From the
Biocenter Oulu, Department of Biochemistry and
Department of Pathology, University of Oulu, FIN-90014 Oulu, Finland and ¶Institut für Physiologische Chemie, Ruhr-Universität Bochum, Bochum, D-44780 Bochum, Germany
Received for publication, March 20, 2003
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ABSTRACT |
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INTRODUCTION |
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The properties of the eukaryotic FASs and their role in metabolism appear to vary depending on the organism and the subcellular organelle; this is analogous to the -oxidation of fatty acids. Although both mitochondrial and peroxisomal
-oxidation perform chain shortening of acyl-CoA substrates, the kinetic properties of the enzymes differ, and they clearly do not represent a metabolic redundancy, as exemplified by inborn errors of fatty acid oxidation (for review see Ref. 9). The disruption of any of the mitochondrial FAS type II genes in yeast results in a respiration-deficient phenotype (1, 2, 10), although the cytosolic FAS is still operational. Also the mitochondrial FAS in the yeast cannot compensate for a defective cytosolic pathway, and the inactivation of the cytosolic FAS is lethal, unless the cells are supplemented with fatty acids in the culture media (11).
We have recently identified a novel type of 2-enoyl thioester reductase from Candida tropicalis (Etr1p) and Saccharomyces cerevisiae (Ybr026p) that is involved in mitochondrial FAS (12). The ybr026c strain exhibits reduced mitochondrial cytochrome contents and an inability to grow on nonfermentable carbon sources (13). The yeast FAS type II 2-enoyl thioester reductases are homodimeric, NADPH-dependent, and are not sequence-related to the homotetrameric reductases of bacterial FAS type II.
Rat nuclear receptor binding factor-1 (RnNrbf-1p), which was described as homolog of the Ybr026p/Mrf1'p by protein data base searches, was observed previously to interact with peroxisome proliferator-activated receptor as well as with a number of other nuclear receptors in the yeast two-hybrid assay (14). In the current work open reading frames (ORF) encoding the human homolog (HsNrbf-1p) of the rat Nrbf-1p was identified in human EST data bank, and the cDNA encoding the bovine homolog (BtNrbf-1p) was cloned. The data demonstrated that Nrbf-1ps are mitochondrial proteins that catalyze an NADPH-dependent reduction of trans-2-enoyl thioesters to the corresponding saturated acyl thioesters. As the expression of human NRBF-1 in the ybr026c
yeast strain rescues the respiration-deficient phenotype, it is concluded that Nrbf-1ps are the orthologs of the yeast 2-enoyl thioester reductases, enzymes previously unidentified in mammals.
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EXPERIMENTAL PROCEDURES |
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Cloning of the NRBF-1 cDNA from Bovine HeartThe total RNA was isolated from bovine heart tissue using QuickPrep system (Amersham Biosciences). cDNA was amplified from isolated RNA with RobustRT-PCR kit (Finnzymes, Espoo, Finland), which contains a high fidelity DyNAzyme EXT, using the primer pair BtNRBF15' (5'-TGG AGG GAA CAT GTG GG-3') and BtNRBF13' (5'-ATC ACA T(G/A)G TGA GAA TCT GCT T-3'). The composition of the BtNRBF15' was based on the sequence of bovine EST clone BE682523 [GenBank] , containing the 5' end of the cDNA obtained by the data base search with the full-length human NRBF-1 sequence. The composition of the BtNRBF13' was based on the consensus sequence obtained from the alignment of human, rat, and mouse sequences. The PCR product was ligated to pSTBlue vector using Acceptor Vector kit (Novagen, Madison, WI), and both strands of the insert were sequenced. The resulting plasmid was named pSTBlue::BtNRBF-1.
