Content uploaded by Susan Rotenberg
Author content
All content in this area was uploaded by Susan Rotenberg on Jan 28, 2015
Content may be subject to copyright.
ARTICLE
Received 20 May 2014 |Accepted 7 Jul 2014 |Published 8 Aug 2014
Export of a single drug molecule in two transport
cycles by a multidrug efflux pump
Nir Fluman1,w, Julia Adler1,w, Susan A. Rotenberg2, Melissa H. Brown3& Eitan Bibi1
Secondary multidrug transporters use ion concentration gradients to energize the removal
from cells of various antibiotics. The Escherichia coli multidrug transporter MdfA exchanges a
single proton with a single monovalent cationic drug molecule. This stoichiometry renders the
efflux of divalent cationic drugs energetically unfavourable, as it requires exchange with at
least two protons. Here we show that surprisingly, MdfA catalyses efflux of divalent cations,
provided that they have a unique architecture: the two charged moieties must be separated
by a long linker. These drugs are exchanged for two protons despite the apparent inability of
MdfA to exchange two protons for a single drug molecule. Our results suggest that these
drugs are transported in two consecutive transport cycles, where each cationic moiety is
transported as if it were a separate substrate. We propose that secondary transport can adopt
a processive-like mode of action, thus expanding the substrate spectrum of multidrug
transporters.
DOI: 10.1038/ncomms5615
1Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel. 2Department of Chemistry and Biochemistry, Queens College—
City University of New York, Flushing, New York 11367, USA. 3School of Biological Sciences, Flinders University, Adelaide South Australia 5001, Australia.
wPresent address: Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel. Correspondence and requests for materials
should be addressed to E.B. (email: e.bibi@weizmann.ac.il).
NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications 1
&2014 Macmillan Publishers Limited. All rights reserved.
Multidrug resistance (Mdr) transporters are ubiquitous
membrane proteins that provide cells with defense
against various cytotoxic compounds. They function as
multispecific efflux pumps, each able to expel a broad spectrum of
chemically dissimilar drugs out of the cell1. Bacteria typically
harbour numerous Mdr transporter-encoding genes in their
genomes, which function as secondary transporters driven by the
proton motive force2. Bacterial Mdr transporters exist in several
phylogenetically and mechanistically distinct protein families3.Of
those, Hþ/multidrug antiporters of the Major Facilitator
Superfamily (MFS) comprise the largest group1.
Different MFS Mdr transporters work with distinct proton
antiport stoichiometries. Although a stoichiometry of one proton
per drug was observed with MdfA from E. coli4,5, QacA from
Staphylococcus aureus and LmrP from Lactococcus lactis use
stoichiometries of up to 2 and 3, respectively6,7. Recent progress
has shed light on the mechanism of proton transport by MdfA, in
which a membrane-embedded acidic residue facilitates proton
binding and transport4. Mutagenesis experiments indicate that
increasing the proton stoichiometry of MdfA from 1 to 2 is
possible by introducing an additional acidic residue in a number
of membrane-embedded locations8. LmrP, which works with a
stoichiometry of up to three protons, contains three membrane-
embedded acidic residues; each functions as a module that
facilitates the transport of a single proton7.
As shown recently, the stoichiometry of one proton per
drug renders MdfA inactive in efflux of divalent cationic
(dicationic) drugs, which require import of two protons per
transport cycle8. The underlying reason is that such an antiport
reaction (export of drug2þin exchange for a single H þ) involves
net charge efflux that opposes the membrane potential (Dc,
interior negative). Indeed, MdfA mutants that are capable of
pumping out two protons per drug are able to confer resistance
against dicationic drugs8. Surprisingly, we show here that also
wild-type MdfA can actively pump certain dicationic drugs.
However, in this case the dicationic drugs must have a distinct
chemical architecture, in which the two cationic moeities are
separated by a long linker. Our studies suggest that each of
these drugs is transported in two successive transport cycles,
where the cationic moieties are treated as if they were separate
monocationic substrates. This unexpected mechanism expands
the Mdr spectrum of MdfA towards a new group of dicationic
drugs and indicates that secondary transporters can operate in a
processive-like manner.
Results
MdfA confers resistance against unique dicationic drugs.
During our characterization of the drug recognition spectrum of
MdfA, we were surprised to find that it confers resistance against
the toxic dicationic compound dequalinium (Dq). The chemical
structure of Dq reveals that it contains two identical cationic
pharmacophores separated by a long, ten-carbon chain linker
(Fig. 1a). To examine whether these characteristics of Dq (sym-
metry and spacer length) are relevant to the unexpected activity,
we challenged MdfA with several dicationic drugs (Fig. 1a). In
some of these compounds, both positive charges are relatively
close (40,6-diamidino-2-phenylindole (DAPI), diminazene (Dmn)
and methyl viologen), while in others the positive charges are
separated by long linkers (Dq, chlorhexidine (Chx) and penta-
midine (Pent)). For simplicity, we term the two groups of com-
pounds SDCs (short dicationic compounds) and LDCs (long
dicationic compounds), respectively. Strikingly, when challenged
against all these drugs, we observed that MdfA could only protect
E. coli against LDCs, demonstrating that symmetry alone is not
sufficient and that the linker length is important (Fig. 1b).
The resistance conferred by MdfA suggests that unlike SDCs,
LDCs are actively extruded by the transporter. To test this, we
characterized the ability of MdfA to prevent Dq accumulation
in vivo in whole cells. The fluorescence of Dq is quenched when it
enters cells (Fig. 2a) and can be de-quenched on lysis of cells that
accumulated Dq (Fig. 2b). Figure 2a,b show that expression of
MdfA from a multicopy plasmid indeed prevented intracellular
accumulation of Dq compared with cells harbouring an empty
vector or expressing the inactive mutant MdfA(R112M)4,9.In
addition, extrusion of Dq was completely inhibited when another
substrate of MdfA, chloramphenicol (CAM), was added as a
competitor (Fig. 2c), as shown previously for another MdfA
substrate10. These results suggest that MdfA mediates active
transport of Dq.
