Facilitated diffusion properties of melibiose permease in Escherichia coli membrane vesicles. Release of co-substrates is rate limiting for permease cycling

Substrate-binding guides individual melibiose permeases MelB to structurally soften and to destabilize cytoplasmic middle-loop C3

The melibiose permease MelB is a well-studied Na+-coupled transporter of the major facilitator superfamily. However, the symport mechanism of galactosides and cations is still not fully understood, especially at structural levels. Here, we use single-molecule force spectroscopy to investigate substrate-induced structural changes of MelB from Salmonella typhimurium. In the absence of substrate, MelB equally populates two different states, from which one shows higher mechanical structural stability with additional stabilization of the cytoplasmic middle-loop C3. In the presence of either melibiose or a coupling Na+-cation, however, MelB increasingly populates the mechanically less stable state, which shows a destabilized middle-loop C3. In the presence of both substrate and co-substrate, this mechanically less stable state of MelB is predominant. Our findings describe how both substrates guide MelB transporters to populate two different mechanically stabilized states, and contribute mechanistic insights to the alternating-access action for the galactoside/cation symport catalyzed by MelB.

X-ray crystallography reveals molecular recognition mechanism for sugar binding in a melibiose transporter MelB

Major facilitator superfamily_2 transporters are widely found from bacteria to mammals. The melibiose transporter MelB, which catalyzes melibiose symport with either Na⁺, Li⁺, or H⁺, is a prototype of the Na⁺-coupled MFS transporters, but its sugar recognition mechanism has been a long-unsolved puzzle. Two high-resolution X-ray crystal structures of a Salmonella typhimurium MelB mutant with a bound ligand, either nitrophenyl-α-d-galactoside or dodecyl-β-d-melibioside, were refined to a resolution of 3.05 or 3.15 Å, respectively. In the substrate-binding site, the interaction of both galactosyl moieties on the two ligands with MelB_St are virturally same, so the sugar specificity determinant pocket can be recognized, and hence the molecular recognition mechanism for sugar binding in MelB has been deciphered. The conserved cation-binding pocket is also proposed, which directly connects to the sugar specificity pocket. These key structural findings have laid a solid foundation for our understanding of the cooperative binding and symport mechanisms in Na⁺-coupled MFS transporters, including eukaryotic transporters such as MFSD2A.

Guan and Hariharan report two crystal structures of melibiose transporter MelB in complex with substrate analogs, nitrophenyl-galactoside, and dodecyl-melibioside. Both structures revealed similar specific site for sugar recognition and resolved the cation-binding pocket, advancing the understanding of MelB and related transporters.

The melREDCA Operon Encodes a Utilization System for the Raffinose Family of Oligosaccharides in Bacillus subtilis

Bacillus subtilis is a heterotrophic soil bacterium that hydrolyzes different polysaccharides mainly found in the decomposed plants. These carbohydrates are mainly cellulose, hemicellulose, and the raffinose family of oligosaccharides (RFOs). RFOs are soluble α-galactosides, such as raffinose, stachyose, and verbascose, that rank second only after sucrose in abundance. Genome sequencing and transcriptome analysis of B. subtilis indicated the presence of a putative α-galactosidase-encoding gene (melA) located in the msmRE-amyDC-melA operon. Characterization of the MelA protein showed that it is a strictly Mn²⁺- and NAD⁺-dependent α-galactosidase able to hydrolyze melibiose, raffinose, and stachyose. Transcription of the msmER-amyDC-melA operon is under control of a σ^A-type promoter located upstream of msmR (P _msmR ), which is negatively regulated by MsmR. The activity of P _msmR was induced in the presence of melibiose and raffinose. MsmR is a transcriptional repressor that binds to two binding sites at P _msmR located upstream of the -35 box and downstream of the transcriptional start site. MsmEX-AmyCD forms an ATP-binding cassette (ABC) transporter that probably transports melibiose into the cell. Since msmRE-amyDC-melA is a melibiose utilization system, we renamed the operon melREDCA IMPORTANCE Bacillus subtilis utilizes different polysaccharides produced by plants. These carbohydrates are primarily degraded by extracellular hydrolases, and the resulting oligo-, di-, and monosaccharides are transported into the cytosol via phosphoenolpyruvate-dependent phosphotransferase systems (PTS), major facilitator superfamily, and ATP-binding cassette (ABC) transporters. In this study, a new carbohydrate utilization system of B. subtilis responsible for the utilization of α-galactosides of the raffinose family of oligosaccharides (RFOs) was investigated. RFOs are synthesized from sucrose in plants and are mainly found in the storage organs of plant leaves. Our results revealed the modus operandi of a new carbohydrate utilization system in B. subtilis.

Structural and functional characterization of protein-lipid interactions of the Salmonella typhimurium melibiose transporter MelB

BACKGROUND

Membrane lipids play critical roles in the structure and function of membrane-embedded transporters. Salmonella typhimurium MelB (MelB_St) is a symporter coupling melibiose translocation with a cation (Na⁺, Li⁺, or H⁺). We present an extensive study on the effects of specific phospholipids on the structure of MelB_St and the melibiose transport catalyzed by this protein.

RESULTS

Lipidomic analysis and thin-layer chromatography (TLC) experiments reveal that at least one phosphatidylethanolamine (PE) and one phosphatidylglycerol (PG) molecule associate with MelB_St at high affinities. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy experiments confirmed the presence of lipid tails and glycerol backbones that co-purified with MelB_St; headgroups of PG were also observed. Studies with lipid-engineered strains, including PE-deficient, cardiolipin (CL)- and PG-deficient, or CL-deficient strains, show that lack of PE or PG, however not CL, largely inhibits both H⁺- and Na⁺-coupled melibiose active transport to different extents. Interestingly, neither the co-substrate binding (melibiose or Na⁺) nor MelB_St folding and stability are affected by changing lipid compositions. Remarkably, the delipidated MelB_St with only 2-3 bound lipids, regardless of the headgroup species, also exhibits unchanged melting temperature values as shown by circular dichroism spectroscopy.

CONCLUSIONS

(1) Lipid tails and glycerol backbones of interacting PE and PG may contribute to the stability of the structure of MelB_St. (2) The headgroups of PE and PG, but not of CL, play important roles in melibiose transport; however, lipid headgroups do not modulate the folding and stability of MelB_St.

Thermodynamic cooperativity of cosubstrate binding and cation selectivity of Salmonella typhimurium MelB

The melibiose symporter MelB couples melibiose transport to that of cations such as Na⁺. Hariharan and Guan show that the binding of Na⁺ and melibiose is thermodynamically cooperative and that Na⁺ coupling is based on ion concentrations and competitive binding, but not ion selectivity.

