Chemical structure of rat liver cytochrome b5. Isolation of peptides by high-pressure liquid chromatography

Production of a recombinant hybrid hemoflavoprotein: engineering a functional NADH:cytochrome c reductase

A gene has been constructed coding for a unique fusion protein, NADH:cytochrome c reductase, that comprises the soluble heme-containing domain of rat hepatic cytochrome b(5) as the amino-terminal portion of the protein and the soluble flavin-containing domain of rat hepatic cytochrome b(5) reductase as the carboxyl terminus. The gene has been expressed in Escherichia coli resulting in the highly efficient production of a functional hybrid hemoflavoprotein which has been purified to homogeneity by a combination of ammonium sulfate precipitation, affinity chromatography on 5'-ADP agarose, and size-exclusion chromatography. The purified protein exhibited a molecular mass of approximately 46 kDa by polyacrylamide gel electrophoresis and 40,875 Da, for the apoprotein, using mass spectrometry which also confirmed the presence of both heme and FAD prosthetic groups. The fusion protein showed immunological cross-reactivity with both anti-rat cytochrome b(5) and anti-rat cytochrome b(5) reductase antibodies indicating the conservation of antigenic determinants from both native domains. Spectroscopic analysis indicated the fusion protein contained both a b-type cytochrome and flavin chromophors with properties identical to those of the native proteins. Amino-terminal and internal amino acid sequencing confirmed the identity of peptides derived from both the heme- and flavin-binding domains with sequences identical to the deduced amino acid sequence. The isolated fusion protein retained NADH:ferricyanide reductase activity (k(cat) = 8.00 x 10(2) s(-1), K(NADH)(m) = 4 microM, K(FeCN(6))(m) = 11 microM) comparable to that of that of native NADH:cytochrome b(5) reductase and also exhibited both NADH:cytochrome c reductase activity (k(cat) = 2.17 x 10(2) s(-1), K(NADH)(m) = 2 microM, K(FeCN(6))(m) = 11 microM, K(Cyt.c)(m) = 1 microM) and NADH:methemoglobin reductase activity (k(cat) = 4.40 x 10(-1) s(-1), K(NADH)(m) = 3 microM, K(mHb)(m) = 47 microM), the latter two activities indicating efficient electron transfer from FAD to heme and retention of physiological function. This work represents the first successful bacterial expression of a soluble, catalytically competent, rat hepatic cytochrome b(5)-cytochrome b(5) reductase fusion protein that retains the functional properties characteristic of the individual heme and flavin domain.

Probing the differences between rat liver outer mitochondrial membrane cytochrome b5 and microsomal cytochromes b5

Two distinct forms of cytochrome b5 exist in the rat hepatocyte. One is associated with the membrane of the endoplasmic reticulum (microsomal, or Mc, cyt b5) while the other is associated with the outer membrane of liver mitochondria (OM cyt b5). Rat OM cyt b5, the only OM cyt b5 identified so far, has a significantly more negative reduction potential and is substantially more stable toward chemical and thermal denaturation than Mc cytochromes b5. In addition, hemin is kinetically trapped in rat OM cyt b5 but not in the Mc proteins. As a result, no transfer of hemin from rat OM cyt b5 to apomyoglobin is observed at pH values as low as 5.2, nor can the thermodyamically favored ratio of hemin orientational isomers be achieved under physiologically relevant conditions. These differences are striking given the similarity of the respective protein folds. A combined theoretical and experimental study has been conducted in order to probe the structural basis behind the remarkably different properties of rat OM and Mc cytochromes b5. Molecular dynamics (MD) simulations starting from the crystal structure of bovine Mc cyt b5 revealed a conformational change that exposes several internal residues to the aqueous environment. The new conformation is equivalent to the "cleft-opened" intermediate observed in a previously reported MD simulation of bovine Mc cyt b5 [Storch, E. M., and Daggett, V. (1995) Biochemistry 34, 9682-9693]. The rat OM protein does not adopt a comparable conformation in MD simulations, thus restricting access of water to the protein interior. Subsequent comparisons of the protein sequences and structures suggested that an extended hydrophobic network encompassing the side chains of Ala-18, Ile-32, Leu-36, and Leu-47 might contribute to the inability of rat OM cyt b5 to adopt the cleft-opened conformation and, hence, stabilize its fold relative to the Mc isoforms. A corresponding network is not present in bovine Mc cyt b5 because positions 18, 32, and 47, are occupied by Ser, Leu, and Arg, respectively. To probe the roles played by Ala-18, Ile-32, and Leu-47 in endowing rat OM cyt b5 with its unusual structural properties, we have replaced them with the corresponding residues in bovine Mc cyt b5. Hence, the I32L (single), A18S/L47R (double), and A18S/L47R/I32L (triple) mutants of rat OM cyt b5 were prepared. The stability of these proteins was found to decrease in the following order: WT rat OM > rat OM I32L > rat OM A18S/L47R > rat OM A18S/L47R/I32L > bovine Mc cyt b5. The decrease in stability of the rat OM protein correlates with the extent to which the hydrophobic cluster involving the side chains of residues 18, 32, 36, and 47 has been disrupted. Complete disruption of the hydrophobic network in the triple mutant is confirmed in a 2.0 A resolution crystal structure of the protein. Disruption of the hydrophobic network also facilitates hemin loss at pH 5.2 for the double and triple mutants, with the less stable triple mutant exhibiting the greater rate of hemin transfer to apomyoglobin. Finally, 1H NMR spectroscopy and side-by-side comparisons of the crystal structures of bovine Mc, rat OM, and rat OM A18S/L47R/I32L cyt b5 allowed us to conclude that the nature of residue 32 plays a key role in controlling the relative stability of hemin orientational isomers A and B in rat OM cyt b5. A similar analysis led to the conclusion that Leu-70 and Ser-71 play a pivotal role in stabilizing isomer A relative to isomer B in Mc cytochromes b5.

