Hydrophobic photolabeling identifies BHA2 as the subunit mediating the interaction of bromelain-solubilized influenza virus hemagglutinin with liposomes at low pH

Free Radical Chain Reactions and Polyunsaturated Fatty Acids in Brain Lipids

Polyunsaturated fatty acyl chains (PUFAs) concentrate in the brain and give rise to numerous oxidative chemical degradation products. It is widely assumed that these products are the result of free radical chain reactions, and reactions of this type have been demonstrated in preparations where a single PUFA substrate species predominates. However, it is unclear whether such reactions can occur in the biologically complex milieu of lipid membranes where PUFA substrates are a minority species, and where diverse free radical scavengers or other quenching mechanisms are present. It is of particular interest to know whether they occur in brain, where PUFAs are concentrated and where PUFA oxidation products have been implicated in the pathogenesis of neurodegenerative disorders. To ascertain whether free radical chain reactions can occur in a complex brain lipid mixture, mouse brain lipids were extracted, formed into vesicles, and treated with a fixed number of hydroxyl radicals under conditions wherein the concentrations and types of PUFA-containing phospholipids were varied. Specific phospholipid species in the mixture were assayed by tandem mass spectrometry to quantify the oxidative losses of endogenous PUFA-containing phospholipids. Results reveal crosstalk between the oxidative degradation of ω3 and ω6 PUFAs that can only be explained by the occurrence of free radical chain reactions. These results demonstrate that PUFAs in a complex brain lipid mixture can participate in free radical chain reactions wherein the extent of oxidative degradation is not limited by the number of reactive oxygen species available to initiate such reactions. These reactions may help explain otherwise puzzling in vivo interactions between ω3 and ω6 PUFAs in mouse brain.

Photolabeling Strategies to Study Membranotropic Peptides Interacting with Lipids and Proteins in Membranes

Membranotropic peptides is a class of peptides that exert their biological action at the level of cell membranes. Understanding how they interact with their different membrane binding partners (lipids, proteins, and/or glycoconjugates) is important to decipher their mechanism of action. Affinity photolabeling is a powerful method to study noncovalent interactions and provide a submolecular picture of the contacts between two interacting partners. In this review, we give a panorama of photolabeling-based studies of the interactions between membranotropic peptides and membranes using either photoreactive lipids or peptides.

A facile, sensitive and quantitative membrane-binding assay for proteins

Soluble proteins that bind membranes function in numerous cellular pathways yet facile, sensitive and quantitative methods that complement and improve sensitivity of widely used liposomes-based assays remain unavailable. Here, we describe the utility of a photoactivable fluorescent lipid as a generic reporter of protein-membrane interactions. When incorporated into liposomes and exposed to ultraviolet (UV), proteins bound to liposomes become crosslinked with the fluorescent lipid and can be readily detected and quantitated by in-gel fluorescence analysis. This modification obviates the requirement for high-speed centrifugation spins common to most liposome-binding assays. We refer to this assay as Proximity-based Labeling of Membrane-Associated Proteins (PLiMAP).

Development and application of diazirines in biological and synthetic macromolecular systems

Many different reagents and methodologies have been utilised for the modification of synthetic and biological macromolecular systems. In addition, an area of intense research at present is the construction of hybrid biosynthetic polymers, comprised of biologically active species immobilised or complexed with synthetic polymers. One of the most useful and widely applicable techniques available for functionalisation of macromolecular systems involves indiscriminate carbene insertion processes. The highly reactive and non-specific nature of carbenes has enabled a multitude of macromolecular structures to be functionalised without the need for specialised reagents or additives. The use of diazirines as stable carbene precursors has increased dramatically over the past twenty years and these reagents are fast becoming the most popular photophors for photoaffinity labelling and biological applications in which covalent modification of macromolecular structures is the basis to understanding structure-activity relationships. This review reports the synthesis and application of a diverse range of diazirines in macromolecular systems.

Oligomerization of fusogenic peptides promotes membrane fusion by enhancing membrane destabilization

A key element of membrane fusion reactions in biology is the involvement of specific fusion proteins. In many viruses, the proteins that mediate membrane fusion usually exist as homotrimers. Furthermore, they contain extended triple-helical coiled-coil domains and fusogenic peptides. It has been suggested that the coiled-coil domains present the fusogenic peptide in a conformation or geometry favorable for membrane fusion. To test the hypothesis that trimerization of fusogenic peptide is related to optimal fusion, we have designed and synthesized a triple-stranded coiled-coil X31 peptide, also known as the ccX31, which mimics the influenza virus hemagglutinin fusion peptide in the fusion-active state. We compared the membrane interactive properties of ccX31 versus the monomeric X31 fusogenic peptide. Our data show that trimerization enhances peptide-induced leakage of liposomal contents and lipid mixing. Furthermore, studies using micropipette aspiration of single vesicles reveal that ccX31 decreases lysis tension, tau(lysis), but not area expansion modulus, Ka, of phospholipid bilayers, whereas monomeric X31 peptide lowers both tau(lysis) and Ka. Our results are consistent with the hypothesis that oligomerization of fusogenic peptide promotes membrane fusion, possibly by enhancing localized destabilization of lipid bilayers.

