The role of the amino-acid sequence on the fusogenicity, secondary structure, and mechanism of membrane fusion was analyzed by comparing a peptide comprising both homologous seg-ments WT
Trang 1Sendai virus N-terminal fusion peptide consists of two similar repeats, both of which contribute to membrane fusion
Sergio G Peisajovich1, Raquel F Epand2, Richard M Epand2and Yechiel Shai1
1
Department of Biological Chemistry, Weizmann Institute of Science, Rehovot, Israel;2Department of Biochemistry, McMaster University Health Sciences Centre, Hamilton, Ontario, Canada
The N-terminal fusion peptide of Sendai virus F1envelope
glycoprotein is a stretch of 14 amino acids, most of which are
hydrophobic Following this region, we detected a segment
of 11 residues that are strikingly similar to the N-terminal
fusion peptide We found that, when anchored to the
mem-brane by palmitoylation of its N-terminus, this segment
(WT-palm-19–33) induces membrane fusion of large
unila-mellar liposomes to almost the same extent as a segment that
includes the N-terminal fusion peptide The activity of
WT-palm-19–33 was dependent on its specific sequence, as a
palmitoylated peptide with the same amino-acid
composi-tion but a scrambled sequence was inactive Interestingly,
two mutations (G7A and G12A) known to increase F1
-induced cell-cell fusion, also increased the homology between
the N-terminal fusion peptide and WT-palm-19–33 The role
of the amino-acid sequence on the fusogenicity, secondary
structure, and mechanism of membrane fusion was analyzed
by comparing a peptide comprising both homologous seg-ments (WT 1–33), a G12A mutant (G12A 1–33), a G7A– G12A double mutant (G7A–G12A 1–33), and a peptide with
a scrambled sequence (SC 1–33) Based on these experiments,
we postulate that replacement of Gly 7 and Gly12 by Ala increases the a helical content of the N-terminal region, with
a concomitant increase in its fusogenic activity Furthermore, the dissimilar abilities of the different peptides to induce membrane negative curvature as well as to promote isotropic
31P NMR signals, suggest that these mutations might also alter the extent of membrane penetration of the 33-residue peptide Interestingly, our results serve to explain the effect of the G7A and G12A mutations on the fusogenic activity of the parent F1protein in vivo
Keywords: viral entry; peptide–lipid interactions; spectro-scopic studies
A key step in the infection by enveloped viruses is the fusion
between the viral and the cellular plasma or endosomal
membranes Most of the specialized viral envelope proteins
directly involved in the fusion process, contain a discrete
region of apolar amino acids, termed the fusion peptide,
which is believed to play an important role in the merging of
the membranes [1] Although much is known about the 3D
structure of fragments of fusion proteins in the absence of
membranes [2–8], the intimate interplay between fusion
peptides and the membrane is still unknown Fusion
peptides’ insertion into the cell membrane [9,10], viral
membrane [11,12], or both [13,14] is believed to facilitate
local dehydration [15] and to promote increased negative
curvature strain in the bilayer (reviewed in [16]), factors that
can help to overcome the energetic barriers associated with
the fusion process In addition, fusion peptides can serve as membrane anchors that facilitate partition of other regions
of the viral envelope proteins to the membrane, which can subsequently participate in membrane merging [17] Viruses from the Paramyxoviridae family are important respiratory tract pathogens of humans [18] A salient feature
of Paramyxoviridae infection is the fusion between infected and noninfected cells [19], a process mediated by the paramyxovirus envelope glycoprotein F The F protein is synthesized as an inactive precursor, which is cleaved by a host protease, producing two fusion-active subunits, F1and F2 [20] F1 remains attached to the membrane by a transbilayer segment, whereas F2 and F1 are disulfide bonded
Although it was initially thought that viral fusion glycoproteins contained a single fusogenic region respon-sible for the actual merging of the membranes, over the last years a more complex view has emerged Both the region consecutive to the N-terminal fusion peptide and the one immediately before the transmembrane domain of
HIV-1 gp4HIV-1 were shown to facilitate membrane fusion [HIV-17,2HIV-1] Furthermore, the F1subunit of Sendai and Measles virus (two distantly related members of the Paramyxovirus family) were shown to contain, in addition to the N-terminal fusion domain, an internal fusogenic segment, located downstream of the N-terminal heptdad repeat [22,23] The structural organization of this internal fusogenic region, postulated based on studies using protein segments [22,24], was recently confirmed by the X-ray determined structure of the prefusion conformation of Newcastle disease virus F protein [25] Both in the cases of Paramyxovirus and
Correspondence to Y Shai, Department of Biological Chemistry,
Weizmann Institute of Science, Rehovot 76100, Israel.
Fax: + 972 8 344112, Tel.: + 972 8 342711,
E-mail: Yechiel.Shai@weizmann.ac.il
Abbreviations: ATR-FTIR, attenuated total reflection Fourier
transformed infrared spectroscopy; BOC, butyloxycarbonyl;
Cho, cholesterol; DiPoPtdEth,
dipalmitoleoylphosphatidylethanol-amine; DOPtdCho, dioleoylphosphatidylcholine; DOPtdEth,
diol-eoylphosphatidylethanolamine; LUV, large unilamellar vesicles;
NBD-PtdEth, N-(7-nitro-2,1,3-benzoxadiazol-4-yl)
phosphatidyleth-anolamine; Rho-PtdEth, N-(lissamine rhodamine B sulfonyl)
phos-phatidylethanolamine; T H , bilayer to hexagonal phase transition
temperature; DTGS, deuterated triglyceride sulfate.
