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At least 19 recordsLinked to original sources

PubMed1991

Electron spin echo envelope modulation spectroscopy supports the suggested coordination of two histidine ligands to the Rieske Fe-S centers of the cytochrome b6f complex of spinach and the cytochrome bc1 complexes of Rhodospirillum rubrum, Rhodobacter sphaeroides R-26, and bovine heart mitochondria.

Electron spin echo envelope modulation (ESEEM) experiments performed on the Rieske Fe-S clusters of the cytochrome b6f complex of spinach chloroplasts and of the cytochrome bc1 complexes of Rhodospirillum rubrum, Rhodobacter sphaeroides R-26, and bovine heart mitochondria show modulation components resulting from two distinct classes of 14N ligands. At the g = 1.92 region of the Rieske EPR spectrum of the cytochrome b6f complex, the measured hyperfine couplings for the two classes of coupled nitrogens are A1 = 4.6 MHz and A2 = 3.8 MHz. Similar couplings are observed for the Rieske centers in the three cytochrome bc1 complexes. These ESEEM results indicate a nitrogen coordination environment for these Rieske Fe-S centers that is similar to that of the Fe-S cluster of a bacterial dioxygenase enzyme with two coordinated histidine ligands [Gurbiel, R. J., Batie, C. J., Sivaraja, M., True, A. E., Fee, J. A., Hoffman, B. M., & Ballou, D. P. (1989) Biochemistry 28, 4861-4871]. The Rieske Fe-S cluster lacks modulation components from a weakly coupled peptide nitrogen observed in water-soluble spinach ferredoxin. Treatment with the quinone analogue inhibitor DBMIB causes a shift in the Rieske EPR spectrum to g = 1.95 with no alteration in the magnetic coupling to the two nitrogen atoms. However, the ESEEM pattern of the DBMIB-altered Rieske EPR signal shows evidence of an additional weakly coupled nitrogen similar to that observed in the spinach ferredoxin ESEEM patterns.

Animals↗

PubMed1994

Structural aspects of the cytochrome b6f complex; structure of the lumen-side domain of cytochrome f.

The following findings concerning the structure of the cytochrome b6f complex and its component polypeptides, cyt b6, subunit IV and cytochrome f subunit are discussed: (1) Comparison of the amino acid sequences of 13 and 16 cytochrome b6 and subunit IV polypeptides, respectively, led to (a) reconsideration of the helix lengths and probable interface regions, (b) identification of two likely surface-seeking helices in cyt b6 and one in SU IV, and (c) documentation of a high degree of sequence invariance compared to the mitochondrial cytochrome. The extent of identity is particularly high (88% for conserved and pseudoconserved residues) in the segments of cyt b6 predicted to be extrinsic on the n-side of the membrane. (2) The intramembrane attractive forces between trans-membrane helices that normally stabilize the packing of integral membrane proteins are relatively weak. (3) The complex isolated in dimeric form has been visualized, along with isolated monomer, by electron microscopy. The isolated dimer is much more active than the monomer, is the major form of the complex isolated and purified from chloroplasts, and is inferred to be a functional form in the membrane. (4) The isolated cyt b6f complex contains one molecule of chlorophyll a. (5) The structure of the 252 residue lumen-side domain of cytochrome f isolated from turnip chloroplasts has been solved by X-ray diffraction analysis to a resolution of 2.3 A.

Amino Acid Sequence↗

PubMed1991

Structural characterization of heme sites in spinach cytochrome b6f complexes: a resonance Raman study.

Resonance Raman spectra of cytochrome b6f complexes isolated from spinach chloroplasts have been obtained. Selective resonance enhancements and partial reductions of the complex by redox mediators were used to isolate and identify the contributions of heme b6 and heme f sites to the observed spectra. Corresponding spectra for turnip cytochrome f have also been obtained. Power-dependent photoreduction was observed in cytochrome f of the complex as well as in the isolated cytochrome f during the course of the Raman experiments.

Cytochromes↗

PubMed1991

Biosynthesis of the chloroplast cytochrome b6f complex: studies in a photosynthetic mutant of Lemna.