Overexpression and Purification of Recombinant Human and Bovine Nrbf-1pTo produce the recombinant, truncated variant of HsNrbf-1p, which was deleted for 31 N-terminal amino acid residues, based on the amino acid sequence comparison to the mature form of Mrf1-p (13), nucleotides 901119 from AA393871
[GenBank]
were amplified with primers HsNRBF1-PET3a-5' (5'-TT TTT TTT CAT ATG GCC TCC TCC TAC TCC GCA TCC-3') and HsNRBF1-PET3a-3' (5'-TTT TTT TTT CAT ATG TCA CAT GGT GAG AAT CTG CTT TGA AGA TAT G-3'), containing NdeI sites (underlined). After digestion with NdeI, the resulting PCR product was cloned into NdeI-digested pET3a vector (Novagen) to generate pET3a::trHsNRBF1. The bovine construct was generated by amplifying nucleotides 951124 from pSTBlue::BtNRBF1, deleting also 31 N-terminal residues, with primers BovETR5' (5'-TT TTT TTT CAT ATG TCC GCC TCC TTC TCT GCC-3') and BovETR3' (5'-TTT GGA TCC TCA CAT GGT GAG AAT CTG CTT TGA AGA CAC-3'), which contained initiation and termination codons (boldface) within the coding sequence and NdeI (BovETR5') and BamHI (BovETR3') sites (underlined). The PCR product was digested with BamHI and NdeI and ligated into similarly digested pET3a vector to obtain pET3a::trBtNRBF1. After verification of the sequences and orientation of the inserts, the plasmid DNA was transformed into E. coli BL21 (DE3) pLysS cells and the expression of the recombinant protein was induced in M9ZB medium by the addition of isopropyl--D-thiogalactoside to a final concentration of 1.0 mM according to the manufacturer's protocol. After incubation for an additional 10 h at 37 °C, the cells were washed and stored at -20 °C until used. Although the immunological cross-reaction of the antibodies raised against Etr1p from C. tropicalis (anti-Etr1pAb; Ref. 12) with mammalian homologs was weak, the antibody could be used for monitoring the purification of recombinant HsNrbf-1p; in the meantime an antibody to HsNrbf-1p (anti-HsETRAb) was raised and used to monitor the purification of recombinant BtNrbf-1p.
Bacterial cells (6 g wet weight) were resuspended in 50 ml of 50 mM N,N-bis(2-hydroxyethyl)glycine, pH 9.1, 50 mM NaCl, 0.1 mM phenylmethylsulfonyl fluoride, 2 mM EDTA, 2 mM EGTA, 0.5 mM benzamide hydrochloride (BA), 0.5 mM dithiothreitol (DTT) (buffer A) and disrupted using a French press (Spectronic Instruments, Rochester, NY) at 32,000 pounds/square inch pressure. The disrupted cell suspension was centrifuged at 20,000 x g (+4 °C) for 30 min, and the supernatant containing 13 mg/ml of protein was applied to an anion exchange Q-Sepharose column (3.0 x 8.0 cm, Amersham Biosciences) equilibrated with buffer A. The flow-through fractions, which contained the protein cross-reacting with anti-Etr1pAb, were dialyzed against 50 mM potassium phosphate, pH 7.8, and applied to a 2',5'-ADP-Sepharose 4B column (1.0 x 10.0 cm, Amersham Biosciences) equilibrated with 50 mM potassium phosphate, pH 7.8, 2 mM EDTA, 2 mM EGTA, 0.5 mM benzamide hydrochloride, 0.5 mM DTT. Bound proteins were eluted with an 80-ml linear NaCl gradient from 0 to 2.0 M, at a flow rate of 1 ml/min. Fractions containing immunodetectable HsNrbf-1p were pooled, dialyzed against 50 mM sodium phosphate, pH 7.1, 50 mM NaCl and applied to a cation exchange Resource S column (volume 6 ml) (Amersham Biosciences) equilibrated with 50 mM sodium phosphate, pH 7.1, 50 mM NaCl, 2 mM EDTA, 2 mM EGTA, 0.5 mM benzamide hydrochloride, 0.5 mM DTT. Bound proteins were eluted with a 60-ml linear NaCl gradient from 50 mM to 1 M at a flow rate of 2 ml/min. The fractions containing HsNrbf-1p were pooled, concentrated with Biomax-10 Ultra-free Centrifugal Filter Unit (Millipore, Bedford, MA), and applied to a Superdex 200 HR 10/30 size exclusion column (Amersham Biosciences) equilibrated with 100 mM sodium phosphate, pH 7.8, 150 mM NaCl, 1 mM EDTA, 1 mM NaN3. The bovine recombinant protein was purified following the protocol used for the human protein.
