Xyloglucan undergoes interpolymeric transglycosylation during binding to the plant cell wall in vivo: evidence from 13C/3H dual labelling and isopycnic centrifugation in caesium trifluoroacetate

Xyloglucan from the walls of Rosa cells that had been cultured on [12C]- or [13C]-glucose formed bands in caesium trifluoroacetate with mean buoyant densities of 1.575 or 1.616 g/ml respectively. Incubation of a mixture of [13C,3H]xyloglucan and [12C,1H]xyloglucan in the presence of xyloglucan endotransglycosylase (XET) activity caused the mean buoyant density of the radioactive material to decrease, indicating that interpolymeric transglycosylation could be detected in vitro. We used two 13C/3H-dual-labelling protocols to look for interpolymeric transglycosylation in vivo. In protocol A, [13C]glucose-grown Rosa cells were transferred into [12C]glucose medium 6 h after a approximately 2 h pulse of l-[1-3H]arabinose (which radiolabels the xylose residues of xyloglucan). The mean buoyant density of the wall-bound [3H]xyloglucan decreased during the following 7 days in culture. This indicates that, during or after the wall-binding of newly synthesized [12C,1H]xyloglucan, it became covalently attached to previously wall-bound [13C, 3H]xyloglucan. In protocol B, [12C]glycerol- or [12C]glucose-grown Rosa cells were transferred into [13C]glucose medium, 20 or 60 min before a approximately 2 h pulse of [3H]arabinose. The buoyant density of the earliest wall-bound [3H]xyloglucan showed that it had a 12C/13C ratio of approximately 1:1. This indicates that, during (or, implausibly, before) wall-binding, the newly synthesized [13C, 3H]xyloglucan became covalently attached to previously synthesized [12C]xyloglucan. During the following 7 days in culture, the mean buoyant density of the [3H]xyloglucan increased, showing that later-synthesized [13C,1H]xyloglucan can be covalently attached to previously wall-bound [12C,13C,3H]xyloglucan. The only known mechanism by which segments of xyloglucans could become covalently attached to each other in the cell wall is by interpolymeric transglycosylation catalysed by XET. We conclude that XET-catalysed interpolymeric transglycosylation accompanies, and probably causes, the integration of newly secreted xyloglucan into the cell-wall architecture.

The proteasome 11S regulator subunit REG alpha (PA28 alpha) is a heptamer

Activity of the 20S proteasome, which performs much of the cytosolic and nuclear proteolysis in eukaryotic cells, is controlled by regulatory complexes that bind to one or both ends of the cylindrical proteasome. One of these complexes, the 11S regulator (REG), is a complex of 28 kDa subunits that is thought to activate proteasomes toward the production of antigenic peptides. REG, purified from red blood cells, is a complex of REG alpha and REG beta subunits. We have crystallized recombinant REG alpha (rREG alpha) and collected diffraction data to 3.0 A resolution. The self-rotation function indicates that rREG alpha forms a heptameric ring in the crystal. Equilibrium sedimentation demonstrates that rREG alpha is a heptamer in solution also.

The concentration-dependent membrane activity of cecropin A

Cecropin A is a naturally occurring, linear, cationic, 37-residue antimicrobial peptide. The precise mechanism by which it kills bacteria is not known, but its site of action is believed to be the cell membrane. To investigate the nature of its membrane activity, we examined the ability of cecropin A to alter membrane permeability in synthetic lipid vesicles and in Gram-negative bacteria. Cecropin A exerted distinctly different types of membrane activity depending on its concentration. In synthetic lipid vesicles, cecropin A dissipated transmembrane electrochemical ion gradients at relatively low concentrations, but much higher concentrations were required to release an encapsulated fluorescent probe. Cecropin A dissipated ion gradients whether or not the vesicle membranes contained anionic lipid, although the presence of anionic lipid dramatically increased peptide binding, and modestly increased the release of an encapsulated probe. Cholesterol did not prevent the dissipation of ion gradients by low concentrations of peptide, but it did inhibit release of the encapsulated probe by high concentrations of peptide. At the highest concentrations examined, cecropin A remained monomeric in solution, and did not aggregate, lyse, or otherwise alter vesicle size. In Gram-negative bacteria, cecropin A was potently bactericidal at concentrations which dissipated ion gradients in lipid vesicles, but much higher concentrations were required to cause the release of cytoplasmic contents. These findings point to the conclusion that cecropin A kills bacteria by dissipating transmembrane electrochemical ion gradients. They weigh against theories comparing the antimicrobial activity of cecropin A to the release of encapsulated probes from lipid vesicles, and against roles for cholesterol or anionic lipid headgroups in the selectivity of peptide action against bacteria.

