Brain spectrin exerts much stronger effect on anionic phospholipid monolayers than erythroid spectrin

Effects of GM1 on brain spectrin-aminophospholipid interactions

Spectrin, a major component of the membrane skeletal meshwork of metazoan cells, is implicated to associate with membrane domains and is known to act as a scaffold for stabilization and activation of different signalling modules. We have studied the effect of GM1 (monosialotetrahexosyl ganglioside), a well-known model ganglioside and a signalling moiety, on the interaction of non-erythroid brain spectrin with both saturated and unsaturated aminophospholipids by spectroscopic methods. We observe that GM1 modulates brain spectrin-aminophospholipid interaction to the greatest degree whereas its effect on erythroid spectrin is not as pronounced. Fluorescence quenching studies show that brain spectrin interacts with DMPC/DMPE-based vesicles with a 10-fold increased affinity in presence of very low amounts of 2% and 5% GM1, and the extent of quenching decreases progressively in presence of increasing amounts of GM1. Interaction of brain spectrin with unsaturated membrane systems of DOPC/DOPE weakens in presence GM1. Increase in the mean lifetime of the Trp residues of brain spectrin in presence of GM1 indicates change in the microenvironment of spectrin, without affecting the secondary structure of the protein significantly. Studies on pressure - area isotherm of Langmuir-Blodgett monolayer and Brewster's angle microscopy show that GM1 has an expanding effect on the aminophospholipid monolayers, and ordered regions in DMPC/DMPE mixed monolayers are formed and are stabilized at higher pressure. GM1-induced fluidization of the phospholipid membranes and probable physical contact between bulky sugar head group of GM1 and spectrin, may explain the modulatory role of GM1 on aminophospholipid interactions with nonerythroid brain spectrin.

EGF domain of coagulation factor IX is conducive to exposure of phosphatidylserine

Lipid rafts are an initiation site for many different signals. Recently, we reported that an EGF domain in activated coagulation factor IX (EGF-F9) increases lipid raft formation and accelerates cell migration. However, the detailed mechanism is not well understood. This study aimed to evaluate the effects of EGF-F9 on the cell membrane. A431 cells (derived from human squamous cell carcinoma) were treated with recombinant EGF-F9. Cells were immunocytochemically stained with probes for lipid rafts or phosphatidylserine (PS). After 3 min of treatment with EGF-F9, cholera toxin subunit B (CTxB) binding domains emerged at the adhesive tips of filopodia. Subsequently, CTxB staining was observed on the filopodial shaft. Finally, large clusters of CTxB domains were observed at the edge of cell bodies. Markers for lipid rafts, such as caveolin-1 and a GPI anchored protein, co-localized with CTxB. Staining with annexin V and XII revealed that PS was exposed at the tips of filopodia, translocated on filopodial shafts, and co-localized with CTxB at the rafts. Immunocytochemistry showed that scramblase-1 protein was present at the filopodial tips. Our data indicates that EGF-F9 accelerates PS exposure around the filopodial adhesion complex and induces clustering of lipid rafts in the cell body. PS exposure is thought to occur on cells undergoing apoptosis. Further study of the function of the EGF-F9 motif in mediating signal transduction is necessary because it is shared by a number of proteins.

Beyond apoptosis: the mechanism and function of phosphatidylserine asymmetry in the membrane of activating mast cells

Loss of plasma membrane asymmetry is a hallmark of apoptosis, but lipid bilayer asymmetry and loss of asymmetry can contribute to numerous cellular functions and responses that are independent of programmed cell death. Exofacial exposure of phosphatidylserine occurs in lymphocytes and mast cells after antigenic stimulation and in the absence of apoptosis, suggesting that there is a functional requirement for phosphatidylserine exposure in immunocytes. In this review we examine current ideas as to the nature of this functional role in mast cell activation. Mechanistically, there is controversy as to the candidate proteins responsible for phosphatidylserine translocation from the internal to external leaflet, and here we review the candidacies of mast cell PLSCR1 and TMEM16F. Finally we examine the potential relationship between functionally important mast cell membrane perturbations and phosphatidylserine exposure during activation.

