Curvature-Dependent Recognition of Ethanolamine Phospholipids by
Duramycin and Cinnamycin
Kunihiko Iwamoto,* Tomohiro Hayakawa,yMotohide Murate,* Asami Makino,* Kazuki Ito,zTetsuro Fujisawa,z
and Toshihide Kobayashi*y§
*Supra-Biomolecular System Research Group, RIKEN (Institute of Physical and Chemical Research) Frontier Research System, Saitama,
Japan;yLipid Biology Laboratory, RIKEN, Saitama, Japan;zRIKEN SPring-8 Center, Hyogo, Japan; and§INSERM UMR 870, INRA U1235,
INSA-Lyon, University Lyon 1 and Hospices Civils de Lyon, Villeurbanne, France
bind phosphatidylethanolamine (PE). The lipid specificity of duramycin was not established. The present study indicates that
both duramycin and cinnamycin exclusively bind to ethanolamine phospholipids (PE and ethanolamine plasmalogen). Model
membrane study indicates that the binding of duramycin and cinnamycin to PE-containing liposomes is dependent on membrane
curvature, i.e., the lantibiotics bind small vesicles more efficiently than large liposomes. The binding of the lantibiotics to
multilamellar liposomes induces tubulation of membranes, as revealed by electron microscopy and small-angle x-ray scattering.
These results suggest that both duramycin and cinnamycin promote their binding to the PE-containing membrane by deforming
Duramycin is a 19-amino-acid tetracyclic lantibiotic closely related to cinnamycin (Ro09-0198), which is known to
Duramycin is a 19-amino-acid tetracyclic peptide produced
by Streptoverticillium cinnamoneus and is closely related to
cinnamycin (Ro09-0198) (Fig. 1 A) (1–5). Both compounds
characterized by the presence of a high proportion of unusual
phosphatidylethanolamine (PE) (6–8). Because of this char-
acteristic, cinnamycin has been employed to study the
distribution and metabolism of PE (9–14). Duramycin is
specificity of duramycin is not well established. Previously it
was proposed that duramycin recognizes a particular mem-
brane conformation determined by the presence of PE or
monogalactosyl diglyceride (MGDG) (15). Analysis of the
membranes of the duramycin-resistant Bacillus subtilis
mutants revealed that they had little or no PE and cardiolipin
(15,16). In contrast, mutation of alkalophilic Bacillus firmus
to duramycin resistance resulted in a substantial replacement
of PE by its plasmalogen form (17).
In eukaryotic cells, PE is mainly restricted to the inner
leaflet of the plasma membrane (19–21). Recently we showed
that cinnamycin induces transbilayer phospholipid move-
ment of target cells in a PE-dependent manner (8). This
causes exposure of the inner leaflet PE to the peptide and
promotes binding of cinnamycin. When the surface concen-
tration of PE is high, cinnamycin induces membrane reor-
ganization such as membrane fusion and the alteration of the
membrane gross morphology (8). However, the detailed
membrane ultrastructure induced by cinnamycin binding is
not well determined. Although duramycin was known to alter
the membrane permeability of mammalian cells (18,22,23),
the precise mechanism(s) of duramycin-induced membrane
damage is not yet determined.
cinnamycin with model membranes. The results indicate that
both duramycin and cinnamycin selectively bind ethanola-
mine phospholipids, irrespective of whether they are of
diacyl- or plasmalogen type. The binding of the lantibiotics
induces reorganization of the membrane into highly curved
tubular structures as revealed by electron microscopy and
small-angle x-ray scattering (SAXS). In addition, we found
the lantibiotics preferentially bind PE in the highly curved
membranes. Thus, both duramycin and cinnamycin promote
ment and by changing membrane curvature.
MATERIALS AND METHODS
The following were purchased from Avanti Polar Lipids (Alabaster, AL):
L-a-phosphatidylcholine (egg, chicken; egg PC).
L-a-phosphatidylethanolamine (egg, chicken; egg PE).
L-a-phosphatidylethanolamine (liver, bovine; liver PE).
Submitted November 21, 2006, and accepted for publication April 27, 2007.
Address reprint requests to Toshihide Kobayashi, Tel.: 81-48-467-9534;
Editor: Michael Edidin.
? 2007 by the Biophysical Society
1608 Biophysical JournalVolume 93 September 20071608–1619
1-oleoyl-2-hydroxy-sn-glycero-3-phosphoethanolamine (18:1 lyso-PE).
L-a-phosphatidylserine (brain, porcine; brain PS).
L-a-phosphatidylinositol (liver, bovine; liver PI).
L-phosphatidyl-DL-glycerol (egg, chicken; egg PG).
L-a-phosphatidic acid (egg, chicken; egg PA).
glycerol (18:1 BMP).
cardiolipin (heart, bovine; heart CL).
chicken; egg SM).
porcine; brain SM).
Total cerebrosides (brain, porcine; GalCer).
From Matreya (Pleasant Gap, PA):
Glucosylceramide (human; GlcCer), lactosylceramide (LacCer), and
monogalactosyl diglyceride (plant, hydrogenated; 18:0 MGDG).
From Larodan Fine Chemicals (Malmo ¨, Sweden):
Monogalactosyl diglyceride (plant MGDG).
From Wako Pure Chemical Industries (Osaka, Japan):
Ganglioside GM1(bovine brain; GM1), ganglioside GM2(NeuAc) (bovine
brain; GM2), and ganglioside GM3(NeuAc) (bovine; GM3).