Dynamic targeting of the agonist-stimulated m2 muscarinic acetylcholine receptor to caveolae in cardiac myocytes

In cardiac myocytes, as well as specialized conduction and pacemaker cells, agonist binding to muscarinic acetylcholine receptors (mAchRs) results in the activation of several signal transduction cascades including the endothelial isoform of nitric-oxide synthase (eNOS) expressed in these cells. Recent evidence indicates that, as in endothelial cells, eNOS in cardiac myocytes is localized to plasmalemma caveolae, specialized lipid microdomains that contain caveolin-3, a muscle-specific isoform of the scaffolding protein caveolin. In this report, using a detergent-free method for isolation of sarcolemmal caveolae from primary cultures of adult rat ventricular myocytes, we demonstrated that the muscarinic cholinergic agonist carbachol promotes the translocation of mAchR into low density gradient fractions containing most myocyte caveolin-3 and eNOS. Following isopycnic centrifugation, the different gradient fractions were exposed to the muscarinic radioligand [3H]quinuclidinyl benzilate (QNB), and binding was determined after membrane filtration or immunoprecipitation. In a direct radioligand binding assay, we found that [3H]QNB binding can be detected in caveolin-enriched fractions only when cardiac myocytes have been previously exposed to carbachol. Furthermore, most of this [3H]QNB binding can be specifically immunoprecipitated by an antibody to the m2 mAchR, indicating that the translocation of this receptor subtype is responsible for the [3H]QNB binding detected in the low density fractions. Moreover, the [3H]QNB binding could be quantitatively immunoprecipitated from the light membrane fractions with a caveolin-3 antibody (but not a control IgG1 antibody), confirming that the m2 mAchR is targeted to caveolae after carbachol treatment. Importantly, atropine, a muscarinic cholinergic antagonist, did not induce translocation of m2 mAchR to caveolae and prevented receptor translocation in response to the agonist carbachol. Thus, dynamic targeting of sarcolemmal m2 mAchR to caveolae following agonist binding may be essential to initiate specific downstream signaling cascades in these cells.

Characterization and crystallization of human uroporphyrinogen decarboxylase

The cytosolic enzyme uroporphyrinogen decarboxylase (URO-D) catalyzes the fifth step in the heme biosynthetic pathway, converting uroporphyrinogen to coproporphyrinogen by decarboxylating the four acetate side chains of the substrate. Recombinant human URO-D has been expressed in Escherichia coli with a histidine tag and has been purified to homogeneity. Purified protein was determined to be a monodisperse dimer by dynamic light scattering. Equilibrium sedimentation analysis confirmed that the protein is dimeric, with a dissociation constant of 0.1 microM. URO-D containing an amino-terminal histidine tag was crystallized in space group P3(1)21 or its enantiomer P3(2)21 with unit cell dimensions a = b = 103.6 A, c = 75.2 A. There is one molecule in the asymmetric unit, consistent with generation of the dimer by the twofold axis of this crystallographic operator. Native data have been collected to 3.0 a resolution.

Design of a leucine zipper coiled coil stabilized 1.4 kcal mol-1 by phosphorylation of a serine in the e position

Using a dimeric bZIP protein, we have designed a leucine zipper that becomes more stable after a serine in the e position is phosphorylated by protein kinase A (delta delta GP = -1.4 kcal mol-1 dimer-1 or -0.7 kcal mol-1 residue-1). Mutagenesis studies indicate that three arginines form a network of inter-helical (i,i' + 5; i, i' + 2) and intra-helical (i, i + 4) attractive interactions with the phosphorylated serine. When the arginines are replaced with lysines, the stabilizing effect of serine phosphorylation is reduced (delta delta GP = -0.5 kcal mol-1 dimer-1). The hydrophobic interface of the leucine zipper needs a glycine in the d position to obtain an increase in stability after phosphorylation. The phosphorylated protein binds DNA with a 15-fold higher affinity. Using a transient transfection assay, we document a PKA dependent four-fold activation of a reporter gene. Phosphorylation of a threonine in the same e position decreases the stability by delta delta GP = +1.2 kcal mol-1 dimer-1. We present circular dichroism (CD) thermal denaturations of 15 bZIP proteins before and after phosphorylation. These data provide insights into the structural determinants that result in stabilization of a coiled coil by phosphorylation.

