Oligonucleotide-directed site-specific mutagenesis of the lac permease of Escherichia coli

Site-directed mutagenesis in photosystem II of the cyanobacterium Synechocystis sp. PCC 6803: Donor D is a tyrosine residue in the D2 protein

The chemical nature of electron donor(s) in photosystem II in photosynthetic membranes was analyzed by site-directed mutagenesis of the gene encoding the protein D2 of the photosystem II reaction center. Mutation of the Tyr-160 residue of the D2 protein into phenylalanine results in the disappearance of the electron paramagnetic resonance signal II(S) originating from D(+), the oxidized form of the slow photosystem II electron donor D. Signal II(S) is still present if a neighboring residue in D2, Met-159, is mutated into arginine. Both mutants have normal rereduction kinetics of the oxidized primary electron donor, P680(+), in octyl glucoside-extracted thylakoids, indicating that D is not directly involved in P680(+) reduction. However, overall photosystem II activity appears to be impaired in the Tyr-160-Phe mutant: photosystem II-dependent growth of this mutant is slowed down by a factor of 3-4, whereas photoheterotrophic growth rates in wild type and mutant are essentially identical. Binding studies of diuron, a photosystem II herbicide, show that there is no appreciable decrease in the number of photosystem II centers in the Tyr-160-Phe mutant. The decrease in photosystem II activity in this mutant may be interpreted to indicate a role of D in photoactivation, rather than one as an important redox intermediate in the photosynthetic electron-transport chain.

Helix packing of lactose permease in Escherichia coli studied by site-directed chemical cleavage

Biotinylated lactose permease from Escherichia coli containing a single-cysteine residue at position 330 (helix X) or at position 147, 148, or 149 (helix V) was purified by avidin-affinity chromatography and derivatized with 5-(alpha-bromoacetamido)-1,10-phenanthroline-copper [OP(Cu)]. Studies with purified, OP(Cu)-labeled Leu-330 --> Cys permease in dodecyl-beta-D-maltopyranoside demonstrate that after incubation in the presence of ascorbate, cleavage products of approximately 19 and 6-8 kDa are observed on immunoblots with anti-C-terminal antibody. Remarkably, the same cleavage products are observed with permease embedded in the native membrane. Comparison with the C-terminal half of the permease expressed independently as a standard indicates that the 19-kDa product results from cleavage near the cytoplasmic end of helix VII, whereas the 6- to 8-kDa fragment probably results from fragmentation near the cytoplasmic end of helix XI. Results are entirely consistent with a tertiary-structure model of the C-terminal half of the permease derived from earlier site-directed fluorescence and site-directed mutagenesis studies. Similar studies with OP(Cu)-labeled Cys-148 permease exhibit cleavage products at approximately 19 kDa and at 15-16 kDa. The larger fragment probably reflects cleavage at a site near the cytoplasmic end of helix VII, whereas the 15- to 16-kDa fragment is consistent with cleavage near the cytoplasmic end of helix VIII. When OP(Cu) is moved 100 degrees to position 149 (Val-149 --> Cys permease), a single product is observed at 19 kDa, suggesting fragmentation at the cytoplasmic end of helix VII. However, when the reagent is moved 100 degrees in the other direction to position 147 (Gly-147 --> Cys permease), cleavage is not observed. The results suggest that helix V is in close proximity to helices VII and VIII with position 148 in the interface between the helices, position 149 facing helix VII, and position 147 facing the lipid bilayer.

A functional superfamily of sodium/solute symporters

Eleven families of sodium/solute symporters are defined based on their degrees of sequence similarities, and the protein members of these families are characterized in terms of their solute and cation specificities, their sizes, their topological features, their evolutionary relationships, and their relative degrees and regions of sequence conservation. In some cases, particularly where site-specific mutagenesis analyses have provided functional information about specific proteins, multiple alignments of members of the relevant families are presented, and the degrees of conservation of the mutated residues are evaluated. Signature sequences for several of the eleven families are presented to facilitate identification of new members of these families as they become sequenced. Phylogenetic tree construction reveals the evolutionary relationships between members of each family. One of these families is shown to belong to the previously defined major facilitator superfamily. The other ten families do not show sufficient sequence similarity with each other or with other identified transport protein families to establish homology between them. This study serves to clarify structural, functional and evolutionary relationships among eleven distinct families of functionally related transport proteins.

