Site-specific mutagenesis of cysteine 148 to serine in the lac permease of Escherichia coli

A mutation in the lactose permease of Escherichia coli that decreases conformational flexibility and increases protein stability

Lactose permease with Cys154 --> Gly (helix V) binds substrate with high affinity but catalyzes little or no transport. The purified, detergent-solubilized mutant protein exhibits much greater thermal stability than the wild type and little tendency to aggregate. Stabilization is also observed in vivo with an unstable mutant that is expressed at significantly higher levels when the Cys154 --> Gly mutation is introduced. In addition, ligand-induced conformational changes are markedly reduced or abolished by the Cys154 --> Gly mutation: (i) Although the fluorescence of purified single Trp33 (helix I) permease is enhanced by ligand binding, introduction of the Cys154 --> Gly mutation abolishes the effect. (ii) The rate of 2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid (MIANS) labeling of permease with a single Cys residue in place of Val331 (helix X) is increased in the presence of ligand but reduced when the Cys154 --> Gly mutation is present. (iii) Fluorescence emission intensity of MIANS-labeled single Cys331 permease is enhanced and blue shifted in the Cys154 --> Gly mutant background, indicating that the latter mutation causes position 331 to become exposed to a less polar environment. The results indicate that the Cys154 --> Gly mutation causes a more compact structure and decreased conformational flexibility, an alteration that specifically blocks the structural changes necessary for substrate translocation with little or no effect on ligand binding.

Recombinant lysine:N(6)-hydroxylase: effect of cysteine-->alanine replacements on structural integrity and catalytic competence

Recombinant lysine:N(6)-hydroxylase, rIucD, catalyzes the hydroxylation of L-lysine to its N(6)-hydroxy derivative, with NADPH and FAD serving as cofactors in the reaction. The five cysteine residues present in rIucD can be replaced, individually or in combination, with alanine without effecting a major change in the thermal stability, the affinity for L-lysine and FAD, as well as the k(cat) for mono-oxygenase activity of the protein. However, when the susceptibility to modification by either 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) or 2,6-dichlorophenol indophenol (DPIP) serves as the criterion for monitoring conformational change(s) in rIucD and its muteins, Cys146-->Ala and Cys166-->Ala substitutions are found to induce an enhancement in the reactivity of one of the protein's remaining cysteine residues with concomitant diminution of mono-oxygenase function. In addition, the systematic study of cysteine-->alanine replacement has led to the identification of rIucD's Cys166 as the exposed residue which is detectable during the reaction of the protein with DTNB but not with iodoacetate. Substitution of Cys51 of rIucD with alanine results in an increase in mono-oxygenase activity (approx. 2-fold). Such replacement, unlike those of other cysteine residues, also enables the covalent DPIP conjugate of the protein to accommodate FAD in its catalytic function. A possible role of rIucD's Cys51 in the modulation of its mono-oxygenase function is discussed.

The sucrose permease of Escherichia coli: functional significance of cysteine residues and properties of a cysteine-less transporter

The sucrose (CscB) permease belongs to the oligosaccharide:H(+) symporter family of the Major Facilitator Superfamily and is homologous to the lactose permease from Escherichia coli. Sucrose transport in cells expressing sucrose permease is completely inhibited by N-ethylmaleimide (NEM), suggesting that one or more of the seven native Cys residues may be important for transport. In this paper, each Cys residue was individually replaced with Ser, and transport activity, membrane expression, and NEM sensitivity are documented. All seven single Cys-->Ser mutants are expressed normally in the membrane and catalyze sucrose transport with activities ranging from 80% to 180% of wild type. Six of the seven Ser mutants are completely inactivated by NEM, while Cys122-->Ser permease is insensitive to the sulfhydryl reagent, indicating that NEM inhibition results from alkylation of Cys122. Subsequently, a sucrose permease devoid of Cys residues (Cys-less) was constructed in which all Cys residues were replaced with Ser simultaneously by using a series of overlap-extension PCRs. Membrane expression and kinetic parameters for Cys-less [K(m) 4.8 mM, V(max) 192 nmol min(-1) (mg of protein)(-1)] are essentially identical to those of wild type [K(m) 5.4 mM, V(max) 196 nmol min(-1) (mg of protein)(-1)]. However, Cys-less permease catalyzes sucrose accumulation to steady-state levels that are approximately 2-fold higher than those of wild type. As anticipated, Cys-less permease is completely resistant to NEM inhibition. The observations demonstrate that Cys residues play no functional role in sucrose permease. Furthermore, the approach described to create the Cys-less transporter is generally applicable to other proteins. An application of Cys-less permease in the study of the substrate binding site is presented in the accompanying paper.

