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Journal of Bacteriology, February 2000, p. 1172-1175, Vol. 182, No. 4
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Visualization of Phospholipid Domains in
Escherichia coli by Using the Cardiolipin-Specific
Fluorescent Dye 10-N-Nonyl Acridine Orange
Eugenia
Mileykovskaya and
William
Dowhan*
Department of Biochemistry and Molecular
Biology, University of Texas
Houston, Medical School, Houston,
Texas 77225
Received 9 August 1999/Accepted 29 November 1999
 |
ABSTRACT |
Cardiolipin (CL)-specific fluorescent dye
10-N-nonyl-acridine orange (NAO) was used to visualize CL
distribution in Escherichia coli cells of different
phospholipid compositions. In a filamentous mutant containing only
anionic phospholipids, green fluorescent spots were observed along the
filaments at approximately regular intervals. Three-dimensional image
reconstruction obtained by optical sectioning and a deconvolution
algorithm revealed NAO-binding domains in the plane of the cell
membrane. Substantial red fluorescence emission of bound NAO supported
labeling of CL-containing domains. These structures were not found in
mutants deficient in CL biosynthesis. The domains were also observed
mostly in the septal region and on the poles in cells of normal size
with wild-type phospholipid composition.
 |
TEXT |
The fluid mosaic model of Singer and
Nicolson (17) assumed membrane lipid homogeneity. However,
cells require a network of membrane domains that produce the specific
environment for the action of membrane proteins (1). The
existence of lipid domains in biological membranes has been
demonstrated by several indirect biophysical techniques such as
fluorescence depolarization (9), differential scanning
calorimetry (2), and fluorescence redistribution after
photobleaching (11). Gel and fluid lipid domains were
directly visualized in mycobacteria by the use of fluorescent
lipophilic probes (4). Uneven distribution of fluorescent steryl dye FM 4-64, which could be a reflection of lateral
heterogeneity of phospholipid distribution in Escherichia
coli membranes, was recently demonstrated by fluorescence
microscopic imaging (6). In our work, we report the first
attempt to visualize the distribution of cardiolipin (CL) in E. coli cells by staining living cells with the fluorescent dye
10-N-nonyl-3,6-bis(dimethylamino)acridine (10-N-nonyl acridine orange [NAO]). It has been shown that
treatment of whole mammalian or yeast cells with NAO resulted in
selective staining of the mitochondrial membrane due to specific
binding of the dye to CL (3, 7, 15, 16). Replacing the NH
group in position 10 of acridine orange by the N-nonyl group
results in an increase in hydrophobicity of the reagent and the
inability to form hydrogen bonds with DNA or RNA. Therefore, NAO only
binds to anionic phospholipids of the cells owing to an interaction between its quaternary amine and the phosphate residue of phospholipids and an intercalation of the hydrophobic acridine moiety into the membrane bilayer (15). With CL, which contains two phosphate groups per molecule, the dye forms a dimer, but with monoacidic phospholipids, the stoichiometry is 1:1. Because of dimer formation, CL
affinity for NAO (Ka = 2 × 106 M
1) is much higher than that of
monoacidic phospholipids (Ka = 7 × 104 M
1) (15). We used NAO for
treatment of E. coli cells of different phospholipid
compositions to monitor CL distribution in bacterial membranes. A short
version of this work was presented previously (E. Mileykovskaya and
W. Dowhan, Abstr. 99th Gen. Meet. Am. Soc. Microbiol. 1999, abstr.
K-134, p. 426, 1999).
Strains and growth conditions.
The E. coli strains
used in this work are described below. AD90
(pss93::Km) is devoid of phosphatidylethanolamine
(PE) and displays filamentous growth unless it carries plasmid pDD72
(pssA+ Cmr), which is temperature
sensitive for replication (5). ADC90/pDD72 is a derivative
of AD90/pDD72 containing the cls::Tn10
allele (18) that makes it deficient in CL biosynthesis.
