Postnuclear Supernatant Fraction Research Articles

Subcellular fractionation by differential centrifugation for mitochondrial studies.

In addition to fluorescence microscopy, the subcellular fractionation of eukaryotic cells remains one of the central methods for the basic characterization of proteins. Here we describe an optimized procedure for the subcellular fractionation of yeast cells, specifically for mitochondrial studies. Major recommendations are to separate the fractions immediately after each centrifugation step, to carefully discard a significant part of the supernatant fractions which is in the direct vicinity to the pellets and, in addition, to perform an extra homogenization step of the post nuclear supernatant fraction. These principles help to collect supernatant fractions with less cross-contaminations from the corresponding pellets. These approaches are scalable and adaptable for the fractionation of other cell types and are also useful for the characterization of other organelles.

Down regulation of Akt/mTOR signaling pathway proteins in hippocampus of Alzheimer’s disease mouse model

AbstractBackgroundAkt and mechanistic target of rapamycin (mTOR) signaling pathways are implicated in AD pathology. Activation of these pathways by external cues facilitates new protein synthesis at the synapse which leads to the synaptic plasticity. We have demonstrated that activity dependent Akt1 mediated new protein synthesis is abolished in primary cortical neurons of APP/PS1 mice. However, the molecular underpinnings for the impairment of new protein synthesis at the synapse are still elusive. Our aim is to understand the molecular mechanisms prior to manifestation of pathological symptoms by examining the Akt1 and mTOR signaling cascades and new protein synthesis in hippocampus of WT and APP/PS1 male mice.MethodsWe isolated post nuclear supernatant (PNS) and synaptosomes from brain cortices of WT and APP/PS1 mice at different ages. Synaptosomes and PNS were resolved on SDS‐PAGE and immunoblotted against respective antibodies. Further, synaptoneurosomes were also prepared from hippocampus and performed 35S methionine incorporation assay without or with KCl stimulation. Statistical comparisons were performed using two‐way ANOVA followed by Tukey’s post‐hoc test or two‐tailed Mann‐Whitney U test.ResultsWe observed that pAkt1, pGSK3β, pmTOR, pS6 ribosomal protein, p4E‐BP1 levels are significantly decreased in both post nuclear supernatant (PNS) and synaptosomal fractions isolated from hippocampus of one month old (presymptomatic) APP/PS1 compared to WT male mice. Further, activity dependent new protein synthesis at the synapse was significantly impaired in synaptoneurosomes isolated from hippocampus of presymptomatic APP/PS1 mice, and this deficit is sustained at young adults (three months). However, no impairment was observed in basal protein synthesis at both ages in hippocampus of APP/PS1 compared to WT male mice. Down regulation of Akt1 precludes synaptic activity dependent protein translation at the dendrites but not in the soma in hippocampal neurons from APP/PS1 mice compared to WT mice.ConclusionsOur finding demonstrate that the dysregulation of Akt1/mTOR and its downstream signaling molecules in hippocampus contributes to synaptic dysfunction in AD through loss of activity dependent new protein synthesis at the synapse in hippocampus which is essential for synaptic plasticity and maintenance.

Autophagosome immunoisolation from GFP-LC3B mouse tissue

ABSTRACTWe describe a protocol for rapid and efficient enrichment of autophagosomes from various tissues of the GFP-LC3 mouse. In order to increase the number of autophagosomes, we block autophagy flux in the GFP-LC3 mouse tissue with a single intraperitoneal injection of leupeptin 4–5 h before tissue harvesting. We homogenize dissected tissue samples using a Dounce homogenizer followed by passing the slurry through needles of different sizes to dissociate the cells and disrupt their outer membranes. The post-nuclear supernatant fraction of the cell lysate is further centrifuged and the supernatant fraction is discarded to remove residual cytosolic GFP-LC3 that is not associated with autophagosomes. The pellet fraction is resuspended and incubated with magnetic microbeads coated with anti-GFP antibodies for 1 h on ice. The lysate-bead mixture is then applied to a column that is placed in a magnetic separator. After washes, the autophagosome fraction is eluted from the column for morphological and protein analysis.Abbreviations: EDTA: ethylenediaminetetraacetic acid; GAPDH: glyceraldehyde 3-phosphate dehydrogenase; GFP: green fluorescent protein; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; LC3: microtubule-associated protein 1 light chain 3 beta; MES: 4-morpholineethanesulfonic acid (MES); SQSTM1: sequestosome 1; TEM: transmission electron microscopy

