An Arabidopsis cytochrome b561 with trans-membrane ferrireductase capability

Ferric reduction by a CYBDOM protein counteracts increased iron availability in root meristems induced by phosphorus deficiency

To mobilize sparingly available phosphorus (P) in the rhizosphere, many plant species secrete malate to release P sorbed onto (hydr)oxides of aluminum and iron (Fe). In the presence of Fe, malate can provoke Fe over-accumulation in the root apoplast, triggering a series of events that inhibit root growth. Here, we identified HYPERSENSITIVE TO LOW P1 (HYP1), a CYBDOM protein constituted of a DOMON and a cytochrome b561 domain, as critical to maintain cell elongation and meristem integrity under low P. We demonstrate that HYP1 mediates ascorbate-dependent trans-plasma membrane electron transport and can reduce ferric and cupric substrates in Xenopus laevis oocytes and in planta. HYP1 expression is up-regulated in response to P deficiency in the proximal zone of the root apical meristem. Disruption of HYP1 leads to increased Fe and callose accumulation in the root meristem and causes significant transcriptional changes in roots. We further demonstrate that HYP1 activity overcomes malate-induced Fe accumulation, thereby preventing Fe-dependent root growth arrest in response to low P. Collectively, our results uncover an ascorbate-dependent metalloreductase that is critical to protect root meristems of P-deficient plants from increased Fe availability and provide insights into the physiological function of the yet poorly characterized but ubiquitous CYBDOM proteins.

A CYBDOM protein impacts iron homeostasis and primary root growth under phosphate deficiency in Arabidopsis

Arabidopsis primary root growth response to phosphate (Pi) deficiency is mainly controlled by changes in apoplastic iron (Fe). Upon Pi deficiency, apoplastic Fe deposition in the root apical meristem activates pathways leading to the arrest of meristem maintenance and inhibition of cell elongation. Here, we report that a member of the uncharacterized cytochrome b561 and DOMON domain (CYBDOM) protein family, named CRR, promotes iron reduction in an ascorbate-dependent manner and controls apoplastic iron deposition. Under low Pi, the crr mutant shows an enhanced reduction of primary root growth associated with increased apoplastic Fe in the root meristem and a reduction in meristematic cell division. Conversely, CRR overexpression abolishes apoplastic Fe deposition rendering primary root growth insensitive to low Pi. The crr single mutant and crr hyp1 double mutant, harboring a null allele in another member of the CYDOM family, shows increased tolerance to high-Fe stress upon germination and seedling growth. Conversely, CRR overexpression is associated with increased uptake and translocation of Fe to the shoot and results in plants highly sensitive to Fe excess. Our results identify a ferric reductase implicated in Fe homeostasis and developmental responses to abiotic stress, and reveal a biological role for CYBDOM proteins in plants.

Spectral and Redox Properties of a Recombinant Mouse Cytochrome b561 Protein Suggest Transmembrane Electron Transfer Function

Cytochrome b561 proteins (CYB561s) are integral membrane proteins with six trans-membrane domains, two heme-b redox centers, one on each side of the host membrane. The major characteristics of these proteins are their ascorbate reducibility and trans-membrane electron transferring capability. More than one CYB561 can be found in a wide range of animal and plant phyla and they are localized in membranes different from the membranes participating in bioenergization. Two homologous proteins, both in humans and rodents, are thought to participate-via yet unidentified way-in cancer pathology. The recombinant forms of the human tumor suppressor 101F6 protein (Hs_CYB561D2) and its mouse ortholog (Mm_CYB561D2) have already been studied in some detail. However, nothing has yet been published about the physical-chemical properties of their homologues (Hs_CYB561D1 in humans and Mm_CYB561D1 in mice). In this paper we present optical, redox and structural properties of the recombinant Mm_CYB561D1, obtained based on various spectroscopic methods and homology modeling. The results are discussed in comparison to similar properties of the other members of the CYB561 protein family.

