Structural and functional conservation of key domains in InsP3 and ryanodine receptors.

Inositol-1,4,5-trisphosphate receptors (InsP(3)Rs) and ryanodine receptors (RyRs) are tetrameric intracellular Ca(2+) channels. In each of these receptor families, the pore, which is formed by carboxy-terminal transmembrane domains, is regulated by signals that are detected by large cytosolic structures. InsP(3)R gating is initiated by InsP(3) binding to the InsP(3)-binding core (IBC, residues 224-604 of InsP(3)R1) and it requires the suppressor domain (SD, residues 1-223 of InsP(3)R1). Here we present structures of the amino-terminal region (NT, residues 1-604) of rat InsP(3)R1 with (3.6 Å) and without (3.0 Å) InsP(3) bound. The arrangement of the three NT domains, SD, IBC-ß and IBC-α, identifies two discrete interfaces (α and ß) between the IBC and SD. Similar interfaces occur between equivalent domains (A, B and C) in RyR1 (ref. 9). The orientations of the three domains when docked into a tetrameric structure of InsP(3)R and of the ABC domains docked into RyR are remarkably similar. The importance of the α-interface for activation of InsP(3)R and RyR is confirmed by mutagenesis and, for RyR, by disease-causing mutations. Binding of InsP(3) causes partial closure of the clam-like IBC, disrupting the ß-interface and pulling the SD towards the IBC. This reorients an exposed SD loop ('hotspot' (HS) loop) that is essential for InsP(3)R activation. The loop is conserved in RyR and includes mutations that are associated with malignant hyperthermia and central core disease. The HS loop interacts with an adjacent NT, suggesting that activation re-arranges inter-subunit interactions. The A domain of RyR functionally replaced the SD in full-length InsP(3)R, and an InsP(3)R in which its C-terminal transmembrane region was replaced by that from RyR1 was gated by InsP(3) and blocked by ryanodine. Activation mechanisms are conserved between InsP(3)R and RyR. Allosteric modulation of two similar domain interfaces within an N-terminal subunit reorients the first domain (SD or A domain), allowing it, through interactions of the second domain of an adjacent subunit (IBC-ß or B domain), to gate the pore.

Effects of nucleotide binding to LmrA: A combined MAS-NMR and solution NMR study.

ABC transporters are fascinating examples of fine-tuned molecular machines that use the energy from ATP hydrolysis to translocate a multitude of substrates across biological membranes. While structural details have emerged on many members of this large protein superfamily, a number of functional details are still under debate. High resolution structures yield valuable insights into protein function, but it is the combination of structural, functional and dynamic insights that facilitates a complete understanding of the workings of their complex molecular mechanisms. NMR is a technique well-suited to investigate proteins in atomic resolution while taking their dynamic properties into account. It thus nicely complements other structural techniques, such as X-ray crystallography, that have contributed high-resolution data to the architectural understanding of ABC transporters. Here, we describe the heterologous expression of LmrA, an ABC exporter from Lactococcus lactis, in Escherichia coli. This allows for more flexible isotope labeling for nuclear magnetic resonance (NMR) studies and the easy study of LmrA's multidrug resistance phenotype. We use a combination of solid-state magic angle spinning (MAS) on the reconstituted transporter and solution NMR on its isolated nucleotide binding domain to investigate consequences of nucleotide binding to LmrA. We find that nucleotide binding affects the protein globally, but that NMR is also able to pinpoint local dynamic effects to specific residues, such as the Walker A motif's conserved lysine residue.

IP3 receptors: some lessons from DT40 cells.

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca2+ channels that are regulated by IP3 and Ca2+ and are modulated by many additional signals. These properties allow them to initiate and, via Ca2+-induced Ca2+ release, regeneratively propagate Ca2+ signals evoked by receptors that stimulate formation of IP3. The ubiquitous expression of IP3R highlights their importance, but it also presents problems when attempting to resolve the behavior of defined IP3R. DT40 cells are a pre-B-lymphocyte cell line in which high rates of homologous recombination afford unrivalled opportunities to disrupt endogenous genes. DT40-knockout cells with both alleles of each of the three IP3R genes disrupted provide the only null-background for analysis of homogenous recombinant IP3R. We review the properties of DT40 cells and consider three areas where they have contributed to understanding IP3R behavior. Patch-clamp recording from the nuclear envelope and Ca2+ release from intracellular stores loaded with a low-affinity Ca2+ indicator address the mechanisms leading to activation of IP(3)R. We show that IP3 causes intracellular IP3R to cluster and re-tune their responses to IP3 and Ca2+, better equipping them to mediate regenerative Ca2+ signals. Finally, we show that DT40 cells reliably count very few IP3R into the plasma membrane, where they mediate about half the Ca2+ entry evoked by the B-cell antigen receptor.

