Erythrocyte spectrin is comprised of many homologous triple helical segments

Plasmodium falciparum selectively degrades α-spectrin of infected erythrocytes after invasion

Remodeling the erythrocyte membrane and skeleton by the malarial parasite Plasmodium falciparum is closely associated with intraerythrocytic development. However, the mechanisms underlying this association remain unclear. In this study, we present evidence that erythrocytic α-spectrin, but not β-spectrin, was dynamically ubiquitinated and progressively degraded during the intraerythrocytic development of P. falciparum, from the ring to the schizont stage. We further observed an upregulated expression of P. falciparum phosphatidylinositol 3-kinase (PfPI3K) in the infected red blood cells during the intraerythrocytic development of the parasite. The data indicated that PfPI3K phosphorylated and activated erythrocytic ubiquitin-protein ligase, leading to increased α-spectrin ubiquitination and degradation during P. falciparum development. We further revealed that inhibition of the activity of PfPI3K impaired P. falciparum development in vitro and Plasmodium berghei infectivity in mice. These findings collectively unveil an important mechanism of PfPI3K-ubiquitin-mediated degradation of α-spectrin during the intraerythrocytic development of Plasmodium species. Proteins in the PfPI3K regulatory pathway are novel targets for effective treatment of severe malaria.

IMPORTANCE

Plasmodium falciparum is the causative agent of severe malaria that causes millions of deaths globally. The parasite invades human red blood cells and induces a cascade of alterations in erythrocytes for development and proliferation. Remodeling the host erythrocytic cytoskeleton is a necessary process during parasitization, but its regulatory mechanisms remain to be elucidated. In this study, we observed that erythrocytic α-spectrin is selectively degraded after P. falciparum invasion, while β-spectrin remained intact. We found that the α-spectrin chain was profoundly ubiquitinated by E3 ubiquitin ligase and degraded by the 26S proteasome. E3 ubiquitin ligase activity was regulated by P. falciparum phosphatidylinositol 3-kinase (PfPI3K) signaling. Additionally, blocking the PfPI3K-ubiquitin-proteasome pathway in P. falciparum-infected red blood cells reduced parasite proliferation and infectivity. This study deepens our understanding of the regulatory mechanisms of host and malarial parasite interactions and paves the way for the exploration of novel antimalarial drugs.

SPTBN1 Mediates the Cytoplasmic Constraint of PTTG1, Impairing Its Oncogenic Activity in Human Seminoma

Seminoma is the most common testicular cancer. Pituitary tumor-transforming gene 1 (PTTG1) is a securin showing oncogenic activity in several tumors. We previously demonstrated that nuclear PTTG1 promotes seminoma tumor invasion through its transcriptional activity on matrix metalloproteinase 2 (MMP-2) and E-cadherin (CDH1). We wondered if specific interactors could affect its subcellular distribution. To this aim, we investigated the PTTG1 interactome in seminoma cell lines showing different PTTG1 nuclear levels correlated with invasive properties. A proteomic approach upon PTTG1 immunoprecipitation uncovered new specific securin interactors. Western blot, confocal microscopy, cytoplasmic/nuclear fractionation, sphere-forming assay, and Atlas database interrogation were performed to validate the proteomic results and to investigate the interplay between PTTG1 and newly uncovered partners. We observed that spectrin beta-chain (SPTBN1) and PTTG1 were cofactors, with SPTBN1 anchoring the securin in the cytoplasm. SPTBN1 downregulation determined PTTG1 nuclear translocation, promoting its invasive capability. Moreover, a PTTG1 deletion mutant lacking SPTBN1 binding was strongly localized in the nucleus. The Atlas database revealed that seminomas that contained higher nuclear PTTG1 levels showed significantly lower SPTBN1 levels in comparison to non-seminomas. In human seminoma specimens, we found a strong PTTG1/SPTBN1 colocalization that decreases in areas with nuclear PTTG1 distribution. Overall, these results suggest that SPTBN1, along with PTTG1, is a potential prognostic factor useful in the clinical management of seminoma.

Dynamics of the axon plasma membrane skeleton

It was recently revealed via super-resolution microscopy experiments that the axon plasma membrane skeleton (APMS) comprises a series of periodically arranged azimuthal actin rings connected via longitudinal spectrin filaments forming an orthotropic network. The common perception is that APMS enhances structural stability of the axon but its impact on axon deformation is unknown. To investigate the response of the APMS to extension, we introduce a coarse-grain molecular dynamics model consisting of actin particles forming rings and chains of particles representing spectrin tetramers with repeats than can unfold. We observe that the shape of force-extension curve is initially linear and the force level depends on the extension rate. Even during the initial deformation stage, unfolding of spectrin repeats occurs, but the saw-tooth shape of the corresponding force-extension curve observed in the case of one spectrin tetramer does not appear in the case of the entire APMS. The reason is that spectrin unfolding is not synchronized across filaments during extension. If actin-spectrin associations remain intact, the force-extension response reaches a perfectly plastic region because of increased spectrin unfolding frequency. However, when actin-spectrin links dissociate, which can happen at moderate and high extension rates, APMS softens and the resistance force decreases linearly as the axon elongates until it reaches a point where the APMS is completely severed. Furthermore, when the ring-to-ring distance is maintained fixed under stretch, the resistance force relaxes exponentially as a function of time due to additional unfolding of spectrin tetramers following the Kelvin-Voigt representation of the Zener model.