MdfA binds both LDCs and SDCs. How does MdfA distinguish
between the two groups of dicationic drugs? One possibility is
that MdfA cannot bind SDCs. To test this, we measured the
ability of SDCs and LDCs to compete with a radiolabelled sub-
strate, the monovalent [3H]-tetraphenylphosphonium (TPP), for
binding to MdfA. The results show that with the exception of
methyl viologen, all the tested dicationic compounds are able to
outcompete [3H]-TPP-binding in a concentration-dependent
manner (Fig. 3). The concentration dependency suggests that
MdfA has similar affinities to these compounds as those reported
for other known substrates10. Thus, the reason for the
discrimination between the two types of dicationic substrates
does not stem from differences in their ability to bind the
transporter.
MdfA catalyses LDC/proton antiport. To further evaluate the
indication that MdfA actively exports LDCs, we performed
transport experiments with everted membrane vesicles. As MdfA-
catalysed active efflux relies on drug/proton antiport, we mea-
sured substrate-dependent proton transport by MdfA in vesicles
from cells expressing MdfA or harbouring an empty vector. The
proton concentration gradient (DpH) across these membranes
was followed by measuring the fluorescence of a DpH-sensitive
indicator, 9-amino-6-chloro-2-methoxyacridine (ACMA). Ener-
gization of everted membrane vesicles by D,L-lactate oxidation
generated a DpH (acidic inside) as observed by quenching of
ACMA fluorescence (Fig. 4). The generated DpH was fully dis-
sipated by adding the proton conducting uncoupler carbonyl
cyanide m-chlorophenyl hydrazone (Fig. 4). Addition of sub-
strates from the LDCs group (Dq, Pent or Chx) or the control
monocationic substrate TPP partially dissipated DpH only in
membranes expressing MdfA (Fig. 4, compare left and right
panels). Thus, MdfA indeed catalyses proton/LDC exchange. In
contrast, addition of the SDC compound Dmn did not trigger
proton translocation. Notably, this assay is not quantitative
because calculating the amount of protons transported by MdfA
in the presence of different compounds might be complicated by
other factors such as the ability of the different substrates to
distribute across the membrane via MdfA-independent routes.
Consequently, as expected, the magnitude to which different
substrates affect DpH (as reflected by the fluorescence change)
did not show any correlation with the charges on the substrates.
MdfA-catalysed transport of LDCs is electroneutral.As
explained earlier, it is energetically unfavourable for any anti-
porter to export dicationic substrates by using a proton/substrate
stoichiometry of 1, as the resulting reaction would involve net
export of a positive charge against the membrane potential.
Therefore, as wild-type MdfA usually works with a stoichiometry
of 1, the transport of LDCs is enigmatic. To better understand
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5615
2NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications
&2014 Macmillan Publishers Limited. All rights reserved.
b
Dq fluorescence (a.u.)
0
100
200
300
Vec MdfA MdfA
(R112M)
0 min
14 min
a
0
50
100
0510
Dq fluorescence (%)
Time (min)
R112M
Vec
wt c
Vec
wt
0
50
100
0510
Time (min)
Dq fluorescence (%)
Figure 2 | MdfA-mediated Dq transport. (a) Characterization of Dq accumulation by E. coli UTL2 mdfAHkan overexpressing MdfA or its inactive mutant
MdfA(R112M), or cells harbouring an empty plasmid. (b) Quantitative Dq accumulation. Cells were suspended in buffer containing glucose and Dq (1mM),
and allowed to accumulate Dq for 0 or 14 min. Dq fluorescence in these cells was determined after the cells were harvested and lysed, relieving their
fluorescence quenching effect. Error bars indicate range of duplicate measurements. (c) Dq accumulation by control or MdfA-expressing cells in the
presence of 0.8 mM CAM. (a,c) The experiments were repeated at least three times and the results shown are representative.
a
Dq
Met-Viol
NH2
+N
N+
+NN+
Dmn
NN
DAPI
NH
Chx
NH Cl
N
H
N
H
N
H
NH
Cl
N
HN
HN
H
Pent
H2N
OO
1600 µg ml–1
TPP
200 µg ml–1
Dmn
400 µg ml–1
EtBr
4 µg ml–1
CAM
10 µg ml–1
DAPI
No drug
180 µg ml–1
Pent
400 µg ml–1
Met-Viol
1 µg ml–1
Chx
b
Vec
MdfA
Vec
MdfA
Vec
MdfA
50 µg ml–1
Dq
Vec
MdfA
NH+
2NH+
2
NH2
NH+
2
NH+
2
H2NH+
2N
NH2
NH+
2
NH2
H+
2N
NH2
NH+
2
NH2
N
H
Figure 1 | MdfA confers resistance only against a unique class of symmetric dicationic compounds (LDC). (a) Structures of the tested dicationic drugs.
Left: long dicationic compounds (LDCs): dequalinium (Dq), chlorhexidine (Chx) and pentamidine (Pent). Right: short dicationic compounds (SDCs):
40,6-diamidino-2-phenylindole (DAPI), diminazene (Dmn) and methyl viologen (Met-Viol). (b)GrowthofE. coli UTmdfAHkan expressing MdfA from a
multicopy plasmid (or harboring empty vector) on solid media supplemented with the indicated drugs. Tenfold serial dilutions of cells were plated from left to
right and grown overnight. The upper panels show controls of growth in the absence of drug or in the presence of previously known monocationic (TPP/
ethidium bromide (EtBr)) or neutral (CAM) MdfA substrates. The experiments were repeated at least three times and the results shown are representative.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5615 ARTICLE
NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications 3
&2014 Macmillan Publishers Limited. All rights reserved.
this important phenomenon, we characterized the electrogenicity
of the LDCs transport reactions. We assumed that MdfA may use
a proton/dicationic substrate stoichiometry of Z2, resulting in
electroneutral or electrogenic transport, respectively, which
involves no export of a net positive charge(s).
