Expression of cytochrome b5 in yeast and characterization of mutants of the membrane-anchoring domain

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Cytochrome b5 modulates multiple reactions in steroidogenesis by diverse mechanisms

Cytochrome b5 (cyt-b5) is a relatively small haemoprotein which plays an important role in the regulation of mammalian steroidogenesis. This unique protein has the ability to modulate the activity of key steroidogenic enzymes via a number of diverse reaction mechanisms. Cyt-b5 can augment the 17,20-lyase activity of CYP17A1 by promoting the interaction of CYP17A1 and POR; enhance the 16-ene-synthase activity of CYP17A1 by acting as an electron donor; and enhance the activity of 3βHSD by increasing the affinity of 3βHSD for its cofactor NAD(+). We review the modulation of CYP17A1 and 3βHSD activity by cyt-b5 and discuss the reaction mechanisms associated with each activity. The physiological importance of cyt-b5 in regulating mammalian steroidogenesis is presented and the impact of inactivating cyt-b5 mutations are reviewed. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

A model of the membrane-bound cytochrome b5-cytochrome P450 complex from NMR and mutagenesis data

Microsomal cytochrome b5 (cytb5) is a membrane-bound protein that modulates the catalytic activity of its redox partner, cytochrome P4502B4 (cytP450). Here, we report the first structure of full-length rabbit ferric microsomal cytb5 (16 kDa), incorporated in two different membrane mimetics (detergent micelles and lipid bicelles). Differential line broadening of the cytb5 NMR resonances and site-directed mutagenesis data were used to characterize the cytb5 interaction epitope recognized by ferric microsomal cytP450 (56 kDa). Subsequently, a data-driven docking algorithm, HADDOCK (high ambiguity driven biomolecular docking), was used to generate the structure of the complex between cytP4502B4 and cytb5 using experimentally derived restraints from NMR, mutagenesis, and the double mutant cycle data obtained on the full-length proteins. Our docking and experimental results point to the formation of a dynamic electron transfer complex between the acidic convex surface of cytb5 and the concave basic proximal surface of cytP4502B4. The majority of the binding energy for the complex is provided by interactions between residues on the C-helix and β-bulge of cytP450 and residues at the end of helix α4 of cytb5. The structure of the complex allows us to propose an interprotein electron transfer pathway involving the highly conserved Arg-125 on cytP450 serving as a salt bridge between the heme propionates of cytP450 and cytb5. We have also shown that the addition of a substrate to cytP450 likely strengthens the cytb5-cytP450 interaction. This study paves the way to obtaining valuable structural, functional, and dynamic information on membrane-bound complexes.

Cytochrome b5: novel roles in steroidogenesis

Cytochrome b(5) (cyt-b(5)) is essential for the regulation of steroidogenesis and as such has been implicated in a number of clinical conditions. It is well documented that this small hemoprotein augments the 17,20-lyase activity of cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). Studies have revealed that this augmentation is accomplished by cyt-b(5) enhancing the interaction between cytochrome P450 reductase (POR) and CYP17A1. In this paper we present evidence that cyt-b(5) induces a conformational change in CYP17A1, in addition to facilitating the interaction between CYP17A1 and POR. We also review the recently published finding that cyt-b(5) allosterically augments the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD), a non cytochrome P450 enzyme, by increasing the enzymes affinity for its cofactor, NAD(+). The physiological importance of this finding, in terms of understanding adrenal androstenedione production, is examined. Finally, evidence that cyt-b(5) is able to form homomeric complexes in living cells is presented and discussed.

Substrate-modulated cytochrome P450 17A1 and cytochrome b5 interactions revealed by NMR

The membrane heme protein cytochrome b5 (b5) can enhance, inhibit, or have no effect on cytochrome P450 (P450) catalysis, depending on the specific P450, substrate, and reaction conditions, but the structural basis remains unclear. Here the interactions between the soluble domain of microsomal b5 and the catalytic domain of the bifunctional steroidogenic cytochrome P450 17A1 (CYP17A1) were investigated. CYP17A1 performs both steroid hydroxylation, which is unaffected by b5, and an androgen-forming lyase reaction that is facilitated 10-fold by b5. NMR chemical shift mapping of b5 titrations with CYP17A1 indicates that the interaction occurs in an intermediate exchange regime and identifies charged surface residues involved in the protein/protein interface. The role of these residues is confirmed by disruption of the complex upon mutagenesis of either the anionic b5 residues (Glu-48 or Glu-49) or the corresponding cationic CYP17A1 residues (Arg-347, Arg-358, or Arg-449). Cytochrome b5 binding to CYP17A1 is also mutually exclusive with binding of NADPH-cytochrome P450 reductase. To probe the differential effects of b5 on the two CYP17A1-mediated reactions and, thus, communication between the superficial b5 binding site and the buried CYP17A1 active site, CYP17A1/b5 complex formation was characterized with either hydroxylase or lyase substrates bound to CYP17A1. Significantly, the CYP17A1/b5 interaction is stronger when the hydroxylase substrate pregnenolone is present in the CYP17A1 active site than when the lyase substrate 17α-hydroxypregnenolone is in the active site. These findings form the basis for a clearer understanding of this important interaction by directly measuring the reversible binding of the two proteins, providing evidence of communication between the CYP17A1 active site and the superficial proximal b5 binding site.

Cytochrome b(5) forms homomeric complexes in living cells

Cytochrome b(5) (cyt-b(5)) is a ubiquitous hemoprotein also associated with microsomal cytochrome P450 17α-hydroxylase/17,20 lyase (CYP17A1). In the steroidogenic pathway CYP17A1 catalyses the metabolism of pregnenolone, yielding both glucocorticoid and androgen precursors. While not affecting the 17α-hydroxylation of pregnenolone, cyt-b(5) augments the 17,20 lyase reaction of 17-hydroxypregnenolone, catalyzing the formation of DHEA, through direct protein-protein interactions. In this study, multimeric complex formation of cyt-b(5) and the possible regulatory role of these complexes were investigated. Cyt-b(5) was isolated from ovine liver and used to raise anti-sheep cyt-b(5) immunoglobulins. Immunochemical studies revealed that, in vivo, cyt-b(5) is primarily found in the tetrameric form. Subsequent fluorescent resonance energy transfer (FRET) studies in COS-1 cells confirmed the formation of homomeric complexes by cyt-b(5) in live cells. Site-directed mutagenesis revealed that the C-terminal linker domain of cyt-b(5) is vital for complex formation. The 17,20-lyase activity of CYP17 was augmented by truncated cyt-b(5), which is unable to form complexes when co-expressed in COS-1 cells, thereby implicating the monomeric form of cyt-b(5) as the active species. This study has shown for the first time that cyt-b(5) forms homomeric complexes in vivo, implicating complex formation as a possible regulatory mechanism in steroidogenesis.

