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1.
Biochemistry ; 56(32): 4219-4234, 2017 08 15.
Article in English | MEDLINE | ID: mdl-28656748

ABSTRACT

Ligand binding is one of the most fundamental properties of proteins. Ligand functions fall into three basic types: substrates, regulatory molecules, and cofactors essential to protein stability, reactivity, or enzyme-substrate complex formation. The regulation of potassium ion movement in bacteria is predominantly under the control of regulatory ligands that gate the relevant channels and transporters, which possess subunits or domains that contain Rossmann folds (RFs). Here we demonstrate that adenosine monophosphate (AMP) is bound to both RFs of the dimeric bacterial Kef potassium efflux system (Kef), where it plays a structural role. We conclude that AMP binds with high affinity, ensuring that the site is fully occupied at all times in the cell. Loss of the ability to bind AMP, we demonstrate, causes protein, and likely dimer, instability and consequent loss of function. Kef system function is regulated via the reversible binding of comparatively low-affinity glutathione-based ligands at the interface between the dimer subunits. We propose this interfacial binding site is itself stabilized, at least in part, by AMP binding.


Subject(s)
Adenosine Monophosphate/chemistry , Potassium-Hydrogen Antiporters/chemistry , Protein Folding , Protein Multimerization , Shewanella/chemistry , Adenosine Monophosphate/genetics , Adenosine Monophosphate/metabolism , Potassium-Hydrogen Antiporters/genetics , Potassium-Hydrogen Antiporters/metabolism , Protein Binding , Protein Domains , Protein Stability , Protein Structure, Quaternary , Shewanella/genetics , Shewanella/metabolism
2.
Biochemistry ; 53(12): 1982-92, 2014 Apr 01.
Article in English | MEDLINE | ID: mdl-24601535

ABSTRACT

The potassium efflux system, Kef, protects bacteria against the detrimental effects of electrophilic compounds via acidification of the cytoplasm. Kef is inhibited by glutathione (GSH) but activated by glutathione-S-conjugates (GS-X) formed in the presence of electrophiles. GSH and GS-X bind to overlapping sites on Kef, which are located in a cytosolic regulatory domain. The central paradox of this activation mechanism is that GSH is abundant in cells (at concentrations of ∼10-20 mM), and thus, activating ligands must possess a high differential over GSH in their affinity for Kef. To investigate the structural requirements for binding of a ligand to Kef, a novel fluorescent reporter ligand, S-{[5-(dimethylamino)naphthalen-1-yl]sulfonylaminopropyl} glutathione (DNGSH), was synthesized. By competition assays using DNGSH, complemented by direct binding assays and thermal shift measurements, we show that the well-characterized Kef activator, N-ethylsuccinimido-S-glutathione, has a 10-20-fold higher affinity for Kef than GSH. In contrast, another native ligand that is a poor activator, S-lactoylglutathione, exhibits a similar Kef affinity to GSH. Synthetic ligands were synthesized to contain either rigid or flexible structures and investigated as ligands for Kef. Compounds with rigid structures and high affinity activated Kef. In contrast, flexible ligands with similar binding affinities did not activate Kef. These data provide insight into the structural requirements for Kef gating, paving the way for the development of a screen for potential therapeutic lead compounds targeting the Kef system.


Subject(s)
Escherichia coli Proteins/chemistry , Glutathione/analogs & derivatives , Potassium-Hydrogen Antiporters/chemistry , Potassium/chemistry , Succinimides/chemistry , Biological Transport, Active/physiology , Escherichia coli Proteins/metabolism , Glutathione/chemistry , Glutathione/metabolism , Ion Channel Gating/physiology , Ligands , Potassium/metabolism , Potassium-Hydrogen Antiporters/metabolism , Protein Binding , Protein Structure, Secondary , Protein Structure, Tertiary , Shewanella/chemistry , Shewanella/metabolism , Succinimides/metabolism
3.
J Bacteriol ; 193(18): 4925-32, 2011 Sep.
Article in English | MEDLINE | ID: mdl-21742892