From Sigma (St. Louis, MO):
1,2-didodecanoyl-sn-glycero-3-phosphoethanolamine (DLPE), 1,2-dimyr-
istoyl-sn-glycero-3-phosphoethanolamine (DMPE), and duramycin and
Abu, a-aminobutyric acid; Ala, alanine; Asn, asparagine; Asp, aspartic acid; Gln, glutamine; Gly, glycine; Lys, lysine; Phe, phenylalanine; Pro, proline; Val,
valine. Ala6is linked to Lys19as lysinoalanine. Ala-S-Ala: lanthionine, Ala-S-Abu: b-methyllanthionine, X2: lysine (duramycin) or arginine (cinnamycin)
(4,5). (B) Rabbit erythrocytes (final 3 3 107cells/ml) were incubated with various concentrations of duramycin for 30 min at 4?C or 37?C. Hemolysis was
measured as described in Materials and Methods.(C) Duramycinwas preincubated with various concentrations of MLVs composed of POPC,90 mol % POPC
and 10 mol % SOPE, or 90 mol % POPC and 10 mol % C18(plasm)-18:1 PE for 1 h at 37?C. The mixtures (final concentration of duramycin was 5 mM) were
then added to rabbit erythrocytes (final 3 3 107cells/ml) and further incubated for 30 min at 37?C, followed by the measurement of hemolysis. Horizontal axis
indicates the final concentration of the total lipids in MLVs. (D) Duramycin was preincubated with MLVs containing 90 mol % POPC and 10 mol % of
indicated lipids, followed by the measurement of hemolysis, as described in panel C. Final concentrations of duramycin and total lipids were 5 mM and
500 mM, respectively. Data are means 6 SD of at least three independent experiments.
Ethanolamine phospholipids inhibit hemolytic activity of duramycin. (A) Structure of duramycin and cinnamycin (Ro09-0198). Abbreviations:
Curvature-Dependent PE-Binding Peptides 1609
Biophysical Journal 93(5) 1608–1619
From Nacalai Tesque (Kyoto, Japan):
Sodium hydrosulfite (sodium dithionite), o-phenylenediamine, and
Dulbecco’s phosphate-buffered saline (?) (PBS). (The PBS, pH
7.4, contained 200 mg/l potassium chloride, 200 mg/l potassium
dihydrogenphosphate, 8000 mg/l sodium chloride, and 1150 mg/l di-
From CovalAb (Lyon, France):
Rabbit polyclonal antisera against duramycin.
Preparation of lipid vesicles
Multilamellar vesicles (MLVs) were prepared by hydrating a lipid film with
20 mM HEPES-NaOH (pH 7.4) and 100 mM NaCl, unless otherwise
indicated, and vortex mixing. To prepare large vesicles, MLVs were sub-
jected to extrusion through polycarbonate filters (Nucleopore, Maidstone,
UK) for 25 times using a two-syringe extruder (Avanti Polar Lipids,
Alabaster, AL). To prepare small vesicles, MLVs were subjected to
sonication using a XL-2020 sonicator (Misonix, Farmingdale, NY) until
peak diameters (nm) of the size distribution by number became ,50 nm.
The vesicle size was examined by dynamic light scattering measurements at
37?C using a Zetasizer Nano ZS (Malvern, Worcestershire, UK). In Fig. 5,
the vesicles were also examined by freeze-fracture electron microscopy.
Measurement of the amount of phospholipids residing in the
outer leaflet of the most external layer of liposomes
Two milliliters of the liposome suspensions (total 50 mM phospholipids)
containing 1 mol % N-NBD-PE were mixed with 20 ml of 1 mM sodium
hydrosulfite at 25?C and the fluorescence was measured using an FP-6500
spectrofluorometer (Jasco, Tokyo, Japan) with excitation and emission
quenches N-NBD-PE, which is localized in the outer leaflet of the most
external layer of the liposomes (24). The fluorescence was monitored until it
Measurement of hemolysis
Rabbit erythrocytes were prepared by washing rabbit whole blood (Nippon
Bio-Supply Center, Tokyo, Japan) with PBS. Measurement of hemolysis of
hemolysis was determined by incubating rabbit erythrocytes (3 3 107cells/
ml) on ice for 30 min, whereas 100% hemolysis was measured after three
on hemolytic activity of duramycin were examined as described previously
(25) with some modifications. In brief, 40 ml of duramycin solution in PBS
without 100 mM NaCl were mixed and incubated for 1 h at 37?C or at 4?C.
After the addition of 160 ml of rabbit erythrocyte suspensions in PBS, the
was measured. Final concentrations of erythrocytes, duramycin and lipo-
somes in the mixture are indicated in the figure legends. In the case of using
determined by measuring phosphorus content (26).
High-sensitivity titration calorimetry
Isothermal titration calorimetry (ITC) was performed using a Microcal
VP-ITC titration calorimeter (MicroCal, Northampton, MA) as described
previously (27) with some modifications. Duramycin was titrated with
vesicles at 37?C. Injection volumes were 8 ml. The calorimeter cell had a
reaction volume of 1.4034 ml. As duramycin was known to give a positive
Biuret test (2), the concentrations of duramycin were measured by using
BCA ProteinAssayReagent(Pierce Biotechnology,Rockford,IL)and/or by
determining dry weight of duramycin. The lipid concentrations of liposomes
were determined by measuring phosphorus content by phosphorus assay
(26). The heats of dilution were determined in the control experiments using
the buffer (20 mM HEPES-NaOH (pH 7.4) and 100 mM NaCl) in place of
duramycin solutions, and were subtracted from the heats determined in the
corresponding duramycin-lipid binding experiments. Binding constants (Ka)
of duramycin and ethanolamine phospholipids were calculated as
assuming duramycin and ethanolamine phospholipid form 1:1 complex (see
Results). Binding constants were estimated from ITC results by curve-fitting
analysis with ‘‘One Set of Sites’’ model from Origin, Ver. 5.0 (MicroCal,
Liposome binding assay using gel filtration
Various liposomes were incubated with duramycin for 30 min at 37?C in 20
mM HEPES-NaOH (pH 7.4), 100 mM NaCl buffer. After incubation,
liposome-bound duramycin was separated from free duramycin by gel fil-
tration as described previously (29,30) with some modifications. Five mil-
liliters polypropylene column (Pierce Biotechnology) was filled with 3 ml of
Bio-Gel A-15m Gel (Bio-Rad, Hercules, CA) equilibrated with the buffer.
