Myosins of Babesia bovis: molecular characterisation, erythrocyte invasion, and phylogeny

Using degenerate primers, three putative myosin sequences were amplified from Australian isolates of Babesa bovis and confirmed as myosins (termed Bbmyo-A, Bbmyo-B, and Bbmyo-C) from in vitro cultures of the W strain of B. bovis. Comprehensive analysis of 15 apicomplexan myosins suggests that members of Class XIV be defined as those with greater than 35% myosin head sequence identity and that these be further subclassed into groups bearing above 50-60% identity. Bbmyo-A protein bears a strong similarity with other apicomplexan myosin-A type proteins (subclass XIVa), the Bbmyo-B myosin head protein sequence exhibits low identity (35-39%) with all members of Class XIV, and 5'-sequence of Bbmyo-C shows strong identity (60%) with P. falciparum myosin-C protein. Domain analysis revealed five divergent IQ domains within the neck of Pfmyo-C, and a myosin-N terminal domain as well as a classical IQ sequence unusually located within the head converter domain of Bbmyo-B. A cross-reacting antibody directed against P. falciparum myosin-A (Pfmyo-A) revealed a zone of approximately 85 kDa in immunoblots prepared with B. bovis total protein, and immunofluorescence inferred stage-specific myosin-A expression since only 25% of infected erythrocytes with mostly paired B. bovis were immuno-positive. Multiplication of B. bovis in in vitro culture was inhibited by myosin- and actin-binding drugs at concentrations lower than those that inhibit P. falciparum. This study identifies and classifies three myosin genes and an actin gene in B. bovis, and provides the first evidence for the participation of an actomyosin-based motor in erythrocyte invasion in this species of apicomplexan parasite.

Identification of a novel C-terminal variant of beta II spectrin: two isoforms of beta II spectrin have distinct intracellular locations and activities

It is established that variations in the structure and activities of betaI spectrin are mediated by differential mRNA splicing. The two betaI spectrin splice forms so far identified have either long or short C-terminal regions. Are analogous mechanisms likely to mediate regulation of betaII spectrins? Thus far, only a long form of betaII spectrin is reported in the literature. Five human expressed sequence tags indicated the existence of a short splice variant of betaII spectrin. The occurrence and DNA sequence of the short C-terminal variant was confirmed by analysis of human and rat cDNA. The novel variant lacks a pleckstrin homology domain, and has 28 C-terminal residues not present in the previously recognized longer form. Transcripts of the short C-terminal variant (7.5 and 7. 0 kb) were most abundant in tissues originating from muscle and nervous system. Antibodies raised to a unique sequence of short C-terminal variant recognized 240 kDa polypeptides in cardiac and skeletal muscle and in nervous tissue; in cerebellum and forebrain, additional 270 kDa polypeptides were detected. In rat heart and skeletal muscle, both long and short C-terminal forms of betaII spectrin localized in the region of the Z line. The central region of the sarcomere, coincident with the M line, was selectively labeled with antibodies to the short C-terminal form. In cerebellum, the short form was not detectable in parallel fibers, structures in which the long form was readily detected. In cultured cerebellar granule neurons, the long form was dominant in neurites, with the short form being most abundant in cell bodies. In vitro, the short form was found to lack the binding activity for the axonal protein fodaxin, which characterizes the C-terminal region of the long form. Subcellular fractionation of brain revealed that the short form was scarcely detectable in post-synaptic density preparations, in which the long form was readily detected. We conclude that variation in the structure of the C-terminal regions of betaII spectrin isoforms correlates with their differential intracellular targeting.