ABSTRACT

Escherichia coli and many other Gram-negative pathogenic bacteria protect themselves from the toxic effects of electrophilic compounds by using a potassium efflux system (Kef). Potassium efflux is coupled to the influx of protons, which lowers the internal pH and results in immediate protection. The activity of the Kef system is subject to complex regulation by glutathione and its S conjugates. Full activation of KefC requires a soluble ancillary protein, KefF. This protein has structural similarities to oxidoreductases, including human quinone reductases 1 and 2. Here, we show that KefF has enzymatic activity as an oxidoreductase, in addition to its role as the KefC activator. It accepts NADH and NADPH as electron donors and quinones and ferricyanide (in addition to other compounds) as acceptors. However, typical electrophilic activators of the Kef system, e.g., N-ethyl maleimide, are not substrates. If the enzymatic activity is disrupted by site-directed mutagenesis while retaining structural integrity, KefF is still able to activate the Kef system, showing that the role as an activator is independent of the enzyme activity. Potassium efflux assays show that electrophilic quinones are able to activate the Kef system by forming S conjugates with glutathione. Therefore, it appears that the enzymatic activity of KefF diminishes the redox toxicity of quinones, in parallel with the protection afforded by activation of the Kef system.


Subject(s)
Escherichia coli Proteins/metabolism , Escherichia coli/enzymology , NAD(P)H Dehydrogenase (Quinone)/metabolism , Oxidoreductases/metabolism , Benzoquinones/metabolism , Escherichia coli/metabolism , Escherichia coli Proteins/genetics , Ferricyanides/metabolism , Humans , Mutagenesis, Site-Directed , NAD/metabolism , NAD(P)H Dehydrogenase (Quinone)/genetics , NADP/metabolism , Oxidoreductases/genetics , Potassium/metabolism , Potassium Channels/metabolism , Protein Subunits/metabolism
4.
Biochem Soc Trans ; 39(3): 733-40, 2011 Jun.
Article in English | MEDLINE | ID: mdl-21599642

ABSTRACT

Mechanosensitive channels sense and respond to changes in bilayer tension. In many respects, this is a unique property: the changes in membrane tension gate the channel, leading to the transient formation of open non-selective pores. Pore diameter is also high for the bacterial channels studied, MscS and MscL. Consequently, in cells, gating has severe consequences for energetics and homoeostasis, since membrane depolarization and modification of cytoplasmic ionic composition is an immediate consequence. Protection against disruption of cellular integrity, which is the function of the major channels, provides a strong evolutionary rationale for possession of such disruptive channels. The elegant crystal structures for these channels has opened the way to detailed investigations that combine molecular genetics with electrophysiology and studies of cellular behaviour. In the present article, the focus is primarily on the structure of MscS, the small mechanosensitive channel. The description of the structure is accompanied by discussion of the major sites of channel-lipid interaction and reasoned, but limited, speculation on the potential mechanisms of tension sensing leading to gating.


Subject(s)
Bacteria/metabolism , Ion Channel Gating/physiology , Lipid Bilayers/chemistry , Mechanotransduction, Cellular/physiology , Bacteria/cytology , Escherichia coli/physiology , Escherichia coli Proteins/chemistry , Escherichia coli Proteins/metabolism , Ion Channels/chemistry , Ion Channels/metabolism , Membrane Lipids/chemistry , Membrane Lipids/metabolism , Models, Molecular , Protein Structure, Tertiary , Stress, Mechanical
5.
Mol Microbiol ; 78(6): 1577-90, 2010 Dec.
Article in English | MEDLINE | ID: mdl-21143325

