Binding of hydrophobic ligands to 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.

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.

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.

Specificity of Prodan for the Self-associating Domain of Spectrin: A Molecular Docking Study

The hydrophobic fluorescent probe Prodan binds to the self-associating domain of spectrin with 1:1 stoichiometry. A model of the self-associating domain was generated based on its homology with other domains of spectrin. Prodan was then docked onto the model, and several sites with low interaction energy were identified. To verify whether the binding of Prodan is specific towards the self-associating domain of spectrin, it was docked on to several other domains of spectrin, having a known three-dimensional structure. Analysis of the docking results suggests that the binding of Prodan to the self-associating domain of spectrin will involve hydrophobic and hydrophilic groups of Prodan. The results clearly indicate the preference of Prodan for a particular binding site of the self-associating domain.

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.

Conformational study of spectrin in presence of submolar concentrations of denaturants

The presence of very low concentrations of the commonly used chemical denaturants, guanidinium chloride (GdmCl) and urea brought about conformational changes in the erythrocyte membrane skeletal protein, spectrin. Evidences in support of changes in the quaternary structure of spectrin have been put forward from quenching study of tryptophan fluorescence, by both steady state and time-resolved measurements, using acrylamide as the quencher. It revealed significant differences between the Stern-Volmer quenching constants (K(SV)) and the fraction of accessible tryptophans (f(e)) observed in absence and presence of GdmCl and urea concentrations below 1 M at which the association of the two subunits remains intact. The steady state anisotropy of both the spectrin tryptophans and the spectrin-bound fluorescence probe, Prodan also indicate changes in the overall flexibility of the spectrin dimer, originating from changes in the quaternary structure of spectrin. Studies on the binding of Prodan, further indicate that conformational changes also occur in spectrin near the Prodan-binding site at the terminal domain of the protein which is reflected in 3-4 fold decrease in the affinity of binding of Prodan to spectrin in the presence of GdmCl and urea compared to that observed in the absence of the denaturants. The dissociation constant (K(d)) of Prodan to spectrin is 0.43 microM at 25 degrees C.

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.

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

The erythrocyte membrane is a composite structure consisting of a lipid bilayer tethered to the spectrin-based membrane skeleton. Two complexes of spectrin with other proteins are known to participate in the attachment. Spectrin has also been shown to interact with phosphatidylserine (PS), a component of the lipid bilayer, which is confined to its inner leaflet. That there may be multiple sites of interaction with PS in the spectrin sequence has been inferred, but they have not hitherto been identified. Here we have explored the interaction of PS-containing liposomes with native alpha- and beta-spectrin chains and with recombinant spectrin fragments encompassing the entire sequences of both chains. We show that both alpha-spectrin and beta-spectrin bind PS and that sites of high affinity are located within 8 of the 38 triple-helical structural repeats which make up the bulk of both chains; these are alpha8, alpha9-10, beta2, beta3, beta4, beta12, beta13, and beta14, and PS affinity was also found in the nonhomologous N-terminal domain of the beta-chain. No other fragments of either chain showed appreciable binding. Binding of spectrin and its constituent chains to mixed liposomes of PS and phosphatidylcholine (PC) depended on the proportion of PS. Binding of spectrin dimers to PS liposomes was inhibited by single repeats containing PS binding sites. It is noteworthy that the PS binding sites in beta-spectrin are grouped in close proximity to the sites of attachment both of ankyrin and of 4.1R, the proteins engaged in attachment of spectrin to the membrane. We conjecture that direct interaction of spectrin with PS in the membrane may modulate its interactions with the proteins and that (considering also the known affinity of 4.1R for PS) the formation of PS-rich lipid domains, which have been observed in the red cell membrane, may be a result.

Erythroid spectrin in miceller detergents

We have studied the interaction of spectrin, the major protein of the erythrocyte cytoskeleton, with four commonly used detergents at concentrations above their critical miceller concentrations (cmc). Fluorescence spectroscopic studies on the emission intensity, steady state polarization, quenching with acrylamide, and time-resolved fluorescence measurements were done with spectrin in anionic detergents, e.g., SDS, deoxycholate, and nonionic detergents, e.g., Triton-X-100 and octylglucoside at concentrations double their respective cmc's. The spectrin-detergent complexes in all four systems have been characterized by far-UV CD and measurements on tryptophan fluorescence in combination with fluorescence of the extrinsic probe, pyrene. Tryptophan fluorescence studies revealed quaternary structural changes due to unzipping of the spectrin subunits in Triton-X-100 without complete dissociation. Both Triton-X-100 and SDS were found to partially denature spectrin indicated by the far-UV CD. Octylglucoside and deoxycholate are shown to have the least structural perturbations on the cytoskeletal protein, rationalizing the use of octylglucoside, in particular and also deoxycholate to be the most effective in preparing cytoskeletal fractions from erythrocytes rather than the Triton-X-100 that has long been used for preparing the Triton shells.

