Activation of erythrocyte membrane Ca2+-ATPase by calpain

RedTell: an AI tool for interpretable analysis of red blood cell morphology

Introduction: Hematologists analyze microscopic images of red blood cells to study their morphology and functionality, detect disorders and search for drugs. However, accurate analysis of a large number of red blood cells needs automated computational approaches that rely on annotated datasets, expensive computational resources, and computer science expertise. We introduce RedTell, an AI tool for the interpretable analysis of red blood cell morphology comprising four single-cell modules: segmentation, feature extraction, assistance in data annotation, and classification. Methods: Cell segmentation is performed by a trained Mask R-CNN working robustly on a wide range of datasets requiring no or minimum fine-tuning. Over 130 features that are regularly used in research are extracted for every detected red blood cell. If required, users can train task-specific, highly accurate decision tree-based classifiers to categorize cells, requiring a minimal number of annotations and providing interpretable feature importance. Results: We demonstrate RedTell's applicability and power in three case studies. In the first case study we analyze the difference of the extracted features between the cells coming from patients suffering from different diseases, in the second study we use RedTell to analyze the control samples and use the extracted features to classify cells into echinocytes, discocytes and stomatocytes and finally in the last use case we distinguish sickle cells in sickle cell disease patients. Discussion: We believe that RedTell can accelerate and standardize red blood cell research and help gain new insights into mechanisms, diagnosis, and treatment of red blood cell associated disorders.

Regulation of Cell Calcium and Role of Plasma Membrane Calcium ATPases

The plasma membrane Ca²⁺ ATPase (PMCA pump) is a member of the superfamily of P-type pumps. It has 10 transmembrane helices and 2 cytosolic loops, one of which contains the catalytic center. Its most distinctive feature is a C-terminal tail that contains most of the regulatory sites including that for calmodulin. The pump is also regulated by acidic phospholipids, kinases, a dimerization process, and numerous protein interactors. In mammals, four genes code for the four basic isoforms. Isoform complexity is increased by alternative splicing of primary transcripts. Pumps 2 and 3 are expressed preferentially in the nervous system. The pumps coexist with more powerful systems that clear Ca²⁺ from the bulk cytosol: their role is thus the regulation of Ca²⁺ in selected subplasma membrane microdomains, where a number of important Ca²⁺-dependent enzymes interact with them. Malfunctions of the pump lead to disease phenotypes that affect the nervous system preferentially.

Calpain-1 inhibitors for selective treatment of rheumatoid arthritis: what is the future?

Effective small-molecule treatment of inflammatory diseases remains an unmet need in medicine. Current treatments are either limited in effectiveness or invasive. The latest biologics prevent influx of inflammatory cells to damaged tissue. Calpain-1 is a calcium-activated cysteine protease that plays an important role in neutrophil motility. It is, therefore, a potential target for intervention in inflammatory disease. Many inhibitors of calpains have been developed but most are unselective and so unsuitable for drug use. However, recent series of α-mercaptoacrylate inhibitors target regulatory domains of calpain-1 and are much more specific. These compounds are effective in impairing the cell spreading mechanism of neutrophils in vitro and raise the possibility of treating rheumatoid arthritis with a pill; however, challenges still remain. Improved bioavailability is needed and solution of their precise mode of action should prompt the development of specific calpain-1 screens for novel classes of inhibitors.

Auto-inhibition of Drs2p, a yeast phospholipid flippase, by its carboxyl-terminal tail

