Studies on influenza haemagglutinin fusion peptide mutants generated by reverse genetics

Influenza haemagglutinin (HA) is responsible for fusing viral and endosomal membranes during virus entry. In this process, conformational changes in the HA relocate the HA(2) N-terminal 'fusion peptide' to interact with the target membrane. The highly conserved HA fusion peptide shares composition and sequence features with functionally analogous regions of other viral fusion proteins, including the presence and distribution of glycines and large side-chain hydrophobic residues. HAs with mutations in the fusion peptide were expressed using vaccinia virus recombinants to examine the requirement for fusion of specific hydrophobic residues and the significance of glycine spacing. Mutant HAs were also incorporated into infectious influenza viruses for analysis of their effects on infectivity and replication. In most cases alanine, but not glycine substitutions for the large hydrophobic residues, yielded fusion-competent HAs and infectious viruses, suggesting that the conserved spacing of glycines may be structurally significant. When viruses containing alanine substitutions for large hydrophobic residues were passaged, pseudoreversion to valine was observed, indicating a preference for large hydrophobic residues at specific positions. Viruses were also obtained with serine, leucine or phenylalanine as the N-terminal residue, but these replicated to significantly lower levels than wild-type virus with glycine at this position.     

Length requirements for membrane fusion of influenza virus hemagglutinin peptide linkers to transmembrane or fusion peptide domains

During membrane fusion, the influenza A virus hemagglutinin (HA) adopts an extended helical structure that contains the viral transmembrane and fusion peptide domains at the same end of the molecule. The peptide segments that link the end of this rod-like structure to the membrane-associating domains are approximately 10 amino acids in each case, and their structure at the pH of fusion is currently unknown. Here, we examine mutant HAs and influenza viruses containing such HAs to determine whether these peptide linkers are subject to specific length requirements for the proper folding of native HA and for membrane fusion function. Using pairwise deletions and insertions, we show that the region flanking the fusion peptide appears to be important for the folding of the native HA structure but that mutant proteins with small insertions can be expressed on the cell surface and are functional for membrane fusion. HA mutants with deletions of up to 10 residues and insertions of as many as 12 amino acids were generated for the peptide linker to the viral transmembrane domain, and all folded properly and were expressed on the cell surface. For these mutants, it was possible to designate length restrictions for efficient membrane fusion, as functional activity was observed only for mutants containing linkers with insertions or deletions of eight residues or less. The linker peptide mutants are discussed with respect to requirements for the folding of native HAs and length restrictions for membrane fusion activity.     

Studies on the mechanism of membrane fusion: site-specific mutagenesis of the hemagglutinin of influenza virus

Oligonucleotide-directed mutagenesis of a cDNA encoding the hemagglutinin of influenza virus has been used to introduce single base changes into the sequence that codes for the conserved apolar "fusion peptide" at the amino-terminus of the HA2 subunit. The mutant sequences replaced the wild-type gene in SV40-HA recombinant virus vectors, and the altered HA proteins were expressed in simian cells. Three mutants have been constructed that introduce single, nonconservative amino acid changes in the fusion peptide, and three fusion phenotypes were observed: substitution of glutamic acid for the glycine residue at the amino-terminus of HA2 abolished all fusion activity; substitution of glutamic acid for the glycine residue at position 4 in HA2 raised the threshold pH and decreased the efficiency of fusion; and, finally, extension of the hydrophobic stretch by replacement of the glutamic acid at position 11 with glycine yielded a mutant protein that induced fusion of erythrocytes with cells with the same efficiency and pH profile as the wild-type protein. However, the ability of this mutant to induce polykaryon formation was greatly impaired. Nevertheless, all the mutant proteins underwent a pH-dependent conformational change and bound to liposomes. These results are discussed in terms of the mechanism of HA-induced membrane fusion.     

