Mutations that simultaneously alter both sugar and cation specificity in the melibiose carrier of Escherichia coli

Sequence-function relationships in MalG, an inner membrane protein from the maltose transport system in Escherichia coli

The malG gene encodes a hydrophobic cytoplasmic membrane protein which is required for the energy-dependent transport of maltose and maltodextrins in Escherichia coli. The MalG protein, together with MalF and MalK proteins, forms a multimeric complex in the membrane consisting of two MalK subunits for each MalF and MalG subunit. Fifteen mutations have been isolated in malG by random linker insertion mutagenesis. Two regions essential for maltose transport have been identified. In particular, a hydrophilic region containing the peptidic motif EAA---G---------I-LP, highly conserved among inner membrane proteins from binding protein-dependent transport systems, is essential for maltose transport. The results also show that several regions of MalG are not essential for function. A region (residues 30-50) encompassing the first predicted transmembrane segment and the first periplasmic loop in MalG may be modified extensively with little effect on maltose transport and no effect on the stability and the localization of the protein. A region located at the middle of the protein (residues 153-157) is not essential for the function of the protein. A region, essential for maltodextrin utilization but not for maltose transport, has been identified near the C-terminus of the protein.

Cloning and sequencing of the melB gene encoding the melibiose permease of Salmonella typhimurium LT2

The nucleotide sequence of the melB gene coding for the Na+ (Li+)/melibiose symporter of Salmonella typhimurium LT2 was determined, and its amino acid sequence was deduced. It consists of 1428 bp, corresponding to a protein of 476 amino acid residues (calculated molecular weight 52,800). The amino acid sequence is homologous to that of the melibiose permease of Escherichia coli K12, with 85% identical residues. All, except one, of the amino acid residues that have been reported to be important for cation or substrate recognition in the melibiose permease of E. coli are conserved in the melibiose permease of S. typhimurium. In addition, part of the sequence resembles the lactose permease of Streptococcus thermophilus, the animal glucose transporter (GLUT1), the plasmid-coded raffinose permease (RafB), and the NADH-ubiquinone oxidoreductase chain 4 (Nuo4) of Aspergillus amstelodami.

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.

Asp-51 and Asp-120 are important for the transport function of the Escherichia coli melibiose carrier

Asp-51 and Asp-120 of the Escherichia coli melibiose carrier on plasmid pKKMB were separately replaced by amber codons and transformed into eight amber suppressor strains, producing eight amino acid substitutions for each site. Glu-51 and Glu-120 were the only replacements in the carrier that allowed the cells to ferment melibiose and that showed transport of melibiose against a concentration gradient. Revertants to Glu-51 and Glu-120 show less activity than the wild type. The Asp-51 position is more crucial for Na(+)-stimulated melibiose accumulation than is the Asp-120 site.

Can the excretion of metabolites by bacteria be manipulated?

Bacteria can release metabolites into the environment by various mechanisms. Excretion may occur by passive diffusion or by the reversal of the uptake process when the internal concentration of the metabolite exceeds the thermodynamic equilibrium level. In other cases, solutes are excreted against the concentration gradient by special extrusion systems. Their mode of energy coupling is different to that of the well-studied group of uptake systems. A thorough understanding of the transport processes will help to improve the excretion of metabolites of commercial interest, allow a more efficient production of metabolites in bulk quantities, and permit their exploitation to establish new markets.

A novel mechanism of cation/substrate cotransport: Na+/H+/adenosine cotransport in Vibrio parahaemolyticus

Adenosine is actively transported with Na+ in Vibrio parahaemolyticus (Sakai, Y., Tsuda, M., Tsuchiya, T. (1987) Biochim, Biophys. Acta 893, 43-48). The proton conductor carbonylcyanide m-chlorophenylhydrazone, CCCP, strongly inhibited active transport of adenosine at pH 8.5 as well as at pH 7.0. This seemed peculiar because the driving force, an electrochemical potential of Na+, is established by the Na(+)-extruding respiratory chain at pH 8.5 in this organism, although it is established by the function of the Na+/H+ antiporter at pH 7.0. This suggested that H+ might be involved in the adenosine transport. We detected H+ uptake induced by adenosine influx in V. parahaemolyticus cells in the presence of Na+, but not in its absence, suggesting the occurrence of Na+/H+/adenosine cotransport. We isolated formycin A-resistant mutants which showed defective adenosine transport. The mutation resulted in simultaneous losses of Na+ uptake and H+ uptake induced by adenosine. In revertants from these mutants the Na+ uptake and H+ uptake were restored simultaneously. The frequencies of reversion were in the order of 10(-7), indicating that the mutations were single mutations; namely that Na+/adenosine cotransport and H+/adenosine cotransport took place via the same carrier. Thus, we conclude that adenosine is transported by the novel mechanism of Na+/H+/adenosine cotransport in V. parahaemolyticus.

