Structural studies on human spectrin. Comparison of subunits and fragmentation of native spectrin

Membrane skeleton of innervated and denervated fast- and slow-twitch muscle

We used confocal microscopy and immunoblotting to study membrane skeletal proteins of fast-twitch (extensor digitorum longus) and slow-twitch (soleus) muscles of the adult rat. In the extensor digitorum longus (EDL), beta-spectrin concentrates in costameres, whereas dystrophin is enriched at costameres but is also present in intercostameric regions. In the soleus, beta-spectrin and dystrophin underlie much of the sarcolemma, and intercostameric regions are difficult to detect. The EDL sarcolemma reorganizes following denervation to resemble soleus sarcolemma, but denervation does not significantly affect the latter. Consistent with these observations, soleus contains similar amounts of dystrophin but more beta-spectrin than EDL. Denervation increases beta-spectrin levels only in the EDL and dystrophin levels in both muscles. Denervation does not affect beta-fodrin, a beta-spectrin homolog expressed in embryonic myofibers. Thus, neuromuscular activity controls sarcolemmal organization and the levels of beta-spectrin and dystrophin, but not postnatal downregulation of beta-fodrin. The differences in organization of the sarcolemma may underlie the differential susceptibility of fast and slow myofibers to dystrophinopathies.

Enhanced Mg(2+)-ATPase activity in ghosts from HS erythrocytes and in normal ghosts stripped of membrane skeletal proteins may reflect enhanced aminophospholipid translocase activity

Hereditary spherocytosis (HS) is a congenital haemolytic anaemia which is characterized by a great variety of structural defects in the red cell's membrane skeleton and/or deficiencies in particular membrane (skeletal) proteins. Enhanced (Mg2+)-dependent adenosine triphosphatase (Mg(2+)-ATPase) activities, varying from 115% to 160%, were invariably found in erythrocyte ghosts derived from 13 HS patients. Similarly, an enhancement of Mg(2+)-ATPase activity by 30% is observed in normal red cell ghosts that have been stripped of the greater part of their membrane skeletal proteins by treatment with a low ionic strength buffer. Reassociation of those stripped ghosts with spectrin reduces the enhanced Mg(2+)-ATPase activity to its original level. Since in both cases, HS ghosts and stripped normal ghosts, the stabilizing effects that the membrane skeleton exerts on the maintenance of an endofacial localization of the aminophospholipids are impaired, the enhanced Mg(2+)-ATPase activity is interpreted to reflect an increased activity of the aminophospholipid translocase. The present observations therefore support a role of the membrane skeleton in the stabilization of phospholipid asymmetry in the red cell membrane and consequently in reducing the energy consumption of the translocase.

Modulation of erythrocyte membrane mechanical function by beta-spectrin phosphorylation and dephosphorylation

The mechanical properties of human erythrocyte membrane are largely regulated by submembranous protein skeleton whose principal components are alpha- and beta-spectrin, actin, protein 4.1, adducin, and dematin. All of these proteins, except for actin, are phosphorylated by various kinases present in the erythrocyte. In vitro studies with purified skeletal proteins and various kinases has shown that while phosphorylation of these proteins can modify some of the binary and ternary protein interactions, it has no effect on certain other interactions between these proteins. Most importantly, at present there is no direct evidence that phosphorylation of skeletal protein(s) alters the function of the intact membrane. To explore this critical issue, we have developed experimental strategies to determine the functional consequences of phosphorylation of beta-spectrin on mechanical properties of intact erythrocyte membrane. We have been able to document that membrane mechanical stability is exquisitely regulated by phosphorylation of beta-spectrin by membrane-bound casein kinase I. Increased phosphorylation of beta-spectrin decreases membrane mechanical stability while decreased phosphorylation increases membrane mechanical stability. Our data for the first time demonstrate that phosphorylation of a skeletal protein in situ can modulate physiological function of native erythrocyte membrane.