ABSTRACT

Survival of exposure to methylglyoxal (MG) in Gram-negative pathogens is largely dependent upon the operation of the glutathione-dependent glyoxalase system, consisting of two enzymes, GlxI (gloA) and GlxII (gloB). In addition, the activation of the KefGB potassium efflux system is maintained closed by glutathione (GSH) and is activated by S-lactoylGSH (SLG), the intermediate formed by GlxI and destroyed by GlxII. Escherichia coli mutants lacking GlxI are known to be extremely sensitive to MG. In this study we demonstrate that a ΔgloB mutant is as tolerant of MG as the parent, despite having the same degree of inhibition of MG detoxification as a ΔgloA strain. Increased expression of GlxII from a multicopy plasmid sensitizes E. coli to MG. Measurement of SLG pools, KefGB activity and cytoplasmic pH shows these parameters to be linked and to be very sensitive to changes in the activity of GlxI and GlxII. The SLG pool determines the activity of KefGB and the degree of acidification of the cytoplasm, which is a major determinant of the sensitivity to electrophiles. The data are discussed in terms of how cell fate is determined by the relative abundance of the enzymes and KefGB.


Subject(s)
Escherichia coli/metabolism , Glutathione/analogs & derivatives , Lactoylglutathione Lyase/metabolism , Pyruvaldehyde/metabolism , Escherichia coli/drug effects , Escherichia coli/enzymology , Escherichia coli Proteins/genetics , Escherichia coli Proteins/metabolism , Glutathione/metabolism , Lactoylglutathione Lyase/genetics , Microbial Viability , Potassium-Hydrogen Antiporters/genetics , Potassium-Hydrogen Antiporters/metabolism , Pyruvaldehyde/pharmacology
6.
Proc Natl Acad Sci U S A ; 107(46): 19784-9, 2010 Nov 16.
Article in English | MEDLINE | ID: mdl-21041667

ABSTRACT

Gram negative pathogens are protected against toxic electrophilic compounds by glutathione-gated potassium efflux systems (Kef) that modulate cytoplasmic pH. We have elucidated the mechanism of gating through structural and functional analysis of Escherichia coli KefC. The revealed mechanism can explain how subtle chemical differences in glutathione derivatives can produce opposite effects on channel function. Kef channels are regulated by potassium transport and NAD-binding (KTN) domains that sense both reduced glutathione, which inhibits Kef activity, and glutathione adducts that form during electrophile detoxification and activate Kef. We find that reduced glutathione stabilizes an interdomain association between two KTN folds, whereas large adducts sterically disrupt this interaction. F441 is identified as the pivotal residue discriminating between reduced glutathione and its conjugates. We demonstrate a major structural change on the binding of an activating ligand to a KTN-domain protein. Analysis of the regulatory interactions suggests strategies to disrupt pathogen potassium and pH homeostasis.


Subject(s)
Escherichia coli/metabolism , Ion Channel Gating/physiology , Potassium/metabolism , Amino Acid Sequence , Biological Transport/drug effects , Escherichia coli Proteins/chemistry , Escherichia coli Proteins/metabolism , Glutathione/analogs & derivatives , Glutathione/metabolism , Glutathione/pharmacology , Ion Channel Gating/drug effects , Ligands , Models, Molecular , Molecular Sequence Data , Mutant Proteins/chemistry , Mutant Proteins/metabolism , Potassium Channels/chemistry , Potassium Channels/metabolism , Protein Binding/drug effects , Protein Multimerization/drug effects , Protein Structure, Tertiary , Succinimides/pharmacology
7.
Proc Natl Acad Sci U S A ; 107(28): 12664-9, 2010 Jul 13.
Article in English | MEDLINE | ID: mdl-20616037