Interaction of modified tail-anchored proteins with liposomes: effect of extensions of hydrophilic segment at the COOH-terminus of holo-cytochromes b₅

A group of membrane proteins having a single COOH-terminal hydrophobic domain capable of post-translational insertion into lipid bilayer is known as tail-anchored (TA) proteins. To clarify the insertion mechanism of the TA-domain of human cytochrome b(5) (Hcytb5) into ER membranes, we produced and purified various membrane-bound forms of Hcytb5 with their heme b-bound, in which various truncated forms of NH(2)-terminal bovine opsin sequence were appended at the COOH-terminus of the native form. We analyzed the integration of the TA-domains of these forms onto protein-free liposomes. The integration occurred efficiently even in the presence of a small amount of sodium cholate and, once incorporated, such proteoliposomes were very stable. The mode of the integration was further analyzed by treatment of the proteoliposomes with trypsin either on the extravesicular side or on the luminal side. LC-MS analyses of the trypsin digests obtained from the proteoliposomes indicated that most of the C-terminal hydrophilic segment of the native Hcytb5 were exposed towards the lumen of the vesicles and, further, a significant part of the population of the extended C-terminal hydrophilic segments of the modified Hcytb5 were exposed in the lumen as well, suggesting efficient translocation ability of the TA-domain without any assistance from other protein factors. Present results opened a route for the use of the C-terminal TA-domain as a convenient tool for the transport of proteins as well as short peptides into artificial liposomes.

Biochemical and spectroscopic characterization of the human mitochondrial amidoxime reducing components hmARC-1 and hmARC-2 suggests the existence of a new molybdenum enzyme family in eukaryotes

The mitochondrial amidoxime reducing component mARC is a newly discovered molybdenum enzyme that is presumed to form the catalytical part of a three-component enzyme system, consisting of mARC, heme/cytochrome b(5), and NADH/FAD-dependent cytochrome b(5) reductase. mARC proteins share a significant degree of homology to the molybdenum cofactor-binding domain of eukaryotic molybdenum cofactor sulfurase proteins, the latter catalyzing the post-translational activation of aldehyde oxidase and xanthine oxidoreductase. The human genome harbors two mARC genes, referred to as hmARC-1/MOSC-1 and hmARC-2/MOSC-2, which are organized in a tandem arrangement on chromosome 1. Recombinant expression of hmARC-1 and hmARC-2 proteins in Escherichia coli reveals that both proteins are monomeric in their active forms, which is in contrast to all other eukaryotic molybdenum enzymes that act as homo- or heterodimers. Both hmARC-1 and hmARC-2 catalyze the N-reduction of a variety of N-hydroxylated substrates such as N-hydroxy-cytosine, albeit with different specificities. Reconstitution of active molybdenum cofactor onto recombinant hmARC-1 and hmARC-2 proteins in the absence of sulfur indicates that mARC proteins do not belong to the xanthine oxidase family of molybdenum enzymes. Moreover, they also appear to be different from the sulfite oxidase family, because no cysteine residue could be identified as a putative ligand of the molybdenum atom. This suggests that the hmARC proteins and sulfurase represent members of a new family of molybdenum enzymes.

The interaction of microsomal cytochrome P450 2B4 with its redox partners, cytochrome P450 reductase and cytochrome b(5)

Cytochrome P450 2B4 is a microsomal protein with a multi-step reaction cycle similar to that observed in the majority of other cytochromes P450. The cytochrome P450 2B4-substrate complex is reduced from the ferric to the ferrous form by cytochrome P450 reductase. After binding oxygen, the oxyferrous protein accepts a second electron which is provided by either cytochrome P450 reductase or cytochrome b(5). In both instances, product formation occurs. When the second electron is donated by cytochrome b(5), catalysis (product formation) is ∼10- to 100-fold faster than in the presence of cytochrome P450 reductase. This allows less time for side product formation (hydrogen peroxide and superoxide) and improves by ∼15% the coupling of NADPH consumption to product formation. Cytochrome b(5) has also been shown to compete with cytochrome P450 reductase for a binding site on the proximal surface of cytochrome P450 2B4. These two different effects of cytochrome b(5) on cytochrome P450 2B4 reactivity can explain how cytochrome b(5) is able to stimulate, inhibit, or have no effect on cytochrome P450 2B4 activity. At low molar ratios (<1) of cytochrome b(5) to cytochrome P450 reductase, the more rapid catalysis results in enhanced substrate metabolism. In contrast, at high molar ratios (>1) of cytochrome b(5) to cytochrome P450 reductase, cytochrome b(5) inhibits activity by binding to the proximal surface of cytochrome P450 and preventing the reductase from reducing ferric cytochrome P450 to the ferrous protein, thereby aborting the catalytic reaction cycle. When the stimulatory and inhibitory effects of cytochrome b(5) are equal, it will appear to have no effect on the enzymatic activity. It is hypothesized that cytochrome b(5) stimulates catalysis by causing a conformational change in the active site, which allows the active oxidizing oxyferryl species of cytochrome P450 to be formed more rapidly than in the presence of reductase.

