Nuclear mRNA export requires complex formation between Mex67p and Mtr2p at the nuclear pores

We have identified between Mex67p and Mtr2p a complex which is essential for mRNA export. This complex, either isolated from yeast or assembled in Escherichia coli, can bind in vitro to RNA through Mex67p. In vivo, Mex67p requires Mtr2p for association with the nuclear pores, which can be abolished by mutating either MEX67 or MTR2. In all cases, detachment of Mex67p from the pores into the cytoplasm correlates with a strong inhibition of mRNA export. At the nuclear pores, Nup85p represents one of the targets with which the Mex67p-Mtr2p complex interacts. Thus, Mex67p and Mtr2p constitute a novel mRNA export complex which can bind to RNA via Mex67p and which interacts with nuclear pores via Mtr2p.

Mtr10p functions as a nuclear import receptor for the mRNA-binding protein Npl3p

MTR10, previously shown to be involved in mRNA export, was found in a synthetic lethal relationship with nucleoporin NUP85. Green fluorescent protein (GFP)-tagged Mtr10p localizes preferentially inside the nucleus, but a nuclear pore and cytoplasmic distribution is also evident. Purified Mtr10p forms a complex with Npl3p, an RNA-binding protein that shuttles in and out of the nucleus. In mtr10 mutants, nuclear uptake of Npl3p is strongly impaired at the restrictive temperature, while import of a classic nuclear localization signal (NLS)-containing protein is not. Accordingly, the NLS within Npl3p is extended and consists of the RGG box plus a short and non-repetitive C-terminal tail. Mtr10p interacts in vitro with Gsp1p-GTP, but with low affinity. Interestingly, Npl3p dissociates from Mtr10p only by incubation with Ran-GTP plus RNA. This suggests that Npl3p follows a distinct nuclear import pathway and that intranuclear release from its specific import receptor Mtr10p requires the cooperative action of both Ran-GTP and newly synthesized mRNA.

Nup116p and nup100p are interchangeable through a conserved motif which constitutes a docking site for the mRNA transport factor gle2p

Nup116p and Nup100p are highly related yeast GLFG nucleoporins, but only Nup116p is stoichiometrically bound to Gle2p, a previously identified mRNA export factor. A short Gle2p-binding sequence within Nup116p (GLEBS; residues 110-166) is sufficient and necessary to anchor Gle2p at the nuclear pores, whereas the carboxy-terminal domain of Nup116p mediates its own nuclear pore complex (NPC) association. The GLEBS is evolutionarily conserved and found in rat/Xenopus Nup98 and an uncharacterized Caenorhabditis elegans ORF, but is absent from Nup100p. When the GLEBS is deleted from Nup116p, Gle2p dissociates from the nuclear envelope and clusters of herniated nuclear pores form. When the GLEBS is inserted into Nup100p, Nup100p-GLEBS complements both the thermosensitive and NPC-herniated phenotype of nup116- cells, and Gle2p is retargeted concomitantly to the NPCs. Thus, the in vivo function of Gle2p is strictly coupled to the short GLEBS within Nup116p which links this putative mRNA transport factor to the nuclear pores.

Yeast genetics to dissect the nuclear pore complex and nucleocytoplasmic trafficking

Eukaryotic cells evolved when their genetic information was packed into the cell nucleus. DNA replication and RNA biogenesis occur inside the nucleus while protein synthesis takes place in the cytoplasm. Bi-directional trafficking between these two compartments is mediated by a single supramolecular assembly, the nuclear pore complex. Nucleocytoplasmic transport is signal mediated, energy dependent, and requires, besides nuclear pore proteins (nucleoporins), a number of soluble transport factors. We review here our current knowledge on the role of nucleoporins, and on the mechanism of nucleocytoplasmic transport, with emphasis on the yeast Saccharomyces cerevisiae.

