A spectrin-dependent ATPase of the human erythrocyte membrane

Role of Spectrin in Endocytosis

Cytoskeletal spectrin is found in (non)erythroid cells. Eukaryotic endocytosis takes place for internalizing cargos from extracellular milieu. The role of spectrin in endocytosis still remains poorly understood. Here, I summarize current knowledge of spectrin function, spectrin-based cytoskeleton and endocytosis of erythrocytes, and highlight how spectrin contributes to endocytosis and working models in different types of cells. From an evolutionary viewpoint, I discuss spectrin and endocytosis in a range of organisms, particularly in plants and yeast where spectrin is absent. Together, the role of spectrin in endocytosis is related to its post-translational modification, movement/rearrangement, elimination (by proteases) and meshwork fencing.

The Contribution of Genetic and Environmental Factors to Quantitative Variability of Erythrocyte Membrane Proteins in Primary Hypotension

Our previous studies have shown that, compared with healthy individuals, patients with primary arterial hypotension (PAH) have significant quantitative changes in erythrocyte membrane proteins. The purpose of the present study was to evaluate the contribution made by genetic and environmental factors to quantitative variation of erythrocyte membrane proteins in PAH. We studied 109 hypotensive patients, 124 normotensive subjects, 222 of their first-degree relatives and 24 twin pairs by sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis. The decomposition of total phenotypic variance of erythrocyte membrane proteins to genetic and environmental components was performed on the basis of correlations among first-degree relatives by the least squares method. The genetic dominance and shared environmental factors were found to influence the variability of cytoskeletal membrane proteins whose contents were changed in PAH. Furthermore, variations in alpha-spectrin, actin and anion exchanger in hypotensives were substantially influenced by major gene and maternal effects. Ankyrin 2.1 and actin content was under the control of common underlying genes. Variations in membrane-associated glutathione-S-transferase and tropomyosin were predominantly affected by polygenes. These findings suggest that the putative major genes with pleiotropic effects appear to be involved in the control of quantitative disorders of erythrocyte membrane proteins in primary hypotension.

Mode of action of lithium in affective disorders. An influence on intracellular calcium functions

The inference that lithium acts by altering intracellular calcium functions is supported by the three areas considered above. First, recent work in other laboratories has broadened the range of lithium actions on calcium-dependent physiological functions. Second, a theoretical analysis of the coupling of calcium transport to the triphosphoinositide response presents a plausible mechanism by which lithium could limit the damage caused by deficient calcium transport. Third, we have recently reported that there is a direct enhancement of the calmodulin-activated membrane-bound calcium pump in lithium-treated bipolar subjects.

Increased content of a minor ankyrin in erythrocyte membranes of bipolar subjects

This preliminary study reports on the increased content of a structurally important protein in erythrocyte membranes of bipolar patients treated with lithium carbonate compared with control subjects. The external portion of the erythrocyte membrane is a lipid bilayer into which various integral proteins are inserted; some of these serve as ion channels or energy-dependent transport systems. Another group of proteins, including those termed "ankyrins," link the bilayer to an underlying network of cytoskeletal proteins, and maintain both cell shape and the arrangement of integral proteins within the bilayer. Structural changes resulting from the observed excess of one of the ankyrins in membranes from bipolar subjects could alter several aspects of membrane function. Ankyrins also occur in neurons, where a corresponding increase would alter central nervous system function.