Proteomic analysis of protein composition of rat forebrain cortex exposed to morphine for 10 days; comparison with animals exposed to morphine and subsequently nurtured for 20 days in the absence of this drug

Proteomic analysis was performed in post-nuclear supernatant fraction (PNS) prepared from forebrain cortex of rats exposed to increasing doses of morphine (10–50mg/kg) for 10days and sacrificed 24h (group +M10) or 20days (group +M10/−M20) after the last dose of morphine. PNS fraction was resolved by 2D-ELFO and stained by CBB. Analysis of the difference between (+M10) and (−M10) samples of PNS by PDQuest accompanied by MALDI-TOF MS/MS indicated the significant change of 28 proteins. Importantly, the number of altered proteins was decreased to 14 after 20days of nurturing animals in the absence of morphine. This new and important finding indicating the ability of mammalian organism to return to physiological norm after removal of the drug was verified by an independent methodology — gel-free & label-free quantification and normalization procedure denominated as MaxLFQ. The 113 proteins were identified as altered by morphine in (+M10) samples when compared with (−M10) samples of PNS and this number was decreased to 19 after 20days of nurturing the animals in the absence of this drug. Biological significanceForebrain cortex of rats exposed to morphine for 10days is severely altered as far as the overall protein composition is involved. Depending on the method used for protein detection and quantification, 28 (MALDI-TOF MS/MS) or 113 (MaxLFQ) altered proteins were identified. Importantly, in rats sacrificed 20days after the last dose of morphine, the number of altered proteins was decreased to 14 (MALDI-TOF MS/MS) and 19 (MaxLFQ), respectively. Our data indicate the high ability of living organism to oppose the drastic, morphine-induced change of the target tissue protein composition with the aim to return to the physiological norm after complete removal of the drug.

Proteomic analysis of post-nuclear supernatant fraction and percoll-purified membranes prepared from brain cortex of rats exposed to increasing doses of morphine

BackgroundProteomic analysis was performed in post-nuclear supernatant (PNS) and Percoll-purified membranes (PM) prepared from fore brain cortex of rats exposed to increasing doses of morphine (10–50 mg/kg) for 10 days.ResultsIn PNS, the 10 up (↑)- or down (↓)-regulated proteins exhibiting the largest morphine-induced change were selected, excised manually from the gel and identified by MALDI-TOF MS/MS: 1-(gi|148747414, Guanine deaminase), ↑2.5×; 2-(gi|17105370, Vacuolar-type proton ATP subunit B, brain isoform), ↑2.6×; 3-(gi|1352384, Protein disulfide-isomerase A3), ↑3.4×; 4-(gi|40254595, Dihydropyrimidinase-related protein 2), ↑3.6×; 5-(gi|149054470, N-ethylmaleimide sensitive fusion protein, isoform CRAa), ↑2.0×; 6-(gi|42476181, Malate dehydrogenase, mitochondrial precursor), ↑1.4×; 7-(gi|62653546, Glyceraldehyde-3-phosphate dehydrogenase), ↑1.6×; 8-(gi|202837, Aldolase A), ↑1.3×; 9-(gi|31542401, Creatine kinase B-type), ↓0.86×; 10-(gi|40538860, Aconitate hydratase, mitochondrial precursor), ↑1.3×. The identified proteins were of cytoplasmic (1, 4, 5, 7, 9), cell membrane (2), endoplasmic reticulum (3) and mitochondrial (6, 8, 10) origin and 9 of them were significantly increased, 1.3-3.6×. The 4 out of 9 up-regulated proteins (4, 6, 7, 10) were described as functionally related to oxidative stress; the 2 proteins participate in genesis of apoptotic cell death.In PM, the 18 up (↑)- or down (↓)-regulated proteins were identified by LC-MS/MS and were of plasma membrane [Brain acid soluble protein, ↓2.1×; trimeric Gβ subunit, ↓2.0x], myelin membrane [MBP, ↓2.5×], cytoplasmic [Internexin, ↑5.2×; DPYL2, ↑4.9×; Ubiquitin hydrolase, ↓2.0×; 60S ribosomal protein, ↑2.7×; KCRB, ↓2.6×; Sirtuin-2, ↑2.5×; Peroxiredoxin-2, ↑2.2×; Septin-11, ↑2.2×; TERA, ↑2.1×; SYUA, ↑2.0×; Coronin-1A, ↓5.4×] and mitochondrial [Glutamate dehydrogenase 1, ↑2.7×; SCOT1, ↑2.2×; Prohibitin, ↑2.2×; Aspartate aminotransferase, ↓2.2×] origin. Surprisingly, the immunoblot analysis of the same PM resolved by 2D-ELFO indicated that the “active”, morphine-induced pool of Gβ subunits represented just a minor fraction of the total signal of Gβ which was decreased 1.2x only. The dominant signal of Gβ was unchanged.ConclusionBrain cortex of rats exposed to increasing doses of morphine is far from being adapted. Significant up-regulation of proteins functionally related to oxidative stress and apoptosis suggests a major change of energy metabolism resulting in the state of severe brain cell “discomfort” or even death.