Properties of normal and mutant polypeptide fragments from the dimer self-association sites of human red cell spectrin

We have examined the properties and interactions of expressed polypeptide fragments from the N-terminus of the alpha-chain and the C-terminus of the beta-chain of human erythroid spectrin. Each polypeptide comprises one complete structural repeating unit, together with the incomplete repeat that interacts with its partner when spectrin tetramers are formed. The shared repeat thus generated is made up of two helices from the C-terminal part of the beta-chain and one helix from the N-terminus of the alpha-chain. Three mutant beta-chain fragments with amino acid substitutions in the incomplete terminal repeat were also studied. The alpha- and beta-chain fragments were both substantially monomeric, as shown by sedimentation equilibrium. Circular dichroism analysis and thermal denaturation profiles revealed that the complete repeat present in each fragment had entered the stable tertiary fold. Unexpectedly, the conformational stability of the folded beta-chain repeat was found to be grossly perturbed by the mutations, all of them well beyond its C-terminal boundary; possible explanations for this phenomenon are considered. Sedimentation equilibrium showed that in equimolar mixtures the wildtype alpha- and beta-chain peptides formed a 1:1 complex. Mixing curves, observed by circular dichroism, revealed that association was accompanied by an increase in alpha-helicity. From continuous-variation profiles an association constant in the range 1-2 x 10(6) M-1 was inferred. The association was unaffected by the apparently unstructured anionic tail of 54 residues, found at the C-terminus of the spectrin beta-chain. Of the three mutations in the beta-chain fragment, one (an Ala-->Val replacement in the A helix segment of the incomplete repeat) had a relatively small effect on the association with the alpha-chain fragment, whereas Trp-->Arg mutations in the A and in the remote B helix segments were much more deleterious. These observations are consistent with the relative severities of the haemolytic conditions associated with the mutations.

Actomyosin motor in the merozoite of the malaria parasite, Plasmodium falciparum: implications for red cell invasion

The genome of the malaria parasite, Plasmodium falciparum, contains a myosin gene sequence, which bears a close homology to one of the myosin genes found in another apicomplexan parasite, Toxoplasma gondii. A polyclonal antibody was generated against an expressed polypeptide of molecular mass 27,000, based on part of the deduced sequence of this myosin. The antibody reacted with the cognate antigen and with a component of the total parasite protein on immunoblots, but not with vertebrate striated or smooth muscle myosins. It did, however, recognise two components in the cellular protein of Toxoplasma gondii. The antibody was used to investigate stage-specificity of expression of the myosin (here designated Pf-myo1) in P. falciparum. The results showed that the protein is synthesised in mature schizonts and is present in merozoites, but vanishes after the parasite enters the red cell. Pf-myo1 was found to be largely, though not entirely, associated with the particulate parasite cell fraction and is thus presumably mainly membrane bound. It was not solubilised by media that would be expected to dissociate actomyosin or myosin filaments, or by non-ionic detergent. Immunofluorescence revealed that in the merozoite and mature schizont Pf-myo1 is predominantly located around the periphery of the cell. Immuno-gold electron microscopy also showed the presence of the myosin around almost the entire parasite periphery, and especially in the region surrounding the apical prominence. Labelling was concentrated under the plasma membrane but was not seen in the apical prominence itself. This suggests that Pf-myo1 is associated with the plasma membrane or with the outer membrane of the subplasmalemmal cisterna, which forms a lining to the plasma membrane, with a gap at the apical prominence. The results lead to a conjectural model of the invasion mechanism.

Actin polymerisation regulates integrin-mediated adhesion as well as rigidity of neutrophils

Activation of adherent neutrophils causes them to convert from selectin-mediated rolling to integrin-mediated immobilisation and migration. Migration is known to depend on formation and redistribution of filamentous (F) actin, but although immobilisation in seconds parallels early cortical actin polymerisation, no link has been proven. We tested the effect of the actin-polymerising agent jasplakinolide (10 microM) on adhesive and mechanical properties of neutrophils. Pretreated cells were able to adhere and roll on immobilised platelets in a flow-based adhesion assay, but whereas untreated rolling cells became immobilised in seconds when chemotactic formyl peptide (fMLP, 0.1 microM) was superfused over them, the cells treated with jasplakinolide continued rolling. Pretreatment with jasplakinolide also blocked de novo expression of integrin CD11b and shape change which otherwise occurred in minutes after treatment with fMLP. Jasplakinolide directly caused actin polymerisation within neutrophils, evidenced by a marked increase in rigidity (resistance to aspiration into a 5 microm micropipette) and increase in association of actin with the Triton-insoluble cytoskeleton. These results indicate that rearrangement of the actin cytoskeleton regulates integrin-mediated adhesion of activated neutrophils, as well as their migration and mechanical properties.

