Large-scale purification, dissociation and functional reassembly of the maltose ATP-binding cassette transporter (MalFGK2) of Salmonella typhimurium

The maltose ATP-binding cassette (ABC) transporter of Salmonella typhimurium is composed of a membrane-associated complex (MalFGK₂) and a periplasmic substrate binding protein. To further elucidate protein-protein interactions between the subunits, we have studied the dissociation and reassembly of the MalFGK₂ complex at the level of purified components in proteoliposomes. First, we optimized the yield in purified complex protein by taking advantage of a newly constructed expression plasmid that carries the malK, malF and malG genes in tandem orientation. Incorporated in proteoliposomes, the complex exhibited maltose binding protein/maltose-dependent ATPase activity with a V_max of 1.25 μmol P_i/min/mg and a K_m of 0.1 mM. ATPase activity was sensitive to vanadate and enzyme IIA^Glc, a component of the enterobacterial glucose transport system. The proteoliposomes displayed maltose transport activity with an initial rate of 61 nmol/min/mg. Treatment of proteoliposomes with 6.6 M urea resulted in the release of medium-exposed MalK subunits concomitant with the complete loss of ATPase activity. By adding increasing amounts of purified MalK to urea-treated proteoliposomes, about 50% of vanadate-sensitive ATPase activity relative to the control could be recovered. Furthermore, the phenotype of MalKQ140K that exhibits ATPase activity in solution but not when associated with MalFG was confirmed by reassembly with MalK-depleted proteoliposomes.     

Functional reassembly of the Escherichia coli maltose transporter following purification of a MalF-MalG subassembly

Taking advantage of a chaperone-like function of MalK, a stable complex of MalF-MalG could be solubilized from the Escherichia coli membrane and purified in high yield in the absence of MalK. This MalF-MalG complex was competent for efficient reassembly of a functional MalFGK(2) maltose transporter complex both in detergent solution and in proteoliposomes.     

Topography of the surface of the signal-transducing protein EIIA(Glc) that interacts with the MalK subunits of the maltose ATP-binding cassette transporter (MalFGK2) of Salmonella typhimurium

The signal-transducing protein EIIA(Glc), a component of the phosphoenolpyruvate-glucose phosphotransferase system, plays a key role in carbon regulation in enteric bacteria, such as Escherichia coli and Salmonella typhimurium. The phosphorylation state of EIIA(Glc) governs transport and metabolism of a number of carbohydrates. When glucose as preferred carbon source is transported, EIIA(Glc) becomes predominantly unphosphorylated and allosterically inhibits several permeases, including the maltose ATP-binding cassette transport system (MalFGK2) in a process termed "inducer exclusion." We have mapped the binding surface of EIIA(Glc) that interacts with the MalK subunits by using synthetic cellulose-bound peptide arrays like pep scan- and substitutional analyses. Three regions constituting two binding sites were identified encompassing residues 69-79 (I), 87-91 (II), and 118-127 (III). Region III is MalK-specific, whereas residues from regions I and II partly overlap but are not identical to the binding interfaces for interaction with glycerol kinase and lactose permease. These results were fully verified by studying the inhibitory effect of purified EIIA(Glc) variants carrying mutations at positions representative of each of the three regions on the ATPase activity of the purified maltose transport complex reconstituted into proteoliposomes. Moreover, a synthetic peptide encompassing residues 69-91 was demonstrated to partially inhibit ATPase activity. We also show for the first time that the N-terminal domain of EIIA(Glc) is essential for inducer exclusion.     

ATP-driven MalK dimer closure and reopening and conformational changes of the "EAA" motifs are crucial for function of the maltose ATP-binding cassette transporter (MalFGK2)