Endocytosis of serum albumin in regenerating rat liver

Lysosomes, isolated from rat liver after 70% partial hepatectomy (PHX), were found, by Western blotting, to contain a considerable amount of serum albumin. The level of intralysosomal serum albumin after PHX showed biphasic patterns: it increased immediately after PHX, peaked at 30 min, rapidly declined within a few hours, rose again with a peak at 15 hr, and gradually declined thereafter. At 15 hr after PHX, the content of lysosomal proteins in the liver increased to twice the level of unoperated control, and the electron-microscopic observation of the isolated lysosomes revealed numerous large membrane-delimited structures with ground substances of variable electron opacities. The increase in the intralysosomal serum albumin at 30 min and 15 hr was accompanied by changes in the buoyant densities of endosomes in Percoll density gradients. At both time points, the density profiles of endosomes isolated from hepatectomized rats shifted to the denser direction, suggesting that PHX activates fusion and/or maturation of endosomes. Formaldehyde-treated bovine serum albumin is known to be taken up by the liver by receptor-mediated endocytosis. The uptake of the modified heterologous albumin was shown to be activated as early as 30 min after PHX. Both the uptake of serum albumin into lysosomes and the shift of buoyant density profile of endosomes after PHX were inhibited by the administration of adrenergic receptor antagonists, particularly by the alpha r-antagonist prazosin. Further, the concentration of catecholamines in rat serum, particularly that of norepinephrine, was found to increase immediately after PHX, relative to that in serum from sham-operated rats. These results suggest that the elevation of serum norepinephrine levels after PHX activates endocytosis and facilitates delivery of endocytosed serum albumin to lysosomes, where albumin is digested to yield amino acids for possible use in protein synthesis during liver regeneration.

Type II DNA topoisomerase from Saccharomyces cerevisiae is a stable dimer

Type II DNA topoisomerases function as homodimeric enzymes in transiently cleaving double-stranded DNA to catalyze unlinking and unknotting reactions. The dimeric enzyme creates a DNA double-strand break by forming a covalent attachment between an active site tyrosine from each monomer and a 5'-phosphate from each strand of DNA. The dimer must be very stable to dissociation or subunit exchange when covalently attached to DNA to prevent directly or indirectly catalyzed rearrangements of the genome. Past studies have indicated conflicting results for the monomer-dimer stability of topoisomerase II in solution. Here, we report results from sedimentation equilibrium studies and two different subunit exchange assays indicating that purified Saccharomyces cerevisiae DNA topoisomerase II exists as a stable dimer in solution, with a Kd estimated to be < or = 10(-11) M. This high dimer stability is not detectably altered by a change of ionic strength or by the presence of ATP, ADP, or DNA.

The use of iodixanol as a density gradient material for separating human sperm from semen

Iodixanol, a new nonionic density gradient material with relatively low osmolality and high density, was evaluated to determine its suitability for the separation of human sperm from semen for their subsequent therapeutic use. Using a three-layer iodixanol gradient (approximately 1.17/1.15/1.05 g/mL), sperm were centrifuged at 1000g for 30 min and collected from the 1.05/1.15 interface. Using this method, a mean of 78% of the motile and 99% of the morphologically normal sperm originally present in the semen were recovered at the interface. There was no significant increase in the percentage of motile or morphologically normal sperm in the final preparation compared to the original semen. Sperm survived iodixanol density gradient centrifugation well, showing only modest declines in motility (18%) and velocity (35%) during a subsequent 24-h incubation period. When iodixanol was compared to Percoll density gradient centrifugation with semen from the same ejaculate, there was no significant difference between methods with regard to sperm yield, enrichment of motile or morphologically normal sperm in the preparation or sperm survival following separation. Iodixanol provides a suitable, nontoxic alternative to Percoll for the preparation of human sperm for therapeutic use.

