Integration of the thylakoid membrane protein cytochrome b6 in the cytoplasmic membrane of Escherichia coli

An overexpression system for spinach apocytochrome b(6) as a fusion protein to a maltose-binding protein in Escherichia coli was established using the expression vector pMalp2. The fusion of the cytochrome b(6) to the periplasmic maltose-binding protein directs the cytochrome on the Sec-dependent pathway. The cytochrome b(6) has a native structure in the bacterial cytoplasmic membrane with both NH(2) and COOH termini on the same, periplasmic side of the membrane but has the opposite orientation compared to that in thylakoid. Our data also show that in the E. coli cytoplasmic membrane, apocytochrome b(6) and exogenic hemes added into a culture media spontaneously form a complex with similar spectroscopic properties to native cytochrome b(6). Reconstituted membrane-bound cytochrome b(6) contain two b hemes (alpha band, 563 nm; average E(m,7) = -61 +/- 0.84 and -171 +/- 1.27 mV).     

On the mode of integration of the thylakoid membrane protein cytochrome b(6) into cytoplasmic membrane of Escherichia coli

In the stroma compartment, several pathways are used for integration/translocation of chloroplast proteins into or across the thylakoid membrane. In this study we investigated the mode of incorporation of the chloroplast-encoded cytochrome b(6) into the bacterial membrane. Cytochrome b(6) naturally comprises of four transmembrane helices (A,B,C,D) and contains two b-type hemes. In the present study, mature cytochrome b(6) or constructed deletion mutants of cytochrome were expressed in E. coli cells. The membrane insertion of cytochrome b(6) in this bacterial model system requires an artificially added presequence that directs the protein to use an E. coli membrane-insertion pathway. This could be accomplished by fusion to maltose-binding protein (MBP) or to the bacterial Sec-dependent signal peptide (SSpelB). The integration of mature cytochrome b(6) into the bacterial cytoplasmic membrane by the Sec pathway has been reported previously by our group (Kroliczewski et al., 2005, Biochemistry, 44: 7570). The results presented here show that cytochrome b(6) devoid of the first helix A can be inserted into the membrane, as can the entire ABCD. On the other hand, the construct devoid of helices A and B is translocated through the membrane into the periplasm without any effective insertion. This suggests the importance of the membrane-anchoring sequences that are likely to be present in only the A and B part, and it is consistent with the results of computational prediction which did not identify any membrane-anchoring sequences for the C or D helices. We also show that the incorporation of hemes into the truncated form of cytochrome b(6) is possible, as long as the B and D helices bearing axial ligands to heme are present.     

The identification of the heat-stable microsomal protein required for methoxyflurane metabolism as cytochrome b5

Methoxyflurane is an anesthetic whose metabolism by cytochrome P-450LM2 has been shown to be dependent upon a heat-stable microsomal protein (Canova-Davis, E., and Waskell, L. A. (1982) Biochem. Biophys. Res. Commun. 108, 1264-1270). Treatment of this protein with diethylpyrocarbonate, which modifies selected amino acids, caused a dose-dependent loss in its ability to effect the metabolism of methoxyflurane by purified cytochrome P-450LM2. This protein factor has been identified as cytochrome b5 by demonstrating that cytochrome b5 and the heat-stable factor coelute during cytochrome b5 purification. Neither ferriheme nor apocytochrome b5 was able to substitute for the activating factor, while cytochrome b5 reconstituted from apocytochrome b5 and heme exhibited an activity similar to that of native b5. Examination of the cytochrome b5 molecule by computer graphics suggested that diethylpyrocarbonate did not inactivate b5 by reacting with the anionic surface of the cytochrome b5 molecule. Maximal rates of methoxyflurane metabolism were obtained at a ratio of 1:1:1 of the three proteins, cytochrome P-450LM2:reductase:cytochrome b5. In summary, it has been demonstrated that the heat-stable protein, cytochrome b5, is obligatory for the metabolism of methoxyflurane by cytochrome P-450LM2. These data also suggest that cytochrome b5 may be acting as an electron donor to P-450LM2 in the O-demethylation of methoxyflurane.     