Mechanical role of the submembrane spectrin scaffold in red blood cells and neurons

Spectrins are large, evolutionarily well-conserved proteins that form highly organized scaffolds on the inner surface of eukaryotic cells. Their organization in different cell types or cellular compartments helps cells withstand mechanical challenges with unique strategies depending on the cell type. This Review discusses our understanding of the mechanical properties of spectrins, their very distinct organization in red blood cells and neurons as two examples, and the contribution of the scaffolds they form to the mechanical properties of these cells.

Spectrins and human diseases

Spectrin, as one of the major components of a plasma membrane-associated cytoskeleton, is a cytoskeletal protein composed of the modular structure of α and β subunits. The spectrin-based skeleton is essential for preserving the integrity and mechanical characteristics of the cell membrane. Moreover, spectrin regulates a variety of cell processes including cell apoptosis, cell adhesion, cell spreading, and cell cycle. Dysfunction of spectrins is implicated in various human diseases including hemolytic anemia, neurodegenerative diseases, ataxia, heart diseases, and cancers. Here, we briefly discuss spectrins function as well as the clinical manifestations and currently known molecular mechanisms of human diseases related to spectrins, highlighting that strategies for targeting regulation of spectrins function may provide new avenues for therapeutic intervention for these diseases.

βII spectrin (SPTBN1): biological function and clinical potential in cancer and other diseases

βII spectrin, the most common isoform of non-erythrocyte spectrin, is a cytoskeleton protein present in all nucleated cells. Interestingly, βII spectrin is essential for the development of various organs such as nerve, epithelium, inner ear, liver and heart. The functions of βII spectrin include not only establishing and maintaining the cell structure but also regulating a variety of cellular functions, such as cell apoptosis, cell adhesion, cell spreading and cell cycle regulation. Notably, βII spectrin dysfunction is associated with embryonic lethality and the DNA damage response. More recently, the detection of altered βII spectrin expression in tumors indicated that βII spectrin might be involved in the development and progression of cancer. Its mutations and disorders could result in developmental disabilities and various diseases. The versatile roles of βII spectrin in disease have been examined in an increasing number of studies; nonetheless, the exact mechanisms of βII spectrin are still poorly understood. Thus, we summarize the structural features and biological roles of βII spectrin and discuss its molecular mechanisms and functions in development, homeostasis, regeneration and differentiation. This review highlight the potential effects of βII spectrin dysfunction in cancer and other diseases, outstanding questions for the future investigation of therapeutic targets. The investigation of the regulatory mechanism of βII spectrin signal inactivation and recovery may bring hope for future therapy of related diseases.

Multiple Functions of Spectrin: Convergent Effects

Spectrin is a multifunctional, multi-domain protein most well known in the membrane skeleton of mature human erythrocytes. Here we review the literature on the crosstalk of the chaperone activity of spectrin with its other functionalities. We hypothesize that the chaperone activity is derived from the surface exposed hydrophobic patches present in individual "spectrin-repeat" domains and show a competition between the membrane phospholipid binding functionality and chaperone activity of spectrin. Moreover, we show that post-translational modifications such as glycation which shield these surface exposed hydrophobic patches, reduce the chaperone function. On the other hand, oligomerization which is linked to increase of hydrophobicity is seen to increase it. We note that spectrin seems to prefer haemoglobin as its chaperone client, binding with it preferentially over other denatured proteins. Spectrin is also known to interact with unstable haemoglobin variants with a higher affinity than in the case of normal haemoglobin. We propose that chaperone activity of spectrin could be important in the cellular biochemistry of haemoglobin, particularly in the context of diseases.

Human erythrocytes: cytoskeleton and its origin

In the last few years, erythrocytes have emerged as the main determinant of blood rheology. In mammals, these cells are devoid of nuclei and are, therefore, unable to divide. Consequently, all circulating erythrocytes come from erythropoiesis, a process in the bone marrow in which several modifications are induced in the expression of membrane and cytoskeletal proteins, and different vertical and horizontal interactions are established between them. Cytoskeleton components play an important role in this process, which explains why they and the interaction between them have been the focus of much recent research. Moreover, in mature erythrocytes, the cytoskeleton integrity is also essential, because the cytoskeleton confers remarkable deformability and stability on the erythrocytes, thus enabling them to undergo deformation in microcirculation. Defects in the cytoskeleton produce changes in erythrocyte deformability and stability, affecting cell viability and rheological properties. Such abnormalities are seen in different pathologies of special interest, such as different types of anemia, hypertension, and diabetes, among others. This review highlights the main findings in mammalian erythrocytes and their progenitors regarding the presence, conformation and function of the three main components of the cytoskeleton: actin, intermediate filaments, and tubulin.

