Isolation and complete sequence of CBR, a gene encoding a putative cytochrome b reductase in Saccharomyces cerevisiae

Analysis of cytochrome b(5) reductase-mediated metabolism in the phytopathogenic fungus Zymoseptoria tritici reveals novel functionalities implicated in virulence

Septoria tritici blotch (STB) caused by the Ascomycete fungus Zymoseptoria tritici is one of the most economically damaging diseases of wheat worldwide. Z. tritici is currently a major target for agricultural fungicides, especially in temperate regions where it is most prevalent. Many fungicides target electron transfer enzymes because these are often important for cell function. Therefore characterisation of genes encoding such enzymes may be important for the development of novel disease intervention strategies. Microsomal cytochrome b5 reductases (CBRs) are an important family of electron transfer proteins which in eukaryotes are involved in the biosynthesis of fatty acids and complex lipids including sphingolipids and sterols. Unlike the model yeast Saccharomyces cerevisiae which possesses only one microsomal CBR, the fully sequenced genome of Z. tritici bears three possible microsomal CBRs. RNA sequencing analysis revealed that ZtCBR1 is the most highly expressed of these genes under all in vitro and in planta conditions tested, therefore ΔZtCBR1 mutant strains were generated through targeted gene disruption. These strains exhibited delayed disease symptoms on wheat leaves and severely limited asexual sporulation. ΔZtCBR1 strains also exhibited aberrant spore morphology and hyphal growth in vitro. These defects coincided with alterations in fatty acid, sphingolipid and sterol biosynthesis observed through GC-MS and HPLC analyses. Data is presented which suggests that Z. tritici may use ZtCBR1 as an additional electron donor for key steps in ergosterol biosynthesis, one of which is targeted by azole fungicides. Our study reports the first functional characterisation of CBR gene family members in a plant pathogenic filamentous fungus. This also represents the first direct observation of CBR functional ablation impacting upon fungal sterol biosynthesis.

Activation of the unfolded protein response pathway causes ceramide accumulation in yeast and INS-1E insulinoma cells

Sphingolipids are not only important components of membranes but also have functions in protein trafficking and intracellular signaling. The LCB1 gene encodes a subunit of the serine palmitoyltransferase, which is responsible for the first step of sphingolipid synthesis. Here, we show that activation of the unfolded protein response (UPR) can restore normal ceramide levels and viability in yeast cells with a conditional defect in LCB1. Dependence on UPR was demonstrated by showing the HAC1-dependence of the suppression. A similar induction of ceramides by UPR seems to take place in mammalian cells. In rat pancreatic INS-1E cells, UPR activation induces the transcription of the CerS6 gene, which encodes a ceramide synthase. This correlates with the specific accumulation of ceramide with a C16 fatty acyl chain upon UPR activation. Therefore, our study reveals a novel connection between UPR induction and ceramide synthesis that seems to be conserved between yeast and mammalian cells.

Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency

Some 60 years ago, Quentin Gibson reported the first hereditary disorder involving an enzyme when he deduced that familial methaemoglobinaemia was caused by an enzymatic lesion associated with the glycolysis pathway in red blood cells. This disorder, now known as recessive congenital methaemoglobinaemia (RCM), is caused by NADH-cytochrome b5 reductase (cb(5)r) deficiency. Two distinct clinical forms, types I and II, have been recognized, both characterized by cyanosis from birth. In type II, the cyanosis is accompanied by neurological impairment and reduced life expectancy. Cytochrome b(5) reductase is composed of one FAD and one NADH binding domain linked by a hinge region. It is encoded by the CYB5R3 (previously known as DIA1) gene and more than 40 mutations have been described, some of which are common to both types of RCM. Mutations associated with type II tend to cause incorrect splicing, disruption of the active site or truncation of the protein. At present the description of the sequence variants of cb(5)r in the literature is confusing, due to the use of two conventions which differ by one codon position. Herein we propose a new system for nomenclature of cb(5)r based on recommendations of the Human Genome Variation Society. The development of a heterologous expression system has allowed the impact of naturally occurring variants of cb(5)r to be assessed and has provided insight into the function of cb(5)r.

