Self-association of cytochrome b5 in aqueous solution. Gel filtration and ultracentirfugational studies

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.

Registration of the protein with compact disk

CD-based optico-acoustical biosensor (OAB) was used for detection of various types of proteins represented by bovine serum albumin (BSA), heme-containing myoglobin (Mb), monoclonal antibody against viral protein marker of hepatitis B (anti-HBsAg) and membrane-bound cytochrome P450scc (P450scc). We applied standard compact disc reader (CD-ROM) as an optical analyzer and a standard compact disc (CD) as a biochip containing immobilized protein molecules. This biosensor can translate into a digital code the changes of optical signal from the proteins and their complexes immobilized on the CD surface. Then, the digital code is translated into an acoustic series or, in other words, into a "music of proteins". We demonstrate the use of the OAB for direct detection of proteins with different molecular weights, such as BSA, Mb, P450scc, anti-HBsAg with the concentration detection limit (DL) about 10(-7)M. By signal amplification achieved with autometallography, a higher sensitivity level (DL∼10(-9)M) for the detection of myoglobin was obtained. The method of OAB-detection of proteins is cheap: it requires no special equipment like spectrometers, refractometers and other devices. Due to the fact that acoustic series of the protein complexes antigen/antibody differs from that of single proteins, the OAB-detection is of particular interest for rapid assay in yes/no data type and for home diagnostics. Combination of the OAB with a mass spectrometer allowed the detection and identification of the target proteins fished out directly onto a standard CD surface.

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.

Chimeric fluorescent fusion proteins to monitor autophagy in plants

Autophagy is induced under nutrient-deficient conditions in both growing tobacco BY-2 cultured cells as well as Arabidopsis and others intact plants. The fluorescent protein-tagged structural protein for autophagosomes, the Atg8 protein, allows nondestructive detection of autophagy induction in plant cells and tissues by fluorescence microscopy. Using this technique, the general operation of autophagy in growing root cells has been observed. A synthetic cargo protein for autophagy consisting of cytochrome b5 and the red fluorescence protein, DsRed, allows for the quantitative assay of autophagy in tobacco cells. This chapter describes methods for detecting autophagy in these plant cells using fluorescent protein fusions in situ with light microscopy, as well as quantification of autophagy.

Sheep adrenal cytochrome b5: active as a monomer or a tetramer in vivo?

Cytochrome b5 (cyt b5) is an ubiquitous hemoprotein also associated with microsomal cytochromes P450. It has been reported that cyt b5 influences cytochrome P450-dependent catalyses through electron transport as well as direct protein-protein interactions. To investigate the influence of cyt b5 on ovine adrenal steroidogenesis, we isolated and characterized cyt b5 from ovine liver. The molecular mass of the purified protein was 15,260 as determined by electrospray mass spectrometry. SDS-Polyacrylamide gel electrophoresis, even after stringent detergent and mercaptoethanol pretreatment, indicated multimeric forms of the protein, the most prominent being the tetramer (+/-60 kDa) with minor bands corresponding to the monomer (+/-16 kDa) and dimer (+/-30 kDa). Trypsin treatment of cyt b5 resulted in a truncated enzyme with a molecular mass of +/-10 kDa. The aggregation of cytochrome b5 was abolished by the tryptic removal of the membrane binding region. In Western blot analyses antibodies against the truncated protein recognised only this low molecular mass form and not the full length cyt b5, or any of the higher molecular complexes, showing the involvement of the membrane binding domain of the protein, not only in aggregation, but also in the quaternary structure which determines epitope presentation for antibody production. Immunoblot analyses of sheep adrenal microsomes with the anti-truncated cyt b5 antibody were also negative. Immunoblot analyses and immunocytochemistry of adrenal tissue with antibodies against the full length cyt b5 indicated that the tetrameric form of the protein was in all probability the dominant specie in vivo.

Protein micelles from lipoxygenase 3

Heat-induced conformational changes in lipoxygenase 3 were characterized by differential scanning calorimetry. The positions of the observed transitions were sensitive to the composition of the buffer. In particular, lipoxygenase 3 heated in carbonate buffer at pH 8.0 formed large soluble aggregates. Variable-temperature circular dichroism revealed that the formation of the aggregates was not accompanied by the unfolding of the C-terminal domain, which is composed primarily of alpha-helix. The aggregates were investigated using size exclusion chromatography, native polyacrylamide gel electrophoresis, dynamic light scattering, and electron microscopy. The data were consistent with the formation of roughly spherical particles with an average hydrodynamic radius of 26 nm and an approximate composite molecular weight of 10,000,000 Da. To account for the formation of soluble aggregates from lipoxygenase 3, we propose that hydrophobic amino acid residues are exposed by unfolding of the N-terminal beta-barrel domain of the protein resulting in the formation of protein micelles with a hydrophilic surface composed of the C-terminal domains.

