Structure and physiological function of calpains

PEST sequences in cAMP-dependent protein kinase subunits of the aquatic fungus Blastocladiella emersonii are necessary for in vitro degradation by endogenous proteases

During Blastocladiella emersonii germination, the regulatory (R) and the catalytic (C) subunits of the cAMP-dependent protein kinase (PKA) are rapidly and concurrently degraded, after PKA activation in response to a transient increase in intracellular cAMP levels. The possibility that PEST sequences could be acting as proteolytic recognition signals in this process was investigated, and high score PEST sequences were found in both B. emersonii R and C subunits. Deletions in the PEST sequences were obtained by site-directed mutagenesis and the different PKA subunits were independently expressed in Escherichia coli. Proteolysis assays of the various R and C recombinant forms, using B. emersonii cell extracts as the source of proteases, showed a strong correlation between the presence of high score PEST sequences and susceptibility to degradation. Furthermore, the amino-terminal sequence of the proteolytic fragments indicated that the cleavage sites in both subunits are located at or near the PEST regions. The PEST sequence in B. emersonii C subunit, which when deleted or disrupted leads to resistance to proteolysis, is entirely contained in the 72-amino-acid extension located in the N-terminus of the protein. C subunit mutants carrying deletions in this region displayed little difference in their kinetic properties or enzyme thermostability. These results suggest that the N-terminal extension may only play a role in C subunit degradation.

Cytoplasmic processing is a prerequisite for presentation of an endogenous antigen by major histocompatibility complex class II proteins

Biochemical and functional studies have demonstrated major histocompatibility complex (MHC) class II-restricted presentation of select epitopes derived from cytoplasmic antigens, with few insights into the processing reactions necessary for this alternate pathway. Efficient presentation of an immunodominant epitope derived from glutamate decarboxylase (GAD) was observed regardless of whether this antigen was delivered exogenously or via a cytoplasmic route into human histocompatibility leukocyte antigen class II-DR4(+) antigen-presenting cells. Presentation of exogenous as well as cytoplasmic GAD required the intersection of GAD peptides and newly synthesized class II proteins. By contrast, proteolytic processing of this antigen was highly dependent upon the route of antigen delivery. Exogenous GAD followed the classical pathway for antigen processing, with an absolute requirement for endosomal/lysosomal acidification as well as cysteine and aspartyl proteases resident within these organelles. Presentation of endogenous GAD was dependent upon the action of cytoplasmic proteases, including the proteasome and calpain. Thus, translocation of processed antigen from the cytoplasm into membrane organelles is necessary for class II-restricted presentation via this alternate pathway. Further trimming of these peptides after translocation was mediated by acidic proteases within endosomes/lysosomes, possibly after or before class II antigen binding. These studies suggest that processing of exogenous and cytoplasmic proteins occurs through divergent but overlapping pathways. Furthermore, two cytoplasmic proteases, the proteasome and calpain, appear to play important roles in MHC class II-restricted antigen presentation.

Proteolysis in Caenorhabditis elegans sex determination: cleavage of TRA-2A by TRA-3

The Caenorhabditis elegans tra-3 gene promotes female development in XX hermaphrodites and encodes an atypical calpain regulatory protease lacking calcium-binding EF hands. We report that despite the absence of EF hands, TRA-3 has calcium-dependent proteolytic activity and its proteolytic domain is essential for in vivo function. We show that the membrane protein TRA-2A, which promotes XX female development by repressing the masculinizing protein FEM-3, is a TRA-3 substrate. Cleavage of TRA-2A by TRA-3 generates a peptide predicted to have feminizing activity. These results indicate that proteolysis regulated by calcium may control some aspects of sexual cell fate in C. elegans.

Proteolysis in Caenorhabditis elegans sex determination: cleavage of TRA-2A by TRA-3

The Caenorhabditis elegans tra-3 gene promotes female development in XX hermaphrodites and encodes an atypical calpain regulatory protease lacking calcium-binding EF hands. We report that despite the absence of EF hands, TRA-3 has calcium-dependent proteolytic activity and its proteolytic domain is essential for in vivo function. We show that the membrane protein TRA-2A, which promotes XX female development by repressing the masculinizing protein FEM-3, is a TRA-3 substrate. Cleavage of TRA-2A by TRA-3 generates a peptide predicted to have feminizing activity. These results indicate that proteolysis regulated by calcium may control some aspects of sexual cell fate in C. elegans.

Two forms of the apoptosis-linked protein ALG-2 with different Ca(2+) affinities and target recognition

The apoptosis-linked gene ALG-2 encodes a Ca(2+)-binding protein of the penta EF-hand family. To investigate the Ca(2+) binding properties of the recombinant ALG-2 protein, we have cloned ALG-2 cDNA from mouse liver mRNA. Sequence analysis showed that two types of clones were present. One (named ALG-2,5) corresponds to the published ALG-2 sequence (Vito, P., Lacana, E., and D'Adamio, L. (1996) Science 271, 521-525); the second (named ALG-2,1) is 6 nucleotides shorter, and the corresponding protein lacks the amino acid residues Gly(121) and Phe(122). Both transcripts are present in mouse tissues in the same 2:1 molar ratio. The ALG-2,5 and ALG-2,1 recombinant proteins are fully soluble in the metal-free form but can be precipitated from bacterial lysates by Ca(2+). In the presence of Tween the Ca(2+) binding profiles display two high affinity sites with [Ca(2+)](0.5) values of 1.2 and 3.1 microM for ALG-2,5 and ALG-2,1, respectively, plus one low affinity site. Using the yeast two-hybrid system we demonstrate that both proteins have a strong tendency to form homo- and heterodimers. In contrast to ALG-2, 5, the ALG-2,1 isoform does not interact with the target protein AIP-1, earlier described to play a role in apoptosis (Vito, P., Pellegrini, L., Guiet, C., and D'Adamio, L. (1999) J. Biol. Chem. 274, 1533-1540). We propose that the minor sequence difference between ALG-2,5 and ALG-2,1 affects the Ca(2+) binding properties and function of the proteins.

Oxidative stress-induced apoptosis of cochlear sensory cells: otoprotective strategies

Apoptosis is an important process, both for normal development of the inner ear and for removal of oxidative-stress damaged sensory cells from the cochlea. Oxidative-stressors of auditory sensory cells include: loss of trophic factor support, ischemia-reperfusion, and ototoxins. Loss of trophic factor support and cisplatin ototoxicity, both initiate the intracellular production of reactive oxygen species and free radicals. The interaction of reactive oxygen species and free radicals with membrane phospholipids of auditory sensory cells creates aldehydic lipid peroxidation products. One of these aldehydes, 4-hydroxynonenal, functions as a mediator of apoptosis for both auditory neurons and hair cells. We present several approaches for the prevention of auditory sensory loss from reactive oxygen species-induced apoptosis: 1) preventing the formation of reactive oxygen species; (2) neutralizing the toxic products of membrane lipid peroxidation; and 3) blocking the damaged sensory cells' apoptotic pathway.

