Myelin basic protein induces hexagonal phase formation in dispersions of diacylphosphatidic acid

Cardiolipin remodeling enables protein crowding in the inner mitochondrial membrane

Mitochondrial cristae are extraordinarily crowded with proteins, which puts stress on the bilayer organization of lipids. We tested the hypothesis that the high concentration of proteins drives the tafazzin-catalyzed remodeling of fatty acids in cardiolipin, thereby reducing bilayer stress in the membrane. Specifically, we tested whether protein crowding induces cardiolipin remodeling and whether the lack of cardiolipin remodeling prevents the membrane from accumulating proteins. In vitro, the incorporation of large amounts of proteins into liposomes altered the outcome of the remodeling reaction. In yeast, the concentration of proteins involved in oxidative phosphorylation (OXPHOS) correlated with the cardiolipin composition. Genetic ablation of either remodeling or biosynthesis of cardiolipin caused a substantial drop in the surface density of OXPHOS proteins in the inner membrane of the mouse heart and Drosophila flight muscle mitochondria. Our data suggest that OXPHOS protein crowding induces cardiolipin remodelling and that remodeled cardiolipin supports the high concentration of these proteins in the inner mitochondrial membrane.

Solid-state NMR spectroscopy of 18.5 kDa myelin basic protein reconstituted with lipid vesicles: spectroscopic characterisation and spectral assignments of solvent-exposed protein fragments

Myelin basic protein (MBP, 18.5 kDa isoform) is a peripheral membrane protein that is essential for maintaining the structural integrity of the multilamellar myelin sheath of the central nervous system. Reconstitution of the most abundant 18.5 kDa MBP isoform with lipid vesicles yields an aggregated assembly mimicking the protein's natural environment, but which is not amenable to standard solution NMR spectroscopy. On the other hand, the mobility of MBP in such a system is variable, depends on the local strength of the protein-lipid interaction, and in general is of such a time scale that the dipolar interactions are averaged out. Here, we used a combination of solution and solid-state NMR (ssNMR) approaches: J-coupling-driven polarization transfers were combined with magic angle spinning and high-power decoupling to yield high-resolution spectra of the mobile fragments of 18.5 kDa murine MBP in membrane-associated form. To partially circumvent the problem of short transverse relaxation, we implemented three-dimensional constant-time correlation experiments (NCOCX, NCACX, CONCACX, and CAN(CO)CX) that were able to provide interresidue and intraresidue backbone correlations. These experiments resulted in partial spectral assignments for mobile fragments of the protein. Additional nuclear Overhauser effect spectroscopy (NOESY)-based experiments revealed that the mobile fragments were exposed to solvent and were likely located outside the lipid bilayer, or in its hydrophilic portion. Chemical shift index analysis showed that the fragments were largely disordered under these conditions. These combined approaches are applicable to ssNMR investigations of other peripheral membrane proteins reconstituted with lipids.

Biophysics of sphingolipids II. Glycosphingolipids: An assortment of multiple structural information transducers at the membrane surface

Glycosphingolipids are ubiquitous components of animal cell membranes. They are constituted by the basic structure of ceramide with its hydroxyl group linked to single carbohydrates or oligosaccharide chains of different complexity. The combination of the properties of their hydrocarbon moiety with those derived from the variety and complexity of their hydrophilic polar head groups confers to these lipids an extraordinary capacity for molecular-to-supramolecular transduction across the lateral/transverse planes in biomembranes and beyond. In our opinion, most of the advances made over the last decade on the biophysical behavior of glycosphingolipids can be organized into three related aspects of increasing structural complexity: (1) intrinsic codes: local molecular interactions of glycosphingolipids translated into structural self-organization. (2) Surface topography: projection of molecular shape and miscibility of glycosphingolipids into formation of coexisting membrane domains. (3) Beyond the membrane interface: glycosphingolipid as modulators of structural topology, bilayer recombination and surface biocatalysis.

