lac permease of Escherichia coli: histidine-205 and histidine-322 play different roles in lactose/H+ symport

Secondary Active Transporters

Transport of solutes across biological membranes is essential for cellular life. This process is mediated by membrane transport proteins which move nutrients, waste products, certain drugs and ions into and out of cells. Secondary active transporters couple the transport of substrates against their concentration gradients with the transport of other solutes down their concentration gradients. The alternating access model of membrane transporters and the coupling mechanism of secondary active transporters are introduced in this book chapter. Structural studies have identified typical protein folds for transporters that we exemplify by the major facilitator superfamily (MFS) and LeuT folds. Finally, substrate binding and substrate translocation of the transporters LacY of the MFS and AdiC of the amino acid-polyamine-organocation (APC) superfamily are described.

Structure of LacY with an α-substituted galactoside: Connecting the binding site to the protonation site

The X-ray crystal structure of a conformationally constrained mutant of the Escherichia coli lactose permease (the LacY double-Trp mutant Gly-46→Trp/Gly-262→Trp) with bound p-nitrophenyl-α-d-galactopyranoside (α-NPG), a high-affinity lactose analog, is described. With the exception of Glu-126 (helix IV), side chains Trp-151 (helix V), Glu-269 (helix VIII), Arg-144 (helix V), His-322 (helix X), and Asn-272 (helix VIII) interact directly with the galactopyranosyl ring of α-NPG to provide specificity, as indicated by biochemical studies and shown directly by X-ray crystallography. In contrast, Phe-20, Met-23, and Phe-27 (helix I) are within van der Waals distance of the benzyl moiety of the analog and thereby increase binding affinity nonspecifically. Thus, the specificity of LacY for sugar is determined solely by side-chain interactions with the galactopyranosyl ring, whereas affinity is increased by nonspecific hydrophobic interactions with the anomeric substituent.

A chemiosmotic mechanism of symport

Lactose permease (LacY), a paradigm for the largest family of membrane transport proteins, catalyzes the coupled translocation of a galactoside and an H(+) across the Escherichia coli membrane (galactoside/H(+) symport). Initial X-ray structures reveal N- and C-terminal domains, each with six largely irregular transmembrane helices surrounding an aqueous cavity open to the cytoplasm. Recently, a structure with a narrow periplasmic opening and an occluded galactoside was obtained, confirming many observations and indicating that sugar binding involves induced fit. LacY catalyzes symport by an alternating access mechanism. Experimental findings garnered over 45 y indicate the following: (i) The limiting step for lactose/H(+) symport in the absence of the H(+) electrochemical gradient (∆µ̃H+) is deprotonation, whereas in the presence of ∆µ̃H+, the limiting step is opening of apo LacY on the other side of the membrane; (ii) LacY must be protonated to bind galactoside (the pK for binding is ∼10.5); (iii) galactoside binding and dissociation, not ∆µ̃H+, are the driving forces for alternating access; (iv) galactoside binding involves induced fit, causing transition to an occluded intermediate that undergoes alternating access; (v) galactoside dissociates, releasing the energy of binding; and (vi) Arg302 comes into proximity with protonated Glu325, causing deprotonation. Accumulation of galactoside against a concentration gradient does not involve a change in Kd for sugar on either side of the membrane, but the pKa (the affinity for H(+)) decreases markedly. Thus, transport is driven chemiosmotically but, contrary to expectation, ∆µ̃H+ acts kinetically to control the rate of the process.