The biosynthesis of the cytochrome b6f complex has been studied in a mutant, no. 1073, of Lemna perpusilla that contained less than 1% of the four protein subunits when compared with a wild-type strain. RNA gel blot analyses of the mutant indicated that the chloroplast genes for cytochrome f, cytochrome b6, and subunit IV (petA, petB, and petD, respectively) are transcribed and that the petB and petD transcripts undergo their normal processing. Analysis of polysomal polyA+ RNA indicated that the level of translationally active mRNA for the nuclear-encoded Rieske Fe-S protein (petC) was reduced by greater than 100-fold in the mutant. Immunoprecipitation of in vivo labeled proteins indicated that both cytochrome f and subunit IV are synthesized and that subunit IV has a 10-fold higher rate of protein turnover in the mutant. These results are discussed in terms of the assembly of the cytochrome complex and the key role of the Rieske Fe-S protein in this process.

Chloroplasts↗

PubMed1994

Characterization of the chloroplast cytochrome b6f complex as a structural and functional dimer.

Size analysis of the cytochrome b6f complex by FPLC Superose-12 chromatography and Blue Native PAGE indicated a predominantly dimeric component with M(r) = (1.9-2.5) x 10(5). The true dimer molecular weight including bound lipid, but not detergent, was estimated to be 2.3 x 10(5). Size and shape analysis by negative-stain single-particle electron microscopy indicated that the preparation of dimeric complexes contains a major population that has a protein cross section 40% larger than the monomer, binds more negative stain, and has a geometry with a distinct 2-fold axis of symmetry compared to the monomeric complex. The dimeric species is more stable at higher ionic strength with respect to conversion to the monomeric species. SDS-PAGE of monomer and dimer preparations indicated that both contain the four major polypeptides in approximately equal stoichiometry and also contain the petG M(r) 4000 subunit. One bound chlorophyll a per monomer, part of the bound lipid, is present in monomer and dimer. The in vitro electron-transport activity (decyl-PQH2-->PC-ferricyanide) of the separated dimer was comparable to that of the isolated b6f complex and was 4-5-fold greater than that of the monomer preparation, whose activity could be attributed to residual dimer. No difference in the properties of the dimer and monomer was detected by SDS-PAGE or redox difference spectrophotometry that could account for the difference in activities. However, the concentration of the Rieske [2Fe-2S] center was found by EPR analysis of the gy = 1.90 signal to be lower in the monomer fraction by a factor of 3.5 relative to the dimer.(ABSTRACT TRUNCATED AT 250 WORDS)

Biopolymers↗

PubMed1986

Quantitation of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone binding sites in chloroplast membranes: evidence for a functional dimer of the cytochrome b6f complex.

The binding of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) to chloroplast thylakoid membranes was investigated by analyzing the inhibition of electron transfer by DBMIB according to a steady-state rate relationship for enzyme-catalyzed reactions in the presence of tightly binding reversible inhibitors. DBMIB interacts with the cytochrome b6f complex in a manner best described by an apparent dissociation constant near 6 nM. The binding site titer is 1 mmol X mol chlorophyll-1. This number of DBMIB binding sites approaches one-half the number of cytochrome b6f complexes present in the membrane. These data suggest that the cytochrome b6f complex may function in electron transfer as a dimer, plastoquinol oxidation being totally inhibited by the binding of a single DBMIB molecule to the dimer.

Chloroplasts↗

PubMed1988

Cytochrome b6f complex is required for phosphorylation of light-harvesting chlorophyll a/b complex II in chloroplast photosynthetic membranes.

The light-harvesting chlorophyll a/b complex (LHC II) and four photosystem II (PS II) core proteins (8.3, 32, 34 and 44 kDa) become phosphorylated in response to reduction of the intersystem electron transport chain of green plant chloroplasts. Previous studies indicated that reduction of the plastoquinone (PQ) pool is the key event in kinase activation. However, we show here that, unlike PS II proteins, LHC II is phosphorylated only when the cytochrome b6f complex is active. Two lines of evidence support this conclusion. (1) 2,5-Dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and the 2,4-dinitrophenyl ether of iodonitrothymol (DNP-INT), which are known to block electron flow into the cytochrome complex, selectively inhibit LHC II phosphorylation in spinach thylakoids. (2) The hcf6 mutant of maize, which contains PQ but lacks the cytochrome b6f complex, phosphorylates the four PS II proteins but fails to phosphorylate LHC II in vivo or in vitro.