Analysis of chimeric spinach/cyanobacterial CP43 mutants of Synechocystis sp. PCC 6803: the chlorophyll-protein CP43 affects the water-splitting system of Photosystem II

Mutants of the cyanobacterium Synechocystis sp. PCC 6803 have been generated in which parts of psbC (the gene encoding the Photosystem II chlorophyll-protein CP43) have been replaced with the homologous gene fragment from spinach. Upon the replacement of all but the 3' 84 bp of the cyanobacterial psbC gene with the homologous fragment from spinach, an obligate photoheterotrophic mutant was generated. Two photoautotrophic derivatives of this mutant were made reincorporating 3' cyanobacterial sequences back into the spinach psbC gene of the mutant. These two mutants are similar to each other, carrying a chimeric CP43 with the N-terminal half from spinach. These mutants are photosynthetically active at a rate of about half that of wild type, which correlates with a decreased Photosystem II/chlorophyll ratio in these mutants. Thylakoids from the chimeric mutants contain a CP43 protein which migrates slightly more slowly on SDS-polyacrylmide gels than the native Synechocystis CP43. Interestingly, these mutants show significant shifts in thermoluminescence peaks, reflecting altered thermodynamic properties of the back reaction between the acceptor side and the water-splitting system. On the basis of the oscillations of these shifts with number of flashes, we conclude that S2 is stabilized and S3 is destabilized in these mutants. This represents evidence for an involvement of CP43 in events associated with water splitting.

What we can learn from the effects of thiol reagents on transport proteins

Many secondary membrane transport systems contain reactive sulfhydryl groups. In this review the applications of SH reagents for analyzing the role of sulfhydryl groups in membrane transport systems will be discussed. First an overview will be given of the more important reagents, that have been used to study SH-groups in membrane transport systems, and examples will be given of transport proteins in which the role of cysteines have been analyzed. An important application of SH-reagents to label transport proteins using various SH-reagents modified with fluorescent- or spin-label moieties will be discussed. Two general models are shown which have been proposed to explain the role of sulfhydryl groups in some membrane transport systems.

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.

Activation and detection of (pro)mutagenic chemicals using recombinant strains of Streptomyces griseus

Two recombinant strains of Streptomyces griseus have been developed to report on the activation of promutagenic chemicals. This activation is monitored by reversion of the bacterial test strains to a kanamycin-resistant phenotype. Strain H69 detects point mutations and was reverted at an increased frequency by acetonitrile, 2-aminoanthracene, 1,2-benzanthracene, benzidine, benzo(a)pyrene, 9,10-dimethyl-1,2-benzanthracene, and glycine. The second strain, FS2, detects frame shift mutations and was reverted at an increased frequency by 1,2-benzanthracene, benzidine, and glycine. Compounds such as butylated hydroxytoluene, catechol, chlorobenzene, hydroquinone, potassium chloride, phenol, cis-stilbene, trans-stilbene, and toluene did not elicit positive responses in either strain. In addition, these strains are capable of detecting direct-acting mutagens such as N-methyl-N'-nitrosoguanidine and ICR-191, providing further evidence of their promise for detecting a wider range of mutagens. To our knowledge, this is the first report of bacterial strains capable of activating promutagenic compounds and detecting their mutagenic metabolites without the benefit of an exogenous activation system such as the rodent liver homogenate (S9).