Interaction of dystrophin fragments with model membranes

The interaction with membrane lipids of recombinant fragments of human dystrophin, corresponding to a single structural repeating unit of the rod domain, was examined. Surface plasmon resonance, constant-pressure isotherms in a Langmuir surface film balance, and interfacial rheology were used to observe binding of the polypeptides and its effects on the properties of the lipid film. Modification of the monolayer properties was found to depend on the presence of phosphatidylserine in the lipid mixture and on the native tertiary fold of the polypeptide; thus a fragment with the minimum chain length required for folding (117 residues) or longer caused a contraction of the surface area at constant pressure, whereas fragments of 116 residues or less had no effect. The full extent of contraction was reached at a surface concentration of lipid corresponding to an average area of about 42 A2 per lipid molecule. A dystrophin fragment with the native, folded conformation induced a large increase in surface shear viscosity of the lipid film, whereas an unfolded fragment had no effect. Within a wide range of applied shear, the shear viscosity remained Newtonian. Binding of liposomes to immobilized dystrophin fragments could be observed by surface plasmon resonance and was again related to the conformational state of the polypeptide and the presence of phosphatidylserine in the liposomes. Our results render it likely that intact dystrophin interacts directly and strongly with the sarcolemmal lipid bilayer and grossly modifies its material properties.

Contractile protein system in the asexual stages of the malaria parasite Plasmodium falciparum

F-actin was detected in asexual-stage Plasmodium falciparum parasites by fluorescence microscopy of blood films stained with fluorescent phalloidin derivatives. F-actin was present at all stages of development and appeared diffusely distributed in trophic parasites, but merozoites stained strongly at the poles and peripheries. No filament bundles could be discerned. A similar distribution was obtained by immunofluorescence with 2 polyclonal anti-actin antibodies, one of which was directed against a peptide sequence present only in parasite actin (as inferred from the DNA sequence of the gene). A monoclonal anti-actin antibody stained very mature or rupturing schizonts but not immature parasites. Myosin was identified in immunoblots of parasite protein extracts by several monoclonal anti-skeletal muscle myosin antibodies, as well as by a polyclonal antiserum directed against a consensus conserved myosin sequence (IQ motif). The identity of the polypeptides recognised by these antibodies was confirmed by overlaying blots with biotinylated F-actin. The antiserum and one of the monoclonal antibodies were used in immunofluorescence studies and were found to stain all blood-stage parasites, with maximal intensity towards the poles of merozoites. Our results are consistent with the presence of an actomyosin motor system in the blood-stage malaria parasite.

Interaction of protein 4.1 with the red cell membrane: effects of phosphorylation by protein kinase C

Phosphorylation with endogenous protein kinase C causes the membrane skeletal protein, band 4.1, to lose its capacity to attach to one of two classes of high-affinity binding sites on the red cell membrane. These sites are the ones eliminated by proteolysis in situ of glycophorin C; the surviving type of site is located in a C-terminal peptide of the glycophorin C, retained on the membrane after proteolysis, which is also the site of attachment of p55. A synthetic peptide, comprising the 28 C-terminal residues of glycophorin C, also binds protein 4.1. Phosphorylation of the intact cells, stimulated by phorbol ester, approximately halves the retention of glycophorin C in the membrane cytoskeletons prepared from these cells and reduces the affinity of extracellular glycophorin C epitopes for their antibody.