As a first attempt to probe transport energetics in living cells,
we determined the effect of the external pH on MdfA-mediated
resistance to neutral, monocationic and LDC drugs. When the
external pH rises above the intracellular level of B7.6, the pH
gradient across the membrane (DpH) reverses its direction and
becomes acidic inside relative to outside11. Thus, electroneutral
transport reactions, which rely energetically only on DpH,
become unfavourable. In contrast, electrogenic transport
reactions are still supported by the electrical potential (Dc,
interior negative)5. Our results confirmed previous experiments
with MdfA5, which showed that resistance against the neutral
substrate CAM is driven by both components of the electro-
chemical potential (Dc and DpH), whereas resistance against the
monovalent cation ethidium bromide is solely dependent on the
DpH (Fig. 5a). Figure 5a also shows clearly that MdfA-mediated
resistance against LDCs (Dq, Chx and Pent), just like ethidium
bromide, is pH dependent, suggesting that the transport reaction
with these drugs is electroneutral. To further assess the proposed
electroneutral mode of antiport in the case of LDCs, we measured
Dq accumulation in living cells depleted of the DpH (by addition
of the ionophore nigericin and external pH 8). These conditions
make drug efflux possible only if it can be driven by Dc.
However, Fig. 5b shows that this was not the case and MdfA
could not catalyse antiport of Dq when Dc (interior negative) is
the sole driving force.
Taken together, the results indicate that the transport of the
dicationic drug Dq is electroneutral and depends solely on the
DpH. These results are intriguing, because Dq and each of the
other LDCs carry two positive charges, meaning that two protons
are exchanged for one drug molecule during the antiport.
However, as MdfA cannot translocate two protons simultaneously,
export of LDCs requires a novel mode of transport (see later).
LDC binding triggers release of a single proton from MdfA.
How does the long linker between the charges in LDCs (Fig. 1a)
allow transport with an increased proton/drug stoichiometry of 2?
Recent studies have indicated that the antiport mechanism of
MdfA relies on competition between substrate and proton on
binding to the transporter. Monocationic substrates were shown
to trigger the release of a single stoichiometric proton from MdfA,
in agreement with an antiport stoichiometry of 1 (ref. 4). Here,
one possible scenario is that the linker in LDCs enables the two
cationic moieties to trigger proton release from two distinct
proton-binding sites, if those exist in MdfA. We therefore tested
whether LDCs compete with only one or maybe two protons for
binding. To this end, we compared the amount of protons
released from MdfA on binding of the LDCs Dq and Pent with
the amount released by binding of the monocationic TPP. Proton
release was inferred from an increase in the solution’s acidity, as
0
20
40
60
80
100
120
25 µM
125 µM
Relative [3H]TPP-binding (%)
LDCsSDCs
No drug
DAPI
Dmn
Met-Viol
Chx
Dq
Pent
*
**
**
**
**
**
**
*
**
Figure 3 | MdfA binds both groups of dicationic drugs (LDCs and SDCs).
[3H]-TPP binding by wild-type MdfA and its inhibition by addition of
increasing concentrations (25 or 125 mM) of dicationic drugs. Error bars
indicate s.d. of triplicate measurements (*Po10 2;**Pr10 4compared
with the no drug sample, one tailed t-test). The experiments were
repeated at least three times and the results shown are representative.
Lactate CCCP
Substrate
Dq
MdfA
vesicles
Control
vesicles
Pent
TPP
Chx
01234
100
200
Time (min)
Dmn
300
Fluorescence (a.u.)
Figure 4 | Proton transport by control or MdfA-containing everted
membrane vesicles. The transmembrane DpH was followed by measuring
ACMA fluorescence. Lactate (added after 0.5 min, blue arrow) and
carbonyl cyanide m-chlorophenyl hydrazone (CCCP; after 3 min, black
arrow) were used to generate or abolish DpH, respectively. Substrates were
added at 2 min (orange arrow), in the following concentrations: TPP
(125 mM), Dq (25 mM), Pent (125 mM), Chx (25 mM) and Dmn (12.5 mM).
The experiments were repeated at least three times and the results shown
are representative.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5615
4NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications
&2014 Macmillan Publishers Limited. All rights reserved.
measured by the fluorescent pH indicator fluorescein4. Notably,
Chx could not be tested in this assay because it generates high
artificial fluorescent signals (not shown). Figure 6 shows that
despite the differences in transport stoichiometry, the dicationic
drugs Dq and Pent, and the monocationic compound TPP, all
induced the release of a single stoichiometric proton from MdfA.
Thus, it appears that MdfA contains a single proton-binding site.
Dq binds to MdfA with 1:1 stoichiometry. Another purely
theoretical possibility is that LDCs bind to an MdfA dimer where
each MdfA molecule binds one of the separable cationic moieties,
thus transporting the substrate cooperatively. We do not favour
this possibility because MdfA and other MFS transporters are
known to function as monomers12,13. In addition, the several
available structures of MFS members show that the binding sites
are located deep within the proteins14–18 and therefore the spacer
separating the two cationic moieties of LDCs would not fit the
length required for binding to separate transporters in the
membrane. Indeed, we saw no evidence for cross-linking of MdfA
dimers on Dq binding, when attempting to cross-link with
formaldehyde, paraformaldehyde or disuccinimidylsuberate (data
not shown). To further examine the binding stoichiometry, we
characterized the interaction of purified MdfA protein with Dq.
Binding was measured fluorimetrically, by following Dq
fluorescence. The results show that on MdfA binding, the
fluorescence of Dq is quenched (Fig. 7a). This effect is not
observed when Dq is mixed with the inactive mutant
MdfA(R112M)4, or when TPP is added as a competitive
inhibitor (Fig. 7a). We next determined the affinity of Dq
binding to MdfA by two complementary approaches, in which
one component (either MdfA or Dq) is kept constant at a low
concentration, while the other component is titrated at increasing
concentrations. Figure 7b shows the effect of increasing [MdfA]
on Dq fluorescence and Fig. 7c shows the competitive inhibition
of TPP binding by MdfA, mediated by increasing [Dq]. The two
approaches yield comparable dissociation constants, with
K
d
B0.7 mM for MdfA at ambient temperature measured by Dq
fluorescence (Fig. 7b) and K
I
of B2mM determined from [3H]-
TPP binding-competition experiments at 4 °C (Fig. 7c). Notably,
both binding curves are hyperbolic, suggesting no cooperativity in
binding, consistent with a binding stoichiometry of 1.
To evaluate the results more directly, we conducted Dq-
mediated cross-linking of MdfA. Dq is a photoactivatable
compound that on ultraviolet irradiation is able to react
covalently with nearby residues19. The results show that Dq can
cross-link a functionally active20 split mutant of MdfA, in which
the 8 amino-terminal (N8) and the 4 carboxy-terminal (C4)
transmembrane helices were expressed as separate polypeptides
with C-terminal hexahistidine tags (Fig. 7d). This indicates that
Dq can interact with two separate regions of MdfA, at least
transiently. In contrast, Dq could not cross-link two molecules of
full-length MdfA (Fig. 7d), suggesting that interaction of Dq with
an MdfA dimer does not take place.
