Download European Journal of Biochemistry

Eur. J. Biochem. 101, 571 -579 (1979)
Architecture of the Outer Membrane of Escherichia coli K12
Phase Transitions of the Bacteriophage K3 Receptor Complex
Loek VAN ALPHEN, Ben LUGTENBERG, Ernst Th. RIETSCHEL, and Chris MOMBERS
Department of Molecular Cell Biology, Section Microbiology and Institute for Molecular Biology, State University of Utrecht ;
Max-Planck-Institute fur Immunologie, Freiburg; and Department of Biochemistry, State University of Utrecht
(Received June 7, 1979)
The adsorption constant of the irreversible adsorption of the bacteriophage K3 to Escherichia
coli K12 bacteria is strongly dependent on the incubation temperature. Two inflection points are
observed in an Arrhenius plot. For cells grown at 37 "C the inflection points are found at 20 "C and
28 "C whereas these inflection points shift to 10 "C and 19 "C for cells grown at 12 "C.
To study the lipid environment of the receptor the temperature dependence of the inactivation
of bacteriophage K3 was measured in vitro in the presence of various lipids. The Arrhenius plots
of the rate of inactivation of phage K3 by complexes of protein d and lipopolysaccharide are very
similar to those observed for whole cells. With lipopolysaccharide isolated from cells grown at
37 "C inflection points are observed at 20 "C and 28 "C. With lipopolysaccharide from cells grown
at 12 "C the inflection points are found at 10 "C and 21 "C. These results show that the environment
of protein d in vivo can be mimicked perfectly in vitro by protein d/lipopolysaccharide complexes.
The fatty acid composition of lipopolysaccharide isolated from cells grown at 37 "C and at 12 "C
differs in that in the latter case the amounts of mono-unsaturated fatty acids (mainly palmitoleic
acid) are increased at the expense of lauric acid. This difference in fatty acid composition probably
explains the difference in the phase transition temperatures caused by the two lipopolysaccharide
preparations. A transition at the inflection point of the highest temperature is also found for
lipopolysaccharide using light-scattering measurements and appears to be a thermal transition,
since it is also observed in differential scanning calorimetry.
Cells of mutant strain CE1071 lacking outer membrane proteins b and c and concomitantly
containing phospholipids in the outer leaflet of the outer membrane, adsorb phage K3 with an almost
normal rate, but the shape of the Arrhenius plot differs from the curve of wild-type cells. The
characteristics of the adsorption of phage K3 to these mutant cells can be mimicked in vitro by the
incorporation of phospholipid into protein d/lipopolysaccharide complexes, indicating that phospholipids are part of the environment of the phage K3 receptor in cells of this mutant, but not in wildtype cells
The outer membrane of gram-negative bacteria
contains lipopolysaccharide, phospholipids and proteins. Lipopolysaccharide is located in the outer
monolayer [l] whereas phospholipids are probably
exclusively present in the inner monolayer [2,3]. One
of the major proteins of the outer membrane is the
heat-modifiable protein d [4,5], which is also referred
to as protein 3a [6], 1I* [7] and 0- 10 [8]. This protein
has been purified in its non-heat modified form [7,9].
In complex with lipopolysaccharide this protein inactivates bacteriophages K3 [9] and TuII* [lo, 111.
In the latter case the lipid A part of lipopolysaccharide
is as active as intact lipopolysaccharide in generating
phage receptor activity [l 11. Phospholipid or detergent
cannot replace lipopolysaccharide nor do they stimulate receptor activity [9,10].
Protein d is a membrane-spanning protein [9,10,
12,131 which interacts with lipopolysaccharide in vitro
[9 - 111. The way in which it interacts with the membrane is largely unknown.
In this paper the temperature dependence of the
adsorption of bacteriophage K3 to intact cells is
described as a biological tool to study the phase
transition of the phage receptor. Moreover, the temperature dependence of the activity of the reconstituted receptor is compared with that in intact cells.
Interaction of Protein d with Lipids in the Outer Membrane of E. coli
512
The results are discussed in relation to properties of
the outer membrane lipids, to lipid-protein interactions and to the architecture of the outer membrane.