Protein kinases in mitochondria of the invertebrate Artemia franciscana

The information concerning protein kinases in animal mitochondria is scarce and related only to mammals. No data are available for invertebrates. We demonstrate here the presence of casein kinase II (CK II) and cAMP-dependent protein kinase (PKA) in the purified mitochondria of the crustacean Artemia franciscana. Whereas the mitochondrial CK II showed the same characteristics of the cytosolic enzyme, mitochondrial PKA had an apparent Km for its substrate Kemptide 1 order of magnitude lower than that of the cytosolic enzyme. CK II and PKA phosphorylate different sets of proteins in Artemia mitochondria in vitro. The use of an activity gel assay has allowed the detection of additional protein kinases, as yet unidentified, in Artemia mitochondria.

Characterization of the biophysical properties of human erythroblasts as a preliminary step to the isolation of fetal erythroblasts from maternal peripheral blood for non invasive prenatal genetic investigation

BACKGROUND AND OBJECTIVE

Fetal erythroblasts in maternal circulation represent a valuable source of fetal cell material which can be obtained with non-invasive procedures that do not endanger the fetus. Physical separation techniques have been invaluable in the isolation and characterization of different cells. There are basically two principles that have been used most successfully: separation according to density and separation according to size. In order to determine whether physical separation procedures are capable of purifying human erythroblasts, the biophysical characteristics of these cells were determined.

METHODS

Bone marrow particles were obtained from formal adults and peripheral blood buffy coats from blood banks. A single cell suspension was initially fractionated by buoyant density gradient centrifugation. Fractions enriched in erythroblasts were pooled and further processed by velocity sedimentation in order to take advantage of the differences in size of erythroblasts and other cells.

RESULTS

Density distribution curves were drawn after density gradient centrifugation for the different cell types present in the starting cell samples. Separation of the erythroblast-enriched density fractions by velocity sedimentation was successful and a highly purified population of erythroblasts was obtained. Cell size distribution of the different cell types was determined.

INTERPRETATION AND CONCLUSIONS

This initial study defines the biophysical properties (size and density) of human erythroblasts in bone marrow and peripheral blood and is a necessary preliminary step in setting up the optimal procedure for the isolation of fetal erythroblasts from maternal peripheral blood in sufficient amounts and purity for prenatal non-invasive genetic investigation.

Rapid incorporation of docosahexaenoic acid from dietary sources into brain microsomal, synaptosomal and mitochondrial membranes in adult mice

This study examined the incorporation of docosahexaenoic acid (DHA) from several dietary sources into the brain tissue and intracellular organelles in mice which had been fed a 5% palm oil (low n-3 fatty acid level) diet for 8 or 11 weeks. The percentages of DHA in the tissues of mice fed 5% representative oils for 30 days or 5% purified n-3 fatty acid diets for 6 days were analyzed using gas chromatography. The percentage of DHA in the brain was ranked in the following order: the salmon oil diet group > the sardine oil diet group > > the perilla oil diet group > > the lard and palm oil diet groups for the 30 day feeding trial; and the DHA diet group > > the eicosapentaenoic acid and alpha-linolenic acid diet groups for the 6 day feeding trial. The percentage of arachidonic acid showed a more dramatic decrease than that of docosapentaenoic acid. These results reflected the plasma fatty acid concentrations, but were not as pronounced as the changes observed in the plasma. The majority of the DHA incorporated into the brain was recovered in microsomal, synaptosomal, and mitochondrial fractions separated by density gradient centrifugation. These membrane fractions took up DHA within several days. These results suggest that the intake of DHA itself increases the DHA level of brain membranes more rapidly than intake of the precursors in animals fed a low n-3 fatty acid level diet.