HDL11 [pgsA::kan
(lacOP-pgsA+) lpp2
zdg::Tn10] carries the pgsA gene,
which is required for phosphatidylglycerol (PG) and CL synthesis, under
lacOP regulation (8, 10). The strain also
contains a mutation in the lpp gene encoding the major outer
membrane lipoprotein. This mutation suppresses the normally lethal
effect of a "leaky" pgsA mutation so that the mutant can
grow in the absence of IPTG
(isopropyl-
-D-thiogalactopyranoside). E614 (lpp2
zdg::Tn10) like HDL11 carries a mutant
lpp allele that results in an outer membrane that is leaky
to macromolecules (20). All strains were grown in
Luria-Bertani (LB) medium at 30°C. In the case of AD90, the medium
was supplemented with 50 mM MgCl2, which is absolutely
required of all PE-deficient strains (5); addition of
MgCl2 to wild-type cells had no effect on the results. In
some experiments, 20 µg of cephalexin per ml was added to the growth
medium to produce filamentous PE-containing cells. IPTG at 200 µM was
used for induction of PG and CL biosynthesis in strain HDL11. Fresh
overnight cultures were diluted between 1:50 and 1:1,000 and grown to
an optical density at 600 nm (OD600) of 0.2 to 0.6.
Microscopic techniques.
For microscopic examination, cells
from liquid cultures were stained directly in the growth medium with
200 nM NAO for 1 h at room temperature. Nucleoids of living cells
were stained with 4',6-diamidino-2-phenylindole (DAPI) at a final
concentration of 1 µg/ml. Cells were immobilized on a cover glass
treated with poly-L-lysine. Cells were viewed in the
presence of the dyes with an Olympus BX60 epifluorescence microscope
equipped with a 100-W HBO lamp, a standard fluorescein isothiocyanate
(FITC) filter set, and a 100× fluorite oil immersion objective. A
standard DAPI filter was used for viewing DAPI stained cells. To
increase the color contrast for overlay images, red-green-blue output
cables were switched for red and blue so that DAPI staining appeared in
red pseudocolor. Images were captured with an Optronics DEI-750 video
camera and manipulated in Adobe PHOTOSHOP 3.0.
Three-dimensional image reconstruction of fluorescence was performed
with a Delta Vision wide-field optical sectioning microscope (Applied
Precision, Issaquah, Wash.) equipped with a 100× oil-immersion objective and visualized with a cooled charge-coupled device camera and
a FITC filter set. Z-axis optical sections were taken at 0.1-µm intervals for a total of 20 sections. Deconvolution of raw data was
performed with five rounds of integration.
Microscopy of NAO-stained E. coli cells.
It was
previously shown that NAO at a nanomolar concentration stained
mitochondria in intact yeast and mammalian cells without inhibition of
their functions or disruption of crista structures (7, 16).
In our study, NAO at 100 to 200 nM in the growth medium resulted in
staining of AD90/pDD72 (wild-type phospholipid composition, 83% PE,
12.7% PG, 3% CL, less than 0.3% PA) and AD90 (containing only
anionic phospholipids, 63% PG, 13% CL, and 8% PA) (5),
without any noticeable influence on their growth rates (data not
shown). Figure 1A shows staining with
NAO of living AD90 cells completely lacking amino-containing
phospholipids and containing only the anionic phospholipids PG and CL.
As found previously, pssA mutants are filamentous due to
inhibition of the cell division process (5, 12). Figure 1A
clearly demonstrates the uneven distribution of the dye in mutant
cells. Green fluorescent spots are observed along the filaments at
approximately regular intervals. The poles of the cells also produced
stronger fluorescent signals. Figure 1B shows a single AD90 cell at a
higher magnification, and Fig. 2A shows a deconvoluted image of an
optical section of an AD90 cell (Fig. 2A). The images (Fig. 1B and 2A)
are consistent with localization of fluorescent spots in the plane of
the cell membranes. In addition, the red fluorescence of these domains (Fig. 2B) colocalizes with the green fluorescent domains (Fig. 2C). As
mentioned above, NAO binding to CL results in dimerization of the dye
(15) in a stacking organization in which dye molecules are
in close proximity. The emitted fluorescence of the complex shifts to
red (emission peak at 640 nm for dimer and 525 nm for monomer) due to
the metachromatic properties of acridine molecules (14).