Nucleolar KKE/D repeat proteins Nop56p and Nop58p interact with Nop1p and are required for ribosome biogenesis

Different point mutations in the nucleolar protein fibrillarin (Nop1p in Saccharomyces cerevisiae) can inhibit different steps in ribosome synthesis. A screen for mutations that are synthetically lethal (sl) with the nop1-5 allele, which inhibits pre-rRNA processing, identified NOP56. An independent sl mutation screen with nop1-3, which inhibits pre-rRNA methylation, identified a mutation in NOP58. Strikingly, Nop56p and Nop58p are highly homologous (45% identity). Both proteins were found to be essential and localized to the nucleolus. A temperature-sensitive lethal mutant allele, nop56-2, inhibited many steps in pre-rRNA processing, particularly on the pathway of 25S/5.8S rRNA synthesis, and led to defects in 60S subunit assembly. Epitope-tagged constructs show that both Nop56p and Nop58p are associated with Noplp in complexes, Nop56p and Nop1p exhibiting a stoichiometric association. These physical interactions presumably underlie the observed sl phenotypes. Well-conserved homologs are present in a range of organisms, including humans (52% identity between human hNop56p and yeast Nop56p), suggesting that these complexes have been conserved in evolution.

Nup93, a vertebrate homologue of yeast Nic96p, forms a complex with a novel 205-kDa protein and is required for correct nuclear pore assembly

Yeast and vertebrate nuclear pores display significant morphological similarity by electron microscopy, but sequence similarity between the respective proteins has been more difficult to observe. Herein we have identified a vertebrate nucleoporin, Nup93, in both human and Xenopus that has proved to be an evolutionarily related homologue of the yeast nucleoporin Nic96p. Polyclonal antiserum to human Nup93 detects corresponding proteins in human, rat, and Xenopus cells. Immunofluorescence and immunoelectron microscopy localize vertebrate Nup93 at the nuclear basket and at or near the nuclear entry to the gated channel of the pore. Immunoprecipitation from both mammalian and Xenopus cell extracts indicates that a small fraction of Nup93 physically interacts with the nucleoporin p62, just as yeast Nic96p interacts with the yeast p62 homologue. However, a large fraction of vertebrate Nup93 is extracted from pores and is also present in Xenopus egg extracts in complex with a newly discovered 205-kDa protein. Mass spectrometric sequencing of the human 205-kDa protein reveals that this protein is encoded by an open reading frame, KIAAO225, present in the human database. The putative human nucleoporin of 205 kDa has related sequence homologues in Caenorhabditis elegans and Saccharomyces cerevisiae. The analyze the role of the Nup93 complex in the pore, nuclei were assembled that lack the Nup93 complex after immunodepletion of a Xenopus nuclear reconstitution extract. The Nup93-complex-depleted nuclei are clearly defective for correct nuclear pore assembly. From these experiments, we conclude that the vertebrate and yeast pore have significant homology in their functionally important cores and that, with the identification of Nup93 and the 205-kDa protein, we have extended the knowledge of the nearest-neighbor interactions of this core in both yeast and vertebrates.

Two functionally distinct domains generated by in vivo cleavage of Nup145p: a novel biogenesis pathway for nucleoporins

Nup145p is an essential yeast nucleoporin involved in nuclear export of polyadenylated RNAs. We demonstrate here that Nup145p is cleaved in vivo to yield two functionally distinct domains: a carboxy-terminal domain (C-Nup145p) which is located at the nuclear pore complex (NPC) and assembles into the Nup84p complex, and a GLFG-containing amino-terminal domain (N-Nup145p) which is not part of this complex. Whereas the essential C-Nup145p accomplishes the functions required for efficient mRNA export and normal NPC distribution, N-Nup145p, which is homologous to the GLFG-containing nucleoporins Nup100p and Nup116p, is not necessary for cell growth. However, the N-Nup145p becomes essential in a nup188 mutant background. Strikingly, generation of a free N-domain is a prerequisite for complementation of this peculiar synthetic lethal mutant. These data suggest that N- and C-domains of Nup145p perform independent functions, and that the in vivo cleavage observed is of functional importance.