Probing conformational stability and dynamics of erythroid and nonerythroid spectrin: effects of urea and guanidine hydrochloride

We have studied the conformational stability of the two homologous membrane skeletal proteins, the erythroid and non-erythroid spectrins, in their dimeric and tetrameric forms respectively during unfolding in the presence of urea and guanidine hydrochloride (GuHCl). Fluorescence and circular dichroism (CD) spectroscopy have been used to study the changes of intrinsic tryptophan fluorescence, anisotropy, far UV-CD and extrinsic fluorescence of bound 1-anilinonapthalene-8-sulfonic acid (ANS). Chemical unfolding of both proteins were reversible and could be described as a two state transition. The folded erythroid spectrin and non-erythroid spectrin were directly converted to unfolded monomer without formation of any intermediate. Fluorescence quenching, anisotropy, ANS binding and dynamic light scattering data suggest that in presence of low concentrations of the denaturants (up-to 1M) hydrogen bonding network and van der Waals interaction play a role inducing changes in quaternary as well as tertiary structures without complete dissociation of the subunits. This is the first report of two large worm like, multi-domain proteins obeying twofold rule which is commonly found in small globular proteins. The free energy of stabilization (ΔGuH20) for the dimeric spectrin has been 20 kcal/mol lesser than the tetrameric from.

Malachite green interacts with the membrane skeletal protein, spectrin

Energy minimized complex of MG with the self association domain of spectrin.

Spectrin and phospholipids - the current picture of their fascinating interplay

The spectrin-based membrane skeleton is crucial for the mechanical stability and resilience of erythrocytes. It mainly contributes to membrane integrity, protein organization and trafficking. Two transmembrane protein macro-complexes that are linked together by spectrin tetramers play a crucial role in attaching the membrane skeleton to the cell membrane, but they are not exclusive. Considerable experimental data have shown that direct interactions between spectrin and membrane lipids are important for cell membrane cohesion. Spectrin is a multidomain, multifunctional protein with several distinctive structural regions, including lipid-binding sites within CH tandem domains, a PH domain, and triple helical segments, which are excellent examples of ligand specificity hidden in a regular repetitive structure, as recently shown for the ankyrin-sensitive lipid-binding domain of beta spectrin. In this review, we summarize the state of knowledge about interactions between spectrin and membrane lipids.

The Protein 4.1 family: hub proteins in animals for organizing membrane proteins

Proteins of the 4.1 family are characteristic of eumetazoan organisms. Invertebrates contain single 4.1 genes and the Drosophila model suggests that 4.1 is essential for animal life. Vertebrates have four paralogues, known as 4.1R, 4.1N, 4.1G and 4.1B, which are additionally duplicated in the ray-finned fish. Protein 4.1R was the first to be discovered: it is a major mammalian erythrocyte cytoskeletal protein, essential to the mechanochemical properties of red cell membranes because it promotes the interaction between spectrin and actin in the membrane cytoskeleton. 4.1R also binds certain phospholipids and is required for the stable cell surface accumulation of a number of erythrocyte transmembrane proteins that span multiple functional classes; these include cell adhesion molecules, transporters and a chemokine receptor. The vertebrate 4.1 proteins are expressed in most tissues, and they are required for the correct cell surface accumulation of a very wide variety of membrane proteins including G-Protein coupled receptors, voltage-gated and ligand-gated channels, as well as the classes identified in erythrocytes. Indeed, such large numbers of protein interactions have been mapped for mammalian 4.1 proteins, most especially 4.1R, that it appears that they can act as hubs for membrane protein organization. The range of critical interactions of 4.1 proteins is reflected in disease relationships that include hereditary anaemias, tumour suppression, control of heartbeat and nervous system function. The 4.1 proteins are defined by their domain structure: apart from the spectrin/actin-binding domain they have FERM and FERM-adjacent domains and a unique C-terminal domain. Both the FERM and C-terminal domains can bind transmembrane proteins, thus they have the potential to be cross-linkers for membrane proteins. The activity of the FERM domain is subject to multiple modes of regulation via binding of regulatory ligands, phosphorylation of the FERM associated domain and differential mRNA splicing. Finally, the spectrum of interactions of the 4.1 proteins overlaps with that of another membrane-cytoskeleton linker, ankyrin. Both ankyrin and 4.1 link to the actin cytoskeleton via spectrin, and we hypothesize that differential regulation of 4.1 proteins and ankyrins allows highly selective control of cell surface protein accumulation and, hence, function. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé

Binding of polarity-sensitive hydrophobic ligands to erythroid and nonerythroid spectrin: fluorescence and molecular modeling studies