After applying 100 ml of the reaction mixture, 200 ml of the buffer was
added and the eluent was collected. This step was repeated until majority of
duramycin was eluted from the column. Each fraction was analyzed for the
amounts of liposomes and duramycin. For quantification of the liposomes,
50 ml of each fraction was diluted with 50 ml of the buffer, followed by the
addition of 10 ml 10% Triton X-100. The fluorescence was measured using
an ARVO SX Multilabel Counter (Wallac, Turku, Finland) with excitation
and emission wavelengths at 485 and 535 nm, respectively. Duramycin was
quantified by enzyme-linked immunosorbent assay.
Enzyme-linked immunosorbent assay of duramycin
A fifty-microliter sample was added to each well of an Immulon 2HB
(Thermo Fisher Scientific, Waltham, MA) microtiter plate. After overnight
(TBS; 10 mM Tris-HCl, pH7.4, 150 mM NaCl). Two-hundred microliters of
30 mg/ml bovine serum albumin (Fraction V; Sigma, St. Louis, MO) in TBS
was then added to each well. After 2-h incubation at room temperature, the
wells were washed with TBS. The bound duramycin was detected by adding
anti-duramycin antiserum followed by incubation with ECL anti-rabbit IgG,
horseradish peroxidase-linked species-specific whole antibody (from don-
key; GE Healthcare UK, Buckinghamshire, UK). The intensity of the color
developed with o-phenylenediamine as a substrate was measured with a
Microplate Reader model 680 (Bio-Rad), reading the absorption at 490 nm
with reference at 630 nm.
Negative staining was performed as reported previously (8) with some
modifications. MLVs containing 2 mM total lipids were incubated with
various concentrations ofduramycinfor 30minat 37?C.After centrifugation
at 19,000 3 g for 30 min at 4?C, the pellet was suspended in the same
volume of the buffer containing 20 mM HEPES-NaOH (pH 7.4) and 100
mM NaCl buffer, fixed with 2.5% glutaraldehyde for 30 min at room
temperature and washed three times with the same buffer by centrifugation
at 19,000 3 g for 30 min at 4?C. For negative staining images, the
1610 Iwamoto et al.
Biophysical Journal 93(5) 1608–1619
suspensions were adsorbed onto poly-D-lysine-treated formvar-coated grids
and negatively stained with 2% sodium phosphotungstic acid. For freeze
fracture images, the samples were frozen in liquid propane cooled by liquid
nitrogen, fractured in a freeze-etching machine (Balzers BAF400T, Balzers,
Liechtenstein) at ?110?C, and replicated by platinum/carbon. Replicated
sampleswereimmersedinhouseholdbleach todissolvethelipids, washedin
water, and then mounted on formvar-coated copper grids. Both specimens
for negative staining and freeze fracture images were examined under trans-
mission electron microscope (Tecnai 12, Philips, Eindhoven, The Netherlands,
or 1200EX-II, JEOL, Tokyo, Japan). Electron micrographs recorded on im-
aging plates were scanned and digitized by an FDL 5000 imaging system
(Fuji Photo Film, Tokyo, Japan).
SAXS measurements were carried out at RIKEN Structural Biology
Beamline I (BL45XU) at SPring-8, 8 GeV synchrotron radiation source
(Hyogo, Japan) (31). The x-ray wavelength used was 0.9 A˚and the beam
size at the sample position was ;0.4 3 0.7 mm2. The distance of sample-to-
detector was 968 mm. Samples were measured in a sample cell with a path
length of 1.5 mm and a pair of thin quartz windows (30 mm thickness). The
sample temperature was controlled to 37 6 0.01?C with a high precision
thermoelectric device. The samples were allowed to equilibrate for at least
5 min and the diffraction collected with 2 min exposure. Buffer profiles were
also taken for background subtraction purposes. The SAXS patterns were
recorded with an imaging plate (30 3 30 cm2) system of RIGAKU R-Axis
IV21(32). The two-dimensional scattering patterns were circularly aver-
aged and reduced to one-dimensional profiles using FIT2D, Ver. 12.012
(http://www.esrf.fr/computing/scientific/FIT2D/), a two-dimensional data
reduction and analysis program. The reciprocal spacing (s) and scattering
s ¼ 1=d ¼ ð2=lÞsinu;
q ¼ 2ps ¼ ð4p=lÞsinu;
where d is the lattice spacing, 2u is the scattering angle, and l is the
wavelength of x ray, were calibrated with silver behenate by the long-period
spacing of 5.838 nm (34).
Modeling analysis of SAXS data
Small-angle scattering intensity, I(q), can be described as
IðqÞ ¼ kNPðqÞ;
where k is an instrument constant, N is the number density of particles, and
P(q) is the particle scatteringfunction.Of possible structuraltypestested, the
SAXS data of duramycin-membrane complexes was well fitted by a concen-
tric cylindrical shell model. The scattering function of a randomly oriented
cylindrical particle composed of ithshells is expressed as
where F(q) is the form factor of the cylinder with ithshells and 2H is the
length of the cylinder. The values Dri, Vi, and Ricorrespond to the average
excess electron density (contrast), volume, and radius of ithshell, respec-
tively. The value x is the angle between the longest particle axis and the
scattering vector q, and J1corresponds to the first-order Bessel function.