Association state of human red blood cell band 3 and its interaction with ankyrin

We have studied the association state of band 3, the anion channel and predominant transmembrane protein of the human red blood cell, and the anomalous stoichiometry and dynamics of its interaction with ankyrin, which acts as a link to the spectrin of the membrane skeletal network. Band 3 exists in benign nonionic detergent solutions as a dimer. Tetramer is formed irreversibly in the course of manipulations, particularly in ion-exchange chromatography. The dimer in solution binds ankyrin without self-associating. In ankyrin-free inside-out membrane vesicles and when incorporated into phosphatidylcholine liposomes, only some 10% to 15% of band 3 chains bind ankyrin at saturation. Moreover, in liposomes this was independent of protein:lipid ratio between 1:2 and 1:40. The bound fraction of band 3 remains with the detergent-extracted membrane cytoskeleton, but is released if the ankyrin has been cleaved with chymotrypsin before detergent treatment; thus, the attachment to the membrane cytoskeleton is entirely through ankyrin and not through other constituents such as protein 4.1. The ratio of band 3 to ankyrin in this complex implies that it consists of two chains of band 3 and one chain of ankyrin, at least after detergent extraction. The bound and free populations of band 3 exchange freely in the membrane. In the artificial liposome membrane binding of ankyrin to band 3 dimers cause association of the band 3 into higher aggregates, as seen in freeze-fracture electron microscopy. Successive manipulations of the red blood cell membrane, which are involved in the preparation of ghosts, of inside-out vesicles, and of inside-out vesicles stripped of peripheral proteins are accompanied by progressive aggregation of intramembrane particles, as judged by freeze-fracture electron microscopy. Thus the intramembrane particles are evidently stabilized in the intact cell by the peripheral protein network and the cytosolic milieu. Aggregation may be expected to limit the number of functional ankyrin binding sites. However, although extraneous ankyrin binds to the unoccupied binding site on the spectrin tetramers in intact ghost membranes, little or no ankyrin can bind to the unoccupied band 3 dimers in situ, perhaps by reason of occlusion of binding sites by the membrane skeletal network.

Concentrated Tris solutions for the preparation, depolymerization, and assay of actin: application to erythroid actin

High concentrations of Tris are effective in dissociating actin-containing complexes, such as the red cell membrane cytoskeleton. A preparative procedure for red cell actin is based on the dissociation of the membrane skeletal complex in a buffer containing 1 M Tris hydrochloride, followed by gel filtration chromatography in the same medium. The actin is recovered as the monomer and is fully native, as judged by its critical concentration of polymerization, inhibition of DNase I, stimulation of myosin ATPase, and the appearance in the electron microscope of filaments, both bare and decorated with heavy meromyosin, and of magnesium ion-induced paracrystals. The Tris solution causes rapid depolymerization of F-actin with no denaturation, and the solution of monomeric actin in this medium is stable for many weeks in the cold; concentrated Tris is more reliable than guanidinium chloride for the depolymerization of F-actin in the estimation of total actin concentration by the DNase I inhibition assay.

Membrane attachment sites for the membrane cytoskeletal protein 4.1 of the red blood cell

The identity of the membrane binding sites for the membrane cytoskeletal protein 4.1 of the human red blood cell has been investigated. Exhaustive proteolysis of the membrane with a range of proteases led to the elimination of only some 60% of all binding sites. The predominant integral membrane protein, band 3, as well as glycophorin A, was totally digested at levels of proteolysis that were essentially without effect on the number of 4.1 binding sites. Proteolysis caused scission of the polypeptide chain of glycophorin C (together with the minor product, glycophorin D, of the same gene), but left a fragment from the region of the C-terminus still attached to the membrane. We have found a low-molecular weight protein, possessing an epitope (recognized by an antibody directed against the cytoplasmic domain of glycophorin C) in common with this proteolytic fragment, in cells of a Leach phenotype, which are characterized by lack of extracellular epitopes of glycophorin C. When these membranes were extracted at low ionic strength to dissociate the membrane cytoskeleton, approximately half the content of 4.1 was liberated, compared with only some 25% from normal membranes. Cells of a different variant of the Leach phenotype, which are totally devoid of glycophorin C, lost close to 70% of their 4.1 under these circumstances. The Rh(D) transmembrane protein, which interacts with the membrane cytoskeleton, is also resistant to proteolysis of the cytoplasmic membrane surface, but Rhnull cells, devoid of this protein, showed no decreased retention of 4.1. The results suggest that glycophorin C (with D) may contain two types of binding site for 4.1, which would be sufficient in number to account for all the strong binding of 4.1 on normal membranes; modulation of binding at one of the sites by another protein or by lipid is not excluded. A possible site for reinitiation of translation overlapping the premature stop codon in the mutant expressing the truncated glycophorin C can be discerned.