An ABC transporter with a secondary-active multidrug translocator domain.

Multidrug resistance, by which cells become resistant to multiple unrelated pharmaceuticals, is due to the extrusion of drugs from the cell's interior by active transporters such as the human multidrug resistance P-glycoprotein. Two major classes of transporters mediate this extrusion. Primary-active transporters are dependent on ATP hydrolysis, whereas secondary-active transporters are driven by electrochemical ion gradients that exist across the plasma membrane. The ATP-binding cassette (ABC) transporter LmrA is a primary drug transporter in Lactococcus lactis that can functionally substitute for P-glycoprotein in lung fibroblast cells. Here we have engineered a truncated LmrA protein that lacks the ATP-binding domain. Surprisingly, this truncated protein mediates a proton-ethidium symport reaction without the requirement for ATP. In other words, it functions as a secondary-active multidrug uptake system. These findings suggest that the evolutionary precursor of LmrA was a secondary-active substrate translocator that acquired an ATP-binding domain to enable primary-active multidrug efflux in L. lactis.

Multidrug transport by the ABC transporter Sav1866 from Staphylococcus aureus.

Sav1866 is an ATP-binding cassette (ABC) protein from the pathogen Staphylococcus aureus and is a homologue of bacterial and human multidrug ABC transporters. Recently, the three-dimensional crystal structure of Sav1866 was determined at 3.0 A resolution [Dawson, R. J., and Locher, K. P. (2006) Nature 443, 180-185]. Although this structure is frequently used to homology model human and microbial ABC multidrug transporters by computational methods, the ability of Sav1866 to transport multiple drugs has not been described. We obtained functional expression of Sav1866 in the drug-sensitive, Gram-positive bacterium Lactococcus lactis Delta lmrA Delta lmrCD lacking major endogenous multidrug transporters. Sav1866 displayed a Hoechst 33342, verapamil, tetraphenylphosphonium, and vinblastine-stimulated ATPase activity. In growing cells, Sav1866 expression conferred resistance to Hoechst 33342. In transport assays in intact cells, Sav1866 catalyzed the translocation of amphiphilic cationic ethidium. Additionally, Sav1866 mediated the active transport of Hoechst 33342 in membrane vesicles and proteoliposomes containing purified and functionally reconstituted protein. Sav1866-mediated resistance and transport were inhibited by the human ABCB1 and ABCC1 modulator verapamil. This work represents the first demonstration of multidrug transport by Sav1866 and suggests that Sav1866 can serve as a well-defined model for studies on the molecular bases of drug-protein interactions in ABC transporters. Our methods for the overexpression, purification, and functional reconstitution of Sav1866 are described in detail.

Functional role of transmembrane helix 6 in drug binding and transport by the ABC transporter MsbA.

The ATP-binding cassette transporter MsbA in Gram-negative bacteria can transport antibiotics and toxic ions. However, the key functional regions in MsbA which determine substrate specificity remain to be identified. We recently examined published mutations in the human MsbA homologue ABCB1 that alter multidrug transport in cells and identified mutations that affect the specificity for individual substrates (termed change-in-specificity mutations). When superimposed on the corrected 3.7 A resolution crystal structure of homodimeric MsbA from S almonella typhimurium, these change-in-specificity mutations colocalize in a major groove in each of the two "wings" of transmembrane helices (TMHs) that point away from one another toward the periplasm. Near the apex of the groove, the periplasmic side of TMH 6 in both monomers contains a hotspot of change-in-specificity mutations and residues which, when replaced with cysteines in ABCB1, covalently interact with thiol-reactive drug analogues. We tested the importance of this region of TMH 6 for drug-protein interactions in Escherichia coli MsbA. In particular, we focused on conserved S289 and S290 residues in the hotspot. Their simultaneous replacement with alanine (termed the SASA mutant) significantly reduced the level of binding and transport of ethidium and Taxol by MsbA, whereas the interactions with Hoechst 33342 and erythromycin remained unaffected. Hence, the SASA mutation is associated with a change-in-specificity phenotype analogous to that of the change-in-specificity mutations in ABCB1. This study demonstrates for the first time the significance of TMH 6 for drug binding and transport by MsbA. Based on these data, a possible mechanism for alternating access of drug-binding surfaces in MsbA is discussed.