A preliminary report of this work was presented
at the symposium 'Functions of Microbial Membranes' held at Tubingen, F.R.G., September 1977.
in which Pt and Po represent the number of phages at
times t and o respectively and N is the number of
bacteria. The adsorption constant k, was plotted
versus 1 / T in an Arrhenius plot. The incubations for
one plot were performed with the same batch of cells
to avoid that variations in the cell population influence the adsorption constant.
MATERIALS AND METHODS
Reconstitution of the Receptorf o r the Bacteriophage K3
and Determination of the Inactivation Constant ki
Strains and Growth Conditions
Cells of the following Escherichia coli K12 strains
were used. For bacteriophage adsorption experiments
cells of strain JC7620 [14], which is wild type with
respect to the structure of lipopolysaccharide and the
composition of the outer membrane proteins, were
grown in yeast broth at 37°C or at 12°C. Cells of
strain CE1071 were grown at 37 "C. The latter strain
contains an ompB mutation, resulting in the absence
of major outer membrane proteins b and c, which is
compensated for by increased amounts of phospholipids and lipopolysaccharide [2]. The composition
of the medium has been described before [15]. Bacteriophage K3 [16] was multiplied and purified as
described before [9]. Bacteriophage TuII* was a
generous gift from Dr U. Henning [17].
Adsorption of Bacteriophages to Cells
+
The rate of irreversible and irreversible
reversible adsorption of phage K3 to cells was measured as
follows. A suspension of cells (2 x lo7- 2 x 109/ml)
in yeast broth was supplemented with chloramphenicol
and KCN, final concentrations 100 pg/ml and 2.5 mM
respectively. After prewarming at the desired temperature 2 x lo7 plaque-forming units of phage K3 were
added per ml of suspension. Samples were taken at
various times, diluted lo4 times (only in the case of
the determination of the rate of irreversible adsorption) and filtered through a membrane filter (0.45 pm
pore size; Millipore, Bedford, Mass., U.S.A.) to
separate non-adsorbed phage from cells and adsorbed
phages. The filtrate was tested for plaque-forming
units after an appropriate dilution with a doublelayer technique.
The temperature dependence of the rate of adsorption of phages K3 and TuII* to cells was determined
as described above for the reversible
irreversible
adsorption of phage K3 to cells with the following
modification. The cell density was chosen appropriately between 5 x lo7 and 2 x lo1' cells/ml during the
adsorption experiments, which were performed at
various temperatures between 0 °C and 37°C. The
adsorption followed pseudo first-order kinetics at all
temperatures. The adsorption rate constant was determined according to the formules : log Pt/Po= - k, .N . t,
+
Protein d and phospholipids were isolated from
E. coli K12 as described previously [9], whereas
E. coli K 12 lipopolysaccharide was isolated according
to the method of Galanos et al. 1181. Ribonucleic
acid could not be detected by measuring the absorbance at 254nm of a solution of 10mg/ml. Phospholipids were removed by repeated extractions of
lipopolysaccharide with chloroform/methanol (2 : 1,
v/v) as was judged from thin-layer chromatograms
(2). Vesicles containing protein d, lipopolysaccharide
and, if indicated, phospholipids were prepared by
sonication and annealing as described [9]. For phage
inactivation experiments these vesicles were used
without purification. The vesicles appear to be homogenous as they sediment as a single peak in a 40- SO %
(v/v) Percoll gradient after centrifugation for 2 h at
125000 x g. This peak contains practically all the
protein d, lipopolysaccharide and, if present, phospholipid as indicated by dodecylsulphate-polyacrylamide gel electrophoresis [5] and after determination
of the radioactivity when 32P-labelled lipopolysaccharide [ 191 and [2-3H]glycerol-labeled phospholipid
were used. The density of vesicles containing phospholipid was considerably lower than that of those
consisting of protein d and lipopolysaccharide only.
The determine the capacity of the vesicles to inactivate the phage, the vesicles were diluted appropriately between twofold and fiftyfold into yeast broth
supplemented with KCN and chloramphenicol in
final concentrations of 2.5 mM and 100 pg/ml respectively. After preincubation at various temperatures
between 0 "C and 37 "C the inactivation of bacteriophage K3 by the reconstituted receptor was determined as described [9]. The dependence of the inactivation constant ki, which is comparable with the
adsorption constant k, for irreversible adsorption to
cells, on the incubation temperature was visualized
by plotting log ki versus 1/T.