A proton nuclear magnetic resonance study of the mobile regions of human erythroid spectrin

The effect of added NaCl (0-150 mM) and temperature (6-65 degrees C) on the conformation of erythrocyte spectrin was investigated using 400 MHz 1H NMR. The relatively narrow resonances (20-40 Hz linewidth) in the spectra arising from protons in regions of the molecule undergoing rapid motions were selectively detected using either the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence without water presaturation or a simple pi/2 pulse sequence with water presaturation. The T2 relaxation of these protons was not influenced by changes in solution conditions (0-150 mM NaCl, 6-37 degrees C) indicating that their motions were independent of the overall shape of the molecule. Significant increases in the areas of the aliphatic peaks for spectrin samples at fixed salt concentrations occurred as the temperature was raised from 6 to 37 degrees C. The increases were independent of the state of polymerization of spectrin and were greater in the absence of added salt above 25 degrees C. The changes reflect increasing numbers of mobile residues, probably due to partial unfolding of spectrin's repeated structural unit. At temperatures above 37 degrees C, sharp increases in the areas of the spectral envelopes reflect cooperative unfolding of spectrin. Comparison with results previously obtained in this laboratory using CD and ORD indicate that at least part of the lost structure is alpha-helical.

Involvement of cytoskeletal proteins in the barrier function of the human erythrocyte membrane. II. Formation of membrane leaks in ghost membranes after limited proteolysis of skeletal proteins by trypsin

Limited proteolysis of human erythrocyte ghost membranes by low levels of trypsin (10-240 ng/ml) added bilaterally at 0 degrees C together with the proteinase inhibitor, phenylmethylsulfonyl fluoride (PMSF) before resealing at 37 degrees C leads to a graded digestion of spectrin and ankyrin and the disappearance of band 4.1 protein, while band 3 is cleaved only to a very low extent. These alterations are accompanied by an increase of membrane permeability of the resealed ghosts to hydrophilic nonelectrolytes (erythritol to sucrose), taken to reflect impaired resealing. Moreover, the membrane begins to vesiculate. Shedding of vesicles during the efflux measurements can not be responsible for the increased release of test solutes, since the ghosts do not loose hemoglobin and discriminate the nonelectrolytes according to their size. Moreover, the vesiculation site itself does not seem to act as the leak site, since ghosts prepared from erythrocytes pretreated with a carbodiimide which induces membrane rigidification still exhibit a pronounced protein degradation and vesiculation while the permeability enhancement induced by trypsination is markedly suppressed. The trypsin-induced leak has the properties of an aqueous pore as indicated, besides size selectivity, by its inhibition by phloretin and the very low activation energy. In analogy with concepts developed in the preceding paper (Klonk, S. and Deuticke, B. (1992) Biochim. Biophys. Acta 1106, 126-136 (Part I in this series)) the impaired resealing after limited proteolysis is assumed to be related to a perturbation of interactions of membrane skeletal elements with themselves and/or with the bilayer domain constituting the permeability barrier.

Structural analysis of homologous repeated domains in alpha-actinin and spectrin

The amino acid sequences of chick and slime mould alpha-actinin each contain four repeats of approximately 122 residues. These repeats are homologous to the 18-22 repeats, each of approximately 106 residues, found in the alpha and beta subunits of spectrin and fodrin, and to the multiple repeats of approximately 110 residues found in the Duchenne muscular dystrophy protein (dystrophin). The repeats correspond to the elongated rod-like portion of these molecules. We present a multiple sequence alignment of 21 repeats from this superfamily (8 alpha-actinin and 13 spectrin/fodrin), based on optimal pairwise alignments, from which a characteristic consensus pattern of amino acid types is deduced. Trp 46 is invariant in all but one repeat, and physicochemical classes of amino acids are conserved at 25 other positions. Secondary structure prediction on both the alpha-actinin and spectrin repeats taken together with the distribution of proline residues in the sequences, strongly suggest that each repeated domain consists of a four-helix structure. Our predictions differ significantly from previous three-helix models based on analyses of fewer sequences. To determine possible interdomain regions, sites of limited proteolysis of the native chick alpha-actinin dimer were determined and located in the amino acid sequence. The majority of these sites were in corresponding positions in different repeats within a segment predicted as a long helix. We propose a model, consistent with the overall dimensions of the rod-like portions of the molecules, in which these long, probably interrupted helices, link adjacent domains.