We have investigated conformational changes of the purified maltose ATP-binding cassette transporter (MalFGK(2)) upon binding of ATP. The transport complex is composed of a heterodimer of the hydrophobic subunits MalF and MalG constituting the translocation pore and of a homodimer of MalK, representing the ATP-hydrolyzing subunit. Substrate is delivered to the transporter in complex with periplasmic maltose-binding protein (MalE). Cross-linking experiments with a variant containing an A85C mutation within the Q-loop of each MalK monomer indicated an ATP-induced shortening of the distance between both monomers. Cross-linking caused a substantial inhibition of MalE-maltose-stimulated ATPase activity. We further demonstrated that a mutation affecting the "catalytic carboxylate" (E159Q) locks the MalK dimer in the closed state, whereas a transporter containing the "ABC signature" mutation Q140K permanently resides in the resting state. Cross-linking experiments with variants containing the A85C mutation combined with cysteine substitutions in the conserved EAA motifs of MalF and MalG, respectively, revealed close proximity of these residues in the resting state. The formation of a MalK-MalG heterodimer remained unchanged upon the addition of ATP, indicating that MalG-EAA moves along with MalK during dimer closure. In contrast, the yield of MalK-MalF dimers was substantially reduced. This might be taken as further evidence for asymmetric functions of both EAA motifs. Cross-linking also caused inhibition of ATPase activity, suggesting that transporter function requires conformational changes of both EAA motifs. Together, our data support ATP-driven MalK dimer closure and reopening as crucial steps in the translocation cycle of the intact maltose transporter and are discussed with respect to a current model.     

Purification and characterization of the heterologously expressed trehalose/maltose ABC transporter complex of the hyperthermophilic archaeon Thermococcus litoralis.

We report the purification of the maltose/trehalose transporter complex MalFGK of the hyperthermophilic archaeon Thermococcus litoralis. The complex was expressed in Escherichia coli, solubilized in dodecyl maltoside and purified with the aid of a histidine tag on one of the membrane proteins. One hundred grams of cells yielded 3 mg of pure complex. The final product showed ATPase activity at 70 degrees C and was soluble at low detergent concentration. ATPase activity was not due to dissociation of the MalK subunit from the integral membrane proteins MalF and MalG but could not be further stimulated by trehalose/maltose binding protein (TMBP), be it the native protein as isolated from T. litoralis or the soluble engineered protein. The purified native TMBP was identified as a glycoprotein.     

Functional reconstitution of a maltose ATP-binding cassette transporter from the thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius

The thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius grows at 60 degrees C and pH 2-3. The organism can utilize maltose and maltodextrins as energy source that are taken up by an ATP-binding cassette (ABC) import system. Genes encoding a maltose binding protein, MalE, and two membrane-integral subunits, MalF and MalG, are clustered on the chromosome but a malK gene translating into a cognate ATPase subunit is lacking. Here we report the cloning of malK from genomic DNA by using the msiK gene of Streptomyces lividans as a probe. Purified MalK exhibited a spontaneous ATPase activity with a Vmax of 0.13 micromol Pi/min/mg and a Km of 330 microM that was optimal at the growth temperature of the organism. Coexpression of malK, malF and malG in Escherichia coli resulted in the formation of a complex that could be coeluted from an affinity matrix after solubilization of membranes with dodecylmaltoside. Proteoliposomes prepared from the MalFGK complex and preformed phospholipid vesicles of A. acidocaldarius displayed a low intrinsic ATPase activity that was stimulated sevenfold by maltose-loaded MalE, thereby indicating coupling of ATP hydrolysis to substrate translocation. These results provide evidence for MalK being the physiological ATPase subunit of the A. acidocaldarius maltose transporter. Moreover, to our knowledge, this is the first report on the functional reconstitution of an ABC transport system from a thermophilic microorganism.     

Mutation of a single MalK subunit severely impairs maltose transport activity in Escherichia coli.

The maltose transport system of Escherichia coli, a member of the ABC transport superfamily of proteins, consists of a periplasmic maltose binding protein and a membrane-associated translocation complex that contains two copies of the ATP-binding protein MalK. To examine the need for two nucleotide-binding domains in this transport complex, one of the two MalK subunits was inactivated by site-directed mutagenesis. Complexes with mutations in a single subunit were obtained by attaching a polyhistidine tag to the mutagenized version of MalK and by coexpressing both wild-type MalK and mutant (His)6MalK in the same cell. Hybrid complexes containing one mutant (His)6MalK subunit and one wild-type MalK subunit were separated from those containing two mutant (His)6MalK proteins based on differential affinities for a metal chelate column. Purified transport complexes were reconstituted into proteoliposome vesicles and assayed for maltose transport and ATPase activities. When a conserved lysine residue at position 42 that is involved in ATP binding was replaced with asparagine in both MalK subunits, maltose transport and ATPase activities were reduced to 1% of those of the wild type. When the mutation was present in only one of the two subunits, the complex had 6% of the wild-type activities. Replacement of a conserved histidine residue at position 192 in MalK with arginine generated similar results. It is clear from these results that two functional MalK proteins are required for transport activity and that the two nucleotide-binding domains do not function independently to catalyze transport.     