(Received 1 April 2002, revised 14 July 2002, accepted 24 July 2002)
Trang 2Retrovirus internal fusogenic regions, the mechanism by
which these segments destabilize membranes remains
unknown
In an attempt to further our understanding of the
process of viral infection and to determine whether
the presence of a fusogenic region consecutive to the
N-terminal fusion peptide is a characteristic common to
unrelated viral families, here we analyzed the role in
membrane merging of the N-terminal domain of Sendai
virus F1 protein The first 14 amino acids of this region
are termed the N-terminal fusion peptide Following this
apolar segment, we detected a stretch of about 11
residues strikingly similar to the N-terminal fusion
peptide (Fig 1) We found that, when anchored to the
membrane by palmitoylation of its N-terminus, this
segment (WT-palm-19–33) induces membrane fusion of
large unilamellar liposomes to almost the same extent as
a longer fragment that also includes the N-terminal
fusion peptide (WT 1–33); whereas a palmitoylated peptide with the same amino-acid composition of WT-palm-19–33, but a scrambled sequence (SC-palm-19–33), was inactive In addition, we analyzed the role of the amino-acid sequence on the fusogenicity, secondary structure, and mechanism of membrane fusion exerted
by the F1N-terminal region
E X P E R I M E N T A L P R O C E D U R E S
Materials BOC-amino acids were purchased from Novabiochem AG (La¨ufelfingen, Switzerland), and BOC-amino acid phenyl-acetamidomethyl (PAM)-resin was obtained from Applied Biosystems (Foster City, CA, USA) Reagents for peptide synthesis were obtained from Sigma Dioleoylphosphatidyl-choline (DOPtdCho), dioleoylphosphatidylethanolamine (DOPtdEth), and dipalmitoleoylphosphatidylethanolamine (DiPoPtdEth) were purchased from Avanti Polar Lipids (Alabaster, AL, USA); cholesterol (Cho) was purchased from Lipid Products (South Nutfield, UK NBD-PtdEth and Rho-PtdEth were purchased from Molecular Probes (Eugene, OR) All other reagents were of analytical grade Buffers were prepared using double glass-distilled water NaCl/KCl/Pi is composed of NaCl (8 gÆL)1), KCl (0.2 gÆL)1), KH2PO4 (0.2 gÆL)1), and Na2HPO4 (1.09 gÆL)1),
pH 7.3
Peptide synthesis The peptides (derived from Sendai virus F1protein Swiss-prot entry P04856) were synthesized by a standard solid phase method using a Boc-strategy on PAM-resin as described [26] The peptides were cleaved from the resin
by HF treatment and purified by RP-HPLC Purity ( 99%) was confirmed by analytical HPLC The peptide compositions were determined by amino-acid analysis and mass spectrometry
Preparation of lipid vesicles Large unilamellar vesicles (LUV) were prepared from DOPtdCho, DOPtdEth, and Cho (1 : 1 : 1) and when necessary with different amounts of Rho-PtdEth and NBD-PtdEth, as follows: dry mixed lipid films were suspended in NaCl/KCl/Pi buffer by vortexing to produce large multi-lamellar vesicles The lipid suspension was freeze-thawed six times and then extruded 20 times through polycarbonate membranes with 0.1 lm-diameter pores (Nuclepore Corp., Pleasanton, CA, USA)
Peptide-Induced lipid mixing Lipid mixing of large unilamellar vesicles was measured using a fluorescence probe dilution assay [27] Lipid vesicles containing 0.6 mol% each of NBD-PtdEth (energy donor) and Rho-PtdEth (energy acceptor) were prepared in NaCl/ KCl/Pias described above A 1 : 4 mixture of labeled and unlabeled vesicles (110 lM total phospholipid concentra-tion) was suspended in 400 lL of NaCl/KCl/Pi, and a small volume of peptide in dimethylsulfoxide was added The increase in NBD fluorescence at 530 nm was monitored
Fig 1 Sendai virus F 1 N-terminal fusion domain is composed of two
repeats Panel A, the segment ranging from residue 7 to residue 17 is
homologous to the segment 21–31 The mutations G7A and G12A,
known to increase cell-cell fusion between cells expressing the F 1
protein and normal cells [30], increase also the homology between the
two segments: G7 matches A21 and G12 matches A26 Panel B,
sequence alignment of the segment 7–31 of Sendai virus with
homol-ogous regions in other paramyxoviruses (HPIV 1, Human
Parainflu-enza virus 1; Measles virus; Rinderpest virus; CDV, Canine Distemper
virus; Mumps virus; SV5, Simian Parainfluenza virus 5; NDV,
New-castle Disease virus) The consensus sequence consists of those amino
acids that are present in at least 50% of the aligned sequences Panel C,
alignment between the segments 7–17 and 21–31 of the consensus
sequence.