Function, Structure, and Evolution of the Major Facilitator Superfamily: The LacY Manifesto

The major facilitator superfamily (MFS) is a diverse group of secondary transporters with members found in all kingdoms of life. A paradigm for MFS is the lactose permease (LacY) of Escherichia coli, which couples the stoichiometric translocation of a galactopyranoside and an H+ across the cytoplasmic membrane. LacY has been the test bed for the development of many methods applied for the analysis of transport proteins. X-ray structures of an inward-facing conformation and the most recent structure of an almost occluded conformation confirm many conclusions from previous studies. Although structure models are critical, they are insufficient to explain the catalysis of transport. The clues to understanding transport are based on the principles of enzyme kinetics. Secondary transport is a dynamic process—static snapshots of X-ray crystallography describe it only partially. However, without structural information, the underlying chemistry is virtually impossible to conclude. A large body of biochemical/biophysical data derived from systematic studies of site-directed mutants in LacY suggests residues critically involved in the catalysis, and a working model for the symport mechanism that involves alternating access of the binding site is presented. The general concepts derived from the bacterial LacY are examined for their relevance to other MFS transporters.

Structure of sugar-bound LacY

Here we describe the X-ray crystal structure of a double-Trp mutant (Gly46→Trp/Gly262→Trp) of the lactose permease of Escherichia coli (LacY) with a bound, high-affinity lactose analog. Although thought to be arrested in an open-outward conformation, the structure is almost occluded and is partially open to the periplasmic side; the cytoplasmic side is tightly sealed. Surprisingly, the opening on the periplasmic side is sufficiently narrow that sugar cannot get in or out of the binding site. Clearly defined density for a bound sugar is observed at the apex of the almost occluded cavity in the middle of the protein, and the side chains shown to ligate the galactopyranoside strongly confirm more than two decades of biochemical and spectroscopic findings. Comparison of the current structure with a previous structure of LacY with a covalently bound inactivator suggests that the galactopyranoside must be fully ligated to induce an occluded conformation. We conclude that protonated LacY binds D-galactopyranosides specifically, inducing an occluded state that can open to either side of the membrane.

Evolutionary mix-and-match with MFS transporters II

One fundamentally important problem for understanding the mechanism of coupling between substrate and H(+) translocation with secondary active transport proteins is the identification and physical localization of residues involved in substrate and H(+) binding. This information is exceptionally difficult to obtain with the Major Facilitator Superfamily (MFS) because of the broad sequence diversity of the members. The MFS is the largest and most diverse group of transporters, many of which are clinically important, and includes members from all kingdoms of life. A wide range of substrates is transported, in many instances against a concentration gradient by transduction of the energy stored in an H(+) electrochemical gradient using symport mechanisms, which are discussed herein. Crystallographic structures of MFS members indicate that a deep central hydrophilic cavity surrounded by 12 mostly irregular transmembrane helices represents a common structural feature. An inverted triple-helix structural symmetry motif within the N- and C-terminal six-helix bundles suggests that the proteins may have arisen by intragenic multiplication. In the work presented here, the triple-helix motifs are aligned in combinatorial fashion so as to detect functionally homologous positions with known atomic structures of MFS members. Substrate and H(+)-binding sites in symporters that transport substrates, ranging from simple ions like phosphate to more complex peptides or disaccharides, are found to be in similar locations. It also appears likely that there is a homologous ordered kinetic mechanism for the H(+)-coupled MFS symporters.

Functional role of conserved transmembrane segment 1 residues in human sodium-dependent vitamin C transporters

Sodium-dependent vitamin C transporters, SVCT1 and SVCT2, are the only two known proteins for the uptake of ascorbate, the active form of vitamin C. Little structural information is available for SVCTs, although a transport activity increase from pH 5.5 to 7.5 suggests a functional role of one or more conserved histidines (p K a approximately 6.5). Confocal fluorescence microscopy and uptake kinetic analyses were used here to characterize cells transfected with mutants of EGFP-tagged hSVCTs. Mutating any of the four conserved histidine residues (His51, 147, 210, or 354) in hSVCT1 to alanine did not affect the apical membrane localization in polarized MDCK cells. His51Ala (in putative transmembrane segment 1, TM1) was the only mutation that resulted in a significant loss of ascorbate transport and an increase in apparent Km with no significant effect on Vmax. The corresponding mutation in hSVCT2, His109Ala, also led to a loss of transport activity. Among eight other mutations of His51 in hSVCT1, significant sodium-dependent ascorbate transport activity was only observed with asparagine or tyrosine replacement. Thus, our results suggest that uncharged His51, directly or indirectly, contributes to substrate binding through the hydrogen bond. His51 cannot account for the observed pH dependence as neutral amino acid substitutions failed to abolish the pH-dependent activity increase. The importance of TM1 is further strengthened by the comparable loss of sodium-dependent ascorbate transport activity upon the mutation of adjacent conserved Gln50 and the apparent change in substrate specificity in the hSVCT1-His51Gln mutation, which showed a specific increase in sodium-independent dehydroascorbate transport.

