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PubMed1990

Automated determination of fluoride using silicone-facilitated diffusion.

An automated procedure for the determination of fluoride in biological fluids that uses silicone-facilitated diffusion at room temperature to separate fluoride from other sample constituents is described. Fluoride is measured by its ability to quench the fluorescence of a morin-thorium complex. The procedure appears to be free of significant interferences and is capable of measuring fluoride in the submicromolar range.

Animals↗
PubMed1972

Importance of facilitated diffusion for effective utilization of glycerol by Escherichia coli.

Wild-type Escherichia coli possesses an inducible permeation system which catalyzes facilitated diffusion of glycerol into the cell. A spectrophotometric method can be used to assess the presence of this mechanism. The structural gene for the facilitator (glpF) and the structural gene for glycerol kinase (glpK) apparently belong to a single operon. The glpF(+) allele permits effective glycerol utilization by the cells, and, at millimolar concentrations of glycerol, cells carrying the glpF(+) allele grow much faster than glpF genotypes. Although the glycerol-scavenging power of the cell depends both on the facilitated entry of the substrate and its subsequent trapping by an adenosine triphosphate-dependent phosphorylation, the two gene products, the facilitator and kinase, function independently. Wild-type Shigella flexneri appears to be glpK(+) but glpF. This organism grows slowly in media at low concentrations of glycerol. When the glpF(+) and glpK(+) alleles of E. coli are inserted into the S. flexneri genome by transduction, the hybrid strain grows rapidly in low glycerol medium. Vice versa, when the glpF and glpK(+) alleles of S. flexneri are incorporated into E. coli, the hybrid strain grows slowly in low glycerol medium.

Alleles↗
PubMed1991

Mechanisms for the facilitated diffusion of substrates across cell membranes.

Two classes of theoretical mechanisms for protein-mediated, passive, transmembrane substrate transport (facilitated diffusion) are compared. The simple carrier describes a carrier protein that exposes substrate influx and efflux sites alternately but never both sites simultaneously. Two-site models for substrate transport describe carrier proteins containing influx and efflux sites simultaneously. Velocity equations describing transport by these mechanisms are derived. These equations take the same general form, being characterized by five experimental constants. Simple carrier-mediated transport is restricted to hyperbolic kinetics under all conditions. Two-site carrier-mediated transport may deviate from hyperbolic kinetics only under equilibrium exchange conditions. When both simple- and two-site carriers display hyperbolic kinetics under equilibrium exchange conditions, these models are indistinguishable by using steady-state transport data alone. Seven sugar transport systems are analyzed. Five of these systems are consistent with both models for sugar transport. Uridine, leucine, and cAMP transport by human red cells are consistent with both simple- and two-site models for transport. Human erythrocyte sugar transport can be modeled by simple- and two-site carrier mechanisms, allowing for compartmentalization of intracellular sugars. In this instance, resolution of the intrinsic properties of the human red cell sugar carrier at 20 degrees C requires the use of submillisecond transport measurements.

Biological Transport↗
PubMed2000

The yeast gene MSC2, a member of the cation diffusion facilitator family, affects the cellular distribution of zinc.

The sequence of the yeast gene YDR205W places it within the family of cation diffusion facilitators: membrane proteins that transport transition metals. Deletion of YDR205W was reported to result in an increase in unequal sister chromatid recombination and was named meiotic sister chromatid recombination 2 (MSC2; Thompson, D. A., and Stahl, F. W. (1999) Genetics 153, 621-641). We report here that a msc2 strain shows a phenotype of decreased viability in glycerol-ethanol media at 37 degrees C. Associated with decreased growth is an abnormal morphology typified by an increase in size of both cells and vacuoles. Addition of extracellular Zn2+ completely suppresses the morphological changes and partially suppresses the growth defect. Regardless of the concentration of Zn2+ in the media, the msc2 strain had a higher Zn2+ content than wild type cells. Zinquin staining also revealed that msc2 had a marked increase in fluorescence compared with the wild type, again reflecting an increase in intracellular Zn2+. The deletion strain accumulated excess Zn2+ in nuclei-enriched membrane fractions, and when grown at 37 degrees C in glycerol-ethanol media, it showed a decreased expression of Zn2+-regulated genes. The expression of genes regulated by either Fe2+ or Cu2+ was not affected. An epitope-tagged Msc2p was localized to the endoplasmic reticulum/nucleus. These results suggest that Msc2p affects the cellular distribution of zinc and, in particular, the zinc content of nuclei.