Simultaneous purification and characterization of cytochrome b5 reductase and cytochrome b5 from sheep liver

Cytochrome b5 was purified from detergent solubilized sheep liver microsomes by using three successive DEAE-cellulose, and Sephadex G-100 column chromatographies. It was purified 54-fold and the yield was 23.5% with respect to microsomes. The apparent Mr of cytochrome b5 was estimated to be 16,200 +/- 500 by SDS-PAGE. Absolute absorption spectrum of the purified cytochrome b5 showed maximal absorption at 412 nm and dithionite-reduced cytochrome b5 gave peaks at 557, 526.5 and 423 nm. The ability of the purified sheep liver cytochrome b5 to transfer electrons from NADH-cytochrome b5 reductase to cytochrome c was investigated. The K(m) and Vmax values were calculated to be 0.088 microM cytochrome b5 and 315.8 microM cytochrome c reduced/min/mg enzyme, respectively. Also the reduction of cytochrome b5 by reductase was studied and K(m) and Vmax values were determined to be 5 microM cytochrome b5 and 5200 nmol cytochrome b5 reduced/min/mg enzyme, respectively. The K(m) and Vmax values for the cofactor NADH in the presence of saturating concentration of cytochrome b5 were found to be 0.0017 mM NADH and 6944 nmol cytochrome b5 reduced/min/mg enzyme, respectively. NADH-cytochrome b5 reductase was also partially purified from the same source, detergent solubilized sheep liver microsomes, by using two successive DEAE-cellulose, and 5'-ADP-agarose affinity column chromatographies. It was purified 144-fold and the yield was 7% with respect to microsomes. The apparent monomer Mr of reductase was estimated to be 34,000 by SDS-PAGE. When ferricyanide was used as an electron acceptor, reductase showed maximum activity between 6.8 and 7.5. The K(m) and Vmax values of the enzyme for ferricyanide were calculated as 0.024 mM ferricyanide and 673 mumol ferricyanide reduced/min/mg enzyme, respectively. The K(m) and Vmax values for the cofactor NADH in the presence of saturating amounts of ferricyanide were found to be 0.020 mM NADH and 699 mumol ferricyanide reduced/min/mg enzyme, respectively.

Charged amino acids at the carboxyl-terminal portions determine the intracellular locations of two isoforms of cytochrome b5

Outer mitochondrial membrane cytochrome b5 (OMb), which is an isoform of cytochrome b5 (cyt b5) in the endoplasmic reticulum, is a typical tail-anchored protein of the outer mitochondrial membrane. We cloned cDNA containing the complete amino acid sequence of OMb and found that the protein has no typical structural feature common to the mitochondrial targeting signal at the amino terminus. To identify the region responsible for the mitochondrial targeting of OMb, various mutated proteins were expressed in cultured mammalian cells, and the subcellular localization of the expressed proteins was analyzed. The deletion of more than 11 amino acid residues from the carboxyl-terminal end of OMb abolished the targeting of the protein to the mitochondria. When the carboxyl-terminal 10 amino acids of OMb were fused to the cyt b5 that was previously deleted in the corresponding 10 residues, the fused protein localized in the mitochondria, thereby indicating that the carboxyl-terminal 10 amino acid residues of OMb have sufficient information to transport OMb to the mitochondria. The replacement of either of the two positively charged residues within the carboxyl-terminal 10 amino acids by alanine resulted in the transport of the mutant proteins to the endoplasmic reticulum. The mutant cyt b5, in which the acidic amino acid in its carboxyl-terminal end was replaced by basic amino acid, could be transported to the mitochondria. It would thus seem that charged amino acids in the carboxyl-terminal portion of these proteins determine their locations in the cell.

