Interactions of the alpha-spectrin N-terminal region with beta-spectrin. Implications for the spectrin tetramerization reaction

Clinical and molecular genetic analysis of a Chinese family with hereditary elliptocytosis caused by a novel mutation in the EPB41 gene

BACKGROUND

Hereditary elliptocytosis (HE) is a heterogeneous red blood cell membrane disorder characterized by the presence of elliptocytes on a peripheral blood smear. Clinical manifestations of HE vary widely from asymptomatic carriers to patients with severe transfusion-dependent anemia. Most patients are asymptomatic or have mild anemia, which hinders diagnosis. The proband in this case had mild anemia and jaundice over a period of 4 years, the etiology of which was unclear. Hence, he was admitted to our hospital for further diagnosis.

METHODS

Peripheral blood smears and routine blood tests were performed and biochemical parameters of the proband, and his family members were determined. To confirm the diagnosis, gene mutations were screened in the proband using next-generation sequencing (NGS) and verified by Sanger sequencing in other family members.

RESULTS

A novel mutation (c.1294delA, p.Ser432 fs) in exon 15 of the EPB41 gene was detected in the proband and his family members. This mutation results in a frameshift and a premature stop codon at position 455, encoding a truncated protein. The variant was likely pathogenic according to the criteria of the American College of Medical Genetics and Genomics. SWISS-MODEL protein structure prediction indicated partial loss of the spectrin and actin binding and C-terminal domains.

CONCLUSION

A heterozygous mutation 1294delA in exon 15 of the EPB41 gene was identified using NGS and Sanger sequencing in members of a Chinese family. This identification expands the spectrum of EPB41 mutations and contributes to the genetic diagnosis of families with HE.

The role of βII spectrin in cardiac health and disease

Spectrins are large, flexible proteins comprised of α-β dimers that are connected head-to-head to form the canonical heterotetrameric spectrin structure. Spectrins were initially believed to be exclusively found in human erythrocytic membrane and are highly conserved among different species. βII spectrin, the most common isoform of non-erythrocytic spectrin, is found in all nucleated cells and forms larger macromolecular complexes with ankyrins and actins. Not only is βII spectrin a central cytoskeletal scaffolding protein involved in preserving cell structure but it has also emerged as a critical protein required for distinct physiologic functions such as posttranslational localization of crucial membrane proteins and signal transduction. In the heart, βII spectrin plays a vital role in maintaining normal cardiac membrane excitability and proper cardiac development during embryogenesis. Mutations in βII spectrin genes have been strongly linked with the development of serious cardiac disorders such as congenital arrhythmias, heart failure, and possibly sudden cardiac death. This review focuses on our current knowledge of the role βII spectrin plays in the cardiovascular system in health and disease and the potential future clinical implications.

Genotype-phenotype correlations in hereditary elliptocytosis and hereditary pyropoikilocytosis

Hereditary elliptocytosis (HE) and hereditary pyropoikilocytosis (HPP) are heterogeneous red blood cell (RBC) membrane disorders that result from mutations in the genes encoding α-spectrin (SPTA1), β-spectrin (SPTB), or protein 4.1R (EPB41). The resulting defects alter the horizontal cytoskeletal associations and affect RBC membrane stability and deformability causing shortened RBC survival. The clinical diagnosis of HE and HPP relies on identifying characteristic RBC morphology on peripheral blood smear and specific membrane biomechanical properties using osmotic gradient ektacytometry. However, this phenotypic diagnosis may not be readily available in patients requiring frequent transfusions, and does not predict disease course or severity. Using Next-Generation sequencing, we identified the causative genetic mutations in fifteen patients with clinically suspected HE or HPP and correlated the identified mutations with the clinical phenotype and ektacytometry profile. In addition to identifying three novel mutations, gene sequencing confirmed and, when the RBC morphology was not evaluable, identified the diagnosis. Moreover, genotypic differences justified the phenotypic differences within families with HE/HPP.