Export of dicationic substrates by other Mdr transporters. The
results of all the previous experiments support the notion that
MdfA translocates two protons in exchange for a single LDC
molecule, but probably not simultaneously. This led us to propose
another explanation for the increased stoichiometry during
transport of LDCs. We hypothesized that MdfA transports these
unique drug molecules in two successive transport cycles.
According to this scenario, MdfA handles each positively charged
group as if it were a separate substrate, which is transported
against a proton (see model in Fig. 8).
The suggestion that transport of a single substrate proceeds in
two consecutive cycles relies only on the substrate properties.
Therefore, we reasoned that this ability to transport LDCs might
not be unique to MdfA but perhaps shared by other MFS Mdr
transporters. To test the generality of this phenomenon, we
studied QacA, a different MFS-related Mdr transporter from
S. aureus. QacA transports both monocationic and dicationic
drugs21, and previous studies showed that the activity of QacA
against the dicationic drugs depends on an acidic residue at
position 323 (ref. 22). Neutralizing D323 by mutagenesis
interrupted the activity towards dications while retaining active
efflux of monocationic drugs6. We speculated that a mechanism
involving two consecutive transport cycles should enable the
D323 mutant of QacA to transport LDCs, but not SDCs. This was
tested by drug resistance assays using E. coli harbouring
heterologously expressed QacA or QacA(D323C). As expected,
the results show that wild-type QacA was able to confer resistance
against all tested dicationic drugs (LDCs and SDCs) (Fig. 9a).
pH 6.5
pH 7.0
pH 7.5
pH 8.0
Vec
MdfA
Vec
MdfA
Vec
MdfA
Vec
MdfA
No drug
4 µg ml–1 CAM
400 µg ml–1 EtBr
50 µg ml–1 Dq
200 µg ml–1 Pent
1 µg ml–1 Chx
0
50
100
0510
Time (min)
Dq fluorescence (%)
Vec
wt
ba
Figure 5 | pH dependence of MdfA-mediated Mdr. (a) pH-dependent drug resistance assay. The experiment was done as in Fig. 1b, with a pH-buffering
agent (Bis-Tris-Propane) added to the medium to generate the pH indicated to the right. (b) Accumulation of Dq in DpH-dissipated cells. Dissipation
of DpH was achieved by conducting the assay at pH 8 in the presence of 2 mM nigericin. The membranes were permeabilized to nigericin by an
EDTA-ethanol treatment, which did not affect the assay when the DpH was not dissipated. The experiments were repeated at least three times and the
results shown are representative.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5615 ARTICLE
NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications 5
&2014 Macmillan Publishers Limited. All rights reserved.
Remarkably, however, although the QacA(D323C) mutant had
partially lost its ability to transport SDCs, it retained resistance
activity against LDCs (Dq and Chx) (Fig. 9a). These results
support our hypothesis that a monocationic Mdr transporter can
transport dicationic substrates that belong to the LDC group. In
support of this notion, previous studies have shown that QacB, a
close variant of QacA harbouring a neutral residue at position 323
and consequently lacked activity against most dications, was
found to retain activity towards several dicationic drugs. The
common denominator of these drugs is that their two cationic
moieties are separated by a long linker6,22,23, and meet our
criteria for LDCs.
Finally, as an independent complementary approach, we
characterized the activity of MdfA(V231E). Unlike wild-type
MdfA, this mutant is able to translocate two protons per
transport cycle8, rendering it active against both LDCs and
Fluorescence (a.u.)
500
540
580
480
520
560
500
540
580
440
480
520
TPP
TPP
2 µM
HCl
Dq
Dq
2 µM
HCl
2 µM
HCl
Pent
Pent
TPP Dq
Pent
2 µM
HCl
123456
Time (min)
0
Pent
TPP
Dq
No
MdfA
Figure 6 | Substrate-induced proton release. MdfA was solubilized and
purified to homogeneity and dialysed to remove buffering agents. The
protein was diluted to 2 mM and the solution acidity was monitored in a
time-dependent manner by measuring the pH-sensitive fluorescence of the
probe fluorescein. Before the assay, the protein was titrated by NaOH or
HCl to a fluorescent signal of B550 (pHB6.5). Notably, the absence of a
buffering agent generates spontaneous slow drifts in the acidity, creating a
background signal. The substrate-induced changes are distinguished
from the background by their much higher rate of changes. The arrows
denote additions of HCl (2 mM), TPP (150 mM), Pent (300 mM) or Dq
(50 mM). HCl was added in an MdfA-equimolar amount to reveal the signal
expected by a stoichiometric release of a single proton per MdfA molecule.
This signal is nearly identical to that of substrates (TPP/Dq/Pent),
suggesting that all substrates release a single proton from MdfA. Substrates
were added at saturating concentrations. As a control, substrates were
added twice to examine the saturability of binding and the related proton
release. In addition, substrates did not appreciably affect fluorescence when
added to a solution without MdfA (lower panel). The experiments were
repeated at least three times and the results shown are representative.
K
d
=0.7±0.1 µM
K
I
=2.2±0.2 µM
0
50
100
150
200
250
02040
60
70
80
90
100
0510
0
Dq
Dq + MdfA
Dq + MdfA + TPP
Dq + MdfA(R112M)
40
80
120
Dq fluorescence (%)
[3H]-TPP binding (counts)
[MdfA] (µM)
[Dq] (µM)
Dq fluorescence (%)
ab
cd
10
17
26
34
43
72
95
N8
C4
Dq
UV
+–+ +–+
–++ –++
MdfA
kDa
*
Figure 7 | Affinity and stoichiometry of Dq binding by purified MdfA.
(a) Quenching of 0.2 mM Dq fluorescence by 2 mM wild-type MdfA or
MdfA(R112M). Effect of adding 1 mM TPP to the mixture is also shown.
