Beta-galactoside transport in E. coli: a functional dissection of lac permease

Chemical Targeting of Membrane Transporters: Insights into Structure/Function Relationships

Chemical modification of proteins is a vintage strategy that is still fashionable due to the information that can be obtained from this approach. An interesting application of chemical modification is linked with membrane transporters. These proteins have peculiar features such as the presence of hydrophobic and hydrophilic domains, which show different degree of accessibility to chemicals. The presence of reactive residues in the membrane transporters is at the basis of the chemical targeting strategy devoted to investigating structure/function relationships; in particular, information on the substrate binding site, regulatory domains, dimerization domains, and the interface between hydrophilic loops and transmembrane domains has been obtained over the years by chemical targeting. Given the difficulty in handling membrane transporters, their study experienced a great delay, particularly concerning structural information. Chemical targeting has been applied with reasonable success to some membrane transporters belonging to the families SLC1, SLC6, SLC7, and SLC22. Furthermore, some data on the potential application of chemical targeting in pharmacology are also discussed.

Influence of lipids on protein-mediated transmembrane transport

Transmembrane proteins are responsible for transporting ions and small molecules across the hydrophobic region of the cell membrane. We are reviewing the evidence for regulation of these transport processes by interactions with the lipids of the membrane. We focus on ion channels, including potassium channels, mechanosensitive and pentameric ligand gated ion channels, and active transporters, including pumps, sodium or proton driven secondary transporters and ABC transporters. For ion channels it has been convincingly shown that specific lipid-protein interactions can directly affect their function. In some cases, a combined approach of molecular and structural biology together with computer simulations has revealed the molecular mechanisms. There are also many transporters whose activity depends on lipids but understanding of the molecular mechanisms is only beginning.

Transport of ions across peritoneal membrane

The electrical conductance of ions across the peritoneal membrane of young buffalo (approximately 18-24 months old) has been recorded. Aqueous solutions of NaF, NaNO3, NaCl, Na2SO4, KF, KNO3, KCl, K2SO4, MgCl2, CaCl2, CrCl3, MnCl2, FeCl3, CoCl2, and CuCl2 were used. The conductance values have been found to increase with increase in concentration as well as with temperature (15 to 35 degrees C) in these cases. The slope of plots of specific conductance, kappa, versus concentration exhibits a decrease in its values at relatively higher concentrations compared to those in extremely dilute solutions. Also, such slopes keep on increasing with increase in temperature. In addition, the conductance also attains a maximum limiting value at higher concentrations in the said cases. This may be attributed to a progressive accumulation of ionic species within the membrane. The kappa values of electrolytes follow the sequence for the anions: SO4(2-)>Cl->NO3->F- while that for the cations: K+>Na+>Ca2+>Mn2+>Co2+>Cu2+>Mg2+>Cr3+>Fe3+. In addition, the diffusion of ions depends upon the charge on the membrane and its porosity. The membrane porosity in relation to the size of the hydrated species diffusing through the membrane appears to determine the above sequence. As the diffusional paths in the membrane become more difficult in aqueous solutions, the mobility of large hydrated ions gets impeded by the membrane framework and the interaction with the fixed charge groups on the membrane matrix. Consequently, the membrane pores reduce the conductance of small ions, which are much hydrated. An increase in conductance with increase in temperature may be due to the state of hydration, which implies that the energy of activation for the ionic transport across the membrane follows the sequence of crystallographic radii of ions accordingly. The Eyring's equation, kappa=(RT/Nh)exp[-DeltaH*/RT]exp[DeltaS*/R], has been found suitable for explaining the temperature dependence of conductance in the said cases. This is apparent from the linear plots of log[kappaNh/RT] versus 1/T. The results indicate that the permeation of ions through the membrane giving negative values of DeltaS* suggest that there may be formation of either covalent linkage between the penetrating ions and the membrane material or else the permeation may not be the rate-determining step. On the one hand, a high DeltaS* value associated with the high value of energy of activation, Ea, for diffusion may suggest the existence of either a large zone of activation or loosening of more chain segments of the membrane. On the other hand, low value of DeltaS* implies that converse is true in such cases, i.e., either a small zone of activation or no loosening of the membrane structure upon permeation.

A bacterial model system for understanding multi-drug resistance

Mankind stands at the crossroads, recognizing the need for a radical change in bacterial disease management. The development of several antimicrobial agents in the 1940s and 1950s allowed man to gain the upper hand in controlling these diseases. However, the horizon is now clouded by the activation in bacteria of cryptic multi-drug resistance (MDR) genes and the spread of plasmid- and integron-born MDR genes through bacterial populations. Unless remedial measures are taken, nearly all currently available antimicrobial agents are likely to soon lose their efficacies. We briefly review the bacterial MDR phenomenon and focus on a recently emerging family of small multi-drug resistance (SMR) pumps which may provide an ideal model system for understanding the MDR phenomenon in general.

