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

Expression of the beta subunit of spectrin in nonerythroid cells

Antibodies raised against electrophoretically purified chicken erythrocyte beta subunit of spectrin, called "beta-spectrin," have been used to demonstrate the presence of an immunoreactive form of this polypeptide in nonerythroid tissues. Immunoautoradiography shows that, in chicken erythrocytes, this antiserum reacts with beta-spectrin (Mr 220,000) and another polypeptide (Mr 230,000) that, by two-dimensional tryptic peptide analysis, shows extensive homology with beta-spectrin but not with the alpha subunit of spectrin, called "alpha-spectrin." Immunoautoradiography and immunoprecipitation of various chicken tissues with this antiserum shows that either one variant or both variants of beta-spectrin are expressed. Indirect immunofluorescence reveals that the antiserum reacts with a plasma membrane-associated component of erythroid and some nonerythroid cells. Particularly strong fluorescence is observed in skeletal and cardiac muscle cells where beta-spectrin appears to form a grid-like network along the inner surface of the sarcolemma. The noncoordinated distribution of alpha- and beta-spectrin variants indicates that their expression may be tailored to the functional requirements of the plasma membrane in different cells.

Isolation and structural properties of a high-molecular-weight actin-binding protein (filamin-like protein) in hog thyroid gland

A high-molecular-weight protein has been isolated from hog thyroid gland. This protein, with a molecular weight of 475,000 determined by ultracentrifugation and gel filtration, is a complex of two polypeptides with apparent molecular weights of 250,000 and 240,000. It may be related to filamin-like proteins by its physicochemical properties and its immunogenic cross-reactivity towards gizzard filamin antibodies. Furthermore it interacts with F-actin in a stoichiometry of 1 mol of high-molecular-weight protein/approximately 12-14 mol actin monomer allowing microfilament association, as shown by electron microscopy.

Actin polymerization induced by calspectin, a calmodulin-binding spectrin-like protein

We have purified from a membrane fraction of bovine brain a calmodulin-binding protein (calspectin) that shares a number of properties with erythrocyte spectrin: It has a heterodimeric structure with Mr 240 000 and 235 000 and binds to (dimeric form) or crosslinks (tetrameric form) F-actin. We show that calspectin (tetramer) is capable of inducing the polymerization of G-actin to actin filaments by increasing nucleation under conditions where actin alone polymerizes at a much slower rate. Thus, brain calspectin behaves in the same manner as erythrocyte spectrin, supporting the idea that, in conjunction with actin oligomers it comprises the cytoskeletal meshwork underlying the cytoplasmic surface of the nerve cell.

Complete exchange of phosphatidylcholine from intact erythrocytes after protein crosslinking

Treatment of human erythrocytes with tetrathionate or diamide resulted in extensive crosslinking of membranous and cytoskeletal proteins. Such treatment was followed by an incubation with phosphatidylcholine specific exchange protein to investigate the rate and extent of exchange of phosphatidylcholine between the erythrocytes and 14C-labeled phosphatidylcholine containing microsomal membranes or vesicles. Exchange profiles showed that the exchange of phosphatidylcholine is facilitated in treated cells when compared to control erythrocytes and, more importantly, that all of the phosphatidylcholine is exchangeable after protein crosslinking whereas in control cells only the phosphatidylcholine pool located in the outer layer of the membrane is exchangeable. These observations demonstrate that crosslinking of cytoskeletal and membraneous proteins enhances the rate of transbilayer movement of phosphatidylcholine considerably.

Calmodulin-dependent spectrin kinase activity in resealed human erythrocyte ghosts

Membrane protein phosphorylation has been studied in resealed human erythrocyte ghosts by measuring the incorporation of 32P into spectrin and band 3. Norepinephrine- and Ca2+-stimulated phosphate incorporation was diminished in ghosts depleted of calmodulin. Ghosts prepared with endogenous calmodulin showed Ca2+- and norepinephrine-stimulated protein phosphorylation only when the ghosts had been resealed in the presence of (gamma-32P)ATP. Ghosts resealed with or without calmodulin in the presence of unlabeled ATP showed no net gain or loss of 32P when exposed to norepinephrine or a Ca2+-specific ionophore. These observations suggest that Ca2+ and norepinephrine stimulation of membrane protein phosphorylation is mediated by calmodulin-dependent spectrin kinase activity, and not by increased turnover of spectrin ATPase or by inhibition of phosphospectrin phosphatase.