A functional steroid-binding element in an ATP-binding cassette multidrug transporter.

The human breast cancer resistance protein is an ATP-binding cassette (ABC) multidrug transporter that affects the bioavailability of chemotherapeutic drugs and can confer drug resistance on cancer cells. It is the second member of the ABCG subfamily, other members of which are associated with human steroid disorders such as hypercholesterolemia, sitosterolemia, and atherosclerosis. The molecular bases of protein-steroid interactions in ABC transporters are unknown. Here, we identify a steroid-binding element in the membrane domain of ABCG2 with a similarity to steroid hormone/nuclear receptors. The element facilitates steroid hormone binding and mediates modulation of ABCG2 activity. The identification of this element might provide an opportunity for the development of new therapeutic ligands for ABCG2.

Caught in the act: ATP hydrolysis of an ABC-multidrug transporter followed by real-time magic angle spinning NMR.

The ATP binding cassette (ABC) transporter LmrA from Lactococcus lactis transports cytotoxic molecules at the expense of ATP. Molecular and kinetic details of LmrA can be assessed by solid-state nuclear magnetic resonance (ssNMR), if functional reconstitution at a high protein-lipid ratio can be achieved and the kinetic rate constants are small enough. In order to follow ATP hydrolysis directly by 31P-magic angle spinning (MAS) nuclear magnetic resonance (NMR), we generated such conditions by reconstituting LmrA-dK388, a mutant with slower ATP turnover rate, at a protein-lipid ration of 1:150. By analysing time-resolved 31P spectra, protein activity has been directly assessed. These data demonstrate the general possibility to perform ssNMR studies on a fully active full length ABC transporter and also form the foundation for further kinetic studies on LmrA by NMR.

New light on multidrug binding by an ATP-binding-cassette transporter.

ATP-binding-cassette (ABC) multidrug transporters confer multidrug resistance to pathogenic microorganisms and human tumour cells by mediating the extrusion of structurally unrelated chemotherapeutic drugs from the cell. The molecular basis by which ABC multidrug transporters bind and transport drugs is far from clear. Genetic analyses during the past 14 years reveal that the replacement of many individual amino acids in mammalian multidrug resistance P-glycoproteins can affect cellular resistance to drugs, but these studies have failed to identify specific regions in the primary amino acid sequence that are part of a defined drug-binding pocket. The recent publication of an X-ray crystallographic structure of the bacterial P-glycoprotein homologue MsbA and an MsbA-based homology model of human P-glycoprotein creates an opportunity to compare the original mutagenesis data with the three-dimensional structures of transporters. Our comparisons reveal that mutations that alter specificity are present in three-dimensional 'hotspot' regions in the membrane domains of P-glycoprotein.

A critical role of a carboxylate in proton conduction by the ATP-binding cassette multidrug transporter LmrA.

The ATP binding cassette (ABC) transporter LmrA from the bacterium Lactococcus lactis is a homolog of the human multidrug resistance P-glycoprotein (ABCB1), the activity of which impairs the efficacy of chemotherapy. In a previous study, LmrA was shown to mediate ethidium efflux by an ATP-dependent proton-ethidium symport reaction in which the carboxylate E314 is critical. The functional importance of this key residue for ABC proteins was suggested by its conservation in a wider family of related transporters; however, the structural basis of its role was not apparent. Here, we have used homology modeling to define the structural environment of E314. The residue is nested in a hydrophobic environment that probably elevates its pKa, accounting for the pH dependency of drug efflux that we report in this work. Functional analyses of wild-type and mutant proteins in cells and proteoliposomes support our proposal for the mechanistic role of E314 in proton-coupled ethidium transport. As the carboxylate is known to participate in proton translocation by secondary-active transporters, our observations suggest that this substituent can play a similar role in the activity of ABC transporters.

ATP-dependent substrate transport by the ABC transporter MsbA is proton-coupled.

ATP-binding cassette transporters mediate the transbilayer movement of a vast number of substrates in or out of cells in organisms ranging from bacteria to humans. Current alternating access models for ABC exporters including the multidrug and Lipid A transporter MsbA from Escherichia coli suggest a role for nucleotide as the fundamental source of free energy. These models involve cycling between conformations with inward- and outward-facing substrate-binding sites in response to engagement and hydrolysis of ATP at the nucleotide-binding domains. Here we report that MsbA also utilizes another major energy currency in the cell by coupling substrate transport to a transmembrane electrochemical proton gradient. The dependence of ATP-dependent transport on proton coupling, and the stimulation of MsbA-ATPase by the chemical proton gradient highlight the functional integration of both forms of metabolic energy. These findings introduce ion coupling as a new parameter in the mechanism of this homodimeric ABC transporter.