Effect of lipid unsaturation on the binding of native and a mutant form of cytochrome b5 to membranes

The partitioning of native cytochrome b5 and a mutant form, where Trp-108 and Trp-112 were both replaced by Leu, into small unilamellar lipid vesicles was examined. The vesicles were made from phosphatidylcholines containing mono- and di-unsaturated acyl chains. As these amphipathic proteins self-associate in aqueous solution, the binding was not monitored by a simple lipid titration experiment but by an exchange assay using fluorescence quenching by brominated lipids. Each protein had a greater affinity for lipids containing mono-unsaturated chains than for vesicles containing di-unsaturated chains, and the affinities of both proteins increased in buffers of higher ionic strength. The native protein had a higher affinity than the mutant protein for all vesicles; the ratio of the affinities was relatively constant at approximately 30. This corresponds to a difference in the free energy of partitioning of 2 kcal mol(-)(1). The fluorescence quantum yields of both proteins were much lower in lipids with di-unsaturated chains whereas a similar lowering was not seen with a simple Trp compound. These data suggest that the decreased membrane hydrophobicity seen by the proteins in di-unsaturated membranes is not an inherent property of the bilayer but is induced by the insertion of the protein. Further, the similar behavior of the two proteins suggests this modulation is not sensitive to the amino acid side chains of the inserted domain.

Effect of protein aggregation in the aqueous phase on the binding of membrane proteins to membranes

Analysis of the binding of hydrophobic peptides or proteins to membranes generally assumes that the solute is monomeric in both the aqueous phase and the membrane. Simulations were performed to examine the effect of solute self-association in the aqueous phase on the binding of monomeric solute to lipid vesicles. Aggregation lowered the initial concentration of monomeric solute, which was then maintained at a relatively constant value at the expense of the aggregated solute, as the lipid concentration was increased. The resultant binding isotherm has a more linear initial portion rather than the classic hyperbolic shape. Although this shape is diagnostic of solute self-association in the aqueous phase, various combinations of values for the membrane partition coefficient and the solute self-association constant will generate similar isotherms. Data for cytochrome b5 were analyzed and, when the self-association constant was estimated by gel filtration, a unique value for the membrane partition coefficient was obtained. Thus, to obtain a true partition coefficient the state of the solute in the aqueous phase must be known. If the concentration of the monomeric solute species in the aqueous phase can be independently determined, then, even with heterogeneous aggregates, the true partition coefficient can be obtained.

Evidence for multiple mechanisms for membrane binding and integration via carboxyl-terminal insertion sequences

Subcellular localization of proteins with carboxyl-terminal insertion sequences requires the molecule be both targeted to and integrated into the correct membrane. The mechanism of membrane integration of cytochrome b5 has been shown to be promiscuous, spontaneous, nonsaturable, and independent of membrane proteins. Thus endoplasmic reticulum localization for cytochrome b5 depends primarily on accurate targeting to the appropriate membrane. Here direct comparison of this mechanism with that of three other proteins integrated into membranes via carboxyl-terminal insertion sequences [vesicle-associated membrane protein 1(Vamp1), polyomavirus middle-T antigen, and Bcl-2] revealed that, unlike cytochrome b5, membrane selectivity for these molecules is conferred at least in part by the mechanisms of membrane integration. Bcl-2 membrane integration was similar to that of cytochrome b5 except that insertion into lipid vesicles was inefficient. Unlike cytochrome b5 and Bcl-2, Vamp1 binding to canine pancreatic microsomes was saturable, ATP-dependent, and abolished by mild trypsin treatment of microsomes. Surprisingly, although the insertion sequence of polyomavirus middle-T antigen was sufficient to mediate electrostatic binding to membranes, binding did not lead to integration into the bilayer. Together these results demonstrate that there are at least two different mechanisms for correct membrane integration of proteins with insertion sequences, one mediated primarily by targeting and one relying on factors in the target membrane to mediate selective integration. Our results also demonstrate that, contrary to expectation, hydrophobicity is not sufficient for insertion sequence-mediated membrane integration. We suggest that the structure of the insertion sequence determines whether or not specific membrane-bound receptor proteins are required for membrane integration.