Major facilitator superfamily

The major facilitator superfamily (MFS) is one of the two largest families of membrane transporters found on Earth. It is present ubiquitously in bacteria, archaea, and eukarya and includes members that can function by solute uniport, solute/cation symport, solute/cation antiport and/or solute/solute antiport with inwardly and/or outwardly directed polarity. All homologous MFS protein sequences in the public databases as of January 1997 were identified on the basis of sequence similarity and shown to be homologous. Phylogenetic analyses revealed the occurrence of 17 distinct families within the MFS, each of which generally transports a single class of compounds. Compounds transported by MFS permeases include simple sugars, oligosaccharides, inositols, drugs, amino acids, nucleosides, organophosphate esters, Krebs cycle metabolites, and a large variety of organic and inorganic anions and cations. Protein members of some MFS families are found exclusively in bacteria or in eukaryotes, but others are found in bacteria, archaea, and eukaryotes. All permeases of the MFS possess either 12 or 14 putative or established transmembrane alpha-helical spanners, and evidence is presented substantiating the proposal that an internal tandem gene duplication event gave rise to a primordial MFS protein prior to divergence of the family members. All 17 families are shown to exhibit the common feature of a well-conserved motif present between transmembrane spanners 2 and 3. The analyses reported serve to characterize one of the largest and most diverse families of transport proteins found in living organisms.

High-efficiency expression and characterization of the synaptic-vesicle monoamine transporter from baculovirus-infected insect cells

The full-length cDNA for the rat synaptic-vesicle monoamine transporter (VMAT2) containing a C-terminal polyhistidine epitope has been engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) insect cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed at levels of 7.8x10(6) transporters per cell, as assessed by [3H]dihydrotetrabenazine binding. A 1l culture of infected cells produced approx. 15 nmol (900 microg) of transporter. rVMAT2 expressed in the Sf9 cells bound [3H]dihydrotetrabenazine with a KD of 31.2 nM and a Bmax of 19.9 pmol/mg. Two polypeptides of 55 and 63 kDa were identified using the photolabel, 7-azido-8-[125I]iodoketanserin ([125I]AZIK). Photoaffinity labelling of rVMAT2 by 1 nM [125I]AZIK was protectable by 10 microM tetrabenazine and 10 microM 7-aminoketanserin. Digitonin-solubilized VMAT2 was purified to greater than 95% homogeneity using immobilized Ni2+-affinity chromatography, followed by lectin (Concanavalin A) chromatography. The purified transporter migrates as a single broad band with a molecular mass of approx. 63kDa, as analyzed by SDS/PAGE. The purified transporter retained the ability to bind ligands ([125I]AZIK and [3H]dihydrotetrabenazine). The purified VMAT2 bound [3H]dihydrotetrabenazine with a KD of 86.2 nM. As is the case with the monoamine transporter from bovine chromaffin granule membranes, purified VMAT2 is covalently modified by dicyclohexylcarbodi-imide (DCCD) and is specifically labelled by [14C]DCCD. This labelling is inhibited by tetrabenazine and ketanserin. These data indicate that VMAT2 can be overexpressed using the baculovirus expression system and purified.

Peptide mapping of the [125I]Iodoazidoketanserin and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine binding sites for the synaptic vesicle monoamine transporter

The full-length cDNA for the rat recombinant synaptic vesicle monoamine transporter (rVMAT2) containing a COOH-terminal polyhistidine epitope was engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed to high levels and purified to >95% homogeneity using immobilized Ni2+-affinity chromatography followed by lectin (concanavalin A) chromatography. Purified transporter was photolabeled using [125I]-7-azido-8-iodoketanserin ([125I]AZIK) and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine ([125I]TBZ-AIPP). Both [125I]AZIK and [125I]TBZ-AIPP photoaffinity labeling of purified rVMAT2 were protectable by 10 microM tetrabenazine (TBZ), 10 microM 7-aminoketanserin, and 1 mM concentrations of the transporter substrates dopamine, norepinephrine, and serotonin. Radiolabeled peptides were generated using enzymatic and chemical methods, purified using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and NH2-terminal microsequenced. Radiosequencing of [125I]AZIK-labeled rVMAT2 indicated derivatization of Lys-20 in the NH2 terminus, just prior to putative transmembrane domain 1 (TMD1). [125I]TBZ-AIPP derivatized a segment of rVMAT2 between Gly-408 and Cys-431 in TMD10 and 11. These data implicate juxtaposition of TMD1 and 10/11.

Proton-dependent multidrug efflux systems

Multidrug efflux systems display the ability to transport a variety of structurally unrelated drugs from a cell and consequently are capable of conferring resistance to a diverse range of chemotherapeutic agents. This review examines multidrug efflux systems which use the proton motive force to drive drug transport. These proteins are likely to operate as multidrug/proton antiporters and have been identified in both prokaryotes and eukaryotes. Such proton-dependent multidrug efflux proteins belong to three distinct families or superfamilies of transport proteins: the major facilitator superfamily (MFS), the small multidrug resistance (SMR) family, and the resistance/ nodulation/cell division (RND) family. The MFS consists of symporters, antiporters, and uniporters with either 12 or 14 transmembrane-spanning segments (TMS), and we show that within the MFS, three separate families include various multidrug/proton antiport proteins. The SMR family consists of proteins with four TMS, and the multidrug efflux proteins within this family are the smallest known secondary transporters. The RND family consists of 12-TMS transport proteins and includes a number of multidrug efflux proteins with particularly broad substrate specificity. In gram-negative bacteria, some multidrug efflux systems require two auxiliary constituents, which might enable drug transport to occur across both membranes of the cell envelope. These auxiliary constituents belong to the membrane fusion protein and the outer membrane factor families, respectively. This review examines in detail each of the characterized proton-linked multidrug efflux systems. The molecular basis of the broad substrate specificity of these transporters is discussed. The surprisingly wide distribution of multidrug efflux systems and their multiplicity in single organisms, with Escherichia coli, for instance, possessing at least nine proton-dependent multidrug efflux systems with overlapping specificities, is examined. We also discuss whether the normal physiological role of the multidrug efflux systems is to protect the cell from toxic compounds or whether they fulfil primary functions unrelated to drug resistance and only efflux multiple drugs fortuitously or opportunistically.

