Phosphatidylserine Binding Sites in Erythroid Spectrin: Location and Implications for Membrane Stability

The effect of the lipid-binding site of the ankyrin-binding domain of erythroid beta-spectrin on the properties of natural membranes and skeletal structures

It was previously shown that the beta-spectrin ankyrin-binding domain binds lipid domains rich in PE in an ankyrin-dependent manner, and that its N-terminal sequence is crucial in interactions with phospholipids. In this study, the effect of the full-length ankyrin-binding domain of β-spectrin on natural erythrocyte and HeLa cell membranes was tested. It was found that, when encapsulated in resealed erythrocyte ghosts, the protein representing the full-length ankyrin-binding domain strongly affected the shape and barrier properties of the erythrocyte membrane, and induced partial spectrin release from the membrane, while truncated mutants had no effect. As found previously (Bok et al. Cell Biol. Int. 31 (2007) 1482–94), overexpression of the full-length GFP-tagged ankyrin-binding domain aggregated and induced aggregation of endogenous spectrin, but this was not the case with overexpression of proteins truncated at their N-terminus. Here, we show that the aggregation of spectrin was accompanied by the aggregation of integral membrane proteins that are known to be connected to spectrin via ankyrin, i.e. Na⁺K⁺ATP-ase, IP3 receptor protein and L1 CAM. By contrast, the morphology of the actin cytoskeleton remained unchanged and aggregation of cadherin E or N did not occur upon the overexpression of either full-length or truncated ankyrin-binding domain proteins. The obtained results indicate a substantial role of the lipid-binding part of the β-spectrin ankyrin-binding domain in the determination of the membrane and spectrin-based skeleton functional properties.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Aggregation of spectrin and PKCtheta is an early hallmark of fludarabine/mitoxantrone/dexamethasone-induced apoptosis in Jurkat T and HL60 cells

It has been shown that changes in spectrin distribution in early apoptosis preceded changes in membrane asymmetry and phosphatidylserine (PS) exposure. PKCtheta was associated with spectrin during these changes, suggesting a possible role of spectrin/PKCtheta aggregation in regulation of early apoptotic events. Here we dissect this hypothesis using Jurkat T and HL60 cell lines as model systems. Immunofluorescent analysis of alphaIIbetaII spectrin arrangement in Jurkat T and HL60 cell lines revealed the redistribution of spectrin and PKCtheta into a polar aggregate in early apoptosis induced by fludarabine/mitoxantrone/dexamethasone (FND). The appearance of an alphaIIbetaII spectrin fraction that was insoluble in a non-ionic detergent (1% Triton X-100) was observed concomitantly with spectrin aggregation. The changes were observed within 2 h after cell exposure to FND, and preceded PS exposure. The changes seem to be restricted to spectrin and not to other cytoskeletal proteins such as actin or vimentin. In studies of the mechanism of these changes, we found that (i) neither changes in apoptosis regulatory genes (e.g., Bcl-2 family proteins) nor changes in cytoskeleton-associated proteins were detected in gene expression profiling of HL60 cells after the first hour of FND treatment, (ii) caspase-3, -7, -8, and -10 had minor involvement in the early apoptotic rearrangement of spectrin/PKCtheta, and (iii) spectrin aggregation was shown to be partially dependent on PKCtheta activity. Our results indicate that spectrin/PKCtheta aggregate formation is related to an early stage in drug-induced apoptosis and possibly may be regulated by PKCtheta activity. These findings indicate that spectrin/PKCtheta aggregation could be considered as a hallmark of early apoptosis and presents the potential to become a useful diagnostic tool for monitoring efficiency of chemotherapy as early as 24 h after treatment.

