Reconstitution of binding protein dependent ribose transport in spheroplasts derived from a binding protein negative escherichia coli K12 mutant and from salmonella typhimurium

Highly purified ribose-binding protein from Escherichia coli has been used to reconstitute a binding-protein-dependent ribose transport in spheroplasts derived from a binding-protein-deficient mutant of E coli K 12, and in spheroplasts derived from Salmonella typhimurium. The cross-species reconstitution was nearly as efficient as the reconstitution of the E coli strain from which the binding protein was derived. Antibody raised against the ribose binding protein completely prevented reconstitution, whereas it had no effect on whole cells. The reconstitution procedure has been improved by generating spheroplasts from cells grown in a rich medium and by reducing the background uptake in spheroplasts through a special washing procedure. Rapid purification of ribose binding protein by high pressure liquid chromatography is also described.     

Exclusion of Induced Bacteriophage from Cells of a Lysogenic Bacillus megaterium Committed to Sporulation

In Escherichia coli ML 308-225, d-ribose is transported into the cell by a constitutive active transport system of high activity. The activity of this transport system is severely reduced in cells subjected to osmotic shock, and the system is not present in membrane vesicles. The mechanism by which metabolic energy is coupled to transport of ribose was investigated. Substrates which generate adenosine 5'-triphosphate primarily through oxidative phosphorylation are poor energy sources for ribose uptake in DL-54, a mutant of ML 308-225 which lacks activity for the membrane-bound Ca(2+), Mg(2+)-dependent adenosine triphosphatase required for oxidative phosphorylation. Arsenate severely inhibits ribose uptake, whereas, under the same conditions, uptake of l-proline is relatively insensitive to arsenate. Anaerobiosis does not significantly inhibit ribose uptake in ML 308-225 or DL-54 when glucose is the energy source. A significant amount of ribose uptake is resistant to uncouplers of oxidative phosphorylation such as 2,4-dinitrophenol. These results indicate that the phosphate bond energy of adenosine 5'-triphosphate, rather than an energized membrane state, couples energy to ribose transport in ML 308-225.     

Unusual binding sites for horseradish peroxidase on the surface of cultured and isolated mammalian cells. Suppression of binding by certain nucleotides and glycoproteins, and a role for calcium

Binding sites for horseradish peroxidase (HRP), with unusual properties, were detected on the surface of cultured and isolated cells after the cells (on cover slips) had been quickly dried, fixed in cold methanol, and post-fixed in a paraformaldehyde solution. The reaction for surface-bound HRP was suppressed by micromolar concentrations of glycoproteins such as invertase, equine luteinizing hormone (eLH) or human chorionic gonadotropin (hCG). The reaction was also suppressed by 20 mM CDP, UDP, GTP, NAD, and ribose 5-phosphate. Two to six times higher concentrations of GMP, fructose 1-phosphate, galactose 6-phosphate, mannose 6-phosphate, fructose 6-phosphate, and glucose 6-phosphate were required to suppress the binding reaction. AMP, ATP, heparin, mannan, and eight non-phosphorylated sugars showed relatively low competing potencies but fucoidin and alpha-lactalbumin were strong inhibitors. No addition of Ca2+ was required for the binding of HRP to the cell surface. However, calcium-depleted, inactive HRP did not compete with the binding of native (calcium-containing) HRP whereas H2O2-inactivated HRP suppressed the binding. GTP, NAD, ribose 5-phosphate, and EGTA accelerated the release of previously-bound HRP from the cell surface whereas glycoproteins (invertase, eLH, and hCG) did not do so. Addition of Ca2+ to GTP, NAD, ribose 5-phosphate or to EGTA prevented the accelerated release of HRP from the cell surface. It is suggested that calcium, present either in the surface membrane or in HRP itself, is involved in the binding of HRP to the cell surface and in the inhibition of binding by GTP, NAD, and ribose 5-phosphate. It is also suggested that alpha-lactalbumin, GTP, UDP, and CDP compete with the binding of HRP to a glycosyltransferase on the cell surface.     