Structural topology of transmembrane helix 10 in the lactose permease of Escherichia coli

In the lactose permease of Escherichia coli, transmembrane helix 10 has been shown to be functionally important. The structure of this helix has been examined in greater detail in this study. A total of 46 substitution and 8 insertional mutants were constructed and analyzed along the entire length of transmembrane helix 10. The results identified amino acids that are tolerant of substitutions by a variety of amino acids. Since a number of these amino acids (Thr-320, Val-331, Phe-325, and Ile-317) are clustered in one region in a helical wheel projection of transmembrane helix 10, it seems likely that this face of helix 10 is interacting with the membrane. The channel lining domain is thought to consist of the helical face containing Glu-325, Leu-318, Leu-329, His-322, Val-315, Cys-333, Val-326, and Lys-319 based on the results here and from earlier findings. Deleterious mutations along this face tended to greatly increase the Km value for lactose transport with only minor effects on the Vmax. Analysis of insertional mutants revealed that perturbation of the spatial relationship between amino acids at the periplasmic edge is less deleterious than perturbation in the center of the helix or the cytoplasmic edge. Using all of the above information, a detailed structural topology of transmembrane helix 10 is proposed.

An overview of membrane transport proteins in Saccharomyces cerevisiae

All eukaryotic cells contain a wide variety of proteins embedded in the plasma and internal membranes, which ensure transmembrane solute transport. It is now established that a large proportion of these transport proteins can be grouped into families apparently conserved throughout organisms. This article presents the data of an in silicio analysis aimed at establishing a preliminary classification of membrane transport proteins in Saccharomyces cerevisiae. This analysis was conducted at a time when about 65% of all yeast genes were available in public databases. In addition to approximately 60 transport proteins whose function was at least partially known, approximately 100 deduced protein sequences of unknown function display significant sequence similarity to membrane transport proteins characterized in yeast and/or other organisms. While some protein families have been well characterized by classical genetic experimental approaches, others have largely if not totally escaped characterization. The proteins revealed by this in silicio analysis also include a putative K+ channel, proteins similar to aquaporins of plant and animal origin, proteins similar to Na+-solute symporters, a protein very similar to electroneural cation-chloride cotransporters, and a putative Na+-H+ antiporter. A new research area is anticipated: the functional analysis of many transport proteins whose existence was revealed by genome sequencing.

Interpreting the effects of specific protein modification on antiport coupling mechanisms: the case of the aspartate/glutamate exchanger

Reaction of two cysteine residues in the aspartate/glutamate carrier of mitochondria is reported to abolish exchange but to actuate a passive one-way exit of extremely low substrate affinity and specificity, but with the same activation energy as antiport (Dierks, T., Salentin, A. and Krämer, R. (1990) Biochim. Biophys. Acta 1028, 281-288). This behaviour, reminiscent of a channel, becomes understandable when the required control over carrier mobility by the substrate is allowed for. Whether the transport mechanism involves a substrate site alternately exposed on opposite sides of the membrane or sites simultaneously exposed on both sides, and whether the substrate acts by converting an immobile carrier conformation to an inherently mobile intermediate or by stabilizing the transition state in carrier movement, the same fundamental relationship emerges: the ratio of coupled to uncoupled rates (antiport relative to net flux) is limited by the ratio of substrate dissociation constants in successive carrier conformations, one immobile, the other mobile; the increment in the binding energy in the two forms must therefore be large. Shifts in the equilibrium between these conformations and shifts in their relative affinities for the substrate can account for the properties of the modified transport system, which, it is concluded, functions as a carrier, not a channel.

Interpreting the effects of site-directed mutagenesis on active transport systems

Single amino acid substitutions in the lactose permease of Escherichia coli are known to elicit behaviour, such as the transformation of an active into a passive system, not explained by current co-transport models. The behaviour, it is shown, can be explained by an expanded reaction scheme that takes account of the required alternation of the carrier, in the course of the coupled reaction, between mobile and immobile conformations or between conformations that bind either only one substrate or both substrates. The extended model links such behaviour to altered conformational equilibria or binding regions. Thus, mutations that affect the equilibrium between a mobile one-site conformation of the free carrier and an immobile conformation having sites for both substrates allow passive transport of the second substrate in an ordered mechanism, and mutations in a secondary substrate binding region that affects this conformational change allow passive transport of the first substrate. Mutations in regions interacting with a substrate in the transition state in carrier movement, as well as in the initial binding sites, can also be distinguished. The analysis applies to both primary and secondary active transport.