Overexpression and reconstitution of a Rieske iron-sulfur protein from the higher plant

The iron-sulfur protein subunit, known as the Rieske protein, is one of the central components of the cytochrome b(6)f complex residing in chloroplast and cyanobacterial thylakoid membranes. We have constructed plasmids for overexpression in Escherichia coli of full-length and truncated Rieske (PetC) proteins from the Spinacia oleracea fused to MalE. Overexpressed fusion proteins were predominantly found (from 55 to 70%) in cytoplasm in a soluble form. The single affinity chromatography step (amylose resine) was used to purify about 15mg of protein from 1 liter of E. coli culture. The isolated proteins were electrophoretically pure and could be used for further experiments. The NifS-like protein IscS from the cyanobacterium Synechocystis PCC 6803 mediates the incorporation of 2Fe-2S clusters into apoferredoxin and cyanobacterial Rieske apoprotein in vitro. Here, we used the recombinant IscS protein for the enzymatic reconstitution of the iron-sulfur cluster into full-length Rieske fusion and truncated Rieske fused proteins. Characterization by EPR spectroscopy of the reconstituted proteins demonstrated the presence of a 2Fe-2S cluster in both full-length and truncated Rieske fusion proteins.     

Purification and characterization of cytosolic liver protein facilitating heme transport into apocytochrome b5 from mitochondria. Evidence for identifying the heme transfer protein as belonging to a group of glutathione S-transferases

The results of the present experiment are shown in terms of the transport of protoheme from mitochondria to apocytochrome b5 when fresh rat liver mitochondria, apocytochrome b5, and cytosol were incubated. The heme transfer protein was purified from rat liver cytosol up to approximately 133-140-fold with a 43% yield by the procedure discussed herein, including Sephadex G-75 and CM-cellulose column chromatography. The final preparation showed apparent homogeneity upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Its native form was found to be a dimeric protein with a Mr = 45,000 which consists of a subunit with a Mr = 23,000. In the transporting system, the heme transfer depended on the concentration of mitochondria (donor), apocytochrome b5 (acceptor), and purified transfer protein, respectively. Omission of one of these components led to an almost complete loss of the transfer activity. The transport of mitochondrial protoheme was a rapid reaction which showed approximate linearity until 1.5 min and after that it became saturated. When the functional capacity was tested by the NADH-cytochrome c reductase system, the reconstituted cytochrome b5 expressed its complete original catalytic properties, as well as its characteristic absorption spectra for the hemoprotein. Furthermore, the detailed physicochemical and immunological characterization of the transfer protein provided evidence that the protein is identical with soluble glutathione S-transferase, which conjugates glutathione with a variety of electrophilic compounds. At least one of the glutathione S-transferase isozymes observed was identified as GST-C2, which comprises the subunit of Yb'Yb' by the immunoprecipitation reaction using various anti-glutathione S-transferase isozyme antibodies.     

Expression and characterization of the diheme cytochrome c subunit of the cytochrome bc complex in Heliobacterium modesticaldum

Heliobacterium modesticaldum is a Gram-positive, anaerobic, anoxygenic photoheterotrophic bacterium. Its cytochrome bc complex (Rieske/cyt b complex) has some similarities to cytochrome b(6)f complexes from cyanobacteria and chloroplasts, and also shares some characteristics of typical bacterial cytochrome bc(1) complexes. One of the unique factors of the heliobacterial cytochrome bc complex is the presence of a diheme cytochrome c instead of the monoheme cytochrome f in the cytochrome b(6)f complex or the monoheme cytochrome c(1) in the bc(1) complex. To understand the structure and function of this diheme cytochrome c protein, we expressed the N-terminal transmembrane-helix-truncated soluble H. modesticaldum diheme cytochrome c in Escherichia coli. This 25kDa recombinant protein possesses two c-type hemes, confirmed by mass spectrometry and a variety of biochemical techniques. Sequence analysis of the H. modesticaldum diheme cytochrome c indicates that it may have originated from gene duplication and subsequent gene fusion, as in cytochrome c(4) proteins. The recombinant protein exhibits a single redox midpoint potential of +71mV versus NHE, which indicates that the two hemes have very similar protein environments.     

