Effect of cytochrome b5 on the size, density, and permeability of phosphatidylcholine vesicles

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.

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.

Construction of protocellular structures under simulated primitive earth conditions

We have developed experimental approaches for the construction of protocellular structures under simulated primitive earth conditions and studied their formation and characteristics. Three types of envelopes; protein envelopes, lipid envelopes, and lipid-protein envelopes are considered as candidates for protocellular structures. Simple protein envelopes and lipid envelopes are presumed to have originated at an early stage of chemical evolution, interaction mutually and then evolved into more complex envelopes composed of both lipids and proteins. Three kinds of protein envelopes were constructed in situ from amino acids under simulated primitive earth conditions such as a fresh water tide pool, a warm sea, and a submarine hydrothermal vent. One protein envelope was formed from a mixture of amino acid amides at 80 degrees C using multiple hydration-dehydration cycles. Marigranules, protein envelope structures, were produced from mixtures of glycine and acidic, basic and aromatic amino acids at 105 degrees C in a modified sea medium enriched with essential transition elements. Thermostable microspheres were also formed from a mixture of glycine, alanine, valine, and aspartic acid at 250 degrees C and above. The microspheres did not form at lower temperatures and consist of silicates and peptide-like polymers containing imide bonds and amino acid residues enriched in valine. Amphiphilic proteins with molecular weights of 2000 were necessary for the formation of the protein envelopes. Stable lipid envelopes were formed from different dialkyl phospholipids and fatty acids. Large, stable, lipid-protein envelopes were formed from egg lecithin and the solubilized marigranules. Polycations such as polylysine and polyhistidine, or basic proteins such as lysozyme and cytochrome c also stabilized lipid-protein envelopes.

Phospholipid dependence of the anion transport system of the human erythrocyte membrane. Studies on reconstituted band 3/lipid vesicles

Band 3 protein extracted from human erythrocyte membranes by Triton X-100 was recombined with the major classes of phospholipid occurring in the erythrocyte membrane. The resulting vesicle systems were characterized with respect to recoveries, phospholipid composition, protein content and vesicle size as well as capacity and activation energy of sulfate transport. Transport was classified into band-3-specific fluxes and unspecific permeability by inhibitors. Transport number (sulfate ions per band 3 per minute) served as a measure of functional therapy after reconstitution. The transport properties of band 3 proved to be insensitive to replacement of phosphatidylcholine by phosphatidylethanolamine, while sphingomyelin and phosphatidylserine gradually inactivated band-3-specific anion transport when present at mole fractions exceeding 30 mol%. The activation energy of transport remained unaltered in spite of the decrease in transport numbers. The results, which are discussed in terms of requirements of band 3 protein function with respect to the fluidity and surface charge of its lipid environment, provide a new piece of evidence that the transport function of band 3 protein depends on the properties of its lipid environment just as the catalytic properties of some other membrane enzymes. The well-established species differences in anion transport (Gruber, W. and Deuticke, B. (1973) J. Membrane Biol. 13, 19-36) may to some extent reflect this lipid dependence.

Characterization of cytochrome P-450scc-containing liposomes

Purified cytochrome P450scc from bovine adrenocortical mitochondria was incorporated into liposomes by the cholate-dilution method utilizing either dialysis or Sephadex gel filtration. Among synthetic phospholipids tested, dioleoylglycerophosphocholine showed the best stability during the incorporation of P450scc into liposomes. A maximum amount of heme was incorporated into liposomes at a molar ratio of phospholipid to the cytochrome of approx. 200. When P450scc was incorporated into the dioleoylglycerophosphocholine liposomes by the cholate-filtration method, the P450scc-containing liposomes showed two major populations on the elution pattern of the Sepharose 4B gel filtration, and were seen at a diameter of 200-600 A and its aggregated forms. When the cytochrome was incorporated into dioleoylglycerophosphocholine liposomes or cholesterol-free adrenocortical mitochondrial liposomes, P450scc was less stable than P450scc in aqueous solution. Cholesterol or adrenodoxin markedly stabilized the liposomal P450scc. Liposomal P450scc required cholesterol for its optimum reduction with adrenodoxin, adrenodoxin reductase, and NADPH in the presence of CO. About 70% of the total heme in the dioleoylglycerophosphocholine liposomes was reduced by the enzymatic reduction in the presence of cholesterol, indicating that 70% of the total molecules are exposed to the surface of the outer monolayer. In order to see the location of the heme in membrane, the dioleoylglycerophosphocholine-liposomal P450scc was subjected to p-chloromercuriphenyl sulfonic acid treatment. This reagent destroyed the liposomal P450scc. These results suggest that the heme is located in the proximity of the p-chloromercuriphenyl sulfonic acid reacting sites which are exposed to the surface, or located on the vincinity of polar heads of the membrane.