Mutational screening of the RB1 gene in Indian patients with retinoblastoma reveals eight novel and several recurrent mutations.

Retinoblastoma is the most common primary intraocular malignancy in children, caused by inactivation of the RB1 gene on chromosome 13. We carried out a mutational screen of the exons and promoter of the RB1 gene in Indian patients with retinoblastoma in order to determine the range of mutations giving rise to disease. Forty-seven patients were screened for mutations in all exons and promoter of the RB1 gene by single strand conformation polymorphism followed by sequencing. Tumors were available from 27 patients (12 bilateral and 15 unilateral retinoblastoma) while only peripheral blood was available from 20 patients, all with bilateral disease. Mutations were found in 22 patients, 9 from the analysis of tumors and 13 from peripheral blood. Eight novel mutations were identified, including 4 single base changes, 2 small deletions and 1 duplication. These are g.64365T>G (Tyr325Ter), g.78131G>A (Trp515Ter), g.150061G>T (Glu587Ter), g.170383C>G (S834X), g.41924A>C (IVS3-2A>C), g.150064ins4, g.160792del22, and g.76940del14 (IVS15 del +20-33). Almost all mutations produced nonsense codons or frameshifts. Recurrent mutations, especially at CpG sites were seen predominantly. Detectable mutations in exons were found in 46% of patients tested. Large deletions, epigenetic changes as well as mutations in non-coding regions may be the cause of disease in the remainder of patients. Knowledge of the full range of mutations can aid in the design of screening tests for individuals at risk.

Tuning the drug efflux activity of an ABC transporter in vivo by in vitro selected DARPin binders.

ABC transporters use the energy from binding and hydrolysis of ATP to import or extrude substrates across the membrane. Using ribosome display, we raised designed ankyrin repeat proteins (DARPins) against detergent solubilized LmrCD, a heterodimeric multidrug ABC exporter from Lactococcus lactis. Several target-specific DARPin binders were identified that bind to at least three distinct, partially overlapping epitopes on LmrD in detergent solution as well as in native membranes. Remarkably, functional screening of the LmrCD-specific DARPin pools in L. lactis revealed three homologous DARPins which, when generated in LmrCD-expressing cells, strongly activated LmrCD-mediated drug transport. As LmrCD expression in the cell membrane was unaltered upon the co-expression of activator DARPins, the activation is suggested to occur at the level of LmrCD activity. Consistent with this, purified activator DARPins were found to stimulate the ATPase activity of LmrCD in vitro when reconstituted in proteoliposomes. This study suggests that membrane transporters are tunable in vivo by in vitro selected binding proteins. Our approach could be of biopharmaceutical importance and might facilitate studies on molecular mechanisms of ABC transporters.

MacB ABC transporter is a dimer whose ATPase activity and macrolide-binding capacity are regulated by the membrane fusion protein MacA.

Gram-negative bacteria utilize specialized machinery to translocate drugs and protein toxins across the inner and outer membranes, consisting of a tripartite complex composed of an inner membrane secondary or primary active transporter (IMP), a periplasmic membrane fusion protein, and an outer membrane channel. We have investigated the assembly and function of the MacAB/TolC system that confers resistance to macrolides in Escherichia coli. The membrane fusion protein MacA not only stabilizes the tripartite assembly by interacting with both the inner membrane protein MacB and the outer membrane protein TolC, but also has a role in regulating the function of MacB, apparently increasing its affinity for both erythromycin and ATP. Analysis of the kinetic behavior of ATP hydrolysis indicated that MacA promotes and stabilizes the ATP-binding form of the MacB transporter. For the first time, we have established unambiguously the dimeric nature of a noncanonic ABC transporter, MacB that has an N-terminal nucleotide binding domain, by means of nondissociating mass spectrometry, analytical ultracentrifugation, and atomic force microscopy. Structural studies of ABC transporters indicate that ATP is bound between a pair of nucleotide binding domains to stabilize a conformation in which the substrate-binding site is outward-facing. Consequently, our data suggest that in the presence of ATP the same conformation of MacB is promoted and stabilized by MacA. Thus, MacA would facilitate the delivery of drugs by MacB to TolC by enhancing the binding of drugs to it and inducing a conformation of MacB that is primed and competent for binding TolC. Our structural studies are an important first step in understanding how the tripartite complex is assembled.