Slow, reversible, coupled folding and binding of the spectrin tetramerization domain

Many intrinsically disordered proteins (IDPs) are significantly unstructured under physiological conditions. A number of these IDPs have been shown to undergo coupled folding and binding reactions whereby they can gain structure upon association with an appropriate partner protein. In general, these systems display weaker binding affinities than do systems with association between completely structured domains, with micromolar K(d) values appearing typical. One such system is the association between α- and β-spectrin, where two partially structured, incomplete domains associate to form a fully structured, three-helix bundle, the spectrin tetramerization domain. Here, we use this model system to demonstrate a method for fitting association and dissociation kinetic traces where, using typical biophysical concentrations, the association reactions are expected to be highly reversible. We elucidate the unusually slow, two-state kinetics of spectrin assembly in solution. The advantages of studying kinetics in this regime include the potential for gaining equilibrium constants as well as rate constants, and for performing experiments with low protein concentrations. We suggest that this approach would be particularly appropriate for high-throughput mutational analysis of two-state reversible binding processes.

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.

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.

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.

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

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

Mutational effects at the tetramerization site of nonerythroid alpha spectrin

Spectrin, a prominent cytoskeletal protein, exerts its fundamental role in cellular function by forming a sub-membrane filamentous network. An essential aspect of spectrin network formation is the tetramerization of spectrin alphabeta heterodimers. We used laboratory methods, the yeast two-hybrid system and random mutagenesis, to investigate, for the first time, effects of amino acid mutations on tetramerization of nonerythroid (brain) spectrin (fodrin). Based on high sequence homology with erythroid spectrin, we assume the putative tetramerization region of nonerythroid alpha-spectrin at the N-terminal region. We introduced mutations in the region consisting of residues 1-45 and studied mutational effects on spectrin alphabeta association to form tetramers. We detected single, double, and triple mutations involving 24 residues in this region. These amino acid mutations of nonerythroid alpha-spectrin exhibit full, partial, or no effect on the association with nonerythroid beta-spectrin. Single amino acid mutations in the region of residues 1-9 (D2Y, G5V, V6D, and V8M) did not affect the association. However, seven single mutations (I15F, I15N, R18G, V22D, R25P, Y26N, and R28P) affected the alphabeta association. These mutations were clustered in the region predicted by sequence alignment to be crucial in nonerythroid alpha-spectrin for tetramerization, a region that spanned residues 12-36, corresponding to the partial domain Helix C' (residues 21-45) in erythroid alpha-spectrin. In addition, two other mutations, one upstream and one downstream of this region at positions 10 (E10D) and 37 (R37P), also affected the alphabeta association. Our results implied nonerythroid alpha-spectrin partial domain helix may be longer than Helix C' (residues 21-45 and a total of 25 residues) in erythroid alpha-spectrin and spanned at least residues 10-37. It is interesting to note that seven out of these nine single mutations (I15F, I15N, R18G, V22D, R25P, Y26N, R37P) were at the a, d, e or g heptad positions based on sequence alignment with erythroid alpha-spectrin. Four of the mutated residues (I15, R18, V22, R25) are conserved in both erythroid and nonerythroid spectrin. These positions were previously identified as hot spots in erythroid alpha-spectrin that lead to severe hematological symptoms. This study clearly demonstrated that single mutation in a region predicted to be critical functionally in nonerythroid alpha-spectrin indeed leads to functional abnormalities and may lead to neurological disorders.

Hereditary elliptocytosis: spectrin and protein 4.1R

Hereditary elliptocytosis (HE) is a common disorder of erythrocyte shape, occurring especially in individuals of African and Mediterranean ancestry, presumably because elliptocytes confer some resistance to malaria. The principle lesion in HE is mechanical weakness or fragility of the erythrocyte membrane skeleton due to defects in alpha-spectrin, beta-spectrin, or protein 4.1. Numerous mutations have been described in the genes encoding these proteins, including point mutations, gene deletions and insertions, and mRNA processing defects. Several mutations have been identified in a number of individuals on the same genetic background, suggesting a "founder effect." The majority of HE patients are asymptomatic, but some may experience hemolytic anemia, splenomegaly, and intermittent jaundice.