Benzoquinones↗

PubMed1993

NAD turnover and utilisation of metabolites for RNA synthesis in a reaction sensing the redox state of the cytochrome b6f complex in isolated chloroplasts.

NAD is normally regarded as a redox molecule or as the substrate for ADP-ribosylation reactions. In this study, we describe the rapid metabolism of NAD by Percoll-gradient-purified lettuce chloroplasts and show that the adenine moiety can be incorporated into RNA in a dark-activated reaction that senses the redox state of the cytochrome b6f complex. Isolated chloroplasts rapidly metabolised radiolabelled NAD+ to 5'-AMP (within seconds) and adenosine during a 60-min incubation in vitro; the products were analysed by high-performance liquid chromatography. No radiolabelled ADP-ribose was detected. Radioactivity was incorporated into trichloroacetic-acid-insoluble material during this period, with approximately 2-4-fold more incorporation occurring in the dark. Most of this radiolabel was rendered acid-soluble by dilute alkaline digestion at 37 degrees C, yielding an approximately equal mixture of 2'-AMP and 3'-AMP, and by RNase digestion, identifying the acid-insoluble radioactive material as RNA. Protein-bound ADP-ribose would have yielded 5'-AMP and/or oligomeric/polymeric ADP-ribose after alkali digestion. The utilisation of NAD metabolites for RNA synthesis was restricted to the thylakoid compartment of the chloroplast. The use of a variety of electron-transport inhibitors such as 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, bromanil (tetrabromo-1,4-benzoquinone), electron donors (dithiothreitol), electron acceptors (ferricyanide) and an uncoupler showed that the incorporation of radiolabel from NAD into acid-insoluble material was favoured when the cytochrome b6f complex was in the oxidised state (as pertaining to incubations in the dark).

Chloroplasts↗

PubMed1991

Electrostatic destabilization of the cytochrome b6f complex in the thylakoid membrane.

Three of the membrane-spanning polypeptides of the chloroplast cytochrome (cyt) b6f complex were sequentially released from the thylakoid membrane, in the order cyt b6, suIV and Rieske iron-sulfur protein, as the pH was increased from 10 to 12, a protocol usually employed to remove peripheral proteins from membranes. The fourth polypeptide of the cyt b6f complex, cyt f, which spans the membrane once, was apparently not released. The pH values for half-release at low ionic strength were approximately 10.7, 11.1 and 11.3 respectively. The separation of the polypeptides of the complex and the sequential release is readily seen at pH 11, where the loss from the membrane of cyt b6, suIV and Fe iron-sulfur center is approximately 90%, 50% and 20%, respectively. the release of cyt b6 from the membrane was reflected by the absence of its characteristic reduced minus oxidized absorbance signal. The pH values at which the release occurred increased as the ionic strength was raised, implying that the release of the b6f polypeptides arises from extrusion due to repulsive electrostatic interactions probably caused by deprotonation of tyrosine and lysine residues. The lipid content of the released polypeptides was very low, consistent with the observation of a non-membranous state. It is proposed that the pH-dependent extrusion requires two electrostatic effects at alkaline pH higher than approximately 10.5: (i) increased electrostatic repulsion between neighbouring polypeptides of the complex, arising from increased net negative charge in the peripheral segments of these polypeptides, which can cause separation of the polypeptides from the complex; and (ii) ionization of residues such as tyrosine in the membrane-spanning alpha-helices, and neutralization of residues such as lysine which can bind to the negative membrane surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Blotting, Western↗

PubMed1993

Identification of two 4-kDa miniproteins in the cytochrome b6f complex from Chlamydomonas reinhardtii.

Two low molecular weight subunits (ca. 4 kDa) have been identified in highly purified preparations of cytochrome b6f from the green unicellular alga Chlamydomonas reinhardtii. The N-terminus of the first one is blocked. It is synthesized in the chloroplast. It cross reacts with an antiserum raised against a synthetic peptide with the C-terminal sequence of the predicted product of the chloroplast gene petG. The homologue of this protein had been previously identified by other authors in the b6f complex from maize. The N-terminal sequence of the second subunit does not correspond to any known protein. This polypeptide, provisionally named petX, is synthesized in the cytosol. An antiserum has been raised against the corresponding synthetic peptide. Immunoblotting experiments show that neither petG nor petX are present in thylakoid membranes from a b6f-less strain of C. reinhardtii.