Genetically probing the regions of ribose-binding protein involved in permease interaction

The ribose-binding protein (RBP) of Escherichia coli , located in the periplasm, binds to ribose and mediates transport and chemotaxis. The regions on the tertiary structure of RBP that interact with the membrane permease, an ABC transporter, were genetically probed by screening a mutation using the chimeric receptor Trz. Trz is a hybrid protein between the periplasmic domain of chemoreceptor Trg and the cytoplasmic portion of osmosensor EnvZ, which provides a system for monitoring the chemotactic interaction of RBP on MacConkey agar plates when coupled with a reporter lacZ fused to an ompC gene. The expression of ompC can be increased by an interaction of ribose-bound RBP with Trz. A transport defect, either in the binding protein or in the membrane permease, causes a signalling-constitutive Lac⁺ phenotype of Trz even in the absence of ribose. This appears to be due to the presence of a small amount of ribose, which is normally taken up by the high-affinity transport system. By taking advantage of this, we have designed a system for genetic screening that permits a selection for mutations in the binding protein, causing specific defects in permease interaction but not in tactic interaction. Mutant RBPs that were isolated were unable to perform normal ribose uptake and to utilize ribose as a carbon source, while other functions such as taxis and sugar-binding properties were not substantially affected. The mutational changes were repeatedly found in several residues of RBP, concentrating on three surface regions and comprising two domains of the tertiary structure. We suggest that the two regions, including residues 52 and 166, are specifically involved in the permease interaction while the third region, including residues 72, 134, and others, recognizes both the permease and the chemosensory receptor.     

Molecular interactions in ribose transport: the binding protein module symmetrically associates with the homodimeric membrane transporter.

The Escherichia coli high-affinity ribose transporter is composed of the periplasmic ribose-binding protein (RBP or RbsB), the membrane component (RbsC) and the ATP-binding protein (RbsA). In order to dissect the molecular interactions initiating the transport process, RbsC suppressors for transport-defective rbsB mutations were isolated. These suppressors are localized in two regions of RbsC, which are allele-specific to N- or C-terminal domain mutations of RBP, suggesting that there are two distinct regions of RbsC, each interacting with one of the two domains of RBP. To demonstrate that these two regions provide a homodimeric binding surface for RBP we constructed a dimeric rbsC in which two genes are joined tandemly from head to tail with the addition of a linker. The dimeric RbsC protein is stable and functional in growth and ribose uptake. By exploiting the allele specificity between the domain-specific mutations and their suppressors, we generated all mutation-suppressor combinations in a single rbsB plus the dimeric rbsC genes. Their phenotypes are consistent with the proposal that the binding protein module interacts symmetrically with homodimeric RbsC. The mode of association proposed here for the ribose transport components could be extended to other ABC transporters with similar structural organizations.     

Reconstitution of binding protein-dependent ribose transport in spheroplasts of Escherichia coli K-12.

Purified Escherichia coli K-12 ribose binding protein was used to reconstitute the high affinity ribose transport system in spheroplasts derived from ribose-induced cells. It was not possible to reconstitute ribose transport in spheroplasts derived from uninduced cells or from transport-negative mutant strains, suggesting that one or more additional inducible components are required for binding protein-dependent ribose transport. It was possible to reconstitute transport in a ribokinase-deficient mutant which constitutively transports but does not utilize ribose.     

Multiple genetic changes determine ribose utilization by Novikoff hepatoma cell variants

In variants of the Novikoff hepatoma cell line, the ability to use D-ribose as a carbon source appeared to be due to changes in the expression of ribokinase. Examination of ribokinase activity was prompted by the finding that uptake of radiolabeled ribose was linear for 30 min in six variants but became saturated within 2 min in nine other variants. The linear uptake of ribose was due to a high rate of phosphorylation by ribokinase. Variants which showed linear uptake kinetics had ribokinase levels of 6.8 +/- 1.7 nm/min per mg protein as compared to the parental levels of 0.90 +/- 0.25 nm/min per mg protein. The nine variants which showed saturable uptake kinetics had low parenteal levels of ribokinase. However, these variants showed a change in the subcellular location of that activity. The enzyme was predominantly membrane-associated in both parental cells and high ribokinase variants. In contrast, the low ribokinase variants had a cytoplasmic form of the enzyme. A more general membrane change probably occurred in these variants, since they showed an increased sensitivity to the unrelated membrane reactive compounds, phytohemagglutinin and ouabain.     