Fluorescent and photoactivable probes in depth-dependent analysis of membranes

This report summarizes our efforts towards depth-dependent analysis of membranes by design of suitable fluorescent and photoactivable lipid probes, which can be incorporated into membranes. The objective of depth-dependent analysis has been two fold, one to obtain information on lipid domains and other on transmembrane domains of membrane-bound proteins. In view of increasing importance of lipid rafts and other localized domain and limited success in case of structure determination of membrane-bound proteins vis-à-vis their soluble counterparts, it is tempting to rapidly attach fluorescent or photoactivable probes to lipids to get a probes where relatively little attention is paid to design of such probes. We have shown here how careful design of such probes is required to immobilize such probes in membranes for effective depth-dependent analysis of membranes. An effective design has become important when identification of putative transmembrane domains predicted primarily from the genome data based on hydropathy plots, often needs confirmation by contemporary methodology.

Factor Va increases the affinity of factor Xa for prothrombin: a binding study using a novel photoactivable thiol-specific fluorescent probe

The multiprotein complex of factor Xa, factor Va, and prothrombin efficiently generates the blood-clotting agent, thrombin. Here, the formation of the factor Xa*prothrombin complex and the effects of factor Va on this complex were examined using a photoactivable thiol-specific fluorescent probe (LWB), which was synthesized and incorporated into the active site of factor Xa. The use of fluorescent LWB illustrated that factor Xa has an increased affinity for prothrombin in the presence of factor Va. Further exposure of these components to UV light resulted in a specific photocrosslinking of LWB-factor Xa to prothrombin, suggesting a physical association between these proteins. These data demonstrate that LWB can successfully function both as a spectroscopic probe and as a photocrosslinking reagent for studying protein-protein interactions.

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.

Consequences of affinity in heterogeneous catalytic reactions: highly chemoselective hydrogenolysis of iodoarenes

The catalytic hydrodeiodination reaction using molecular hydrogen and Pd/C has been revisited. It is shown, for the first time, that the chemoselectivity of this reaction is controlled by the high affinity of the iodinated compound for the catalyst. This reaction is compatible with most easily reducible functional groups (nitro, aldehyde, olefin, etc.). Using this reaction, the first general method for tritium labeling of 3-(trifluoromethyl)-3-phenyldiazirine is described.

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.

Membrane perturbation and fusion pore formation in influenza hemagglutinin-mediated membrane fusion. A new model for fusion

Low pH-induced fusion mediated by the hemagglutinin (HA) of influenza virus involves conformational changes in the protein that lead to the insertion of a "fusion peptide" domain of this protein into the target membrane and is thought to perturb the membrane, triggering fusion. By using whole virus, purified HA, or HA ectodomains, we found that shortly after insertion, pores of less than 26 A in diameter were formed in liposomal membranes. As measured by a novel assay, these pores stay open, or continue to close and open, for minutes to hours and persist after pH neutralization. With virus and purified HA, larger pores, allowing the leakage of dextrans, were seen at times well after insertion. For virus, dextran leakage was simultaneous with lipid mixing and the formation of "fusion pores," allowing the transfer of dextrans from the liposomal to the viral interior or vice versa. Pores did not form in the viral membrane in the absence of a target membrane. Based on these data, we propose a new model for fusion, in which HA initially forms a proteinaceous pore in the target, but not in the viral membrane, before a lipidic hemifusion intermediate is formed.

The central proline of an internal viral fusion peptide serves two important roles

The fusion peptide of the avian sarcoma/leukosis virus (ASLV) envelope protein (Env) is internal, near the N terminus of its transmembrane (TM) subunit. As for most internal viral fusion peptides, there is a proline near the center of this sequence. Robson-Garnier structure predictions of the ASLV fusion peptide and immediate surrounding sequences indicate a region of order (beta-sheet), a tight reverse turn containing the proline, and a second region of order (alpha-helix). Similar motifs (order, turn or loop, order) are predicted for other internal fusion peptides. In this study, we made and analyzed 12 Env proteins with substitutions for the central proline of the fusion peptide. Env proteins were expressed in 293T cells and in murine leukemia virus pseudotyped virions. We found the following. (i) All mutant Envs form trimers, but when the bulky hydrophobic residues phenylalanine or leucine are substituted for proline, trimerization is weakened. (ii) Surprisingly, the proline is required for maximal processing of the Env precursor into its surface and TM subunits; the amount of processing correlates linearly with the propensity of the substituted residue to be found in a reverse turn. (iii) Nonetheless, proteolytically processed forms of all Envs are preferentially incorporated into pseudotyped virions. (iv) All Envs bind receptor with affinity greater than or equal to wild-type affinity. (v) Residues that support high infectivity cluster with proline at intermediate hydrophobicity. Infectivity is not supported by mutant Envs in which charged residues are substituted for proline, nor is it supported by the trimerization-defective phenylalanine and leucine mutants. Our findings suggest that the central proline in the ASLV fusion peptide is important for the formation of the native (metastable) Env structure as well as for membrane interactions that lead to fusion.

