Accumulation on the Cytoplasmic Membrane of the Precursor to Dimethyl Sulfoxide Reductase in Molybdenum Cofactor-Deficient Mutants of Rhodobacter sphaeroides f. sp. denitrificans

The role of the molybdenum cofactor (Mo cofactor) in the translocationof dimethyl sulfoxide (DMSO) reductase to the periplasmic spacewas studied in vivo by isolating chlorate-resistant mutantsof Rhodobacter sphaeroides f. sp. denitrificans. More than 50%of the chlorate-resistant mutants isolated were defective inthe biosynthesis of the Mo cofactor and all of these mutantsaccumulated the precursor form of the enzyme. About 45% of themutants contained the same level of Mo cofactor as the parentstrain and exhibited normal levels of DMSO reductase and nitratereductase activities when chlorate was absent from the medium,but the activities of these enzymes were depressed when chloratewas present. Much of the accumulated precursor form of the enzymein a Mo cofactor-deficient mutant was bound to the cytoplasmicmembrane and was sensitive to treatment with proteinase K fromthe periplasmic side of the membrane, an indication that theprecursor was exposed on the periplasmic surface of the membrane.The precursor accumulated on the membrane of the parent strainwhen molybdate was removed from the medium or upon additionof tungstate and this precursor was also sensitive to the treatmentwith proteinase K from the periplasmic side. These results suggestthat the Mo cofactor is necessary for proteolytic processingof the precursor to the mature enzyme on the periplasmic sideof the membrane, whereas binding of the precursor to the membraneand translocation across it can occur in the absence of thecofactor. Almost all of the Mo cofactor available for directreconstitution in vitro of nitrate reductase activity from thenit-l mutant of Neurospora crassa was present in the cytoplasmicfractions. (Received December 11, 1991; Accepted March 25, 1992)     

A Nitrate-Inducible Plasma Membrane Protein of a Marine Alga, Heterosigma akashiwo

The rate of nitrate uptake by Heterosigma akashiwo cells thathad been cultured in medium with nitrate or ammonium ions asthe source of nitrogen was measured using¹⁵NO_3– The ratioof ¹⁵N/¹⁴N increased dramatically in nitrate-grown cells. Inammonium-grown cells, the ratio of ¹⁵N/¹⁴N did not increasefor 3 h but then it began to increase. Even when nitrate reductaseactivity was inhibited by tungstate, nitrate-grown cells couldtake up nitrate. Plasma membranes from nitrate-grown and ammonium-grown cellswere purified by the silica-microbead method, and polypeptidesassociated with the membranes were analyzed by SDS-PAGE andimmunostaining. A major polypeptide with a molecular mass of26 kDa appeared 3 h after the transfer of ammonium-grown cellsto nitrate-containing medium, and it disappeared 2 d after thetransfer of nitrate-grown cells to ammonium-containing medium.The 26 kDa polypeptide also appeared when cell growth shiftedfrom the logarithmic phase to the stationary phase and the ammoniumcontent of the medium decreased, even when the cells were culturedin ammonium-containing medium. (Received April 10, 1992; Accepted July 30, 1992)     

Phosphorylation of hepatitis C virus-encoded nonstructural protein NS5A.

Two proteins, a 56-kDa protein (p56) and a 58-kDa protein (p58), are produced from the hepatitis C virus (HCV) nonstructural region 5A (NS5A). Recently, we found that both proteins are phosphorylated at serine residues and that p58 is a hyperphosphorylated form of p56. Furthermore, hyper-phosphorylation depends on the production of an intact form of the HCV NS4A protein. To clarify the nature of NS5A phosphorylation, pulse-chase analysis was performed with a transient protein production system in cultured cells. The study indicated that basal and hyperphosphorylation of NS5A occurred after proteolytic production of NS5A was complete. In an attempt to identify the location of the hyperphosphorylation sites in p58, proteins with sequential deletions from the C-terminal region of NS5A and with mutations of possible phosphorylated serine residues to a neutral amino acid, alanine, were constructed. The deleted or mutated proteins were then tested for hyperphosphorylation in the presence of the NS4A product. Here, we report that serine residues 2197, 2201, and/or 2204 are important for hyper-phosphorylation. Important sites for basal phosphorylation were identified in the region from residues 2200 to 2250 and in the C-terminal region of the NS5A product. A subcellular localization study showed that most of the NS5A products were localized in the nuclear periplasmic membrane fraction.     