Proteomics of isolated sieve tubes from Nicotiana tabacum: sieve element-specific proteins reveal differentiation of the endomembrane system

Symplasmicly connected cells called sieve elements form a network of tubes in the phloem of vascular plants. Sieve elements have essential functions as they provide routes for photoassimilate distribution, the exchange of developmental signals, and the coordination of defense responses. Nonetheless, they are the least understood main type of plant cells. They are extremely sensitive, possess a reduced endomembrane system without Golgi apparatus, and lack nuclei and translation machineries, so that transcriptomics and similar techniques cannot be applied. Moreover, the analysis of phloem exudates as a proxy for sieve element composition is marred by methodological problems. We developed a simple protocol for the isolation of sieve elements from leaves and stems of Nicotiana tabacum at sufficient amounts for large-scale proteome analysis. By quantifying the enrichment of individual proteins in purified sieve element relative to bulk phloem preparations, proteins of increased likelyhood to function specifically in sieve elements were identified. To evaluate the validity of this approach, yellow fluorescent protein constructs of genes encoding three of the candidate proteins were expressed in plants. Tagged proteins occurred exclusively in sieve elements. Two of them, a putative cytochrome b561/ferric reductase and a reticulon-like protein, appeared restricted to segments of the endoplasmic reticulum (ER) that were inaccessible to green fluorescent protein dissolved in the ER lumen, suggesting a previously unknown differentiation of the endomembrane system in sieve elements. Evidently, our list of promising candidate proteins ( SI Appendix, Table S1) provides a valuable exploratory tool for sieve element biology.

AtDTX25, a member of the multidrug and toxic compound extrusion family, is a vacuolar ascorbate transporter that controls intracellular iron cycling in Arabidopsis

Iron (Fe) is an essential element, its transport is regulated by the cell redox balance. In seeds, Fe enters the embryo as Fe²⁺ and is stored in vacuoles as Fe³⁺ . Through its ferric reduction activity, ascorbate plays a major role in Fe redox state and therefore Fe transport within the seed. We searched for ascorbate membrane transporters responsible for controlling Fe reduction by screening the yeast ferric reductase-deficient fre1 strain and isolated AtDTX25, a member of the Multidrug And Toxic compound Extrusion (MATE) family. AtDTX25 was shown to mediate ascorbate efflux when expressed in yeast and Xenopus oocytes, in a pH-dependent manner. In planta, AtDTX25 is highly expressed during germination and encodes a vacuolar membrane protein. Isolated vacuoles from AtDTX25-1 knockout mutant contained less ascorbate and more Fe than wild-type (WT), and mutant seedlings were highly sensitive to Fe deficiency. Iron imaging further showed that the remobilisation of Fe from vacuoles was highly impaired in mutant seedlings. Taken together, our results established AtDTX25 as a vacuolar ascorbate transporter, required during germination to promote the reduction of the pool of stored Fe³⁺ and its remobilisation to feed the developing seedling.

The transferred translocases: An old wine in a new bottle

The role of translocases was underappreciated and was not included as a separate class in the enzyme commission until August 2018. The recent research interests in proteomics of orphan enzymes, ionomics, and metallomics along with high-throughput sequencing technologies generated overwhelming data and revamped this enzyme into a separate class. This offers a great opportunity to understand the role of new or orphan enzymes in general and specifically translocases. The enzymes belonging to translocases regulate/permeate the transfer of ions or molecules across the membranes. These enzyme entries were previously associated with other enzyme classes, which are now transferred to a new enzyme class 7 (EC 7). The entries that are reclassified are important to extend the enzyme list, and it is the need of the hour. Accordingly, there is an upgradation of entries of this class of enzymes in several databases. This review is a concise compilation of translocases with reference to the number of entries currently available in the databases. This review also focuses on function as well as dysfunction of translocases during normal and disordered states, respectively.

Functional Assembly of Caenorhabditis elegans Cytochrome b-2 (Cecytb-2) into Phospholipid Bilayer Nanodisc with Enhanced Iron Reductase Activity

Among seven homologs of cytochrome b₅₆₁ in a model organism C. elegans, Cecytb-2 was confirmed to be expressed in digestive organs and was considered as a homolog of human Dcytb functioning as a ferric reductase. Cecytb-2 protein was expressed in Pichia pastoris cells, purified, and reconstituted into a phospholipid bilayer nanodisc. The reconstituted Cecytb-2 in nanodisc environments was extremely stable and more reducible with ascorbate than in a detergent-micelle state. We confirmed the ferric reductase activity of Cecytb-2 by analyzing the oxidation of ferrous heme upon addition of ferric substrate under anaerobic conditions, where clear and saturable dependencies on the substrate concentrations following the Michaelis–Menten equation were observed. Further, we confirmed that the ferric substrate was converted to a ferrous state by using a nitroso-PSAP assay. Importantly, we observed that the ferric reductase activity of Cecytb-2 became enhanced in the phospholipid bilayer nanodisc.