The Na⁺-coupled melibiose symporter MelB, which can also be coupled to H⁺ or Li⁺ transport, is a prototype for the glycoside-pentoside-hexuronide:cation symporter family. Although the 3-D x-ray crystal structure of Salmonella typhimurium MelB (MelB_St) has been determined, the symport mechanisms for the obligatory coupled transport are not well understood. Here, we apply isothermal titration calorimetry to determine the energetics of Na⁺ and melibiose binding to MelB_St, as well as protonation of this transporter. Studies of the thermodynamic cycle for the formation of the Na⁺–MelB_St–melibiose ternary complex at pH 7.45 reveal that the binding of Na⁺ and melibiose is cooperative. The binding affinity for one substrate (Na⁺ or melibiose) is increased by the presence of the other by about eightfold. The coupling free energies (ΔΔG) of either substrate binding are ∼5 kJ/mol, and binding of both substrates releases a free energy of ∼35 kJ/mol. Measurements of the Na⁺-binding enthalpy at three different pH values, including the pK_a value of MelB, indicate that the binding of one Na⁺ displaces one H⁺ per MelB_St molecule. In addition, the absolute dissociation constants for Na⁺ and H⁺, determined by competitive binding, show that MelB_St is selective for H⁺ over Na⁺ by ∼1,000-fold at a pK_a of 6.25. Thus, the Na⁺ coupling in MelB_St is based not on ion selectivity but on ion concentrations and competitive binding because of a much higher Na⁺ concentration under physiological conditions. Such a selectivity feature seems to be common for membrane transport proteins that can bind both H⁺ and Na⁺ at a common site.

Effect of Detergents on Galactoside Binding by Melibiose Permeases

The effect of various detergents on the stability and function of the melibiose permeases of Escherichia coli (MelBEc) and Salmonella typhimurium (MelBSt) was studied. In n-dodecyl-β-d-maltoside (DDM) or n-undecyl-β-d-maltoside (UDM), WT MelBSt binds melibiose with an affinity similar to that in the membrane. However, with WT MelBEc or MelBSt mutants (Arg141 → Cys, Arg295 → Cys, or Arg363 → Cys), galactoside binding is not detected in these detergents, but binding to the phosphotransferase protein IIA(Glc) is maintained. In the amphiphiles lauryl maltose neopentyl glycol (MNG-3) or glyco-diosgenin (GDN), galactoside binding with all of the MelB proteins is observed, with slightly reduced affinities. MelBSt is more thermostable than MelBEc, and the thermostability of either MelB is largely increased in MNG-3 or GDN. Therefore, the functional defect with DDM or UDM likely results from the relative instability of the sensitive MelB proteins, and stability, as well as galactoside binding, is retained in MNG-3 or GDN. Furthermore, isothermal titration calorimetry of melibiose binding with MelBSt shows that the favorable entropic contribution to the binding free energy is decreased in MNG-3, indicating that the conformational dynamics of MelB is restricted in this detergent.

Characterization of the l-alanine exporter AlaE of Escherichia coli and its potential role in protecting cells from a toxic-level accumulation of l-alanine and its derivatives

We previously reported that the alaE gene of Escherichia coli encodes the l-alanine exporter AlaE. The objective of this study was to elucidate the mechanism of the AlaE exporter. The minimum inhibitory concentration of l-alanine and l-alanyl-l-alanine in alaE-deficient l-alanine-nonmetabolizing cells MLA301ΔalaE was 4- and >4000-fold lower, respectively, than in the alaE-positive parent cells MLA301, suggesting that AlaE functions as an efflux pump to avoid a toxic-level accumulation of intracellular l-alanine and its derivatives. Furthermore, the growth of the alaE-deficient mutant derived from the l-alanine-metabolizing strain was strongly inhibited in the presence of a physiological level of l-alanyl-l-alanine. Intact MLA301ΔalaE and MLA301ΔalaE/pAlaE cells producing plasmid-borne AlaE, accumulated approximately 200% and 50%, respectively, of the [³H]l-alanine detected in MLA301 cells, suggesting that AlaE exports l-alanine. When 200 mmol/L l-alanine-loaded inverted membrane vesicles prepared from MLA301ΔalaE/pAlaE were placed in a solution containing 200 mmol/L or 0.34 μmol/L l-alanine, energy-dependent [³H]l-alanine accumulation occurred under either condition. This energy-dependent uphill accumulation of [³H]l-alanine was strongly inhibited in the presence of carbonyl cyanide m-chlorophenylhydrazone but not by dicyclohexylcarbodiimide, suggesting that the AlaE-mediated l-alanine extrusion was driven by proton motive force. Based on these results, physiological roles of the l-alanine exporter are discussed.

Thermodynamic mechanism for inhibition of lactose permease by the phosphotransferase protein IIAGlc

In a variety of bacteria, the phosphotransferase protein IIA(Glc) plays a key regulatory role in catabolite repression in addition to its role in the vectorial phosphorylation of glucose catalyzed by the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). The lactose permease (LacY) of Escherichia coli catalyzes stoichiometric symport of a galactoside with an H(+), using a mechanism in which sugar- and H(+)-binding sites become alternatively accessible to either side of the membrane. Both the expression (via regulation of cAMP levels) and the activity of LacY are subject to regulation by IIA(Glc) (inducer exclusion). Here we report the thermodynamic features of the IIA(Glc)-LacY interaction as measured by isothermal titration calorimetry (ITC). The studies show that IIA(Glc) binds to LacY with a Kd of about 5 μM and a stoichiometry of unity and that binding is driven by solvation entropy and opposed by enthalpy. Upon IIA(Glc) binding, the conformational entropy of LacY is restrained, which leads to a significant decrease in sugar affinity. By suppressing conformational dynamics, IIA(Glc) blocks inducer entry into cells and favors constitutive glucose uptake and utilization. Furthermore, the studies support the notion that sugar binding involves an induced-fit mechanism that is inhibited by IIA(Glc) binding. The precise mechanism of the inhibition of LacY by IIA(Glc) elucidated by ITC differs from the inhibition of melibiose permease (MelB), supporting the idea that permeases can differ in their thermodynamic response to binding IIA(Glc).

Insights into the inhibitory mechanisms of the regulatory protein IIA(Glc) on melibiose permease activity

The phosphotransfer protein IIA(Glc) of the bacterial phosphoenolpyruvate:carbohydrate phosphotransferase system plays a key role in the regulation of carbohydrate metabolism. Melibiose permease (MelB) is one among several permeases subject to IIA(Glc) regulation. The regulatory mechanisms are poorly understood; in addition, thermodynamic features of IIA(Glc) binding to other proteins are also unknown. Applying isothermal titration calorimetry and amine-specific cross-linking, we show that IIA(Glc) directly binds to MelB of Salmonella typhimurium (MelB(St)) and Escherichia coli MelB (MelB(Ec)) at a stoichiometry of unity in the absence or presence of melibiose. The dissociation constant values are 3-10 μM for MelB(St) and 25 μM for MelB(Ec). All of the binding is solely driven by favorable enthalpy forces. IIA(Glc) binding to MelB(St) in the absence or presence of melibiose yields a large negative heat capacity change; in addition, the conformational entropy is constrained upon the binding. We further found that the IIA(Glc)-bound MelB(St) exhibits a decreased binding affinity for melibiose or nitrophenyl-α-galactoside. It is believed that sugar binding to the permease is involved in an induced fit mechanism, and the transport process requires conformational cycling between different states. Thus, the thermodynamic data are consistent with the interpretation that IIA(Glc) inhibits the induced fit process and restricts the conformational dynamics of MelB(St).