Carrier Proteins↗
PubMed2004

Cation diffusion facilitator proteins modulate Raf-1 activity.

The Ras-extracellular signal-regulated kinase (ERK) cascade is a critical intracellular signaling pathway that regulates growth, survival, and differentiation. Previous work established that Ras-GTP binds to, and facilitates the activation of, the protein kinase Raf-1. Recently, it was demonstrated that the cation diffusion facilitator (CDF) proteins are involved in Ras-ERK signaling by use of a Caenorhabditis elegans genetic screen that identified suppressors of activated Ras. In the current work, we demonstrate that CDF proteins may function downstream of Ras, but upstream of Raf-1 in Xenopus oocytes. We also show that the C. elegans protein CDF-1 and its mammalian homologue ZnT-1 bind to the amino-terminal regulatory portion of Raf-1 and promote the biological and enzymatic activity of Raf-1. Furthermore, we show that Zn(2+) inhibits Raf-1 binding to ZnT-1. We propose a model in which CDF protein binding facilitates Raf-1 activation.

14-3-3 Proteins↗
PubMed1993

Interaction between facilitated diffusion of glucose across the plasma membrane and its metabolism in Trichomonas vaginalis.

The parasitic protist Trichomonas vaginalis transports glucose across the plasma membrane by facilitated diffusion. The Km of the transporter for glucose was 1.6 mM. The uptake of labelled glucose in a minimal medium not allowing growth reached saturation only after 2.5 h, indicating the turnover of storage carbohydrate. Organisms grown on glucose showed higher activities both of the transporter and of the subsequent metabolic pathway than organisms grown on maltose. At low external glucose concentrations the transport step was rate limiting, at higher levels a subsequent enzymatic step. The uptake mechanism for glucose of T. vaginalis resembled that of parasitic kinetoplastid protists and Entamoeba histolytica.

Animals↗
PubMed1973

Facilitated diffusion of monosaccharides in Saccharomyces cerevisiae: experimental investigation of kinetic parameters without the assumptions of symmetry.

Until the question of symmetry or asymmetry in the facilitated diffusion of monosaccharides by Saccharomyces cerevisiae is resolved, attempts to study the transport process cannot be based on assumptions of symmetry, such as equal concentrations at equilibrium or kinetic parameters that are equal in opposite directions. The assumptions of symmetry may be circumvented by measuring efflux against water and against various external concentrations of sugar. The measurement of efflux against water eliminates any involvement of influx, and the separate determinations of influx and efflux parameters do not require that the parameters be equal. Furthermore, the use of relative internal concentrations eliminates any necessity of assuming that the equilibrium concentrations are equal. Since the influx and efflux parameters are to be compared, the measurement of influx on effluxing cells allows both sets of parameters to be determined on cells which are physiologically the same. This procedure has been tested by obtaining the kinetic parameters of l-sorbose transport. The validity of these parameters was demonstrated by using them to generate theoretical efflux curves that fit the experimental data and by showing that they give the best fit curve to the relationship of velocity and permeant concentration. Although the question of symmetry remains unanswered, this procedure has opened the way for experimental evaluation of the situation and further investigation of the transport process in yeast.

Biological Transport↗
PubMed2004

Characteristics of zinc transport by two bacterial cation diffusion facilitators from Ralstonia metallidurans CH34 and Escherichia coli.

CzcD from Ralstonia metallidurans and ZitB from Escherichia coli are prototypes of bacterial members of the cation diffusion facilitator (CDF) protein family. Expression of the czcD gene in an E. coli mutant strain devoid of zitB and the gene for the zinc-transporting P-type ATPase zntA rendered this strain more zinc resistant and caused decreased accumulation of zinc. CzcD, purified as an amino-terminal streptavidin-tagged protein, bound Zn2+, Co2+, Cu2+, and Ni2+ but not Mg2+, Mn2+, or Cd2+, as shown by metal affinity chromatography. Histidine residues were involved in the binding of 2 to 3 mol of Zn2+ per mol of CzcD. ZitB transported 65Zn2+ in the presence of NADH into everted membrane vesicles with an apparent Km of 1.4 microM and a Vmax of 0.57 nmol of Zn2+ min(-1) mg of protein(-1). Conserved amino acyl residues that might be involved in binding and transport of zinc were mutated in CzcD and/or ZitB, and the influence on Zn2+ resistance was studied. Charged or polar amino acyl residues that were located within or adjacent to membrane-spanning regions of the proteins were essential for the full function of the proteins. Probably, these amino acyl residues constituted a pathway required for export of the heavy metal cations or for import of counter-flowing protons.