Analysis of the Composition of Lipopolysaccharide
32P-labeled lipopolysaccharide was isolated and
analyzed by paper chromatography as described [19].
Quantitative determinations of the alditol acetates
of hydrolyzed lipopolysaccharide were performed with
gas-liquid chromatography [20]. Methods for the
L. van Alphen, B. Lugtenberg, E:. Th. Rietschel, and C. Mombers
determination of 3-deoxy-~-mannooctulosonic acid
[21], glucosamine [22] and phosphate [23] have been
described.
Fatty acids were liberated by treating lipopolysaccharide first with acid (4 M HCl, 4 h, 100'C) and
then with alkali (1 M KOH, 1 h, 100°C) [24]. They
were, in the form of their methyl esters, analyzed by
gas-liquid chromatography on SE 30 (10% on gaschrom Q, 100- 120 mesh, 170 "C) using a glass column
(200 x 0.3 cm) and nitrogen as carrier gas (30 ml/min).
Individual fatty acid methyl esters were characterized
by comparing their retention time relatively to that
of authentic standards as well as by mass spectrometry
which was performed under the conditions described
[25,26].
For quantitative assays heptadecanoic acid served
as an internal standard.
Measurement of the Phase Transition
of Lipopolysaccharide
Lipopolysaccharide was suspended in 10 mM TrisHCljl50 mM NaCI, pH 7.4, at a concentration of
30 mg/ml, by heating for 5 min at 55 "C, followed by
brief sonication and cooling to 0 "C. Phase transitions
were determined (a) by measuring light scattering at
455 nm as a function of the temperature by increasing
the temperature of a fivefold diluted preparation at a
rate of 1 "C/min in a Philips SP1700 double-beam
spectrophotometer and (b) by differential scanning
calorimetry of the pellet obtained after centrifugation
of 1ml of the lipopolysaccharide suspension for 10 min
at 12000 x g in a Perkin Elmer DSC 2 apparatus at a
rate of 1 "Cimin.
RESULTS
General Characteristics
of the Adsorption of Bacteriophage K3
Adsorption of bacteriophages to cells can occur
reversibly and/or irreversibly [27]. When after adsorption of phages to cells, free and adsorbed phages
are separated prior to dilution, the reduction in the
number of plaques represents both the irreversibly and
reversibly adsorbed phage. The amount of phage
which remains attached to the cells after extensive
dilution (10 000 times) at the incubation temperature
prior to separation of free and adsorbed phages represents the fraction of irreversibly adsorbed phage.
In order to exclude the possibility that changes in
adsorption rates of phage K3 to cells with temperature
are due to a shift in the ratio between reversible and
irreversible adsorption, both adsorption rates were
determined at various temperatures. Fig. 1 shows that
the curves for irreversible and irreversible plus reversible adsorption of bacteriophage K3 to cells of strain
573
Time (min)
10
20
0
-0.2
- 0.4
- 0.6
e
.
e-
-m
-on
- 1.c
- 1:.
-1.1
Fig. 1. Rate ojirreversible and reversible adsorption of bacteriophage
K3 to cells. Late exponential phase cells of strain JC7620 were
tested for rates of irreversible (closed symbols) and irreversible
reversible (open symbols) adsorptions of phage K3. (A) cells grown
at 37 "C (2 x lo7 cells/ml), tested at 37 "C (H, O), (B) cells grown at
37°C tested at 25°C ( 2 x lo7 cells/ml) (A,A) and (C) cells grown
at 12 "C tested at 18 "C (2 x lo9 cells/ml) (0,0)
+
JC7620 at 18 "C, 25 "C and 37 "C do not differ significantly, indicating that the rate of irreversible adsorption is hardly affected by the rate of reversible adsorption. It should be noted that the adsorption of phage
K3 at 18 "C was measured using cells grown at 12 "C,
since below 20°C phage K3 does not adsorb to cells
grown at 37 "C (see below).
The adsorption of phage K3 showed both in vivo
(Fig.1) and in vitro [9] first-order kinetics under the
experimental conditions used, in which no more than
95% of the phage adsorbed to cells or reconstituted
receptor. A deviation from the first-order kinetics
occurs when more than 95% of the phages adsorbs
and is probably due to inhomogeneity of phage
stocks [28]. Therefore in these experiments the adsorption constant can be regarded as a reaction constant of a first-order reaction. Consequently the
temperature dependence of the adsorption rate can
be expressed in an Arrhenius plot in order to detect
phase transitions of the receptor for phage K3.