The tertiary amine local anesthetic dibucaine binds to the membrane skeletal protein spectrin

The quinoline-based tertiary amine dibucaine has been shown to bind the membrane skeletal protein spectrin with a dissociation constant of 3.5x10(-5) M at 25 degrees C. Such binding is detected by monitoring the quenching of the tryptophan fluorescence intensity with increasing concentrations of dibucaine only and not with the benzene-based local anesthetics procaine, tetracaine and lidocaine. Binding of dibucaine also indicated changes in the tertiary structure of spectrin indicated by a circular dichroism spectrum in the near-UV region due to absorption of the aromatic side chains. The thermodynamic parameters associated with the binding indicated the interaction of dibucaine and spectrin to be enthalpy-driven and insensitive to an increase in the ionic strength of the buffer.

Rapid diffusion of spectrin bound to a lipid surface

Human erythrocyte spectrin was labelled with the probe 5, 5'-disulfato-1-(6-hexanoic acid N-hydroxysuccinimide ester)-1'-ethyl-3,3,3',3'-tetramethylindocarbocyanine (Cy3). Cy3-spectrin was bound to the outer surface of dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles and its diffusion measured by fluorescence recovery after photobleaching (FRAP). It was found that at 30 degrees C, above the lipid gel to liquid-crystalline phase transition of the lipids, Cy3-spectrin had an unexpectedly high diffusion coefficient D=(2.1+/-0.6)x10(-7)) cm2/s. At the phase transition, diffusion of Cy3-spectrin was only slightly lower; D=(1.3+/-0.3)x10(-7) cm2/s, whereas at 14 degrees C, well below the lipid phase transition, diffusion was found to be much slower with D=(3.1+/-0.12)x10(-9) cm2/s. The fast diffusion of Cy3-spectrin on the lipid surface implies that the individual bonds which bind spectrin to the lipid surface must rapidly be made and broken. In the light of these results, spectrin-lipid interactions alone appear unlikely to have any significant role in supporting the cell membrane. Probably, the interactions serve only to localise the spectrin at the inner lipid surface in order to facilitate formation of the cytoskeleton.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Fluorescence studies of spectrin and its subunits

To better understand the solution structure of spectrin, the environment of its tryptophan residues have been examined by fluorescence spectroscopy. The spectra and the extent of quenching by several quenching agents have been determined for intact spectrin and its alpha and beta subunits. The arsenal of quenchers used in the study represented both hydrophilic and hydrophobic species including anionic, cationic and neutral compounds. Effects on spectrin fluorescence of ethanol and ionic strength, which extend and/or rigidify spectrin, and of glycerol, which is commonly used in electron microscopy of the protein, have also been assessed in the presence and absence of quenchers. Most of the tryptophans of spectrin are either internally quenched or are sequestered, hindering the approach of hydrophilic quenching agents. Both the spectral shape and the extent of quenching by acrylamide indicate that some tryptophans of the beta subunit are slightly more exposed in the isolated chain than in the dimer. Similar effects on spectra and on quenching of the intact dimer and of the isolated beta chain are seen when the ionic strength is reduced. Ethanol and glycerol reduce spectrin tryptophan accessibility to 2-p-toluidinyl napthalene-6-sulfonic acid (TNS). It therefore appears that low ionic strength, alpha-beta association and neutral solute (or lowered dielectric constant) all induce a similar, but modest conformational change in the domain structure. The extent of TNS binding is not increased by lowering the ionic strength, suggesting that the expansion and/or stiffening of the molecule in low electrolyte solution does not involve exposure of significant numbers of hydrophobic sites.

Fluorescence quenching of spectrin and other red cell membrane cytoskeletal proteins. Relation to hydrophobic binding sites

The intrinsic fluorescence of spectrin is strongly quenched by low concentrations of 2-bromostearate. This results from binding at a series of hydrophobic sites. Analysis of dynamic fluorescence quenching by acrylamide, iodide and caesium ions, separately and in conjunction with 2-bromostearate, leads to the conclusion that most of the tryptophan side-chains are exposed to solvent. The sites at which the fatty-acid-quenched tryptophans are located apparently interact with the lipid bilayer in the cell, as judged by quenching by bromostearate dissolved in the lipid phase. A minor proportion of the side-chains in native spectrin give rise to sharp proton magnetic resonance signals, indicative of segmental mobility; these chain elements contain some tryptophan residues, as revealed by weak downfield signals from the heterocyclic ring protons. These signals are not appreciably perturbed by stearic acid or by phosphatidylserine liposomes, suggesting that the hydrophobic binding sites are not in mobile chain elements. By contrast with a series of globular proteins which, with the exception of serum albumins, show little or no quenching by 2-bromostearate, the peripheral red cell membrane skeletal proteins ankyrin (and its spectrin-binding domain), protein 4.1 and (to a lesser extent) actin show evidence of a high affinity for the hydrophobic ligand and may, like spectrin, interact directly with the bilayer in situ.

