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

Nanoerythrosomes tailoring: Lipid induced protein scaffolding in ghost membrane derived vesicles

A peculiar polygonal protein scaffolding that resembles to spectrin-based skeleton of red blood cells can be reconstructed on the outer surface of vesicle-like nanoerythrosomes. The approximately 130 nm sized nanoerythrosomes are produced from red blood cell ghosts by addition of phospholipids (dipalmitoylphosphatidylcholine, DPPC). The scaffolding, constructed from the structural proteins of the cell membrane skeleton, covers the whole object resulting an enhanced stiffness. The protein pattern of the scaffolding is thermosensitive, reversible transformable in the biologically relevant temperature range. When the lipid additive is changed from DPPC to lysophospholipid (LPC), the protein network/scaffolding ceases to exist. By the variation of lipid type and ratio, a tailoring of the nanoerythrosomes can be achieved. During the tailoring process nanoerythrosomes or micelles, in a wide size range from 200 to 30 nm, are produced.

The role of spectrin in cell adhesion and cell-cell contact

Spectrins are proteins that are responsible for many aspects of cell function and adaptation to changing environments. Primarily the spectrin-based membrane skeleton maintains cell membrane integrity and its mechanical properties, together with the cytoskeletal network a support cell shape. The occurrence of a variety of spectrin isoforms in diverse cellular environments indicates that it is a multifunctional protein involved in numerous physiological pathways. Participation of spectrin in cell–cell and cell–extracellular matrix adhesion and formation of dynamic plasma membrane protrusions and associated signaling events is a subject of interest for researchers in the fields of cell biology and molecular medicine. In this mini-review, we focus on data concerning the role of spectrins in cell surface activities such as adhesion, cell–cell contact, and invadosome formation. We discuss data on different adhesion proteins that directly or indirectly interact with spectrin repeats. New findings support the involvement of spectrin in cell adhesion and spreading, formation of lamellipodia, and also the participation in morphogenetic processes, such as eye development, oogenesis, and angiogenesis. Here, we review the role of spectrin in cell adhesion and cell–cell contact.

Impact statement

This article reviews properties of spectrins as a group of proteins involved in cell surface activities such as, adhesion and cell–cell contact, and their contribution to morphogenesis. We show a new area of research and discuss the involvement of spectrin in regulation of cell–cell contact leading to immunological synapse formation and in shaping synapse architecture during myoblast fusion. Data indicate involvement of spectrins in adhesion and cell–cell or cell–extracellular matrix interactions and therefore in signaling pathways. There is evidence of spectrin’s contribution to the processes of morphogenesis which are connected to its interactions with adhesion molecules, membrane proteins (and perhaps lipids), and actin. Our aim was to highlight the essential role of spectrin in cell–cell contact and cell adhesion.

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.

Dystrophin and Spectrin, Two Highly Dissimilar Sisters of the Same Family

Dystrophin and Spectrin are two proteins essential for the organization of the cytoskeleton and for the stabilization of membrane cells. The comparison of these two sister proteins, and with the dystrophin homologue utrophin, enables us to emphasise that, despite a similar topology with common subdomains and a common structural basis of a three-helix coiled-coil, they show a large range of dissimilarities in terms of genetics, cell expression and higher level structural organisation. Interactions with cellular partners, including proteins and membrane phospholipids, also show both strikingly similar and very different behaviours. The differences between dystrophin and spectrin are also illustrated by the large variety of pathological anomalies emerging from the dysfunction or the absence of these proteins, showing that they are keystones in their function of providing a scaffold that sustains cell structure.

Nanomechanical properties of composite protein networks of erythroid membranes at lipid surfaces

Erythrocyte membranes have been particularly useful as a model for studies of membrane structure and mechanics. Native erythroid membranes can be electroformed as giant unilamellar vesicles (eGUVs). In the presence of ATP, the erythroid membrane proteins of eGUVs rearrange into protein networks at the microscale. Here, we present a detailed nanomechanical study of individual protein microfilaments forming the protein networks of eGUVs when spread on supporting surfaces. Using Peak Force tapping Atomic Force Microscopy (PF-AFM) in liquid environment we have obtained the mechanical maps of the composite lipid-protein networks supported on solid surface. In the absence of ATP, the protein pool was characterized by a Young's Modulus E_pool≈5-15MPa whereas the complex filaments were found softer after protein supramolecular rearrangement; E_fil≈0.4MPa. The observed protein softening and reassembling could be relevant for understanding the mechanisms of cytoskeleton reorganization found in pathological erythrocytes or erythrocytes that are affected by biological agents.