Cytochrome b is necessary for the effective processing of core protein I and the iron-sulfur protein of complex III in the mitochondria

The effect of cytochrome b on the assembly of the subunits of complex III into the inner mitochondrial membrane has been studied in a mutant of yeast (W-267, Box 6-2) that lacks a spectrally detectable cytochrome b and synthesizes a shortened form of apocytochrome b. We recently reported that several cytochrome b-deficient mutants contained significantly diminished amounts of core proteins I and II as well as the iron-sulfur protein, but contained equal amounts of cytochrome c1 compared to the wild type (K. Sen and D. S. Beattie, Arch. Biochem. Biophys. 242, 393-401, 1985). In the present study, the time course of processing of precursors of both core protein I and the iron-sulfur protein which had accumulated in cells treated with the uncoupler carbonyl m-chlorophenyl hydrazone (CCCP) was noted to be significantly lower in the mutant compared to the wild type. The amounts of the mature forms of these proteins in mitochondria pulse labeled under different conditions was also considerably decreased at all times studied. The synthesis of both proteins appeared to be unaffected in the mutant, as the precursor forms of both proteins accumulated to the same extent when processing in vivo was blocked by CCCP. Furthermore, translation of RNA in a reticulocyte lysate in vitro indicated that the messenger RNAs for both proteins were present in the mutant and translated with equal efficiency. The import into isolated mitochondria of the precursor forms of the iron-sulfur protein synthesized in the cell-free system was also decreased in the mutant mitochondria. In addition, the precursor form was bound to the exterior of the mitochondrial membrane where it was sensitive to digestion with proteases. By contrast, the synthesis and processing of cytochrome c1 appeared to be unaffected in these mutants. These results suggest that cytochrome b is necessary for the proper processing and assembly of both core protein I and the iron-sulfur protein, but not for cytochrome c1, into complex III of the inner mitochondrial membrane.     

Fusion of phospholipid vesicles mediated by cytochrome c

Fusion of phosphatidylserine/phosphatidylethanolamine (1/1) vesicles induced by cytochrome c is studied at a wide range of pH values. A pH profile for the fusion with maximum values at pH 5 and pH 8 is obtained and this is found to be similar to the profile for cytochrome c binding to the vesicles. The binding property of apocytochrome c to the same phospholipid vesicles is found to be about the same as that of the cytochrome c at low ionic strength, but very different at high salt concentrations. No appreciable fusion of vesicles by apocytochrome c is observed. Proteolytic treatment and dansyl chloride labeling of cytochrome c- and apocytochrome c-vesicle complexes show that the C-terminal segments of these proteins with molecular weights of about 3000 and 5000, respectively, penetrate the bilayer. The hydrophobic labeling studies with photoreactive phosphatidylcholine in the bilayer show that segments of both cytochrome c and apocytochrome c go deep into the bilayer.     

Decreased amounts of core proteins I and II and the iron-sulfur protein in mitochondria from yeast lacking cytochrome b but containing cytochrome c1

The effect of cytochrome b on the assembly of the subunits of complex III into the inner mitochondrial membrane has been studied in four mutants of yeast that lack a spectrally detectable cytochrome b and do not synthesize apocytochrome b. Quantitative analysis of intact mitochondria by immunoprecipitation or immunoblotting techniques with specific antisera revealed that the core proteins and the iron-sulfur protein were decreased 50% or more in the mitochondria from the mutants as compared to the wild type. Sonication of wild-type mitochondria did not result in any decrease in any of these proteins from the membrane; however, sonication of mitochondria from the four mutants resulted in a further decrease in the amount of these proteins suggesting that they are not as tightly bound to the mitochondrial membrane in the absence of cytochrome b. By contrast, the amounts of cytochrome c1 in the mitochondria, as determined both spectroscopically and immunologically, were not significantly affected by the absence of cytochrome b. In addition, no loss of cytochrome c1 was observed after sonication of the mitochondria suggesting that this protein is tightly bound to the membrane. These results suggest that the processing and/or assembly of these subunits of complex III into the mitochondrial membrane is affected by the absence of cytochrome b.     