Peroxiredoxin II Restrains DNA Damage-induced Death in Cancer Cells by Positively Regulating JNK-dependent DNA Repair

The 2-Cys peroxiredoxins (Prx) belong to a family of antioxidant enzymes that detoxify reactive oxygen and nitrogen species and are distributed throughout the intracellular and extracellular compartments. However, the presence and role of 2-Cys Prxs in the nucleus have not been studied. This study demonstrates that the PrxII located in the nucleus protects cancer cells from DNA damage-induced cell death. Although the two cytosolic 2-Cys Prxs, PrxI and PrxII, were found in the nucleus, only PrxII knockdown selectively and markedly increased cell death in the cancer cells treated with DNA-damaging agents. The increased death was completely reverted by the nuclearly targeted expression of PrxII in an activity-independent manner. Furthermore, the antioxidant butylated hydroxyanisole did not influence the etoposide-induced cell death. Mechanistically, the knockdown of Prx II expression impaired the DNA repair process by reducing the activation of the JNK/c-Jun pathway. These results suggest that PrxII is likely to be attributed to a tumor survival factor positively regulating JNK-dependent DNA repair with its inhibition possibly sensitizing cancer cells to chemotherapeutic agents.

Novel Thioredoxin-related Transmembrane Protein TMX4 Has Reductase Activity

In the endoplasmic reticulum (ER), a number of thioredoxin (Trx) superfamily proteins are present to enable correct disulfide bond formation of secretory and membrane proteins via Trx-like domains. Here, we identified a novel transmembrane Trx-like protein 4 (TMX4), in the ER of mammalian cells. TMX4, a type I transmembrane protein, was localized to the ER and possessed a Trx-like domain that faced the ER lumen. A maleimide alkylation assay showed that a catalytic CXXC motif in the TMX4 Trx-like domain underwent changes in its redox state depending on cellular redox conditions, and, in the normal state, most of the endogenous TMX4 existed in the oxidized form. Using a purified recombinant protein containing the Trx-like domain of TMX4 (TMX4-Trx), we confirmed that this domain had reductase activity in vitro. The redox potential of this domain (-171.5 mV; 30 degrees C at pH 7.0) indicated that TMX4 could work as a reductase in the environment of the ER. TMX4 had no effect on the acceleration of ER-associated degradation. Because TMX4 interacted with calnexin and ERp57 by co-immunoprecipitation assay, the role of TMX4 may be to enable protein folding in cooperation with these proteins consisting of folding complex in the ER.

