Spectrin: present status of a putative cyto-skeletal protein of the red cell membrane

The relevance of the aldehyde bisulfite toluidine blue reaction and its variants in the submicroscopic carbohydrate research

Carbohydrates are chemical compounds that contain only oxygen, hydrogen and carbon. They are classified by their number of sugar units: monosaccharides (such as glucose and fructose), and disaccharides (such as sucrose and lactose) are simple carbohydrates; oligosaccharides and polysaccharides (such as starch, glycogen and cellulose) are complex carbohydrates. Carbohydrates play a crucial role in diverse biological systems [Hricovín M. Structural aspects of carbohydrates and the relation with their biological properties. Curr Med Chem 2004;11:2565-83]. According to Roseman [Sugars of the cell membrane. In: Weissmann G, Clairborn E, editors. Cell membranes. Biochemistry, Cell Biology, Pathology. New York: H. P. Publ. Co; 1975. p. 55-64], two classes of glycoproteins are described. Free glycoproteins are localised in the surface coat of the membranes and form a thick mobile layer, without any association to the membrane itself. Functionally, however, they are located in a close association with the membrane (e.g. in the duodenal mucosa). The other group consists of the membrane glycoproteins, which are integral to the membranes and are located in the outer layer. The oligosaccharide chains are bound to the N-terminal part of proteins, and are situated in the hydrophilic zone. Glycoproteins have diverse functions. They are important in specific receptor functions, in immunological cell destruction and play a significant role in reactions with lectins, antibodies, as well as in cell association and mutual recognition of the cells. This paper focuses on aspects of a summary of polarisation optical investigations and biological functions of the following three groups of carbohydrates: oligosaccharides, glycoproteins and glycosaminoglycans.

Erythroid spectrin in miceller detergents

We have studied the interaction of spectrin, the major protein of the erythrocyte cytoskeleton, with four commonly used detergents at concentrations above their critical miceller concentrations (cmc). Fluorescence spectroscopic studies on the emission intensity, steady state polarization, quenching with acrylamide, and time-resolved fluorescence measurements were done with spectrin in anionic detergents, e.g., SDS, deoxycholate, and nonionic detergents, e.g., Triton-X-100 and octylglucoside at concentrations double their respective cmc's. The spectrin-detergent complexes in all four systems have been characterized by far-UV CD and measurements on tryptophan fluorescence in combination with fluorescence of the extrinsic probe, pyrene. Tryptophan fluorescence studies revealed quaternary structural changes due to unzipping of the spectrin subunits in Triton-X-100 without complete dissociation. Both Triton-X-100 and SDS were found to partially denature spectrin indicated by the far-UV CD. Octylglucoside and deoxycholate are shown to have the least structural perturbations on the cytoskeletal protein, rationalizing the use of octylglucoside, in particular and also deoxycholate to be the most effective in preparing cytoskeletal fractions from erythrocytes rather than the Triton-X-100 that has long been used for preparing the Triton shells.

A comparative study of the effect of freeze-thawing on peripheral and integral membrane proteins

The effect of rapid freeze-thawing on the structural rearrangement of integral and peripheral membrane proteins was studied using fluorescent spectroscopy. It was discovered that a long wave shift occurred in the fluorescence spectra of cytochrome P450, both in solution and within phospholipid vesicles. The efficiency of quenching of membrane-bound cytochrome P450 with acrylamide also increased after freezing. Similarly, a long wave shift in the fluorescence, spectra occurred after freezing spectrin solution and erythrocyte ghosts. These data are interpreted in terms of physical and chemical factors in the medium affecting the spectral properties of protein chromophores, and suggest that an increase occurs in the polarity of the microenvironment of the protein molecules, resulting in conformational modification. Study of the temperature dependence of spectrin fluorescence in solution, and within erythrocyte ghosts, suggests that modification of protein dynamics occurs both during freezing and during thawing. Despite various differences in the structure of the protein molecules, and in the phospholipid environment and solvent composition, freeze-thawing results in unilateral, irreversible conformation changes, more prominent in peripheral than integral proteins. The possibility that these changes contribute to cryoinjury to cell membranes is discussed.

Ultrastructural plasticity of the dentate gyrus granule cells following recurrent limbic seizures: I. Increase in somatic spines

Various paradigms have been used to assess the capacity of the adult brain to undergo activity-dependent morphological plasticity. In this report we have employed recurrent limbic seizures as a means of studying the effects of this form of enhanced neuronal activity on cellular morphology and, in particular, on the incidence of somatic spines on the dentate gyrus granule cells. Seizure activity was induced by the placement of focal, unilateral electrolytic lesions in the dentate gyrus hilus of adult rats. At various intervals postlesion, rats with behaviorally verified seizures were sacrificed, and the hippocampi contralateral to the lesions were removed and prepared for electron microscopy. Quantitative analysis showed that as early as 5 hours postlesion there was a dramatic increase in the density and morphological complexity of spines on the perikarya of the granule cells in rats that received seizure-producing hilus lesions when compared to granule cells from control rats. Many of the somatic spines received asymmetric synapses. The increase in somatic spines was dependent on seizure activity and persisted for at least 1 month following a single recurrent seizure episode. CA1 pyramidal neurons, which exhibit changes in gene expression in response to hilus lesion-induced seizures but do not normally possess somatic spines, did not exhibit an activity-dependent elaboration of somatic spines. Thus, the seizure-induced elaboration of somatic spines represents an amplification of an existing feature of the granule cells and not an effect occurring throughout hippocampus. These data provide evidence for very rapid and long-lasting structural plasticity in response to brief episodes of seizure activity in the adult brain.