Hyperthermia stimulates energy-proteasome-dependent protein degradation in cultured myotubes

Previous studies suggest that elevated temperature stimulates protein degradation in skeletal muscle, but the intracellular mechanisms are not fully understood. We tested the role of different proteolytic pathways in temperature-dependent degradation of long- and short-lived proteins in cultured L6 myotubes. When cells were cultured at different temperatures from 37 to 43 degrees C, the degradation of both classes of proteins increased, with a maximal effect noted at 41 degrees C. The effect of high temperature was more pronounced on long-lived than on short-lived protein degradation. By using blockers of individual proteolytic pathways, we found evidence that the increased degradation of both long-lived and short-lived proteins at high temperature was independent of lysosomal and calcium-mediated mechanisms but reflected energy-proteasome-dependent degradation. mRNA levels for enzymes and other components of different proteolytic pathways were not influenced by high temperature. The results suggest that hyperthermia stimulates the degradation of muscle proteins and that this effect of temperature is regulated by similar mechanisms for short- and long-lived proteins. Elevated temperature may contribute to the catabolic response in skeletal muscle typically seen in sepsis and severe infection.

pH regulation of gene expression in fungi

A system for the regulation of gene expression by ambient (extracellular) pH was first identified in Aspergillus nidulans. This system consists of the products of the pacC and palA, B, C, F, H, and I genes. pacC encodes a zinc finger transcription factor and these pal genes encode components of an ambient pH signal transduction pathway. pH regulatory systems have also been identified in other fungi. Components of these regulatory systems are homologous to those in A. nidulans. This review describes the pH regulatory system in A. nidulans and the history of this research and how it relates to other systems. pH regulation in bacteria and animal cells is also briefly discussed.

Direct cleavage by the calcium-activated protease calpain can lead to inactivation of caspases

Caspases, a unique family of cysteine proteases involved in cytokine activation and in the execution of apoptosis can be sub-grouped according to the length of their prodomain. Long prodomain caspases such as caspase-8 and caspase-9 are believed to act mainly as upstream caspases to cleave downstream short prodomain caspases such as caspases-3 and -7. We report here the identification of caspases as direct substrates of calcium-activated proteases, calpains. Calpains cleave caspase-7 at sites distinct from those of the upstream caspases, generating proteolytically inactive fragments. Caspase-8 and caspase-9 can also be directly cleaved by calpains. Two calpain cleavage sites in caspase-9 have been identified by N-terminal sequencing of the cleaved products. Cleavage of caspase-9 by calpain generates truncated caspase-9 that is unable to activate caspase-3 in cell lysates. Furthermore, direct cleavage of caspase-9 by calpain blocks dATP and cytochrome-c induced caspase-3 activation. Therefore our results suggest that calpains may act as negative regulators of caspase processing and apoptosis by effectively inactivating upstream caspases.

Solh, the mouse homologue of the Drosophila melanogaster small optic lobes gene: organization, chromosomal mapping, and localization of gene product to the olfactory bulb

The Drosophila melanogaster small optic lobes gene (sol) is required for normal development of the neuropiles of the medulla and lobula complexes of the adult optic lobes. The predicted protein products of sol and its human homologue SOLH contain zinc-finger-like repeats, a calpain-like protease domain, and a C-terminal domain of unknown function. Long-distance PCR was used to amplify genomic DNA for Solh, the mouse homologue of sol, following the identification of mouse Solh expressed sequence tags. The nucleotide sequence of the Solh coding region (6.0 kb) was determined. The predicted Solh protein of 1095 amino acid residues shows 89% identity (93% similarity) to the human homologue. Solh was localized by in situ hybridization to band A3.3 on mouse Chromosome 17, in a region of maintained homology with human 16p13.3. Antipeptide antibodies were prepared and verified by demonstration of specific reactivity with recombinant human SOLH protein prepared by in vitro transcription/translation and expression in insect cells using the baculovirus system. The antibodies were used to show that the Solh protein localizes to the olfactory bulb in mouse and rat brain, suggesting that it could have an analogous role in development of sensory system neurons in Drosophila and in mammals.

Epidermal growth factor receptor activation of calpain is required for fibroblast motility and occurs via an ERK/MAP kinase signaling pathway

To become migratory, cells must reorganize their connections to the substratum, and during locomotion they must break rear attachments. The molecular and biochemical mechanisms underlying these biophysical processes are unknown. Recent studies have implicated both extracellular signal-regulated kinase/mitogen-activated protein (ERK/MAP) kinase and calpain (EC 3.4.22.17) in these processes, but it is uncertain whether these are two distinct pathways acting on different modes of motility. We report that cell deadhesion involved in epidermal growth factor (EGF) receptor-mediated fibroblast motility requires activation of M-calpain downstream of ERK/MAP kinase signaling. NR6 fibroblasts expressing full-length wild type epidermal growth factor receptor required both calpain and ERK activation, as demonstrated by pharmacological inhibitors (calpeptin and calpain inhibitor I and PD98059, respectively) for EGF-induced deadhesion and motility. EGF induced rapid activation of calpain that was preventable by molecular inhibition of the Ras-Raf-MEK but not phospholipase Cgamma signaling pathway, and calpain was stimulated by transfection of constitutively active MEK. Enhanced calpain activity was not mirrored by increased calpain protein levels or decreased levels of its endogenous inhibitor calpastatin. The link between ERK/MAP kinase signaling and cell motility required the M-isoform of calpain (calpain II), as determined by specific antisense-mediated down-regulation. These data promote a previously undescribed signaling pathway of ERK/MAP kinases activating calpain to destabilize cell-substratum adhesions in response to EGF stimulation.

The crystal structure of calcium-free human m-calpain suggests an electrostatic switch mechanism for activation by calcium

Calpains (calcium-dependent cytoplasmic cysteine proteinases) are implicated in processes such as cytoskeleton remodeling and signal transduction. The 2.3-A crystal structure of full-length heterodimeric [80-kDa (dI-dIV) + 30-kDa (dV+dVI)] human m-calpain crystallized in the absence of calcium reveals an oval disc-like shape, with the papain-like catalytic domain dII and the two calmodulin-like domains dIV+dVI occupying opposite poles, and the tumor necrosis factor alpha-like beta-sandwich domain dIII and the N-terminal segments dI+dV located between. Compared with papain, the two subdomains dIIa+dIIb of the catalytic unit are rotated against one another by 50 degrees, disrupting the active site and the substrate binding site, explaining the inactivity of calpains in the absence of calcium. Calcium binding to an extremely negatively charged loop of domain dIII (an electrostatic switch) could release the adjacent barrel-like subdomain dIIb to move toward the helical subdomain dIIa, allowing formation of a functional catalytic center. This switch loop could also mediate membrane binding, thereby explaining calpains' strongly reduced calcium requirements in vivo. The activity status at the catalytic center might be further modulated by calcium binding to the calmodulin domains via the N-terminal linkers.