Quantitative detection of rapid motions in spectrin by NMR

Previous high resolution proton NMR data on human erythrocyte spectrin molecules has indicated the existence of regions exhibiting rapid internal motions within the intact molecules [L. W.-M. Fung, H.-Z. Lu, R. P. Hjelm, jr, M. E. Johnson, FEBS Lett., 197, 234 (1986)]. We have extended the studies by developing quantitative NMR methods to determine the fraction of spectrin protons exhibiting rapid internal motions, in both the isolated molecule and within the spectrin-actin network. Using both one-pulse and spin echo pulse sequences, we find that the fraction of the protons in rapid motion is about 15% of the total protons in the spectrin molecule at 37 degrees C in phosphate buffer with 150 mM NaCl at pH 7.4. Quantitative information on these rapid motions will be important in understanding the structural, mechanical and functional properties of spectrin molecules, as well as in understanding filamentous protein structures in general.

Selective detection of rapid motions in spectrin by NMR

Human erythrocyte spectrin molecules exhibit relatively sharp (30-50 Hz) proton NMR signals in the aliphatic region. A standard solvent presaturation pulse sequence that also partially suppresses the broad envelope from protons with rigid structures in spectrin and selectively enhances the sharp resonances has been used to characterize the behavior of these resonances. The overall resonance pattern strongly resembles that of the denatured spectrin. The observed spectra are also quite similar to the line-broadened spectrum from a mixture of amino acids that corresponds to the composition of the spectrin molecule. These data indicate the existence of regions exhibiting rapid internal motions within the intact spectrin molecule, and suggest that the amino acid composition of the residues giving rise to the sharp resonances is quite similar to that of the full spectrin molecule.

The present status of erythrocyte spectrin structure: the 106-residue repetitive structure is a basic feature of an entire class of proteins

Spectrin, the major component of the erythroid membrane skeleton, is a long, asymmetrical rodlike protein that interacts with several other proteins to form a two-dimensional membrane skeleton. Progress in several laboratories over the past few years including substantial partial peptide and nucleotide sequence determination has greatly enhanced our knowledge of the structural properties of this large molecule (heterodimer = 465,000 daltons). The alpha and beta subunits are homologous with approximately 30% identity. They are aligned in an antiparallel side-to-side orientation with the amino- and carboxy-termini near opposite physical ends of the molecule. The predominant structural feature elucidated from sequencing this large molecule is the nearly universal occurrence in both subunits of a single type of repetitive structure. The periodicity of this homologous structure is exactly 106 amino acid residues. As many as 36 homologous, but nonidentical, repeats exist and comprise more than 90% of the mass of the heterodimer. Each of these repetitive units is folded into a triple-stranded structure that is highly helical. Peptide maps, antibody crossreactivity, peptide sequence analysis, and more recently nucleic acid sequences have defined several major properties of the erythroid molecule and related proteins in other tissues. Tissue-specific spectrins have the same 106-residue repetitive structure and show sequence homology to erythroid spectrin.

Degradation of unassembled alpha- and beta-spectrin by distinct intracellular pathways: regulation of spectrin topogenesis by beta-spectrin degradation

Analysis of the turnover of unassembled proteins during the assembly of the erythroid membrane skeleton has revealed that alpha- and beta-spectrin, two structurally related, high molecular weight proteins, are degraded in a selective manner by two distinct intracellular pathways. Unassembled alpha-spectrin (t1/2 approximately equal to 2 hr) is degraded by a system with all the pharmacological characteristics of a membrane-bound, lysosomal-type pathway. This result illustrates for the first time the selective degradation of an intracellular short-lived, unassembled protein by a lysosomal pathway. In contrast, unassembled beta-spectrin is degraded extremely rapidly (t1/2 approximately equal to 15-20 min at 38 degrees C) by a soluble cytoplasmic system in an apparently ATP-independent manner. These observations suggest that the selective and rapid degradation of beta-spectrin serves an important regulatory role in the topogenesis of the spectrin-based membrane skeleton in the chicken erythrocyte.