Requirement of cytoplasmic components for lidocaine-induced shape change in human erythrocytes

To clarify the mechanism underlying local anesthetic-induced changes in the shape of human erythrocytes from discocytes to stomatocytes, we treated erythrocytes with lidocaine, a cationic drug. Analysis of the erythrocyte membrane and cytoplasm by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed that the intensities of the stained bands of 62 kDa, 28 kDa and 22 kDa depended on the extent of the shape change induced by lidocaine. The change in the intensity of the 28 kDa band was particularly marked. We identified the cytoplasmic substances, i.e., the 28 kDa and 22 kDa peptides, as carbonic anhydrase (CA) and glutathione peroxidase (GSH Px)1, respectively, by immunoblotting. The 62 kDa peptide was identified as Hb by column chromatography and SDS-PAGE analysis. To identify the protein responsible for the lidocaine-induced shape change, we incorporated CA and GSH Px into ATP-MgCl2-resealed ghosts. The shape of the resealed ghosts changed upon addition of lidocaine, but only in the presence of CA. These results suggest that ATP and CA are required for the shape changes induced by lidocaine.

Glutamate in the mammalian CNS

The excitatory amino acid glutamate plays an important role in the mammalian CNS. Studies conducted from 1940 to 1950 suggested that oral administration of glutamate could have a beneficial effect on normal and retardate intelligence. The neurotoxic nature of glutamate resulting in excitotoxic lesions (neuronal death) is thought possibly to underlie several neurological diseases including Huntington's disease, status epilepticus. Alzheimer's dementia and olivopontocerebellar atrophy. This neurodegenerative effect of glutamate also appears to regulate the formation, modulation and degeneration of brain cytoarchitecture during normal development and adult plasticity, by altering neuronal outgrowth and synaptogenesis. In addition to its function as a neurotransmitter in several regions of the CNS, glutamate seems to be specifically implicated in the memory process. Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity associated with learning and memory, both involve glutamate receptors. Studies with antagonists of glutamate receptors reveal a highly selective dependency of LTP and LTD on the N-methyl-D-aspartate and quisqualate receptors respectively. The therapeutic value of glutamate receptor antagonists is being actively investigated. The most promising results have been obtained in epilepsy and to some extent in ischaemia and stroke. The major drawback remains the inability of antagonists to permeate the blood-brain barrier when administered systemically. Efforts should be directed towards finding antagonists that are lipid soluble and able to cross the blood-brain barrier and to find precursors that would yield the antagonist intracerebrally.

Calcium-induced cleavage and breakdown of spectrin in the rat lens

Incubation of intact rat lenses under conditions that stimulated a net influx of calcium resulted in a pronounced loss of transparency and a major decrease in the levels of spectrin. The progressive loss of this cytoskeletal component coincided with the appearance of polypeptides of approximately 150 kDa which showed immunoreactivity with an antibody raised to spectrin. These bands disappeared on further incubation. It is, therefore, suggested that a calcium-activated protease is present in the lens which is capable of degrading spectrin by the initial removal of approximately 90 kDa fragments. This process calcium-induced proteolysis may be the basis for the cytoskeletal reorganisation observed during the differentiation of lens fibre cells and may be involved in cataract development.

On the relationships between the rate of cytoskeletal stable assemblies turnover, stability of the differentiated state, development and aging

There is a general consensus that biological specificity is a structure-derived property. If a living system is going to maintain its structure and function then the newly synthesized molecules should replace the faulty ones at the correct time and in the correct places so that the previously established cellular topology will be preserved. In addition, pre-existing spatial determinants which will direct the asymmetrical assembly of the newly synthesized molecules should be available. Therefore, regulation of turnover of cellular architecture represents an essential feature of living systems. In considering the underlying causes of cellular senescence it seemed reasonable to focus on the relationship between development of a stable phenotype and the turnover of cellular and extracellular stable assemblies, currently thought to be involved in maintaining the stability of the differentiated state. In recent years evidence has accumulated suggesting a reciprocal relationship between cytoarchitecture turnover rate and achievement of a stable structure. The lack of a feedback control on the turnover of cellular stable assemblies and/or a low turnover rate of cytoarchitecture components would mean that they will be subjected to damaging processes such as oxidation, cross-linking, aminoacid racemization or non-enzymatic browning which are known to occur in other long-lived proteins. The consequence would be the generation, with advancing age, of faulty cellular structures which, in turn, would alter the deposition of newly synthesized molecules. This process may lead to a progressive breakdown in cellular and extracellular stable structures. The process of directed assembly seems to be general for biological systems displaying history-dependent development. We believe that it is this strategy which imposes severe limitations on presegregated spatial determinants turnover rates and, therefore plays a major role in initiating the aging process. We also suggest that species-specific life-span might be determined by the species-specific regulatory networks which governs the cell-specific cytoarchitecture damaging rate. Moreover, aging appears to be an intrinsic feature of biological systems displaying history-dependent development and should be absent in systems displaying history-independent life-cycles, such as bacteria, some species of protozoa, and certain transformed cell lines. An important feature of protein turnover is that this process requires metabolic energy. Therefore, we can expect that structure preservation strategy is a part of a more general energy-saving strategy, a view previously expressed by T.B.L. Kirkwood (Nature, Lond., 1977, 270, 301-304).