Experimental titration curves of CL with NAO in isolated rat liver
mitochondria using flow cytometry showed that at an NAO
concentration of about 100 to 200 nM, the intensities of the red and
green fluorescent signals were almost the same (14). In our
experiments (Fig. 2A and C), the intensities of green (emission at 528 nm) and red (emission at 617 nm) fluorescence of NAO bound to the cells
described above were also found to be about the same, consistent with
NAO binding to CL rather than monoacidic phospholipids, which do not
induce a red shift.

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FIG. 1.
Staining of living AD90 filamentous cells with NAO.
Cells were grown in LB medium supplemented with 50 mM
MgCl2. (A) NAO was added to the cells in the exponential
phase of growth, and pictures were taken after 1 h of incubation
with the dye. (B) A part of filamentous AD90 cell at a higher
magnification is shown. Bars, 2.5 µM. Exposure time, 0.25 s.
FIG. 2.
Deconvoluted images of an optical section of an AD90 cell.
Cells were stained as described in the legend to Fig. 1. (A and B)
Excitation was at 490 nm, and emission was at either 528 (A) or 617 (B)
nm. Bar, 2.5 µM. Exposure time, 0.5 s. (C) Colocalization of
green and red fluorescent domains.
FIG. 3.
Staining of living ADC/pDD72 (A), HDL11 (B), and E614 (C)
cells with NAO. Cells were grown in LB medium supplemented with
cephalexin and stained with NAO. Bars, 2.5 µM. Exposure times, 2 (A)
and 1 (B and C) s.
FIG. 4.
Staining of living AD90/pDD72 cells with NAO. (A to F)
Individual AD90/pDD72 cells. (G and H) Deconvoluted images of an
optical section of an AD90/pDD72 cell. Excitation was at 490 nm and
emission was at either 528 (G) or 617 (H) nm. (I) Three-dimensional
picture of an AD90/pDD72 cell stained with NAO obtained by
reconstruction of deconvolved optical sections. The same cell is shown
at three different angles of rotation. Bars, 2.5 µM. Exposure time,
1 s.
FIG. 5.
AD90/pDD72 (A to C) and AD90 (D) cells stained with NAO and
DAPI. Panels A and B show the same cell stained with NAO and DAPI,
respectively. Panel C shows the overlay of the images in panels A
and B. Panel D shows the overlay of NAO and DAPI staining. Stains, NAO
(green) and DAPI (red). Bar, 2.5 µM.
|
|
We demonstrated previously that disruption of the
CLS (now
named
CRD) gene in yeast resulted in loss of CL and the
inability
of NAO to stain mitochondria in intact cells (
3).
In the present
work, we applied the same control to
E. coli
cells. ADC90/pDD72
has about a 10- to 30-fold reduced level of CL
(
18), and growth
in the presence of NAO revealed no
fluorescent structures (Fig.
3A). Cells were grown in the presence of
an inhibitor of cell
division, cephalexin, to produce filaments. These
data strongly
suggest that CL is required for formation of the
NAO-binding domains.
We also used strain HDL11, in which the level of
anionic phospholipids
(PG and CL) is dependent on the level of IPTG in
the growth medium
(
8,
10). In the cells grown in the absence
of IPTG (90.5%
PE, 1.8% PG, 1.3% CL, 6.3% phosphatidic acid)
(
10), the level
of total fluorescence was similar to that
seen in Fig.
3A (data
not shown). Addition of IPTG (79.2% PE, 15.7%
PG, 3.2% CL, 1.5%
phosphatidic acid) (
10) induced
regularly distributed fluorescent
structures along the filamentous
cells (Fig.