Mex67p, a novel factor for nuclear mRNA export, binds to both poly(A)+ RNA and nuclear pores

An essential cellular factor for nuclear mRNA export called Mex67p which has homologous proteins in human and Caenorhabditis elegans was identified through its genetic interaction with nucleoporin Nup85p. In the thermosensitive mex67-5 mutant, poly(A)+ RNA accumulates in intranuclear foci shortly after shift to the restrictive temperature, but NLS-mediated nuclear protein import is not inhibited. In vivo, Mex67p tagged with green fluorescent protein (GFP) is found at the nuclear pores, but mutant mex67-5-GFP accumulates in the cytoplasm. Upon purification of poly(A)+ RNA derived from of UV-irradiated yeast cells, Mex67p, but not nucleoporins Nup85p and Nup57p, was crosslinked to mRNA. In a two-hybrid screen, a putative RNA-binding protein with RNP consensus motifs was found to interact with the Mex67p carboxy-terminal domain. Thus, Mex67p is likely to participate directly in the export of mRNA from the nucleus to the cytoplasm.

From nucleoporins to nuclear pore complexes

One of the largest supramolecular assemblies in the eukaryotic cell, the nuclear pore complex, is now being dissected into its numerous molecular constituents. The combined use of biochemistry and genetics in yeast has made this rapid development possible. Although less is known about vertebrate nucleoporins, the first clues are now emerging about their in vivo function also. Much remains to be learned about nuclear pore complex assembly and function, however.

MHP1, an essential gene in Saccharomyces cerevisiae required for microtubule function

The gene for a microtubule-associated protein (MAP), termed MHP1 (MAP-Homologous Protein 1), was isolated from Saccharomyces cerevisiae by expression cloning using antibodies specific for the Drosophila 205K MAP. MHP1 encodes an essential protein of 1,398 amino acids that contains near its COOH-terminal end a sequence homologous to the microtubule-binding domain of MAP2, MAP4, and tau. While total disruptions are lethal, NH2-terminal deletion mutations of MHP1 are viable, and the expression of the COOH-terminal two-thirds of the protein is sufficient for vegetative growth. Nonviable deletion-disruption mutations of MHP1 can be partially complemented by the expression of the Drosophila 205K MAP. Mhp1p binds to microtubules in vitro, and it is the COOH-terminal region containing the tau-homologous motif that mediates microtubule binding. Antibodies directed against a COOH-terminal peptide of Mhp1p decorate cytoplasmic microtubules and mitotic spindles as revealed by immunofluorescence microscopy. The overexpression of an NH2-terminal deletion mutation of MHP1 results in an accumulation of large-budded cells with short spindles and disturbed nuclear migration. In asynchronously growing cells that overexpress MHP1 from a multicopy plasmid, the length and number of cytoplasmic microtubules is increased and the proportion of mitotic cells is decreased, while haploid cells in which the expression of MHP1 has been silenced exhibit few microtubules. These results suggest that MHP1 is essential for the formation and/or stabilization of microtubules.

Docosahexaenoic acid synthesis in human skin fibroblasts involves peroxisomal retroconversion of tetracosahexaenoic acid

The purpose of this study was to determine whether the formation of docosahexaenoic acid in human cells occurs through a pathway that involves 24-carbon n-3 fatty acid intermediates and retroconversion. Normal human skin fibroblasts synthesized radiolabeled docosahexaenoic acid from [1-(14)C]18:3n-3, [3-(14)C]22:5n-3, [3-(14)C]24:5n-3, and [3-(14)C]24:6n-3. The amount of docosahexaenoate formed was reduced in fibroblasts defective in peroxisomal biogenesis, by 90-100% in Zellweger's syndrome and by 50-75% in infantile Refsum's disease. Fatty acid elongation and desaturation were intact in these mutant cells. No decrease in radiolabeled docosahexaenoic acid production occurred in mutant fibroblasts defective in peroxisomal alpha-oxidation or mitochondrial beta-oxidation, or in normal fibroblasts treated with methyl palmoxirate to inhibit mitochondrial beta-oxidation. Therefore, the retroconversion step in docosahexaenoic acid formation occurs through peroxisomal beta-oxidation in normal human cells. These results demonstrate that the pathway for docosahexaenoic acid synthesis in human cells involves 24-carbon intermediates. The limited ability to synthesize docosahexaenoic acid may underlie some of the pathology that occurs in genetic diseases involving peroxisomal beta-oxidation.

