Intramembranous particles and ripples in lipid-cytochrome c bilayers

The invariant ARG91 is required for the rupture of liposomes by cytochrome C

Lipid binding properties of cytochrome c (cyt c) were investigated by using semisynthetic mutant protein having amino acid substitution on the evolutionary invariant residue Arg91. We demonstrate here that the membrane binding properties of cyt c are dramatically altered by substituting norleucine for the invariant Arg91. More specifically, while the binding of this mutant to liposomes per se is indistinguishable from the wild type protein, it completely lacks the ability of the native cyt c to rupture liposome bilayers.

Persistence of cytochrome c binding to membranes at physiological mitochondrial intermembrane space ionic strength

We have shown that cytochrome c (cyt c) diffuses primarily in three dimensions in the intermembrane space (IMS) of intact mitochondria at physiological ionic strength (I). Recently, we found that a small percentage (11.2 +/- 2.1%) of endogenous cyt c remains bound to inner mitochondrial membranes (IMM) at high, physiological I (I = 150 mM), even after extensive washing with solutions at physiological I, overnight dialysis, changes in medium osmolarity, or further purification of IMM at high I using self-generating Percoll gradients. Measurements of heme c/heme a ratios, and electron transport (ET) reactions in which cyt c participates, confirmed the presence of a low amount of this I-resistant, membrane-bound form of cyt c (MB-cyt c), that had one third of the ET activity of electrostatically-bound cyt c (EB-cyt c), and which could not account for maximal ET rates. The amount of MB-cyt c was significantly increased above endogenous MB-cyt c by exposing KCl-washed IMM to increasing concentrations of exogenous cyt c. Also, subjecting large unilamellar vesicles (LUV) to successive cycles of cyt c binding/high I KCl-washes gave progressive increases in MB-cyt c. These protocols allowed in vitro characterization of MB-cyt c. The I at which binding takes place affects the affinity of cyt c for membranes, and oxidized cyt c had a greater intrinsic affinity for IMM or SUV than reduced cyt c. MB-cyt c appears to be bound partially by hydrophobic interactions since MB-cyt c was detected on negatively charged (asolectin) LUV and also on neutral, zwitterionic (phosphatidylcholine) LUV at high I. Consistent with the concentration-dependent changes in MB-cyt c, decreasing the IMS-volume of intact mitochondria (i.e., increasing th endogenous IMS-cyt c concentration) by metabolic or osmotic means increased the amount of MB-cyt c. After cyt c was delivered into the IMS by liposome-mediated low pH-induced fusion, resonance energy transfer showed a time-dependent cyt c-membrane proximity which was consistent with slow exchange of soluble IMS-entrapped cyt c molecules with a population bound to membranes at I = 150 mM. We conclude that, even though the majority of functional IMS-cyt c diffuses in three dimensions, a small portion remains firmly bound on the surface of the IMM under I conditions that are physiological for intact mitochondria. The occurrence of MB-cyt c may reflect an intrinsic conformational flexibility in cyt c, that allows a degree of membrane penetration and the formation of hydrophobic interactions which stabilize the membrane-bound form. The persistence of cyt c-membrane interactions under physiological I conditions indicates that cyt c-mediated ET in the IMS involves both fast (3D-diffusion) and slow (2D-diffusion) pathways for electron transfer.

Advantages of the use of intact plant tissues in freeze-fracture electron microscopy

The use of whole, intact plant tissue for freeze-fracture electron microscopy provides important information that cannot be obtained from the use of isolated biological membranes or of artificial (phospholipid) membrane preparations. This is not to imply that these examinations of such preparations are not useful, since it would be difficult to interpret our observations of intact cells and tissues without the analysis of these model systems. Analysis of intact tissue and cells reveals the relative densities of membrane proteins of the different membranes within a cell; the three-dimensional organization of various organelles, especially the endoplasmic reticulum; changes in intramembranous particle (IMP) distribution due to stress or injury; and, in conjunction with the use of filipin, membrane sterol content and relative distribution. It is our intention that this survey of freeze-fracture images of intact plant tissues will illustrate the uniqueness of the information gained from an analysis of whole plant tissues compared to isolated membrane fractions.

