Biochemistry of ATP-dependent red cell membrane shape change

Volumetric Semantic Instance Segmentation of the Plasma Membrane of HeLa Cells

In this work, an unsupervised volumetric semantic instance segmentation of the plasma membrane of HeLa cells as observed with serial block face scanning electron microscopy is described. The resin background of the images was segmented at different slices of a 3D stack of 518 slices with 8192 × 8192 pixels each. The background was used to create a distance map, which helped identify and rank the cells by their size at each slice. The centroids of the cells detected at different slices were linked to identify them as a single cell that spanned a number of slices. A subset of these cells, i.e., the largest ones and those not close to the edges were selected for further processing. The selected cells were then automatically cropped to smaller regions of interest of 2000 × 2000 × 300 voxels that were treated as cell instances. Then, for each of these volumes, the nucleus was segmented, and the cell was separated from any neighbouring cells through a series of traditional image processing steps that followed the plasma membrane. The segmentation process was repeated for all the regions of interest previously selected. For one cell for which the ground truth was available, the algorithm provided excellent results in Accuracy (AC) and the Jaccard similarity Index (JI): nucleus: JI =0.9665, AC =0.9975, cell including nucleus JI =0.8711, AC =0.9655, cell excluding nucleus JI =0.8094, AC =0.9629. A limitation of the algorithm for the plasma membrane segmentation was the presence of background. In samples with tightly packed cells, this may not be available. When tested for these conditions, the segmentation of the nuclear envelope was still possible. All the code and data were released openly through GitHub, Zenodo and EMPIAR.

Phosphoinositide metabolism and shape control in sheep red blood cells

Metabolic depletion of sheep red blood cells leads to decreased intracellular concentrations of ATP and reduced glutathione as well as degradation of phosphoinositides. In sheep red blood cells, depletion of ATP induced two types of shape transformation: one early phase involving formation of protrusions on the cell surface similar to those observed upon depletion of human red blood cells; and one late phase, in which the sheep red blood cells develop long, rod-shaped projections. During the initial stages of shape changes, degradation of the phosphoinositides parallels the discocyte-echinocyte transformation, thus giving further support to a shape-controlling mechanism based on the bilayer-couple hypothesis. However, formation of the long projections does not coincide with turnover of the phosphoinositides but rather with the level of reduced glutathione. This indicates that development of these rod-like extensions on the cell surface is induced by oxidative processes that may well involve cross-linking of membrane skeleton proteins.

Human erythrocyte shape regulation: interaction of metabolic and redox status

The echinocyte-to-discocyte shape recovery of metabolically depleted erythrocytes is compromised by sulfhydryl reducing agents (Truong, H.-T.N., Ferrell, J.E., Jr. and Huestis, W.H. (1986) Blood 67, 214-221). In the presence of dithiothreitol (DTT) and sugars, crenated cells recover normal discoid shape transiently, but then develop the invaginations and intracellular inclusions of stomatocytes. The stomatogenic effects of DTT were investigated in erythrocytes recovering from crenation induced by several independent mechanisms. Cells crenated by direct manipulation of the membrane bilayer (lysophosphatidylcholine incorporation) recovered discoid shape similarly in the presence and absence of the reducing agent. In contrast, resealed ghosts and cells crenated by Mg2+ depletion or Ca2+ loading did not maintain stable discoid morphology in the presence of DTT, proceeding further to form stomatocytes. Thus cell crenation by expedients that involve cellular metabolic processes develop a redox-related morphological instability that is not found in amphipath-crenated cells.

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

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

Absence of phosphorylation-induced gelation of erythrocyte membrane skeletons: a diagnostic tool for hereditary spherocytosis

As yet there is no single test specific for the diagnosis of hereditary spherocytosis. In the search for a specific test, a method described by Pinder et al. [14] using a cAMP-independent protein kinase extracted from normal erythrocyte membranes was used. Membrane skeletons were prepared from erythrocyte ghosts by extraction with a non-ionic detergent, i.e., Triton X-100. Upon phosphorylation with c-AMP-independent protein kinase the suspension of normal membrane skeletons set to a gelatinous mass. Membrane skeletons from patients with spherocytosis failed to show this phenomenon. In order to clarify whether this phenomenological difference can be used as a diagnostic tool for hereditary spherocytosis, a semiquantitative method of observing the gelation process was used under definite shear stress conditions. We investigated 33 patients with different hemolytic anemias (spherocytosis, hereditary elliptocytosis, hereditary stomatocytosis, homozygous beta-thalassemia and enzymopenic hemolytic anemias). With the exception of spherocytosis, all preparations of membrane skeletons showed gelation after 30-50 min. Spherocytosis membrane skeletons did not show a significant gelation even after 12 h of incubation. Thus, the failing gelation is specific for the diagnosis of hereditary spherocytosis. The "gelation assay" might be a valuable method for defining patients with hemolytic anemias due to erythrocyte membrane defects. Its molecular basis and the possible importance for the pathogenesis of spherocytosis require further investigations.