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

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

Translation initiation in Drosophila melanogaster is reduced by mutations upstream of the AUG initiator codon

The importance to in vivo translation of sequences immediately upstream of the Drosophila alcohol dehydrogenase (Adh) start codon was examined at two developmental stages. Mutations were introduced into the Adh gene in vitro, and the mutant gene was inserted into the genome via germ line transformation. An A-to-T substitution at the -3 position did not affect relative translation rates of the ADH protein at the second-instar larval stage but resulted in a 2.4-fold drop in translation of ADH at the adult stage. A second mutant gene, containing five mutations in the region -1 to -9, was designed to completely block translation initiation. However, transformant lines bearing these mutations still exhibit detectable ADH, albeit at substantially reduced levels. The average fold reduction at the second-instar larval stage was 5.9, while at the adult stage a 12.5-fold reduction was observed.

Translation initiation in Drosophila melanogaster is reduced by mutations upstream of the AUG initiator codon

The importance to in vivo translation of sequences immediately upstream of the Drosophila alcohol dehydrogenase (Adh) start codon was examined at two developmental stages. Mutations were introduced into the Adh gene in vitro, and the mutant gene was inserted into the genome via germ line transformation. An A-to-T substitution at the -3 position did not affect relative translation rates of the ADH protein at the second-instar larval stage but resulted in a 2.4-fold drop in translation of ADH at the adult stage. A second mutant gene, containing five mutations in the region -1 to -9, was designed to completely block translation initiation. However, transformant lines bearing these mutations still exhibit detectable ADH, albeit at substantially reduced levels. The average fold reduction at the second-instar larval stage was 5.9, while at the adult stage a 12.5-fold reduction was observed.

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)

A rapid method for reconstitution of bacterial membrane proteins

We have devised a simple method for the reconstitution of bacterial membrane proteins directly from small (1-20 ml) volumes of cell culture, thus eliminating the preparation of membrane vesicles. Cells are subjected to simultaneous lysozyme digestion and osmotic lysis, and after brief centrifugation ghosts are solubilized in 1.2% octyl-beta-D-glucopyranoside (octylglucoside) in the presence of added carrier lipid and an osmolyte. Aliquots of the clarified supernatant are suitable for reconstitution, as documented by using extracts from three different Gram-negative cells to recover both inorganic phosphate (Pi)-linked antiport and oxalate:formate exchange activities in proteoliposomes. These proteoliposomes are physically stable, non-leaky and can sustain a membrane potential and, because functional porins do not reconstitute, the artificial system has transport characteristics similar to those found when proteoliposomes are obtained using standard methods. This method should become an important tool for the screening and characterization of large numbers of strains, both wild-type and mutant.

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.

Design of a membrane transport protein for fluorescence spectroscopy

To modify the lac permease of Escherichia coli for fluorescence spectroscopy, six tryptophan residues at positions 10, 33, 78, 151, 171, and 223 were first replaced individually with phenylalanine by using oligonucleotide-directed site-specific mutagenesis. None of the tryptophan residues is critical for activity, as evidenced by the finding that the mutant permease molecules catalyze lactose/H+ symport almost as well as wild-type permease. Subsequently, a permease molecule was designed in which all of the tryptophan residues were replaced with phenylalanine. Remarkably, the lac permease harboring all six mutations catalyzes active lactose transport about 75% as well as wild-type permease. The fluorescence emission spectrum of purified wild-type permease solubilized in octyl beta-D-glucopyranoside and phospholipid exhibits a broad maximum centered at 350 nm, and the peak is almost completely absent from the spectrum of permease devoid of tryptophan. Furthermore, a new maximum centered at about 306 nm is apparent in the spectrum of the modified permease, suggesting that tyrosine fluorescence in the native protein is quenched by internal energy transfer to tryptophan residues. By using site-directed mutagenesis to replace specified residues in the molecule without tryptophan, it should now be possible to utilize tryptophan fluorescence spectroscopy to study static and dynamic aspects of permease structure and function.