Uptake and utilization of nucleosides for energy repletion

In this paper, we report that cells undergoing metabolic stress conditions may use the ribose moiety of nucleosides as energy source to slow down cellular damage. In fact, the phosphorolytic cleavage of the N-glycosidic bond of nucleosides generates, without energy expense, the phosphorylated pentose, which through pentose phosphate pathway and glycolysis, can be converted to energetic intermediates. In this respect, nucleosides may be considered as energy source, alternative or supplementary to glucose, which may become of primary importance especially in conditions of cellular stress. In accordance with the role of these compounds in energy repletion, we also show that the uptake of nucleosides is increased when the energetic demand of the cell is enhanced. As cell model, we have used a human colon carcinoma cell line, LoVo, and the depletion of ATP, with a concomitant fall in the cell energy charge, has been induced by exclusion of glucose from the medium and pre-incubation with oligomycin, an inhibitor of oxidative phosphorylation. In these conditions of energy starvation, we show that the uptake of 2'-deoxyadenosine in LoVo cells is significantly enhanced, and that the phosphorylated ribose moiety of inosine can be used for energy repletion through anaerobic glycolysis. Our data support previous reports indicating that the phosphorylated ribose stemming from the intracellular catabolism of nucleosides may be used in eukaryots as energy source, and advance our knowledge on the regulation of the uptake of nucleosides in eukaryotic cells.     

TRAIL expression in vascular smooth muscle

TRAIL is a cell-associated tumor necrosis factor-related apoptosis-inducing ligand originally identified in immune cells. The ligand has the capacity to induce apoptosis after binding to cell surface receptors. To examine TRAIL expression in murine vascular tissue, we employed in situ hybridization and immunohistochemistry. In these studies, we found that TRAIL mRNA and protein were specifically localized throughout the medial smooth muscle cell layer of the pulmonary artery. Notably, a similar pattern of expression was observed in the mouse aorta. Consistent with these findings, we found that cultures of primary human aorta and pulmonary artery smooth muscle cells express abundant TRAIL mRNA and protein. We also found that these cells and endothelial cells undergo cell lysis in response to exogenous addition of TRAIL. Last, we confirmed that TRAIL specifically activated a death program by confirming poly(ADP ribose) polymerase cleavage. Overall, we believe that these findings are relevant to understanding the factors that regulate cell turnover in the vessel wall.     

Ribose utilization with an excess of mutarotase causes cell death due to accumulation of methylglyoxal

Methylglyoxal (MG) is a highly reactive metabolic intermediate, presumably accumulated during uncontrolled carbohydrate metabolism. The major source of MG is dihydroxyacetone phosphate, which is catalyzed by MG synthase (the mgs product) in bacteria. We observed Escherichia coli cell death when the ribose transport system, consisting of the RbsDACBK proteins, was overproduced on multicopy plasmids. Almost 100% of cell death occurs a few hours after ribose addition (>10 mM), due to an accumulation of extracellular MG as detected by (1)H-nuclear magnetic resonance ((1)H-NMR). Under lethal conditions, the concentration of MG produced in the medium reached approximately 1 mM after 4 h of ribose addition as measured by high-performance liquid chromatography. An excess of the protein RbsD, recently characterized as a mutarotase that catalyzes the conversion between the beta-pyran and beta-furan forms of ribose, was critical in accumulating the lethal level of MG, which was also shown to require ribokinase (RbsK). The intracellular level of ribose 5-phosphate increased with the presence of the protein RbsD, as determined by (31)P-NMR. As expected, a mutation in the methylglyoxal synthase gene (mgs) abolished the production of MG. These results indicate that the enhanced ribose uptake and incorporation lead to an accumulation of MG, perhaps occurring via the pentose-phosphate pathway and via glycolysis with the intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. It was also demonstrated that a small amount of MG is synthesized by monoamine oxidase.     