Mechanical Unfolding of Spectrin Repeats Induces Water-Molecule Ordering

Mechanical processes are involved at many stages of the development of living cells, and often external forces applied to a biomolecule result in its unfolding. Although our knowledge of the unfolding mechanisms and the magnitude of the forces involved has evolved, the role that water molecules play in the mechanical unfolding of biomolecules has not yet been fully elucidated. To this end, we investigated with steered molecular dynamics simulations the mechanical unfolding of dystrophin's spectrin repeat 1 and related the changes in the protein's structure to the ordering of the surrounding water molecules. Our results indicate that upon mechanically induced unfolding of the protein, the solvent molecules become more ordered and increase their average number of hydrogen bonds. In addition, the unfolded structures originating from mechanical pulling expose an increasing amount of the hydrophobic residues to the solvent molecules, and the uncoiled regions adapt a convex surface with a small radius of curvature. As a result, the solvent molecules reorganize around the protein's small protrusions in structurally ordered waters that are characteristic of the so-called "small-molecule regime," which allows water to maintain a high hydrogen bond count at the expense of an increased structural order. We also determined that the response of water to structural changes in the protein is localized to the specific regions of the protein that undergo unfolding. These results indicate that water plays an important role in the mechanically induced unfolding of biomolecules. Our findings may prove relevant to the ever-growing interest in understanding macromolecular crowding in living cells and their effects on protein folding, and suggest that the hydration layer may be exploited as a means for short-range allosteric communication.

Fine mapping of hydrophobic contacts reassesses the organization of the first three dystrophin coiled-coil repeats

Coiled-coil domain is a structural motif found in proteins crucial for achievement of central biological processes, such as cellular cohesion or neuro-transmission. The coiled-coil fold consists of alpha-helices bundle that can be repeated to form larger filament. Hydrophobic residues, distributed following a regular seven-residues' pattern, named heptad pattern, are commonly admitted to be essential for the formation and the stability of canonical coiled-coil repeats. Here we investigated the first three coiled-coil repeats (R1-R3) of the central domain of dystrophin, a scaffolding protein in muscle cells whose deficiency leads to Duchenne and Becker Muscular Dystrophies. By an atomic description of the hydrophobic interactions, we highlighted (i) that coiled-coil filament conformational changes are associated to specific patterns of inter-helices hydrophobic contacts, (ii) that inter-repeat hydrophobic interactions determine the behavior of linker regions including filament kinks, and (iii) that a non-strict conservation of the heptad patterns is leading to a relative plasticity of the dystrophin coiled-coil repeats. These structural features and modulations of the coiled-coil fold could better explain the mechanical properties of the central domain of dystrophin. This contribution to the understanding of the structure-function relationship of dystrophin, and especially of the R1-R3 fragment frequently used in the design of protein for gene therapies, should help in the improvement of the strategies for the cure of muscular dystrophies.

α-Helix Unwinding as Force Buffer in Spectrins

Spectrins are cytoskeletal proteins located at the inner face of the plasma membrane, making connections between membrane anchors and the actin cortex, and between actin filaments. Spectrins share a common structure forming a bundle of 3 α-helices and play a major role during cell deformation. Here, we used high-speed force spectroscopy and steered molecular dynamics simulations to understand the mechanical stability of spectrin, revealing a molecular force buffering function. We find that spectrin acts as a soft spring at short extensions (70-100 Å). Under continuous external stretching, its α-helices unwind, leading to a viscous mechanical response over larger extensions (100-300 Å), represented by a constant-force plateau in force/extension curves. This viscous force buffering emerges from a quasi-equilibrium competition between disruption and re-formation of α-helical hydrogen bonds. Our results suggest that, in contrast to β-sheet proteins, which unfold in a catastrophic event, α-helical spectrins dominantly unwind, providing a viscous force buffer over extensions about 5 times their folded length.

Affinity of IDPs to their targets is modulated by ion-specific changes in kinetics and residual structure

Intrinsically disordered proteins (IDPs) are characterized by a lack of defined structure. Instead, they populate ensembles of rapidly interconverting conformations with marginal structural stabilities. Changes in solution conditions such as temperature and crowding agents consequently affect IDPs more than their folded counterparts. Here we reveal that the residual structure content of IDPs is modulated both by ionic strength and by the type of ions present in solution. We show that these ion-specific structural changes result in binding affinity shifts of up to sixfold, which happen through alteration of both association and dissociation rates. These effects follow the Hofmeister series, but unlike the well-established effects on the stability of folded proteins, they already occur at low, hypotonic concentrations of salt. We attribute this sensitivity to the marginal stability of IDPs, which could have physiological implications given the role of IDPs in signaling, the asymmetric ion profiles of different cellular compartments, and the role of ions in biology.