ATP modulates subunit-subunit interactions in an ATP-binding cassette transporter (MalFGK2) determined by site-directed chemical cross-linking

The binding protein-dependent maltose transport system of enterobacteria (MalFGK(2)), a member of the ATP-binding cassette (ABC) transporter superfamily, is composed of two integral membrane proteins, MalF and MalG, and of two copies of an ATPase subunit, MalK, which hydrolyze ATP, thus energizing the translocation process. In addition, an extracellular (periplasmic) substrate-binding protein (MalE) is required for activity. Ligand translocation and ATP hydrolysis are dependent on a signaling mechanism originating from the binding protein and traveling through MalF/MalG. Thus, subunit-subunit interactions in the complex are crucial to the transport process but the chemical nature of residues involved is poorly understood. We have investigated the proximity of residues in a conserved sequence ("EAA" loop) of MalF and MalG to residues in a helical segment of the MalK subunits by means of site-directed chemical cross-linking. To this end, single cysteine residues were introduced into each subunit at several positions and the respective malF and malG alleles were individually co-expressed with each of the malK alleles. Membrane vesicles were prepared from those double mutants that contained a functional transporter in vivo and treated with Cu(1,10-phenanthroline)(2)SO(4) or bifunctional cross-linkers. The results suggest that residues Ala-85, Lys-106, Val-114, and Val-117 in the helical segment of MalK, to different extents, participate in constitution of asymmetric interaction sites with the EAA loops of MalF and MalG. Furthermore, both MalK monomers in the complex are in close contact to each other through Ala-85 and Lys-106. These interactions are strongly modulated by MgATP, indicating a structural rearrangement of the subunits during the transport cycle. These data are discussed with respect to current transport models.     

Inducer exclusion in Firmicutes: insights into the regulation of a carbohydrate ATP binding cassette transporter from Lactobacillus casei BL23 by the signal transducing protein P‐Ser46‐HPr

Catabolite repression is a mechanism that enables bacteria to control carbon utilization. As part of this global regulatory network, components of the phosphoenolpyruvate:carbohydrate phosphotransferase system inhibit the uptake of less favorable sugars when a preferred carbon source such as glucose is available. This process is termed inducer exclusion. In bacteria belonging to the phylum Firmicutes, HPr, phosphorylated at serine 46 (P‐Ser46‐HPr) is the key player but its mode of action is elusive. To address this question at the level of purified protein components, we have chosen a homolog of the Escherichia coli maltose/maltodextrin ATP‐binding cassette transporter from Lactobacillus casei (MalE1‐MalF1G1K1₂) as a model system. We show that the solute binding protein, MalE1, binds linear and cyclic maltodextrins but not maltose. Crystal structures of MalE1 complexed with these sugars provide a clue why maltose is not a substrate. P‐Ser46‐HPr inhibited MalE1/maltotetraose‐stimulated ATPase activity of the transporter incorporated in proteoliposomes. Furthermore, cross‐linking experiments revealed that P‐Ser46‐HPr contacts the nucleotide‐binding subunit, MalK1, in proximity to the Walker A motif. However, P‐Ser46‐HPr did not block binding of ATP to MalK1. Together, our findings provide first biochemical evidence that P‐Ser‐HPr arrests the transport cycle by preventing ATP hydrolysis at the MalK1 subunits of the transporter.     

Maltose-binding protein is open in the catalytic transition state for ATP hydrolysis during maltose transport

The maltose transport complex of Escherichia coli, a member of the ATP-binding cassette superfamily, mediates the high affinity uptake of maltose at the expense of ATP. The membrane-associated transporter consists of two transmembrane subunits, MalF and MalG, and two copies of the cytoplasmic ATP-binding cassette subunit, MalK. Maltose-binding protein (MBP), a soluble periplasmic protein, delivers maltose to the MalFGK(2) transporter and stimulates hydrolysis by the transporter. Site-directed spin labeling electron paramagnetic resonance spectroscopy is used to monitor binding of MBP to MalFGK(2) and conformational changes in MBP as it interacts with MalFGK(2). Cysteine residues and spin labels have been introduced into the two lobes of MBP so that spin-spin interaction will report on ligand-induced closure of the protein (Hall, J. A., Thorgeirsson, T. E., Liu, J., Shin, Y. K., and Nikaido, H. (1997) J. Biol. Chem. 272, 17610-17614). At least two different modes of interaction between MBP and MalFGK(2) were detected. Binding of MBP to MalFGK(2) in the absence of ATP resulted in a decrease in motion of spin label at position 41 in the C-terminal domain of MBP. In a vanadate-trapped transition state intermediate, all free MBP became tightly bound to MalFGK(2), spin label in both lobes became completely immobilized, and spin-spin interactions were lost, suggesting that MBP was in an open conformation. Binding of non-hydrolyzable MgATP analogs or ATP in the absence of Mg is sufficient to stabilize a complex of open MBP and MalFGK(2). Taken together, these data suggest that closure of the MalK dimer interface coincides with opening of MBP and maltose release to the transporter.     