Unusual binding sites for horseradish peroxidase on the surface of cultured and isolated mammalian cells

Summary Binding sites for horseradish peroxidase (HRP), with unusual properties, were detected on the surface of cultured and isolated cells after the cells (on cover slips) had been quickly dried, fixed in cold methanol, and postfixed in a paraformaldehyde solution. The reaction for surface-bound HRP was suppressed by micromolar concentrations of glycoproteins such as invertase, equine luteinizing hormone (eLH) or human chorionic gonadotropin (hCG). The reaction was also suppressed by 20 mM CDP, UDP, GTP, NAD, and ribose 5-phosphate. Two to six times higher concentrations of GMP, fructose 1-phosphate, galactose 6 phosphate, mannose 6-phosphate, fructose 6-phosphate, and glucose 6-phosphate were required to suppress the binding eaction. AMP, ATP, heparin, mannan, and eight non-phosphorylated sugars showed relatively low competing potencies but fucoidin and -lactalbumin were strong inhibitors. No addition of Ca²⁺ was required for the binding of HRP to the cell surface. However, calcium-depleted, inactive HRP did not compete with the binding of native (calcium-containing) HRP whereas H₂O₂-inactivated HRP suppressed the binding. GTP, NAD, ribose 5-phosphate, and EGTA accelerated the release of previously-bound HRP from the cell surface whereas glycoproteins (invertase, cLH, and hCG) did not do se. Addition of Ca²⁺ to GTP, NAD, ribose 5-phosphate or to EGTA prevented the accelerated release of HRP from the cell surface. It is suggested that calciam, present either in the surface membrane or in HRP itself, is involved in the binding of HRP to the cell surface and in the inhibition of binding by GTP, NAD, and ribose 5-phosphate. It is also suggested that -lactalbumin, GTP, UDP, and CDP compete with the binding of HRP to a glycosyltransferase on the cell surface.     

Cadmium uptake by cells of renal origin

We compared the ability of rat glomerular mesangial cells and LLC-PK1 cells to take up Cd2+ from solution. The former are smooth muscle-like cells of mesenchymal origin, the latter an established line of proximal tubular epithelium. Both cells, as well as primary glomerular epithelia, accumulated Cd2+ against a concentration gradient in a time-dependent manner. Uptake by mesangial cells obeyed a Michaelis model with an apparent Km of 19 microM and could be described by an initial rapid step of surface binding followed by rate-limiting internalization. In contrast, uptake by LLC-PK1 cells was non-saturable under accessible concentrations of Cd2+ and internalization was not a necessary consequence of association with the cell surface. In several other cell types, Cd2+ uptake has been shown to be inhibited by blockage of cell-surface sulfhydryl groups. In contrast, uptake by neither mesangial nor LLC-PK1 cells was depressed by N-ethylmaleimide, which actually enhanced the surface binding and to a lesser extent the uptake by the LLC-PK1 cell line. Neither depended on metabolic energy for uptake or utilized Ca2+ channels. The internalization process was temperature dependent and was obliterated at 2 degrees C. In mesangial cells, this allowed direct observation of the internalization event from a presaturated surface pool. The rate of this process was consistent with the Vmax calculated from the Michaelis model. Surface binding and uptake were decreased by binding of Cd2+ to serum proteins and albumin and were much less dependent on the presence of low molecular weight components of serum. Therefore, these cells may be especially sensitive to Cd2+ at concentrations encountered in vivo because of the low protein content of the plasma ultrafiltrate. Surface binding of Cd2+ to mesangial cells was suppressed by competing divalent ions following the order of the Irving-Williams series (Mn less than Co less than Ni less than Cu greater than Zn), although Zn2+ showed the greatest effect on internalization. In LLC-PK1 cells, Zn2+ and Cu2+ were both effective in decreasing Cd2+ uptake. We conclude that Cd2+ uptake by the tubular epithelial cells is rapid and independent of specific cell surface interactions, whereas uptake by rat mesangial cells follows binding to a specific surface ligand saturating at about 1.5 x 10(7) copies/cell. In both types of cells the uptake appears quite specific for Cd2+ and shows some cross-reactivity with other metal cations explicable by competitive ligand binding.     

