Spectrin tetramer-dimer equilibrium and the stability of erythrocyte membrane skeletons

Characteristics of membrane protein phosphorylation in Plasmodium berghei-infected mouse erythrocytes

Membrane protein phosphorylation in Plasmodium berghei-infected erythrocytes was studied by incubating intact cells with (32P)orthophosphate and incubating isolated membrane with (gamma-32P)ATP. Phosphorylated proteins were detected by autoradiography after sodium dodecylsulfate (SDS)-polyacrylamide gel electrophoresis or isoelectric focusing followed by gel electrophoresis. New phosphorylated proteins were found in membrane from infected erythrocytes, including a protein with electrophoretic mobility identical to band 5, with Mr 43,000. The molar ratio of phosphate to protein ranged between 0.1 and 0.5. Isoelectric focusing-SDS polyacrylamide gel electrophoresis, peptide mapping, extractability properties, and reduction of susceptibility to DNase I inhibition suggested that this protein is phosphorylated actin. In contrast, spectrin phosphorylation in infected erythrocytes was mostly unchanged.

Restriction of the lateral motion of band 3 in the erythrocyte membrane by the cytoskeletal network: dependence on spectrin association state

The effects of incubation of erythrocyte ghosts under various conditions (ionic strength or addition of ankyrin, diamines, or ATP) on the lateral motion of band 3 in the membranes were studied by using the fluorescence photobleaching recovery technique. Incubation of ghosts with exogenous ankyrin increased the immobile fraction of band 3, from 0.6 in intact ghosts to 0.8-0.9 when an average of 0.2 mol of extra ankyrin was bound per mole of band 3. Ankyrin-free band 3 proteins were mobile, but their mobility was governed by the spectrin association state in the cytoskeletal network. The diffusion constant was 5.3 X 10(-11) cm2 s-1 at a spectrin tetramer mole fraction of 0.3-0.4 in 10 mM NaCl/5 mM sodium phosphate, pH 7.8, and decreased 1 order of magnitude when the tetramer fraction increased to 0.5 in higher NaCl concentration (150 mM NaCl). A similar decrease was observed when the spectrin tetramer fraction was increased by 0.2 mM spermine in 10 mM NaCl/10 mM tris(hydroxymethyl)aminomethane hydrochloride, pH 7.6. On the other hand, the rotational motion of band 3 in the membranes was not affected by the spectrin association state. Trypsin treatment of ghosts cleaved off the cytoplasmic domain of band 3 and caused a marked (8-fold) increase in the lateral mobility, D = 4.0 X 10(-10) cm2 s-1. These results indicate that the lateral mobility of ankyrin-free band 3 protein is restricted by interactions of their cytoplasmic domain with the cytoskeletal network. A model is presented that band 3 can pass the network when spectrins are in dissociated dimers and cannot pass when they are tetramers. The lateral diffusion constant is thus determined by the spectrin dimer population in the network.

Erythrocyte membrane deformability and stability: two distinct membrane properties that are independently regulated by skeletal protein associations

Skeletal proteins play an important role in determining erythrocyte membrane biophysical properties. To study whether membrane deformability and stability are regulated by the same or different skeletal protein interactions, we measured these two properties, by means of ektacytometry, in biochemically perturbed normal membranes and in membranes from individuals with known erythrocyte abnormalities. Treatment with 2,3-diphosphoglycerate resulted in membranes with decreased deformability and decreased stability, whereas treatment with diamide produced decreased deformability but increased stability. N-ethylmaleimide induced time-dependent changes in membrane stability. Over the first minute, the stability increased; but with continued incubation, the membranes became less stable than control. Meanwhile, the deformability of these membranes decreased with no time dependence. Biophysical measurements were also carried out on pathologic erythrocytes. Membranes from an individual with hereditary spherocytosis and a defined abnormality in spectrin-protein 4.1 association showed decreased stability but normal deformability. In a family with hereditary elliptocytosis and an abnormality in spectrin self-association, the membranes had decreased deformability and stability. Finally, membranes from several individuals with Malaysian ovalocytosis had decreased deformability but increased stability. Our data from both pathologic membranes and biochemically perturbed membranes show that deformability and stability change with no fixed relationship to one another. These findings imply that different skeletal protein interactions regulate membrane deformability and stability. In light of these data, we propose a model of the role of skeletal protein interactions in deformability and stability.