Vanadate-induced trapping of nucleotides by purified maltose transport complex requires ATP hydrolysis

The maltose transport system in Escherichia coli is a member of the ATP-binding cassette superfamily of transporters that is defined by the presence of two nucleotide-binding domains or subunits and two transmembrane regions. The bacterial import systems are unique in that they require a periplasmic substrate-binding protein to stimulate the ATPase activity of the transport complex and initiate the transport process. Upon stimulation by maltose-binding protein, the intact MalFGK(2) transport complex hydrolyzes ATP with positive cooperativity, suggesting that the two nucleotide-binding MalK subunits interact to couple ATP hydrolysis to transport. The ATPase activity of the intact transport complex is inhibited by vanadate. In this study, we investigated the mechanism of inhibition by vanadate and found that incubation of the transport complex with MgATP and vanadate results in the formation of a stably inhibited species containing tightly bound ADP that persists after free vanadate and nucleotide are removed from the solution. The inhibited species does not form in the absence of MgCl(2) or of maltose-binding protein, and ADP or another nonhydrolyzable analogue does not substitute for ATP. Taken together, these data conclusively show that ATP hydrolysis must precede the formation of the vanadate-inhibited species in this system and implicate a role for a high-energy, ADP-bound intermediate in the transport cycle. Transport complexes containing a mutation in a single MalK subunit are still inhibited by vanadate during steady-state hydrolysis; however, a stably inhibited species does not form. ATP hydrolysis is therefore necessary, but not sufficient, for vanadate-induced nucleotide trapping.     

Full engagement of liganded maltose‐binding protein stabilizes a semi‐open ATP‐binding cassette dimer in the maltose transporter

MalFGK₂ is an ATP‐binding cassette (ABC) transporter that mediates the uptake of maltose/maltodextrins into Escherichia coli. A periplasmic maltose‐binding protein (MBP) delivers maltose to the transmembrane subunits (MalFG) and stimulates the ATPase activity of the cytoplasmic nucleotide‐binding subunits (MalK dimer). This MBP‐stimulated ATPase activity is independent of maltose for purified transporter in detergent micelles. However, when the transporter is reconstituted in membrane bilayers, only the liganded form of MBP efficiently stimulates its activity. To investigate the mechanism of maltose stimulation, electron paramagnetic resonance spectroscopy was used to study the interactions between the transporter and MBP in nanodiscs and in detergent. We found that full engagement of both lobes of maltose‐bound MBP unto MalFGK₂ is facilitated by nucleotides and stabilizes a semi‐open MalK dimer. Maltose‐bound MBP promotes the transition to the semi‐open state of MalK when the transporter is in the membrane, whereas such regulation does not require maltose in detergent. We suggest that stabilization of the semi‐open MalK₂ conformation by maltose‐bound MBP is key to the coupling of maltose transport to ATP hydrolysis in vivo, because it facilitates the progression of the MalK dimer from the open to the semi‐open conformation, from which it can proceed to hydrolyze ATP.     

The detergent-soluble maltose transporter is activated by maltose binding protein and verapamil

The maltose transporter FGK2 complex of Escherichia coli was purified with the aid of a glutathione S-transferase molecular tag. In contrast to the membrane-associated form of the complex, which requires liganded maltose binding protein (MBP) for ATPase activity, the purified detergent-soluble complex exhibited a very high level of ATPase activity. This uncoupled activity was not due to dissociation of the MalK ATPase subunit from the integral membrane protein MalF and MalG subunits. The detergent-soluble ATPase activity of the complex could be further stimulated by wild-type MBP but not by a signaling-defective mutant MBP. Wild-type MBP increased the V_max of the ATPase 2.7-fold but had no effect on the K_m of the enzyme for ATP. When the detergent-soluble complex was reconstituted in proteoliposomes, it returned to being dependent on MBP for activation of ATPase, consistent with the idea that the structural changes induced in the complex by detergent that result in activation of the ATPase are reversible. The uncoupled ATPase activity resembled the membrane-bound activity of the complex also with respect to sensitivity to NaN₃, as well as a mercurial, p-chloromercuribenzosulfonic acid. Verapamil, a compound that activates the ATPase activity of the multiple drug resistance P-glycoprotein, activated the maltose transporter ATPase as well. The activation of this bacterial transporter by verapamil suggests that a structural feature that is conserved among both eukaryotic and prokaryotic ATP binding cassette transporters is responsible for this activation.     

