Polymerisation of G-actin by spectrin preparations: identification of the active constituent

The human erythrocyte plasma membrane: a Rosetta Stone for decoding membrane-cytoskeleton structure

The mammalian erythrocyte, or red blood cell (RBC), is a unique experiment of nature: a cell with no intracellular organelles, nucleus or transcellular cytoskeleton, and a plasma membrane with uniform structure across its entire surface. By virtue of these specialized properties, the RBC membrane has provided a template for discovery of the fundamental actin filament network machine of the membrane skeleton, now known to confer mechanical resilience, anchor membrane proteins, and organize membrane domains in all cells. This chapter provides a historical perspective and critical analysis of the biochemistry, structure, and physiological functions of this actin filament network in RBCs. The core units of this network are nodes of ~35-37 nm-long actin filaments, interconnected by long strands of (α1β1)₂-spectrin tetramers, forming a 2D isotropic lattice with quasi-hexagonal symmetry. Actin filament length and stability is critical for network formation, relying upon filament capping at both ends: tropomodulin-1 at pointed ends and αβ-adducin at barbed ends. Tropomodulin-1 capping is essential for precise filament lengths, and is enhanced by tropomyosin, which binds along the short actin filaments. αβ-adducin capping recruits spectrins to sites near barbed ends, promoting network formation. Accessory proteins, 4.1R and dematin, also promote spectrin binding to actin and, with αβ-adducin, link to membrane proteins, targeting actin nodes to the membrane. Dissection of the molecular organization within the RBC membrane skeleton is one of the paramount achievements of cell biological research in the past century. Future studies will reveal the structure and dynamics of actin filament capping, mechanisms of precise length regulation, and spectrin-actin lattice symmetry.

Actin assembly and filament cross-linking in the presence of TW 260/240, the tissue-specific spectrin of the chicken intestinal brush border

TW 260/240 is a tissue-specific spectrin found in the terminal web region of the chicken intestinal brush border. We have examined the effects of TW 260/240 on assembly rates and critical concentrations (Co's) for monomer addition at the barbed and pointed ends of the actin filament. For these studies, acrosomal processes (AP) from Limulus sperm were used as nuclei for actin assembly. Under conditions which favor the interaction of TW 260/240 for actin (20-75 mM KCl, 2 mM Mg++) no effect on either elongation rates or Co's at either end of the actin filament was observed in the presence of this spectrinlike protein. The Limulus AP nucleation assay also allowed visualization of the kinetics of filament binding and cross-linking by TW 260/240. Ultrastructural analysis of TW 260/240 binding to actin filaments at their growing ends indicates that TW 260/240 tetramers bind laterally to the filament. Finally, evidence is presented that indicates that filaments cross-linked by TW 260/240 are stabilized against shear-dependent breakage.

Structural and dynamic states of actin in the erythrocyte

Analysis of the nucleotide tightly associated with isolated erythrocyte cytoskeletons show it to be ADP, rather then ATP. This confirms that at least a major part of the erythrocyte actin is in the F-form. A re-evaluation of the stoichiometry of spectrin and actin in the erythrocyte (taking account of a gross difference between the color responses of the two proteins on staining of electrophoretic gels) leads to values of 1x10(5) and 5x10(5) for the number of molecules of spectrin tetramer and actin respectively per cell. It has been found possible to perform spectrophotometric DNAase I assays fro actin on lysed whole cells. The concentration of monomeric actin at 0 degrees C is approximately 16 mug/ml packed cells. After washing the lysed cells the monomer pool is not re-established, indicating that only a small proportion of the actin subunits are free to dissociate. The actin monomer concentration in the cytosol remains unchanged after equilibration of the cells with cytochalasin E. The ability of actin-containing complexes in the membrane to nucleate the polymerization of added G-actin was measured fluorimetrically; it was found that membranes incubated with cytochalasin E were completely inert with respect to nucleating activity under conditions that favor appreciable growth at the slowly-growing ("pointed") ends of free actin filaments. This suggests that these ends of the actin "protofilaments" in the red cell are blocked or sterically obstructed. After treatment of the membranes with guanidine hydrochloride under conditions that dissociate F-actin, the measured concentration of actin monomer rises to approximately 180 mug/ml of packed cells, which is nearly 70 percent of the total actin content. On treatment with trypsin in the presence of DNAase, the spectrin and 4.1 are extensively degraded, but the actin remains undamaged. This treatment, followed by exposure to guanidine hydrochloride, causes a further rise in the concentration of actin responsive to the DNAase assay to 250 mug/ml of cells, compared with 270 mug/ml estimated by densitometry of stained gels. The oligomeric complex, consisting of actin, spectrin, and 4.1, that is extracted from the membrane at low ionic strength, generates no detectable actin monomer after the same treatment. From literature data on the number of cytochalasin binding sites per cell and our value for the total actin content, we obtain a number-average degree of polymerization for actin in the membrane of 12-17. The results lead to a model for the structure of the cytoskeletal network and suggest some consequences of metabolic depletion.

