Effect of metal ions on the structure and activity of calcium-activated neutral protease (CANP)

Calcium negatively regulates an intramolecular interaction between the N-terminal recoverin homology and EF-hand motif domains and the C-terminal C1 and catalytic domains of diacylglycerol kinase α

The type I diacylglycerol kinase (DGK) isozymes (α, β and γ) contain a shared recoverin homology (RVH) domain, a tandem repeat of Ca2+-binding EF-hand motifs, two cysteine-rich C1 domains, and the catalytic domain. We previously reported that a DGKα mutant lacking the RVH domain and EF-hands was constitutively active, implying that the N-terminal region (NTR) of DGKα, consisting of the RVH domain and EF-hand motifs, intramolecularly interacts with and masks the activity of the C-terminal region (CTR), containing the C1 and catalytic domains. In this study, we demonstrate that a glutathione S-transferase (GST)-fused DGKα-NTR construct physically binds to a green fluorescent protein (GFP)-fused DGKα-CTR construct. Moreover, co-precipitation of GFP-DGKα-CTR with GST-DGKα-NTR was clearly attenuated by the addition of 1 μM Ca2+. This result indicates that Ca2+ induces dissociation of the physical interaction between DGKα-NTR and DGKα-CTR. In addition to previously reported calcium-dependent changes in the hydrophobicity and net surface charge, Ca2+ also appeared to induce a decrease in the α-helical content of DGKα-NTR. These results suggest that Ca2+-induced conformational changes in the NTR release the intramolecular association between the NTR and the CTR of DGKα.

Calpain chronicle--an enzyme family under multidisciplinary characterization

Calpain is an intracellular Ca2+-dependent cysteine protease (EC 3.4.22.17; Clan CA, family C02) discovered in 1964. It was also called CANP (Ca2+-activated neutral protease) as well as CASF, CDP, KAF, etc. until 1990. Calpains are found in almost all eukaryotes and a few bacteria, but not in archaebacteria. Calpains have a limited proteolytic activity, and function to transform or modulate their substrates' structures and activities; they are therefore called, "modulator proteases." In the human genome, 15 genes--CAPN1, CAPN2, etc.--encode a calpain-like protease domain. Their products are calpain homologs with divergent structures and various combinations of functional domains, including Ca2+-binding and microtubule-interaction domains. Genetic studies have linked calpain deficiencies to a variety of defects in many different organisms, including lethality, muscular dystrophies, gastropathy, and diabetes. This review of the study of calpains focuses especially on recent findings about their structure-function relationships. These discoveries have been greatly aided by the development of 3D structural studies and genetic models.

Characteristic properties of endoplasmic reticulum membrane m-calpain, calpastatin and lumen m-calpain: a comparative study between membrane and lumen m-calpains

Previously, we reported that bovine pulmonary smooth muscle endoplasmic reticulum (ER) membrane possesses associated m-calpain and calpastatin and ER lumen contains only m-calpain. Herein, we report characteristic properties of ER membrane m-calpain (MCp), calpastatins and lumen m-calpain (LCp) and a brief comparative study between MCp and LCp. MCp containing 80 kDa large and 28 kDa small subunit is non-phosphorylated, whereas LCp containing only 80 kDa large subunit is phosphorylated. Optimum pH, Ca(2+) concentration and pI value of both MCp and LCp are 7.5, 5 mM and 4.5, respectively. MCp and LCp have similar kinetic parameters and circular dichroism (CD) spectra. Autolysis of MCp and LCp are different. Coimmunoprecipitation studies revealed that LCp is associated with ERp57 in the ER lumen, which suggests that the regulation of LCp differs from the regulation of MCp. In presence of Ca(2+), the activated LCp cleaves inositol 1,4,5-trisphosphate receptor-1 (IP(3)R1) in the ER lumen, whereas the activated MCp cleaves Na(+)/Ca(2+) exchanger-1 (NCX1) in the ER membrane. We have determined pI (4.6 and 4.7, respectively) and IC(50) (0.52 and 0.8 nM, respectively) values of 110 and 70 kDa calpastatins. For first time, we have determined the characteristic properties, regulation and functional activity of LCp in the ER lumen.