Trang 3with the excitation set at 467 nm The fluorescence intensity
before the addition of the peptide was referred to as zero
percent lipid mixing, and the fluorescence intensity upon the
addition of Triton X-100 (0.05% v/v) was referred to as
100% lipid mixing
Electron microscopy
The effects of the peptides on liposomal suspensions were
examined by negative-staining electron microscopy A drop
containing DOPtdCho/DOPtdEth/Cho (1 : 1 : 1) LUV
alone or a mixture of LUV and peptide was deposited onto
a carbon-coated grid and negatively stained with 2% uranyl
acetate The grids were examined using a JEOL JEM 100B
electron microscope (Japan Electron Optics Laboratory
Co., Tokyo, Japan)
Differential scanning calorimetry
A Nanocal instrument from Calorimetry Sciences
Corpora-tion (Spanish Fork, UT, USA) was used for all scans Films
composed of DiPoPtdEth and increasing mole fractions of
peptide were prepared by dissolving the lipid in chloroform:
methanol (2 : 1) and adding appropriate amounts of a
dilute methanolic solution of peptide The lipid DiPoPtdEth
was used for determining the effects of the peptides on
curvature because this lipid has a sharp bilayer to hexagonal
phase transition at a moderate temperature of 43C so that
small shifts in the temperature of the transition of this lipid
can easily be measured This is not the case for the
DOPtdCho/DOPtdEth/Cholesterol mixture used for other
purposes in this manuscript The films were dried in a test
tube under a stream of nitrogen and then kept for 2–3 h in a
vacuum dessicator They were hydrated with Pipes buffer
pH 7.40 (20 mM Pipes, 0.15M NaCl, 1 mM EDTA and
20 mgÆL)1 NaN3) to give a final lipid concentration of
7 mgÆmL)1, vortexed extensively and loaded into the
calorimeter sample cell The same buffer was placed in the
reference cell Heating scan rates of 0.75C min)1 were
used The bilayer to hexagonal phase transition was fitted
using parameters to describe an equilibrium with a single
van’t Hoff enthalpy and the transition temperature reported
as that for the fitted curve Data was analyzed with the
programORIGIN5.0
31P NMR Spectroscopy
The 31P NMR spectra were measured using suspensions
of about 10 mg of a lipid mixture containing equimolar
amounts of DOPtdCho, DOPtdEth and cholesterol, with
or without the addition of peptide at a lipid to peptide
molar ratio of 200 : 1 The lipids and peptide were mixed
in organic solvent and dried, as described for the DSC
The lipid film was hydrated with 200 lL of 20 mM Pipes,
1 mM EDTA, 150 mM NaCl with 20 mgÆL)1 NaN3,
pH 7.40 Spectra were obtained using a Bruker AV-500
spectrometer operating at 202.456 MHz in a 5-mm
broadband inverse probe with triple axis gradient
capa-bility The spectra were acquired over a 48.544-kHz sweep
width in 32K data points (0.338 s acquisition time) A 90
pulse width of 9.9 ls (90 flip angle) and a relaxation
delay of 3.0 s were used Composite pulse decoupling was
used to remove any proton coupling Generally, 700 free
induction decays were processed using an exponential line broadening of 100 Hz and were zero-filled to 64K prior to Fourier transformation Probe temperature was main-tained at 25C by a Bruker B-VT 3000 variable tem-perature unit Temtem-peratures were monitored with a calibrated thermocouple probe placed in the cavity of the NMR magnet
ATR-FTIR Measurements Spectra were obtained with a Bruker equinox 55 FTIR spectrometer equipped with a deuterated triglyceride sulfate (DTGS) detector and coupled with an ATR device For each spectrum, 150 scans were collected, with resolution of
4 cm)1 Samples were prepared as previously described [28] Briefly, DOPtdCho/DOPtdEth/Cho (1 : 1 : 1) (0.78 mg) alone or with peptide (23 lg) were deposited on a ZnSe horizontal ATR prism (80· 7 mm) Prior to sample preparation the trifluoroacetate (CF3COO–) counterions, which strongly associate with the peptide, were replaced with chloride ions through several washings of the peptides
in 0.1MHCl and lyophilizations This allowed the elimin-ation of the strong C¼O stretching absorption band near
1673 cm)1[29] Peptides were dissolved in methanol, and lipids in a 1 : 2 methanol/CHCl3 mixture Lipid-peptide mixtures or lipids with the corresponding volume of methanol were spread with a Teflon bar on the ZnSe prism Drying under vacuum for 30 min eliminated the solvents Polarized spectra were recorded and the respective spectra corresponding to pure phospholipids in each polarization were subtracted from the sample spectra to yield the difference spectra The background for each spectrum was a clean ZnSe prism Hydration of the sample was achieved by introduction of excess of deuterium oxide (2H2O) into a chamber placed on top the ZnSe prism in the ATR casting and incubation for 30 min prior to acquisition of spectra Any contribution of2H2O vapor to the absorbance spectra near the amide I peakregion was eliminated by subtraction
of the spectra of pure lipids equilibrated with2H2O under the same conditions