Investigation of the actin-deoxyribonuclease I interaction using a pyrene-conjugated actin derivative

The interaction of deoxyribonuclease I with muscle actin was studied with the aid of a pyrenyl derivative of the actin [Kouyama, T., & Mihashi, K. (1981) Eur. J. Biochem. 114, 33-38] that increases its quantum yield by an order of magnitude on polymerization. It is shown that this derivative copolymerizes with unlabeled G-actin in a random manner and will also bind to deoxyribonuclease with inhibition of enzymic activity. The derivative affords a highly sensitive means of following nucleated polymerization. Preincubation of F-actin with deoxyribonuclease at a concentration of 5% or less of that of total subunits causes inhibition of polymerization of additional G-actin onto the filaments. In red cell membranes that contain stabilized short filaments of actin such that the concentration of filament ends is large relative to monomers, complete inhibition of nucleated polymerization of G-actin is achieved by preincubation with deoxyribonuclease. The results indicate that binding of DNase occurs at the "plus" ends of the actin filaments. Competition with cytochalasin E, which is known to have a high affinity for the plus or preferentially growing ends of F-actin, can be observed. Whereas the activity of deoxyribonuclease in the 1:1 complex with G-actin is inhibited, the enzyme attached to the ends of filaments appears to be fully active. This causes a reduction in the inhibition of enzymic activity with increasing F-actin concentration, presumably by reason of a change in the partition of the enzyme between monomers and filament ends. The degree of inhibition increases with time, however, as the actin depolymerizes. Implications for measurements of actin monomer concentrations by the deoxyribonuclease assay procedure are considered.

Nucleation of actin polymerization from profilactin. Opposite effects of different nuclei

The lag in polymerization of calf spleen profilactin in response to addition of MgCl2 can be overcome by small amounts of spectrin-actin-band 4.1 complex, covalently crosslinked actin oligomers or sonicated F-actin. All of these factors also nucleate polymerization of pure actin. Another nucleator of actin polymerization, villin, delays filaments formation from profilactin. A simple model of the interaction of profilin with actin can explain these apparently conflicting results in terms of the polarity with which actin filaments elongate from the different nuclei.

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

We studied the binding of actin to the erythrocyte membrane by a novel application of falling ball viscometry. Our approach is based on the notion that if membranes have multiple binding sites for F-actin they will be able to cross-link and increase the viscosity of actin. Spectrin- and actin-depleted inside-out vesicles reconstituted with purified spectrin dimer or tetramer induce large increases in the viscosity of actin. Comparable concentrations of spectrin alone, inside-out vesicles alone, inside-out vesicles plus heat-denatured spectrin dimmer or tetramer induce large increases in the viscosity of actin. Comparable concentrations of spectrin alone, inside-out vesicles alone, inside-out plus heat denatured spectrin, ghosts, or ghosts plus spectrin have no effect on the viscosity of actin. Centrifugation experiments show that the amount of actin bound to the inside-out vesicles is enhanced in the presence of spectrin. The interactions detected by low-shear viscometry reflect actin interaction with membrane- bound spectrin because (a) prior removal of band 4.1 and ankyrin (band 2.1, the high- affinity membrane attachment site for spectrin) reduces both spectrin binding to the inside-out vesicles and their capacity to stimulate increase in viscosity of actin in the presence of spectrin + actin are inhibited by the addition of the water-soluble 72,000- dalton fragment of ankyrin, which is known to inhibit spectrin reassociation to the membrane. The increases in viscosity of actin induced by inside-out vesicles reconstituted with purified spectrin dimer or tetramer are not observed when samples are incubated at 0 degrees C. This temperature dependence may be related to the temperature-dependent associations we observe in solution studies with purified proteins: addition of ankyrin inhibits actin cross-linking by spectrin tetramer plus band 4.1 at 0 degrees C, and enhances it at 32 degrees C. We conclude (a) that falling ball viscometry can be used to assay actin binding to membranes and (b) that spectrin is involved in attaching actin filaments or oligomers to the cytoplasmic surface of the erythrocyte membrane.

Spectrin plus band 4.1 cross-link actin. Regulation by micromolar calcium

A low-salt extract prepared from human erythrocyte membranes forms a solid gel when purified rabbit muscle G- or F-actin is added to it to give a concentration of approximately 1 mg/ml. This extract contains spectrin, actin, band 4.1, band 4.9, hemoglobin, and several minor components. Pellets obtained by centrifugation of the gelled material at 43,000 g for 10 min contain spectrin, actin, band 4.1, and band 4.9. Although extracts that are diluted severalfold do not gel when actin is added to them, the viscosity of the mixtures increases dramatically over that of G-actin alone, extract alone, or F-actin alone at equivalent concentrations. Heat-denatured extract is completely inactive. Under conditions of physiological ionic strength and pH, information of this supramolecular structure is inhibited by raising the free calcium ion concentration to micromolar levels. Low-salt extracts prepared by initial extraction at 37 degrees C (and stored at 0 degree C) gel after actin is added to them only when warmed, whereas extracts prepared by extraction at 0 degree C are active on ice as well as after warming. Preincubation of the 37 degrees C low-salt extract under conditions that favor conversion of spectrin dimer to tetramer greatly enhances gelation activity at 0 degree C. Conversely, preincubation of the 0 degree C low-salt extract under conditions that favor conversion of spectrin tetramer to dimer greatly diminishes gelation activity at 0 degree C. Spectrin dimers or tetramers are purified from the 37 dgrees or 0 degree C low-salt extract by gel filtration at 4 degrees C over Sepharose 4B. The addition of actin to either purified spectrin dimer (at 32 degrees C) or tetramer (at 0 degree C or 32 degrees C) results in relatively small increases in viscosity, whereas the addition of actin to a high-molecular-weight complex (HMW complex) containing spectrin, actin, band 4.1, and band 4.9 results in dramatic, calcium-sensitive increases in viscosity. These viscosities are comparable to those obtained with the 37 degrees or 0 degree C low-salt extracts. The addition of purified band 4.1 to either purified spectrin dimer (at 32 degrees C) or purified spectrin tetramer (at 0 degree C) plus actin results in large increases in viscosity similar to those observed for the HMW complex and the crude extract, which is in agreement with a recent report by E. Ungewickell, P. M. Bennett, R. Calvert, V. Ohanian, and W. B. Gratzer. 1979 Nature (Lond.) 280:811-814. We suggest that this spectrin-actin-band 4.1 gel represents a major structural component of the erythrocyte cytoskeleton.