2-Enoyl Thioester Reductase Activity and Reaction Product AnalysisEnoyl reductase activity was measured using trans-2-hexenoyl-CoA (60 µM), synthesized by the mixed anhydride method (16) as described previously (17). For end product analysis 3 µg of recombinant trHsNrbf-1p was incubated in the presence of 60 µM trans-2-hexenoyl-CoA and 300 µM NADPH in 1 ml of 20 mM potassium phosphate, pH 7.8, at 22 °C. Taken aliquots of 100 µl were acidified with 10 µl of 2 M HCl, and samples of 20 µl from the aliquots were subjected to a reversed phase chromatography (Waters Novapak C18, 2 x 150 mm, Waters 626 pump, Waters 996 photodiode array detector, Waters Millipore, Millford, MA). The column was eluted with an isocratic flow of 0.2 ml/min with buffers A (25 mM CH3COONH4, pH 5.5) and B (80% CH3CN and 25 mM CH3COONH4, pH 5.5) in 83:17 ratio. Metabolites in the eluted peaks were identified by matrix-assisted laser desorption ionization-time of flight-mass spectroscopy (MALDI-TOF MS) (ABI Voyager-DETM STR Biospectrometry Workstation).
Determination of Kinetic Parameters for HsNrbf-1pThe kinetic constants for reductase activities were determined using substrate concentrations of 1.25200 µM for trans-2-hexenoyl-CoA, trans-2-decenoyl-CoA, and trans-2-hexadecenoyl-CoA. Otherwise the assay mixture contained 125 µM NADPH and 100 µg of bovine serum albumin in 50 mM potassium phosphate, pH 7.5, in a final volume of 1 ml at 22 °C. The kinetic data were transferred to Lineweaver-Burk plots by using GraFit computer software (Sigma). The catalytic turnover numbers were obtained by dividing the maximum velocities with the enzyme (subunit) concentrations in the reaction mixtures.
pEGFP-N1::HsNBRF-1 Construct for Localization StudiesFull-length HsNRBF-1 cDNA was amplified from pYE352::HsNRBF-1 with the primer pair HsNRBF-EGFP5' (5'-TT GAA TTC ATG TGG GTC TGC AGT ACC CTG-3') and HsNRBF-EGFP3' (5'-TTT GGT ACC CAC ATG GTG AGA ATC TGC TTT GAA G-3'), with EcoRI and KpnI sites (underlined) at the 5' and 3' ends of the cDNA sequence, respectively. The resulting PCR product was digested with EcoRI and KpnI, gel-purified, and subcloned in-frame with the 5' end of the green fluorescent protein (GFP) cDNA in the expression vector pEGFP-N1 (Clontech) resulting in pEGFP-N1::HsNRBF-1.
Fluorescence MicroscopyAttached HeLa cells (CCL-2; American Type Culture Collection) were grown on glass coverslips in Dulbecco's modified Eagle's media (Invitrogen) supplemented with 10% fetal bovine serum, 100 units/ml penicillin, and 100 µg/ml streptomycin (Invitrogen). The transfection of the HeLa cells was done using FuGENE 6 reagent (Roche Diagnostics), according to the manufacturer's protocol, and using 6 µl of FuGENE 6 with 2 µg of either pEGFP-N1::HsNRBF-1 or pEGFP-N1. After transfection the cells were grown for 36 h and treated with the mitochondrion-selective fluorescent dye MitoTracker Red CMXRos (Molecular Probes Europe BV, Leiden, Netherlands) according to the manufacturer's instructions. Cells were washed in phosphate-buffered saline (Invitrogen) and fixed with 4% paraformaldehyde in 14 mM sodium phosphate, pH 7.4, 150 mM NaCl (PBS) for 20 min at 22 °C. To minimize the background fluorescence, the fixed cells were incubated in 50 mM NH4Cl/PBS for 10 min before treatment with 0.2% (v/v) Triton X-100/PBS for 10 min at 22 °C. Nuclear DNA was counterstained with 1 µg/ml 4',6-diamidino-2'-phenylindole dihydrochloride (DAPI), and the mounted (IMMU-MOUNT; Thermo Shandon, Pittsburgh, PA) slides were examined using a fluorescence microscope (Olympus Optica, Hamburg, Germany). Images were recorded with a CCD camera and processed with the Jasc Software Paint Shop Pro.