Functional conservation in the putative substrate binding site of the sucrose permease from Escherichia coli

The sucrose (CscB) permease is the only member of the oligosaccharide:H(+) symporter family in the Major Facilitator Superfamily that transports sucrose but not lactose or other galactosides. In lactose permease (lac permease), the most studied member of the family, three residues have been shown to participate in galactoside binding: Cys148 hydrophobically interacts with the galactosyl ring, while Glu126 and Arg144 are charge paired and form H-bonds with specific galactosyl OH groups. In the present study, the role of the corresponding residues in sucrose permease, Asp126, Arg144, and Ser148, is investigated using a functional Cys-less mutant (see preceding paper). Replacement of Ser148 with Cys has no significant effect on transport activity or expression, but transport becomes highly sensitive to the sulfhydryl reagent N-ethylmaleimide (NEM) in a manner similar to that of lac permease. However, in contrast to lac permease, substrate affords no protection whatsoever against NEM inactivation of transport or alkylation with [(14)C]NEM. Neutral (Ala, Cys) mutations of Asp126 and Arg144 abolish sucrose transport, while membrane expression is not affected. Similarly, combination of two Ala mutations within the same molecule (Asp126-->Ala/Arg144-->Ala) yields normally expressed, but completely inactive permease. Conservative replacements result in highly active molecules: Asp126-->Glu permease catalyzes sucrose transport comparable to Cys-less permease, while mutant Arg144-->Lys exhibits decreased but significant activity. The observations demonstrate that charge pair Asp126-Arg144 plays an essential role in sucrose transport and suggest that the overall architecture of the substrate binding sites is conserved between sucrose and lac permeases.

Effect of selective cysteine --> alanine replacements on the catalytic functions of lysine: N6-hydroxylase

Recombinant lysine: N6-hydroxylase, rIucD, catalyzes the conversion of L-lysine to its N6-hydroxy derivative. Re-examination of the nucleotide sequence of iucD, the gene encoding for the enzyme, has revealed a few discrepancies in the data documented in literature and the corrected version is presented. The revised nucleotide sequence predicts the presence of five cysteine residues in the primary structure of IucD. Two of these residues, cysteine 51 and cysteine 158 are alkylatable by iodoacetate in the native conformation of the protein resulting in a loss of monooxygenase activity while their replacement with alanine has no such adverse effect. Site directed mutagenesis studies have enabled an assessment of the reactivity of these cysteine residue(s) towards thiol modifying agents.

Probing the conformation of the lactose permease of Escherichia coli by in situ site-directed sulfhydryl modification

By using site-directed chemical labeling of lactose permease, conformational changes induced by ligand binding are observed in the native membrane of Escherichia coli. Membranes containing permease mutants with a single-Cys residue and a biotin-acceptor domain were labeled with radioactive N-ethylmaleimide (NEM) in the presence or absence of beta-D-galactopyranosyl 1-thio-beta-D-galactopyranoside (TDG) or a proton electrochemical gradient, followed by solubilization in n-dodecyl beta-D-maltopyranoside and adsorption to avidin. TDG-induced enhancement of the reactivity of membrane-embedded Val315-->Cys (helix X) permease is observed, while the reactivity of Val331-->Cys (helix X) permease is inhibited by ligand binding or imposition of a proton electrochemical gradient. In contrast, the reactivity of permease with a single native Cys residue at position 148 (helix V) is blocked by TDG, but unaffected by the proton electrochemical gradient. Furthermore, as shown with right-side-out and inside-out membrane vesicles, the accessibility of Cys148 to either NEM or impermeant methanethiosulfonate derivatives is comparable from both sides of the membrane. On the other hand, TDG protects Cys148 from alkylation more effectively in right-side-out vesicles (apparent KD = 20-50 microM) than inside-out vesicles (apparent KD ca. 1.0 microM). The findings provide strong support for the conclusion that the permease retains close to native conformation in n-dodecyl-beta-D-maltopyranoside. In addition, the results are consistent with the idea that lactose permease has two binding sites: one with higher affinity on the periplasmic surface of the membrane and another with lower affinity on the cytoplasmic surface.

