Spectrin promotes the association of F-actin with the cytoplasmic surface of the human erythrocyte membrane

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

5-fluorouracil interferes with actin organization, stress fiber formation and cell migration in corneal endothelial cells during wound repair along the natural basement membrane

Corneal endothelial cells respond to a circular freeze wound by undergoing actin cytoskeletal reorganization that is mainly characterized by the disappearance of circumferential microfilament bundles (CMBs) and the subsequent appearance of distinct stress fibers. This cytoskeletal rearrangement is associated with changes in cell shape as migrating cells lose their polyhedral appearance, spread out, and assume a stellate morphology with cell processes extending outward into the injured area. We report here that in the presence of low concentrations (0.01-0.l mM) of the anti-metabolite 5-fluorouracil (5-FU), characteristic actin organization becomes disrupted and migrating cells do not display elongated processes typical of control tissues and translocation into the injury zone is retarded, but not inhibited. Rhodamine phalloidin staining revealed no evidence of stress fiber formation. A higher concentration of 5-FU (1.0 mM) not only prevented formation of discernible stress fibers but also resulted in a more restricted cell movement during wound repair. That this was not a cytotoxic effect was demonstrated by transferring tissues back into standard medium allowing endothelia to reinitiate migration and undergo complete wound healing by 72 h post-transfer. Overnight incubation of endothelia in 4 muM phallacidin resulted in limited CMB disruption the extent of which was dependent on the 5-FU concentration. The effects of 5-FU on the actin cytoskeleton are reversible and by 24 h after placing treated endothelia into medium without 5-FU, actin begins to become re-established and by 48 h microfilament patterns in the tissue resemble those of non-treated endothelia. Similarly, when non-injured tissues are cultured in the presence of 5-FU for 24 h, subsequently injured and returned to standard medium, they exhibit no stress fibers when observed at 24 h post-wounding. However, by 48 h post-injury these cells now display stress fibers and extend processes into the wound area. Biochemical studies on isolated muscle actin demonstrated that actin polymerization is unaffected in the presence of either 0.01 or 1 mM 5-FU as determined by the F-actin sedimentation and falling ball viscosity techniques. Thus, the mechanism(s) by which 5-FU exerts its actions on the actin cytoskeleton appears to be one of an indirect nature.

F-actin and myosin II binding domains in supervillin

Detergent-resistant membranes contain signaling and integral membrane proteins that organize cholesterol-rich domains called lipid rafts. A subset of these detergent-resistant membranes (DRM-H) exhibits a higher buoyant density ( approximately 1.16 g/ml) because of association with membrane skeleton proteins, including actin, myosin II, myosin 1G, fodrin, and an actin- and membrane-binding protein called supervillin (Nebl, T., Pestonjamasp, K. N., Leszyk, J. D., Crowley, J. L., Oh, S. W., and Luna, E. J. (2002) J. Biol. Chem. 277, 43399-43409). To characterize interactions among DRM-H cytoskeletal proteins, we investigated the binding partners of the novel supervillin N terminus, specifically amino acids 1-830. We find that the supervillin N terminus binds directly to myosin II, as well as to F-actin. Three F-actin-binding sites were mapped to sequences within amino acids approximately 280-342, approximately 344-422, and approximately 700-830. Sequences with combinations of these sites promote F-actin cross-linking and/or bundling. Supervillin amino acids 1-174 specifically interact with the S2 domain in chicken gizzard myosin and nonmuscle myosin IIA (MYH-9) but exhibit little binding to skeletal muscle myosin II. Direct or indirect binding to filamin also was observed. Overexpression of supervillin amino acids 1-174 in COS7 cells disrupted the localization of myosin IIB without obviously affecting actin filaments. Taken together, these results suggest that supervillin may mediate actin and myosin II filament organization at cholesterol-rich membrane domains.

Binding of actin to liver cell membranes: the state of membrane-bound actin

Previous work has shown that actin binds specifically and saturably to liver membranes stripped of endogenous actin (Tranter, M. P., S. P. Sugrue, and M. A. Schwartz. 1989. J. Cell Biol. 109:2833-2840). Scatchard plots of equilibrium binding data were linear, indicating that binding is not cooperative, as would be expected for F- or G-actin. To determine the state of membrane-bound actin, we have analyzed the binding of F- and G-actin to liver cell membranes. G-actin in low salt depolymerization buffer and EF-actin, a derivative that polymerizes very poorly in solution, bind to liver cell membranes as well as untreated actin in polymerization buffer. Phalloidin-stabilized F-actin binds, but to a lesser extent. The binding of F- and G-actins are mutually competitive and are inhibited by ATP, suggesting that both forms of actin bind to the same sites. For untreated actin in polymerization buffer, the time course of binding is biphasic, with an initial rapid component which is followed by a plateau phase, then a second, slower component. The binding kinetics of pure F-actin and pure G-actin are both monophasic and match the fast and slower components, respectively, of untreated actin. In the reconstituted system, membrane-bound actin does not stain with rhodamine-phalloidin, nor are actin filaments detected by EM. Distinct regions of amorphous material, however, are visible, which stain with an anti-actin antibody. The exact nature of this material has yet to be determined. A model of actin binding is presented.