Error bars indicate s.d. of triplicate measurements (*Po10 4, two-tailed t-
test). (b) Quenching of 0.2 mM Dq fluorescence by increasing MdfA
concentrations. The binding of Dq to MdfA was analysed by nonlinear
regression fitting (line), yielding the indicated K
d
.(c) Inhibition of [3H]-TPP
binding to MdfA by increasing Dq concentrations. The results were
analysed by non-linear regression fitting (line), yielding the indicated K
I
.
Error bars indicate s.d. of triplicate measurements. (d) Dq-mediated
photoinduced cross-linking of MdfA. Membranes expressing functional
split-MdfA (C-terminally tagged N8-His
6
and C4-His
6
, left lanes) or wild-
type MdfA-His
6
(right lanes) were incubated in the absence or presence of
100 mM Dq and irradiated with ultraviolet. Proteins were separated by SDS–
PAGE and western blotting against the His tag. Cross-linked product of N8
and C4 is indicated by a star.
+
+
+
+
+
+
+
+
+
+
Periplasm
Cytoplasm
Figure 8 | Model for transport of divalent cations in two successive
transport cycles. The transporter first binds and transports only one
cationic moiety (sphere) of the substrate. Once this moiety is expelled, the
transporter binds and then transports the second half of the substrate. Each
transport cycle is depicted as an interconversion between inward- and
outward-facing conformations.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5615
6NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications
&2014 Macmillan Publishers Limited. All rights reserved.
SDCs (as QacA) (Fig. 9b, upper panel). If MdfA(V231E) exports
LDCs in two successive cycles, in exchange for 2 protons per cycle
(a total of 4 protons), the antiport would be electrogenic. In
contrast, this mutant might catalyse electroneutral SDC/proton
antiport, as these compounds are exported in a single cycle in
exchange for two protons. To assess this prediction in vivo,we
tested the pH dependence of MdfA(V231E) resistance activity
against dications. The results show that this mutant could not
confer resistance against SDCs (DAPI and Dmn) at elevated pH,
suggesting electroneutral transport of these drugs (exchange of
two protons per one dicationic substrate) (Fig. 9b, lower panel).
In contrast to SDCs, MdfA(V231E) conferred resistance against
LDCs (Chx and Dq) even at pH 8 (Fig. 9b, lower panel),
suggesting that their transport is indeed electrogenic. When
considered in the context of our hypothesis that LDCs are
transported in two consecutive cycles, the indication that even in
the absence of DpH the mutant can energize Dq efflux suggests
that the transport is likely to be electrogenic and each of the two
positive moieties of Dq might be exchanged for two protons.
Together, the results suggest that MdfA(V231E) catalyses efflux of
LDCs with an overall higher proton exchange stoichiometry than
SDCs, in support of the hypothesis that LDCs are exported by
MFS secondary Mdr transporters in two consecutive cycles.
Discussion
Recently, important aspects of the mechanism underlying MdfA-
mediated antiport were elucidated4. A key principle in the
mechanism is that substrate binding triggers release of a single
stoichiometric proton from MdfA. This offers an explanation for
the proton/drug antiport stoichiometry of 1 that was observed
with MdfA5. Different MFS Mdr transporters work with distinct
proton antiport stoichiometries. For example, QacA from
S. aureus and LmrP from L. lactis use stoichiometries of up to
2 or 3 (refs 6,7). Compared with most Mdr transporters, the
proton/drug stoichiometry of 1 (as shown for MdfA) is relatively
low, rendering it generally inactive in efflux of dicationic drugs8.
Indeed, MdfA mutants that are capable of pumping two protons
per drug molecule acquire resistance also against dicationic
drugs8. Therefore, in the case of wild-type MdfA, the discovery of
efflux of certain dicationic drugs (LDCs) was surprising and
posed a mechanistic riddle. An important clue to solving this was
that MdfA seems to confer resistance only to LDCs, whose two
positive charges are separated by a long linker.
Our studies showed that LDCs are transported in a manner
similar to the monocationic drugs. Namely, they are exchanged
with protons, bind the transporter with a 1:1 stoichiometry and,
most importantly, their transport is electroneutral. This suggests
that they are probably exchanged for two protons. Nevertheless,
binding of LDCs induces the release of only a single proton from
MdfA, just like monocationic drugs4. If so, why is their proton/
drug antiport stoichiometry twice higher? The unique
architecture of these substrates, with two charges that are
separated by a long linker, offers a hypothetical solution. We
propose that MdfA exports LDCs in two successive transport
cycles, in which each charged moiety is transported as if it were a
separate substrate. In each cycle, one proton is transported, thus
providing a total of two protons imported during the process,
which ends after the full release of the LDC molecule from MdfA.
According to this scenario, the carbon chain that separates the
two charged domains of the LDCs must remain buried inside the
transporter, or in the lipid-transporter interface, just before the
second transport cycle (Fig. 8). Such a hydrophobic linker may
behave as a carbon chain of lipids and integrate into the lipid–
protein structure without interrupting the ability of MdfA to
undergo transport-related conformational changes. Through
additional studies with QacA and its D323C mutant and with
the MdfA mutant V231E, we obtained evidence in support of the
proposed mechanism and the notion that it might be universal in
secondary Mdr transporters. Briefly, the results show that
QacA(D323C), which is in principal a monocationic drug
pump, can confer resistance against LDCs. Furthermore, we
showed that an MdfA mutant that generally exchanges two
protons per dicationic drugs from the SDC group can transport
also LDCs; however, these compounds are exchanged for more
than two protons. This is consistent with the hypothesis that
LDCs are transported in two sequential cycles also by this MdfA
mutant. It seems that the simplest explanation for these
observations is that MFS Mdr transporters handle certain
dicationic drugs as if they were two separate monocations,
which are transported separately.