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.

Spectrin domains: proteolytic susceptibility as a probe of protein structure

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

Mapping functional sites on biological macromolecules

Electron microscopy of macromolecules dried from glycerol and rotary shadowed from a low angle can reveal the structure of individual molecules, or groups of molecules, with remarkable clarity. We used this technique to examine the interaction of the red blood cell cytoskeletal proteins spectrin, a 500,000 dalton protein which is long (750 A) and flexible;actin, a 43,000 dalton protein capable of polymerizing into double helical filaments; and band 4.1, an 82,000 dalton globular protein. By examining binary and ternary complexes of these molecules, the binding sites for actin, band 4.1 and a fourth protein ankyrin, which links the cytoskeleton to the membrane, have been mapped along the length of the spectrin molecule. These findings, which have enabled us to construct a model of the red cell cytoskeleton, show that low angle shadowing is a powerful but simple method for investigating associations among macromolecules.

Consequences of alkaline treatment for the ultrastructure of the acetylcholine-receptor-rich membranes from Torpedo marmorata electric organ

After fixation with glutaraldehyde and impregnation with tannic acid, the membrane that underlies the nerve terminals in Torpedo marmorata electroplaque presents a typical asymmetric triple-layered structure with an unusual thickness; in addition, it is coated with electron-dense material on its inner, cytoplasmic face. Filamentous structures are frequently found attached to these "subsynaptic densities." The organization of the subsynaptic membrane is partly preserved after homogenization of the electric organ and purification of acetylcholine-receptor (AchR)-rich membrane fragments. In vitro treatment at pH 11 and 4 degrees C of these AchR-rich membranes releases an extrinsic protein of 43,000 mol wt and at the same time causes the complete disappearance of the cytoplasmic condensations. Freeze-etching of native membrane fragments discloses remnants of the ribbonlike organization of the AchR rosettes. This organization disappears ater alkaline treatment and is replaced by a network which is not observed after rapid freezing and, therefore, most likely results from the lateral redistribution of the AchR rosettes during condition of slow freezing. A dispersion of the AchR rosettes in the plane of the membrane also occurs after fusion of the pH 11-treated fragments with phospholipid vesicles. These results are interpreted in terms of a structural stabilization and immobilization of the AchR by the 43,000-Mr protein binding to the inner face of the subsynaptic membrane.

Electron microscopic study of reassociation of spectrin and actin with the human erythrocyte membrane

Reassociation of spectrin and actin with human erythrocyte membranes was studied by stereoscopic electron microscopy of thin sections combined with tannic acid- glutaraldehyde fixation. Treatment of the erythrocyte membrane with 0.1 mM EDTA (pH 8.0) extracted more than 90 percent of the spectrin and actin and concomitantly removed filamentous meshworks underlying the membranes, followed by fragmentation into small inside-out vesicles. When such spectrin-depleted vesicles were incubated with the EDTA extract (crude spectrin), a filamentous meshwork, similar to those of the original membranes, was reformed on the cytoplasmic surface of the vesicles. The filamentous components, with a uniform thickness of 9 nm, took a tortuous course and joined one another often in an end-to-end fashion to form a irregular but continuous meshwork parallel to the membrane. Purified spectrin was also reassociated with the vesicles in a population density of filamentous components almost comparable to that of the crude spectrin-reassociated vesicles. However, the meshwork formation was much smaller in extent, showing many independent filamentous components closely applied to the vesicle surface. When muscle G-actin was added to the crude spectrin- or purified spectrin- reassociated vesicles under conditions which favor actin polymerization, actin filaments were seen to attach to the vesicles through the filamentous components. Two modes of association of actin filaments with the membrane were seen: end-to-membrane and side-to- membrane associations. In the end-to-membrane association, each actin filament was bound with several filamentous components exhibiting a spiderlike configuration, which was considered to be the unit of the filamentous meshwork of the original erythrocyte membrane.