Quantifying apoptosis in banked human brains using flow cytometry

Fragmentation of genomic DNA, a major biochemical feature of programmed cell death (apoptosis), is easily detected when apoptosis is prevalent. In brain tissue apoptotic cells are usually scarce and detection requires more sensitive techniques. We describe a highly sensitive method to quantify apoptosis in frozen human brain tissue using flow cytometry. Nuclei from homogenized brain specimens were isolated to purity using discontinuous isopyknic centrifugation through 2.2 M sucrose. DNA strand breaks in apoptotic nuclei were conjugated with biotinylated-dUTP using terminal deoxynucleotidyl transferase (TdT) and tagged with streptavidin-conjugated FITC (TUNEL). Negative controls excluding the TdT step, and positive controls using DNAase pretreatment to create 3'-OH strand breaks were run in parallel. The proportion of nuclei with TdT-dependent labeling in adult brain specimens was < 0.01% in 6 out of 7 specimens. In 3 fetal brains it averaged 0.86 +/- 0.11%. Apoptotic cells were readily detected in 2 malignant glial neoplasms and in a patient with HIV encephalitis. Comparable frequencies of stained nuclei were present in adjacent specimens embedded in paraffin and labeled in situ. By screening millions of nuclei cytometry detected very rare apoptotic events, producing quantitative results using banked frozen brains. The method has potential applications to studies of human brain development, neurodegenerative diseases, and brain tumors.

The fragmentation of incoming Semliki Forest virus nucleocapsids in mosquito (Aedes albopictus) cells might be coupled to virion uncoating

The fate of Semliki Forest virus (SFV) nucleocapsid, especially the capsid protein (C-protein), was investigated during the early stages of a productive infection in mosquito Aedes albopictus cells. Infection of the cells resulted in a time dependent accumulation of a C-protein derived fragment. This fragmentation of incoming viral nucleocapsid was prevented by NH4Cl, an agent generally used to elevate the pH in acidic intracellular compartments, suggesting that a low intravesicular pH is required for this process. Density gradient analysis of the postnuclear cell lysate demonstrated that the fragmentation was associated with a cellular compartment showing a density of 1.14 +/- 0.02 g/ml. This cellular compartment was devoid from a lysosomal marker enzyme and represented the timely preceding cellular fraction through which SFV passed before encountering a lysosomal fraction. Furthermore, the intracellular distribution of the viral, 3H-uridine-labeled RNA suggested that the same fraction might represent a key cellular compartment in which the separation of the viral RNA from the viral structural proteins is primed. In conclusion, these data lead to the suggestion that the fragmentation of incoming SFV nucleocapsids in Aedes albopictus cells might be the part of the mechanism leading to the release of viral RNA into the cytosol during early stages of productive infection.

The relative positions of alanine residues in the hydrophobic core control the formation of two-stranded or four-stranded alpha-helical coiled-coils

The objective of this study was to investigate the positional effect of hydrophobic interactions in the alpha-helical interface in controlling the formation of two-stranded and four-stranded coiled-coils. Two disulfide-bridged antiparallel coiled-coils were designed which differ only in the position of a single Ala residue in the middle heptad: in peptide 2H the Ala residues are in register (in the same rung), while in peptide 4H they are not. Data from size-exclusion chromatography and sedimentation equilibrium experiments showed that under benign conditions peptides 2H and 4H were two-stranded and four-stranded coiled-coils respectively. These results, in conjunction with molecular modeling studies, suggests that when four Ala residues are in the same plane of a potential four-stranded coiled-coil, the small side chains of Ala would create a large cavity in the hydrophobic interface of the potential four-stranded structure which is destabilizing and favors the two-stranded, disulfide-bridged coiled-coil. In contrast, an alternating Leu-Ala hydrophobic packing in the two planes distributes the potential cavity over a larger region, which may be partially filled by minor adjustments of the neighboring Leu side chains. As a result, there is still sufficient hydrophobic contact to maintain the four stranded structure.