Membrane structures of the hemifusion-inducing fusion peptide mutant G1S and the fusion-blocking mutant G1V of influenza virus hemagglutinin suggest a mechanism for pore opening in membrane fusion

Influenza virus hemagglutinin (HA)-mediated membrane fusion is initiated by a conformational change that releases a V-shaped hydrophobic fusion domain, the fusion peptide, into the lipid bilayer of the target membrane. The most N-terminal residue of this domain, a glycine, is highly conserved and is particularly critical for HA function; G1S and G1V mutant HAs cause hemifusion and abolish fusion, respectively. We have determined the atomic resolution structures of the G1S and G1V mutant fusion domains in membrane environments. G1S forms a V with a disrupted "glycine edge" on its N-terminal arm and G1V adopts a slightly tilted linear helical structure in membranes. Abolishment of the kink in G1V results in reduced hydrophobic penetration of the lipid bilayer and an increased propensity to form beta-structures at the membrane surface. These results underline the functional importance of the kink in the fusion peptide and suggest a structural role for the N-terminal glycine ridge in viral membrane 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.     

The structure of a membrane fusion mutant of the influenza virus haemagglutinin.

The haemagglutinin glycoprotein (HA) of influenza virus specifically mediates fusion of the viral and host cell endosomal membranes at the acidic pH of endosomes. The HAs from mutant viruses with raised fusion pH optima contain amino acid substitutions in regions of the HA structure thought to be involved in the fusion process [Daniels et al. (1985b) Cell, 40, 431-439]. We have determined the neutral pH crystal structure of one such mutant, HA2 112 Asp----Gly. A water molecule appears to partially replace the aspartate side chain, and no changes are observed in the surrounding structure. It appears that four intra-chain hydrogen bonds that stabilize the location of the N-terminus of HA2 are lost in the mutant, resulting in a local destabilization that facilitates the extrusion of the N-terminus at higher pH.     

The transmembrane domain of influenza hemagglutinin exhibits a stringent length requirement to support the hemifusion to fusion transition

Glycosylphosphatidylinositol-anchored influenza hemagglutinin (GPI-HA) mediates hemifusion, whereas chimeras with foreign transmembrane (TM) domains mediate full fusion. A possible explanation for these observations is that the TM domain must be a critical length in order for HA to promote full fusion. To test this hypothesis, we analyzed biochemical properties and fusion phenotypes of HA with alterations in its 27-amino acid TM domain. Our mutants included sequential 2-amino acid (Delta2-Delta14) and an 11-amino acid deletion from the COOH-terminal end, deletions of 6 or 8 amino acids from the NH(2)-terminal and middle regions, and a deletion of 12 amino acids from the NH(2)-terminal end of the TM domain. We also made several point mutations in the TM domain. All of the mutants except Delta14 were expressed at the cell surface and displayed biochemical properties virtually identical to wild-type HA. All the mutants that were expressed at the cell surface promoted full fusion, with the notable exception of deletions of >10 amino acids. A mutant in which 11 amino acids were deleted was severely impaired in promoting full fusion. Mutants in which 12 amino acids were deleted (from either end) mediated only hemifusion. Hence, a TM domain of 17 amino acids is needed to efficiently promote full fusion. Addition of either the hydrophilic HA cytoplasmic tail sequence or a single arginine to Delta12 HA, the hemifusion mutant that terminates with 15 (hydrophobic) amino acids of the HA TM domain, restored full fusion activity. Our data support a model in which the TM domain must span the bilayer to promote full fusion.     

Membrane fusion mediated by coiled coils: a hypothesis

A molecular model of the low-pH-induced membrane fusion by influenza hemagglutinin (HA) is proposed based upon the hypothesis that the conformational change to the extended coiled coil creates a high-energy hydrophobic membrane defect in the viral envelope or HA expressing cell. It is known that 1) an aggregate of at least eight HAs is required at the fusion site, yet only two or three of these HAs need to undergo the "essential" conformational change for the first fusion pore to form (Bentz, J. 2000. Biophys. J. 78:000-000); 2) the formation of the first fusion pore signifies a stage of restricted lipid flow into the nascent fusion site; and 3) some HAs can partially insert their fusion peptides into their own viral envelopes at low pH. This suggests that the committed step for HA-mediated fusion begins with a tightly packed aggregate of HAs whose fusion peptides are inserted into their own viral envelope, which causes restricted lateral lipid flow within the HA aggregate. The transition of two or three HAs in the center of the aggregate to the extended coiled coil extracts the fusion peptide and creates a hydrophobic defect in the outer monolayer of the virion, which is stabilized by the closely packed HAs. These HAs are inhibited from diffusing away from the site to admit lateral lipid flow, in part because that would initially increase the surface area of hydrophobic exposure. The other obvious pathway to heal this hydrophobic defect, or some descendent, is recruitment of lipids from the outer monolayer of the apposed target membrane, i.e., fusion. Other viral fusion proteins and the SNARE fusion protein complex appear to fit within this hypothesis.     

