Requirement of N-terminal amino acid residues of gp41 for human immunodeficiency virus type 1-mediated cell fusion

Hemifusion activity of a chimeric influenza virus hemagglutinin with a putative fusion peptide from hepatitis B virus

Entry of enveloped viruses is often mediated by an aminoterminal hydrophobic fusion peptide of a viral surface protein. The S domain of the hepatitis B virus surface protein contains a putative fusion peptide at position 7-18, but no systems are available to study its function directly. We tested the functionality of this peptide and a related peptide from another hepadnavirus in the context of the well-characterized influenza virus hemagglutinin H7 using gene mutation. The chimeric hemagglutinins could be expressed stably in CV 1 cells and were transported to the cell surface. The chimeras were incompletely cleaved by cellular proteases but cleavage could be completed by trypsin treatment of the cells. The chimeras did not differ in receptor binding, i.e. erythrocyte binding. Hemifusion and fusion pore formation were detected with membrane or cytosolic fluorescent dye-labeled erythrocytes as target structures of the hemagglutinin. Five of six different chimeras mediated hemifusion in 20-54% of the hemagglutinin-expressing cells, complete fusion and syncytium formation was not observed. The data suggest that the sequence 7-18 of the hepatitis B S domain may indeed initiate the first step of viral entry, i.e. hemifusion.

The polar region consecutive to the HIV fusion peptide participates in membrane fusion

The fusion peptide of HIV-1 gp41 is formed by the 16 N-terminal residues of the protein. This 16-amino acid peptide, in common with several other viral fusion peptides, caused a reduction in the bilayer to hexagonal phase transition temperature of dipalmitoleoylphosphatidylethanolamine (T(H)), suggesting its ability to promote negative curvature in membranes. Surprisingly, an elongated peptide corresponding to the 33 N-terminal amino acids raised T(H), although it was more potent than the 16-amino acid fusion peptide in inducing lipid mixing with large unilamellar liposomes of 1:1:1 dioleoylphosphatidylethanolamine/dioleoylphosphatidylcholine/choleste rol. The 17-amino acid C-terminal fragment of the peptide can induce membrane fusion by itself, if it is anchored to a membrane by palmitoylation of the amino terminus, indicating that the additional 17 hydrophilic amino acids contribute to the fusogenic potency of the peptide. This is not solely a consequence of the palmitoylation, as a random peptide with the same amino acid composition with a palmitoyl anchor was less potent in promoting membrane fusion and palmitic acid itself had no fusogenic activity. The 16-amino acid N-terminal fusion peptide and the longer 33-amino acid peptide were labeled with NBD. Fluorescence binding studies indicate that both peptides bind to the membrane with similar affinities, indicating that the increased fusogenic activity of the longer peptide was not a consequence of a greater extent of membrane partitioning. We also determined the secondary structure of the peptides using FTIR spectroscopy. We find that the amino-terminal fusion peptide is inserted into the membrane as a beta-sheet and the 17 C-terminal amino acids lie on the surface of the membrane, adopting an alpha-helical conformation. It was further demonstrated with the use of rhodamine-labeled peptides that the 33-amino acid peptide self-associated in the membrane while the 16-amino acid N-terminal peptide did not. Thus, the 16-amino acid N-terminal fusion peptide of HIV inserts into the membrane and, like other viral fusion peptides, lowers T(H). In addition, the 17 consecutive amino acids enhance the fusogenic activity of the fusion peptide presumably by promoting its self-association.

Role of the N-terminal peptides of viral envelope proteins in membrane fusion

Membrane fusion is an important biological process that is observed in a wide variety of intra and intercellular events. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion is highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. While the Influenza hemagglutinin is currently the best understand fusion protein, there is still much to be learned about the key events in enveloped virus fusion reactions. This review compares our current understanding of the membrane fusion activity of Influenza and retrovirus viruses. We shall be concerned especially with the studies that lead to interpretations at the molecular level, so we shall concentrate on model membrane systems where the molecular components of the membrane and the environment are strictly controlled.

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.