Functional consequences of mutations in the conserved 'signature sequence' of the ATP-binding-cassette protein MalK.

The binding-protein-dependent maltose-transport system of enterobacteria, a member of the ATP-binding-cassette (ABC) transporter superfamily, is composed of two integral membrane proteins, MalF and MalG, and two copies of an ATPase subunit, MalK, which hydrolyze ATP, thus energizing the translocation process. Isolated MalK displays spontaneous ATPase activity, whereas in the assembled MalFGK2 complex, reconstituted in liposomes, ATP hydrolysis requires stimulation by the substrate-loaded extracellular maltose-binding protein, MalE. The ATPase domains of ABC transporters, including MalK, share a unique sequence motif ('LSGGQ', 'signature sequence' or 'linker peptide') with as yet unknown function. To elucidate its role in the transport process, we investigated the consequences of mutations affecting two highly conserved residues (G137, Q140) in the MalK-ATPase of Salmonella typhimurium, by biochemical means. Residues corresponding to Q140 in other ABC proteins have not yet been studied. All mutant alleles (G137--> A, V, T; Q140--> L, K, N) fail to restore a functional transport complex in vivo. In addition, the mutations increase the repressing activity of MalK on other maltose-regulated genes when compared with wild-type MalK. Purified variants of G137 have lost the ability to hydrolyze ATP but still display nucleotide-binding activity, albeit with reduced affinity. Binding of MgATP results in similar protection against trypsin, as observed with wild-type, indicating no major change in protein structure. In contrast, the variants of Q140 differ in their properties, depending on the chemical nature of the replacement residue. MalKQ140L fails to hydrolyze ATP and exhibits a strong intrinsic resistance to trypsin in the absence of MgATP, suggesting a drastically altered conformation. In contrast, the purified mutant proteins Q140K and Q140N display ATPase activities and MgATP-induced changes in the tryptic cleavage pattern similar to those of wild-type. However, mutant transport complexes containing the Q140K or Q140N variants, when studied in proteoliposomes, are severely impaired in MalE-maltose-stimulated ATPase activity. These results are discussed with respect to the crystal structure of the homologous HisP protein [Hung, L.-W., Wang, I.X., Nikaido, K., Liu, P.-Q., Ames, G.F.-L. & Kim, S.-H. (1998) Nature (London) 396, 703-707] and are interpreted in favor of a role of the signature sequence in activating the hydrolyzing activity of MalK upon substrate-initiated conformational changes in MalF/MalG.     

Maltose binding protein (MalE) interacts with periplasmic loops P2 and P1 respectively of the MalFG subunits of the maltose ATP binding cassette transporter (MalFGK(2)) from Escherichia coli/Salmonella during the transport cycle

The ATP binding cassette (ABC-) transporter mediating the uptake of maltose/maltodextrins in Escherichia coli/Salmonella enterica serovar Typhimurium is one of the best characterized systems and serves as a model for studying the molecular mechanism by which ABC importers exert their functions. The transporter is composed of a periplasmic maltose binding protein (MalE), and a membrane-bound complex (MalFGK(2)), comprising the pore-forming hydrophobic subunits, MalF and MalG, and two copies of the ABC subunit, MalK. We report on the isolation of suppressor mutations within malFG that partially restore transport of a maltose-negative mutant carrying the malK809 allele (MalKQ140K). The mutation affects the conserved LSGGQ motif that is involved in ATP binding. Three out of four suppressor mutations map in periplasmic loops P2 and P1 respectively of MalFG. Cross-linking data revealed proximity of these regions to MalE. In particular, as demonstrated in vitro and in vivo, Gly-13 of substrate-free and substrate-loaded MalE is in close contact to Pro-78 of MalG. These data suggest that MalE is permanently in close contact to MalG-P1 via its N-terminal domain. Together, our results are interpreted in favour of the notion that substrate availability is communicated from MalE to the MalK dimer via extracytoplasmic loops of MalFG, and are discussed with respect to a current transport model.     