3B, cells grown with
cephalexin). Similar structures were
seen in an NAO-treated
lpp strain (E614) with wild-type
phospholipid metabolism and composition
(Fig.
3C). The results of these
experiments show NAO-binding domains
in cells with wild-type
phospholipid composition as well as a
correlation between the level of
CL and the presence of these
domains.
In strain AD90/pDD72 with a wild-type phospholipid composition, the
NAO-binding structures were also observed in many cells
of normal size
(Fig.
4A to F), but they were better seen after
additional manipulation
of the contrast of the images. Fluorescent
spots in these cells were
localized at the poles and septal areas
(Fig.
4). Figures
4G and H
represent green and red fluorescence
of a deconvoluted image of an
optical section of such a cell.
The localization of the fluorescent
spots is consistent with the
NAO-labeled domains being in the plane of
the membrane. Figure
4I shows NAO fluorescence in a pole and septal
area of another
cell after three-dimensional reconstruction and
rotation of the
image at three different angles. In this cell, all of
the septal
area produces a high level of fluorescent signal. These
results
make it unlikely that NAO is labeling inclusion bodies but
rather
is labeling structures in the cell
membrane.
Next, we attempted to localize the position of NAO-binding domains
relative to the position of nucleoids by using double staining
with NAO
and DAPI. As can be seen in Fig.
5, NAO domains are apparently
localized to the areas between nucleoids. This localization is
more
clear in the case of wild-type cells (Fig.
5A to C, septal
localization) than in the case of the AD90 mutant cells, where
some
aberrations are seen (Fig.
5D). However, we previously showed
that
positioning of FtsZ in AD90 filamentous cells also exhibited
some
aberrations (
12).
Conclusions.
Utilizing NAO as a tag for CL, we demonstrated
for the first time the uneven distribution of this lipid along the
E. coli cell with apparent localization in the plane of the
cell membrane. Using NAO as a tool for localization of CL domains, we
cannot completely exclude the possibility that treatment with the dye induces CL domain formation. However, we used extremely low
concentrations of NAO, which for yeast mitochondria resulted in a ratio
of CL to dye of about 1,000 (7) and in our experiments did
not inhibit E. coli growth. In any case, the regular
distribution of NAO-binding domains in cells is more consistent with
the existence of specific zones in the membrane rather than random
domain formation induced by the dye. The specificity for CL binding by
NAO was established in eukaryotic cells where the complexity of
membrane composition is at least on the level with the E. coli cell envelope. The red shift in the fluorescence spectrum and
the lack of fluorescence in cells deficient in CL further validate the
detection of CL-enriched domains. We can propose several possible
explanations for the observed phenomenon. (i) There are CL-enriched
domains in the E. coli membranes. (ii) There are
phospholipid-enriched domains with a higher ratio of lipid to protein
in these regions of membranes, and the NAO-CL complex is a marker of
the discontinuous enrichment in phospholipid content over the surface
of the membrane. (iii) There are sites in the E. coli
envelope with higher permeability for NAO. In this case, we would have
to propose a limited lateral diffusion of the lipids in these domains.
This seems less likely since the outer membranes of lpp and
pssA mutants are leaky to macromolecules and stained no
differently than wild-type cells. Anionic phospholipid-enriched domains
in E. coli have been implicated in protein translocation
across the inner membrane (10) and initiation of DNA
replication (19). A hypothetical model for participation of
anionic phospholipids in formation of the septal domain in E. coli cells has been suggested (13). Our present experiments are directed toward the elucidation of the nature of
NAO-binding domains and their possible functional role in E. coli.
 |
ACKNOWLEDGMENTS |
This work was supported by NIH grant GM 20478 to W.D.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Biochemistry and Molecular Biology, University of Texas
Houston,
Medical School, P. O. Box 20708, Houston, TX 77225. Phone: (713)
500-6051. Fax: (713) 500-0652. E-mail:
wdowhan{at}bmb.med.uth.tmc.edu.
 |
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Journal of Bacteriology, February 2000, p. 1172-1175, Vol. 182, No. 4
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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