Lactose permease H+-lactose symporter: mechanical switch or Brownian ratchet?

Lactose permease structure is deemed consistent with a mechanical switch device for H(+)-coupled symport. Because the crystallography-assigned docking position of thiodigalactoside (TDG) does not make close contact with several amino acids essential for symport; the switch model requires allosteric interactions between the proton and sugar binding sites. The docking program, Autodock 3 reveals other lactose-docking sites. An alternative cotransport mechanism is proposed where His-322 imidazolium, positioned in the central pore equidistant (5-7 A) between six charged amino acids, Arg-302 and Lys-319 opposing Glu-269, Glu-325, Asp-237, and Asp-240, transfers a proton transiently to an H-bonded lactose hydroxyl group. Protonated lactose and its dissociation product H(3)O+ are repelled by reprotonated His-322 and drift in the electrostatic field toward the cytosol. This Brownian ratchet model, unlike the conventional carrier model, accounts for diminished symport by H322N mutant; how H322 mutants become uniporters; why exchanging Lys-319 with Asp-240 paradoxically inactivates symport; how some multiple mutants become revertant transporters; the raised export rate and affinity toward lactose of uncoupled mutants; the altered specificity toward lactose, melibiose, and galactose of some mutants, and the proton dissociation rate of H322 being 100-fold faster than the symport turnover rate.

Conservation of residues involved in sugar/H(+) symport by the sucrose permease of Escherichia coli relative to lactose permease

Building a three-dimensional model of the sucrose permease of Escherichia coli (CscB) with the X-ray crystal structure lactose permease (LacY) as template reveals a similar overall fold for CscB. Moreover, despite only 28% sequence identity and a marked difference in substrate specificity, the structural organization of the residues involved in sugar-binding and H(+) translocation is conserved in CscB. Functional analyses of mutants in the homologous key residues provide strong evidence that they play a similar critical role in the mechanisms of CscB and LacY.

Lessons from lactose permease

An X-ray structure of the lactose permease of Escherichia coli (LacY) in an inward-facing conformation has been solved. LacY contains N- and C-terminal domains, each with six transmembrane helices, positioned pseudosymmetrically. Ligand is bound at the apex of a hydrophilic cavity in the approximate middle of the molecule. Residues involved in substrate binding and H+ translocation are aligned parallel to the membrane at the same level and may be exposed to a water-filled cavity in both the inward- and outward-facing conformations, thereby allowing both sugar and H+ release directly into either cavity. These structural features may explain why LacY catalyzes galactoside/H+ symport in both directions utilizing the same residues. A working model for the mechanism is presented that involves alternating access of both the sugar- and H+-binding sites to either side of the membrane.

Suppressor scanning at positions 177 and 236 in the Escherichia coli lactose/H+ cotransporter and stereotypical effects of acidic substituents that suggest a favored orientation of transmembrane segments relative to the lipid bilayer

Acidic substituents for Ala-177 (helix 6) or Tyr-236 (helix 7) in LacY cause effects on sugar recognition and cosubstrate coupling that are stereotypical of neutral substituents. Thus, helices 6 and 7 are probably oriented to produce little side-chain contact with the low dielectric lipid bilayer at positions 177 and 236.

