The interaction of the “Folch-Lees” protein with lipids at the air-water interface

PI(4,5)P2 and Cholesterol: Synthesis, Regulation, and Functions

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is the most abundant membrane phosphoinositide and cholesterol is an essential component of the plasma membrane (PM). Both lipids play key roles in a variety of cellular functions including as signaling molecules and major regulators of protein function. This chapter provides an overview of these two important lipids. Starting from a brief description of their structure, synthesis, and regulation, the chapter continues to describe the primary functions and signaling processes in which PI(4,5)P₂ and cholesterol are involved. While PI(4,5)P₂ and cholesterol can act independently, they often act in concert or affect each other's impact. The chapters in this volume on "Cholesterol and PI(4,5)P₂ in Vital Biological Functions: From Coexistence to Crosstalk" focus on the emerging relationship between cholesterol and PI(4,5)P₂ in a variety of biological systems and processes. In this chapter, the next section provides examples from the ion channel field demonstrating that PI(4,5)P₂ and cholesterol can act via common mechanisms. The chapter ends with a discussion of future directions.

On the mechanism of cholesterol interaction with apolipoproteins A-I and E

It is shown that cholesterol may interact with some substances containing the guanidine group (guanidine itself, arginine, metformin and dodecylguanidine bromide) and with arginine-rich proteins--apoproteins A-I and E. In the latter case the interaction produces the formation of cholesterol-apoprotein complexes. Analysis of such complexes has shown that one apo A-I molecule binds 17-22 and one apo E molecule binds 30-35 sterol molecules, which approximately corresponds to the amount of arginine residues in these proteins. Formation of cholesterol-apoprotein complexes has been suggested to occur due to: (1) formation of hydrogen bond and/or ion-dipole interaction between cholesterol hydroxyl and guanidine groups of the apoprotein arginine residues and (2) hydrophobic interaction of the cholesterol aliphatic chain with nonpolar side chains of the amino acids occupying the third position from arginine in the protein molecule.

ESR imaging of myelin basic protein induced vesicle aggregation

Using a modulated magnetic field gradient technique, the conventional ESR spectrum of well-defined spatial sections and the one-dimensional-ESR image of the nitroxide centre line of spin-labeled stearic acid in phospholipid vesicles were recorded with a spatial resolution of 4.10(-5) m after pelleting the vesicles inside 1 mm (i.d.) sample capillaries in a slow centrifuge (2500 X g). The sedimentation characteristics of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol vesicles were quantitatively compared with particular reference to vesicle aggregation induced by myelin basic protein. Protein-induced changes in the effective molecular mass were determined from ESR images of sedimentation profiles. The present data lend further support to the notion that the primary target of myelin basic protein-lipid interaction is the acidic lipid pool of myelin.

An electron spin resonance spin-label study of lipophilin in oriented phospholipid bilayers

Model membranes consisting of dimyristoyl phosphatidylcholine and a hydrophobic protein from bovine myelin, lipophilin, were studied using the cholesterol-resembling cholestane ESR spin label. Orientation of the membranes made it possible to deconvolute the spectra into two fractions, one of oriented spin labels reflecting phospholipid bilayer of high order, and one of isotropically tumbling spin labels ascribed to the lipid fraction surrounding the protein molecule (boundary lipid). This isotropic tumbling is different from the behavior of phospholipid molecules near the protein, which retain some degree of order, and indicates that the boundary lipid fraction in our model system forms a rather fluid environment for the protein. A nonlinear relation was found between protein concentration and amount of boundary spin labels. Addition of cholesterol decreases the amount of boundary spin labels. Both findings form evidence for a preferential binding of cholesterol by the membrane protein.

Lipophilin (proteolipid apoprotein) of brain white matter. Purification and amino acid sequence studies of the four tryptophan fragments

The chemical cleavage of lipophilin (proteolipid apoprotein) from bovine brain white matter with HBr/dimethyl sulfoxide at the tryptophan residues, under conditions adapted to this hydrophobic protein, releases four fragments with approximate molecular masses 14 kDa (Trp I), 6.8 kDa (Trp IV), 5.2 kDa (Trp III) and 2.1 kDa (Trp II). These fragments have been separated and purified by a combination of solvent distribution, molecular sieve chromatography (Bio-Gel P-150) and high-performance liquid chromatography for automated Edman degradation and combined gas-liquid chromatography/mass spectroscopy. The complete amino acid sequences of Trp II and III and large sequences of Trp I are reported in this communication. The amino acid sequence of Trp IV and the sequences of peptides releasable from lipophilin by proteolytic enzymes (trypsin, thermolysin, subtilisin, chymotrypsin) have been described in previous reports from this laboratory. Despite two small gaps in the complete primary structure of lipophilin from myelin of central nervous system, our sequence data suggest the arrangement of four long hydrophobic sequences (30-40 apolar amino acid residues) within the hydrophobic core of the myelin lipid bilayer, linked by three hydrophilic regions at the aqueous membrane interphase. These features lend lipophilin the properties of a polytopic membrane protein.