Isolation of Rat Heart MitochondriaCrude mitochondria fraction from rat heart was isolated as described (18). The bacterial protease (Nagarse) used was from Sigma. Mitochondrial pellet was suspended to a buffer containing 10 mM Tris-Cl, pH 7.8, 250 mM sucrose, 0.2 mM EDTA, giving a protein concentration of 23 mg/ml. An aliquot of mitochondrial fraction (46 mg of protein) was purified further by isopycnic density gradient centrifugation (19) on a self-generated Percoll (Sigma) gradient (20) with an initial Percoll concentration of 32% (v/v). Gradient was generated by ultracentrifugation at 100,000 x g for 30 min in Beckman VTi50 rotor using slow acceleration/deceleration mode. Gradient was unloaded in 2-ml fractions from the bottom of the tube, and marker enzyme activities, cytochrome c oxidase (21), esterase (22), and lactate dehydrogenase (23) for mitochondria, microsomes, and cytosol, respectively, were measured.
Isolation of Wild-type BtNrbf-1pThe mitochondria were isolated from bovine heart, obtained from a local slaughterhouse as described earlier (24), and stored at -70 °C until used. The soluble mitochondrial extract was prepared by disrupting 2 g of mitochondria by sonication in 20 ml of 50 mM potassium phosphate, pH 7.8, containing 70 mM NaCl at 0 °C, followed by ultracentrifugation at 100,000 x g for 1 h. A sample of supernatant containing 120 mg of protein was dialyzed against 20 mM potassium phosphate, pH 7.8, and applied to a 2',5'-ADP-Sepharose 4B column (1.0 x 7.0 cm) equilibrated with dialysis buffer at the flow rate of 0.5 ml/min. The bound proteins were eluted with a NaCl gradient from 0 to 0.4 M (10 ml) and from 0.4 to 2 M (10 ml). The steep part of the gradient was implemented to prevent the excess dilution of BtNrbf-1p, as detected by immunoblotting with anti-HsETRAb. The fractions containing BtNrbf-1p were pooled, dialyzed against PBS, and subjected to immunoisolation. A suspension of 100 µl of protein A-coated Dynabeads (Dynal, Oslo, Norway) was washed three times with 100 mM sodium phosphate buffer, pH 8.2, and incubated for 1 h at room temperature with 10 µl of anti-HsETRAb or preimmune serum. The antibody-loaded beads were washed three times with PBS containing 0.1% Tween 20 (PBST) and transferred using the PickPen magnetic particle transfer device (Bio-Nobile, Turku, Finland) to a 400-µl aliquot of the dialyzed sample containing BtNrbf-1p (catalyzing the reduction of 60 µM trans-2-decenoyl-CoA at the rate of 0.6 nmol/min). After incubation at 4 °C with gentle shaking the beads were separated, washed three times with PBST, and transferred to 20 µl of SDS sample buffer containing 50 mM DTT. After 30 min of incubation at 37 °C, the beads were removed, and the sample was acetylamidated with 110 mM iodoacetamide for 1 h at 37 °C. After electrophoresis, bands were excised, destained, and dried. The gel pieces were rehydrated with trypsin solution (20 µg/ml porcine trypsin in 40 mM NH4CO3 containing 10% (v/v) CH3CN and 0.5% (w/v) -octyl-D-glucoside) and incubated overnight at 37 °C. Digested peptides were recovered by extraction with four 20-µl aliquots of 0.1% trifluoroacetic acid in water containing 5, 30, 50, and 70% CH3CN, respectively. Extracts were pooled, dried, and dissolved into 5 µl of saturated
-cyano-4-hydroxycinnamic acid in 40% CH3CN, 0.1% trifluoroacetic acid, and subjected to MALDI-TOF MS.
Transmission EM of ybr026c Cells Expressing HsNrbf-1pFor the electron microscopy studies S. cerevisiae ybr026c
cells overexpressing HsNrbf-1p or trHsNrbf-1p were transferred from overnight cultures in synthetic medium lacking uracil and containing 2% (w/v) D-glucose (25) to oleic acid medium (26) and grown for 17 h at 30 °C. BJ 1991 wild-type cells and ybr026c
cells were grown overnight on 1% (w/v) yeast extract, 2% (w/v) peptone, and 2% (w/v) D-glucose and then transferred to oleic acid medium. The cells were fixed in 2.5% glutaraldehyde in 100 mM sodium phosphate buffer, pH 7.4, pelleted, and immersed in 2% agar (w/v) in PBS. Yeast cells in agar blocks were further post-fixed in 1% osmium tetroxide (w/v), dehydrated in acetone, and embedded in Embed 812 (Electron Microscopy Sciences, Fort Washington, PA). Thin sections of 80 nm were cut with a Reichert Ultracut ultramicrotome and examined using a Philips CM100 transmission electron microscope.