Preparation of a biologically active apo-cytochrome b5 via heterologous expression in Escherichia coli

Cytochrome b(5) (b(5)) has been shown to modulate many cytochrome P450 (CYP)-dependent reactions. In order to elucidate the mechanism of such modulations, it is necessary to evaluate not only the effect of native b(5) on CYP-catalyzed reactions, but also that of the apo-cytochrome b(5) (apo-b(5)). Therefore, the apo-b(5) protein was prepared using a heterologous expression in Escherichia coli. The gene for rabbit b(5) was constructed from synthetic oligonucleotides using polymerase chain reaction (PCR), cloned into pUC19 plasmid and amplified in DH5 alpha cells. The gene sequence was verified by DNA sequencing. The sequence coding b(5) was cleaved from pUC19 by NdeI and XhoI restriction endonucleases and subcloned to the expression vector pET22b. This vector was used to transform E. coli BL-21 (DE3) Gold cells by heat shock. Expression of b(5) was induced with isopropyl beta-D-1-thiogalactopyranoside (IPTG). The b(5) protein, produced predominantly in its apo-form, was purified from isolated membranes of E. coli cells by chromatography on a column of DEAE-Sepharose. Using such procedures, the homogenous preparation of apo-b(5) protein was obtained. Oxidized and reduced forms of the apo-b(5) reconstituted with heme exhibit the same absorbance spectra as native b(5). The prepared recombinant apo-b(5) reconstituted with heme can be reduced by NADPH:CYP reductase. The reconstituted apo-b(5) is also fully biologically active, exhibiting the comparable stimulation effect on the CYP3A4 enzymatic activity towards oxidation of 1-phenylazo-2-hydroxynaphthalene (Sudan I) as native rabbit and human b(5).

Inter-species complementation of the translocon beta subunit requires only its transmembrane domain

In eukaryotes, proteins enter the secretory pathway through the translocon pore of the endoplasmic reticulum. This protein translocation channel is composed of three major subunits, called Sec61alpha, beta and gamma in mammals. Unlike the other subunits, the beta subunit is dispensable for translocation and cell viability in all organisms studied. Intriguingly, the knockout of the Sec61beta encoding genes results in different phenotypes in different species. Nevertheless, the beta subunit shows a high level of sequence homology across species, suggesting the conservation of a biological function that remains ill-defined. To address its cellular roles, we characterized the homolog of Sec61beta in the fission yeast Schizosaccharomyces pombe (Sbh1p). Here, we show that the knockout of sbh1(+) results in severe cold sensitivity, increased sensitivity to cell-wall stress, and reduced protein secretion at 23 degrees C. Sec61beta homologs from Saccharomyces cerevisiae and human complement the knockout of sbh1(+) in S. pombe. As in S. cerevisiae, the transmembrane domain (TMD) of S. pombe Sec61beta is sufficient to complement the phenotypes resulting from the knockout of the entire encoding gene. Remarkably, the TMD of Sec61beta from S. cerevisiae and human also complement the gene knockouts in both yeasts. Together, these observations indicate that the TMD of Sec61beta exerts a cellular function that is conserved across species.

Analysis of the interactions of cytochrome b5 with flavocytochrome P450 BM3 and its domains

Interactions between a soluble form of microsomal cytochrome b(5) (b(5)) from Musca domestica (housefly) and Bacillus megaterium flavocytochrome P450 BM3 and its component reductase (CPR), heme (P450) and FAD/NADPH-binding (FAD) domains were analyzed by a combination of steady-state and stopped-flow kinetics methods, and optical spectroscopy techniques. The high affinity binding of b(5) to P450 BM3 induced a low-spin to high-spin transition in the P450 heme iron (K(d) for b(5) binding = 0.44 microM and 0.72 microM for the heme domain and intact flavocytochrome, respectively). The b(5) had modest inhibitory effects on steady-state turnover of P450 BM3 with fatty acids, and the ferrous-carbon monoxy P450 complex was substantially stabilized on binding b(5). Single turnover reduction of b(5) by BM3 using stopped-flow absorption spectroscopy (k(lim) = 116 s(-1)) was substantially faster than steady-state reduction of b(5) by P450 BM3 (or its CPR and FAD domains), indicating rate-limiting step(s) other than BM3 flavin-to-b(5) heme electron transfer in the steady-state reaction. Steady-state b(5) reduction by P450 BM3 was considerably accelerated at high ionic strength. Pre-reduction of P450 BM3 by NADPH decreased the k(lim) for b(5) reduction approximately 10-fold, and also resulted in a lag phase in steady-state b(5) reduction that was likely due to BM3 conformational perturbations sensitive to the reduction state of the flavocytochrome. Ferrous b(5) could not reduce the ferric P450 BM3 heme domain under anaerobic conditions, consistent with heme iron reduction potentials of the two proteins. However, rapid oxidation of both hemoproteins occurred on aeration of the ferrous protein mixture (and despite the much slower autoxidation rate of b(5) in isolation), consistent with electron transfer occurring from b(5) to the oxyferrous P450 BM3 in the complex. The results demonstrate that strong interactions occur between a eukaryotic b(5) and a model prokaryotic P450. Binding of b(5) perturbs BM3 heme iron spin-state equilibrium, as is seen in many physiologically relevant b(5) interactions with eukaryotic P450s. These results are consistent with the conservation of structure of P450s (particularly at the heme proximal face) between prokaryotes and eukaryotes, and may point to as yet undiscovered roles for b(5)-like proteins in the control of activities of certain prokaryotic P450s.

Molecular Characterization of Specifically Active Recombinant Fused Enzymes Consisting of CYP3A4, NADPH-Cytochrome P450 Oxidoreductase, and Cytochrome b5

Microsomal cytochrome P450 3A4 (CYP3A4) catalyzes monooxygenase reactions toward a diverse group of exogenous and endogenous substrates and requires cytochrome b5 (b5) in the oxidation of the typical substrate testosterone. To analyze the molecular interaction among CYP3A4, NADPH-cytochrome P450 oxidoreductase (P450 reductase), and b5, we constructed several fused enzyme genes and expressed them in Saccharomyces cerevisiae. The recombinant fused enzymes CYP3A4-truncated (t)-P450 reductase-t-b5 (3RB) and CYP3A4-t-b5-t-P450 reductase (3BR) in yeast microsomes showed a higher specific activity in 6beta-hydroxylation of testosterone than did the reconstitution premixes of CYP3A4, P450 reductase, and b5. The purified fused enzymes exhibited lower Km values and substantially increased Vmax values in 6beta-hydroxylation of testosterone and oxidation of nifedipine. Moreover, the fused enzymes showed significantly higher activities in cytochrome c reduction than the reconstitution premixes. Although the affinity of 3RB toward cytochrome c was twice as high as that of 3BR, 3BR and 3RB showed nearly the same affinity toward NADPH/NADH. In addition, the heme of the CYP3A4 moiety of 3RB was reduced preferentially and more rapidly than that of 3BR, whereas the heme of the b5 moiety of 3BR was selectively reduced compared with that of 3RB. These results suggest that the conformation of the 3RB molecule was the most suitable for high activity because of appropriate ordering of the CYP3A4, P450 reductase, and b5 moieties for efficient electron flow. Thus, we believe that the b5 moiety plays an important role in the efficient transfer of the second electron in the vicinity of the CYP3A4 moiety.