Self-assembly of spectrin oligomers in vitro: a basis for a dynamic cytoskeleton

Purified human erythrocyte spectrin is able to form large oligomeric species without the collaboration of any other proteins. This reversible self-assembly process is both temperature and concentration dependent and seems to be mediated by the same kinds of low affinity noncovalent associations between spectrin monomers that promote tetramer formation. Low ionic strength extracts of erythrocyte membranes also contain these oligomeric species. These results support the idea that spectrin oligomers and the factors that regulate their formation may be responsible for both the stability and the versatility of the erythrocyte membrane cytoskeleton. It is postulated that the high concentrations of spectrin necessary for oligomerization are maintained in vivo by a high-affinity interaction with ankyrin. Such a coupling of high and low affinity interactions in multifunctional proteins may have significant implications for membrane structure and function.

Spectrin oligomers: a structural feature of the erythrocyte cytoskeleton

Spectrin reversibly self-associates to high molecular weight oligomers through a concentration-driven process characterized by association constants of about 10(5) mol(-1). This association if prominent under physiological conditions of pH, ionic strength, and temperature. It is disrupted by urea, but not Triton X-100. The process of spectrin association appears mathematically to resemble that for tropomyosin, although the mechanism is probably different. Spectrin association is weak compared to other prominent protein-protein associations in the red cell membrane skeleton. The linkage of these weak and strong associations suggests a process whereby the membrane skeleton spontaneously assembles. Such affinity-modulated assembly involving weak associations is likely to abe the focus of numerous membrane.

Identification of proteolytically resistant domains of human erythrocyte spectrin

Digestion of purified human erthrocyte spectrin with proteolytic enzymes at 0 degrees C results in the production of intermediate-size peptides that resist further cleavage at 0 degrees C. By two-dimensional peptide analysis of these intermediate peptides it has been determined that five unique peptides are produced by tryptic cleavage of the alpha subunit of spectrin (band 1); these have apparent molecular weights of 80,000, 46,000, 46,000, 41,000, and 30,000 and account for 97% of the alpha subunit. Similarly, four unique peptides having apparent molecular weights of 74,000, 65,000, 33,000, and 38,000 account for 90% of the beta subunit (band 2). By examining larger peptide fragments, the linear alignment of the unique peptides along each of the spectrin subunits has been established. These results indicate that spectrin is composed of two nonidentical subunits, each containing multiple proteolytically resistant domains. These domains, which may be largely alpha-helical, seem to be connected by small protease-sensitive segments. The proteolytic resistance of these domains is not influenced by the multimeric state of the spectrin molecule.

Expression of red cell membrane proteins in erythroid precursor cells

Specific antibodies to human glycophorin A and spectrin were used to study the expression of these membrane proteins in normal and pathologic human bone marrow. In immunofluorescence experiments spectrin and glycophorin A are found in 50-60% of the nucleated cells in normal bone marrow. These two proteins are expressed at all stages of red cell differentiation and can be traced at least to the earliest morphologically recognizable nucleated red cell precursor, the proerythroblast; the two proteins are specific for cells of the red cell series and are not found to be expressed in lymphocytic, granulocytic cells or platelets. These conclusions were drawn from studies on bone marrow in patients with a temporary block in erythropoiesis at the level of stem cells or of the pronormoblast. Bone marrow from these individuals either lacked all nucleated cells stainable for glycophorin A and spectrin or contained only pronormoblasts. Similar findings were obtained on spleen cells from mice which were made severely anemic by multiple injections with N-acetyl-phenylhydrazine. Antibodies to a sialoglycoprotein isolated from mouse red cell membranes stain 70-80% of all cells in the spleen of anemic animals, while only 1-2% of such cells are seen in the spleen of normal animals. Spectrin and glycophorin A could be labeled metabolically and isolated using specific antibodies. The human tumor cell line K562 expresses both membrane proteins, but induction experiments with various agents thus far have failed to change their expression.