Analysis of a putative voltage-gated prokaryotic potassium channel

The Potassium Efflux System Kef: Bacterial Protection against Toxic Electrophilic Compounds

Kef couples the potassium efflux with proton influx in gram-negative bacteria. The resulting acidification of the cytosol efficiently prevents the killing of the bacteria by reactive electrophilic compounds. While other degradation pathways for electrophiles exist, Kef is a short-term response that is crucial for survival. It requires tight regulation since its activation comes with the burden of disturbed homeostasis. Electrophiles, entering the cell, react spontaneously or catalytically with glutathione, which is present at high concentrations in the cytosol. The resulting glutathione conjugates bind to the cytosolic regulatory domain of Kef and trigger activation while the binding of glutathione keeps the system closed. Furthermore, nucleotides can bind to this domain for stabilization or inhibition. The binding of an additional ancillary subunit, called KefF or KefG, to the cytosolic domain is required for full activation. The regulatory domain is termed K⁺ transport-nucleotide binding (KTN) or regulator of potassium conductance (RCK) domain, and it is also found in potassium uptake systems or channels in other oligomeric arrangements. Bacterial RosB-like transporters and K⁺ efflux antiporters (KEA) of plants are homologs of Kef but fulfill different functions. In summary, Kef provides an interesting and well-studied example of a highly regulated bacterial transport system.

Bioremediation potential of hexavalent chromium-resistant Arthrobacter globiformis 151B: study of the uptake of cesium and other alkali ions

Cesium (Cs⁺) enters environments largely because of global release into the environment from weapons testing and accidents such as Fukushima Daiichi and Chernobyl nuclear waste. Even at low concentrations, Cs⁺ is highly toxic to ecological receptors because of its physicochemical similarity to macronutrient potassium (K⁺). We investigated the uptake and accumulation of Cs⁺ by Arthrobacter globiformis strain 151B in reference to three similar alkali metal cations rubidium (Rb⁺), sodium (Na⁺), and potassium (K⁺). The impact of hexavalent chromium (Cr⁺⁶) as a co-contaminant was also evaluated. A. globiformis 151B accumulated Cs⁺ and Cr⁶⁺ in a time-dependent fashion. In contrast, the uptake and accumulation of Rb⁺ did not exhibit any trends. An exposure to Cs⁺, Rb⁺, and Cr⁺⁶ triggered a drastic increase in K⁺ and Na⁺ uptake by the bacterial cells. That was followed by the efflux of K⁺ and Na⁺, suggesting a Cs⁺ "substitution." Two-dimensional gel-electrophoresis of bacterial cell proteomes with the following mass-spectrometry of differentially expressed bands revealed that incubation of bacterial cells with Cs⁺ induced changes in the expression of proteins involved in the maintenance of cellular homeostasis and reactive oxygen species removal. The ability of A. globiformis 151B to mediate the uptake and accumulation of cesium and hexavalent chromium suggests that it possesses wide-range bioremediation potential.

Molecular Mechanisms for Bacterial Potassium Homeostasis

Potassium ion homeostasis is essential for bacterial survival, playing roles in osmoregulation, pH homeostasis, regulation of protein synthesis, enzyme activation, membrane potential adjustment and electrical signaling. To accomplish such diverse physiological tasks, it is not surprising that a single bacterium typically encodes several potassium uptake and release systems. To understand the role each individual protein fulfills and how these proteins work in concert, it is important to identify the molecular details of their function. One needs to understand whether the systems transport ions actively or passively, and what mechanisms or ligands lead to the activation or inactivation of individual systems. Combining mechanistic information with knowledge about the physiology under different stress situations, such as osmostress, pH stress or nutrient limitation, one can identify the task of each system and deduce how they are coordinated with each other. By reviewing the general principles of bacterial membrane physiology and describing the molecular architecture and function of several bacterial K⁺-transporting systems, we aim to provide a framework for microbiologists studying bacterial potassium homeostasis and the many K⁺-translocating systems that are still poorly understood.