OthersThe generation of the S. cerevisiae BJ1991 ybr026c strain used in the transformations with pYE352::HsNRBF-1 or pYE352::trHsNRBF-1 has been described previously (27). Transformed yeast strains were maintained on synthetic medium lacking uracil (SC-U) and supplemented with 2% (w/v) D-glucose and tested for complementation on SC-U containing 3% (w/v) glycerol as a nonfermentable carbon source. Antibody against human Nrbf-1 was raised by immunizing rabbits with purified recombinant HsNrbf-1p. The IgG fraction from the antisera was purified by using the Econo-Pac Serum IgG purification kit (Bio-Rad). Restriction enzymes were purchased from New England Biolabs (Hertfordshire, UK) or from Amersham Biosciences. The use of laboratory animals was approved by the committee on animal experimentation of the University of Oulu.
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RESULTS |
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The expression of HsNRBF-1 was analyzed in different human tissues by Northern blot analysis. The strongest signals of 1.4 kb for HsNRBF-1 were observed in the samples from skeletal and heart muscles (Fig. 2A). Weak signals of the same size were observed in the samples from brain, placenta, liver, kidney, and pancreas, but no signal was observed in the sample from lung. Hybridization with a -actin probe was used as a control for the amount of mRNA on the nylon blot (Fig. 2B). Two mRNAs of
-actin were visualized in heart and skeletal muscle, namely a band of 2 kb size and a smaller one of 1.61.8 kb size as observed previously (29, 30).
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Because the Northern blot analysis indicated that the NRBF-1 was expressed in heart muscle, this tissue was chosen as a material for characterization of the mammalian Nrbf-1p originating from a wild-type source. The BtNRBF-1 cloned from bovine heart was 1133 bp long and contained an open reading frame of 1122 nucleotides, encoding a polypeptide of 373 amino acid residues. The similarities between human and bovine Nrbf-1p sequences were 88 and 85% at the nucleotide and amino acid level, respectively.
The amino acid sequence identities between HsNrbf-1p (Bt- Nrbf-1p) and Ybr026p and Etr1p were 40 (35%) and 36% (38%), respectively, and 80% (83%) to Rattus norvegicus Nrbf-1p (14). An amino acid identity of 42% (44%) was obtained with the annotated Ybr026p homolog in Caenorhabditis elegans. Fig. 1 shows that the identical/similar amino acid residues in these proteins formed clusters, which were found along the length of the polypeptides excluding the extreme N and C termini. The tyrosine residue critical for catalysis in the yeast enzymes (31) is also conserved in the mammalian protein (Tyr-94 in HsNrbf-1p).
Overexpression and Purification of HsNrbf-1p and BtNrbf-1pTo characterize further the functions of HsNrbf-1p and BtNrbf-1p, their variants deleted for the predicted mitochondrial targeting signals were overexpressed from a pET3a vector in Escherichia coli and purified to apparent homogeneity as described under "Experimental Procedures," yielding from 6.0 to 8.8 mg of protein from 2 liters of the bacterial culture. SDS-PAGE analysis showed a single protein band of 37 kDa, which is in agreement with the mass of predicted polypeptides (37,052 Da and 36,973 Da) encoded by HsNRBF-1 and Bt- NRBF-1 in pET3a. Size exclusion chromatography using a Superdex 200 HR column gave an estimated native mass of 65 kDa (Fig. 3), indicating that both proteins are homodimers.