Cooperativity mutants of bacteriophage lambda cI repressor: temperature dependence of self-assembly

Analytical ultracentrifugation was used to study higher order self-assembly of lambda cI repressors, including eight mutants whose monomer to dimer reactions were recently characterized [Burz et al. (1994) Biochemistry 33, 8399]. Six of the mutants were found to remain dimeric up to 50 microM total protein; the remaining mutants (EK102 and PT158) were found to undergo higher order oligomerization, as does wild-type cI. For these three repressors, we determined the stoichiometries and energetics of higher order assembly over the temperature range 5-40 degrees C. Weak dimerization exhibited by two other mutants, SN228 and SR228, was also evaluated by sedimentation equilibrium over this same temperature range. The end-state for higher order assembly of wild-type cI was determined to be octameric, in agreement with Senear et al. [(1993) Biochemistry 32, 6179-6189]. The assembly free energies resolved by the sedimentation analysis program NONLIN [Johnson, M. L., et al. (1981) Biophys. J. 36, 575-588] leads to the prediction that tetramers may contribute significantly to the intermediate populations during assembly. This analysis of the species populations is in accord with recent conclusions from fluorescence anisotropy studies [Banik et al. (1993) J. Biol. Chem. 268, 3938]. It was found that two of the mutant repressors (EK102 and PTI58) assemble into octamers, but with differing possible intermediates. PT158 satisfies the stoichiometry 8M1 <--> 2M4 <--> M8, while the EK102 data conforms to a 4M2 <--> 2M4 <--> M8 model, similar to WT (both the EK102 and WT data could also be described by a dimer-octamer model with no intermediates). Of the six repressors found in this study to remain dimeric, three exhibit non-cooperative DNA binding (GD147, KN192, YH210), two express intermediate cooperativity (EK188, SR228), and one is fully cooperative (SN228). The three octamerizing repressors are fully cooperative [Burz & Ackers (1994) Biochemistry 33, 8399], suggesting a correlation between their ability to form higher order assemblies and to engage in cooperative DNA binding. Linear van't Hoff plots were obtained for overall assembly of wild-type and EK102 dimers, while that of PT158 monomers was curved, indicating a negative heat capacity change. The van't Hoff analyses of dimerization constants for SN228 and SR228 were distinctly different from each other and also from that of wild type; such differences might be related to the disparate cooperative behavior found previously for these mutants (Burz & Ackers, 1994).

The uptake of calcium by isolated chromaffin granules of the adrenal medulla

Bovine chromaffin secretory granules were purified by isopycnic Metrizamide gradient centrifugation and their Ca2+ sequestration pathways were characterized. The rate of Ca2+ sequestration at 37 degrees C was first order, with a maximal uptake of 26.9 +/- 0.46 (mean +/- S.D., n = 3) nmol Ca2+/mg protein and a first order rate constant (k) of 0.046 +/- 0.002 min-1. At 4 degrees C the rate of uptake was substantially attenuated, with only 2.47 +/- 0.2 (mean +/- S.D, n = 3) nmol Ca2+/mg protein sequestered in 60 min. Ca2+ sequestration was 93% inhibited by 180 mM NaCl [I50% of 78.7 +/- 9.3 mM NaCl (mean +/- S.D., n = 11)] but only slightly inhibited by KCl or MgCl2. Ca2+ sequestration was not stimulated by incubation with MgATP but was inhibited by 57% after incubation with 30 microM monensin. Ca2+ sequestration was dependent on extravesicular Ca2+ with half-maximal sequestration at pCa2+ 6.81 +/- 0.028 (mean +/- S.D., n = 3). Sequestered Ca2+ could be exchanged with external 45Ca2+, the exchange rate was first order (k of 0.042 +/- 0.004: mean +/- S.D., n = 3) and saturated at 27.7 +/- 1.1 nmol Ca2+/mg (mean +/- S.D., n = 3). The Ca2+/Ca2+ exchange system was totally inhibited by NaCl or KCl but only slightly by MgCl2. About 75% of sequestered 45Ca2+ could be released by incubation with NaCl, but only 8% was released by incubation with KCl. Half-maximal release of sequestered 45Ca2+ required 69.3 +/- 12.2 mM NaCl (mean +/- S.D., n = 3). The Na+-induced release of sequestered 45Ca2+ was rapid, t0.5 of 2.80 +/- 0.63 min (mean +/- S.D., n = 3) and inhibited at 4 degrees C. The concurrent incubation of chromaffin granules with 45Ca2+ and either annexin proteins V or VI resulted in attenuated uptake of 45Ca2+. These results suggest that Ca2+ uptake in adrenal chromaffin granules is regulated by Na+ and Ca2+ gradients and also possibly by annexins V and VI.

