strand) and the conformational distribution is determined in part by the membrane headgroup and cholesterol composition. In the helical conformation, the 15N spectra suggest that the influenza peptide adopts an orientation approximately parallel to the membrane surface while the HIV peptide adopts an orientation closer to the membrane bilayer normal. For the beta strand conformation, there appears to be a broader peptide orientational distribution. Overall, the data suggest that the solid-state NMR experiments can test models which correlate peptide orientation with their fusogenic function.

Oligomeric beta-structure of the membrane-bound HIV-1 fusion peptide formed from soluble monomers

The human immunodeficiency virus type 1 (HIV-1) fusion peptide serves as a useful model system for understanding viral/target cell fusion, at least to the lipid mixing stage. Previous solid-state NMR studies have shown that the peptide adopts an oligomeric beta-strand structure when associated with a lipid and cholesterol mixture close to that of membranes of host cells of the virus. In this study, this structure was further investigated using four different peptide constructs. In aqueous buffer solution, two of the constructs were primarily monomeric whereas the other two constructs had significant populations of oligomers/aggregates. NMR measurements for all membrane-associated peptide constructs were consistent with oligomeric beta-strand structure. Thus, constructs that are monomeric in solution can be converted to oligomers as a result of membrane association. In addition, samples prepared by very different methods had very similar NMR spectra, which indicates that the beta-strand structure is an equilibrium rather than a kinetically trapped structure. Lipid mixing assays were performed to assess the fusogenicities of the different constructs, and there was not a linear correlation between the solution oligomeric state and fusogenicity. However, the functional assays do suggest that small oligomers may be more fusogenic than either monomers or large aggregates.

Inhibition of HIV-1 envelope glycoprotein-mediated cell fusion by a DL-amino acid-containing fusion peptide: possible recognition of the fusion complex

The N-terminal fusion peptide (FP) of human immunodeficiency virus-1 (HIV-1) is a potent inhibitor of cell-cell fusion, possibly because of its ability to recognize the corresponding segments inside the fusion complex within the membrane. Here we show that a fusion peptide in which the highly conserved Ile(4), Phe(8), Phe(11), and Ala(14) were replaced by their d-enantiomers (IFFA) is a potent inhibitor of cell-cell fusion. Fourier transform infrared spectroscopy confirmed that despite these drastic modifications, the peptide preserved most of its structure within the membrane. Fluorescence energy transfer studies demonstrated that the diastereomeric peptide interacted with the wild type FP, suggesting this segment as the target site for inhibition of membrane fusion. This is further supported by the similar localization of the wild type and IFFA FPs to microdomains in T cells and the preferred partitioning into ordered regions within sphingomyelin/phosphatidyl-choline/cholesterol giant vesicles. These studies provide insight into the mechanism of molecular recognition within the membrane milieu and may serve in designing novel HIV entry inhibitors.

Structural and functional properties of an unusual internal fusion peptide in a nonenveloped virus membrane fusion protein

The avian and Nelson Bay reoviruses are two of only a limited number of nonenveloped viruses capable of inducing cell-cell membrane fusion. These viruses encode the smallest known membrane fusion proteins (p10). We now show that a region of moderate hydrophobicity we call the hydrophobic patch (HP), present in the small N-terminal ectodomain of p10, shares the following characteristics with the fusion peptides of enveloped virus fusion proteins: (i) an abundance of glycine and alanine residues, (ii) a potential amphipathic secondary structure, (iii) membrane-seeking characteristics that correspond to the degree of hydrophobicity, and (iv) the ability to induce lipid mixing in a liposome fusion assay. The p10 HP is therefore predicted to provide a function in the mechanism of membrane fusion similar to those of the fusion peptides of enveloped virus fusion peptides, namely, association with and destabilization of opposing lipid bilayers. Mutational and biophysical analysis suggested that the internal fusion peptide of p10 lacks alpha-helical content and exists as a disulfide-stabilized loop structure. Similar kinked structures have been reported in the fusion peptides of several enveloped virus fusion proteins. The preservation of a predicted loop structure in the fusion peptide of this unusual nonenveloped virus membrane fusion protein supports an imperative role for a kinked fusion peptide motif in biological membrane fusion.