The induction of microsomal NADPH:cytochrome P450 and NADH:cytochrome b(5) reductases by long-term salt treatment of cotton (Gossypium hirsutum L.) and bean (Phaseolus vulgaris L.) plants

We studied the effect of salinity on the activity of microsomal NADPH:cytochrome P450 reductase (CPR, EC 1.6.2.4) and NADH:ferricytochrome b(5) oxidoreductase (B5R, EC 1.6.2.2) in two dicotyledonous plant species differing in their sensitivity to salt, cotton (Gossypium hirsutum L. cv Ogosta) and common bean (Phaseolus vulgaris L. cv Dobrujanski 7). A significant inhibition of fresh weight of salt-treated bean plants was observed, while cotton was affected to a much lesser degree. NaCl application resulted in a significant increase in the activity of both reductases, but was more pronounced in salt-tolerant cotton. We suppose that alterations in B5R and CPR activities may be targeted to the maintenance of membrane lipids. Most probably, plants use both enzymes (B5R and CPR) and their respective electron donors (NADH and NADPH) to reduce cytochrome b(5), which can donate reducing equivalents to a series of lipid-modification reactions such as desaturation and hydroxylation.

Structure of Physarum polycephalum cytochrome b5 reductase at 1.56 A resolution

Physarum polycephalum cytochrome b(5) reductase catalyzes the reduction of cytochrome b(5) by NADH. The structure of P. polycephalum cytochrome b(5) reductase was determined at a resolution of 1.56 A. The molecular structure was compared with that of human cytochrome b(5) reductase, which had previously been determined at 1.75 A resolution [Bando et al. (2004), Acta Cryst. D60, 1929-1934]. The high-resolution structure revealed conformational differences between the two enzymes in the adenosine moiety of the FAD, the lid region and the linker region. The structural properties of both proteins were inspected in terms of hydrogen bonding, ion pairs, accessible surface area and cavity volume. The differences in these structural properties between the two proteins were consistent with estimates of their thermostabilities obtained from differential scanning calorimetry data.

Cytochrome b5 reductase: the roles of the recessive congenital methemoglobinemia mutants P144L, L148P, and R159*

Recessive congenital methemoglobinemia (RCM, OMIM 250800) arises from defects in either the erythrocytic or microsomal forms of the flavoprotein, cytochrome b5 reductase (cb5r) and was the first disease to be directly associated with a specific enzyme deficiency. Of the 33 verified mutations in cb5r that give rise to either the type I (erythrocytic) or type II (generalized) forms of RCM, three of the mutations, corresponding to P144L, L148P, and R159*, are located in a segment of the primary sequence composed of residues G143 to V171 which serves as a "hinge" or "linker" region between the FAD- and NADH-binding lobes of the protein. With the exception of R159*, which produces a truncated non-functional cb5r resulting in type II RCM, the type I methemoglobinemias resulting from the P144L or L148P mutations have been proposed to be due to decreased enzyme stability. Utilizing a recombinant form of the rat cb5r enzyme, we have generated the P144L, L148P, and P144L/L148P mutants, purified the resulting proteins to homogeneity and characterized their spectroscopic, kinetic, and thermodynamic properties. The three mutant proteins retained full complements of FAD with the P144L and L148P variants being spectroscopically indistinguishable from wild-type cb5r. In contrast, kinetic analyses revealed that the P144L, L148P, and P144L/L148P variants retained only 28, 31, and 8% of wild-type NADH:cytochrome b5 reductase activity, respectively, together with significant alterations in affinity for both NADH and NAD+. In addition, FAD oxidation-reduction potentials were 32, 19, and 65 mV more positive for the mutants than the corresponding FAD/FADH2 couple in native cb5r (E0'=-272 mV). Thermal and proteolytic stability measurements indicated that all three mutants were less stable than the wild-type protein while differential spectroscopy indicated altered pyridine nucleotide binding in all three variants. These results demonstrate that the "hinge" region is important in maintaining the correct orientation of the flavin- and pyridine nucleotide-binding lobes within the protein for efficient electron transfer and that the P144L and L148P mutations disrupt the normal registration of the FAD- and NADH-binding lobes resulting in altered affinities for both the physiological reducing substrate, NADH and its product, NAD+.

Cytochrome b5 oxidoreductase: expression and characterization of the original familial ideopathic methemoglobinemia mutations E255- and G291D