Association of vanadate-sensitive Mg(2+)-ATPase and shape change in intact red blood cells

Intact human erythrocytes, initially depleted of Mg2+ by EDTA incubation in the presence of A23187, exhibit Mg(2+)-dependent phosphate production of around 1.5 mmol per liter cells.h, half-maximally activated at around 0.4 mM added free Mg2+. This appears to correspond to Mg(2+)-stimulated adenosine triphosphatase (Mg(2+)-ATPase) activity found in isolated membranes, which is known to have a similar activity and affinity for Mg2+. Vanadate (up to 100 microM) inhibited Mg(2+)-dependent phosphate production and ATP breakdown in intact cells. Over a similar concentration range vanadate (3-100 microM) transformed intact cells from normal discocytes to echinocytes within 4-8 h at 37 degrees C, and more rapidly in Mg(2+)-depleted cells. The rate of Ca(2+)-induced echinocytosis was also enhanced in Mg(2+)-depleted cells. These results support previous studies in erythrocyte ghosts suggesting that vanadate-induced shape change is associated with inhibition of Mg(2+)-ATPase activity localized in the plasma membrane of the red blood cell.

Relationship of hemolysis buffer structure, pH and ionic strength to spontaneous contour smoothing of isolated erythrocyte membranes

Isolated human erythrocyte membranes crenate when suspended in isotonic medium, but can use MgATP to reduce their net positive curvature, yielding smooth discs and cup forms that eventually undergo endocytosis. An earlier report from this laboratory (Patel, V.P. and Fairbanks, G. (1981) J. Cell Biol. 88, 430-440), has described a phenomenon of ATP-independent shape change in which ghosts prepared by hemolysis and washing in synthetic zwitterionic buffers crenated at 0 degree C, but underwent conversion to smooth discs and cups when warmed in the absence of MgATP. We have further explored the effect of the hemolysis condition on the requirement for ATP in ghost shape change. 25 hemolysis buffers were applied at 10 mM (pH 7.4, 0 degree C). Eight anionic buffers with relatively high ionic strength (e.g., phosphate and diethylmalonic acid (DMA] yielded ghosts requiring ATP for shape change, while two cationic buffers (Bistris and imidazole) and ten synthetic zwitterionic buffers (e.g., Tricine and Hepes) with lower ionic strength produced ghosts that smoothed spontaneously at 30 degrees C. Hemolysis at intermediate ionic strength yielded mixed populations in which spontaneous smoothing was expressed in all-or-none fashion. Maximal ATP-independent shape change was induced by hemolysis at pH 7.3-7.7, while ATP was required after hemolysis at pH less than or equal to 7.1 even when the ionic strength at hemolysis was low. Ghosts requiring ATP could be converted to ATP independence by washing at low ionic strength, but ATP independence could not be reversed readily by washing at high ionic strength. Exposure to low ionic strength at pH greater than 7.1 presumably changes membrane organization in a way that alters the temperature dependence of tensions within the bilayer or skeleton of the composite membrane.