The covalent modification of spectrin in red cell membranes by the lipid peroxidation product 4-hydroxy-2-nonenal

Spectrin strengthens the red cell membrane through its direct association with membrane lipids and through protein-protein interactions. Spectrin loss reduces the membrane stability and results in various types of hereditary spherocytosis. However, less is known about acquired spectrin damage. Here, we showed that alpha- and beta-spectrin in human red cells are the primary targets of the lipid peroxidation product 4-hydroxy-2-nonenal (HNE) by immunoblotting and mass spectrometry analyses. The level of HNE adducts in spectrin (particularly alpha-spectrin) and several other membrane proteins was increased following the HNE treatment of red cell membrane ghosts prepared in the absence of MgATP. In contrast, ghost preparation in the presence of MgATP reduced HNE adduct formation, with preferential beta-spectrin modification and increased cross-linking of the HNE-modified spectrins. Exposure of intact red cells to HNE resulted in selective HNE-spectrin adduct formation with a similar preponderance of HNE-beta-spectrin modifications. These findings indicate that HNE adduction occurs preferentially in spectrin at the interface between the skeletal proteins and lipid bilayer in red cells and suggest that HNE-spectrin adduct aggregation results in the extrusion of damaged spectrin and membrane lipids under physiological and disease conditions.

Membrane domains based on ankyrin and spectrin associated with cell-cell interactions

Nodes of Ranvier and axon initial segments of myelinated nerves, sites of cell-cell contact in early embryos and epithelial cells, and neuromuscular junctions of skeletal muscle all perform physiological functions that depend on clustering of functionally related but structurally diverse ion transporters and cell adhesion molecules within microdomains of the plasma membrane. These specialized cell surface domains appeared at different times in metazoan evolution, involve a variety of cell types, and are populated by distinct membrane-spanning proteins. Nevertheless, recent work has shown that these domains all share on their cytoplasmic surfaces a membrane skeleton comprised of members of the ankyrin and spectrin families. This review will summarize basic features of ankyrins and spectrins, and will discuss emerging evidence that these proteins are key players in a conserved mechanism responsible for assembly and maintenance of physiologically important domains on the surfaces of diverse cells.

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.

Mapping of the lipid-binding and stability properties of the central rod domain of human dystrophin

Dystrophin is a cytoskeletal protein that confers resistance to the sarcolemma against the stress of contraction-relaxation cycles by interacting with cytoskeletal and membrane partners. Apart from several proteins, membrane phospholipids are a partner of the central rod domain made up of 24 spectrin-like repeats, separated into sub-domains by four hinges. We previously showed that repeats 1 to 3 bind to membrane anionic phospholipids, while repeats 20 to 24 are not able to do so. We focus here on the phospholipid-binding properties of the major part of the central rod domain, namely, the sub-domain delineated by hinges 2 and 3 comprising 16 repeats ranging from repeat 4 to 19 (R4-19). We designed and produced multirepeat proteins comprising three to five repeats and report their lipid-binding properties as well as their thermal stabilities. When these proteins are mixed with liposomes including the anionic lipid phosphatidylserine, they form stable protein-vesicle complexes as determined by gel-filtration chromatography. The absence of an anionic lipid precludes the formation of such complexes. Spectroscopic analyses by circular dichroism and tryptophan fluorescence show that, while the alpha-helical secondary structures are not modified by the binding, protein trans conformation leads to the movement of tryptophan residues into more hydrophobic environments. In addition, the decrease in the molar ellipticity ratio at 222/208 nm as observed by circular dichroism indicates that lipid binding reduces the inter-helical interactions of multirepeat proteins, thus suggesting partly "opened" coiled-coil structures. Combining these results with data from our previous studies, we propose a new model of the dystrophin molecule lying along the membrane bilayer, in which the two sub-domains R1-3 and R4-19 interact with lipids and F-actin, while the distal sub-domain R20-24 does not exhibit any interaction. These lipid-binding domains should thus maintain a structural link between cytoskeletal actin and sarcolemma via the membrane phospholipids.