Quantification and characterization of regulin, a Mr-230,000 highly elongated protein of rabbit reticulocytes

Procedures are described by which regulin in rabbit reticulocytes was quantified and isolated in relatively large amounts. In these cells the protein occurs at a ratio of about 1.1-1.6 regulin monomers/spectrin tetramer, corresponding to 80,000-100,000 molecules of Mr-230,000 regulin/cell. Erythrocytes contain less than 12% of the amount of regulin in reticulocytes and the protein has not been detected in non-erythroid cells. Regulin was found primarily in the cytosolic fraction of lysed reticulocytes. It appears to be unusually sensitive to proteolysis by Ca2+-activated thiol proteases. Isolation of Mr-230,000 undegraded regulin was accomplished by the use of protease inhibitors including N-ethylmaleimide. A striking characteristic of regulin is its tendency to aggregate in neutral solution of low ionic strength. Physical studies of the isolated protein indicate that it has a highly elongated form in solution. The protein has no known enzymatic activity but was shown previously to interact with and increase the enzymatic activity of a protein phosphatase. The properties of regulin suggest that it may have a structural function but it appears to be physically and immunologically distinct from known proteins. It is suggested that regulin may contribute to a gel matrix within the cytoplasm of reticulocytes.

Influence of red cell surface charge on red cell membrane curvature

The wellknown biconcave shape of resting RBC reflects an intrinsic overall negative spontaneous membrane curvature. It depends to a high degree on the integrity of the spectrin-actin-ankyrin-Band-3 hetero-complex. Alterations of this complex have previously been shown to be associated with shape transitions, which have been abolished by treatment with enzymes reducing the surface charge of the RBC. In this report we show that treatment of erythrocytes with enzymes reducing the surface charge (e.g. neuraminidase, trypsin, chymotrypsin and pronase), consistently exerts a "stomatocytogenic" effect, i.e. it reduces mean mean curvature. Also the time dependency for the charge reduction and for the correlated decrease of mean mean curvature is shown. So-called stomatocytogenic agents (e.g. clorpromazine, tetracain and triton X100) and so-called echinocytogenic agents (e.g. dinitrophenol and Na-salicylate) are known to change membrane curvature in a dose dependent manner. It is further shown that by prior reduction of surface charge by various enzymes the dose response curves of all stomatocytogenic and echinocytogenic agents tested is shifted towards higher degrees of stomatocytosis or lesser degrees of echinocytosis. The data show, that in RBC pronounced curvature influences are produced by the surface charge located on the sialic acid residues in the glycocalix.

Direct involvement of spectrin thiols in maintaining erythrocyte membrane thermal stability and spectrin dimer self-association

Human erythrocytes vesiculate upon exposure to temperatures of 49 degrees C and above. Pretreatment of the cells with the thiol-alkylating agent N-ethylmaleimide (NEM) lowers the temperature needed to produce the same effect. Concomitant with the cells' heat susceptibility, skeletal mechanical instability and an increase in spectrin dissociation have been reported (Smith and Palek (1983) Blood 62, 1190). In the present study, similar results were achieved by preincubation of the cells with diamide, which could be reversed by reduction with dithiothreitol. Another oxidative agent, sodium tetrathionate, could only induce the temperature susceptibility, with little effect on spectrin dissociation. Incubation of spectrin solutions with NEM or diamide caused decreased association of spectrin dimers and increased dissociation of spectrin tetramers. Estimation of membrane and spectrin thiols in the treated cells showed that NEM was effective while blocking less than 20% of the thiols. Diamide and tetrathionate blocked more than 50% of the thiols, but were less effective than NEM. It is suggested that some very defined population of thiols is essential for spectrin self-association and for membrane thermal stability. They are more available to NEM than to diamide and less so to tetrathionate. Other thiols participate in maintaining the membrane thermal stability only.