Protein-mediated surface structuring in biomembranes

The lipids and proteins of biomembranes exhibit highly dissimilar conformations, geometrical shapes, amphipathicity, and thermodynamic properties which constrain their two-dimensional molecular packing, electrostatics, and interaction preferences. This causes inevitable development of large local tensions that frequently relax into phase or compositional immiscibility along lateral and transverse planes of the membrane. On the other hand, these effects constitute the very codes that mediate molecular and structural changes determining and controlling the possibilities for enzymatic activity, apposition and recombination in biomembranes. The presence of proteins constitutes a major perturbing factor for the membrane sculpturing both in terms of its surface topography and dynamics. We will focus on some results from our group within this context and summarize some recent evidence for the active involvement of extrinsic (myelin basic protein), integral (Folch-Lees proteolipid protein) and amphitropic (c-Fos and c-Jun) proteins, as well as a membrane-active amphitropic phosphohydrolytic enzyme (neutral sphingomyelinase), in the process of lateral segregation and dynamics of phase domains, sculpturing of the surface topography, and the bi-directional modulation of the membrane biochemical reactivity.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

Molecular parameters involved in aminoglycoside nephrotoxicity

Aminoglycoside antibiotics are hydrophilic molecules consisting of an animated cyclitol associated with amino sugar. They bind in vivo as well as in vitro to negatively charged membranes. Their use as chemotherapeutic agents is unfortunately accompanied by oto- and nephrotoxic reactions, and the purpose of this review is to examine the role of the molecular interactions between aminoglycosides and membranes in the development of nephrotoxicity. 31P Nuclear magnetic resonance (NMR) and fluorescence depolarization have been used to characterize the effect of aminoglycosides on phosphate heads and fatty acyl chains of phospholipids. 15N NMR has been used to obtain interesting information on regioselective interactions of amino groups of antibiotics with phospholipids. The binding of aminoglycosides with negatively charged membranes is associated with impairment of phospholipid catabolism, change in membrane permeability, and membrane aggregation. Biochemical analysis and 1H NMR spectroscopy have brought information on the molecular mechanism involved in the impairment of phospholipid catabolism. Nephrotoxic aminoglycosides could induce sequestration of phosphatidylinositol and therefore reduce the amount of negative charge available for optimal lysosomal phospholipase activity toward phosphatidylcholine included in liposomes that also contain cholesterol and sphingomyelin. Conformational analysis shows that aminoglycosides, which have a high potency to inhibit lysosomal phospholipase activity, adopt an orientation parallel to the lipid/water interface. This orientation of the aminoglycoside molecule at the interface is also critical to explain the marked increase of membrane permeability induced by less nephrotoxic aminoglycosides such as isepamicin and amikacin. This effect is indeed only observed with aminoglycosides oriented perpendicular to this interface, probably related to the creation of a local condition of disorder. The impairment of phospholipid catabolism, which is considered to be an early and significant step in the development of aminoglycoside toxicity, is therefore not related to the change in membrane permeability. However, the role of this latter phenomenon and of membrane aggregation for aminoglycoside nephrotoxicity could be further investigated.

Modulation by gangliosides of the lamellar-inverted micelle (hexagonal II) phase transition in mixtures containing phosphatidylethanolamine and dioleoylglycerol

We studied the effect of gangliosides GD1a and GM1 on the lamellar-to-hexagonal II phase transition of mixtures of dioleoylphosphatidylethanolamine/dioleoylphosphatidyl choline, 3:1, and of transphosphatidylated phosphatidylethanolamine with dioleoylglycerol by high-sensitivity differential scanning calorimetry, 31P-NMR, and pyrene fluorescence of a phosphatidylcholine probe. Gangliosides had a dual effect. Below 1 mol % ganglioside the hexagonal II phase transition was affected but still occurred at lower temperature than in the absence of gangliosides. The presence of between 1 and 2 mol % gangliosides increased the temperature for formation of the hexagonal II phase and progressively decreased its cooperativity. Above 3 mol % gangliosides totally inhibited the formation of both the temperature-induced and composition-induced hexagonal phase, probably by opposing the geometric distortions necessary for the inverted micellar structures.

