Erythrocyte Ca2(+)-ATPase: activation by enzyme oligomerization versus by calmodulin

Insights into the oligomerization process of the C-terminal domain of human plasma membrane Ca²+-ATPase

Plasma membrane calcium pumps (PMCAs) sustain a primary transport system for the specific removal of cytosolic calcium ions from eukaryotic cells. PMCAs are characterized by the presence of a C-terminal domain referred to as a regulatory domain. This domain is target of several regulatory mechanisms: activation by Ca²+-calmodulin complex and acidic phospholipids, phosphorylation by kinase A and C, proteolysis by calpain and oligomerization. As far as oligomerization is concerned, the C-terminal domain seems to be crucial for this process. We have cloned the C-terminal domain of the human PMCA isoform 1b, and characterized its properties in solution. The expressed protein maintains its tendency to oligomerize in aqueous solutions, but it is dissociated by amphipathic molecules such as diacylglycerol and sodium dodecyl sulphate. The presence of sodium dodecyl sulphate stabilizes the domain as a compact structure in monomeric form retaining the secondary structure elements, as shown by small angle neutron scattering and circular dichroism measurements. The importance of oligomerization for the regulation of PMCA activity and intracellular calcium concentration is discussed.

Isoform-specific distribution of the plasma membrane Ca2+ ATPase in the rat brain

Regulation of cytoplasmic calcium is crucial both for proper neuronal function and cell survival. The concentration of Ca2+ in cytoplasm of a neuron at rest is 10,000 times lower than in the extracellular space, pointing to the importance of the transporters that extrude intracellular Ca2+. The family of plasma membrane calcium-dependent ATPases (PMCAs) represent a major component of the Ca2+ regulatory system. However, little information is available on the regional and cellular distribution of these calcium pumps. We used immunohistochemistry to investigate the distribution of each of the four PMCA isoforms (PMCA1-4) in the rat brain. Each isoform exhibited a remarkably precise and distinct pattern of distribution. In many cases, PMCA isoforms in a single brain structure were differentially expressed within different classes of neurons, and within different subcellular compartments. These data show that each isoform is independently organized and suggest that PMCAs may play a more complex role in calcium homeostasis than generally recognized.

Oligomerization of the plasma membrane calcium pump involves two regions with different thermal stability

Ca(2+) pump dimerization was studied by using a combined approach of thermal denaturation and fluorescence resonance energy transfer. The measurement of calcium pump ability to dimerize after the unfolding of individual functional domains of the enzyme demonstrated the existence of two different regions involved in the self-association process. One of these regions is highly susceptible to thermal unfolding and was identified as the calmodulin (CaM)-binding domain. The other region whose thermal stability is higher than those of the catalytic and CaM-binding domains could be related with the previously found C28W-binding regions.

The active species of plasma membrane Ca2+-ATPase are a dimer and a monomer-calmodulin complex

The purified plasma membrane Ca2+-ATPase is fully activated through the enzyme concentration-dependent self-association at physiologically relevant Ca2+ concentrations (Kosk-Kosicka, D., and Bzdega, T. (1988) J. Biol. Chem. 263, 18184-18189; Kosk-Kosicka, D., Bzdega, T., and Wawrzynow, A. (1989) J. Biol. Chem. 264, 19495-19499). We have previously shown that the Ca2+-ATPase activity of the oligomeric enzyme is independent of calmodulin, in contrast to another active enzyme species, a presumable monomer, that is activated by calmodulin binding. Presently, we have succeeded in determining the molecular mass of the two active enzyme species by equilibrium ultracentrifugation. For the calmodulin-dependent species, the molecular mass is 170 +/- 30 kDa, which is consistent with predominantly monomeric Ca2+-ATPase with bound calmodulin. The molecular mass of calmodulin-independent oligomers is 260 +/- 34 kDa, indicating that they are dimers. Results of experiments performed under different calcium and potassium concentrations and in the presence of dextran that causes molecular crowding verify a strict Ca2+ requirement of the dimerization process. We conclude that the active species of the Ca2+-ATPase are a monomer-calmodulin complex and a dimer.

The plasma membrane calcium pump--a physiological perspective on its regulation

This review focuses on the physiological role of the plasma membrane Ca(2+)+ Mg(2+)-dependent adenosine triphosphatase (PM Ca(2+)-ATPase) in cellular signalling. Particular attention has been paid to the regulation of the PM Ca(2+)-ATPase (PM Ca2+ pump) by calmodulin, proteases, protein kinases, acidic phospholipids and oligomerization in intact cells. We also review recent work investigating the possible regulation of the PM Ca2+ pump by G proteins and agonists. The source of adenosine triphosphate (ATP) and Ca2+ in fueling and activating the Ca2+ pump is discussed, as well as the possible role of the PM Ca(2+)-ATPase in subplasma membrane Ca2+ regulation. The physiological implication of the localisation of the PM Ca2+ pump in caveolae is also considered.