Radiolabel-transfer cross-linking demonstrates that protein 4.1 binds to the N-terminal region of beta spectrin and to actin in binary interactions

Erythrocyte protein 4.1 plays a major role in stabilizing the spectrin-actin junction of the erythrocyte membrane skeleton. The particular sites on spectrin responsible for the binding of actin and protein 4.1 have not been specifically defined, although the general region of the 'tail' end, opposite the self-association site, has been deduced by electron microscopy. Using a photoactivatable, radiolabel-transfer cross-linker, 1-[N-(2-hydroxy-5-azidobenzoyl)-2-aminoethyl]-4-(N-hydroxysuccinimidyl)- succinate, we have determined that the binding site for protein 4.1 on spectrin resides in the N-terminal region of beta spectrin within a sequence homologous to the actin-binding region of alpha actinin. Moreover, this technique provided clear evidence for a direct binding interaction between actin filaments and protein 4.1 that was confirmed by rapid-sedimentation assays. In summary, use of radiolabel-transfer cross-linking has enabled assignment of the protein-4.1-binding site on erythrocyte spectrin and has identified a previously ill-defined binary interaction between protein 4.1 and F-actin.

Identification of an unusual deletion within homologous repeats of human reticulocyte beta-spectrin and probable peptide polymorphism

We screened two different human reticulocyte cDNA libraries with beta-spectrin(beta Sp)-specific polyclonal antibodies and with our original radiolabeled human BSP cDNA probe (encoding beta Sp). Of the 20 independent clones, the largest had about a 2.5-kb insert corresponding to the deduced amino acid (aa) sequence of the beta-7 to beta-14 repetitive segments. Among these segments, segment 12 was 7 aa shorter than the other repetitive segments. We showed that this truncation was not a result of (i) cloning artifact, (ii) alternate splicing, or (iii) common genomic polymorphism by additional examination of 14 individual human chromosomes. Recently, another laboratory described the BSP nucleotide (nt) sequence overlapping partially with our sequence. These overlapping sequences were homologous with the exception of two nt differences at the positions 1342 and 1514. The discrepancy at nt 1342 changes the His to Arg. This newly derived probe has been used to find an additional example of BSP restriction fragment length polymorphism.

The spectrin-actin junction of erythrocyte membrane skeletons

High-resolution electron microscopy of erythrocyte membrane skeletons has provided striking images of a regular lattice-like organization with five or six spectrin molecules attached to short actin filaments to form a sheet of five- and six-sided polygons. Visualization of the membrane skeletons has focused attention on the (spectrin)5,6-actin oligomers, which form the vertices of the polygons, as basic structural units of the lattice. Membrane skeletons and isolated junctional complexes contain four proteins that are stable components of this structure in the following ratios: 1 mol of spectrin dimer, 2-3 mol of actin, 1 mol of protein 4.1 and 0.1-0.5 mol of protein 4.9 (numbers refer to mobility on SDS gels). Additional proteins have been identified that are candidates to interact with the junction, based on in vitro assays, although they have not yet been localized to this structure and include: tropomyosin, tropomyosin-binding protein and adducin. The spectrin-actin complex with its associated proteins has a key structural role in mediating cross-linking of spectrin into the network of the membrane skeleton, and is a potential site for regulation of membrane properties. The purpose of this article is to review properties of known and potential constituent proteins of the spectrin-actin junction, regulation of their interactions, the role of junction proteins in erythrocyte membrane dysfunction, and to consider aspects of assembly of the junctions.

How actin binds and assembles onto plasma membranes from Dictyostelium discoideum

We have shown previously (Schwartz, M. A., and E. J. Luna. 1986. J. Cell Biol. 102: 2067-2075) that actin binds with positive cooperativity to plasma membranes from Dictyostelium discoideum. Actin is polymerized at the membrane surface even at concentrations well below the critical concentration for polymerization in solution. Low salt buffer that blocks actin polymerization in solution also prevents actin binding to membranes. To further explore the relationship between actin polymerization and binding to membranes, we prepared four chemically modified actins that appear to be incapable of polymerizing in solution. Three of these derivatives also lost their ability to bind to membranes. The fourth derivative (EF actin), in which histidine-40 is labeled with ethoxyformic anhydride, binds to membranes with reduced affinity. Binding curves exhibit positive cooperativity, and cross-linking experiments show that membrane-bound actin is multimeric. Thus, binding and polymerization are tightly coupled, and the ability of these membranes to polymerize actin is dramatically demonstrated. EF actin coassembles weakly with untreated actin in solution, but coassembles well on membranes. Binding by untreated actin and EF actin are mutually competitive, indicating that they bind to the same membrane sites. Hill plots indicate that an actin trimer is the minimum assembly state required for tight binding to membranes. The best explanation for our data is a model in which actin oligomers assemble by binding to clustered membrane sites with successive monomers on one side of the actin filament bound to the membrane. Individual binding affinities are expected to be low, but the overall actin-membrane avidity is high, due to multivalency. Our results imply that extracellular factors that cluster membrane proteins may create sites for the formation of actin nuclei and thus trigger actin polymerization in the cell.