Mechanism of cholesterol exchange between phospholipid vesicles

The kinetics of cholesterol exchange between two populations of small unilamellar vesicles has been investigated. There is no change in the initial rate of this exchange process over a 100-fold change in the acceptor vesicle concentration at a constant donor concentration. These results are not consistent with a collision-dependent exchange mechanism. In support of transfer via the aqueous phase, the inclusion of a negatively charged lipid into the vesicles did not affect the exchange rate. Evidence for a water-soluble pool of cholesterol that had partitioned ut of the vesicle was obtained. Finally, cholesterol exchange was observed when donor and acceptor membranes were separated by a barrier through which neither could pass. These data together support our contention that the exchange of cholesterol between these vesicles involves a water-soluble intermediate.

The binding of cytochrome b5 to plasma membranes of rat liver: its implication for membrane specificity and biogenesis

The in vitro incorporation of cytochrome b5 into purified plasma membranes was investigated by biochemical and immunological methods. Plasma membrane preparations incorporated three times less cytochrome b5 than did microsomal preparations; 60% of this cytochrome b5 could not be reduced by the NADH-cytochrome b5 reductase and considered as being bound to the plasma membrane. The morphological observations made after the immunochemical labeling of cytochrome b5 clearly showed a good but asymmetrical distribution of the ferritin labeling: only the inner face of the plasma membrane incorporated cytochrome b5. These results are discussed with respect to theories which concern the subcellular membrane relationships in the cell.

Block poly(Ala)-poly(Lys). A water-soluble model for intrinsic membrane proteins?

Block poly(Ala)16-poly(Lys)13.5 was synthesized by the Leuchs anhydride method. This polypeptide is water soluble in a largely monomeric form, but binds rapidly and spontaneously to unilamellar vesicles of dimyristoyl phosphatidylcholine at pH 7.4. The interaction is evidently of a hydrophobic nature since the complex is not disrupted by salt and no similar reaction is given by polylysine. Evidence for the interaction was obtained by ultrafiltration, chromatography on Sepharose 4B, and sedimentation velocity ultracentrifugation. While direct information on the molecular structure of the complex is still lacking, we propose that this amphipathic block copolymer binds to lipids in a similar manner as intrinsic membrane proteins and hence can be used to study the interactions of intrinsic proteins with lipids.

Incorporation of rat histocompatibility (AgB) antigens into liposomes, and their susceptibility to immune lysis

The purification of detergent-solubilized rat histocompatibility antigens is described. The antigen may readily be incorporated into synthetic vesicles (liposomes) which appear to be unilamellar and between 0.1 microbeter and 0.2 micrometer in diameter. The addition of specific antibody leads to the agglutination and precipitation of the liposomes. An enzyme, horseradish peroxidase, may be incorporated into the trapped aqueous phase of the liposome, and its release by antibody and complement can be detected.

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.

Asymmetric binding of cytochrome b5 to the membrane of human erythrocyte ghosts

The intact, amphipatic form of cytochrome b5 could bind to unsealed ghosts, but not to resealed ghosts, suggesting that the cytochrome could bind only to the inner (cytoplasmic) surface of the ghost membrane. This was further confirmed by the finding that the cytochrome could bind to closed, inside-out vesicles prepared from the ghosts. This asymmetric binding was not due to the exclusive localization of sialic acid and sugar chains on the outer surface of the ghosts membrane, because the cytochrome could not bind to ghosts even after enzymatic removal of these components. Although liposomes consisting of phosphatidylcholine or both phosphatidylcholine and sphingomyelin could effectively bind the cytochrome, this binding capacity was progressively decreased as increasing amount of cholesterol was included in the composition of phosphatidylcholine liposomes. Removal of cholesterol from resealed ghosts by incubation with egg phosphatidylcholine liposomes resulted in the binding of cytochrome b5 to the outer surface of the treated ghosts. The possibility is discussed that the asymmetric binding is due to preferential localization of cholesterol in the outer leaflet of the lipid bilayer that constitutes the ghost membrane.

The reactivities of tyrosine and tryptophan residues in lipid-bound cytochrome b5

Purified cytochrome b5 from rabbit liver microsomes was bound to liposomes prepared from microsomal lipids. Tyrosyl and tryptophyl side chains of the protein were modified by water-soluble reagents and the reactivities of these amino acid residues in the liposome-bound cytochrome b5 were compared to those of the free protein. At pH 13, 80% of the tyrosines in lipid-free cytochrome b5 ionized immediately, whereas in the lipid-bound protein only 65% ionized within the first minute. In contrast, acetylation with acetylimidazole resulted in the conversion of all 5 tyrosine groups of lipid-free as well as lipid-bound cytochrome b5 into O-acetylated derivatives, which upon treatment with hydroxylamine were completely deacetylated. Reaction with N-bromosuccinimide revealed that only 60% of the 4 tryptophan residues present in cytochrome b5 were accessible to the reagent in the lipid-bound protein, although all tryptophans could be modified in lipid free cytochrome b5. It was concluded that the two tyrosines in the region linking the protein to the membrane are not shielded by lipid bilayer but that of the three tryptophans in the same region one is completely buried in the membrane, whereas the remaining two tryptophans may be both partly exposed to the solvent or alternatively, one may be partially and the other completely exposed.