Coupling of spectrin and polylysine to phospholipid monolayers studied by specular reflection of neutrons

The technique of specular reflection of neutrons is applied for the first time to study the charge-dependent interaction of the protein spectrin and the polypeptide poly-L-lysine with model phospholipid monolayers in the condensed phase state. We first established the structure of a pure monolayer of dimyristolyphosphatidylcholine (DMPC) in both the expanded and condensed fluid phase states without protein in the subphase. The thickness of the hydrocarbon chains increases from 11.4 +/- 1.5 A in the expanded state to 15.8 +/- 1.5 A in the condensed state, whereas the head group region is approximately 10 A thick for both phase states. When spectrin is present in the subphase, the dimensions of DMPC in the condensed state are not significantly affected, but there is approximately 0.09 volume fraction spectrin in the head group region. Lipid-spectrin coupling is enhanced by electrostatic interaction, as the volume fraction of spectrin in the head group region increases to 0.22 in a mixed monolayer of DMPC and negatively charged dimyristolyphosphatidylglycerol in the condensed state. In contrast to spectrin, polylysine does not penetrate the head group region, but forms a layer electrostatically adsorbed to the charged head groups.

EPR study of the hydrophobic interaction of spectrin with fatty acids

The hydrophobic interaction between spin-labelled stearic acid and spectrin was studied by electron paramagnetic resonance (EPR) and fluorescence quenching. The results are quantitatively interpreted in terms of two types of binding site on spectrin. A comparison between the results of the EPR and fluorescence experiments show the drawback of the fluorescence method in binding studies.

Dark interaction of 5-methoxypsoralen and 8-methoxypsoralen with erythrocyte ghosts: a fluorescence and circular dichroism study

The dark interaction of 5-methoxypsoralen (5-MOP) and 8-methoxypsoralen (8-MOP) with plasma membranes was studied using human erythrocyte ghosts as a model. In the presence of ghosts, modifications of the fluorescence characteristics of 5-MOP were observed, together with a quenching of the fluorescence of the tryptophan (Trp) residues of membrane proteins (up to 25%). Moreover, the appearance of an induced circular dichroism indicates that 5-MOP is located in a chiral environment. In contrast, only slight effects were observed in the case of 8-MOP. It is concluded that 5-MOP molecules are located partly within chiral protein sites of the membrane in such a way that a Förster energy transfer can occur from the Trp residues to the psoralen molecules.

Properties of the spectrin-like structural element of smooth-muscle alpha-actinin

The fragment of smooth muscle alpha-actinin, comprising the four spectrin-like structural repeating units, has a high alpha-helix content, similar to that of spectrin, and a hydrodynamic frictional coefficient, indicative of an elongated, probably bent or kinked rod-like structure, as found for spectrin dimer and tetramer. The fragment exists in solution as an extremely stable dimer, which is dissociated only under denaturing conditions and is much more resistant to dissociation by urea than is the spectrin heterodimer. High-resolution proton magnetic resonance spectra reveal that a part of the polypeptide chain gives rise to sharp resonances; this is also true of spectrin and it implies that the individual structural repeating units contain segmentally mobile elements, which may be required to generate the elastic properties of the spectrin family of proteins. Again like spectrin, the alpha-actinin fragment contains multiple binding sites for long-chain fatty acids, as revealed by quenching of tryptophan fluorescence by 2-bromostearate (though not by 9(10)-bromostearate). The results point to extensive structural and functional similarities between the repeating units of all the proteins of the spectrin family.

Elasticity of the human red cell membrane skeleton. Effects of temperature and denaturants

The molecular basis for the elasticity of the human erythrocyte membrane was explored. Skeletons were released from ghosts in Triton X-100 and their dimensions followed by dark-field microscopy and packed volume. The rest size of skeletons was assumed to reflect the balance point between expansion (deformation) driven by electrostatic repulsions among the excess of fixed negative charges on the proteins and contraction (recovery) driven by their elasticity. The size of skeletons decreased with increasing temperature. This finding suggests that entropy drives elasticity. The requisite entropy change could be associated with either the configurational freedom of flexible protein chains or with the solvation of side chains exposed during protein dissociation (hydrophobic effects). To distinguish between these two alternatives, we tested the impact of two weak denaturants, 10% ethanol and 20 nM lithium 3,5-diiodosalicylate. Both agents reversibly promoted the expansion of skeletons, presumably by reducing their elasticity. Since the conformation of random coils and globular proteins should not be significantly altered by these mild treatments, this finding strongly suggests a role for weak interdomain and/or interprotein associations. We conclude that the elasticity of the red cell membrane skeleton may not derive from the configurational entropy of flexible coils. Rather, the elastic energy may arise from reversible dissociations of weak but specific intramolecular and/or intermolecular contacts, presumably within deformed spectrin filaments.