Ultrastructure of matrix vesicles in chick growth plate as revealed by quick freezing and freeze substitution

The ultrastructure of extracellular membrane-bound matrix vesicles (MVs), their biogenesis, and the surrounding matrix in chick tibial growth plate were studied after quick freezing and freeze substitution (FS) in an organic solvent. There were several notable differences in the ultrastructural preservation of cartilage when FS was used as compared with conventional fixation. The ultrastructural appearance of MVs after FS was extremely variable. Within the MVs, intravesicular filaments, amorphous material, and membrane-associated undercoat structures were observed. Intravesicular filaments, similar in diameter to microfilaments seen in the cytoplasm, were attached to the inside of MV membranes. This observation indicates the similarity of MV membranes and the plasma membrane. In some MVs in the proliferative zone an electron-dense material was present along the inner side of the MV membrane. In the prehypertrophic zone, crystalline material often appeared within the electron-dense material, which may be a precursor form of hydroxyapatite. The earliest crystals observed were in MVs but not in the extracellular matrix. Regarding MV formation, in addition to budding from cell surfaces and to cellular disintegration, this study also indicates that a sequential process of extrusion of preformed cytoplasmic structures may occur. Also, small MVs measuring 25-40 nm seem to arise from the disruption of large MVs. This is a previously unreported observation on MV biogenesis. FS preserves proteoglycans in the cartilage matrix as a fine, filamentous network. Initial extracellular calcification was not associated with this network.

The effect of microinfusions of drugs into the accessory olfactory bulb on the olfactory block to pregnancy

Female mice which have mated and are subsequently exposed to the odour (pheromones) of a strange male undergo hormonal changes resulting in a block to their pregnancy. The fact that the stud male's odours can also block pregnancies, that is other than his own, implies the formation of a memory or some form of recognition process by the female for this male's pheromones at the time of mating. The purpose of this study was to evaluate the effect of microinfusions of drugs which interfere with neural transmission, into the accessory olfactory bulbs. This was carried out immediately after mating over a 4-h period during which the "memory" to the stud male's pheromones is formed. Infusions of the alpha-blocker, phentolamine, blocked the formation of the olfactory memory, while the GABA receptor blocker, bicuculline, itself blocked pregnancy, but was without effect on memory formation. Protein synthesis inhibition or calpain inactivation in the accessory bulb was without effect on memory formation at any of the doses used. These studies demonstrate that GABAergic transmitter blockade in the accessory olfactory bulb at the time of mating can prevent subsequent blastocyst implantation some 3 days later, while alpha-noradrenergic blockade can prevent the formation of an olfactory memory to the stud male.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.

Heat-induced dissociation of human erythrocyte spectrin dimer into monomers

Human erythrocyte spectrin heated above 49 degrees C could be separated into two fractions by DEAE-Toyopearl column chromatography at room temperature. The first fraction eluting with the salt gradient was predominantly the alpha subunit, indicating a heat-induced dissociation of the spectrin alpha beta dimer into monomers. The second fraction, obtained with 0.5 M NaOH after salt elution, consisted of high-molecular-weight proteins in addition to alpha and beta subunits, which were visualized by gel electrophoresis with sodium dodecyl sulfate. The isolated beta subunit when heated above 48 degrees C could also be separated into two fractions by column chromatography. About 30% of the protein eluted with the salt solution and the rest of the proteins were in the alkali eluate in which high molecular weight protein bands also appeared, indicating a heat-induced aggregation of the beta subunits. Almost all the isolated alpha subunit, however, eluted out with the salt solution, even though the subunit was heated at 52 degrees C. Studies of the binding of subunits to inside-out vesicles indicate that the isolated beta subunit was denatured irreversibly by heating; on the other hand, the alpha subunit kept its binding ability after heating above 50 degrees C. These findings are attributed to the heat-induced dissociation of the spectrin molecules into alpha and beta subunits at 49-50 degrees C, and eventual aggregation of the denatured beta subunits.

The plasma membrane of yeast protoplasts exposed to hypotonicity becomes porous but does not disintegrate in the presence of protons or polyvalent cations

Protoplasts of Saccharomyces cerevisiae swelled, lysed and disintegrated when exposed to hypotonic solutions at neutral pH. At pH 4.5 or lower the hypotonically treated protoplasts did not disintegrate and they retained their intracellular proteins, nucleic acids and nucleotides. However, they became leaky for K+ and Ca2+, indicating that pores had been created in the surface membrane, relaxing the osmotic stress. Upon readjustment of pH to neutral, the hypotonically treated protoplasts released the intracellular content and disintegrated. Also, at low pH, protoplasts did not swell in isotonic ammonium acetate and were refractory to the permeabilizing effect of nystatin and to lysis with low concentrations of detergents. Protoplasts were similarly protected against lysis and disintegration by hypotonic treatment or by detergents, even at neutral pH, if the incubation media contained polyvalent cations, especially Zn2+, La3+, spermine, and Ca2+ chelated with EDTA. The protoplasts exposed to hypotonic stress at low pH did not respire and could not regenerate into viable cells. Effects of H+ and polyvalent cations on intramembrane forces acting between molecules of membrane phospholipids are considered along with possible changes in interactions between membrane proteins.