人细胞色素c基因在大肠杆菌中的克隆和表达及活性测定

通过PCR方法 ,从人胎儿心肌基因组DNA中得到precytc基因 (有 1个内含子 ) ,剔除内含子后 ,得到细胞色素c基因的编码序列 .测序结果表明 ,该基因与GenBank中报道的人细胞色素c基因核苷酸顺序完全一致 .将其插入原核表达载体pET3a的NdeⅠ和HindⅢ位点之间构建pET3a cytc重组质粒 ,并成功转化入E .coliBL2 1(DE3)中 .经IPTG诱导表达后 ,15 %SDS PAGE分析 ,可观察到1条与细胞色素c蛋白分子量相符的电泳条带 .Western印迹结果显示 ,该条带与小鼠抗人cytc单克隆抗体IgG2b发生特异反应 ,证实为人细胞色素c的前体蛋白 .体外使血红素与该前体蛋白结合生成完整的人细胞色素c蛋白 ,其耗氧量在不同浓度具有与反应时间的线性关系 ,为研究人细胞色素c结构和功能关系奠定了基础     

Bilayer-penetrating properties enable apocytochrome c to follow a special import pathway into mitochondria.

In this study, we have investigated the protein/lipid interactions of two mitochondrial precursor proteins, apocytochrome c and pCOX IV-DHFR, which exhibit mitochondrial import pathways with different characteristics. In-vitro-synthesized apocytochrome c was found to bind efficiently and specifically to liposomes composed of negatively charged phospholipids and showed a (at least partial) translocation across a lipid bilayer, as reported previously for the chemically prepared precursor protein [Rietveld, A. & de Kruijff, B. (1984) J. Biol. Chem. 259, 6704-6707; Dumont, M. E. & Richards, F. M. (1984) J. Biol. Chem. 259, 4147-4156]. Negatively charged liposomes were shown to efficiently compete with mitochondria for import of in-vitro-synthesized apocytochrome c into the organelle, suggesting an important role for negatively charged phospholipids in the initial binding of apocytochrome c to mitochondria. In contrast, the purified and in-vitro-synthesized precursor fusion protein pCOX IV-DHFR, consisting of the presequence of yeast cytochrome oxidase subunit IV fused to mouse dihydrofolate reductase was unable to translocate across a pure lipid bilayer. The data indicate that the ability of apocytochrome c to spontaneously translocate across the bilayer is not shared by all mitochondrial precursor proteins. The implications of the special protein/lipid interaction of apocytochrome c for import into mitochondria will be discussed.     

Iron-sulfur cluster reconstitution of spinach chloroplast Rieske protein requires a partially prefolded apoprotein

The Rieske 2Fe-2S protein is a central component of the photosynthetic electron transport cytochrome b6f complex in chloroplast and cyanobacterial thylakoid membranes. We have constructed plasmids for expression in Escherichia coli of full-length and truncated Spinacia oleracea Rieske (PetC) proteins fused to the MalE, maltose binding protein. The expressed Rieske fusion proteins were found predominantly in soluble form in the E. coli cytoplasm. These proteins could be readily purified for further experimentation. In vitro reconstitution of the characteristic, "Rieske-type" 2Fe-2S cluster into these fused proteins was accomplished by a chemical method employing reduced iron and sulfide. Cluster incorporation was monitored by electron paramagnetic resonance and optical circular dichroism (CD) spectroscopy. CD spectral analysis in the ultraviolet region suggests that the spinach Rieske apoprotein must be in a partially folded conformation to incorporate an appropriate iron-sulfur cluster. These data further suggest that upon cluster integration, further folding occurs, allowing the Rieske protein to attain a final, native structure. The data presented here are the first to demonstrate successful chemical reconstitution of the 2Fe-2S cluster into a Rieske apoprotein from higher plant chloroplasts.     

Biogenesis of yeast mitochondrial cytochrome c: a unique relationship to the TOM machinery

The import of cytochrome c into the mitochondrial intermembrane space is not understood at a mechanistic level. While the precursor apocytochrome c can insert into protein-free lipid bilayers, the purified translocase of the outer membrane (TOM) complex supports the translocation of apocytochrome c into proteoliposomes. We report an in organello analysis of cytochrome c import into yeast mitochondria from wild-type cells and different mutants cells, each defective in one of the seven Tom proteins. The import of cytochrome c is not affected by removal of the receptor Tom20 or Tom70. Moreover, neither the transfer protein Tom5 nor the assembly factors Tom6 and Tom7 are needed for import of cytochrome c. When the general import pore (GIP)-protein Tom40 is blocked, the import of cytochrome c is moderately affected. Mitochondria lacking the central receptor and organizing protein Tom22 contain greatly reduced levels of cytochrome c. We conclude that up to two components of the TOM complex, Tom22 and possibly the GIP, are involved in the biogenesis of cytochrome c.     