Animals↗

PubMed1988

Quinol-cytochrome c oxidoreductase from the thermophilic bacterium PS3. Purification and properties of a cytochrome bc1(b6f) complex.

A quinol-cytochrome c oxidoreductase (cytochrome bc1 complex) has been purified from plasma membranes of a thermophilic Bacillus, PS3, by ion-exchange chromatography in the presence of Triton X-100. The purified enzyme shows absorption bands at 561-562 nm and 553 nm at room temperature, and 560, 551, and 547 nm at 80 K upon reduction, and gives an ESR signal similar to that of a Rieske-type iron sulfur center. Its contents of protohemes, heme c, and non-heme iron are about 23, 10, and 21 nmol/mg of protein, respectively. The enzyme consists of four polypeptides with molecular masses of 29, 23, 21, and 14 kDa judging from their electrophoretic mobilities in the presence of sodium lauryl sulfate. Since the staining intensities of the respective bands are almost proportional to their molecular masses, the monomer complex (87 kDa) of the subunits probably consists of a cytochrome b having two protohemes, a cytochrome c1 and an Fe2-S2-type iron sulfur center. The 29 and 21 kDa subunits were identified as cytochromes c1 and b, respectively, and the 23-kDa subunit is probably an iron-sulfur protein, since the 14-kDa polypeptide can be removed with 3 M urea without reducing the content of non-heme iron. Several characteristics of the subunits and chromophores indicate that the PS3 enzyme is rather similar to cytochrome b6f (a bc1 complex equivalent) of chloroplasts and Cyanobacteria. The PS3 complex catalyzes reduction of cytochrome c with various quinol compounds in the presence of P-lipids and menaquinone. The turnover number at pH 6.8 was about 5 s-1 at 40 degrees C and 50 s-1 at 60 degrees C. The enzyme is heat-stable up to 65 degrees C.

Bacillus↗

PubMed1988

Evidence for a cytochrome f-Rieske protein subcomplex in the cytochrome b6f system from spinach chloroplasts.

The cytochrome b6f complex of spinach chloroplasts was prepared with minor modification according to the method of E. Hurt and G. Hauska (1981) Eur. J. Biochem. 117, 591-599) replacing, however, the final ultracentrifugation step by hydroxyapatite chromatography as suggested by M. F. Doyle and C.-A Yu (1985) Biochem. Biophys. Res. Commun. 131, 700-706). The purified complex was partially dissociated by treatment with 4 M urea or 0.1% sodium dodecyl sulfate (SDS) in the absence of reducing agents. A binary subcomplex consisting of cytochrome f and the Rieske iron-sulfur protein was observed under these conditions by three different methods: (a) hydroxyapatite chromatography; (b) extraction with an isopropanol/water/trifluoroacetic acid mixture; and (c) gel filtration in the presence of low SDS concentrations. The subcomplex dissociated into its components by treatment with mercaptoethanol. These results suggest a close interaction of the cytochrome f with the Rieske protein involving SH groups which under reducing conditions leads to complete dissociation of the subcomplex.

Chlorophyll↗

PubMed1994

Crystal structure of chloroplast cytochrome f reveals a novel cytochrome fold and unexpected heme ligation.

BACKGROUND: Cytochrome f is the high potential electron acceptor of the chloroplast cytochrome b6f complex, and is the electron donor to plastocyanin. The 285-residue cytochrome f subunit is anchored in the thylakoid membrane of the chloroplast by a single membrane-spanning segment near the carboxyl terminus. A soluble redox-active 252-residue lumen-side polypeptide with native spectroscopic and redox properties, missing the membrane anchor and carboxyl terminus, was purified from turnip chloroplasts for structural studies. RESULTS: The crystal structure of cytochrome f, determined to 2.3 A resolution, has several unexpected features. The 252-residue polypeptide is organized into one large and one small domain. The larger heme-binding domain is strikingly different from known structures of other c-type cytochromes and has the same fold as the type III domain of the animal protein, fibronectin. Cytochrome f binds heme with an unprecedented axial heme iron ligand: the amino terminus of the polypeptide. CONCLUSION: The first atomic structure of a subunit of either the cytochrome b6f complex or of the related cytochrome bc1 complex has been obtained. The structure of cytochrome f allows prediction of the approximate docking site of plastocyanin and should allow systematic studies of the mechanism of intra- and inter-protein electron transfer between the cytochrome heme and plastocyanin copper, which are approximately isopotential. The unprecedented axial heme iron ligand also provides information on the sequence of events (i.e. cleavage of signal peptide and ligation of heme) associated with translocation of the cytochrome across the membrane and its subsequent folding.