Histidine-94 is the only important histidine residue in the melibiose permease of Escherichia coli

Oligonucleotide-directed, site-specific mutagenesis has been utilized to modify the melB gene of Escherichia coli such that each of the seven His residues in the melibiose permease has been replaced with Arg. Replacement of His-213, His-442, or His-456 has no significant effect on permease activity, while permease with Arg in place of His-198, His-318, or His-357 retains more than 70% of wild-type activity. In striking contrast, replacement of His-94 with Arg causes a complete loss of sugar binding and transport, although the cells contain a normal complement of permease molecules. Thus, as shown previously with lac permease, only a single His residue is important for activity, but, in the case of mel permease, the critical His residue is present in the 3rd putative transmembrane helix rather than the 10th.

A five-residue sequence near the carboxyl terminus of the polytopic membrane protein lac permease is required for stability within the membrane

The lac permease (lacY gene product) of Escherichia coli contains 417 amino acid residues and is predicted to have a short hydrophilic amino terminus on the inner surface of the cytoplasmic membrane, multiple transmembrane hydrophobic segments in alpha-helical conformation, and a 17-amino acid residue hydrophilic carboxyl-terminal tail on the inner surface of the membrane. To assess the importance of the carboxyl terminus, the properties of several truncation mutants were studied. The mutants were constructed by site-directed mutagenesis such that stop codons were placed at specified positions, and the altered lacY genes were expressed at a relatively low rate from plasmid pACYC184. Permease truncated at position 407 or 401 retains full activity, and a normal complement of molecules is present in the membrane, as judged by immunoblot analyses. Thus, it is apparent that the carboxyl-terminal tail plays no direct role in membrane insertion of the permease, its stability, or in the mechanism of lactose/H+ symport. In marked contrast, when truncations are made at residues 396 (i.e., 4 amino acid residues from the carboxyl terminus of putative helix XII), 389, 372, or 346, the permease is no longer found in the membrane. Remarkably, however, when each of the mutated lacY genes is expressed at a high rate by means of the T7 RNA polymerase system [Tabor, S. & Richardson, C. C. (1985) Proc. Natl. Acad. Sci. USA 82, 1074-1079], all of the truncated permeases are present in the membrane, as indicated by [35S]methionine incorporation studies; however, permease truncated at residue 396, 389, 372, or 346 is defective with respect to lactose/H+ symport. Finally, pulse-chase experiments indicate that wild-type permease or permease truncated at residue 401 is stable, whereas permease truncated at or prior to residue 396 is degraded at a significant rate. The results are consistent with the notion that residues 396-401 in putative helix XII are important for protection against proteolytic degradation and suggest that this region of the permease may be necessary for proper folding.

Characterization of site-directed mutants in the lac permease of Escherichia coli. 1. Replacement of histidine residues

Wild-type lac permease from Escherichia coli and two site-directed mutant permeases containing Arg in place of His35 and His39 or His322 were purified and reconstituted into proteoliposomes. H35-39R permease is indistinguishable from wild type with regard to all modes of translocation. In contrast, purified, reconstituted permease with Arg in place of His322 is defective in active transport, efflux, equilibrium exchange, and counterflow but catalyzes downhill influx of lactose without concomitant H+ translocation. Although permease with Arg in place of His205 was thought to be devoid of activity [Padan, E., Sarkar, H. K., Viitanen, P. V., Poonian, M. S., & Kaback, H. R. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 6765], sequencing of lac Y in pH205R reveals the presence of two additional mutations in the 5' end of the gene, and replacement of this portion of lac Y with a restriction fragment from the wild-type gene yields permease with normal activity. Permeases with Asn, Gln, or Lys in place of His322, like H322R permease, catalyze downhill influx of lactose without H+ translocation but are unable to catalyze active transport, equilibrium exchange, or counterflow. Unlike H322R permease, however, the latter mutants catalyze efflux at rates comparable to that of wild-type permease, although the reaction does not occur in symport with H+. Finally, as evidenced by flow dialysis and photoaffinity labeling experiments, replacement of His322 appears to cause a marked decrease in the affinity of the permease for substrate. The results confirm and extend the contention that His322 is the only His residue in the permease involved in lactose/H+ symport and that an imidazole moiety at position 322 is obligatory.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of site-directed mutants in the lac permease of Escherichia coli. 2. Glutamate-325 replacements