Properties of permease dimer, a fusion protein containing two lactose permease molecules from Escherichia coli

An engineered fusion protein containing two tandem lactose permease molecules (permease dimer) exhibits high transport activity and is used to test the phenomenon of negative dominance. Introduction of the mutation Glu-325-->Cys into either the first or the second half of the dimer results in a 50% decrease in activity, whereas introduction of the mutation into both halves of the dimer abolishes transport. Lactose transport by permease dimer is completely inactivated by N-ethylmaleimide; however, 40-45% activity is retained after N-ethylmaleimide treatment when either the first or the second half of the dimer is replaced with a mutant devoid of cysteine residues. The observations demonstrate that both halves of the fusion protein are equally active and suggest that each half may function independently. To test the possibility that oligomerization between dimers might account for the findings, a permease dimer was constructed that contains two different deletion mutants that complement functionally when expressed as untethered molecules. Because this construct does not catalyze lactose transport to any extent whatsoever, it is unlikely that the two halves of the dimer interact or that there is an oligomeric interaction between dimers. The approach is consistent with the contention that the functional unit of lactose permease is a monomer.

Molecular cloning of a maltose transport gene from Bacillus stearothermophilus and its expression in Escherichia coli K-12

Genes responsible for maltose utilization from Bacillus stearothermophilus ATCC7953 were cloned in the plasmid vector pBR325 and functionally expressed in Escherichia coli. The 4.2 kb Bacillus DNA insert in clone pAM1750 suppressed the growth defects on maltose caused by mutations in E. coli maltose transport genes (malE, malK or complete malB deletion) but not mutations in genes affecting intracellular maltose metabolism (malA region). Transport studies in E. coli and B. stearothermophilus suggested that pAM1750 codes for a high affinity transport system, probably one of two maltose uptake systems found in B. stearothermophilus ATCC7953. Nucleotide sequence analysis of a 3.6 kb fragment of pAM1750 revealed three open reading frames (ORFs). One of the ORFs, malA, encoded a putative hydrophobic protein with 12 potential transmembrane segments. MalA showed amino acid sequence similarity to proteins in the superfamily containing LacY lactose permease and also some similarity to MalG protein, a member of a binding protein-dependent transport system in E. coli. The products of two other ORFs were not hydrophobic, did not show similarity to other known sequences and were found not to be essential for maltose utilization in transport-defective E. coli mutants. Hence MalA protein was the only protein necessary for maltose transport, but despite giving a detectable but low level of transport function in E. coli, the protein was very poorly expressed and could not be identified.

Cysteine scanning mutagenesis of the N-terminal 32 amino acid residues in the lactose permease of Escherichia coli

Using a functional lactose permease mutant devoid of Cys residues (C-less permease), each amino acid residue in the hydrophilic N-terminus and the first putative transmembrane helix was systematically replaced with Cys (from Tyr-2 to Trp-33). Twenty-three of 32 mutants exhibit high lactose accumulation (70-100% or more of C-less), and an additional 8 mutants accumulate to lower but highly significant levels. Surprisingly, Cys replacement for Gly-24 or Tyr-26 yields fully active permease molecules, and permease with Cys in place of Pro-28 also exhibits significant transport activity, although previous mutagenesis studies on these residues suggested that they may be required for lactose transport. As expected, Cys replacement for Pro-31 completely inactivates, in agreement with previous findings indicating that "helix-breaking" propensity at this position is necessary for full activity (Consler TG, Tsolas O, Kaback HR, 1991, Biochemistry 30:1291-1297). Twenty-nine mutants are present in the membrane in amounts comparable to C-less permease, whereas membrane levels of mutants Tyr-3-->Cys and Phe-12-->Cys are slightly reduced, as judged by immunological techniques. Dramatically, mutant Phe-9-->Cys is hardly detectable when expressed from the lac promoter/operator at a relatively low rate, but is present in the membrane in a stable form when expressed at a high rate from the T7 promoter. Finally, studies with N-ethylmalemide show that 6 Cys-replacement mutants that cluster at the C-terminal end of putative helix I are inactivated significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of glutamate-269 in the lactose permease of Escherichia coli

Glu-269, which is located on the hydrophilic face of putative helix VIII in the lactose permease of Escherichia coli, has been replaced with Asp, Gln or Cys by oligonucleotide-directed, site specific mutagenesis. Cells expressing Asp-269 permease exhibit no lactose accumulation or lactose-induced H+ translocation, but retain some ability to mediate lactose influx down a concentration gradient at high substrate concentrations. Furthermore, right-side-out membrane vesicles containing Asp-269 permease do not catalyse active lactose transport, facilitated lactose efflux or equilibrium exchange. Remarkably, however, Asp-269 permease accumulates beta, D-galactopyranosyl 1-thio-beta,D-galactopyranoside in a partially uncoupled fashion, whereas no transport of methyl-beta,D-thiogalactopyranoside, sucrose or maltose is detectable. Mutant permeases containing neutral replacements (Gln or Cys) or Glu-269 are completely devoid of activity, although the proteins are present in the membrane at concentrations comparable with wild-type or Asp-269 permease. The observations demonstrate that a carboxylate at position 269 is essential for transport activity, and Glu-269 is important for substrate binding and/or recognition.