Drs2p, a yeast type IV P-type ATPase (P4-ATPase), or flippase, couples ATP hydrolysis to phosphatidylserine translocation and the establishment of membrane asymmetry. A previous study has shown that affinity-purified Drs2p, possessing an N-terminal tandem affinity purification tag (TAPN-Drs2), retains ATPase and translocase activity, but Drs2p purified using a C-terminal tag (Drs2-TAPC) was inactive. In this study, we show that the ATPase activity of N-terminally purified Drs2p associates primarily with a proteolyzed form of Drs2p lacking the C-terminal cytosolic tail. Truncation of most of the Drs2p C-terminal tail sequence activates its ATPase activity by ∼4-fold. These observations are consistent with the hypothesis that the C-terminal tail of Drs2p is auto-inhibitory to Drs2p activity. Phosphatidylinositol 4-phosphate (PI(4)P) has been shown to positively regulate Drs2p activity in isolated Golgi membranes through interaction with the C-terminal tail. In proteoliposomes reconstituted with purified, N-terminally TAP-tagged Drs2p, both ATPase and flippase activity were significantly higher in the presence of PI(4)P. In contrast, PI(4)P had no significant effect on the activity of a truncated form of Drs2p, which lacked the C-terminal tail. This work provides the first direct evidence, in a purified system, that a phospholipid flippase is subject to auto-inhibition by its C-terminal tail, which can be relieved by a phosphoinositide to stimulate flippase activity.

Inhibition of Na+/Ca2+ exchanger by peroxynitrite in microsomes of pulmonary smooth muscle: role of matrix metalloproteinase-2

Treatment of bovine pulmonary artery smooth muscle microsomes with peroxynitrite (ONOO-) (100 microM) markedly stimulated matrix metalloproteinase-2 (MMP-2) activity and also enhanced Ca2+ATPase activity and ATP-dependent Ca2+ uptake. Pretreatment of the microsomes with vitamin E (1 mM) and TIMP-2 (50 microg/ml) preserved the increase in MMP-2 activity, Ca2+ATPase activity and also ATP-dependent Ca2+ uptake in the microsomes. In contrast, Na(+)-dependent Ca2+ uptake in the microsomes was inhibited by ONOO- and this was found to be reversed by vitamin E (1 mM) and TIMP-2 (50 microg/ml). However, changes caused by ONOO- in MMP-2 activity, ATP-dependent Ca2+ uptake and Na(+)-dependent Ca2+ uptake were not reversed upon pretreatment of the microsomes with a low concentration of 5 microg/ml of TIMP-2 which, on the contrary, reversed MMP-2 (1 microg/ml)-mediated alteration on these parameters. The inhibition of Na(+)-dependent Ca2+ uptake by ONOO- and MMP-2 overpowered the stimulation of ATP-dependent Ca2+ uptake in the microsomes. Treatment with ONOO- abolished the inhibitory effect of TIMP-2 (5 microg/ml) on MMP-2 (1 microg/ml) causing 14C-gelatin degradation. Overall, the present study suggests that ONOO- inactivated TIMP-2, the ambient inhibitor of MMP-2, leading to activation of the ambient proteinase, MMP-2, and subsequently stimulated Ca2+ATPase activity and ATP-dependent Ca2+ uptake, but inhibited Na(+)-dependent Ca2+ uptake, resulting in a marked decrease in Ca2+ uptake in microsomes of bovine pulmonary artery smooth muscle.

Role of membrane-associated Ca+ dependent matrix metalloprotease-2 in the oxidant activation of Ca2+Atpase by tertiary butylhydroperoxide

Treatment of bovine pulmonary artery smooth muscle plasma membrane suspension with the oxidant tert-butylhydroperoxide (t-buOOH) increases Ca2+ATPase activity. The smooth muscle plasma membrane possesses a Ca2+ dependent protease activity in the gelatin containing zymogram having an apparent molecular mass of 72 kDa. The 72 kDa protease activity was found to be inhibited by EGTA and the tissue inhibitor of metalloprotease-2 (TIMP-2). Since 72 kDa is the molecular mass of MMP-2 and since in our present study the 72 kDa protease in the gelatin containing zymogram is inhibited by matrix metalloprotease inhibitors, EGTA and TIMP-2, it may be suggested that the 72 kDa protease is the MMP-2. In addition to the increasing Ca2+ATPase activity, t-buOOH also enhances the activity of the membrane associated Ca2+ dependent protease that degrades 14C-gelatin. The oxidant triggered protease activity and the Ca2+ATPase activity were found to be prevented by the antioxidant vitamin E, and also by the Ca2+ dependent matrix metalloprotease inhibitors: EGTA and TIMP-2. Adding MMP-2 to the smooth muscle plasma membrane suspension caused an increase in Ca2+ATPase activity and pretreatment with TIMP-2 prevents the increase in Ca2+ATPase activity. Combined treatment of the smooth muscle plasma membrane with low doses of MMP-2 and t-buOOH augments further the Ca2+ATPase activity caused by the respective doses of either t-buOOH or MMP-2. Pretreatment with TIMP-2 prevents the increase in Ca2+ATPase activity elicited by the low doses of MMP-2 and/or t-buOOH.