Analysis of amino acid residues involved in catalysis of polyethylene glycol dehydrogenase from Sphingopyxis terrae, using three-dimensional molecular modeling-based kinetic characterization of mutants

Polyethylene glycol dehydrogenase (PEGDH) from Sphingopyxis terrae (formerly Sphingomonas terrae) is composed of 535 amino acid residues and one flavin adenine dinucleotide per monomer protein in a homodimeric structure. Its amino acid sequence shows 28.5 to 30.5% identity with glucose oxidases from Aspergillus niger and Penicillium amagasakiense. The ADP-binding site and the signature 1 and 2 consensus sequences of glucose-methanol-choline oxidoreductases are present in PEGDH. Based on three-dimensional molecular modeling and kinetic characterization of wild-type PEGDH and mutant PEGDHs constructed by site-directed mutagenesis, residues potentially involved in catalysis and substrate binding were found in the vicinity of the flavin ring. The catalytically important active sites were assigned to His-467 and Asn-511. One disulfide bridge between Cys-379 and Cys-382 existed in PEGDH and seemed to play roles in both substrate binding and electron mediation. The Cys-297 mutant showed decreased activity, suggesting the residue's importance in both substrate binding and electron mediation, as well as Cys-379 and Cys-382. PEGDH also contains a motif of a ubiquinone-binding site, and coenzyme Q10 was utilized as an electron acceptor. Thus, we propose several important amino acid residues involved in the electron transfer pathway from the substrate to ubiquinone.

Expression of outer mitochondrial membrane cytochrome b 5 in Escherichia coli. Purification of the recombinant protein and studies of its interaction with electron-transfer partners

In the present work, we report expression in Escherichia coli, purification, and characterization of recombinant full-length cytochrome b(5) from outer mitochondrial membrane. Optimization of expression conditions for cytochrome b(5) from outer mitochondrial membrane allowed reaching expression level up to 10(4) nmol of the hemeprotein per liter of culture. Recombinant cytochrome b(5) from outer mitochondrial membrane was purified from cell lysate by using metal-affinity chromatography. It has physicochemical, spectral, and immunochemical properties similar to those of cytochrome b(5) from rat liver outer mitochondrial membrane. Immobilized recombinant mitochondrial cytochrome b(5) was used as affinity ligand to study its interaction with electron transfer proteins. By using this approach, it is shown that in interaction of NADPH:cytochrome P450 reductase with both forms of cytochrome b(5) an important role is played by hydrophobic interactions between proteins, although the contribution of these interactions in complex formation with NADPH:cytochrome P450 reductase is different for isoforms of cytochrome b(5).

A cytosolic cytochrome b5-like protein in yeast cell accelerating the electron transfer from NADPH to cytochrome c catalyzed by Old Yellow Enzyme

A 410-nm absorbing species which enhanced the reduction rate of cytochrome c by Old Yellow Enzyme (OYE) with NADPH was found in Saccharomyces cerevisiae. It was solubilized together with OYE by the treatment of yeast cells with 10% ethyl acetate. The purified species showed visible absorption spectra in both oxidized and reduced forms, which were the same as those of the yeast microsomal cytochrome b5. At least 14 amino acid residues of the N-terminal region coincided with those of yeast microsomal b5, but the protein had a lower molecular weight determined to be 12,600 by SDS-PAGE and 9775 by mass spectrometry. The cytochrome b5-like protein enhanced the reduction rate of cytochrome c by OYE, and a plot of the reduction rates against its concentration showed a sigmoidal curve with an inflexion point at 6x10(-8) M of the protein.

Bipartite signals mediate subcellular targeting of tail-anchored membrane proteins in Saccharomyces cerevisiae

Tail-anchored proteins have an NH(2)-terminal cytosolic domain anchored to intracellular membranes by a single, COOH-terminal, transmembrane segment. Sequence analysis identified 55 tail-anchored proteins in Saccharomyces cerevisiae, with several novel proteins, including Prm3, which we find is required for karyogamy and is tail-anchored in the nuclear envelope. A total of six tail-anchored proteins are present in the mitochondrial outer membrane and have relatively hydrophilic transmembrane segments that serve as targeting signals. The rest, by far the majority, localize via a bipartite system of signals: uniformly hydrophobic tail anchors are first inserted into the endoplasmic reticulum, and additional segments within the cytosolic domain of each protein can dictate subsequent sorting to a precise destination within the cell.

Bcl-2 and porin follow different pathways of TOM-dependent insertion into the mitochondrial outer membrane

The bcl-2 gene encodes a 26kDa protein which functions as a central regulator of apoptosis. Here we investigated the pathway of Bcl-2alpha into the mitochondrial outer membrane using the yeast Saccharomyces cerevisiae as a model organism. We found that interactions of Bcl-2alpha with the mitochondrial import receptor Tom20 are dependent on two positively charged lysine residues in the immediate vicinity of the carboxy-terminal hydrophobic membrane anchor. The targeting function of these residues is independent of Tom22. Subsequent insertion of Bcl-2alpha into the mitochondrial outer membrane does not require Tom5 or Tom40, indicating that Bcl-2alpha bypasses the general import pore (GIP). Bcl-2alpha shows a unique pattern of interactions with the components of the mitochondrial TOM complex, demonstrating that at least two different pathways lead from the import receptor Tom20 into the mitochondrial outer membrane.