Structural Analysis of the αN-Terminal Region of Erythroid and Nonerythroid Spectrins by Small-Angle X-ray Scattering

We used SpalphaI-1-156 peptide, a well-characterized model peptide of the alphaN-terminal region of erythrocyte spectrin, and SpalphaII-1-149, an alphaII brain spectrin model peptide similar in sequence to SpalphaI-1-156, to study their association affinities with a betaI-spectrin peptide, SpbetaI-1898-2083, by isothermal titration calorimetry. We also determined their conformational flexibilities in solution by small-angle X-ray scattering (SAXS) methods. These two peptides exhibit sequence homology and could be expected to exhibit similar association affinities with beta-spectrin. However, our studies show that the affinity of SpalphaII-1-149 with SpbetaI-1898-2083 is much higher than that of SpalphaI-1-156. Our SAXS findings also indicate a significantly more extended conformation for SpalphaII-1-149 than for SpalphaI-1-156. The radius of gyration values obtained by two different analyses of SAXS data and by molecular modeling all show a value of about 25 A for SpalphaI-1-156 and of about 30 A for SpalphaII-1-149, despite the fact that SpalphaI-1-156 has seven amino acid residues more than SpalphaII-1-149. For SpalphaI-1-156, the SAXS results are consistent with a flexible junction between helix C' and the triple helical bundle that allows multiple orientations between these two structural elements, in good agreement with our published NMR analysis. The SAXS findings for SpalphaII-1-149 support the hypothesis that this junction region is rigid (and probably helical) for alphaII brain spectrin. The nature of the junction region, from one extreme as a random coil (conformationally mobile) segment in alphaI to another extreme as a rigid segment in alphaII, determines the orientation of helix C' relative to the first structural domain. We suggest that this particular junction region in alpha-spectrin plays a major role in modulating its association affinity with beta-spectrins, and thus regulates spectrin tetramer levels. We also note that these are the first conformational studies of brain spectrin.

Spectrin alpha II and beta II isoforms interact with high affinity at the tetramerization site

Spectrin tetramers form by the interaction of two alpha-beta dimers through two helices close to the C-terminus of a beta subunit and a single helix at the N-terminus of an alpha subunit. Early work on spectrin from solid tissues (typified by alphaII and betaII polypeptides) indicated that it forms a more stable tetramer than erythroid spectrin (alphaI-betaI). In the present study, we have probed the molecular basis of this phenomenon. We have quantified the interactions of N-terminal regions of two human alpha polypeptides (alphaI and alphaII) with the C-terminal regions of three beta isoforms (betaISigma1, betaIISigma1 and betaIISigma2). alphaII binds either betaII form with a much higher affinity than alphaI binds betaISigma1 ( K (d) values of 5-9 nM and 840 nM respectively at 25 degrees C). betaIISigma1 and betaIISigma2 are splice variants with different C-terminal extensions outside the tetramerization site: these extensions affect the rate rather than the affinity of alpha subunit interaction. alphaII spectrin interacts with each beta subunit with higher affinity than alphaI, and the betaII polypeptides have higher affinities for both alpha chains than betaISigma1. The first full repeat of the alpha subunit has a major role in determining affinity. Enthalpy changes in the alphaII-betaIISigma2 interaction are large, but the entropy change is comparatively small. The interaction is substantially reduced, but not eliminated, by concentrated salt solutions. The high affinity and slow overall kinetics of association and dissociation of alphaII-betaII spectrin may suit it well to a role in strengthening cell junctions and providing stable anchor points for transmembrane proteins at points specified by cell-adhesion molecules.

Solution structural studies on human erythrocyte alpha-spectrin tetramerization site

We have determined the solution NMR structure of a recombinant peptide that consists of the first 156 residues of erythroid alpha-spectrin. The first 20 residues preceding the first helix (helix C') are in a disordered conformation. The subsequent three helices (helices A1, B1, and C1) form a triple helical bundle structural domain that is similar, but not identical, to previously published structures for spectrin from Drosophila and chicken brain. Paramagnetic spin label-induced NMR resonance broadening shows that helix C', the partial domain involved in alpha- and beta-spectrin association, exhibits little interaction with the structural domain. Surprisingly, helix C' is connected to helix A1 of the structural domain by a segment of 7 residues (the junction region) that exhibits a flexible disordered conformation, in contrast to the predicted rigid helical structure. We suggest that the flexibility of this particular junction region may play an important role in modulating the association affinity of alpha- and beta-spectrin at the tetramerization site of different isoforms, such as erythroid spectrin and brain spectrin. These findings may provide insight for explaining various physiological and pathological conditions that are a consequence of varying alpha- and beta-subunit self-association affinities in their formation of the various spectrin tetramers.