Reconstitution of nuclear protein transport with semi-intact yeast cells

We have developed an in vitro nuclear protein import reaction from semi-intact yeast cells. The reaction uses cells that have been permeabilized by freeze-thaw after spheroplast formation. Electron microscopic analysis and antibody-binding experiments show that the nuclear envelope remains intact but the plasma membrane is perforated. In the presence of ATP and cytosol derived from yeast or mammalian cells, a protein containing the nuclear localization sequence (NLS) of SV40 large T-antigen is transported into the nucleus. Proteins with mutant NLSs are not imported. In the absence of cytosol, binding of NLS-containing proteins occurs at the nuclear envelope. N-ethylmaleimide treatment of the cytosol as well as antibodies to the nuclear pore protein Nsp1 inhibit import but not binding to the nuclear envelope. Yeast mutants defective in nuclear protein transport were tested in the in vitro import reaction. Semi-intact cells from temperature-sensitive nsp1 mutants failed to import but some binding to the nuclear envelope was observed. On the other hand, no binding and thus no import into nuclei was observed in semi-intact nsp49 cells which are mutated in another nuclear pore protein. Np13 mutants, which are defective for nuclear protein import in vivo, were also deficient in the binding step under the in vitro conditions. Thus, the transport defect in these mutants is at the level of the nucleus and the point at which nuclear transport is blocked can be defined.

Modification of low density lipoprotein association with the arterial intima. A possible environment for the antiatherogenic action of beta-blockers

The accumulation of lipoproteins and lipids associated with the progression of atherosclerotic lesions appears to be a sequential process involving interactions with the intimal extracellular matrix and, subsequently, with cells. Apolipoprotein B-containing lipoproteins appear first to be retained and modified focally by proteoglycans of the extracellular matrix. The association with the extracellular matrix may lead to further modifications. In vitro resident macrophages take up the modified apolipoprotein B lipoproteins via a nonsaturable mechanism that may contribute to their transformation into foam cells characteristic of the atherosclerotic lesion. The affinity of low density lipoprotein (LDL) for arterial proteoglycans in vitro is related to the charge and triglyceride content. Small, triglyceride-poor, cholesterol-rich particles interact more efficiently with proteoglycans and are taken up faster by cultured macrophages than larger triglyceride-rich ones. beta-Blockers increase the relative triglyceride content of circulating LDL, and in vitro this LDL has a lower affinity for arterial proteoglycans. These results suggest that some of the experimental antiatherogenic actions of beta-blockers may be related to a reduced LDL affinity for extracellular intima components associated with the small changes induced by the drugs in the lipoprotein structure. Hypothetically, this reduced affinity could diminish the focal accumulation of LDL in lesion-prone sites.

Modifications of low-density lipoprotein induced by arterial proteoglycans and chondroitin-6-sulfate

Association of low-density lipoproteins (LDL) with arterial chondroitin sulfate proteoglycans (CSPG) appears to contribute to their deposition in the extracellular intimal compartment and to its internalization by macrophages. CSPG and LDL interact by ionic bridges with formation of soluble and insoluble complexes. We studied the alterations on LDL structure induced by its association with arterial CSPG and other glycosaminoglycans (GAG). In soluble complexes, at low and at physiological ionic strength, arterial CSPG and sulfated GAG modify the kinetics of apoB-100 proteolysis by trypsin. However, less marked alterations in the peptide patterns were observed with proteinase V8 and almost none with thermolysin. This is indirect evidence that the presence of CSPG and GAG modified the exposure of polar regions of apoB-100 in LDL. Competitive binding experiments with agarose-bound heparin and soluble GAG also suggest that after formation of insoluble complexes with arterial CSPG and resolubilization the exposure of Lys, Arg-rich segments of apoB-100 is increased. Results from differential scanning calorimetry and differential thermal spectrophotometry showed that the CSPG and GAG-induced modifications reduced the thermal stability of the surface and core in LDL. If present in vivo, the structural alterations of polar segments of the LDL protein moiety may influence the outcome of its interaction with the arterial mesenchyma.