Expression, purification, and physical properties of recombinant flavocytochrome fusion proteins containing rat cytochrome b(5) linked to NADPH-cytochrome P450 reductase by different membrane-binding segments

Reconstitution of the enzymatic activities using purified microsomal cytochrome P450s (P450) requires the presence of a membrane-binding segment in the mammalian flavoprotein, NADPH--cytochrome P450 reductase (CPR), and the hemeprotein, cytochrome b(5) (b(5)). The mechanism(s) by which the membrane-binding segments of these proteins exert such a critical role in influencing the reconstitution of the NADPH-supported activity of a P450 remains undefined. In the present work we describe the construction, expression, and purification of four different types of recombinant flavocytochromes containing rat b(5) and rat CPR linked by various membrane-binding segments. The physical properties of these artificial fusion proteins have been studied to determine their ability to serve as electron transfer agents. These studies are a prelude to the subsequent study (accompanying paper) evaluating the functional roles of the hydrophobic (membrane-binding) sequences of b(5) and CPR in the reconstitution of P450 activities. The present study shows that the purified recombinant fusion proteins can serve as active electron transport carriers from NADPH to cytochrome c as well as b(5) by intramolecular as well as intermolecular reactions. It is shown here that the electron transport properties of these purified fusion proteins are influenced by high concentrations of KCl, suggesting a role for charged amino acids in protein-protein interactions. The present study illustrates the application of artificial recombinant flavocytochromes as useful proteins for the study of intramolecular electron transport reactions for comparison with intermolecular interactions.

Reconstitution of the enzymatic activities of cytochrome P450s using recombinant flavocytochromes containing rat cytochrome b(5) fused to NADPH--cytochrome P450 reductase with various membrane-binding segments

The role of the hydrophobic membrane-binding segments of NADPH-cytochrome P450 reductase (CPR) and cytochrome b(5) remain undefined. We have expressed four different recombinant flavocytochromes containing b(5) linked to CPR with different hydrophobic segments as linkers. These fusion proteins have been expressed in Escherichia coli and purified and some of their physical properties and electron transfer activities described in the accompanying paper. Of interest is the presence of internal "membrane-binding" hydrophobic segments in these flavocytochromes. This paper describes the ability of these flavocytochromes to reconstitute in vitro two P450 activities that have been reported to be stimulated by the addition of b(5) (the 17,20-lyase activity of CYP17A and the 6 beta hydroxylation of testosterone catalyzed by CYP3A4) and two P450 reactions that do not respond to the presence of b(5) (the 17 alpha-hydroxylation of progesterone catalyzed by CYP17A and the omega hydroxylation of lauric acid catalyzed by CYP4A1). The present study shows that a hydrophobic "membrane-binding" segment must be present in the artificial flavocytochromes in order to successfully reconstitute in vitro hydroxylation activities with P450s. Differences in the effectiveness of the different flavocytochromes to reconstitute enzymatic activities depends on the P450 tested and the nature of the hydrophobic linker segment present in the purified recombinant flavocytochromes. The hypothesis is proposed that differences in the surface topology of a P450 may dictate differences in their docking with the CPR or b(5) component of a fusion protein, resulting in differences in the rates of electron transfer to the P450.

Interaction of apo-cytochrome b5 with cytochromes P4503A4 and P45017A: relevance of heme transfer reactions

Maximal activity of CYP3A4 is obtained using a reconstitution system consisting of NADPH-P450 reductase (CPR), dioleoylphosphatidylcholine (DOPC), an ionic detergent, and cytochrome b(5) (b(5)). The mechanism by which b(5) stimulates the catalytic activity of CYP3A4 is controversial. Recent data report that apo-cytochrome b(5) (apo-b(5)) can substitute for holo-b(5) by serving as an allosteric effector. These authors concluded that b(5) is not directly involved in electron transfer reactions to CYP3A4. We have studied the effect of apo-b(5) on the ability of purified CYP3A4 to catalyze the 6beta-hydroxylation of testosterone and horse CYP17A to catalyze the 17,20-lyase reaction. The high molecular weight form of holo-b(5) (HMW-holo-b(5)) stimulates the 6beta-hydroxylation of testosterone while the low molecular weight (truncated) form of holo-b(5) (LMW-holo-b(5)) does not. When added to the reconstituted system, HMW-apo-b(5) stimulates the activity of CYP3A4 to a level 50-60% of that obtained with HMW-holo-b(5). A similar stimulation of 17alpha-hydroxyprogesterone metabolism is seen when studying the CYP17A-catalyzed reaction. Neither LMW-holo-b(5) nor LMW-apo-b(5) stimulates the activity of CYP3A4 or CYP17A. CYP3A4 forms a complex during affinity chromatography with immobilized HMW-holo-b(5) but not with immobilized HMW-apo-b(5). Incubation of apo-b(5) with CYP3A4, using conditions required for reconstitution of enzymatic activities, results in the transfer of heme from the CYP3A4 preparation to apo-b(5), thereby forming holo-b(5). The separation of heme proteins by thiol-disulfide exchange chromatography confirms the formation of holo-b(5). A His67Ala mutant of HMW-b(5) as well as the Zn-substituted protoporphyrin derivative of HMW-b(5) do not stimulate the activity of either CYP3A4 or CYP17A. These data show that the mechanism of stimulation of CYP3A4 and CYP17A activities by apo-b(5) results from the formation of holo-b(5) by a heme transfer reaction.

Targeting of C-terminal (tail)-anchored proteins: understanding how cytoplasmic activities are anchored to intracellular membranes

A class of integral membrane proteins, referred to as 'tail-anchored proteins', are inserted into phospholipid bilayers via a single segment of hydrophobic amino acids at the C-terminus, thereby displaying a large functional domain in the cytosol. This membrane attachment strategy allows eukaryotic cells to position a wide range of cytoplasmic activities close to the surface of an intracellular membrane. Tail-anchored proteins often, but not always, demonstrate a selective distribution to specific intracellular organelles. This membrane-specific distribution is required for the large number of targeting proteins that are tail-anchored, but may or may not be critical for the numerous tail-anchored pro-apoptotic and anti-apoptotic proteins of the Bcl-2 family. Recent work has begun to address the mechanism for targeting tail-anchored proteins to their resident membranes, but questions remain. What targeting signals determine each protein's intracellular location? Are there receptors for these signals and, if so, how do they function? What steps are required to integrate tail-anchored proteins into the phospholipid bilayers? In this Traffic interchange, we summarise what is known about tail-anchored proteins, and outline the areas that are currently under study.