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.

Gelsolin is expressed in early erythroid progenitor cells and negatively regulated during erythropoiesis

We have identified an approximately 85-kD protein in chicken erythrocytes which is immunologically, structurally, and functionally related to the gelsolin found in many muscle and nonmuscle cell types. Cell fractionation reveals a Ca2+-dependent partitioning of gelsolin into the soluble cytoplasm and the membrane-associated cytoskeleton of differentiating or mature erythrocytes. Depending on either the presence of Ca2+ during cell lysis or on the preincubation of the intact cells with the Ca2+-ionophore A23187, up to 40% of the total cellular gelsolin is found associated with the membrane skeleton. Expression of gelsolin shows a strong negative regulation during erythroid differentiation. From quantitations of its steady-state molar ratio to actin, gelsolin is abundant in early progenitor cells as revealed from avian erythroblastosis virus- and S13 virus-transformed cells which are arrested at the colony forming unit erythroid (CFU-e) stage of erythroid development. In these cells, which have a rudimentary and unstable membrane skeleton, gelsolin remains quantitatively cytoplasmic, irrespective of the Ca2+ concentration. During chicken embryo development and maturation, the expression of gelsolin decreases by a factor of approximately 10(3) in erythroid cells. This down regulation is independent from that of actin, which is considerably less, and is observed also when S13-transformed erythroid progenitor cells are induced to differentiate under conditions where the actin content of these cells does not change. In mature erythrocytes of the adult the amount of gelsolin is low, and significantly less than required for potentially capping of all membrane-associated actin filaments. We suggest that the gelsolin in erythroid cells is involved in the assembly of the actin filaments present in the membrane skeleton, and that it may provide for a mechanism, by means of its severing action on actin filaments, to extend the meshwork of the spectrin-actin-based membrane skeleton in erythroid cells during erythropoiesis.

Abnormal oxidant sensitivity and beta-chain structure of spectrin in hereditary spherocytosis associated with defective spectrin-protein 4.1 binding

Hereditary spherocytosis (HS) is an inherited disorder of erythrocyte shape associated with spectrin deficiency and hemolytic anemia. In a subset of patients with the autosomal dominant form of HS, spectrin displays a reduced capacity to bind protein 4.1 and, therefore, actin; both functions that are critical to the membrane skeleton. A specific structural defect has not been identified in the spectrin from these patients. Chymotryptic digestion of the isolated spectrin chains shows impaired cleavage of the distal peptide of the beta subunit, the beta IV domain. In previous work, we have shown that mild oxidation markedly diminishes the binding capacity of normal spectrin for protein 4.1. Here we observe that chemical reduction of freshly isolated, untreated HS spectrin dramatically improves its function. Thus, a primary structural defect in the beta subunit of spectrin in this subtype of HS may lead to oxidant sensitivity, and secondarily, to a functional defect in the binding of spectrin to protein 4.1 and actin.

Calcium-sensitive, lipid-binding cytoskeletal proteins of the human placental microvillar region

In this study we describe a group of Ca2+-sensitive proteins located in the microvillar region of the human placental syncytiotrophoblast. By following the distribution of proteins between the particulate and supernatant phases of detergent-solubilized microvilli in the presence of defined concentrations of free Ca2+, we demonstrate a class of proteins of subunit molecular weights 72,000, 69,000, 38,000, 36,000, and 32,000 that associate with both the cytoskeleton and lipid at high concentrations of free Ca2+. These proteins can be released from microvilli using EGTA-containing buffers. Although they do not bind to phenyl-Sepharose, they will bind to phospholipids immobilized on phenyl-Sepharose columns in a Ca2+-dependent manner and show a marked preference for phospholipids with negatively charged headgroups. The results provide evidence for a sequence of events which may occur within the microvillus as the localized concentration of intracellular free Ca2+ rises.