Ca(2+)-ATPase pump forms and an endogenous inhibitor in bovine brain synaptosomes

Two forms of Ca(2+)-pump were identified in bovine brain synaptic membranes as aspartylphosphate intermediates and were characterized. The 140 kDa and 97 kDa phosphoproteins were digested by calpain, producing two phosphorylated fragments, of M.W. 124 and 80 kDa respectively, not inhibited by thapsigargin, and displayed a trypsin digestion pattern with the formation of one phosphorylatable fragment of about 80 kDa. These results suggest that both pumps belong to the Plasma Membrane-type of Ca2+ ATPases, differing from the Sarco- or Endoplasmic Reticulum kind. A plasma membrane Ca(2+)-ATPase proteinaceous inhibitor with molecular weight between 6,000 and 10,000 Da was resolved from synaptic terminal cytosol, where it is enriched by fourfold with respect to frontal cortex brain cytosol. Such enrichment is already evident in the correspondent crude fractions. The presence of calcium pump and its proteinaceous inhibitor inside the synaptic terminals from bovine brain is discussed in terms of free calcium level regulation in neuron synaptoplasm.

Role of an aprotinin-sensitive protease in the activation of Ca(2+)-ATPase by superoxide radical (O2-.) in microsomes of pulmonary vascular smooth muscle

We have investigated the role of an aprotinin-sensitive protease in regulating Ca(2+)-ATPase activity and Ca2+ uptake (ATP-dependent and Na(+)-dependent) in microsomes of bovine pulmonary vascular smooth muscle during treatment with the O2(-.)-generating system hypoxanthine plus xanthine oxidase. Treatment of the smooth muscle microsomes with the O2(-.)-generating system produced a protease in a gelatin-containing zymogram with an apparent molecular mass of 16 kDa. This 16 kDa proteolytic protein was found to be inhibited by superoxide dismutase (SOD) and aprotinin but not by PMSF. Using polyclonal antiserum to aprotinin, we found that it is an ambient antiprotease of the smooth muscle microsomes. Treatment of the microsomes with the O2(-.)-generating system stimulated protease activity tested with a synthetic substrate N-benzoyl-DL-arginine p-nitroanilide and also enhanced Ca(2+)-ATPase activity. It also stimulated ATP-dependent Ca2+ uptake. In contrast, Na(+)-dependent Ca2+ uptake was found to be inhibited by the O2(-.)-generating system. Pretreatment of the microsomes with SOD and aprotinin preserved the increase in protease activity, Ca(2+)-ATPase activity and ATP-dependent Ca2+ uptake. In addition, O2(-.)-caused inhibition of the Na(+)-dependent Ca2+ uptake which was reversed by SOD and aprotinin. Pretreatment with PMSF did not cause any discernible alteration in the protease activity, Ca(2+)-ATPase activity. ATP-dependent Ca2+ uptake and Na(+)-dependent Ca2+ uptake in the microsomes caused by the O2(-.)-generating system. These results suggest that an aprotinin-sensitive protease plays a pivotal role in regulating Ca(2+)-ATPase and Ca(2+)-uptake activities in microsomes of pulmonary vascular smooth muscle under oxidant O2(-.)-triggered conditions.