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Enzymatic Properties of 2-Enoyl Thioester ReductaseThe amino acid sequence similarity of HsNrbf-1p and BtNrbf-1p with C. tropicalis Etr1p and S. cerevisiae Ybr026p prompted us to test whether they have a similar enzymatic activity. Therefore, HsNrbf-1p was incubated in the presence of trans-2-hexenoyl-CoA and NADPH, and samples were taken from the reaction at various time points for HPLC analysis (Fig. 4, panel 1). The oxidation of NADPH was accompanied by the disappearance of trans-2-hexenoyl-CoA with a concomitant accumulation of a metabolite (Fig. 4, panel 2, AC) having the same retention time as hexanoyl-CoA (Fig. 4, panel 3, AC). The accumulating metabolite was 2 mass units larger than trans-2-hexenoyl-CoA (863.7 m/z) in MALDI-TOF MS analysis consistent with the end product being hexanoyl-CoA (865.7 m/z). The data show that HsNrbf-1p is a 2-enoyl thioester reductase, reducing trans-2-enoyl thioesters to their saturated counter-parts. NADH could not replace NADPH in the assay.
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The kinetic parameters for the HsNrbf-1p were determined using trans-2-hexenoyl- and trans-2-decenoyl-CoA as a substrates. The Km values (kcat values) of 37.0 ± 5.4 (0.53 ± 0.03 s-1) and 7.1 ± 0.9 µM (0.93 ± 0.03 s-1) were obtained, respectively. These values translate to the catalytic efficiency (kcat/Km) of 1.44 x 104 and 1.31 x 105 M-1 s-1 for C6 and C10 substrates. The assays with trans-2-hexadecenoyl-CoA gave a bell-shaped curve when the kcat values versus substrate concentrations were blotted. The maximum kcat of 0.2 s-1 was obtained at 6 µM trans-hexadecenoyl-CoA. The detected low activity under tested conditions hampered the estimation of kinetic parameters. When the BtNrbf-1p was tested with trans-2-decenoyl-CoA, the kcat was 0.52 ± 0.02 s-1, the Km 4.5 ± 0.6 µM-1, and the kcat/Km 1.15 x 105 M-1 s-1, values that are comparable with those obtained for HsNrbf-1p.
Subcellular Localization of Mammalian and Rat Nrbf-1p The N-terminal extensions of human, bovine, and rat Nrbf-1p are rich in hydrophobic and positively charged amino acid residues, contain hydroxylated residues, and lack negatively charged residues; these are features of a mitochondrial targeting signal (32). Subcellular localization prediction of HsNrbf-1p, BtNrbf-1p, and rat Nrbf-1p gave 8097% probability that they are mitochondrial proteins (TargetP V1.0 (33) and Mito-ProtII 1.0a4 (34)). This prediction challenges the previous suggestion that rat Nrbf-1 is a cytosolic/nuclear protein. To reinvestigate the subcellular localization of mammalian Nrbf-1p, pEGFP-N1::HsNRBF-1 was expressed in HeLa cells (Fig. 5, AC); the mitochondria were stained with MitoTracker Red (Fig. 5, C and G), and the nuclear DNA was counterstained with DAPI (Fig. 5, B and F). The cells transfected with pEGFP-N1::HsNRBF-1 show a well defined punctated fluorescence pattern (Fig. 5A) that superimposes with MitoTracker staining (Fig. 5D). In contrast, a diffuse uniform cellular fluorescent signal, not superimposable with MitoTracker staining, was observed in the cells transfected with control plasmid (pEGFP-N1) (Fig. 5, E and H). Separated experiments were carried out with HeLa cells expressing the GFP attached N-terminally to HsNrbf-1p and the outcome was diffuse fluorescent signal, similar to that obtained with control plasmid (data not shown). The localization of wild-type Nrbf-1p was also studied with subcellular fractionation of rat heart organelles by differential and isopycnic centrifugations in self-generated Percoll gradient. Cytochrome c oxidase was concentrated at the density of 1.081.1 g/ml (Fig. 5I). Immunoblotting of the Percoll gradient fractions with anti-HsETRAb (Fig. 5I) visualized, in the fractions containing cytochrome c oxidase activity, a 37-kDa band that corresponds to the polypeptide size predicted from NRBF-1_Rn (Fig. 1). Because 2-enoyl thioester reductase activity was detected in the same fractions, the findings suggested mitochondrial localization of Nrbf-1p also in rat heart.