Nuclear magnetic resonance studies of mutations at the tetramerization region of human alpha spectrin

Many spectrin mutations that destabilize tetramer formation and lead to hereditary hemolytic anemias are located at the N-terminal region of alpha-spectrin, with the Arg28 position considered to be a mutation hot spot. We have introduced mutations at positions 28 and 45 into a model peptide, Sp alpha 1-156, consisting of the first 156 residues in the N-terminal region of alpha-spectrin (alpha N). The association of these alpha-spectrin peptides that have single amino acid replacements with a beta-spectrin model peptide, consisting of the C-terminal region of beta-spectrin (beta C), was determined, and structural changes due to amino acid replacements were monitored by nuclear magnetic resonance (NMR). We found evidence for similar and very localized structural changes in Sp alpha 1-156Arg45Thr and Sp alpha 1-156Arg45Ser, although these 2 mutant peptides associated with beta-spectrin peptide with significantly differing affinities. The Sp alpha 1-156Arg28Ser peptide showed an affinity for the beta-spectrin peptide comparable to that of Sp alpha 1-156Arg45Ser, but it exhibited substantial and widespread spectral changes. Our results suggest that both Arg45 replacements induce only minor structural perturbations in the first helix of Sp alpha 1-156, but the Arg28Ser replacement affects both the first helix and the following structural domain. Our results also indicate that the mechanism for reduced spectrin tetramerization is through mutation-induced changes in molecular recognition at the alpha beta-tetramerization site, rather than through conformational disruption, as has been suggested in prior literature.

The interferon-inducible gene, Ifi204, acquires malignant transformation capability upon mutation at the Rb-binding sites

p204 overexpression in retinoblastoma (Rb)-/- mouse embryo fibroblasts or transfection of p204 mutated at both Rb-binding sites confer growth advantages, resulting in a significantly higher number of foci in a cell focus assay. To investigate the possibility that mutated p204 acquires malignant transformation capability, NIH3T3 cells were stably transfected with the expression vector pRcRSV204 double-mutant (p204dm) harboring both the C-terminal deletion up to amino acid 568 and the point mutation from glutamic acid to lysine at position 427, and analyzed for markers typical of cell immortalization and transformation. We detected a greater abundance of cell colonies in soft agar with p204dm-expressing cells than vector control cells. The p204dm-transfected cells also displayed two other characteristics associated with malignant transformation, i.e. growth under low-serum conditions and formation of tumors in athymic nude mice. Moreover, their telomerase activity was significantly higher than in the vector control cells. It would thus seem that p204, devoid of functional Rb-binding motifs, can become oncogenic.

Important region in the beta-spectrin C-terminus for spectrin tetramer formation

Many hereditary hemolytic anemias are due to spectrin mutations at the C-terminal region of beta-spectrin (the betaC region) that destabilize spectrin tetramer formation. However, little is known about the betaC region of spectrin. We have prepared four recombinant beta-peptides of different lengths from human erythrocyte spectrin, all starting at position 1898 of the C-terminal region, but terminating at position 2070, 2071, 2072 or 2073. Native polyacrylamide gel electrophoresis showed that the two peptides terminating at positions 2070 and 2071 did not associate with an N-terminal region alpha-peptide (Spalpha1-156) in the micromolar range. However, the peptides that terminated at positions 2072 and 2073 associated with the alpha-peptide. Circular dichroism results showed that the unassociated helices in both alpha- and beta-peptides became associated, presumably to form a helical bundle, for those beta-peptides that formed an alphabeta complex, but not for those beta-peptides that did not form an alphabeta complex. In addition, upon association, an increase in the alpha-helical content was observed. These results showed that the beta-peptides ending prior to residue 2072 (Thr) would not associate with alpha-peptide, and that no helical bundling of the partial domains was observed. Thus, we suggest that the C-terminal segment of beta-spectrin, starting from residue 2073 (Thr), is not critical to spectrin tetramer formation. However, the C-terminal region ending with residue 2072 is important for its association with alpha-spectrin in forming tetramers.