Based on the observation by the electron microscopy, the length of the
straight part of the duramycin-membrane rod was estimated to be ;150 nm,
which is much larger than the rod radius (;10 nm) and is out of limit of our
experimental resolution. Therefore, we set the 2H value with 150 nm for
this fitting analysis. In case of such large H, the axial factor in Eq. 4 drops
to zero very rapidly, unless those orientations where x is very small. This
means that the rods make a contribution to the scattering only when they
are nearly perpendicular to the scattering vector q. That is, the I(q) remains
nearly unaltered with a slight change of H (35,36) in q that we observed.
Duramycin specifically binds
Whereas the specific binding of cinnamycin to PE is estab-
lished, the lipid specificity of duramycin is obscure. Previ-
ously it was proposed that duramycin recognizes a particular
membrane conformation determined by the presence of PE
or MGDG (15). Studies on duramycin-resistant bacteria sug-
gest that duramycin binds to PE (15–17), but not to ethanol-
amine plasmalogen (17). In the present study, we examined
the lipid specificity of duramycin by 1), examination of the
inhibitory effects of liposomes from various lipids on the
toxicity of duramycin; and 2), heat measurement during in-
teraction of duramycin and the liposomes by ITC. Similar to
cinnamycin (6), duramycin exhibited temperature-dependent
hemolytic activity against rabbit erythrocytes (Fig. 1 B).
When duramycin was preincubated with POPC MLVs, the
hemolytic activity was not affected (Fig. 1, C and D). In
contrast, preincubation with PE-containing MLVs inhibited
the hemolytic activity, suggesting that duramycin binds PE
(Fig. 1, C and D). Similar inhibitory effects were observed
with ethanolamine plasmalogen-containing MLVs (Fig. 1, C
and D). Similar to duramycin, cinnamycin-induced hemo-
lysis was inhibited by PE- and ethanolamine plasmalogen-
containing liposomes (Supplementary Material, Fig. S1),
suggesting that cinnamycin also binds the plasmalogen form
of ethanolamine phospholipids. In Fig. 1 D, we examined the
effects of MLVs containing 90 mol % POPC and 10 mol %
of various lipids on duramycin-induced hemolysis. Hemo-
lysis was inhibited by the presence of PEs with different fatty
acids, ethanolamine plasmalogens, and lyso-PE. However,
other lipids, including MGDG, did not affect the hemolysis,
suggesting that duramycin specifically binds ethanolamine
The interaction of duramycin with ethanolamine phos-
pholipids was then examined by ITC. Whereas injections of
POPC (Fig. 2 A) or POPC/MGDG (9:1) (Fig. 2 D) lipo-
somes to duramycin solution exhibited only slight exother-
mic reactions, each injection of POPC/POPE (9:1) (Fig. 2 B)
or POPC/C16(plasm)-18:1 PE (9:1) (Fig. 2 C) liposome
suspension caused a distinct exothermic reaction. The re-
action enthalpy DH? was calculated from the ITC profile. In
Fig. 2 B, the total amount of duramycin in the sample cell
Curvature-Dependent PE-Binding Peptides 1611
Biophysical Journal 93(5) 1608–1619
was nd0¼ 28:2 nmol and the total heat measured until the
equivalent point was +12
thalpy was thus calculated as DH? ¼ +hi=nd0¼ ?5:0 kcal/
mol duramycin. Similarly, the reaction enthalpy DH? was
calculated as DH? ¼ +hi=nd0¼ ?3:8 kcal/mol duramycin
for POPC/C16(plasm)-18:1 PE liposome suspension (Fig.
2 C). In Fig. 2 B, the amount of PE injected in the first 12
steps was 53.3 nmol. Thus, duramycin/PE ratio was 1:1.89
when the peptide was consumed. Cinnamycin is reported to
form 1:1 complex with PE (37,38). Assuming duramycin also
forms 1:1 complex, our results suggest that only the PE in
the outer leaflet is accessible to duramycin when the PE con-
tent was 10 mol %. When the PE content of POPC/POPE
liposomes exceeded 20 mol %, duramycin bound more than
50 mol % of the total PE (data not shown), as reported on
cinnamycin (38). The binding constant of duramycin and
POPE and that of duramycin and C16(plasm)-18:1 PE were
estimated as Ka¼ (2.1 6 0.4) 3 108M?1and Ka¼ (1.1 6
0.2) 3 108M?1, respectively.
1hi¼ ?142 mcal. The reaction en-
The binding of duramycin and cinnamycin to PE is
dependent on the physical properties and the
curvature of the membrane
We examined the effect of physical properties of PE-containing
liposomes on the inhibition of duramycin-induced hemoly-
sis. As shown in Fig. 1 D, POPC/DSPE MLVs inhibited the
hemolytic activity of duramycin. In contrast, DSPC/DSPE
(9:1) MLVs did not inhibit duramycin-induced hemolysis
(Fig. 3 A). DSPC/POPE (Tm¼ 20?C (39)) and DSPC/DOPE
(Tm¼ ?16?C (39)) inhibited hemolysis (Fig. 3 A). It is
speculated that DSPC (Tm¼ 54.5?C)/DSPE (Tm¼ 74?C)
(39) provides tightly packed surfaces at 37?C. These results
suggest that the binding of duramycin to PE is dependent on
the physical properties of the membrane. In Fig. 3 B, the
effect of membrane curvature of DSPC/DSPE liposomes on
the inhibition ofduramycin-induced hemolysis wasexamined.
The parentheses in the figure legend indicate the diameters
of the liposomes examined by dynamic light scattering. In
contrast to MLVs, both small vesicles and large vesicles
inhibited hemolysis, small vesicles being more effective than
large vesicles. Similar results were obtained with cinnamycin
(Supplementary Material, Fig. S2).