Oxidant-mediated proteolytic activation of Ca(+)-ATPase in microsomes of pulmonary smooth muscle

Treatment of bovine pulmonary artery smooth muscle tissue microsomes with H2O2 (1 mM) markedly stimulated protease activity tested with a synthetic substrate N-benzoyl-DL-arginine p-nitroanilide (BAPNA), and also enhanced Ca(2+)-ATPase activity. ATP-dependent Ca(2+) uptake was found to be stimulated upon treatment of the microsomes with H2O2. Pretreatment of the microsomes with vitamin E and aprotinin prevented the H2O2-induced stimulation of Ca(2+)-ATPase activity and also ATP-dependent Ca(2+) uptake. In contrast, H2O2-induced inhibition of Na(+)-dependent Ca(2+) uptake was reversed by vitamin E and aprotinin.

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.

Inhibition of erythrocyte membrane (Ca2+ + Mg2+)-ATPase by the organophosphorus insecticides parathion and methylparathion

Organophosphorus insecticides parathion and methylparathion non-competitively inhibited the activity of (Ca2+ + Mg2+)-ATPase bound to and solubilized from pig erythrocyte membrane. Both enzyme preparations exhibited biphasic substrate curves displaying the existence of two functional active sites with low and high affinity to ATP. Also, the relationship between the activity of bound enzyme and Ca2+ concentration was biphasic. The activity reached maximum at 20 microM then dropped progressively as the Ca2+ concentration was raised. The inhibition of the activity was more pronounced for parathion than for methylparathion and the solubilized enzyme preparation was more affected than the bound one. The inhibition constants (Ki) for parathion for bound enzyme were 55 and 158 microM for high- and low-affinity active sites, respectively; for methylparathion these values equalled 74 and 263 microM, respectively. Ki values for parathion were 36 and 118 microM for solubilized enzyme (high- and low-affinity sites, respectively), for methylparathion -62 and 166 microM, respectively. The magnitude of the effect was greater for a low Ca2+ concentration, which could arise from different conformational states of the enzyme at different calcium concentrations. The results of the experiment suggest that the insecticides inhibited the ATPase by binding to a site on the enzyme rather than by the interaction with associated lipids, although lipids could weaken the action of the compounds due to the strong affinity of organophosphorus insecticides to lipids.

The plasma membrane calcium pump: functional domains, regulation of the activity, and tissue specificity of isoform expression

The plasma membrane Ca2+ pump is responsible for the fine regulation of the intracellular Ca2+ level and is thus involved in the control of several cellular processes. The activity of the pump is regulated by a multiplicity of mechanisms, among which are calmodulin, acidic phospholipids, kinase-mediated phosphorylation, or an oligomerization process. The C-terminal part of the molecule interacts with the region of the pump close to the active site, leading to the decrease of the activity in the resting state. Four genes coding for different isoforms of the plasma membrane Ca2+ ATPase are known in humans. Isoform 1 and 4 represent housekeeping isoforms, whereas isoforms 2 and 3 are only present in specialized tissues. The variability of the protein is further increased by alternative RNA splicing at two sites (A, C). Alternative splicing occurs within (splice site C) or near (splice site A) regions coding for regulatory domains of the protein. In all isoforms a corresponding splice form exists at both splice sites. These common splice forms are present in all tissues, whereas isoform unique splice forms are normally only present in specialized tissues. In neuronal tissues all isoforms and almost the complete set of splice forms are found. The transcripts of the different isoforms are distributed in a region-specific manner in neuronal tissues.

The plasma membrane calcium pump is the preferred calpain substrate within the erythrocyte

The activation of calpain in normal human erythrocytes incubated in the presence of Ca2+ and the Ca2+ ionophore A23187 led to the decline of the Ca(2+)-dependent ATPase activity of the cells. Preloading of the erythrocyte with an anticalpain antibody prevented the decline. The pump was also inactivated by applied to isolated erythrocyte plasma membranes. The decline of the pump activity corresponded to the degradation of the pump protein and was inversely correlated to the amount of the natural inhibitor of calpain, calpastatin, present in the cells. In erythrocytes containing only 50% of the normal level the degradation started at a concentration of Ca2+ significantly lower than in normal cells. A comparison of the concentrations of Ca2+ required for the degradation of a number of erythrocyte membrane proteins showed that the Ca2+ pump and band 3 were the most sensitive. All other membrane proteins tested were attacked at higher levels of intracellular Ca2+. Thus, the degradation of the Ca2+ pump protein may be a simple and sensitive means to monitor calpain activation in vivo. Furthermore, the results have shown that the calpastatin level correlated directly with the amount of activable calpain and with the concentration of Ca2+ required to trigger the activation process.