Phosphorylation reduces the affinity of protein 4.1 for spectrin

The phosphorylation of protein 4.1 by the membrane kinase and casein kinase A has been investigated. Each of these kinases catalyzed the incorporation of 2 mol of phosphate per mole of protein 4.1. The presence of both kinases in the reaction mixture did not lead to an increase in the incorporation of phosphates into the protein. An analysis of the acid hydrolysis products of the 32P-labeled protein 4.1 indicated that the radioactivities were distributed between phosphothreonine and phosphoserine in a ratio of about 2 to 1. The effects of phosphorylation on the binding of protein 4.1 to spectrin were investigated by using sucrose density gradient centrifugation. The affinity of protein 4.1 for spectrin was reduced about 5-fold, from a KD of 2 X 10(-6) M to a KD of 9.4 X 10(-6) M, by phosphorylation. The phosphorylation of spectrin, on the other hand, appeared to increase slightly its affinity for protein 4.1. The results suggest that phosphorylation may lead to a relaxation of the cytoskeletal network and the formation of a more flexible membrane structure that is important to red cell function.

The human erythrocyte membrane skeleton may be an ionic gel. I. Membrane mechanochemical properties

Biochemical and biophysical observations indicate that the erythrocyte membrane skeleton is composed of a swollen network of long, flexible and ionizable macromolecules located at the cytoplasmic surface of the fluid membrane lipid bilayer. We have analyzed the mechanochemical properties of the erythrocyte membrane assuming that the membrane skeleton constitutes an ionic gel (swollen ionic elastomer). Using recently established statistical thermodynamic theory for such gels, our analysis yields mathematical expressions for the mechanochemical properties of erythrocyte membranes that incorporate membrane molecular parameters to an extent not achieved previously. The erythrocyte membrane elastic shear modulus and maximum elastic extension ratio predicted by our membrane model are in quantitative agreement with reported values for these parameters. The gel theory predicts further that the membrane skeleton modulus of area compression, KG, may be small as well as large relative to the membrane elastic shear modulus, G, depending on the environmental conditions. Our analysis shows that the ratio between these two parameters affects both the geometry and the stability of the favoured cell shapes.

Ultrastructure of the intact skeleton of the human erythrocyte membrane

Filamentous skeletons were liberated from isolated human erythrocyte membranes in Triton X-100, spread on fenestrated carbon films, negatively stained, and viewed intact and unfixed in the transmission electron microscope. Two forms of the skeleton were examined: (a) basic skeletons, stripped of accessory proteins with 1.5 M NaCl so that they contain predominantly polypeptide bands 1, 2, 4.1, and 5; and (b) unstripped skeletons, which also bore accessory proteins such as ankyrin and band 3 and small plaques of residual lipid. Freshly prepared skeletons were highly condensed. Incubation at low ionic strength and in the presence of dithiothreitol for an hour or more caused an expansion of the skeletons, which greatly increased the visibility of their elements. The expansion may reflect the opening of spectrin from a compact to an elongated disposition. Expanded skeletons appeared to be organized as networks of short actin filaments joined by multiple (5-8) spectrin tetramers. In unstripped preparations, globular masses were observed near the centers of the spectrin filaments, probably corresponding to complexes of ankyrin with band 3 oligomers. Some of these globules linked pairs of spectrin filaments. Skeletons prepared with a minimum of perturbation had thickened actin protofilaments, presumably reflecting the presence of accessory proteins. The length of these actin filaments was highly uniform, averaging 33 +/- 5 nm. This is the length of nonmuscle tropomyosin. Since there is almost enough tropomyosin present to saturate the F-actin, our data support the hypothesis that tropomyosin may determine the length of actin protofilaments in the red cell membrane.

Influence of preparative procedures on the membrane viscoelasticity of human red cell ghosts

The effects of systematic variations in the preparative procedures on the membrane viscoelastic properties of resealed human red blood cell ghosts have been investigated. Ghosts, prepared by hypotonic lysis at 0 degrees C and resealing at 37 degrees C, were subjected to: measurement of the time constant for extensional recovery (tc); measurement of the membrane shear elastic modulus (mu) via three separate techniques; determination of the membrane viscosity (eta m) via a cone-plate Rheoscope. Membrane viscosity was also determined as eta m = mu X tc. Compared to intact cells, ghosts had shorter tc, regardless of their residual hemoglobin concentration (up to 21.6 g/dl). However, prolonged exposure to hypotonic media did increase their recovery time toward the intact cell value. The shear elastic modulus, as judged by micropipette aspiration of membrane tongues (mu p), was similar for all ghosts and intact cells. This result, taken with the tc data, indicates that ghosts have reduced membrane viscosity. Rheoscopic analysis also showed that eta m was reduced for ghosts, with the degree of reduction (approx. 50%) agreeing well with that estimated by the product mu p X tc. However, flow channel and pipette elongation estimates indicated that the ghost membrane elastic modulus was somewhat elevated compared to intact cells. We conclude that: ghosts have reduced membrane viscosity; ghosts have membrane rigidities close to intact cells, except possibly when the membrane is subjected to very large strains; the reduction in eta m is not directly related to the loss of hemoglobin; prolonged exposure of ghosts to low-ionic strength media increases the membrane viscosity toward its initial cellular level. These data indicate that the mechanical characteristics of ghost membranes can be varied by changing the methods of preparation and thus have potential application to further studies of the structural determinants of red cell membrane viscoelasticity.