NADH:cytochrome b5 oxidoreductase catalyzes the transfer of reducing equivalents from the physiological electron donor, NADH, to two molecules of cytochrome b5. Utilizing a heterologous expression system for the soluble, catalytic domain of the rat microsomal enzyme, we have produced two mutants, corresponding to E255- and G291D. These mutants correspond to the two specific mutations that were identified over a half century later following diagnosis of the original cases of type I recessive congenital methemoglobinemia (RCM). We have purified both the E255- and G291D variants to homogeneity to determine the molecular basis for type I RCM in these individuals. Both the E255- and G291D variants retained a full complement of FAD and exhibited absorption and CD spectroscopic properties comparable to those of the wild-type protein. Oxidation-reduction potentiometric titrations yielded standard midpoint potentials (E0') for the FAD/FADH2 couple of -271 and -273 mV for the E255- and G291D variants, respectively, which were comparable to the value of -268 mV obtained for the wild-type protein and confirmed that the redox potential of the flavin was unaffected by either mutation. Thermal and proteolytic stability studies revealed that while the G291D variant exhibited stability comparable to that of wild-type, the E255- variant was markedly less stable, indicative of an altered conformation. Initial-rate kinetic studies revealed that both mutants had decreased catalytic activity (kcat), with the E255- and G291D variants retaining approximately 38 and 58% of wild-type activity, respectively. However, the affinity for NADH (KmNADH) was decreased approximately 100-fold for E255- compared to only approximately 1.3-fold for G291D, results supported by the spectroscopic binding constant (Ks) obtained for G291D. These results indicate that the properties of both the E255- and G291D cytochrome b5 oxidoreductase mutants are similar to those of other variants that have been identified as resulting in the type I form of RCM.

Proteolytic processing of CmPP36, a protein from the cytochrome b(5) reductase family, is required for entry into the phloem translocation pathway

Cucurbita maxima (pumpkin) phloem sap contains a 31 kDa protein that cross-reacts with antibodies directed against the red clover necrotic mosaic virus movement protein (RCNMV MP). Microsequence data from phloem-purified 31 kDa protein were used to isolate a complementary DNA: the open reading frame encodes a 36 kDa protein belonging to the cytochrome b(5) reductase (Cb5R) family; the gene was termed CmPP36. Western analyses established that CmPP36, RCNMV MP and CmPP16 (Xoconostle-Cázares et al., 1999, Science 283, 94-98) are immunologically related, probably due to a common epitope, represented by the NADH(+)-binding domain of CmPP36. An N-terminal 5 kDa membrane-targeting domain is cleaved to produce the 31 kDa Delta N-CmPP36 detected in the phloem sap. Microinjection experiments established that Delta N-CmPP36, but not CmPP36, is able to interact with plasmodesmata to mediate its cell-to-cell transport. Thus, intercellular movement of CmPP36 requires proteolytic processing in the companion cell to produce a soluble, movement-competent, protein. In contrast to RCNMV and CmPP16, Delta N-CmPP36 interacts with but does not mediate the trafficking of RNA. Northern and in situ RT-PCR studies established that CmPP36 mRNA is present in all plant organs, being highly abundant within vascular tissues. In roots of hydroponically grown pumpkin plants, CmPP36 mRNA levels respond to changes in available iron in the culture solution. Finally, enzymatic assays established that both CmPP36 and Delta N-CmPP36 could reduce Fe(3+)-citrate and Fe(3+)-EDTA in the presence of NADH(+). These findings are discussed in terms of the possible roles played by CmPP36 in phloem function.

Biochemical and molecular characterization of the [NiFe] hydrogenase from the hyperthermophilic archaeon, Thermococcus litoralis

Thermococcus litoralis is a hyperthermophilic archaeon that grows at temperatures up to 98 degrees C by fermentative metabolism and reduces elemental sulfur (S0) to H2S. A [NiFe] hydrogenase, responsible for H2S or H2 production, has been purified and characterized. The enzyme is composed of four subunits with molecular mass 46, 42, 34 and 32 kDa. Elemental analyses gave approximate values of 22 Fe, 22 S and 1 Ni per hydrogenase. EPR spectra at 70 and 5 K indicated the presence of four or five [4Fe-4S] and one [2Fe-2S] type clusters. The optimal temperature for both H2 evolution and oxidation, using artificial electron carriers, was around 80 degrees C. The operon encoding the T. litoralis enzyme is composed of four genes forming one transcriptional unit, and transcription is not regulated by S0. An unusual transcription-initiation site is located 139 bp upstream from the translational start point. Sequence analyses indicated the presence of new putative nucleotide-binding domains. Upstream from the hydrogenase operon, ORFs probably encoding a molybdopterin oxidoreductase enzyme have been identified. Based on sequence, biochemical and biophysical analyses, a model of the enzyme and the pathway of electron flow during catalysis is proposed.