Suppression of conformation-compromised mutants of Salmonella enterica serovar Typhimurium MelB

The crystal structure of the Na(+)-coupled melibiose permease of Salmonella enterica serovar Typhimurium (MelBSt) demonstrates that MelB is a member of the major facilitator superfamily of transporters. Arg residues at positions 295, 141, and 363 are involved in interdomain interactions at the cytoplasmic side by governing three clusters of electrostatic/polar interactions. Insertion of (one at a time) Glu, Leu, Gln, or Cys at positions R295, R141, and R363, or Lys at position R295, inhibits active transport of melibiose to a level of 2 to 20% of the value for wild-type (WT) MelBSt, with little effect on binding affinities for both sugar and Na(+). Interestingly, a spontaneous suppressor, D35E (periplasmic end of helix I), was isolated from the R363Q MelBSt mutant. Introduction of the D35E mutation in each of the mutants at R295, R141 (except R141E), or R363 rescues melibiose transport to up to 91% of the WT value. Single-site mutations for the pair of D35 and R175 (periplasmic end of helix VI) were constructed by replacing Asp with Glu, Gln, or Cys and R175 with Gln, Asn, or Cys. All mutants with mutations at R175 are active, indicating that a positive charge at R175 is not necessary. Mutant D35E shows reduced transport; D35Q and D35C are nearly inactivated. Surprisingly, the D35Q mutation partially rescues both R141C and R295Q mutations. The data support the idea that Arg at position 295 and a positive charge at positions 141 and 363 are required for melibiose transport catalyzed by MelBSt, and their mutation inhibits conformational cycling, which is suppressed by a minor modification at the opposite side of the membrane.

Structure-based mechanism for Na(+)/melibiose symport by MelB

The bacterial melibiose permease (MelB) belongs to the glycoside-pentoside-hexuronide:cation symporter family, a part of the major facilitator superfamily (MFS). Structural information regarding glycoside-pentoside-hexuronide:cation symporter family transporters and other Na(+)-coupled permeases within MFS has been lacking, although a wealth of biochemical and biophysical data are available. Here we present the three-dimensional crystal structures of Salmonella typhimurium MelBSt in two conformations, representing an outward partially occluded and an outward inactive state of MelBSt. MelB adopts a typical MFS fold and contains a previously unidentified cation-binding motif. Three conserved acidic residues form a pyramidal-shaped cation-binding site for Na(+), Li(+) or H(+), which is in close proximity to the sugar-binding site. Both cosubstrate-binding sites are mainly contributed by the residues from the amino-terminal domain. These two structures and the functional data presented here provide mechanistic insights into Na(+)/melibiose symport. We also postulate a structural foundation for the conformational cycling necessary for transport catalysed by MFS permeases in general.

Reduced Na+ affinity increases turnover of Salmonella enterica serovar Typhimurium MelB

The melibiose permease of Salmonella enterica serovar Typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+). Bioinformatics and mutational analyses indicate that a conserved Gly117 (helix IV) is a component of the Na(+)-binding site. In this study, Gly117 was mutated to Ser, Asn, or Cys. All three mutations increase the maximum rate (V(max)) for melibiose transport in Escherichia coli DW2 and greatly decrease Na(+) affinity, indicating that intracellular release of Na(+) is facilitated. Rapid melibiose transport, particularly by the G117N mutant, triggers osmotic lysis in the lag phase of growth. The findings support the previous conclusion that Gly117 plays an important role in cation binding and translocation. Furthermore, a spontaneous second-site mutation (P148L between loop(4-5) and helix V) in the G117C mutant prevents cell lysis. This mutation significantly decreases V(max) with little effect on cosubstrate binding in G117C, G117S, and G117N mutants. Thus, the P148L mutation specifically inhibits transport velocity and thereby blocks the lethal effect of elevated melibiose transport in the Gly117 mutants.

Role of Gly117 in the cation/melibiose symport of MelB of Salmonella typhimurium

The melibiose permease of Salmonella typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+), and bioinformatics analysis indicates that a conserved Gly117 (helix IV) is part of the Na(+)-binding site. We mutated Gly117 to Ala, Pro, Trp, or Arg; the effects on melibiose transport and binding of cosubstrates depended on the physical-chemical properties of the side chain. Compared with WT MelB(St), the Gly117 → Ala mutant exhibited little difference in either cosubstrate binding or stimulation of melibiose transport by Na(+) or Li(+), but all other mutations reduced melibiose active transport and efflux, and decreased the apparent affinity for Na(+). The bulky Trp at position 117 caused the greatest inhibition of melibiose binding, and Gly117 → Arg yielded less than a 4-fold decrease in the apparent affinity for melibiose at saturating Na(+) or Li(+) concentration. Remarkably, the mutant Gly117 → Arg catalyzed melibiose exchange in the presence of Na(+) or Li(+), but did not catalyze melibiose translocation involving net flux of the coupling cation, indicating that sugar is released prior to release of the coupling cation. Taken together, the findings are consistent with the notion that Gly117 plays an important role in cation binding and translocation.

Mechanism of melibiose/cation symport of the melibiose permease of Salmonella typhimurium

The MelB permease of Salmonella typhimurium (MelB-ST) catalyzes the coupled symport of melibiose and Na(+), Li(+), or H(+). In right-side-out membrane vesicles, melibiose efflux is inhibited by an inwardly directed gradient of Na(+) or Li(+) and stimulated by equimolar concentrations of internal and external Na(+) or Li(+). Melibiose exchange is faster than efflux in the presence of H(+) or Na(+) and stimulated by an inwardly directed Na(+) gradient. Thus, sugar is released from MelB-ST externally prior to the release of cation in agreement with current models proposed for MelB of Escherichia coli (MelB-EC) and LacY. Although Li(+) stimulates efflux, and an outwardly directed Li(+) gradient increases exchange, it is striking that internal and external Li(+) with no gradient inhibits exchange. Furthermore, Trp → dansyl FRET measurements with a fluorescent sugar (2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside) demonstrate that MelB-ST, in the presence of Na(+) or Li(+), exhibits (app)K(d) values of ∼1 mM for melibiose. Na(+) and Li(+) compete for a common binding pocket with activation constants for FRET of ∼1 mM, whereas Rb(+) or Cs(+) exhibits little or no effect. Taken together, the findings indicate that MelB-ST utilizes H(+) in addition to Na(+) and Li(+). FRET studies also show symmetrical emission maximum at ∼500 nm with MelB-ST in the presence of 2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside and Na(+), Li(+), or H(+), which implies a relatively homogeneous distribution of conformers of MelB-ST ternary complexes in the membrane.