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.

Dissecting the molecular mechanism of ion-solute cotransport: substrate specificity mutations in the putP gene affect the kinetics of proline transport

Rare mutations that alter the substrate specificity of proline permease cluster in discrete regions of the putP gene, suggesting that they may replace amino acids at the active site of the enzyme. If putP substrate specificity mutations directly after the active site of proline permease, the mutants should show specific defects in the kinetics of proline transport. In order to test this prediction, we examined the kinetics of three putP substrate specificity mutants. One class of mutation increases the Km over 120 fold but only decreases the Vmax fourfold. Such Km mutants may be specifically defective in substrate recognition, thus identifying an amino acid critical for substrate binding. Another class of mutation decreases the Vmax 80-fold without changing the Km. Vmax mutants appear to alter the rate of substrate translocation without affecting the substrate binding site. The last class of mutation alters both the Km and Vmax of proline transport. These results indicate that substrate specificity mutations alter amino acids critical for Na+/proline symport.

Towards an understanding of the structural basis of 'forbidden' transport pathways in the Escherichia coli lactose carrier: mutations probing the energy barriers to uncoupled transport

Recent progress in the analysis of mutants of the Escherichia coli lactose carrier function is reviewed, with special emphasis on the structural basis for energy barriers which prevent 'forbidden' conformational changes. Mutations which break down the barriers to forbidden isomerizations involving the binary carrier:sugar (CS) and carrier:proton (CH) complexes have been obtained in several laboratories. These mutants allow uncoupled transport of H+ or galactoside in the lactose carrier which normally couples cation and sugar movement in a 1:1 stoichiometry. These uncoupled mutants appear to be associated with changes in both sugar and cation recognition, suggesting that the physical interactions forming the basis for co-substrate recognition and uncoupling are not independently variable. By postulating that translocation involves transformation of the stable intermediate of the co-transport cycle to unstable transition state conformations of the carrier, it is possible to consider the consequences of mutagenesis in terms of transition state theory. Consistent with several experimental observations, the analysis predicts in each mutant the occurrence of more than one abnormality in the transport cycle (such as changes in sugar recognition, cation recognition or the coupling reaction). We have called the general phenomenon a 'mutational double-effect' because any mutation which alters the Gibbs free energy change of one reaction in the transport cycle must affect the free energy change of at least one other reaction in this cycle.

Proton-linked sugar transport systems in bacteria

The cell membranes of various bacteria contain proton-linked transport systems for D-xylose, L-arabinose, D-galactose, D-glucose, L-rhamnose, L-fucose, lactose, and melibiose. The melibiose transporter of E. coli is linked to both Na+ and H+ translocation. The substrate and inhibitor specificities of the monosaccharide transporters are described. By locating, cloning, and sequencing the genes encoding the sugar/H+ transporters in E. coli, the primary sequences of the transport proteins have been deduced. Those for xylose/H+, arabinose/H+, and galactose/H+ transport are homologous to each other. Furthermore, they are just as similar to the primary sequences of the following: glucose transport proteins found in a Cyanobacterium, yeast, alga, rat, mouse, and man; proteins for transport of galactose, lactose, or maltose in species of yeast; and to a developmentally regulated protein of Leishmania for which a function is not yet established. Some of these proteins catalyze facilitated diffusion of the sugar without cation transport. From the alignments of the homologous amino acid sequences, predictions of common structural features can be made: there are likely to be twelve membrane-spanning alpha-helices, possibly in two groups of six; there is a central hydrophilic region, probably comprised largely of alpha-helix; the highly conserved amino acid residues (40-50 out of 472-522 total) form discrete patterns or motifs throughout the proteins that are presumably critical for substrate recognition and the molecular mechanism of transport. Some of these features are found also in other transport proteins for citrate, tetracycline, lactose, or melibiose, the primary sequences of which are not similar to each other or to the homologous series of transporters. The glucose/Na+ transporter of rabbit and man is different in primary sequence to all the other sugar transporters characterized, but it is homologous to the proline/Na+ transporter of E. coli, and there is evidence for its structural similarity to glucose/H+ transporters in Plants. In vivo and in vitro mutagenesis of the lactose/H+ and melibiose/Na+ (H+) transporters of E. coli has identified individual amino acid residues alterations of which affect sugar and/or cation recognition and parameters of transport. Most of the bacterial transport proteins have been identified and the lactose/H+ transporter has been purified. The directions of future investigations are discussed.