Phosphatidylinositol glycan (PI-G) anchored membrane proteins. Amino acid requirements adjacent to the site of cleavage and PI-G attachment in the COOH-terminal signal peptide.

Secreted proteins are processed from a nascent form that contains an NH2-terminal signal peptide. During processing, the latter is cleaved by a specific NH2-terminal signal peptidase. The nascent form of phosphatidylinositol glycan (PI-G) tailed proteins contain both an NH2- and a COOH-terminal signal peptide. The two signal peptides have much in common, such as size and hydrophobicity. The COOH-terminal peptide is also cleaved during processing. We propose that the amino acid in a nascent protein that ultimately combines with the PI-G moiety be designated the omega site. Amino acids adjacent and COOH-terminal to the omega site would then be omega + 1, omega + 2, etc. In previous studies, we showed that allowable substitutions at the omega site of an engineered form of placental alkaline phosphatase (miniPLAP) are limited to 6 small amino acids. In the present study, mutations were made at the omega + 1 and omega + 2 sites. At the omega + 1 site, processing to varying degrees was observed with 8 of the 9 amino acids substituted for alanine, the normal constituent. Only the proline mutant showed no processing. By contrast, the only substituents permitted at the omega + 2 site were glycine and alanine, with only trace activity observed with serine and cysteine. Thus, just as there is a -1, -3 rule for predicting cleavage by NH2-terminal signal peptidase, there appears to be a comparable omega, omega + 2 rule for predicting cleavage/PI-G addition by COOH-terminal signal transamidase.     

Fusion induced aggregation of model vesicles studied by dynamic and static light scattering

Membrane fusion is a key step in the virus mediated cell fusion. The vesicular dispersion serves as a model system to study the membrane fusion. We employed dynamic and static light scattering to study the fusion of phosphatidylcholine vesicles in the presence of model fusion peptide fragments from the hemagglutinin HA2 protein. The fusion-induced aggregation under the present experimental setup exhibited strong pH dependence, similar to the parental viral protein. Replacement of the glycine residue at the extreme amino terminus by glutamic acid (G1E) abolished fusion activity. The average molecular mass and diameter of vesicular dispersion obtained from static and dynamic light scattering measurements respectively at neutral and acidic pH showed about three fold increase in acidic solution containing wild type fusion peptide. The light scattering data are consistent with lipid mixing results. The present work demonstrates the utility of light scattering as a facile means to monitor the fusion process.     

Structure of an analog of fusion peptide from hemagglutinin

A 20-residue peptide E5 containing five glutamates, an analog of the fusion peptide of influenza virus hemagglutinin (HA) exhibiting fusion activity at acidic pH lower than 6.0-6.5 was studied by circular dichroism (CD), Fourier transform infrared, and 1H-NMR spectroscopy in water, water/trifluoroethanol (TFE) mixtures, dodecylphosphocholine (DPC) micelles, and phospholipid vesicles. E5 became structurally ordered at pH < or = 6 and the helical content in the peptide increased in the row: water < water/TFE < DPC approximately = phospholipid vesicle while the amount of beta-structure was approximately reverse. 1H-NMR data and line-broadening effect of 5-, 16-doxylstearates on proton resonances of DPC bound peptide showed E5 forms amphiphilic alpha-helix in residues 2-18, which is flexible in 11-18 part. The analysis of the proton chemical shifts of DPC bound and CD intensity at 220 nm of phospholipid bound E5 showed that the pH dependence of helical content is characterized by the same pKa approximately 5.6. Only Glu11 and Glu15 in DPC bound peptide showed such elevated pKas, presumably due to transient hydrogen bond(s) Glu11 (Glu15) deltaCOO- (H+)...HN Glu15 that dispose(s) the side chain of Glu11 (Glu15) residue(s) close to the micelle/water interface. These glutamates are present in the HA-fusion peptide and the experimental half-maximal pH of fusion for HA and E5 peptides is approximately 5.6. Therefore, a specific anchorage of these peptides onto membrane necessary for fusion is likely driven by the protonation of the carboxylate group of Glu11 (Glu15) residue(s) participating in transient hydrogen bond(s).     