Lipid polymorphism of model and cellular membranes as revealed by electron microscopy

This review surveys the current state of knowledge relating to lipid polymorphism within both model lipid membrane and cellular membrane systems. Emphasis is placed upon the contribution of data obtained by transmission electron microscopy of freeze-fractured specimens. Some consideration is also given to the other important methods for the study of lipid polymorphism, namely X-ray diffraction and NMR spectroscopy. A detailed account of the different phases found in lipid mixtures within model membranes (bilayer, cubic or tetragonal, tubular or hexagonal) provides the background to the understanding of the factors involved in polymorphic phase transitions. The sequential steps involved in lipid polymorphism are defined from electron microscopical data and are related to the structural changes that can be detected within biological membranes. It is proposed that the fine structural changes detected at the initial stages of polymorphic transition in vivo may be highly relevant in relation to membrane fusion events, to the formation of tight junctions, and even to physiological transport processes. Since the later stages of polymorphic transition generally destroy the permeability barrier of model and cellular membranes, extensive rather than localized phase transition of the lipid bilayer is not at the moment considered to be compatible with cellular viability.

Interaction of cytochrome c with liposomes: covalent labeling of externally bound protein by the fluorescent probe, azidonaphthalenedisulfonic acid, enclosed in the inner aqueous compartment of unilamellar vesicles

The photoreactive fluorescent probe, 3-azidonaphthalene-2,7-disulfonic acid (ANDS) was encapsulated in the inner aqueous compartment of small unilamellar liposomes, prepared from egg phosphatidylcholine (PC) +/- 20 mol% dihexadecylphosphate (DHP). After adding cytochrome c externally to a suspension of these vesicles, the probe was activated by ultraviolet irradiation, and the protein was separated from the lipids. When negatively charged (egg PC/DHP) vesicles at low ionic strength were used, which form an electrostatic complex with cytochrome c, the protein was labeled by ANDS. This process depended on irradiation time, and was inhibited by increasing the ionic strength of the medium. Labeling was not observed with isoelectric (egg PC) vesicles. These observations suggest that electrostatic binding of cytochrome c to the bilayer is accompanied by intramembrane penetration to such a depth that the protein can communicate with the inner membrane-water interface.

Model membrane morphology and crosslinking of oxidized lipids with proteins

Model membranes composed of thion-phosphatidylcholine, cardiolipin, and cytochrome c have been studied by 31P NMR, polyacrylamide gel electrophoresis, gel filtration, fluorescence, and freeze-fracturing. Covalent binding of oxidized phospholipids to cytochrome c was shown to result in the formation of high-molecular-weight oligomeric complexes via Schiff base formation between a protein molecule and aldehydes produced upon peroxidation of phospholipids. The initial steps of the protein oligomerization lead to the appearance of intramembranous particles (IMPs) of various size and distribution on freeze-fractured faces of these model membranes. In the final phase of the crosslinking between cytochrome c and oxidized products of cardiolipin there is a breakdown of membrane vesicles and formation of globular lipoprotein complexes which are seen as globular particles. It is believed that the covalent linking between the products of phospholipid peroxidation and membrane proteins causes the oligomerization of membrane proteins and structural alteration in the hydrophobic region of other models also and, perhaps, in biological membranes.

1H-nuclear magnetic resonance study of the association of the basic protein of central nervous system myelin with lysophosphatidylcholine

High-resolution 270 MHZ 1H-nuclear magnetic resonance spectroscopy has been used to follow the interaction of myristoyllysophosphatidylcholine with bovine myelin basic protein. At lipid/protein ratios up to 30:1 it proved possible to follow changes in the spectra of both the protein and the lipid. Lysophosphatidylcholine induced several changes in the protein spectrum. Foremost amongst these changes were downfield shifts of histidine C2 protons, and upfield shifts and broadening of the phenylalanine aromatic proteins. Several other resonances assigned to nonpolar amino acid side chains also broadened. But even at a lipid/protein molar ratio of 30:1 the majority of the protein appeared to remain in a loosely coiled conformation. In the presence of the protein the lipid acyl chain peaks were moved upfield and broadened, whereas the resonances associated with the head-group protons were unaffected. These changes were consistent with partial immobilization of the acyl chain of lysophosphatidylcholine on binding to the basic protein, with hydrophobic interactions providing the predominant attraction between this lipid and the basic protein.