Mutational analysis of histidine residues in the rabbit Na+/dicarboxylate co-transporter NaDC-1

Succinate transport by the rabbit Na+/dicarboxylate co-transporter, NaDC-1, expressed in Xenopus oocytes was inhibited by the histidyl-selective reagent diethyl pyrocarbonate (DEPC). Therefore the role of histidine residues in the function of NaDC-1 was examined by site-directed mutagenesis. All 11 histidine residues in NaDC-1 were converted to alanine, but only mutant H106A exhibited a decrease in succinate transport. Additional mutations of NaDC-1 at position 106 showed that aspartic acid and asparagine, but not arginine, can substitute for histidine. Examination of succinate and citrate kinetics of H106A revealed a decrease in Vmax with no change in Km. Cell surface biotinylation experiments showed that the transport activity of all four mutants at position 106 was correlated with the amount of cell surface expression, suggesting a role of His-106 in membrane expression rather than function. Two of the histidine mutants, H153A and H569A, exhibited insensitivity to inhibition by DEPC, indicating that these residues are involved in binding DEPC. Neither of these residues is required for transport activity; thus DEPC probably inhibits NaDC-1 function by hindrance of the mobility of the carrier. We conclude that histidine residues are not critical for transport function in NaDC-1, although His-106 might be involved in determining protein expression or stability in the membrane.

Primary structure and properties of the Na+/glucose symporter (Sg1S) of Vibrio parahaemolyticus

Previously, we cloned and sequenced a DNA fragment from Vibrio parahaemolyticus and found four open reading frames (ORFs). Here, we clearly demonstrate that one of the ORFs, ORF1, is the gene (sglS) encoding a Na+/glucose symporter (SglS). We characterize the Na+/glucose symporter produced in Escherichia coli mutant (JM1100) cells which lack original glucose transport activity and galactose transport activity. We also show that phlorizin, a potent inhibitor of the SGLT1 Na+/glucose symporter of animal cells, inhibited glucose transport, but not galactose transport, via the SglS system.

Physiological evidence for an interaction between Glu-325 and His-322 in the lactose carrier of Escherichia coli

Site-directed mutagenesis and second-site suppressor analysis have proven to be useful approaches to examine the role of charged amino acids in the structure and function of the lactose carrier of Escherichia coli. A lactose carrier mutant Glu-325 --> Ser failed to ferment melibiose and showed white clones on melibiose MacConkey indicator plates. Several red revertants were isolated from these plates. Two of these revertants showed a double mutation, the original mutation (Glu-325 --> Ser) plus His-322 --> Asp. Seven revertants showed a second site mutation His-322 --> Asn. Although the second site revertants failed to accumulate sugars they do show more rapid uptake of melibiose into cells containing alpha-galactosidase than the original mutant Glu-325 --> Ser. The complete loss of transport activity due to the removal of the negative charge at 325 can be partially compensated for by the introduction of a new negative charge at 322. A site-directed double mutant His-322 --> Asn/Glu-325 --> Asn showed a greater rate of lactose uptake (Vmax) than either of the single mutants His-322 --> Asn or Glu 325 --> Asn. It was concluded that there is some type of physiological interaction (possibly a salt bridge) between His-322 and Glu-325.