SNAT2 Amino Acid Transporter Is Regulated by Amino Acids of the SLC6 γ-Aminobutyric Acid Transporter Subfamily in Neocortical Neurons and May Play No Role in Delivering Glutamine for Glutamatergic Transmission

System A transporters SNAT1 and SNAT2 mediate uptake of neutral alpha-amino acids (e.g. glutamine, alanine, and proline) and are expressed in central neurons. We tested the hypothesis that SNAT2 is required to support neurotransmitter glutamate synthesis by examining spontaneous excitatory activity after inducing or repressing SNAT2 expression for prolonged periods. We stimulated de novo synthesis of SNAT2 mRNA and increased SNAT2 mRNA stability and total SNAT2 protein and functional activity, whereas SNAT1 expression was unaffected. Increased endogenous SNAT2 expression did not affect spontaneous excitatory action-potential frequency over control. Long term glutamine exposure strongly repressed SNAT2 expression but increased excitatory action-potential frequency. Quantal size was not altered following SNAT2 induction or repression. These results suggest that spontaneous glutamatergic transmission in pyramidal neurons does not rely on SNAT2. To our surprise, repression of SNAT2 activity was not limited to System A substrates. Taurine, gamma-aminobutyric acid, and beta-alanine (substrates of the SLC6 gamma-aminobutyric acid transporter family) repressed SNAT2 expression more potently (10x) than did System A substrates; however, the responses to System A substrates were more rapid. Since ATF4 (activating transcription factor 4) and CCAAT/enhancer-binding protein are known to bind to an amino acid response element within the SNAT2 promoter and mediate induction of SNAT2 in peripheral cell lines, we tested whether either factor was similarly induced by amino acid deprivation in neurons. We found that glutamine and taurine repressed the induction of both transcription factors. Our data revealed that SNAT2 expression is constitutively low in neurons under physiological conditions but potently induced, together with the taurine transporter TauT, in response to depletion of neutral amino acids.

In vitro import of peroxisome-targeting signal type 2 (PTS2) receptor Pex7p into peroxisomes

Pex7p, the peroxisome-targeting signal type 2 (PTS2) receptor, transports PTS2 proteins to peroxisomes from the cytosol. We here established a cell-free Pex7p translocation system. In assays using post-nuclear supernatant fractions each from wild-type CHO-K1 and pex7 ZPG207 cells, 35S-labeled Pex7p was imported into peroxisomes. 35S-Pex7p import was also evident using rat liver peroxisomes. 35S-Pex7p was not imported into peroxisomal remnants from a pex5 ZPG231 defective in PTS2 import and pex2 Z65. When the import of 35S-Pex5pL was inhibited with an excess amount of recombinant Pex5pS, 35S-Pex7p import was concomitantly abrogated, suggesting that Pex5pL was a transporter for Pex7p, unlike a yeast cochaperone, Pex18p. 35S-Pex7p as well as 35S-Pex5p was imported in an ATP-independent manner, whilst the import of PTS1 and PTS2 cargo-proteins was ATP-dependent. Thereby, ATP-independent import of Pex7p implicated that Pex5p export requiring ATP hydrolysis is not a limiting step for its cargo recruitment to peroxisomes. PTS1 protein import was indeed insensitive to N-ethylmaleimide, whereas Pex5p export was N-ethylmaleimide-sensitive. Taken together, the cargo-protein translocation through peroxisomal membrane more likely involves another ATP-requiring step in addition to the Pex5p export. Moreover, upon concurrent import into peroxisomes, 35S-Pex5pL and 35S-Pex7p were detected at mutually distinct ratios in the immunoprecipitates each of the import machinery peroxins including Pex14p, Pex13p, and Pex2p, hence suggesting that Pex7p as well as Pex5p translocated from the initial docking complex to RING complex on peroxisomes.

The AAA-ATPase p97 facilitates degradation of apolipoprotein B by the ubiquitin-proteasome pathway