How Structure Correlates to Function for Membrane Associated HIV-1 gp41 Constructs Corresponding to the N-terminal Half of the Ectodomain

To address the structure-function relationship of discrete regions within the gp41 ectodomain, 70-residue peptide constructs corresponding to the N-terminal subdomain of the HIV-1 gp41 ectodomain were examined in a membrane-associated context. These fragments encompass both fusion peptide (FP) and N-terminal heptad repeat (NHR) regions, and model the N-terminal half of the pre-hairpin intermediate (PHI), which is believed to be the target of the potent entry inhibitor DP-178, recently approved by the FDA. Using mutants, we attempted to map the structural organization of the N-terminal subdomain. Our results suggest that the N-terminal subdomain contains two discrete structural regions: the FP adopts a beta-sheet conformation and the NHR is alpha-helical. This structural make-up is essential for fusogenic function, since loss of function mutants exhibit both a significant reduction in region-specific secondary structure as well as significant impairment in lipid mixing of large unilamellar vesicles. Our results, delineating membrane-associated structure of the FP region differ from previous ones by inclusion of the autonomous oligomerization domain (NHR), which likely contributes to stabilization of the FP structure. Correspondingly, the alpha-helical structure for the NHR, in context of the FP, correlates with structural predictions for this region in both the hairpin and PHI conformations during fusion. Based on our results, we postulate how oligomerization of regions in this sub-domain is essential for fusion pore formation.

Are fusion peptides a good model to study viral cell fusion?

Fusion peptides are hydrophobic and conserved sequences located within glycoprotein ectodomains that protrude from the virion surface. Direct participation of fusion peptides in the viral membrane fusion phenomenon has been inferred from genetic analyses showing that even a single residue substitution or a deletion within these sequences may completely block the process. However, the specific fusion peptide activities associated to the multi-step fusion mechanism are not well defined. Based on the assumption that fusion peptides are transferred into target membranes, biophysical methodologies have been applied to study integration into model membranes of synthetic fragments representing functional and non-functional sequences. From these studies, it is inferred that, following insertion, functional sequences generate target membrane perturbations and adopt specific structural arrangements within. Further characterization of these artificial systems may help in understanding the molecular processes that bring initial bilayer destabilizations to the eventual opening of a fusion pore.

Membrane structure of the human immunodeficiency virus gp41 fusion peptide by molecular dynamics simulation. II. The glycine mutants

In this work, molecular dynamics (MD) simulation of the interaction of three mutants, G3V, G5V and G10V, of the human immunodeficiency virus (HIV) gp41 16-residue fusion peptide (FP) with an explicit palmitoyloleoylphosphatidyl-ethanolamine (POPE) lipid bilayer was performed. The goals of this work are to study the correlation of the fusogenic activity of the FPs with the mode of their interaction with the bilayer and to examine the roles of the many glycine residues in the FP in the fusion process. The results of this work corroborate the main conclusion of our earlier MD work of the WT FP and several mutants with polar substitution. These two studies provide correlation between the mode of insertion and the fusogenic activity of these peptides and support the hypothesis that an oblique insertion of the fusion domain of the viral protein is required for fusogenic activity. Inactive mutants interact with the bilayer by a surface-binding mode. The results of this work, combined with the results of our earlier work, show that, while the secondary structures of the wild-type FP and its mutants do not affect the fusogenic activities, the conformational flexibility appears to be an important factor. The active WT FP and its partially active mutants, G3V and G5V, all have significant conformational transitions at one of the glycine sites. They occur at Gly(5) in FP-wt, at Gly(10) in FP-G5V and at Gly(13) in FP-G3V. Thus, a glycine site in each of these active (or partially active) FPs provides conformational flexibility. On the other hand, the inactive mutants FP-G10V, FP-L9R and FP-V2E do not have any conformational transitions except at either terminus and thus possess no conformational flexibility. Thus, the results of this work support the suggestion that the role of glycine residues in the fusion domain is to provide the necessary conformational flexibility for fusion activity. The glycines also form a "glycine strip" in the FP that locates on one (the less hydrophobic) face of the helix (the "sided helix"). However, whether this "glycine strip" is disrupted or not does not seem to correlate with the retention of fusogenic activities. Finally, although the FLGFL (8-12) motif is absolutely conserved in the HIV fusion domain, a well-structured motif stabilized by hydrogen bonding does not appear to be required for activity. In fact, hydrogen bonding in this motif was found to be missing in FP-G3V and FP-G5V. Both of these mutants are partially active.