Spectrin, human erythrocyte shapes, and mechanochemical properties

Physical studies of human erythrocyte spectrin indicate that isolated spectrin dimers and tetramers in solution are worm-like coils with a persistence length of approximately 20 nm. This finding, the known polyelectrolytic nature of spectrin, and other structural information about spectrin and the membrane skeleton molecular organization have lead us to the hypothesis that the human erythrocyte membrane skeleton constitutes a two-dimensional ionic gel (swollen ionic elastomer). This concept is incorporated in what we refer to as the protein gel-lipid bilayer membrane model. The model accounts quantitatively for red elastic shear modulus and the maximum elastic extension ratio reported for the human erythrocytes membrane. Gel theory further predicts that depending on the environmental conditions, the membrane skeleton modulus of area compression may be small or large relative to the membrane elastic shear modulus. Our analyses show that the ratio between these two parameters affects both the geometry and the stability of the favored cell shapes and that the higher the membrane skeleton compressibility the smaller the values of the gel tension needed to induce cell shape transformations. The main virtue of the protein gel-lipid bilayer membrane model is that it offers a novel theoretical and molecular basis for the various mechanical properties of the membrane skeleton such as the membrane skeleton modulus of area compression and osmotic tension, and the effects of these properties on local membrane skeleton density, cell shape, and shape transformations.

Clinical disorders of the red cell membrane skeleton

The lipid bilayer of the adult red cell is supported on its inner surface by a complex arrangement of proteins known as the membrane skeleton. This filamentous network, a major component of which is a multifunctional protein called spectrin, has an essential role in determining the shape, structural integrity, and deformability of the red cell. A significant achievement of modern biochemistry and hematology has been the elucidation of the organization of the components of the membrane skeleton and their relationship to other membrane proteins and lipids. This article reviews current concepts of membrane skeleton structure and function and emphasizes recent advances which have been made in characterizing and classifying molecular defects of the skeleton which manifest clinically with changes in the shape and stability of the red cell. The pathobiology of hereditary skeletal defects associated with hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary pyropoikilocytosis (HPP) are comprehensively discussed. Secondary defects of the membrane skeleton occurring in glucose-6-phosphate dehydrogenase deficiency and sickle cell anemia are also briefly considered.

ESR studies of the erythrocyte membrane skeletal protein network: influence of the state of aggregation of spectrin on the physical state of membrane proteins, bilayer lipids, and cell surface carbohydrates

The stability of the human erythrocyte membrane skeletal network is reported to be dependent on the state of aggregation of spectrin and decreased or increased by polyphosphate anions or the polyamine, spermine, respectively. We have employed polyacrylamide gel electrophoresis and electron spin resonance (ESR) utilizing spin labels specific for membrane proteins, bilayer lipids, or cell-surface sialic acid in order to gain insight into these observations and into the reliability of the ESR spectra of the protein-specific spin label used to correctly report the interactions of the skeletal protein network. The major findings are: (1) We confirm previous reports that the preferred state of spectrin aggregation in the skeletal network is tetrameric and that spectrin can be reversibly transformed to dimeric spectrin and back to tetrameric spectrin on the membrane. (2) The ESR spectra of the protein specific maleimide spin label employed accurately reflect the state of aggregation of spectrin. (3) As dimeric spectrin is increased on the membrane or when 2,3-bisphosphoglycerate was added to spin-labeled membranes, increased segmental motion of protein spin label binding sites reflecting decreased protein-protein interactions in the skeletal network is observed (P less than 0.002 and P less than 0.005, respectively). (4) Conversely, as protein-protein interactions between skeletal proteins or between skeletal proteins and the bilayer are increased by spermine (reflected in the total inability to extract spectrin from the membrane in contrast to control membranes), highly decreased segmental motion of the protein specific spin label binding site is observed (P less than 0.005). (5) The dimeric-tetrameric state of spectrin aggregation on the membrane does not have influence on the order or motion of bilayer lipids nor on the rotational rate of spin-labeled, cell-surface sialic acid, a result also observed when protein-protein interactions were decreased by 2,3-bisphosphoglycerate. In contrast, increased protein-protein interactions by addition of spermine produced a small, but significant, increase in order and decrease in motion of bilayer lipids near the membrane surface as well as a nearly 40% decrease in the apparent rotational correlation time of spin labeled, cell surface sialic acid (P less than 0.002). These latter observations are discussed with reference to possible associations of phospholipids and the major, transmembrane sialoglycoprotein with the skeletal protein network.