Regulation of the erythrocyte Ca(2+)-ATPase at high pH

The activation of the Ca(2+)-ATPase from erythrocyte membranes at high pH has been investigated. Following alkalinization and in the absence of regulators, the enzyme exhibits a very high affinity for Ca2+ and a decreased maximal velocity. Either addition of calmodulin, addition of acidic phospholipids, or controlled trypsinization decreases the concentration of effector required to elicit half-maximal activation of the enzyme for calcium to similar values. The increase in affinity for Ca2+, however, is smaller than that observed at neutral pH. The maximal velocity at high pH becomes insensitive to both calmodulin and controlled proteolysis, although calmodulin binds to the protein with similar affinities at pH 7.0 and 8.0, as indicated by similarity in binding to a calmodulin-Sepharose resin and in dependence on calmodulin concentrations when the pH is increased. In contrast to the attenuated effects of calmodulin and proteolysis, at pH 8.0 the enzyme is susceptible to stimulation by phospholipids, indicating that the pathway for transduction of the signal from phospholipids is distinct from that pathway engaged by calmodulin and/or trypsinization. At pH 8.0, phosphatidylinositol induces the modulatory effect of ATP at the regulatory site but calmodulin does not. We suggest that the intraenzymic connection between the calmodulin-binding, autoinhibitory peptide and the nucleotide domain of the enzyme is impaired upon alkalinization, which would account for the differing abilities of the activators to modulate the ATP effects.

Effects of deoxygenation on active and passive Ca2+ transport and on the cytoplasmic Ca2+ levels of sickle cell anemia red cells

Elevated [Ca2+]i in deoxygenated sickle cell anemia (SS) red cells (RBCs) could trigger a major dehydration pathway via the Ca(2+)-sensitive K+ channel. But apart from an increase in calcium permeability, the effects of deoxygenation on the Ca2+ metabolism of sickle cells have not been previously documented. With the application of 45Ca(2+)-tracer flux methods and the combined use of the ionophore A23187, Co2+ ions, and intracellular incorporation of the Ca2+ chelator benz-2, in density-fractionated SS RBCs, we show here for the first time that upon deoxygenation, the mean [Ca2+]i level of SS discocytes was significantly increased, two- to threefold, from a normal range of 9.4 to 11.4 nM in the oxygenated cells, to a range of 21.8 to 31.7 nM in the deoxygenated cells, closer to K+ channel activatory levels. Unlike normal RBCs, deoxygenated SS RBCs showed a two- to fourfold increase in pump-leak Ca2+ turnover. Deoxygenation of the SS RBCs reduced their Ca2+ pump Vmax, more so in reticulocyte- and discocyte-rich than in dense cell fractions, and decreased their cytoplasmic Ca2+ buffering. Analysis of these results suggests that both increased Ca2+ influx and reduced Ca2+ pump extrusion contribute to the [Ca2+]i elevation.

Ca(2+)-calmodulin-dependent modification of smooth-muscle myosin light-chain kinase leading to its co-operative activation by calmodulin

It has recently been shown that at relatively high molar ratios of myosin light-chain kinase (MLCKase) to calmodulin (CM) almost complete inhibition of the kinase activity occurs [Sobieszek (1991) J. Mol. Biol. 220, 947-957]. This inhibition resulted in a highly co-operative activation of MLCKase by CM, whereas the opposite activation (of CM by kinase) was hyperbolic, as expected (unco-operative). This difference in activation was observed only for kinase preparations preincubated with sub-stoichiometric amounts of CM, and only when micromolar concentrations of Ca2+ were present. The inhibitory effect was variable and depended not only on the concentration ratio of kinase to CM but also on the MLCKase preparation. For most of the preparations full inhibition required 5-15 min of preincubation at 25 degrees C and a 3-6-fold molar excess of kinase over CM. The inhibition was reversible, since full activity could be obtained after saturation of the kinase by additional CM. The inhibitory effect did not require ATP (excluding phosphorylation-type modifications of the kinase), and dephosphorylation of the kinase was not involved, since inhibition of an endogenous MLCK phosphatase by microcystin-LR did not decrease the inhibitory effect. Since the co-operative activation by CM was observed for cross-linked MLCKase preparations enriched in kinase dimers, but was absent for the analogous preparations enriched in the oligomers, we concluded that Ca(2+)-CM-dependent changes in the oligomeric state of the kinase were responsible for the modification observed. The exact nature of these modifications remains to be established.