High-level expression in Escherichia coli and purification of the membrane-bound form of cytochrome b(5)

Expression of the membrane-bound form of rabbit cytochrome b(5) in Escherichia coli has been significantly improved through the use of the T7 expression vector pLW01 (A. Bridges, L. Gruenke, Y.-T. Chang, I. Vakser, G. Loew, and L. Waskell, 1998, J. Biol. Chem. 273, 17036-17049) in conjunction with strain C41(DE3) (B. Miroux and J. Walker, 1996, J. Mol. Biol. 260, 289-298). Cell cultures expressing the cytochrome b(5) contained an average of 820 mg/liter of culture and reached peak levels as high as 1100 mg/liter when higher antibiotic concentrations were used. Maximal levels were obtained from cultures when expression was induced with 10 microM IPTG. Approximately 90% of the cytochrome b(5) was expressed as apoprotein which was reconstituted by addition of exogenous heme. The cytochrome b(5) was purified from detergent-solubilized bacterial membranes using anion-exchange chromatography on DEAE-Sepharose followed by size-exclusion chromatography on Superdex-75. Purification of cytochrome b(5) from a 500-ml culture yielded 121 mg of protein which had a specific content of 50 nmol of heme per milligram of protein with an overall recovery of 35%. The final cytochrome b(5) was free of any detectable contaminants when analyzed by SDS-PAGE.

Role of highly conserved residues in the reaction catalyzed by recombinant Delta7-sterol-C5(6)-desaturase studied by site-directed mutagenesis

The role of 15 residues in the reaction catalyzed by Arabidopsis thaliana Delta7-sterol-C5(6)-desaturase (5-DES) was investigated using site-directed mutagenesis and expression of the mutated enzymes in an erg3 yeast strain defective in 5-DES. The mutated desaturases were assayed in vivo by sterol analysis and quantification of Delta5,7-sterols. In addition, the activities of the recombinant 5-DESs were examined directly in vitro in the corresponding yeast microsomal preparations. One group of mutants was affected in the eight evolutionarily conserved histidine residues from three histidine-rich motifs. Replacement of these residues by leucine or glutamic acid completely eliminated the desaturase activity both in vivo and in vitro, in contrast to mutations at seven other conserved residues. Thus, mutants H203L, H222L, H222E, P201A, G234A, and G234D had a 5-DES activity reduced to 2-20% of the wild-type enzyme, while mutants K115L, P175V, and P175A had a 5-DES activity and catalytical efficiency (V/K) that was similar to that of the wild-type. Therefore, these residues are not essential for the catalysis but contribute to the activity through conformational or other effects. One possible function for the histidine-rich motifs would be to provide the ligands for a presumed catalytic Fe center, as previously proposed for a number of integral membrane enzymes catalyzing desaturations and hydroxylations [Shanklin et al. (1994) Biochemistry 33, 12787-12794]. Another group of mutants was affected in residue 114 based on previous in vivo observations in A. thaliana indicating that mutant T114I was deficient in 5-DES activity. We show that the enzyme T114I has an 8-fold higher Km and 10-fold reduced catalytic efficiency. Conversely, the functionally conservative substituted mutant enzyme T114S displays a 28-fold higher Vmax value and an 8-fold higher Km value than the wild-type enzyme. Consequently, V/K for T114S was 38-fold higher than that for T114I. The data suggest that Thr 114 is involved in stabilization of the enzyme-substrate complex with a marked discrimination between the ground-state and the transition state of a rate-controlling step in the catalysis by the 5-DES.

Cytochrome P450 enzyme systems in fungi

The involvement of cytochrome P450 enzymes in many complex fungal bioconversion processes has been characterized in recent years. Accordingly, there is now considerable scientific interest in fungal cytochrome P450 enzyme systems. In contrast to S. cerevisiae, where surprisingly few P450 genes have been identified, biochemical data suggest that many fungi possess numerous P450 genes. This review summarizes the current information pertaining to these fungal cytochrome P450 systems, with emphasis on the molecular genetics. The use of molecular techniques to improve cytochrome P450 activities in fungi is also discussed.

Fah1p, a Saccharomyces cerevisiae cytochrome b5 fusion protein, and its Arabidopsis thaliana homolog that lacks the cytochrome b5 domain both function in the alpha-hydroxylation of sphingolipid-associated very long chain fatty acids

A search of the Saccharomyces cerevisiae genome data base for cytochrome b5-like sequences identified a 1.152-kilobase pair open reading frame, located on chromosome XIII at locus YMR272C (FAH1). That gene encodes a putative 384-amino acid protein with an amino-terminal cytochrome b5 domain. The b5 core domain shows a 52% identity and 70% similarity to that of the yeast microsomal cytochrome b5 and a 35% identity and 54% similarity to the b5 core domain of OLE1, the S. cerevisiae Delta-9 fatty acid desaturase. Expression of the S. cerevisiae FAH1 cytochrome b5 domain in Escherichia coli produces a soluble protein that exhibits the typical oxidized versus reduced differential absorbance spectra of cytochrome b5. Sequence analysis of Fah1p reveals other similarities to Ole1p. Both proteins are predicted to have two hydrophobic domains, each capable of spanning the membrane twice, and both have the HX(2-3)(XH)H motifs that are characteristic of membrane-bound fatty acid desaturases. These similarities to Ole1p suggested that Fah1p played a role in the biosynthesis or modification of fatty acids. Disruption of the FAH1 gene in S. cerevisiae did not give any visible phenotype, and there was no observable difference in content or distribution of the most abundant long chain saturated and unsaturated 14-18-carbon fatty acid species. Northern blot analysis, however, showed that this gene is expressed at much lower levels ( approximately 150-fold) than the OLE1 gene, suggesting that it might act on a smaller subset of fatty acids. Analysis of sphingolipid-derived very long chain fatty acids revealed an approximately 40-fold reduction of alpha-HO 26:0 and a complementary increase in 26:0 in the gene-disrupted fah1Delta strain. GAL1 expression of the S. cerevisiae FAH1 genes in the fah1Delta strain restores alpha-HO 26:0 fatty acids to wild type levels. Also identified are a number of homologs to this gene in other species. Expression of an Arabidopsis thaliana FAH1 gene, which does not contain the cytochrome b5 domain, in the fah1Delta strain produced an approximately 25-fold increase in alpha-HO 26:0 and reduced the levels of its 26-carbon precursor, suggesting that it functions in very long chain fatty acid hydroxylation using an alternate electron transfer mechanism.