Biophysical characterization of the structure of the amino-terminal region of gp41 of HIV-1. Implications on viral fusion mechanism

A peptide of 51 amino acids corresponding to the NH2-terminal region (5-55) of the glycoprotein gp41 of human immunodeficiency virus type 1 was synthesized to study its conformation and assembly. Nuclear magnetic resonance experiments indicated the sequence NH2-terminal to the leucine zipper-like domain of gp41 was induced into helix in the micellar solution, in agreement with circular dichroism data. Light scattering experiment showed that the peptide molecules self-assembled in water into trimeric structure on average. That the peptide molecules oligomerize in aqueous solution was supported by gel filtration and diffusion coefficient experiments. Molecular dynamics simulation based on the NMR data revealed a flexible region adjacent to the hydrophobic NH2 terminus of gp41. The biological significance of the present findings on the conformational flexibility and the propensity of oligomerization of the peptide may be envisioned by a proposed model for the interaction of gp41 with membranes during fusion process.

Structural properties of the putative fusion peptide of fertilin, a protein active in sperm-egg fusion, upon interaction with the lipid bilayer

We recently demonstrated that a peptide representing the putative fusion domain of fertilin, a surface membrane protein of sperm involved in sperm-egg fusion, induces fusion of large unilamellar vesicles containing negatively charged lipids [Martin, I., and Ruysschaert, J. M. (1997) FEBS Lett. 405, 351-355]. In the present work, we demonstrate that increasing the concentration in negatively charged lipids strongly enhances the binding of the fertilin fusion peptide to the membrane, suggesting that electrostatic attractions play a crucial role in the binding process. While no significant change of the secondary structure content is observed by increasing the amounts of negatively charged lipids in the bilayer, the orientation of the alpha-helix changes from a parallel to an oblique orientation in the membrane. This topological change is confirmed by amide II hydrogen/deuterium exchange measurements that monitor the accessibility of the peptide to the water medium. Differential scanning calorimetry data also suggest that the fertilin fusion peptide lowers the bilayer to hexagonal phase transition temperature of model membranes composed of mixtures of dipalmitoleoylphosphatidylethanolamine and 1-palmitoyl-2-oleoylphosphatidylserine and therefore promotes negative curvature in lipid vesicles. A comparison of the biophysical properties and the membrane-perturbing activities of fertilin and of viral fusion peptides is discussed in terms of sperm-egg fusion and virus cell fusion.

Mutational analysis of the fusion peptide of Moloney murine leukemia virus transmembrane protein p15E

Fusion peptides are hydrophobic sequences located at the N terminus of the transmembrane (TM) envelope proteins of the orthomyxoviruses and paramyxoviruses and several retroviruses. The Moloney murine leukemia virus TM envelope protein, p15E, contains a hydrophobic stretch of amino acids at its N terminus followed by a region rich in glycine and threonine residues. A series of single amino acid substitutions were introduced into this region, and the resulting proteins were examined for their abilities to be properly processed and transported to the cell surface and to induce syncytia in cells expressing the ecotropic receptor. One substitution in the hydrophobic core and several substitutions in the glycine/threonine-rich region that prevented both cell-cell fusion and the transduction of NIH 3T3 cells when incorporated into retroviral vector particles were identified. In addition, one mutation that enhanced the fusogenicity of the resulting envelope protein was identified. The fusion-defective mutants trans dominantly interfered with the ability of the wild-type envelope protein to cause syncytium formation in a cell-cell fusion assay, although no trans-dominant inhibition of transduction was observed. Certain substitutions in the hydrophobic core that prevented envelope protein processing were also found. These data indicate that the N-terminal region of p15E is important both for viral fusion and for the correct processing and cell surface expression of the viral envelope protein.

The FLG motif in the N-terminal region of glucoprotein 41 of human immunodeficiency virus type 1 adopts a type-I beta turn in aqueous solution and serves as the initiation site for helix formation

NMR and CD studies were carried out on a peptide representing the hydrophobic N-terminal domain of envelope glycoprotein of human immunodeficiency virus type-1 in solutions of varying polarity. It was found that in aquaeous solution the amide proton of glycine in the FLG motif resonated at a considerably high field and its chemical shift, within the limit of experimental precision, had a temperature coefficient of zero in the range studied. The upfield shift of NH of the glycine could be largely attributed to the ring-current effect of phenylalanine in the FLG motif that participated in a type-1 beta turn with a short Cbeta(i)-NH(i+2) distance. The slower proton-deuterion exchange for the glycine amide proton relative to that of other glycines was consistent with a folded structure for the motif in aquaeous solution. Results of the molecular simulation showed that this proton was shielded from the solvent by non-polar side chains of the amino acid residues surrounding the turn stabilized by hydrophobic interactions, thus explaining the zero temperature coefficient of the proton chemical shift. The structural stabilizing effect of the hydrophobic interaction was supported by the behavior of the proton in less polar Me2SO solution, in which the anomaly in the chemical shift and its temperature coefficient was less prominent. Detailed secondary-structure analysis suggested that the beta turn of the FLG motif may act as an initiation core for helix formation, probably because the turn readily transforms into helical form.