Proton-Coupled Electron Transport in Two Distinct CYBASC Paralogs of Arabidopsis thaliana: A Comparative Characterization of Highly Conserved Tyrosine and Lysine Residues

CYBASC proteins are ascorbate (AscH^-) reducible, diheme b-containing integral membrane cytochrome b₅₆₁ proteins (cytb₅₆₁), which are proposed to be involved in AscH^- recycling and facilitation of iron absorption. Two distinct CYBASC paralogs from the plant Arabidopsis thaliana, Atcytb₅₆₁-A (A-paralog) and Atcytb₅₆₁-B (B-paralog), have been found to differ in their visible-spectral characteristics and their interaction with AscH^- and ferric iron chelates. A previously determined crystal structure of the B-paralog provides the first insights into the structural organization of a CYBASC member and implies hydrogen bonding between the substrate AscH^- and the conserved lysine residues at positions 77 (B-K77) and 81 (B-K81). The function of the highly conserved tyrosine at position 70 (B-Y70) is not obvious in the crystal structure, but its localization indicates the possible involvement in proton-coupled electron transfer. Here we show that B-Y70 plays a major role in the modulation of the oxidation-reduction midpoint potential of the high-potential heme, E_M(b_H), as well as in AscH^- oxidation. Our results support the involvement of the functionally conserved B-K77 in the stabilization of the dianion Asc^2-. These findings are supported by the crystal structure of the B-paralog, but a comparative biochemical and biophysical characterization of the A- and B-paralogs implied distinct and more complex functions of the corresponding residues A-Y69 and A-K76 in the A-paralog. Our results emphasize the need for a high-resolution crystal structure of the A-paralog to illuminate the differences in functional organization between the two paralogs.

Direct measurements of ferric reductase activity of human 101F6 and its enhancement upon reconstitution into phospholipid bilayer nanodisc

We studied human 101F6 protein to clarify its physiological function as a ferric reductase and its relationship to tumor suppression activity. We found for the first time that purified 101F6 both in detergent micelle state and in phospholipid bilayer nanodisc state has an authentic ferric reductase activity by single turnover kinetic analyses. The kinetic analysis on the ferrous heme oxidation of reduced 101F6 upon the addition of a ferric substrate, ferric ammonium citrate (FAC), showed concentration-dependent accelerations of its reaction with reasonable values of K_M and V_max. We further verified the authenticity of the ferric reductase activity of 101F6 using nitroso-PSAP as a Fe²⁺-specific colorimetric chelator. 101F6 in nanodisc state showed higher efficiency for FAC than in detergent micelle state.

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Human tumor suppressor 101F6 protein was reconstituted into nanodisc.

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101F6 functions as a ferric reductase both in detergent micelle and in nanodisc.

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101F6 in nanodisc showed higher efficiency in the reductase activity.

Ostreococcus tauri is a new model green alga for studying iron metabolism in eukaryotic phytoplankton

Background

Low iron bioavailability is a common feature of ocean surface water and therefore micro-algae developed original strategies to optimize iron uptake and metabolism. The marine picoeukaryotic green alga Ostreococcus tauri is a very good model for studying physiological and genetic aspects of the adaptation of the green algal lineage to the marine environment: it has a very compact genome, is easy to culture in laboratory conditions, and can be genetically manipulated by efficient homologous recombination. In this study, we aimed at characterizing the mechanisms of iron assimilation in O. tauri by combining genetics and physiological tools. Specifically, we wanted to identify and functionally characterize groups of genes displaying tightly orchestrated temporal expression patterns following the exposure of cells to iron deprivation and day/night cycles, and to highlight unique features of iron metabolism in O. tauri, as compared to the freshwater model alga Chalamydomonas reinhardtii.

Results

We used RNA sequencing to investigated the transcriptional responses to iron limitation in O. tauri and found that most of the genes involved in iron uptake and metabolism in O. tauri are regulated by day/night cycles, regardless of iron status. O. tauri lacks the classical components of a reductive iron uptake system, and has no obvious iron regulon. Iron uptake appears to be copper-independent, but is regulated by zinc. Conversely, iron deprivation resulted in the transcriptional activation of numerous genes encoding zinc-containing regulation factors. Iron uptake is likely mediated by a ZIP-family protein (Ot-Irt1) and by a new Fea1-related protein (Ot-Fea1) containing duplicated Fea1 domains. The adaptation of cells to iron limitation involved an iron-sparing response tightly coordinated with diurnal cycles to optimize cell functions and synchronize these functions with the day/night redistribution of iron orchestrated by ferritin, and a stress response based on the induction of thioredoxin-like proteins, of peroxiredoxin and of tesmin-like methallothionein rather than ascorbate. We briefly surveyed the metabolic remodeling resulting from iron deprivation.