Spectraplakin family proteins - cytoskeletal crosslinkers with versatile roles

The different cytoskeletal networks in a cell are responsible for many fundamental cellular processes. Current studies have shown that spectraplakins, cytoskeletal crosslinkers that combine features of both the spectrin and plakin families of crosslinkers, have a critical role in integrating these different cytoskeletal networks. Spectraplakin genes give rise to a variety of isoforms that have distinct functions. Importantly, all spectraplakin isoforms are uniquely able to associate with all three elements of the cytoskeleton, namely, F-actin, microtubules and intermediate filaments. In this Review, we will highlight recent studies that have unraveled their function in a wide range of different processes, from regulating cell adhesion in skin keratinocytes to neuronal cell migration. Taken together, this work has revealed a diverse and indispensable role for orchestrating the function of different cytoskeletal elements in vivo.

The nesprin-cytoskeleton interface probed directly on single nuclei is a mechanically rich system

The cytoskeleton provides structure and plays an important role in cellular function such as migration, resisting compression forces, and transport. The cytoskeleton also reacts to physical cues such as fluid shear stress or extracellular matrix remodeling by reorganizing filament associations, most commonly focal adhesions and cell-cell cadherin junctions. These mechanical stimuli can result in genome-level changes, and the physical connection of the cytoskeleton to the nucleus provides an optimal conduit for signal transduction by interfacing with nuclear envelope proteins, called nesprins, within the LINC (linker of the nucleus to the cytoskeleton) complex. Using single-molecule on single nuclei assays, we report that the interactions between the nucleus and the cytoskeleton, thought to be nesprin-cytoskeleton interactions, are highly sensitive to force magnitude and direction depending on whether cells are historically interfaced with the matrix or with cell aggregates. Application of ∼10-30 pN forces to these nesprin linkages yielded structural transitions, with a base transition size of 5-6 nm, which are speculated to be associated with partial unfoldings of the spectrin domains of the nesprins and/or structural changes of histones within the nucleus.

Effect of pH on stability, conformation, and chaperone activity of erythroid & non-erythroid spectrin

Spectrin, a major component of the eukaryotic membrane skeleton, has been shown to have chaperone like activity. Here we investigate the pH induced changes in the structure and stability of erythroid and brain spectrin by spectroscopic methods. We also correlate these changes with modulations of chaperone potential at different pH. We have followed the pH induced structural changes by circular dichroism spectroscopy and intrinsic tryptophan fluorescence. It is seen that lowering the pH from 9 has little effect on structure of the proteins till about pH6. At pH4, there is significant change of the secondary structure of the proteins, along with a 5nm hypsochromic shift of the emission maxima. Below pH4 the proteins undergo acid denaturation. Probing exposed hydrophobic patches on the proteins using protein-bound 8-anilinonaphthalene-1-sulfonate fluorescence demonstrates that there is higher solvent accessibility of hydrophobic surfaces in both forms of spectrin at around pH4. Dynamic light scattering and 90° light scattering studies show that the both forms of spectrin forms oligomers at pH~4. Chemical unfolding data shows that these oligomers are less stable than the tetrameric form. Aggregation studies with BSA show that at pH4, both spectrins exhibit better chaperone activity. This enhancement of chaperone like activity appears to result from an increase in regions of solvent-exposed hydrophobicity and oligomeric state of the spectrins which in turn are induced by moderately acid pH. This may have in-vivo implications in cells facing stress conditions where cytoplasmic pH is lowered.

Dielectric relaxations on erythrocyte membrane as revealed by spectrin denaturation

We studied the effect of spectrin denaturation at 49.5°C (TA) on the dielectric relaxations and related changes in the complex impedance, Z*, complex capacitance, C*, and dielectric loss curve of suspensions containing human erythrocytes, erythrocyte ghost membranes (EMs) and Triton-X-100 residues of EMs. The loss curve prior to, minus the loss curve after TA, resulted in a bell-shaped peak at 1.5MHz. The changes in the real and imaginary components of Z* and C* at TA, i.e., ΔZre, ΔZim, ΔCre and ΔCim, calculated in the same way, strongly varied with frequency. Between 1.0 and 12MHz the -ΔZim vs ΔZre, and ΔCim vs ΔCre plots depicted semicircles with critical frequencies, fcr, at 2.5MHz expressing recently reported relaxation of spectrin dipoles. Between 0.02 and 1.0MHz the -ΔZim vs ΔZre plot exhibited another relaxation whose fcr mirrored that of beta relaxation. This relaxation was absent on Triton-X-shells, while on erythrocytes and EMs it was inhibited by selective dissociation of either attachment sites between spectrin and bilayer. Considering above findings and inaccessibility of cytosole to outside field at such frequencies, the latter relaxation was assumed originating from a piezoelectric effect on the highly deformable spectrin filaments.