Sendai virus N-terminal fusion peptide consists of two similar repeats, both of which contribute to membrane fusion

The N-terminal fusion peptide of Sendai virus F1 envelope glycoprotein is a stretch of 14 amino acids, most of which are hydrophobic. Following this region, we detected a segment of 11 residues that are strikingly similar to the N-terminal fusion peptide. We found that, when anchored to the membrane by palmitoylation of its N-terminus, this segment (WT-palm-19-33) induces membrane fusion of large unilamellar liposomes to almost the same extent as a segment that includes the N-terminal fusion peptide. The activity of WT-palm-19-33 was dependent on its specific sequence, as a palmitoylated peptide with the same amino-acid composition but a scrambled sequence was inactive. Interestingly, two mutations (G7A and G12A) known to increase F1- induced cell-cell fusion, also increased the homology between the N-terminal fusion peptide and WT-palm-19-33. The role of the amino-acid sequence on the fusogenicity, secondary structure, and mechanism of membrane fusion was analyzed by comparing a peptide comprising both homologous segments (WT 1-33), a G12A mutant (G12A 1-33), a G7A-G12A double mutant (G7A-G12A 1-33), and a peptide with a scrambled sequence (SC 1-33). Based on these experiments, we postulate that replacement of Gly 7 and Gly12 by Ala increases the alpha helical content of the N-terminal region, with a concomitant increase in its fusogenic activity. Furthermore, the dissimilar abilities of the different peptides to induce membrane negative curvature as well as to promote isotropic 31P NMR signals, suggest that these mutations might also alter the extent of membrane penetration of the 33-residue peptide. Interestingly, our results serve to explain the effect of the G7A and G12A mutations on the fusogenic activity of the parent F1 protein in vivo.

Membrane structure of the human immunodeficiency virus gp41 fusion domain by molecular dynamics simulation

The structures of the 16-residue fusion domain (or fusion peptide, FP) of the human immunodeficiency virus gp41 fusion protein, two of its mutants, and a shortened peptide (5-16) were studied by molecular dynamics simulation in an explicit palmitoyloleoylphosphoethanolamine bilayer. The simulations showed that the active wild-type FP inserts into the bilayer approximately 44 degrees +/- 6 degrees with respect to the bilayer normal, whereas the inactive V2E and L9R mutants and the inactive 5 to 16 fragment lie on the bilayer surface. This is the first demonstration by explicit molecular dynamics of the oblique insertion of the fusion domain into lipid bilayers, and provides correlation between the mode of insertion and the fusogenic activity of these peptides. The membrane structure of the wild-type FP is remarkably similar to that of the influenza HA(2) FP as determined by nuclear magnetic resonance and electron spin resistance power saturation. The secondary structures of the wild-type FP and the two inactive mutants are quite similar, indicating that the secondary structure of this fusion domain plays little or no role in affecting the fusogenic activity of the fusion peptide. The insertion of the wild-type FP increases the thickness of the interfacial area of the bilayer by disrupting the hydrocarbon chains and extending the interfacial area toward the head group region, an effect that was not observed in the inactive FPs.

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.

Conformation and interaction with the membrane models of the amino-terminal peptide of influenza virus hemagglutinin HA2 at fusion pH

Conformations of a 48-mer peptide corresponding to the amino-terminal region of influenza HA2 in aqueous and membranous environments were studied. In aqueous solution the peptide was found to be oligomeric and its helicity was enhanced at higher concentrations. The conformation in phospholipid bilayer and insertion depth into the sodium dodecyl sulfate (SDS) micelle for the fusion peptide were in line with those determined for the amino-terminal 25-mer analog. The turn of residues 28-31 found in the crystal structure of hemagglutinin at neutral pH persisted in the presence of SDS at pH 5.0. Except for the turn, conformational lability of the amino portion of HA2 is suggested by comparison of the secondary structure determined herein with that obtained with the influenza fusion protein crystallized in the aqueous phase at neutral pH. The backbone amide proton exchange experiment suggested an interaction with the micellar surface for the segment carboxy-terminal to the fusion peptide domain.