Filamin 2 (FLN2): A muscle-specific sarcoglycan interacting protein

Mutations in genes encoding for the sarcoglycans, a subset of proteins within the dystrophin-glycoprotein complex, produce a limb-girdle muscular dystrophy phenotype; however, the precise role of this group of proteins in the skeletal muscle is not known. To understand the role of the sarcoglycan complex, we looked for sarcoglycan interacting proteins with the hope of finding novel members of the dystrophin-glycoprotein complex. Using the yeast two-hybrid method, we have identified a skeletal muscle-specific form of filamin, which we term filamin 2 (FLN2), as a gamma- and delta-sarcoglycan interacting protein. In addition, we demonstrate that FLN2 protein localization in limb-girdle muscular dystrophy and Duchenne muscular dystrophy patients and mice is altered when compared with unaffected individuals. Previous studies of filamin family members have determined that these proteins are involved in actin reorganization and signal transduction cascades associated with cell migration, adhesion, differentiation, force transduction, and survival. Specifically, filamin proteins have been found essential in maintaining membrane integrity during force application. The finding that FLN2 interacts with the sarcoglycans introduces new implications for the pathogenesis of muscular dystrophy.

The sensitivity of c-Jun and c-Fos proteins to calpains depends on conformational determinants of the monomers and not on formation of dimers

Milli- and micro-calpains are ubiquitous cytoplasmic cysteine proteases activated by calcium. They display a relatively strict specificity for their substrates which they usually cleave at only a limited number of sites. Motifs responsible for recognition by calpains have not been characterized yet, and recently a role for PEST motifs in this process has been ruled out. c-Fos and c-Jun transcription factors are highly sensitive to calpains in vitro. They thus provide favourable protein contexts for studying the structural requirements for recognition and degradation by these proteases. Using in vitro degradation assays and site-directed mutagenesis, we report here that susceptibility to calpains is primarily determined by conformational determinants of the monomers and not by the quaternary structure of c-Fos and c-Jun proteins. The multiple cleavage sites borne by both proteins can be divided into at least two classes of sensitivity, the most sensitive ones being easily visualized in the presence of rate-limiting amounts of calpains. One site located at position 90-91 in c-Fos protein is extremely sensitive. However, efficient proteolysis did not have any strict dependence on the nature of the amino acids on either side of the scissile bond in the region extending from P2 to P'2. The structural integrity of the monomers is not crucial for recognition by calpains. Rather, sensitive sites can be recognized independently and their recognition is dependent on the local conformation of peptide regions that may span several tens of amino acids and maybe more in the case of the identified c-Fos hypersensitive site.

Behavioral efficacy of posttraumatic calpain inhibition is not accompanied by reduced spectrin proteolysis, cortical lesion, or apoptosis

Administration of the selective calpain inhibitor AK295 has been shown to attenuate motor and cognitive dysfunction after brain trauma in rats. To explore mechanisms underlying the behavioral efficacy of posttraumatic calpain inhibition, we investigated histologic consequences of AK295 administration. Anesthetized Sprague-Dawley rats received lateral fluid percussion brain injury of moderate severity (2.2 to 2.4 atm) or served as uninjured controls. At 15 minutes after injury, animals were randomly assigned to receive a 48-hour infusion of either 2 mmol/L AK295 (120 to 140 mg/kg) or saline via the carotid artery. At 48 hours and 1 week after injury, spectrin fragments generated specifically by calpain were detected by Western blotting and immunohistochemistry, respectively, in saline-treated, brain-injured animals. Interestingly, equivalent spectrin breakdown was observed in AK295-treated animals when cortical and hippocampal regions were evaluated. Similarly, administration of the calpain inhibitor did not attenuate cortical lesion size or the numbers of apoptotic cells in the cortex, subcortical white matter, or hippocampus, as verified by terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick-end labeling and morphology, at 48 hours after injury. These data suggest that an overt reduction in spectrin proteolysis, cortical lesion, or apoptotic cell death at 48 hours or 1 week is not required for behavioral improvements associated with calpain inhibition by AK295 after experimental brain injury in rats.

Calpain inhibitors protect auditory sensory cells from hypoxia and neurotrophin-withdrawal induced apoptosis

Inhibitors of calpain have been shown to protect nerve growth factor (NGF)-deprived ciliary ganglion neurons and hypoxic cortical neurons. Calpains have been identified in the cochlea and are active during ischemic injury. Since apoptosis can be initiated by loss of neurotrophic support, hypoxia, and ototoxins (e.g., cisplatin, CDDP), the role of calpain inhibitors under these conditions was examined in auditory hair cells and neurons. Dissociated spiral ganglion neuron (SGN) cell cultures and organ of Corti explants from P3 rats were used to test the efficacy of calpain inhibitors as otoprotective molecules. Our results indicate that calpain inhibitor I, calpain inhibitor II, and leupeptin all provided significant protection of SGNs against neurotrophin-withdrawal and hypoxia-induced apoptosis. The increase in neuronal survival ranged from 2.16 to 2.31 times greater than in untreated neurotrophin-withdrawn SGN cell cultures. BOC-Asp(Ome)-Fluoromethyl Ketone (B-D-FMK), a general caspase inhibitor, increased neuronal survival 2.16 times more. Neuronal survival rates were from 1.88 to 2.27 times greater than in untreated, hypoxic neurons and hair cell survival rates were from 1.98 to 2.03 times greater than untreated, hypoxic organ of Corti explants. However, protection of auditory hair cells and neurons from CDDP-induced damage (10 and 6 micrograms/ml, respectively) was limited with any of these calpain inhibitors. Apoptotic pathways initiated by neurotrophin-deprivation and ototoxic stress (e.g., CDDP) have been shown to be different. Our results agree with this finding, with neurotrophin-withdrawal and hypoxia, but not CDDP damage-induced apoptosis being calpain-dependent.

The calcium-activated neutral protease calpain I is present in normal foetal skin and is decreased in neonatal harlequin ichthyosis

Calcium concentration is a critical factor for epidermal differentiation and is implicated in the expression and post-translational modification of numerous proteins in suprabasal cells of the epidermis. Calpains (calcium-activated neutral proteases) are believed to participate in signal transduction via highly regulated cytoplasmic protease activity. Here we investigate the expression of calpain I in normal human skin development and in neonatal harlequin ichthyosis (HI), a disorder of altered epidermal differentiation, especially the transition from the granular to the fully differentiated cornified layer. Calpain I was detected in developing foetal epidermis at 54 days estimated gestational age in the basal layer and the periderm of the developing foetal epidermis. By 125 days, calpain I was also detected in the granular layer. This pattern was maintained in newborn skin, but expression was significantly weaker in HI biopsies (n = 7). Reduced expression of calpain was specific to HI and was not observed in other skin diseases. Calpain was also normally expressed in the outer root sheath of hair follicles, in sebaceous glands and in sweat ducts and glands. Immunoblots of epidermal and keratinocyte extracts showed that the 78-kDa and 76-kDa active forms were generated via limited proteolysis of the 80-kDa inactive subunit; however, all forms were diminished in HI, consistent with findings in tissue sections. Our results show that calpain is present throughout the epidermis and is expressed from the early stages of development. These findings implicate calcium-mediated signalling events in the alteration of differentiation that occurs in HI.