lac permease with Ala in place of Glu325 was solubilized from the membrane, purified, and reconstituted into proteoliposomes. The reconstituted molecule is completely unable to catalyze lactose/H+ symport but catalyzes exchange and counterflow at least as well as wild-type permease. In addition, Ala325 permease catalyzes downhill lactose influx without concomitant H+ translocation and binds p-nitrophenyl alpha-D-galactopyranoside with a KD only slightly higher than that of wild-type permease. Studies with right-side-out membrane vesicles demonstrate that replacement of Glu325 with Gln, His, Val, Cys, or Trp results in behavior similar to that observed with Ala in place of Glu325. On the other hand, permease with Asp in place of Glu325 catalyzes lactose/H+ symport about 20% as well as wild-type permease. The results indicate that an acidic residue at position 325 is essential for lactose/H+ symport and that hydrogen bonding at this position is insufficient. Taken together with previous results and those presented in the following paper [Lee, J. A., Püttner, I. B., & Kaback, H. R. (1989) Biochemistry (third paper of three in this issue)], the findings are consistent with the idea that Arg302, His322, and Glu325 may be components of a H+ relay system that plays an important role in the coupled translocation of lactose and H+.

Effect of distance and orientation between arginine-302, histidine-322, and glutamate-325 on the activity of lac permease from Escherichia coli

lac permease of Escherichia coli was modified by site-directed mutagenesis in order to investigate the effects of polarity, distance, and orientation between the components of a putative H+ relay system (Arg302/His322/Glu325) postulated to be involved in lactose-coupled H+ translocation. The importance of polarity between His322 and Glu325 was studied by interchanging the residues, and the modified permease--H322E/E325H--is inactive in all modes of translocation. The effect of distance and/or orientation between His322 and Glu325 was investigated by interchanging Glu325 with Val326, thereby moving the carboxylate one residue around putative helix X. The resulting permease molecule--E325V/V326E--is also completely inactive; control mutations, E325V [Carrasco, N., Püttner, I. B., Antes, L. M., Lee, J. A., Larigan, J. D., Lolkema, J. S., Roepe, P. D., & Kaback, H. R. (1989) Biochemistry (second paper of three in this issue)], and E325A/V326E, indicate that a Glu residue at position 326 inactivates the permease. The wild-type orientation between His and Glu was then restored by further mutation of E325V/V326E to introduce a His residue into position 323 or by interchanging Met323 with His322. The resulting permease molecules--M323H/E325V/V326E and H322M/M323H/E325V/V326E--contain the wild-type His/Glu orientation, but the His/Glu ion pair is rotated about the helical axis by 100 degrees relative to Arg302 in putative helix IX. Both mutants are inactive with respect to all modes of translocation. The results provide strong support for the contention that the polarity between His322 and Glu325 and the geometric relationship between Arg302, His322, and Glu325 are critical for permease activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the human erythrocyte glucose transporter in Escherichia coli

The gene encoding the human erythrocyte glucose transporter, cloned from HepG2 hepatoma cells, was expressed in Escherichia coli by introducing a prokaryote-type ribosome binding site, subcloning the gene into the T7 promoter/T7 polymerase expression system, and transforming a strain that is defective in glucose transport. Cells bearing plasmids with the transporter gene take up 2-deoxy-D-glucose and D-glucose, unlike cells bearing plasmids without the transporter gene. Moreover, 2-deoxy-D-glucose uptake is inhibited by unlabeled D-glucose, cytochalasin B, or mercuric chloride but not by L-glucose. The glucose transport protein is inserted into the membrane of E. coli, as evidenced by immunoblotting experiments with two site-directed polyclonal antibodies, one directed against the COOH terminus of the glucose transporter and the other directed against a synthetic peptide containing amino acid residues 225-238. As detected with both antibodies, the protein migrates with apparent molecular mass of 34 kDa in sodium dodecyl sulfate/12% polyacrylamide, a size similar to that of the unglycosylated glucose-transport protein synthesized in vitro.