Identification of methionine Nalpha-acetyltransferase from Saccharomyces cerevisiae

N alpha-Acetylation is the most frequently occurring chemical modification of the alpha-NH2 group of eukaryotic proteins and was believed until now to be catalyzed by a single N alpha-acetyltransferase. The transfer of an acetyl group from acetyl coenzyme A to the alpha-amino group of five NH2-terminal residues (serine, alanine, methionine, glycine, and threonine) in proteins accounts for approximately 95% of acetylated residues. We have found that a crude lysate from Saccharomyces cerevisiae mutant (aaa1) deficient in N alpha-acetyltransferase activity can effectively transfer an acetyl group to peptides containing NH2-terminal methionine but not to serine or alanine. This methionine N alpha-acetyltransferase has been extensively purified, and this purified enzyme can selectively transfer an acetyl group to various model peptides containing an NH2-terminal methionine residue and a penultimate aspartyl, asparaginyl, or glutamyl residue. Such specificity of N alpha-acetylation of methionine has been previously observed based on the analysis of eukaryotic protein sequences (Persson, B., Flinta, C., Heijne, G., and Jornvall, H. (1985) Eur. J. Biochem. 152, 523-527; Arfin, S.M., and Bradshaw, R. A. (1988) Biochemistry 27, 7979-7984). The indentification of this methionine N alpha-acetyltransferase provides an explanation as to why two distinct classes of N alpha-acetylated proteins exist in nature: (i) those whose initiator methionine is acetylated and (ii) those whose penultimate residue is acetylated after cleavage of the initiator methionine.     

Effects of alterations of the amino-terminal glycine of influenza hemagglutinin fusion peptide on its structure,organization and membrane interactions

Mutations of the glycine residue at the amino terminus of HA2 have been shown to have a large effect on the fusion activity of HA2, the extent of which apparently correlates with the side chain bulkiness of the substituting amino acids. To investigate into the cause of abrogation in fusogenicity and virus-promoted fusion mechanism, we synthesized several peptides in which this glycine was substituted by serine, glutamic acid, or lysine. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dimyristoyl sn-glycero-3-phosphoglycerol (DMPG) were used as model membranes in the fluorescence, circular dichroism (CD), and FTIR measurements while sodium dodecyl sulfate was used in NMR studies. We found that, for the less active variants, affinity to membrane, degree of solvent dehydration, lipid perturbation, depth of insertion, and helicity were less. Comparison of affinity to membrane bilayer among these analogs revealed that binding of the fusion peptide is determined largely by the hydrophobic effect. Additionally, the orientation is closer to the membrane normal for the wild-type fusion peptide in the helix form while the inactive analogs inserted more parallel to the membrane surface.     

Functional analysis of the putative fusion domain of the baculovirus envelope fusion protein F