Purification and characterization of two forms of microsomal cytochrome b5 from sheep lung

Two forms of cytochrome b5 were purified from detergent solubilized sheep lung microsomes by three successive DEAE-cellulose, Sephadex G-100 and Sephadex G-200 column chromatographies. The specific contents of cytochromes b5-I and b5-II were determined to be 45.4 and 43.8 nmol b5/mg protein, which represented up to 567 and 547-fold purification compared with that of the lung microsomes. The most striking difference between b5-I and b5-II was observed in their elution pattern from the third DEAE-cellulose column. Cytochromes gave one protein band with a Mr of 16400 +/- 500 on SDS-PAGE. Both forms of reduced b5 showed a major peak at 423 nm while reduced b5-I had two minor peaks at 527 and 556 and reduced b5-II gave two well-defined peaks at 526 and 555 nm. The ability of the purified b5-I and b5-II fractions to transfer the electrons from NADH-cytochrome b5 reductase to cytochrome c was investigated. Apparent Km, 0.055 microM, of b5-II was found to be 38% lower than that of b5-I. In addition, cytochrome b5-I was found to be more sensitive to heat treatment than b5-II when cytochromes were subjected to 62 degrees C for varying periods of time and the coupling of b5 reduction to cytochrome c reduction was determined. These results may indicate that two forms may exist in lung endoplasmic reticulum.

Chromatographic separation and behavior of microsomal cytochrome P450 and cytochrome b5

The methods used for separation of the multiple mammalian cytochrome P450 enzymes by liquid chromatography are reviewed. In addition to the chromatographic techniques, preparation and handling of samples and prefractionation procedures are considered. Conditions that affect stability and chromatographic resolution of cytochromes P450 are also discussed. Special emphasis is put on useful methods which are not routinely used for P450 separation, such as immobilized metal affinity or hydrophobic-interaction chromatography. Applications of low- and high-pressure methods with regard to preparative and analytical separations are compared. It is shown that high- and medium-pressure ion-exchange chromatography are suitable tools for separation of closely related P450 enzymes, especially when specific detection methods are available. In addition to fractionation of cytochromes P450, the isolation and chromatographic behavior of cytochrome b5 is discussed.

Membrane topology and retention of microsomal aldehyde dehydrogenase in the endoplasmic reticulum

Microsomal aldehyde dehydrogenase (msALDH) is anchored to the endoplasmic reticulum (ER) membrane by the hydrophobic domain at its carboxyl terminus, and most of the molecule is exposed to the cytoplasm (Masaki, R., Yamamoto, A., and Tashiro, Y.(1994) J. Cell Biol. 126, 1407-1420). To determine the membrane topology and the intracellular localization of msALDH, the amino-terminal region of bovine opsin containing N-glycosylation sites was fused to the carboxyl terminus of msALDH, and three chimeric proteins with extensions of different sizes were expressed in COS cells. Indirect immunofluorescence microscopy showed the ER localization of all of the chimeric proteins similar to wild-type msALDH. Immunoblotting revealed that the two chimeric proteins containing longer extensions, those with the N-glycosylation site at distances of 13 and 21 amino acids from the membrane anchor, respectively, were glycosylated. These results indicate that the membrane binding domain of msALDH spans the bilayer of the ER. The carbohydrate chain of the chimeras was sensitive to endoglycosidase H but resistant to endoglycosidase D. Upon treatment of transfected COS cells with brefeldin A, the carbohydrate chain was processed to an endoglycosidase H-resistant form, presumably by cis/medial Golgi-specific enzymes redistributed in the ER. These biochemical results in addition to immunofluorescence microscopic observations suggest that msALDH is retained in the ER by blockading of the exit from the ER.