Properties of interacting aspartic acid and lysine residues in the lactose permease of Escherichia coli

The side chains of the interacting pair Asp237(helix VII)-Lys358(helix XI) or Asp240(helix VII)-Lys319(helix X) in the lactose permease of Escherichia coli were extended by replacement with Glu and/or Arg or by site-specific derivatization of single-Cys replacement mutants. Iodoacetic acid was used to carboxymethylate Cys, or methanethiosulfonate derivatives [Akabas, M. H., Stauffer, D. A., Xu, M., & Karlin, A. (1992) Science 258, 307] were used to attach negatively charged ethylsulfonate or positively charged ethylammonium groups. Replacement of Asp237 with Glu, carboxymethyl-Cys, or sulfonylethylthio-Cys yields active permease with Lys or Arg at position 358. Similarly, the permease tolerates replacement of Lys358 with Arg or ammonioethylthio-Cys with Asp or Glu at position 237. Remarkably, moreover, permease with Lys, Arg, or ammonioethylthio-Cys in place of Asp237 is highly active when Lys358 is replaced with Asp or Glu, in agreement with the conclusion that the polarity of the charge interaction can be reversed without loss of activity [Sahin-Tóth, M., Dunten, R. L., Gonzalez, A., & Kaback, H. R. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 10547]. In contrast, replacement of Asp240 with Glu abolishes lactose transport, and permease with carboxymethyl-Cys, at position 240 is inactive when paired with Lys319, but it exhibits significant activity with Arg319. Interestingly, sulfonylethylthio-Cys substitution for Asp240 also results in significant transport activity. Permease with Arg or ammonioethylthio-Cys in place of Lys319 exhibits high activity with Asp240 as the negative counterion, but no lactose transport is observed when either of these modifications is paired with Glu240.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of transport systems in yeast by photodynamic treatment with toluidine blue

Photodynamic treatment of yeast with the sensitizer Toluidine blue results in loss of cell viability. In previous investigations it was suggested that plasma membrane damage might be responsible for the loss of colony forming capacity. In this context the influence of photodynamic treatment on transmembrane transport systems was studied. It appeared that the uptake of the sugars glucose, lactose and galactose, the amino acids arginine, phenylalanine, glycine and aspartic acid and of the inorganic compound phosphate was inhibited by photodynamic treatment. The different elements of the energy providing system necessary for active transport, viz. the plasma membrane ATPase and the protonmotive force, were not significantly affected by Toluidine blue and light, indicating that inhibition of transport is not caused by a reduction of the membrane potential or the transmembrane pH gradient. These observations suggest that the transport carriers themselves were damaged by treatment with Toluidine blue and light. This could be confirmed in experiments, in which the lactose and galactose transport proteins of treated and untreated cells were reconstituted in plasma membrane vesicles. It appeared that the carriers, obtained from photodynamically treated Kluyveromyces marxianus cell, had lost their transport capacity.

The Rhizobium meliloti rhizopine mos locus is a mosaic structure facilitating its symbiotic regulation

The Rhizobium meliloti L5-30 mos locus, encoding biosynthesis of the rhizopine 3-O-methyl-scyllo-inosamine, is shown to be a mosaic structure. The mos locus consists of four open reading frames (ORFs) (ORF1 and mosABC) arranged in an operon structure. Within this locus, several domains of homology with other prokaryotic symbiotic genes (nifH, fixA, fixU, and nifT) are present, suggesting that this locus may represent a hot spot for rearrangement of symbiotic genes. Unusually, these domains are present in the coding as well as noncoding regions of the mos locus. Proteins corresponding to those encoded by mosABC, but not ORF1, have been detected in nodule extracts by using antibodies. As ORF1 shows extensive homology with the 5' region of the nifH gene (P.J. Murphy, N. Heycke, S.P. Trenz, P. Ratet, F.J. de Bruijn, and J. Schell, Proc. Natl. Acad. Sci. USA 85:9133-9137, 1988) and a frameshift mutation indicates that expression of this ORF is not required for mos activity, we propose that the mos locus has acquired a duplicated copy of nifH, including the promoter region, in order to become symbiotically regulated. Surprisingly, since the functions are likely different, MosA has an amino acid sequence similar to that of the DapA protein of Escherichia coli. The central domain of MosB has extensive homology with a range of diverse proteins involved with carbohydrate metabolism in either antibiotic or outer-cell-wall biosynthesis. This region is also common to the regulatory proteins DegT and DnrJ, suggesting a regulatory role for MosB. The structure of MosC is consistent with its being a membrane transport protein.