The curvature-dependent binding of duramycin to DSPC/
DSPE membranes was then measured by ITC. Injections of
DSPC/DSPE (9:1) large vesicles of ;700 nm diameters into
duramycin solution resulted in slight exothermic reactions
(Fig. 4 B). In contrast, injections of DSPC/DSPE small
vesicles of ;40 nm diameters revealed distinct exothermic
reactions until the sixth injection (Fig. 4 D). We also found
that injections of DSPC small vesicles revealed, to some
extent, exothermic reactions, whereas those of DSPC large
vesicles did not (Fig. 4, A and C). We next investigated
whether duramycin preferentially binds highly curved mem-
branes when lipids contain unsaturated fatty acid. Fig. 3 C
shows the inhibition of duramycin-induced hemolysis by
liposomes with different sizes. POPC/POPE small vesicles
were slightly more effective than large vesicles and MLVs
when the liposomes were preincubated with duramycin at
37?C. When POPC/POPE liposomes were preincubated with
duramycin at 4?C, a clear difference was observed in the
inhibitory effects of small vesicles, large vesicles, and MLVs
with the small vesicles showing the most prominent effect
(Fig. 3 D). When duramycin was titrated with POPC/POPE
(9:1) large vesicles, the peak diameter of which was evaluated
as ;700 nm by dynamic light scattering, the binding constant
of duramycin to POPE was estimated to be Ka¼ (4.0 6 1.9) 3
107M?1from the ITC result (data not shown). This value is
five times smaller than that of the titration with POPC/POPE
vesicles in Fig. 2 B. The peak diameter of the POPC/POPE
vesicles used in Fig. 2 B was evaluated as ;100 nm by
dynamic light scattering. These results suggest that duramycin
preferentially binds PE in fluid and highly curved membranes.
In Fig. 5, A–D, large vesicles and small vesicles were
further characterized by freeze-fracture electron microscopy.
lipids. Lipid vesicles were prepared by extrusion through polycarbonate
filters with 100-nm pore size. ITC was performed as described in Materials
and Methods. The values 20.1 mM (A, B, and D) or 13.7 mM (C) duramycin
in the reaction cell (1.4034 ml) was titrated with 5.30 mM POPC (A), 5.55
mM POPC/POPE (9:1) (B), 5.05 mM POPC/C16(plasm)-18:1 PE (9:1) (C)
or ;6 mM POPC/plant MGDG (9:1) (D) at 37?C. Each peak corresponds to
the injection of 8 ml of liposomes. Data are representatives of three inde-
pendent experiments (A and B).
Duramycin specifically interacts with ethanolamine phospho-
1612 Iwamoto et al.
Biophysical Journal 93(5) 1608–1619
Freeze fracture replica indicates that most of the small
vesicles are unilamellar (Fig. 5, C and D), whereas large
vesicles sometimes contain several layers (Fig. 5, A and B).
The average of diameter of DSPC/DSPE small vesicles was
estimated to be 49.8 nm whereas that of large vesicles was
410 nm (Supplementary Material, Table S1). Dynamic light
scattering estimated the diameters of the same samples as
34.1 nm (small vesicles) and 797 nm (large vesicles), re-
spectively (Supplementary Material, Table S2). Although
the sizes of small vesicles were comparable between two
methods, freeze fracture replica gave smaller size of large
liposomes than dynamic light scattering. This could be be-
cause specimens were not always fractured in the plane of
the center. In Fig. 5 E, the amount of phospholipids in the
outer leaflet of most external layer of liposomes was esti-
mated by measuring the quenching of N-NBD-PE incorpo-
rated into liposomes with sodium hydrosulfite. N-NBD-PE
becomes nonfluorescent when reduced with sodium hydro-
sulfite. Since sodium hydrosulfite penetrate membranes very
slowly, it is possible to estimate the amount of N-NBD-PE
on the outer leaflet of most external layer of liposomes using
this method. Fig. 5 E indicates that almost 50% of N-NBD-PE
was quenched by sodium hydrosulfite in small vesicles,
indicating that most of small vesicle preparations were
unilamellar as shown in freeze fracture replica. In contrast,
only 30% of N-NBD-PE fluorescence was quenched in
DSPC/DSPE large vesicles. This result indicates that DSPC/
DSPE large vesicles are in average 1.5 layers in these
to small and large liposomes was directly measured. In this
assay, duramycin/(phospholipids in the outer leaflet of most
external layer of liposomes) ratio was adjusted to 1:10. After
incubation with liposomes, the mixture was separated by a
Bio-Gel A-15m gel filtration column (BioGel Company,
Cleveland, OH). Both small and large liposomes were not
retained in the column and recovered in the void fraction
whereas free duramycin was retained in the column (data not
shown). When duramycin was incubated with DSPC lipo-
somes, duramycin and liposomes were segregated after gel
filtration, irrespective of the size of liposomes (Fig. 5, F and
H). In contrast, when duramycin was incubated with DSPC/
DSPE small vesicles, duramycin eluted as a single peak
together with the liposomes, indicating the binding of
duramycin to the small liposomes (Fig. 5 I). The incubation
of duramycin with DSPC/DSPE large vesicles resulted in the
recovery of duramycin in both bound and unbound fraction
(Fig. 5 G). These results suggest that duramycin preferen-
tially bind to highly curved membrane.