We have used three polarity-sensitive fluorescence probes, 6-propionyl 2-(N,N-dimethyl-amino) naphthalene (Prodan), pyrene and 8-anilino 1-naphthalene sulphonic acid, to study their binding with erythroid and nonerythroid spectrin, using fluorescence spectroscopy. We have found that both bind to prodan and pyrene with high affinities with apparent dissociation constants (Kd) of .50 and .17 μM, for prodan, and .04 and .02 μM, for pyrene, respectively. The most striking aspect of these bindings have been that the binding stoichiometry have been equal to 1 in erythroid spectrin, both in dimeric and tetrameric form, and in tetrameric nonerythroid spectrin. From an estimate of apparent dielectric constants, the polarity of the binding site in both erythroid and nonerythroid forms have been found to be extremely hydrophobic. Thermodynamic parameters associated with such binding revealed that the binding is favored by positive change in entropy. Molecular docking studies alone indicate that both prodan and pyrene bind to the four major structural domains, following the order in the strength of binding to the Ankyrin binding domain > SH3 domain > Self-association domain > N-terminal domain of α-spectrin of both forms of spectrin. The binding experiments, particularly with the tetrameric nonerythroid spectrin, however, indicate more toward the self association domain in offering the unique binding site, since the binding stoichiometry have been 1 in all forms of dimeric and tetrameric spectrin, so far studied by us. Further studies are needed to characterize the hydrophobic binding sites in both forms of spectrin.

A monolayer assay tailored to investigate lipid-protein systems

Model membrane systems have become invaluable tools to investigate specific features of cellular membranes. Although a variety of different experimental assays does exist, many of them are rather complicated in their preparation and difficult in their practical realisation. Here, we propose a new simple miniaturised monolayer assay that can easily be combined with standard analytical techniques such as confocal fluorescence microscopy and fluorescence correlation spectroscopy (FCS).

Specific anchoring modes of two distinct dystrophin rod sub-domains interacting in phospholipid Langmuir films studied by atomic force microscopy and PM-IRRAS

Dystrophin rod repeats 1-3 sub-domain binds to acidic phosphatidylserine in a small vesicle binding assay, while the repeats 20-24 sub-domain does not. In the present work, we studied the adsorption behaviour of both sub-domains at the air/liquid interface and at the air/lipid interface in a Langmuir trough in order to highlight differences in interfacial properties. The adsorption behaviour of the two proteins at the air/liquid interface shows that they display surface activity while maintaining their alpha-helical secondary structure as shown by PM-IRRAS. Strikingly, R20-24 needs to be highly hydrated even at the interface, while this is not the case for R1-3, indicating that the surface activity is dramatically higher for R1-3 than R20-24. Surface-pressure measurements, atomic force microscopy and PM-IRRAS are used in a Langmuir experiment with DOPC-DOPS monolayers at two different surface pressures, 20 mN/m and 30 mN/m. At the lower surface pressure, the proteins are adsorbed at the lipid film interface while maintaining its alpha-helical structure. After an increase of the surface pressure, R1-3 subsequently produces a stable film, while R20-24 induces a reorganization of the lipid film with a subsequent decrease of the surface pressure close to the initial value. AFM and PM-IRRAS show that R1-3 is present in high amounts at the interface, being arranged in clusters representing 3.3% of the surface at low pressure. By contrast, R20-24 is present at the interface in small amounts bound only by a few electrostatic residues to the lipid film while the major part of the molecule remains floating in the sub-phase. Then for R1-3, the electrostatic interaction between the proteins and the film is enhanced by hydrophobic interactions. At higher surface pressure, the number of protein clusters increases and becomes closer in both cases implying the electrostatic character of the binding. These results indicate that even if the repeats exhibit large structural similarities, their interfacial properties are highly contrasted by their differential anchor mode in the membrane. Our work provides strong support for distinct physiological roles for the spectrin-like repeats and may partly explain the effects of therapeutic replacement of dystrophin deficiency by minidystrophins.

Mitoxantrone changes spectrin-aminophospholipid interactions

Understanding drug-membrane and drug-membrane protein interactions would be a crucial step towards understanding the action and biological properties of anthracyclines, as the cell membrane with its integral and peripheral proteins is the first barrier encountered by these drugs. In this paper, we briefly describe mitoxantrone-monolayer and mitoxantrone-bilayer interactions, focusing on the effect of mitoxantrone on the interactions between erythroid or nonerythroid spectrin with phosphatidylethanolamine-enriched mono- and bilayers. We found that mitoxantrone markedly modifies the interaction of erythroid and nonerythroid spectrins with phosphatidylethanolamine/phosphatidylcholine (PE/PC) monolayers. The change in delta pi induced by spectrins is several-fold larger in the presence of 72 nM mitoxantrone than in its absence: spectrin/mitoxantrone complexes induced a strong compression of the monolayer. Spin-labelling experiments showed that spectrin/mitoxantrone complexes caused significant changes in the order parameter measured using a 5'-doxyl stearate probe in the bilayer, but they practically did not affect the mobility of 16'-doxyl stearate. These results indicate close-to-surface interactions/penetrations without significant effect on the mid-region of the hydrophobic core of the bilayer. The obtained apparent equilibrium dissociation constants indicated relatively similar mitoxantrone-phospholipid and mitoxantrone-spectrin (erythroid and nonerythroid) binding affinities. These results might in part, explain the effect of mitoxantrone on spectrin distribution in the living cells.