Cysteine scanning mutagenesis of putative transmembrane helices IX and X in the lactose permease of Escherichia coli

Using a functional lactose permease mutant devoid of Cys residues (C-less permease), each amino-acid residue in putative transmembrane helices IX and X and the short intervening loop was systematically replaced with Cys (from Asn-290 to Lys-335). Thirty-four of 46 mutants accumulate lactose to high levels (70-100% or more of C-less), and an additional 7 mutants exhibit lower but highly significant lactose accumulation. As expected (see Kaback, H.R., 1992, Int. Rev. Cytol. 137A, 97-125), Cys substitution for Arg-302, His-322, or Glu-325 results in inactive permease molecules. Although Cys replacement for Lys-319 or Phe-334 also inactivates lactose accumulation, Lys-319 is not essential for active lactose transport (Sahin-Tóth, M., Dunten, R.L., Gonzalez, A., & Kaback, H.R., 1992, Proc. Natl. Acad. Sci. USA 89, 10547-10551), and replacement of Phe-334 with leucine yields permease with considerable activity. All single-Cys mutants except Gly-296 --> Cys are present in the membrane in amounts comparable to C-less permease, as judged by immunological techniques. In contrast, mutant Gly-296 --> Cys is hardly detectable when expressed at a relatively low rate from the lac promoter/operator but present in the membrane in stable form when expressed at a high rate from T7 promoter. Finally, studies with N-ethylmaleimide (NEM) show that only a few mutants are inactivated significantly. Remarkably, the rate of inactivation of Val-315 --> Cys permease is enhanced at least 10-fold in the presence of beta-galactopyranosyl 1-thio-beta-D-galactopyranoside (TDG) or an H+ electrochemical gradient (delta mu-H+). The results demonstrate that only three residues in this region of the permease -Arg-302, His-322, and Glu-325-are essential for active lactose transport. Furthermore, the enhanced reactivity of the Val-315 --> Cys mutant toward NEM in the presence of TDG or delta mu-H+ probably reflects a conformational alteration induced by either substrate binding or delta mu-H+.

Alamethicin and related peptaibols--model ion channels

Peptaibols are considered as models of those ion channels which consist of a bundle of transbilayer helices surrounding a central pore. X-Ray diffraction and NMR studies have yielded high resolution structures for several peptaibols. In conjunction with other spectroscopic investigations and molecular dynamics simulations, these studies suggest that peptaibols form proline-kinked alpha-helices, and that there may be "hinge-bending" movement of the helix in the region of the central proline residue. The amphipathicity of peptaibol helices is analyzed in relation to their channel-forming properties. Studies of the interactions of peptaibols with lipid bilayers suggest that they are helical when in a membrane-like environment, and that the helix orientation relative to the bilayer is sensitive to the peptaibol:lipid ratio, and to the degree of hydration of the bilayer. Electrical studies reveal that many peptaibols form multiple-conductance level channels in a voltage-dependent fashion. Analysis of conductance levels provides support for the "barrel stave" model of channel formation, whereby different conductance levels correspond to different numbers of monomers in a helix bundle. Alternative models for voltage-activation are discussed, and the roles of molecular dipoles and of hinge-bending in this process are considered. Two molecular models for an N = 6 bundle of alamethicin helices are presented and their electrostatic properties analyzed. The relevance of studies of peptaibols to channel and transport proteins in general is considered.

Role of the charge pair aspartic acid-237-lysine-358 in the lactose permease of Escherichia coli

Using a lactose permease mutant devoid of Cys residues (C-less permease), Asp237 and Lys358 were replaced with Cys or other amino acids to pursue the proposal that the two residues form a charge pair [King, S. C., Hansen, C. L., & Wilson, T.H. (1991) Biochim. Biophys. Acta 1062, 177-186]. Individual replacement of Asp237 with Cys, Ala, or Lys or replacement of Lys358 with Cys, Ala, or Asp virtually abolishes active lactose transport. However, simultaneous replacement of both residues with Cys and/or Ala yields permease with high activity. Therefore, neutral amino acid substitutions at either position are detrimental only because they leave the opposing charge unpaired. Strikingly, moreover, when Asp237 is interchanged with Lys358, high activity is observed. The results indicate strongly that Asp237 and Lys358 interact to form a salt bridge and that neither residue nor the salt bridge per se is important for activity. Immunoblots reveal low membrane levels of the active mutants lacking the putative salt bridge, suggesting a role for the salt bridge in either permease folding or stability and raising the possibility that the salt bridge may exist in a folding intermediate but not in the mature protein. Remarkably, however, a mutant with Cys in place of Asp237 is restored to full activity by carboxymethylation which recreates a negative charge at position 237. Pulse-chase analysis and heat-inactivation studies indicate that the stability of the double mutant with Cys at positions 237 and 358 is comparable to C-less. Therefore, the interaction between Asp237 and Lys358 is likely to be important for permease folding and is maintained in the mature protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and expression of the UGA4 gene coding for the inducible GABA-specific transport protein of Saccharomyces cerevisiae

Transport of 4-aminobutyric acid (GABA) in Saccharomyces cerevisiae is mediated by three transport systems: the general amino acid permease (GAP1 gene), the proline permease (PUT4 gene), and a specific GABA permease (UGA4 gene) which is induced in the presence of GABA. The UGA4 gene encoding the inducible GABA-specific transporter was cloned and sequenced and its expression analyzed. The predicted amino acid sequence shows that UGA4 encodes a 62 kDa protein having 9-12 putative membrane-spanning regions. The predicted UGA4 protein shares significant sequence similarity with the yeast choline transporter (CTR gene), exhibiting but limited similarity to the previously reported GABA transporters, i.e. the yeast GAP1 and PUT4 permeases and the rat brain GAT-1 transporter. Induction of UGA4 in the presence of GABA is exerted at the level of UGA4 mRNA accumulation, most probably at the level of transcription itself. This induction is conferred by the 5' flanking region and requires the integrity of two positive regulatory proteins, the inducer-specific factor UGA3 and the pleiotropic factor UGA35/DURL/DAL81. In the absence of the pleiotropic UGA43/DAL80 repressor, UGA4 is constitutively expressed at high level.