Friction analysis of kinetic schemes: the friction coefficient

Friction analysis is proposed as the application of general control analysis to single enzymes to describe the control of elementary kinetic steps on the overall catalytic rate. For each transition, a friction coefficient is defined that measures the sensitivity of the turnover rate to the free energy of the transition state complex of the transition. The latter is captured in a single property of the transition, termed friction, as the geometrical mean of the inverse of the forward and backward rate constants. By definition, the friction coefficient measures the relative change in the turnover rate in response to a small change in the friction. The friction coefficient is the sum of the flux control coefficients of the forward and backward rate constants from general control theory and measures the extent to which an elementary step is rate determining. Two basic rules apply to the friction coefficients: (i) the summation theorem states that summation of the friction coefficients over all the steps in a scheme results in a value of 1, and (ii) the group rule states that grouping of rate constants of similar transitions results in a friction coefficient for the group that is the sum of the friction coefficients of the individual steps in the group. The friction coefficients are derived for a number a kinetic schemes taking the rate equations as the starting point and both rules are demonstrated. In fully coupled systems the friction coefficients of individual steps lie between 0 and 1. In partially uncoupled systems the summation theorem applies to all the rates in the system, however, the summation of subsets of friction coefficients may exceed the value of one, implying negative values for other steps in the scheme. The values of individual friction coefficients lie between -1 and 1. The friction coefficient is redefined in a numerical treatment of the steady state of more complex enzymatic schemes.

Dynamics of lactose permease of Escherichia coli determined by site-directed chemical labeling and fluorescence spectroscopy

Mutants with a single Cys residue in place of Phe27, Pro28, Phe29, Phe30, or Pro31 at the periplasmic end of putative transmembrane helix I were used to study the interaction of lactose permease with ligand by site-directed chemical modification or fluorescence spectroscopy. With permease embedded in the native membrane, mutant Phe27-->Cys or Phe28-->Cys is readily labeled with [14C]-N-ethylmaleimide (NEM), while mutant Phe29-->Cys, Phe30-->Cys, or Phe31-->Cys reacts less effectively. beta,D-Galactopyranosyl 1-thio-beta,D-galactopyranoside (TDG) has little or no effect on the reactivity of Phe27-->Cys, Phe29-->Cys, or Phe30-->Cys permease. Remarkably, however, Pro31-->Cys permease which is essentially unreactive in the absence of ligand becomes highly reactive in the presence of TDG. Ligand also enhances the NEM reactivity of the mutant with Cys in place of Pro28 which is presumably on the same face of helix I as position 31. The five single-Cys mutants which also contain a biotin acceptor domain in the middle cytoplasmic loop were purified by monomeric avidin-affinity chromatography in dodecyl beta,D-maltoside and subjected to site-directed fluorescence spectroscopy. Mutants Phe27-->Cys, Phe29-->Cys, and Phe30-->Cys react rapidly with 2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid (MIANS), and reactivity is not altered in the presence of TDG. In striking contrast, mutants Pro28-->Cys and Pro31-->Cys react extremely slowly with MIANS in the absent of ligand, and TDG dramatically enhances reactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic analysis of lactose and proton coupling in Glu379 mutants of the lactose transport protein of Streptococcus thermophilus

The role of Glu379 in the lactose-H+ symport protein (LacS) of Streptococcus thermophilus was studied by analyzing the kinetic mechanism of transport of wild-type and Ala379, Asp379, and Gln379 mutant proteins. Glu379 forms part of the sequence motif Lys-X-X-His-X-X-Glu that is present in a number of sugar transport proteins, including LacY of Escherichia coli. The E379A and E379Q mutants were defective in the uptake of lactose against a concentration gradient and lactose-dependent proton uptake, but catalyzed facilitated influx of lactose down a concentration gradient and equilibrium exchange with rates similar to that of the wild-type enzyme. The E379D mutant was partially defective in the coupled transport of lactose and protons. These results suggest that an acidic residue at position 379 is required for the coupled uptake of lactose and protons and are consistent with a mechanism in which lactose transport in the E379A and E379Q mutants occurs by uniport rather than proton symport. Lactose efflux down a concentration gradient in wild-type LacS and LacS-E379D increased with pH with apparent pK (pKa) values of > or = 8.5 and 8.0, respectively, whereas efflux in the E379Q mutant increased sigmoidally with a pKa of about 6.0. Imposition of an artificial membrane potential (inside negative) in membrane vesicles bearing wild-type LacS or LacS-E379Q not only inhibited the lactose efflux mediated by wild-type but also that of the mutant enzyme. To associate the role of Glu379 with specific step(s) in the translocation cycle of LacS, the properties of wild-type LacS and the Glu379 mutants have been evaluated by numerical analysis of simple kinetic schemes for translocation catalysis by solute H+ symport proteins. The properties of the wild-type enzyme are consistent with a mechanism in which the order of ligand binding on the inside is substrate first and proton last, whereas the order is random (or proton first, substrate last) at the outer surface of the membrane. The wild-type enzyme is asymmetric with regard to proton binding; the pK for proton binding on the outside is at least 4 units higher than the pK on the inside. The properties of the Glu379 mutants correspond with a lowering of the pK on the outside (pKOUT approximately pKIN), and the induction of a leak pathway in which the binary enzyme-substrate complex becomes mobile.