Temperature Dependence of the Rate
of the Adsorption of Bacteriophage K3 to Cells
The rate of irreversible adsorption of phage K3
to cells of strain JC7620 grown at 37°C showed a
Interaction of Protein d with Lipids in the Outer Membrane of E. coli
514
-7.0
B
A
19OC
-8.0
28OC
-11.0 -
-12.0 -
3.2
3.3
-
3.4
03/7
3.5
3.2
3.3
-
34
lo3/,'
3.5
3.6
Fig. 2. Temperature dependence of the rate ofadsorption ofphage K3 to cells. Cells of strain JC7620 were grown at 37 "C (A) or 12 "C (B) and cells
of strain CE1071 were grown at 37°C (C). Adsorption experiments were performed at various temperatures and the adsorption rate
constant k , versus 1/T is visualized in an Arrhenius plot
strong dependence on the incubation temperature as
shown in an Arrhenius plot (Fig. 2A). Inflection points
were observed at 20°C and 28°C. Below 20°C adsorption of phage K3 to cells was not observed, even
not in highly concentrated cell suspensions (2 x 10"
cells/ml) over prolonged periods (2 h). Between 28 "C
and 37°C the adsorption rate constant was not dependent on the temperature, suggesting that the receptor is maximally accessible for the phage. Temperatures above 37°C were avoided as at these temperatures significant phage denaturation occurred.
The inflection points in the Arrhenius plot (Fig. 2A)
suggest the possibility that the activity of the phage K3
receptor is influenced by phase transitions in its lipid
environment. To test this idea an increase in the
fluidity of the outer membrane at lower temperatures
was introduced by lowering the growth temperature
of the cells from 37 "C to 12 "C, since it is known that
thereby the relative amount of unsaturated fatty acids
of the phospholipids is increased [29-311. This
temperature shift did not significantly influence the
amount of protein d as judged from dodecylsulphate/
polyacrylamide gel electrophoresis (not shown). When
cells of strain JC7620 grown at 12 "C were used in the
adsorption experiments, the Arrhenius plot had the
same shape as that for cells grown at 37 "C. However,
the inflection point temperatures were about 10 "C
lower (10 "C and 19 "C) (Fig. 2B). Again adsorption
of phage K3 to cells was not observed below the lower
inflection point temperature and the adsorption rate
is independent on the temperature above the highest
inflection point temperature (19 "C). The results clearly
show that in wild type cells the phase transition of the
phage K3 receptor decreases coinciding with decreased
growth temperature of the cells.
The phage adsorption characteristics of cells of
mutant strain CE1071 were studied (Fig.2C) as these
cells contain increased amounts of phospholipids
which are at least partly present in the outer leaflet
[2]. The characteristics of the Arrhenius plot differ
considerably from the ones observed with cells of
strain JC7620. Some phage adsorption occurs below
the inflection point temperature at 18 "C compared to
the adsorption of phage K3 to cells of strain JC7620.
Moreover, an inflection point around 28 "C could not
be observed, possible due to the low slope of the curve
above 18 "C.