Group II nucleopolyhedroviruses (NPVs), e.g., Spodoptera exigua MNPV, lack a GP64-like protein that is present in group I NPVs but have an unrelated envelope fusion protein named F. In contrast to GP64, the F protein has to be activated by a posttranslational cleavage mechanism to become fusogenic. In several vertebrate viral fusion proteins, the cleavage activation generates a new N terminus which forms the so-called fusion peptide. This fusion peptide inserts in the cellular membrane, thereby facilitating apposition of the viral and cellular membrane upon sequential conformational changes of the fusion protein. A similar peptide has been identified in NPV F proteins at the N terminus of the large membrane-anchored subunit F(1). The role of individual amino acids in this putative fusion peptide on viral infectivity and propagation was studied by mutagenesis. Mutant F proteins with single amino acid changes as well as an F protein with a deleted putative fusion peptide were introduced in gp64-null Autographa californica MNPV budded viruses (BVs). None of the mutations analyzed had an major effect on the processing and incorporation of F proteins in the envelope of BVs. Only two mutants, one with a substitution for a hydrophobic residue (F152R) and one with a deleted putative fusion peptide, were completely unable to rescue the gp64-null mutant. Several nonconservative substitutions for other hydrophobic residues and the conserved lysine residue had only an effect on viral infectivity. In contrast to what was expected from vertebrate virus fusion peptides, alanine substitutions for glycines did not show any effect.     

Effects of alterations of the amino-terminal glycine of influenza hemagglutinin fusion peptide on its structure, organization and membrane interactions

Mutations of the glycine residue at the amino terminus of HA2 have been shown to have a large effect on the fusion activity of HA2, the extent of which apparently correlates with the side chain bulkiness of the substituting amino acids. To investigate into the cause of abrogation in fusogenicity and virus-promoted fusion mechanism, we synthesized several peptides in which this glycine was substituted by serine, glutamic acid, or lysine. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dimyristoyl sn-glycero-3-phosphoglycerol (DMPG) were used as model membranes in the fluorescence, circular dichroism (CD), and FTIR measurements while sodium dodecyl sulfate was used in NMR studies. We found that, for the less active variants, affinity to membrane, degree of solvent dehydration, lipid perturbation, depth of insertion, and helicity were less. Comparison of affinity to membrane bilayer among these analogs revealed that binding of the fusion peptide is determined largely by the hydrophobic effect. Additionally, the orientation is closer to the membrane normal for the wild-type fusion peptide in the helix form while the inactive analogs inserted more parallel to the membrane surface.     

Charged residues are involved in membrane fusion mediated by a hydrophilic peptide located in vesicular stomatitis virus G protein

Membrane fusion is an essential step of the internalization process of the enveloped animal viruses. Vesicular stomatitis virus (VSV) infection is mediated by virus spike glycoprotein G, which induces membrane fusion at the acidic environment of the endosomal compartment. In a previous work, we identified a specific sequence in VSV G protein, comprising the residues 145 to 164, directly involved in membrane interaction and fusion. Unlike fusion peptides from other viruses, this sequence is very hydrophilic, containing six charged residues, but it was as efficient as the virus in catalyzing membrane fusion at pH 6.0. Using a carboxyl-modifying agent, dicyclohexylcarbodiimide (DCCD), and several synthetic mutant peptides, we demonstrated that the negative charges of peptide acidic residues, especially Asp153 and Glu158, participate in the formation of a hydrophobic domain at pH 6.0, which is necessary to the peptide-induced membrane fusion. The formation of the hydrophobic region and the membrane fusion itself were dependent on peptide concentration in a higher than linear fashion, suggesting the involvement of peptide oligomerization. His148 was also necessary to hydrophobicity and fusion, suggesting that peptide oligomerization occurs through intermolecular electrostatic interactions between the positively-charged His and a negatively-charged acidic residue of two peptide molecules. Oligomerization of hydrophilic peptides creates a hydrophobic region that is essential for the interaction with the membrane that results in fusion.     

Role of hemagglutinin surface density in the initial stages of influenza virus fusion: lack of evidence for cooperativity

Membrane fusion mediated by influenza virus hemagglutinin (HA) is believed to proceed via the cooperative action of multiple HA trimers. To determine the minimal number of HA trimers required to trigger fusion, and to assess the importance of cooperativity between these HA trimers, we have generated virosomes containing coreconstituted HAs derived from two strains of virus with different pH dependencies for fusion, X-47 (optimal fusion at pH 5.1; threshold at pH 5.6) and A/Shangdong (optimal fusion at pH 5.6; threshold at pH 6.0), and measured fusion of these virosomes with erythrocyte ghosts by a fluorescence lipid mixing assay. Virosomes with different X-47-to-A/Shangdong HA ratios, at a constant HA-to-lipid ratio, showed comparable ghost-binding activities, and the low-pH-induced conformational change of A/Shangdong HA did not affect the fusion activity of X-47 HA. The initial rate of fusion of these virosomes at pH 5.7 increased directly proportional to the surface density of A/Shangdong HA, and a single A/Shangdong trimer per virosome appeared to suffice to induce fusion. The reciprocal of the lag time before the onset of fusion was directly proportional to the surface density of fusion-competent HA. These results support the notion that there is no cooperativity between HA trimers during influenza virus fusion.     