Red cell membrane: past, present, and future

As a result of natural selection driven by severe forms of malaria, 1 in 6 humans in the world, more than 1 billion people, are affected by red cell abnormalities, making them the most common of the inherited disorders. The non-nucleated red cell is unique among human cell type in that the plasma membrane, its only structural component, accounts for all of its diverse antigenic, transport, and mechanical characteristics. Our current concept of the red cell membrane envisions it as a composite structure in which a membrane envelope composed of cholesterol and phospholipids is secured to an elastic network of skeletal proteins via transmembrane proteins. Structural and functional characterization of the many constituents of the red cell membrane, in conjunction with biophysical and physiologic studies, has led to detailed description of the way in which the remarkable mechanical properties and other important characteristics of the red cells arise, and of the manner in which they fail in disease states. Current studies in this very active and exciting field are continuing to produce new and unexpected revelations on the function of the red cell membrane and thus of the cell in health and disease, and shed new light on membrane function in other diverse cell types.

Adhesive activity of Lu glycoproteins is regulated by interaction with spectrin

The Lutheran (Lu) and Lu(v13) blood group glycoproteins function as receptors for extracellular matrix laminins. Lu and Lu(v13) are linked to the erythrocyte cytoskeleton through a direct interaction with spectrin. However, neither the molecular basis of the interaction nor its functional consequences have previously been delineated. In the present study, we defined the binding motifs of Lu and Lu(v13) on spectrin and identified a functional role for this interaction. We found that the cytoplasmic domains of both Lu and Lu(v13) bound to repeat 4 of the alpha spectrin chain. The interaction of full-length spectrin dimer to Lu and Lu(v13) was inhibited by repeat 4 of alpha-spectrin. Further, resealing of this repeat peptide into erythrocytes led to weakened Lu-cytoskeleton interaction as demonstrated by increased detergent extractability of Lu. Importantly, disruption of the Lu-spectrin linkage was accompanied by enhanced cell adhesion to laminin. We conclude that the interaction of the Lu cytoplasmic tail with the cytoskeleton regulates its adhesive receptor function.

Disorders of red cell membrane

Studies during the last three decades have enabled the development of detailed molecular insights into the structural basis of altered function in various inherited red cell membrane disorders. This review highlights our current understanding of molecular and mechanistic insights into various inherited red cell membrane disorders involving either altered membrane structural organization (hereditary spherocytosis, hereditary elliptocytosis and hereditary ovalocytosis) or altered membrane transport function (hereditary stomatocytosis). The molecular basis for the vast majority of cases of hereditary spherocytosis, elliptocytosis and ovalocytosis have been fully defined while little progress has been made in defining the molecular basis for hereditary stomatocytosis. Mutations in a number of distinct genes account for hereditary spherocytosis and elliptocytosis, while a single genetic defect accounts for all cases of hereditary ovalocytosis. Based on these molecular insights, a comprehensive understanding of the structural basis for altered membrane function has been developed. Loss of vertical linkage between membrane skeleton and lipid bilayer leads to membrane loss in hereditary spherocytosis, while weakening of lateral linkages between skeletal proteins leads to membrane fragmentation and surface area loss in hereditary elliptocytosis. Importantly, the severity of anaemia in both these disorders is directly related to extent of membrane surface area loss. Splenectomy results in amelioration of anaemia.

Organizing the fluid membrane bilayer: diseases linked to spectrin and ankyrin

Ankyrin and spectrin were first discovered as binding partners in the membrane skeleton of human erythrocytes. Mutations in genes encoding these proteins cause hereditary spherocytosis. Recent advances reveal that ankyrin and spectrin are required for organization of a surprisingly diverse set of proteins, including ion channels and cell adhesion molecules that are localized in specialized membrane domains in many cell types. New insights into the cell biology of ankyrin and spectrin reveal that these proteins actively participate in assembly of specialized membrane domains in addition to their conventional maintenance role as scaffolding proteins. Recently described inherited human diseases due to defects in spectrin or ankyrin include spinocerebellar ataxia type 5 and a cardiac arrhythmia, termed sick sinus syndrome with bradycardia or ankyrin-B syndrome. Together, these studies identify an emerging paradigm for pathogenesis of human disease where failure in cellular localization of membrane-spanning proteins results in loss of physiological function.