Dynamic association of band 3 with triton shells in human erythrocyte ghosts

To test a possibility that free band 3 and ankyrin-linked band 3 are exchanged in situ, band 3 was labeled with 125I, using intact red blood cells and lactoperoxidase. The cytoplasmic surface of this labeled band 3 was considered to be intact. When Triton shells were incubated with Triton supernatants prepared from 125I-labeled intact erythrocytes at 37 degrees C in the presence of Mg-ATP under isotonic conditions, the incorporation of free 125I-labeled band 3 to shells was observed. This incorporation was affected by the presence of Triton X-100 in the incubation mixture, and significantly decreased when the content of Triton X-100 was less than 0.04% (v/v). On the other hand, ankyrin-linked 125I-labeled band 3 was released when shells prepared from 125I-labeled intact erythrocytes were incubated with the Triton supernatants at 37 degrees C under the same condition as when free 125I-labeled band 3 incorporation was observed. These results strongly suggest that free and ankyrin-linked band 3 exchanged with each other in the presence of Triton X-100. A water-soluble 43 kDa fragment of band 3 inhibited the incorporation of free 125I-labeled band 3 to the shells and also inhibited the Mg-ATP-dependent shape change of ghosts in the absence of Triton X-100. Both of these inhibitory effects remained, even after 10 min of heat treatment at 100 degrees C, but drastically decreased by treatment with trypsin. Our results strongly suggest that a dynamic exchange of the free band 3 for ankyrin-linked band 3 may occur in intact erythrocytes, and it may even contribute to the shape change of erythrocytes.

The human erythrocyte membrane skeleton may be an ionic gel. II. Numerical analyses of cell shapes and shape transformations

In the first paper in this series (Stokke et al. Eur Biophys J 1986, 13:203-218) we developed the general theory of the mechanochemical properties and the elastic free energy of the protein gel--lipid bilayer membrane model. Here we report on an extensive numerical analysis of the human erythrocyte shapes and shape transformations predicted by this new cell membrane model. We have calculated the total elastic free energy of deformation of four different cell shape classes: disc-shaped cells, cup-shaped cells, crenated cells, and cells with membrane invaginations. We find that which of these shape classes is favoured depends strongly on the spectrin gel osmotic tension, IIGu, and the surface tensions, IIEu and IIPu, of the extracellular and protoplasmic halves of the membrane lipid bilayer, respectively. For constant ratio IIEu/IIPu greater than O large negative or positive values of IIGu favour respectively the crenated and invaginated cell shape classes. For small absolute values of IIGu, IIEu, and IIPu, biconcave or cup-shaped cells are the stable ones. Our numerical analysis shows that the higher the membrane skeleton compressibility is, the smaller are the values of IIGu needed to induce cell shape transformation. We find that the stable and metastable shapes of discocytes and stomatocytes generally depend both on the shape of the stressfree membrane skeleton and the membrane skeleton compressibility.

Calmodulin inhibits the phosphorylation of spectrin in vitro

In vitro phosphorylation of purified spectrin dimer was studied in the presence of Ca2+-calmodulin (CaM). CaM inhibited autophosphorylation of the beta subunit of spectrin. The inhibitory effect (65% at a 32-fold molar excess) appeared to be due to a weak interaction of CaM with spectrin. CaM was similarly effective in a phosphatase-stimulated autothiophosphorylation of the beta subunit with [gamma-35S]ATP. Hence, its inhibitory effect was not due to stimulation of a spectrin-associated phosphatase activity. Phosphorylation of spectrin by the catalytic subunit of a cAMP-dependent protein kinase occurred in both subunits (1984, FEBS Lett. 169, 323). CaM selectively inhibited a cAMP-dependent phosphorylation of the alpha subunit of spectrin to 30% at two CaM per spectrin. It was ineffective on the cAMP-dependent phosphorylation of the beta subunit up to a 32-fold molar excess. These results yield functional evidence for a CaM-spectrin interaction. They further suggest that CaM can regulate the extent of a cAMP-dependent phosphorylation of the alpha subunit of spectrin.

Spectrin, red cell shape and deformability. II. The antagonistic action of spectrin and sialic acid residues in determining membrane curvature in genetic spectrin deficiency in mice

In a companion paper, the shapes of spectrin deficient mouse erythrocytes were described; in contrast to previous assumptions, spherules with tethered microvesicles rather than true "spherocytes" were found. Thence, spectrin deficient mouse erythrocytes are endowed with an excess of surface area for the given volume but the membrane is assuming a highly positive curvature. Observations during and after the action of enzymes cleaving the red cell surface charge (Neuraminidase, Trypsin, Chymotrypsin) showed that the previously positive membrane curvature, as well as the tendency of the membrane to flow into fingerlike protrusions was completely abolished. The erythrocytes of the spectrin deficient, desialylated mouse erythrocytes assumed a variety of shapes, often discocytic or even stomatocytic, i.e. their membrane presented with negative curvature. However, while these desialylated membranes could be easily deformed (elongated) by shear flow they did not recoil elastically into any definitive configuration after removal of the deforming forces. It is concluded from these observations that spectrin (acting on the inner interface between membrane and cytoplasm) and sialic acid residues (acting on the outer interface between membrane and plasma) exert antagonizing effects on membrane curvature and membrane bending elasticity. Sialic acid residues, strongly charged and situated on the outer side of the cell, produce positive membrane curvature; this observation can most readily be explained by assuming that this mechanical effect is caused by repulsive coulombic forces expanding the outer half of the bilayer. To explain the effect of the spectrin-complex in counteracting positive or in producing negative membrane curvature, a similar expansive coulombic force acting between the highly charged residues has been postulated. Thence, a model for explaining the overall elastic behaviour of the normal mammalian red cell is developed which is based on the assumption of elastic interactions of proteinacous membrane components coupled to the lipid bilayer of the membrane.