Tumor invasion: role of growth factor-induced cell motility

Cancer progression to the invasive and metastatic stage represents the most formidable barrier to successful treatment. To develop rational therapies, we must determine the molecular bases of these transitions. Cell motility is one of the defining characteristics of invasive tumors, enabling tumors to migrate into adjacent tissues or transmigrate limiting basement membranes and extracellular matrices. Invasive tumor cells have been demonstrated to present dysregulated cell motility in response to extracellular signals from growth factors and cytokines. Recent findings suggest that this growth factor receptor-mediated motility is one of the most common aberrations in tumor cells leading to invasiveness and represents a cellular behavior distinct from-adhesion-related haptokinetic and haptotactic migration. This review focuses on the emerging understanding of the biochemical and biophysical foundations of growth factor-induced cell motility and tumor cell invasiveness, and the implications for development of targeted agents, with particular emphasis on signaling from the epidermal growth factor (EGF) and hepatocyte growth factor (HGF) receptors, as these have most often been associated with tumor invasion. The nascent models highlight the roles of various intracellular signaling pathways including phospholipase C-gamma (PLC gamma), phosphatidylinositol (PI)3'-kinase, mitogen-activated protein (MAP) kinase, and actin cytoskeleton-related events. Development of novel agents against tumor invasion will require not only a detailed appreciation of the biochemical regulatory elements of motility but also a paradigm shift in our approach to and assessment of cancer therapy.

The PEST domain of IkappaBalpha is necessary and sufficient for in vitro degradation by mu-calpain

Polypeptide sequences enriched in proline (P), glutamate (E), serine (S), and threonine (T), dubbed PEST domains, are proposed to expedite the degradation of proteins. The proteolysis of one PEST-containing protein, IkappaBalpha, is prerequisite to the activation of the transcription factor NF-kappaB. Two mechanisms of IkappaBalpha degradation in vivo have been described, one well characterized through the ubiquitin-proteasome pathway, and another less characterized through calpain. In this report, a mutational analysis was done to identify any regions of IkappaBalpha that facilitate its recognition and proteolysis by calpain in vitro. These studies revealed that the PEST sequence of IkappaBalpha is critical for its calpain-dependent degradation. Furthermore, the IkappaBalpha-PEST domain binds to the calmodulin-like domain of the large subunit of mu-calpain (muCaMLD). Transfer of the IkappaBalpha-PEST domain to a protein incapable of either binding to or being degraded by mu-calpain allowed for the interaction of the chimeric protein with muCaMLD and resulted in its susceptibility to calpain proteolysis. Moreover, the muCaMLD of calpain acts as a competitive inhibitor of calpain-dependent IkappaBalpha degradation. Our data demonstrate that the IkappaBalpha-PEST sequence acts as a modular domain to promote the physical association with and subsequent degradation by mu-calpain and suggest a functional role for PEST sequences in other proteins as potential calpain-targeting units.

Calpain cleavage of integrin beta cytoplasmic domains

We showed previously that the calcium-dependent protease, calpain, cleaves the cytoplasmic domain of the integrin beta3 subunit. To investigate whether susceptibility to calpain is a common feature of all integrin beta subunits, and to map calpain cleavage sites in different integrin beta tails, we treated recombinant cytoplasmic domains of integrin beta1A, beta1D, beta2, beta3 and beta7 subunits with purified calpain in vitro. We found that the cytoplasmic domains of all these integrin chains were cleaved by calpain. HPLC followed by mass spectrometry was used to identify calpain cleavage sites. These sites were clustered in the C-terminal half of the integrin beta cytoplasmic domains in regions flanking the two NXXY motifs, suggesting the possibility that the structural framework provided by these motifs is recognized by calpain. We used the knowledge of these cleavage sites to develop cleavage site-specific antibodies and to demonstrate cleavage of the beta1A cytoplasmic domain in intact platelets stimulated with calcium ionophore or thrombin. Thus susceptibility to calpain cleavage is common to integrin beta subunits, can be induced in intact cells, and appears to favor regions surrounding two conserved NXXY motifs.

Expression, partial purification and functional properties of themuscle-specific calpain isoform p94

The muscle-specific calpain isoform p94 has high propensity to autocatalytic degradation, thus no significant amounts of the intact active protein have been available so far. As a result, aspects like its regulation (via Ca2+ and other factors) and its intracellular localization are unknown or obscure. In this work, large amounts of human p94 have been produced in insect cells using a recombinant baculovirus expression system. Although most of the protease was recovered in an insoluble and catalytically inactive form, the soluble fraction contained amounts of intact active p94 adequate for its characterization. His-tagged recombinant p94, obtained by the same expression system, was partially purified as an active product. Both the unmodified and the partially purified His-tagged p94 bound calcium with high affinity, and their autolytic activity required Ca2+. The sensitivity of the catalytic activity of the recombinant protease to Ca2+ was very high. In fact, p94 in soluble cell extracts autolysed to a significant extent even in the presence of submicromolar Ca2+ levels. Thus, in analogy to what demonstrated for the ubiquitous m- and micro-calpain isoforms, intracellular Ca2+ might be one of the factors controlling the activity of this muscle-specific calpain isoform.

Ambient pH signal transduction in Aspergillus: completion of gene characterization

Completing the molecular analysis of the six pal genes of the ambient pH signal transduction pathway in Aspergillus nidulans, we report the characterization of palC and palH. The derived translation product of palH contains 760 amino acids with prediction of seven transmembrane domains in its N-terminal moiety. Remarkably, a palH frameshift mutant lacking just over half the PalH protein, including almost all of the long hydrophilic region C-terminal to the transmembrane domains, retains some PalH function. The palC-derived translation product contains 507 amino acids, and the null phenotype of a frameshift mutation indicates that at least one of the C-terminal 142 residues is essential for function. Uniquely among the A. nidulans pH-signalling pal genes, palC appears to have no Saccharomyces cerevisiae homologue, although it does have a Neurospora crassa expressed sequence tag homologue. In agreement with findings for the palA, palB and palI genes of this signalling pathway, levels of the palC and palH mRNAs do not appear to be pH regulated.

Sepsis stimulates release of myofilaments in skeletal muscle by a calcium-dependent mechanism

Sepsis is associated with a pronounced catabolic response in skeletal muscle, mainly reflecting degradation of the myofibrillar proteins actin and myosin. Recent studies suggest that sepsis-induced muscle proteolysis may reflect ubiquitin-proteasome-dependent protein breakdown. An apparently conflicting observation is that the ubiquitin-proteasome pathway does not degrade intact myofibrils. Thus, it is possible that actin and myosin need to be released from the myofibrils before they can be ubiquitinated and degraded by the proteasome. We tested the hypothesis that sepsis results in disruption of Z-bands, increased expression of calpains, and calcium-dependent release of myofilaments in skeletal muscle. Sepsis induced in rats by cecal ligation and puncture resulted in increased gene expression of micro-calpain, m-calpain, and p94 and in Z-band disintegration in the extensor digitorum longus muscle. The release of myofilaments from myofibrillar proteins was increased in septic muscle. This response to sepsis was blocked by treating the rats with dantrolene, a substance that inhibits the release of calcium from intracellular stores to the cytoplasm. The present results provide evidence that sepsis is associated with Z-band disintegration and a calcium-dependent release of myofilaments in skeletal muscle. Release of myofilaments may be an initial and perhaps rate-limiting component of sepsis-induced muscle breakdown.