The ATPase associated with various cellular activities (AAA-ATPase) p97 (p97) has been implicated in the retrotranslocation of target proteins for delivery to the cytosolic proteasome during endoplasmic reticulum-associated degradation (ERAD). Apolipoprotein B-100 (apoB-100) is an ERAD substrate in liver cells, including the human hepatoma, HepG2. We studied the potential role of p97 in the ERAD of apoB-100 in HepG2 cells using cell permeabilization, coimmunoprecipitation, and gene silencing. Degradation was abolished when HepG2 cytosol was removed by digitonin permeabilization, and treatment of intact cells with the proteasome inhibitor MG132 caused accumulation of ubiquitinated apoB protein in the cytosol. Cross-linking of intact cells with the thiol-cleavable agent dithiobis(succinimidylpropionate) (DSP), as well as nondenaturing immunoprecipitation, demonstrated an interaction between p97 and intracellular apoB. Small interfering ribonucleic acid (siRNA)-mediated reduction of p97 protein increased the intracellular levels of newly synthesized apoB-100, predominantly because of a decrease in the turnover of newly synthesized apoB-100 protein. However, although the posttranslational degradation of newly synthesized apoB-100 was delayed by p97 knockdown, secretion of apoB-100 was not affected. Knockdown of p97 also impaired the release of apoB-100 and polyubiquitinated apoB into the cytosol. In summary, our results suggest that retrotranslocation and proteasomal degradation of apoB-100 can be dissociated in HepG2 cells, and that the AAA-ATPase p97 is involved in the removal of full-length apoB from the biosynthetic pathway to the cytosolic proteasome.

The Epithelial Sodium Channel (ENaC) Traffics to Apical Membrane in Lipid Rafts in Mouse Cortical Collecting Duct Cells

We previously showed that ENaC is present in lipid rafts in A6 cells, a Xenopus kidney cell line. We now demonstrate that ENaC can be detected in lipid rafts in mouse cortical collecting duct ((MPK)CCD(14)) cells by detergent insolubility, buoyancy on density gradients using two distinct approaches, and colocalization with caveolin 1. Less than 30% of ENaC subunits were found in raft fractions. The channel subunits also colocalized on sucrose gradients with known vesicle targeting and fusion proteins syntaxin 1A, Vamp 2, and SNAP23. Hormonal stimulation of ENaC activity by either forskolin or aldosterone, short or long term, did not alter the lipid raft distribution of ENaC. Methyl-beta-cyclodextrin added apically to (MPK)CCD(14) cells resulted in a slow decline in amiloride-sensitive sodium transport with short circuit current reductions of 38.1 +/- 9.6% after 60 min. The slow decline in ENaC activity in response to apical cyclodextrin was identical to the rate of decline seen when protein synthesis was inhibited by cycloheximide. Apical biotinylation of (MPK)CCD(14) cells confirmed the loss of ENaC at the cell surface following cyclodextrin treatment. Acute stimulation of the recycling pool of ENaC was unaffected by apical cyclodextrin application. Expression of dominant negative caveolin isoforms (CAV1-eGFP and CAV3-DGV) which disrupt caveolae, reduced basal ENaC currents by 72.3 and 78.2%, respectively; but, as with cyclodextrin, the acute response to forskolin was unaffected. We conclude that ENaC is present in and regulated by lipid rafts. The data are consistent with a model in which rafts mediate the constitutive apical delivery of ENaC.

Dual Subcellular Localization in the Endoplasmic Reticulum and Peroxisomes and a Vital Role in Protecting against Oxidative Stress of Fatty Aldehyde Dehydrogenase Are Achieved by Alternative Splicing

Fatty aldehyde dehydrogenase (FALDH, ALDH3A2) is thought to be involved in the degradation of phytanic acid, a saturated branched chain fatty acid derived from chlorophyll. However, the identity, subcellular distribution, and physiological roles of FALDH are unclear because several variants produced by alternative splicing are present in varying amounts at different subcellular locations. Subcellular fractionation experiments do not provide a clear-cut conclusion because of the incomplete separation of organelles. We established human cell lines heterologously expressing mouse FALDH from each cDNA without tagging under the control of an inducible promoter and detected the variant FALDH proteins using a mouse FALDH-specific antibody. One variant, FALDH-V, was exclusively detected in peroxisomal membranes. Human FALDH-V with an amino-terminal Myc sequence also localized to peroxisomes. The most dominant form, FALDH-N, and other variants examined, however, were distributed in the endoplasmic reticulum. A gas chromatography-mass spectrometry-based analysis of metabolites in FALDH-expressing cells incubated with phytol or phytanic acid showed that FALDH-V, not FALDH-N, is the key aldehyde dehydrogenase in the degradation pathway and that it protects peroxisomes from oxidative stress. In contrast, both FALDHs had a protective effect against oxidative stress induced by a model aldehyde for lipid peroxidation, dodecanal. These results suggest that FALDH variants are produced by alternative splicing and share an important role in protecting against oxidative stress in an organelle-specific manner.