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

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

Microwave & magnetic (M2) proteomics reveals CNS-specific protein expression waves that precede clinical symptoms of experimental autoimmune encephalomyelitis

Central nervous system-specific proteins (CSPs), transported across the damaged blood-brain-barrier (BBB) to cerebrospinal fluid (CSF) and blood (serum), might be promising diagnostic, prognostic and predictive protein biomarkers of disease in individual multiple sclerosis (MS) patients because they are not expected to be present at appreciable levels in the circulation of healthy subjects. We hypothesized that microwave &magnetic (M(2)) proteomics of CSPs in brain tissue might be an effective means to prioritize putative CSP biomarkers for future immunoassays in serum. To test this hypothesis, we used M(2) proteomics to longitudinally assess CSP expression in brain tissue from mice during experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. Confirmation of central nervous system (CNS)-infiltrating inflammatory cell response and CSP expression in serum was achieved with cytokine ELISPOT and ELISA immunoassays, respectively, for selected CSPs. M(2) proteomics (and ELISA) revealed characteristic CSP expression waves, including synapsin-1 and α-II-spectrin, which peaked at day 7 in brain tissue (and serum) and preceded clinical EAE symptoms that began at day 10 and peaked at day 20. Moreover, M(2) proteomics supports the concept that relatively few CNS-infiltrating inflammatory cells can have a disproportionally large impact on CSP expression prior to clinical manifestation of EAE.

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.

Mechanism of Assembly of the Non-Covalent Spectrin Tetramerization Domain from Intrinsically Disordered Partners

Interdomain interactions of spectrin are critical for maintenance of the erythrocyte cytoskeleton. In particular, “head-to-head” dimerization occurs when the intrinsically disordered C-terminal tail of β-spectrin binds the N-terminal tail of α-spectrin, folding to form the “spectrin tetramer domain”. This non-covalent three-helix bundle domain is homologous in structure and sequence to previously studied spectrin domains. We find that this tetramer domain is surprisingly kinetically stable. Using a protein engineering Φ-value analysis to probe the mechanism of formation of this tetramer domain, we infer that the domain folds by the docking of the intrinsically disordered β-spectrin tail onto the more structured α-spectrin tail.

Structural organization of the nine spectrin repeats of Kalirin

Sequence analysis suggests that KALRN, a Rho GDP/GTP exchange factor genetically linked to schizophrenia, could contain as many as nine tandem spectrin repeats (SRs). We expressed and purified fragments of Kalirin containing from one to five putative SRs to determine whether they formed nested structures that could endow Kalirin with the flexible rodlike properties characteristic of spectrin and dystrophin. Far-UV circular dichroism studies indicated that Kalirin contains nine SRs. On the basis of thermal denaturation, sensitivity to chemical denaturants, and the solubility of pairs of repeats, the nine SRs of Kalirin form nested structures. Modeling studies confirmed this conclusion and identified an exposed loop in SR5; consistent with the modeling, this loop was extremely labile to proteolytic cleavage. Analysis of a direpeat fragment (SR4:5) encompassing the region of Kalirin known to interact with NOS2, DISC-1, PAM, and Arf6 identified this as the least stable region. Analytical ultracentrifugation indicated that SR1:3, SR4:6, and SR7:9 were monomers and adopted an extended conformation. Gel filtration suggested that ΔKal7, a natural isoform that includes SR5:9, was monomeric and was not more extended than SR5:9. Similarly, the nine SRs of Kal7, which was also monomeric, were not more extended than SR5:9. The rigidity and flexibility of the nine SRs of Kal7, which separate its essential N-terminal Sec14p domain from its catalytic domain, play an essential role in its contribution to the formation and function of dendritic spines.

Intertwined αβ spectrin meeting helical actin protofilament in the erythrocyte membrane skeleton: wrap-around vs. point-attachment

Our 3-D model for a junctional complex (JC) in the erythrocyte membrane skeleton proposed that the helical actin protofilament functions as a mechanical axis for three pairs of αβ spectrin (Sp), and each pair wraps around the protofilament in a back-to-back fashion. The distal end of each Sp is further associated with the lipid bilayer by a suspension complex (SC). Here, we detail how splitting and rejoining of αβ Sp around a protofilament may form a loop that sustains and equilibrates tension. Sequential association of β and α Sp solves the challenge of constructing multiple loops along the protofilament, and topological connection facilitates their re-association. The wrap-around model minimizes the strain of the actin binding site on β Sp due to tension, redirection, or sliding of intertwined Sp. Pairing Sp balances the opposing forces and provides a mechanism for elastic recovery. The wrap-around junction thus provides mechanical advantages over a point-attachment junction in maintaining the integrity and functionality of the network. Severing α or β Sp may convert a wrapping-around junction to a point-attachment junction. In that case, a “bow up” motion of JC during deformation may disturb or flip the overlaid lipid bilayer, and mark stressed erythrocytes for phagocytosis.