Amino Acid Substitution↗
PubMed2002

Zinc ions and cation diffusion facilitator proteins regulate Ras-mediated signaling.

C. elegans cdf-1 was identified in a genetic screen for regulators of Ras-mediated signaling. CDF-1 is a cation diffusion facilitator protein that is structurally and functionally similar to vertebrate ZnT-1. These proteins have an evolutionarily conserved function as positive regulators of the Ras pathway, and the Ras pathway has an evolutionarily conserved ability to respond to CDF proteins. CDF proteins regulate Ras-mediated signaling by promoting Zn(2+) efflux and reducing the concentration of cytosolic Zn(2+), and cytosolic Zn(2+) negatively regulates Ras-mediated signaling. Physiological concentrations of Zn(2+) cause a significant inhibition of Ras-mediated signaling. These findings suggest that Zn(2+) negatively regulates a conserved element of the signaling pathway and that Zn(2+) regulation is important for maintaining the inactive state of the Ras pathway.

Amino Acid Sequence↗
PubMed2006

Facilitated diffusion of DNA-binding proteins: Simulation of large systems.

The recently introduced method of excess collisions to estimate reaction times of protein-DNA systems in the presence of facilitated diffusion ("sliding") requires a cell of full system size. This bottleneck is avoided with a modification, by which a set of empirical parameters is calibrated using numerical simulations of a small test system. Once this is done, reaction times for systems of arbitrary dimensions are derived by extrapolation. It is shown that at physiological sliding lengths a test system of the order of 100 nm radius suffices to extract accurate reaction times for realistic cell dimensions. The achieved speedup, when compared to explicit simulations of the reaction process, is increasing in third order of the extrapolated radius of the cell.

Algorithms↗
PubMed1985

Facilitated diffusion of carbon dioxide in whole blood and hemoglobin solutions.

The values of effective permeability (Krogh's diffusion coefficient) for carbon dioxide have been measured in horizontal stationary layers of whole blood and hemoglobin solutions in quasi-steady state, with the goal of understanding the specific nature of facilitated diffusion of carbon dioxide occurring in these media. The average partial pressure of carbon dioxide within the layer ranged from 0.74 kPa (5.6 mm Hg) to 15.7 kPa (118 mm Hg). Facilitation effects were significant in hemolysed blood and in hemoglobin solutions at low pCO2; the facilitation factor was up to 2.3. Facilitation effects were considerably less for intact blood; the facilitation factor of intact blood with hematocrit 45% was 0.3. The presence of the red cell membrane appears to have a negative effect on facilitation of carbon dioxide transport.

Animals↗
PubMed2004

Characterization of a Glomus intraradices gene encoding a putative Zn transporter of the cation diffusion facilitator family.

A full-length cDNA (GintZnT1) encoding a putative Zn transporter was isolated from the extraradical mycelium of Glomus intraradices. Based on its sequence analysis, GintZnT1 was classified as a member of the cation diffusion facilitator (CDF) family of heavy metal transporters. Functional analysis of GintZnT1 was performed by heterologous expression in yeast mutants defective in different CDFs. Although Zn sensitivity of the mutants was not reverted, an effect of GintZnT1 on the labile regulatory Zn pool was detected by using a Zn-regulated beta-galactosidase reporter gene. GintZnT1 expression was studied in the extraradical mycelium obtained from a symbiotic root organ culture. Gin +/- ZnT1 was up-regulated in the extraradical mycelium of G. intraradices upon short-time exposure to Zn and when the mycelia were developed in 75 microM Zn supplemented plates. These data suggest a role of GintZnT1 in Zn compartmentalization and in the protection of G. intraradices against Zn stress.

Amino Acid Sequence↗
PubMed2004

The chromosomally encoded cation diffusion facilitator proteins DmeF and FieF from Wautersia metallidurans CH34 are transporters of broad metal specificity.