lac permease of Escherichia coli containing a single histidine residue is fully functional

Arg-302, His-322, and Glu-325, neighboring residues in putative helices IX and X of the lac permease (lacY gene product) of Escherichia coli, play an important role in lactose/H+ symport, possibly as components of a catalytic triad similar to that postulated for the serine proteases [Kaback, H. R. (1987) Biochemistry 26, 2071-2076]. By using restriction fragments of lacY genes harboring specific site-directed mutations, a fusion gene has been constructed that encodes a permease in which His-35 and His-39 are replaced with arginine, and His-205 with glutamine (RQHE permease). The resultant molecule contains a single histidine residue at position 322 and exhibits all of the properties of the wild-type permease. In addition, an analogous single-histidine permease was engineered with alanine at position 325 in place of glutamic acid (RQHA permease). This construct is defective in active transport but catalyzes exchange and counterflow normally. RQHA permease, like the single-histidine permease with Glu-325, also shows normal behavior with respect to N-ethylmaleimide inactivation, substrate protection, and binding. In addition to providing strong support for previous experiments, the engineered permease molecules should be useful for determining the apparent pK of His-322 under various conditions.

Mammalian and bacterial sugar transport proteins are homologous

The uptake of a sugar across the boundary membrane is a primary event in the nutrition of most cells, but the hydrophobic nature of the transport proteins involved makes them difficult to characterize. Their amino-acid sequences can, however, be determined by cloning and sequencing the corresponding gene (or complementary DNA). We have determined the sequences of the arabinose-H+ and xylose-H+ membrane transport proteins of Escherichia coli. They are homologous with each other and, unexpectedly, with the glucose transporters of human hepatoma and rat brain cells. All four proteins share similarities with the E. coli citrate transporter. Comparisons of their sequences and hydropathic profiles yield insights into their structure, functionally important residues and possible evolutionary relationships. There is little apparent homology with the lactose-H+ (LacY) or melibiose-Na+ (MelB) transport proteins of E. coli.

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

The lac permease of Escherichia coli was modified by site-directed mutagenesis such that His-205 or His-322 is replaced with either Asn or Gln. Permease with Asn or Gln in place of His-205 exhibits normal activity, while permease with Asn or Gln in place of His-322 exhibits no activity. The results are consistent with the interpretation that His-205 and His-322 play different roles in lactose/H+ symport, the former involving hydrogen bonding of the imidazole nitrogens and the latter requiring positive charge in the imidazole ring. In addition, it is demonstrated that permease with Arg in place of His-322 does not catalyze efflux, exchange, or counterflow. The observations, in conjunction with those in the accompanying paper [Carrasco, N., Antes, L. M., Poonian, M. S., & Kaback, H. R. (1986) Biochemistry (following paper in this issue)], suggest that His-322 plays an important role in H+ translocation, possibly as a component of a charge-relay system with Glu-325, a neighboring residue in helix 10.

lac permease of Escherichia coli: histidine-322 and glutamic acid-325 may be components of a charge-relay system

When Glu-325 in the lac permease of Escherichia coli is replaced with Ala, lactose/H+ symport is abolished. Thus, the altered permease catalyzes neither uphill lactose accumulation nor efflux. Remarkably, however, permease with Ala-325 catalyzes exchange and counterflow at completely normal rates. Taken together with the results presented in the accompanying paper [Püttner, I. B., Sarkar, H. K., Poonian, M. S., & Kaback, H. R. (1986) Biochemistry (preceding paper in this issue)], the findings suggest that the His-322 and Glu-325 may be components of a charge-relay system that plays an important role in the coupled translocation of lactose and H+.