Utilization of exogenous purine compounds in Bacillus cereus. Translocation of the ribose moiety of inosine.

Intact cells of Bacillus cereus catalyze the breakdown of exogenous AMP to hypoxanthine and ribose 1-phosphate through the successive action of 5'-nucleotidase, adenosine deaminase, and inosine phosphorylase. Inosine hydrolase was not detectable, even in crude extracts. Inosine phosphorylase causes a "translocation" of the ribose moiety (as ribose 1-phosphate) inside the cell, while hypoxanthine remains external. Even though the equilibrium of the phosphorolytic reaction favors nucleoside synthesis, exogenous inosine (as well as adenosine and AMP) is almost quantitatively transformed into external hypoxanthine, since ribose 1-phosphate is readily metabolized inside the cell. Most likely, the translocated ribose 1-phosphate enters the sugar phosphate shunt, via its prior conversion into ribose 5-phosphate, thus supplying the energy required for the subsequent uptake of hypoxanthine in B. cereus.     

Role of the membrane-associated folate binding protein (folate receptor) in methotrexate transport by human KB cells

The uptake of methotrexate by KB cells was observed to be dependent on time, temperature, and concentration of extracellular methotrexate. The Kd for methotrexate surface binding to KB cells was approximately 200 nM. Following exposure of KB cells to trace quantities of [3H]methotrexate for periods ranging from 6 min to 24 h, the cellular methotrexate was progressively formed into methotrexate polyglutamates and was bound to dihydrofolate reductase as well as to a particulate folate binding protein. To further study the mechanism of methotrexate uptake in KB cells, the N-hydroxysuccinimide ester of methotrexate was used to covalently label the surface of KB cells and to inhibit transport of methotrexate. The N-hydroxysuccinimide ester of methotrexate was bound to a species of protein with an apparent molecular weight of 160,000 in 1% (v/v) Triton X-100 that bound folic acid and was specifically precipitated by antiserum raised against the previously purified high-affinity folate binding protein (the folate receptor) from human KB cells. In addition, trypsin was utilized to remove surface-accessible covalently bound methotrexate. The amount of covalently bound methotrexate that could be released by trypsin initially decreased on incubation at 37 degrees C, suggesting that the methotrexate and binding protein were internalized. However, with time, trypsin could again release the covalently bound methotrexate, suggesting that the binding protein cycles from the external cell surface to the inside of the cell and out again.     

The effects of abdominal irradiation on intestinal transport in the rat as assessed with isolated epithelial cells

The time course of the effect of exposure to sublethal irradiation on transport of several substrates by the intestine has been studied using isolated enterocytes. Rats received a single dose of 6 Gy to the abdomen, and isolated intestinal epithelial cells were prepared 3, 7, and 14 days later. The ability of the cells to take up D-glucose, L-leucine, and glycyl-L-leucine was assessed using 2.5-min incubation periods and was compared with the uptake in control cells. It was found that the protein content of the cells increased after irradiation, and although some of this was the result of increased binding of albumin to the cells there was also a nonspecific increase in most cell proteins. Consequently uptake data were expressed per unit number of cells and not per milligram of cell protein. Comparison of uptake expressed in this way showed that D-glucose and glycyl-L-leucine uptake was elevated 3 days after irradiation while that of L-leucine was unaffected. By 14 days after irradiation the glucose and glycyl-L-leucine uptake had returned to normal but the L-leucine transport was depressed. These data indicate that the effects of irradiation upon substrate transport in the intestines are not uniform and that although the cell population is initially reduced the remaining cells can compensate by increasing their transport capacity.     