Hereditary ovalocytosis with compensated haemolysis

The clinical and laboratory phenotype of compensated haemolysis in a patient with hereditary ovalocytosis is reported. Clinical presentation was intermittent jaundice and abdominal pain due to pigment gall stones. Haematological analysis revealed an absolute reticulocytosis with an otherwise normal full blood count and biochemical evidence of haemolysis. Variable results were observed with blood grouping reagents. The patient's red cells were stomatocytic ovalocytic, rigid, resistant to malarial parasite invasion, defective in anion transport, and had the characteristic two linked mutations in the red cell band 3 gene.

Actin in the merozoite of the malaria parasite, Plasmodium falciparum

Merozoites of the human malaria parasite, Plasmodium falciparum, when treated with cytochalasin B, will attach irreversibly to red cells with formation of a vestigial internal (parasitophorous) vacuole, but they are inhibited from moving into the cell. The existence of an actin-based motile mechanism is implied. Immunoblotting, peptide mapping and the DNase inhibition assay have been used to show that the merozoite contains actin. It makes up an estimated 0.3% of the total parasite protein and is partitioned in the ratio of about 1:2 between the cytosolic and particulate protein fractions. In the former it is unpolymerised and in the latter filamentous. Most of the anti-actin-reactive protein in the soluble fraction and about 20% of that in the pellet has an apparent molecular weight of 55,000 and reacts with an anti-ubiquitin antibody; it is thus evidently ubiquitinyl actin, or arthrin, which has so far been detected only in insect flight muscle.

Basis of unique red cell membrane properties in hereditary ovalocytosis

Hereditary ovalocytes from a Mauritian subject are extremely rigid, with a shear elastic modulus about three times that of normal cells, and have increased resistance to invasion by the malaria parasite Plasmodium falciparum in vitro. The genetic anomaly resides in band 3; the protein gives rise to chymotryptic fragments with reduced mobility in SDS/polyacrylamide gel electrophoresis, but this is a result of anomalous binding of SDS and not a higher molecular weight. Analysis of the band 3 gene reveals (1) a point mutation (Lys56----Glu), which also occurs in a common asymptomatic band 3 (Memphis) variant and governs the electrophoretic properties, and (2) a deletion of nine amino acid residues, including a proline residue, encompassing the interface between the membrane-associated and the N-terminal cytoplasmic domains. The interaction of the mutant band 3 with ankyrin appears unperturbed. The fraction of band 3 capable of undergoing translation diffusion in the membrane is greatly reduced in the ovalocytes. Cells containing the asymptomatic band 3 variant were normal with respect to all the properties that we have studied. Possible mechanisms by which a structural change in band 3 at the membrane interface could regulate rigidity are examined.

Fluorescence quenching of spectrin and other red cell membrane cytoskeletal proteins. Relation to hydrophobic binding sites

The intrinsic fluorescence of spectrin is strongly quenched by low concentrations of 2-bromostearate. This results from binding at a series of hydrophobic sites. Analysis of dynamic fluorescence quenching by acrylamide, iodide and caesium ions, separately and in conjunction with 2-bromostearate, leads to the conclusion that most of the tryptophan side-chains are exposed to solvent. The sites at which the fatty-acid-quenched tryptophans are located apparently interact with the lipid bilayer in the cell, as judged by quenching by bromostearate dissolved in the lipid phase. A minor proportion of the side-chains in native spectrin give rise to sharp proton magnetic resonance signals, indicative of segmental mobility; these chain elements contain some tryptophan residues, as revealed by weak downfield signals from the heterocyclic ring protons. These signals are not appreciably perturbed by stearic acid or by phosphatidylserine liposomes, suggesting that the hydrophobic binding sites are not in mobile chain elements. By contrast with a series of globular proteins which, with the exception of serum albumins, show little or no quenching by 2-bromostearate, the peripheral red cell membrane skeletal proteins ankyrin (and its spectrin-binding domain), protein 4.1 and (to a lesser extent) actin show evidence of a high affinity for the hydrophobic ligand and may, like spectrin, interact directly with the bilayer in situ.