β-III spectrin underpins ankyrin R function in Purkinje cell dendritic trees: protein complex critical for sodium channel activity is impaired by SCA5-associated mutations

Beta III spectrin is present throughout the elaborate dendritic tree of cerebellar Purkinje cells and is required for normal neuronal morphology and cell survival. Spinocerebellar ataxia type 5 (SCA5) and spectrin associated autosomal recessive cerebellar ataxia type 1 are human neurodegenerative diseases involving progressive gait ataxia and cerebellar atrophy. Both disorders appear to result from loss of β-III spectrin function. Further elucidation of β-III spectrin function is therefore needed to understand disease mechanisms and identify potential therapeutic options. Here, we report that β-III spectrin is essential for the recruitment and maintenance of ankyrin R at the plasma membrane of Purkinje cell dendrites. Two SCA5-associated mutations of β-III spectrin both reduce ankyrin R levels at the cell membrane. Moreover, a wild-type β-III spectrin/ankyrin-R complex increases sodium channel levels and activity in cell culture, whereas mutant β-III spectrin complexes fail to enhance sodium currents. This suggests impaired ability to form stable complexes between the adaptor protein ankyrin R and its interacting partners in the Purkinje cell dendritic tree is a key mechanism by which mutant forms of β-III spectrin cause ataxia, initially by Purkinje cell dysfunction and exacerbated by subsequent cell death.

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.

Spectrins: a structural platform for stabilization and activation of membrane channels, receptors and transporters

This review focuses on structure and functions of spectrin as a major component of the membrane skeleton. Recent advances on spectrin function as an interface for signal transduction mediation and a number of data concerning interaction of spectrin with membrane channels, adhesion molecules, receptors and transporters draw a picture of multifaceted protein. Here, we attempted to show the current depiction of multitask role of spectrin in cell physiology. 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.

Artificial Spectrin Shells Reconstituted on Giant Vesicles

In the experimental approach to a synthetic minimal cell, the membrane compartment is a main component. Lipid vesicles represent the natural host for the artificial reconstruction of a cytomimetic membrane skeleton able to support mechanical function. Using the membrane component of human erythroid cells, we have reconstructed a membrane shell composed of a spectrin skeleton and fed by ATP. The structural and mechanical analysis reveals this spectrin skeleton as topological network supporting mechanical rigidity. Such an artificial shell would define a membrane compartment mechanically stable under physiological conditions.

Key amino acid residues of ankyrin-sensitive phosphatidylethanolamine/phosphatidylcholine-lipid binding site of βI-spectrin

It was shown previously that an ankyrin-sensitive, phosphatidylethanolamine/phosphatidylcholine (PE/PC) binding site maps to the N-terminal part of the ankyrin-binding domain of β-spectrin (ankBDn). Here we have identified the amino acid residues within this domain which are responsible for recognizing monolayers and bilayers composed of PE/PC mixtures. In vitro binding studies revealed that a quadruple mutant with substituted hydrophobic residues W1771, L1775, M1778 and W1779 not only failed to effectively bind PE/PC, but its residual PE/PC-binding activity was insensitive to inhibition with ankyrin. Structure prediction and analysis, supported by in vitro experiments, suggests that “opening” of the coiled-coil structure underlies the mechanism of this interaction. Experiments on red blood cells and HeLa cells supported the conclusions derived from the model and in vitro lipid-protein interaction results, and showed the potential physiological role of this binding. We postulate that direct interactions between spectrin ankBDn and PE-rich domains play an important role in stabilizing the structure of the spectrin-based membrane skeleton.

On the association of lipid rafts to the spectrin skeleton in human erythrocytes

Lipid rafts are local inhomogeneities in the composition of the plasma membrane of living cells, that are enriched in sphingolipids and cholesterol in a liquid-ordered state, and proteins involved in receptor-mediated signalling. Interactions between lipid rafts and the cytoskeleton have been observed in various cell types. They are isolated as a fraction of the plasma membrane that resists solubilization by nonionic detergents at 4°C (detergent-resistant membranes, DRMs). We have previously described that DRMs are anchored to the spectrin-based membrane skeleton in human erythrocytes and can be released by increasing the pH and ionic strength of the solubilization medium with sodium carbonate. It was unexplained why this carbonate treatment was necessary and why this requirement was not reported by other workers in this area. We show here that when contaminating leukocytes are present in erythrocyte preparations that are subjected to detergent treatment, the isolation of DRMs can occur without the requirement for carbonate treatment. This is due to the uncontrolled breakdown of erythrocyte membrane components by hydrolases that are released from contaminating neutrophils that lead to proteolytic disruption of the supramolecular assembly of the membrane skeleton. Results presented here corroborate the concept that DRMs are anchored to the membrane skeleton through electrostatic interactions that most likely involve the spectrin molecule.