In vivo calcification induced by a proteolipid complex (lysozyme-acidic phospholipid)

A synthetically prepared proteolipid complex was inserted into experimentally produced osseous wounds in the tibias of rats. Similar control wounds were made in the humeri and no proteolipid was inserted. At 7-day intervals for 28 days, rats were sacrificed and both the control and experimental sites were evaluated by gross inspection after the overlying soft tissues had been surgically removed. Specimens were then processed for histologic evaluation employing a Zeiss Videoplan Image Analysis System with Osteoplan for quantitative bone morphometrics. At the 7 and 14 day levels, the rate of osseous wound healing at the experimental sites greatly exceeded the rate at the control sites (p less than 0.0005). By 21 days the experimental areas still showed an accelerated healing response compared with the controls (p less than 0.005). By 28 days the experimental and the control wounds demonstrated almost complete osseous healing.

Dinosterol in model membranes: fluorescence polarization studies

The interaction between the phospholipids of Crypthecodinium cohnii, a heterotropic marine dinoflagellate, and its major sterol dinosterol is less soluble in model membranes and as effective in increasing the microviscosity. These results indicate that the unique side chain of dinosterol does not play a special role in terms of complementary interaction with the phospholipids of this organism.

Dual effect of membrane cholesterol on simple and mediated transport processes in human erythrocytes

The influence of cholesterol on simple and facilitated transport processes across the membrane of intact human erythrocytes was studied after graded depletion or enrichment of membrane cholesterol by incubation of the cells in phospholipid or phospholipid/cholesterol suspensions. 1. The carrier-mediated transfer of L-lactate and of L-arabinose proved to be enhanced in this effect. In contrast, the self-exchange of SO4(2-), mediated by the inorganic anion-exchange system, and the simple diffusion of erythritol via the lipid phase of the membrane are inhibited by cholesterol. 2. Reversibility of these two opposite effects of cholesterol was demonstrated by measurements on cells depleted again after cholesterol enrichment and enriched again after previous depletion. 3. Certain phospholipids used for preparing the lipid dispersions that are required for cholesterol variation have effects on permeability of their own, due, for example, to traces of contaminants. A discrimination of such artifacts due, for example, to traces of contaminants. A discrimination of such artifacts from the effects of cholesterol is only possible by demonstrating reversibility. 4. The opposite effects of cholesterol on various facilitated transfer processes, which have a correlation in the opposite effects of other modifications of the membrane lipid phase (Deuticke, B., Grunze, M. and Haest, C.W.M. (1979) Alfred Benzon Symposium 14, Munksgaard, Copenhagen, in the press), are indicative of different types of lipid-protein interaction in the erythrocyte membrane.

Interactions of Folch-Lees proteolipid apoprotein with planar lipid bilayers

Water-soluble Folch-Lees proteolipid apoprotein from bovine CNS white matter induces a voltage-dependent conductance in black lipid membranes. Na+ is required for the induced conductance change but the established conductance has very low ionic selectivity. The induced conductance fluctuates with a minimum amplitude of 10(-11)--10(-10) mho. The magnitude of the conductivity change is dependent on protein concentration and on the composition of lipid bilayers. At a fixed voltage the induced conductance of a phosphatidylcholine-cholesterol membrane is proportional to the sixth power of the protein concentration and the first power of Na+ concentration. The interactions between the apoprotein and the lipids are both electrostatic and hydrophobic, but the interaction leading to the conductance increase appears to be mainly hydrophobic. Both the increase in conductance and the current fluctuations remain after extensive washing of the chambers to remove the protein. Furthermore, pronase or glutaraldehyde added to either the cis or trans side of the membrane does not affect the apoprotein-established conductance. However, if the bilayer is formed in the presence of both the apoprotein and pronase or if the apoprotein is treated with pronase prior to its addition to the chamber, no conductance change is observed. The association of the apoprotein with the membrane thus appears to render the protein inaccessible to proteolytic digestion, suggesting that the apoprotein is at least partially imbedded in the membrane interior.