ABSTRACT

We describe a mechanosensitive (MS) channel that has mechanosensitive channel of miniconductance (MscM) activity, and displays unique properties with respect to gating. Mechanosensitive channels respond to membrane tension, are ubiquitous from bacteria to man, and exhibit a great diversity in structure and function. These channels protect Bacteria and Archaea against hypoosmotic shock and are critical determinants of shape in chloroplasts. Given the dominant roles played in bacteria by the mechanosensitive channel of small conductance (MscS) and the mechanosensitive channel of large conductance (MscL), the role of the multiple MS channel homologs observed in most organisms remains obscure. Here we demonstrate that a MscS homolog, YbdG, extends the range of hypoosmotic shock that Escherichia coli cells can survive, but its expression level is insufficient to protect against severe shocks. Overexpression of the YbdG protein provides complete protection. Transcription and translation of the ybdG gene are enhanced by osmotic stress consistent with a role for the protein in survival of hypoosmotic shock. Measurement of the conductance of the native channel by standard patch clamp methods was not possible. However, a fully functional YbdG mutant channel, V229A, exhibits a conductance in membrane patches consistent with MscM activity. We find that MscM activities arise from more than one gene product because ybdG deletion mutants still exhibit an occasional MscM-like conductance. We propose that ybdG encodes a low-abundance MscM-type MS channel, which in cells relieves low levels of membrane tension, obviating the need to activate the major MS channels, MscS and MscL.


Subject(s)
Bacteria/genetics , Bacteria/metabolism , Escherichia coli/genetics , Escherichia coli/physiology , Archaea/genetics , Archaea/metabolism , Sequence Deletion
8.
Structure ; 17(6): 893-903, 2009 Jun 10.
Article in English | MEDLINE | ID: mdl-19523906

ABSTRACT

KTN (RCK) domains are nucleotide-binding folds that form the cytoplasmic regulatory complexes of various K+ channels and transporters. The mechanisms these proteins use to control their transmembrane pore-forming counterparts remains unclear despite numerous electrophysiological and structural studies. KTN (RCK) domains consistently crystallize as dimers within the asymmetric unit, forming a pronounced hinge between two Rossmann folds. We have previously proposed that modification of the hinge angle plays an important role in activating the associated membrane-integrated components of the channel or transporter. Here we report the structure of the C-terminal, KTN-bearing domain of the E. coli KefC K+ efflux system in association with the ancillary subunit, KefF, which is known to stabilize the conductive state. The structure of the complex and functional analysis of KefC variants reveal that control of the conformational flexibility inherent in the KTN dimer hinge is modulated by KefF and essential for regulation of KefC ion flux.


Subject(s)
Cell Membrane/metabolism , Membrane Transport Proteins/metabolism , Potassium Channels, Inwardly Rectifying/chemistry , Potassium Channels, Inwardly Rectifying/metabolism , Amino Acid Motifs , Amino Acid Sequence , Binding Sites , Conserved Sequence , Dimerization , Escherichia coli Proteins/genetics , Escherichia coli Proteins/isolation & purification , Escherichia coli Proteins/metabolism , Helix-Turn-Helix Motifs , Hydrophobic and Hydrophilic Interactions , Molecular Sequence Data , Mutation , Potassium Channels/genetics , Potassium Channels/isolation & purification , Potassium Channels/metabolism , Protein Binding , Protein Conformation , Protein Structure, Tertiary , Recombinant Fusion Proteins/metabolism , Sequence Homology, Amino Acid
9.
Science ; 321(5893): 1179-83, 2008 Aug 29.
Article in English | MEDLINE | ID: mdl-18755969

ABSTRACT

How ion channels are gated to regulate ion flux in and out of cells is the subject of intense interest. The Escherichia coli mechanosensitive channel, MscS, opens to allow rapid ion efflux, relieving the turgor pressure that would otherwise destroy the cell. We present a 3.45 angstrom-resolution structure for the MscS channel in an open conformation. This structure has a pore diameter of approximately 13 angstroms created by substantial rotational rearrangement of the three transmembrane helices. The structure suggests a molecular mechanism that underlies MscS gating and its decay of conductivity during prolonged activation. Support for this mechanism is provided by single-channel analysis of mutants with altered gating characteristics.