Structural insight into an ankyrin-sensitive lipid-binding site of erythroid beta-spectrin

It was recently shown that the region within beta-spectrin responsible for interactions with ankyrin includes a lipid-binding site which displayed sensitivity to inhibition by ankyrin. We studied its structure by constructing a series of single and double spin-labeled beta-spectrin-derived peptides and analyzing their spin-spin distances via electron paramagnetic resonance spectroscopy and the Fourier deconvolution method. The results indicate that the whole ankyrin-sensitive lipid-binding site of beta-spectrin exhibits a helical conformation revealing a distinct 3(10)-helix contribution at its N-terminus. The start of the helix was located five residues upstream along the sequence compared to the theoretical predictions. A model based on the obtained data provides direct evidence that the examined lipid-binding site is a highly amphipathic helix, which is correlated with the specific conformation of its N-terminal fragment.

Plasmodium falciparum erythrocyte membrane protein 3 (PfEMP3) destabilizes erythrocyte membrane skeleton

Plasmodium falciparum erythrocyte membrane protein 3 (PfEMP3) is a parasite-derived protein that appears on the cytoplasmic surface of the host cell membrane in the later stages of the parasite's development where it associates with membrane skeleton. We have recently demonstrated that a 60-residue fragment (FIa1, residues 38-97) of PfEMP3 bound to spectrin. Here we show that this polypeptide binds specifically to a site near the C terminus of alpha-spectrin at the point that spectrin attaches to actin and protein 4.1R in forming the junctions of the membrane skeletal network. We further show that this polypeptide disrupts formation of the ternary spectrin-actin-4.1R complex in solution. Importantly, when incorporated into the cell, the PfEMP3 fragment causes extensive reduction in shear resistance of the cell. We conjecture that the loss of mechanical cohesion of the membrane may facilitate the exit of the mature merozoites from the cell.

The ring-infected erythrocyte surface antigen (RESA) of Plasmodium falciparum stabilizes spectrin tetramers and suppresses further invasion

The malaria parasite Plasmodium falciparum releases the ring-infected erythrocyte surface antigen (RESA) inside the red cell on entry. The protein migrates to the host cell membrane, where it binds to spectrin, but neither the nature of the interaction nor its functional consequences have previously been defined. Here, we identify the binding motifs involved in the interaction and describe a possible function. We have found that spectrin binds to a 108-amino acid fragment (residues 663-770) of RESA, and that this RESA fragment binds to repeat 16 of the beta-chain, close to the labile dimer-dimer self-association site. We further show that the RESA fragment stabilizes the spectrin tetramer against dissociation into its constituent dimers, both in situ and in solution. This is accompanied by enhanced resistance of the cell to both mechanical and thermal degradation. Resealed erythrocytes containing RESA(663-770) display resistance to invasion by merozoites of P falciparum. We infer that the evolutionary advantage of RESA to the parasite lies in its ability to prevent invasion of cells that are already host to a developing parasite, as well as possibly to guard the cell against thermal damage at the elevated body temperatures prevailing in febrile crises.

Spectrin organization and dynamics: new insights

Spectrin is the major constituent protein of the erythrocyte cytoskeleton which forms a filamentous network on the cytoplasmic face of the membrane by providing a scaffold for a variety of proteins. In this review, several aspects of spectrin organization are highlighted, particularly with respect to its ability to bind hydrophobic ligands and its interaction with membrane surfaces. The characteristic binding of the fluorescent hydrophobic probes Prodan and pyrene to spectrin, which allows an estimation of the polarity of the hydrophobic probe binding site, is illustrated. In addition, the contribution of uniquely localized and conserved tryptophan residues in the 'spectrin repeats' in these processes is discussed. A functional implication of the presence of hydrophobic binding sites in spectrin is its recently discovered chaperone-like activity. Interestingly, spectrin exhibits residual structural integrity even after denaturation which could be considered as a hallmark of cytoskeletal proteins. Future research could provide useful information about the possible role played by spectrin in cellular physiology in healthy and diseased states.