Revealing the ultrastructure of the membrane pores of intact Serratia marcescens cells by atomic force microscopy

This study aimed to characterize the surface ultrastructure of intact Serratia marcescens cells under physiological conditions. Topographic information of membrane pores of the cells was obtained by atomic force microscope (AFM). Three types of membrane pores (CH-1-Pore A, CH-1-Pore B and CH-1-Pore C) were observed and the spatial arrangements of membrane-spanning subunits in membranes were defined. High-resolution images revealed that the doughnut-shaped structures of CH-1-Pore A and CH-1-Pore B were composed of six-to-eight and four transmembrane subunits. The inverted teepee-shaped structure of CH-1-Pore C was segmented into two transmembrane subunits straddling a single funnel-like pore. This study, to the best of authors' knowledge, represents the first direct characterization of the surface ultrastructure of the membrane pores of Serratia marcescens CH-1 cells at the nanometer scale and offers new prospects of mapping membrane pores on intact prokaryotic cells.

Assembly of Kch, a putative potassium channel from Escherichia coli

Attempts to explore the structure and function of Kch, a putative potassium channel of Escherichia coli have yielded varying results; potassium-associated functions have been found in vivo but not in vitro. Here the kch gene is shown to produce two proteins, full-length Kch and the large C-terminal cytosolic domain (the RCK domain). Further, these two proteins are associated at the initial stages of purification. Previous structural studies of full-length Kch claim that the isolated protein forms large aggregates that are not suitable for analysis. The results presented here show that the purified protein sample, although heterogeneous, has one major population with a mass of about 400kDa, implying the presence of two Kch tetramers in a complex form. A three dimensional reconstruction at 25A based on electron microscopy data from negatively stained particles, revealed a 210A long and 95A wide complex in which the two tetrameric Kch units are linked by their RCK domains, giving rise to a large central ring of density. The formation of this dimer of tetramers on expression or during purification, may explain why attempts to reconstitute Kch into liposomes for activity measurements have failed.

Light- and pH-dependent structural changes in the PsbS subunit of photosystem II

In higher plants, the PsbS subunit of photosystem II (PSII) plays a crucial role in pH- and xanthophyll-dependent nonphotochemical quenching of excess absorbed light energy, thus contributing to the defense mechanism against photoinhibition. We determined the amino acid sequence of Zea mays PsbS and produced an antibody that recognizes with high specificity a region of the protein located in the stroma-exposed loop between the second and third putative helices. By means of this antiserum, the thylakoid membranes of various higher plant species revealed the presence of a 42-kDa protein band, indicating the formation of a dimer of the 21-kDa PsbS protein. Crosslinking experiments and immunoblotting with other antisera seem to exclude the formation of a heterodimer with other PSII protein components. The PsbS monomer/dimer ratio in isolated thylakoid membranes was found to vary with luminal pH in a reversible manner, the monomer being the prevalent form at acidic and the dimer at alkaline pH. In intact chloroplasts and whole plants, dimer-to-monomer conversion is reversibly induced by light, known to cause luminal acidification. Sucrose-gradient centrifugation revealed a prevalent association of the PsbS monomer and dimer with light-harvesting complex and PSII core complexes, respectively. The finding of the existence of a light-induced change in the quaternary structure of the PsbS subunit may contribute to understanding the mechanism of PsbS action during nonphotochemical quenching.

Gain-of-function mutations indicate that Escherichia coli Kch forms a functional K+ conduit in vivo

Although Kch of Escherichia coli is thought to be a K(+) channel by sequence homology, there is little evidence that it actually conducts K(+) ions in vitro or in vivo. We isolated gain-of-function (GOF) Kch mutations that render bacteria specifically sensitive to K(+) ions. Millimolar added K(+), but not Na(+) or sorbitol, blocks the initiation or continuation of mutant growth in liquid media. The mutations are mapped at the RCK (or KTN) domain, which is considered to be the cytoplasmic sensor controlling the gate. Additional mutations directed to the K(+)-filter sequence rescue the GOF mutant. The apparent K(+)-specific conduction through the 'loose-cannon' mutant channel suggests that the wild-type Kch channel also conducts, albeit in a regulated manner. Changing the internal ATG does not erase the GOF toxicity, but removes kch's short second product, suggesting that it is not required for channel function in vivo. The mutant phenotypes are better explained by a perturbation of membrane potential instead of internal K(+) concentration. Possible implications on the normal function of Kch are discussed.