Alteration of sugar-induced conformational changes of the melibiose permease by mutating Arg141 in loop 4-5

The melibiose permease (MelB) from Escherichia coli couples the uptake of melibiose to that of Na+, Li+, or H+. In this work, we applied attenuated total reflection Fourier transform infrared (ATR-FTIR) difference spectroscopy to obtain information about the structural changes involved in substrate interaction with the R141C mutant and with the wild-type MelB reacted with N-ethylmaleimide (NEM). These modified permeases have the ability to bind the substrates but fail to transport them. It is shown that the sugar-induced ATR-FTIR difference spectra of the R141C mutant are different from those corresponding to the Cys-less permease from which it is derived. There are alterations of peaks assigned to turns and beta-structures located most likely in loop 4-5. In addition, and quite notably, a peak at 1659 cm(-1), assigned to changes at the level of one alpha-helix subpopulation, disappears in the melibiose-induced difference spectrum in the presence of Na+, suggesting a reduction of the conformational change capacity of the mutated MelB. These helices may involve structural components that couple the cation- and sugar-binding sites. On the other hand, MelB-NEM difference spectra are proportionally less disrupted than the R141C ones. Hence, the transport cycle of these two permeases, modified at two different loops, is most likely impaired at a different stage. It is proposed that the R141C mutant leads to the generation of a partially defective ternary complex that is unable to catalyze the subsequent conformational change necessary for substrate translocation.

A 3D structure model of the melibiose permease of Escherichia coli represents a distinctive fold for Na+ symporters

The melibiose permease of Escherichia coli (MelB) catalyzes the coupled stoichiometric symport of a galactoside with a cation (either Na(+), Li(+), or H(+)), using free energy from the downhill translocation of one cosubstrate to catalyze the accumulation of the other. Here, we present a 3D structure model of MelB threaded through a crystal structure of the lactose permease of E. coli (LacY), manually adjusted, and energetically minimized. The model contains 442 consecutive residues ( approximately 94% of the polypeptide), including all 12 transmembrane helices and connecting loops, with no steric clashes and superimposes well with the template structure. The electrostatic surface potential calculated from the model is typical for a membrane protein and exhibits a characteristic ring of positive charges around the periphery of the cytoplasmic side. The 3D model indicates that MelB consists of two pseudosymmetrical 6-helix bundles lining an internal hydrophilic cavity, which faces the cytoplasmic side of the membrane. Both sugar and cation binding sites are proposed to lie within the internal cavity. The model is consistent with numerous previous mutational, biochemical/biophysical characterizations as well as low-resolution structural data. Thus, an alternating access mechanism with sequential binding is discussed. The proposed overall fold of MelB is different from the available crystal structures of other Na(+)-coupled transporters, suggesting a distinctive fold for Na(+) symporters.

Functional reconstitution of SdcS, a Na+-coupled dicarboxylate carrier protein from Staphylococcus aureus

In Staphylococcus aureus, the transport of dicarboxylates is mediated in part by the Na+-linked carrier protein SdcS. This transporter is a member of the divalent-anion/Na+ symporter (DASS) family, a group that includes the mammalian Na+/dicarboxylate cotransporters NaDC1 and NaDC3. In earlier work, we cloned and expressed SdcS in Escherichia coli and found it to have transport properties similar to those of its eukaryotic counterparts (J. A. Hall and A. M. Pajor, J. Bacteriol. 187:5189-5194, 2005). Here, we report the partial purification and subsequent reconstitution of functional SdcS into liposomes. These proteoliposomes exhibited succinate counterflow activity, as well as Na+ electrochemical-gradient-driven transport. Examination of substrate specificity indicated that the minimal requirement necessary for transport was a four-carbon terminal dicarboxylate backbone and that productive substrate-transporter interaction was sensitive to substitutions at the substrate C-2 and C-3 positions. Further analysis established that SdcS facilitates an electroneutral symport reaction having a 2:1 cation/dicarboxylate ratio. This study represents the first characterization of a reconstituted Na+-coupled DASS family member, thus providing an effective method to evaluate functional, as well as structural, aspects of DASS transporters in a system free of the complexities and constraints associated with native membrane environments.

The Inner Interhelix Loop 4–5 of the Melibiose Permease from Escherichia coli Takes Part in Conformational Changes after Sugar Binding

Cytoplasmic loop 4-5 of the melibiose permease from Escherichia coli is essential for the process of Na+-sugar translocation (Abdel-Dayem, M., Basquin, C., Pourcher, T., Cordat, E., and Leblanc, G. (2003) J. Biol. Chem. 278, 1518-1524). In the present report, we analyze functional consequences of mutating each of the three acidic amino acids in this loop into cysteines. Among the mutants, only the E142C substitution impairs selectively Na+-sugar translocation. Because R141C has a similar defect, we investigated these two mutants in more detail. Liposomes containing purified mutated melibiose permease were adsorbed onto a solid supported lipid membrane, and transient electrical currents resulting from different substrate concentration jumps were recorded. The currents evoked by a melibiose concentration jump in the presence of Na+, previously assigned to an electrogenic conformational transition (Meyer-Lipp, K., Ganea, C., Pourcher, T., Leblanc, G., and Fendler, K. (2004) Biochemistry 43, 12606-12613), were much smaller for the two mutants than the corresponding signals in cysteineless MelB. Furthermore, in R141C the stimulating effect of melibiose on Na+ affinity was lost. Finally, whereas tryptophan fluorescence spectroscopy revealed impaired conformational changes upon melibiose binding in the mutants, fluorescence resonance energy transfer measurements indicated that the mutants still show cooperative modification of their sugar binding sites by Na+. These data suggest that: 1) loop 4-5 contributes to the coordinated interactions between the ion and sugar binding sites; 2) it participates in an electrogenic conformational transition after melibiose binding that is essential for the subsequent obligatory coupled translocation of substrates. A two-step mechanism for substrate translocation in the melibiose permease is suggested.

State-dependent conformations of the translocation pathway in the tyrosine transporter Tyt1, a novel neurotransmitter:sodium symporter from Fusobacterium nucleatum

The gene of a novel prokaryotic member (Tyt1) of the neurotransmitter:sodium symporter (NSS) family has been cloned from Fusobacterium nucleatum. In contrast to eukaryotic and some prokaryotic NSSs, which contain 12 transmembrane domains (TMs), Tyt1 contains only 11 TMs, a characteristic shared by approximately 70% of prokaryotic NSS homologues. Nonetheless upon heterologous expression in an engineered Escherichia coli host, Tyt1 catalyzes robust Na+-dependent, highly selective l-tyrosine transport. Genetic engineering of Tyt1 variants devoid of cysteines or with individually retained endogenous cysteines at positions 18 or 238, at the cytoplasmic ends of TM1 and TM6, respectively, preserved normal transport activity. Whereas cysteine-less Tyt1 was resistant to the inhibitory effect of sulfhydryl-alkylating reagents, N-ethylmaleimide inhibited transport by Tyt1 variants containing either one or both of the endogenous cysteines, and this inhibition was altered by the substrates sodium and tyrosine, consistent with substrate-induced dynamics in the transport pathway. Our findings support a binding model of Tyt1 function in which an ordered sequence of substrate-induced structural changes reflects distinct conformational states of the transporter. This work identifies Tyt1 as the first functional bacterial NSS member putatively consisting of only 11 TMs and shows that Tyt1 is a suitable model for the study of NSS dynamics with relevance to structure/function relationships of human NSSs, including the dopamine, norepinephrine, serotonin, and gamma-aminobutyric acid transporters.