Dimerization of the extracellular domain of the erythropoietin (EPO) receptor by EPO: one high-affinity and one low-affinity interaction

Although there is considerable evidence that signaling by the erythropoietin (EPO) receptor is initiated when it is dimerized by binding EPO, it has been previously reported that the soluble extracellular domains of the EPO receptor (sEPOR) are not dimerized in the presence of EPO and are able to form only 1:1 complexes with EPO. We have now shown unambiguously by light scattering, sedimentation equilibrium, and titration calorimetry that two molecules of sEPOR can bind to a single EPO monomer but that the binding of the second sEPOR is approximately 1000-fold weaker than that of the first. Because this second binding interaction is quite weak (Kd of approximately 1 microM), the 2:1 sEPOR.EPO complexes are easily dissociated during chromatography (forming the 1:1 complexes reported previously) and cannot be isolated in pure form. Global analysis of the sedimentation equilibrium data has enabled us to determine the binding constants and is consistent with a model in which EPO has two independent binding sites for sEPOR but cannot exclude anticooperative or sequential binding models. The influence of glycosylation of EPO and/or sEPOR on the binding affinities has also been investigated. Titration calorimetry is consistent with the sedimentation data and shows that the weaker binding site has a more negative delta H. The relation of these results to the binding of EPO to membrane-bound receptors and to the phenomenon of apparent high-affinity and low-affinity classes of receptors is discussed.

Structural studies of Escherichia coli UDP-N-acetylmuramate:L-alanine ligase

Uridine diphosphate N-acetylmuramate:L-alanine ligase (EC 6.3.2.8, UNAM:L-Ala ligase or MurC gene product) adds the first amino acid to the sugar moiety of the peptidoglycan precursor, catalyzing one of the essential steps in cell wall biosynthesis for both gram-positive and gram-negative bacteria. Here, we report our studies on the secondary and quaternary structures of UNAM:L-Ala ligase from Escherichia coli. The molecular weight of the purified recombinant enzyme determined by electrospray ionization mass spectrometry agreed well with the molecular weight deduced from the DNA sequence. Through sedimentation equilibrium analysis, we show that the enzyme exists in equilibrium between monomeric and dimeric forms and that the dissociation constant of the dimer, Kd, was determined to be 1.1 +/- 0.4 microM at 37 degrees C and 0.58 +/- 0.30 microM at 4 degrees C. A very similar Kd value was also obtained at 37 degrees C by gel filtration chromatography. The secondary structure of the enzyme was characterized by circular dichroism spectroscopy. No change in the secondary structure was observed between the monomeric and dimeric forms of the enzyme. The activity assays at enzyme concentrations both below and above the determined Kd value lead to the conclusion that the enzyme is active both as dimers and as monomers and that the specific activity is independent of the oligomerization state.

A soluble protein is imported into Euglena chloroplasts as a membrane-bound precursor

The Euglena precursor to the small subunit of ribulose-15-bisphosphate carboxylase/oxygenase (pSSU) is a polyprotein. To determine the transport route from cytoplasm to chloroplast, Euglena was pulse labeled with 35S-sulfate and the organelles were separated on sucrose gradients. After a pulse, pSSU was found in the endoplasmic reticulum (ER) and Golgi apparatus. During a chase, ER-and Golgi-localized pSSU decreased concomitant with the appearance of SSU in chloroplasts. SSU was not found in pSSU-containing ER and Golgi fractions. Na2CO3 did not remove pSSU from ER or Golgi membranes, indicating that it was an integral membrane protein. pSSU was inserted in vitro into canine microsomes, and Na2CO3 did not remove pSSU from the microsomal membrane. The in vivo and in vitro experiments show that Euglena pSSU is inserted into the ER membrane and transported as an integral membrane protein to the Golgi apparatus before chloroplast import and polyprotein processing.