Thermal stabilities of brain spectrin and the constituent repeats of subunits

The different genes that encode mammalian spectrins give rise to proteins differing in their apparent stiffness. To explore this, we have compared the thermal stabilities of the structural repeats of brain spectrin subunits (alphaII and betaII) with those of erythrocyte spectrin (alphaI and betaI). The unfolding transition midpoints (T(m)) of the 36 alphaII- and betaII-spectrin repeats extend between 24 and 82 degrees C, with an average higher by some 10 degrees C than that of the alphaI- and betaI-spectrin repeats. This difference is reflected in the T(m) values of the intact brain and erythrocyte spectrins. Two of three tandem-repeat constructs from brain spectrin exhibited strong cooperative coupling, with elevation of the T(m) of the less stable partner corresponding to coupling free energies of approximately -4.4 and -3.5 kcal/mol. The third tandem-repeat construct, by contrast, exhibited negligible cooperativity. Tandem-repeat mutants, in which a part of the "linker" helix that connects the two domains was replaced with a corresponding helical segment from erythroid spectrin, showed only minor perturbation of the thermal melting profiles, without breakdown of cooperativity. Thus, the linker regions, which tolerate few point mutations without loss of cooperative function, have evidently evolved to permit conformational coupling in specified regions. The greater structural stability of the repeats in alphaII- and betaII-spectrin may account, at least in part, for the higher rigidity of brain compared to erythrocyte spectrin.

Ankyrin-G and β2-Spectrin Collaborate in Biogenesis of Lateral Membrane of Human Bronchial Epithelial Cells

Ankyrins are a family of adapter proteins required for localization of membrane proteins to diverse specialized membrane domains including axon initial segments, specialized sites at the transverse tubule/sarcoplasmic reticulum in cardiomyocytes, and lateral membrane domains of epithelial cells. Little is currently known regarding the molecular basis for specific roles of different ankyrin isoforms. In this study, we systematically generated alanine mutants of clusters of charged residues in the spectrin-binding domains of both ankyrin-B and -G. The corresponding mutants were evaluated for activity in either restoration of abnormal localization of the inositol trisphosphate receptor in the sarcoplasmic reticulum in mutant mouse cardiomyocytes deficient in ankyrin-B or in prevention of loss of lateral membrane in human bronchial epithelial cells depleted of ankyrin-G by small interfering RNA. Interestingly, ankyrin-B and -G share two homologous sites that result in loss of function in both systems, suggesting that common molecular interactions underlie diverse roles of these isoforms. Ankyrins G and B also exhibit differences; mutations affecting spectrin binding had no effect on ankyrin-B function but did abolish activity of ankyrin-G in restoring lateral membrane biogenesis. Depletion of beta(2)-spectrin by small interfering RNA phenocopied depletion of ankyrin-G and resulted in a failure to form new lateral membrane in interphase and mitotic cells. These results demonstrate that ankyrin-G and beta(2)-spectrin are functional partners in biogenesis of the lateral membrane of epithelial cells.