Subject(s)
Cell Membrane/chemistry , Escherichia coli Proteins/chemistry , Escherichia coli Proteins/physiology , Escherichia coli/chemistry , Ion Channel Gating , Ion Channels/chemistry , Ion Channels/physiology , Crystallography, X-Ray , Electric Conductivity , Escherichia coli/physiology , Escherichia coli Proteins/genetics , Hydrophobic and Hydrophilic Interactions , Ion Channels/genetics , Models, Molecular , Mutant Proteins/chemistry , Mutation , Patch-Clamp Techniques , Pressure , Protein Conformation , Protein Structure, Secondary , Protein Structure, Tertiary
10.
Biophys J ; 94(8): 3003-13, 2008 Apr 15.
Article in English | MEDLINE | ID: mdl-18065458

ABSTRACT

Mechanosensitive channels rescue bacterial cells from a fate of lysis when they transfer from a high- to low-osmolarity environment. Of three Escherichia coli mechanosensitive proteins studied to date, only MscS-Ec demonstrates a small anionic preference and a desensitized, nonconducting state under sustained pressure. Little is known about the mechanisms generating these distinctive properties. Eliminating the sole positive charge in the MscS-Ec pore region (Arg(88)) did not alter anionic preference. Adding positive charges at either end of the pore did not augment anionic preference, and placing negative charges within the pore did not diminish it. Thus, pore charges do not control this characteristic. However, from this analysis we identified mutations in the hinge region of the MscS-Ec pore helix (at Gly(113)) that profoundly affected ability of the channel to desensitize. Substitution with nonpolar (Ala, Pro) or polar (Asp, Arg, Ser) residues inhibited transition to the desensitized state. Interestingly, Gly(113) replaced with Met did not impede desensitization. Thus, although Gly is not specifically required at position 113, MscS desensitization is strongly influenced by the residue situated here. Mutations at residues further into the pore also regulated desensitization. Transition to this unique mechanosensitive channel state is discussed in terms of existing data.


Subject(s)
Cell Membrane/physiology , Escherichia coli Proteins/genetics , Ion Channel Gating/physiology , Ion Channels/genetics , Mechanotransduction, Cellular/physiology , Models, Biological , Mutagenesis, Site-Directed , Amino Acid Substitution , Cell Membrane/chemistry , Computer Simulation , Escherichia coli Proteins/chemistry , Ion Channels/chemistry , Lipid Bilayers/chemistry , Models, Chemical , Models, Molecular , Porosity , Stress, Mechanical
11.
Methods Enzymol ; 428: 47-61, 2007.
Article in English | MEDLINE | ID: mdl-17875411

ABSTRACT

Bacterial mechanosensitive (MS) channels play a significant role in protecting cells against hypoosmotic shock. Bacteria that have been diluted from high osmolarity medium into dilute solution are required to cope with sudden water influx associated with an osmotic imbalance equivalent to 10 to 14 atm. The cell wall is only poorly expansive and the cytoplasmic membrane even less so. Thus, swelling is not an option and the cell must rapidly eject solutes to diminish the osmotic gradient and thereby preserve structural integrity. This chapter describes cellular assays of MS channel function and their interpretation.


Subject(s)
Bacterial Physiological Phenomena , Ion Channels/physiology , Mechanotransduction, Cellular/physiology , Ion Channel Gating , Ion Channels/genetics , Osmotic Pressure
12.
Nat Struct Mol Biol ; 12(2): 113-9, 2005 Feb.
Article in English | MEDLINE | ID: mdl-15665866

ABSTRACT

The crystal structure of an open form of the Escherichia coli MscS mechanosensitive channel was recently solved. However, the conformation of the closed state and the gating transition remain uncharacterized. The pore-lining transmembrane helix contains a conserved glycine- and alanine-rich motif that forms a helix-helix interface. We show that introducing 'knobs' on the smooth glycine face by replacing glycine with alanine, and substituting conserved alanines with larger residues, increases the pressure required for gating. Creation of a glycine-glycine interface lowers activation pressure. The importance of residues Gly104, Ala106 and Gly108, which flank the hydrophobic seal, is demonstrated. A new structural model is proposed for the closed-to-open transition that involves rotation and tilt of the pore-lining helices. Introduction of glycine at Ala106 validated this model by acting as a powerful suppressor of defects seen with mutations at Gly104 and Gly108.