Mammalian nuclei contain foci which are highly enriched in components of the pre-mRNA splicing machinery

The organization of the major snRNP particles in mammalian cell nuclei has been analysed by in situ labelling using snRNA-specific antisense probes made of 2'-OMe RNA. U3 snRNA is exclusively detected in the nucleolus while all the spliceosomal snRNAs are found in the nucleoplasm outside of nucleoli. Surprisingly, U2, U4, U5 and U6 snRNAs are predominantly observed in discrete nucleoplasmic foci. U1 snRNA is also present in foci but in addition is detected widely distributed throughout the nucleoplasm. An anti-peptide antibody specific for the non-snRNP splicing factor U2AF reveals it to have a similar distribution to U1 snRNA. Co-localization studies using confocal fluorescence microscopy prove that U2AF is present in the snRNA-containing foci. Antibody staining also shows the foci to contain snRNP-specific proteins and m3G-cap structures. The presence of major components of the nuclear splicing apparatus in foci suggests that these structures may play a role in pre-mRNA processing.

Interaction of LDL with human arterial proteoglycans stimulates its uptake by human monocyte-derived macrophages

The aim of this work was to investigate the possible mechanisms for uptake by human monocyte-derived macrophages (HMDM) of low density lipoprotein (LDL) pretreated with human arterial chondroitin-6-SO4-rich proteoglycan (LDL-PG). HMDM were incubated with 125I-labeled tyramine cellobiose-labeled LDL-PG, native LDL, and acetylated LDL (Ac-LDL). The results showed that two to four times more LDL-PG than LDL was bound and internalized by the HMDM. Competition experiments showed that LDL-PG competed with native LDL for the apoB,E (LDL) receptor, but not for the Ac-LDL scavenger receptor. Both the LDL and LDL-PG uptake were reduced after preincubation of the macrophages with unlabeled native LDL, though to a lesser extent with LDL-PG. The specific binding of 125I-labeled LDL and 125I-labeled LDL-PG at 4 degrees C was both saturable and concentration-dependent. The dissociation constant (Kd) for binding was 8.6 x 10(-9) M for LDL and 9.4 x 10(-9) M for LDL-PG, but the maximum binding (Bmax) was 1.5-times higher for LDL-PG. Cholesterol derived from LDL-PG was less effective than native LDL in suppressing HMG-CoA reductase activity. The results indicate that the uptake of LDL-PG is mediated not only by the LDL-receptor, but also by another unspecific pathway, which may not be subjected to regulation. These results provide further support for the hypothesis that LDL modifications induced by arterial PG may contribute to the formation of foam cells.

Effect of arterial proteoglycans on the interaction of LDL with human monocyte-derived macrophages

The effect of human arterial proteoglycans (PG1) on the interaction of low density lipoprotein (LDL) with cultured human monocyte-derived macrophages (HMDM) was studied. LDL was insolubilized by treatment with chondroitin-6-sulfate-rich, partially purified PG1. The LDL, resolubilized in culture medium, was added to HMDM. The PG1 pretreated LDL induced lipid accumulation in the HMDM, converting them into foam cells. Mass determination of lipids by spectrophotometric and chromatographic procedures showed a 2-4-fold accumulation of triglycerides, phospholipids, unesterified cholesterol and cholesterol esters in 48 h, in the HMDM incubated with PG pretreated LDL, when compared to those incubated with native LDL. Incorporation of [14C]oleic acid into the HMDM lipid esters correlated with the accumulation. Association of 125I-labeled LDL and of fluorescent labeled LDL (3,3-octadecyl indocarbocyanine) to HMDM also indicated that the PG1-pretreatment of LDL increased its uptake. Density gradient centrifugation, isoelectric focusing and electron microscopy showed that, when added to the cells, the PG1 pretreated LDL was not aggregated or altered in its surface charge. However, controlled trypsin treatment and polypeptide pattern analysis indicate that the accessibility of apoB has been altered. The results suggest that changes in the surface of LDL, induced by the arterial PG1, lead to increased endocytosis of the lipoprotein and stimulation of lipid synthesis in the macrophages. The possibility that a similar process may cause lipid accumulation in arterial macrophages is discussed.