The relationship to knobs of the 92,000 D protein specific for knobby strains of Plasmodium falciparum

A 92,000 D protein was identified associated with the membrane of host erythrocytes infected with the FCB1 Plasmodium falciparum strain from Colombia. The same protein was identified in the knob-forming Gambian (and the Malayan Camp) strain, but was not present in all the corresponding knobless strains. In the FCB1 strain as well as in the FCR3 strain the protein is synthesized during the ring-stage period. The cleavage products of the 92,000 D protein were investigated by peptide mapping following limited proteolytic digestion with Staphylococcus aureus V8 protease. The 92,000 D protein cleavage products from both the Colombian and the Gambian strains were identical. Moreover, both the proteins were sensitive to trypsin and chymotrypsin and also to treatment with neuraminidase. Enzymatic removal of the protein from the erythrocyte membrane by trypsin or chymotrypsin did not affect parasite maturation. The merozoites thus produced were fully invasive and the morphology of the knobs was unaltered. When the erythrocyte membrane was treated with trypsin before the time of synthesis of the 92,000 D protein, it was not possible to identify the protein in membranes of later stages of infected erythrocytes, indicating that the protein cannot be inserted into the membrane cytoskeleton compartment. Knobs, however, were formed more or less normally, suggesting that it is not the accumulation of this protein which products the knobs.

Specific molybdenum binding to spectrin subunits

Molybdenum in the form of its pentavalent complex binds primarily to spectrin when incubated with erythrocytes. Only the band 1 subunit is involved in this interaction thus indicating some structural differences between spectrin subunits.

Synthesis and assembly of spectrin during avian erythropoiesis: stoichiometric assembly but unequal synthesis of alpha and beta spectrin

The synthesis and assembly of spectrin was investigated in erythroid cells during chicken embryo development. Immunoprecipitation of Triton X-100-soluble and -insoluble cytoskeletal fractions with alpha- and beta-spectrin antisera show that, at steady state, alpha and beta spectrin are present in stoichiometric amounts, and exclusively, in the cytoskeleton. However, pulse labeling of cells and in vitro translation of total erythroid cell RNA reveal that alpha spectrin is synthesized in a two to three fold excess over beta spectrin. Pulse-chase experiments show that newly synthesized alpha and beta spectrin are present in both the cytoskeletal and soluble fractions, and that stoichiometric amounts are stably assembled in the cytoskeleton. On the other hand, there is a severalfold excess of alpha relative to beta spectrin in the soluble fraction, both of which turn over with a half-life of 50 min. In cells from 4 day old embryos, more than 80% of the newly synthesized beta spectrin, but only 10% of the alpha spectrin, are present in the cytoskeleton. Thus, early in development, the association of alpha and beta spectrin with the membrane-cytoskeleton may be rate-limited by the amount of beta spectrin synthesized. Later on in erythroid development, progressively lesser proportions of newly synthesized beta spectrin are present in the cytoskeleton, suggesting that during development, the rate of association of beta spectrin with the membrane-cytoskeleton becomes limited by some other membrane-cytoskeletal component.

Spectrin, fodrin, and TW260/240: a family of related proteins lining the plasma membrane

Recently, molecules highly related to erythrocyte spectrin have been identified in nonerythroid cells. Here we summarize our current understanding of these molecules and suggest a model for their organization. Significant differences exist between this family of proteins isolated from mammalian cells and avian cells, and this may explain the variability in antibody preparations as well as differences in peptide maps of these subunits which have been reported. We have prepared antibodies specific for the variant subunits of the spectrinlike proteins fodrin, spectrin, and TW260/240 and analyzed the distribution of these variant subunits in different chicken cell types as well as their developmental distribution in the intestine. The results suggest that fodrin is the general member of this family of proteins and can even coexist with other spectrinlike proteins in the same cells.