The targeting information of the mitochondrial outer membrane isoform of cytochrome b5 is contained within the carboxyl-terminal region

Two isoforms of mammalian cytochrome b5, which have homologous cytosolic amino-terminal catalytic domains, are located one on endoplasmic reticulum (ER b5) the other on mitochondrial outer membranes (OM b5). A cDNA coding for the previously unknown carboxyl-terminal domain of OM b5 was cloned and a chimera between the catalytic domain of ER b5 and the carboxyl-terminal region of OM b5 was expressed in cultured mammalian cells. The chimera localized to mitochondria, indicating that the carboxyl-terminal 43 amino acids of OM b5 contain sufficient information to target the catalytic domain of ER b5 to the mitochondrial outer membrane.

Purification and characterization of two forms of cytochrome b5 from an arachidonic acid-producing fungus, Mortierella hygrophila

Two forms of cytochrome b5 have been purified from the microsomes of an arachidonic acid-producing fungus, Mortierella hygrophila IFO 5941, after detergent solubilization. They have monomeric molecular masses of about 16 kDa and 19 kDa. Their absorption spectra are similar to those of mammalian cytochrome b5s. Their amino acid compositions show some similarity to those of mammalian cytochrome b5s, but the contents of some amino acids (glycine, alanine, aspartic acid + asparagine, glutamic acid + glutamine, arginine, proline, histidine, leucine and lysine) are unique to the cytochrome b5s of M. hygrophila. Some of their internal peptide sequences also show close homology with those of some mammals (approx. 65 to 67%), while some others show no or little homology. The addition of various acyl-CoAs to NADH-reduced microsomes caused an abrupt shiftdown of the steady state reduction level of cytochrome b5. This indicates the increased utilization of electrons for the desaturation process and may suggest that the cytochrome b5s of this fungus actually take part in its microsomal desaturation system for polyunsaturated fatty acid biosynthesis as electron carriers.

Microsomal aldehyde dehydrogenase is localized to the endoplasmic reticulum via its carboxyl-terminal 35 amino acids

Rat microsomal aldehyde dehydrogenase (msALDH) has no amino-terminal signal sequence, but instead it has a characteristic hydrophobic domain at the carboxyl terminus (Miyauchi, K., R. Masaki, S. Taketani, A. Yamamoto, A. Akayama, and Y. Tashiro. 1991. J. Biol. Chem. 266:19536-19542). This membrane-bound enzyme is a useful model protein for studying posttranslational localization to its final destination. When expressed from cDNA in COS-1 cells, wild-type msALDH is localized exclusively in the well-developed ER. The removal of the hydrophobic domain results in the cytosolic localization of truncated proteins, thus suggesting that the portion is responsible for membrane anchoring. The last 35 amino acids of msALDH, including the hydrophobic domain, are sufficient for targeting of E. coli beta-galactosidase to the ER membrane. Further studies using chloramphenicol acetyltransferase fusion proteins suggest that two hydrophilic sequences on either side of the hydrophobic domain play an important role in ER targeting.

Differential expression of the mRNAs for the soluble and membrane-bound forms of rabbit cytochrome b5

Total RNA was extracted from a variety of rabbit tissues and reverse transcribed for use in the polymerase chain reaction technique. Using primers designed to amplify the membrane-bound liver cytochrome b5 cDNA, products of two sizes were observed. Both hybridized strongly to a radiolabelled liver cytochrome b5 probe. Sequencing confirmed that the two types of cDNA product encoded the membrane-bound and the soluble forms of b5. Messenger RNA corresponding to the soluble cytochrome was detected in the lung, gallbladder and the adrenal gland, as well as in reticulocytes and bone marrow. This was an unexpected finding since the protein has been isolated only from erythrocytes. In contrast, membrane-bound cytochrome b5 mRNA was detected in all tissues tested, suggesting that the corresponding protein is ubiquitous in tissue distribution.

Fluorescence study of a mutant cytochrome b5 with a single tryptophan in the membrane-binding domain

Fluorescence studies of cytochrome b5 are complicated by the presence of three tryptophans, at positions 108, 109, and 112, in the membrane-binding domain. The cDNA for rabbit liver cytochrome b5, isolated from a lambda gt11 library, was used to generate a mutated mRNA where the codons for tryptophans-108 and -112 were replaced by codons for leucine. The sequence was expressed in Escherichia coli and the mutant protein was isolated. This mutant protein had the expected absorption spectrum, and its amino acid composition was confirmed by amino acid analysis and by DNA sequencing of the construct. The fluorescence emission spectrum of the mutant is blue-shifted and is narrower than that of the native protein. The quantum yield of the mutant protein, per molecule, is only 60% of that of the native protein, and the enhancement when bound to lipid vesicles or detergent micelles is higher for the mutant. Fluorescence anisotropy measurements and quenching studies using brominated lipids suggest that the fluorescence of the native protein is due to tryptophans-109 and -108 while tryptophan-112 does not emit but undergoes nonradiative energy transfer to tryptophan-108. With this mutant, it was shown that incomplete energy transfer from tyrosines-126 and -129 to tryptophan-109 occurs when the membrane binding domain is inserted into lipid vesicles, which suggests that the membrane-binding domain does not exist in a tight hairpin loop.