Phenylhydrazine-induced changes in erythrocyte membrane surface lipid packing

Phenylhydrazine-induced oxidative damage in red cells results in increased binding of merocyanine 540, a fluorescence probe sensitive to changes in lipid packing. Fluorescence polarization studies with diphenylhexatriene did not reveal major changes in order parameters both in intact red cells and lysates treated with phenylhydrazine. These fluorescence studies indicate that major changes are observed in membrane lipids. Analytical studies of membrane phospholipids revealed a significant decrease in phosphatidylethanolamine. The results of the fluorescence and lipid studies, taken in association with our previously reported findings on spectrin and other cytoskeletal protein degradation in red cells exposed to phenylhydrazine, suggests that degradation of cytoskeleton membrane proteins is also responsible for changes in the lipid bilayer surface of the red cell membrane.

Biosynthesis of spectrin and its assembly into the cytoskeletal system of Friend erythroleukemia cells

Friend erythroleukemia cells, grown in the presence of dimethyl sulfoxide for 3 d, synthesize unequal amounts of the two chains (alpha and beta) of spectrin with approximately 15-30% more beta than alpha spectrin. When cells were ruptured by nitrogen cavitation, nascent alpha and beta spectrin were found to be associated with a membranous cell fraction and were not detected in the soluble cytoplasmic cell fraction. Nascent membrane-bound spectrin appeared not to be protected by membranes, since it was susceptible to trypsin degradation in the absence of detergent. On fractionation of cells with 1% Triton X-100, more (1.75-fold) nascent spectrin was found in the Triton-soluble fraction than in the Triton-insoluble fraction (cytoskeleton). In the Triton-soluble fraction, there was 55% more nascent beta spectrin than alpha spectrin, while the cytoskeleton contained nearly equal amounts of alpha and beta spectrin. Cells were pulse-labeled with L-[35S]methionine for 2 min and chase incubated for varying periods of time from 15 to 90 min with nonradioactive L-methionine. Radioactive spectrin accumulated in the Triton-soluble fraction for the first 15 min of chase incubation and then dropped by 25% in the next hour. By contrast, the amount of radioactive spectrin in the Triton-insoluble fraction rose gradually for 1 h of the chase period. This indicates that, in Friend erythroleukemia cells, a pool of membrane-bound spectrin containing an excess of the beta polypeptide is used to form the cytoskeletal system which is composed of equal molar amounts of alpha and beta spectrin. The location of spectrin was determined by immunoelectron microscopy. Small amounts of spectrin were detected in cells not treated with dimethyl sulfoxide and in these cells it was located on the surface membrane and within the cytoplasm. On treatment with dimethyl sulfoxide, complex vacuolar structures containing viruses appeared in the cells. In cells treated with dimethyl sulfoxide for 3 d 30% of the spectrin was near the outer membrane and 25% was associated with vacuolar structures, whereas in cells treated for 5 and 7 d the majority of spectrin (57-61%) was located in the vacuolar areas.

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.

Studies on the fluorescence labeling of human red blood cell membrane ghosts with 4'-(9-acridinylamino)methanesulfon-m-anisidide

4'-(9-Acridinylamino)methanesulfon-m-anisidide (mAMSA) interacts with red cell membranes, resulting in the formation of fluorescent protein adducts. The mAMSA-membrane protein adducts exhibited an emission fluorescence maximum at 445 nm, with two shoulders at approximately 425 and 470 nm. The major labeled proteins were identified as spectrins 1 and 2 and bands 3, 4.1, 4.2 and 5. The fluorescence intensity increased with increasing mAMSA concentrations (0.03 to 1.5 mM), time (15-120 min), and temperature of the reaction. Results from sodium dodecyl sulfate gel electrophoresis show that mAMSA caused no detectable change in the molecular weight of membrane proteins. This indicates that mAMSA is a monofunctional, noncrosslinking agent. Other acridine analogs, 9-aminoacridine and acridine, did not fluorescently label membrane proteins, suggesting that the presence of the acridine nucleus is not sufficient for labeling. Addition of 2-mercaptoethanol to the mAMSA-membrane reaction mixtures reversed the fluorescence labeling. Furthermore, pretreatment of membrane proteins with N-ethylmaleimide or iodoacetamide prevented the formation of fluorescent mAMSA-membrane protein adducts. These data suggest that mAMSA interacts with sulfhydryl groups of the membrane proteins. When the membrane sulfhydryl groups were assayed by labeling with N-[ethyl-2-3H]ethylmaleimide, it was shown that the accessible membrane sulfhydryl groups were reduced after the mAMSA treatment. The above results suggest that mAMSA covalently binds to the sulfhydryl groups in the red cell membrane, with the production of fluorescent mAMSA-protein adducts.

Olfactory discrimination learning is blocked by leupeptin, a thiol protease inhibitor

Rats were trained on successive two-odor discriminations with the cues randomly located in an 8-arm radial maze. After several days of training using different odor pairs, the thiol protease inhibitor leupeptin was infused into the ventricles and testing continued. Leupeptin caused a pronounced, dose-dependent and reversible deficit in performance in this task. Previous studies have shown that these drug concentrations do not influence spontaneous activity, feeding and drinking, or the acquisition and retention of avoidance conditioning. The results are interpreted as supporting the hypothesis that a calcium-sensitive proteinase is involved in certain forms of memory that require modification of telencephalic circuitries.