What studies of fusion peptides tell us about viral envelope glycoprotein-mediated membrane fusion (review)

This review describes the numerous and innovative methods used to study the structure and function of viral fusion peptides. The systems studied include both intact fusion proteins and synthetic peptides interacting with model membranes. The strategies and methods include dissecting the fusion process into intermediate stages, comparing the effects of sequence mutations, electrophysiological patch clamp methods, hydrophobic photolabelling, video microscopy of the redistribution of both aqueous and lipophilic fluorescent probes between cells, standard optical spectroscopy of peptides in solution (circular dichroism and fluorescence) and attenuated total reflection-Fourier transform infrared spectroscopy of peptides bound to planar bilayers. Although the goal of a detailed picture of the fusion pore has not been achieved for any of the intermediate stages, important properties useful for constraining the development of models are emerging. For example, 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. The functional role of the fusion peptides (which tend to be either nonpolar or aliphatic) is then to bind to and dehydrate the outer bilayers at a localized site; and thus reduce the energy barrier for the formation of highly curved, lipidic 'stalk' intermediates. In addition, the importance of the formation of specific, 'higher-order' fusion peptide complexes has also been shown. Recent crystallographic structures of core domains of two more fusion proteins (in addition to influenza haemagglutinin) has greatly facilitated the development of prototypic models of the fusion site. This latter effort will undoubtedly benefit from the insights and constraints gained from the studies of fusion peptides.

Exon 1 leader sequences downstream of U5 are important for efficient human immunodeficiency virus type 1 gene expression

In previous studies, little attention has been paid to maintaining the native HIV-1 leader sequence in reporter constructs analyzing the human immunodeficiency virus type 1 (HIV-1) promoter activity. To investigate a possible influence of the leader sequence on HIV-1-driven gene expression in the presence as well as in the absence of Tat, an expression vector was designed for transcripts consisting of the native HIV-1 tat 1.4 mRNA leader followed by the open reading frame for the bacterial chloramphenicol acetyltransferase (CAT). Deletion mutants with mutations within the leader sequence downstream of U5 (lsdU5) were constructed, as well as a mutant containing a mutation with a reverse orientation of this region. Quantification of CAT protein in HeLa-T4+ cells transiently transfected with wild-type and mutant leader constructs showed that the exon 1-derived lsdU5 region has an influence on basal as well as Tat-induced protein expression. The dramatic decrease in the level of CAT protein upon deletion of lsdU5 was paralleled by a drop in the steady-state level of CAT mRNA. Deletion of the exon 1-derived lsdU5 region also decreased the expression of mRNAs containing authentic HIV-1 sequences instead of CAT. The effect observed with the reporter constructs was not due to the loss of binding sites for nuclear factors, as could be shown with DBF1 and Sp1 mutant constructs. Nuclear run-on transcription assays showed that the presence or absence of lsdU5 did not influence the rate of transcription. This indicates that the exon 1 lsdU5 element functions at the posttranscriptional level in the processing, nucleocytoplasmic export, or stabilization of HIV-1 transcripts.

Human immunodeficiency virus type 1 envelope glycoprotein oligomerization requires the gp41 amphipathic alpha-helical/leucine zipper-like sequence

Human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) oligomerization was investigated by coexpressing wild-type and truncated envelope glycoproteins to determine the minimum sequence required for mutant-wild-type hetero-oligomerization. The gp41 putative amphipathic alpha-helix, Leu-550 to Leu-582, was essential for hetero-oligomer formation. Alanine substitution of 9 of the 10 residues composing the gp41 amphipathic alpha-helix 4-3 hydrophobic repeat sequence was required to inhibit mutant-wild-type hetero-oligomerization and to render the envelope glycoprotein precursor, gp160, monomeric. This indicates that multiple hydrophobic contacts contribute to the stable envelope glycoprotein oligomeric structure. Single alanine substitutions within the hydrophobic repeat sequence did not affect gp160 oligomeric structure but abolished syncytium-forming function. Some mutations also diminished gp160 processing efficiency and the association between gp120 and gp41 in a position-dependent manner. These results indicate that the gp41 amphipathic alpha-helix 4-3 hydrophobic repeat sequence plays a central role in HIV-1 envelope glycoprotein oligomerization and fusion function.