Temperature Dependence of the Rate of Inactivation
of Phage K3 by the Reconstituted Receptor
In order to obtain further insight in the characteristics of the Arrhenius plot for phage K3 adsorption
to intact cells, similar adsorption experiments were
carried out with protein d/lipopolysaccharide complexes. Previously we have found that complexes of
protein d and lipopolysaccharide inactivate bacteriophage K3 irreversibly. Phospholipids could not replace lipopolysaccharide and moreover they did not
attribute to the ability of the reconstituted receptor to
inactivate the phage at 25°C [9]. 'The temperature
dependence of the phage inactivation by the reconstituted receptor is shown in Fig.3. The Arrhenius
plots of the reconstituted receptors resemble those of
the corresponding whole cells. Inactivation of phage
K3 by protein d/lipopolysaccharide (37 "C) vesicles
(Fig.3A) showed inflection points at the same temperatures as were found for adsorption of phage K3
to cells grown at 37°C (Fig.2A). Similarly, when
protein d/lipopolysaccharide (12 "C) vesicles were
used as receptor (Fig. 3 B) inflection points were found
at the same temperatures as in the case of adsorption
of phage K3 to cells grown at 12°C (Fig.2B). The
results clearly show that the growth temperature of
L. van Alphen, B. Lugtenberg, E. Th. Rietschel, and C. Mombers
-94-
3.2
33
-
3.4
1031~
35
3.6 52
515
3.3
3.4
o31r-
3.5
:
3.2
3.3
3.4
10~1~-
3.5
:3
Fig. 3. Temperature dependence of the rute of adsorption of phage K3 to reconstituted receptor complexes. Vesicles containing protein d(37 "C)
were prepared with lipopolysaccharide(37 "C) (A), with lipopolysaccharide(l2 "C) (B) and with lipopolysaccharide(37 "C) plus phospholipids(37 "C) (C). Inactivation of phage K3 by the reconstituted receptors was measured at various temperatures. The dependence of the
inactivation rate constant ki on the temperature is visualized in an Arrhenius plot
the cells which serve as the source for lipopolysaccharide determines the temperatures of the inflection
points. The only striking difference between the
adsorption characteristics of the phage to the reconstituted receptor and to whole cells is that some phage
inactivation by protein d/lipopolysaccharide vesicles
could be observed below the lowest inflection point
temperatures. The reason for this phenomenon is not
clear.
The presence of phospholipids in protein d/lipopolysaccharide vesicles strongly influences the characteristics of the Arrhenius plot. The curve shows a
strong resemblance with the one obtained for the
phage adsorption to cells of strain CE1071. The slopes
of the curve both above and below 20 "C became lower
than in the absence of phospholipids (Fig.3C) and
the inflection point at 28°C could not be demonstrated.
Lipopolysaccharide Analyses
Since lipopolysaccharide(l2 "C) and lipopolysaccharide(37 "C) produce a difference in the temperature
dependence of phage K3 inactivation, the composition of lipopolysaccharide(12 "C) and lipopolysaccharide(37 "C) was examined. Analysis of the fatty
acid composition revealed significant differences as
shown in Table 1. In lipopolysaccharide(37 "C) the
amount of total fatty acids was 21.0% (wiw). As
found previously for E. coli [26] mainly lauric
(0.16 pmol/mg), myristic (0.19 pmol/mg) and 3-hydroxymyristic (0.51 pmol/mg) with smaller amounts
of palmitic acid (0.03 pmol/mg) were found. In lipopolysaccharide( 12 "C), the amount of total fatty acids
was 22.3 % (w/w). In this lipopolysaccharide comparable amounts of myristic (0.17 pmol/mg), palmitic
Table 1. Fatty acid composition of lipopolysaccharide(l2 " C ) and
lipopolysaccharide(37 " C )
Lipopolysaccharide was isolated from cells grown at 12°C or
37°C. Fatty acids were liberated (HCI, KOH) and analyzed as
methyl esters by gas-liquid chromatography (SE-30, 10 %, 170 'C).
Results are an average of three experiments k S.D. Total amount
of fatty acids in lipopolysaccharide(12 "C) is 22.3 f 2.1 % (w/w).
Total amount of fatty acids in lipopolysaccharide(37 "C) is
21.0 f 1.8 (wiw). Values for 3-hydroxymyristic represent the sum
of 3-OH-14:O and A2-14:1
Amount of fatty acid in
Fatty acid
Lauric
Myristic
Myristoleic
Palmitic
Palmitoleic
Stearic
Oleic
3-Hydroxymyristic
(12:O)
(14:O)
(14: 1)
(16:O)
(16: 1)
(18:O)
(18: 1)
(3-OH-1410)
lipopolysaccharide(12°C)
lipopolysaccharide(37 "C)
0.05 f 0.02
0.17 f 0.01
trace
0.06 f 0.01
0.10 0.02
0.01
0.01
0.51 0.3
0.16 k 0.02
0.19 f 0.01
*
-
0.03 f 0.03
trace
0.01
trace
0.51 f 0.04
(0.06 pmol/mg) and 3-hydroxymyristic acid (0.51 pmol/
mg) were detected as in lipopolysaccharide(37 "C).