Enrichment of Penicillium chrysogenum microbodies by isopycnic centrifugation in nycodenz as visualized with immuno-electron microscopy

A procedure to enrich microbodies from Penicillium chrysogenum and a method to evaluate the purity and integrity of the microbodies are described. As a P. chrysogenum microbody marker acyltransferase (AT) was used. The P. chrysogenum hyphae were converted into protoplasts with Novozym 234. In Percoll-sucrose buffer the protoplasts were separated from mycelial debris after 10,000 x g centrifugation. Purified protoplasts were lysed, and the cell homogenate was centrifuged to form a 14,000 x g pellet. After 2 h, 45,000 x g isopycnic centrifugation of the 14,000 x g pellet on a continuous 20-60% nycodenz gradient, ten fractions were collected. The fractions were analyzed for AT containing microbodies by immuno-blotting and immuno-electron microscopy. The results showed that AT-microbodies are enriched in the 38% nycodenz fraction. The microbodies had a diameter of 400 to 500 nm, revealed an intact single membrane and confined AT. The estimated equilibrium density of the P. chyrsogenum microbodies was 1.20 g ml-1 as deduced from the 38% (w/v) nycodenz concentration.

Subcellular distribution of receptor sites in human brain: differentiation between heavy and light structures of high and low density

Studies of the subcellular localization of neuroreceptors in the rat brain have shown that most of them are associated with light and low density subcellular fractions. In two human brain areas, quite different subcellular distributions were observed. After fractionation by differential centrifugation of frontal cortex homogenates, benzodiazepine and serotonin 5-HT2 receptors were mainly found in the heavy mitochondrial (M) fraction, whereas mu-opiate and muscarinic cholinergic receptors were mainly concentrated in the microsomal (P) fraction. In human putamen, the presynaptic markers of dopaminergic nerve terminals (neurotensin receptors, dopamine uptake sites and amine vesicular transporter-binding sites), benzodiazepine receptors and serotonin uptake sites were recovered both in the high and low density fractions, whereas the muscarinic, opiate and, to a lesser extent, dopamine D2 receptors were mostly concentrated in the microsomal fraction. In the cerebral cortex, after isopycnic centrifugation in sucrose gradients, neuroreceptors were found in the high density fractions where the peaks of cytochrome oxidase and that of nerve endings, as identified by amine uptake and by means of electron microscopy were also found. A single peak of benzodiazepine receptors was observed in high density (1.15-1.17 g/ml) fractions suggesting that these receptors are much more concentrated in the nerve terminals or dendrites rather than in the dendritic spines or vesicles. The fact that muscarinic and opiate receptors were recovered in the P fraction with plasma membrane constituents and also in M and L fractions, which is confirmed by a bimodal distribution in sucrose gradient, suggests that they are localized in both the nerve terminals or dendrites and in the small vesicles or dendritic spines. In the putamen, much of the specific binding to uptake sites for dopamine and serotonin was recovered in the high density fractions, but the existence of another peak at a lower density indicates the presence of microsomal uptake sites. The results indicate that differential and isopycnic fractionation methods performed on human brain samples, make it possible to separate tissue fractions enriched in nerve endings, dendrites, dendritic spines, plasma membranes or vesicles.

KCl cotransport activity in light versus dense transferrin receptor-positive sickle reticulocytes

A subset of sickle cells becomes K(+)-depleted and dehydrated before or soon after leaving the bone marrow. These young cells may be identified in blood as transferrin receptor-positive (TfR+) dense reticulocytes. KCl cotransport, which is normally active in young erythroid cells with a maximum at pH 6.8, is a candidate pathway for K+ depletion of sickle reticulocytes. In this investigation, KCl cotransport activity was evaluated in young, TfR+ cells which had become dense in vivo and in age-matched cells which had retained normal hydration. Sickle erythrocytes were first separated into three primary density fractions, with care taken to preserve the in vivo hydration state. After normalization of intracellular hemoglobin concentration with nystatin, the cells were incubated at 37 degrees C for 20 min at pH 6.8 and 7.4. Before and after incubation, each primary fraction was separated into four secondary density fractions. The percentage of TfR+ cells in each secondary fraction was measured and a density distribution for TfR+ cells was determined for each primary fraction before and after incubation. The density shift during incubation was a measure of KCl cotransport. TfR+ cells from the denser primary fractions II and III had significantly more density shift than TfR+ cells from the light fraction I. Although the shifts were larger at low pH, differences between primary fractions were also observed at pH 7.4. These data indicate that the cells which become dense quickly in vivo have more KCl cotransport activity than those which remain light in vivo, and support this pathway as a primary mechanism for dehydration of young sickle cells.