Comparative studies of Ca2+-dependent proteases (CDP I and CDP II) from Allomyces arbuscula

Allomyces arbuscula, an aquatic fungus, contains two Ca2+-dependent neutral cysteine proteases (CDP I and CDP II), eluting respectively, at 0.07 and 0.2 M NaCl from DEAE cellulose columns. The purified CDP I has a Mr of 39 kDa whereas CDP II appears as a doublet of 43 and 40 kDa. Both enzymes require free thiol, the same concentration of Ca2+ for half maximal activation, and are inactivated by thiol protease inhibitors. Our results show that despite these similarities the two enzymes are different because affinity-purified CDP II antibodies do not cross-react with CDP I antigen in Western blots. In contrast, there is a strong cross-reaction between the two 43 and 40 kDa CDP II peptides and their respective antibodies. Both enzymes cleave preferentially the carboxy terminus of Arg and to a limited extent Lys on the cleavage site. This primary specificity is governed by the nature of the amino acids in the P2 and P3 positions. In general either Pro or Gly in P2 is required, with preference for Pro and in P3 position, Gly over Val. CDP II has higher catalytic activity than CDP I. The sulfhydryl reagent NEM is a more potent inhibitor of CDP I than CDP II. Although the function of the phosphorylable site(s) is not clear, both CDP I and CDP II contain phosphorylable serine residue(s).

Expression and functional characteristics of calpain 3 isoforms generated through tissue-specific transcriptional and posttranscriptional events

Calpain 3 is a nonlysosomal cysteine protease whose biological functions remain unknown. We previously demonstrated that this protease is altered in limb girdle muscular dystrophy type 2A patients. Preliminary observations suggested that its gene is subjected to alternative splicing. In this paper, we characterize transcriptional and posttranscriptional events leading to alterations involving the NS, IS1, and IS2 regions and/or the calcium binding domains of the mouse calpain 3 gene (capn3). These events can be divided into three groups: (i) splicing of exons that preserve the translation frame, (ii) inclusion of two distinct intronic sequences between exons 16 and 17 that disrupt the frame and would lead, if translated, to a truncated protein lacking domain IV, and (iii) use of an alternative first exon specific to lens tissue. In addition, expression of these isoforms seems to be regulated. Investigation of the proteolytic activities and titin binding abilities of the translation products of some of these isoforms clearly indicated that removal of these different protein segments affects differentially the biochemical properties examined. In particular, removal of exon 6 impaired the autolytic but not fodrinolytic activity and loss of exon 16 led to an increased titin binding and a loss of fodrinolytic activity. These results are likely to impact our understanding of the pathophysiology of calpainopathies and the development of therapeutic strategies.

Diverse stimuli induce calpain overexpression and apoptosis in C6 glioma cells

Calpain, a Ca2+-activated cysteine protease, has been implicated in apoptosis of immune cells. Since central nervous system (CNS) is abundant in calpain, the possible involvement of calpain in apoptosis of CNS cells needs to be investigated. We studied calpain expression in rat C6 glioma cells exposed to reactive hydroxyl radical (.OH) [formed via the Fenton reaction (Fe2++H2O2+H+-->Fe3++H2O+.OH)], interferon-gamma (IFN-gamma), and calcium ionophore (A23187). Cell death, cell cycle, calpain expression, and calpain activity were examined. Diverse stimuli induced apoptosis in C6 cells morphologically (chromatin condensation as detected by light microscopy) and biochemically [DNA fragmentation as detected by TdT-mediated dUTP Nick-End Labeling (TUNEL) assay]. Oxidative stress arrested a population of C6 cells at the G2/M phase of cell cycle. The levels of mRNA expression of six genes were analyzed by the reverse transcriptase-polymerase chain reaction (RT-PCR). Diverse stimuli did not alter beta-actin (internal control) expression, but increased calpain expression, and the upregulated bax (pro-apoptotic)/bcl-2 (anti-apoptotic) ratio. There was no significant increase in expression of calpastatin (endogenous calpain inhibitor). Western blot analysis showed an increase in calpain content and degradation of myelin-associated glycoprotein (MAG), a calpain substrate. Pretreatment of C6 cells with calpeptin (a cell-permeable calpain inhibitor) blocked calpain overexpression, MAG degradation, and DNA fragmentation. We conclude that calpain overexpression due to.OH stress, IFN-gamma stimulation, or Ca2+ influx is involved in C6 cell death, which is attenuated by a calpain-specific inhibitor.

Gene for a protein capable of enhancing lateral root formation

Analysis of genes preferentially expressed in hairy roots caused by infection with Agrobacterium rhizogenes has provided insights into the regulation of lateral root formation. A hairy root preferential cDNA, HR7, has been cloned from hairy roots of Hyoscyamus niger. HR7 encodes a novel protein partially homologous to a metallocarboxypeptidase inhibitor and is expressed exclusively in the primordium and base of lateral roots in hairy roots. Overexpression of HR7 in transgenic roots of H. niger dramatically enhances the frequency of lateral root formation. The results of this study indicate that expression of HR7 plays a critical role in initiating lateral root formation.

Down-regulation of types I, II and III inositol 1,4,5-trisphosphate receptors is mediated by the ubiquitin/proteasome pathway

Activation of certain phosphoinositidase-C-linked cell-surface receptors is known to cause an acceleration of the proteolysis of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptors and, thus, lead to Ins(1,4,5)P3-receptor down-regulation. In the current study we have sought to determine whether the ubiquitin/proteasome pathway is involved in this adaptive response. The data presented show (i) that activation of phosphoinositidase-C-linked receptors causes Ins(1,4,5)P3-receptor ubiquitination in a range of cell types (AR4-2J cells, INS-1 cells and rat cerebellar granule cells), (ii) that the Ins(1,4,5)P3-receptor down-regulation induced by activation of these receptors is blocked by proteasome inhibitors, (iii) that all known Ins(1,4,5)P3 receptors (types I, II and III) are substrates for ubiquitination, (iv) that ubiquitination occurs while Ins(1,4,5)P3 receptors are membrane-bound, (v) that Ins(1,4, 5)P3-receptor ubiquitination and down-regulation are stimulated only by those agonists that elevate Ins(1,4,5)P3 concentration persistently, and (vi) that a portion of cellular Ins(1,4,5)P3 receptors (those that are not type-I-receptor-associated) can be resistant to ubiquitination and degradation. In total these data indicate that the ubiquitin/proteasome pathway mediates Ins(1,4, 5)P3-receptor down-regulation and suggest that ubiquitination is stimulated by the binding of Ins(1,4,5)P3 to its receptor.

