Isolation and Characterization of Lactose Permease Mutants with an Enhanced Recognition of Maltose and Diminished Recognition of Cellobiose

Engineering of a new Escherichia coli strain efficiently metabolizing cellobiose with promising perspectives for plant biomass-based application design

The necessity to decrease our fossil energy dependence requests bioprocesses based on biomass degradation. Cellobiose is the main product released by cellulases when acting on the major plant cell wall polysaccharide constituent, the cellulose. Escherichia coli, one of the most common model organisms for the academy and the industry, is unable to metabolize this disaccharide. In this context, the remodeling of E. coli to catabolize cellobiose should thus constitute an important progress for the design of such applications. Here, we developed a robust E. coli strain able to metabolize cellobiose by integration of a small set of modifications in its genome. Contrary to previous studies that use adaptative evolution to achieve some growth on this sugar by reactivating E. coli cryptic operons coding for cellobiose metabolism, we identified easily insertable modifications impacting the cellobiose import (expression of a gene coding a truncated variant of the maltoporin LamB, modification of the expression of lacY encoding the lactose permease) and its intracellular degradation (genomic insertion of a gene encoding either a cytosolic β-glucosidase or a cellobiose phosphorylase). Taken together, our results provide an easily transferable set of mutations that confers to E. coli an efficient growth phenotype on cellobiose (doubling time of 2.2 ​h in aerobiosis) without any prior adaptation.

Evidence for the transport of maltose by the sucrose permease, CscB, of Escherichia coli

The purpose of this study was to examine the sugar recognition and transport properties of the sucrose permease (CscB), a secondary active transporter from Escherichia coli. We tested the hypothesis that maltose transport is conferred by the wild-type CscB transporter. Cells of E. coli HS4006 harboring pSP72/cscB were red on maltose MacConkey agar indicator plates. We were able to measure "downhill" maltose transport and establish definitive kinetic behavior for maltose entry in such cells. Maltose was an effective competitor of sucrose transport in cells with CscB, suggesting that the respective maltose and sucrose binding sites and translocation pathways through the CscB channel overlap. Accumulation ("uphill" transport) of maltose by cells with CscB was profound, demonstrating active transport of maltose by CscB. Sequencing of cscB encoded on plasmid pSP72/cscB used in cells for transport studies indicate an unaltered primary CscB structure, ruling out the possibility that mutation conferred maltose transport by CscB. We conclude that maltose is a bona fide substrate for the sucrose permease of E. coli. Thus, future studies of sugar binding, transport, and permease structure should consider maltose, as well as sucrose.

Amino acids that confer transport of raffinose and maltose sugars in the raffinose permease (RafB) of Escherichia coli as implicated by spontaneous mutations at Val-35, Ser-138, Ser-139, Gly-389 and Ile-391

In order to identify amino acid residues in the Escherichia coli raffinose-H(+) permease (RafB) that play a role in sugar selection and transport, we first incubated E. coli HS4006 containing plasmid pRU600 (expresses inducible raffinose permease and alpha-galactosidase) on maltose MacConkey indicator plates overnight. Initially, all colonies were white, indicating no fermentation of maltose. Upon further incubation, 100 mutants appeared red. pRU600 DNA was prepared from 55 mutants. Five mutants transferred the phenotype for fermentation of maltose (red). Plasmid DNA from five maltose-positive phenotype transformants was prepared and sequenced, revealing three distinct types of mutations. Two mutants exhibited Val-35-->Ala (MT1); one mutant had Ile-391-->Ser (MT2); and two mutants had Ser-138-->Asp, Ser-139-->Leu and Gly-389-->Ala (MT3). Transport studies of [(3)H]-maltose showed that cells harboring MT1, MT2 and MT3 had greater uptake (P <or= 0.05) than cells harboring wild-type RafB. However, [(14)C]-raffinose uptake was reduced in all mutant cells (P <or= 0.05) with MT1, MT2 and MT3 mutants compared to cells harboring wild-type RafB. Kinetic analysis showed enhanced apparent K (m) values for maltose and reduced V (max)/ K (m) ratios for raffinose compared to wild-type values. The apparent K (i) value of maltose for RafB indicates a competitive relationship between maltose and raffinose. Maltose "uphill" accumulation was greater for mutants (P <or= 0.05) than for cells with wild-type RafB. Thus, we implicate residues in RafB that are responsible for raffinose transport and suggest that the substituted residues in RafB dictate structures that enhance transport of maltose.