Substrate-induced conformational changes of melibiose permease from Escherichia coli studied by infrared difference spectroscopy

Fourier transform infrared difference spectroscopy has been used to obtain information about substrate-induced structural changes of the melibiose permease (MelB) from Escherichia coli reconstituted into liposomes. Binding of the cosubstrate Na(+) gives rise to several peaks in the amide I and II regions of the difference spectrum Na(+).MelB minus H(+).MelB, that denote the presence of conformational changes in all types of secondary structures (alpha-helices, beta-sheets, loops). In addition, peaks around 1400 and at 1740-1720 cm(-1) are indicative of changes in protonation/deprotonation or in environment of carboxylic groups. Binding of the cosubstrate Li(+) produces a difference spectrum that is also indicative of conformational changes, but that is at variance as compared to that induced by Na(+) binding. To analyze the following transport steps, the melibiose permease with either H(+), Na(+), or Li(+) bound was incubated with melibiose. The difference spectra obtained by subtracting the spectrum cation.MelB from the respective complex cation.melibiose.MelB were roughly similar among them, but different from those induced by cation binding, and more intense. Therefore, major conformational changes that are induced during melibiose binding/substrate translocation, like those denoted by intense peaks at 1668 and 1645 cm(-)(1), are similar for the three cotransporting cations. Changes in the protonation state and/or in the environment of given carboxylic residues were also induced by melibiose-MelB interaction in the presence of cations.

Electrophysiological characterization of a recombinant human Na+-coupled nucleoside transporter (hCNT1) produced in Xenopus oocytes

Human concentrative nucleoside transporter 1 (hCNT1) mediates active transport of nucleosides and anticancer and antiviral nucleoside drugs across cell membranes by coupling influx to the movement of Na(+) down its electrochemical gradient. The two-microelectrode voltage-clamp technique was used to measure steady-state and presteady-state currents of recombinant hCNT1 produced in Xenopus oocytes. Transport was electrogenic, phloridzin sensitive and specific for pyrimidine nucleosides and adenosine. Nucleoside analogues that induced inwardly directed Na(+) currents included the anticancer drugs 5-fluorouridine, 5-fluoro-2'-deoxyuridine, cladribine and cytarabine, the antiviral drugs zidovudine and zalcitabine, and the novel thymidine mimics 1-(2-deoxy-beta-d-ribofuranosyl)-2,4-difluoro-5-methylbenzene and 1-(2-deoxy-beta-d-ribofuranosyl)-2,4-difluoro-5-iodobenzene. Apparent K(m) values for 5-fluorouridine, 5-fluoro-2'-deoxyuridine and zidovudine were 18, 15 and 450 microm, respectively. hCNT1 was Na(+) specific, and the kinetics of steady-state uridine-evoked Na(+) currents were consistent with an ordered simultaneous transport model in which Na(+) binds first followed by uridine. Membrane potential influenced both ion binding and carrier translocation. The Na(+)-nucleoside coupling stoichiometry, determined directly by comparing the uridine-induced inward charge movement to [(14)C]uridine uptake was 1: 1. hCNT1 presteady-state currents were used to determine the fraction of the membrane field sensed by Na(+) (61%), the valency of the movable charge (-0.81) and the average number of transporters present in the oocyte plasma membrane (6.8 x 10(10) per cell). The hCNT1 turnover rate at -50 mV was 9.6 molecules of uridine transported per second.

Evidence for intraprotein charge transfer during the transport activity of the melibiose permease from Escherichia coli

Electrogenic activity associated with the activity of the melibiose permease (MelB) of Escherichia coli was investigated by using proteoliposomes containing purified MelB adsorbed onto a solid-supported membrane. Transient currents were selectively recorded by applying concentration jumps of Na+ ions (or Li+) and/or of different sugar substrates of MelB (melibiose, thio-methyl galactoside, raffinose) using a fast-flow solution exchange system. Characteristically, the transient current response was fast, including a single decay exponential component (tau approximately 15 ms) on applying a Na+ (or Li+) concentration jump in the absence of sugar. On imposing a Na+ (or Li+) jump on proteoliposomes preincubated with the sugar, a sugar jump in a preparation preincubated with the cation, or a simultaneous jump of the cation and sugar substrates, the electrical transients were biphasic and comprised both the fast and an additional slow (tau approximately 350 ms) decay components. Finally, selective inactivation of the cosubstrate translocation step by acylation of MelB cysteins with N-ethyl maleimide suppressed the slow response components and had no effect on the fast transient one. We suggest that the fast transient response reflects charge transfer within MelB during cosubstrate binding while the slow component is associated with charge transfer across the proteoliposome membrane. From the time course of the transient currents, we estimate a rate constant for Na+ binding in the absence and presence of melibiose of k > 50 s(-1) and one for melibiose binding in the absence of Na+ of k approximately 10 s(-1).

Consequences of hydrophobic mismatch between lipids and melibiose permease on melibiose transport

The structural and functional consequences of a mismatch between the hydrophobic thickness d(P) of a transmembrane protein and that d(L) of the supporting lipid bilayer were investigated using melibiose permease (MelB) from Escherichia coli reconstituted in a set of bis saturated and monounsaturated phosphatidylcholine species differing in acyl-chain length. Influence of MelB on the midpoint gel-to-liquid-phase transition temperature, T(m), of the saturated lipids was investigated through fluorescence polarization experiments, with 1,6-diphenyl-1,3,5-hexatriene as the probe, for varying protein/lipid molar ratio. Diagrams in temperature versus MelB concentration showed positive or negative shifts in T(m) with the short-chain lipids DiC12:0-PC and DiC14:0-PC or the long-chain lipids DiC16:0-PC and DiC18:0-PC, respectively. Theoretical analysis of the data yielded a d(L) value of 3.0 +/- 0.1 nm for the protein, similar to the 3.02 nm estimated from hydropathy profiles. Influence of the acyl chain length on the carrier activity of MelB was investigated in the liquid phase, using the monounsaturated PCs. Binding of the sugar to the transporter showed no dependence on the acyl chain length. In contrast, counterflow and Deltapsi-driven experiments revealed strong dependence of melibiose transport on the lipid acyl chain length. Similar bell-shaped transport versus acyl chain length profiles were obtained, optimal activity being supported by diC16:1-PC. On account of a d(P) value of 2.65 nm for the lipid and of various local constraints which would all tend to elongate the acyl chains in contact with the protein, one can conclude that maximal activity was obtained when the hydrophobic thickness of the bilayer matched that of the protein.