Amino Acid Sequence↗

PubMedDate not supplied

Membrane association of the Rieske iron-sulfur protein.

The mode of membrane attachment of the Rieske iron-sulfur proteins from cytochrome b6f complex of pea thylakoids and from cytochrome bc1 complex of yeast mitochondria has been studied using biochemical approaches. The relative sensitivity of the Rieske protein to trypsin in the thylakoid membrane shows that all trypsin sites of the Rieske protein are on the lumen side of the thylakoid membrane. In contrast to cytochrome f the chloroplast Rieske protein was extracted from thylakoids using chaotropic agents (NaSCN, urea), an alkaline pH and relatively low concentrations of Trinon X-100. The cytochrome bc1 complex Rieske protein from mitochondrial membranes of yeast was also released by NaSCN and alkaline treatment. The results presented here led us to the conclusion that the mitochondrial and chloroplast Rieske proteins are extrinsic and that their association with the rest of the complex involves hydrophobic interactions.

Antibodies↗

PubMed1987

Sulfide-dependent photosynthetic electron flow coupled to proton translocation in thylakoids of the cyanobacterium Oscillatoria limnetica.

Light-induced proton translocation coupled to sulfide-dependent electron transport has been studied in isolated thylakoids of the cyanobacterium Oscillatoria limnetica. The thylakoids are obtained by osmotic shock of washed spheroplasts, prepared with glycine-betaine as the osmotic stabilizer. 13C NMR studies suggests that betaine is the major osmoregulator in O. limnetica. Thylakoid preparations obtained from both sulfide-induced anoxygenic cells and noninduced oxygenic cells are capable of proton pumping coupled to phenazinemethosulfate-mediated cyclic electron flow. However, only in the induced thylakoids can sulfide-dependent proton gradient (delta pH) formation be measured, using either NADP or methyl viologen as the terminal acceptor. Sulfide-dependent delta pH formation correlates with a high-affinity electron donation site (apparent Km 44 microM at pH 7.9). This site is not lost upon washing of the thylakoids. In addition, both sulfide-dependent electron transport and delta pH formation are sensitive to inhibitors of the cytochrome b6f complex such as 2-n-nonyl-4-hydroxyquinoline-N-oxide, 2,4-dinitrophenyl ether of 2-iodo-4-nitrothymol, or stigmatellin. Sulfide-dependent NADP photoreduction of low affinity (which does not saturate by as much as 7 mM sulfide) is detected in both induced and noninduced thylakoids, but this activity is insensitive to the inhibitors and is not coupled to proton transport. It is suggested that the adaptation of O. limnetica to anoxygenic photosynthesis involves the induction of a thylakoid factor(s) which creates a high-affinity site for sulfide, and the transfer of its electrons via the cytochrome b6f complex, coupled to proton translocation.

Cyanobacteria↗

PubMed1992

The biosynthesis of membrane and soluble plastidic c-type cytochromes of Chlamydomonas reinhardtii is dependent on multiple common gene products.

Cytochrome c6 functions in the thylakoid lumen to catalyze electron transfer from reduced cytochrome f of the cytochrome b6f complex to P700+ of photosystem I. The biogenesis of mature cyt c6 from cytosolically translated pre-apocytochrome c6 involves numerous post-translational modifications including the proteolytic removal of a transit sequence and the covalent attachment of heme to two cysteinyl thiols on the apoprotein. Here, we report on the characterization of a previously unrecognized class of non-allelic mutants of Chlamydomonas reinhardtii that are blocked at the conversion of apocyt c6 to holocyt c6. The mutants are acetate requiring since they are also deficient in cyt f, cyt b and the Rieske FeS protein. Pulse-chase studies indicate that heme attachment is not required for the two-step processing of pre-apocytochrome c6 to apocyt c6, but is required for the stability of the mature protein. This is in contrast to the biosynthesis of mitochondrial cyt c1 where heme attachment is required for the second processing step. We propose that the assembly of both holocytochrome c6 and the cytochrome b6f complex are dependent on common gene products, possibly those involved in heme delivery or metabolism. This is the first suggestion that multiple loci are involved in the biosynthesis of both plastidic c-type cytochromes.