Mechanism and evolution of the uncoupling protein of brown adipose tissue

The uncoupling protein found in mitochondria from thermogenic brown adipose tissue is structurally very similar to two other mitochondrial carrier proteins transporting ADP/ATP and phosphate, respectively. Similarities are also seen with the mechanism of these carriers, which are part of a family of H+/OH(-)-substrate anion co-transporters, further strengthening the evidence that the uncoupling protein has evolved from this family of mitochondrial carrier proteins.

A consensus structure for membrane transport

Combined information from biochemical and molecular biological experiments reveals a consistent structural rhythm that underlies the construction of all membrane carriers and perhaps all transport systems. Biochemical work shows that while some carrier proteins function as monomers, others operate as dimers. But despite this variation, all examples can be modelled as having a pair of membrane-embedded domains, each of which contains an array of (about) six transmembrane helical elements. This pattern is best documented among membrane carriers, where the minimal functional unit is known in a reasonable number of cases. Nevertheless, the same conclusion is likely to characterize other solute transporters. These unexpected correlations suggest that all membrane carriers, including those that take part in "energy coupling", have a uniform structural design on which is superimposed a variety of kinetic and biochemical mechanisms.

The melibiose/Na+ symporter of Escherichia coli: kinetic and molecular properties

The role of the co-transported cation in the coupling mechanism of the melibiose permease of Escherichia coli has been investigated by analysing its sugar-binding activity, facilitated diffusion reactions and energy-dependent transport reactions catalysed by the carrier functioning either as an H+, Na+ or Li(+)-sugar symporter. The results suggest that the coupling cation not only acts as an activator for sugar-binding on the carrier but also regulates the rate of dissociation of the co-substrates in the cytoplasm by controlling the stability of the ternary complex cation-sugar-carrier facing the cell interior. Furthermore, there is some evidence that the membrane potential enhances the rate of symport activity by increasing the rate of dissociation of the co-substrates from the carrier in the cellular compartment. Identification of the melibiose permease as a membrane protein of 39 kDa by using a T7 RNA polymerase/promoter expression system is described. Site-directed mutagenesis has been used to replace individual carrier histidine residues by arginine to probe the functional contribution of each of the seven histidine residues to the symport mechanism. Only substitution of arginine for His94 greatly interferes with the carrier function. It is finally shown that mutations affecting the glutamate residue in position 361 inactivate translocation of the co-substrates but not their recognition by the permease.

Molecular and genetical analysis of the fructose-glucose transport system in the cyanobacterium Synechocystis PCC6803

Complementation for glucose transport capacity of deficient mutants from Synechocystis PCC6803 allowed the cloning of the corresponding gene, glcP. The protein predicted from one open reading frame (ORF) in the DNA sequence was 468 residues long. It showed 46-60% amino acid sequence homology and similarity in size and predicted structure (including twelve probable membrane-spanning regions) with a group of non-phosphorylating sugar transporters from mammals, yeasts and Escherichia coli. A second ORF, 64 base pairs downstream from glcP, was detected. Its function, dispensable under auto- and heterotrophic conditions, could not be determined. Genetic analysis of mutants confirmed that the resistance to fructose, acquired simultaneously with the deficiency in glucose transport, resulted from mutations in the glcP gene, whose approximate location could be determined.

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.