Structural studies of the melibiose permease of Escherichia coli by fluorescence resonance energy transfer. I. Evidence for ion-induced conformational change

Further insight into the cosubstrate-induced structural change of the melibiose permease (MelB) of Escherichia coli has been sought by investigating the binding and spectroscopic properties of the fluorescent sugar 2'-(N-5-dimethylaminonaphthalene-1-sulfonyl)aminoethyl 1-thio-beta-D-galactopyranoside (Dns2-S-Gal) and related analogs (Dns3-S-Gal or Dns6-S-Gal with a propyl or hexyl instead of an ethyl linker, respectively) interacting with MelB in membrane vesicles or in proteoliposomes. The three analogs efficiently inhibit melibiose transport and bind to MelB in a sodium-dependent fashion. Their dissociation constants (Kd) are in the micromolar range in the presence of NaCl and an order of magnitude higher in its absence. In the presence of NaCl and Dns2-S-Gal, sample excitation at 335 or 297 nm gives rise to a fluorescent signal at around 465 nm, whereas Dns3-S-Gal or Dns6-S-Gal emits a fluorescence light at 490 or 506 nm, respectively. Detailed study of the Dns2-S-Gal signal elicited by a 297 nm illumination indicates that a tryptophan-mediated fluorescence resonance energy transfer phenomenon is involved in the response. All fluorescence signals below 500 nm are prevented by addition of melibiose in excess, and the kinetic constants describing their dependence on the probe or NaCl concentrations closely correlate with the probe binding constants. Finally, the Dns2-S-Gal signal recorded in sodium-free medium is red shifted by up to 25 nm from that recorded in the presence of NaCl. Taken together, these results suggest (i) that the fluorescence signals below 500 nm arise from Dns-S-Gal molecules bound to MelB, (ii) the presence of a highly hydrophobic environment close to or at the sugar-binding site, the polarity of which increases on moving away from the sugar-binding site, and (iii) that the interaction of sodium ions with MelB enhances the hydrophobicity of this environment. These results are consistent with the induction of a cooperative change of the structure of the sugar-binding site or of its immediate vicinity by the ions.

Proteolytic mapping and substrate protection of the Escherichia coli melibiose permease

The topology and substrate-induced conformational change(s) of the Na+ (Li+ or H+)-melibiose cotransporter (MelB) of Escherichia coli were investigated by limited protease digestion. To facilitate these analyses, MelB was epitope-tagged both at its carboxyl-terminus and at its amino-terminus. Limited digestion with different proteases indicates that the cytoplasmic loops connecting transmembrane domains 4-5, 6-7, and 10-11 together with the carboxyl-terminus of MelB are exposed in the cytoplasm. In contrast, periplasmic loops are highly resistant to all the proteases examined, including nonspecific proteases such as proteinase K and thermolysin. The effect of Na+ or Li+ and/or melibiose on the rate of protease digestion of the cytoplasmic loops was also analyzed. The rate of protease digestion of loop 4-5 is specifically reduced, by approximately 3-fold, by the presence of Na+ or Li+. These results suggest that loop 4-5 is near or part of the cation binding site. Moreover, the presence of both melibiose and either Na+ or Li+ further reduced the rate of protease digestion of this loop 4-5 by up to 9-fold, although no protection from protease digestion was observed when melibiose was added alone. The increase in resistance to proteases observed in the presence of the cation alone or the cation plus melibiose suggests that the interaction of the two cosubstrate with MelB results in change(s) of MelB conformation.

Melibiose permease of Escherichia coli: structural organization of cosubstrate binding sites as deduced from tryptophan fluorescence analyses

Binding of the coupling ion (Na+ or Li+) and sugars to the purified melibiose permease of Escherichia coli, reconstituted in proteoliposomes, produces selective and cooperative changes of the transporter tryptophan fluorescence. To assess the individual contribution of N- or C-terminal domains of the permease to these substrate-induced fluorescence variations, we replaced the two tryptophans located in its C-terminal half (W299 and W342) by a phenylalanine and compared the signal change in mutants and wild-type permease. None of the mutations significantly impairs transport activity. Persistence of the ion-induced signal quenching in a permease carrying only the six other tryptophans of the N-terminal domain is consistent with a previous suggestion that this domain accommodates the ion-binding site. On the other hand, the sugar-induced fluorescence increase varies from mutant to mutant in a sugar-specific fashion. While alpha-galactosides increase essentially the fluorescence of W299 and W342, beta-galactosides enhance the signal of W299 and of one (or more) of the N-terminal tryptophans but quench that of W342. Moreover, addition of sugars producers a 10 nm blue shift of both W299 and W342 emission spectra, suggesting reduced accessibility of these residues to solvent following substrate binding. These data suggest that W299 and W342 are at or close to the sugar binding site and that this latter is lined by the C-terminal helices IX and X. Moreover, as sugars with the beta-configuration also enhance the fluorescence of the N-terminal tryptophans, it is suggested that one (or more) helix of the N-terminal half may be also at or near the sugar binding site. This implies close proximity and/or tight functional linkage between some N-terminal helices and helices IX and X of the C-terminal domain of the transporter.

Alteration of Na(+)-coupled transport in site-directed mutants of the melibiose carrier of Escherichia coli

Asn-58 of the Escherichia coli melibiose carrier was replaced by Ala, Leu, Ser, and Gln. Trp-54 was replaced by Leu and a double mutant Leu-54/Ala-58 was constructed using site-directed mutagenesis. Cation/sugar cotransport and sugar-induced cation uptake were studied for each mutant. The change of Asn-58 to Ala results in a nearly complete loss of Na(+)-stimulated galactoside transport as well as sugar-stimulated Na+ uptake. Substitutions of Leu, Gln, and Ser for Asn-58 were also defective in Na(+)-stimulated sugar transport. The Trp-54 to Leu mutant shows moderate sugar accumulation with cation selectivity similar to wild-type. The double mutant Leu-54/Ala-58 shows elevated H(+)-melibiose cotransport as well as reduced Na(+)-stimulated melibiose cotransport. These results suggest that Asn-58 is important for Na+ recognition.