Interactions of band 3-protein from human erythrocyte membranes with cholesterol and cholesterol analogues

The interaction between cholesterol and band 3-protein from human erythrocyte membranes was studied by incorporating the solubilized protein into monolayers of cholesterol and related sterols at the air-water interface and measuring the changes in surface pressure which accompanied protein incorporation. The following results were obtained: 1) Band 3-protein shows a very strong interaction with cholesterol monolayers. Both apolar and polar bonds contribute to this interaction. 2) Steroids with a structure slightly different from that of cholesterol (especially with respect to the polar group and the side chain) in most cases show a reduced affinity for band 3-protein. Thus, the protein-sterol interaction is highly specific. It is assumed that the protein-cholesterol interaction can be subidivided into two parts: an unspecific one which results from contributions from several sterol molecules, and a specific one which is due to the high affinity binding of the protein and cholesterol. The structural element responsible for the high affinity interaction is assumed to be a sterol-binding niche on the surface of band 3-protein. The sterol is thought to be held in the niche by a hydrogen bond at its polar head and a variety of hydrophobic bonds along its ring system and side chain.

Interaction of the acetylcholine (nicotinic) receptor protein from Torpedo marmorata electric organ with monolayers of pure lipids

Membrane fragments rich in cholinergic (nicotinic) receptor protein were purified from the electric organ of Torpedo marmorata. Their lipid composition is essentially characterized by the prominence of cholesterol, phosphatidylethanolamine and phosphatidylcholine, long-chain fatty acyl constituents, and the absence of sphingomyelin. Solubilised receptor was purified from these fragments and the concentration of sodium cholate lowered by dialysis to 0.01% (w/v). When this preparation was injected under a lipid monolayer, an increase of surface pressure developed, which was not observed with the detergent alone nor in the absence of lipid film. When covalently radiolabelled receptor preparations were injected at a constant surface pressure the radioactivity recovered with the film was proportional to the increase in area. It is concluded that the pressure or area increases are due to the penetration of the cholinergic receptor protein into the lipid film. Incorporation experiments into films formed from various pure lipids showed that the protein interacts more readily with cholesterol than with ergosterol, phosphatidylcholine, or other phospholipids. Its affinity is also higher for long-chain phosphatidylcholines than for short-chain ones. The degree of unsaturation and fluidity of the 3-sn-phosphatidylcholine (lecithin) films are of secondary importance. Parallel experiments with covalently and non-covalently labelled receptor preparations showed that part of the protein recovered with the film lost its alpha-toxin binding ability during the penetration. Similar data were obtained with the receptor purified from Electrophorus electricus electric organ.

Interactions of acetylcholine receptor and acetylcholinesterase with lipid monolayers

The interaction of acetylcholine receptor and acetylcholinesterase with lipid monolayers was followed by measuring changes in surface pressure. When injected into the subphase of a lipid monolayer, the proteins caused increases in surface pressure from 5 to 10 dynes/cm, indicating a penetration of protein into the monolayer. At pH values below the isoelectric point of the proteins the incorporation was improved. The same was observed when Ca2+ (2mM) was added. The presence of the enzyme in the mixed film could be demonstrated by using diiso [3H] propyl fluorophosphate-labelled acetylcholinesterase as well as by measuring enzyme activity. Acetylcholine receptor was shown to be present in the mixed film by using a complex made of the receptor and alpha-[3H]neurotoxin.

Effect of lipids on enzymatic activity of pig heart mitochondrial malate dehydrogenase monomolecular films

The effect of various lipids on the enzymatic activity of pig heart mitochondrial malate dehydrogenase monomolecular films was studied using the subphase exchange technique described previously. Surface pressure-surface area (pi-A) curves of mixed films of enzyme with dipalmitoyllecithin, egg lecithin, cholesterol, and phospholipids extracted from pig hearts showed that the enzyme interacted with all of the lipids and that the enzyme remained in the film at pressures well above the collapse pressure of malate dehydrogenase in the absence of lipid. The surface enzyme activity was dependent on surface pressure for each lipid; in all cases, the lipids greatly broadened the range of surface pressures where surface enzyme activity was observed. The pi-A and enzyme activity data showed good correlation. Although the simple model system employed does not simulate the complexity of the biological membrane, it gives some evidence for the role of lipids in the stability of membrane-bound enzymes.