Genomic sequencing of the beta-proteobacterium Wautersia (previously Ralstonia) metallidurans CH34 revealed the presence of three genes encoding proteins of the cation diffusion facilitator (CDF) family. One, CzcD, was previously found to be part of the high-level metal resistance system Czc that mediates the efflux of Co(II), Zn(II), and Cd(II) ions catalyzed by the CzcCBA cation-proton antiporter. The second CDF protein, FieF, is probably mainly a ferrous iron detoxifying protein but also mediated some resistance against other divalent metal cations such as Zn(II), Co(II), Cd(II), and Ni(II) in W. metallidurans or Escherichia coli. The third CDF protein, DmeF, showed the same substrate spectrum as FieF, but with different preferences. DmeF plays the central role in cobalt homeostasis in W. metallidurans, and a disruption of dmeF rendered the high-level metal cation resistance systems Czc and Cnr ineffective against Co(II). This is evidence for the periplasmic detoxification of substrates by RND transporters of the heavy metal efflux family subgroup.

Bacterial Proteins↗
PubMed1986

Reduction of anoxia through myoglobin-facilitated diffusion of oxygen.

At relatively low perfusion rates, anoxic regions may occur in tissue even though oxygen remains in the blood as it leaves the capillary at the venous end. In this paper a mathematical theory of facilitated diffusion is developed and used to determine the extent to which myoglobin increases the removal of oxygen from blood and aids in the reduction or elimination of regions of anoxia.

Anaerobiosis↗
PubMed2003

Genes encoding proteins of the cation diffusion facilitator family that confer manganese tolerance.

The yeast Saccharomyces cerevisiae expressing a cDNA library prepared from Stylosanthes hamata was screened for enhanced Mn(2+) tolerance. From this screen, we identified four related cDNAs that encode membrane-bound proteins of the cation diffusion facilitator (CDF) family. One of these cDNAs (ShMTP1) was investigated in detail and found to confer Mn(2+) tolerance to yeast by internal sequestration rather than by efflux of Mn(2+). Expression of ShMTP1 in a range of yeast mutants suggested that it functions as a proton:Mn(2+) antiporter on the membrane of an internal organelle. Similarly, when expressed in Arabidopsis, ShMTP1 conferred Mn(2+) tolerance through internal sequestration. The ShMTP1 protein fused to green fluorescent protein was localized to the tonoplast of Arabidopsis cells but appeared to localize to the endoplasmic reticulum of yeast. We suggest that the ShMTP1 proteins are members of the CDF family involved in conferring Mn(2+) tolerance and that at least one of these proteins (ShMTP1) confers tolerance by sequestering Mn(2+) into internal organelles.

Adaptation, Physiological↗
PubMed1987

Facilitated diffusion properties of melibiose permease in Escherichia coli membrane vesicles. Release of co-substrates is rate limiting for permease cycling.

The mechanism of melibiose symport by the melibiose permease of Escherichia coli was studied by looking at the modifications of the facilitated diffusion properties of the permease which arise upon substitution of the coupled cations (H+, Na+, or Li+). Kinetic analysis of melibiose influx and efflux down a concentration gradient, exchange at equilibrium, and counterflow were examined in de-energized membrane vesicles resuspended in media allowing melibiose to be co-transported with either H+, Na+, or Li+. The data show that the maximal rates of melibiose efflux coupled to either H+, Na+, or Li+ are between 10 and 40 times faster than the corresponding influxes. This suggests that the permease functions asymmetrically. Cross-comparison between the rates of net [3H]melibiose entry during the influx reactions coupled to either cation and corresponding unidirectional sugar inflow during exchange and counterflow reactions leads to the conclusions that: 1) the step involving release of the co-substrates from the permease on the inner surface of the membrane is sequenced (sugar first and then coupled cation); 2) this step is rate determining for cycling of the permease. The Na+-melibiose passive flux data indicate in particular that release of Na+ ions rather than release of sugar into the intravesicular space is the slowest step during permease cycling. This property would hamper net passive Na+-melibiose influx but should allow exchange of sugar without concomitant exchange of the coupled cation. Finally, evidence is provided suggesting that the relative rates of release of the two co-substrates from the permease on the inner membrane surface varied considerably in relation to the identity of the coupled cation.

Diffusion↗
PubMed1980

Facilitated diffusion as a method for selective accumulation of materials from the primordial oceans by a lipid-vesicle protocell.

A model is proposed for the selective accumulation of amino acids, sugars, nucleotides, cations and protons from the primordial oceans into a lipid vesicle type of protocell. The model is built on facilitated diffusion using simple, primordial, lipid-soluble carriers. The advantages a lipid vesicle protocell would have had over the other potential types of protocells are discussed.

Amino Acids↗