The reaction of Mg2+ with the Ca2+-ATPase from human red cell membranes and its modification by Ca2+

Media prepared with CDTA and low concentrations of Ca2+, as judged by the lack of Na+-dependent phosphorylation and ATPase activity of (Na+ +K+)-ATPase preparations are free of contaminant Mg2+. In these media, the Ca2+-ATPase from human red cell membranes is phosphorylated by ATP, and a low Ca2+-ATPase activity is present. In the absence of Mg2+ the rate of phosphorylation in the presence of 1 microM Ca2+ is very low but it approaches the rate measured in Mg2+-containing media if the concentration of Ca2+ is increased to 5 mM. The KCa for phosphorylation is 2 microM in the presence and 60 microM in the absence of Mg2+. Results are consistent with the idea that for catalysis of phosphorylation the Ca2+-ATPase needs Ca2+ at the transport site and Mg2+ at an activating site and that Ca2+ replaces Mg2+ at this site. Under conditions in which it increases the rate of phosphorylation, Ca2+ is without effect on the Ca2+-ATPase activity in the absence of Mg2+ suggesting that to stimulate ATP hydrolysis Mg2+ accelerates a reaction other than phosphorylation. Activation of the E1P----E2P reaction by Mg2+ is prevented by Ca2+ after but not before the synthesis of E1P from E1 and ATP, suggesting that Mg2+ stabilizes E1 in a state from which Mg2+ cannot be removed by Ca2+ and that Ca2+ stabilizes E1P in a state insensitive to Mg2+. The response of the Ca2+-ATPase activity to Mg2+ concentration is biphasic, activation with a KMg = 88 microM is followed by inhibition with a Ki = 9.2 mM. Ca2+ at concentration up to 1 mM acts as a dead-end inhibitor of the activation by Mg2+, and Mg2+ at concentrations up to 0.5 mM acts as a dead-end inhibitor of the effects of Ca2+ at the transport site of the Ca2+-ATPase.

Is there any connection between heat inactivation of spectrin-dependent ATPase and loss of smooth biconcave shape of red cells?

Spectrin-dependent ATPase activity was measured in membranes from native human erythrocytes and erythrocytes heated for 20 min at different temperatures. This activity was found to decline when the erythrocytes were heated at 48 degrees C and higher. The break in ATPase activity corresponds to morphological changes in erythrocytes found by Crome and Mollison [Brit. J. Haematol. (1964) 10, 137]. The role of spectrin-dependent ATPase in erythrocyte shape maintenance is discussed.

Effects of divalent metal ions on the calcium pump and membrane phosphorylation in human red cells

In inside-out red cell membrane vesicles ATP-dependent calcium transport is activated by the divalent metal ions Mg2+, Mn2+, Co2+, Ni2+ and Fe2+. This activation is based on the formation of Me2+ -ATP complexes which can serve as energy-donor substrates for the calcium pump, and probably, satisfy the requirement for free Me2+ in this transport process. Higher Me2+ concentrations inhibit calcium transport with various efficiencies. Mn2+ directly competes with Ca2+ at the transport site, while other divalent metal ions investigated have no such effect. The formation of the hydroxylamine-sensitive phosphorylated intermediate (EP) of the red cell membrane calcium pump from [gamma-32P]ATP is induced by Ca2+ while rapid dephosphorylation requires the presence of Mg2+. At higher concentrations Mn2+ and Ni2+ inhibit predominantly the formation of EP, while Co2+ and Fe2+ block dephosphorylation. The possible sites and nature of the divalent metal interactions with the red cell calcium pump are discussed. Hydroxylamine-insensitive membrane phosphorylation in inside-out vesicles from [gamma-32P]ATP is significantly stimulated by Mn2+ and Co2+, as compared to that produced by Mg2+, Fe2+ and Ni2+. Part of this labelling is found in phospholipids, especially in phosphatidylinositol. The results presented for the metal dependency of protein and lipid phosphorylation in red cell membranes may help in the characterization of ATP consumptions directly related to the calcium pump and those involved in various regulatory processes.