Association and dissociation of the calcium-binding domains of calpain by Ca2+

The calmodulin-like domain of calpain is important for the association of the calpain large and small subunits. We expressed the calmodulin-like domains of the large subunits of rabbit mu- and m-calpains and their small subunits in E. coli and purified them to homogeneity. Unlike the full-length subunits, the calmodulin-like domains are soluble in buffer containing Ca2+. We performed gel filtration chromatography of the purified proteins and found that all three calmodulin-like domains exist as homodimers in the absence of Ca2+ and dissociate into monomers upon the addition of Ca2+.

Are there multiple proteolytic pathways contributing to c-Fos, c-Jun and p53 protein degradation in vivo?

The c-Fos and c-Jun oncoproteins and the p53 tumor suppressor protein are short-lived transcription factors. Several catabolic pathways contribute to their degradation in vivo. c-Fos and c-Jun are thus mostly degraded by the proteasome, but there is indirect evidence that, under certain experimental/physiological conditions, calpains participate in their destruction, at least to a limited extent. Lysosomes have also been reported to participate in the destruction of c-Fos. Along the same lines, p53 is mostly degraded following the ubiquitin/proteasome pathway and calpains also seem to participate in its degradation. Moreover, c-Fos, c-Jun and p53 turnovers are regulated upon activation of intracellular signalling cascades. All taken together, these observations underline the complexity of the mechanisms responsible for the selective destruction of proteins within cells.

Pathophysiological role of calpain in experimental demyelination

Calcium-activated neutral proteinase (calpain) has been extensively studied over the past three decades such that many enzymatic and structural properties of this enzyme are well understood. However, the pathophysiological roles of calpain remain poorly defined. In addition to recent studies delineating a role for calpain in various pathological conditions, this proteinase has been implicated in the degradation of myelin proteins in autoimmune demyelinating diseases such as multiple sclerosis and experimental allergic encephalomyelitis (EAE). In EAE, calpain translational expression is significantly increased in activated glial/inflammatory cells that participate in myelinolysis while calpain substrates (axonal and myelin proteins) are lost. Thus, since all major myelin proteins are calpain substrates, early studies suggest calpain may play an important role in demyelination of the central nervous system.

Increased intracellular calpain detection in experimental focal cerebral ischemia

Calpains are intracellular proteinases whose proteolytic activity is directed mainly against the cytoskeleton and regulatory proteins. We studied the presence of calpain by immunohistochemistry in a rat model of reversible focal cerebral ischemia (3 h) at various times of reperfusion. The numbers of calpain-positive cells on the ischemic side were compared with the non-ischemic side. In controls only 2 +/- 1% cells were positive, whereas the cortex of the ischemic vs the non-ischemic side showed 88 +/- 3% vs 13 +/- 4% calpain-positive cells (p < 0.001), and the basal ganglia 47 +/- 3% vs 13 +/- 4% (p < 0.01) after 3 h ischemia and 24 h reperfusion. This is the first demonstration of elevated intracellular levels of calpains in areas of cerebral ischemia. Longer reperfusion resulted in an increase in calpain positivity.

Protein-protein interactions in intracellular Ca2+-release channel function

Release of Ca2+ ions from intracellular stores can occur via two classes of Ca2+-release channel (CRC) protein, the inositol 1,4, 5-trisphosphate receptors (InsP3Rs) and the ryanodine receptors (RyRs). Multiple isoforms and subtypes of each CRC class display distinct but overlapping distributions within mammalian tissues. InsP3Rs and RyRs interact with a plethora of accessory proteins which modulate the activity of their intrinsic channels. Although many aspects of CRC structure and function have been reviewed in recent years, the properties of proteins with which they interact has not been comprehensively surveyed, despite extensive current research on the roles of these modulators. The aim of this article is to review the regulation of CRC activity by accessory proteins and, wherever possible, to outline the structural details of such interactions. The CRCs are large transmembrane proteins, with the bulk of their structure located cytoplasmically. Intra- and inter-complex protein-protein interactions between these cytoplasmic domains also regulate CRC function. Some accessory proteins modulate channel activity of all CRC subtypes characterized, whereas other have class- or even isoform-specific effects. Certain accessory proteins exert both direct and indirect forms of regulation on CRCs, occasionally with opposing effects. Others are themselves modulated by changes in Ca2+ concentration, thereby participating in feedback mechanisms acting on InsP3R and RyR activity. CRCs are therefore capable of integrating numerous signalling events within a cell by virtue of such protein-protein interactions. Consequently, the functional properties of InsP3Rs and RyRs within particular cells and subcellular domains are 'customized' by the accessory proteins present.

Specificity determinants of proteolytic processing of Aspergillus PacC transcription factor are remote from the processing site, and processing occurs in yeast if pH signalling is bypassed

The Aspergillus nidulans transcription factor PacC, which mediates pH regulation, is proteolytically processed to a functional form in response to ambient alkaline pH. The full-length PacC form is unstable in the presence of an operational pH signal transduction pathway, due to processing to the relatively stable short functional form. We have characterized and used an extensive collection of pacC mutations, including a novel class of "neutrality-mimicking" pacC mutations having aspects of both acidity- and alkalinity-mimicking phenotypes, to investigate a number of important features of PacC processing. Analysis of mutant proteins lacking the major translation initiation residue or truncated at various distances from the C terminus showed that PacC processing does not remove N-terminal residues, indicated that processing yields slightly heterogeneous products, and delimited the most upstream processing site to residues approximately 252 to 254. Faithful processing of three mutant proteins having deletions of a region including the predicted processing site(s) and of a fourth having 55 frameshifted residues following residue 238 indicated that specificity determinants reside at sequences or structural features located upstream of residue 235. Thus, the PacC protease cuts a peptide bond(s) remote from these determinants, possibly thereby resembling type I endonucleases. Downstream of the cleavage site, residues 407 to 678 are not essential for processing, but truncation at or before residue 333 largely prevents it. Ambient pH apparently regulates the accessibility of PacC to proteolytic processing. Alkalinity-mimicking mutations L259R, L266F, and L340S favor the protease-accessible conformation, whereas a protein with residues 465 to 540 deleted retains a protease-inaccessible conformation, leading to acidity mimicry. Finally, not only does processing constitute a crucial form of modulation for PacC, but there is evidence for its conservation during fungal evolution. Transgenic expression of a truncated PacC protein, which was processed in a pH-independent manner, showed that appropriate processing can occur in Saccharomyces cerevisiae.