Expression of enzymatically active CYP3A4 by Caco-2 cells grown on extracellular matrix-coated permeable supports in the presence of 1alpha,25-dihydroxyvitamin D3

The human colon carcinoma cell line, Caco-2, is widely used as a model for oral absorption of xenobiotics. The usefulness of Caco-2 cells has been limited, however, because they do not express appreciable quantities of CYP3A4, the principle cytochrome P450 present in human small bowel epithelial cells. We report that treatment of Caco-2 cells with 1 alpha,25-dihydroxyvitamin D3, beginning at confluence, results in a dose- and duration-dependent increase in CYP3A4 mRNA and protein, with little apparent effect on the expression of CYP3A5 or CYP3A7. This treatment also results in increases in NADPH cytochrome P450 reductase and P-glycoprotein (the MDR1 gene product) but has no detectable effect on expression of CYP1A1, CYP2D6, cytochrome b5, liver or intestinal fatty acid binding proteins, or villin. Maximal expression of CYP3A4 requires an extracellular matrix on a permeable support and the presence of serum. In the treated cells, the intrinsic formation clearance of 1'-hydroxymidazolam (a reaction characteristically catalyzed by CYP3A enzymes) was estimated to be somewhat lower than that of human jejunal mucosa (1.14 and 3.67 ml/min/g of cells, respectively). The 1'-OH-midazolam/4-OH-midazolam product ratio produced by the cells (approximately 5.3) is comparable to, but somewhat lower than, that observed in human jejunal microsomes (7.4-15.4), which may reflect the presence of CYP3A7 in the Caco-2 cells. 25-Hydroxyvitamin D3 is less efficacious but reproduces the effects of the dihydroxy compound, whereas unhydroxylated vitamin D is without appreciable effect. These observations, together with the time course of response, suggest that the vitamin D receptor may be involved in CYP3A4 regulation. The culture model we describe should prove useful in defining the role of CYP3A4 in limiting the oral bioavailability of many xenobiotics.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

The carboxyl terminus of the membrane-binding domain of cytochrome b5 spans the bilayer of the endoplasmic reticulum

Preliminary studies (Vergères, G., and Waskell, L. (1992) J. Biol. Chem. 267, 12583-12591) have suggested that the carboxyl-terminal membrane-binding domain of cytochrome b5 traverses the membrane and that the carboxyl terminus is in the lumen of the endoplasmic reticulum. In order to confirm and extend these studies, additional experiments were conducted. The gene coding for rat cytochrome b5 was transcribed and the resulting mRNA was translated in vitro in a rabbit reticulocyte lysate in the presence of microsomes. The binding and topology of cytochrome b5 were investigated by treating microsomes containing the newly incorporated cytochrome b5 with carboxypeptidase Y and trypsin. Our studies indicate that cytochrome b5 is inserted both co- and post-translationally into microsomes in a topology in which the membrane-binding domain spans the bilayer with its COOH terminus in the lumen. Cytochrome b5 is also incorporated into microsomes pretreated with trypsin in a topology indistinguishable from the one resulting from the insertion of the protein into untreated microsomes, reconfirming that cytochrome b5 does not use the signal recognition particle-dependent translocation machinery. Our results do not allow a distinction to be made between a spontaneous insertion mode or some other trypsin-resistant receptor-mediated mechanism. A role for Pro115 in the middle of the membrane-binding domain of cytochrome b5 was also examined by mutating it to an alanine and subsequently characterizing the ability of the mutant protein to be incorporated into membranes. The mutant protein inserted more slowly in vitro into microsomes as well as into pure lipid bilayers by a factor of 2 to 3.

Cytochrome b5, its functions, structure and membrane topology

The first part of the present communication reviews recent advances in our understanding of the known physiological functions of cytochrome b5. In addition, one section is devoted to a description of a recently discovered function of cytochrome b5, namely its involvement in the synthesis of the oncofetal antigen N-glycolylneuraminic acid. The second part of the article summarizes site-directed mutagenesis studies, primarily conducted in the author's laboratory, in both the catalytic heme-binding and membrane-binding domain of cytochrome b5. These studies have shown that: 1) the membrane binding domain of cytochrome b5 spans the bilayer; 2) cytochrome b5 lacking 19 COOH-terminal amino acids does not bind to membrane bilayers; and 3) specific amino acids in the membrane binding domain have been mutated and shown not to be essential for the function of cytochrome b5 with its redox partners.

Cloning and characterization of a yeast cytochrome b5-encoding gene which suppresses ketoconazole hypersensitivity in a NADPH-P-450 reductase-deficient strain

Cytochrome P-450 (Cyp) 51 or lanosterol-C14-demethylase is the main target for antifungal compounds of the triazole family like ketoconazole (Kz). Disruption of the associated NADPH-P-450 reductase-encoding gene (YRED) is not lethal, but decreases by about 20-fold the Kz resistance (KzR) of wild-type (wt) Saccharomyces cerevisiae. Transformation of a YRED-disrupted strain by a yeast genomic library based on a multicopy vector allowed us to identify a suppressor of Kz hypersensitivity. Deletion analysis of the 5-kb cloned fragment indicated that yeast cytochrome b5-encoding gene (CYB5), which encodes a 120-amino-acid (aa) protein, is required and sufficient for the suppressor effect. The encoded polypeptide shares about 30% aa identity with mammalian cytochromes b5 (Cyb5). CYB5 disruption and tetrad analysis demonstrate that yeast Cyb5 is not required for growth in a Yred+ strain. Determination of the microsomal content of b-type cytochromes by differential spectra indicated the presence of a strongly decreased or null Cyb5 level in the disrupted strain. This confirms that we have cloned the gene encoding the major microsomal form of Cyb5 which appears not to be essential. Minor Cyb5 isoforms could also be present in yeast or other redox proteins could substitute for the pleiotropic roles of Cyb5 in the sterol and lipid biosynthesis pathways.