Lipopolysaccharide(l2 "C) however differred from
lipopolysaccharide(37 "C) in that it contained significantly less lauric acid (0.05 pmol/mg) and, instead
larger amounts of a fatty acid, which migrated on gasliquid chromatographic analysis like palmitoleic acid
(methylester) (0.10 pmol/mg). The mass spectrum of
the corresponding material exhibited characteristic
fragments (inter aha) at mje = 268 (M),236 (M-32,
base peak), 194 (M-74) and 152 (M-116). These data
516
Interaction of Protein d with Lipids in the Outer Membrane of E. coli
Table 2. Sugar composition of lipopolysaccharide isolated from cells
grown at 12 " C and 37°C
Results for galactose, glucose and L-glycero-D-mannoheptose were
determined by gas-liquid chromatography of alditol acetates.
acid, glucosamine and
Results for 3-deoxy-~-mannooctulosonic
phosphate were determined by colorimetric methods
Sugar
Lipopolysaccharide(12°C)
Lipopolysaccharide(37°C)
Clmol/mg
__
0.13
0.37
0.34
0.23
0.13
0.35
0.33
0.33
~~~
Galactose
Glucose
L-Glycero-D-mannoheptose
Glucosamine
3-Deoxy-~-mannooctu~osonic
acid
0.20
Phosphate
1.23
~
atures in the Arrhenius plot of the phage adsorption
(Fig.2 and 3). No transition could be discovered
corresponding with the lower inflection point temperature. The transition at the higher temperature is a
thermal one since in differential scanning calorimetry
of lipopolysaccharide(37 "C) a transition was seen
from 29-38°C in a heating curve (Fig.5) and from
34 - 24 'C in a cooling curve (not shown).
~~
0.27
1.48
are consistent for palmitoleic acid (d-16: 1). It therefore appears that the amount of fatty acids in both
lipopolysaccharide-preparations is similar ( z 21 22 %) and that lipopolysaccharide(l2 T)as compared
to lipopolysaccharide(37 "C) contains significant
amounts of palmitoleic and less lauric acid (Table 1).
The composition of the carbohydrate chain of
lipopolysaccharide( 12 "C) and lipopolysaccharide(37 "C) is roughly the same (Table 2). The numbers of
galactose, glucose, heptose and phosphate residues
per lipopolysaccharide molecule are not significantly
different, but lipopolysaccharide(37 "C) contains one
more glucosamine residue, probably at the terminal
position [32], than lipopolysaccharide(l2 T).
Paper chromatography of both 32P-labeled lipopolysaccharides showed that the mobility of lipopolysaccharide( 12 "C) and lipopolysaccharide(37 "C) was
the same, indicating that no strong differences exist
in the hydrophilicity of the two lipopolysaccharide
preparations.
Phase Transition of Lipopolysaccharide
Light scattering, which can be used to measure
phase transitions in liposomes [33,34], was applied
to detect phase transitions in the different lipopolysaccharide preparations as a function of the temperature. The result is shown in Fig.4. The transition
temperatures of lipopolysaccharide(12 " C ) and lipopolysaccharide(37 OC) are 20 "C (range 7.6 "C) and
29.6 "C (range 3 "C) respectively. The difference in
range and the differences in the extent of the light
scattering below and above the phase transitions can
be explained by differences in hydration of lipopolysaccharide and in the amount of cations present in
the molecule [35]. The transition temperatures correspond well with the higher inflection point temper-
DISCUSSION
The study of the temperature dependence of the
activity of membrane-bound enzymes in vivo and in
reconstituted enzyme systems is succesfully used to
understand the influence of the lipid environment of
the membrane on the enzyme activity. Although in the
outer membrane of E. coii the major proteins are
transmembranous proteins [36], the lack of any
known enzymatic activity of these proteins hampers
a study of the lipid environment of these proteins.
However, in complex with lipopolysaccharide [9 - 11,
371, the outer membrane proteins serve as receptors
for bacteriophages. This property appeared to be
useful to study the interaction of protein d with lipopolysaccharide and the phospholipids of the outer
membrane.