Tight binding of influenza virus hemagglutinin to its receptor interferes with fusion pore dilation

Deletion of oligosaccharide side chains near the receptor binding site of influenza virus A/USSR/90/77 (H1N1) hemagglutinin (HA) enhanced the binding of HA to erythrocyte receptors, as was also observed with A/FPV/Rostock/34 (H7N1). Correlated with the enhancement of binding activity, the cell fusion activity of HA was reduced. A mutant HA in which three oligosaccharide side chains were deleted showed the highest level of binding and the lowest level of fusion among the HAs tested. The cell fusion activity of the oligosaccharide deletion mutant of HA, however, was drastically elevated when the binding activity was reduced by deletion of four amino acids adjacent to the receptor binding site. Thus, a reciprocal relationship was observed between the receptor binding and the cell fusion activities of H1/USSR HA. No difference was observed, however, in lipid mixing activity, so-called hemifusion, between wild-type (WT) and oligosaccharide deletion mutant HAs. Soluble dye transfer testing showed that even the HA with the lowest cell fusion activity was able to form fusion pores through which a small molecule such as calcein could pass. However, electron microscopic studies revealed that a large molecule such as hemoglobin hardly passed through the fusion pores formed by the mutant HA, whereas hemoglobin did efficiently pass through those formed by the WT HA. These results suggested that interference in the process of dilation of fusion pores occurs when the binding of HA to the receptor is too tight. Since the viral nucleocapsid is far larger than hemoglobin, appropriate receptor binding affinity is important for virus entry.     

Viral fusion peptides and identification of membrane-interacting segments

Viral envelope glycoproteins promote infection by mediating fusion between viral and cellular membranes. Fusion occurs after dramatic conformational changes within fusion proteins, leading to the exposure of a short stretch of mostly apolar residues, termed the fusion peptide, which is presumed to insert into the membrane and initiate the fusion process. The typical global composition of fusion peptides, rich in hydrophobic but also in small amino acids such as alanine and glycine, was used here as bait to detect other peptidic segments that can insert into membranes. We so evidenced a similar composition in several cytotoxic peptides, which promote pore formation such as peptides involved in amyloidoses and hydrophobic alpha-hairpins of pore-forming toxins. It is suggested that the structural plasticity observed for several membrane active peptides can be conferred by this particular global amino acid composition, which could be thus used to predict such functional behavior from genome data.     

Fatty acids on the A/USSR/77 influenza virus hemagglutinin facilitate the transition from hemifusion to fusion pore formation

Influenza virus hemagglutinin (HA) has three highly conserved acylation sites close to the carboxyl terminus of the HA2 subunit, one in the transmembrane domain and two in the cytoplasmic domain. Each site is modified by palmitic acid through a thioester linkage to cysteine. To elucidate the biological significance of HA acylation, the acylation sites of HA of influenza virus strain A/USSR/77 (H1N1) were changed by site-directed mutagenesis, and the membrane fusion activity of mutant HAs lacking the acylation site(s) was examined quantitatively using transfer assays of lipid (R18) and aqueous (calcein) dyes. Lipid mixing, so-called hemifusion, activity was not affected by deacylation, whereas transfer of aqueous dye, so-called fusion pore formation, was dramatically restricted. When the fusion reaction was induced by a lower pH than the optimal one, calcein transfer with the mutant HAs was improved, but simultaneously a considerable calcein leakage into the medium was observed. From these results, we conclude that the palmitic acids on the H1 subtype HA facilitate the transition from hemifusion to fusion pore formation.