Poly(ethylene glycol) (PEG)-mediated fusion between pure lipid bilayers: a mechanism in common with viral fusion and secretory vesicle release?

Membrane fusion is fundamental to the life of eukaryotic cells. Cellular trafficking and compartmentalization, import of food stuffs and export of waste, inter-cellular communication, sexual reproduction, and cell division are all dependent on this basic process. Yet, little is known about the molecular mechanism(s) by which fusion occurs. It is known that fusing membranes must somehow be docked and brought into close contact. Specific proteins, many of which have been identified within the past decade, accomplish this. An electrical connection or 'fusion pore' is established between compartments surrounded by the fusing membranes. Three primary views of the mechanism of pore formation during secretory and viral fusion have been proposed within the past decade. In one view, a protein ring forms an initial transient connection that expands slowly by recruiting lipid so as to form a lipidic junction. In another view, the initial fusion pore consists of a protein-lipid complex that transforms slowly until the fusion proteins dissociate from the complex to form an irreversible lipidic pore. In a third view, the initial pore is a transient lipid pore that fluctuates between open and closed states before either expanding irreversibly or closing. Recent work has helped define the mechanism by which poly(ethylene glycol) (PEG) mediates fusion of highly curved model membranes composed only of synthetic phospholipids. PEG is a highly hydrated polymer that can bring vesicle membranes to near molecular contact by making water between them thermodynamically unfavourable. Disrupted packing in the contacting monolayers of these vesicle membranes is necessary to induce fusion. The time course and sequence of molecular events of the ensuing fusion process have also been defined. This sequence of events involves the formation of an initial, transient intermediate in which outer leaflet lipids have mixed and small transient pores join fusing compartments ('stalk'). The transient intermediate transforms in 1-3 min to a fusion-committed, second intermediate ('septum') that then 'pops' to form the fusion pore. Inner leaflet mixing, which is shown to be distinct from outer leaflet mixing, accompanies contents mixing that marks formation of the fusion pore. Both the sequence of events and the activation energies of these events correspond well to those observed in viral membrane fusion and secretory granule fusion. These results strongly support the contention that both viral and secretory fusion events occur by lipid molecule rearrangements that can be studied and defined through the use of PEG-mediated vesicle fusion as a model system. A possible mechanism by which fusion proteins might mediate this lipidic process is described.

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.

Analysis of the membrane-interacting domains of myelin basic protein by hydrophobic photolabeling

Myelin basic protein is a water soluble membrane protein which interacts with acidic lipids through some type of hydrophobic interaction in addition to electrostatic interactions. Here we show that it can be labeled from within the lipid bilayer when bound to acidic lipids with the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and by two lipid photolabels. The latter included one with the reactive group near the apolar/polar interface and one with the reactive group linked to an acyl chain to position it deeper in the bilayer. The regions of the protein which interact hydrophobically with lipid to the greatest extent were determined by cleaving the TID-labeled myelin basic protein (MBP) with cathepsin D into peptides 1-43, 44-89, and 90-170. All three peptides from lipid-bound protein were labeled much more than peptides from the protein labeled in solution. However, the peptide labeling pattern was similar for both environments. The two peptides in the N-terminal half were labeled similarly and about twice as much as the C-terminal peptide indicating that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half. MBP can be modified post-translationally in vivo, including by deamidation, which may alter its interactions with lipid. However, deamidation had no effect on the TID labeling of MBP or on the labeling pattern of the cathepsin D peptides. The site of deamidation has been reported to be in the C-terminal half, and its lack of effect on hydrophobic interactions of MBP with lipid are consistent with the conclusion that the N-terminal half interacts hydrophobically more than the C-terminal half. Since other studies of the interaction of isolated N-terminal and C-terminal peptides with lipid also indicate that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half, these results from photolabeling of the intact protein suggest that the N-terminal half of the intact protein interacts with lipid in a similar way as the isolated peptide. The similar behavior of the intact protein to that of its isolated peptides suggests that when the purified protein binds to acidic lipids, it is in a conformation which allows both halves of the protein to interact independently with the lipid bilayer. That is, it does not form a hydrophobic domain made up from different parts of the protein.

Membrane fusion promoted by increasing surface densities of the paramyxovirus F and HN proteins: comparison of fusion reactions mediated by simian virus 5 F, human parainfluenza virus type 3 F, and influenza virus HA

The membrane fusion reaction promoted by the paramyxovirus simian virus 5 (SV5) and human parainfluenza virus type 3 (HPIV-3) fusion (F) proteins and hemagglutinin-neuraminidase (HN) proteins was characterized when the surface densities of F and HN were varied. Using a quantitative content mixing assay, it was found that the extent of SV5 F-mediated fusion was dependent on the surface density of the SV5 F protein but independent of the density of SV5 HN protein, indicating that HN serves only a binding function in the reaction. However, the extent of HPIV-3 F protein promoted fusion reaction was found to be dependent on surface density of HPIV-3 HN protein, suggesting that the HPIV-3 HN protein is a direct participant in the fusion reaction. Analysis of the kinetics of lipid mixing demonstrated that both initial rates and final extents of fusion increased with rising SV5 F protein surface densities, suggesting that multiple fusion pores can be active during SV5 F protein-promoted membrane fusion. Initial rates and extent of lipid mixing were also found to increase with increasing influenza virus hemagglutinin protein surface density, suggesting parallels between the mechanism of fusion promoted by these two viral fusion proteins.