Cellular membrane-binding ability of the C-terminal cytoplasmic domain of human immunodeficiency virus type 1 envelope transmembrane protein gp41

The amphipathic alpha-helices located in the cytoplasmic tail of the envelope (Env) transmembrane glycoprotein gp41 of human immunodeficiency virus type 1 have been implicated in membrane association and cytopathicity. Deletion of the last 12 amino acids in the C terminus of this domain severely impairs infectivity. However, the nature of the involvement of the cytoplasmic tail in Env-membrane interactions in cells and the molecular basis for the defect in infectivity of this mutant virus are still poorly understood. In this study we examined the interaction of the cytoplasmic tail with membranes in living mammalian cells by expressing a recombinant cytoplasmic tail fragment and an Escherichia coli beta-galactosidase/cytoplasmic tail fusion protein, both of them lacking gp120, the gp41 ectodomain, and the transmembrane region. We found through cell fractionation, in vivo membrane flotation, and confocal immunofluorescence studies that the cytoplasmic tail contained determinants to be routed to a perinuclear membrane region in cells. Further mapping showed that each of the three lentivirus lytic peptide (LLP-1, LLP-2, and LLP-3) sequences conferred this cellular membrane-targeting ability. Deletion of the last 12 amino acids from the C terminus abolished the ability of the LLP-1 motif to bind to membranes. High salt extraction, in vitro transcription and translation, and posttranslational membrane binding analyses indicated that the beta-galactosidase/LLP fusion proteins were inserted into membranes via the LLP sequences. Subcellular fractionation and confocal microscopy studies revealed that each of the LLP motifs, acting in a position-independent manner, targeted non-endoplasmic reticulum (ER)-associated beta-galactosidase and enhanced green fluorescence protein to the ER. Our study provides a basis for the involvement of the gp41 cytoplasmic tail during Env maturation and also supports the notion that the membrane apposition of the C-terminal cytoplasmic tail plays a crucial role in virus-host interaction.

Mutational evidence for an internal fusion peptide in flavivirus envelope protein E

The envelope protein E of the flavivirus tick-borne encephalitis (TBE) virus promotes cell entry by inducing fusion of the viral membrane with an intracellular membrane after uptake by endocytosis. This protein differs from other well-studied viral and cellular fusion proteins because of its distinct molecular architecture and apparent lack of involvement of coiled coils in the low-pH-induced structural transitions that lead to fusion. A highly conserved loop (the cd loop), which resides at the distal tip of each subunit and is mostly buried in the subunit interface of the native E homodimer at neutral pH, has been hypothesized to function as an internal fusion peptide at low pH, but this has not yet been shown experimentally. It was predicted by examination of the X-ray crystal structure of the TBE virus E protein (F. A. Rey et al., Nature 375:291-298, 1995) that mutations at a specific residue within this loop (Leu 107) would not cause the native structure to be disrupted. We therefore introduced amino acid substitutions at this position and, using recombinant subviral particles, investigated the effects of these changes on fusion and related properties. Replacement of Leu with hydrophilic amino acids strongly impaired (Thr) or abolished (Asp) fusion activity, whereas a Phe mutant still retained a significant degree of fusion activity. Liposome coflotation experiments showed that the fusion-negative Asp mutant did not form a stable interaction with membranes at low pH, although it was still capable of undergoing the structural rearrangements required for fusion. These data support the hypothesis that the cd loop may be directly involved in interactions with target membranes during fusion.

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.

Common properties of fusion peptides from diverse systems

Although membrane fusion occurs ubiquitously and continuously in all eukaroytic cells, little is known about the mechanism that governs lipid bilayer fusion associated with any intracellular fusion reactions. Recent studies of the fusion of enveloped viruses with host cell membranes have helped to define the fusion process. The identification and characterization of key proteins involved in fusion reactions have mainly driven recent advances in our understanding of membrane fusion. The most important denominator among the fusion proteins is the fusion peptide. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion will be highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. Much of what is known regarding the molecular mechanism of viral membrane fusion has been gained using liposomes as model systems in which the molecular components of the membrane and the environment are strictly controlled. Many amphilphilic peptides have a high affinity for lipid bilayers, but only a few sequences are able to induce membrane fusion. The presence of alpha-helical structure in at least part of the fusion peptide is strongly correlated with activity whereas, beta-structure tends to be less prevalent, associated with non-native experimental conditions, and more related to vesicle aggregation than fusion. The specific angle of insertion of the peptides into the membrane plane is also found to be an important characteristic for the fusion process. A shallow penetration, extending only to the central aliphatic core region, is likely responsible for the destabilization of the lipids required for coalescence of the apposing membranes and fusion.