Localization of the N-terminal methionine of rat liver cytochrome P-450 in the lumen of the endoplasmic reticulum

Recent cumulative evidence suggests that liver microsomal cytochrome P-450 (P-450) is exposed to the cytosol with the exception of the N-terminal peptide (amino acid residues 1 to 21), or two peptides (residues 1 to 60). We tested the localization of the N-terminal methionine residue of P-450IIB1 of rat liver microsomes in the natural membrane with the site-specific reagent fluorescein isothiocyanate. The N-terminus of isolated P-450 was stoichiometrically modified in solution with fluorescein isothiocyanate. In intact microsomes, the N-terminus was not modified but became accessible to the reagent when the membrane was dissolved with Triton X-100. Our results indicate that the N-terminus faces the lumen of the endoplasmic reticulum, and we propose that P-450 spans the membrane only once with amino acid residues 1 to 21.

Mass spectrometric analysis of rabbit and bovine trypsin-solubilized cytochrome b5

The sequence and blocking group of the amino-terminal 15 amino acids of rabbit trypsin-solubilized cytochrome b5 were determined by liquid secondary ion mass spectrometry (LSIMS) and tandem mass spectrometry (MS/MS). The molecular weights of peptides generated from a Staphylococcus aureus V8 protease digest of this protein were determined by LSIMS analysis and the two peptides containing the blocked amino-terminus were sequenced by tandem mass spectrometry to yield the sequence; N-acetyl-Ala-Ala-Glu-Ser-Asp-Lys-Asp-Val-Lys-Tyr-Tyr-Thr-Leu-Glu-Glu. Comparison of this sequence with a recently reported cDNA sequence (Dariush et al., 1988) indicates that Gln at position 3 is selectively deamidated, although no other discrepancies were found. Intact rabbit and bovine trypsin-solubilized cytochrome b5 were also analyzed by LSIMS on a high-field mass spectrometer equipped with a diode array detector. Mass measurement of the unresolved protonated molecular ion peak tops gave average molecular weights of 9462.2 +/- 2 and 9502.3 +/- 2 for bovine and rabbit trypsin-solubilized cytochrome b5, respectively. In both cases, these molecular weights correspond to a cytochrome b5 fragment consisting of amino acids Asp(7)-Arg(88). The average molecular weight for the rabbit amino-terminal-blocked form of trypsin-solubilized cytochrome b5 was found to be 10,144.5 +/- 2, which was consistent with the molecular weight predicted for the extended N-acetylated form (residues 1-88) of Mr 10,146.1.

Soluble and membrane-bound forms of cytochrome b5 are the products of a single gene in chicken

In order to determine the relationship of the soluble cytochrome b5 found in erythrocytes to the membrane-bound form found in other tissues, a cDNA clone encoding cytochrome b5 in chicken erythrocytes was isolated by using mixed oligonucleotides based on a partial amino acid sequence of the protein. Complete nucleotide sequence identity between the erythrocyte cDNA and the sequence of a cDNA clone of the liver protein suggests that they are transcribed from the same gene. The isolation and structural analysis of genomic clones was also consistent with the presence of only one cytochrome b5 gene in chicken. These results suggest that the formation of soluble erythrocyte cytochrome b5 occurs by proteolytic processing of the membrane-bound form. Thus, previous reports indicating that the carboxyl terminal amino acid residue of the erythrocyte form differs from the corresponding residue of the membrane-bound form may suggest the existence of a novel post-translational modification.