Membrane topology model of Escherichia coli alpha-ketoglutarate permease by phoA fusion analysis

Escherichia coli alpha-ketoglutarate permease (KgtP) is a 432-amino-acid protein that symports alpha-ketoglutarate and protons. KgtP was predicted to contain 12 membrane-spanning domains on the basis of a calculated hydropathy profile. The membrane topology model of KgtP was analyzed by using kgtP-phoA gene fusions and measuring alkaline phosphatase activities in cells expressing the chimeric proteins. Comparisons of the phosphatase activity levels and the locations of the KgtP-PhoA junctions are consistent with the predicted membrane topology model of KgtP.

Insertional mutagenesis of hydrophilic domains in the lactose permease of Escherichia coli

The lactose permease of Escherichia coli is a membrane transport protein postulated to contain a hydrophilic N terminus (hydrophilic domain 1), 12 hydrophobic transmembrane alpha-helices that traverse the membrane in zigzag fashion connected by hydrophilic domains, and a hydrophilic C terminus (hydrophilic domain 13). To test whether the hydrophilic domains are important for function, each domain was independently disrupted by insertion of two or six contiguous histidine residues, and the mutants were characterized with respect to initial rate of lactose transport and steady-state level of accumulation. Remarkably, histidine insertions into 10 out of 13 hydrophilic domains result in molecules that catalyze lactose accumulation effectively, although the initial rate of transport is compromised in certain cases. In contrast, insertions into hydrophilic domain 3, 9, or 10 cause a marked decrease in transport activity. As judged by immunoblots and [35S]methionine pulse-chase experiments, diminished activity is not due to decreased expression of the mutated permeases, defective insertion into the membrane, or increased rates of proteolysis after insertion. The results (i) suggest that most of the hydrophilic domains in the permease do not play an essential role in the transport mechanism and (ii) focus on the region of the permease containing putative helices IX and X as being particularly important for activity.

Structure and dynamics of polypeptides and proteins in lipid membranes

The elucidation of the molecular mechanisms whereby ions and polar molecules are translocated across the hydrophobic barrier of a lipid bilayer in biological membranes is one of the most challenging problems in biological research. Specific membrane proteins, such as pumps, carriers and channels, play the central role in the various translocation pathways. Recent progress in expression cloning has provided the sequence of a number of biologically important membrane proteins and in principle the door is open to investigate every protein which might be of importance in the central signal transduction and transport processes. Unfortunately, to date there are only a few examples where the three-dimensional structure of membrane proteins are known at atomic resolution. The photosynthetic reaction centres from purple bacteria (Deisenhoferet al.1985), bacteriorhodopsin (Hendersonet al.1990) and the large porin channel ofRhodobacter capsulata(Weisset al.1991). According to these structural data membrane proteins seem to fold in general in membrane-spanning α-helices and β-strands in order to saturate hydrogen bonds. Only these two motifs seem to form stable structures which can be in contact with the hydrophobic lipid interior of a membrane.

Functional interactions between putative intramembrane charged residues in the lactose permease of Escherichia coli

Using a lactose permease mutant devoid of Cys residue ("C-less permease"), we systematically replaced putative intramembrane charged residues with Cys. Individual replacements for Asp-237, Asp-240, Glu-269, Arg-302, Lys-319, His-322, Glu-325, or Lys-358 abolish active lactose transport. When Asp-237 and Lys-358 are simultaneously replaced with Cys and/or Ala, however, high activity is observed. Therefore, when either Asp-237 or Lys-358 is replaced with a neutral residue, leaving an unpaired charge, the permease is inactivated, but neutral replacement of both residues yields active permease [King, S. C., Hansen, C. L. & Wilson, T. H. (1991) Biochim. Biophys. Acta 1062, 177-186]. Remarkably, moreover, when Asp-237 is interchanged with Lys-358, high activity is observed. The observations provide a strong indication that Asp-237 and Lys-358 interact to form a salt bridge. In addition, the data demonstrate that neither residue nor the salt bridge plays an important role in the transport mechanism. Thirteen additional double mutants were constructed in which a negative and a positively charged residue were replaced with Cys. Only Asp-240-->Cys/Lys-319-->Cys exhibits significant activity, accumulating lactose to 25-30% of the steady state observed with C-less permease. Replacing either Asp-240 or Lys-319 individually with Ala also inactivates the permease, but double mutants with neutral substitutions (Cys and/or Ala) at both positions exhibit essentially the same activity as Asp-240-->Cys/Lys-319-->Cys. In marked contrast to Asp-237 and Lys-358, interchanging Asp-240 and Lys-319 abolishes active lactose transport. The results demonstrate that Asp-240 and Lys-319, like Asp-237 and Lys-358, interact functionally and may form a salt bridge. However, the interaction between Asp-240 and Lys-319 is clearly more complex than the interaction between Asp-237 and Lys-358. In any event, the findings suggest that putative transmembrane helix VII lies next to helices X and XI in the tertiary structure of lactose permease.