Conclusions

The mechanisms of iron uptake and utilization by O. tauri differ fundamentally from those described in C. reinhardtii. We propose this species as a new model for investigation of iron metabolism in marine microalgae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2666-6) contains supplementary material, which is available to authorized users.

RNA-seq analysis of the effect of kanamycin and the ABC transporter AtWBC19 on Arabidopsis thaliana seedlings reveals changes in metal content

Plants are exposed to antibiotics produced by soil microorganisms, but little is known about their responses at the transcriptional level. Likewise, few endogenous mechanisms of antibiotic resistance have been reported. The Arabidopsis thaliana ATP Binding Cassette (ABC) transporter AtWBC19 (ABCG19) is known to confer kanamycin resistance, but the exact mechanism of resistance is not well understood. Here we examined the transcriptomes of control seedlings and wbc19 mutant seedlings using RNA-seq analysis. Exposure to kanamycin indicated changes in the organization of the photosynthetic apparatus, metabolic fluxes and metal uptake. Elemental analysis showed a 60% and 80% reduction of iron uptake in control and wbc19 mutant seedlings respectively, upon exposure to kanamycin. The drop in iron content was accompanied by the upregulation of the gene encoding for FERRIC REDUCTION OXIDASE 6 (FRO6) in mutant seedlings but not by the differential expression of other transport genes known to be induced by iron deficiency. In addition, wbc19 mutants displayed a distinct expression profile in the absence of kanamycin. Most notably the expression of several zinc ion binding proteins, including ZINC TRANSPORTER 1 PRECURSOR (ZIP1) was increased, suggesting abnormal zinc uptake. Elemental analysis confirmed a 50% decrease of zinc content in wbc19 mutants. Thus, the antibiotic resistance gene WBC19 appears to also have a role in zinc uptake.

Structure and mechanism of a eukaryotic transmembrane ascorbate-dependent oxidoreductase

Vitamin C, also known as ascorbate, is required in numerous essential metabolic reactions in eukaryotes. The eukaryotic ascorbate-dependent oxidoreductase cytochrome b561 (Cyt b561), a family of highly conserved transmembrane enzymes, plays an important role in ascorbate recycling and iron absorption. Although Cyt b561 was identified four decades ago, its atomic structure and functional mechanism remain largely unknown. Here, we report the high-resolution crystal structures of cytochrome b561 from Arabidopsis thaliana in both substrate-free and substrate-bound states. Cyt b561 forms a homodimer, with each protomer consisting of six transmembrane helices and two heme groups. The negatively charged substrate ascorbate, or monodehydroascorbate, is enclosed in a positively charged pocket on either side of the membrane. Two highly conserved amino acids, Lys(81) and His(106), play an essential role in substrate recognition and catalysis. Our structural and biochemical analyses allow the proposition of a general electron transfer mechanism for members of the Cyt b561 family.

Cytochromes b561: ascorbate-mediated trans-membrane electron transport

SIGNIFICANCE

Cytochromes b561 (CYB561s) constitute a family of trans-membrane (TM), di-heme proteins, occurring in a variety of organs and cell types, in plants and animals, and using ascorbate (ASC) as an electron donor. CYB561s function as monodehydroascorbate reductase, regenerating ASC, and as Fe³⁺-reductases, providing reduced iron for TM transport. A CYB561-core domain is also associated with dopamine β-monooxygenase redox domains (DOMON) in ubiquitous CYBDOM proteins. In plants, CYBDOMs form large protein families. Physiological functions supported by CYB561s and CYBDOMs include stress defense, cell wall modifications, iron metabolism, tumor suppression, and various neurological processes, including memory retention. CYB561s, therefore, significantly broaden our view on the physiological roles of ASC.

RECENT ADVANCES

The ubiquitous nature of CYB561s is only recently being recognized. Significant advances have been made through the study of recombinant CYB561s, revealing structural and functional properties of a unique "two-heme four-helix" protein configuration. In addition, the DOMON domains of CYBDOMs are suggested to contain another heme b.

CRITICAL ISSUES

New CYB561 proteins are still being identified, and there is a need to provide an insight and overview on the various roles of these proteins and their structural properties.

FUTURE DIRECTIONS

Mutant studies will reveal in greater detail the mechanisms by which CYB561s and CYBDOMs participate in cell metabolism in plants and animals. Moreover, the availability of efficient heterologous expression systems should allow protein crystallization, more detailed (atomic-level) structural information, and insights into the intra-molecular mechanism of electron transport.