Spectrin domains: proteolytic susceptibility as a probe of protein structure

Mild treatment of human erythrocyte spectrin with trypsin produces discrete intermediate-sized peptides. The effects of buffer composition, enzyme-substrate ratio, temperature, and other experimental parameters on the resulting peptide pattern have been examined. Spectrin is capable of regaining its proteolytic resistance after NaDodSO4-induced denaturation, permitting the use of isolated subunits to study spectrin structure and function. Tryptic digestion of isolated subunits also has greatly facilitated the identification of the subunit origin of the intermediate-sized peptides. Isolated subunits could also be recombined to form functional units similar but not identical to the native dimeric form of the regions or domains connected by small protease sensitive segments. The structural integrity and accessibility of these sites is minimally affected by oligomeric state or proteolytic digestion conditions. The similarities of sizes, isoelectric points, and amino acid compositions of many intermediate-size peptides from areas of both subunits suggest that at least part of spectrin's structure may have evolved via replication of a single gene. A possible structural repeat of approximately 50,000 daltons is hypothesized.

The molecular basis of the defect in phosphorylation of spectrin in human hereditary spherocytosis

The molecular basis for the depressed phosphorylation of the smaller polypeptide of spectrin (band 2) in the erythrocytes of patients suffering from hereditary spherocytosis is investigated. Comparison of healthy and spherocytic spectrin polypeptides by controlled proteolysis reveals no abnormality in the degradation pattern or in the sites of phosphorylation. It is concluded that the lesion is a consequence of a defective control of phosphorylation. The defect can be mimicked in healthy cells by the introduction of calcium into the erythrocyte and the possibility that the primary pathological lesion is a deficient control of the calcium content of the erythrocyte is discussed.

Purification of a trypsin-insensitive fragment of spectrin from human erythrocyte membranes

When spectrin is treated with trypsin, a series of polypeptide fragments is generated, One particular fragment having an approximate molecular weight of 80 000 constitutes 18% of the trypsin-digested mixture and is trypsin-insensitive. This fragment has been isolated and purified by gel filtration followed by ion-exchange chromatography. The molecular weight of the fragment, as seen from sedimentation equilibrium measurements and from gel electrophoresis, both in the presence and absence of detergent, is close to 80 000. There was no evidence of self-association under the conditions used. Changes in the specific rotation at 365 nm were used to detect temperature-dependent conformation changes in the fragment and to compare these changes with those in the intact spectrin molecule. The fragment undergoes temperature-dependent transitions centered at 46 and 58 degrees C, similar to those in intact spectrin (49 and 55 degrees C). Although the thermal transitions exhibited by intact spectrin are markedly salt-dependent, those shown by the fragment are not. ORD (optical rotary dispersion) measurements indicate 53% apparent alpha-helix in the fragment, compared to 68% in intact spectrin. Antibodies raised against the fragment cross-react only with band 1, the largest polypeptide of spectrin, indicating that the fragment is derived from band 1.

Topology of membrane sulfhydryl groups in the human erythrocyte. Demonstration of a non-reactive population in intrinsic proteins

A major fraction of the protein sulfhydryl groups of human erythrocyte membranes can be oxidized to disulfide bonds by the lipid soluble reagent, diamide, and the hydrophilic reagent, tetrathionate. Furthermore, the same fraction also reacts with the monofunctional reagent, N-ethylmaleimide. About 20% of the SH groups, however, do not react with any of these agents even upon prolonged treatment and increased concentrations. These 'non-reacting' SH groups were now localized by a procedure involving blockage of the accessible SH groups by non-labeled N-ethylmaleimide or by diamide, subsequent isolation and solubilization of the membranes in SDS and labelling of the now accessible, residual SH groups with N-[ethyl-2-3H]ethylmaleimide. The distribution of the radioactivity over the peptide fractions shows that the non-reacting SH groups are mainly localized in the intrinsic proteins, while essentially all of the SH groups of the extrinsic protein, spectrin, are reactive. After solubilization of the membranes with Triton X-100 the non-reacting SH groups became reactive towards N-ethylmaleimide. It is proposed that lack of reaction of SH groups in the native membranes is due to their localization within the hydrophobic core of the membrane.