Design of membrane-inserting peptides: spectroscopic characterization with and without lipid bilayers

This paper reports the spectroscopic characterization of two de novo peptides. The first sequence, Ala peptide = H2N-Ala27-Tyr-Lys6-CONH2, gives circular dichroism (CD) and Fourier transform infrared (FTIR) spectra characteristic of beta structure in solution, binds to lipid bilayer vesicles poorly, and tends to precipitate in buffered 0.1 M salt solutions. In the second sequence, Leu peptide = H2N-Ala2-Leu3-Ala22-Tyr-Lys6-CONH2, three leucines are substituted for three alanine residues. This small sequence change results in CD spectra that are characteristic of helical structures, while the FTIR spectra give evidence for complex equilibria between multiple structures in solution. The Leu peptide does not precipitate in buffered salt solutions and binds to lipid bilayers. The polarized attenuated total reflectance infrared spectra provide evidence of a transmembrane orientation for the helical peptide in lipid bilayers. The collective spectroscopic results are summarized in a tentative model in which the Leu peptide exhibits multiple equilibria between extended unordered, helix, and coiled-coil structures in solution; when lipid vesicles are added, the peptide binds to the lipid surface and then inserts into the lipid in a transmembrane orientation. The slow kinetics exhibited by the peptide suggest multiple conformational changes during the lipid-peptide interactions. The design rationale for the peptides is included in an appendix.

Effect of cholesterol on the tight insertion of cytochrome b5 into large unilamellar vesicles

When cytochrome b5 is added to large unilamellar vesicles (LUVs) of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), it binds predominantly in a 'loose,' or transferable form. Prolonged incubation of 30 degrees C leads to insertion in the physiological 'tight,' nontransferable form, with a halftime for the loose --> tight conversion of approx. 9 days. In this study, the effect of cholesterol on the rate of tight insertion was determined. Tight binding was assayed by depleting the LUVs of loose cytochrome b5 with an excess of SUV acceptors and then separating the liposome populations by gel-filtration or velocity sedimentation. Incorporation of cholesterol into the LUVs was found to markedly increase the rate of tight insertion, even though cholesterol decreases the equilibrium binding constant and saturation level of protein binding. The effect is not a continuously increasing function of cholesterol content, but attains a maximum at 20-25% mol%, where the rate enhancement is approx. 10-fold over baseline. At higher cholesterol levels, the rate decreases, returning to baseline at 40 mol% cholesterol. These observations are highly unusual in that cholesterol generally decreases the membrane binding affinity and the permeability of solutes, and does so as a monotonic function of cholesterol concentration (above the liquid-crystalline phase transition of the phospholipids). It is suggested that tight insertion is enhanced by lipid-protein packing mismatches and by bilayer fluidity; the former increases monotonically with increasing cholesterol whereas the latter decreases monotonically. At 20-25 mol% cholesterol the optimum balance of these physical properties is obtained for tight insertion.

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.

CMP-N-acetylneuraminic acid hydroxylase from mouse liver and pig submandibular glands. Interaction with membrane-bound and soluble cytochrome b5-dependent electron transport chains

In this report, the nature of the protein components involved in the functioning of cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5 Ac) hydroxylase in high-speed supernatants of mouse liver has been investigated. Fractionation and reconstitution experiments showed that this enzyme system consists of NADH-cytochrome b5 reductase, cytochrome b5 and a 56-kDa terminal electron acceptor having the CMP-Neu5 Ac hydroxylase activity. This enzyme system is extracted in a soluble protein fraction; however, the amphipathic, usually membrane-associated, forms of cytochrome b5 and the reductase were found to predominate and are presumably the forms which support the turnover of the hydroxylase in vivo. Although the majority of cellular cytochrome b5 and cytochrome b5 reductase is membrane-bound, the addition of intact microsomes elicited no significant increase in the hydroxylase activity of supernatants. Detergent-solubilised microsomes, however, potently activated the hydroxylase, probably due to the greater accessibility of the cytochrome b5. Accordingly, in reconstitution experiments, pure hydrophilic cytochrome b5 interacts more effectively with the hydroxylase than isolated amphipathic cytochrome b5. Studies on the CMP-Neu5 Ac hydroxylase system in fractionated porcine submandibular glands and bovine liver suggest that the composition of this enzyme system is conserved in all mammals possessing sialoglycoconjugates containing N-glycolylneuraminic acid.