Plasma membrane electron pathways and oxidative stress

SIGNIFICANCE

Several redox compounds, including respiratory burst oxidase homologs (Rboh) and iron chelate reductases have been identified in animal and plant plasma membrane (PM). Studies using molecular biological, biochemical, and proteomic approaches suggest that PM redox systems of plants are involved in signal transduction, nutrient uptake, transport, and cell wall-related processes. Function of PM-bound redox systems in oxidative stress will be discussed.

RECENT ADVANCES

Present knowledge about the properties, structures, and functions of these systems are summarized. Judging from the currently available data, it is likely that electrons are transferred from cytosolic NAD(P)H to the apoplast via quinone reductases, vitamin K, and a cytochrome b561. In tandem with these electrons, protons might be transported to the apoplastic space.

CRITICAL ISSUES

Recent studies suggest localization of PM-bound redox systems in microdomains (so-called lipid or membrane rafts), but also organization of these compounds in putative and high molecular mass protein complexes. Although the plant flavocytochrome b family is well characterized with respect to its function, the molecular mechanism of an electron transfer reaction by these compounds has to be verified. Localization of Rboh in other compartments needs elucidation.

FUTURE DIRECTIONS

Plant members of the flavodoxin and flavodoxin-like protein family and the cytochrome b561 protein family have been characterized on the biochemical level, postulated localization, and functions of these redox compounds need verification. Compositions of single microdomains and interaction partners of PM redox systems have to be elucidated. Finally, the hypothesis of an electron transfer chain in the PM needs further proof.

The physiological roles and metabolism of ascorbate in chloroplasts

Ascorbate is a multifunctional metabolite in plants. It is essential for growth control, involving cell division and cell wall synthesis and also involved in redox signaling, in the modulation of gene expression and regulation of enzymatic activities. Ascorbate also fulfills crucial roles in scavenging reactive oxygen species, both enzymatically and nonenzymatically, a well-established phenomenon in the chloroplasts stroma. We give an overview on these important physiological functions and would like to give emphasis to less well-known roles of ascorbate, in the thylakoid lumen, where it also plays multiple roles. It is essential for photoprotection as a cofactor for violaxanthin de-epoxidase, a key enzyme in the formation of nonphotochemical quenching. Lumenal ascorbate has recently also been shown to act as an alternative electron donor of photosystem II once the oxygen-evolving complex is inactivated and to protect the photosynthetic machinery by slowing down donor-side induced photoinactivation; it is yet to be established if ascorbate has a similar role in the case of other stress effects, such as high light and UV-B stress. In bundle sheath cells, deficient in oxygen evolution, ascorbate provides electrons to photosystem II, thereby poising cyclic electron transport around photosystem I. It has also been shown that, by supporting linear electron transport through photosystem II in sulfur-deprived Chlamydomonas reinhardtii cells, in which oxygen evolution is largely inhibited, externally added ascorbate enhances hydrogen production. For fulfilling its multiple roles, Asc has to be transported into the thylakoid lumen and efficiently regenerated; however, very little is known yet about these processes.

Roles of conserved Arg(72) and Tyr(71) in the ascorbate-specific transmembrane electron transfer catalyzed by Zea mays cytochrome b561

Cytochromes b561, novel transmembrane electron transport proteins residing in eukaryotic cells, have a number of common features including six transmembrane α-helices and two heme ligation sites. Our recent studies on recombinant Zea mays cytochrome b561 suggested that concerted proton/electron transfer mechanism was functioning in plant cytochromes b561 as well and that conserved Lys(83) on a cytosolic loop had important roles for ascorbate-binding and a succeeding electron transfer. In the present study, we conducted site-directed mutagenesis analyses on conserved Arg(72) and Tyr(71). Removal of a positive charge at Arg(72) did not affect significantly on the final heme reduction level with ascorbate as reductant. However, characteristic pH-dependent initial time-lag upon electron acceptance from ascorbate was completely lost for R72A and R72E mutants. Substitution of Tyr(71) with Ala or Phe affected both on the final heme reduction level and on the pH-dependent initial time-lag, causing acceleration of the electron transfer. These observations were interpreted as existence of specific interactions of Tyr(71) and Arg(72) with ascorbate. However, their mechanistic roles were distinctly different from that of Lys(83), as exemplified by K83A/Y71A double mutant, and might be related for expelling of monodehydroascorbate radical from the substrate-binding site to prevent a back-flow of electrons.