Effects of prazosin and propranolol on blood lipids and lipoproteins in hypertensive patients

Prazosin and propranolol were compared in an open, crossover study to determine their effects on plasma lipids and lipoproteins. After a four-week placebo period, 10 hypertensive patients were randomly assigned to prazosin treatment (Group I) and another 10 to propranolol treatment (Group II) for eight weeks. After a second four-week placebo period, treatment in each group was switched to the alternative drug for eight weeks. The mean blood pressure was reduced to normal levels (diastolic blood pressure less than or equal to 90 mm Hg) by both drugs--prazosin (1 to 8 mg per day) and propranolol (40 to 240 mg per day). The results of the study indicate that prazosin decreases serum cholesterol levels. In contrast, propranolol not only increases serum triglyceride levels and very-low-density lipoprotein cholesterol, but decreases total high-density lipoprotein cholesterol, high-density lipoprotein2 cholesterol, high-density lipoprotein2, and apoprotein A-I. The data suggest that propranolol may induce significant, potentially atherogenic changes in lipid metabolism, whereas prazosin may represent an advantageous alternative as an antihypertensive agent, especially in subjects with an already atherogenic lipoprotein profile.

Partial structure of the active moiety of a lipoprotein complexing proteoglycan from human aorta

Proteoglycans and glycosaminoglycans of the intima-media extracellular matrix have been stated to play a role in lipoprotein deposition associated with atherogenesis. It is therefore important to characterize the active lipoprotein-complexing moiety of these macromolecular aggregates. We have isolated a soluble proteoglycan aggregate of approximately 5 X 10(6) molecular weight after homogenization of human aortic intima-media in an isosmotic sucrose solution, sequential differential centrifugation, dialysis, exclusion chromatography and preparative electrophoresis. This proteoglycan aggregate, labelled lipoprotein-complexing proteoglycan (LCP), has been previously shown to form specific complexes with low density lipoproteins, either isolated or in sera. Density gradient centrifugation in dissociative conditions of the LCP, cellulose acetate acetate electrophoresis of the subfractions, chondroitinases treatment and high performance liquid chromatography of the unsaturated disaccharides indicated that the glycosaminoglycan moiety was composed of 56% chondroitin-6-SO4, 26% hyaluronate and/or undersulfated chondroitin and 17% chondroitin-4-SO4. In pore-gradient polyacrylamide gel electrophoresis, the hyaluronate monomer appeared to have a molecular weight of 250000 while that of the chondroitin sulfates ranged between 50000 and 70000 after extensive treatment with protease. The fractions enriched in the chondroitin sulfate monomers were the most reactive towards LDL and their reactivity was abolished by chondroitinase AC indicating that the lipoprotein-complexing capacity of the LCP aggregate is associated to these molecules.

Genes and transcripts for the polypeptides of the cytochrome b6/f complex from spinach thylakoid membranes

Cytochrome b6/f complex was prepared from washed thylakoid membranes by a procedure involving detergent treatment and centrifugation in sucrose gradients. The complex is composed of at least four polypeptide species, cytochrome f which occurs in two variant forms (mol. wt. 34/33 kd), cytochrome b6 (23 kd), the high-potential Rieske iron-sulfur protein (19 kd) and a fourth subunit (17 kd) of unknown function. Transcripts for the cytochromes f, b6 and subunit 4 were found in plastid RNA, those for the Rieske iron-sulfur protein in cytosolic poly(A) RNA. Transcripts for cytochrome b6 and subunit 4 are translated in rabbit reticulocyte lysates into products of correct length. The Rieske iron-sulfur protein and the cytochrome f apoprotein appear to be made as precursors with excess sequences of 7 and 4 kd, respectively. Cytochrome f, cytochrome b6 and subunit 4 are encoded by uninterrupted plastid genes that are located in the large single-copy region of the circular DNA molecule. Each of these genes is present once per chromosome. Their location and direction of transcription have been determined by hybrid-selection mapping and by cell-free transcription/translation of various recombinant DNAs. The genes for cytochrome b6 and for subunit 4 lie near each other, but do not overlap. They are transcribed into a single message. The gene for cytochrome f maps 15 kbp away from this cluster, close to the 3' end of the gene for the large subunit of ribulosebisphosphate carboxylase/oxygenase, and is transcribed into a separate 4 kb long RNA. All these genes have the same polarities with respect to each other.