The mitochondrial precursor protein apocytochrome c strongly influences the order of the headgroup and acyl chains of phosphatidylserine dispersions. A 2H and 31P NMR study

Deuterium and phosphorus nuclear magnetic resonance techniques were used to study the interaction of the mitochondrial precursor protein apocytochrome c with headgroup-deuterated (dioleoylphosphatidyl-L-[2-2H1]serine) and acyl chain deuterated (1,2-[11,11-2H2]dioleoylphosphatidylserine) dispersions. Binding of the protein to dioleoylphosphatidylserine liposomes results in phosphorus nuclear magnetic resonance spectra typical of phospholipids undergoing fast axial rotation in extended liquid-crystalline bilayers with a reduced residual chemical shift anisotropy and an increased line width. 2H NMR spectra on headgroup-deuterated dioleoylphosphatidylserine dispersions showed a decrease in quadrupolar splitting and a broadening of the signal on interaction with apocytochrome c. Addition of increasing amounts of apocytochrome c to the acyl chain deuterated dioleoylphosphatidylserine dispersions results in the gradual appearance of a second component in the spectra with a 44% reduced quadrupolar splitting. Such large reduction of the quadrupolar splitting has never been observed for any protein studied yet. The lipid structures corresponding to these two components could be separated by sucrose gradient centrifugation, demonstrating the existence of two macroscopic phases. In mixtures of phosphatidylserine and phosphatidylcholine similar effects are observed. The induction of a new spectral component with a well-defined reduced quadrupolar splitting seems to be confined to the N-terminus since addition of a small hydrophilic amino-terminal peptide (residues 1-38) also induces a second component with a strongly reduced quadrupolar splitting. A chemically synthesized peptide corresponding to amino acid residues 2-17 of the presequence of the mitochondrial protein cytochrome oxidase subunit IV also has a large perturbing effect on the order of the acyl chains, indicating that the observed effects may be a property shared by many mitochondrial precursor proteins. In contrast, binding of the mature protein, cytochrome c, to acyl chain deuterated phosphatidylserine dispersions has no effect on the deuterium and phosphorus nuclear magnetic resonance spectra, thereby demonstrating precursor-specific perturbation of the phospholipid order. The inability of holocytochrome c to perturb the phospholipid order is due to folding of this protein, since unfolding of cytochrome c by heat or urea treatment results in similar effects on dioleoylphosphatidylserine bilayers, as observed for the unfolded precursor. Implications of these data for the import of apocytochrome c into mitochondria will be discussed.     

A water-lipid interface induces a highly dynamic folded state in apocytochrome c and cytochrome c, which may represent a common folding intermediate.

In this study, we have used CD and NMR techniques to investigate the secondary structure of (apo-) cytochrome c both in solution and when associated with micelles. In aqueous solution, the holoprotein cytochrome c is tightly folded at secondary and tertiary levels and differs strongly from its random-coiled precursor. However, in the presence of 12-PN/12-Pglycol (9:1) micelles, we observed a remarkable resemblance between the CD spectra of these partially helical proteins. The water-lipid interface induces a secondary folding of apocytochrome c, whereas cytochrome c is suggested to partially lose its tertiary structure. The exchange of all amide protons and, using deuterium-labeled proteins, of all amide deuterons with the solvent was monitored by NMR. A rapid exchange rate was observed, indicating that these folding states are highly dynamic. Saturation-transfer NMR of micelle-associated apocytochrome c showed that the exchange takes place at the (sub-) second time scale. The holoprotein in the presence of micelles was found to have two distinct exchange rates: (1) a fast rate, comparable to that found for the micelle-associated precursor and 4.5 times slower than that of the random-coiled apocytochrome c, and (2) a slow rate which is 75 times slower than the precursor in solution. Urea denaturation studies showed the micelle-bound proteins to have a low helix stability, which explains the inability of the lipid-induced secondary structure to prevent its labile protons from rapid exchange. The uniqueness of this lipid-induced highly dynamic folding state of (apo-) cytochrome c is demonstrated by comparison with amphiphilic polypeptides like melittin, and its implications for membrane translocation and functioning are discussed.     