Spectrin-phospholipid interactions. Existence of multiple kinds of binding sites?

The object of this paper is to review briefly the studies on the interactions of erythroid and non-erythroid spectrins with lipids in model and natural membranes. An important progress on the identification of lipid-binding sites has recently been made although many questions remain still unanswered. In particular, our understanding of the physiological role of such interactions is still limited. Another important issue is the occurrence of spectrins in membrane rafts, how they are attached to the raft and what is their function in rafts.

Cholesterol affects spectrin–phospholipid interactions in a manner different from changes resulting from alterations in membrane fluidity due to fatty acyl chain composition

We previously showed that erythrocyte and brain spectrins bind phospholipid vesicles and monolayers prepared from phosphatidylethanolamine and phosphatidylserine and their mixtures with phosphatidylcholine (Review: A.F. Sikorski, B. Hanus-Lorenz, A. Jezierski, A. R. Dluzewski, Interaction of membrane skeletal proteins with membrane lipid domain, Acta Biochim. Polon. 47 (2000) 565). Here, we show how changes in the fluidity of the phospholipid monolayer affect spectrin-phospholipid interaction. The presence of up to 10%-20% cholesterol in the PE/PC monolayer facilitates the penetration of the monolayer by both types of spectrin. For monolayers constructed from mixtures of PI/PC and cholesterol, the effect of spectrins was characterised by the presence of two maxima (at 5 and 30% cholesterol) of surface pressure for erythroid spectrin, and a single maximum (at 20% cholesterol) for brain spectrin. The binding assay results indicated a small but easily detectable decrease in the affinity of erythrocyte spectrin for FAT-liposomes prepared from a PE/PC mixture containing cholesterol, and a 2- to 5-fold increase in maximal binding capacity (B(max)) depending on the cholesterol content. On the other hand, the results from experiments with a monolayer constructed from homogenous synthetic phospholipids indicated an increase in deltapi change with the increase in the fatty acyl chain length of the phospholipids used to prepare the monolayer. This was confirmed by the results of a pelleting experiment. Adding spectrins into the subphase of raft-like monolayers constructed from DOPC, SM and cholesterol (1/1/1) induced an increase in surface pressure. The deltapi change values were, however, much smaller than those observed in the case of a natural PE/PC (6/4) monolayer. An increased binding capacity for spectrins of liposomes prepared from a "raft-like" mixture of lipids could also be concluded from the pelleting assay. In conclusion, we suggest that the effect of membrane lipid fluidity on spectrin-phospholipid interactions is not simple but depends on how it is regulated, i.e., by cholesterol content or by the chemical structure of the membrane lipids.

Phospholipid binding by proteins of the spectrin family: a comparative study

Erythroid and neuronal spectrin (fodrin) are both known to interact strongly with the aminophospholipids that occur in the inner leaflet of plasma membranes. In erythroid spectrin the positions of the binding sites within the constituent (alphaI and betaI) polypeptide chains have been defined, and also the importance of the lipid interaction in regulating the properties of the membrane. Here we report the locations of the corresponding binding sites in the alphaII and betaII chains that make up the fodrin molecule. Of the 10 lipid-binding repeats in the erythroid spectrin chains 5 are conserved in fodrin; one cluster of 3 consecutive structural repeating units in alphaI erythroid spectrin (repeats 8-10) is displaced by one repeat in alphaII fodrin (repeats 9-11). Fodrin also contains one binding site at the N-terminus of the alphaII chain, not present in the erythroid protein. The regions of the two spectrins containing equivalent lipid-binding sites show a much higher degree of sequence identity than corresponding repeats that do not share this property. The evolutionary conservation of the distribution of a large proportion of strong lipid-binding sites in the polypeptide chains of these two proteins of disparate character argues for a specific function of fodrin-phospholipid interactions in the neuron.