Transport of a substrate in more than a single cycle has never
been demonstrated for secondary or primary active solute
transporters. Transported substrates are usually small molecules
that do not justify such a mechanism, because they can be
transported in a single step. Transport of larger molecules, such as
proteins across the membrane is usually catalysed by machineries
that generate a continuous transmembrane channel24. The
pH 6.5 Vec
MdfA
100 µg ml–1 Dmn 4 µg ml–1 DAPI
No drug 0.5 µg ml–1 Chx 20 µg ml–1 Dq
MdfA(V231E)
10 µg ml–1 Dmn 1 µg ml–1 DAPI
No drug 0.5 µg ml–1 Chx 20 µg ml–1 Dq
pH 8 Vec
MdfA
MdfA(V231E)
100 µg ml–1 Dmn 10 µg ml–1 DAPI
No drug 2 µg ml–1 Chx 25 µg ml–1 Dq
Vec
QacA
QacA(D323C)
a
b
Figure 9 | Effect of acidic residue substitutions on resistance against dications by QacA and MdfA. The experiments were done as in
Figs 1b and 5. (a) Mdr by E. coli expressing QacA or its D323C mutant. (b) pH dependence of multidrug resistance by E. coli UTmdfAHkan expressing
MdfA or its V231E mutant. Resistance to Dmn and DAPI at pH 8 was tested at lower drug concentrations than pH 6.5 to make sure that even
low-level resistance is not conferred. The experiments were repeated at least three times and the results shown are representative.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5615 ARTICLE
NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications 7
&2014 Macmillan Publishers Limited. All rights reserved.
continuous channel enables such large substrates to be transport-
ed in a step-wise, processive manner. In active transporters, the
substrate-conducting channel is always blocked on either side of
the membrane. The transport mechanism works such that when
the blockade is opened on one side, it closes on the other side of
the membrane, enabling the substrate to bind or dissociate from
either side of the membrane alternatively. This blockade should
hinder processive transport; however, our results suggest that
relatively narrow, hydrophobic linkers may still be threaded
through the blocked channel. This mechanism might be shared
by other active transporters that translocate large substrates with
long linkers. The unusual processive mechanism broadens the
substrate profile of Mdr transporters and may extend the
spectrum of substrates that are accessible for active transporters
in general.
Methods
Drug resistance assay.Antibacterial resistance was assayed as described25.
Briefly, E. coli UTmdfA::Kan or UTL2mdfA::Kan25 harbouring empty vector or
expressing MdfA were grown aerobically to A
600
of 1 and a series of 10-fold
dilutions was prepared. Four microlitres of the serial dilution were spotted on drug-
containing LB-agar plates and tested for growth after overnight incubation at 37 °C.
pH-dependent resistance was tested by adding 75 mM bis-Tris-propane buffer at
the indicated pH to the media as described5. Resistance conferred by QacA was
assayed similarly with the following modifications: (i) the E. coli strain used was
DH5a; (ii) the control vector was pBluescriptII, on which the plasmids encoding
QacA (pSK4322) and QacA(D323C) (pSK4432) are based; (iii) liquid cultures were
grown in 2YT media supplemented with 0.1 mg ml 1ampicillin. All drugs were
from Sigma Aldrich.
MdfA overexpression and purification.E. coli cells harbouring plasmid pUC18/
Para/mdfA- His6 were grown at 37 °C in LB medium supplemented with ampicillin
(200 mgml1). Overnight cultures were diluted to 0.07 A
600
units, grown to 1.0
A
600
units and induced with 0.2% arabinose for 1 h. A typical 10-l culture yielded
15 g (wet weight) of cells. Cell pellets were washed once in 150 ml of 50 mM KPi
(pH 7.3) supplemented with 5 mM MgSO
4
and collected by centrifugation (10 min,
10,000 g). Next, the cells were suspended in 150 ml of the same buffer containing
10 mgml1DNAse and 1 mM phenylmethylsulphonyl fluoride and passed three
times through a liquidizer (Emulsiflex-C5, Avestin) (10,000 psi) for disruption. Cell
debris was removed by centrifugation (30 min, 10,000 g) and the membranes were
collected by ultracentrifugation (1 h, 100,000 g). The membranes were homo-
genized in 27 ml of urea buffer (20mM Tris-HCl pH 8, 0.5 M NaCl, 5 M urea, 10%
glycerol, 28 mM b-mercaptoethanol and 1 mM phenylmethylsulp honyl fluoride),
incubated for 30 min at 4 °C and collected by ultracentrifugation (2.5 h, 100,000g).
The membranes were washed with 27 ml of membrane buffer (20 mM Tris–HCl
pH 8, 0.5 M NaCl, 10% glycerol and 3.5 mM b-mercaptoethanol). Finally, the
membranes were suspended by homogenization in 7 ml of the same buffer and
aliquots of 1 ml were snap-frozen in liquid nitrogen and stored at 80 °C.
For MdfA purification, thawed membranes were diluted fivefold in
solubilization buffer (20 mM Tris–HCl pH 8, 0.5 M NaCl, 10% glycerol, 0.1%
b-dodecyl maltopyranoside (DDM), 5 mM imidazole) and solubilized by gradual
addition of 10% DDM (Anatrace) to a final concentration of 1.1% and
homogenized. Insoluble material was discarded by ultracentrifugation (30 min,
100,000g) and the soluble fraction was mixed with solubilization buffer-
equilibrated Talon beads (Clontech) (typically 0.5 ml for 2 ml of thawed
membranes). Next, the mixture was agitated for 3 h at 4 °C and the suspension was
poured into a column. The column was then washed twice with ten-column
volumes of solubilization buffer. MdfA was eluted in elution buffer (20 mM
Tris–HCl pH 7.2, 0.12 M NaCl, 10% glycerol, 0.1% DDM, 100 mM imidazole). The
protein was collected after initial discard of 0.75 column volume. The protein was
then dialysed overnight against dialysis buffer (20 mM Tris-HCl pH 7.2, 0.12 M
NaCl, 10% glycerol, 0.01% DDM) at 4 °C. Protein concentration was determined by
measuring A
280
(1 mg ml 1B2.1 A
280
)13.
Measurement of [3H]-TPP binding.Radiolabelled TPP binding was carried out
by immobilizing purified MdfA-His
6
on Ni2þbeads, allowing it to bind [3H]-TPP
(American Radiolabeled Chemicals) and counting the MdfA-associated radio-
activity as described13. For assessment of competition, the indicated concentrations
of competitors were added during [3H]-TPP binding. To determine K
I
, the data
were fitted to the equation B¼B
max
B
max
([Dq]/([Dq þK
I
)) using nonlinear
regression, where Bis the amount of bound [3H]-TPP (in counts) and B
max
is the
maximal amount of B(without added Dq).