Structure and physiological function of calpains

For a long time now, two ubiquitously expressed mammalian calpain isoenzymes have been used to explore the structure and function of calpain. Although these two calpains, mu- and m-calpains, still attract intensive interest because of their unique characteristics, various distinct homologues to the protease domain of mu- and m-calpains have been identified in a variety of organisms. Some of these 'novel' calpain homologues are involved in important biological functions. For example, p94 (also called calpain 3), a mammalian calpain homologue predominantly expressed in skeletal muscle, is genetically proved to be responsible for limb-girdle muscular dystrophy type 2A. Tra-3, a calpain homologue in nematodes, is involved in the sex determination cascade during early development. PalB, a key gene product involved in the alkaline adaptation of Aspergillus nidulans, is the first example of a calpain homologue present in fungi. These findings indicate various important functional roles for intracellular proteases belonging to the calpain superfamily.

Biologically active monomeric and heterodimeric recombinant human calpain I produced using the baculovirus expression system

Calpain I is a heterodimeric protein that is part of a family of calcium-activated intracellular cysteine proteases presumed to play a role in mediating signals transduced by calcium. Expression of bioactive recombinant human calpain I has been achieved using the baculovirus expression system, by either co-infection with two viruses, each expressing one of the subunits, or infection with a single virus containing both subunits. The approximately 80 kDa catalytic subunit exhibited calcium-dependent proteolytic activity when expressed alone or with the approximately 30 kDa regulatory subunit. Baculoviral recombinant calpain I appeared fully active in that the catalytic subunit in unpurified cell extracts exhibited calcium-dependent autocatalytic cleavage at the correct locus. The amount of approximately 80 kDa subunit accumulated at steady state was greatly increased by co-expression of the approximately 30 kDa subunit, suggesting a possible role for enzyme stabilization by the latter subunit. The recombinant human calpain I was purified to near homogeneity and compared with purified native human erythrocyte calpain I. The recombinant and native enzymes had equivalent inhibition constants for structurally diverse calpain inhibitors, identical calcium activation profiles, and similar specific activities, demonstrating the suitability of using the recombinant protein for studies of the native enzyme.

Studies on the autolysis of m-calpain from the skeletal muscle of the amphibian Rana ridibunda

The autolytic mechanisms responsible for the regulation of m-calpain purified from the skeletal muscle of the amphibian Rana ridibunda were examined. Both subunits of the calpain molecule were found to undergo autolysis in the presence of Ca2+. Various divalent cations were examined for their ability to induce calpain autolysis. The concentrations of these cations required for the complete calpain autolysis were: 500 microM Ca2+, 800 microM Mn2+, 2 mM Sr2+, 10 mM Ba2+, whereas Mg2+, even at 10 mM did not induce any autolysis. Calpain autolysis induced by the above divalent cations is a temperature dependent process. Presence of Mn2+ or Sr2+ reduces the Ca2+ requirement of calpain for autolysis. The rate of autolysis depends on the protease concentration; protease inhibitors such as E-64, leupeptin, antipain, and iodoacetic acid inhibit the autolysis of calpain; E-64 inhibits irreversibly while leupeptin inhibits reversibly the autolysis; and irreversibly inactivated by E-64 calpain is fully digested by native calpain. Autolysis of calpain in the presence of alkali denatured casein increases the Ca2+ sensitivity of the protease for its half maximal and maximal caseinolytic activity. Limited autolysis of calpain is also induced in the presence of the endogenous substrate G-actin, and the rate of autolysis is slower than that obtained in the absence of substrates.

Purification and characterization of m-calpain from the skeletal muscle of the amphibian Rana ridibunda

Calpain was purified to apparent homogeneity from the skeletal muscle of the amphibian Rana ridibunda. It is composed of two subunits of 78 and 28 kDa, respectively. The enzyme exhibits kinetic properties similar to those of mammalian and avian skeletal muscle m-calpains. Ca2+ requirements for half and maximum activities are 400 microM and 1.5 mM, respectively. It is strongly inhibited by thiol protease inhibitors such as leupeptin, E-64, and antipain and by alkylating thiol group agents such as iodoacetic acid (IAA), iodoacetamide (IAM), and N-ethylmaleimide (NEM). Its activity is enhanced by reduced thiols such as dithiothreitol (DTT), cysteine, and 2-mercaptoethanol. The enzyme is stable in the absence of Ca2+ at 55 degrees C, it displays maximum activity at 25 degrees C, and it shows a broad pH optimum between 6.5 and 7.8. In the absence of Ca2+, various divalent cations such as Sr2+, Mn2+, and Ba2+ strongly activate, while other divalent cations such as Ni2+, Co2+, Cd2+, Zn2+, and Cu2+ have no effect on its activity. In the presence of Ca2+, the cations Sr2+, Mn2+, and Ba2+ show a synergistic effect, while the cations of the other group strongly inhibit the calpain activity. The above data demonstrate that calpain from the skeletal muscle of the amphibian Rana ridibunda is a neutral, Ca(2+)-activated thiol protease and that it belongs to the class of m-calpains.

