A Kdp-like,high-affinity,K+-translocating ATPase is expressed during growth of Rhodobacter sphaeroides in low potassium media

Cells of the purple non-sulphur bacterium Rhodobacter sphaeroides express a high-affinity K⁺ uptake system when grown in media with low K⁺ concentrations. Antibodies againts the catalytic KdpB protein or the whole KdpABC complex of Escherichia coli crossreact with a 70.0 kDa R. sphaeroides protein that was expressed only in cells grown in media with low K⁺ concentrations. In membranes derived from R. sphaeroides cells grown with low K⁺ concentrations (induced cells), a high ATPase activity could be detected when assayed in Tris-HCl pH 8.0 containing 1 mM MgSO₄. This ATPase activity increased upon addition of 1 mM KCl from 166 to 289 mol ATP hydrolysed x min^-1 x g protein^-1 (1.7-fold stimulation). The K⁺-stimulated ATPase activity was inhibited approximately 93% by 0.5 mM vanadate but hardly by N,N-dicyclohexylcarbo-diimide (DCCD). These results indicate that the inducible K⁺-ATPase in R. sphaeroides resembles the Kdp K⁺-translocating ATPase of Escherichia coli. This Kdp-like transport system is also expressed in R. capsulatus and Rhodospirillum rubrum during growth in media with low K⁺ concentrations suggesting a wide distribution of this transport system among phototrophic bacteria.Abbreviations electrical potential difference across the cytoplasmic membrane - pH pH difference across the cytoplasmic membrane - BSA bovine serum albumine - PAGE polyacrylamide gel electrophoresis - HEPES 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid - PMSF phenyl-methyl-sulfonyl fluoride - DCCD N,N-dicyclohexylcarbodiimide - AIB 2--aminoisobutyric acid - TMG methyl--d-thiogalactopyranoside     

Density gradient characterization of the high density lipoproteins in cholesterol-fed hyper- and hyporesponding patas monkeys (Erythrocebus patas)

Diet-induced changes in high density lipoprotein (HDL) density and size were studied in patas monkeys. When the animals were switched from a moderate fat-low cholesterol diet to a high fat-high cholesterol (HFHC) diet, the plasma apoA-I levels increased initially in all of the animals. The apoA-I levels remained elevated in monkeys able to maintain their plasma cholesterol concentrations near basal levels (hyporesponders), but began to decrease in monkeys who became severely hypercholesterolemic (hyperresponders), reaching levels as low as 65-70% of their basal value by 24 weeks. The larger, lipid-rich HDL (HDL2) was shown by density gradient ultracentrifugation and gradient-PAGE (polyacrylamide gel electrophoresis) to be the HDL fraction responsible for these changes in apoA-I, completely accounting for the increase in apoA-I in hyporesponders and the decrease in apoA-I in hyperresponders. The HDL3 levels remained unchanged in hyporesponders but increased markedly in hyperresponders, partially compensating for the decrease of HDL2 in those animals. Gradient-PAGE showed the HDL3 to be heterogeneous, containing at least two populations of particles of the same density but differing significantly in size. The smaller of these HDL3 were most prominent in the HFHC-fed hyperresponders. These data show that nonhuman primate HDL is both physically and metabolically heterogeneous, and indicate that a high fat-high cholesterol diet-induced hypercholesterolemia severely depresses the HDL2 levels.     

Mode of action of LciA, the lactococcin A immunity protein

Monoclonal antibodies were raised against a fusion between the Escherichia coli maltose-binding protein and LciA, the immunity protein that protects Lactococcus lactis against the effects of the bacteriocin lactococcin A. One of the antibodies directed against the LciA moiety of the fusion protein was used to locate the immunity protein in the L. lactis producer cell. LciA was present in the cytosolic. the membrane-associated, and the membrane fractions in roughly equal amounts, irrespective of the production by the cells of lactococcin A. The monoclonal antibody specifically reacted with right-side-out vesicles obtained from a strain producing the immunity protein. It did not react with inside-out vesicles of the same strain, or with right-side-out vesicles obtained from a strain producing both LciA and lactococcin A. Also, externally added lactococcin A blocked the interaction between the antibody and right-side-out vesicles obtained from a strain producing only LciA. The epitope in LciA was localized between amino acid residues 60 and 80. As the epitope could be removed from right-side-out vesicles by proteinase K, it is located at the outside of the cell. The immunity protein contains a putative a-amphiphilic helix from residue 29 to 47. A model is proposed in which this helix is thought to traverse the membrane in such a way that the C-terminal part of the protein, containing the epitope, is on the outside of the cell. Vesicle-fusion studies together with leucine-uptake experiments suggest that the immunity protein interacts with the putative receptor for lactococcin A, thus preventing pore formation by the bacteriocin.     

Characterization of two phosphate transport systems in Acinetobacter johnsonii 210A.