Animals↗

PubMed1995

Biosynthesis of cytochrome f in Chlamydomonas reinhardtii: analysis of the pathway in gabaculine-treated cells and in the heme attachment mutant B6.

Chlamydomonas reinhardtii uses two c-type cytochromes for photosynthetic electron transfer: the thylakoid membrane-bound cytochrome f of the cytochrome b6f complex and the soluble cytochrome c6. Previously, a class of photosynthesis-minus, acetate-requiring mutants was identified which were deficient in both c-type cytochromes, and biochemical analyses of cytochrome c6 biosynthesis in these strains indicated that they were each blocked at the step of heme attachment to apocytochrome c6. In order to demonstrate that the deficiency in cytochrome f results from the same biochemical and genetic defect, cytochrome f biosynthesis was examined in the B6 mutant (a representative of this phenotypic class) and in spontaneous suppressor strains derived from B6. Pulse-radiolabeling experiments show that B6 synthesizes a form of cytochrome f that is rapidly degraded in vivo. This polypeptide is membrane associated and migrates with an electrophoretic mobility identical to that of standard apocytochrome f produced in vitro but slightly greater than that of standard holocytochrome f produced in vivo by wild-type cells. These findings suggest that the B6 strain is unable to convert apocytochrome f to holocytochrome f and that apocytochrome f is unstable in vivo. In the suppressed strains, accumulation of both holocytochrome f and holocytochrome c6 is restored. One suppressor mutation (strain B6R) displays uniparental inheritance whereas another (B6T3) displays Mendelian inheritance. In both cases, the three phenotypes, photosynthesis-plus, b6f+ and cyt c6+ co-segregate in genetic crosses. This study therefore confirms that the dual cyt b6f-/cytc6- deficiency in B6 results from a single mutation that affects a step in holocytochrome formation that is common to the biosynthetic pathways of both plastidic c-type cytochromes. The study also confirms that pre-apocytochrome f synthesis, processing and association with the membrane is not dependent on heme attachment. Synthesis of cytochrome f does, however, appear to be dependent on heme availability. In cells depleted of tetrapyrrole pathway intermediates by gabaculine treatment, cytochrome f synthesis was significantly reduced. Since gabaculine treatment did not affect the stability of cytochrome f nor the accumulation of cytochrome f-encoding transcripts, the reduction is attributed to post-transcriptional regulation of preapocytochrome f synthesis via a pathway that is sensitive to the availability of heme or a tetrapyrrole pathway intermediate.

Animals↗

PubMed1994

Membrane association of cytochrome b6f subunits. The Rieske iron-sulfur protein from Chlamydomonas reinhardtii is an extrinsic protein.

The mode of membrane attachment of five subunits from Chlamydomonas reinhardtii cytochrome b6f complex has been studied using biochemical approaches. Antisera specific for cytochrome f, cytochrome b6, the Rieske iron-sulfur protein, subunit IV, and a 4-kDa subunit (product of the petG gene) were used to quantify the degree of extraction of each of these polypeptides following various treatments. In contrast to the other four subunits, the Rieske protein was extracted to extents varying between 50 and 100% following two cycles of freezing and thawing in the presence of chaotropic agents (KSCN, urea, or NaI). The Rieske protein was not extracted by 2 M NaCl and was rather resistant to alkaline treatments, being extracted by 20 mM 3-(cyclohexylamino)propanesulfonic acid buffer only at pH > 11.5. The hydrodynamic behavior of the isolated Rieske protein was examined in the absence and presence of detergent by ultracentrifugation and by molecular sieving. The extracted protein bound neither to laurylmaltoside nor to C12E8 micelles. Its sedimentation coefficient (D20,w = 9.6 x 10(-11) m2 x s-1), diffusion coefficient (s20,w = 2S), an deduced molecular mass (20.0 +/- 1.7 kDa) are those expected for the monomeric protein. We conclude that the Rieske protein is extrinsic and therefore does not cross the membrane, although its association with the rest of the complex involves primarily hydrophobic interactions, and that the other four subunits analyzed are intrinsic.

Animals↗