Pathologic and nonpathologic variants of the spectrin molecule in two black families with hereditary elliptocytosis

Five patients with hereditary elliptocytosis (HE) from two unrelated black families were studied. The patients had prominent elliptocytosis and a decreased erythrocyte resistance to heat treatment. In one infant blood smears showed elliptocytosis and poikilocytosis; his erythrocytes fagmented at a lower temperature than those of his mother and sister, both having typical mild HE. Defective dimer-dimer association was present in all patients. Limited tryptic digestion of spectrin and subsequent analysis by one- and two-dimensional electrophoresis revealed a similar and reproducible decrease in the 80,000-dalton peptide (alpha I domain) and the concomitant appearance of a 46,000-dalton peptide. All the patients had the polymorphism of the spectrin alpha II domain commonly observed in black populations. In addition, modifications relative to the alpha III domain were detected; similar variants were found in one black control subject out of 136 and are likely related to a genetic polymorphism of the alpha III domain. No differences were observed between the peptide patterns in the infant with poikilocytosis and those of his HE sister and mother.

Crude spectrin extraction from reticulocyte-rich blood samples

Crude spectrin was extracted from the isolated red cell ghosts with low ionic strength buffer at 37 degrees C for 30 min. No significant alterations in crude spectrin extractability in wide range of patients with various hematologic diseases were observed. However, blood samples characterized by elevated reticulocytosis provided crude extracts with increased amount of non-heme membrane skeletal proteins. The presence of ribose in the crude spectrin extracts obtained from reticulocyte-rich blood samples indicates also the presence of nucleic acids which causes the shift of protein peak in the extract from 280 nm towards lower wavelengths. A model experiment with a normal crude spectrin extract mixed with various amounts of RNA allowed us to obtain the correction curve which served for determination of non-heme protein (crude spectrin) extractability.

Altered spectrin association and membrane fragility without abnormal spectrin heat sensitivity in a case of congenital hemolytic anemia

In hereditary pyropoikilocytosis (HPP) and one type of hereditary elliptocytosis (HE), spectrin self-association is abnormal [5,7]. Spectrin extracted from normal erythrocyte membranes at 0 degree C is nearly all tetrameric, while in HPP and HE (type 1) a substantial amount of the extracted spectrin is dimeric. Abnormal reassociation of spectrin dimers to tetramers can also be demonstrated. We here report the case of a family in which the child has moderately severe hemolysis, with extreme microcytosis and poikilocytosis. The spectrin extracted at 0 degree C was predominately dimer. Parents had levels of dimer intermediate between patient and control values. The temperature dependence was normal for erythrocyte fragmentation; spectrin extractability; and circular dichroism of purified spectrin. Neither the patient nor either parent had elliptocytic red cells as judged from smears and scanning electron microscopy. The presence of substantial amounts of dimeric spectrin in the parents is consistent with a model in which each parent is heterozygous for a different nonassociating mutant spectrin, while the child has inherited a nonassociating molecule from each parent. In each individual, the degree of mechanical stability of the erythrocyte membrane, determined by ektacytometry, was proportional to the amount of tetramer found in the membrane. The description of this case is consistent with either HPP or a form of homozygous HE which is asymptomatic in the carrier state.

N-ethylmaleimide causes mechanical fragility and accumulation of spectrin dimers in the rat erythrocyte membrane

Treatment of rat erythrocytes with N-ethylmaleimide is found to render them mechanically fragile. Membranes of the lysed cells show degradation of band 3 and, to a lesser extent, of spectrin; as well as considerable accumulation of dimeric spectrin. The predominant action of N-ethylmaleimide on isolated membranes, however, is the conversion of spectrin to its dimeric form.

Brain spectrin: a review

Red blood cell spectrin, along with actin and several other proteins, forms a skeletal meshwork on the cytoplasmic surface of the erythrocyte plasma membrane. This structure is thought to maintain red blood cell shape, membrane structural stability, and cellular elasticity, as well as controlling the lateral mobility of integral membrane proteins and the transbilayer movement of phospholipids. It is now clearly established that spectrin-related molecules are ubiquitous structural elements subjacent to the plasma membrane of mammalian and avian nonerythroid cells. In this review, we present the current knowledge concerning brain spectrin. Brain spectrin is an approximately 11S, approximately 1,000,000 molecular weight (alpha beta)2 tetramer containing subunits of 240,000 (alpha) and 235,000 (beta) molecular weight. It is present in the cortical cytoplasm of all neuronal cell bodies and processes, and to a lesser extent in glial cells. Its involvement in the actin-membrane interaction, as well as other proposed functions in the nervous system is discussed.