ATR-FTIR Data analysis Prior to curve fitting, a straight base line passing through the ordinates at 1700 cm)1and 1600 cm)1was subtracted To resolve overlapping bands, the spectra were processed using PEAKFITTM (Jandel Scientific, San Rafael, CA, USA) software Second-derivative spectra were calculated to identify the positions of the component bands in the spectra These wavenumbers were used as initial parameters for curve fitting with Gaussian component peaks Positions, bandwidths, and amplitudes of the peaks were varied until good agreement between the calculated sum of all compo-nents and the experimental spectra were achieved (r2> 0.995), under the following constraints: (a) the resulting bands shifted by no more than 2 cm)1from the initial parameters, and (b) all the peaks had reasonable half-widths (< 20–25 cm)1) The relative contents of different secondary structure elements were estimated by dividing the areas of individual peaks, assigned to particular secondary structure, by the whole area of the resulting amide I band The experiments were repeated twice and were found to be
in good agreement
Trang 4R E S U L T S
Sendai virus F1N-terminal fusion domain is composed
of two repeats
The N-terminal fusion peptide of Sendai virus F1envelope
glycoprotein is formed by the 14 most N-terminal amino
acids Following this apolar region, we detected a segment
of 11 residues strikingly similar to the N-terminal fusion
peptide As shown in Fig 1A, the segment ranging from
residue 7 to residue 17 is similar to the segment 21–31
Interestingly, the mutations G7A and G12A, known to
increase cell–cell fusion between cells expressing the F1
protein and normal cells [30], increase also the identity
between the two segments: G7 matches A21 and G12
matches A26 Furthermore, as shown in Fig 1B,C,
homo-logous regions exist in other paramyxoviruses This
intrigu-ing findintrigu-ing prompted us to investigate the role played by the
region consecutive to the Sendai virus N-terminal fusion
peptide in membrane fusion To this end, we synthesized a
peptide corresponding to amino acids 19–33 from Sendai F1
protein (WT-palm-19–33, see Table 1) and compared its
ability to induce lipid mixing of large unilamellar liposomes
with that of a longer segment (WT 1–33) that includes the
N-terminal fusion peptide In order to facilitate partition of
the short WT-palm-19–33 to the membrane, its N-terminus
was palmitoylated To ensure that palmitoylation did not
cause lipid mixing per se, we used as a control a
palmitoy-lated peptide with the same amino-acid composition of
WT-palm-19–33, but with a scrambled sequence
(SC-palm-19–33)
WT-palm-19–33 induces lipid mixing of large unilamellar
vesicles
The ability of the peptides to induce lipid mixing of
DOPtdCho/DOPtdEth/Cho (1 : 1 : 1) large unilamellar
vesicles (LUV) was determined by the probe-dilution assay
[27] As depicted in Fig 2, WT-palm-19–33 induces lipid
mixing of LUV in a dose-dependent manner, although it is
not as potent as the longer WT 1–33 On the contrary,
SC-palm-19–33 is poorly active and palmitic acid alone did not
induce any significant lipid mixing (not shown), reflecting
that the WT-palm-19–33¢s potency is not solely a
conse-quence of palmitoylation It has been previously reported
that the mutations G7A and G12A increase the cell–cell
fusion activity of the full-length F protein [30] Accordingly,
the G12A mutation enhanced the lipid mixing ability of a
peptide corresponding to the N-terminal segment of Sendai
F1protein toward negatively charged LUV composed of PS [31] As we wanted to determine the role of these mutations
on the mechanism of membrane fusion exerted by Sendai F1 protein, we also tested here the ability to induce lipid mixing
of zwitterionic DOPtdCho/DOPtdEth/Cho (1 : 1 : 1) LUV
of the mutant G12A 1–33, the double mutant G7A G12A 1–33, and a peptide with the same amino-acid composition
of WT 1–33, but a scrambled sequence (SC 1–33) We found that successive replacement of Gly 7 and Gly 12 by Ala results in higher fusogenic activity (Fig 2), whereas scram-bling of the wild-type sequence renders the SC 1–33 peptide inactive, indicating that the fusogenicity of the peptides
Table 1 Sequences of the peptides and lipopeptides.
WT 1–33 FFGAVIGTIALGVATSAQITAGIALAEAR
EAKR
G12A 1–33 FFGAVIGTIALAVATSAQITAGIALAEAR
EAKR
G7A–G12A 1–33 FFGAVIATIALAVATSAQITAGIALAEAR
EAKR
SC 1–33 VILEQRAFAVGGAILTSKFAIGGRTAAIA
TAEA
WT-palm-19–33 palmitoyl – ITAGIALAEAREAKR
SC-palm-19–33 palmitoyl -AERATAELGIKAIAR
Fig 2 Peptide-promoted membrane fusion of DOPtdCho/DOPtdEth/ Cho (1 : 1 : 1) LUV as determined by lipid mixing Panel A, dose dependence of lipid mixing Peptide aliquots were added to mixtures of LUV (22 l M ), containing 0.6% NBD-PtdEth and Rho-PtdEth, and unlabeled LUV (88 l M ) in NaCl/KCl/P i The increase in the fluores-cence was measured 15 min after the addition of the peptide The fluorescence intensity upon the addition of reduced Triton-X-100 (0.25% v/v) was referred to as 100% Symbols: WT 1–33, empty cir-cles; G12A 1–33, empty triangles; G7A–G12A 1–33, filled squares; SC 1–33, empty squares; WT-palm-19–33, filled circles; SC-palm-19–33, filled triangles Panel B, kinetics of lipid mixing for a peptide to lipid ratio of 0.06.