Calpain modulates capacitation and acrosome reaction through cleavage of the spectrin cytoskeleton

Research on fertilization in mammalian species has revealed that Ca(2+) is an important player in biochemical and physiological events enabling the sperm to penetrate the oocyte. Ca(2+) is a signal transducer that particularly mediates capacitation and acrosome reaction (AR). Before becoming fertilization competent, sperm must experience several molecular, biochemical, and physiological changes where Ca(2+) plays a pivotal role. Calpain-1 and calpain-2 are Ca(2+)-dependent proteases widely studied in mammalian sperm; they have been involved in capacitation and AR but little is known about their mechanism. In this work, we establish the association of calpastatin with calpain-1 and the changes undergone by this complex during capacitation in guinea pig sperm. We found that calpain-1 is relocated and translocated from cytoplasm to plasma membrane (PM) during capacitation, where it could cleave spectrin, one of the proteins of the PM-associated cytoskeleton, and facilitates AR. The aforementioned results were dependent on the calpastatin phosphorylation and the presence of extracellular Ca(2+). Our findings underline the contribution of the sperm cytoskeleton in the regulation of both capacitation and AR. In addition, our findings also reveal one of the mechanisms by which calpain and calcium exert its function in sperm.

Do we already know how spectrin attracts ankyrin?

The interaction of ankyrin and spectrin yields the major anchor between the membrane skeleton and the lipid bilayer. It is critical for red cell deformability and stability, and it is also involved in the cellular localization of several proteins, in cell differentiation, and in neuron activity. Therefore, its nature is of great interest, and recently, several researchers have had varying degrees of success in elucidating the structural basis of ankyrin-spectrin recognition. In this short paper, we briefly summarize the data obtained and compare the resulting conclusions.

Control of erythrocyte membrane-skeletal cohesion by the spectrin-membrane linkage

Spectrin tetramer is the major structural member of the membrane-associated skeletal network of red cells. We show here that disruption of the spectrin-ankyrin-band 3 link to the membrane leads to dissociation of a large proportion of the tetramers into dimers. Noncovalent perturbation of the linkage was induced by a peptide containing the ankyrin-binding site of the spectrin beta-chain, and covalent perturbation by treatment with the thiol reagent, N-ethylmaleimide (NEM). This reagent left the intrinsic self-association capacity of the spectrin dimers unaffected and disturbed only the ankyrin-band 3 interaction. The dissociation of spectrin tetramers on the membrane into functional dimers was confirmed by the binding of a spectrin peptide directed against the self-association sites. Dissociation of the tetramers resulted, we infer, from detachment of the proximal ends of the constituent dimers from the membrane, thereby reducing their proximity to one another and thus weakening their association. The measured affinity of the interaction of the peptides with the free dimer ends on the membrane permits an estimate of the equilibrium between intact and dissociated tetramers on the native membrane. This indicates that in the physiological state the equilibrium proportion of the dissociated tetramers may be as high as 5-10%. These findings enabled us to identify an additional important functional role for the spectrin-ankyrin-band 3 link in regulating spectrin self-association in the red cell membrane.

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.

Structural basis for spectrin recognition by ankyrin

Maintenance of membrane integrity and organization in the metazoan cell is accomplished through intracellular tethering of membrane proteins to an extensive, flexible protein network. Spectrin, the principal component of this network, is anchored to membrane proteins through the adaptor protein ankyrin. To elucidate the atomic basis for this interaction, we determined a crystal structure of human betaI-spectrin repeats 13 to 15 in complex with the ZU5-ANK domain of human ankyrin R. The structure reveals the role of repeats 14 to 15 in binding, the electrostatic and hydrophobic contributions along the interface, and the necessity for a particular orientation of the spectrin repeats. Using structural and biochemical data as a guide, we characterized the individual proteins and their interactions by binding and thermal stability analyses. In addition to validating the structural model, these data provide insight into the nature of some mutations associated with cell morphology defects, including those found in human diseases such as hereditary spherocytosis and elliptocytosis. Finally, analysis of the ZU5 domain suggests it is a versatile protein-protein interaction module with distinct interaction surfaces. The structure represents not only the first of a spectrin fragment in complex with its binding partner, but also that of an intermolecular complex involving a ZU5 domain.

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 intracellular interactions of the L1 family of cell adhesion molecules

The L1 family of CAMs (cell adhesion molecules) has long aroused the interest of researchers, but primarily the extracellular interactions of these proteins have been elucidated. More recently, attention has turned to the intracellular signalling potentiated by transmembrane proteins and the cytoplasmic proteins with which they can interact. The present review brings up to date the current body of published knowledge for the intracellular interactions of L1-CAM family proteins and the potential importance of these interactions for the mechanisms of L1-CAM action.