Preparation and properties of human red-cell ankyrin

We describe a procedure for the preparation of ankyrin from human red cells with a yield of 2-3 mg of protein from 30 ml of packed cells. This represents an improvement of an order of magnitude over the usual earlier procedure. Moreover, the product is, in our hands, much more stable against adsorption and proteolysis, and can in general be stored for at least 2 months at 4 degrees C without significant decrease in concentration and binding activity. The preparation depends on the release of the ankyrin-band-3 complex from the membrane cytoskeleton when intact cells are lysed in a medium containing concentrated Triton X-100. The complex is dissociated at high ionic strength, and the final purification is achieved by gel filtration in a medium containing 2 M-Tris or 0.6 M-NaBr. The ankyrin contains all the progression of components present in the intact membrane. All react with affinity-purified polyclonal anti-ankyrin antibodies, and all give widely similar patterns of peptides in partial proteolytic digests. The ankyrin is fully active, as judged by its capacity to bind to band-3-containing membrane vesicles and to Sepharose-coupled spectrin. All components bind to the membrane vesicles. Purified components 2.1 and 2.2, as well as the calmodulin-binding cytoskeletal constituent adducin, can be isolated in pure form by a single anion-exchange column step.

Composition of the ternary protein complex of the red cell membrane cytoskeleton

The red cell membrane skeletal network is constructed from actin, spectrin and protein 4.1 in a molar ratio of actin subunits/spectrin heterodimer/protein 4.1 of 2:1:1. This represents saturation of the actin filaments, since incubation with extraneous spectrin and protein 4.1 leads to no binding of additional spectrin, either to the inner surface of ghost membranes or to lipid-free membrane cytoskeletons. Partial extraction of spectrin from the membrane is accompanied by release of actin under all conditions. Regardless of the proportion of spectrin extracted, the molar ratio of spectrin dimers/actin subunits is constant at 1:2. This is not the result of release or cooperative breakdown of whole lattice junctions from the network, for the number of actin filaments, judged by capacity to nucleate polymerisation of added G-actin, remains unchanged even when as much as 60% of the total spectrin has been lost. A similar 1:2:1 stoichiometry characterises the complex formed when G-actin is allowed to polymerise in the presence of varying amounts of spectrin and protein 4.1. When this complex is treated with the depolymerising agent, 1 M guanidine hydrochloride, it breaks down into smaller units of the same stoichiometry. After cross-linking these can be recovered from a gel-filtration column. Complexes prepared starting from G-actin appear to be much more stable than those formed when spectrin and protein 4.1 are bound to F-actin.