Structural properties of apocytochrome b5: presence of a stable native core

Upon removal of the heme group, the water-soluble fragment of cytochrome b5 adopts a conformation less stable and compact than that of the holoprotein [Huntley, T. E., & Strittmatter, P. (1972) J. Biol. Chem. 247, 4641-4647]. This conformation, imposed by the amino acid sequence alone, has not been described in detail. One- and two-dimensional proton nuclear magnetic resonance spectroscopy techniques were applied to the apoprotein of the soluble fragment of rat liver cytochrome b5 in an effort to characterize the structure of the apoprotein. Nuclear Overhauser spectroscopy revealed a number of short interresidue distances and demonstrated that, in spite of the increased flexibility, at least one cluster of side chains exists on a time scale long enough for study. Several residues participating in the cluster, in particular the only Trp (Trp 22), were identified. Similarities with the spectrum of the reduced holoprotein were observed that led to the inspection of the cytochrome b5 crystal structure for assigning resonances. It appeared that the environment of this residue maintains its integrity in the apoprotein. Since in the holoprotein Trp 22 belongs to a hydrophobic core formed in part by beta-strands, it is proposed that some of this beta-structure is stable in the absence of the heme-protein interactions. Implications for structure and folding are discussed.

Structure of cytochrome b5 and its topology in the microsomal membrane

The complete amino acid sequence of human and chicken liver microsomal cytochrome b5 was determined. The amino termini of cytochrome b5 from four other mammalian species were examined in order to determine their complete covalent structure. As in the rat species, cytochrome b5 preparations from man, rabbit, calf and horse had an acetylated alanine as the first residue. In contrast, the pig cytochrome had alanine at the amino terminus. The amino terminus of the chicken cytochrome b5 was also unmodified, and extended three residues absent in the mammalian species. In order to investigate whether the carboxy-terminal segment of cytochrome b5 is located on the cytosolic or the luminal side of the microsomal membrane, rabbit liver microsomes were treated with trypsin and subjected to gel filtration and high-pressure liquid chromatography. The nonpolar peptide isolated from these microsomes lacked the terminal hexapeptide, indicating that when cytochrome b5 is bound to intact microsomes, the carboxy terminus is located on the cytosolic side of the membrane and does not extend in the lumen of the endoplasmic reticulum.

Rat hepatic microsomal cytochrome b5. A simple large-scale purification procedure and antibody production by antigen-containing liposomes

Cytochrome b5 from rat liver microsomes (microsomal fractions) was purified in its native form. The procedure described has great capacity, is fast, and the final product is pure as judged from SDS/polyacrylamide-gel electrophoresis. Antibodies to cytochrome b5 are obtained after administration of the antigen inserted into small unilamellar lipid vesicles.

The complete nucleotide sequence of human liver cytochrome b5 mRNA

We have isolated and sequenced a cDNA clone corresponding to human liver cytochrome b5 mRNA. The 760 base pair (bp) sequence contains the complete coding and 3' non-translated regions plus 52 bp of 5' non-translated sequence. The derived amino acid sequence showed that the previous assignment of several amino acids was in error. In addition, the sequence of the previously unknown COOH hydrophobic region has been obtained.

The primary structure of chicken liver cytochrome b5 deduced from the DNA sequence of a cDNA clone

A cDNA clone encoding the chicken liver cytochrome b5 was isolated by probing a library with synthetic oligonucleotides based on a partial amino acid sequence of the protein. Determination of the DNA sequence indicated a 414-nucleotide open reading frame which encodes a 138-amino acid residue polypeptide. The open reading frame contains 6 amino acids at the amino terminus which were not present on any of the cytochrome b5 polypeptides for which the amino acid sequence has been determined directly, suggesting that the protein is proteolytically processed to the mature form. The results of genomic Southern analysis were consistent with the presence of two structurally different genes in the chicken genome, raising the possibility that the soluble and membrane-bound forms of the protein are the products of separate genes.

Cloning of DNA fragments complementary to tobacco nitrate reductase mRNA and encoding epitopes common to the nitrate reductases from higher plants