Dq transport in whole cells.Overnight cultures of E. coli UTL2mdfA::kan cells
harbouring empty vector (pT7-5) or plasmid pT7-5/mdfA-His
6
were diluted
50-fold and grown at 37 °C in LB broth supplemented with ampicillin
(100 mgml1) and kanamycin (30 mgml1)uptoanA
600
of B1 and transferred
to ice. Ice-cold cell suspensions (0.3 A
600
units) were pelleted and washed once with
1 ml of wash buffer (50 mM KPi pH 7, 0.2% glucose, 2mM MgSO
4
), centrifuged
and resuspended in 110 ml of the same buffer. From these cells, 100 ml were added
to a cuvette containing 1,900 ml of pre-warmed (37 °C) wash buffer supplemented
with 1 mM Dq (Sigma). The suspension was continuously stirred and warmed while
measuring Dq fluorescence quenching (excitation 330 nm, emission 370 nm).
For experiments with CAM, the wash buffers were supplemente d with 0.8 mM
CAM (Sigma).
For dissipation of DpH, the protocol was modified as follows: cell suspensions
were pelleted, resuspended and incubated for 10 min in modified wash buffer
(50 mM KPi pH 8, 1% EtOH, 2 mM EDTA, 2 mM nigericin (Fermentek)). Cells
were centrifuged and resuspended in 110 ml of the same buffer. From these cells,
100 ml were added to cuvette containing 1,900 ml of pre-warmed (37 °C) modified
wash buffer supplemented with 1 mM Dq and 0.2% glucose, and the fluorescence
was recorded.
Estimation of Dq fluorescence inside cells was done similarly with
modifications. Cells (A
600
of 1.5) were pelleted and resuspended in 100 ml wash
buffer. To this suspension, 1.5 ml of Dq-supplemented wash buffer was added and
the cells were allowed to accumulate Dq for 0 or 14 min at 37 °C. Cells were
centrifuged (1 min, 13,000 r.p.m., tabletop centrifuge, Eppendorf 5424)
immediately after incubation and the supernatant was discarded. Pellets were
resuspended in 100 ml water, mixed with 1.5 ml of 1% SDS and incubated at
ambient temperature for several minutes. Dq fluorescence in these lysates was
determined fluorimetrically.
Proton transport in everted membrane vesicles.Proton antiport was assayed as
described4.E. coli UT5600/mdfA::kan cells overexpressing MdfA (from plasmid
pT7-5/araP/mdfA-His6) or harbouring empty vector were grown to A
600
of B1.2,
induced for 1 h by arabinose and everted membrane vesicles were prepared as
described26. Membranes were frozen by liquid N
2
in 10% glycerol, 5 mM MgSO
5
,
50 mM KPi pH 7.3 in a concentration of 20 mg ml 1protein and stored at
80 °C. Membranes were thawed quickly at 46 °C for experiments. For each
measurement, 20 ml membranes were diluted into 2 ml of pre-warmed (30 °C)
proton transport solution (50 m mM K-gluconate, 50 mM KCl, 10mM MgSO4,
titrated to pH 6 and supplemented with 1 mM ACMA (Sigma-Aldrich)).
Membranes were equilibrated for 4 min before fluorescence measurement
(excitation and emission wavelengths of 409 and 474, respectively). The samples
were continuously stirred during measurement. At the indicated times,
D,L-lactate (2 mM), drugs (at the indicated concentrations) or carbonyl cyanide
m-chlorophenyl hydrazone (0.01 mM, Sigma-Aldrich) were added. Lactate stocks
(200 mM) were titrated with NaOH to pH 7 before experiments.
Measurement of substrate-induced proton release.Proton release was mea-
sured as described4. Purified MdfA was dialysed 4 times against 100 volumes of
unbuffered solution containing 120 mM NaCl, 10% glycerol and 0.01% DDM. The
protein was diluted to 2 mM in 120 mM NaCl, 10% glycerol, 0.1% DDM and 1 mM
fluorescein was added. The mixture was titrated to fluorescence of B550
(pHB6.5) before the experiment. During the experiment, the fluorescence was
measured (excitation and emission wavelengths of 488 and 513, respectively), while
samples were continuously stirred at ambient temperature. Substrates were added
at concentrations of 150 mM (TPP), 50 mM (Dq) or 300 mM (Pent). HCl was added
at a concentration of 2 mM.
Dq binding measured by fluorescence.Purified MdfA was concentrated using
Vivaspin ultrafiltration spin columns (100 kDa cutoff, Sartorius). MdfA and Dq
were mixed at concentra tions of 10 mM MdfA and 0.2 mM Dq. Next, this mixture
was serially diluted (1.42-fold each dilution) into a solution containing only 0.2 mM
Dq, keeping [Dq] constant while diluting MdfA. The fluorescence of each
increasingly diluted sample was measured in a quartz cuvette (excitation and
emission wavelengths of 330 and 370, respectively). To determine K
d
, the data were
fitted to the equation: fluorescence (%) ¼100% ((DF[MdfA])/(K
d
þ[MdfA]))
using nonlinear regression, where DFis the percent fluorescence difference upon
MdfA binding.
Dq-mediated cross-linking of MdfA.Everted membrane vesicles were prepared
from cells overexpressing MdfA or the functional split mutant N8-His
6
/C4-His
6
(ref. 20) as described above for proton transport measurements. Membrane vesicles
were thawed, collected by ultracentrifugation (100,000 g,30min,4°C) and
resuspended in 0.5 M NaCl, 10% glycerol, 20 mM Tris-HCl pH 8 at a total protein
concentration of 1 mg ml 1. Dq (100 mM final) or buffer wa s added to 40 ml
vesicles and was allowed to bind during 20 min incubation at ambient temperature
in the dark. Samples were transferred to ice-cold 96-well plates and irradiated on
ice for 20 min with ultraviolet at 366 nm from a distance of 2.7 cm using Model
UVGL-25 Minervalight lamp (San Gabriel, CA). Irradiated samples (40 ml) were
mixed with 13.3 ml4protein sample buffer and 10 ml samples were analysed by
SDS–PAGE and western blotting as described4.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5615
8NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications
&2014 Macmillan Publishers Limited. All rights reserved.
References
1. Fluman, N. & Bibi, E. Bacterial multidrug transport through the lens of the
major facilitator superfamily. Biochim. Biophys. Acta 1794, 738–747 (2009).