Identification of the polypeptides in the cytochrome b6/f complex from spinach chloroplasts with redox-center-carrying subunits

An improved procedure for the isolation of the cytochrome b6/f complex from spinach chloroplasts is reported. With this preparation up to tenfold higher plastoquinol-plastocyanin oxidoreductase activities were observed. Like the complex obtained by our previous procedure, the complex prepared by the modified way consisted of five polypeptides with apparent molecular masses of 34, 33, 23, 20, and 17 kD, which we call Ia, Ib, II, III, and IV, respectively. In addition, one to three small components with molecular masses below 6 kD were now found to be present. These polypeptides can be extracted with acidic acetone. Cytochrome f, cytochrome b6, and the Rieske Fe-S protein could be purified from the isolated complex and were shown to be represented by subunits Ia + Ib, II, and III, respectively. The heterogeneity of cytochrome f is not understood at present. Estimations of the stoichiometry derived from relative staining intensities with Coomassie blue and amido black gave 1:1:1:1 for the subunits Ia + Ib/II/III/IV, which is interesting in of the presence of two cytochromes b6 per cytochrome f. Cytochrome f titrated as a single-electron acceptor with a pH-independent midpoint potential of +339 mV between pH 6.5 and 8.3, while cytochrome b6 was heterogeneous. With the assumption of two components present in equal amounts, two one-electron transitions with Em(1) = 40 mV and Em(2) = -172 at pH 6.5 were derived. Both midpoint potentials were pH-dependent.

A cytochrome b/c1 complex with ubiquinol--cytochrome c2 oxidoreductase activity from Rhodopseudomonas sphaeroides GA

A cytochrome b/c1 complex which catalyses the reduction of cytochrome c by ubiquinol has been isolated from Rhodopseudomonas sphaeroides GA. It contains two hemes b and substoichiometric amounts of ubiquinone-10 and of the Rieske Fe-S center per cytochrome c1, and is essentially free of reaction center and bacteriochlorophyll. The complex consists of three major polypeptides with apparent molecular masses of 40, 34 and 25 kDa. The 34-kDa polypeptide carries heme. Cytochrome c1 has a midpoint potential of 285 mV. For cytochrome b two midpoint potentials, at 50 and -60 mV, at pH 7.4, can be derived if one assumes two components of equal amount. Ubiquinol--cytochrome c oxidoreductase activity is specific for ubiquinol and bacterial cytochromes c, and is inhibited by antimycin A and 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole. The complex shows oxidant-induced reduction of cytochrome b.

Reduction of cytochromes b6 and f in isolated plastoquinol-plastocyanin oxidoreductase driven by photochemical reaction centers from Rhodopseudomonas sphaeroides

Photochemical reaction centers isolated from the bacterium Rhodopseudomonas sphaeroides are able to donate electrons to cytochromes b6 and f in the plastoquinol,-platocyanin oxidoreductase isolated from spinach chloroplasts. The reduction reactions occur only after the second single turnover flash, in a reaction which is sensitive to inhibitors of the reactions in the chloroplast membranes. When all the components of the b6f complex are oxidized prior to activation, both cytochromes b6 and f are reduced after the second flash. If cytochrome f is reduced prior to activation, cytochrome b6 is still reduced after the second flash, but as the potential is lowered so that the Rieske iron-sulfur cluster is reduced prior to activation, the reduction of cytochrome b6 fails. The b6f complex thus functions in such a way that a two-electron redox couple, probably a quinone, is capable of reducing both cytochrome b6 and cytochrome f, the latter via the Rieske iron-sulfur cluster, in a coupled reaction where both electrons must leave the two-electron carrier. Cytochrome b6 is thus reduced in a manner analogous to "oxidant-induced reduction."