FRET imaging of diatoms expressing a biosilica-localized ribose sensor

Future materials are envisioned to include bio-assembled, hybrid, three-dimensional nanosystems that incorporate functional proteins. Diatoms are amenable to genetic modification for localization of recombinant proteins in the biosilica cell wall. However, the full range of protein functionalities that can be accommodated by the modified porous biosilica has yet to be described. Our objective was to functionalize diatom biosilica with a reagent-less sensor dependent on ligand-binding and conformational change to drive FRET-based signaling capabilities. A fusion protein designed to confer such properties included a bacterial periplasmic ribose binding protein (R) flanked by CyPet (C) and YPet (Y), cyan and yellow fluorescent proteins that act as a FRET pair. The structure and function of the CRY recombinant chimeric protein was confirmed by expression in E. coli prior to transformation of the diatom Thalassiosira pseudonana. Mass spectrometry of the recombinant CRY showed 97% identity with the deduced amino acid sequence. CRY with and without an N-terminal Sil3 tag for biosilica localization exhibited characteristic ribose-dependent changes in FRET, with similar dissociation constants of 123.3 μM and 142.8 μM, respectively. The addition of the Sil3 tag did not alter the affinity of CRY for the ribose substrate. Subsequent transformation of T. pseudonana with a vector encoding Sil3-CRY resulted in fluorescence localization in the biosilica and changes in FRET in both living cells and isolated frustules in response to ribose. This work demonstrated that the nano-architecture of the genetically modified biosilica cell wall was able to support the functionality of the relatively complex Sil3-CyPet-RBP-YPet fusion protein with its requirement for ligand-binding and conformational change for FRET-signal generation.     

In situ labeling of the adipocyte lipid binding protein with 3-[125I]iodo-4-azido-N-hexadecylsalicylamide. Evidence for a role of fatty acid binding proteins in lipid uptake

Differentiating 3T3-L1 cells have been used to investigate the process of fatty acid uptake, its cellular specificity, and the involvement of cytoplasmic carrier proteins. The profile of fatty acid uptake in both differentiated and undifferentiated cells was biphasic, consisting of an initial rapid phase (0-20 s) followed by a second slower phase (60-480 s). In both cell types the initial phase of fatty acid (FA) uptake was temperature-insensitive whereas the rate of uptake during the second phase decreased 4-fold when measurements were made at 4 degrees C. The rate of [9,10-3H]oleate uptake in 3T3-L1 adipocytes was 10-fold greater than in the fibroblastic precursor cells. The acquisition of a differentially expressed cytoplasmic fatty acid binding protein (adipocyte lipid binding protein (ALBP] occurs coincident with the increased ability of these cells to take up FAs. Uptake experiments with 3-[125I]iodo-4-azido-N-hexadecylsalicylamide demonstrated that this photoactivatable FA analogue accumulated intracellularly in a time-, temperature-, and cell-specific fashion. Moreover, when 3T3-L1 adipocytes were presented with 3-[125I]iodo-4-azido-N-hexadecylsalicylamide and then irradiated, a single cytoplasmic 15-kDa protein was labeled. The in situ-labeled 15-kDa protein was identified as ALBP by its ability to be immunoprecipitated with anti-ALBP antisera. Taken together these results indicate that fatty acids traverse the plasma membrane and are bound by ALBP in the cytoplasmic compartment. It is likely that lipid uptake in other cell systems, such as liver, heart, intestine, and nerve tissue, proceeds by a similar process and that this represents a general mechanism for cell-specific FA uptake and utilization.     

Lipid free apolipoprotein E binds to the class B Type I scavenger receptor I (SR-BI) and enhances cholesteryl ester uptake from lipoproteins

The Class B type I scavenger receptor I (SR-BI) is a physiologically relevant high density lipoprotein (HDL) receptor that can mediate selective cholesteryl ester (CE) uptake by cells. Direct interaction of apolipoprotein E (apoE) with this receptor has never been demonstrated, and its implication in CE uptake is still controversial. By using a human adrenal cell line (NCI-H295R), we have addressed the role of apoE in binding to SR-BI and in selective CE uptake from lipoproteins to cells. This cell line does not secrete apoE and SR-BI is its major HDL-binding protein. We can now provide evidence that 1) free apoE is a ligand for SR-BI, 2) apoE associated to lipids or in lipoproteins does not modulate binding or CE-selective uptake by the SR-BI pathway, and 3) the direct interaction of free apoE to SR-BI leads to an increase in CE uptake from lipoproteins of both low and high densities. We propose that this direct interaction could modify SR-BI structure in cell membranes and potentiate CE uptake.