Mammalian Sec16/p250 Plays a Role in Membrane Traffic from the Endoplasmic Reticulum

Coat protein complex II (COPII)-coated vesicles/carriers, which mediate export of proteins from the endoplasmic reticulum (ER), are formed at special ER subdomains in mammals, termed ER exit sites or transitional ER. The COPII coat consists of a small GTPase, Sar1, and two protein complexes, Sec23-Sec24 and Sec13-Sec31. Sec23-Sec24 and Sec13-Sec31 appear to constitute the inner and the outermost layers of the COPII coat, respectively. We previously isolated two mammalian proteins (p125 and p250) that bind to Sec23. p125 was found to be a mammalian-specific, phospholipase A(1)-like protein that participates in the organization of ER exit sites. Here we show that p250 is encoded by the KIAA0310 clone and has sequence similarity to yeast Sec16 protein. Although KIAA0310p was found to be localized at ER exit sites, subcellular fractionation revealed its predominant presence in the cytosol. Cytosolic KIAA0310p was recruited to ER membranes in a manner dependent on Sar1. Depletion of KIAA0310p mildly caused disorganization of ER exit sites and delayed protein transport from the ER, suggesting its implication in membrane traffic out of the ER. Overexpression of KIAA0310p affected ER exit sites in a manner different from that of p125. Binding experiments suggested that KIAA0310p interacts with both the inner and the outermost layer coat complexes, whereas p125 binds principally to the inner layer complex. Our results suggest that KIAA0310p, a mammalian homologue of yeast Sec16, builds up ER exit sites in cooperation with p125 and plays a role in membrane traffic from the ER.

The Dynamin-like Protein Vps1p of the Yeast Saccharomyces cerevisiae Associates with Peroxisomes in a Pex19p-dependent Manner

Dynamins and dynamin-like proteins play important roles in organelle division. In Saccharomyces cerevisiae, the dynamin-like protein Vps1p (vacuolar protein sorting protein 1) is involved in peroxisome fission, as cells deleted for the VPS1 gene contain reduced numbers of enlarged peroxisomes. What relationship Vps1p has with peroxisomes remains unclear. Here we show that Vps1p interacts with Pex19p, a peroxin that acts as a shuttling receptor for peroxisomal membrane proteins or as a chaperone assisting the assembly/stabilization of proteins at the peroxisome membrane. Vps1p contains two putative Pex19p recognition sequences at amino acids 509-523 and 633-647. Deletion of the first (but not the second) sequence results in reduced numbers of enlarged peroxisomes in cells, as in vps1delta cells. Deletion of either sequence has no effect on vacuolar morphology or vacuolar protein sorting, suggesting that the peroxisome and vacuole biogenic functions of Vps1p are separate and separable. Substitution of proline for valine at position 516 of Vps1p abrogates Pex19p binding and gives the peroxisome phenotype of vps1delta cells. Microscopic analysis showed that overexpression of Pex19p or redirection of Pex19p to the nucleus does not affect the normal cellular distribution of Vps1p in the cytosol and in punctate structures that are not peroxisomes, suggesting that Pex19p does not function in targeting Vps1p to peroxisomes. Subcellular fractionation showed that a fraction of Vps1p is associated with peroxisomes and that deletion or mutation of the first Pex19p recognition sequence abrogates this association. Our results are consistent with Pex19p acting as a chaperone to stabilize the association of Vps1p with peroxisomes and not as a receptor involved in targeting Vps1p to peroxisomes.