Binding of the dystrophin second repeat to membrane di-oleyl phospholipids is dependent upon lipid packing

Dystrophin is the genetically deficient protein in Duchenne Muscular Dystrophy. Its C- and N-terminal ends interact with cytoskeletal and membrane proteins, establishing a link between the cytoskeleton and the extracellular matrix. In a previous study, we showed that there is an interaction between the second repeat of the rod domain and membrane phospholipids, which places tryptophan residues in close contact with the membrane. Here, we examine the binding of the dystrophin repeat-2 to small unilamellar vesicles with varying composition. We find that the protein binds predominantly to di-oleyl-phosphatidylserine. The binding as a function of increasing mol% of DOPS appears to be cooperative due to reduction of dimensionality, greatly enhanced in the absence of salts, and partly modulated by pH. Substituting small by large unilamellar vesicles induces a 30-fold lower affinity of the protein for the membrane phospholipids. However, modifying the packing of the acyl chains by introducing lipids such as phosphatidylethanolamine and cholesterol to the vesicle leads to an approximately 7-fold increase in affinity. Taken together, these results show that the binding involves electrostatic forces in addition to hydrophobic ones.

Structure of the calmodulin alphaII-spectrin complex provides insight into the regulation of cell plasticity

AlphaII-spectrin is a major cortical cytoskeletal protein contributing to membrane organization and integrity. The Ca2+-activated binding of calmodulin to an unstructured insert in the 11th repeat unit of alphaII-spectrin enhances the susceptibility of spectrin to calpain cleavage but abolishes its sensitivity to several caspases and to at least one bacterially derived pathologic protease. Other regulatory inputs including phosphorylation by c-Src also modulate the proteolytic susceptibility of alphaII-spectrin. These pathways, acting through spectrin, appear to control membrane plasticity and integrity in several cell types. To provide a structural basis for understanding these crucial biological events, we have solved the crystal structure of a complex between bovine calmodulin and the calmodulin-binding domain of human alphaII-spectrin (Protein Data Bank ID code 2FOT). The structure revealed that the entire calmodulin-spectrin-binding interface is hydrophobic in nature. The spectrin domain is also unique in folding into an amphiphilic helix once positioned within the calmodulin-binding groove. The structure of this complex provides insight into the mechanisms by which calmodulin, calpain, caspase, and tyrosine phosphorylation act on spectrin to regulate essential cellular processes.

Continuous membrane-cytoskeleton adhesion requires continuous accommodation to lipid and cytoskeleton dynamics

The plasma membrane of most animal cells conforms to the cytoskeleton and only occasionally separates to form blebs. Previous studies indicated that many weak interactions between cytoskeleton and the lipid bilayer kept the surfaces together to counteract the normal outward pressure of cytoplasm. Either the loss of adhesion strength or the formation of gaps in the cytoskeleton enables the pressure to form blebs. Membrane-associated cytoskeleton proteins, such as spectrin and filamin, can control the movement and aggregation of membrane proteins and lipids, e.g., phosphoinositol phospholipids (PIPs), as well as blebbing. At the same time, lipids (particularly PIPs) and membrane proteins affect cytoskeleton and signaling dynamics. We consider here the roles of the major phosphatidylinositol-4,5-diphosphate (PIP2) binding protein, MARCKS, and PIP2 levels in controlling cytoskeleton dynamics. Further understanding of dynamics will provide important clues about how membrane-cytoskeleton adhesion rapidly adjusts to cytoskeleton and membrane dynamics.

Changes in cerebral membrane lipid composition and fluidity during thioacetamide-induced hepatic encephalopathy