Characterization of a human digestive tract-specific calpain, nCL-4, expressed in the baculovirus system

Human nCL-4, a digestive tract-specific calpain, was stably produced with 30K, a regulatory subunit for ubiquitous calpain as a fully active form using the baculovirus-expression system. nCL-4 showed an activity only when it was coexpressed with 30K. Expressed heterodimeric recombinant nCL-4 was purified to near homogeneity by sequential column chromatographies. Purified nCL-4 showed a calcium-dependent activity (calcium concentration at 50% maximum activity (Ka): 0.125 mM) with a sp act of 21 U/mg, which is distinct from those of ubiquitous calpains. nCL-4 exhibited calcium-dependent autolysis, but the cleavage pattern of nCL-4 was clearly different from ubiquitous calpains. Although it was inhibited by leupeptin, E-64, and calpastatin, and exhibited an optimal pH at 7.3 like other ubiquitous calpains, its optimal temperature was much lower. When overexpressed in COS-7 cells, clear asymmetric juxtanuclear, and/or nuclear staining rather than typical cytoplasmic staining was observed. Moreover, a translation product of nCL-4 was detected in rat stomach tissue by immunofluorescence analysis. In conclusion, human nCL-4 resembles ubiquitous calpain in some enzymatic properties and interacts with 30K for its activity. This is the first report on biochemical and enzymatic properties of a fully active tissue-specific calpain species expressed in the baculovirus system.

Reovirus-induced apoptosis is preceded by increased cellular calpain activity and is blocked by calpain inhibitors

The cellular pathways of apoptosis have not been fully characterized; however, calpain, a cytosolic calcium-activated cysteine protease, has been implicated in several forms of programmed cell death. Reoviruses induce apoptosis both in vitro and in vivo and serve as a model for studying virus-induced cell death. We investigated the potential role of calpain in reovirus-induced apoptosis in vitro by measuring calpain activity as well as evaluating the effects of calpain inhibitors. L929 cells were infected with reovirus type 3 Abney (T3A), and calpain activity, measured as cleavage of the fluorogenic calpain substrate Suc-Leu-Leu-Val-Tyr-AMC, was monitored. There was a 1.6-fold increase in calpain activity in T3A-infected cells compared to mock-infected cells; this increase was completely inhibited by preincubation with calpain inhibitor I (N-acetyl-leucyl-leucyl-norleucinal [aLLN]), an active-site inhibitor. Both aLLN and PD150606, a specific calpain inhibitor that interacts with the calcium-binding site, inhibited reovirus-induced apoptosis in L929 cells by 54 to 93%. Apoptosis induced by UV-inactivated reovirus was also reduced 65 to 69% by aLLN, indicating that inhibition of apoptosis by calpain inhibitors is independent of effects on viral replication. We conclude that calpain activation is a component of the regulatory cascade in reovirus-induced apoptosis.

Calpain involvement in calphostin C-induced apoptosis

A major problem in assessing the role of calpains in apoptosis induction concerns the fact that calpain inhibitors can also impair the activity of the proteasome, also reported to be involved in apoptosis. Herein we showed that apoptosis induced by calphostin C in U937 human promonocytic leukemia cells was associated, at its onset, with enhanced protein (poly)ubiquitination. This observation prompted us to study whether protein degradation through the ubiquitin/proteasome pathway was involved in apoptosis induction. We found that N-acetyl-Leu-Leu-norleucinal (50 microM), a proteasome as well as a calpain inhibitor, was able to reduce calphostin C-induced apoptosis by approximately 60%, whereas lactacystin (10 microM), a specific proteasome inhibitor, was ineffective. These results suggest that calphostin C-induced apoptosis is partly calpain-mediated, but does not require protein degradation through the ubiquitin/proteasome pathway.

Epidermal growth factor receptor-mediated motility in fibroblasts

Cell motility is induced by many growth factors acting through cognate receptors with intrinsic tyrosine kinase activity (RPTK). However, most of the links between receptor activation and the biophysical processes of cell motility remain undeciphered. We have focused on the mechanisms by which the EGF receptor (EGFR) actuates fibroblast cell motility in an attempt to define this integrated process in one system. Our working model is that divergent, but interconnected pathways lead to the biophysical processes necessary for cell motility: cytoskeleton reorganization, membrane extension, formation of new adhesions to substratum, cell contraction, and release of adhesions at the rear. We postulate that for any given growth factor some of the pathways/processes will be actively signaled and rate-limiting, while others will be permissive due to background low-level activation. Certain couplings have been defined, such as PLCgamma and actin modifying proteins being involved in cytoskeletal reorganization and lamellipod extension and MEK being implicated in detachment from substratum. Others are suggested by complementary investigations in integrin-mediated motility, including rac in membrane protrusion, rho in new adhesions, myosin II motors in contraction, and calpain in detachment, but have yet to be placed in growth factor-induced motility. Our model postulates that many biochemical pathways will be shared between chemokinetic and haptokinetic motility but that select pathways will be activated only during RPTK-enhanced motility.

Low levels of inorganic lead noncompetitively inhibit mu-calpain

Calpain is a ubiquitous calcium-dependent cysteine protease, whose cytoskeletal protein substrates suggest that it may be important in neuronal differentiation. Lead (Pb2+) is known to substitute for Ca2+ in a variety of intracellular processes, and interferes with the development of hippocampal neurons in vitro. We found that free Pb2+ at 1 nM does not activate calpain in the absence of Ca2+. Pb2+ inhibited the activity of calpain; the degree of calpain inhibition was dependent on an interaction between concentrations of both Ca2+ and Pb2+. In the presence of 1 microM free Ca2+, 10 pM free Pb2+ reduced calpain activity, but in the presence of 100 microM free Ca2+, 1 nM free Pb2+ failed to inhibit calpain. This provides evidence that Pb2+ competes for the Ca2+ binding sites on calpain. In the presence of 40 microM free Ca2+, 1 nM free Pb2+ significantly reduces Vmax without altering Km, suggesting that Pb2+ acts as a noncompetitive inhibitor of calpain. Inhibition of calpain is one mechanism by which Pb2+ may interfere with neuronal development.

Glutamate receptor antagonists inhibit calpain-mediated cytoskeletal proteolysis in focal cerebral ischemia

Excitatory amino acids may promote microtubular proteolysis observed in ischemic neuronal degeneration by calcium-mediated activation of calpain, a neutral protease. We tested this hypothesis in an animal model of focal cerebral ischemia without reperfusion. Spontaneously hypertensive rats were treated with 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)quinoxaline (NBQX), a competitive antagonist of the neuronal receptor for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), or cis-4-[phosphono-methyl]-2-piperidine carboxylic acid (CGS 19755), a competitive antagonist of the N-methyl-d-aspartate (NMDA) receptor. After treatment, all animals were subjected to permanent occlusion of the middle cerebral artery for 6 or 24 h. Infarct volumes measured in animals pretreated with CGS 19755 after 24 h of ischemia were significantly smaller than those quantified in ischemic controls. Rats pretreated with NBQX showed partial amelioration of cytoskeletal injury with preserved immunolabeling of microtubule-associated protein 2 (MAP 2) at 6 and 24 h and reduced accumulation of calpain-cleaved spectrin byproducts only at 6 h. Prevention of cytoskeletal damage was more effective after pretreatment with CGS 19755, as shown by retention of MAP 2 immunolabeling and significant restriction of calpain activity at both 6 and 24 h. Preserved immunolabeling of tau protein was observed at 6 and 24 h only in animals pretreated with CGS 19755. Western analysis performed on ischemic cortex taken from controls or rats pretreated with either NBQX or CGS 19755 suggested that loss of tau protein immunoreactivity was caused by dephosphorylation, rather than proteolysis. These results demonstrate a crucial link between excitotoxic neurotransmission, microtubular proteolysis, and neuronal degeneration in focal cerebral ischemia.