alpha beta Spectrin coiled coil association at the tetramerization site

On the basis of sequence homology studies, it has been suggested that the association of human erythrocytes alpha and beta spectrin at the tetramerization site involves interactions between helices. However, no empirical details are available, presumably due to the experimental difficulties in studying spectrin molecules because of its size and/or its structural flexibility. It has been speculated that erythrocyte tetramerization involves helical bundling rather than coiled coil association. We have used recombinant spectrin peptides to model alpha and beta spectrin to study their association at the tetramerization site. Two alpha peptides, Sp alpha 1-156 and Sp alpha 1-368, and one beta peptide, Sp beta 1898-2083, were used as model peptides to demonstrate the formation of the alpha beta complex. We also found that the replacement of R28 in Sp alpha 1-368 to give Sp alpha 1-368R28C abolished complex formation with the beta peptide. Circular dichroism techniques were used to monitor the secondary structures of the individual peptides and of the complex, and the results showed that both Sp alpha 1-156 and Sp beta 1898-2083 peptides in solution, separately, included helices that were not paired with other helices in the absence of their binding partners. However, in a mixture of Sp alpha 1-156 and Sp beta 1898-2083 and formation of the alpha beta complex, the unpaired helices associated to form coiled coils. Since the sequences of these two peptides that are involved in the coiled coil association are derived from a native protein, the information obtained from this study also provides insight toward a better understanding of naturally occurring coiled coil subunit-subunit association.

Dynamic molecular modeling of pathogenic mutations in the spectrin self-association domain

Disruption of spectrin self-association underlies many inherited hemolytic disorders. Using dynamic modeling and energy minimization, the 3-dimensional structure of the self-association domain has been estimated in human erythrocyte spectrin and the structural consequences of 17 elliptogenic mutations determined. The predicted structure of the normal self-association domain was remarkably similar to the crystal structure of the Drosophila alpha-spectrin 14th repeat unit, despite replacement in the human sequence of over 70% of the amino acids relative to fly spectrin, including 2 prolines in the human sequence that appear in helical regions of the fly structure. The predicted structure placed all hydrophilic residues at the surface and identified 4 salt bridges, 9 hydrophobic interactions, and 4 H-bonds that stabilize the native self-association unit. Remarkably, every pathologic point mutation, including seemingly conservative substitutions such as G for A, A for V, or K for R (single-letter amino acid codes), led to conformational rearrangements in the predicted structure. The degree of structural disruption, as measured by root-mean-square deviation of the predicted backbone structure from the Drosophila structure, correlated strongly with the severity of clinical disease associated with each mutation. This approach thus enables an accurate prediction, from the primary sequence, of the clinical consequences of specific point mutations in spectrin. The 3-dimensional structure of the self-association domain derived here is likely to be accurate. It provides a powerful heuristic model for understanding how point mutations disrupt cytoskeletal function in a variety of hemolytic disorders.

Laboratory method to study mutational effects on human erythrocyte spectrin tetramerization

We have developed a laboratory method combining a random mutagenesis method and a yeast two-hybrid system to study effects of mutation on human erythrocyte spectrin tetramerization. A PCR-based procedure was used to generate random mutations in DNA fragments of the first 55 residues of alpha-spectrin. Each of the DNA fragments from random mutagenesis was fused with a DNA fragment of native spectrin consisting of residues 56 to 368 to give a DNA fragment of the first 368 residues in alpha-spectrin. The alpha-spectrin DNA fragment and a DNA fragment containing the last 449 residues in beta-spectrin were introduced into the yeast two-hybrid system for rapid screening of alpha- and beta-spectrin interaction. Yeast colonies with interacting alpha- and beta-peptides were blue, and those with non-interacting alpha- and beta-peptides were white. Six single amino acid mutations (R27G, Y35N, F38S, L49H, Y53N, and Y53C) and a double amino acid mutation (K16M, I24N) were identified from 8 white colonies, but no mutations were found in the DNA fragments of 14 blue colonies. Thus this simple laboratory method allows us to study effects of mutation on interactions of alpha- and beta-spectrin at the tetramerization site.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

Spectrin oligomerization is cooperatively coupled to membrane assembly: a linkage targeted by many hereditary hemolytic anemias?