The transport of neurotransmitters into synaptic vesicles

As investigations identify additional plasma membrane neurotransmitter transporters, attention has focused on the molecular basis of neurotransmitter transport into synaptic vesicles. The transport of biogenic amines into chromaffin granules has served as the paradigm for understanding vesicular transport. Recent work now describes the vesicular transport of other classical neurotransmitters, which occur by distinct but related mechanisms. To determine their biochemical basis, several of the transporters have been functionally reconstituted in liposomes. The ability of vesicular amine transport to protect against the neurotoxin MPP+ has permitted the isolation of the first cDNA clone for a member of this family, and the sequence establishes a relationship with drug-resistance transporters in bacteria.

Membrane transport proteins: implications of sequence comparisons

Analyses of the sequences and structures of many transport proteins that differ in substrate specificity, direction of transport and mechanism of transport suggest that they form a family of related proteins. Their sequence similarities imply a common mechanism of action. This hypothesis provides an objective basis for examining their mechanisms of action and relationships to other transporters.

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.

Membrane protein spanning segments as export signals

Escherichia coli lac permease is a polytopic integral membrane protein with six translocated (periplasmic) domains. Individual N-terminal cytoplasmic regions and membrane-spanning segments adjacent to each of the periplasmic domains acted as export signals for an attached sensor protein (alkaline phosphatase). However, the export activity of one of the spanning segments was considerably lower than that of the others, and was limited by the presence of a positively charged residue (Arg302). These observations are compatible with models of membrane protein insertion in which hydrophilic domains are translocated independently. However, the results suggest that efficient translocation may sometimes require interaction between individual spanning segments.

Functional complementation of internal deletion mutants in the lactose permease of Escherichia coli

Using the lactose permease of Escherichia coli, a well-characterized membrane protein with 12 transmembrane domains, we demonstrated that certain paired in-frame deletion constructs complement each other functionally. Although cells expressing the deletion mutants individually are unable to catalyze active lactose accumulation, cells simultaneously expressing specific pairs of deletions catalyze transport up to 60% as do cells expressing wild-type permease. Moreover, complementation clearly does not occur at the level of DNA but probably occurs at the protein level. Remarkably, functional complementation is observed only with pairs of permease molecules containing large deletions and is not observed with missense mutations or point deletions. Although the mechanism of functional complementation is obscure, the findings indicate that certain pairs of permease molecules containing specific internal deletions can interact to form a functional complex in the same way phenomenologically as do independently expressed polypeptides corresponding to different N- and C-terminal portions of the permease.

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.

Construction of a functional lactose permease devoid of cysteine residues

By use of oligonucleotide-directed, site-specific mutagenesis, a lactose (lac) permease molecule was constructed in which all eight cysteinyl residues were simultaneously mutagenized (C-less permease). Cys154 was replaced with valine, and Cys117, -148, -176, -234, -333, -353, and -355 were replaced with serine. Remarkably, C-less permease catalyzes lactose accumulation in the presence of a transmembrane proton electrochemical gradient (interior negative and alkaline). Thus, in intact cells and right-side-out membrane vesicles containing comparable amounts of wild-type and Cys-less permease, the mutant protein catalyzes lactose transport at a maximum velocity and to a steady-state level of accumulation of about 35% and 55%, respectively, of wild-type with a similar apparent Km (ca. 0.3 mM). As anticipated, moreover, active lactose transport via C-less permease is completely resistant to inactivation by N-ethylmaleimide. Finally, C-less permease also catalyzes efflux and equilibrium exchange at about 35% of wild-type activity. The results provide definitive evidence that sulfhydryl groups do not play an essential role in the mechanism of lactose/H+ symport. Potential applications of the C-less mutant to studies of static and dynamic aspects of permease structure/function are discussed.

Organization and stability of a polytopic membrane protein: deletion analysis of the lactose permease of Escherichia coli

The overall topology of polytopic membrane proteins is thought to result from either the oriented insertion of the N-terminal alpha-helical domain followed by passive insertion of subsequent helices or from the function of independent topogenic determinants dispersed throughout the molecules. By using the lactose permease of Escherichia coli, a well-characterized membrane protein with 12 transmembrane domains and the N and C termini on the cytoplasmic surface of the membrane, we have studied the insertion and stability of in-frame deletion mutants. So long as the first N-terminal and the last four C-terminal putative alpha-helical domains are retained, stable polypeptides are inserted into the membrane, even when an odd number of helical domains is deleted. Moreover, even when an odd number of helices is deleted, the C terminus remains on the cytoplasmic surface of the membrane, as judged by lacY-phoA fusion analysis. In addition, permease molecules devoid of even or odd numbers of putative transmembrane helices retain a specific pathway for downhill lactose translocation. The findings imply that relatively short C-terminal domains of the permease contain topological information sufficient for insertion in the native orientation regardless of the orientation of the N terminus.