Functional characterization of the recombinant human tumour suppressor 101F6 protein, a cytochrome b(561) homologue

Candidate human tumour suppressor gene product, 101F6 protein, is a highly hydrophobic transmembrane protein and a member of cytochrome b(561) family. Purified 101F6 protein expressed in Pichia pastoris cells showed visible absorption spectra similar but distinct from those of cytochrome b(561). Haem content analysis indicated presence of two haems B per molecule. Midpoint potentials of the purified protein were found as +109 and +26 mV for two haems, slightly lower than those for bovine chromaffin granule or plant Zea mays cytochromes b(561). Electron paramagnetic resonance (EPR) spectra in oxidized state at 5 K showed only a highly anisotropic low-spin (HALS) signal at g(z) = 3.75. However, at 15 and 20 K, another HALS-type signal appeared at g(z) = 3.65 being overlapped with that of g(z) = 3.75. The rhombic EPR signal at g(z) = 3.16 previously seen in other cytochromes b(561) was not observed, suggesting distinct haem environments. Absence of the inhibition in the electron transfer from ascorbate by a treatment of 101F6 protein with diethylpyrocarbonate showed a remarkable contrast from those of other cytochromes b(561) where the 'concerted H(+)/e(-) transfer mechanism' at the cytosolic haem centre was blocked by specific Nε-carbethoxylation of haem-coordinating imidazole, suggesting that 101F6 protein might accept electrons via a mechanism distinct from other cytochromes b(561).

Heterologous production and characterisation of two distinct dihaem-containing membrane integral cytochrome b(561) enzymes from Arabidopsis thaliana in Pichia pastoris and Escherichia coli cells

Cytochrome (cyt) b(561) proteins are dihaem-containing membrane proteins, belonging to the CYBASC (cytochrome-b(561)-ascorbate-reducible) family, and are proposed to be involved in ascorbate recycling and/or the facilitation of iron absorption. Here, we present the heterologous production of two cyt b(561) paralogs from Arabidopsis thaliana (Acytb(561)-A, Acytb(561)-B) in Escherichia coli and Pichia pastoris, their purification, and initial characterisation. Spectra indicated that Acytb(561)-A resembles the best characterised member of the CYBASC family, the cytochrome b(561) from adrenomedullary chromaffin vesicles, and that Acytb(561)-B is atypical compared to other CYBASC proteins. Haem oxidation-reduction midpoint potential (E(M)) values were found to be fully consistent with ascorbate oxidation activities and Fe(3+)-chelates reductase activities. The ascorbate dependent reduction and protein stability of both paralogs were found to be sensitive to alkaline pH values as reported for the cytochrome b(561) from chromaffin vesicles. For both paralogs, ascorbate-dependent reduction was inhibited and the low-potential haem E(M) values were affected significantly by incubation with diethyl pyrocarbonate (DEPC) in the absence of ascorbate. Modification with DEPC in the presence of ascorbate left the haem E(M) values unaltered compared to the unmodified proteins. However, ascorbate reduction was inhibited. We concluded that the ascorbate-binding site is located near the low-potential haem with the Fe(3+)-chelates reduction-site close to the high-potential haem. Furthermore, inhibition of ascorbate oxidation by DEPC treatment occurs not only by lowering the haem E(M) values but also by an additional modification affecting ascorbate binding and/or electron transfer. Analytical gel filtration experiments suggest that both cyt b(561) paralogs exist as homodimers.

The Bradyrhizobium japonicum frcB gene encodes a diheme ferric reductase

Iron utilization by bacteria in aerobic environments involves uptake as a ferric chelate from the environment, followed by reduction to the ferrous form. Ferric iron reduction is poorly understood in most bacterial species. Here, we identified Bradyrhizobium japonicum frcB (bll3557) as a gene adjacent to, and coregulated with, the pyoR gene (blr3555) encoding the outer membrane receptor for transport of a ferric pyoverdine. FrcB is a membrane-bound, diheme protein, characteristic of eukaryotic ferric reductases. Heme was essential for FrcB stability, as were conserved histidine residues in the protein that likely coordinate the heme moieties. Expression of the frcB gene in Escherichia coli conferred ferric reductase activity on those cells. Furthermore, reduced heme in purified FrcB was oxidized by ferric iron in vitro. B. japonicum cells showed inducible ferric reductase activity in iron-limited cells that was diminished in an frcB mutant. Steady-state levels of frcB mRNA were strongly induced under iron-limiting conditions, but transcript levels were low and unresponsive to iron in an irr mutant lacking the global iron response transcriptional regulator Irr. Thus, Irr positively controls the frcB gene. FrcB belongs to a family of previously uncharacterized proteins found in many proteobacteria and some cyanobacteria. This suggests that membrane-bound, heme-containing ferric reductase proteins are not confined to eukaryotes but may be common in bacteria.