Binding and assembly of actin filaments by plasma membranes from Dictyostelium discoideum

The binding of native, 125I-Bolton-Hunter-labeled actin to purified Dictyostelium discoideum plasma membranes was measured using a sedimentation assay. Binding was saturable only in the presence of the actin capping protein, gelsolin. In the presence of gelsolin, the amount of actin bound at saturation to three different membrane preparations was 80, 120, and 200 micrograms/mg of membrane protein. The respective concentrations of actin at half-saturation were 8, 12, and 18 micrograms/ml. The binding curves were sigmoidal, indicating positive cooperativity at low actin concentrations. This cooperativity appeared to be due to actin-actin associations during polymerization, since phalloidin converted the curve to a hyperbolic shape. In kinetic experiments, actin added as monomers bound to membranes at a rate of 0.6 microgram ml-1 min-1, while pre-polymerized actin bound at a rate of 3.0 micrograms ml-1 min-1. Even in the absence of phalloidin, actin bound to membranes at concentrations well below the normal critical concentration. This membrane-bound actin stained with rhodamine-phalloidin and was cross-linked by m-maleimidobenzoyl succinimide ester, a bifunctional cross-linker, into multimers with the same pattern observed for cross-linked F-actin. We conclude that D. discoideum plasma membranes bind actin specifically and saturably and that these membranes organize actin into filaments below the normal critical concentration for polymerization. This interaction probably occurs between multiple binding sites on the membrane and the side of the actin filament, and may be related to the clustering of membrane proteins.

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.

Reorganization of alpha-fodrin induced by stimulation in secretory cells

Spectrin is an ubiquitous protein composed of heterodimers with alpha and beta subunits. It was first described in erythrocyte cell membranes (see ref. 1 for review) and subsequently in brain, intestinal brush borders, kidney, liver and adrenals. Brain spectrin (fodrin) alpha-subunit, responsible for actin binding, has a relative molecular mass (Mr) of 240,000, whereas the beta-subunit, involved in membrane attachment, has an Mr of 235,000 (refs 1, 3, 9-13). The membrane of secretory granules from adrenal chromaffin cells membrane of secretory granules from adrenal chromaffin cells increases the viscosity of F-actin solution, and spectrin-like protein is associated with storage granule and plasma membranes. Here, we report the localization of fodrin in secretory cells using monospecific antibodies against the alpha-subunit of fodrin using indirect immunofluorescence. We find that the alpha-subunit forms an intensely stained continuous ring in the subplasmalemmal region of resting chromaffin cells. On stimulation of the cell with nicotine, high potassium or ionophores in the presence of calcium, fodrin forms patches. This aggregation is inhibited by trifluoperazine, hence the entrance of calcium into cells following cell depolarization seems to be the calmodulin-dependent stimulus initiating patch formation.

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.

Partial purification and characterization of an actin-bundling protein, band 4.9, from human erythrocytes

Band 4.9 (a 48,000-mol-wt polypeptide) has been partially purified from human erythrocyte membranes. In solution, band 4.9 polypeptides exist as trimers with an apparent molecular weight of 145,000 and a Stokes radius of 50 A. Electron microscopy shows that the protein is a three-lobed structure with a radius slightly greater than 50 A. When gel-filtered rabbit muscle actin is polymerized in the presence of band 4.9, actin bundles are generated that are similar in appearance to those induced by "vinculin" or fimbrin. The bundles appear brittle and when they are centrifuged small pieces of filaments break off and remain in the supernatant. At low band 4.9 to actin molar ratios (1:30), band 4.9 lowers the apparent steady-state low-shear falling ball viscosity by sequestering filaments into thin bundles; at higher ratios, the bundles become thicker and obstruct the ball's movement leading to an apparent increase in steady-state viscosity. Band 4.9 increases the length of the lag phase and decreases the rate of elongation during actin polymerization as measured by high-shear Ostwald viscometry or by the increase in the fluorescence of pyrene-labeled actin. Band 4.9 does not alter the critical actin monomer concentration. We hypothesize that band 4.9, together with actin, erythrocyte tropomyosin, and spectrin, forms structures in erythroid precursor cells analogous to those formed by fimbrin, actin, tropomyosin, and TW 260/240 in epithelial brush borders. During erythroid development and enucleation, the actin filaments may depolymerize up to the membrane, leaving a membrane skeleton with short stubs of actin bundled by band 4.9 and cross-linked by spectrin.

Interaction of rat liver lysosomal membranes with actin

Membranes were prepared from lysosomes purified 80-fold by centrifugation in a discontinuous metrizamide gradient. When salt-washed membranes were combined with rabbit muscle actin, an increase in viscosity could be measured using a falling ball viscometer. The lysosomal membrane-actin interaction was actin- and membrane-concentration dependent and appeared to be optimal under presumed physiological conditions (2 mM MgCl2, 1 mM MgATP, neutral pH, and free calcium concentration less than 10(-8) M). The actin cross-linking activity of the membrane was optimal at pH 6.4. The interaction was maximal between 10(-7) and 10(-9) M free calcium ions and inhibited by approximately 50% at concentrations of calcium greater than 0.5 x 10(-7) M. The actin-lysosomal membrane interaction was destroyed if the membranes were pretreated with Pronase, or if the membranes were purified in the absence of protease inhibitors. The interaction was not destroyed if membranes were washed with high salt or extracted with KCl and urea. In addition, a sedimentation assay for the actin-lysosomal membrane interaction was also performed to corroborate the viscometry data. The results suggest the existence of an integral lysosomal membrane actin-binding protein.