A General Channel Model Accounts for Channel, Carrier, Countertransport and Cotransport Kinetics

In this work we propose a unifying model of mediated membrane transport, based upon the idea that the integral membrane proteins involved in these processes operate via complex channel mechanisms. In the first part, we briefly review literature about the structural aspects of membrane transporters. We conclude that there is a substantial amount of evidence suggesting that most membrane proteins performing transport are embodied with channel-like structures that may constitute the translocation paths. This includes cases where the phenomenological transport kinetics do not correspond to the classical channel behavior. In the second part of this article we introduce the general channel model of mediated transport and employ it to derive specific examples, like simple one- or two-ligand channels, water-ligand channels, simple carriers, co- and counter-transport systems and more complex water-ligand carriers. We show that, for the most part, these particular cases can be obtained by the application of the techniques of diagram reduction to the full model. The necessary conditions for diagram reduction reflect physical properties of the protein and its surroundings.

Control of H+/Lactose Coupling by Ionic Interactions in the Lactose Permease ofEscherichia coli

A combinatorial approach was used to study putative interactions among six ionizable residues (Asp-240, Glu-269, Arg-302, Lys-319, His-322, and Glu-325) in the lactose permease. Neutral mutations were made involving five ion pairs that had not been previously studied. Double mutants, R302L/E325Q and D240N/H322Q, had moderate levels of downhill [(14)C]-lactose transport. Mutants in which only one of these six residues was left unchanged (pentuple mutants) were also made. A Pent269(-) mutant (in which only Glu-269 remains) catalyzed a moderate level of downhill lactose transport. Pent240(-) and Pent 322(+) also showed low levels of downhill lactose transport. Additionally, a Pent240(-) mutant exhibited proton transport upon addition of melibiose, but not lactose. This striking result demonstrates that neutralization of up to five residues of the lactose permease does not abolish proton transport. A mutant with neutral replacements at six ionic residues (hextuple mutant) had low levels of downhill lactose transport, but no uphill accumulation or proton transport. Since none of the mutants in this study catalyzes active accumulation of lactose, this is consistent with other reports that have shown that each residue is essential for proper coupling. Nevertheless, none of the six ionizable residues is individually required for substrate-induced proton cotransport. These results suggest that the H(+) binding domain may be elsewhere in the permease or that cation binding may involve a flexible network of charged residues.

A revised model for the structure and function of the lactose permease. Evidence that a face on transmembrane segment 2 is important for conformational changes

The lactose permease is an integral membrane protein that cotransports H(+) and lactose into the bacterial cytoplasm. Previous work has shown that bulky substitutions at glycine 64, which is found on the cytoplasmic edge of transmembrane segment 2 (TMS-2), cause a substantial decrease in the maximal velocity of lactose uptake without significantly affecting the K(m) values (Jessen-Marshall, A. E., Parker, N. J., and Brooker, R. J. (1997) J. Bacteriol. 179, 2616-2622). In the current study, mutagenesis was conducted along the face of TMS-2 that contains glycine-64. Single amino acid substitutions that substantially changed side-chain volume at codons 52, 57, 59, 63, and 66 had little or no effect on transport activity, whereas substitutions at codons 49, 53, 56, and 60 were markedly defective and/or had lower levels of expression. According to helical wheel plots, Phe-49, Ser-53, Ser-56, Gln-60, and Gly-64 form a continuous stripe along one face of TMS-2. Several of the TMS-2 mutants (S56Y, S56L, S56Q, Q60A, and Q60V) were used as parental strains to isolate mutants that restore transport activity. These mutations were either first-site mutations or second-site suppressors in TMS-1, TMS-2, TMS-7 or TMS-11. A kinetic analysis showed that the suppressors had a higher rate of lactose transport compared with the corresponding parental strains. Overall, the results of this study are consistent with the notion that a face on TMS-2, containing Phe-49, Ser-53, Ser-56, Gln-60, and Gly-64, plays a critical role in conformational changes associated with lactose transport. We hypothesize that TMS-2 slides across TMS-7 and TMS-11 when the lactose permease interconverts between the C1 and C2 conformations. This idea is discussed within the context of a revised model for the structure of the lactose permease.