Excess Peroxisomes Are Degraded by Autophagic Machinery in Mammals

Peroxisomes are degraded by autophagic machinery termed "pexophagy" in yeast; however, whether this is essential for peroxisome degradation in mammals remains unknown. Here we have shown that Atg7, an essential gene for autophagy, plays a pivotal role in the degradation of excess peroxisomes in mammals. Following induction of peroxisomes by a 2-week treatment with phthalate esters in control and Atg7-deficient livers, peroxisomal degradation was monitored within 1 week after discontinuation of phthalate esters. Although most of the excess peroxisomes in the control liver were selectively degraded within 1 week, this rapid removal was exclusively impaired in the mutant liver. Furthermore, morphological analysis revealed that surplus peroxisomes, but not mutant hepatocytes, were surrounded by autophagosomes in the control. Our results indicated that the autophagic machinery is essential for the selective clearance of excess peroxisomes in mammals. This is the first direct evidence for the contribution of autophagic machinery in peroxisomal degradation in mammals.

Shuttling Mechanism of Peroxisome Targeting Signal Type 1 Receptor Pex5: ATP-Independent Import and ATP-Dependent Export

Peroxisomal matrix proteins are posttranslationally imported into peroxisomes with the peroxisome-targeting signal 1 receptor, Pex5. The longer isoform of Pex5, Pex5L, also transports Pex7-PTS2 protein complexes. After unloading the cargoes, Pex5 returns to the cytosol. To address molecular mechanisms underlying Pex5 functions, we constructed a cell-free Pex5 translocation system with a postnuclear supernatant fraction from CHO cell lines. In assays using the wild-type CHO-K1 cell fraction, (35)S-labeled Pex5 was specifically imported into and exported from peroxisomes with multiple rounds. (35)S-Pex5 import was also evident using peroxisomes isolated from rat liver. ATP was not required for (35)S-Pex5 import but was indispensable for export. (35)S-Pex5 was imported neither to peroxisome remnants from RING peroxin-deficient cell mutants nor to those from pex14 cells lacking a Pex5-docking site. In contrast, (35)S-Pex5 was imported into the peroxisome remnants of PEX1-, PEX6-, and PEX26-defective cell mutants, including those from patients with peroxisome biogenesis disorders, from which, however, (35)S-Pex5 was not exported, thereby indicating that Pex1 and Pex6 of the AAA ATPase family and their recruiter, Pex26, were essential for Pex5 export. Moreover, we analyzed the (35)S-Pex5-associated complexes on peroxisomal membranes by blue-native polyacrylamide gel electrophoresis. (35)S-Pex5 was in two distinct, 500- and 800-kDa complexes comprising different sets of peroxins, such as Pex14 and Pex2, implying that Pex5 transited between the subcomplexes. Together, results indicated that Pex5 most likely enters peroxisomes, changes its interacting partners, and then exits using ATP energy.

Modulation of Folate Dependent DNA Methyltransferase 1 (dnmt1) Activity in Whole Body γ-irradiated Mice.

Abstract The methylation pattern of DNA is established during embryogenesis and is subsequently maintained by maintenance methylation activity of the enzyme DNA methyltransferase 1 (dnmtl). The modulation of nuclear dnmtl activity in nuclear and post-nuclear supernatant fractions was studied after various time intervals in 1, 3. 5 and 7 Gy whole body γ-irradiated mice. Significant changes in enzyme activity were observed in nuclear fraction of liver at various stages after irradiation. Our results show a dose-dependent increase in nuclear dnmtl activity after 24 hours al 1 to 5 Gy, with the maximum increase in activity at 7 Gy. An attenuation of enzyme activity was observed after 48 hours, which was continued at 96 hours post-iradiation, where the activity approached subnormal levels at higher doses. The dnmtl activity was less conspicuous in supernatant fraction and no significant change in activity was observed 24 hours after irradiation. Present studies suggest the possible role of radiation as a stressor in modulating the folate dependent dnmtl activity, which in turn may affect global DNA methylation pattern.