Functional reconstitution of a maltose ATP-binding cassette transporter from the thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius

The thermoacidophilic gram-positive bacterium Alicyclobacillus acidocaldarius grows at 60 °C and pH 2-3. The organism can utilize maltose and maltodextrins as energy source that are taken up by an ATP-binding cassette (ABC) import system. Genes encoding a maltose binding protein, MalE, and two membrane-integral subunits, MalF and MalG, are clustered on the chromosome but a malK gene translating into a cognate ATPase subunit is lacking. Here we report the cloning of malK from genomic DNA by using the msiK gene of Streptomyces lividans as a probe. Purified MalK exhibited a spontaneous ATPase activity with a V_max of 0.13 μmol P_i/min/mg and a K_m of 330 μM that was optimal at the growth temperature of the organism. Coexpression of malK, malF and malG in Escherichia coli resulted in the formation of a complex that could be coeluted from an affinity matrix after solubilization of membranes with dodecylmaltoside. Proteoliposomes prepared from the MalFGK complex and preformed phospholipid vesicles of A. acidocaldarius displayed a low intrinsic ATPase activity that was stimulated sevenfold by maltose-loaded MalE, thereby indicating coupling of ATP hydrolysis to substrate translocation. These results provide evidence for MalK being the physiological ATPase subunit of the A. acidocaldarius maltose transporter. Moreover, to our knowledge, this is the first report on the functional reconstitution of an ABC transport system from a thermophilic microorganism.     

A Cys-less variant of the bacterial ATP binding cassette protein MalK is functional in maltose transport and regulation

The cysteine residues of the ABC protein MalK from Salmonella typhimurium maltose transport system (C40, C350, C360) were consecutively replaced by serines. Cys-less MalK was fully functional in maltose transport in vivo. Moreover, the activity of MalK as a repressor of other maltose-regulated genes was also retained. The absence of cysteine residues in the purified protein was verified by its failure to react with fluorescein-5-maleimide. In contrast to purified wild-type MalK, the ATPase activity of the C40S variant was insensitive to inhibition by N-ethylmaleimide.     

Unliganded maltose-binding protein triggers lactose transport in an Escherichia coli mutant with an alteration in the maltose transport system.

Escherichia coli accumulates malto-oligosaccharides by the maltose transport system, which is a member of the ATP-binding-cassette (ABC) superfamily of transport systems. The proteins of this system are LamB in the outer membrane, maltose-binding protein (MBP) in the periplasm, and the proteins of the inner membrane complex (MalFGK2), composed of one MalF, one MalG, and two MalK subunits. Substrate specificity is determined primarily by the periplasmic component, MBP. However, several studies of the maltose transport system as well as other members of the ABC transporter superfamily have suggested that the integral inner membrane components MalF and MalG may play an important role in determining the specificity of the system. We show here that residue L334 in the fifth transmembrane helix of MalF plays an important role in determining the substrate specificity of the system. A leucine-to-tryptophan alteration at this position (L334W) results in the ability to transport lactose in a saturable manner. This mutant requires functional MalK-ATPase activity and the presence of MBP, even though MBP is incapable of binding lactose. The requirement for MBP confirms that unliganded MBP interacts with the inner membrane MalFGK2 complex and that MBP plays a crucial role in triggering the transport process.     

The ABC transporter MalFGK(2) sequesters the MalT transcription factor at the membrane in the absence of cognate substrate

MalK, the cytoplasmic component of the maltose ABC transporter from Escherichia coli is known to control negatively the activity of MalT, the activator of the maltose regulon, through complex formation. Here we further investigate this regulatory process by monitoring MalT activity and performing fluorescence microscopy analyses under various conditions. We establish that, under physiological conditions, the molecular entity that interacts with MalT is not free MalK, but the maltose transporter, MalFGK(2) , which sequesters MalT to the membrane. Furthermore, we provide compelling evidence that the transporter's ability to bind MalT is not constitutive, but strongly diminished when MalFGK(2) is engaged in sugar transport. Notably, the outward-facing transporter, i.e. the catalytic intermediate, is ineffective in inhibiting MalT compared to the inward-facing state, i.e. the resting form. Analyses of available genetic and structural data suggest how the interaction between one inactive MalT molecule and MalFGK(2) would be sensitive to the transporter state, thereby allowing MalT release upon maltose entrance. A related mechanism may underpin signalling by other ABC transporters.