Trang 5depends on their specific sequences Note that when the
experiments were repeated using different liposome
prepa-rations, differences of 15–20% were observed However,
with any given single liposome preparation the relative
activities of the different peptides remained unchanged and
the error was never higher than 5–10%
Lipid mixing is a result of membrane fusion
In order to confirm that the observed intervesicular lipid
mixing was the result of membrane fusion, suspensions of
LUV were directly visualized under an electron microscope,
before and after the treatment with the peptides Briefly,
DOPtdCho/DOPtdEth/Cho (1 : 1 : 1) LUV of 100
nm-diameter (200 lM) were incubated for 15 min alone, or with
each of the peptides (peptide lipid)1molar ratio of 0.05) in
NaCl/KCl/Pi, before examination by electron microscopy
Figure 3 shows representative micrographs of the LUV
without any peptide (Panel A), with WT 1–33 (Panel B),
with G12A 1–33 (Panel C), with G7A G12A 1–33 (Panel
D), with WT-palm-19–33 (Panel E), and with SC 1–33
(Panel F) The activity of SC-palm-19–33 was similar to that
of SC 1–33 and therefore it is not shown It is evident from
the micrographs that the lipid mixing observed with G7A
G12A 1–33, G12A 1–33, WT 1–33, and WT-palm-19–33
appear concurrently with an increase in the size of the
vesicles, confirming that the ability of the peptides to induce
lipid mixing is the result of membrane fusion In order to
shed light into their mechanism of action, we analyzed the
ability of the peptides to lower the THof DiPoPtdEth, to
give rise to isotropic31P NMR signals, and determined the
secondary structure of the membrane-bound full-length
peptides by ATR-FTIR spectroscopy
Peptide effects on DiPoPtdEth transition temperature
THis a measure of the relative stability of the Laand HII lipid phases A reduction in TH with the addition of a peptide can be interpreted as a tendency of the peptide to promote negative curvature of the membrane As indicated
by shifts in TH, at high peptide concentrations, WT 1–33 is the most potent in lowering the transition temperature, followed by G12A 1–33, SC 1–33, and by G7A–G12A 1–33
On the other hand, WT-palm-19–33 slightly increases TH (Fig 4)
31
P NMR spectroscopy The shape of the31P NMR powder pattern of lipid mixtures serves as a good criterion for their morphology The mixture
of DOPtdCho:DOPtdEth:cholesterol (1 : 1 : 1) exhibits a spectrum typical of a membrane bilayer (see Fig 5) Upon addition of only 0.05 mol% peptide, WT 1–33 and to a lower extent G12A 1–33 cause the formation of a structure that gives rise to an isotropic component at the chemical shift of phosphoric acid (Fig 5) This is typical for highly curved membrane structures and the appearance of such peaks has been associated with higher rates of membrane fusion [32,33] Structures such as hemifusion intermediates and fusion pores have highly curved surfaces that would allow for the motional averaging of the chemical shift anisotropy of the phospholipid Interestingly, G7A G12A 1–33 and Palm-WT 19–33, although active in lipid mixing, did not give rise to significant isotropic components A similar lackof isotropic component was observed for the scrambled peptide, SC 1–33, as well as the short and lipid-mixing inactive peptides
Fig 3 Electron micrographs of negatively stained vesicles Panel A, DOPtdCho/DOPtd-Eth/Cho (1 : 1 : 1) LUV alone; Panel B, WT 1–33; Panel C, G12A 1–33; Panel D, G7A– G12A 1–33; Panel E, WT-palm-19–33; Panel
F, SC 1–33 The vesicles were incubated with the peptides ([peptide]/[lipid] 0.05) for 15 min prior to visualization The bar represents
200 nm The effect of SC-palm-19–33 was similar to that of SC 1–33, therefore it is not shown.
Trang 6Replacement of Gly7 and Gly12 by Ala results in higher
a-helical content in the membrane
As structure has been shown to be important for the activity
of fusion peptides [31,34,35] and in order to investigate
whether the different behavior of the peptides in inducing
lipid mixing of LUV, lowering the THof DiPoPtdEth, and
promoting isotropic31P NMR signals could be related to
differences in their structures, we determined the secondary
structure of the membrane-bound peptides by ATR-FTIR
spectroscopy The spectra of the different peptides and the
respective second derivatives calculated to identify the
positions of the components bands are shown in Fig 6
The percentages corresponding to the different structures
are listed in Table 2 As expected, the combined G7A and G12A mutations significantly increase the a-helical content
of the peptides, likely lowering the conformational flexibility associated with the higher amount of random structures observed in WT 1–33 On the other hand, scrambling of the wild-type sequence resulted in a very different spectrum with
a predominance of aggregated b strands
D I S C U S S I O N
The region consecutive to the N-terminal fusion peptide participates in the fusion process
In this study we report that the N-terminal domain of Sendai F1 protein can be considered as two consecutive repeats (Fig 1) Remarkably, the mutations G7A and G12A, known to enhance F1-induced cell-cell fusion [30], augment also the identity between the two segments Interestingly, we found that, when partitioned into the membrane by palmitoylation of its N-terminus, the most C-terminal repeat induces fusion of large unilamellar liposomes (Figs 2 and 3) It should be noted that membrane anchoring via a C-terminal cysteine coupled
to a modified phospholipid has been shown before to lead
to fusogenic properties in a short synthetic model peptide [36] However, the fusogenic activity of WT-palm-19–33 is not solely a consequence of its palmitoylation, as a palmitoylated peptide with the same amino-acid compo-sition of WT-palm-19–33, but a scrambled sequence, as well as palmitic acid alone, are not active These results suggest that the Sendai virus F1N-terminal domain is composed of two repeats, both of which participate in the actual merging of the viral and cellular membranes We
do not believe that palmitoylation fulfills all of the functions of the initial 18 amino acids, as addition of these residues results in a longer peptide with a substan-tially increased fusogenic activity The finding that homologous regions exist in other paramyxoviruses (Fig 1, panels B and C) suggests that the structural
Fig 5.31PNMR spectra of an equimolar
mixture of DOPtdCho:DOPtdEth:cholesterol
with and without the addition of 0.05 mol
per-centage peptide at 25 °C The samples were
hydrated with 20 m M Pipes, 1 m M EDTA,
150 m NaCl with 0.002% NaN , pH 7.40.
Fig 4 Shift of the bilayer to the hexagonal phase transition temperature
of DiPoPtdEth as a function of the mole fraction of peptide Symbols:
WT 1–33, filled circles; G12A 1–33, filled squares; SC 1–33, filled
diamonds; G7A–G12A 1–33, empty triangles; WT-palm-19–33, empty
circles.