The role of eukaryotic subtilisin-like endoproteases for the activation of human immunodeficiency virus glycoproteins in natural host cells

Proteolytic activation of the precursor envelope glycoproteins gp160 of human immunodeficiency virus type 1 (HIV-1) and gp140 of HIV-2, a prerequisite for viral infection, results in the formation of gp120/gp41 and gp125/gp36, respectively. Cleavage is mediated by cellular proteases. Furin, a member of the eukaryotic subtilisin family, has been shown to be an activating protease for HIV. Here, we compared the presence of furin and other mammalian subtilisins in lymphatic cells and tissues. Northern blot analyses revealed that furin and the recently discovered protease LPC/PC7 were the only subtilisin-like enzymes transcribed in such cells. Furin was identified as an enzymatically active endoprotease present in different lymphocytic, as well as monocytic, cell lines. When expressed from vaccinia virus vectors, the proprotein convertases were correctly processed, transported, and secreted into the media and enzymatically active. Coexpression of different subtilisins with the HIV envelope precursors revealed that furin and LPC/PC7 are able to cleave HIV-1 gp160. Moreover, both enzymes proteolytically processed the envelope precursor of HIV-2. gp140 was also cleaved to some extent by PC1, which is not, however, present in lymphatic cells. Furin- and LPC/PC7-catalyzed cleavage of HIV-1 gp160 resulted in biologically active envelope protein. In conclusion, among the known members of the subtilisin family, only furin and LPC/PC7 fulfill the requirements of a protease responsible for in vivo activation of HIV envelope glycoproteins.

Role of complement in HIV infection

In human plasma, HIV activates the complement system, even in the absence of specific antibodies. Complement activation would, however, be harmful to the virus if the reactions were allowed to go to completion, since their final outcome would be virolysis. This is avoided by complement regulatory molecules, which either are included in the virus membrane upon budding from the infected cells (e.g. DAF/CD55) or are secondarily attached to HIV envelope glycoproteins as in the case of factor H. By using this strategy of interaction with complement components, HIV takes advantage of human complement activation for enhancement of infectivity, for follicular localization, and for broadening its target cell range at the same time that it displays an intrinsic resistance against the lytic action of human complement. This intrinsic resistance to complement-mediated virolysis can be overcome by monoclonal antibodies inhibiting recruitment of human factor H to the virus surface, suggesting a new therapeutic principle.

Lipid membrane fusion induced by the human immunodeficiency virus type 1 gp41 N-terminal extremity is determined by its orientation in the lipid bilayer

The amino-terminal extremity of the human immunodeficiency virus type 1 transmembrane protein (gp41) is thought to play a pivotal role in the fusion of virus membranes with the plasma membrane of the target cell and in syncytium formation. Peptides with sequences taken from the human immunodeficiency virus type 1 gp41 fusogenic (synthetic peptides SPwt and SP-2) and nonfusogenic (SP-3 and SP-4) glycoproteins adopt mainly a beta-sheet conformation in the absence of lipid, as determined by attenuated total reflection Fourier transform infrared spectroscopy, and after interaction with large unilamellar liposomes, the beta-sheet is partly converted into an alpha-helical conformation. Peptides SPwt and SP-2 but not SP-3 or SP-4 were able to promote lipid mixing as assessed by fluorescence energy transfer assay and dye leakage in a vesicle leakage assay. By using polarized attenuated total reflection Fourier transform infrared spectroscopy, SPwt and SP-2 were found to adopt an oblique orientation in the lipid membrane whereas SP-3 and SP-4 were oriented nearly parallel to the plane of the membrane. These findings confirm the correlation between the membrane orientation of the alpha-helix and the lipid mixing ability in vitro. Interestingly, the data provide a direct correlation with the fusogenic activity of the parent glycoproteins in vivo.