A membrane cytoskeleton from Dictyostelium discoideum. III. Plasma membrane fragments bind predominantly to the sides of actin filaments

The binding between sonicated Dictyostelium discoideum plasma membrane fragments and F-actin on Sephacryl S-1000 beads was found to be competitively inhibited by myosin subfragment-1. This inhibition is MgATP-sensitive, exhibits a Ki of approximately 5 X 10(-8) M, and is reciprocal, since membranes inhibit the binding of 125I-heavy meromyosin to F-actin on beads. These experiments demonstrate that membrane binding and S-1 binding to F-actin on beads are mutually exclusive and, therefore, that the membrane fragments bind predominantly to the sides, rather than to the ends, of the actin filaments. This conclusion is supported by electron micrographs that show many lateral associations between membrane fragments and bead-associated actin filaments. Such lateral associations could play an important role in the organization and lateral movement of membrane proteins by the cytomusculature.

A membrane cytoskeleton from Dictyostelium discoideum. II. Integral proteins mediate the binding of plasma membranes to F-actin affinity beads

In novel, low-speed sedimentation assays, highly purified, sonicated Dictyostelium discoideum plasma membrane fragments bind to F-actin beads (fluorescein-labeled F-actin on antifluorescein IgG-Sephacryl S-1000 beads). Binding was found to be (a) specific, since beads containing bound fluorescein-labeled ovalbumin or beads without bound fluorescein-labeled protein do not bind membranes, (b) saturable at approximately 0.6 microgram of membrane protein per microgram of bead-bound F-actin, (c) rapid with a t1/2 of 4-20 min, and (d) apparently of reasonable affinity since the off rate is too slow to be measured by present techniques. Using low-speed sedimentation assays, we found that sonicated plasma membrane fragments, after extraction with chaotropes, still bind F-actin beads. Heat-denatured membranes, proteolyzed membranes, and D. discoideum lipid vesicles did not bind F-actin beads. These results indicate that integral membrane proteins are responsible for the binding between sonicated membrane fragments and F-actin on beads. This finding agrees with the previous observation that integral proteins mediate interactions between D. discoideum plasma membranes and F-actin in solution (Luna, E.J., V. M. Fowler, J. Swanson, D. Branton, and D. L. Taylor, 1981, J. Cell Biol., 88:396-409). We conclude that low-speed sedimentation assays using F-actin beads are a reliable method for monitoring the associations between F-actin and membranes. Since these assays are relatively quantitative and require only micrograms of membranes and F-actin, they are a significant improvement over other existing techniques for exploring the biochemical details of F-actin-membrane interactions. Using F-actin beads as an affinity column for actin-binding proteins, we show that at least 12 integral polypeptides in D. discoideum plasma membranes bind to F-actin directly or indirectly. At least four of these polypeptides appear to span the membrane and are thus candidates for direct transmembrane links between the cytoskeleton and the cell surface.

Chromaffin granule membrane-F-actin interactions and spectrin-like protein of subcellular organelles: a possible relationship

The membrane of chromaffin granule, the secretory vesicle of adrenal medullary cells storing catecholamines, enkephalins, and many other components, interacts with F-actin. Using low shear falling ball viscometry to estimate actin binding to membranes, we demonstrated that mitochondrial and plasma membranes from chromaffin cells also provoked large increases in viscosity of F-actin solutions. Mitochondrial membranes also had the capacity to cause complete gelation of F-actin. In addition, vasopressin-containing granules from neurohypophysial tissue were shown to bind F-actin and to increase the viscosity of F-actin solutions. Using an antibody directed against human erythrocyte spectrin, it was found that a spectrin-like protein was associated with secretory granule membrane, mitochondrial membrane, and plasma membrane. The chromaffin granule membrane-associated spectrin-like protein faces the cytoplasmic side, is composed of two subunits (240 kD and 235kD ), the alpha-subunit (240 kD, pHi5 .5) being recognized by the antibody. Nonionic detergents such as Triton X-100 or Nonidet P40 failed to release fully active spectrin-like protein. In contrast, Kyro EOB , a different nonionic detergent, was found to release spectrin-like protein while keeping intact F-actin binding capacity, at least below 0.5% Kyro EOB concentration. Chromaffin cells in culture were stained with antispectrin antibody, showing the presence of spectrin-like protein in the cell periphery close to the cell membrane but also in the cytoplasm. We conclude that in living cells the interaction of F-actin with chromaffin granule membrane spectrin observed in vitro is important in controlling the potential function of secretory vesicles.

Association of actin with the platelet membrane

Human platelet membrane-actin associations were studied by means of differential extraction of purified membranes and low-shear viscometry of membrane-F-actin mixtures. As indicated by resistance to extraction with 0.6 M potassium iodide, a significant amount of platelet actin appears to be tightly associated with the membrane. When tested by falling-ball viscometry, both whole and KI-extracted membranes increased the low-shear viscosity of preformed rabbit skeletal muscle F-actin at physiologically reasonable pH and ionic conditions. This membrane-associated actin gelation activity was dependent upon low free calcium concentration (10(-8)-10(-7) M). The results are consistent with specific associations between actin and platelet membranes and may be relevant to membrane-cytoskeletal interactions believed to occur in the intact cell.