A functional-phylogenetic classification system for transmembrane solute transporters

A comprehensive classification system for transmembrane molecular transporters has been developed and recently approved by the transport panel of the nomenclature committee of the International Union of Biochemistry and Molecular Biology. This system is based on (i) transporter class and subclass (mode of transport and energy coupling mechanism), (ii) protein phylogenetic family and subfamily, and (iii) substrate specificity. Almost all of the more than 250 identified families of transporters include members that function exclusively in transport. Channels (115 families), secondary active transporters (uniporters, symporters, and antiporters) (78 families), primary active transporters (23 families), group translocators (6 families), and transport proteins of ill-defined function or of unknown mechanism (51 families) constitute distinct categories. Transport mode and energy coupling prove to be relatively immutable characteristics and therefore provide primary bases for classification. Phylogenetic grouping reflects structure, function, mechanism, and often substrate specificity and therefore provides a reliable secondary basis for classification. Substrate specificity and polarity of transport prove to be more readily altered during evolutionary history and therefore provide a tertiary basis for classification. With very few exceptions, a phylogenetic family of transporters includes members that function by a single transport mode and energy coupling mechanism, although a variety of substrates may be transported, sometimes with either inwardly or outwardly directed polarity. In this review, I provide cross-referencing of well-characterized constituent transporters according to (i) transport mode, (ii) energy coupling mechanism, (iii) phylogenetic grouping, and (iv) substrates transported. The structural features and distribution of recognized family members throughout the living world are also evaluated. The tabulations should facilitate familial and functional assignments of newly sequenced transport proteins that will result from future genome sequencing projects.

A K319N/E325Q double mutant of the lactose permease cotransports H+ with lactose. Implications for a proposed mechanism of H+/lactose symport

In this study, we have examined the transport characteristics of the wild-type lactose permease, single mutants in which Lys-319 was changed to asparagine or alanine or Glu-325 was changed to glutamine or alanine, and the corresponding double mutant strains. The wild-type and Asn-319 mutant showed high levels of lactose uptake, with Km values of 0.42 and 1.30 mM and Vmax values of 102.6 and 48.3 nmol of lactose/min/mg of protein, respectively. The Asn-319/Gln-325 strain had a normal Km of 0.36 mM and a moderate Vmax of 18.5 nmol of lactose/min/mg of protein. By comparison, the single E325Q strain had a normal Km of 0.27 mM but a very defective Vmax of 1.3 nmol of lactose/min/mg of protein. A similar trend was observed among the alanine substitutions at these positions, although the Vmax values were lower for the Ala-319 mutations. When comparing the Vmax values between the single position 325 mutants with those of the double mutants, these results indicate that neutral 319 mutations substantially alleviate a defect in Vmax caused by neutral 325 mutations. With regard to H+/lactose coupling, the wild-type permease is normally coupled and can transport lactose against a gradient. The position 325 single mutants showed no evidence of H+ transport with lactose or thiodigalactoside (TDG) and were unable to facilitate uphill lactose transport. The single Asn-319 mutant and double Asn-319/Gln-325 mutant were able to transport H+ upon the addition of lactose or TDG. In addition, both of these strains catalyzed a sugar-dependent H+ leak that inhibited cell growth in the presence of TDG. These two strains were also defective in uphill transport, which may be related to their sugar-dependent leak pathway. Based on these and other results in the literature, a model is presented that describes how the interactions among several ionizable residues within the lactose permease act in a concerted manner to control H+/lactose coupling. In this model, Lys-319 and Glu-325 play a central role in governing the ability of the lactose permease to couple the transport of H+ and lactose.

Microbial genome analyses: global comparisons of transport capabilities based on phylogenies, bioenergetics and substrate specificities