Fluorescence spectroscopic analysis of calpain II interactions with calcium and calmodulin antagonists

1. The intrinsic fluorescence of epoxysuccinyl-inhibited calpain II undergoes a Ca2(+)-dependent decrease which contrasts with the increase observed for calmodulin. 2. Calpain II was inhibited by the calmodulin antagonist toluidinylnaphthalenesulfonate (TNS), and a Ca2(+)-dependent increase in TNS fluorescence intensity was observed for epoxysuccinyl-inhibited calpain II. 3. The calmodulin antagonists calmidazolium CDZ and felodipine both caused decreases in the intrinsic fluorescence of epoxysuccinyl-inhibited calpain II. 4. Increasing concentrations of Ca2+ caused an increase in the fluorescence intensity of the inhibited enzyme in the presence of (CDZ), and a decrease in the presence of felodipine. 5. It is concluded from these studies that Ca2+ and calmodulin antagonists induce conformational changes in calpain II, and that changes occur in regions other than the Ca2(+)-binding domains.

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.

Tissue distribution of calcium-activated neutral proteinases in rat

A simple separation method involving a minimum number of essential steps was established to separate the two kinds of calcium-activated neutral proteinase (CANP) activity from each other and from endogenous CANP inhibitor activity. Determination of CANP activity in rat tissues using this method revealed a ubiquitous distribution of two CANPs. The level of CANP activity, however, differs between tissues. Low-calcium-requiring CANP (microCANP) is plentiful in spleen and kidney, while high-calcium-requiring CANP (mCANP) is plentiful in lung and brain. In all of the tissues examined, mCANP activity predominates over microCANP activity.

Changes in the activity of skeletal muscle calcium-activated neutral proteinase (EC 3.4.22.17) and its specific inhibitor in chickens grown at different rates in response to graded levels of dietary protein

1. Two experiments are reported in which the effect of alteration in growth rate on the levels of avian skeletal muscle calcium-activated neutral proteinase (EC 3.4.22.17) (CANP or calpain) and its specific inhibitor (calpastatin), a system thought to be implicated in myofibrillar catabolism, was studied by means of manipulation of dietary protein concentration. 2. In Expt 1 broiler chicks were given free access to diets containing 105, 149, 197 and 212 g protein/kg for 20 d. In Expt 2 the four dietary treatments were 119, 141, 182 and 227 g protein/kg diet given for 16 d. Chick growth rate and total leg skeletal muscle weight significantly increased (P less than 0.001) with increasing dietary protein concentration in both experiments. Total skeletal muscle protein increased with the level of dietary protein, the effect being significant (P less than 0.01 and P less than 0.001 in Expts 1 and 2 respectively). 3. Minced leg muscle was homogenized in low-salt buffers, and the extract chromatographed on DEAE-cellulose to separate proteinase and inhibitor activity. The partially purified CANP enzyme and inhibitor proteins were present at a concentration broadly consistent with literature reports, and their elution characteristics and Ca2+ concentration dependence were not varied by dietary protein concentration. 4. Both the muscle CANP and CANP inhibitor activities (units/kg muscle) exhibited upward trends with growth rate and increased muscle weight. However, these differences were not statistically significant (P greater than 0.05) and were not present at all when the results were expressed as units/g muscle protein.

Regulation of activity of calcium activated neutral protease

Various lines of evidence suggest that calcium dependent protease (CANP, calpain) exists in the cytosol as an inactive proenzyme which is converted to an active form by autolysis. During autolysis only the N-terminal regions of both subunits of proCANP are modified. About 20 and 90 residues are removed from the large and small subunits, respectively. The N-terminal region (domain I) of the large subunit modified during autolysis precedes the protease domain and corresponds to the propeptides of various cysteine proteinases. Analyses of the autocatalytic activation of CANP in the presence of plasma membranes reveal that proCANP translocates to the membrane in the presence of microM Ca2+ and is activated at the membrane. The CANP inhibitor and Ca2+ are the most important factors for the regulation of CANP activity. The primary translation product of the mRNA for rabbit liver CANP inhibitor contains four internal repeats. Structural analyses of the liver and erythrocyte inhibitors reveal that they contain four and three repeats, respectively. The repeating unit was identified as the functional unit of the inhibitor and each unit inhibits one mole of CANP. On the basis of these results, an activation mechanism for proCANP at the membrane was proposed. The native enzyme, which has been called CANP or calpain, should now be called proCANP or calpainogen. CANP and calpain should be used for the autolyzed active form.