Effects of monovalent cations on red cell shape and size

Human erythrocytes were incubated in isotonic solutions of different monovalent cations. The apparent size of the red cells measured on scanning electron microscopic pictures decreases in the order Li+ greater than Na+ = K+ greater than Rb+. These differences in size are abolished after pretreatment with trypsin, which removes a large part of the charges associated with membrane glycoproteins. Shape alterations are also observed. Normal biconcave shapes are visible after Na+ or K+ incubation, whereas Li+ leads to flabby, flattened cells with a certain tendency to crenation, and Rb+ causes more pronounced biconcavity with a certain tendency to cupping. The overall effects of pretreatment with trypsin are similar to those of Li+. Our results provide evidence that the electrostatic repulsion of glycoproteins and other charged membrane components may play an essential role in maintaining red cell shape.

Interaction between the subunits of human erythrocyte spectrin using a fluorescence probe

Fluorescence labeling of spectrin subunits was performed with N-(1-anilinonaphthyl-4)maleimide (ANM) to study the interaction between alpha and beta subunits. The fluorescence anisotropy of both ANM alpha and ANM beta increased linearly with the addition of nonfluorescent beta or alpha subunit, and saturated at a protein ratio about 1, indicating that 1 mol alpha subunit binds to 1 mol beta subunit with high affinity in vitro. Furthermore, this binding seemed to be reversible, because the anisotropy value decreased when an excess fo nonfluorescent alpha was added to the ANM alpha/beta mixture. The anisotropy of ANM alpha attained a maximum level within l min after addition of the same quantity of nonfluorescent beta at 12 degrees C, and the anisotropy of this mixture decreased rapidly when an excess of nonfluorescent alpha was added. These findings suggested that both the binding process of beta to ANM alpha and the dissociation step of ANM alpha from the ANM alpha-beta complex were quite rapid. The results obtained here imply that dynamic interaction between alpha and beta subunits of spectrin should be taken into account in understanding the role of the spectrin molecule in the cytoskeletal mesh.

Sequence comparison of human and murine erythrocyte alpha-spectrin cDNA

The results of hybridization analyses using cDNA probes for mouse and human alpha-spectrin mRNA indicate that a single gene encodes the alpha-subunit of erythrocyte spectrin. Sequencing of the cDNA clones showed that they code for 370 amino acids (aa) covering three repeat domains close to the C terminus of alpha-spectrin. The cloned cDNAs will now permit the isolation of the alpha-spectrin gene and should lead to the characterization of the genetic aspects in human hereditary anemias in which alpha-spectrin has been characterized as the site of the molecular defect.

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.

The biochemistry of memory: a new and specific hypothesis

Recent studies have uncovered a synaptic process with properties required for an intermediate step in memory storage. Calcium rapidly and irreversibly increases the number of receptors for glutamate (a probable neurotransmitter) in forebrain synaptic membranes by activating a proteinase (calpain) that degrades fodrin, a spectrin-like protein. This process provides a means through which physiological activity could produce long-lasting changes in synaptic chemistry and ultrastructure. Since the process is only poorly represented in the brain stem, it is hypothesized to be responsible for those forms of memory localized in the telencephalon.

Loss of adhesion of murine erythroleukemia cells to fibronectin during erythroid differentiation

Uninduced murine erythroleukemia cells specifically attached to fibronectin-coated dishes but not to dishes coated with laminin or type I or IV collagen. Dimethyl sulfoxide-induced differentiation of these cells caused a dramatic decrease in adhesion to fibronectin that was correlated with synthesis of the erythrocyte glycoprotein "band III," a membrane marker of the differentiated erythrocyte. Loss or modification of fibronectin binding sites on the cell surface during erythroid differentiation may cause the release of reticulocytes from the interstitial matrix of bone marrow into the blood.

A high molecular weight antigen in Plasmodium falciparum recognized by inhibitory monoclonal antibodies

Inhibitory monoclonal antibodies which bind to some isolates of Plasmodium falciparum from Papua New Guinea, but not from other areas, bound to a 220 kD antigen. By immunofluorescence microscopy this antigen was shown to be located both within the schizont cytoplasm and also within the schizont infected erythrocyte, but external to the schizont itself. Even at antibody concentrations which caused greater than 70% inhibition of parasite multiplication, accumulation of schizont stages or aggregates of merozoites were not seen, consistent with inhibition occurring at a point after the release of merozoites. While this suggests that the antigen may be present on merozoites, the quantity was below the limit of detection. It is suggested that the large amount of antigen released by rupturing schizonts may be a mechanism used by the parasite to evade immunological attack.

Structural and functional aspects of domain motions in proteins

Three distinct categories of large-scale flexibility in proteins have been documented by single-crystal X-ray diffraction studies: the relatively free movement of essentially rigid globular domains that are connected by a flexible segment of polypeptide, the reorientation of essentially rigid domains among a few distinct conformations, and the concerted transition of a contiguous region of the surface of a protein from a disordered state to an ordered state. In a number of examples, well-defined functions can be assigned to these large-scale structural changes. The occurrence of such motions in proteins of known structure is reviewed, and the best-studied examples are discussed in detail to allow a critical evaluation of the methods used to identify and study these motions.