We have conducted genome sequence analyses of seven prokaryotic microorganisms for which completely sequenced genomes are available (Escherichia coli, Haemophilus influenzae, Helicobacter pylori, Bacillus subtilis, Mycoplasma genitalium, Synechocystis PCC6803 and Methanococcus jannaschii). We report the distribution of encoded known and putative polytopic cytoplasmic membrane transport proteins within these genomes. Transport systems for each organism were classified according to (1) putative membrane topology, (2) protein family, (3) bioenergetics, and (4) substrate specificities. The overall transport capabilities of each organism were thereby estimated. Probable function was assigned to greater than 90% of the putative transport proteins identified. The results show the following: (1) Numbers of transport systems in eubacteria are approximately proportional to genome size and correspond to 9.7 to 10.8% of the total encoded genes except for H. pylori (5.4%), Synechocystis (4.7%) and M. jannaschii (3.5%) which exhibit substantially lower proportions. (2) The distribution of topological types is similar in all seven organisms. (3) Transport systems belonging to 67 families were identified within the genomes of these organisms, and about half of these families are also found in eukaryotes. (4) 12% of these families are found exclusively in Gram-negative bacteria, but none is found exclusively in Gram-positive bacteria, cyanobacteria or archaea. (5) Two superfamilies, the ATP-binding cassette (ABC) and major facilitator (MF) superfamilies account for nearly 50% of all transporters in each organism, but the relative representation of these two transporter types varies over a tenfold range, depending on the organism. (6) Secondary, pmf-dependent carriers are 1.5 to threefold more prevalent than primary ATP-dependent carriers in E. coli, H. influenzae, H. pylori and B. subtilis while primary carriers are about twofold more prevalent in M. genitalium and Synechocystis. M. jannaschii exhibits a slight preference for secondary carriers. (7) Bioenergetics of transport generally correlate with the primary forms of energy generated via available metabolic pathways but ecological niche and substrate availability may also be determining factors. (8) All organisms display a similar range of transport specificities with quantitative differences presumably reflective of disparate ecological niches. (9) M. jannaschii and Synechocystis have a two to threefold increased proportion of transporters for inorganic ions with a concomitant decrease in transporters for organic compounds. (10) 6 to 18% of all transporters in these bacteria probably function as drug export systems showing that these systems are prevalent in non-pathogenic as well as pathogenic organisms. (11) All seven prokaryotes examined encode proteins homologous to known channel proteins, but none of the channel types identified occurs in all of these organisms. (12) The phosphoenolpyruvate:sugar phosphotransferase system is prevalent in the large genome organisms, E. coli and B. subtilis, and is present in the small genome organisms, H. influenzae and M. genitalium, but is totally lacking in H. pylori, Synechocystis and M. jannaschii. Details of the information summarized in this article are available on our web sites, and this information will be periodically updated and corrected as new sequence and biochemical data become available.

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.

Lactose carrier mutants of Escherichia coli with changes in sugar recognition (lactose versus melibiose)

The purpose of this research was to identify amino acid residues that mediate substrate recognition in the lactose carrier of Escherichia coli. The lactose carrier transports the alpha-galactoside sugar melibiose as well as the beta-galactoside sugar lactose. Mutants from cells containing the lac genes on an F factor were selected by the ability to grow on succinate in the presence of the toxic galactoside beta-thio-o-nitrophenylgalactoside. Mutants that grew on melibiose minimal plates but failed to grow on lactose minimal plates were picked. In sugar transport assays, mutant cells showed the striking result of having low levels of lactose downhill transport but high levels of melibiose downhill transport. Accumulation (uphill) of melibiose was completely defective in all of the mutants. Kinetic analysis of melibiose transport in the mutants showed either no change or a greater than normal apparent affinity for melibiose. PCR was used to amplify the lacY DNA of each mutant, which was then sequenced by the Sanger method. The following six mutations were found in the lacY structural genes of individual mutants: Tyr-26-->Asp, Phe-27-->Tyr, Phe-29-->Leu, Asp-240-->Val, Leu-321-->Gln, and His-322-->Tyr. We conclude from these experiments that Tyr-26, Phe-27, Phe-29 (helix 1), Asp-240 (helix 7), Leu-321, and His-322 (helix 10) either directly or indirectly mediate sugar recognition in the lactose carrier of E. coli.