The molecular basis for membrane - cytoskeleton association in human erythrocytes

Spectrin, the major cytoskeletal protein in erythrocytes, is localized on the inner membrane surface in association with membrane-spanning glycoproteins and with intramembrane particles. The presence of a specific, high-affinity protein binding site for spectrin on the cytoplasmic surface of the membrane has been established by measurement of reassociation of spectrin with spectrin-depleted inside-out vesicles. A 72,000 Mr proteolytic fragment of this attachment protein has been purified, which bound to spectrin in solution and competed for reassociation of spectrin with vesicles. A 215,000 Mr polypeptide has been identified as the precursor of the spectrin-binding fragment. The membrane attachment protein for spectrin was named ankyrin, and has been purified and characterized. Ankyrin has been demonstrated to be tightly associated in detergent extracts of vesicles with band 3, a major membrane-spanning polypeptide, and to bind directly to a proteolytic fragment derived from the cytoplasmic domain of band 3. Ankyrin is thus an example of a protein that directly links a cytoplasmic structural protein to an integral membrane protein. The organization of the erythrocyte membrane has implications for more complex cell types since immunoreactive forms of ankyrin distinct from myosin or filamin have been detected by radioimmunoassay in a variety of cells and tissues. Indirect immunofluorescent staining of cultured cells reveals immunoreactive forms of ankyrin in a cytoplasmic meshwork and in a punctate distribution over nuclei. The staining changes dramatically during mitosis, with concentration of stain at the spindle poles in metaphase and intense staining of the cleavage furrow during cytokinesis.

Calcium ions, drug action and the red cell membrane

Intracellular calcium regulates a number of membrane functions in the erythrocyte, including control of shape, membrane lipid composition and cation permeability. Measurement of total red cell calcium has yielded values between 5 and 15 nmol/ml cells, and these low values in part reflect the absence of Ca2+ -containing organelles. Most intracellular Ca2+ is bound and the low cell ionized Ca2+ concentration (approximately 0.2 microM) is maintained by a combination of low membrane permeability and a powerful Ca2+ -pump. This pump has been identified with a (Ca2+ + Mg2+)-stimulated ATPase, and both Ca2+ transport and ATP splitting are stimulated by calmodulin, a low molecular weight protein which binds Ca2+ avidly and activates many Ca2+ -dependent enzymes. Both high and low affinity kinetics for Ca2+ pumping have been demonstrated, depending on the extent of binding of calmodulin to the pump. A stoichiometry of either 1 or 2 Ca2+ ions pumped per ATP molecule split has been shown, and the value may vary with the level of intracellular Ca2+. Phenothiazines, such as chlorpromazine inhibit the Ca2+ -pump by antagonizing the increment in activity produced by calmodulin. The passive inward leak of Ca2+ into erythrocytes can be quantitated by 45Ca2+ uptake into red cells whose Ca2+ -pump has been inhibited. Estimates of the Ca2+ permeability, based on unidirectional influx, yield values many orders of magnitude lower than for nucleated cells. Influx of Ca2+ into human erythrocytes occurs by a facilitated diffusion process, which can be inhibited by phenothiazines and the cinchona alkaloids. Calcium affects many membrane functions including cation permeability, lipid composition and some cytoskeletal interactions which may determine cell shape. Any rise in intracellular Ca2+ activates a specific K+ channel which normally makes little contribution to K+ fluxes. Kinetic studies of this process demonstrate either high or low affinity Ca2+ -activation of K+ efflux, with low affinity of the channel to Ca2+ being the probable state in vivo. Propranolol is the best known activator of Ca2+ -stimulated K+ efflux, although the mechanism of stimulation is unclear. Like other tissues, red cells possess a Ca2+ -activated phosphoinositol phosphodiesterase. Although it has been suggested that the echinocytic shape change induced by Ca2+ is due to the hydrolysis of polyphosphoinositides, it seems more likely that this shape change results from an effect of Ca2+ on the macromolecular interactions of the cytoskeleton. Abnormal Ca2+ permeability may contribute to red cell destruction in a variety of diseases. For example, in sickle cell anemia a large Ca2+ influx occurs when cells are sickled under deoxy conditions, and moreover, the ability of the Ca2+ -pump to extrude the increment of cell Ca2+ is impaired. Thus, red cell Ca2+ is increased 3-7-fold above normal and this may contribute to the short survival of sickle red cells...