The cytochrome b5-fold: an adaptable module

The family of b5-like cytochromes encompasses, besides cytochrome b5 itself, hemoprotein domains covalently associated with other redox proteins, in flavocytochrome b2 (L-lactate dehydrogenase), sulfite oxidase and assimilatory nitrate reductase. A comparison of about 40 amino acid sequences deposited in data banks shows that eight residues are invariant and about 15 positions carry strongly conservative substitutions. Examination of the location of these invariant and conserved positions in the light of the three-dimensional structures of beef cytochrome b5 and S cerevisiae flavocytochrome b2 suggests a strongly conserved protein structure for the b5-like heme-binding domain throughout evolution. Numerous NMR studies have demonstrated the existence of a positional isomerism for the heme, which involves both a 180 degree-rotation around the heme alpha,gamma-meso carbon atoms and a rotation through an axis normal to the heme plane at the iron. NMR studies did not detect significant differences in protein structure between reduced and oxidized states, or between species. The role of a number of side chains was probed by site-directed mutagenesis. Studies of complex formation and of electron transfer rates between cytochrome b5 and redox partners have led to the idea that complexation is driven by electrostatic forces, that it is generally the exposed heme edge which makes contact with electron donors and acceptors, but that there are multiple overlapping sites within this general area. For the bi- and trifunctional members of the family, extrapolation of available data would suggest a mobile heme-binding domain within a complex structure. In these cases the existence of a single interaction area for both electron donor and acceptor, or of two different ones, remains open to discussion.

Amino acid substitutions in the membrane-binding domain of cytochrome b5 alter its membrane-binding properties

The structure-function relationships of the 43-amino-acid membrane-binding domain of cytochrome b5 have been examined in two mutant forms of the protein. In one mutant, two tryptophans in the membrane-binding domain, at positions 108 and 112, were replaced by leucines, and in the second mutant, in addition, aspartic acid 103 was also replaced by leucine. The fluorescence emission spectra of the three proteins and their degree of quenching by brominated lipids indicate that the mutations are not producing major conformational changes or allowing a deeper degree of penetration of the domain into the bilayer. The hydrophobicities of the three proteins were compared, by determining strengths of self-association and membrane affinities, and it was found that the protein with two additional leucines was much less hydrophobic and the one with three additional leucines was much more hydrophobic than the native cytochrome. It appears that small changes in amino acid composition, which produce no gross changes in the structure of the membrane-binding domain, will nevertheless produce very large changes in the strengths of self- and membrane-association. These differences in self-association had profound effects on the times required for membrane-association to reach equilibrium.

A model system for studying membrane biogenesis. Overexpression of cytochrome b5 in yeast results in marked proliferation of the intracellular membrane

Cytochrome b5 is an amphipathic microsomal protein that is anchored to the endoplasmic reticulum by a single hydrophobic transmembrane alpha-helix located near the carboxyl terminus of the protein. In yeast, cytochrome b5 provides electrons for fatty acid desaturation and ergosterol biosynthesis. High level expression of cytochrome b5 in Saccharomyces cerevisiae was achieved using the yeast metallothionein promoter and a synthetic cytochrome b5 gene. In order to accommodate the markedly increased amount of the membrane-bound cytochrome b5, the yeast cell proliferated its nuclear membrane. As many as 20 pairs of stacked membranes could be observed to partially encircle the nucleus. This morphological arrangement of membrane around the nucleus is known as a karmella. In an effort to understand which part of the cytochrome b5 molecule, i.e. the membrane anchor or the soluble heme domain, which is competent in electron transfer, provided the signal for the de novo membrane biogenesis, a series of studies, including site-directed mutagenesis, was undertaken. The results of these experiments demonstrated that the inactive hemedeficient apo form of the membrane-bound protein stimulates membrane proliferation to the same extent as the holo wild-type protein, whereas cytosolic forms of cytochrome b5 did not induce membrane synthesis. These data demonstrate that membrane proliferation is a consequence of the cell's ability to monitor the level of membrane proteins and to compensate for alterations in these levels rather than the result of the ability of the extra cytochrome b5 to catalyze synthesis of extra lipid that had to be accommodated in new membrane. Site-directed mutagenesis studies of the membrane binding domain of cytochrome b5 provided additional clues about the nature of the signal for membrane proliferation. Replacement of the membrane anchor by a non-physiological nonsense sequence of 22 leucines gave rise to a mutant protein that triggered membrane biosynthesis. The conclusion from these experiments is clear; the signal for membrane proliferation does not reside in some specific amino acid sequence but instead in the hydrophobic properties of the proliferant. Interestingly, these membranes are somewhat diminished in quantity and have a slightly altered morphology compared to those induced by the wild-type protein. It was also observed that disruption of the putative alpha helix of the membrane anchor by an Ala116Pro mutation, which gives rise to two sequential prolines at positions 115 and 116 results in a protein with diminished capacity to induce membrane formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Mammalian PC-12 cell genetically engineered for human cytochrome P450 2E1 expression

The stable expression of the human cytochrome CYP2E1 (P450 alcohol) was performed in the mammalian cell line PC-12. This cell line expressed cytochrome b5 (58 +/- 12 pmol/mg microsomal protein vs 528 +/- 80 pmol/mg in microsomal human liver) and a high level of NADPH: cytochrome P450 reductase (140 +/- 20 nmol.min-1.mg microsomal protein-1 vs 68 +/- 48 nmol.min-1.mg-1 in microsomal human liver). An expression plasmid was constructed using the cDNA for the human CYP2E1 mRNA and the Rous sarcoma virus (RSV) promoter. This plasmid was co-transfected with the plasmid RSVneo into PC-12 cells. Clones were selected for resistance to the neomycin analog, G418, and then screened for expression of the CYP2E1 isozyme by testing for 6-hydroxylation of chlorzoxazone, a specific substrate for CYP2E1. Expression of CYP2E1 was confirmed in one clone, DB-7, by Western blot analysis and by measurement of monooxygenase activities which were not detectable in PC-12 cells. Chlorzoxazone 6-hydroxylation, n-butanol oxidation and dimethylnitrosamine N-demethylation were localized in microsomes (62, 60 and 63 pmol.min-1.mg microsomal protein-1, respectively) and were inhibited by carbon monoxide and diethyldithiocarbamate, both inhibitors of P450 enzymes. Although the level of the enzyme activities was about a tenth of that measured in human liver microsomes, CYP2E1 expressed in DB-7 cells has catalytic competence similar to human liver CYP2E1. DB-7 cells metabolized acetaminophen and this metabolic activation was shown to be toxic to these cells by release of lactate dehydrogenase. Construction of recombinant cell lines expressing CYP2E1 provides a useful tool for studying the catalytic properties of this enzyme and the consequent cytotoxic effects of substrates metabolized by this enzyme.