Functional analysis of mutated purine-cytosine permease from Saccharomyces cerevisiae. A possible role of the hydrophilic segment 371-377 in the active carrier conformation

The purine-cytosine permease (PCP) is an active transporter located in the plasma membrane of the yeast Saccharomyces cerevisiae. This protein mediates purine (adenine, guanine, and hypoxanthine) and cytosine accumulation in the cell by using an electrochemical potential difference in proton as the energy source. Various mutant strains, with altered Kt(app) (apparent Michaelis constant of transport) of uptake for one or several bases, have already been selected. Their cloning and sequencing revealed that three of them presented substitutions in the same region of the putative sequence of the PCP: this region might correspond to the hydrophilic segment 371-377 (I-A-N-N-I-P-N). Two mutants displayed single mutations, resulting in only one amino acid residue change (N377I and N374I, respectively), and the other displayed three amino acid substitutions (I371V, I375V, and N377G). Therefore, to analyze the contribution of individual amino acid changes to the phenotype of the complex mutant, single (N377G) and double (I371V,I375V) mutants were constructed by site-directed mutagenesis. The influence of single mutations in this region was studied by measuring, for adenine, hypoxanthine, and cytosine, the uptake constants on cells and equilibrium binding parameters on plasma membrane-enriched fractions. Uptake and binding constant determinations showed that all the variations observed for the Kt(app) of uptake were correlated with variations of the binding Kd(app) for the corresponding solutes. Thus, our results emphasize the role of the two asparagine residues, located at positions 374 and 377, respectively, in the binding of the bases. In addition, the sole substitution of the 377 asparagine residue by glycine is responsible for the phenotype of the triple mutant. The effect of pH on the apparent hypoxanthine binding dissociation constant showed that the effects of N377G and N377I changes were, at least partially, due to a shift of the pKa of an ionizable amino acid residue of the unliganded permease. These two amino acid residue changes induced a shift of the pKa of this group in the unliganded, deprotonated permease about two units toward acidic pH. This result suggests that the 371-377 segment might play a key role in the proper three-dimensional structure of the active purine-cytosine permease.

Suppressor analysis of mutations in the loop 2-3 motif of lactose permease: evidence that glycine-64 is an important residue for conformational changes

A superfamily of transport proteins, which includes the lactose permease of Escherichia coli, contains a highly conserved motif, G-X-X-X-D/E-R/K-X-G-R/K-R/K, in the loops that connect transmembrane segments 2 and 3 and transmembrane segments 8 and 9. Previous analysis of this motif in the lactose permease (A. E. Jessen-Marshall, N. J. Paul, and R. J. Brooker, J. Biol. Chem. 270:16251-16257, 1995) has shown that the conserved glycine residue found at the first position in the motif (i.e., Gly-64) is important for transport function. Every substitution at this site, with the exception of alanine, greatly diminished lactose transport activity. In this study, three mutants in which glycine-64 was changed to cysteine, serine, and valine were used as parental strains to isolate 64 independent suppressor mutations that restored transport function. Of these 64 isolates, 39 were first-site revertants to glycine or alanine, while 25 were second-site mutations that restored transport activity yet retained a cysteine, serine, or valine at position 64. The second-site mutations were found to be located at several sites within the lactose permease (Pro-28 --> Ser, Leu, or Thr; Phe-29 --> Ser; Ala-50 --> Thr, Cys-154 --> Gly; Cys-234 --> Phe; Gln-241 --> Leu; Phe-261 --> Val; Thr-266 --> Iso; Val-367 --> Glu; and Ala-369 --> Pro). A kinetic analysis was conducted which compared lactose uptake in the three parental strains and several suppressor strains. The apparent Km values of the Cys-64, Ser-64, and Val-64 parental strains were 0.8 mM, 0.7 mM, and 4.6 mM, respectively, which was similar to the apparent Km of the wild-type permease (1.4 mM). In contrast, the Vmax values of the Cys-64, Ser-64, and Val-64 strains were sharply reduced (3.9, 10.1, and 13.2 nmol of lactose/min x mg of protein, respectively) compared with the wild-type strain (676 nmol of lactose/min x mg of protein). The primary effect of the second-site suppressor mutations was to restore the maximal rate of lactose transport to levels that were similar to the wild-type strains. Taken together, these results support the notion that Gly-64 in the wild-type permease is at a site in the protein which is important in facilitating conformational changes that are necessary for lactose translocation across the membrane. According to our tertiary model, this site is at an interface between the two halves of the protein.

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.

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.

Structural features of the uniporter/symporter/antiporter superfamily

The uniporter/symporter/antiporter superfamily is an evolutionarily related group of solute transporters. For the entire superfamily, we have used a new predictive program to identify the transmembrane domains. These transmembrane domains were then analyzed with regard to their overall hydrophobicity and amphipathicity. In addition, the lengths of the hydrophilic loops connecting the transmembrane domains were calculated. These data, together with structural information in the literature, were collectively used to produce a general model for the three-dimensional arrangement of the transmembrane domains.