Allocation of Fe and ferric chelate reductase activities in mesophyll cells of barley and sorghum under Fe-deficient conditions

Although the photosynthetic apparatus requires large amounts of Fe, the adaptive mechanisms of mesophyll cells for Fe acquisition under Fe-deficient conditions are unknown. Barley and sorghum, which are tolerant and susceptible to Fe deficiency, respectively, have similar Fe and chlorophyll contents in their leaves. However, the Fe-deficient barley photosynthetic apparatus was functional while that of sorghum was not. We show that barley preferentially allocates Fe to thylakoid membranes under Fe-deficient conditions. On the other hand, in sorghum, the proportion of leaf Fe allocated to thylakoids was not altered by Fe deficiency. The relationship between the maintenance of photosynthesis and light-dependent ferric chelate reductase activity on plasma membranes and chloroplast envelopes is also discussed.

A cytochrome b561 with ferric reductase activity from the parasitic blood fluke, Schistosoma japonicum

BACKGROUND

Iron has an integral role in numerous cellular reactions and is required by virtually all organisms. In physiological conditions, iron is abundant in a largely insoluble ferric state. Ferric reductases are an essential component of iron uptake by cells, reducing iron to the soluble ferrous form. Cytochromes b561 (cyts-b561) are a family of ascorbate reducing transmembrane proteins found in most eukaryotic cells. The identification of the ferric reductase duodenal cytochrome b (dcytb) and recent observations that other cyts-b561 may be involved in iron metabolism have opened novel perspectives for elucidating their physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

Here we have identified a new member of the cytochrome b561 (Sjcytb561) family in the pathogenic blood fluke Schistosoma japonicum that localises to the outer surface of this parasitic trematode. Heterologous expression of recombinant Sjcyt-b561 in a Saccharomyces cerevisiae mutant strain that lacks plasma membrane ferrireductase activity demonstrated that the molecule could rescue ferric reductase activity in the yeast.

SIGNIFICANCE/CONCLUSIONS

This finding of a new member of the cytochrome b561 family further supports the notion that a ferric reductase function is likely for other members of this protein family. Additionally, the localisation of Sjcytb561 in the surface epithelium of these blood-dwelling schistosomes contributes further to our knowledge concerning nutrient acquisition in these parasites and may provide novel targets for therapeutic intervention.

Spectral characterization of the recombinant mouse tumor suppressor 101F6 protein

Tumor suppressor protein 101F6, a gene product of the 3p21.3 (human) and 9F1 (mouse) chromosomal region, has recently been identified as a member of the cytochrome b561 (Cyt-b561) protein family by sequence homology. The His(6)-tagged recombinant mouse tumor suppressor Cyt-b561 protein (TSCytb) was recently expressed in yeast and purified, and the ascorbate reducibility was determined. TSCytb is auto-oxidizable and has two distinct heme b centers with redox potentials of approximately 40 and approximately 140 mV. Its split alpha-band in the dithionite-reduced spectrum at both 295 and 77 K is well resolved, and the separation between the two alpha-peaks is approximately 7 nm (approximately 222 cm(-1)). Singular value decomposition analysis of the split alpha-band in the ascorbate-reduced spectra revealed the presence of two major spectral components, each of them with split alpha-band but with different peak separations (6 and 8 nm). Similar minor differences in peak separation were obtained when the split alpha-bands in ascorbate-reduced difference spectra at low (<1 mM) and high (>10 mM) ascorbate concentrations were analysed. According to low-temperature electron paramagnetic resonance (EPR) spectroscopy, the two heme b centers are in the low-spin ferric state with maximum principal g values of 3.61 and 2.96, respectively. These values differ from the ones observed for other members of the Cyt-b561 family. According to resonance Raman spectroscopy, the porphyrin rings are in a relaxed state. The spectroscopic results are only partially in agreement with those obtained earlier for the native chromaffin granule Cyt-b561.

Importance of the conserved lysine 83 residue of Zea mays cytochrome b(561) for ascorbate-specific transmembrane electron transfer as revealed by site-directed mutagenesis studies