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.

The cell surface and its metabolism

The cell surface structure is highly dynamic. In particular, binding of ligand induces the redistribution of receptors on the cell surface as well as the internalisation of ligand-receptor complexes. Internalisation in turn leads to a recycling of the receptor or to a decrease in the cell's responsiveness to the ligand. Modulation of the cell surface structure is apparently regulated intracellularly by components of the cell's cytoskeleton. A crucial component in this respect is likely to be a sub-membranous filamentous network that is linked directly to the cytoplasmic face of the surface membrane. In erythrocytes this network can be separated from purified preparations of the plasma membrane by virtue of its insolubility in nonionic detergents. Application of this procedure to the plasma membrane fraction of human B lymphoblastoid cells has yielded a detergent-insoluble residue comprising actin and a 68,000-Mr polypeptide as major components, together with polypeptides of 28,000-, 33,000- and 120,000-Mr as prominent but more minor components. The association of the 68,000-Mr protein with the detergent-insoluble residue and the original plasma membrane is Ca2+-dependent. Burkitt lymphoma cells differ noticeably from lymphoblastoid cells in that the 68,000-Mr protein is not associated with the inner face of the surface membrane. This difference may reflect the malignant phenotype of Burkitt lymphomas or the hypothetical sub-population of normal B lymphocytes from which the lymphomas are derived.

alpha m-Crystallin: the native form of the protein?

Evidence is presented that alpha-crystallin isolated at 37 degrees C exists as a species, alpha m, which has a minimum molecular weight of about 320000 and a sedimentation coefficient of about 12 S. The amino acid composition, subunit distribution, near- and far-UV CD spectra and immunochemical properties were identical to those of the previously studied, 19 S protein, alpha c-crystallin (minimum molecular weight, 635000). It was demonstrated that only alpha m-crystallin was present in 37 degrees C lens extracts and that cooling of lenses or extracts resulted in a conversion of alpha m- to alpha c-crystallin. This conversion appears to be a general phenomenon, independent of age or species. It was concluded that alpha c-crystallin is an aggregate, produced by cooling, and that alpha m-crystallin is more likely to represent the in vivo form of the protein.

Tannic acid improves the visualization of the human erythrocyte membrane skeleton by freeze-etching

The effect of fixatives on the membrane skeleton underlying the human erythrocyte membrane was examined by freeze-etching. An anastomosing fibrillar network was readily observed on the protoplasmic surface of the erythrocyte membrane treated with tannic acid. Such structure was much less defined in unfixed membrane or membrane fixed with glutaraldehyde or glutaraldehyde followed by osmium tetraoxide. Tannic acid caused a marked increase in diameter of the fibrillar components of the membrane skeleton and of the protoplasmic surface particles of inside-out vesicles prepared by alkali treatment but did not affect the size of intramembranous particles seen on fracture faces nor the appearance of exoplasmic surfaces. The improved visualization of the membrane skeleton after treatment with tannic acid resulted from interactions between tannic acid and exposed membrane proteins.

Formation of aqueous pores in the human erythrocyte membrane after oxidative cross-linking of spectrin by diamide

Oxidation of erythrocyte membrane SH-groups by diamide and tetrathionate induces cross-linking of spectrin (Haest, C.W.M., Kamp, D., Plasa, G. and Deuticke, B. (1977) Biochim. Biophys. Acta 469, 226-230). This cross-linking was now shown to go along with a concentration- and time-dependent enhancement of membrane permeability for hydrophilic nonelectrolytes and ions. The enhancement is specific for oxidative SH-group modifications, is reversible by reduction of the induced disulfides, can be suppressed by a very brief pre-treatment of the cells with low concentrations of N-ethylmaleimide and is strongly temperature-dependent. The pathway of the induced permeability discriminates nonelectrolytes on the basis of molecular size and exhibits a very low activation energy (Ea 3-8 kcal/mol). These findings are reconcilable with the formation of a somewhat inhomogeneous population of aqueous pores with radii probably less than or equal to 0.65 nm. Estimated pore numbers vary with the size of the probe molecule. Assuming a diffusion coefficient as in bulk water within the pore, at least 20 pores per cell have to be postulated; more realistic lower diffusion coefficients increase that number. Alterations of the lipid domain by changes of cholesterol contents and insertion of hexanol or nonionic detergents alter the number or size of the pores. Since aggregation of skeletal and intrinsic membrane proteins also occurs after the SH-oxidation, in parallel to the formation of membrane leaks, one may consider (a) defects in the disturbed bilayer interface, (b) a mismatch between lipid and intrinsic proteins or (c) channels in between aggregated intrinsic proteins as structures forming the pores induced by diamide treatment.