A glycine to alanine substitution in the paramyxovirus SV5 fusion peptide increases the initial rate of fusion

Simian virus 5 fusion (F) protein mutant F-G3A, which contains a glycine-to-alanine substitution at position 3 in the conserved hydrophobic fusion peptide at the N-terminus of the F1 subunit, has been shown previously to cause increased syncytium formation compared to wild-type (wt) F protein, when expressed using an SV40 recombinant virus vector system (C. M. Horvath and R. A. Lamb (1992) J. Virol. 66, 2443-2455). The wt F and the F-G3A proteins were expressed in eukaryotic cells using the vaccinia virus-bacteriophage T7 RNA polymerase (vac-T7) expression system, and they showed similar cell surface expression levels as determined by flow cytometry. The final extent of fusion when the vac-T7 expression system was used was not found to be greatly different when examined with a reporter gene activation assay. However, the initial rate of fusion was found to be five- to sixfold higher for the F-G3A mutant protein than the wt F protein, when examined using a quantitative assay for lipid mixing based on relief of self-quenching of fluorescence of the lipid probe octadecyl rhodamine (R18). A microscopic fluorescent dye transfer assay also showed a much earlier spread of dye from R18-labeled red blood cells to the cells expressing the mutant F-G3A protein than the wt F protein. Thus, these data indicate that a single gly-to-ala mutation in the fusion peptide domain, although not affecting the final extent of fusion, significantly increased the rate of fusion. Possible mechanisms for the increased rate of fusion are discussed.

Pores formed by influenza hemagglutinin

Low pH-induced fusion mediated by the hemagglutinin (HA) of influenza virus involves a conformational change in the protein that leads to the insertion of a "fusion peptide" of the protein into the target membrane. It has been suggested that this insertion, aided by the formation of a complex of multiple HA trimers, would lead to perturbation of the bilayer structure of the membrane, initiating fusion. Here we present data showing that the interaction of the bromelain released ectodomain of the protein (BHA) with liposomal membranes at low pH leads to pore formation, at least at low temperatures. Strongly temperature-dependent low pH-induced inactivation of BHA resulted in a complete lack of activity of BHA above 10 degrees C. Even at 0 degrees C, only about 5% of the BHA participated in pore formation. Viral HA was less rapidly inactivated and still induced pores at 37 degrees C. BHA-induced pore formation showed a sigmoidal time course. Once BHA had formed a pore in one liposome, it did not form a pore in a further liposome. Quantitative analysis of pore formation indicated that one single BHA trimer sufficed to produce a pore. These data indicate that fusion peptide insertion perturbs the membrane and that the formation of a complex of trimers is not a prerequisite for the perturbation.

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.

An amphipathic alpha-helix is the principle membrane-embedded region of CTP:phosphocholine cytidylyltransferase. Identification of the 3-(trifluoromethyl)-3-(m-[125I]iodophenyl) diazirine photolabeled domain

CTP:phosphocholine cytidylyltransferase (CT), the rate controlling enzyme in phosphatidylcholine biosynthesis, is activated by reversible membrane binding. To investigate the membrane binding mechanism of CT, we have used the photoreactive hydrophobic probe 3-(trifluoromethyl)-3-(m-[l25I]iodophenyl)diazirine ([125I]TID). Association of CT with phosphatidylcholine/oleic acid (1:1) vesicles was first demonstrated by gel filtration analysis. Upon irradiation, CT was covalently labeled by [125I]TID presented in phosphatidylcholine/oleic acid vesicles. This demonstrates an intercalation of part of the protein into the hydrophobic core of the membrane. To identify the membrane-embedded domain, the chymotrypsin digestion products of [125I]TID labeled CT were analysed. Chymotrypsin digestion produced a set of previously defined N-terminal fragments (Craig, L., Johnson, J.E. and Cornell, R.B. (1994) J. Biol. Chem. 269, 3311), as well as several small C-terminal fragments which react with an anti-peptide antibody raised against the proposed amphipathic alpha-helix. All fragments containing the amphipathic helical region of the enzyme had [125I]TID label associated, while the chymotryptic fragment which lacked this region was not highly labeled. Similar fragment labeling patterns were produced when [125I]TID was presented in phosphatidylcholine/oleic acid or phosphatidylcholine/diacylglycerol vesicles, suggesting that the same domain of CT mediates binding to membranes containing either of the two lipid activators. A 62-residue synthetic peptide corresponding in sequence to the amphipathic helical region of CT was labeled with [125I]TID, demonstrating its ability to intercalate independently of the rest of the protein. These results indicate a membrane-binding mechanism for cytidylyltransferase involving the intercalation of the amphipathic alpha-helix region into the hydrophobic acyl chain core of the activating membrane.