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.

Tight insertion of cytochrome b5 into large unilamellar vesicles

Cytochrome b5 spontaneously binds to liposomes in a 'loose', or transferable form, whereas in vivo b5 binds post-translationally to the ER in the 'tight' or nontransferable form. The mechanism of tight insertion is unknown, except that it does not require SRP or energy input. The present study shows that prolonged incubation of b5 with large unilamellar vesicles (LUVs) of phosphatidylcholine results in slow conversion of the loose to the tight form, with a halftime of days. However, the process is complex. When the b5-LUVs are depleted of loose b5, by transfer of b5 to sonicated vesicles, the tight b5 is found to be concentrated to near saturating levels in a small fraction of the LUVs. If the LUVs devoid of tight b5 are recovered and then reincubated with fresh b5, the same slow transformation recurs. Apparently, a new population of vesicles, containing tight b5, is generated during the prolonged incubation with the protein. The b5-enriched LUVs contain about the same level of trapped sucrose as does the original vesicle preparation, indicating that vesicle integrity is maintained throughout the process. When fresh b5 is added to these tight b5-containing LUVs, all the freshly bound protein rapidly inserts (< 2 h) into the tight configuration. Apparently, the newly formed tight-b5/LUV vesicle population is 'insertion-active'. A model for these complex transformations is proposed.

Biophysical characterization of alpha-crystallin aggregates: validation of the micelle hypothesis

The size of alpha-crystallin aggregates, as well as the structural organization of each particle's subunits, is currently unknown, although a number of different laboratories have suggested both structures and average molecular weights (Thomson, J.A. and Augusteyn, R.C. (1984) Proc. Int. Soc. Eye Res. 3, 152). One hypothesis, compatible with literature reports and consistent with what is known of subunit primary structure and physiological function, is that alpha-crystallin exists in vivo as a naturally occurring protein micelle (Sen, A.C. and Chakrabarti, B. (1991) Biophysical J. 59, 108a.) To test this hypothesis, experiments were performed on this protein to determine its behavior under increased hydrostatic pressure and the effect of its concentration on aqueous surface tension. With increasing hydrostatic pressure, the turbidity of an alpha-crystallin solution increases exponentially to a plateau at about 6000-8000 psi; upon release of pressure, the samples slowly return to their original turbidity level. Other naturally aggregating proteins, such as skeletal muscle myosin, demonstrate a decrease in turbidity under the same conditions. The surface tension of alpha-crystallin in aqueous solution decreases to a plateau with increasing protein subunit concentration, with an inflection point over the range 0.18-0.25 mM; cholate and other amphiphiles exhibit similar behavior. In contrast, plots of surface tension over the equivalent concentration range for other protein aggregates in the same buffer more closely approximate the types of curve obtained with short chain aliphatic acids. These results indicate that alpha-crystallin behaves like the protein version of a micelle.

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.

Covalent modification of serum transferrin with phospholipid and incorporation into liposomal membranes

A method is described for incorporation of water-soluble proteins into liposomal membranes using covalent protein-phospholipid conjugates in detergent solution. A disulfide derivative of phosphatidylethanolamine containing a reactive N-hydroxysuccinimide ester group is synthesized, and the derivative is reacted with serum transferrin in deoxycholate-containing buffer. Disulfide-linked transferrin-phosphatidylethanolamine conjugates containing up to 6 mol phospholipid/mol protein are prepared. The amphiphilic conjugates have solubility properties very similar to integral membrane proteins. The conjugates self-associate to form protein micelles of narrow size distribution (Stokes radii 6-7 nm), and in the presence of excess phospholipid (egg phosphatidylcholine), they readily incorporate into liposomal membranes upon removal of detergent. Stable incorporation into liposomes requires the introduction of two molecules of phosphatidylethanolamine into the transferrin. Using the disulfide linker to release transferrin from the liposomes, evidence is presented for a function of the phosphatidylethanolamine as an anchor-molecule into the liposomal lipid. Optimal conditions for preparation of homogeneous liposomes with diameters in the range 30-125 nm and with a varying content of transferrin are defined. The liposomes appear well suited for studies on liposome-cell membrane interactions.