Direct preparation of giant proteo-liposomes by in vitro membrane protein synthesis

We investigated the direct constitution of membrane proteins into giant liposomes in cell-free (in vitro) protein synthesis. Giant liposomes were present in a translation reaction cocktail of a wheat germ cell-free protein translation system. Apo cytochrome b(5) (b5) and its fusion proteins were synthesized and directly localized in the liposomes. After the translation reaction, the proteo-liposomes were isolated by simplified discontinuous density-gradient centrifugation. Apo cytochrome b(5) conjugated dihydrofolate reductase (DHFR) was synthesized in the same procedure and the protein was directly displayed on the liposome surface. b5 acts as a "hydrophobic tag" for recruitment to the liposome surface.     

Heterologous expression and in vitro assembly of the transmembrane cytochrome b6

Folding and assembly studies with alpha-helical membrane proteins are often hampered by the absence of high-level expression systems as well as by missing suitable in vitro refolding procedures. Experimental constraints and requirements for heterologous expression and in vitro assembly of cytochrome b6 have been examined and conditions for in vitro reconstitutions of the protein have been optimized. Cytochrome b6 can serve as an excellent model system for in vitro studies on the dynamic interplay of an apo-protein and heme cofactors during assembly of a transmembrane b-type cytochrome. In vitro assembled cytochrome b6 binds two hemes with different midpoint potentials and both ferri as well as ferro heme bind to the apo-cytochrome. However, the ferro cytochrome appears to be less stable than the ferri form.     

Import of a DHFR hybrid protein into glycosomes in vivo is not inhibited by the folate-analogue aminopterin

Dihydrofolate reductase fusion proteins have been widely used to study conformational properties of polypeptides translocated across membranes. We have studied the import of dihydrofolate reductase fusion proteins into glycosomes and mitochondria of Trypanosoma brucei. As signal sequences we used the last 22 carboxy-terminal amino acids of glycosomal phosphoglycerate kinase for glycosomes, and the cleavable presequences of yeast cytochrome b2 or cytochrome oxidase subunit IV for mitochondria. Upon addition of aminopterin, a folate analogue that stabilizes the dihydrofolate reductase moiety, import of the fusion protein targeted to glycosomes was not inhibited, although the results of protease protection assays showed that the fusion protein could bind the drug. Under the same conditions, import of a DHFR fusion protein targeted to mitochondria was inhibited by aminopterin. When DHFR fusion proteins targeted simultaneously to both glycosomes and mitochondria were expressed, import into mitochondria was inhibited by aminopterin, whereas uptake of the same proteins into glycosomes was either unaffected or slightly increased. These findings suggest that the glycosomes possess either a strong unfolding activity or an unusually large or flexible translocation channel.     

Amphitropic proteins: regulation by reversible membrane interactions (Review)

The ability of apocytochrome c and the heme containing respiratory chain component, cytochrome c, to induce fusion of phosphatidylcholine (PC) small unilamellar vesicles containing 0–50 mol% negatively charged lipids was examined. Both molecules mediated fusion of phosphatidylserine (PS):PC 1:1 vesicles as measured by energy transfer changes between fluorescent lipid probes in a concentration- and pH-dependent manner, although cytochrome c was less potent and interacted over a more limited pH range than the apocytochrome c. Maximal fusion occurred at pH 3, far below the pK_a of the 19 lysine groups contained in the protein (pl = 10.5). A similar pH dependence was observed for vesicles containing 50 mol% cardiolipin (CL), phosphatidylglycerol (PG), and phosphatidylinositol (PI) in PC but the apparent pK_a values varied somewhat. In the absence of vesicles, the secondary structure of apocytochrome c was unchanged over this pH range, but in the presence of negatively charged vesicles, the polypeptide underwent a marked conformational change from random coil to α-helix. By comparing the pH dependencies of fusion induced by poly-L-lysine and apocytochrome c, we concluded that the pH dependence derived from changes in the net charge on both the vesicles and apocytochrome c. Aggregation could occur under conditions where fusion was imperceptible. Fusion increased with increasing mole ratio of PS. Apocytochrome c did induce some fusion of vesicles composed only of PC with a maximum effect at pH 4. Biosynthesis of cytochrome c involves translocation of apocytochrome c from the cytosol across the outer mitochondrial membrane to the outer mitochondrial space where the heme group is attached. The ability of apocytochrome c to induce fusion of both PS-containing and PC-only vesicles may reflect characteristics of protein/membrane interaction that pertain to its biological translocation.