Lipids are an essential structural and functional component of cellular membranes. Changes in membrane lipid composition are known to affect the activities of many membrane-associated enzymes, endocytosis, exocytosis, membrane fusion and neurotransmitter uptake, and have been implicated in the pathophysiology of many neurodegenerative disorders. In the present study, we investigated changes in the lipid composition of membranes isolated from the cerebral cortex of rats treated with thioacetamide (TAA), a hepatotoxin that induces fulminant hepatic failure (FHF) and thereon hepatic encephalopathy (HE). HE refers to acute neuropsychiatric changes accompanying FHF. The estimation of membrane phospholipids, cholesterol and fatty acid content in cerebral cortex membranes from TAA-treated rats revealed a decrease in cholesterol, phosphatidylserine, sphingomyelin, a monounsaturated fatty acid, namely oleic acid, and the polyunsaturated fatty acids gamma-linolenic acid, decosa hexanoic acid and arachidonic acid compared with controls. Assessment of membrane fluidity with pyrene, 1,6-diphenyl-1,3,5-hexatriene and 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene revealed a decrease in the annular membrane fluidity, whereas the global fluidity was unaffected. The level of the thiobarbituric acid reactive species marker for lipid peroxidation also increased in membranes from TAA-treated rats, thereby indicating the prevalence of oxidative stress. Results from the present study demonstrate gross alterations in cerebral cortical membrane lipid composition and fluidity during TAA-induced HE, and their possible implications in the pathogenesis of this condition are also discussed.

Conformational Stabilities of the Structural Repeats of Erythroid Spectrin and Their Functional Implications

The two polypeptide chains of the erythroid spectrin heterodimer contain between them 36 structural repeating modules, which can function as independently folding units. We have expressed all 36 and determined their thermal stabilities. These vary widely, with unfolding transition mid-points (T(m)) ranging from 21 to 72 degrees C. Eight of the isolated repeats are largely unfolded at physiological temperature. Constructs comprising two or more adjacent repeats show inter-repeat coupling with coupling free energies of several kcal mol(-1). Constructs comprising five successive repeats from the beta-chain displayed cooperativity and strong temperature dependence in forced unfolding by atomic force microscopy. Analysis of aligned sequences and molecular modeling suggests that high stability is conferred by large hydrophobic side chains at position e of the heptad hydrophobic repeats in the first helix of the three-helix bundle that makes up each repeat. This inference was borne out by the properties of mutants in which the critical residues have been replaced. The marginal stability of the tertiary structure at several points in the spectrin chains is moderated by energetic coupling with adjoining structural elements but may be expected to permit adaptation of the membrane to the large distortions that the red cell experiences in the circulation.

Structural cooperativity in spectrin type repeats motifs of dystrophin

Dystrophin is a member of the spectrin family of proteins, which are characterized as being predominantly composed the spectrin-type-repeat, a triple alpha-helical bundle motif present in multiple tandem copies, producing a rod-like shape. Whether or not this motif, which is determined by sequence homology, is correlated with biophysical domains in the intact protein is uncertain. The nature of the domain structure impacts the flexibility and shape of the rod region of this protein, which is a target for modification in several therapeutic approaches aimed at Duchenne Muscular Dystrophy, a common and fatal genetic disease caused by defective dystrophin. We examined three such motifs in dystrophin, expressing them recombinantly both singly and in tandem, and studying their thermodynamic properties by solvent and thermal denaturation. We have found that the degree to which they are independently stable and expressible varies considerably. The fourth motif appears to be largely stable and independent, whereas the third and second motifs interact strongly.

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.

AlphaII-spectrin interacts with Tes and EVL, two actin-binding proteins located at cell contacts

The spectrin-based membrane skeleton, a multi-protein scaffold attached to diverse cellular membranes, is presumed to be involved in the stabilization of membranes, the establishment of membrane domains as well as in vesicle trafficking and nuclear functions. Spectrin tetramers made of alpha- and beta-subunits are linked to actin microfilaments, forming a network that binds a multitude of proteins. The most prevalent alpha-spectrin subunit in non-erythroid cells, alphaII-spectrin, contains two particular spectrin repeats in its central region, alpha9 and alpha10, which host an Src homology 3 domain, a tissue-specific spliced sequence of 20 residues, a calmodulin-binding site and major cleavage sites for caspases and calpains. Using yeast two-hybrid screening of kidney libraries, we identified two partners of the alpha9-alpha10 repeats: the potential tumour suppressor Tes, an actin-binding protein mainly located at focal adhesions; and EVL (Ena/vasodilator-stimulated phosphoprotein-like protein), another actin-binding protein, equally recruited at focal adhesions. Interactions between spectrin and overexpressed Tes and EVL were confirmed by co-immunoprecipitation. In vitro studies showed that the interaction between Tes and spectrin is mediated by a LIM (Lin-11, Isl-1 and Mec3) domain of Tes and by the alpha10 repeat of alphaII-spectrin whereas EVL interacts with the Src homology 3 domain located within the alpha9 repeat. Moreover, we describe an in vitro interaction between Tes and EVL, and a co-localization of these two proteins at focal adhesions. These interactions between alphaII-spectrin, Tes and EVL indicate new functions for spectrin in actin dynamics and focal adhesions.