Bacterial transporters

Recent experiments in bacterial systems have established an extended database of sequences broadly relevant to all membrane transporters, allowing serious study of evolutionary relationships. The database will be especially useful in integrating conclusions derived from work with proteins in the major facilitator superfamily, because this kinship includes both eukaryotic and prokaryotic model systems. Even among carriers not linked by evolution, clear hints of functional homology have been note. Advances are also evident in the structural analysis of membrane carriers. Site-directed mutagenesis in a bacterial antiporter has shown how the translocation pathway might be identified; this should complement recent progress in preparing two-dimensional crystals of the eukaryotic anion-exchange protein, band 3. Together, these studies could soon verify or reject the idea that the transport pathway lies at the interface between the amino-terminal and carboxy-terminal helical bundles found in the hydrophobic core of most carrier proteins. If verified, the argument might allow construction of informed three-dimensional models in the absence of crystallographic evidence.

Lactose permease mutants which transport (malto)-oligosaccharides

Lactose permease mutants, which were previously isolated in sugar specificity studies, were screened for their abilities to transport the trisaccharide maltotriose. Six multiple mutants (e.g., five double mutants and one triple mutant) were identified as forming fermentation-positive colonies on maltotriose MacConkey plates and were also shown to grow on maltotriose minimal plates. All of these multiple mutants contained a combination of two or three amino acid substitutions at position 177, 236, 306, or 322 within the permease. In contrast, none of the corresponding single mutants at these locations were observed to exhibit an enhanced rate of maltotriose transport. In whole-cell assays, the multiple mutants were shown to transport relatively long alpha-nitrophenylglucoside (alpha NPG) molecules. In certain cases, alpha NPG molecules containing up to four glucose residues in addition to the nitrophenyl group were shown to be transported to a significant degree. Overall, the abilities of lactose permease mutants to transport maltotriose and long alpha NPGs are discussed with regard to the dimensions of the sugar and the mechanism of sugar transport.

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+.

Isolation of lactose permease mutants which recognize arabinose

In the present study lactose permease mutants were isolated which recognize the monosaccharide, L-arabinose. Although the wild-type permease exhibits a poor recognition for L-arabinose, seven independent mutants were identified by their ability to grow on L-arabinose minimal plates. When subjected to DNA sequencing, it was found that all seven of these mutants were single-site mutations in which alanine 177 was changed to valine. The wild type and valine 177 mutant were then analyzed with regard to their abilities to recognize and transport monosaccharides and disaccharides. Free L-arabinose was shown to competitively inhibit [14C]-lactose transport yielding a Ki value of 121 mM for the Val177 mutant and a much higher value of 320 mM for the wild-type. Among several monosaccharides, D-glucose as well as L-arabinose inhibited lactose transport in the Val177 mutant to a significantly greater extent, while D-arabinose and D-xylose only caused a slight inhibition. On the other hand, kinetic studies with sugars which are normally recognized by the wild-type permease such as [14C]-galactose and [14C]-lactose revealed that the Val177 mutant and wild-type strains had similar transport characteristics for these two sugars. Overall, these results are consistent with the notion that the Val177 substitution causes an enhanced recognition for particular sugars (i.e. L-arabinose) but does not universally affect the recognition and unidirectional transport for all sugars. This idea is further supported by the observation that site-directed mutants containing isoleucine, leucine, phenylalanine, or proline at position 177 also were found to possess an enhanced recognition for L-arabinose.

Serine residues responsible for tetracycline transport are on a vertical stripe including Asp-84 on one side of transmembrane helix 3 in transposon Tn10-encoded tetracycline/H+ antiporter of Escherichia coli

Putative transmembrane helix 3 of the tetracycline/H+ antiporter encoded by a transposon, Tn10, contains four serine residues, Ser-77, Ser-82, Ser-91 and Ser-92. Each of these serine residues was replaced by site-directed mutagenesis. Of these four serine residues, Ser-77 was important for the transport function, and a bulky side chain at position 91 hindered substrate translocation, whereas Ser-82 and Ser-92 did not play any role. Ser-77 and Ser-91 are on the same vertical stripe, that includes the essential Asp-84, on the hydrophilic side of putative helix 3. These observations suggest that helix 3 is part of the tetracycline translocation channel across the membrane.