Soluble sugar permeases of the phosphotransferase system in Escherichia coli: evidence for two physically distinct forms of the proteins in vivo

The bacterial phosphoenolpyruvate-dependent phosphotransferase system (PTS) consists of a set of cytoplasmic energy-coupling proteins and various integral membrane permeases/sugar phosphotransferases, each specific for a different sugar. We have conducted biochemical analyses of three PTS permeases (enzymes II), the glucose permease (IIGlc), the mannitol permease (IIMtl) and the mannose permease (IIMan). These enzymes each catalyse two vectorial/chemical reactions, sugar phosphorylation using phosphoenolpyruvate (PEP) as the phosphoryl donor, dependent on enzyme I, HPr and IIA as well as IIBC (the PEP reaction), and transphosphorylation using a sugar phosphate (glucose-6-P for IIGlc and IIMan; mannitol-1-P for IIMtl) as the phosphoryl donor, dependent only on IIBC (the TP reaction). When crude extracts of French-pressed or osmotically shocked Escherichia coli cells are centrifuged in an ultracentrifuge at high speed, 5-20% of the enzyme II activity remains in the high-speed supernatant, and passage through a gel filtration column gives two activity peaks, one in the void volume exhibiting high PEP-dependent and TP activities, and a second included peak with high PEP-dependent activity and high (IIMan), moderate (IIGlc) or negligible (IIMtl) TP activities. Both log and stationary phase cells exhibit comparable relative amounts of pelletable and soluble enzyme II activities, but long-term exposure of cells to chloramphenicol results in selective loss of the soluble fraction with retention of much of the pelleted activity concomitant with extensive protein degradation. Short-term exposure of cells to chloramphenicol results in increased activities in both fractions, possibly because of increased lipid association, with more activation in the soluble fraction than in the pelleted fraction. Western blot analyses show that the soluble IIGlc exhibits a subunit size of about 45 kDa, and all three soluble enzymes II elute from the gel filtration column with apparent molecular weights of 40-50 kDa. We propose that enzymes II of the PTS exist in two physically distinct forms in the E. coli cell, one tightly integrated into the membrane and one either soluble or loosely associated with the membrane. We also propose that the membrane-integrated enzymes II are largely dimeric, whereas the soluble enzymes II, retarded during passage through a gel filtration column, are largely monomeric.

A novel aldo-keto reductase from Escherichia coli can increase resistance to methylglyoxal toxicity

A novel aldo-keto reductase (AKR) from Escherichia coli has been cloned, expressed and purified. This protein, YghZ, is distantly related (<40%) to mammalian aflatoxin dialdehyde reductases of the aldo-keto reductase AKR7 family and to potassium channel beta-subunits in the AKR6 family. The enzyme has been placed in a new AKR family (AKR14), with the designation AKR14A1. Sequences encoding putative homologues of this enzyme exist in many other bacteria. The enzyme can reduce several aldehyde and diketone substrates, including the toxic metabolite methylglyoxal. The K(m) for the model substrate 4-nitrobenzaldehyde is 1.06 mM and for the endogenous dicarbonyl methylglyoxal it is 3.4 mM. Overexpression of the recombinant enzyme in E. coli leads to increased resistance to methylglyoxal. It is possible that this enzyme plays a role in the metabolism of methylglyoxal, and can influence its levels in vivo.

Functional properties of Kch, a prokaryotic homologue of eukaryotic potassium channels

To test the hypothesis that the Kch gene of Escherichia coli encodes a potassium channel, we have transformed E. coli with an expression vector containing the Kch sequence and observed the effect of over-expression of Kch on E. coli. We found that: (i) over-expression of Kch is toxic to E. coli, but the toxicity could be prevented by supplementing the growth medium with K(+), Rb(+), and NH(4)(+), but not Na(+), consistent with the properties of a potassium selective pore; (ii) Cs(+), a blocker of potassium channels, rescues the growth of Kch over-expressing cells; and (iii) when the putative pore-forming region of Kch, containing the signature sequence, was replaced with the corresponding region of the eukaryotic Shaker potassium channel, and the resultant construct expressed in E. coli, the cells became critically dependent on K(+) supply for survival. These data are consistent with the proposed function of Kch, i.e., K(+) conduction.