2. Nishino, K. & Yamaguchi, A. Analysis of a complete library of putative drug
transporter genes in Escherichia coli.J. Bacteriol. 183, 5803–5812 (2001).
3. Saier, Jr M. H. & Paulsen, I. T. Phylogeny of multidrug transporters. Semin. Cell
Dev. Biol. 12, 205–213 (2001).
4. Fluman, N., Ryan, C. M., Whitelegge, J. P. & Bibi, E. Dissection of mechanistic
principles of a secondary multidrug efflux protein. Mol. Cell 47, 777–787
(2012).
5. Lewinson, O. et al. The Escherichia coli multidrug transporter MdfA catalyzes
both electrogenic and electroneutral transport reactions. Proc. Natl Acad. Sci.
USA 100, 1667–1672 (2003).
6. Mitchell, B. A., Paulsen, I. T., Brown, M. H. & Skurray, R. A. Bioenergetics of
the staphylococcal multidrug export protein QacA. Identification of distinct
binding sites for monovalent and divalent cations. J. Biol. Chem. 274,
3541–3548 (1999).
7. Schaedler, T. A. & van Veen, H. W. A flexible cation binding site in the
multidrug major facilitator superfamily transporter LmrP is associated with
variable proton coupling. FASEB J. 24, 3653–3661 (2010).
8. Tirosh, O. et al. Manipulating the drug/proton antiport stoichiometry of the
secondary multidrug transporter MdfA. Proc. Natl Acad. Sci. USA 109,
12473–12478 (2012).
9. Sigal, N. et al. 3D model of the Escherichia coli multidrug transporter MdfA
reveals an essential membrane-embedded positive charge. Biochemistry 44,
14870–14880 (2005).
10. Lewinson, O. & Bibi, E. Evidence for simultaneous binding of dissimilar
substrates by the Escherichia coli multidrug transporter MdfA. Biochemistry 40,
12612–12618 (2001).
11. Zilberstein, D., Schuldiner, S. & Padan, E. Proton electrochemical gradient in
Escherichia coli cells and its relation to active transport of lactose. Biochemistry
18, 669–673 (1979).
12. Guan, L., Murphy, F. D. & Kaback, H. R. Surface-exposed positions in the
transmembrane helices of the lactose permease of Escherichia coli determined
by intermolecular thiol cross-linking. Proc. Natl Acad. Sci. USA 99, 3475–3480
(2002).
13. Sigal, N., Lewinson, O., Wolf, S. G. & Bibi, E. E. coli multidrug transporter
MdfA is a monomer. Biochemistry 46, 5200–5208 (2007).
14. Abramson, J. et al. Structure and mechanism of the lactose permease of
Escherichia coli.Science 301, 610–615 (2003).
15. Huang, Y., Lemieux, M. J., Song, J., Auer, M. & Wang, D. N. Structure and
mechanism of the glycerol-3-phosphate transporter from Escherichia coli.
Science 301, 616–620 (2003).
16. Newstead, S. et al. Crystal structure of a prokaryotic homologue of the
mammalian oligopeptide-proton symporters, PepT1 and PepT2. EMBO J. 30,
417–426 (2011).
17. Dang, S. et al. Structure of a fucose transporter in an outward-open
conformation. Nature 467, 734–738 (2010).
18. Yin, Y., He, X., Szewczyk, P., Nguyen, T. & Chang, G. Structure of the
multidrug transporter EmrD from Escherichia coli.Science 312, 741–744
(2006).
19. Rotenberg, S. A. & Sun, X. G. Photoinduced inactivation of protein kinase C by
dequalinium identifies the RACK-1-binding domain as a recognition site.
J. Biol. Chem. 273, 2390–2395 (1998).
20. Adler, J. & Bibi, E. Promiscuity in the geometry of electrostatic interactions
between the Escherichia coli multidrug resistance transporter MdfA and
cationic substrates. J. Biol. Chem. 280, 2721–2729 (2005).
21. Brown, M. H. & Skurray, R. A. Staphylococcal multidrug efflux protein QacA.
J. Mol. Microbiol. Biotechnol. 3, 163–170 (2001).
22. Paulsen, I. T., Brown, M. H., Littlejohn, T. G., Mitchell, B. A. & Skurray, R. A.
Multidrug resistance proteins QacA and QacB from Staphylococcus aureus:
membrane topology and identification of residues involved in substrate
specificity. Proc. Natl Acad. Sci. 93, 3630–3635 (1996).
23. Mitchell, B. A., Brown, M. H. & Skurray, R. A. QacA multidrug efflux pump
from Staphylococcus aureus: comparative analysis of resistance to diamidines,
biguanidines, and guanylhydrazones. Antimicrob. Agents Chemother. 42,
475–477 (1998).
24. Rapoport, T. A. Protein translocation across the eukaryotic endoplasmic
reticulum and bacterial plasma membranes. Nature 450, 663–669 (2007).
25. Sigal, N., Fluman, N., Siemion, S. & Bibi, E. The secondary multidrug/proton
antiporter MdfA tolerates displacements of an essential negatively charged side
chain. J. Biol. Chem. 284, 6966–6971 (2009).
26. Reenstra, W. W., Patel, L., Rottenberg, H. & Kaback, H. R. Electrochemical
proton gradient in inverted membrane vesicles from Escherichia coli.
Biochemistry 19, 1–9 (1980).
Acknowledgements
This work was supported by a grant from the Israel Science Foundation (1128/06) to E.B.
and by the Weizmann Australia programme to E.B. and M.H.B.
Author contributions
N.F. and E.B. designed research and wrote the paper. N.F. performed experiments. J.A.
performed preliminary studies. M.H.B. provided QacA expressing plasmids. S.A.R. was
consulted and provided Dq and analogues.
Additional information
Competing financial interests: The authors declare no competing financial interests.
Reprints and permission information is available online at http://npg.nature.com/
reprintsandpermissions/
How to cite this article: Fluman, N. et al. Export of a single drug molecule
in two transport cycles by a multidrug efflux pump. Nat. Commun. 5:4615
doi: 10.1038/ncomms5615 (2014).
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5615 ARTICLE
NATURE COMMUNICATIONS | 5:4615 | DOI: 10.1038/ncomms5615 | www.nature.com/naturecommunications 9
&2014 Macmillan Publishers Limited. All rights reserved.