A cytochrome f/b6 complex of five polypeptides with plastoquinol-plastocyanin-oxidoreductase activity from spinach chloroplasts

The isolation of a cytochrome f/b6 complex from spinach chloroplasts, with high yield and purity is reported. The complex consists of five polypeptides with a molecular mass of 34, 33, 23.5, 20 and 17.5 kDa, and contains one cytochrome f, two cytochromes b6 and the Rieske Fe-S center with two non-heme irons. It does not contain plastocyanin and is almost completely devoid of chlorophyll and carotenoids, but lipid and detergent are present. It is lacking cytochrome b-559, although three of the five polypeptides seem to carry heme groups. The preparation has plastoquinol-plastocyanin oxidoreductase activity with plastoquinol-1 and plastoquinol-9, which is sensitive to 2,5-dibromomethylisopropyl-p-benzoquinone, to 2-iodo-6-isopropyl-3-methyl-2',4',4'-trinitrodiphenyl ether, to 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole, and to bathophenanthroline. Characteristics of this activity with respect to substrate concentrations, pH, detergent effect and other parameters are described.

Structural requirements of quinone coenzymes for endogenous and dye-mediated coupled electron transport in bacterial photosynthesis

Electron transport in continuous light has been investigated in chromatophores of Rhodopseudomonas capsulata. Ala pho+, depleted in ubiquinone-10 and subsequently reconstituted with various ubiquinone homologs and analogs. In addition the restoration of electron transport in depleted chromatophores by the artificial redox compounds N-methylphenazonium methosulfate and N,N,N',N'-tetramethyl-p-phenylenediamine was studied. The following pattern of activities was obtained: (1) Reconstitution of cyclic photophosphorylation with ubiquinone-10 was saturated at about 40 ubiquinone molecules per reaction center. (2) Reconstitution by ubiquinone homologs was dependent on the length of the isoprenoid side chain and the amount of residual ubiquinone in the extracted chromatophores. If two or more molecules of ubiquinone-10 per reaction center were retained, all homologs with a side chain longer than two isoprene units were as active as ubiquinone-10 in reconstitution, and the double bonds in the side chain were not required. If less than two molecules per reaction center remained, an unsaturated side chain longer than five units was necessary for full activity. Plastoquinone, alpha-tocopherol, and naphthoquinones of the vitamin K series were relatively inactive in both cases. (3) All ubiquinone homologs, also ubiquinone-1 and -2, could be reduced equally well by the photosynthetic reaction center, as measured by light-induced proton binding in the presence of antimycin A and uncoupler. Plastoquinone was found to be a poor electron acceptor. (4) Photophosphorylation could be reconstituted by N-methylphenazonium methosulfate as well as by N,N,N',N'-tetramethyl-p-phenylenediamine in an antimycin-insensitive way, if more than two ubiquinones per reaction center remained. These compounds were active also in more extensively extracted particles reconstituted with ubiquinone-1, which itself was inactive.

Vectorial redox reactions of physiological quinones. I. Requirement of a minimum length of the isoprenoid side chain

Physiological quinones carrying isoprenoid side chains have been compared with homologues lacking the side chain, for their ability to carry electrons and protons from dithionite to ferricyanide, trapped in liposomes. Six differential observations were made: (1) Plastoquinone and ubiquinones, with a side chain of more than two isoprene units, are by far better mediators than their short-chain homologues. Also other benzoquinones lacking a long side chain are poor catalysts, except dimethyl-methylenedioxy-p-benzoquinone, a highly autooxidizable compound. Tocopherol is a good catalyst. (2) Vitamin K-1 and K-2 are poor mediators compared to vitamin K-3. (3) The reaction catalyzed by quinones carrying long isoprenoid side chains has an about three-fold higher activation energy, irrespective of the catalytic efficiency. (4) The reaction catalyzed by quinones lacking a long side chain follows pseudo first-order kinetics, while the reaction with quinones carrying a long side chain is of apparently higher order. (5) The rate with ubiquinone-1 is increasing pH, while with ubiquinone-9 it is decreasing. (6) The reaction mediated by short-chain quinones seems to be satuarated at lower dithionite concentration. We conclude that isoprenoid quinones are able to translocate electrons and protons in lipid membranes, and that the side chain has a strong impact on the mechanism. This and the relevance of the model reaction for electron and proton transport in photosynthesis and respiration is discussed.