Rapid modulation of N-formyl chemotactic peptide receptors on the surface of human granulocytes: formation of high-affinity ligand-receptor complexes in transient association with cytoskeleton

When human granulocytes were exposed to 50 nM N-formyl-Met-Leu-[3H]Phe at 37 degrees C they rapidly formed ligand-receptor complexes that dissociated 50-100 times more slowly than those on cells initially exposed to the peptide at 4 degrees C. These complexes of apparent higher affinity were stable after detergent solubilization of the cells with Triton X-100. The complexes co-isolated with the detergent insoluble cytoskeletal residues and were free of the cytosolic and Golgi markers, lactate dehydrogenase and galactosyl transferase, respectively. After 5 s of exposure to f-Met-Leu-Phe, 2,000-3,000 molecules of ligand per cell were trapped in such complexes. Continued exposure resulted in capture of a maximum of 14,000 molecules per cell by 5 min. Exposure at 15 degrees C, a temperature at which endocytosis of the receptor is prevented, resulted in complex formation at a linear rate for at least 20 min to levels twice those measured at 37 degrees C. At 4 degrees C, complex formation was approximately 10% of the maximum amount formed at 37 degrees C. Pulse-chase experiments revealed that the complex was in transient association with the cytoskeleton with a half life ranging between 30 s to 4 min depending on the length of the original incubation. Electron microscopic autoradiography indicated that after 1 min of incubation at 37 degrees C, the majority of the specific autoradiographic grains were localized to the outer circumference of the cellular cytoskeleton. After 4 min of incubation, the grains were less frequent at the cytoskeleton periphery but still threefold enriched over a random cellular distribution. We conclude that a metabolically controlled modulation of the state of the N-formyl chemotactic peptide receptor occurs in the plasma membrane which may be the result of transient association of ligand-receptor complex and the cell cytoskeleton.

A phenomenological difference between membrane skeletal protein complexes isolated from normal and hereditary spherocytosis erythrocytes

Membrane skeletons may be obtained from human erythrocytes by extraction with non-ionic detergent. When treated under defined conditions with a cAMP-independent kinase preparation from normal membranes, a suspension of these membrane skeletons sets to a gelatinous mass. Membrane skeletons from the cells of hereditary spherocytosis patients fail to show this response. Those from subjects with some other haemolytic anaemias do not share the abnormality. The gelation process could be shown also to occur with normal membrane skeletons, extracted at high ionic strength, and containing essentially only the structural protein constituents, spectrin, actin, 4.1 and 4.9. It also occurred rapidly when a column-purified kinase preparation was used, so that no significant amounts of contaminating proteins were introduced. Added spectrin, 4.1 or actin in moderate amounts did not induce gelation in the presence of ATP. Cytochalasin E did not perturb the gelation process. Gelation required ATP as well as kinase, and did not occur when the non-hydrolysable analogue, AMP X PNP, was used instead. Gelation was accompanied by phosphorylation of the spectrin alone, and is thus evidently a consequence of the modification of its properties by this means. Inhibition of phosphorylation by added adenosine retarded gelation. It may be inferred that phosphorylation of spectrin generates new, probably weak, non-covalent interactions between cytoskeletal constituents that cause association of the isolated cytoskeletons. A semi-quantitative method of observing the gelation process, based on the time of incubation before the membrane skeleton suspension ceases to flow under gravity at a low shear, is described.

Isolation and characterization of large (0.5 - 1.0 micron) cytoskeleton-free vesicles from human and rabbit erythrocytes

Large (0.5 - 1.0 micron) cytoskeleton-free vesicles were obtained, by 'budding', from fresh human and rabbit erythrocytes incubated at 45 degrees C and titrated with EDTA and CaCl2. This process occurs without hemolysis. The isolated vesicles maintain their cytoplasmic integrity and normal membrane orientation, and are resistant to hemolysis over the pH range 5.0 - 11.0 and temperature range 4-50 degrees C. The only membrane proteins detected in vesicles from human erythrocytes were band 3 region polypeptides and bands PAS-1, PAS-2 and PAS-3. Vesicles obtained from rabbit erythrocytes were similarly simple. Because of their size and stability these vesicles are amenable to both kinetic and quantitative analysis using the same experimental techniques employed in studies of synthetic lipid membranes. The results obtained in this study indicate that these vesicles are essentially markedly simplified biological cells, and thus may be useful as a biologically relevant model membrane system for examining the molecular interactions which occur within, across and between cell membranes.