The N-terminal region of hepatitis C virus nonstructural protein 3 (NS3) is essential for stable complex formation with NS4A.

Hepatitis C virus proteins are produced by proteolytic processing of the viral precursor polyprotein that is encoded in the largest open reading frame of the viral genome. Processing of the nonstructural viral polyprotein requires the viral serine-type proteinase present in nonstructural protein 3 (NS3). The cleavage of the junction between NS4B and NS5A is mediated by NS3 only when NS4A is present. NS4A is thought to be a cofactor that enhances the cleavage efficiency of NS3 in hepatitis C virus protein-producing cells. Stable NS3-NS4A complex formation required the N-terminal 22 amino acid residues of NS3. This interaction contributed to stabilization of the NS3 product as well as increased the efficiency of cleavage at the NS4B/5A site. The N-terminal 22 amino acid residues fused to Escherichia coli dihydrofolate reductase also formed a stable complex with NS4A. NS3 derivatives which lacked the N-terminal 22 amino acid residues showed drastically reduced cleavage activity at the NS4B/5A site even in the presence of NS4A. These data suggested that the interaction with NS4A through the 22 amino acid residues of NS3 is primarily important for the NS4A-dependent processing of the NS4B/5A site by NS3.     

Ceramide Prevents Neuronal Programmed Cell Death Induced by Nerve Growth Factor Deprivation

Abstract: We examined the ability of ceramide and sphingomyelinase (SMase) to prevent neuronal programmed cell death (PCD). We found that a cell-permeable ceramide analogue prevented neuronal PCD when applied to established sympathetic neuron primary cultures at the time of nerve growth factor (NGF) deprivation. Other amphiphilic lipids such as oleic acid failed to prevent cell death. Exogenous SMase also showed the same effect, probably by raising the intracellular ceramide level by sphingomyelin (SM) breakdown. Phosphocholine, another hydrolytic product of SM by SMase, did not prevent cell death. Other phospholipases, such as phospholipase C and phospholipase A₂, could not prevent cell death. Given the recent findings that the SM cycle is activated to increase the intracellular ceramide level on NGF binding to the low-affinity NGF receptor (LNGFR) and that NGF binding to LNGFR suppresses apoptosis in neural cell lines, our results suggest the possibility of the SM cycle as a signaling mechanism transducing the PCD-preventing activity of NGF.     

Light-Induced CREB Phosphorylation and Gene Expression in Rat Retinal Cells

Abstract: The signal pathway for light-induced expression of c- fos and the neuropeptide somatostatin (SS) in rat retinal cells was investigated. Flashing light induced c- fos and SS mRNA in the inner nuclear layer and the ganglion cell layer. As both c- fos and SS genes have a cyclic AMP response element (CRE) in their promoters, CRE-binding protein (CREB) phosphorylation in retinal cells was examined with a phospho-CREB-specific antibody. Both flashing light and administration of the L-type Ca²⁺ channel activator Bay K 8644 induced phosphorylation of CREB in the nuclei of the amacrine cells and the ganglion cells where c- fos /SS mRNAs were expressed. These cells could be double-stained with anti-calmodulin kinase II (anti-CaM kinase II) monoclonal antibody and phospho-CREB-specific polyclonal antiserum after Bay K 8644 administration, indicating the colocalization of phosphorylated CREB at Ser¹³³ and CaM kinase II in the neural retina.     

Putative morphogen, DIF, of Dictyostelium discoideum induces apoptosis in rat pancreatic AR42J cells

We have recently shown that differentiation-inducing factor-1 (DIF-1) of Dictyostelium discoideum is capable of raising intracellular calcium concentration ([Ca²⁺]_i) and suppressing cell proliferation of rat pancreatic AR42J cells in a dose-dependent manner, and that DIF-1 at a concentration of 40 μmol/L is toxic to the cells. In this study, we have further characterized the cytotoxic effect of DIF-1 on AR42J cells and have analyzed the effect of DIF-1 on [Ca²⁺]_i. In the presence of 40 μmol/L DIF-1, cells began to bleb after approximately 6 h, and most had died within 48 h. Biochemical analysis revealed that DNA fragmentation was accompanied by cell death. Monitoring the changes in [Ca²⁺]_i induced by DIF-1, it was found that cells were able to adapt to stimulation with DIF-1 so that they did not respond to subsequent stimulation by DIF-1. These results indicate that DIF-1 induced apoptosis in AR42J cells probably via a cell signaling system.     