Analysis of the ternary interaction of the red cell membrane skeletal proteins spectrin, actin, and 4.1

Spectrin dimers interact weakly with F-actin under physiological solvent conditions (with an association constant of about 5 X 10(3) M-1 at 20 degrees C). In the presence of the membrane skeletal constituent, protein 4.1, strong binding is observed; an analysis of the profiles for formation of a ternary complex leads to an association constant of about 1 X 10(12) M-2. This association becomes weaker at low ionic strength, whereas the opposite applies to the spectrin-actin interaction. The stability of the ternary complex is maximal at physiological ionic strength and somewhat above. The effect of temperature in the range 0-20 degrees C on the formation of the ternary complex is small, whereas the spectrin-actin interaction almost vanishes at low temperature. There is no detectable calcium sensitivity in either the binary or the ternary system within the limits of precision of our assay. The ternary complex resembles the natural system in the membrane in that the actin is resistant to dissociation and unavailable in the deoxyribonuclease assay; after selective proteolytic destruction of spectrin and 4.1, all the actin becomes available. In the absence of 4.1, spectrin dimers do not measurably protect the actin against dissociation.

Ultrastructure of unit fragments of the skeleton of the human erythrocyte membrane

We have examined fragments of the filamentous network underlying the human erythrocyte membrane by high-resolution electron microscopy. Networks were released from ghosts by extraction with Triton X-100, freed of extraneous proteins in 1.5 M NaCl, and collected by centrifugation onto a sucrose cushion. These preparations contained primarily protein bands 1 + 2 (spectrin), band 4.1 and band 5 (actin). The networks were partially disassembled by incubation at 37 degrees C in 2 mM NaPi (pH 7), which caused the preferential dissociation of spectrin tetramers to dimers. The fragments so generated were fractionated by gel filtration chromatography and visualized by negative staining with uranyl acetate on fenestrated carbon films. Unit complexes, which sedimented at approximately 40S, contained linear filaments approximately 7-8 nm diam from which several slender and convoluted filaments projected. The linear filaments had a mean length of 52 +/- 17 nm and a serrated profile reminiscent of F-actin. They could be decorated in an arrowhead pattern with S1 fragments of muscle heavy meromyosin which, incidentally, displaced the convoluted filaments. Furthermore, the linear filaments nucleated the polymerization of rabbit muscle G-actin, predominantly but not exclusively from the fast-growing ends. On this basis, we have identified the linear filaments as F-actin; we infer that the convoluted filaments are spectrin. Spectrin molecules were usually attached to actin filaments in clusters that showed a preference for the ends of the F-actin. We also observed free globules up to 15 nm diam, usually associated with three spectrin molecules, which also nucleated actin polymerization; these may be simple junctional complexes of spectrin, actin, and band 4.1. In larger ensembles, spectrin tetramers linked actin filaments and/or globules into irregular arrays. Intact networks were an elaboration of the basic pattern manifested by the fragments. Thus, we have provided ultrastructural evidence that the submembrane skeleton is organized, as widely inferred from less direct information, into short actin filaments linked by multiple tetramers of spectrin clustered at sites of association with band 4.1.

Defective binding of spectrin to ankyrin in a kindred with recessively inherited hereditary elliptocytosis

The interaction of spectrin with spectrin-depleted inside-out membrane vesicles was studied in a kindred with an atypical variant of hereditary elliptocytosis inherited in a recessive manner. The probands are characterized by prominent elliptocytosis, decreased erythrocyte thermal stability, an altered limited tryptic peptide pattern of spectrin digested at low ionic strength, and defective spectrin dimer-dimer association. The parents are normal. The spectrin/band 3 ratio determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of isolated membranes of the probands was decreased to approximately 70% of control values, and total erythrocyte spectrin content in one proband was also decreased on SDS-PAGE. When a monospecific antispectrin antibody was used, a faintly labeled fragment of molecular weight approximately 28,000 was detected on immunoblots of whole cell lysates of one proband and a control, but could not account for the decreased erythrocyte spectrin content of the proband on SDS-PAGE. Binding and competitive inhibition studies revealed an alteration in the spectrin-ankyrin interaction due to an abnormality of spectrin in the probands. No defect was found in the mother; the father's spectrin showed decreased binding affinity, although it was not so severe as in the probands. Separation of bound and unbound spectrin dimers from one proband and subsequent conversion to tetramers showed that the self-association defect was detectable only on the bound subpopulation of her spectrin. These findings demonstrate a hitherto undescribed functional abnormality of spectrin in this kindred which could result in decreased stability of the membrane skeleton and contribute to the elliptocytic shape of these erythrocytes.