Crystalline patterns of myelin lipids visualized by freeze fracture

Freeze fracture of rat optic nerve reveals smooth, particle-free regions on the lammellar fracture faces of myelin when prepared by standard procedures. When the fixed, glycerin-impregnated tissue is incubated at 6 degrees C for two or more days, crystalline patterns indicative of a phase transition can be seen in the particle-free regions. The crystalline patterns can be destroyed by subsequent incubation at 37 degrees C and are not seen when the initial incubation is at room temperature or 37 degrees C. Butylated hydroxytoluene has no effect on the formation of the crystalline patterns. The time course of the formation of the crystalline patterns suggest that the rate-limiting step in the process is not the phase transition itself. We propose that the lipids associated with the particles in vivo are involved in the formation of the crystalline patterns.

Lipid-protein interactions with native and modified myelin basic protein

The basic protein of central nervous system myelin has been shown to form complexes with acidic lipids in vitro. We measured the interaction of myelin basic protein with several charged and neutral lipids in a biphasic chloroform/methanol/water system and investigated the effect of decreasing the electrical charge of the basic amino groups of the myelin basic protein by acetylation. The modified myelin basic protein, which has an average of eight acetyl residues incorporated, was characterised by gel electrophoresis and circular dichroism. Complexes formed between the acetylated myelin basic protein and acidic lipids exhibited a reduction in the amount of lipids bound, a value that could be correlated with the number of modified amino groups. The significance of these experiments with reference to protein-lipid interaction in the myelin membrane is discussed.

Preparation and properties of vesicles of a purified myelin hydrophobic protein and phospholipid. A spin label study

Lipophilin, a hydrophobic protein purified from the proteolipid of normal hupid and protein in 2-chloro-ethanol followed by dialysis against buffer. This method resulted in homogeneous incorporation of the protein into lipid vesicles as judged by sedimentation on a sucrose gradient and freeze fracture electreter and the freeze fracture faces contained intramembrane particles. The effect of lipophilin on the organization of the lipid was studied by use of spin label probes. Two distinct components were present in the spectrum of fatty acid spin labels in the lipid-protein vesicles. One was immobilized presumably due to the presence of boundary lipid around the protein and the second component waicles and probably due to a lamellar phase but with a slightly greater order parameter. Lipophilin was found to increase the order parameter linearly with increasing concentration of protein incorporated into the vesicles. However, the phase transition temperature as measured from the 2,2,6,6-tetramethyl piperidine-1-oxyl (TEMPO) solubility parameter was unchanged.

Lipid-protein interactions in model membranes from bovine brain white matter. An ESR spin label and electron microscopy study

Lipid-protein model membranes, prepared from bovine brain white matter and containing all the lipids and Folch-Lees proteolipids, have been studied in macroscopically oriented multibilayers. To examine the lipid environment the membranes were spin labeled with the cholestane spin label (3'-spiro(2'=(N-oxyl-4',4'-dimethyl-oxazolidine))5alpha-cholestane) and a fatty acid spin label (4',4'-dimethyloxazolidine-N-oxyl derivative of 5-ketostearic acid). The ESR spectra exhibit two components arising from fairly well oriented and completely unoriented lipids. Up to a temperature of 55 degrees C the amount of oriented lipids is almost constant, being about 35%. At higher temperatures this percentage drops rapidly to zero. It is shown that the presence of unoriented lipids arises mainly from disrupted areas in the lipid bilayer structure. This is confirmed by electron miccroscopy and from an analysis of the temperature dependence of the order parameters of the spin labels. The presence of locally disrupted lipid parts in the bilayer is discussed in relation to the interaction of the brain white matter lipids with Folch-Lees protein.

Protein-liposome interactions and their relevance to the structure and function of cell membranes

Recent studies on the interactions of soluble proteins, membrane proteins and enzymes with phospholipid model membranes are reviewed. Similarities between the properties of such systems and the behavior of biomembranes, such as alterations in the redox potential of cytochrome c after binding to membranes and effects of phospholipid fluidity on (Na+K) ATPase activity, are emphasized. The degree of correspondence between the behavior of model systems and natural membranes encourages the continuing use of model membranes in studies on protein-lipid interactions. However, some of the data on the increase of surface pressure of phospholipid monolayers by proteins and increases in the permeability of liposomes indicate that many soluble proteins also have a capability to interact hydrophobically with phospholipids. Thus a sharp distinction between both peripheral and integral membrane proteins and non-membrane proteins are not seen by these techniques. Cautious use of such studies, however, should lead to greater understanding of the molecular basis of cell membrane structure and function in normal and pathological states. Studies implicating protein-lipid interactions and (Na+K) ATPase activity in membrane alterations in disease states are also briefly discussed.