Photosensitized cross-linking of erythrocyte membrane proteins. Evidence against participation of amino groups in the reaction

Exposure of human erythrocyte ghosts (pH 8, 10 degrees C) to visible light in the presence of the photosensitizer, methylene blue, results in a relatively rapid loss of spectrin (bands 1 and 2 on sodium dodecyl sulfate gel electropherograms) and the appearance of high molecular weight cross-linked derivatives. Isolated spectrin also undergoes photosensitized cross-linking, indicating that the reaction is not lipid-dependent. Extensive cross-linking was neither reversed by dithiothreitol nor prevented by prior blocking of SH groups with N-ethylmaleimide, suggesting that cysteine residues are not crucial bridging sites. The possible requirement for NH2 groups, as suggested by previous model studies (Dubbelman, T.M.A.R., de Goeij, A.F.P.M. and van Steveninck, J. (1978) Biochim. Biophys. Acta 511, 141--151), was tested. Succinylation of spectrin protected against cross-linking, but this effect is attributed to the disruption of quaternary structure, as deduced from sedimentation measurements. However, virtually complete blocking of NH2 groups by amidination perturbed overall structure relatively little, and had no effect on cross-linking. Moreover, exogenous amines such as ethylamine, added in large excess to spectrin prior to irradiation, did not interfere with cross-link formation. These results suggest that NH2 groups are not involved in the reaction.

Spin labeling of human spectrin. Effects of temperature, divalent cations and reassociation with erythrocyte membrane

Spectrin extracted from human red blood cells has been spin labeled in its dimeric and tetrameric forms with five different nitroxide derivatives of increasing chain length between their maleimide binding group and their nitroxide reporter group. Three molecules of spin label are bound per spectrin dimer. Electron spin resonance spectra show the simultaneous presence of strongly and weakly immobilized spin labels. Their relative proportion depends on the label length and is suddenly modified when it reaches 12 A This indicates the presence of cavities of approximately this size in the tertiary structure of spectrin in solution at 0 degrees C. The conformation of spectrin varies greatly with temperature. Reversible changes occur between 0 and 35 degrees C. At higher temperatures, partial denaturation is observed. Divalent cations (Mg2+ and Ca2+) stabilize spectrin in a more constrained conformation and protect it against thermal denaturation. The same behavior is observed when spin-labeled spectrin is reassociated with spectrin-depleted inside-out erythrocyte vesicles. When fatty acid spin labels are incorporated in the phospholipidic structure of these vesicles, the reassociation of spectrin does not change their electron spin resonance spectra. This result confirms the fact that spectrin interacts predominantly with proteins on erythrocyte membranes.

Properties and structural role of the subunits of human spectrin

The subunits of spectrin from human erythrocytes were separated by ion-exchange chromatography on hydroxyapatite in the presence of urea. When renatured from the urea solution they are found to be monomeric, although the smaller subunit (band 2) is prone to aggregation. In shape, solubility and secondary structure the subunits resemble the native spectrin dimer, indicating that subunit interaction is not essential for maintaining the native conformation. When the subunits are recombined, a dimer with the sedimentation coefficient of the native species is formed. This constitutes direct evidence that native spectrin is a heterodimer, rather than a mixture containing homologous and heterologous species. The interaction of the separated subunits with the chymotryptic fragment of the spectrin-binding protein (protein 2.1, or ankyrin) of the erythrocyte membrane was studied. Only the smaller subunit has the ability to bind, and thus presumably contains the site by which the cytoskeleton is attached to the plasma membrane. On the other hand, the formation of a complex with F-actin and protein 4.1 requires the presence of both subunits. A complex of these proteins with band 2 is formed, however, when traces of an additional, as yet unidentified, protein are present.