Changes in morphology and in polyphosphoinositide turnover of human erythrocytes after cholesterol depletion

Human erythrocytes were cholesterol-depleted (5-25%) by incubation with phosphatidylcholine vesicles in media containing Ca2+ at different concentrations (0, 28 nM, 5 microM or 1 mM). After removal of the vesicles, the cells were reincubated with [32P]phosphate in the same media. Control (incubated in buffer alone) and cholesterol-maintained erythrocytes (incubated with cholesterol/phosphatidylcholine vesicles) were treated similarly. Cholesterol depletion induced the conversion of the cells into stomatocytes III and spherostomatocytes and decreased the turnover rate of phosphatidylinositol phosphate and of phosphatidylinositol bisphosphate. None of these effects were observed in cholesterol-maintained cells. In cholesterol-depleted cells, they occurred without changes in the ATP specific activity or in the polyphosphoinositide concentrations. Moreover, these modifications of shape and of lipid metabolism were proportional to the extent of the cholesterol depletion and were independent of the external Ca2+ concentration. In contrast, other effects of cholesterol depletion, a decrease in the turnover rate of phosphatidic acid, a decrease in diacylglycerol and in phosphatidic acid concentrations were dependent on the external Ca2+ concentration. Thus it appears that the shape change was not correlated with a change in the concentrations of these phospholipids or of diacylglycerol and therefore cannot be explained by a bilayer couple mechanism involving these phospholipids. However, the spherostomatocytic transformation was correlated with the decrease in the turnover rate of the polyphosphoinositides, but not with the turnover rate of phosphatidic acid, suggesting a role for the turnover of the polyphosphoinositides in the maintenance of the erythrocyte shape.

The role of ankyrin in shape and deformability change of human erythrocyte ghosts

Human erythrocyte membranes (ghosts) from acid/citrate/dextrose preserved blood were digested with trypsin (protein/trypsin = 100:1) under hypotonic conditions and then analyzed by SDS-polyacrylamide gel electrophoresis. After digestion for about 20-30 s at 0 degree C, only ankyrin had disappeared and other bands including spectrin, actin, band 4.1 and band 3 remained intact. This observation was supported by electron micrographs showing that the horizontally disposed, filamentous structure was a little apart from the lipid bilayer and its components were not destroyed. In contrast to intact ghosts, treatment with chlorpromazine, or Mg-ATP did not induce shape change in these trypsin-treated ghosts. The number of transformable cells correlated closely with the amount of remaining ankyrin in the SDS-polyacrylamide gel electrophoresis pattern. Furthermore, the chlorpromazine- and Mg-ATP-induced decreases in viscosity of suspensions of erythrocyte ghosts were also prevented by trypsin treatment for 20-30 s at 0 degree C. These findings suggest that ankyrin plays an important role in the change in shape and deformability of erythrocyte ghosts. The molecular mechanism of drug-induced shape change and the role of undermembrane structure in regulating erythrocyte shape and deformability are discussed.

Phosphoinositide metabolism and the morphology of human erythrocytes

ATP-depleted human erythrocytes lose their smooth discoid shape and adopt a spiny, crenated form. This shape change coincides with the conversion of phosphatidylinositol-4,5-bisphosphate to phosphatidylinositol and phosphatidic acid to diacylglycerol. Both crenation and lipid dephosphorylation are accelerated by iodoacetamide, and both are reversed by nutrient supplementation. The observed changes in lipid populations should shrink the membrane inner monolayer by 0.6%, consistent with estimates of bilayer imbalance in crenated cells. These observations suggest that metabolic crenation arises from a loss of inner monolayer area secondary to the degradation of phosphatidylinositol-4,5-bisphosphate and phosphatidic acid. A related process, crenation after Ca2+ loading, appears to arise from a loss inositides by a different pathway.