Trang 7and functional organization reported here for Sendai
N-terminal fusion peptide, may be common to other
members of the Paramyxoviridae family Furthermore,
recently it was shown that the polar region consecutive to the HIV-1 gp41 N-terminal fusion peptide also enhances its fusogenic activity, presumably by promoting self-association of the fusion peptide [17] The similarity between what was found in HIV-1, a retrovirus, and what
we report here in Sendai, a paramyxovirus, suggests that the N-terminal fusogenic domains from these distantly related viruses share a common mechanism
Secondary structure modulates the fusogenic activity
of the peptides Changes in the secondary structure of fusogenic peptides have been shown to alter their activity [34] Interestingly, the mutations Gly7 to Ala and Gly12 to Ala were shown to increase F1-induced cell-cell fusion [30] Here, we analyzed how these mutations, which increase the identity between the two repeats, affect the structure and activity of the N-terminal region of Sendai F1 protein We found that replacement of Gly7 and Gly12 by Ala, which increased the
a helical content of the peptide when bound to DOPtdEth/ DOPtdCho/Cho (1 : 1 : 1) membranes, enhanced the fuso-genicity of the peptide On the contrary, scrambling of its amino-acid sequence resulted in an inactive peptide with a significantly reduced amount of a helix The presence of Gly
at position 7 and 12 in the wild-type Sendai F1 protein imparts a greater flexibility to this region Replacement of the two Gly by Ala, a residue with a higher helical propensity, may result in a longer or more stable helix The G7A and G12A mutations are associated in vivo with a severe cytopathic effect, thus glycine may have been selected
to balance high fusion activity with successful viral repli-cation [30] It should be mentioned that Rapaport and Shai [31] did not observed a significant difference in the a helical content of WT 1–33 and its G12A mutant, as determined
by circular dichroism in 70% TFE and methanol However, these are only membrane mimetic environments, whereas here we measured the peptides’ secondary structure in the presence of phospholipid membranes Unlike organic solvents, aqueous dispersions of phospholipids allow seg-ments of peptides to partition simultaneously into both aqueous and nonpolar solvent environments As observed with HIV-1 gp41 fusion peptide, we cannot rule out that other secondary structures play also some role during the fusion process [17,35,37,38]
The mechanism of membrane fusion According to the stalkmodel [36] in its modified form [39,40], both a membrane fusion pore and the inverted
Fig 6 FTIR spectra deconvolution of the fully deuterated amide I band
(1600–1700 cm 1 ) and their respective second derivatives Panel A, WT
1–33; panel B, G12A 1–33; panel C, G7A–G12A 1–33; panel D, SC
1–33 The component peaks are the result of a curve fitting using a
Gauss line shape The amide I frequencies characteristic of the various
secondary-structure elements were taken from [41] The sums of the
fitted components superimpose on the experimental amide I region
spectra The solid lines represent the experimental FTIR spectra after
Savitzky-Golay smoothing; the broken lines represent the fitted
com-ponents of the spectra A 100 : 1 lipid:peptide molar ratio was used.
Table 2 Secondary structure of the membrane-bound peptides according to FTIR spectroscopy A 100 : 1 lipid:peptide molar ratio was used The amide I frequencies characteristic of the various secondary-structure elements were taken from Jackson and Mantsch [41], mean values ± standard deviation are given.
Sample
Secondary Structure (%)
Trang 8hexagonal phase arise through a common intermediate The
first step is hemifusion between the outer leaflets of two
opposing membranes that results in a stalkwith high
negative curvature Subsequently, joining of the opposing
monolayers leads to formation of the more stable trans
monolayer contact (TMC) intermediate The pathway after
formation of TMCs diverges, leading either to membrane
fusion or to the formation of an inverted hexagonal phase
Rupture of a single TMC produces a fusion pore, whereas
transition to inverted hexagonal phase requires aggregation
of numerous TMCs [40] Several fusion peptides have been
shown to lower the transition temperature from lamellar to
inverted hexagonal phases (reviewed in [16]), indicating that
they promote negative curvature in the membrane, thus
favoring formation of the highly curved stalkintermediate
This property correlated well with the infectivity of influenza
virus containing single amino-acid mutations in the fusion
peptide segment of hemagglutinin [16] Here we observed an
inverse correlation between the lipid mixing ability of WT
1–33, G12A 1–33, and G7A G12A 1–33 and their ability to
lower THor to give rise to an isotropic peakin the31P NMR
spectra It should be noted that in the NMR and DSC
experiments the peptide was added to both sides of the
bilayer starting from a solution in organic solvent This is
different from the procedure for the lipid mixing assay in
which the peptide is added to one side of the bilayer in
buffer This difference in the methodology that had to be
used could contribute to a different behaviour of the
peptides in the different system An additional factor,
however, that could contribute to the higher fusogenic
activity of the single mutant G12A and the double mutant
G7A G12A, despite their smaller effect on THand on the
31P NMR isotropic peak, as compared to the wild-type
peptide, may be related to their more shallow penetration
into the membrane, as shown for different constructs of the
HIV-1 fusion peptide [17] This possibility is supported by
the effect of WT-palm-19–33, which due to the polar nature
of its amino-acid composition, is likely to be located on the
surface and, indeed, causes a slight increase in TH As
noticed before for other viral fusion peptides [1], when the
amino-acid sequence of Sendai F1 N-terminal region is
represented as an a helix, it forms a sided helix, with most of
the Gly and Ala residues lying on the same face We can
speculate that the presence of Ala at position 7 and/or 12
reduces the flexibility of G12A 1–33 and G7A G12A 1–33
by extending an a helix that runs closer to the membrane
surface, thus diminishing the insertion into the membrane
Then, G12A 1–33 and more markedly G7A G12A 1–33
protrude from the membrane more than WT 1–33; thus, at
high mole fraction, G12A 1–33 and even more G7A G12A
1–33 may sterically prevent aggregation of TMCs and the
concomitant transition to the inverted hexagonal phase
Fusion between two opposing membranes requires the
formation of only one fusion pore and therefore it is not
affected by a protruding peptide Alternatively, this
intrigu-ing observation might be related to their different potency in
facilitating the rupture of the dimple in the center of the
TMC The peptide that better promotes the rupture of the
TMC will favor formation of fusion pore-like structures
more easily, therefore lowering the chances of TMC
aggregation and subsequent hexagonal phase formation
The current study generalizes the finding that, consecutive
to their N-terminal fusion peptides, the envelope
glycopro-teins from Paramyxo- and Lentiviruses have a relatively polar helical segment that facilitates membrane fusion but
do not insert deeply into the membrane, suggesting that unrelated viral families share common mechanisms of cell entry How such surface seeking helices promote fusion remains to be determined but could include lowering the degree of hydration of the membrane surface
A C K N O W L E D G E M E N T S
Sergio G Peisajovich is supported by fellowships from The Mifal Ha’paiys Foundation of Israel and the Feinberg Graduate School of the Weizmann Institute of Science This workwas supported in part by the Canadian Institutes of Health Research (grant MT-7654).