Structures of the spectrin-ankyrin interaction binding domains

As key components of the erythrocyte membrane skeleton, spectrin and ankyrin specifically interact to tether the spectrin cytoskeleton to the cell membrane. The structure of the spectrin binding domain of ankyrin and the ankyrin binding domain of spectrin have been solved to elucidate the structural basis for ankyrin-spectrin recognition. The structure of repeats 14 and 15 of spectrin shows that these repeats are similar to all other spectrin repeats. One feature that could account for the preference of ankyrin for these repeats is the presence of a conserved, negatively charged patch on one side of repeat 14. The structure of the ankyrin ZU5 domain shows a novel structure containing a beta core. The structure reveals that the canonical ZU5 consensus sequence is likely to be missing an important region that codes for a beta strand that forms part of the core of the domain. In addition, a positively charged region is suggestive of a binding surface for the negatively charged spectrin repeat 14. Previously reported mutants of ankyrin that map to this region lie mostly on the surface of the protein, although at least one is likely to be part of the core.

Molecular epitopes of the ankyrin-spectrin interaction

Isoforms of ankyrin and its binding partner spectrin are responsible for a number of interactions in a variety of human cells. Conflicting evidence, however, had identified two different, non-overlapping human erythroid ankyrin subdomains, Zu5 and 272, as the minimum binding region for beta-spectrin. Complementary studies on the ankyrin-binding domain of spectrin have been somewhat more conclusive yet have not presented binding in terms of well-phased, integral numbers of spectrin repeats. Thus, the objective of this study was to clearly define and characterize the minimal ankyrin-spectrin binding epitopes. Circular dichroism (CD) wavelength spectra of the aforementioned ankyrin subdomains show that these fragments are 30-60% unstructured. In contrast, human erythroid beta-spectrin repeats 13, 14, 15, and 16 (prepared in all combinations of two adjacent repeats) demonstrated proper folding and stability as determined by CD and tryptophan wavelength and heat denaturation scans. Native polyacrylamide gel electrophoresis (PAGE) gel shifts as well as affinity pull-down assays implicated Zu5 and beta-spectrin repeats 14-15 as the minimum binding epitopes. These results were confirmed by analytical ultracentrifugation to sedimentation equilibrium by which a 1:1 complex was obtained if and only if Zu5 was mixed with beta-spectrin constructs containing repeats 14 and 15 in tandem. Surface plasmon resonance yielded a K D of 15.2 nM for binding of beta-spectrin fragments to the ankyrin subdomain Zu5, accounting for all of the binding observed between the intact molecules. Collectively, these results show the 14th and 15th beta-spectrin repeats comprise the minimal, phased region of beta-spectrin, which binds ankyrin at the Zu5 subdomain with high affinity.

The 22.5 kDa spectrin-binding domain of ankyrinR binds spectrin with high affinity and changes the spectrin distribution in cells in vivo

It was previously shown that ankyrins play a crucial role in the membrane skeleton arrangement. Purifying ankyrinR obtained from erythrocytes is a time-consuming process. Therefore, cloned and bacterially expressed ankyrinR-spectrin-binding domain (AnkSBD) is a demanded tool for studying spectrin-ankyrin interactions. In this communication, we report on the cloning and purification of AnkSBD and describe the results of binding experiments, in which we showed high-affinity interactions between the AnkSBD construct and isolated erythrocyte or non-erythroid spectrins. pEGFP-AnkSBD-transfected cells co-localised with non-erythroid spectrin in HeLa cells. The functional interactions of the AnkSBD construct in vivo and in vitro open many possibilities to study the structure and function of this domain, which has not yet been as extensively studied when compared to the aminoterminal domain of this protein.

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.

Mutation of a single residue, beta-glutamate-20, alters protein-lipid interactions of light harvesting complex II

It is well established that assembly of the peripheral antenna complex, LH2, is required for proper photosynthetic membrane biogenesis in the purple bacterium Rhodobacter sphaeroides. The underlying interactions are, as yet, not understood. Here we examined the relationship between the morphology of the photosynthetic membrane and the lipid–protein interactions at the LH2–lipid interface. The non-bilayer lipid, phosphatidylethanolamine, is shown to be highly enriched in the boundary lipid phase of LH2. Sequence alignments indicate a putative lipid binding site, which includes β-glutamate-20 and the adjacent carotenoid end group. Replacement of β-glutamate-20 with alanine results in significant reduction of phosphatidylethanolamine and concomitant raise in phosphatidylcholine in the boundary lipid phase of LH2 without altering the lipid composition of the bulk phase. The morphology of the LH2 housing membrane is, however, unaffected by the amino acid replacement. In contrast, simultaneous modification of glutamate-20 and exchange of the carotenoid sphaeroidenone with neurosporene results in significant enlargement of the vesicular membrane invaginations. These findings suggest that the LH2 complex, specifically β-glutamate-20 and the carotenoids' polar head group, contribute to the shaping of the photosynthetic membrane by specific interactions with surrounding lipid molecules.

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.

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.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.