Interaction of a peripheral protein of the erythrocyte membrane, band 4.1, with phosphatidylserine-containing liposomes and erythrocyte inside-out vesicles

Interactions of band 4.1 with mixed phospholipid membranes [phosphatidylserine (PtdSer), phosphatidylethanolamine, phosphatidylcholine, etc.] and erythrocyte inside-out vesicles were studied. Band 4.1 showed a higher affinity to PtdSer-containing membranes. The amount of binding to PtdSer-containing liposomes was larger than that to PtdSer-lacking liposomes. The amount of binding to inside-out vesicles did not change significantly on a protease treatment of the vesicles. The amount of band 4.1 bound on inside-out vesicles decreased on PtdSer-decarboxylase treatment of the vesicles. Ca2+ acted inhibitory to the binding of band 4.1. Band 4.1 together with PtdSer-containing vesicles but not with PtdSer-lacking vesicles induced gelation of spectrin-actin copolymer solution. Ca2+ inhibited the gelation. Fluorescence energy transfer from PtdSer-containing vesicles to band 4.1 was larger than that from PtdSer-lacking vesicles. Band 4.1 caused a marked release of tempocholine from preloaded PtdSer-containing liposomes but not from PtdSer-lacking liposomes. The release was larger from liposomes containing more PtdSer. Ca2+ was inhibitory to the tempocholine release. We suggest from these results that band 4.1 provides another anchoring site for the cytoskeletal spectrin-actin network to PtdSer domains in the inner layer of erythrocyte membrane. This anchoring may be involved in functional regulation since the interaction causes the membrane permeability change that is dependent on Ca2+.

Particle assemblies in astrocytic plasma membranes are rearranged by various agents in vitro and cold injury in vivo

Distinct aggregates of small intramembranous particles and assemblies characterize the P-face of freeze-fractured astrocytic membranes. To test the lability of the assemblies, astrocytes were treated in vitro with different chemical agents and in vivo by cold injury. The assemblies appeared either to contain or be associated with protein because exposure to medium containing cycloheximide, an inhibitor of protein synthesis, led to a sharp decrease in assemblies, down to 1% of the control levels within three hours. To ascertain whether the assemblies were tethered to the cytoskeleton, the cells were treated in vitro with disruptors of microtubules (colchicine) or microfilaments (cytochalasins); the assemblies became consistently rearranged. Protein denaturants, urea and guanidine HCl, brought about a selective aggregation of assembly with assembly. The lectin, concanavalin A, did not alter the distribution of the assemblies within the plane of the membrane fracture. Surface replicas of in vitro, non-fractured, astrocytes revealed surface particles which did not resemble assemblies. In vivo, the plasma membranes of astrocytes were altered within minutes of cold injury to the brain surface. In the centre of the lesions, damaged astrocytes had assemblies that were clumped like those of in vitro astrocytes exposed to denaturants. In the periphery of the lesions, however, the assemblies did not aggregate but increased in number. These results provide indirect evidence that assemblies may consist of protein, that the recognizable particle constituent of the assembly is confined to the interior of the membrane and is not present on the uncleaved cell surface, and that assemblies are connected with the cytoskeleton. Therefore, certain changes in the environment of the astrocyte caused by injury in vivo or addition of chemical agents in vitro alter the distribution of assemblies in the astrocytic plasma membrane either by a direct effect on the assemblies or indirectly by an alteration of the cytoplasmic proteins.

Osmotic hemolysis and fragility. A new model based on membrane disruption, and a potential clinical test

Red cell osmotic hemolysis has traditionally been defined by the loss of hemoglobin, in response to reduced osmotic pressure, as measured spectroscopically. Previous work from this laboratory using resistive pulse spectroscopy (RPS) has shown that in a mixed population of hemolyzing cells, ghosts can be detected as being more deformable, and hence appearing distinctly smaller, than the remaining intact cells. Other researchers using similar methods have reported detection of ghosts as apparently smaller objects, resulting from their greater sensitivity to dielectric breakdown. We now confirm both of these results, and demonstrate by kinetic studies that changes which occur in the rheological and electrical properties of ghosts are independent phenomena. We include in our analysis the explicit calculation of ghost and intact spherocyte resistivity after dielectric breakdown. The two different characterizations for ghosts are integrated into a proposed model of osmotic hemolysis based on known red blood cell membrane and cytoplasmic properties. This work provides both a theoretical and a practical foundation for RPS-based measures of osmotic fragility, including a potential new clinical test, measures which provides very early detection of the ultimate fate of osmotically stressed red cells.

The role of band 4.1 in the association of actin with erythrocyte membranes

Spectrin stimulates the association of F-actin with erythrocyte inside-out vesicles. Although inside-out vesicles are nearly devoid of two of the three major cytoskeletal proteins, spectrin and actin, they retain nearly all of the cytoskeletal protein designated band 4.1. Inside-out vesicles which have been substantially depleted of band 4.1 by extraction in 1 M KCl, 0.4 M urea and then reconstituted with spectrin show a markedly diminished ability to bind actin by comparison with vesicles containing normal amounts of band 4.1. This diminution is not due to an impaired ability of the vesicles to bind spectrin. Addition of purified band 4.1 to vesicles either before or after they have been reconstituted with spectrin restores their actin binding capacity to near normal levels as does addition of a spectrin-band 4.1 complex prepared by sucrose gradient centrifugation. Band 4.1 bound to vesicles in the absence of added spectrin has no effect on actin binding. Our results suggest that a spectrin band 4.1 complex is responsible for binding actin to erythrocyte membranes.