A molecular defect of spectrin in a subset of patients with hereditary elliptocytosis. Alterations in the alpha-subunit domain involved in spectrin self-association

Hereditary elliptocytosis (HE) is a clinically and biochemically heterogenous group of diseases characterized by elliptically shaped erythrocytes and an autosomal dominant mode of inheritance. Whereas the self-association of spectrin heterodimers to tetramers is defective in a subpopulation of HE patients, designated HE[SpD-SpD], it is normal in others. We have examined the peptide pattern produced by limited tryptic digestion of spectrin extracts from patients with HE[SpD-SpD] to determine if the functional defects in spectrin self-association could be correlated with structural changes in the spectrin molecule. Although the peptide pattern produced by limited tryptic digestion of spectrin extracts from those HE patients with normal spectrin self-association was indistinguishable from the pattern from control normal volunteers, digestion of the spectrin extracts from the HE[SpD-SpD] patients showed a reproducible diminution in the 80,000-D domain of the alpha-subunit, which is involved in spectrin dimer self-association. The decrease in the 80,000-D fragment was associated with an increase in a 74,000-D fragment in eight of nine families, or, in one family, with an increase of fragments at 46,000 and 17,000 D. These atypical peptide patterns were similar to those previously reported in two variants of hereditary pyropoikilocytosis (HPP), which also had defective self-association of spectrin. These data indicate that two distinct structural variants of spectrin alpha-subunit are associated with the defective spectrin heterodimer self-association in a subpopulation of HE patients.

A dynamical study on the interactions between the cytoskeleton components in the human erythrocyte as detected by saturation transfer electron paramagnetic resonance of spin-labeled spectrin, ankyrin, and protein 4.1

Isolated human erythrocyte spectrin, ankyrin, and protein 4.1 have been labeled with the maleimide spin label, 3-maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl, and studied by saturation transfer electron paramagnetic resonance spectroscopy. The presence of the labels does not affect the reassociation of these proteins with erythrocyte membranes selectively depleted of either spectrin-actin or of all the extrinsic proteins. When maleimide spin-labeled spectrin is reassociated with the erythrocyte membrane in presence of all the cytoskeleton components, including endogeneous or purified muscle actin, spectrin still preserves its flexible character. The rotational mobilities of maleimide spin-labeled ankyrin and maleimide spin-labeled protein 4.1 are of the same order of magnitude (tau c (L"/L) approximately 5 X 10(-5) and 8 X 10(-5) s, respectively, at 2 degrees C), while protein 4.1 is almost three times smaller in size than ankyrin. This result indicates that the movements of membrane-bound maleimide spin-labeled protein 4.1 are more restricted than those of ankyrin. This suggests that their respective binding sites have different structural properties. The rotational movements of both proteins are slowed down on the addition of spectrin indicating that protein 4.1 as well as ankyrin also represents one of the links of the cytoskeleton to the membrane.

Defective spectrin dimer-dimer association in a family with transfusion dependent homozygous hereditary elliptocytosis

Red cell membrane proteins have been examined in a family in which three children have severe transfusion-dependent homozygous hereditary elliptocytosis. The membranes in all three show a considerable excess of spectrin dimers over tetramers in spectrin extracts. The red cell membranes of their parents with heterozygous hereditary elliptocytosis show a lesser but significant increase in spectrin dimers. Some of the family members also have an alpha-globin gene deletion and haemoglobin D trait. The present results are the first demonstration of a defect of spectrin dimer-dimer association in homozygous elliptocytosis and provide strong support for the concept that this defect is the primary cause of the red cell abnormality in at least some families of hereditary elliptocytosis.

The spectrin membrane skeleton of normal and abnormal human erythrocytes: a review

The erythrocyte membrane skeleton composed of spectrin, actin, and several other proteins is essential for the maintenance of the erythrocyte shape, reversible deformability, and membrane structural integrity in addition to controlling the lateral mobility of integral membrane proteins. In this review, we shall give an historical development of the current model of the erythrocyte membrane skeleton. We will then describe how the experimental technology developed to study the normal membrane skeleton has paved the way for the recent identification of alterations of skeletal protein interactions in hereditary spherocytosis, hereditary elliptocytosis, and hereditary pyropoikilocytosis. We will conclude with a discussion of some of the more exciting and promising directions for future research that are currently being initiated in this vanguard field of cell biology.