A comprehensive model of the spectrin divalent tetramer binding region deduced using homology modeling and chemical cross-linking of a mini-spectrin

Spectrin dimer-tetramer interconversion is a critical contributor to red cell membrane stability, but some properties of spectrin tetramer formation cannot be studied effectively using monomeric recombinant domains. To address these limitations, a fused αβ mini-spectrin was produced that forms wild-type divalent tetramer complexes. Using this mini-spectrin, a medium-resolution structure of a seven-repeat bivalent tetramer was produced using homology modeling coupled with chemical cross-linking. Inter- and intramolecular cross-links provided critical distance constraints for evaluating and optimizing the best conformational model and appropriate docking interfaces. The two strands twist around each other to form a super-coiled, rope-like structure with the AB helix face of one strand associating with the opposing AC helix face. Interestingly, two tetramer site hereditary anemia mutations that exhibit wild-type binding in univalent head-to-head assays are located in the interstrand region. This suggests that perturbations of the interstrand region can destabilize spectrin tetramers and the membrane skeleton. The α subunit N-terminal cross-links to multiple sites on both strands, demonstrating that this non-homologous tail remains flexible and forms heterogeneous structures in the tetramer complex. Although no cross-links were observed involving the β subunit non-homologous C-terminal tail, several cross-links were observed only when this domain was present, suggesting it induces subtle conformational changes to the tetramer site region. This medium-resolution model provides a basis for further studies of the bivalent spectrin tetramer site, including analysis of functional consequences of interstrand interactions and mutations located at substantial molecular distances from the tetramer site.

ATP-dependent mechanism protects spectrin against glycation in human erythrocytes

Human erythrocytes are continuously exposed to glucose, which reacts with the amino terminus of the β-chain of hemoglobin (Hb) to form glycated Hb, HbA1c, levels of which increase with the age of the circulating cell. In contrast to extensive insights into glycation of hemoglobin, little is known about glycation of erythrocyte membrane proteins. In the present study, we explored the conditions under which glucose and ribose can glycate spectrin, both on the intact membrane and in solution and the functional consequences of spectrin glycation. Although purified spectrin could be readily glycated, membrane-associated spectrin could be glycated only after ATP depletion and consequent translocation of phosphatidylserine (PS) from the inner to the outer lipid monolayer. Glycation of membrane-associated spectrin led to a marked decrease in membrane deformability. We further observed that only PS-binding spectrin repeats are glycated. We infer that the absence of glycation in situ is the consequence of the interaction of the target lysine and arginine residues with PS and thus is inaccessible for glycation. The reduced membrane deformability after glycation in the absence of ATP is likely the result of the inability of the glycated spectrin repeats to undergo the obligatory unfolding as a consequence of interhelix cross-links. We thus postulate that through the use of an ATP-driven phospholipid translocase (flippase), erythrocytes have evolved a protective mechanism against spectrin glycation and thus maintain their optimal membrane function during their long circulatory life span.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

Binding of alphaII spectrin to 14-3-3beta is involved in NCAM-dependent neurite outgrowth

Members of the 14-3-3 protein family have been implicated in neuronal migration, synaptic plasticity and learning. Using affinity chromatography followed by mass spectrometry analysis, we show here that the cytoskeletal protein alphaII spectrin is a novel ligand of 14-3-3beta. We found that 14-3-3beta interacts with alphaII spectrin via the mode 2 14-3-3 binding motif RLIQS(1302)HP. Binding required phosphorylation of Ser(1302) by casein kinase II and was enhanced in the presence of calmodulin. Co-immunoprecipitation of alphaII spectrin and 14-3-3beta with the neural cell adhesion molecule NCAM suggested that the 14-3-3-spectrin-interaction affects NCAM function. Indeed, disruption of the 14-3-3beta/alphaII spectrin interaction by mutating Ser(1302) to Ala enhanced NCAM-dependent neurite outgrowth. Our results indicate that the phosphorylation-dependent interaction between 14-3-3beta and alphaII spectrin acts as a switch between positive and negative regulation of neurite outgrowth stimulated by NCAM, representing a novel and acute mechanism preventing uncontrolled elongation of neuronal processes.