Cysteine scanning mutagenesis of helix V in the lactose permease of Escherichia coli

Using a functional lactose permease mutant devoid of Cys (C-less permease), each amino acid residue in putative transmembrane helix V was replaced individually with Cys (from Met145 to Thr163). Of the 19 mutants, 13 are highly functional (60-125% of C-less permease activity), and 4 exhibit lower but significant lactose accumulation (15-45% of C-less permease). Cys replacement of Gly147 or Trp151 essentially inactivates the permease (< 10% of C-less); however, previous studies [Menezes, M. E., Roepe, P. D., & Kaback, H. R. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 1638; Jung, K., Jung, H., et al. (1995) Biochemistry 34, 1030] demonstrate that neither of these residues is important for activity. Immunoblots reveal that all of the mutant proteins are present in the membrane in amounts comparable to C-less permease with the exception of Trp151-->Cys and single Cys154 permeases which are present in reduced amounts. Finally, only three of the single-Cys mutants are inactivated significantly by N-ethylmaleimide (Met145-->Cys, native Cys148, and Gly159-->Cys), and the positions of the three mutants fall on the same face of helix V.

Cysteine 148 in the lactose permease of Escherichia coli is a component of a substrate binding site. 1. Site-directed mutagenesis studies

Cys 148 in the lactose permease of Escherichia coli has been replaced with hydrophobic (Ala, Val, Ile, Phe), hydrophilic (Ser, Thr), or charged (Asp, Lys) residues, and the properties of the replacement mutants have been analyzed. Although Cys 148 is not essential for transport, the size and polarity of the side chain at this position modifies transport activity and substrate specificity. Thus, small hydrophobic side chains (Ala, Val) generally increase the apparent affinity of the permease for substrate, while hydrophilic side chains (Ser, Thr, Asp) decrease apparent affinity and bulky or positively charged side chains (Phe, Lys) virtually abolish activity. In addition, hydrophilic substitutions (Ser, Thr, Asp) alter the specificity of the permease toward monosaccharides relative to disaccharides. On the basis of these and other observations, it is concluded that Cys 148 is located in a sugar binding site of lac permease and probably interacts hydrophobically with the galactosyl moiety. The postulate receives more direct support from site-directed fluorescence labeling studies presented in the following paper in this issue [Wu, J., & Kaback, H. R. (1994) Biochemistry (following paper in this issue)].

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.

The lactose carrier of Klebsiella pneumoniae M5a1; the physiology of transport and the nucleotide sequence of the lacY gene

A comparison has been made between the physiology and amino acid sequence of the lactose carriers of Klebsiella pneumoniae M5a1 and Escherichia coli K-12. The membrane transport of lactose was much weaker in Klebsiella than in E. coli. On the other hand o-nitrophenylgalactoside uptake by Klebsiella was distinctly greater than with E. coli. In spite of the differences in sugar transport between the two organisms, the amino acid sequences of the respective lactose carriers were remarkably similar (60% of the amino acids are identical).

Arginine modification with butanedione inhibits the potassium ATPase of Streptococcus faecalis

The K+-ATPase of Streptococcus faecalis is inhibited by incubation with the arginine-modifying reagent 2,3-butanedione. The inactivation proceeds by pseudo - first order kinetics and a double-logarithmic plot of the pseudo - first order rate constants versus reagent concentrations yields a reaction order of 1.14 with respect to butanedione. Partially inactivated ATPase exhibits a decreased maximal velocity but the same affinity for ATP, as compared to the native enzyme. Butanedione modification is inhibited by adenine nucleotides. These results indicate the involvement of most likely one crucial arginyl residue in adenine nucleotide binding by the ATPase.