In the erythrocyte, ankyrin is the major adapter protein linking tetramers of band 3 to the spectrin-actin cytoskeleton. This linkage involves a direct interaction between ankyrin and the 14th-15th repeat unit of beta-spectrin. The spectrin cytoskeleton itself is stabilized by the self-association of spectrin heterodimers into tetramers and larger oligomers, a process mediated by the 17th repeat unit of beta-spectrin and a short NH(2) -terminal sequence in alpha-spectrin. The self-association of spectrin and its ankyrin-mediated membrane binding have generally been considered independent events. We now demonstrate that spectrin self-association, the binding of spectrin to ankyrin, and the binding of ankyrin to the 43-kDa cytoplasmic domain of band 3 (cdb3) are coupled in a positively cooperative way. In solution, [(125)I]-labeled ankyrin was found by ND-PAGE3 to enhance the affinity of spectrin self-association by 10-fold. The reciprocal process was also true, in that spectrin tetramers and oligomers bound ankyrin with enhanced affinity relative to dimer spectrin. Saturation of the beta-spectrin self-association site by an NH(2) -terminal 80-kDa alpha-spectrin peptide enhanced the affinity of spectrin dimer for ankyrin, indicating a direct relationship between ankyrin binding and the occupancy of the beta-spectrin self-association site. cdb3 accentuated these cooperative interactions. Several inherited spectrin mutations that cause hemolytic disease but that do not directly destabilize the self-association or ankyrin-binding sites can be explained by these results. Three classes of mutations appear to disrupt cooperative coupling between self-association and ankyrin binding: (i) mutation of the linker sequences that join helices C and A in repeat units that intervene between the two functional sites, mutations that presumably block repeat-to-repeat transfer of conformational information; (ii) mutations in alpha-spectrin repeats 4 to 6 that disrupt the ability of this region to trans-regulate ankyrin binding by the adjacent beta-spectrin repeats 14-15; and (iii) exon-skipping mutations that shorten alpha-spectrin and force repeats 4 to 6 to fall out-of-register with the ankyrin-binding motif in beta-spectrin. Collectively, these results demonstrate a molecular mechanism whereby a membrane receptor can directly promote cytoskeletal assembly.

NMR analysis of secondary structure and dynamics of a recombinant peptide from the N-terminal region of human erythroid alpha-spectrin

We have studied the nuclear magnetic resonance solution secondary structure of the N-terminal region in human erythroid alpha-spectrin using a recombinant model peptide of alpha-spectrin consisting of residues 1-156. Pulsed field gradient diffusion coefficient measurements show that the model peptide exists as a monomer under the solution conditions used. The first 20 residues are in a random coil conformation, followed by a helix of 25 residues and then a random coil segment before the next helix. The random coil nature of this linker was confirmed by the presence of fast internal motion from (15)N relaxation measurements. The second, third and fourth helices are thought to form the triple helical bundle structural domain, consistent with previous studies. Our study shows that the N-terminal region of alpha-spectrin prior to the first structural domain forms a well behaved helix without its beta-spectrin partner.

Flexibility of the alpha-spectrin N-terminus by EPR and fluorescence polarization

The structure and flexibility of the biologically important alpha-spectrin amino terminal region was examined by the use of fluorescence and EPR spectroscopy. The region studied has been previously demonstrated to be essential for the alpha-spectrin:beta-spectrin association of the tetramerization site. Appropriate spectroscopic probe moieties were coupled to this region in a recombinant fragment of human erythroid alpha-spectrin. There was good agreement between the EPR and fluorescence techniques in most of this region. Mobility determinations indicated that a portion of the region was relatively immobilized. This is significant, since although predictive methods have indicated that this region should be alpha-helical, previous experimental evidence obtained on smaller synthetic peptides had indicated that this region was disordered. Observed rigidity appears to be incompatible with such a disordered state, and has important ramifications for the flexibility of this molecule that is so integral to its role in stabilizing erythrocyte membranes.

Spin label EPR structural studies of the N-terminus of alpha-spectrin

Spectrin, a vital component in human erythrocyte, is composed of alpha- and beta-subunits, which associate to form (alphabeta)2 tetramers. The tetramerization site is believed to involve the alpha-spectrin N-terminus and the beta-spectrin C-terminus. Abnormal interactions in this region may lead to blood disorders. It has been proposed that both termini consist of partial structural domains and that tetramerization involves the association of these partial domains. We have studied the N-terminal region of a model peptide for alpha-spectrin by making a series of double spin-labeled peptides and studying their dipolar interaction by electron paramagnetic resonance methods. Our results indicate that residues 21-42 of the N-terminus region exhibit an alpha-helical conformation, even in the absence of B-spectrin.