Activation induces a rapid reorganization of spectrin in lymphocytes

Lymphocyte activation results in a rapid reorganization of the cytoskeletal protein spectrin. Immediately following an activation signal, there is fragmentation of a spectrin-rich cytoplasmic structure and subsequent translocation of the fragments to defined areas of the plasma membrane in both antigen-specific T cell hybridomas and lymph node T cells. These dramatic changes have been documented by light and electron microscopic immunolocalization and by immunoblot analysis of plasma membrane-enriched preparations. A T cell hybridoma variant lacking the spectrin-rich cytoplasmic structure of the parental line does not redistribute spectrin and produces little or no IL-2 in response to antigen-dependent activation. This suggests a functional link between spectrin distribution and activation potential. We propose that the cytoplasmic structure functions as an organizing center or reservoir for spectrin that is sensitive to signaling at the cell surface.

Interaction of spectrin with phospholipids. Quenching of spectrin intrinsic fluorescence by phospholipid suspensions

Phospholipid suspensions prepared of phosphatidylethanolamine, phosphatidylserine and their mixtures are able to influence the intrinsic protein fluorescence of spectrin. In the case of phosphatidylethanolamine suspension up to 75% of protein fluorescence can be quenched. The interaction of phospholipid aggregates with spectrin is modulated by pH and ionic strength. Phospholipids, particularly phosphatidylethanolamine display a 'stabilizing' effect against the changes of protein fluorescence induced by increasing ionic strength and by thermal denaturation.

Labelling of erythrocyte spectrin in situ with phenylisothiocyanate

The labelling of erythrocyte spectrin in situ with the hydrophobic reagent phenylisothiocyanate (Sigrist, H. and Zahler, P. (1978) FEBS Lett. 95, 116-120) is studied. Spectrin isolated from erythrocytes which have been incubated with phenylisothiocyanate is covalently modified by the probe. The modification in the spectrin molecule is stable under an excess of nucleophile in alkaline conditions. The labelling is very little or not affected by preincubation of erythrocytes of membranes with the polar, structural analogue of phenylisothiocyanate, p-sulfophenylisothiocyanate. When erythrocyte ghosts are subjected to labelling, a substantial increase in the degree of spectrin modification is observed. Subunits of labelled spectrin separated electrophoretically show similar amounts of attached label.

Interaction of anilinonaphtyl labeled spectrin with fatty acids and phospholipids: a fluorescence study

Anilinonaphtyl labeled spectrin exhibits a fluorescence emission spectrum characteristic of a highly hydrophobic environment. Quenching of the fluorescence intensity by nitroxide analogs of fatty acids of affinity 10(4) M-1 reveals that the sites of interaction of fatty acids lie very close to the anilinonaphtyl groups. Similar experiments performed with a nitroxide analog of phosphatidylserine yield a 30% quenching of fluorescence while the same phosphatidylcholine analog has essentially no effect. The changes in the fluorescence emission spectrum exhibited in the presence of sonicated phosphatidylserine vesicles further outline the specificity of interaction towards phosphatidylserine, with one spectrin binding site per about 750 exposed phospholipids. Moreover, they suggest a penetration of the anilinonaphtyl group into the lipid bilayer.

Quantitative composition and characterization of the proteins in membrane vesicles released from erythrocytes by dimyristoylphosphatidylcholine. A membrane system without cytoskeleton

Membrane vesicles were prepared by incubation of human erythrocytes with dimyristoylphosphatidylcholine [3] and isolated by isopycnic centrifugation on Dextran density gradients. Protein analyses were carried out with crossed immunoelectrophoresis and dodecylsulfate polyacrylamide gel electrophoresis. The right-side-out-oriented membrane vesicles contained membrane and cytoplasmic proteins of the erythrocyte but lacked cytoskeletal components. Comparison of proteins in vesicles and erythrocyte membranes showed that acetylcholinesterase was enriched two to six times in the vesicles relative to both membrane-spanning proteins, band 3, and glycophorin. Two further, hitherto unidentified, sialic acid-containing membrane antigens were found in the vesicles. Both faced the outside of the membranes and were enriched two to seven times. Ankyrin was not present in the membrane vesicles and spectrin could not be detected by dodecylsulfate polyacrylamide gel electrophoresis. We suggest that the redistribution of proteins in the vesicles reflects differences in their interactions with other membrane components and their relative mobility within the erythrocyte membrane.