Some perspectives on the viscosity of actin filaments

Measurements of the dynamic viscosity of various actin filament preparations under conditions of low and controlled shear: (a) confirm the shear rate dependence of F-actin viscosities and show that this dependence obeys the power law relationship observed for entangled synthetic polymers; (b) permit estimation of the extent to which shear artifact amplifies changes in the apparent viscosity of F-actin measured in a falling ball viscometer; (c) show that gel-filtration chromatography of actin and the addition of cytochalasin B to F-actin bring about small (20-40%) changes in the viscosity of the F-actin solutions. These variations are consistent with alterations in the actin-binding protein concentrations required for incipient gelation, a parameter inversely related to average filament length. Therefore: (a) changes in the viscosity of F-actin can be magnified by use of the falling ball viscometer, and may exaggerate their biological importance; (b) chromatography of actin may not be required to obtain meaningful information about the rheology of actin filaments; (c) changes in actin filament length can satisfactorily explain alterations in F-actin viscosity exerted by cytochalasin B and by chromatography, obviating the need to postulate specific interfilament interactions.

In vitro reconstitution of chromaffin granule-cytoskeleton interactions: ionic factors influencing the association of F-actin with purified chromaffin granule membranes

Chromaffin granules are the secretory vesicles directly involved in exocytosis of catecholamines, enkephalins, and other components from adrenal medullary cells. The granules occupy a large portion of the cytoplasmic volume and thus may interact extensively with cytoskeletal elements such as actin. Indeed, using both sedimentation techniques and falling ball viscometry [Fowler et al: J Cell Biol 88: 388, 1981] to measure actin binding by membranes, we were able to show that chromaffin granules bind F-actin via a protein site on the membrane, and that these interactions are reversibly inhibited by raising the free calcium ion concentration to micromolar levels ([Ca++]free for half-maximal inhibition approximately 2.6 x 10(-7)M)[Fowler and Pollard: Nature 295:336, 1982]. Here, we show that F-actin-chromaffin granule interactions are unaffected by changes in pH between about pH 6.4 and 7.4 but are about 50% inhibited by raising the pH from 7.5 to 8.0. They are also 50% inhibited by increasing the KCl concentration to about 200 mM but are not significantly affected by increasing concentrations of K-glutamate up to 500 mM or by varying the MgCl2 concentration between 0 and 6 mM. The interactions between chromaffin granule membranes and F-actin are also reduced in the presence of ATP, AMP-PNP, or free pyrophosphate; cAMP and AMP are without effect. The ability of chromaffin granule membranes to interact with F-actin under conditions that may approximate the resting intracellular environment (neutral pH, low KCl, 1-2 mM MgCl2, 1 mM ATP, [Ca++]free less than 10(-7)M, 30 degrees C) suggests that these interactions may partially reconstitute naturally occurring associations between chromaffin granules and the cytoskeleton. Further, regulation of chromaffin granule membrane-actin interactions by ionic factors (pH, calcium, chloride ions, nucleotides) that vary intracellularly leads us to propose that associations between actin and the chromaffin granule membrane could influence the location and dislocation of these organelles in the cytoplasm.

A membrane cytoskeleton from Dictyostelium discoideum. I. Identification and partial characterization of an actin-binding activity

Dictyostelium discoideum plasma membranes isolated by each of three procedures bind F-actin. The interactions between these membranes and actin are examined by a novel application of falling ball viscometry. Treating the membranes as multivalent actin-binding particles analogous to divalent actin-gelation factors, we observe large increases in viscosity (actin cross-linking) when membranes of depleted actin and myosin are incubated with rabbit skeletal muscle F-actin. Pre-extraction of peripheral membrane proteins with chaotropes or the inclusion of Triton X-100 during the assay does not appreciably diminish this actin cross-linking activity. Lipid vesicles, heat-denatured membranes, proteolyzed membranes, or membranes containing endogenous actin show minimal actin cross-linking activity. Heat-denatured, but not proteolyzed, membranes regain activity when assayed in the presence of Triton X-100. Thus, integral membrane proteins appear to be responsible for some or all of the actin cross-linking activity of D. discoideum membranes. In the absence of MgATP, Triton X-100 extraction of isolated D. discoideum membranes results in a Triton-insoluble residue composed of actin, myosin, and associated membrane proteins. The inclusion of MgATP before and during Triton extraction greatly diminishes the amount of protein in the Triton-insoluble residue without appreciably altering its composition. Our results suggest the existence of a protein complex stabilized by actin and/or myosin (membrane cytoskeleton) associated with the D. discoideum plasma membrane.