Irreversible effects of calcium ions on the plasma membrane calcium pump

The calcium pump of human red cells can be irreversibly activated by preincubation of the membranes in the presence of calcium ions, with a pattern reminiscent of that produced by controlled trypsin attack. With 1 mM Ca2+, the activity of the basal enzyme increases three to fourfold over 30 to 60 min, to levels about half those obtained in the presence of calmodulin. On the whole, the effect occurs slowly, with a very low Ca2+ affinity at 37 degrees C and is unaffected by serine-protease inhibitors. The activation caused by 1 mM Ca2+ is little affected by leupeptin (a thiol-protease inhibitor) and that obtained at 10 microM Ca2+ is not inhibited. Preincubations at 0 degrees C also lead to activation, to a level up to half that seen at 37 degrees C, and the effect is not affected by leupeptin or antipain. No activation is observed by preincubating soluble purified Ca,Mg-ATPase in Ca(2+)-containing solutions at 37 degrees C. Instead, calcium ions protect the detergent-solubilized enzyme from thermal inactivation, the effect being half-maximal between 10 and 20 microM Ca2+. We conclude that the activation of the membrane-bound Ca,Mg-ATPase by Ca2+ should result from an irreversible conformational change in the enzyme and not from attack by a membrane-bound protease, and that this change presumably arises from the release of inhibitory particles existing in the original membrane preparations.

Stimulation of human platelet Ca(2+)-ATPase and Ca2+ restoration by calpain

To clarify the possible role of calpain (calcium activated neutral protease; EC 3.4.22.17) in Ca2+ homeostasis of human platelets, we investigated the effects of cell permeable calpain inhibitors, calpeptin and E-64d (EST), on the restoration of cytoplasmic Ca2+ ([Ca2+]i) in both Fura-2 and aspirin (ASA) loaded platelets. Although neither calpeptin (30 microM) nor EST (250 microM) altered the increase of [Ca2+]i in thrombin (1 U/ml) stimulated platelets, both calpain inhibitors delayed the decrease of [Ca2+]i back towards the basal level. These observations suggested that calpain might be involved in Ca2+ restoration. Then, the activity of Ca(2+)-ATPase was examined in thrombin (2 U/ml) stimulated platelets. Thrombin produced a rapid rise in Ca(2+)-ATPase activity by 2-fold at 8 s of incubation, which then returned to below the basal activity within 2 min. Calpeptin inhibited transient Ca(2+)-ATPase activation induced by thrombin in a dose related manner. Ca(2+)-ATPase of isolated platelet membranes was digested by purified human platelet calpain-I and Ca(2+)-ATPase activity was investigated. With a short incubation (8-15 s), Ca(2+)-ATPase activity was increased about 2-fold and then it decreased below the basal level at longer incubations or at a higher calpain/membrane ratio. The initial rate of Ca2+ uptake was also increased by about 2-fold with a short incubation (8-15 s). For molecular characterization of the Ca(2+)-ATPase, the formation of the enzyme-phosphate complex (EP) was investigated. The membrane bound intact 105 kD Ca(2+)-ATPase was converted by calpain to a fragment of approximately 50 kD.(ABSTRACT TRUNCATED AT 250 WORDS)