The temperature dependence of the inactivation by
reconstituted receptor complexes consisting of protein d and lipopolysaccharide strongly resembles the
adsorption of phage K3 to cells in that (a) the inflection point temperatures and the shape of the Arrhenius plot correspond and (b) a shift in the inflection
point temperature is observed when the cells are grown
at 12 "C instead of 37 "C as well as when the receptor
is reconstituted with lipopolysaccharide isolated from
cells grown at 12 "C instead of at 37 "C (Fig. 2 and 3).
The phage K3 receptor activity of protein d/lipopolysaccharide complexes therefore strongly reflects that
in cells indicating that protein dllipopolysaccharide
complexes occur in the outer membrane of a living
cell.
Two inflection points can be observed in Arrhenius plots for phage K3 adsorption (Fig.2A, B and
3A, B). Adsorption does not occur below the lower
inflection point temperature, which indicates that the
conformation of the receptor changes at this temperature, resulting in total loss of activity. This loss of
activity can be attributed to lipopolysaccharide. Neither protein d nor temperature-dependent alterations
in phage K3 can be responsible for.it since with the
reconstituted receptor composed of protein d(37 "C)
and lipopolysaccharide(12 "C), phage K3 inactivation
could well be measured below 20 "C (Fig. 3 B), the
lower inflection point temperature for protein d/
lipopolysaccharide(37 "C) vesicles (Fig. 3A). It is interesting to note that, in contrast to the inflection point
L. van Alphen, B. Lugtenberg, E. Th. Rietschel, and C. Mombers
0.8
-
0.7
-
0
0
577
I
5
25
-
,
I
I
30
35
40
45
50
("c)
Fig. 4. Phase transition of' lipopolysaccharide as measured by light scattering. Lipopolysaccharide was suspended at 30 mg/ml in 10 mM
10
15
20
Temperature
Tris-HCl/ISO mM NaCl pH 7.4 by Vortex mixing and annealed at 55 "C for 5 min. Light scattering of a fivefold diluted lipopolysaccharide
suspension of lipopolysaccharide(37"C) (0) or lipopolysaccharide(12 "C) ( x ) was performed at a wave length of 455 nm at various
temperatures in a heating curve (1 T jm i n )
I
I
I
I
0
D
20
30
I
40
50
I
60
Temperature ('C)
Fig. 5. Thermal transition of lipopolysaccharide measured by dijjerential scanning calorimetry. Lipopolysaccharide(37 "C) was suspended in a buffer as in Fig. 3 . 1 ml suspension was centrifuged for
10 min at 12000 x g and differential scanning calorimetry was
performed with the pellet by heating at a rate of 1 "C/min
at 28 "C, the inflection point at 20 "C (Fig. 2A and 3A)
was not observed for the adsorption to cells by phage
TuII* (unpublished results), which also used protein d/
lipopolysaccharide complexes as its receptor [lo, 111.
In light scattering as well as differential scanning
calorimetry a phase transition corresponding with the
higher inflection point temperature of the phage
receptor was observed (Fig.4 and 5). This probably
means that the lower transition does not depend on
strong alterations in lipopolysaccharide-lipopolysaccharide interactions. A change in the specific proteinlipopolysaccharide interaction possibly causes the re-
ceptor for phage K3 to be inactive below the lower
inflection point temperature without influencing the
phage TuII* receptor site in the complex. Above the
higher inflection point temperature the adsorption rate
of phage K3 is independent on the temperature (Fig. 2
and 3), suggesting that every collision of the phage
with the receptor results in adsorption. In agreement
with this idea are the values measured for the adsorption constant above the highest inflection point
temperature
lO-'rnl x bacterium-' x min-') (cf.
Fig.2), which are in the same order of magnitude as
the maximal theoretical adsorption rate constants for
phages to cells in that temperature range [28,38].
The results of Fig. 3A and B and Tables 1 and 2
show that the difference in phase transition temperatures of the phage K3 receptor activity in cells grown
either at 12 'C or at 37 "C can be due to differences in
the fatty acid composition or to minor differences in
the sugar chain. The latter possibility is very unlikely
since lipid A probably is the part of lipopolysaccharide
required for receptor activity [ll] whereas the difference in sugar chain mainly occurs at the hydrophilic
end of the lipopolysaccharide molecule. Moreover,
in lipopolysaccharide( 12 "C) the amount of unsaturaked fatty acids (mainly palmitoleic acid) is significantly
higher than in lipopolysaccharide(37 "C). On the other
hand lipopolysaccharide(l2 "C) contains significantly
less lauric acid than lipopolysaccharide(37 "C)
578
(Table 1). An increase in the amount of unsaturated
fatty acids in lipopolysaccharide at lower growth
temperatures has also been observed in Proteus mirabilis, but changes in the chain length did not occur
[391.