H+-induced membrane insertion of influenza virus hemagglutinin involves the HA2 amino-terminal fusion peptide but not the coiled coil region

Fusion of influenza virus with target membranes is induced by acid and involves complex changes in the viral envelope protein hemagglutinin (HA). In a first, kinetically distinct step, the HA polypeptide chain 2 (HA2) is inserted into the target membrane bilayer. Using hydrophobic photolabeling with the phospholipid analogue 1-O-hexadecanoyl-2-O-[9-[[[2-[125I]iodo-4(trifluoromethyl-3H-diazirin -3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, we identified the segment within HA2 that interacts with the membrane. The sole part of the HA2 ectodomain that was labeled with the membrane-restricted reagent is the NH2-terminal fusion peptide (residues 1-22). No labeling occurred within the long coiled coil region generated during the acid-induced conformational transition (Bullough, P. A., Hughson, F. M., Skehel, J. J., and Wiley, D. C. (1994) Nature 371, 37-43). These data strongly suggest that the coiled coil region of HA2 does not insert into the lipid bilayer. This conclusion is at variance with the recent suggestion (Yu, Y. G., King, D. S., and Shin, Y.-K.(1994) Science 266, 274-276) that the coiled coil of HA may splay apart and insert into the target membrane, providing a mechanism by which the viral and the target membrane may come in close apposition.

Fluorescence studies of the effect of pH, guanidine hydrochloride and urea on equinatoxin II conformation

The solvent denaturation of equinatoxin II (EqTxII) in aqueous solutions of urea, guanidine hydrochloride (Gu-HCl) and at various pH values was examined by monitoring changes in the protein intrinsic emission fluorescence spectra and in the fluorescence spectra of the added external probe ANS. It has been observed that EqTxII denaturation is reflected in a strong red shift of intrinsic fluorescence emission maxima accompanied by a simultaneous decrease in fluorescence intensity and that guanidine hydrochloride is significantly more powerful denaturant than urea or changing of pH. Comparison of intrinsic fluorescence spectra of EqTxII denatured by one of the three denaturing agents has shown that the fully denatured states of the protein in Gu-HCl and urea are similar and substantially different from those induced by changing of pH. Furthermore, according to the measurements of the ANS-fluorescence in EqTxII solutions as a function of pH the protein exists at pH values below 2.0 in an acid-denatured compact state.

Spectroscopic characterization of the alkylated alpha-sarcin cytotoxin: analysis of the structural requirements for the protein-lipid bilayer hydrophobic interaction

alpha-Sarcin is a ribosome-inactivating protein that translocates across lipid bilayers, these two abilities explaining its cytotoxic character. This protein is composed of a single polypeptide chain with two disulfide bridges. Reduction and carboxyamidomethylation of alpha-sarcin results in protein unfolding, based on the results of the spectroscopic characterization of the chemically modified protein. The absorption and fluorescence emission bands of the tryptophan residues of the modified protein appear blue- and red-shifted, respectively. Far-UV circular dichroism analysis reveals the presence of residual secondary structure (beta-strands and turns) in the alkylated protein. This retains its ability to interact with lipid bilayers. It promotes vesicle aggregation, lipid-mixing between bilayers and leakage of the intravesicular aqueous contents. The modified protein tends to abolish the phase transition of acid phospholipids as detected by differential scanning calorimetry and depolarization measurements of fluorescence-labelled vesicles. The protein gain access to vesicle-entrapped trypsin. The fluorescence emission of the tryptophan residues is blue-shifted upon interaction of the protein with the bilayers, and anthracene incorporated into the hydrophobic core of the membranes quenches the tryptophan fluorescence emission of the protein. The secondary structure of the alkylated protein interacting with lipid vesicles has been studied by infrared spectroscopy. An increase in the alpha-helix and turn contents and a concomitant decrease in the beta-structure content are observed upon interaction with the bilayers. The results obtained are discussed in terms of the structural requirements for the interaction of alpha-sarcin with lipid membranes.

Structural characterization of viral fusion proteins

Infection by enveloped viruses is initiated by the fusion of viral and cellular membranes. In many cases, the viral membrane proteins that mediate fusion must undergo conformational changes to become active. Influenza hemagglutinin, for example, is activated by a dramatic conformational rearrangement, triggered by the low pH of the intracellular compartment in which fusion occurs. Structural characterization of this rearrangement has led to a reconsideration of how hemagglutinin mediates membrane fusion.