A multidrug ABC transporter with a taste for salt.

BACKGROUND: LmrA is a multidrug ATP-binding cassette (ABC) transporter from Lactococcus lactis with no known physiological substrate, which can transport a wide range of chemotherapeutic agents and toxins from the cell. The protein can functionally replace the human homologue ABCB1 (also termed multidrug resistance P-glycoprotein MDR1) in lung fibroblast cells. Even though LmrA mediates ATP-dependent transport, it can use the proton-motive force to transport substrates, such as ethidium bromide, across the membrane by a reversible, H(+)-dependent, secondary-active transport reaction. The mechanism and physiological context of this reaction are not known. METHODOLOGY/PRINCIPAL FINDINGS: We examined ion transport by LmrA in electrophysiological experiments and in transport studies using radioactive ions and fluorescent ion-selective probes. Here we show that LmrA itself can transport NaCl by a similar secondary-active mechanism as observed for ethidium bromide, by mediating apparent H(+)-Na(+)-Cl(-) symport. Remarkably, LmrA activity significantly enhances survival of high-salt adapted lactococcal cells during ionic downshift. CONCLUSIONS/SIGNIFICANCE: The observations on H(+)-Na(+)-Cl(-) co-transport substantiate earlier suggestions of H(+)-coupled transport by LmrA, and indicate a novel link between the activity of LmrA and salt stress. Our findings demonstrate the relevance of investigations into the bioenergetics of substrate translocation by ABC transporters for our understanding of fundamental mechanisms in this superfamily. This study represents the first use of electrophysiological techniques to analyze substrate transport by a purified multidrug transporter.

On the energy-dependence of Hoechst 33342 transport by the ABC transporter LmrA.

LmrA is an ATP-binding cassette (ABC) multidrug transporter from Lactococcus lactis, and is a structural homologue of the human multidrug resistance P-glycoprotein (ABCB1), the overexpression of which is associated with multidrug resistance in tumours. We recently observed that a truncated version of LmrA lacking the nucleotide-binding domain mediates a proton motive force-dependent ethidium transport reaction by catalyzing proton-ethidium symport. This finding raised the question whether proton motive force-dependent transport can also be observed for other drugs, and whether this reaction is also relevant for full-length LmrA. Furthermore, the observations on LmrA-MD raised the question whether ATP-dependent transport by LmrA in intact cells could be due to the activity of independent ABC transporters that might become upregulated in the lactococcal cells due to the overexpression of LmrA; the recently identified ABC multidrug transporter LmrCD was put forward as a possible candidate. Here, we investigated the energy coupling to the transport of the amphiphilic dye Hoechst 33342 in proteoliposomes containing purified LmrA. For this purpose, LmrA was obtained from lactococcal cells lacking the genomic lmrA and lmrCD genes, in which LmrA was expressed from a plasmid. To separate ATP-dependence from proton motive force-dependence, we also used mutant LmrA proteins, which were affected in their ability to hydrolyse ATP. Our studies in proteoliposomes demonstrate that LmrA can catalyze Hoechst 33342 transport independent of auxiliary proteins, in an ATP-dependent fashion and a transmembrane chemical proton gradient (interior acidic)-dependent fashion.

ABCG transporters: structure, substrate specificities and physiological roles : a brief overview.

The ATP-binding cassette (ABC) transporter superfamily is one of the largest protein families with representatives in all kingdoms of life. Members of this superfamily are involved in a wide variety of transport processes with substrates ranging from small ions to relatively large polypeptides and polysaccharides. The G subfamily of ABC transporters consists of half-transporters, which oligomerise to form the functional transporter. While ABCG1, ABCG4 and ABCG5/8 are involved in the ATP-dependent translocation of steroids and, possibly, other lipids, ABCG2 (also termed the breast cancer resistance protein) has been identified as a multidrug transporter that confers resistance on tumor cells. Evidence will be summarized suggesting that ABCG2 can also mediate the binding/transport of non-drug substrates, including free and conjugated steroids. The characterization of the substrate specificities of ABCG proteins at a molecular level might provide further clues about their potential physiological role(s), and create new opportunities for the modulation of their activities in relation to human disease.

Two proton translocation pathways in a secondary active multidrug transporter.