Messenger RNAs encoding the nitrate reductase apoenzyme from tobacco can be translated in a cell-free system. Poly(A)+ mRNA fractions from the 23-32 S area of a sucrose gradient were used to build a cDNA library in the expression vector gt11 with an efficiency of cloning of approximately 10(4) recombinants/ng mRNA. Recombinant clones were screened with a rabbit polyclonal antibody directed against the corn nitrate reductase, which cross reacts specifically with the nitrate reductases from dicotyledons. Among 240000 recombinant plaques, eight clones were isolated containing inserts of sizes ranging from 1.6 kb to 2.1 kb and sharing sequence homologies. Seven of these clones contained a common internal 1.6 kb EcoRI fragment. The identity of these clones was confirmed as follows. A fusion protein of 170 kDa inducible by IPTG and recognized by the rabbit nitrate reductase antibody was expressed by a lysogen derived from one of the recombinants. The antibodies binding the fused protein were eluted and shown to be inhibitory to the catalytic activity of tobacco nitrate reductase. Two monoclonal antibodies directed against nitrate reductase were also able to bind the hybrid protein. The 1.6 kb EcoRI fragment was sequenced by the method of Sanger. The open reading frame corresponding to a translational fusion with the -galactosidase coding sequence of the vector shared strong homology at the amino acid level with the heme-binding domain of proteins of the cytochrome b5 superfamily and with human erythrocyte cytochrome b5 reductase. When the 1.6 kb EcoRI fragment was used as a probe for Northern blot experiments a signal corresponding to a 3.5 kb RNA was detected in tobacco and in Nicotiana plumbaginifolia mRNA preparations but no cross-hybridization with corn mRNAs was detected. The probe hybridized with low copy number sequences in genomic blots of tobacco DNA.

Synthesis, bacterial expression, and mutagenesis of the gene coding for mammalian cytochrome b5

We have totally synthesized a gene that codes for rat hepatic cytochrome b5. The 5' flanking region was designed for efficient expression of this gene in Escherichia coli by incorporating an optimum ribosome binding site and spacer region. Both a soluble form, analogous to the protease-treated microsomal protein, as well as the complete cytochrome with hydrophobic membrane anchor, was constructed and expressed. Transformants with the gene for the soluble protein overproduce authentic cytochrome b5 to a level of 8% of the total cell protein. The complete cytochrome is expressed to a lesser extent with most of the protein found in the cell membrane fraction. This represents complete synthesis and bacterial expression of a mammalian metalloprotein gene. Cytochrome b5 is normally a six-coordinate low spin heme protein with histidine-39 and histidine-63 as axial ligands. We have replaced histidine-63 with a methionine residue by cassette mutagenesis, utilizing specific restriction enzyme sites engineered into the synthetic gene. The resultant protein has histidine-39 as sole axial ligand and is five-coordinate high spin in the ferric resting state, as indicated by optical and electron spin resonance spectroscopy. The ability to generate mutant cytochrome b5 in high yield is a crucial step in understanding heme protein folding, protein-protein recognition and binding, and biological electron transfer processes.

Mussel glue from Mytilus californianus Conrad: a comparative study

Marine mussels secrete a byssus in order to attach to solid surfaces in the sea. The polyphenolic protein is the "glue" in the adhesive plaques of the byssus. In Mytilus californianus, the polyphenolic protein has an apparent molecular weight of 85,000 +/- 5,000 and is rich in the amino acids lysine, 3,4-dihydroxyphenylalanine, serine, threonine, and hydroxyproline. In composition it resembles the polyphenolic protein of M. edulis (Mr = 125,000), although the M. edulis protein contains significantly less isoleucine and more alanine. Tryptic digestion of M. californianus polyphenolic protein revealed two types of repeating decapeptides (1) (Ser/Thr)-Thr-(Tyr/Dopa)-Hyp-Hyp-Thr-Dopa-Lys-Hyp-Lys and (2) Ile-(Thr/Ser)-(Tyr/Dopa)-Hyp-Hyp-Thr-Dopa-Lys-Hyp-Lys. Residues 2 to 8 are identical with residues 4-10 in M. edulis decapeptides.

Hamster hepatic cytochrome b5: purifications, immunochemical properties, and in vitro synthesis

Cytochrome b5 has been purified from hamster liver microsomes. Both Ouchterlony double-diffusion and rocket immunoelectrophoresis experiments indicate that no immuno-cross-reactivity exists between guinea-pig anti-rabbit cytochrome b5 antibody and hamster cytochrome b5. However, anti-rabbit b5 IgG inhibited both hamster microsomal NADH-cytochrome c reductase and NADPH-dependent 7-ethoxycoumarin-O-deethylase activities. Hamster cytochrome b5 stimulated several reconstituted hamster cytochrome P-450-dependent monooxygenase activities and this stimulatory effect could be inhibited by antibody against rabbit cytochrome b5. Two-dimensional iodinated tryptic peptide mapping experiments provided evidence that the polypeptide fingerprint of hamster cytochrome b5 is substantially different from the fingerprints of cytochrome b5 isolated from rabbit, rat and bovine. We also studied the in vitro synthesis of hamster cytochrome b5 from liver mRNA using a wheat germ lysate system. A 16 kDa polypeptide, which is the same size as hamster cytochrome b5, was immunoprecipitated by antibody against rabbit b5. This experiment suggested that in vitro synthesized hamster cytochrome b5 is recognized by a heterologous antibody. Thus, hamster and rabbit cytochrome b5 do share some common immuno-determinants which may be located close to the heme-binding active site.