The transport of P(i) was characterized in Acinetobacter johnsonii 210A, which is able to accumulate an excessive amount of phosphate as polyphosphate (polyP) under aerobic conditions. P(i) is taken up against a concentration gradient by energy-dependent, carrier-mediated processes. A. johnsonii 210A, grown under P(i) limitation, contains two uptake systems with Kt values of 0.7 +/- 0.2 microM and 9 +/- 1 microM. P(i) uptake via the high-affinity component is drastically reduced by N,N'-dicyclohexylcarbodiimide, an inhibitor of H(+)-ATPase, and by osmotic shock. Together with the presence of P(i)-binding activity in concentrated periplasmic protein fractions, these results suggest that the high-affinity transport system belongs to the group of ATP-driven, binding-protein-dependent transport systems. Induction of this transport system upon transfer of cells grown in the presence of excess P(i) to P(i)-free medium results in a 6- to 10-fold stimulation of the P(i) uptake rate. The constitutive low-affinity uptake system for P(i) is inhibited by uncouplers and can mediate counterflow of P(i), indicating its reversible, secondary nature. The presence of an inducible high-affinity uptake system for P(i) and the ability to decrease the free internal P(i) pool by forming polyP enable A. johnsonii 210A to reduce the P(i) concentration in the aerobic environment to micromolar levels. Under anaerobic conditions, polyP is degraded again and P(i) is released via the low-affinity secondary transport system.     

Betaine and L-carnitine transport by Listeria monocytogenes Scott A in response to osmotic signals.

The naturally occurring compatible solutes betaine and L-carnitine allow the food-borne pathogen Listeria monocytogenes to adjust to environments of high osmotic strength. Previously, it was demonstrated that L. monocytogenes possesses an ATP-dependent L-carnitine transporter (A. Verheul, F. M. Rombouts, R. R. Beumer, and T. Abee, J. Bacteriol. 177:3205-3212, 1995). The present study reveals that betaine and L-carnitine are taken up by separate highly specific transport systems and support a secondary transport mechanism for betaine uptake in L. monocytogenes. The initial uptake rates of betaine and L-carnitine are not influenced by an osmotic upshock, but the duration of transport of both osmolytes is directly related to the osmotic strength of the medium. Regulation of uptake of both betaine and L-carnitine is subject to inhibition by preaccumulated solute. Internal betaine inhibits not only transport of external betaine but also that of L-carnitine and, similarly, internal L-carnitine inhibits transport of both betaine and L-carnitine. The inhibition is alleviated upon osmotic upshock, which suggests that alterations in membrane structure are transmitted to the allosteric binding sites for betaine and L-carnitine of both transporters at the inner surface of the membrane. Upon osmotic downshock, betaine and L-carnitine are rapidly released by L. monocytogenes as a consequence of activation of a channel-like activity. The osmolyte-sensing mechanism described is new and is consistent with various unexplained observations of osmoregulation in other bacteria.     

Malolactic fermentation: electrogenic malate uptake and malate/lactate antiport generate metabolic energy.

The mechanism of metabolic energy production by malolactic fermentation in Lactococcus lactis has been investigated. In the presence of L-malate, a proton motive force composed of a membrane potential and pH gradient is generated which has about the same magnitude as the proton motive force generated by the metabolism of a glycolytic substrate. Malolactic fermentation results in the synthesis of ATP which is inhibited by the ionophore nigericin and the F0F1-ATPase inhibitor N,N-dicyclohexylcarbodiimide. Since substrate-level phosphorylation does not occur during malolactic fermentation, the generation of metabolic energy must originate from the uptake of L-malate and/or excretion of L-lactate. The initiation of malolactic fermentation is stimulated by the presence of L-lactate intracellularly, suggesting that L-malate is exchanged for L-lactate. Direct evidence for heterologous L-malate/L-lactate (and homologous L-malate/L-malate) antiport has been obtained with membrane vesicles of an L. lactis mutant deficient in malolactic enzyme. In membrane vesicles fused with liposomes, L-malate efflux and L-malate/L-lactate antiport are stimulated by a membrane potential (inside negative), indicating that net negative charge is moved to the outside in the efflux and antiport reaction. In membrane vesicles fused with liposomes in which cytochrome c oxidase was incorporated as a proton motive force-generating mechanism, transport of L-malate can be driven by a pH gradient alone, i.e., in the absence of L-lactate as countersubstrate. A membrane potential (inside negative) inhibits uptake of L-malate, indicating that L-malate is transported an an electronegative monoanionic species (or dianionic species together with a proton). The experiments described suggest that the generation of metabolic energy during malolactic fermentation arises from electrogenic malate/lactate antiport and electrogenic malate uptake (in combination with outward diffusion of lactic acid), together with proton consumption as result of decarboxylation of L-malate. The net energy gain would be equivalent to one proton translocated form the inside to the outside per L-malate metabolized.     

arcD, the first gene of the arc operon for anaerobic arginine catabolism in Pseudomonas aeruginosa, encodes an arginine-ornithine exchanger.