Ca2+- and Mg-ATP-dependent shape change of human erythrocyte ghosts and triton shells

Human erythrocyte membranes (ghosts) prepared from fresh blood changed in shape from spherical to crenated, when suspended in 10(-7)-10(-6) M Ca2+-EGTA buffers. Although the ghosts from long-stored ACD blood (10 weeks) were less sensitive to 10(-7)-10(-6) M Ca2+, the ghosts obtained from this blood after it had been preincubated with adenine and inosine for 3 h at 37 degrees C were highly sensitive to Ca2+. When these highly sensitive ghosts were incubated in 10 mM Tris-Cl buffer (pH 7.4) or 1 mM MgCl2 (pH 7.4) at 0 degrees C, they gradually lost Ca2+ sensitivity within 60 min, but they recovered Ca2+ sensitivity again after re-incubation with 2 mM Mg-ATP for 20 min at 37 degrees C followed by washing with 1 mM MgCl2 (pH 7.4). The shape of these highly Ca2+-sensitive ghosts immediately changed from crenate to disc on addition of 1 mM Mg-ATP even at 6 degrees C in the presence of 10(-7)-10(-6) M Ca2+. A similar shape change was also observed when ghosts treated with 0.5% Triton X-100 (Triton shells) were used. Triton shells from fresh blood ghosts or from long-stored blood ghosts which had been preincubated with 2 mM Mg-ATP for 20 min at 37 degrees C shrank immediately in the presence of 10(-6) M Ca2+ and then swelled on addition of 1 mM Mg-ATP. The specificity to ATP and the dependency on ATP concentration are in agreement with those of the ghost shape change at step 2 (Jinbu, Y. et al., Biochem biophys res commun 112 (1983) 384-390) [18]. These results suggest that cytoskeletal protein phosphorylation enhances sensitivity to Ca2+ and induces erythrocyte shape change in the presence of physiological concentrations of ATP and Ca2+.

Reversible shape change of Triton-treated erythrocyte ghosts induced by Ca2+ and Mg-ATP

The shape of the human erythrocyte depends on intracellular ATP content and echinocytic erythrocyte ghosts obtained in the presence of Mg-ATP acquire a diskocytic shape after incubation at 37 degrees C; however, agreement is lacking about the molecular basis of the shape changes. The suggestion that phosphorylation of cytoskeletal structures underlying the membrane is involved has been disputed and alternative explanations based on lipid bilayer theory and metabolism of phospholipid have been proposed. Recently, we re-examined the effect of ATP on the shape of ghosts and found that it consists of two distinct steps. We have, therefore, now examined the effect of physiological concentrations of Ca2+ and ATP on the cytoskeleton of the erythrocyte membrane directly using Triton-treated ghosts which had lost this permeability barrier. Our findings suggest that a noncovalent effect of ATP on the cytoskeleton is a prerequisite for shape change.

Inhibition of erythrocyte membrane shape change by band 3 cytoplasmic fragment

The ATP-dependent transformation of crenated white human erythrocyte ghosts into smoothed disc and cup forms is inhibited by the soluble 40-45-kilodalton (kDa) cytoplasmic portion of the major transmembrane protein, band 3. The band 3 fragment was prepared by chymotryptic treatment of inverted vesicles stripped of peripheral proteins. When present at greater than or equal to 0.2 mg per mg membrane protein (ie, greater than or equal to 2 mol fragment per mol endogenous band 3), the fragment significantly reduced the rate of shape change but did not alter the proportion of membranes that were ultimately converted into smoothed forms (greater than 90%). The inhibitory activity of the fragment could not be attributed to contamination of the fragment preparation by actin or proteolytic enzymes. ATP-independent shape transformation was not inhibited. The band 3 fragment may compete with endogenous, intact band 3 for an association with the spectrin-actin network required for ATP-dependent smoothing of crenated membranes.

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.