Identification of an NADH-cytochrome b(5) reductase gene from an arachidonic acid-producing fungus, Mortierella alpina 1S-4, by sequencing of the encoding cDNA and heterologous expression in a fungus, Aspergillus oryzae

Based on the sequence information for bovine and yeast NADH-cytochrome b(5) reductases (CbRs), a DNA fragment was cloned from Mortierella alpina 1S-4 after PCR amplification. This fragment was used as a probe to isolate a cDNA clone with an open reading frame encoding 298 amino acid residues which show marked sequence similarity to CbRs from other sources, such as yeast (Saccharomyces cerevisiae), bovine, human, and rat CbRs. These results suggested that this cDNA is a CbR gene. The results of a structural comparison of the flavin-binding beta-barrel domains of CbRs from various species and that of the M. alpina enzyme suggested that the overall barrel-folding patterns are similar to each other and that a specific arrangement of three highly conserved amino acid residues (i.e., arginine, tyrosine, and serine) plays a role in binding with the flavin (another prosthetic group) through hydrogen bonds. The corresponding genomic gene, which was also cloned from M. alpina 1S-4 by means of a hybridization method with the above probe, had four introns of different sizes. These introns had GT at the 5' end and AG at the 3' end, according to a general GT-AG rule. The expression of the full-length cDNA in a filamentous fungus, Aspergillus oryzae, resulted in an increase (4.7 times) in ferricyanide reduction activity involving the use of NADH as an electron donor in the microsomes. The M. alpina CbR was purified by solubilization of microsomes with cholic acid sodium salt, followed by DEAE-Sephacel, Mono-Q HR 5/5, and AMP-Sepharose 4B affinity column chromatographies; there was a 645-fold increase in the NADH-ferricyanide reductase specific activity. The purified CbR preferred NADH over NADPH as an electron donor. This is the first report of an analysis of this enzyme in filamentous fungi.

Mutations in the Saccharomyces cerevisiae gene SAC1 cause multiple drug sensitivity

Wild-type yeast Saccharomyces cerevisiae are surprisingly resistant to a wide range of drugs and agents. We had previously isolated novobiocin-sensitive mutants to aid the study of the intracellular target for this drug. Characterization of one of these mutants, mds1, revealed that it was sensitive not only to novobiocin but also to a wide range of drugs. The nature of this multiple drug-sensitive phenotype was shown to be different from that of previously isolated multiple drug-sensitive mutants. We have shown that the multiple drug-sensitivity of mds1 is due to mutations within the gene SAC1 and have identified a variety of mutations within the gene from the Mds1 strain. SAC1 encodes a protein which has been previously implicated in the correct function of the actin cytoskeleton, in inositol metabolism, in ATP transport in the endoplasmic reticulum and in Sec14p (PI-TP) function. We have shown that multiple drug-sensitivity is a new phenotype seen in some, but not all, of the previously characterized sac1 mutants. Based on our findings, we propose a mechanism by which Sac1p could affect drug resistance and also mediate other effects on cell growth.

Characterization of the second form of NADH-Cytochrome b5 reductase gene from arachidonic acid-producing fungus Mortierella alpina 1S-4

The second type of cytochrome b5 reductase (Cb5R-II) gene was characterized in the arachidonic acid-producing fungus Mortierella alpina 1S-4. Its cDNA (897 bp) and predicted amino acid (298 aa) sequences show more than 70% similarity to the previously isolated first type of Cb5R. Highly conserved exon-intron organization suggests that the two genes evolved from the duplication of a common ancestral gene. Cb5R-II has a flavin-binding domain at its highly hydrophobic N-terminal and an NADH-binding domain at the C-terminal. In comparison with Cb5R genes from other sources, high homology (46-54%) was found for yeast and plant genes. Phylogenetic analysis revealed that microbial and plant Cb5R genes represent a gene family evolved from one prototype and are different from mammalian Cb5R genes.

Microsomal electron transfer in higher plants: cloning and heterologous expression of NADH-cytochrome b5 reductase from Arabidopsis

AtCBR, a cDNA encoding NADH-cytochrome (Cyt) b5 reductase, and AtB5-A and AtB5-B, two cDNAs encoding Cyt b5, were isolated from Arabidopsis. The primary structure deduced from the AtCBR cDNA was 40% identical to those of the NADH-Cyt b5 reductases of yeast and mammals. A recombinant AtCBR protein prepared using a baculovirus system exhibited typical spectral properties of NADH-Cyt b5 reductase and was used to study its electron-transfer activity. The recombinant NADH-Cyt b5 reductase was functionally active and displayed strict specificity to NADH for the reduction of a recombinant Cyt b5 (AtB5-A), whereas no Cyt b5 reduction was observed when NADPH was used as the electron donor. Conversely, a recombinant NADPH-Cyt P450 reductase of Arabidopsis was able to reduce Cyt b5 with NADPH but not with NADH. To our knowledge, this is the first evidence in higher plants that both NADH-Cyt b5 reductase and NADPH-Cyt P450 reductase can reduce Cyt b5 and have clear specificities in terms of the electron donor, NADH or NADPH, respectively. This substrate specificity of the two reductases is discussed in relation to the NADH- and NADPH-dependent activities of microsomal fatty acid desaturases.