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Purification of Nrbf-1 from Bovine HeartThe BtNrbf-1p from the soluble extract of isolated bovine heart mitochondria, partially purified with 2',5'-ADP-Sepharose affinity column, was subjected to immunoaffinity purification using anti-HsEtrAb bound to protein A-coated magnetic beads as described under "Experimental Procedures." The reductase activity in the sample decreased with anti-HsETRAb in a dose-dependent manner but not in the control experiments carried out with the beads treated with preimmune serum (Fig. 6A). Analysis of immunoisolated proteins by SDS-PAGE revealed a band of 37 kDa (Fig. 6B) that was excised and in-gel digested. With the peptide fingerprint measured by MALDI-TOF MS (Fig. 6C), the corresponding protein was unambiguously identified as Bt- Nrbf-1p. The peptides detected covered a large part of the sequence between amino acids 45 and 325. N-terminal peptides were not found, in agreement with mitochondrial targeting signal cleavage required for mitochondrial import.
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Physiological Function of HsNrbf-1p Complementation Studies with YeastIn order to study whether HsNrbf-1p is the human ortholog of the yeast Ybr026p and Etr1p, ybr026 cells were transformed with pYE352::HsNBRF-1 or pYE352::trHsNBRF-1, encoding HsNrbf-1p and trHsNrbf-1p, respectively, and tested for growth on glycerol. The ybr026c
strain and ybr026c
expressing catalase A were unable to grow on glycerol. Expression of HsNRBF-1 was able to rescue the growth of ybr026
cells (Fig. 7) similar to YBR026c (12). Growth on glycerol was not rescued by expression of the trHsNrbf-1, a variant of the human protein truncated for mitochondrial targeting signal (Fig. 7), although it was expressed as indicated by immunoblotting with anti-Etr1pAb (data not shown).
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When transmission electron microscopy was carried out, rudimentary mitochondria were visualized in ybr026c cells as well as in the ybr026c
expressing trHsNrbf-1p (Fig. 8, B and C). Compared with mitochondria in wild-type cells (Fig. 8A), enlarged mitochondria were developed upon HsNRBF-1 expression (Fig. 8D). These data are similar to that obtained by expression of yeast 2-enoyl thioester reductase in ybr02c6
cells with or without N-terminal mitochondrial targeting signal (12).
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DISCUSSION |
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From the evolutionary point of view, HsNrbf-1p and its orthologs in other eukaryotes are unique among monofunctional, non-flavin-containing 2-enoyl thioester reductases as they are members of the medium chain alcohol dehydrogenase/reductase superfamily, whereas other known reductases belong to the short chain alcohol dehydrogenase/reductase family. The others include, as prototypes of the family, triclosan-sensitive FabI from E. coli and isoniazid-sensitive InhA from Mycobacterium tuberculosis (36, 37). TSC13/YDL015c in S. cerevisiae has also been identified to encode a 2-enoyl reductase, which catalyzes the last reaction of each cycle of fatty acid chain elongation in the endoplasmic reticulum (38). A novel NADPH-dependent 2-enoyl-CoA reductase from the short chain alcohol dehydrogenase/reductase family has been characterized recently from human and mouse peroxisomes, and it has been hypothesized to contribute to peroxisomal fatty acid chain elongation (39). Both peroxisomal and mitochondrial 2,4-dienoyl-CoA reductases, which catalyze the NADPH-dependent reduction of 2,4-dienoyl-CoA to trans-3-enoyl-CoA and are auxiliary enzymes of -oxidation of (poly)unsaturated fatty acids, are also short chain alcohol dehydrogenase/reductase proteins (40, 41). Recently, Heath and Rock (42) identified as a third type of 2-enoyl thioester reductase from Streptococcus pneumoniae an NADH-dependent flavin enzyme FabK that has no similarity to FabI at the amino acid sequence level and is resistant to triclosan.
Mitochondrial targeting of HsNrbf-1-GFPp in cultured HeLa cells, immunological localization of Nrbf-1p in subcellular fractions of rat heart, and heterologously expressed HsNRBF-1 in yeast disagree with earlier reports (14) on cytosolic/nuclear localization and coactivator hypothesis of mammalian Nrbf-1p. This disagreement can be explained in terms of differences in experimental conditions, because the previous studies were carried out using rat Nrbf-1 with N-terminally attached hemagglutinin (HANrbf-1). In this work the expression of the wild-type HsNrbf-1p or C-terminally tagged HsNrbf-1p leaves the potential N-terminal mitochondrial targeting signal functioning. In addition to the cytosolic/nuclear localization of HANrbf-1 in HeLa cells, the coactivator hypothesis is based on the findings that rat Nrbf-1 interacts with peroxisome proliferator-activated receptor and other nuclear receptors, in the presence of respective ligands, in the yeast two-hybrid assays (14). However, the mitochondrial localization observed in this study necessitates a re-evaluation of the physiological significance of these interactions and the suggestion that the Nrbf-1p acts as a nuclear receptor coactivator.