Interaction of a purified hydrophobic protein from myelin with phospholipid membranes: studies on ultrastructure, phase transitions and permeability

A purified protein fraction from the proteolipids of human brain myelin was recombined with different lipids either in aqueous buffer or in a chloroform-methanol-water (10:5:1, v/v/v) mixture. It was found that under both conditions it binds strongly to phospholipids irrespective of surface charge, the presence of cholesterol or double bonds on the fatty acyl chains. The buoyant density of the resulting lipoprotein membranes is intermediate to that of pure lipids, and proteins. The lipoproteins formed by either of these methods were observed by either freeze-fracture or negative stain electron-microscopy. The overall morphology was similar to that of pure phospholipids, showing large closed multilamellar vesicles. The presence of the protein was detected by the appearance of intramembrane particles in freeze-fracture. The addition of the N-2 protein generally increases the permeability vesicles to 22-Na-+ by 2-3 orders of magnitude depending on the concentration. The presence of calcium in the aqueous medium further increases the Na-+ efflux through negatively charged vesicles. Changes in lipid composition, surface charge, cholesterol, etc., have no appreciable influence on the effect of the protein. Differential scanning calorimetry indicates that the presence of small amounts of N-2 have no effect on the lipid phase transition from solid to liquid crystalline. As the amount of protein bound to the phospholipid increases, the enthalpy of the transition decreases, the main endothermic peak broadens, but there is no change on the midpoint temperature. Membranes containing 50% by weight of protein still show a transition with an enthalpy approximately one half that of the original lipid.

Relation between various phospholipase actions on human red cell membranes and the interfacial phospholipid pressure in monolayers

The action of purified phospholipases on monomolecular films of various interfacial pressures is compared with the action on erythrocyte membranes. The phospholipases which cannot hyorolyse phospholipids of the intact erythrocyte membrane, phospholipase C from Bacillus cereus, phospholipase A2 from pig pancreas and Crotalus adamanteus and phospholipase D from cabbage, can hydrolyse phospholipid monolayers at pressure below 31 dynes/cm only. The phospholipases which can hydrolyse phospholipids of the intact erythrocyte membrane, phospholipase C from Clostridium welchii phospholipase A2 from Naja naja and bee venom and sphingomyelinase from Staphylococcus aureus, can hydrolyse phospholipid monolayers at pressure above 31 dynes/cm. It is concluded that the lipid packing in the outer monolayer of the erythrocyte membrane is comparable with a lateral surface pressure between 31 and 34.8 dynes/cm.

Changes in brain hydrolytic enzyme activities in rats treated with cholesterol biosynthesis inhibitor, AY9944

Intraperitoneal administration of AY9944 causes accumulation of 7-dehydrocholesterol, appearance of abnormal neuronal cytoplasmic lamellar inclusions containing acid phosphatase activities, and degeneration of oligodendroglial cells. In the present study, we attempted to correlate appearance and disappearance of abnormal inclusions, oligodendroglial degeneration, activities of lysosomal enzymes, and accumulation of 7-dehydrocholesterol. One group of 5-day-old rats received daily injection of AY9944, 30 mg/kg, for 30 days. Other groups received the same daily dosage but only in 10 consecutive days, starting from 5, 15, 25, and 35 days, respectively. Activities of 13 brain hydrolytic enzymes were determined. In the first group, activities of most enzymes increased over the controls, reaching the peak between the 15th and 25th day, corresponding to the maximum appearance of the neuronal inclusions and degenerating oligodendroglia. Despite the continued administration of AY9944 and the high tissue concentration of 7-dehydrocholesterol, activities of most enzymes declined after 25 days to relatively steady levels somewhat higher than controls. In the group which received 10-day injections, enzyme activities reached the peak at the end of the injections and then rapidly returned to normal within 10 days thereafter, corresponding to the appearance and disappearance of the abnormal inclusions. However, 7-dehydrocholesterol continued to increase for 10 days after the drug administration was discontinued. AY9944 had no direct effect on any of the enzymes in vitro. There appears to be generalized lysosomal activation concomitant with formation of abnormal neuronal inclusions in the experimental conditions.