Molecular and functional changes in spectrin from patients with hereditary pyropoikilocytosis

The structural and functional properties of spectrin from normal and hereditary pyropoikilocytosis (HPP) donors from the two unrelated families were studied. The structural domains of the spectrin molecule were generated by mild tryptic digestion and analyzed by two-dimensional electrophoresis (isoelectric focusing; sodium dodecyl sulfate-polyacrylamide gel electrophoresis). The alpha I-T80 peptide (Mr 80,000) is not detectable in two related HPP donors; instead, two new peptides (Mr 50,000 and 21,000) are generated and have been identified as fragments of the normal alpha I-T80. A third sibling has reduced levels of both the normal alpha I-T80 and the two new peptides. A similar analysis of spectrin from another HPP family indicates that their spectrins contain reduced amounts of the alpha I-T80 and the 50,000 and 21,000 fragments of the alpha I domain. The HPP donor also has other structural variations in the alpha I, alpha II, and alpha III domains. The alpha I-T80 domain of normal spectrin has been shown to be an important site for spectrin oligomerization (J. Morrow and V.T. Marchesi. 1981. J. Cell Biol. 88: 463-468), and in vitro assays indicate that HPP spectrin has an impaired ability to oligomerize. Ghost membranes from HPP donors are also more fragile than membranes from normal erythrocytes when measured by ektacytometry. In both the oligomerization and fragility assays, the degree of impairment is correlated with the amount of normal alpha I-T80 present in the spectrin molecule. We believe that a structural alteration in the alpha I-T80 domain perturbs normal in vivo oligomerization of spectrin, producing a marked decrease in erythrocyte stability.

Selective phenylglyoxalation of functionally essential arginyl residues in the erythrocyte anion transport protein

The red cell anion transport protein, band 3, can be selectively modified with phenylglyoxal, which modifies arginyl residues (arg) in proteins, usually with a phenylglyoxal: arg stoichiometry of 2:1. Indiscriminate modification of all arg in red cell membrane proteins occurred rapidly when both extra- and intracellular pH were above 10. Selective modification of extracellularly exposed arg was achieved when ghosts with a neutral or acid intracellular pH were treated with phenylglyoxal in an alkaline medium. The rate and specificity of modification depend on the extracellular chloride concentration. At 165 mM chloride maximum transport inactivation was accompanied by the binding of four phenylglyoxals per band 3 molecule. After removal of extracellular chloride, maximum transport inhibition was accompanied by the incorporation of two phenylglyoxals per band 3, which suggests that transport function is inactivated by the modification of a single arg. After cleavage of band 3 with extracellular chymotrypsin, [14C]phenylglyoxal was located almost exclusively in a 35,000-dalton peptide. In contrast, the primary covalent binding site of the isothiocyanostilbenedisulfonates is a lysyl residue in the second cleavage product, a 65,000-dalton fragment. This finding supports the view that the transport region of band 3 is composed of strands from both chymotryptic fragments. The binding of phenylglyoxal and the stilbene inhibitors interfered with each other. The rate of phenylglyoxal binding was reduced by a reversibly binding stilbenedisulfonate (DNDS), and covalent binding of [3H]DIDS to phenylglyoxal-modified membranes was strongly delayed. At DIDS concentrations below 10 10 micrometers, only 50% of the band 3 molecules were labeled with [3H]-DIDS during 90 min at 38 degrees C, thereby demonstrating an interaction between binding of the two inhibitors to the protomers of the oligomeric band 3 molecules.

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.

Immunolocalization of a novel, cytoskeleton-associated polypeptide of Mr 230,000 daltons (p230)

Antibodies were raised against a cytoskeleton-associated, nonphosphorylated, 230,000-dalton bovine lens polypeptide (designated p230), and rendered monospecific by using a novel immunoaffinity technique. In immunofluorescence and electron microscopy of cultured fibroblasts, as well as of various other cells (endothelial, epithelial, lenticular, monocytes, neuroblastoma cells) and tissues (human kidney and liver), p230 was localized as a distinct subplasmalemmal layer in the peripheral cytoplasm of the cells. It constituted less than 0.3% of the total cellular protein in cultured fibroblasts and was not extractable with Triton X-100. In detergent-extracted cytoskeletal preparations of cultured fibroblasts, p230 remained as an elaborate peripheral network that showed a distribution distinctly different from that of the major cytoskeletal structures, stress fibers, cortical myosin, vinculin, and intermediate filaments (IF). The distribution was not dependent on the presence of intact stress fibers or microtubules, as shown by double-fluorescence microscopy of cells exposed to cytochalasin B or cultured in the presence of monensin and of cold-treated cells. Upon demecolcine-induced reorganization of intermediate filaments, however, the localization of p230 was rapidly altered to a dense plaque underneath the perinuclear aggregate of intermediate filaments. On the other hand, p230 seemed to colocalize with the detergent-resistant cell surface lamina, visualized in fluorescence microscopy with fluorochrome-coupled wheat germ agglutinin-lectin. The results suggest that p230 is part of a cell surface- and cytoskeleton-associated subplasmalemmal structure that may play an important role in cell surface-cytoskeleton interaction in various cells both in vitro and in vivo.