Dominant Negative Rab3D Mutants Reduce GTP-bound Endogenous Rab3D in Pancreatic Acini

Two dominant negative mutants of Rab3D, N135I and T36N were recently reported to inhibit the early phase of regulated amylase secretion from mouse pancreatic acini (Chen, X., Edwards, J. A., Logsdon, C. D., Ernst, S. A., and Williams, J. A. (2002) J. Biol. Chem. 277, 18002-18009). Immunocytochemical studies showed that while the wild-type Rab3D localized to zymogen granules, the two dominant negative mutants did not localize to granules and were primarily in the basolateral regions of the cell. The present study, therefore, evaluated the potential mechanisms by which the dominant negative mutants might act. An affinity precipitation assay based on the property of the Rab3 effector Rim1 to interact only with GTP-bound Rab3D was developed. 78.9 +/- 4.5% of total endogenous Rab3D was found in the GTP-bound form. Overexpression of HA-tagged Rab3D, and its Q81L, N135I, and T36N mutants had no effect on the total amount of endogenous Rab3D. However, the dominant negative mutants, T36N and N135I, reduced GTP-bound endogenous Rab3D by 70.0 +/- 3.5% and 72.7 +/- 1.2%, respectively, while the wild-type Rab3D and Q81L mutant had no effect. Triton X-114 phase separation and cell fractionation studies showed that dominant negative Rab3D mutants did not alter isoprenylation or membrane association of endogenous Rab3D. The dominant negative Rab3D did not affect the amount of endogenous Rab3D on purified zymogen granules as assessed by either Western blotting or immunocytochemistry, but reduced the GTP-bound form by 78.6 +/- 3.3%. The two dominant negative Rab3D mutants, therefore, interfere with endogenous Rab3D function by blocking the GDP/GTP exchange but not zymogen granule targeting of endogenous Rab3D.

Subcellular localization of neuronal nitric oxide synthase in rat small intestine

The subcellular localization of neuronal nitric oxide synthase (NOS I, EC 1.14.13.39) was investigated in the longitudinal muscle/myenteric plexus (LM/MP) preparation of rat small intestine. The presence of NOS I, inducible nitric oxide synthase (NOS II), and endothelial nitric oxide synthase (NOS III) was assessed after homogenization and low-speed centrifugation in a postnuclear supernatant by immunological detection after PAGE and Western blotting. Only NOS I was clearly present, whereas NOS II and NOS III were below detection limits. After high-speed centrifugation of the postnuclear supernatant, soluble and particulate fractions were obtained, and the presence of NOS I in these fractions was investigated by measurement of NOS I immunoreactivity and enzyme activity. We found that 90 ± 1% of NOS I immunoreactivity and 97 ± 1% of NOS enzyme activity were confined to the soluble fraction of the tissue. Further immunological analysis demonstrated that washing the particulate fraction revealed detectable amounts of NOS I only after concentration of the washing supernatant. Most particulate NOS I remained in the pellet and therefore represents cell organelle-associated enzyme. No NOS I immunoreactivity could be detected as a soluble protein within organelles of the cell. Particulate NOS I could in part be solubilized by Triton X-100 treatment, and the detection of Triton X-100-soluble NOS I was dependent on the antibody used. In conclusion, our results indicate that NOS I in the LM/MP preparation of rat small intestine is mainly soluble and that the particulate NOS I is partly an intrinsic membrane protein and can partly be solubilized by detergent treatment.

Protein-A Activates Membrane Bound Multicomponent Enzyme Complex, Nadph Oxie in Human Neutrophils®

Protein-A, 42KD cell wall glycoprotein of S. aureus Cowan I enhance mononuclear and polymorphonuclear cell counts in vivo and possesses antitoxic, antitumor, properties. In order to explain the mechanism of its function, the respiratory burst phenomenon in cell and cell free system was studied using lucigenin-dependent chemiluminescence technique. A dose dependent increase in protein A-mediated generation of superoxide radical was observed in resting and PMA stimulated neutrophils. Superoxide dismutase (SOD) was used to confirm the production of superoxide radicals (O−2). To understand the mechanism of protein-A induced O−2 generation; NADPH oxidase activity was measured in cell free system using NADPH as a substrate. A significant increase in NADPH oxidase activity was observed in the membrane and post-nuclear supernatant fraction of activated human neutrophils. Cytosolic fraction showed slight enzyme activation. Protein A (SpA)-induced NADPH oxidase activation in the membrane fraction was observed even in the absence of the substrate NADPH. These data indicate that protein A attenuate the NADPH oxidase system to produce O−2 radicals.