The role of transmembrane domain III in the lactose permease of Escherichia coli

Deletion of putative transmembrane helix III from the lactose permease of Escherichia coli results in complete loss of transport activity. Similarly, replacement of this region en bloc with 23 contiguous Ala, Leu, or Phe residues abolishes active lactose transport. The observations suggest that helix III may contain functionally important residues; therefore, this region was subjected to Cys-scanning mutagenesis. Using a functional mutant devoid of Cys residues (C-less permease) each residue from Tyr 75 to Leu 99 was individually replaced with Cys. Twenty-one of the 25 mutants accumulate lactose to > 70% of the steady-state exhibited by C-less permease, and an additional 3 mutants transport to lower, but significant levels (40-60% of C-less). Cys replacement for Leu 76 results in low transport activity (18% of C-less). However, when placed in the wild-type background, mutant Leu 76-->Cys exhibits highly significant rates of transport (55% of wild type) and steady-state levels of lactose accumulation (65% of wild type). Immunoblots reveal that the mutants are inserted into the membrane at concentrations comparable to wild type. Studies with N-ethylmaleimide show that mutant Gly 96-->Cys is rapidly inactivated, whereas the other single-Cys mutants are not altered significantly by the alkylating agent. Moreover, the rate of inactivation of Gly 96-->Cys permease is enhanced at least 2-fold in the presence of beta-galactopyranosyl 1-thio-beta, D-galactopyranoside. The observations demonstrate that although no residue per se appears to be essential, structural properties of helix III are important for active lactose transport.

Cysteine-scanning mutagenesis of putative helix VII 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 helix VII and the flanking cytoplasmic and periplasmic regions (from Leu212 to Glu255) was replaced individually with Cys. Of the 44 single-Cys mutants, 40 exhibit high transport activity, accumulating lactose to > 50% of the steady-state observed with C-less permease. In contrast, permease with Cys in place of Ala213 or Tyr236 exhibits low but significant activity, and Cys substitution for Asp237 or Asp240 yields permease molecules with little or no activity due to disruption of charge-neutralizing interactions between Asp237 and Lys358 or Asp240 and Lys319, respectively. Immunological analysis reveals that membrane levels of the mutant proteins are comparable to that of C-less permease with the exception of Tyr228-->Cys, which exhibits reduced but significant levels of permease. Finally, the effect of N-ethylmaleimide (NEM) was tested on each mutant, and the results indicate that the transport activity of the great majority of the mutants is not affected by the alkylating agent. Remarkably, the six positions where Cys replacements render the permease highly sensitive to inactivation by NEM are confined to the C-terminal half of helix VII, a region that is strongly conserved among transport proteins homologous to lactose permease. The results demonstrate that although no residue per se in the region scanned is essential, structural features of the C terminus of helix VII may be important for transport activity.