Polymer-induced membrane fusion: potential mechanism and relation to cell fusion events

Poly(ethylene glycol) (PEG) is used widely to mediate cell-cell fusion in the production of somatic cell hybrids and in the fusion injection of macromolecules into cultured cells from erythrocytes or liposomes. However, little is known about the mechanisms by which PEG induces fusion of cell membranes, making its use much more an art than a science. This article considers possible molecular events involved in biomembrane fusion and summarizes what we have learned about these in recent years from studies of fusion of well-defined model membranes. In addition, it recounts observations made over the past several years about the process of PEG-mediated fusion of model membranes. These observations have defined the process to an extent sufficient to allow us to propose a model for the molecular events involved in the process. It is suggested that dehydration leads to asymmetry in the lipid packing pressure in the two leaflets of the membrane bilayer leading to formation of a single bilayer septum at a point of close apposition of two membranes. The single bilayer septum then decays during formation of the initial fusion pore. Agents that enhance or alleviate the dehydration-induced asymmetric packing stress will favor or inhibit fusion. Although the proposed picture is consistent with much accumulated data, it is not yet proven; experiments must now be devised to test its details. Finally, the proposed model is discussed in terms of potential implications for the mechanisms available to a cell in controlling more complex in vivo cell fusion processes such as endocytosis, exocytosis, protein sorting/transport, and viral budding/infection.

Entry and uncoating of enveloped viruses

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pH-dependent binding of the fluorophore bis-ANS to influenza virus reflects the conformational change of hemagglutinin

Binding of the fluorophore 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) to influenza virus A/PR 8/34 is strongly enhanced at low pH. Binding is accompanied by a significant increase in fluorescence intensity. The binding and the fluorescence increase are associated with the low-pH induced conformational change of the viral spike protein, hemagglutinin, exposing hydrophobic binding sites. The data indicate that in addition to the hydrophobic N-terminus of HA2 other hydrophobic sequences of the HA ectodomain become accessible to bis-ANS at low pH. It is shown that the time course of the fluorescence increase of bis-ANS at low pH is determined by the conformational change of HA. The application of this assay for continuously monitoring the kinetics of the structural alteration in HA is discussed and its relevance for elucidating the temporal relationship between the conformational change of HA and virus-membrane fusion is outlined.

Mutations in the membrane-spanning domain of the human immunodeficiency virus envelope glycoprotein that affect fusion activity

A chimeric protein consisting of the human immunodeficiency virus type 1 (HIV-1) envelope protein (Env) ectodomain joined to the transmembrane and cytoplasmic-tail domains of vesicular stomatitis virus G protein lost the ability to fuse CD4+ HeLa cells yet was transported to the cell surface and cleaved normally. These results suggested some critical role of the HIV gp41 transmembrane or cytoplasmic domain in fusion. Subsequent mutagenic analysis of the HIV-1 Env transmembrane domain revealed that the sequence of amino acid residues from positions 696 to 707 of the transmembrane domain was important for fusion function but was not required for anchoring of the Env protein in the lipid bilayer or for transport to the cell surface. Further analysis indicated that the basic residues at positions 696 and 707 were critical for membrane fusion activity, as was the spacing between these residues. These results demonstrate that in addition to providing an anchoring function, the specific amino acid sequence in the transmembrane domain plays a crucial role in the membrane fusion process.

Evidence for a ternary interaction between alpha-actinin, (meta)vinculin and acidic-phospholipid bilayers

The cytoskeletal component vinculin has been demonstrated by hydrophobic photoradiolabelling, to insert into bilayers containing acidic phospholipids and trace amounts of a photoactivatable analogue of lecithin. It is shown in this study that the higher-molecular-mass variant metavinculin and alpha-actinin, also share this property. alpha-Actinin and vinculin were also shown to associate with phosphatidylserine liposomes by chromatography of protein/lipid mixtures on a Bio-Gel A-5m column. Furthermore, interesting differences in the behaviour of binary mixtures of these proteins, in the presence of phosphatidylserine liposomes, are shown. Thus, incubation of alpha-actinin with vinculin or metavinculin, prior to the addition of liposomes, strongly inhibited the photoradiolabelling of alpha-actinin under conditions in which the liposome surface was non-limiting, but enhanced the labelling of vinculin. In contrast, vinculin and metavinculin did not mutually influence their labelling. Using gel-filtration chromatography, it was shown that alpha-actinin still bound to the vinculin-liposome complex, under conditions similar to those used for hydrophobic photolabelling with a non-limiting lipid surface. In the presence of limiting amounts of liposomes, the alpha-actinin/vinculin ratio was markedly decreased in the liposome fractions. Our results suggest the formation of a ternary complex consisting of vinculin, alpha-actinin and phospholipids. In this complex, both proteins interact at the bilayer, resulting in an altered conformation of the two proteins and, as a consequence, in modified bilayer interactions.