Purification of native p94, a muscle-specific calpain, and characterization of its autolysis

p94, a skeletal muscle-specific calpain, has attracted much attention because its gene is responsible for limb-girdle muscular dystrophy type 2A. p94, however, has not been characterized at the protein and enzyme levels, owing to its very rapid autolysis. In the present study, a purification procedure for p94 was first established by using a recombinant inactive p94 expressed in COS cells in which the active site cysteine residue was changed to serine [p94(C129S)]. The isolation of native p94 from rabbit skeletal muscle by the established method with conventional procedures was extremely difficult because p94 became highly unstable in a crude extract on the addition of NaCl for separation. Purification of native p94 was possible with an antibody-affinity column but only as an inactive enzyme; p94(C129S) was purified as a homodimer. Characterization of p94, especially autolysis, was performed with partly purified native p94 and p94(C129S). The autolysis of p94, which consisted at least partly of an intermolecular reaction, proceeded in three consecutive steps; 60 and 58 kDa fragments were produced as intermediates before a stable 55 kDa fragment appeared. Autolysis of p94 was regarded as a degradative step rather than for the activation of the enzyme. All the autolysis cleavage sites were located in the p94-specific insertion sequence 1 region, which explains why p94 is unstable compared with the other calpains. The autolysis sites in p94 clearly showed a different specificity relative to the autolytic and proteolytic cleavage sites of the ubiquitous mu- and m-calpains, in its preference for residues at the P3 to P1' sites, indicating a distinct substrate specificity and function for the muscle enzyme.

In vitro generation of an active calmodulin-independent phosphodiesterase from brain calmodulin-dependent phosphodiesterase (PDE1A2) by m-calpain

In the present study we have shown that bovine brain 60-kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme (CaMPDE - PDE1A2) is proteolyzed by a Ca2+-dependent cysteine protease, m-calpain. The proteolysis of PDE1A2 by m-calpain results in its conversion to a totally calmodulin (CaM)-independent form accompanied by degradation of PDE1A2 into a 45-kDa catalytic fragment and a 15-kDa fragment. The activity of PDE1A2 is unaffected by the presence or absence of CaM during cleavage, suggesting that the interaction between CaM and PDE1A2 does not alter substrate recognition by calpain. Furthermore, we provide evidence, based on the studies of CaM overlay and phosphorylation, that the cleavage site is not present either in the CaM-binding domain or phosphorylation site. N-terminal sequence analysis of the 45-kDa fragment indicated that cleavage occurs between residues 126Gln and 127Ala, and eliminates the CaM-dependent activity of carboxy termini PDE1A2. The present findings suggest that limited proteolysis in the brain through calpains could be an alternate mechanism for activating CaMPDE(s) and for regulating intracellular levels of cAMP.

m-Calpain subunits remain associated in the presence of calcium

The hypothesis that calpain subunits dissociate in the presence of Ca2+ has been tested by methods which avoid interference by Ca2+-induced aggregation and large subunit autolysis. Inactive Cys105Ser-m-calpain, bound either to Ni-NTA-agarose or to immobilized casein, after incubation with Ca2+, could be recovered in high yield as a heterodimer. Natural bovine m-calpain, after irreversible inhibition with Z-LLY-CHN2, also bound to immobilized casein and was eluted as a heterodimer. The Ca2+ requirements of calpain containing a small subunit with EF-hand mutations were higher, both before and after autolysis, than those of wild-type calpain. In mixtures of wild-type and mutant enzymes, subunit exchange did not occur in the presence of Ca2+. The results demonstrate that the subunits in both natural and recombinant m-calpain, in the given experimental conditions, remain associated in the presence of Ca2+ both before and after autolysis.

Characterization of monoclonal antibodies to calpain 3 and protein expression in muscle from patients with limb-girdle muscular dystrophy type 2A

Monoclonal antibodies were raised to two regions of calpain 3 (muscle-specific calcium-activated neutral protease), which is the product of the gene that is defective in limb-girdle muscular dystrophy type 2A. The antibodies produced characteristic patterns of bands on Western blots: normal calpain 3 protein was represented by bands at 94 kd, plus additional fragments at approximately 60 or 30 kd, according to the antibody used. Specificity was confirmed by the loss of all bands in patients with null gene mutations. The "normal" profile of bands was observed in muscle from 33 control subjects and 70 disease-control patients. Calpain 3 protein was found to be extremely stable in fresh human muscle, with full-size protein being detected 8 hours after the muscle had been removed. Blots of muscle from nine limb-girdle muscular dystrophy type 2A patients with defined mutations showed variation in protein expression, with seven showing a clear reduction in the abundance of protein detected. No simple relationship was found between the abundance and clinical severity. Two patients showed normal expression of the full-size 94 kd band accompanied by a clear reduction in the smaller fragments. This pattern was also observed in one patient with an undefined form of limb-girdle dystrophy. These results indicate that immunodiagnosis is feasible, but caution will need to be exercised with the interpretation of near-normal protein profiles.

Putative membrane components of signal transduction pathways for ambient pH regulation in Aspergillus and meiosis in saccharomyces are homologous

The zinc finger regions of the Aspergillus nidulans PacC transcription factor, mediating regulation of gene expression by ambient pH, and the Saccharomyces cerevisiae Rim1p transcription factor, mediating control of meiosis and invasiveness, are homologous and both transcription factors undergo proteolytic processing of the C-terminus for conversion to the functional form. In both cases, functioning of a signal transduction pathway involving several gene products is a necessary prerequisite for processing. We now show that the Aspergillus PalI pH signal transduction component is homologous to the Saccharomyces Rim9p meiotic signal transduction component throughout a region containing four hydrophobic, putative membrane-spanning segments. This suggests that PalI might be a membrane sensor for ambient pH. Deletion of the palI gene established that the less extreme phenotype of palI mutations compared with mutations in the other five genes of the pH signalling pathway is a general feature of palI mutations.

Mechanism of action of a new component of the Ca(2+)-dependent proteolytic system in rat brain: the calpain activator

Rat brain contains a calpain activator specific for the mu-form of the proteinase. We now report that this protein factor binds to the catalytic 80 kDa calpain subunit, promoting the dissociation of the heterodimer structure of the proteinase. The successive steps of the activation process, namely the two autoproteolytic steps producing the 78 kDa and the 75 kDa calpain forms, result in a 100 times faster rate. The activator competes with calpastatin and associates with the inner surface of plasma membranes. Based on its properties, the calpain activator can be visualised as the molecule indicating the sites for calpain activation at which the proteinase can also elude the negative control exerted by calpastatin.