Crystal structure and functional interpretation of the erythrocyte spectrin tetramerization domain complex

As the principal component of the membrane skeleton, spectrin confers integrity and flexibility to red cell membranes. Although this network involves many interactions, the most common hemolytic anemia mutations that disrupt erythrocyte morphology affect the spectrin tetramerization domains. Although much is known clinically about the resulting conditions (hereditary elliptocytosis and pyropoikilocytosis), the detailed structural basis for spectrin tetramerization and its disruption by hereditary anemia mutations remains elusive. Thus, to provide further insights into spectrin assembly and tetramer site mutations, a crystal structure of the spectrin tetramerization domain complex has been determined. Architecturally, this complex shows striking resemblance to multirepeat spectrin fragments, with the interacting tetramer site region forming a central, composite repeat. This structure identifies conformational changes in alpha-spectrin that occur upon binding to beta-spectrin, and it reports the first structure of the beta-spectrin tetramerization domain. Analysis of the interaction surfaces indicates an extensive interface dominated by hydrophobic contacts and supplemented by electrostatic complementarity. Analysis of evolutionarily conserved residues suggests additional surfaces that may form important interactions. Finally, mapping of hereditary anemia-related mutations onto the structure demonstrate that most, but not all, local hereditary anemia mutations map to the interacting domains. The potential molecular effects of these mutations are described.

A fused alpha-beta "mini-spectrin" mimics the intact erythrocyte spectrin head-to-head tetramer

Head-to-head assembly of two spectrin heterodimers to form an actin-cross-linking tetramer is a physiologically dynamic interaction that contributes to red cell membrane integrity. Recombinant beta-spectrin C-terminal and alpha-spectrin N-terminal peptides can form tetramer-like univalent complexes, but they cannot evaluate effects of the open-closed dimer interactions or lateral associations of the two-spectrin strands on tetramer formation. In this study we produced and characterized a fused "mini-spectrin dimer" containing the beta-spectrin C-terminal region linked to the alpha-spectrin N-terminal region. This fused mini-spectrin mimics structural and functional properties of intact, full-length dimers and tetramers, including lateral association of the alpha and beta subunits in the dimer and formation of a closed dimer. High performance liquid chromatography gel filtration analyses of this mini-spectrin provide the first direct non-imaging experimental evidence for open and closed spectrin dimers and show that dimer-tetramer-oligomer interconversion is slow at low temperatures and accelerated at 30 degrees C, analogous to full-length spectrin. This protein exhibits wild type dimer-tetramer dissociation constants of approximately 1 mum at 30 degrees C, independent of initial oligomeric state. Conformational states of the mini-spectrin dimer were probed further using chemical cross-linking, which identified distinct groups of cross-links for "open" and "closed" dimers and confirmed the N-terminal region of alpha-spectrin remains highly flexible in the complex, exhibiting closely analogous structures to those observed for the isolated alpha-spectrin N-terminal using NMR (Park, S., Caffrey, M. S., Johnson, M. E., and Fung, L. W. (2003) J. Biol. Chem. 278, 21837-21844). This fusion protein should serve as a useful template for structural and functional studies of the divalent tetramer site.

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.

Evolution of spectrin function in cytoskeletal and membrane networks

Spectrin is a cytoskeletal protein thought to have descended from an alpha-actinin-like ancestor. It emerged during evolution of animals to promote integration of cells into tissues by assembling signalling and cell adhesion complexes, by enhancing the mechanical stability of membranes and by promoting assembly of specialized membrane domains. Spectrin functions as an (alphabeta([H]))(2) tetramer that cross-links transmembrane proteins, membrane lipids and the actin cytoskeleton, either directly or via adaptor proteins such as ankyrin and 4.1. In the present paper, I review recent findings on the origins and adaptations in this system. (i) The genome of the choanoflagellate Monosiga brevicollis encodes alpha-, beta- and beta(Heavy)-spectrin, indicating that spectrins evolved in the immediate unicellular precursors of animals. (ii) Ankyrin and 4.1 are not encoded in that genome, indicating that spectrin gained function during subsequent animal evolution. (iii) Protein 4.1 gained a spectrin-binding activity in the evolution of vertebrates. (iv) Interaction of chicken or mammal beta-spectrin with PtdInsP(2) can be regulated by differential mRNA splicing, which can eliminate the PH (pleckstrin homology) domain in betaI- or betaII-spectrins; in the case of mammalian betaII-spectrin, the alternative C-terminal region encodes a phosphorylation site that regulates interaction with alpha-spectrin. (v) In mammalian evolution, the single pre-existing alpha-spectrin gene was duplicated, and one of the resulting pair (alphaI) neo-functionalized for rapid make-and-break of tetramers. I hypothesize that the elasticity of mammalian non-nucleated erythrocytes depends on the dynamic rearrangement of spectrin dimers/tetramers under the shearing forces experienced in circulation.

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.