Study of actin filament ends in the human red cell membrane

There is conflicting evidence concerning the state of the actin protofilaments in the membrane cytoskeleton of the human red cell. To resolve this uncertainty, we have analysed their characteristics with respect to nucleation of G-actin polymerization. The effects of cytochalasin E on the rate of elongation of the protofilaments have been measured in a medium containing 0.1 M-sodium chloride and 5 mM-magnesium chloride, using pyrene-labelled G-actin. At an initial monomer concentration far above the critical concentration for the negative ("pointed") end of F-actin, high concentrations of cytochalasin reduce the elongation rate of free F-actin by about 70%. The residual rate is presumed to correspond to the elongation rate at the negative ends. By contrast, the elongation rate on red cell ghosts or cytoskeletons falls to zero, allowing for the background of self-nucleated polymerization of the G-actin. The critical concentration of the actin in the red cell membrane has been measured after elongation of the filaments by added pyrenyl-G-actin in the same solvent. It was found to be 0.07 microM, compared with 0.11 microM under the same conditions for actin alone. This is consistent with prediction for the case of blocked negative ends on the red cell actin. The rate of elongation of actin filaments, free and in the red cell membrane cytoskeleton, has been measured as a function of the concentration of an added actin-capping protein, plasma gelsolin, with a high affinity for the positive ends. The elongation rate falls linearly with increasing gelsolin concentration until it approaches a minimum when the gelsolin has bound to all positive filament ends. The elongation rate at this point corresponds to the activity of the negative ends, and its ratio to the unperturbed polymerization rate (in the absence of capping proteins) is indistinguishable from zero in the case of ghosts, but about 1 : 4 in the case of F-actin. When ATP is replaced in the system by ADP, so that the critical concentrations at the two filament ends are equalized, the difference is equally well-marked: for F-actin, the rate at the equivalence point is about 40% of that in the absence of capping protein, whereas for ghosts the nucleated polymerization rate at the equivalence point is again zero, indicating that under these conditions the negative ends contribute little or not at all to the rate of elongation.(ABSTRACT TRUNCATED AT 400 WORDS)

An examination of the soluble oligomeric complexes extracted from the red cell membrane and their relation to the membrane cytoskeleton

A part of the spectrin extracted from red cell membranes at low ionic strength occurs in the form of a high-molecular weight oligomeric complex with actin and proteins 4.1 and 4.9. When the extraction is performed at 35 degrees, the spectrin is present in this complex as the dimer, all higher forms being dissociated. We have been unable to establish any correlation between the fraction of the spectrin thus complexed and the metabolic state of the cell. At least a large part of the complex appears to be a defined monodisperse species, sedimenting at 31S. The actin is present as short protofilaments. The average number of spectrin molecules associated with each molecule of complex has been studied by cytochalasin binding and electron microscopy. The complexes present the appearance in the electron microscope of spiders, in which the legs are spectrin dimers, attached to a globular element, containing by inference, actin and proteins 4.1 and 4.9; they are active in nucleating the polymerization of G-actin. The complexes are extremely stable, being resistant to dissociation under the conditions of the deoxyribonuclease assay, even after treatment with trypsin to degrade the actin-associated proteins. It is suggested that the complexes represent intact junctions of the membrane cytoskeletal network. Relevant structural features of the network are revealed by electron microscopy. The results lead to inferences concerning the mechanism of dissociation of the network from the membrane.

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

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

Spectrin and protein 4.1 as an actin filament capping complex

Spectrin and protein 4.1, when added to G- or F-actin, cause the formation of short filaments, as judged by the appearance of powerful nucleating activity for G-actin polymerisation. F-Actin filaments are rapidly fragmented under physiological solvent conditions. The effect of cytochalasin E on the polymerisation reaction and the extent of reduction in the critical monomer concentration of actin when spectrin and 4.1 are added suggest that these proteins form a capping system for the more slowly growing, or 'pointed' ends of actin filaments. The interaction is not affected by calcium or by 4.9, the remaining constituent of the purified red cell membrane 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.

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.

An erythrocyte membrane-protein anomaly in march haemoglobinuria

In three patients with march haemoglobinuria a well-defined protein abnormality has been revealed by high-resolution sodium-dodecyl-sulphate/polyacrylamide-gel electrophoresis.

Gel electrophoretic analysis of poly(riboadenylic acid)

It is shown that molecular weights and molecular-weight distributions of poly(rA), and by implication other single-stranded polynucleotides, and synthetic and natural polyelectrolytes in general, can be determined by electrophoresis in polyacrylamide gels. It is shown that fractions of very narrow molecular-weight distribution can be obtained by preparative electrophoresis of polydisperse samples. Molecular-weight calibrations based on sedimentation coefficients of such fractions are given, and in aqueous systems do not coincide with calibrations for partially base-paired RNA species. Poly(rU) fractions fall on the same calibration as poly(rA). Relations between mobilities, relative to standard markers, and molecular weight for poly(rA) over a wide range of molecular weights are given, which allow rapid molecular-weight determination on poly(rA) samples, such as the segments found in many types of messenger RNA.