Melibiose permease of Escherichia coli: substrate-induced conformational changes monitored by tryptophan fluorescence spectroscopy

Tryptophan fluorescence spectroscopy has been used to investigate the effects of sugars and coupling cations (H+, Na+, or Li+) on the conformational properties of purified melibiose permease after reconstitution in liposomes. Melibiose permease emission fluorescence is selectively enhanced by sugars, which serve as substrates for the symport reaction, alpha-galactosides producing larger variations (13-17%) than beta-galactosides (7%). Moreover, the sugar-dependent fluorescence increase is specifically potentiated by NaCl and LiCl (5-7 times), which are well-established activators of sugar binding and transport by the permease. The potentiation effect is greater in the presence of LiCl than NaCl. On their own, sodium and lithium ions produce quenching of the fluorescence signal (2%). Evidence suggesting that sugars and cations compete for their respective binding sites is also given. Both the sugar-induced fluorescence variation and the NaCl(or LiCl)-dependent potentiation effect exhibit saturation kinetics. In each ionic condition, the half-maximal fluorescence change is found at a sugar concentration corresponding to the sugar-binding constant. Also, half-maximal potentiation of the fluorescence change by sodium or lithium occurs at a concentration comparable to the activation constant of sugar binding by each ion. The sugar- and ion-dependent fluorescence variations still take place after selective inactivation of the permease substrate translocation capacity by N-ethylmaleimide. Taken together, the data suggest that the changes in permease fluorescence reflect conformational changes occurring upon the formation of ternary sugar/cation/permease complexes.

Translocation of metal phosphate via the phosphate inorganic transport system of Escherichia coli

Pi transport via the phosphate inorganic transport system (Pit) of Escherichia coli was studied in natural and artificial membranes. Pi uptake via Pit is dependent on the presence of divalent cations, like Mg2+, Ca2+, Co2+, or Mn2+, which form a soluble, neutral metal phosphate (MeHPO4) complex. Pi-dependent uptake of Mg2+ and Ca2+, equimolar cotransport of Pi and Ca2+, and inhibition by Mg2+ of Ca2+ uptake in the presence of Pi, but not of Pi uptake in the presence of Ca2+, indicate that a metal phosphate complex is the transported solute. Metal phosphate is transported in symport with H+ with a mechanistic stoichiometry of 1. Pit mediates efflux and homologous exchange of metal phosphate, but not heterologous metal phosphate exchange with Pi, glycerol-3P, or glucose-6P. The metal phosphate efflux rate increased with pH, whereas the rate of metal phosphate exchange was essentially pH independent. Metal phosphate uptake was inhibited at low internal pH. Efflux was inhibited by a proton motive force (interior negative and alkaline), whereas exchange was inhibited by the membrane potential only. These results have been evaluated in terms of ordered binding and dissociation of metal phosphate and proton on the outer and inner surface of the cytoplasmic membrane.

Coupling mechanisms in active transport

In primary and secondary active transport, the mobility and specificity of the carrier are controlled, over the course of the transport reaction, in accordance with a set of 'rules'. The rules are shown to depend on two mechanisms: a substrate--either the driving substrate (a transported ion or ATP) or the driven substrate--may shift a conformational equilibrium or accelerate a rate-limiting conformational change. From an analysis of coupling mechanisms the following conclusions emerge. (i) The ratio of coupled to uncoupled flux, which should be large, cannot be greater than the ratio of substrate dissociation constants in an initial complex and a conformationally altered state. A minimum value for the increased binding force can be estimated from steady-state constants. (ii) In an ordered mechanism, slippage is expected at high concentrations of the substrate adding to the carrier second, while slippage of the first substrate should remain low. (iii) Slippage in coupled transport is minimized if the driven substrate is last on in loading the carrier and last off in unloading, while the reverse order makes the affinity high in loading and low in unloading, as required for efficient transfer from one compartment to another; hence the preferred mechanism may depend on prevailing physiological conditions. (iv) A coupled transport system can be transformed into a facilitated system for one substrate or both if the control of carrier mobility is undermined through modification of the carrier.

Melibiose permease of Escherichia coli: mutation of histidine-94 alters expression and stability rather than catalytic activity

Previous studies utilizing site-directed mutagenesis [Pourcher et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 468-472] indicate that out of seven histidinyl residues in the melibiose (mel) permease of Escherichia coli, only His94 is important. The role of His94 has now been investigated by replacing the residue with Asn, Gln, or Arg. Cells expressing mel permease with Asn94 or Gln94 retain 30% or 20% of wild-type activity, respectively, and surprisingly, immunological assays demonstrate that diminished transport activity is due to a proportional reduction in the amount of permease in the membrane. Moreover, kinetic analyses of transport and ligand binding studies with right-side-out membrane vesicles indicate that both substrate recognition and turnover (kcat) are comparable in the mutant permeases and the wild-type. Mel permease with Arg in place of His94 also binds ligand and catalyzes sugar accumulation, but only when the cells are grown at 30 degrees C, and evidence is presented that Arg94 permease is inactivated at 37 degrees C. Finally, labeling studies demonstrate that expression and/or insertion of the permease, but not degradation, is strongly dependent on the amino acid present at position 94 and temperature. The findings indicate that an imidazole group at position 94 is required for proper insertion and stability of mel permease, but not for transport activity per se. Since replacement of the other six histidinyl residues in mel permease with Arg has little or no effect on transport activity, it is concluded that histidinyl residues do not play a direct role in the mechanism of this secondary transport protein.

Ordered binding model as a general mechanistic mechanism for secondary active transport systems

The mechanistic mechanism of secondary active transport processes has not been fully elucidated. Based on substrate binding studies dependent on coupling cation concentrations of the glutamate, melibiose, lactose and proline transport carriers in Escherichia coli, the ordered binding mechanism was proposed as the energy coupling mechanism of the transport systems. This ordered binding mechanism satisfactorily explained the properties of the secondary active transport systems. Thus, this mechanism as the general energy coupling mechanism for the transport systems is discussed.

Melibiose permease of Escherichia coli: mutation of aspartic acid 55 in putative helix II abolishes activation of sugar binding by Na+ ions

An aspartic residue (Asp55) located in the putative transmembrane alpha-helix II of the melibiose(mel) permease of Escherichia coli was replaced by Cys using oligonucleotide-directed, site-specific mutagenesis. Although D55C permease is expressed at 0.7 times the level of wild type permease, the mutated mel permease loses the ability to catalyse Na+ or H+ coupled melibiose transport against a concentration gradient. (3H) p-nitrophenyl-alpha-D-galactoside (NPG) binding studies demonstrated that D55C permease binds the sugar co-substrate but Na+ (or Li+) ions do no longer enhance the affinity of D55C permease for the co-transported sugar. In addition sugar binding on D55C permease but not on wild type permease is inactivated by sulfhydryl reagents and the inhibition protected by an excess of melibiose. These observations suggest 1) that the negatively-charged Asp55 residue, expected to be within the membrane embedded domain near the NH2 extremity of mel permease, is in or near the Na(+)-binding site and 2) that the cation and sugar binding sites may be overlapping.