Duramycin alters the organization of
Previously, weshowedthat cinnamycinalters thestructureof
PE-containing liposomes and induces leakage of the contents
and exposure of thelipids of the inner leaflet of theliposomes
by PE-containing liposomes is dependent on the physical
properties of the membrane. Large and small vesicles were
prepared by extrusion through polycarbonate filters with
1.0-mm pore size and by sonication, respectively. Dura-
mycin was preincubated with various concentrations of
liposomes for 1 h at 37?C (A–C) or at 4?C (D). The
liposomes were composed of indicated lipids and con-
tained 10 mol % PE when included. Peak diameters (nm)
of the vesicles evaluated by dynamic light scattering are
given in parentheses. After the preincubation, the mixtures
(final concentration of duramycin was 5 mM) were then
added to rabbit erythrocytes (final 3 3 107cells/ml) and
incubated for 30 min at 37?C. Hemolysis was measured as
described in Materials and Methods. The horizontal axis
indicates the final concentrations of total lipids in lipo-
Inhibition of duramycin-induced hemolysis
Curvature-Dependent PE-Binding Peptides1613
Biophysical Journal 93(5) 1608–1619
(8). Using duramycin, we confirmed that duramycin also
exhibits these properties (Supplemental Fig. S3). These re-
sults indicate that duramycin alters the structure of ethanol-
amine phospholipid-containing membranes, as observed for
cinnamycin. Duramycin-induced alteration of membranes
electron microscopy. Whereas duramycin did not alter the
structure of POPC MLVs (Fig. 6, C and E), it dramatically
deformed POPC/POPE (9:1) MLVs (Fig. 6, D and F) and
POPC/C16(plasm)-18:1 PE (9:1) MLVs (data not shown)
into a rodlike structure with branching and inflection points.
In addition, sometimes small (diameter 20–25 nm) round
shape images were obtained in freeze fracture micrograph
as flattened disks or vesicles, Fig. 6 F suggests the possibility
that duramycin induces the tubules with 20–25 nm diam-
eter. This structure was further supported by SAXS measure-
18:1 PE MLVs were treated with lower concentrations of
observed, but at a lower frequency (data not shown). In
contrast, when treated with 500 mM duramycin, the rodlike
shape was the major structure. The average length of the rod
the maximum length observed was .1 mm. The average
width was 21 nm (n¼ 177). A similar structure was observed
to be performed.
To further explore the ultrastructure of the duramycin-
induced rodlike structure, we measured SAXS of MLV
suspensions that were treated with various concentrations of
duramycin. The SAXS of POPC and POPC/POPE (9:1)
MLVs displayed prominent periodic peaks (Fig. 7) corre-
sponding to lamellar distances of 6.28 and 5.90 nm, respec-
duramycin upto atleast 500 mM (Fig. 7 A).Incontrast, thatof
POPC/POPE MLVs was dramatically altered by duramycin
treatment, i.e., the lamellar structure-derived peaks decreased
with increases in the duramycin concentration and, moreover,
new scattering curves appeared (Fig. 7 B). These scattering
curves, which have well-defined maxima, are attributable to
the form factor from the rodlike structure observed in elec-
tron microscopy. Similar alteration of the SAXS pattern by
duramycin treatment was also observed in POPC/SAPE,
POPC/SDPE, POPC/C16(plasm)-18:1 PE, POPC/1-O-19-
anolamine (C18(plasm)-20:4 PE), POPC/1-O-19-(Z)-octade-
(C18(plasm)-22:6 PE), 1,2-dioleoyl-sn-glycero-3-phospho-
choline (DOPC)/DOPE, and DOPC/C18(plasm)-18:1 PE
MLVs (data not shown). The SAXS pattern of cinnamycin-
To obtain quantitative insight into the organization of the
complexes of duramycin and the POPC/POPE membrane,
we analyzed the SAXS data with a model fitting technique.
Based on the observation by electron microscopy, it was
assumed that the duramycin-membrane complexes form rod-
like structure with a ;20 nm diameter. Moreover, the SAXS
patterns in Fig. 7 B exhibit well-defined maxima, suggesting
thatthe sizedistributionofthe roddiameterisnarrow.Fig. 8 A
shows the result of the fitting and the radial excess electron
density profile of the best fit model. Although neither ho-
mogeneous solid rod nor core/shell type hollow cylinder
models agreed with the experimental data (data not shown),
the model of a hollow cylinder with multishells simulated the
data very well (Fig. 8 A). This model fitting suggests that the
structure formed by duramycin and the POPC/POPE mem-
brane is not a solid rod with a homogeneous electron density,
but rather, a hollow tubule consisting of a POPC/POPE
bilayer (Fig. 8 B).
containing membranes. Small and large vesicles were prepared by soni-
cation and by extrusion through polycarbonate filters with 1.0-mm pore size,
respectively. ITC was performed as described in Materials and Methods.
The values 22.1 mM (A–C) or 26.0 mM (D) duramycin in the reaction cell
(1.4034 ml) was titrated with 6.64 mM DSPC large vesicles (A), 6.21 mM
DSPC/DSPE (9:1) large vesicles (B), 6.70 mM DSPC small vesicles (C), or
6.50 mM DSPC/DSPE small vesicles (D) at 37?C. Peak diameters (nm) of
the liposomes evaluated by dynamic light scattering were ;800 nm (A),
;700 nm (B), ;30 nm (C), and ;40 nm (D), respectively. Each peak cor-
responds to the injection of 8 ml of liposome suspension. Data are rep-
resentatives of two independent experiments.