In the absence of oxygen and nitrate, Pseudomonas aeruginosa metabolizes arginine via the arginine deiminase pathway, which allows slow growth on rich media. The conversion of arginine to ornithine, CO2, and NH3 is coupled to the production of ATP from ADP. The enzymes of the arginine deiminase pathway are organized in the arcDABC operon. The arcD gene encodes a hydrophobic polytopic membrane protein. Translocation of arginine and ornithine in membrane vesicles derived from an Escherichia coli strain harboring a recombinant plasmid carrying the arcD gene was studied. Arginine and ornithine uptake was coupled to the proton motive force with a bias toward the transmembrane electrical potential. Accumulated ornithine was readily exchangeable for external arginine or lysine. The exchange was several orders of magnitude faster than proton motive force-driven transport. The ArcD protein was reconstituted in proteoliposomes after detergent solubilization of membrane vesicles. These proteoliposomes mediate a stoichiometric exchange between arginine and ornithine. It is concluded that the ArcD protein is a transport system that catalyzes an electroneutral exchange between arginine and ornithine to allow high-efficiency energy conversion in the arginine deiminase pathway.     

Mechanism of action of the peptide antibiotic nisin in liposomes and cytochrome c oxidase-containing proteoliposomes.

The interaction of the peptide antibiotic nisin with liposomes has been studied. The effect of this interaction was analyzed on the membrane potential (inside negative) and the pH gradient (inside alkaline) in liposomes made from Escherichia coli phosphatidylethanolamine and egg phosphatidylcholine (9:1, wt/wt). The membrane potential and pH gradient were generated by artificial ion gradients or by the oxidation of ascorbate, N,N,N',N'-tetramethyl-p-phenylenediamine, and cytochrome c by the beef heart cytochrome c oxidase incorporated in the liposomal membranes. Nisin dissipated the membrane potential and the pH gradient in both types of liposomes and inhibited oxygen consumption by cytochrome c oxidase in proteoliposomes. The dissipation of the proton motive force in proteoliposomes was only to a minor extent due to a decrease of the oxidase activity by nisin. The results in these model systems show that a membrane potential and/or a pH gradient across the membrane enhances the activity of nisin. Nisin incorporates into the membrane and makes the membrane permeable for ions. As a result, both the membrane potential and pH gradient are dissipated. The activity of nisin was found to be influenced by the phospholipid composition of the liposomal membrane.     

Biochemical genetic markers of squirrel monkeys and their use for pedigree validation

Family data for 14 biochemical genetic markers of squirrel monkeys (genusSaimiri) were derived from 73 pedigreed progeny and both parents of each, as well as from 16 additional progeny and one parent of each. The data for each marker and the phenotypic patterns were consistent with autosomal codominant inheritance. It was concluded from the genetic marker data that the pedigree records of seven progeny were incorrect. Retrospective investigations of colony records followed by typing of animals that might possibly have been a parent enabled five of the pedigree records to be corrected. Although five of the pedigree errors were cases of mistaken paternity, the other two apparently were the consequence of infant swapping between dams shortly after birth. Because squirrel monkeys exhibit a high degree of allomaternal behavior, infant swapping between dams may occur more frequently than in many other nonhuman primate species.This research was supported in part by NIH Grant P40 RR01254.     

Characterization of histatin 5 with respect to amphipathicity, hydrophobicity, and effects on cell and mitochondrial membrane integrity excludes a candidacidal mechanism of pore formation

Histatin 5 is a 24-residue peptide from human saliva with antifungal properties. We recently demonstrated that histatin 5 translocates across the yeast membrane and targets to the mitochondria, suggesting an unusual antifungal mechanism (Helmerhorst, E. J., Breeuwer, P., van't Hof, W., Walgreen-Weterings, E., Oomen, L. C. J. M., Veerman, E. C. I., Nieuw Amerongen, A. V., and Abee, T. (1999) J. Biol. Chem. 274, 7286-7291). The present study used specifically designed synthetic analogs of histatin 5 to elucidate the role of peptide amphipathicity, hydrophobicity, and the propensity to adopt alpha-helical structures in relation to membrane permeabilization and fungicidal activity. Studies included circular dichroism measurements, evaluation of the effects on the cytoplasmic transmembrane potential and on the respiration of isolated mitochondria, and analysis of the peptide hydrophobicity/amphipathicity relationship (Eisenberg, D. (1984) Annu. Rev. Biochem. 53, 595-623). The 14-residue synthetic peptides used were dh-5, comprising the functional domain of histatin 5, and dhvar1 and dhvar4, both designed to maximize amphipathic characteristics. The results obtained show that the amphipathic analogs exhibited a high fungicidal activity, a high propensity to form an alpha-helix, dissipated the cytoplasmic transmembrane potential, and uncoupled the respiration of isolated mitochondria, similar to the pore-forming peptide PGLa (Peptide with N-terminal Glycine and C-terminal Leucine-amide). In contrast, histatin 5 and dh-5 showed fewer or none of these features. The difference in these functional characteristics between histatin 5 and dh-5 on the one hand and dhvar1, dhvar4, and PGLa on the other hand correlated well with their predicted affinity for membranes based on hydrophobicity/amphipathicity analysis. These data indicate that the salivary protein histatin 5 exerts its antifungal function through a mechanism other than pore formation.