R E F E R E N C E S
1 White, J., Kielian, M & Helenius, A (1983) Membrane fusion pro-teins of enveloped animal viruses Q Rev Biophys 16, 151–195.
2 Bullough, P.A., Hughson, F.M., Skehel, J.J & Wiley, D.C (1994) Structure of influenza haemagglutinin at the pH of membrane fusion Nature 371, 37–43.
3 Chan, D.C., Fass, D., Berger, J.M & Kim, P.S (1997) Core structure of gp41 from the HIV envelope glycoprotein Cell 89, 263–273.
4 Weissenhorn, W., Dessen, A., Harrison, S.C., Skehel, J.J & Wiley, D.C (1997) Atomic structure of the ectodomain from HIV-1 gp41 Nature 387, 426–430.
5 Malashkevich, V.N., Chan, D.C., Chutkowski, C.T & Kim, P.S (1998) Crystal structure of the simian immunodeficiency virus (SIV) gp41 core: conserved helical interactions underlie the broad inhibitory activity of gp41 peptides Proc Natl Acad Sci USA 95, 9134–9139.
6 Yang, Z.N., Mueser, T.C., Kaufman, J., Stahl, S.J., Wingfield, P.T & Hyde, C.C (1999) The crystal structure of the SIV gp41 ectodomain at 1.47 A resolution J Struct Biol 126, 131–144.
7 Kobe, B., Center, R.J., Kemp, B.E & Poumbourios, P (1999) Crystal structure of human T cell leukemia virus type 1 gp21 ectodomain crystallized as a maltose-binding protein chimera reveals structural evolution of retroviral transmembrane proteins Proc Natl Acad Sci USA 96, 4319–4324.
8 Baker, K.A., Dutch, R.E., Lamb, R.A & Jardetzky, T.S (1999) Structural basis for paramyxovirus-mediated membrane fusion Mol Cell 3, 309–319.
9 Harter, C., James, P., Bachi, T., Semenza, G & Brunner, J (1989) Hydrophobic binding of the ectodomain of influenza hemagglu-tinin to membranes occurs through the fusion peptide J Biol Chem 264, 6459–6464.
10 Stegmann, T., Doms, R.W & Helenius, A (1989) Protein-medi-ated membrane fusion Annu Rev Biophys Biophys Chem 18, 187–211.
11 Wharton, S.A., Martin, S.R., Ruigrok, R.W., Skehel, J.J & Wiley, D.C (1988) Membrane fusion by peptide analogues of influenza virus haemagglutinin J Gen Virol 69, 1847–1857.
12 Ruigrok, R.W., Aitken, A., Calder, L.J., Martin, S.R., Skehel, J.J., Wharton, S.A., Weis, W & Wiley, D.C (1988) Studies on the structure of the influenza virus haemagglutinin at the pH of membrane fusion J Gen Virol 69, 2785–2795.
13 Guy, H.R., Durell, S.R., Schoch, C & Blumenthal, R (1992) Analyzing the fusion process of influenza hemagglutinin by mutagenesis and molecular modeling Biophys J 62, 95–97.
14 Hughson, F.M (1995) Structural characterization of viral fusion proteins Curr Biol 5, 265–274.
15 Zimmerberg, J., Curran, M & Cohen, F.S (1991) A lipid/protein complex hypothesis for exocytotic fusion pore formation Ann.
NY Acad Sci 635, 307–317.
Trang 916 Epand, R.M (1998) Lipid polymorphism and protein–lipid
interactions Biochim Biophys Acta 1376, 353–368.
17 Peisajovich, S.G., Epand, R.F., Pritsker, M., Shai, Y & Epand,
R.M (2000) The polar region consecutive to HIV-1 fusion peptide
participates in membrane fusion Biochemistry 39, 1826–1833.
18 Collins, P.L., Chanock, R.M & McIntosh, K (1996)
Parainflu-enza Viruses In Virology (Fields, B.N., Knipe, D.M & Howley,
P.M., eds), pp 1205–1241 Lippincott-Raven, Philadelphia, PA,
USA.