Missing band 7 membrane protein in two patients with high Na, low K erythrocytes

We investigated the erythrocyte membrane proteins of two patients with congenital hemolytic anemia due to increased permeability of the erythrocyte membrane to Na and K (hereditary stomatocytosis and cryohydrocytosis). One-dimensional sodium dodecyl sulfate (SDS) gel electrophoresis resolved the band 7 erythrocyte membrane proteins into three components with approximate molecular weights of 30,000, 28,000, and 26,000. The 28,000-dalton component was decreased in both patients with permeability disorders. Two-dimensional electrophoresis (nonequilibrium pH gradient electrophoresis in the first dimension combined with SDS gel electrophoresis in the first dimension combined with SDS gel electrophoresis in the second dimension) resolved the 28,000-dalton component from normal erythrocyte membranes into two proteins with different isoelectric points, designated 22 x 8 and 60 x 8. In the patients with hereditary stomatocytosis and cryohydrocytosis, 22 x 8 was completely absent, whereas 60 x 8 was detected as usual. In contrast, all the band 7 proteins (including 22 x 8) were invariably present in a survey of normal subjects and reticulocytosis controls. The unique finding of a missing band 7 protein in the patients with hereditary stomatocytosis and cryohydrocytosis raises the possibility that the absence of this protein is responsible for the increased Na and K permeability in these disorders.

Molecular defect of spectrin in hereditary pyropoikilocytosis. Alterations in the trypsin-resistant domain involved in spectrin self-association

In hereditary pyropoikilocytosis (HPP) the erythrocyte membrane skeleton exhibits mechanical instability that can be correlated to defective self-association of spectrin heterodimers. To detect structural changes in the functional domains of HPP spectrin we have examined the peptide pattern produced by limited tryptic digestion of spectrin extracts from two families that contain three HPP patients. Limited tryptic digestion of all three HPP patients revealed a similar and reproducible decrease in the staining intensity of an 80,000-, and 22,000-, and an 88,000-dalton polypeptide with a concomitant increase in a 74,000- and a 90,000-dalton polypeptide as compared with controls. Only changes in the 80,000-, and 74,000-, and 22,000-dalton polypeptides could be correlated to defective spectrin self-association and the amount of spectrin dimers in 0 degrees C extracts of the HPP patients and their affected kindred. Similar results were obtained when the tryptic digests were analyzed by two-dimensional isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis with the affected 74,000- and 80,000-dalton polypeptides focusing into multiple spots ranging in isoelectric point from 5.3-5.4. When HPP spectrin dimers and tetramers were separated and subjected to trypsin digestion, changes in the 80,000-, 74,000-, and 22,000-dalton polypeptides were found predominantly in the spectrin dimer pool. Similar results were obtained for spectrin from two of the probands' mother, whom we have identified as an HPP carrier. We conclude that these HPP patients contain a population of normal, (principally tetrameric) and mutant (principally dimeric) spectrin. The latter is characterized by a defective spectrin dimer self-association due to conformational changes that affect the 80,000-dalton domain.

Spectrin beta-chain variant associated with hereditary elliptocytosis

An electrophoretically fast-moving variant of the spectrin beta-chain was discovered in the erythrocyte membranes of a woman and her father who both exhibited elliptocytosis and mild hemolytic anemia. This abnormal beta'-subunit (Mr = 214,000) co-existed with a decreased normal beta-chain and represented about half of the total beta-chains in the membrane. In contrast to the spectrin beta-chain, the beta'-chain was phosphorylated neither in the membrane by endogenous protein kinases nor in solution by pure membrane casein kinase whether or not the spectrin was dephosphorylated by erythrocyte cytosolic spectrin phosphatase. The presence of the beta'-chain was associated with a defective self-association of spectrin dimer to form tetramer as manifested by: (a) an excess of spectrin dimer in the 4 degrees C spectrin crude extract, (b) a defective self-association of the spectrin dimer in the 37 degrees C crude spectrin extracts. Gel electrophoretic analysis of the tetramer and dimer species isolated from the proband's 4 degrees C extract showed that the tetramer contained trace amounts of the beta'-chain, whereas in contrast, a large proportion of beta'-chain was present in the dimer. These results demonstrated the responsibility of the beta'-chain for the defective reassociation of spectrin dimer into tetramer. The study of this abnormal spectrin confirms the participation of spectrin beta-chain in dimer-dimer association and strongly suggests that the phosphorylation sites of the normal beta-chain are located at the end of the molecule involved in the dimer-dimer interactions.

Role of the reticulum in the stability and shape of the isolated human erythrocyte membrane