Structural and functional studies of interaction between Plasmodium falciparum knob-associated histidine-rich protein (KAHRP) and erythrocyte spectrin

Plasmodium falciparum dramatically modifies the structure and function of the membrane of the parasitized host erythrocyte. Altered membrane properties are the consequence of the interaction of a group of exported malaria proteins with host cell membrane proteins. KAHRP (the knob-associated histidine-rich protein), a member of this group, has been shown to interact with erythrocyte membrane skeletal protein spectrin. However, the molecular basis for this interaction has yet to be defined. In the present study, we defined the binding motifs in both KAHRP and spectrin and identified a functional role for this interaction. We showed that spectrin bound to a 72-amino-acid KAHRP fragment (residues 370-441). Among nine-spectrin fragments, which encompass the entire alpha and beta spectrin molecules (four alpha spectrin and five beta spectrin fragments), KAHRP bound only to one, the alpha N-5 fragment. The KAHRP-binding site within the alpha N-5 fragment was localized uniquely to repeat 4. The interaction of full-length spectrin dimer to KAHRP was inhibited by repeat 4 of alpha spectrin. Importantly, resealing of this repeat peptide into erythrocytes mislocalized KAHRP in the parasitized cells. We concluded that the interaction of KAHRP with spectrin is critical for appropriate membrane localization of KAHRP in parasitized erythrocytes. As the presence of KAHRP at the erythrocyte membrane is necessary for cytoadherence in vivo, our findings have implications for the development of new therapies for mitigating the severity of malaria infection.

Mapping of an ankyrin-sensitive, phosphatidylethanolamine/phosphatidylcholine mono- and bi-layer binding site in erythroid beta-spectrin

It has been shown previously that binding of vesicles and monolayers containing PE (phosphatidylethanolamine) by either erythroid or non-erythroid spectrin proved sensitive to inhibition by purified erythrocyte ankyrin. We tested the lipid-binding affinities of the purified ankyrin-binding domain of beta-spectrin and of its truncated mutants in four ways, by analysing: (1) penetration of 'loose' PE/PC (phosphatidylcholine) monolayers; (2) binding to liposomes in suspension; (3) competition with spectrin for liposomes; and (4) binding of a PE/PC monolayer in a surface plasmon resonance system. The results obtained indicated that the full-length ankyrin-binding domain bound PE/PC mono- and bi-layers with moderate affinity, penetrated monolayers and competed with spectrin for liposomes. Moreover, its truncated mutants that retained the N-terminal part, in contrast with those lacking eight or 38 N-terminal residues (which bound lipid mono- and bi-layers with lower affinity), bound PE/PC mono- and bi-layers with an affinity and capacity comparable with those of the full-length ankyrin-binding domain, and this activity was inhibited by purified erythrocyte ankyrin. The full-length domain, in contrast with the mutant lacking 38 N-terminal residues, induced a small increase in the fluidity of PE/PC membranes when probed with 5'-doxyl stearate, similar to the effect of purified spectrin. Therefore we conclude that the binding site for PE-rich lipids, which is sensitive to ankyrin inhibition, is located in a 38-residue N-terminal fragment of the beta-spectrin ankyrin-binding domain, and that the first eight residues play a key role in this activity.

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.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.