High-performance liquid chromatographic separation of glycopeptides from Nereis cuticle collagen

To facilitate the structural studies of invertebrate collagens, a sensitive and effective method was developed, using reverse-phase high-performance liquid chromatography for preparative isolation of the collagen subunits and their clostridial collagenase-derived peptides; the methods have been applied to Nereis cuticle collagen. The two subunits of denatured Nereis cuticle collagen, termed A and B, were initially separated by high-performance liquid chromatography. These polypeptides, with Mr of about 0.5 million, were each exhaustively digested with clostridial collagenase. The digest of the A subunit, which contains all of the uronic acid, was enriched for the uronic acid-containing glycopeptides by means of gel filtration. These glycopeptides were resolved into 23 major peaks, using reverse-phase HPLC, over a 5-h elution time, with an acetonitrile gradient (0-20%) containing 0.1% TFA. The amino acid composition data suggests that the peptides are of variable length, from 5 to 17 residues, while beta-elimination studies show that the uronic acid-containing moieties are all O-glycosidically linked to threonine residues, in the peptides examined. The amino acid sequence of one of the major glycopeptides was determined and found to be Gly-Hyp-Ala-Gly-Gly-Ile-Gly-Glu-Thr-Gly-Ala-Val-Gly-Leu-Hyp. The amino acid compositions of glycosylated and nonglycosylated peptides which had eluted, numbering about 100, showed a correspondence between hydrophobicity or hydrophilicity and emergence time from the column. We also found that the peptides most enriched in 4-hydroxyproline emerged earliest. These studies provide a foundation for elucidating the detailed structures of the large, unusual subunits of a well-characterized cuticle collagen.

Differences in C-terminal amino acid sequences between erythrocyte and liver cytochrome b5 isolated from pig and human. Evidence for two tissue-specific forms of cytochrome b5

Two forms of cytochrome b5, a soluble erythrocyte form and a membrane-bound liver form, were purified from pig and human, and structural differences between them were analyzed. Porcine and human erythrocyte cytochrome b5 consisted of 97 amino acid residues and contained the same catalytic domain structure (residues 1-96) as that of the corresponding liver cytochrome b5, but had one amino acid replacement at the C-terminus (residue 97). These results suggest that erythrocyte cytochrome b5 is not derived from the liver protein by proteolysis but a translational product from another distinct mRNA of cytochrome b5.

Purification and structural studies of rabbit erythrocyte cytochrome b5

A single form of cytochrome b5 has been isolated in highly purified form from the cytosolic fraction of rabbit erythrocytes by sequential chromatography on DE-52 cellulose, Sephadex G-75, and DEAE-Sephadex A50. The cytochrome is structurally similar to the N-terminal, heme-binding domain of rabbit liver microsomal cytochrome b5. Like the liver protein, it is blocked at the amino terminus. Its amino acid composition is similar to that of residues 1-97 of the microsomal protein. With one exception, tryptic peptides derived from apo-cytochrome b5 of rabbit erythrocytes co-elute with the tryptic peptides obtained from a soluble hemepeptide fragment of microsomal cytochrome b5. These findings, together with amino acid sequence analysis of the carboxyl terminal tryptic peptides, identify the erythrocyte cytochrome b5 as a 97-residue peptide.

Two homologous cytochromes b5 in a single cell

The amino acid sequence of the heme-binding domains of rat liver cytochromes b5 from outer mitochondrial membranes and from microsomes has been determined by a combination of automatic and manual degradation of fragments generated by trypsin digestion and by cleavage at tryptophan. Tryptic peptides were separated by high-pressure liquid chromatography. The sequence of microsomal cytochrome b5 is identical with the one published by Ozols and Heinemann after completion of this study [Biochim. Biophys. Acta (1982) 704, 163-173]. The sequence of outer membrane cytochrome b5 differs from the microsomal one at 38 positions out of 91. There are 40 positions invariant between this sequence and the eight microsomal sequences published thus far. The non-conservative substitutions are located at the surface of the known three-dimensional structure of calf microsomal cytochrome b5 except for the substitution of histidine-15 by arginine. This paper brings the final proof that two iso-cytochromes b5 exist in the same cell. Their high degree of similarity as well as their differential cellular localization raise some questions which are briefly discussed.