In order to examine the widely held hypothesis that the reticulum of proteins which covers the cytoplamsic surface of the human erythrocyte membrane controls cell stability and shape, we have assessed some of its properties. The reticulum, freed of the bilayer by extraction with Triton X-100, was found to be mechanically stable at physiological ionic strength but physically unstable at low ionic strength. The reticulum broke down after a characteristic lag period which decreased 500-fold between 0 degrees and 37 degrees C. The release of polypeptide band 4.1 from the reticulum preceded that of spectrin and actin, suggesting that band 4.1 might stabilize the ensemble but is not essential to its integrity. The time-course of breakdown was similar for ghosts, the reticulum inside of ghosts, and the isolated reticulum. However, at very low ionic strength, the reticulum was less stable within the ghost than when free; at higher ionic strength, the reverse was true. Over a wide range of conditions the membrane broke down to vesicles just as the reticulum disintegrated, presumably because the bilayer was mechanically stabilized by this network. The volume of both ghosts and naked reticula varied inversely and reversibly with ionic strength. The volume of the naked reticulum varied far more widely than the ghost, suggesting that its deformation was normally limited by the less extensible bilayer. The contour of the isolated reticulum was discoid and often dimpled or indented, as visualized in the fluorescence microscope after labeling of the ghosts with fluoroscein isothiocyanate. Reticula derived from ghosts which had lost the ability to crenate in isotonic saline were shriveled, even though the bilayer was smooth and expanded. Conversly, ghosts crenated by dinitrophenol yielded smooth, expanded reticula. We conclude that the reticulum is a durable, flexible, and elastic network which assumes and stabilizes the contour of the membrane but is not responsible for its crenation.

Defective spectrin dimer-dimer association with hereditary elliptocytosis

We examined erythrocytes from 18 patients with hereditary elliptocytosis. Spectrin from eight patients (referred to as type 1) was defective in dimer-dimer association as demonstrated in two ways. First, there was an increased amount of spectrin dimer with a concomitant decrease in tetramer as measured in erythrocyte membrane preparations extracted at 0 degrees C under low-salt conditions (the amount of spectrin dimer was 15-33% of total spectrin species compared with a normal range of 3-7%). Second, the equilibrium constants of spectrin dimer-dimer association were decreased in both solution and in situ membrane. Spectrin from the remaining 10 patients (referred to as type 2) showed normal dimer-dimer association. Membrane skeletons, produced from ghosts of both types of hereditary elliptocytosis by Triton X-100 extraction, were unstable when mechanically shaken. Because spectrin tetramers, but not dimers, can crosslink actin, we postulate that the defective spectrin dimer-dimer association in type 1 diminishes actin crosslinking and thus is responsible for membrane skeletal instability. A defective protein-protein association in type 2, however, remains to be identified.

A molecular defect in two families with hemolytic poikilocytic anemia: reduction of high affinity membrane binding sites for ankyrin

Patients from two families with chronic hemolytic anemia have been studied. The erythrocytes are very fragile and appear microcytic with a great variety of shapes. Clinical evaluation failed to identify traditionally recognized causes of hemolysis. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) showed no significant abnormality of the major polypeptide bands. Erythrocytes spectrin-ankyrin and ankyrin-membrane interactions were analyzed with 125I-labeled spectrin, 125I-labeled ankyrin, and inside-out vesicles. Patients' vesicles bound 125I-spectrin normally. Likewise, patients' spectrin and ankyrin competed normally for the binding sites on control membranes. None of the individual components appeared to have abnormal thermal sensitivity. Ankyrin-stripped, inside-out vesicles prepared from the patients bound less 125I-ankyrin than did vesicles prepared from normals (P less than 0.05 for all corresponding points in the high-affinity region). Scatchard analysis showed the most significant abnormality to be a 50% reduction in the high affinity ankyrin binding sites. Similar experiments were performed with blood from patients with spherocytosis and splenectomized controls, but no abnormalities were detected. The water soluble 43,000-dalton fragments of band 3 (the high-affinity ankyrin binding sites) were prepared from one of the patients and competed normally for 125I-ankyrin binding in solution. This suggests that the primary structural defect is a reduction in the number of high affinity membrane binding sites for ankyrin, and is consistent with an abnormal organization of band 3 in the membrane.

Altered spectrin dimer-dimer association and instability of erythrocyte membrane skeletons in hereditary pyropoikilocytosis

Hereditary pyropoikilocytosis (HPP) is a hemolytic anemia characterized by microspherocytosis, poikilocytosis, and an unusual thermal sensitivity of erythrocytes. We have investigated the contribution of abnormal membrane skeletal assembly to these abnormal HPP erythrocyte properties. Skeletons prepared from fresh HPP ghosts with Triton X-100 were considerably more fragile than skeletons from control erythrocytes. Spectrin, the major skeleton component, extracted at 0 degrees C from normal erythrocytes, was present primarily as tetramers and high molecular weight complexes. In contrast, spectrin extracted from HPP erythrocytes under identical conditions contained a significant amount of dimers with a concomitant decrease of tetramers. Furthermore, spectrin dimers from HPP erythrocytes differed from normal spectrin dimers in their failure to reassociate into tetramers both in solution and in the membrane. Presumptive HPP carriers (asymptomatic mothers of the two patients) exhibited a mild but reproducible increase of spectrin dimers in 0 degrees C extracts and a defective reassociation of spectrin dimers of tetramers both in solution and in the membrane. We conclude that in HPP, self-association of spectrin dimers into tetramers is defective, which accounts for the instability of membrane skeletons.