A conformational change in the lactose permease of Escherichia coli is induced by ligand binding or membrane potential

Lactose transport in membrane vesicles containing lactose permease with a single Cys residue in place of Val 315 is inactivated by N-ethylmaleimide in a manner that is stimulated by substrate or by a H+ electrochemical gradient (delta microH+; Sahin-Tóth M, Kaback HR, 1993, Protein Sci 2:1024-1033). The findings are confirmed and extended in this communication. Purified, reconstituted Val 315-->Cys permease reacts with N-ethylmaleimide or hydrophobic fluorescent maleimides but not with a membrane impermeant thiol reagent, and beta-galactosides specifically stimulate the rate of labeling. Furthermore, the reactivity of purified Val 315-->Cys permease is enhanced by imposition of a membrane potential (delta psi, interior negative). The results indicate that either ligand binding or delta psi induces a conformational change in the permease that brings the N-terminus of helix X into an environment that is more accessible from the lipid phase.

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.

What's new with lactose permease

The lactose permease of Escherichia coli is a paradigm for polytopic membrane transport proteins that transduce free energy stored in an electrochemical ion gradient into work in the form of a concentration gradient. Although the permease consists of 12 hydrophobic transmembrane domains in probable alpha-helical conformation that traverse the membrane in zigzag fashion connected by hydrophilic "loops", little information is available regarding the folded tertiary structure of the molecule. In a recent approach site-directed fluorescence labeling is being used to study proximity relationships in lactose permease. The experiments are based upon site-directed pyrene labeling of combinations of paired Cys replacements in a mutant devoid of Cys residues. Since pyrene exhibits excimer fluorescence if two molecules are within about 3.5A, the proximity between paired labeled residues can be determined. The results demonstrate that putative helices VIII and IX are close to helix X. Taken together with other findings indicating that helix VII is close to helices X and XI, the data lead to a model that describes the packing of helices VII to XI.

Use of site-directed fluorescence labeling to study proximity relationships in the lactose permease of Escherichia coli

The lactose permease of Escherichia coli is a paradigm for polytopic membrane transport proteins that transduce free energy stored in an electrochemical ion gradient into work in the form of a concentration gradient. Although the permease consists of 12 hydrophobic transmembrane domains in probable alpha-helical conformation that traverse the membrane in zigzag fashion connected by hydrophilic "loops", little information is available regarding the folded tertiary structure of the molecule. In this paper, we describe an approach to studying proximity relationships in lactose permease that is based upon site-directed pyrene labeling of combinations of paired Cys replacements in a mutant devoid of Cys residues. Since pyrene exhibits excimer fluorescence if two molecules are within about 3.5 A, the proximity between paired labeled residues can be determined. The results demonstrate that putative helices VIII and IX are close to helix X. Taken together with other findings indicating that helix VII is close to helices X and XI, the data lead to a model that describes the packing of helices VII-XI.

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+.

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)

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.

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.

Lac permease of Escherichia coli: on the path of the proton

Lactose/H+ symport in Escherichia coli is catalysed by a hydrophobic transmembrane protein encoded by the lacY gene that has been purified to homogeneity, reconstituted into proteoliposomes and shown to be completely functional as a monomer. Circular dichroic studies and hydropathy profiling of the amino-acid sequence of this 'lac' permease suggest a secondary structure in which the polypeptide consists of 12 hydrophobic segments in alpha-helical conformation that traverse the membrane in zig-zag fashion connected by shorter, hydrophilic domains with most of the charged residues and many of the residues commonly found in beta-turns. Support for certain general aspects of the model has been obtained from other biophysical studies, as well as biochemical, immunological and genetic approaches. Oligonucleotide-directed, site-specific mutagenesis is currently being utilized to probe the structure and function of the permease. Application of the technique provides an indication that Arg302 (putative helix IX), His322 (putative helix X) and Glu325 (putative helix X) may be sufficiently close to hydrogen-bond and that these residues play a critical role in lactose-coupled H+ translocation, possibly as components of a charge-relay type of mechanism. In contrast, Cys residues, which were long thought to play a central role in the mechanism of lactose/H+ symport, do not appear to be involved in either substrate binding or H+ translocation.

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

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