Modulation by glycosphingolipids of membrane-membrane interactions induced by myelin basic protein and melittin

The effect of glycosphingolipids (GSLs) with oligosaccharide chains of different length and charge on membrane-membrane interactions induced by myelin basic protein (MBP) or melittin (Mel) was comparatively investigated with small unilamellar vesicles. MBP induces a fast vesicle aggregation and close membrane apposition. Merging of lipid bilayers and vesicle fusion induced by MBP are slower and less extensive processes compared to membrane apposition. The changes of membrane permeability concomitant to these phenomena are small. The Trp region of MBP remains in a rather polar environment when interacting with vesicles; its accessibility to NO3- or acrylamide quenching depends on the type of GSLs in the membrane. The Trp region of Mel is inserted more deeply into the lipid bilayer and its accessibility to the aqueous quenchers is less dependent on variations of the oligosaccharide chain of the GSLs. Mel induces a faster and more extensive membrane apposition and bilayer merging than does MBP. Extensive vesicle disruption occurs in the presence of Mel. Negatively charged GSLs facilitate membrane proximity and vesicle aggregation but an increase of the oligosaccharide chain length of either neutral or acidic GSLs decreases the interaction among vesicles that are induced by either protein. This effect is independent of the different mode of insertion of MBP and Mel into the membrane. Our results suggest that the modulation by the oligosaccharide chain on the protein-induced interactions between bilayers containing GSLs is probably exerted beyond the level of local molecular interactions between the basic proteins and the lipids.

The basic protein of CNS myelin: its structure and ligand binding

Consideration of the evidence presented in this review leads to the following conclusions: (a) Isolated MBP in aqueous solution has little ordered secondary or tertiary structure. (b) In this state, the protein can associate with a wide range of hydrophobic and amphiphilic compounds, these interactions involving limited sections of the protein. (c) The strength of binding to bilayers and the accompanying conformational changes in the protein are greatest for systems containing acidic lipids, presumably because of the involvement of ionic interactions. (d) When bound to bilayers of acidic lipids, MBP will have substantially more ordered secondary structure than it manifests in aqueous solution, and it is likely to be oligomeric (possibly hexameric). (e) MBP does affect the organization of lipid aggregates. It influences strongly the separation of bilayers in multilayers of purified lipids, and at present this must be viewed as its prime role within myelin. The greatest impediment to our understanding of MBP is the lack of an assayable biological activity. In contrast to the situation with enzymes, for example, we have no functional test for changes in protein structure or changes accompanying interactions with other molecules. Current evidence suggests that the protein has a structural role within myelin and that its own three-dimensional structure is strongly dependent on the molecules with which it is associated. If this picture is correct, studies of the isolated protein or of the protein in reconstituted lipid systems may yield, at best, a rough guide to the structure within its biological environment. Further clarification of the structure and function of MBP may have to await development of more powerful techniques for studying proteins bound to large molecular aggregates, such as lipid bilayers. The paucity of generally applicable methods is reflected in the fact that even low resolution structures are known for only a handful of intrinsic membrane proteins, and even more limited information exists for proteins associated with membrane surfaces. However, the increasing use of a combination of electron microscopy and diffraction on two-dimensional arrays of proteins formed on lipid bilayers (Henderson et al., 1990) offers the hope that it may not be too long before it will be possible to study at moderate resolution the three-dimensional structure of MBP bound to a lipid membrane.

Polymorphic phospholipid phase transitions as tools to understand peptide-lipid interactions

The effect of peptides on bilayer----non-bilayer phase transitions can be used as a tool to investigate the molecular aspects of peptide-lipid interactions. In this contribution the action on membranes of the peptide antibiotic gramicidin A and the bee venom component melittin are compared. Although the known structures and locations of these peptides upon membrane binding are very different, their actions on membranes show striking parallels. A general model is proposed that explains the seemingly complex peptide-lipid interactions by making use of simple concepts.

Bilayer-stabilizing properties of myelin basic protein in dioleoylphosphatidylethanolamine systems