A cytoplasmic requirement of red cells for invasion by malarial parasites

Human red cells, when lysed by dialysis at high haematocrit against a medium of low ionic strength and then dialysed back to physiological saline at 37 degrees C, give rise to resealed ghosts that are invaded with high efficiency by Plasmodium falciparum parasites. When the haematocrit is reduced, a critical concentration is reached, such that the resealed ghosts no longer support invasion. This indicates that a constituent of the cytoplasm becomes diluted to a concentration below a critical level. This constituent is evidently ATP, for when extraneous ATP is added to the diluent and the dialysate, the susceptibility to invasion is fully restored. This does not occur when the non-hydrolysable analogue, adenylyl-imidodiphosphate (AMP-PNP) is substituted for ATP, whereas the hydrolysable ATP analogue, adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S), which can be utilised by kinases, can partly replace ATP. Stimulation of invasion by the addition of 2,3-diphosphoglycerate was also associated with a perceptible rise in ATP concentration. The invasion process does not appear to involve intracellular calcium, for EGTA introduced into the resealed ghost has no detectable effect. Moreover, vanadate in the medium does not appreciably inhibit invasion, and it is thus unlikely that the requirement for ATP is linked to the activity of membrane ion-pump enzymes. An inhibitor of phosphorylation, adenosine, introduced into the cells at high concentration, causes significant inhibition of invasion. The results suggest that ATP is required for maintaining the turnover of phosphoryl groups of membrane-associated proteins, such as spectrin. A basic scheme for the mechanism of the invasion process is suggested. In addition to the effect of ATP, it is also shown that with greater dilution, and in the presence of ATP, there is an abrupt loss of susceptibility to invasion. It is inferred that this is due to the dilution of another essential cytoplasmic constituent to below a critical concentration. This second constituent has not yet been identified.

Two steps in ATP-dependent shape change of human erythrocyte ghosts

Most human erythrocyte membranes (ghosts) prepared with 10 mM Tris-Cl buffer were spherocytic and changed shape through crenated to discoidal in the presence of 2 mM Mg-ATP at 37 degrees C during 30 min under hypotonic conditions. These discocytic ghosts reverted to spherical form after being washed with 1 mM MgCl2, although their membranes were phosphorylated and they were converted to discocytes again, immediately on addition of 2 mM Mg-ATP, even at 6 degrees C. There seem to be 2 steps in the shape change of the ghosts; the first step proceeds gradually during incubation at 37 degrees C for 30 min in the presence of a physiological concentration of Mg-ATP and the second step occurs rapidly after addition of Mg-ATP even at 6 degrees C. This suggests that not only membrane phosphorylation but also specific ATP-binding (or hydrolysis) is necessary for erythrocyte shape change.

Properties of phosphatidylinositol kinase activities in rabbit erythrocyte membranes

Properties of phosphatidylinositol kinase activities in rabbit erythrocyte membranes were studied by measuring 32P incorporation into di- and triphosphoinositide from Mg-[gamma-32P]ATP. The Km's for 32P incorporation into di- and triphosphoinositide were 110 and 48 microM ATP, respectively. The optimal temperature for 32P incorporation into diphosphoinositide was at 32 degrees C, whereas the optimum for triphosphoinositide labeling occurred at 43 degrees C. Differences in the effects of pH on the rate of 32P incorporation into di- and triphosphoinositide were also found. At 37 degrees C but not at 25 degrees C 32P-labeled diphosphoinositide was phosphorylated to triphosphoinositide in the presence of Mg-ATP. Triton X-100 partially inhibited 32P incorporation into diphosphoinositide but completely inhibited the synthesis of triphosphoinositide. At physiological concentrations, 0.4 mM MgCl2 half-maximally activated di- and triphosphoinositide synthesis. Higher concentrations of MgCl2 (5 to 50 mM) decreased 32P incorporation into diphosphoinositide and greatly enhanced 32P incorporation into triphosphoinositide. NaCl or KCl (less than or equal to 100 mM) did not have any effects on polyphosphoinositide synthesis, whereas 150 to 300 mM NaCl or KCl decreased synthesis of diphosphoinositide and increased synthesis of triphosphoinositide. Further studies showed that 50 mM MgCl2 and 200 mM NaCl or KCl stimulate kinase-mediated phosphorylation of diphosphoinositide to triphosphoinositide. Triton X-100 inhibited the ability of 50 mM MgCl2 and neomycin to stimulate phosphorylation of diphosphoinositide to triphosphoinositide. The pathways for synthesis of di- and triphosphoinositides in erythrocyte membranes are discussed.