The Ca(2+)-transport ATPases from the plasma membrane

The initial studies on the plasma membrane (PM) Ca(2+)-transport ATPases were made in the erythrocyte, a structure that can not be taken as representing a typical eukaryotic cell. In other cell types however, the study of the PM Ca(2+)-transport ATPase is complicated by the simultaneous expression of related Ca(2+)-pumps in intracellular stores. Whereas there are as yet no known specific inhibitors for the PM Ca(2+)-transport ATPase, a number of selective inhibitors for the endo(sarco)plasmic reticulum Ca2+ pumps have been described: thapsigargin, cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone. With the recent introduction of the molecular biological approach, it became quickly obvious that a family of at least 5 different PM Ca(2+)-transport ATPase genes govern the tissue-dependent expression of PM Ca2+ pumps. Moreover alternative splicing of the primary gene transcripts was found to further enhance the number of pump variants. The PM Ca(2+)-transport ATPase are subject to modulatory control by calmodulin, by acidic phospholipids, and by the known families of protein kinases. Each of the ensuing effects are mutually related and interdependent. The wide variety PM Ca2+ pump isoforms and their regulation by such an intricate modulatory network allows the distinct tissues to adapt most adequately to the prevailing tissue and stimulus specific requirements.

The plasma membrane calcium pump: a multiregulated transporter

Activation of many cells, especially nonexcitable cells, results in a Ca(2+) transient that is influenced in part by the kinetics of active extrusion of Ca(2+) across the plasma membrane. The molecular cloning of the plasma membrane Ca(2+)-pump has helped to clarify the relationship between its structure and function. The Ca(2+)-pump is controlled by multiple regulators, including calmodulin, phospholipids and various kinases. Longer term control is achieved through regulation of its gene expression, and the presence of a number of Ca(2+)-pump isoforms that differ in their regulatory domains provides potential functional diversity. In this review, we focus on the mechanisms that regulate the function of the Ca(2+)-pump, and their physiological significance.

Persistent Ca2(+)-induced activation of erythrocyte membrane Ca2(+)-ATPase unrelated to calpain proteolysis

Preincubation of human erythrocyte membranes with calcium in the submillimolar to millimolar concentration range resulted in an increase of the Ca2+ affinity and apparent maximum velocity of the Ca2(+)-stimulated Mg2(+)-dependent ATPase (Ca2(+)-ATPase). The activation was persistent, as it was not reversed when the Ca2(+)-preincubated membranes were washed with ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid-containing buffers. Magnesium was not required for the activation, whereas greater than 2 mM Mg2+ partially antagonized the activation by Ca2+. In some membrane preparations ATP was required in addition to Ca2+ for activation of the Ca2(+)-ATPase, but nonhydrolyzable analogs of ATP had the same effect. Calmodulin prevented the activation by Ca2+ over the same concentration range in which it interacts with the Ca2(+)-ATPase. Taken together the results obtained provided strong evidence that the Ca2+ activation of the enzyme was not due to proteolytic cleavage by endogenous calpain. Thus, activation by Ca2+ was not blocked by leupeptin (100-200 microM), did not require dithiothreitol, and occurred at Ca2+ concentrations greater than those required for activation of calpain I. Furthermore, Ca2+ activation did not result in change in the mobility the native 136-kDa species of the Ca2(+)-ATPase on SDS-gel electrophoresis. Moreover, solubilization of the Ca2(+)-pretreated membranes with Triton X-100 reversed the Ca2+ activation of the Ca2(+)-ATPase. On the other hand, Ca2(+)-pretreatment of the membranes modified the susceptibility of the Ca2(+)-ATPase to both cleavage and activation by exogenously added calpain I. We conclude that pretreatment of Ca2(+)-ATPase in erythrocyte membranes with millimolar Ca2+ activates the enzyme by inducing a persistent conformational change of the enzyme which is, however, subsequently reversed by detergent solubilization.

In vitro digestion of spectrin, protein 4.1 and ankyrin by erythrocyte calcium dependent neutral protease (calpain I)

1. In whole ghosts, ankyrin, protein 4.1, protein band 3 and spectrin are lysed by purified calpain I in the presence of calcium. 2. Limited calpain lysis of purified ankyrin results in several peptides, including a 85 kD peptide bearing the ankyrin interaction site for the protein band 3 internal fragment (43 kD), and a 55 kD peptide carrying the ankyrin-spectrin interaction site. 3. These peptides are differently phosphorylated: the 85 kD by cytosol casein kinase, and the 55 kD by membrane casein kinase. 4. Protein 4.1 lysis mainly produces a 30 kD peptide resistant to proteolysis. 5. The spectrin beta-chain is more sensitive to calpain cleavage than the alpha chain; both chains seem to be cleaved in a similar sequential manner. 6. Limited proteolysis of spectrin dimer does not impede tetramerization in vitro.