Interactions of the HIV-1 fusion peptide with large unilamellar vesicles and monolayers. A cryo-TEM and spectroscopic study

We have examined the interaction of the human immunodeficiency virustype 1 fusion peptide (23 amino acid residues) and of a Trp-containing analog with vesicles composed of dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine and cholesterol (molar ratio, 1:1:1). Both the native and the Trp-substituted peptides bound the vesicles to the same extent and induced intervesicular lipid mixing with comparable efficiency. Infrared reflection-absorption spectroscopy data are compatible with the adoption by the peptide of a main beta-sheet structure in a cospread lipid/peptide monolayer. Cryo-transmission electron microscopy observations of peptide-treated vesicles reveal the existence of a peculiar morphology consisting of membrane tubular elongations protruding from single vesicles. Tryptophan fluorescence quenching by brominated phospholipids and by water-soluble acrylamide further indicated that the peptide penetrated into the acyl chain region closer to the interface rather than into the bilayer core. We conclude that the differential partition and shallow penetration of the fusion peptide into the outer monolayer of a surface-constrained bilayer may account for the detected morphological effects. Such single monolayer-restricted interaction and its structural consequences are compatible with specific predictions of current theories on viral fusion.

The amino-terminal region of the fusion peptide of influenza virus hemagglutinin HA2 inserts into sodium dodecyl sulfate micelle with residues 16-18 at the aqueous boundary at acidic pH. Oligomerization and the conformational flexibility

The conformation and interactions with membrane mimics of the NH(2)-terminal fragment 1-25 of HA2, HA2-(1-25), of influenza virus were studied by spectroscopic methods. Secondary structure analysis of circular dichroism data revealed 45% helix for the peptide at pH 5.0. Tryptophan fluorescence quenching by acrylamide and NMR experiments established that the Trp(14) is inside the vesicular interior and residues 16-18 are at the micellar aqueous boundary. NBD fluorescence enhancement of the NH(2)-terminal labeled fluorophore on the vesicle-bound peptide indicated that the NH(2) terminus of the fusion peptide was located in the hydrophobic region of the lipid bilayer. No significant change in insertion depth was observed between pH 5.0 and 7.4. Collectively, these spectroscopic measurements pointed to an equilibrium between helix and non-helix conformations, with helix being the dominant form, for the segment in the micellar interior. The conformational transition may be facilitated by the high content of glycine, a conformationally flexible amino acid, within the fusion peptide sequence. Self-association of the 25-mer peptide was observed in the N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine SDS-gel electrophoresis experiments. Incorporating the NMR signal attenuation, fluorescence, and gel electrophoresis data, a working model for the organization of the fusion peptide in membrane bilayers was proposed.

Multimerization potential of the cytoplasmic domain of the human immunodeficiency virus type 1 transmembrane glycoprotein gp41

We previously demonstrated that an envelope mutant of human immunodeficiency virus type 1 lacking the entire cytoplasmic domain interferes in trans with the production of infectious virus by inclusion of the mutant envelope into the wild-type envelope complex. We also showed that the envelope incorporation into virions is not affected when the wild-type envelope is coexpressed with the mutant envelope. These results suggest that an oligomeric structure of the cytoplasmic domain is functionally required for viral infectivity. To understand whether the cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein gp41 has the potential to self-assemble as an oligomer, in the present study we fused the coding sequence of the entire cytoplasmic domain at 3' to the Escherichia coli malE gene, which encodes a monomeric maltose-binding protein. The expressed fusion protein was examined by chemical cross-linking, sucrose gradient centrifugation, and gel filtration. The results showed that the cytoplasmic domain of gp41 assembles into a high-ordered structural complex. The intersubunit interaction of the cytoplasmic domain was also confirmed by a mammalian two-hybrid system that detects protein-protein interactions in eucaryotic cells. A cytoplasmic domain fragment expressed in eucaryotic cells was pulled down by glutathione-Sepharose 4B beads via its association with another cytoplasmic domain fragment fused to the C terminus of the glutathione S-transferase moiety. We also found that sequences encompassing the lentiviral lytic peptide-1 and lentiviral lytic peptide-2, which are located within residues 828-856 and 770-795, respectively, play a critical role in cytoplasmic domain self-assembly. Taken together, the results from the present study indicate that the cytoplasmic domain of gp41 by itself is sufficient to assemble into a multimeric structure. This finding supports the hypothesis that a multimeric form of the gp41 cytoplasmic domain plays a crucial role in virus infectivity.