Subject(s)
Escherichia coli Proteins/chemistry , Escherichia coli Proteins/metabolism , Escherichia coli/chemistry , Glycine/metabolism , Ion Channels/chemistry , Ion Channels/metabolism , Alanine/genetics , Alanine/metabolism , Electrophysiology , Escherichia coli/genetics , Escherichia coli/metabolism , Escherichia coli Proteins/genetics , Glycine/genetics , Ion Channels/genetics , Models, Molecular , Mutation/genetics , Phenotype , Protein Structure, Tertiary
13.
Proc Natl Acad Sci U S A ; 100(26): 15959-64, 2003 Dec 23.
Article in English | MEDLINE | ID: mdl-14671322

ABSTRACT

The mechanosensitive (MS) channels MscS and MscL are essential for the survival of hypoosmotic shock by Escherichia coli cells. We demonstrate that MscS and MscL are induced by osmotic stress and by entry into stationary phase. Reduced levels of MS proteins and reduced expression of mscL- and mscS-LacZ fusions in an rpoS mutant strain suggested that the RNA polymerase holoenzyme containing sigmaS is responsible, at least in part, for regulating production of MS channel proteins. Consistent with the model that the effect of sigmaS is direct, the MscS and MscL promoters both use RNA polymerase containing sigmaS in vitro. Conversely, clpP or rssB mutations, which cause enhanced levels of sigmaS, show increased MS channel protein synthesis. RpoS null mutants are sensitive to hypoosmotic shock upon entry into stationary phase. These data suggest that MscS and MscL are components of the RpoS regulon and play an important role in ensuring structural integrity in stationary phase bacteria.


Subject(s)
Bacterial Proteins/physiology , Escherichia coli Proteins/genetics , Escherichia coli/genetics , Gene Expression Regulation, Bacterial/genetics , Ion Channels/genetics , Ion Channels/physiology , Mechanotransduction, Cellular/physiology , Sigma Factor/physiology , Base Sequence , DNA Primers , Genes, Reporter , Mechanotransduction, Cellular/genetics , Plasmids/genetics , Transcription, Genetic , beta-Galactosidase/genetics
14.
EMBO J ; 22(1): 36-46, 2003 Jan 02.
Article in English | MEDLINE | ID: mdl-12505982

ABSTRACT

The major structural features of the Escherichia coli MscS mechanosensitive channel protein have been explored using alkaline phosphatase (PhoA) fusions, precise deletions and site-directed mutations. PhoA protein fusion data, combined with the positive-inside rule, strongly support a model in which MscS crosses the membrane three times, adopting an N(out)-C(in) configuration. Deletion data suggest that the C-terminal domain of the protein is essential for the stability of the MscS channel, whereas the protein will tolerate small deletions at the N-terminus. Four mutants that exhibit either gain-of-function (GOF) or loss-of-function have been identified: a double mutation I48D/S49P inactivates MscS, whereas the MscS mutants T93R, A102P and L109S cause a strong GOF phenotype. The similarity of MscS to the last two domains of MscK (formerly KefA) is reinforced by the demonstration that expression of a truncated MscK protein can substitute for MscL and MscS in downshock survival assays. The data derived from studies of the organization, conservation and the influence of mutations provide significant insights into the structure of the MscS channel.


Subject(s)
Escherichia coli Proteins/chemistry , Escherichia coli/physiology , Ion Channels/chemistry , Alkaline Phosphatase/chemistry , Alkaline Phosphatase/metabolism , Amino Acid Sequence , Amino Acid Substitution , Base Sequence , Cell Membrane/enzymology , DNA Primers , Escherichia coli Proteins/metabolism , Ion Channels/metabolism , Kinetics , Molecular Sequence Data , Mutagenesis , Mutagenesis, Site-Directed , Protein Conformation , Protein Structure, Secondary , Restriction Mapping , Sensitivity and Specificity , Sequence Deletion
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