Quantitative contribution of the acid production to the intracellular acidification in human neutrophils stimulated by N-formyl-methionyl-leucyl-phenylalanine

A chemotactic peptide, N-formyl-methionyl-leucyl-phenylalanine (fMLP), induced an acidification of cytosol by about 0.05 pH units in 30 sec followed by an alkalinization in human neutrophils. The quantitative contribution of acid production to the acidification was studied. The superoxide (O₂ ^–) production stimulated by fMLP was not involved in the acidification because the production of acids in neutrophils from patients with chronic granulomatous disease who do not produce O₂ ^–, was the same as that in normal neutrophils. The intracellular acidification was completely inhibited by deoxyglucose, suggesting that energy metabolism enhanced upon stimulation by fMLP might be the main source of the acidification. Although enhancement of the lactate formation by fMLP was 0.8 nmol/10⁶ cells, which could lower intracellular pH by 0.08 pH units, the lactate production could not explain the initial acidification because the production of lactate started at 1 min after the stimulation while the intracellular acidification began immediately after the stimulation. Mitochondrial respiratory inhibitors such as KCN and rotenone had no effects on the fMLP-induced intracellular acidification. The fMLP-induced production of CO₂ in 30 sec through the hexose monophosphate shunt was only 2.6 pmol/10⁶ cells, which was calculated to decrease intracellular pH by only 0.0014. Thus, changes of energy metabolism induced by fMLP does not explain the acidification.Abbreviations fMLP N-formyl-methionyl-leucyl-phenylalanine - BCECF-AM 2,7-bis(carboxyethyl)carboxyfluorescein acetoxymethyl ester - PMA phorbol 12-myristate 13-acetate - CGD chronic granulomatous disease - HMP hexose monophosphate - pHi intracellular pH     

Involvement of L-Type-Like Amino Acid Transporters in S-Nitrosocysteine-Stimulated Noradrenaline Release in the Rat Hippocampus

Abstract: Nitrogen oxides, such as nitric oxide, have been shown to regulate neuronal functions, including neurotransmitter release. We investigated the effect of S-nitroso-l -cysteine (SNC) on noradrenaline (NA) release in the rat hippocampus in vivo and in vitro. SNC stimulated [³H]NA release from prelabeled hippocampal slices in a dose-dependent manner. SNC stimulated endogenous NA release within 30 min to almost five times the basal level in vivo (microdialysis in freely moving rats). In a Na⁺-containing Tyrode's buffer, SNC-stimulated [³H]NA release was inhibited 30% by the coaddition of l -leucine. In the Na⁺-free, choline-containing buffer, SNC-stimulated [³H]NA release, which was similar to that in the Na⁺-containing buffer, was inhibited markedly by l -leucine, l -alanine, l -methionine, l -phenylalanine, and l -tyrosine. The effects of the other amino acids examined were smaller or very limited. The effect of l -leucine was stronger than that of d -leucine. A specific inhibitor of the L-type amino acid transporter, 2-aminobicyclo[2.2.1]-heptane-2-carboxylate (BCH), inhibited the effects of SNC on [³H]NA release in the Na⁺-free buffer. Uptake of l -[³H]leucine into the slices in the Na⁺-free buffer was inhibited by SNC, BCH, and l -phenylalanine, but not by l -lysine. The effect of SNC on cyclic GMP accumulation was not inhibited by l -leucine, although SNC stimulated cyclic GMP accumulation at concentrations up to 25 µM, much less than the concentration that stimulates NA release. These findings suggest that SNC is incorporated into rat hippocampus via the L-type-like amino acid transporter, at least in Na⁺-free conditions, and that SNC stimulates NA release in vivo and in vitro in a cyclic GMP-independent manner.     

Yersinia pseudotuberculosis in China

Thirty strains of Yersinia pseudotuberculosis were isolated from rabbits (17 strains), wild rats (9 strains) and house rats (4 strains) in China between 1990 and 1993. The biochemical properties of these isolates were identical with those of Y. pseudotuberculosis and no special characteristics were found in these strains. Serologically, serogroups 4b and 5b were identical to isolates found in Japan, and a new serogroup 1c and unclassified strains have also been detected. The existence of virulence-associated properties were different among strains. The pYV plasmid was detected from 6 strains of 30 isolates. This report documents the presence of Y. pseudotuberculosis in China, providing important epidemiological information.