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.

Studies of translocation catalysis

There is a symbiotic relationship between the evolution of fundamental theory and the winning of experimentally-based knowledge. The impact of the General Chemiosmotic Theory on our understanding of the nature of membrane transport processes is described and discussed. The history of experimental studies on transport catalysed by ionophore antibiotics and the membrane proteins of mitochondria and bacteria are used to illustrate the evolution of knowledge and theory. Recent experimental approaches to understanding the lactose-H+ symport protein of Escherichia coli and other sugar porters are described to show that the lack of experimental knowledge of the three-dimensional structures of the proteins currently limits the development of theories about their molecular mechanism of translocation catalysis.

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.

Role of proline residues in the structure and function of a membrane transport protein

By use of site-directed mutagenesis, each prolyl residue in the lac permease of Escherichia coli at positions 28 (putative helix I), 31 (helix I), 61 (helix II), 89 (helix III), 97 (helix III), 123 (helix IV), 192 (putative hydrophilic region 7), 220 (helix VII), 280 (helix VIII), and 327 [helix X; Lolkema, J. S., et al. (1988) Biochemistry 27, 8307] was systematically replaced with Gly, Ala, or Leu or deleted by truncation of the C-terminus [i.e., Pro403 and Pro405; Roepe, P.D., et al. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 3992]. Replacements were chosen on the basis of side-chain helical propensity: Gly, like Pro, is thought to be a "helix breaker", while Ala and Leu are "helix makers". With the exception of Pro28, each prolyl residue can be replaced with Gly or Ala, and Pro403 and -405 can be deleted with the C-terminal tail, and significant lac permease activity is retained. In contrast, when Pro28 is replaced with Gly, Ala, or Ser, lactose transport is abolished, but permease with Ser28 binds p-nitrophenyl alpha-D-galactopyranoside and catalyzes active transport of beta-galactopyranosyl-1-thio-beta-D- galactopyranoside. Replacement of Pro28, -31, -123, -280, or -327 with Leu abolishes lactose transport, while replacement of Pro61, -89, -97, or -220 with Leu has relatively minor effects. None of the alterations in permease activity is due to inability of the mutant proteins to insert into the membrane or to diminished lifetimes after insertion, since the concentration of each mutant permease in the membrane is comparable to that of wild-type permease as judged by immunological analyses.(ABSTRACT TRUNCATED AT 250 WORDS)

Information content of amino acid residues in putative helix VIII of the lac permease from Escherichia coli

Mutants in putative helix VIII of lactose permease that retain the ability to accumulate lactose were created by cassette mutagenesis. A mutagenic insert encoding amino acid residues 259-278 was synthesized chemically by using reagents contaminated with 1% each of the other three bases and ligated into a KpnI/BclI site in the lacY gene in plasmid pGEM-4. Mutants that retain transport activity were selected by transforming a strain of Escherichia coli containing a wild-type lacZ gene, but deleted in lacY, with the mutant library and identifying colonies that transport lactose on indicator plates. Sequencing of the mutated region in lacY in 129 positive colonies reveals 43 single amino acid mutations at 26 sites and 26 multiple mutations. The variable amino acid positions are largely on one side of the putative alpha-helix, a stripe opposite Glu269. This mutable stripe of low information content is probably in contact with the membrane phospholipids.

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.

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

The polytopic membrane protein lac permease harnesses energy from the electrochemical H+ gradient to transport beta-galactosidases against a concentration gradient. Although high-resolution structural information is still lacking, the permease is thought to possess 12 membrane-spanning alpha-helical segments. Various experimental strategies, including site-directed mutagenesis, have been employed to probe the function of this membrane protein at the molecular level.

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 lac permease of Escherichia coli: site-directed mutagenesis studies on the mechanism of beta-galactoside/H+ symport

In this communication, we summarize site-directed mutagenesis studies of the lac permease from Escherichia coli, a prototypic H(+)-coupled active transport protein. We classify mutant permeases by phenotype, and suggest possible roles for some individual residues in the mechanism of H+/lactose symport. Although high-resolution structural information is not presently available, kinetic analysis of the partial reactions catalysed by the mutant permeases, as well as biophysical studies, suggest an evolving model for the mechanism of H+/lactose symport.