Calpains (intracellular calcium-activated cysteine proteinases): structure-activity relationships and involvement in normal and abnormal cellular metabolism

1. Calpains (calcium-activated cysteine proteinases) have evolved by gene fusion events involving calmodulin-like genes, cysteine proteinase genes and other sequences of unknown origin. 2. The enzymes are composed of two non-identical subunits, each of which contains functional calcium-binding sequences. 3. Calpains are inhibited by the endogenous protein inhibitor, calpastatin and some calmodulin antagonists are also inhibitors of calpain. A number of synthetic proteinase inhibitors also inhibit calpains. 4. Calpains can be activated by phospholipids, an endogenous protein activator and some amino acid derivatives. 5. Various protein substrates for calpains have been recognized in vitro, but the identity of in situ substrates remains unclear. 6. Proposals have been made for calpain function, including involvement in signal transduction, platelet activation, cell fusion, mitosis and cytoskeleton and contractile protein turnover. 7. Calpain and calpastatin expression is altered in a number of abnormal states including muscular dystrophy, muscle denervation and tenotomy, hypertension and platelet abnormalities.

Calmodulin-binding proteins as calpain substrates

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Ca2+-mediated activation of human erythrocyte membrane Ca2+-ATPase

Ca2+-ATPase of human erythrocyte membranes, after being washed to remove Ca2+ after incubation with the ion, was found to be activated. Stimulation of the ATPase was related neither to fluidity change nor to cytoskeletal degradation of the membranes mediated by Ca2+. Activation of the transport enzyme was also unaffected by detergent treatment of the membrane, but was suppressed when leupeptin was included during incubation of the membranes with Ca2+. Stimulation of the ATPase by a membrane-associated Ca2+-dependent proteinase was thus suggested. Much less 138 kDa Ca2+-ATPase protein could be harvested from a Triton extract of membranes incubated with Ca2+ than without Ca2+. Activity of the activated enzyme could not be further elevated by exogenous calpain, even after treatment of the membranes with glycodeoxycholate. There was also an overlap in the effect of calmodulin and the Ca2+-mediated stimulation of membrane Ca2+-ATPase. While Km(ATP) of the stimulated ATPase remained unchanged, a significant drop in the free-Ca2+ concentration for half-maximal activation of the enzyme was observed.

Further characterization of calpain-mediated proteolysis of the human erythrocyte plasma membrane Ca2+-ATPase

The membrane-bound form and a solubilized and purified form of the Ca2+-ATPase from human erythrocyte have been proteolyzed under controlled conditions by highly purified Ca2+-dependent neutral cysteine-protease, calpain I, in the absence and in the presence of the calmodulin-calcium complex. In the absence of calmodulin the 136-kDa enzyme was transformed into a group of fragments of 125-124 kDa, followed by the slower formation of a second group of fragments of 82-80 kDa. These heterogeneous fragments were capable of forming an acylphosphate intermediate. The 125- and 82-kDa minor components of each heterogeneous group of fragments (125-124 and 82-80 kDa) were capable of binding calmodulin, whereas the 124- and the 80-kDa major components did not. In the presence of calmodulin, however, the native enzyme was transformed into a 127-kDa fragment followed by the slower formation of an 85-kDa fragment. Both fragments (127 and 85 kDa) formed an acylphosphate intermediate and were capable of binding calmodulin. The presence of calmodulin during calpain action effectively protected the Ca2+-ATPase from proteolytic activation (K.K.W. Wang, A. Villalobo, and B.D. Roufogalis (1988) Arch. Biochem. Biophys. 260, 696-704) and prevented the formation of the calmodulin-insensitive 124- and 80-kDa fragments. Smaller fragments not capable of forming the acylphosphate intermediate were also produced, in particular a 39-37 kDa doublet band retaining the capacity to bind calmodulin. In contrast to the membrane-bound form, the purified form of the Ca2+-ATPase was proteolyzed by calpain at a slower rate.