Modification of trypsin-solubilized cytochrome b5, apocytochrome b5, and liposome-bound cytochrome b5 by diethylpyrocarbonate

The interactions of diethylpyrocarbonate (DEP) with the various forms of cytochrome b5 were studied to gain a better understanding of the factors that influence the extent of modification of the axial histidines of cytochrome b5. Very low concentrations of DEP were able to decrease the heme binding capacity of apocytochrome b5. Moreover, it was shown that two additional histidines, presumed to be the axial ligands (His 39 and 63), were modified in the apo but not the holo form of a given preparation of cytochrome b5. Trypsin-solubilized bovine cytochrome b5 was resistant to the effects of DEP. A 200-fold molar excess of DEP displaced only 15% of the heme in the trypsin-solubilized protein in contrast to an 84% displacement of the heme in the detergent-solubilized protein. However, detergent-solubilized cytochrome b5 which had been incorporated into phospholipid vesicles exhibited the same reactivity with DEP as did the trypsin-solubilized protein. This is attributed to the fact that the two resistant preparations of cytochrome b5 are monomeric in their respective environments while detergent-solubilized cytochrome b5 is known to exist as an octamer in aqueous solutions. Our studies suggest that dissociation of the octamer to the monomer results in a conformational change that decreases the reactivity of the axial ligands of the hydrophilic heme-containing domain of cytochrome b5. Examination of the cytochrome b5 molecule by computer graphics indicates that a tunnel leads from the surface of the molecule to axial histidine 63 and that axial histidine 39 is buried.

Thermodynamic investigations of cytochrome b5 unfolding. II. Detergent-solubilized cytochrome b5 in solution and in a reconstituted system with dimyristoyl phosphatidylcholine

Thermal unfolding of the detergent-solubilized cytochrome b5 was investigated by scanning calorimetry. The protein shows different thermostability in the presence and absence of detergent, and it achieves the maximal transition temperature after incorporation into dimyristoyl phosphatidylcholine liposomes. However, transition temperature and Gibbs energy change at unfolding are still lower than that of the tryptic fragment of cytochrome b5 in aqueous solution. Cytochrome b5 undergoes in aqueous solution in the absence of detergent an irreversible, complicated transition, but it remains in the associated state after thermal denaturation. Half transition temperature, enthalpy and heat capacity changes of cytochrome b5 unfolding under various external conditions are reported and compared with the corresponding values of the tryptic fragment of the protein. The thermodynamic data and independent results are suitable for detailing a model proposed by Tanford (The Hydrophobic Effect (1980), pp. 205-211, John Wiley & Sons, New York) for the spatial arrangement of the protein within the membrane.

Different modes of membrane interactions of the signal sequence of carp preproinsulin and of the insertion sequence of rabbit cytochrome b5

The signal segment of the secretory protein carp preproinsulin is shown to be bound by a protein receptor present in the rough endoplasmic reticulum membrane. The receptor does not bind the insertion segment of the integral membrane protein cytochrome b5. On the other hand, the insertion sequence, in contrast to the signal sequence, is dissolved into the lipid bilayer of natural and artificial membranes. Hydrophobicity or length per se of the two types of peptides cannot be responsible for their different behaviour. We rather propose that the difference resides in their tertiary structure. Insertion peptides may form a compact structure with a diffuse hydrophobic surface, presumably by internal hydrogen bonding. Signal peptides would form hydrogen bonds with a membrane-bound receptor protein, presumably by producing a beta-sheet structure, but their extended structure in aqueous solution would not allow them to dissolve into lipid bilayers.

Association of Pseudomonas cytochrome oxidase with liposomes

Purified Pseudomonas cytochrome oxidase has been associated with asolectin liposomes by two different methods. Firstly, the enzyme was attached to liposomic membranes by adding it to a cholate-phospholipid dispersion and subsequently dialyzing the detergent out of suspension. In the second case the enzyme was adsorbed on the preformed liposomes when added to them after the dialysis. A stimulation of the cytochrome oxidase activity approximately twenty-fold was observed by the first method. In contrast, the activation was absent in the second type of preparation, indicating that interaction between the enzyme and phospholipids is very different in the two types of vesicles. The cholate-dialysis method for reconstitution of protein-phospholipid vesicles seems to lead to rather heterogenous preparations. These can be further fractionated, not only according to their size but also to the protein/phospholipid ratio, by gel chromatography.