Orientation and structure of the NH2-terminal HIV-1 gp41 peptide in fused and aggregated liposomes

For several retroviruses, the N-terminal hydrophobic sequence of the viral envelope glycoprotein has been shown to play a crucial role in the interaction between the virus and the host cell membrane. We report here on the interaction of a synthetic 16 residues peptide corresponding to the gp41 NH2-terminal sequence of Human Immunodeficiency Virus with the phospholipid bilayer. Fluorescence energy transfer measurements show that this peptide can induce lipid mixing of large unilamellar vesicles (LUV) of various compositions at pH 7.4 and 37 degrees C. LUV undergo fusion, provided they contained phosphatidylethanolamine (PE) in their lipid composition. To provide insight into the mechanism of the fusion event, the peptide secondary structure and orientation in the lipid bilayer were determined using Fourier Transform Infrared Spectroscopy (FTIR). The peptide adopts mainly a beta-sheet conformation in the absence of lipids. After interaction with LUV the beta-sheet is partly converted into alpha-helix. The orientation of the peptide with respect to the lipid acyl chains depends on the presence of PE in the lipid bilayer. The peptide is inserted into the lipid bilayer with the helix axis oriented parallel to the lipid acyl chains in the fused vesicles, whereas it is adsorbed parallel to the lipid/water interface in the aggregated vesicles. The role of the two kinds of orientation during the fusion event is discussed.

Membrane fusion

Common themes are emerging from the study of viral, cell-cell, intracellular, and liposome fusion. Viral and cellular membrane fusion events are mediated by fusion proteins or fusion machines. Viral fusion proteins share important characteristics, notably a fusion peptide within a transmembrane-anchored polypeptide chain. At least one protein involved in a cell-cell fusion reaction resembles viral fusion proteins. Components of intracellular fusion machines are utilized in multiple membrane trafficking events and are conserved through evolution. Fusion pores develop during and intracellular fusion events suggesting similar mechanisms for many, if not all, fusion events.

Assessing hydrophobic regions of the plasma membrane H(+)-ATPase from Saccharomyces cerevisiae

The hydrophobic, photoactivatable probe TID [3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine] was used to label the plasma membrane H(+)-ATPase from Saccharomyces cerevisiae. The H(+)-ATPase accounted for 43% of the total label associated with plasma membrane protein and incorporated 0.3 mol of [125I]TID per mol of 100 kDa polypeptide. The H(+)-ATPase was purified by octyl glucoside extraction and glycerol gradient centrifugation, and was cleaved by either cyanogen bromide digestion or limited tryptic proteolysis to isolate labeled fragments. Cyanogen bromide digestion resulted in numerous labeled fragments of mass less than 21 kDa. Seven fragments suitable for microsequence analysis were obtained by electrotransfer to poly(vinylidene difluoride) membranes. Five different regions of amino-acid sequence were identified, including fragments predicted to encompass both membrane-spanning and cytoplasmic protein structure domains. Most of the labeling of the cytoplasmic domain was concentrated in a region comprising amino acids 347 to 529. This catalytic region contains the site of phosphorylation and was previously suggested to be hydrophobic in character (Goffeau, A. and De Meis, L. (1990) J. Biol. 265, 15503-15505). Complementary labeling information was obtained from an analysis of limited tryptic fragments enriched for hydrophobic character. Six principal labeled fragments, of 29.6, 20.6, 16, 13.1, 11.4 and 9.7 kDa, were obtained. These fragments were found to comprise most of the putative transmembrane region and a portion of the cytoplasmic region that overlapped with the highly labeled active site-containing cyanogen bromide fragment. Overall, the extensive labeling of protein structure domains known to lie outside the bilayer suggests that [125I]TID labeling patterns cannot be unambiguously interpreted for the purpose of discerning membrane-embedded protein structure domains. It is proposed that caution should be applied in the interpretation of [125I]TID labeling patterns of the yeast plasma membrane H(+)-ATPase and that new and diverse approaches should be developed to provide a more definitive topology model.

Probing actin and liposome interaction of talin and talin-vinculin complexes: a kinetic, thermodynamic and lipid labeling study

Talin purified from human platelets and chicken gizzard smooth muscle is an actin and lipid binding protein. Here, we have investigated the effect of vinculin on (a) talin-nucleated actin polymerization and (b) insertion of talin into lipid bilayers. Calorimetric data show ternary complex formation between talin, vinculin, and actin. Actin-talin, actin-vinculin and actin-(talin-vinculin) binding and rate constants as well as actin polymerization rates for all three protein species have been determined by steady state titration, stopped-flow, and fluorescence assay. In contrast to an increase of the polymerization rate by a factor of less than 2 for actin-talin and actin-(talin-vinculin) when lowering the temperature, we measured a decrease in rates for actin alone and actin-vinculin. The overall equilibrium constants (Keq) in the van't Hoff plot proved linear and were of one-step reactions. Thermodynamic data exhibited signs of van der Waal's binding forces. Using the photoactivatable lipid analogue [3H]PTPC/11, which selectively labels membrane-embedded hydrophobic domains of proteins, we also show that talin partially inserts into the hydrophobic bilayer of liposomes. This insertion occurs in a similar manner irrespective of preincubation with vinculin.