Cytochromes b(561), a novel class of transmembrane electron transport proteins residing in a large variety of eukaryotic cells, have a number of common structural features including six hydrophobic transmembrane alpha-helices and two heme ligation sites. We found that recombinant Zea mays cytochrome b(561) obtained by a heterologous expression system using yeast Pichia pastoris cells could utilize the ascorbate/mondehydroascorbate radical as a physiological electron donor/acceptor. We found further that a concerted proton/electron transfer mechanism might be operative in Z. mays cytochrome b(561) as well upon the electron acceptance from ascorbate to the cytosolic heme center. The well-conserved Lys(83) residue in a cytosolic loop was found to have a very important role(s) for the binding of ascorbate and the succeeding electron transfer via electrostatic interactions based on the analyses of three site-specific mutants, K83A, K83E, and K83D. Further, unusual behavior of the K83A mutant in pulse radiolysis experiments indicated that Lys(83) might also be responsible for the intramolecular electron transfer to the intravesicular heme. On the other hand, pulse radiolysis experiments on two site-specific mutants, S118A and W122A, for the well-conserved residues in the putative monodehydroascorbate radical binding site showed that their electron transfer activities to the monodehydroascorbate radical were very similar to those of the wild-type protein, indicating that Ser(118) and Trp(122) do not have major roles for the redox events on the intravesicular side.

A new system for heterologous expression of membrane proteins: Rhodospirillum rubrum

Heterologous expression of membrane proteins has met with only limited success. This work presents a new host/vector system for the production of heterologous membrane proteins based on a mutant of the facultatively phototrophic bacterium Rhodospirillum rubrum. Under certain growth conditions, R. rubrum forms an intracytoplasmic membrane (ICM) that houses the photosynthetic apparatus, the structural proteins of which are encoded by puhA and pufBALM. The mutant R. rubrum H2, which was constructed by allelic exchange deleting puhA and pufBALM, does not form ICM. This strain was used as a host for a plasmid expressing the Pseudomonas aeruginosa membrane protein MscL from the Rhodobacter capsulatus puc promoter. ICM was formed in the H2 strain producing MscL but not in the vector control strain. These results suggest that a heterologous membrane protein stimulates ICM formation in R. rubrum and indicate that the capacity to form an ICM that can accommodate heterologous proteins makes R. rubrum a host that will be useful for membrane protein production. P. aeruginosa MscL, which forms inclusion bodies when produced in Escherichia coli, was expressed in R. rubrum H2 and purified from membranes with a yield of 22.8-23.4 mg/L culture (5.53-5.60 mg/g cell paste). Additionally Streptomyces lividans KcsA and P. aeruginosa CycB were produced and purified from R. rubrum H2 with yields of 13.7-14.4 mg/L culture (2.19-2.55 mg/g cell paste) and 6.6-7.4 mg/L culture (1.1-1.2mg/g cell paste), respectively.

Plant lipid rafts: fluctuat nec mergitur

Lipid rafts in plasma membranes are hypothesized to play key roles in many cellular processes including signal transduction, membrane trafficking and entry of pathogens. We recently documented the biochemical characterization of lipid rafts, isolated as detergent-insoluble membranes, from Medicago truncatula root plasma membranes. We evidenced that the plant-specific lipid steryl-conjugates are among the main lipids of rafts together with free sterols and sphingolipids. An extensive proteomic analysis showed the presence of a specific set of proteins common to other lipid rafts, plus the presence of a redox system around a cytochrome b(561) not previously identified in lipid rafts of either plants or animals. Here, we discuss the similarities and differences between the lipids and proteins of plant and animal lipid rafts. Moreover we describe the potential biochemical functioning of the M. truncatula root lipid raft redox proteins and question whether they may play a physiological role in legume-symbiont interactions.

Axial ligation and stoichiometry of heme centers in adrenal cytochrome b561

Cytochrome (cyt) b561 transports electrons across the membrane of chromaffin granules (CG) present in the adrenal medulla, supporting the biosynthesis of norepinephrine in the CG matrix. We have conducted a detailed characterization of cyt b561 using electron paramagnetic resonance (EPR) and optical spectroscopy on the wild-type and mutant forms of the cytochrome expressed in insect cells. The gz = 3.7 (low-potential heme) and gz = 3.1 (high-potential heme) signals were found to represent the only two authentic hemes of cyt b561; models that propose smaller or greater amounts of heme can be ruled out. We identified the axial ligands to hemes in cyt b561 by mutating four conserved histidines (His54 and His122 at the matrix-side heme center and His88 and His161 at the cytoplasmic-side heme center), thus confirming earlier structural models. Single mutations of any of these histidines produced a constellation of spectroscopic changes that involve not one but both heme centers. We hypothesize that the two hemes and their axial ligands in cyt b561 are integral parts of a structural unit that we term the "kernel". Histidine to glutamine substitutions in the cytoplasmic-side heme center but not in the matrix-side heme center led to the retention of a small fraction of the low-potential heme with gz = 3.7. We provisionally assign the low-potential heme to the matrix side of the membrane; this arrangement suggests that the membrane potential modulates electron transport across the CG membrane.