Autolytic activation of calcium-activated neutral protease

Degradation of vimentin by native low calcium ion-requiring protease (mu CANP) was compared to that by autodigested mu CANP. On activation with 5 mM barium ions, a lag time was observed for the case of native mu CANP. This provides direct evidence that native mu CANP is inactive as a protease and must be autolyzed to be activated. Most of the protease activity can be accounted for by autodigested mu CANP with a 76 K polypeptide but another species with 50 K polypeptide may also be active.

Reticulocyte lipoxygenase, ingensin, and ATP-dependent proteolysis

Lipoxygenase purified from rabbit reticulocyte lysate has a molecular mass of 68 kDa on SDS gel and a pI of 5.97. Lipoxygenase is inhibited by nordihydroguaiaretic acid (NDGA), 3-amino-1-(m-(trifluoromethyl)phenyl)-2-pyrazoline (BW755C), 5,8,11,14-eicosatetraynoic acid (ETYA), salicylhydroxamate (SHAM) or hemin. Metal ions or nucleotides do not affect its activity. The addition of certain of these inhibitors to the reticulocyte extract also inhibited the ATP-dependent proteolysis of casein, one of the distinct characteristics of reticulocytes. No clear correlation between lipoxygenase activity and ATP-dependent proteolysis could be detected. Hemin and NDGA inhibited both processes, but the concentrations necessary for inhibition were quite different. SHAM completely inhibited lipoxygenase, but not proteolysis. o-Phenanthroline inhibited ATP-dependent proteolysis, but had no effect on lipoxygenase activity. We have also purified a high-molecular-mass protease, ingensin, from reticulocyte extract. This protease accounted for more than 90% of the casein-degrading activity in reticulocyte extract. NDGA inhibited ingensin at the same concentrations required for inhibition of ATP-dependent proteolysis. These results suggest that lipoxygenase is not indispensable for the ATP-dependent proteolysis and the novel high-molecular-mass protease, ingensin, may be involved in the process.

Cross-reactivity of a monoclonal antibody to the catalytic subunit of chicken gizzard desmin-specific calpain

The desmin-specific calpain I from chicken gizzard smooth muscle is a dimer of 83 and 35 kDalton subunits. A monoclonal antibody to the large subunit did not cross-react with chicken gizzard and hamster skeletal muscle calpain II, but it did recognize hamster skeletal muscle desmin-specific calpain I and the denatured calpain II from chicken gizzard smooth muscle. These results indicate that different desmin-specific calpains have similar large subunits which differ significantly from the large subunit of calpain II in the same tissue.

A putative Ca2+-binding protein: structure of the light subunit of porcine calpain elucidated by molecular cloning and protein sequence analysis

cDNA clones specific for the light subunit of porcine calpain I have been isolated from a porcine kidney cDNA library. The complete primary structure of the light subunit has been revealed by nucleotide sequence analysis of the cDNA clones isolated and amino acid sequence analysis of peptides isolated from the purified mature protein. We found that the light subunit contains two distinct domains. Domain I, the amino-terminal half, has two unusually long, paired polyglycyl sequences and may serve as a binding site to the heavy subunit. Domain II, the carboxyl-terminal half, is a region highly homologous to the putative Ca2+-binding domain of the heavy subunit of chicken calpain elucidated recently. This region has four potential Ca2+-binding sites, each having the "E-F hand" structure. Our results suggest that the Ca2+-mediated proteolytic activity of calpain is controlled through the cooperative and/or sequential actions of multiple Ca2+-binding sites present in both two-subunit molecules, heavy and light subunits of calpain.

Evolutionary origin of a calcium-dependent protease by fusion of genes for a thiol protease and a calcium-binding protein?

Calcium-dependent protease (calcium protease) is apparently involved in a variety of cellular processes. Here we have attempted to clarify the role and regulatory mechanism of calcium protease by analysing its structure. The complete primary structure of calcium protease (relative molecular mass (Mr) 80,000 (80K), 705 amino acids) was deduced from the nucleotide sequence of cloned complementary DNA. The protein contains four distinct domains, and we have observed a marked similarity between the second and fourth domains and the papain-like thiol proteases and calmodulin-like calcium-binding proteins, respectively. This finding suggests that calcium protease arose from the fusion of genes for proteins of completely different function and evolutionary origin. Further, it provides functional insight into cellular regulatory mechanisms mediated by Ca2+ through calcium-binding proteins.