The interaction between aspartic acid 237 and lysine 358 in the lactose carrier of Escherichia coli

The lacY from Escherichia coli strains 020 and AE43 have been cloned on plasmids which were designated p020-K358T and pAE43-D237N. These lacY mutants contain amino acid substitutions changing Lys-358 to Thr or Asp-237 to Asn, respectively. The charge neutralizing effect of each mutation is associated with a functional defect in melibiose transport which we exploited in order to isolate second site revertants to the melibiose-positive phenotype. Eleven melibiose-positive revertants of p020-K358T were isolated. All contained a second-site mutation converting Asp-237 to a neutral amino acid (8 to Asn, 1 to Gly, and 2 to Tyr). Twelve melibiose-positive revertants of pAE43-D237N were isolated. Two were second-site revertants converting Lys-358 to a neutrally Gln residue, while the remainder directly reverted Asn-237 to the wild-type Asp-237. We conclude that the functional intimate relationship between Asp-237 and Lys-358 suggests that these residues may be closely juxtaposed in three-dimensional space, possibly forming a 'charge-neutralizing' salt bridge.

Sodium ion-dependent amino acid transport in membrane vesicles of Bacillus stearothermophilus

Amino acid transport in membrane vesicles of Bacillus stearothermophilus was studied. A relatively high concentration of sodium ions is needed for uptake of L-alanine (Kt = 1.0 mM) and L-leucine (Kt = 0.4 mM). In contrast, the Na(+)-H(+)-L-glutamate transport system has a high affinity for sodium ions (Kt less than 5.5 microM). Lithium ions, but no other cations tested, can replace sodium ions in neutral amino acid transport. The stimulatory effect of monensin on the steady-state accumulation level of these amino acids and the absence of transport in the presence of nonactin indicate that these amino acids are translocated by a Na+ symport mechanism. This is confirmed by the observation that an artificial delta psi and delta mu Na+/F but not a delta pH can act as a driving force for uptake. The transport system for L-alanine is rather specific. L-Serine, but not L-glycine or other amino acids tested, was found to be a competitive inhibitor of L-alanine uptake. On the other hand, the transport carrier for L-leucine also translocates the amino acids L-isoleucine and L-valine. The initial rates of L-glutamate and L-alanine uptake are strongly dependent on the medium pH. The uptake rates of both amino acids are highest at low external pH (5.5 to 6.0) and decline with increasing pH. The pH allosterically affects the L-glutamate and L-alanine transport systems. The maximal rate of L-glutamate uptake (Vmax) is independent of the external pH between pH 5.5 and 8.5, whereas the affinity constant (Kt) increases with increasing pH. A specific transport system for the basic amino acids L-lysine and L-arginine in the membrane vesicles has also been observed. Transport of these amino acids occurs most likely by a uniport mechanism.

Mechanism of Na+/proline symport in Escherichia coli: reappraisal of the effect of cation binding to the Na+/proline symport carrier

The proton and sodium ion dependences of the proline binding and transport activities of the proline carrier in Escherichia coli were investigated in detail. The binding activity in cytoplasmic membrane vesicles from a carrier over-producing strain (PT21/pTMP5) was absolutely dependent on the presence of Na+, but did not necessarily require protonation of the carrier, in contrast to the model previously reported (Mogi, T., Anraku, Y. 1984. J. Biol. Chem. 259:7797-7801). Based on this and previous observations, we propose a modified model of the proline binding reaction of the proline carrier, in which a proton is supposed to be a regulatory factor for the binding activity. The apparent Michaelis constant of proline (Kt) of the transport activity of cytoplasmic membrane vesicles from the wild type E. coli strain driven by a respiratory substrate, ascorbate, showed dependence on a low concentration of sodium ion. The Michaelis constant of sodium ion for transport (KtNa) was estimated to be 25 microM. The proline transport activities in membrane vesicles and intact cells were modulated by H+ concentration, the inhibitory effect of protons (pKa approximately equal to 6) being similar to that observed previously (Mogi, T., Anraku, Y. 1984. J. Biol. Chem. 259:7802-7806). Based on these observations and the modified model of substrate binding to the proline carrier, a model of the proline/Na+ symport mechanism is proposed, in which a proton is postulated to be a regulatory factor of the transport activity.

Mechanism of enhanced melibiose transport rate catalyzed by an Escherichia coli lactose carrier mutant with leucine substituted for serine-306. The pH-dependence of melibiose efflux

The mechanism of melibiose transport was studied in cells containing plasmid pAA22 which expresses the mutant lactose carrier (serine-306 to leucine) cloned from Escherichia coli AA22. These studies were of interest because several lines of evidence suggested that the AA22 mutation conferred novel properties upon the lactose carrier, decreasing turnover with several beta-galactoside substrates, increasing turnover with melibiose, and abolishing active accumulation even though equilibration occurred via symport with H+. Although severely defective in active melibiose accumulation, the present study indicates that in cells poisoned with azide the AA22 carrier does in fact equilibrate melibiose across the membrane more rapidly than the normal lactose carrier. Similarly, melibiose efflux from cells preloaded with melibiose was more rapidly catalyzed by the AA22 carrier than by the normal carrier (pH 7.0). Furthermore, although external H+ did reduce net melibiose efflux to a rate slower than seen in equilibrium exchange, a lower than normal pH was required to achieve this effect. Therefore, at pH 7.0, the AA22 carrier (but not the normal carrier) catalyzed net efflux at a rate approaching that for the exchange process (which was pH-resistant in both the mutant and the parent). At pH 8.0 both the AA22 carrier and the normal carrier catalyzed net melibiose efflux at a rate identical to the equilibrium exchange rate. We suggest (i) that the sensitivity of melibiose efflux to external pH indicates that during efflux the AA22 carrier interacts with protons in a manner similar to the normal carrier (i.e., sugar is cotransported with H+) and hence the absence of accumulation is not explained by internal leak via a binary carrier-melibose complex; and (ii) that the modest increase in rate constants for melibiose exit reflect small changes in activation energy (1 kcal/mol) consistent with a steric mechanism possibly involving van der Waals contacts.

Galactoside-dependent proton transport by mutants of the Escherichia coli lactose carrier: substitution of tyrosine for histidine-322 and of leucine for serine-306

The lac Y genes from two Escherichia coli mutants, MAB20 and AA22, have been cloned in a multicopy plasmid by a novel 'sucrose marker exchange' method. Characterization showed that the plasmids express a lactose carrier with poor affinity for lactose. Neither mutant carried out concentrative uptake with methyl beta-D-galactopyranoside, lactose, or melibiose as the substrate. Nor did the mutants catalyze counterflow or exchange with methyl beta-D-galactopyranoside. Both mutants did, however, retain the capacity to carry out facilitated diffusion with lactose or melibiose. DNA sequencing revealed that MAB20 (histidine-322 to tyrosine) and AA22 (serine-306 to leucine) have amino acid substitutions within the putative 'charge-relay' domain thought to be responsible for proton transport. Galactoside-dependent H+ transport was readily measured in both mutants. We conclude, therefore, that the presence of a histidine residue at position 322 of the lactose carrier is not obligatory for H+ transport per se.