Different members of a simple three-helix bundle protein family have very different folding rate constants and fold by different mechanisms

The 15th, 16th, and 17th repeats of chicken brain alpha-spectrin (R15, R16, and R17, respectively) are very similar in terms of structure and stability. However, R15 folds and unfolds 3 orders of magnitude faster than R16 and R17. This is unexpected. The rate-limiting transition state for R15 folding is investigated using protein engineering methods (Phi-value analysis) and compared with previously completed analyses of R16 and R17. Characterisation of many mutants suggests that all three proteins have similar complexity in the folding landscape. The early rate-limiting transition states of the three domains are similar in terms of overall structure, but there are significant differences in the patterns of Phi-values. R15 apparently folds via a nucleation-condensation mechanism, which involves concomitant folding and packing of the A- and C-helices, establishing the correct topology. R16 and R17 fold via a more framework-like mechanism, which may impede the search to find the correct packing of the helices, providing a possible explanation for the fast folding of R15.

A Two-amino Acid Mutation Encountered in Duchenne Muscular Dystrophy Decreases Stability of the Rod Domain 23 (R23) Spectrin-like Repeat of Dystrophin

Lack of functional dystrophin causes severe Duchenne muscular dystrophy. The subsarcolemmal location of dystrophin, as well as its association with both cytoskeleton and membrane, suggests a role in the mechanical regulation of muscular membrane stress. In particular, phenotype rescue in a Duchenne muscular dystrophy mice model has shown that some parts of the central rod domain of dystrophin, constituted by 24 spectrin-like repeats, are essential. In this study, we made use of rare missense pathogenic mutations in the dystrophin gene and analyzed the biochemical properties of the isolated repeat 23 bearing single or double mutations E2910V and N2912D found in muscle dystrophy with severity grading. No dramatic effect on secondary and tertiary structure of the repeat was found in mutants compared with wild type as revealed by circular dichroism and NMR. Thermal and chemical unfolding data from circular dichroism and tryptophan fluorescence show significant decrease of stability for the mutants, and stopped-flow spectroscopy shows decreased refolding rates. The most deleterious single mutation is the N2912D replacement, although we observe additive effects of the two mutations on repeat stability. Based on three-dimensional structures built by homology molecular modeling, we discuss the modifications of the mutation-induced repeat stability. We conclude that the main forces involved in repeat stability are electrostatic inter-helix interactions that are disrupted following mutations. This study represents the first analysis at the protein level of the consequences of missense mutations in the human dystrophin rod domain. Our results suggest that it may participate in mechanical weakening of dystrophin-deficient muscle.

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.

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.

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.

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.

Fifty years of coiled-coils and alpha-helical bundles: a close relationship between sequence and structure

alpha-Helical coiled coils are remarkable for the diversity of related conformations that they adopt in both fibrous and globular proteins, and for the range of functions that they exhibit. The coiled coils are based on a heptad (7-residue), hendecad (11-residue) or a related quasi-repeat of apolar residues in the sequences of the alpha-helical regions involved. Most of these, however, display one or more sequence discontinuities known as stutters or stammers. The resulting coiled coils vary in length, in the number of chains participating, in the relative polarity of the contributing alpha-helical regions (parallel or antiparallel), and in the pitch length and handedness of the supercoil (left- or right-handed). Functionally, the concept that a coiled coil can act only as a static rod is no longer valid, and the range of roles that these structures have now been shown to exhibit has expanded rapidly in recent years. An important development has been the recognition that the delightful simplicity that exists between sequence and structure, and between structure and function, allows coiled coils with specialized features to be designed de novo.

Structural and functional effects of hereditary hemolytic anemia-associated point mutations in the alpha spectrin tetramer site

The most common hereditary elliptocytosis (HE) and hereditary pyropoikilocytosis (HPP) mutations are alpha-spectrin missense mutations in the dimer-tetramer self-association site. In this study, we systematically compared structural and functional properties of the 14 known HE/HPP mutations located in the alpha-spectrin tetramer binding site. All mutant alpha-spectrin recombinant peptides were well folded, stable structures, with only the R34W mutant exhibiting a slight structural destabilization. In contrast, binding affinities measured by isothermal titration calorimetry were greatly variable, ranging from no detectable binding observed for I24S, R28C, R28H, R28S, and R45S to approximately wild-type binding for R34W and K48R. Binding affinities for the other 7 mutants were reduced by approximately 10- to 100-fold relative to wild-type binding. Some sites, such as R28, were hot spots that were very sensitive to even relatively conservative substitutions, whereas other sites were only moderately perturbed by nonconservative substitutions. The R34W and K48R mutations were particularly intriguing mutations that apparently either destabilize tetramers through mechanisms not probed by the univalent tetramer binding assay or represent polymorphisms rather than the pathogenic mutations responsible for observed clinical symptoms. All alpha0 HE/HPP mutations studied here appear to exert their destabilizing effects through molecular recognition rather than structural mechanisms.