19 Lamb, R.A & Kolakofsky, D (1996) Paramyxoviridae: The
Viruses and Their Replication In Virology (Fields, B.N., Knipe,
D.M & Howley, P.M., eds), pp 1177–1204 Lippincott-Raven,
Philadelphia, PA, USA.
20 Homma, M & Ohuchi, M (1973) Trypsin action on the growth of
sendai virus in tissue culture cells J Virol 12, 1457–1465.
21 Suarez, T., Gallaher, W.R., Agirre, A., Goni, F.M & Nieva, J.L.
(2000) Membrane interface-interacting sequences within the
ectodomain of the human immunodeficiency virus type 1 envelope
glycoprotein: putative role during viral fusion J Virol 74, 8038–
8047.
22 Peisajovich, S.G., Samuel, O & Shai, Y (2000) The F 1 protein of
paramyxoviruses has two fusion peptides: implications for
the mechanism of membrane fusion J Mol Biol 296, 1353–1365.
23 Samuel, O & Shai, Y (2001) Participation of two fusion peptides
in measles virus-induced membrane fusion: emerging similarity
with other Paramyxoviruses Biochemistry 40, 1340–1349.
24 Ghosh, J.K., Peisajovich, S.G & Shai, Y (2000) Sendai virus
internal fusion peptide: structural and functional characterization
and a plausible mode of viral entry inhibition Biochemistry 39,
11581–11592.
25 Chen, L., Gorman, J.J., McKimm-Breschkin, J., Lawrence, L.J.,
Tulloch, P.A., Smith, B.J., Colman, P.M & Lawrence, M.C.
(2001) The structure of the fusion glycoprotein of Newcastle
dis-ease virus suggests a novel paradigm for the molecular mechanism
of membrane fusion Structure 9, 255–266.
26 Merrifield, R.B., Vizioli, L.D & Boman, H.G (1982) Synthesis of
the antibacterial peptide cecropin A(1–33) Biochemistry 21, 5020–
5031.
27 Struck, D.K., Hoekstra, D & Pagano, R.E (1981) Use of
resonance energy transfer to monitor membrane fusion
Bio-chemistry 20, 4093–4099.
28 Gazit, E., Miller, I.R., Biggin, P.C., Sansom, M.S & Shai, Y.
(1996) Structure and orientation of the mammalian antibacterial
peptide cecropin P1 within phospholipid membranes J Mol Biol.
258, 860–870.
29 Surewicz, W.K., Mantsch, H.H & Chapman, D (1993)
Determi-nation of protein secondary structure by Fourier transform
infrared spectroscopy: a critical assessment Biochemistry 32, 389– 394.
30 Horvath, C.M & Lamb, R.A (1992) Studies on the fusion peptide of a paramyxovirus fusion glycoprotein: roles of conserved residues in cell fusion J Virol 66, 2443–2455.
31 Rapaport, D & Shai, Y (1994) Interaction of fluorescently labeled analogues of the amino-terminal fusion peptide of Sendai virus with phospholipid membranes J Biol Chem 269, 15124– 15131.
32 Siegel, D.P., Banschbach, J., Alford, D., Ellens, H., Lis, L.J., Quinn, P.J., Yeagle, P.L & Bentz, J (1989) Physiological levels of diacylglycerols in phospholipid membranes induce membrane fusion and stabilize inverted phases Biochemistry 28, 3703–3709.
33 Ellens, H., Siegel, D.P., Alford, D., Yeagle, P.L., Boni, L., Lis, L.J., Quinn, P.J & Bentz, J (1989) Membrane fusion and inverted phases Biochemistry 28, 3692–3703.
34 Lee, S., Aoki, R., Oishi, O., Aoyagi, H & Yamasaki, N (1992) Effect of amphipathic peptides with different alpha-helical con-tents on liposome-fusion Biochim Biophys Acta 1103, 157–162.
35 Nieva, J.L., Nir, S., Muga, A., Goni, F.M & Wilschut, J (1994) Interaction of the HIV-1 fusion peptide with phospholipid vesicles: different structural requirements for fusion and leakage Bio-chemistry 33, 3201–3209.
36 Markin, V.S., Kozlov, M.M & Borovjagin, V.L (1984) On the theory of membrane fusion The stalkmechanism Gen Physiol Biophys 5, 361–377.
37 Pritsker, M., Jones, P., Blumenthal, R & Shai, Y (1998) A syn-thetic all D -amino acid peptide corresponding to the N-terminal sequence of HIV-1 gp41 recognizes the wild-type fusion peptide in the membrane and inhibits HIV-1 envelope glycoprotein-mediated cell fusion Proc Natl Acad Sci USA 95, 7287–7292.
38 Pritsker, M., Rucker, J., Hoffman, T.L., Doms, R.W & Shai, Y (1999) Effect of nonpolar substitutions of the conserved Phe 11 in the fusion peptide of HIV-1 gp41 on its function, structure, and organization in membranes Biochemistry 38, 11359–11371.
39 Siegel, D.P & Epand, R.M (1997) The mechanism of lamellar-to-inverted hexagonal phase transitions in phosphatidylethanol-amine: implications for membrane fusion mechanisms Biophys J.
73, 3089–3111.
40 Siegel, D.P (1999) The modified stalkmechanism of lamellar/ inverted phase transitions and its implications for membrane fusion Biophys J 76, 291–313.
41 Jackson, M & Mantsch, H.H (1995) The use and misuse of FTIR spectroscopy in the determination of protein structure Crit Rev Biochem Mol Biol 30, 95–120.