The most significant function of mitochondrial FAS in fungi (2, 43) and in plants (3) is regarded as the generation of octanoyl-ACP for lipoic acid synthesis by lipoic acid synthase. Although a dietary supply of lipoic acid is crucial for the well being of animals, a mouse cDNA (mLIP1) and a clone from human EST data bases have been characterized recently as encoding for mitochondrial lipoic acid synthase (44). The expression of LIP1 in Mammalia points to the possibility that dietary uptake of lipoic acid is insufficient (44). Acylated ACP in Mammalia and Neurospora crassa mitochondria is linked to the complex I (45, 46, 47), and the disruption of the ACP-1 in N. crassa results in a respiration-deficient phenotype due to lack of complex I (47). However, the disruption does not affect the cellular lipoic acid content, and it has been proposed that N. crassa has another soluble intramitochondrial pool of ACP (48). One interesting observation is that ACP (46), Nrbf-1 (this study), and lipoic acid synthase (44) in mammals are highly expressed in tissues with a high rate of oxidative phosphorylation.
Bp11-C25/17 S. cerevisiae cells carrying an amber mutation in BPL1 gene encoding biotin:protein ligase required for the formation of the acetyl-CoA carboxylase (ACC) holoenzyme are respiration-deficient although lipoic acid levels are essentially normal (49). This suggests that products other than octanoyl-ACP from mitochondrial FAS and lipoic acid are crucial for the respiratory competence. In line with this, label from [14C]malonic acid was incorporated in vitro into long chain fatty acyl ACP esters in N. crassa (50). The Km, kcat, and Km/kcat values of the recombinant HsNrbf-1p showed that the mammalian enzyme can catalyze reduction of a broad range of trans-2-enoyl substrates to the corresponding acyl esters, although it had a preference for medium (C10) over short (C6) or long chain (C16) substrates. However, the kcat values of mammalian reductase for C6 and C10 substrates are about 10 times lower than those of Etr1p from C. tropicalis (the kcat of 12.3 and 7.2 s-1 for trans-2-hexenoyl-CoA and trans-2-decenoyl-CoA, respectively).2 Interestingly, at least two components of the inner mitochondrial membrane-embedded proteins of the respiratory chain, ND5 of complex I and a subunit of the cytochrome c oxidase in N. crassa, are myristoylated (47, 51).
Although current information on the mammalian mitochondrial FAS is fragmentary, several of its components have been described in the literature, but often the context does not link them to fatty acid synthesis. To examine the physiological role of the mammalian mitochondrial FAS and Nrbf-1p, we have initiated experiments to generate transgenic mice, to elucidate what role FAS and Nrbf-1p play in maintaining mitochondrial respiratory competence.
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FOOTNOTES |
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* This work was supported in part by grants from the Academy of Finland and Sigrid Jusélius Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
|| Supported by a fellowship from Alexander von Humboldt Foundation.
** To whom correspondence should be addressed: Biocenter Oulu, Dept. of Biochemistry, P. O. 3000, FIN-90014 University of Oulu, Finland. Tel.: 358-8-5531150; Fax: 358-85531141; E-mail: Kalervo.Hiltunen{at}oulu.fi.
1 The abbreviations used are: FAS, fatty acid synthesis; ORF, open reading frame; Nrbf-1, nuclear receptor binding factor 1; ACP, acyl carrier protein; GFP, green fluorescent protein; DAPI, 4',6-diamidino-2'-phenylindole dihydrochloride; DTT, dithiothreitol; MALDI-TOF MS, matrix-assisted laser desorption ionization-time of flight-mass spectroscopy; HPLC, high pressure liquid chromatography.
2 J. M. Torkko and J. K. Hiltunen, unpublished data.
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