The fatty acid composition of phospholipids
strongly influences the fluidity of biological membranes [31,40-421. It is tempting to speculate that a
change in the fatty acid composition of lipopolysaccharide enables the bacterium to maintain a proper
fluidity in the outer monolayer of the outer membrane
at different growth temperatures. Both an increase in
the amount of unsaturated and of shorter fatty acids
results in a decrease of the thermotropic phase transition temperature of phospholipids [43]. However,
the degree of the shift of the transition temperature is
not the same. The thermotropic phase transition
temperatures of e.g. the di-palmitoleic and di-oleic
derivatives of phosphatidylethanolamine and phosphatidylcholine are considerably lower than those of
the corresponding di-lauric or di-myristic phospholipids [43]. Therefore it is likely that lipopolysaccharide( 12 "C) is more fluid than lipopolysaccharide(37 T ) at a certain temperature and that the transition
temperatures are lower in the first case. Thus, the
difference in fatty acid composition of the lipopolysaccharides might explain the difference in phase
behaviour of the K3 receptor in cells grown at 12°C
and 37°C. The experiments with light scattering
(Fig. 4) and differential scanning calorimetry (Fig. 5)
on lipopolysaccharide(37 "C) and lipopolysaccharide(12 'C) are consistent with this explanation. Moreover
these measurements confirm that lipopolysaccharide
undergoes a phase transition as seen in X-ray diffraction [35]. However, the inflection point temperatures
for the 37°C receptor fall within the thermotropic
phase transition of the outer membrane isolated from
cells grown at 37°C (E. Burnell et al., unpublished
results).
Phospholipids influence the transition behaviour
of the receptor activity strongly, both in cells of
mutant strain CE1071, which have increased amounts
of phospholipids in their outer membrane and in the
reconstituted receptor (compare Fig. 2A, C and 3A, C
respectively). Since most of these 'extra' phospholipids
are probably located in the outer monolayer of the
outer membrane of this mutant [2], and since this
effect is imitated by protein d/lipopolysaccharide/
phospholipid vesicles, these 'extra' phospholipids influence the receptor site in the outer monolayer,
resulting in a relatively higher receptor activity below
20 "C compared with that in wild-type cells (Fig. 2A
and C), thereby probably creating a more fluid
environment of the receptor. Therefore it is likely
that in wild-type cells the receptor for phage K3, a
protein d/lipopolysaccharide complex, is not influenced by phospholipids, suggesting that phospholipids
Interaction of Protein d with Lipids in the Outer Membrane of E. coli
are not occurring in those domains of the outer monolayer where protein d/lipopolysaccharide complexes
are located. In order to obtain a better insight in the
distribution of lipopolysaccharide species over the
various outer membrane proteins if: would be interesting to examine whether the phase transition temperatures of other phage receptors differ from those
of protein d/lipopolysaccharide complexes.
We thank R. Beeftink for the growth of fermentor cultures at
12 "C, and Drs J. A. F. Op den Kamp and J. de Gier for stimulating discussions. The technical assistance of Nelke van Selm and
Anne-Marie Hack is gratefully acknowledged.
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L. van Alphen, Laboratorium voor de Gezondheidsleer, Mauritskade 57, NL-1092 A D Amsterdam, The Netherlands
B. Lugtenberg *, Laboratorium voor Moleculaire Celbiologie, Rijksuniversiteit te Utrecht, Transitorium 3,
Universiteit’s Centrum ‘De Uithof‘, Padualaan 8, NL-3584 CH Utrecht, The Netherlands
E. Th. Rietschel, Max-Planck-lnstitut fur Immunologie,
Stubewcg 51, D-7800 Freiburg-Zahringen, Federal Republic of Germany
C. Mombers, Laboratorium voor Biochemie, Rijksuniversiteit te Utrecht, Transitorium 3 ,
Universiteit’s Centrum ‘De Uithof, Padualaan 8, NL-3584 CH Utrecht, The Netherlands
*
To whom correspondence should be addressed.