LmrP is a secondary active multidrug transporter from Lactococcus lactis. The protein belongs to the major facilitator superfamily and utilizes the electrochemical proton gradient (inside negative and alkaline) to extrude a wide range of lipophilic cations from the cell. Previous work has indicated that ethidium, a monovalent cationic substrate, is exported by LmrP by electrogenic antiport with two (or more) protons. This observation raised the question whether these protons are translocated sequentially along the same pathway, or through different routes. To address this question, we constructed a 3-D homology model of LmrP based on the high-resolution structure of the glycerol-3P/Pi antiporter GlpT from Escherichia coli, and we tested by mutagenesis the possible proton conduction points suggested by this model. Similar to the template, LmrP is predicted to contain an internal cavity formed at the interface between the two halves of the transporter. On the surface of this cavity lie two clusters of polar, aromatic and carboxylate residues with potentially important function in proton shuttling. Cluster 1 in the C-terminal half contains D235 and E327 in immediate proximity of each other, and is located near the apex of the cavity. Cluster 2 in the N-terminal half contains D142. Analyses of LmrP mutants containing charge-conservative or carboxyl-to-amide replacements at positions 142, 235 and 327 suggest that D142 is part of a dedicated proton translocation pathway in the ethidium translocation reaction. In contrast, D235 and E327 are part of an independent pathway, in which D235 interacts with protons. E327 appears to modulate the pKa of D235 and plays a role in the interaction with ethidium. These results are consistent with the proposal that major facilitator superfamily proteins consist of two membrane domains, one of which is involved in substrate binding and the other in ion coupling, and they indicate that there are two proton conduction pathways at play in the transport mechanism.

New structure model for the ATP-binding cassette multidrug transporter LmrA.

Multidrug resistance of pathogenic microorganisms and mammalian tumors can be associated with the overexpression of multidrug transporters. These integral membrane proteins are capable of extruding a wide range of structurally unrelated compounds from the cell. Among the different classes of multidrug transporters are the ATP binding cassette (ABC) transporters, which are dependent on the binding and hydrolysis of ATP. In the past five years, many researchers have built homology models of ABC extrusion systems using the atomic coordinates of crystallized MsbA, a lipopolysaccharide transporter in Gram-negative bacteria. Likewise, we have previously used the Vibrio cholera MsbA structure as a template in the modeling of the multidrug transporter LmrA from Lactococcus lactis. In view of the recently discovered inaccuracies in the MsbA structure, we have remodelled LmrA using the atomic coordinates of the MsbA homologue Sav1866 from Staphylococcus aureus. To compare and test our MsbA-based and Sav1866-based LmrA models we performed cysteine cross-linking at three key positions in LmrA. The pattern of cross-linking at these positions was consistent with the overall topology of transmembrane helices in Sav1866, suggesting that its crystal structure might be physiologically relevant. We recently identified E314 as a residue important in proton conduction by LmrA. The predicted location of this residue at the interface between the two half-transporters in the Sav1866-based homodimer, within the inner leaflet of the phospholipid bilayer, provides a new structural basis for the role of E314 in LmrA-mediated transport.

Drug-lipid A interactions on the Escherichia coli ABC transporter MsbA.

MsbA is an essential ATP-binding cassette half-transporter in the cytoplasmic membrane of the gram-negative Escherichia coli and is required for the export of lipopolysaccharides (LPS) to the outer membrane, most likely by transporting the lipid A core moiety. Consistent with the homology of MsbA to the multidrug transporter LmrA in the gram-positive Lactococcus lactis, our recent work in E. coli suggested that MsbA might interact with multiple drugs. To enable a more detailed analysis of multidrug transport by MsbA in an environment deficient in LPS, we functionally expressed MsbA in L. lactis. MsbA expression conferred an 86-fold increase in resistance to the macrolide erythromycin. A kinetic characterization of MsbA-mediated ethidium and Hoechst 33342 transport revealed apparent single-site kinetics and competitive inhibition of these transport reactions by vinblastine with K(i) values of 16 and 11 microM, respectively. We also detected a simple noncompetitive inhibition of Hoechst 33342 transport by free lipid A with a K(i) of 57 microM, in a similar range as the K(i) for vinblastine, underscoring the relevance of our LPS-less lactococcal model for studies on MsbA-mediated drug transport. These observations demonstrate the ability of heterologously expressed MsbA to interact with free lipid A and multiple drugs in the absence of auxiliary E. coli proteins. Our transport data provide further functional support for direct LPS-MsbA interactions as observed in a recent crystal structure for MsbA from Salmonella enterica serovar Typhimurium (C. L. Reyes and G. Chang, Science 308:1028-1031, 2005).