Curvature-dependent interaction between duramycin and PE-
1614 Iwamoto et al.
Biophysical Journal 93(5) 1608–1619
N-NBD-PE were prepared by sonication and by extrusion through polycarbonate filters with 1.0-mm pore size, respectively. (A–D) Representative freeze
fracture images and the size distribution of large (A and B) and small (C and D) vesicles composed of DSPC (A and C) and DSPC/DSPE (9:1) (B and D)
containing 1 mol % N-NBD-PE are shown. Bar, 500 nm (A and B) and 100 nm (C and D). (E) Sodium hydrosulfite (final 10 mM) was added to 50 mM (total
lipids) vesicles prepared above and the fluorescence was monitored while stirring at 25?C, as described in Materials and Methods. The percentage of the
phospholipid residing in the outer leaflet of the most external layer of liposomes was estimated from the decrease of fluorescence. (F–I) Large (F and G) and
small (H and I) vesicles composedof DSPC (F and H) and DSPC/DSPE (9:1) (G and I) containing 1 mol % N-NBD-PE were incubated with 30 mM duramycin
for 30 min at 37?C. In this assay, duramycin/(phospholipids in the outer leaflet of most external layer of liposomes) ratio was adjusted to 1:10, using the results
of panel E. After incubation, the liposome-bound duramycin was separated from free duramycin by gel filtration as described in Materials and Methods. The
amounts of liposomes and duramycin in each fraction are shown. Data are representatives of three independent experiments.
Curvature-dependent binding of duramycin to PE-containing membranes. Small and large vesicles (1 mM total phospholipids) containing 1 mol %
Curvature-Dependent PE-Binding Peptides1615
Biophysical Journal 93(5) 1608–1619
Duramycin and cinnamycin specifically bind
Duramycin was reported to induce aggregation of PE or
MGDG-containing liposomes (15). Duramycin also induces
high membrane conductance of PE-containing black lipid
membranes (18). These results suggest that the target of
duramycin is PE and/or MGDG. Consistent with this, several
independently isolated duramycin-resistant mutants of Ba-
cillus strains revealed remarkable reduction of PE and
cardiolipin (15–17). One of the mutants, instead, contained
plasmenylethanolamine (ethanolamine plasmalogen), which
was not detected in the parental strain, B. firmus (17).
Plasmalogen is a glycerophospholipid that has an alk-19-
enylether bond at position sn-1. These reports raise the pos-
form of PE. In this study, we examined the binding of
duramycin to various lipids indirectly by examination of the
inhibitory effects of lipids on duramycin-induced hemolysis
duramycin binds both the diacyl- and plasmalogen-form of
ethanolamine phospholipids. We did not observe the interac-
tion of duramycin with MGDG. It has not been examined
whether cinnamycin binds ethanolamine plasmalogen. Our
results indicate that cinnamycin also binds the diacyl- and
plasmalogen-form of ethanolamine phospholipids.
The binding of the lantibiotics to PE is dependent
on the curvature of the membrane
Whereas duramycin-induced hemolysis was inhibited by
small vesicles composed of DSPC/DSPE, MLVs with the
same lipid composition did not affect the hemolysis. ITC
studies revealed that duramycin interacts with ;40 nm
DSPC/DSPE vesicles, but not with ;700 nm vesicles.
Curvature-dependent inhibition by PE-containing liposomes
was observed both in duramycin- and cinnamycin-induced
hemolysis. These results suggest that the binding to PE-
containing liposomes is dependent on membrane curvature
and the lantibiotics preferentially bind to highly curved
membrane. This idea was further supported by the direct
measurement of the binding of duramycin to different li-
posomes. Previously it was shown that both the hydrophilic
headgroup and hydrophobic tail of PE are required for the
recognition by cinnamycin (6,7,40). It is speculated that
branes into a rodlike structure. Two millimolar (total
lipids) MLVs composedof POPC (A, C, and E) and POPC/
POPE (9:1) (B, D, and F) were incubated in the absence
(A and B) or the presence (C–F) of 500 mM duramycin for
30 min at 37?C. (A–D) Negative staining images, and
(E, F) freeze fracture replica. Arrows indicate round-shape
images, whose diameters were 20–25 nm. Bar, 100 nm.
Duramycin deforms PE-containing mem-
1616 Iwamoto et al.
Biophysical Journal 93(5) 1608–1619
the hydrophobic region of PE is more easily exposed to
duramycin and cinnamycin in small vesicles than in large
The titration of DSPC small vesicles to duramycin re-
vealed, to some extent, an exothermic pattern, whereas that
of DSPC large vesicles did not. These results imply that
duramycin weakly interacts with the highly curved lipid
bilayer irrespective of the lipid composition. Since DSPC
small vesicles did not exhibit inhibitory effects on duramycin-
induced hemolysis, this weak interaction is reversible and
is thus distinct from the interaction between duramycin
Duramycin and cinnamycin induce
The lantibiotics not only bind to PE in high curvature
membranes but also induce high curvature upon binding.