31P-NMR and X-ray diffraction techniques are used to study the comparative ability of myelin basic protein (MBP) vs. other basic proteins to convert hexagonal (HII) phases to stable lamellar (L alpha) structures. Pure dioleoylphosphatidylethanolamine (DOPE) at pH 9 and 7, and mixtures of DOPE/phosphatidylserine (PS) (95:5 and 80:20% w/w) at pH 7 were employed for this investigation. The polymorphic behavior of the lipid suspensions was evaluated in the presence and absence of several basic proteins (MBP, calf thymus histone, lysozyme, melittin) and the cationic polypeptide, polylysine (PL). Each of the proteins and PL was capable of binding the pure DOPE HII phase at pH 9 but with varying morphological consequences, i.e., lamellar stabilization (MBP, histone, PL), formation of new protein-DOPE HII phases (lysozyme) or lipid disordering/vesiculation (melittin). Reduction to pH 7 resulted in the dissociation of protein from DOPE - with the exception of melittin - and the reformation of a pure lipid HII phase. Additions of PS to DOPE at pH 7 facilitated protein binding, but among the proteins examined, only MBP was capable of converting the lipid suspension into a stable multilamellar form. Differences in the lipid morphology produced by each protein are discussed in terms of protein physicochemical characteristics. In addition, a possible relationship between MBP-lipid interactions and the stability of myelin sheath lipid multilayers is inferred from the significant bilayer-stabilizing capacity of MBP.

Modulation of membrane surface curvature by peptide-lipid interactions

Recent reports on the interaction of cardiotoxin and melittin with phospholipid model membranes are reviewed and analyzed. These types of peptide toxins are able to modulate lipid surface curvature and polymorphism in a highly lipid-specific way. It is demonstrated that the remarkable variety of effects of melittin on the organization of different membrane phospholipids can be understood in a relatively simple model, based on the shape-structure concept of lipid polymorphism and taking into account the position of the peptide molecule with respect to the lipids. Based on the strong preference of the peptides for negatively charged lipids and the structural consequences thereof, and on preliminary studies of signal peptide-lipid interaction, a role of inverted or concave lipid structures in the process of protein translocation across membranes is suggested.

Modulation of myelin basic protein-induced aggregation and fusion of liposomes by cholesterol, aliphatic aldehydes and alkanes

The effect of cholesterol on myelin basic protein-induced aggregation of zwitterionic phospholipid vesicles was studied by turbidimetry, quasi-elastic light scattering and centrifugation techniques. Without cholesterol, the degree of vesicle aggregation caused by myelin basic protein is relatively low and is only slightly increased using cholesterol concentrations up to approx. 25-30 mol%. When the cholesterol content in the bilayer exceeds approx. 30 mol%, there is a dramatic increase in the susceptibility of the vesicles to aggregation in the presence of myelin basic protein. Palmitoyl aldehyde and eicosane, substances resembling products of lipid degradation, increase myelin basic protein promoted fusion of vesicles. The fusion is accompanied by increased leakage of entrapped carboxyfluorescein. In the presence of cholesterol, myelin basic protein-induced fusion of the liposomes becomes much more sensitive to the presence of aliphatic aldehydes or alkanes. The results suggest that cholesterol has an important role in promoting membrane adhesion in biological systems but these structures become unstable in the presence of small amounts of products of lipid degradation. The findings have important implications to the understanding of the stability of the myelin membrane.

Association of ferri- and ferro-cytochrome c with lipid multilayers: a 31P solid-state NMR study

The 31P nuclear magnetic resonance anisotropies of dispersions of diacylphosphatidic acid and diacylphosphatidylserine were slightly increased in the presence of cytochrome c: no increase was observed with cardiolipin. However, the 31P spin-lattice relaxation times (T1) for all of these lipids were reduced markedly by the protein. As similar effects were observed with ferri-cytochrome c and with the reduced protein, which is diamagnetic, we suggest that the changes in T1 reflect a reduction in the spectral density of fast motions for the lipid headgroups attendant on binding of protein, rather than paramagnetic relaxation of the phosphorus nuclear spin.

The influence of proteins and peptides on the phase properties of lipids

This paper reviews model membrane studies on the modulation of the macroscopic structure of lipids by lipid-protein interactions, with particular emphasis on the gramicidin molecule. This hydrophobic peptide has three main effects on lipid polymorphism: (1) in lysophosphatidylcholine it triggers a micellar to bilayer transition, (2) in phosphatidylethanolamine it lowers the bilayer to hexagonal HII phase transition temperature and (3) in phosphatidylcholine and other bilayer preferring lipids it is able to induce the formation of an HII phase. From experiments in which the gramicidin molecule was chemically modified it can be concluded that the tryptophan residues play a determining role in the peptide-induced changes in polymorphism. The experimental data lead to the proposal that gramicidin molecules have a tendency to self-associate, possibly mediated by tryptophan-tryptophan interactions and organize into tubular structures such as found in the HII phase.