Membrane fusion activity of the influenza virus hemagglutinin: interaction of HA2 N-terminal peptides with phospholipid vesicles

We have investigated the interaction of a number of synthetic 20-residue peptides, corresponding to the HA2 N-terminus of the influenza virus hemagglutinin (X31 strain), with phospholipid vesicles and monolayers. Besides the wild-type sequence, two peptides were studied with mutations corresponding to those previously studied in entire HA's expressed in transfected cells [Gething et al., (1986) J. Cell. Biol. 102, 11-23]. These mutations comprised a single Glu replacement for Gly at the N-terminus ("El" mutant) or at position 4 ("E4") of the HA2 subunit and were shown to produce striking alterations in virus-induced hemolysis and syncytia formation, especially for E1. The X31 "wild-type" (wt) peptide and its E4 variant are shown here to have the capacity to insert into phosphatidylcholine (POPC) large unilamellar vesicle (LUV) membranes in a strictly pH-dependent manner, penetration being marginal at pH 7.4 and significant at pH 5.0. Bilayer insertion was evident from a shift in the intrinsic Trp fluorescence of the wt and E4 peptides and from the induction of calcein leakage from POPC LUV and correlated well with the peptides' ability at pH 5.0 to penetrate into POPC monolayers at initial surface pressures higher than 30 mN/m. By contrast, the E1 peptide was found, at pH 5.0, to bind less tightly to vesicles (assessed by a physical separation method) and to cause much less leakage of POPC LUV than the wt, even under conditions where the peptides were bound to approximately the same extent. Consistent with the correlation between leakage and penetration observed for the wt peptide at pH 5 versus 7, the E1 peptide, even at low pH, showed much less lipid-vesicle-induced shift of its Trp fluorescence than wt, caused a much slower rate of leakage of vesicle contents, and did not insert into POPC monolayers at surface pressures beyond 28.5 mN/m. Circular dichroism spectroscopy measurements of peptides in POPC SUV showed that the conformations of all three peptides are sensitive to pH, but only the wt and E4 peptides became predominantly alpha-helical at acid pH.

Fusion of influenza virions with a planar lipid membrane detected by video fluorescence microscopy

The fusion of individual influenza virions with a planar phospholipid membrane was detected by fluorescence video microscopy. Virion envelopes were loaded with the lipophilic fluorescent marker octadecylrhodamine B (R18) to a density at which the fluorescence of the probe was self-quenched. Labeled virions were ejected toward the planar membrane from a micropipette in a custom-built video fluorescence microscope. Once a virion fused with the planar membrane, the marker was free to diffuse, and its fluorescence became dequenched, producing a flash of light. This flash was detected as a transient spot of light which increased and then diminished in brightness. The diffusion constants calculated from the brightness profiles for the flashes are consistent with fusion of virus to the membrane with consequent free diffusion of probe within the planar membrane. Under conditions known to be fusigenic for influenza virus (low pH and 37 degrees C), flashes appeared at a high rate and the planar membrane quickly became fluorescent. To further establish that these flashes were due to fusion, we showed that red blood cells, which normally do not attach to planar membranes, were able to bind to membranes that had been exposed to virus under fusigenic conditions. The amount of binding correlated with the amount of flashing. This indicates that flashes signaled the reconstitution of the hemagglutinin glycoprotein (HA) of influenza virus, a well-known erythrocyte receptor, into the planar membrane, as would be expected in a fusion process. The flash rate on ganglioside-containing asolectin membranes increased as the pH was lowered. This is also consistent with the known fusion behavior of influenza virus with cell membranes and with phospholipid vesicles. We conclude that the flashes result from the fusion of individual virions to the planar membrane.

Fusion activity of influenza virus PR8/34 correlates with a temperature-induced conformational change within the hemagglutinin ectodomain detected by photochemical labeling

Fusion of influenza viruses with membranes is catalyzed by the viral spike protein hemagglutinin (HA). Under mildly acidic conditions (approximately pH 5) this protein undergoes a conformational change that triggers the exposure of the "fusion peptide", the hydrophobic N-terminal segment of the HA2 polypeptide chain. Insertion of this segment into the target membrane (or viral membrane?) is likely to represent a key step along the fusion pathway, but the details are far from being clear. The photoreactive phospholipid 1-palmitoyl-2-[11-[4-[3-(trifluoromethyl)diazirinyl]phenyl] [2-3H]undecanoyl]-sn-glycero-3-phosphocholine ([3H]PTPC/11), inserted into the bilayer of large unilamellar vesicles (LUVs), allowed us to investigate both the interaction of viruses with the vesicles under "prefusion" conditions (pH 5; 0 degrees C) and the fusion process itself occurring at elevated temperatures (greater than 15-20 degrees C) only. Despite the observed binding of viruses to LUVs at pH 5 and 0 degrees C, labeling of HA2 was very weak (less than 0.002% of the radioactivity originally present). In contrast, fusion could be readily monitored by the covalent labeling of that polypeptide chain. We have studied also the effect of temperature on the acid-induced (pH 5) interaction of bromelain-solubilized HA (BHA) with vesicles. Labeling of the BHA2 polypeptide chain was found to show a remarkable correlation with the temperature dependence of the fusion activity of whole viruses. A temperature-induced structural change appears to be critical for both the interaction of BHA with membranes and the expression of fusion activity of intact viruses.