According to the obtained model in Fig. 8, the radius of the
core region, which has zero contrast, corresponding to water,
is 4 nm and the thickness of the three shells is in total 4.6 nm
(two 0.8-nm headgroup regions and a 3-nm hydrophobic
region). These obtained dimensions of the hydrophilic and
hydrophobic regions on the membrane are fairly consistent
with the dimensions previously reported on the POPC and
POPE membranes (41–43). The estimated diameter of the
tubular structure of the duramycin-membrane complex is
17.2 nm. This value is smaller than the value observed with
deformation of the duramycin-membrane tubules adsorbed
on the grid during the sample preparation in electron mi-
croscopy, this value (17.2 nm) may be reasonable for the
actual diameter in the aqueous solution. For duramycin and
cinnamycin, the electron density estimated based on the
partial molar volumes of the amino acids (44,45) was ;0.43
electrons/A˚3. Since the electron density of the lantibiotics
is close to that of the POPC/POPE headgroups (;0.42
electrons/A˚3) (46–48), the presence of the lantibiotic in the
hydrophilic region is difficult to evaluate in the present
analysis. On the other hand, the contrast in the hydrophobic
region differs significantly from the expected value for lipid
acyl chains. Generally, the hydrophobic region of the mem-
brane shows negative contrast since the electron density of
hydrocarbon chains is lower than that of water. However, in
the present analysis, the model which has a contrast cor-
responding to the electron density of the acyl chains of
phospholipids (;0.16–0.3 electrons/A˚3) (46–48) in the hy-
drophobic regiondid not fit to the experimental data (data not
shown). The best-fit model in this analysis showed a higher
density in the hydrophobic region, which is even higher than
that of water (0.33 electrons/A˚3) (Fig. 8 A). The high contrast
in the hydrophobic region can be interpreted as a penetra-
tion of duramycin, which has a high electron density, into
the hydrophobic region of the membrane. Recent results of
Machaidze and Seelig (40) indicate that both the PE head-
group and hydrocarbon chains are important in the binding
of cinnamycin to PE. In both cinnamycin and duramycin,
lipophilic amino acids are positioned at one side of the
peptide, whereas the hydrophilic ones are located on the
opposite side (3,4). The results of Machaidze and Seelig
suggest that the first 8–10 segments of each hydrocarbon
chain of PE are in direct contact with a hydrophobic peptidic
SAXS data of POPC/POPE (9:1) membrane incubated with 500 mM
duramycin were fitted with the theoretical model function. The inset shows
the radial excess electron density (contrast, Dr) map of the best-fit model.
The zero level corresponds to the solvent electron density (rwater¼ 0.33
electrons/A˚3). (B) A schematic tubular model formed by duramycin and
POPC/POPE membrane. The core region (light blue), which has zero
contrast, corresponds to water. The two high contrast regions (pink) and the
intermediate region (yellow) correspond to the hydrophilic and hydrophobic
regions of the lipid bilayer, respectively. Duramycin would penetrate into
the hydrophobic region (yellow) (see Discussion).
Model-fitting analysis of SAXS data. (A) The experimental
Two millimolar (total lipids) MLVs composed of POPC (A) and POPC/
POPE(9:1)(B) wereincubatedwith indicatedconcentrations (0–500 mM) of
duramycin for 30 min at 37?C before SAXS measurements. All measure-
ments were performed at 37?C. SAXS measurement was performed as
described in Materials and Methods. The splitting of the lamellar structure-
derived peak in panel A could be the cation-induced phase separation of
POPC in liquid crystalline Laphase (52). While the lamellar peak pattern of
POPC MLVs was not affected by duramycin, the pattern of POPC/POPE
MLVs was dramatically altered by duramycin treatment.
Duramycin alters SAXS pattern of PE-containing membranes.
Curvature-Dependent PE-Binding Peptides 1617
Biophysical Journal 93(5) 1608–1619
surface of cinnamycin. Since the hydrophobic amino acids
are identical between cinnamycin and duramycin, one can
expect the penetration of duramycin to the hydrophobic
region of PE membrane.
Duramycin and cinnamycin promote membrane
binding by inducing transbilayer lipid movement
and by changing membrane curvature
PE mainly resides in the inner layer of the plasma membrane
(19,21,49). To induce cell lysis, duramycin and cinnamycin
the cell surface. Previously we showed that cinnamycin
promotes cell binding by inducing transbilayer lipid move-
ment (8). Transbilayer lipid movement in the plasma mem-
brane causes the exposure of PE to the outer leaflet. The
present study indicates that, in addition to inducing trans-
bilayer lipid movement, duramycin and cinnamycin alter
the membrane to highly curved tubular structures. Since
duramycin and cinnamycin prefer high curvature, the lanti-
biotics promote further binding of the peptides by inducing
tubulation. The mechanism of the lantibiotics-induced mem-
tubulation is accompanied by the transbilayer lipid move-
ment, and the resultant high curvature may be accounted for
by a biased outward-directed transbilayer lipid movement.
Exposure or high curvature?
Our results indicate that the binding of both duramycin and
cinnamycin to PE is dependent on the curvature of the mem-
tion of PE. It is reported that PE is exposed at restricted sites
of the cell surface. Using an amino-reactive probe, trinitro-
benzene sulfonic acid, it has been shown that in steady-state
fibroblasts, 2–2.5 mol % of total PE is exposed on the cell
surface (50,51). From the present results, one cannot exclude
the possibility that PE is evenly distributed on the cell sur-
face and cinnamycin recognizes the high curvature, instead
of the exposure of PE. Further studies are required to under-
stand cellular distribution of PE.
To view all of the supplemental files associated with this
article, visit www.biophysj.org.
We are grateful to H. Iwase (Japan Atomic Energy Agency (JAEA)) and H.
Takahashi (Gunma University) for fruitful discussions on the analysis of
SAXS data, to A. Yamaji-Hasegawa (RIKEN) for the technical help in
measurement of hemolysis and helpful discussions, to T. Zimmer (Friedrich
Schiller Univ. Jena) for valuable discussions, to R. Ishitsuka, H.
Shogomori, Y. Ueda, K. Ishii, M. Abe, and K. Tamada for their help in
SAXS measurements at SPring-8, to K. Tamada, Y. Ueda, F. Hullin-
Matsuda, and R. Ishitsuka for critically reading the manuscript, and to all
members of the Kobayashi labs for valuable discussions.
This work was supported by grants from the Ministry of Education, Science,
Sports and Culture of Japan (Nos. 17390025 and 18050040 to T.K., No.
17659058 to M.M.), grants from RIKEN Frontier Research System, Chem-
ical Biology Project of RIKEN, RIKEN Presidential Research Grant for
Intersystem Collaboration (to T.K.), and a grant from the Hayashi Memorial
Foundation for Female Natural Scientists (to A.M.). K.I. is a Special
Postdoctoral Researcher of RIKEN.
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