A new twist on plasma membrane repair

Cells in multicellular organisms are under constant mechanical stress, and often the plasma membrane (PM) is compromised. Fortunately, there is a vigorous repair mechanism that rapidly (within seconds) reseals the wound site by fusion with an internal membrane patch. Downstream events, remodeling of the injury site and forming replacement PM, must be carried out quickly (within minutes) if a cell is to survive multiple sequential injuries. The repertoire of proteins required to repair breaks (the PM repairome) is one of the major unknowns in this area of research. As an initial approach to defining the PM repairome, a cell surface biotinylation protocol was developed to identify intracellular proteins that become exposed at the site of reversible PM injury. It is likely that at least some of these proteins are important mediators of repair. These initial studies led to a surprising finding, namely the identification of some nuclear and endoplasmic reticulum resident proteins transiently exposed at the surface of cells that ultimately recovered from PM damage. Thus, in reversible mechanical damage to the PM, underlying cellular structures may also be injured, and will also require mechanisms for repair. Other proteins at wound sites were previously identified docking partners for pathogenic bacteria and viruses (vimentin and nucleolin), or found to be upregulated and exposed on the surface of cancer cells (nucleolin and nucleophosmin-1). The new information from these studies may lead to development of novel antimicrobial and antineoplastic drugs.

A plasma membrane wound proteome: reversible externalization of intracellular proteins following reparable mechanical damage

Cells in mechanically active tissues undergo constant plasma membrane damage that must be repaired to allow survival. To identify wound-associated proteins, a cell-impermeant, thiol-reactive biotinylation reagent was used to label and subsequently isolate intracellular proteins that become exposed on the surface of cultured cells after plasma membrane damage induced by scraping from substratum or crushing with glass beads. Scrape-damaged cells survived injury and were capable of forming viable colonies. Proteins that were exposed to the cell surface were degraded or internalized a few seconds to several minutes after damage, except for vimentin, which was detectable on the cell surface for at least an hour after injury. Seven major biotinylated protein bands were identified on SDS-PAGE gels. Mass spectrometric studies identified cytoskeletal proteins (caldesmon-1 and vimentin), endoplasmic reticulum proteins (ERp57, ERp5, and HSP47), and nuclear proteins (lamin C, heterogeneous nuclear ribonucleoprotein F, and nucleophosmin-1) as major proteins exposed after injury. Although caldesmon was a major wound-associated protein in calpain small subunit knock-out fibroblasts, it was rapidly degraded in wild-type cells, probably by calpains. Lamin C exposure after wounding was most likely the consequence of nuclear envelope damage. These studies document major intracellular proteins associated with the cell surface of reversibly damaged somatic cells. The studies also show that externalization of some proteins reported to have physiologic or pathologic roles on the cell surface can occur in cells undergoing plasma membrane damage and subsequent repair.

Foraging in a simulated natural environment: There's a rat loose in the lab

Rats were required to earn their food in a large room having nine boxes placed in it, each of which contained food buried in sand. In different phases of the experiment the amount of time allowed for foraging, the amount of food available in each food patch, and the location of the different available amounts were varied. The rats exhaustively sampled all patches each session but seemed to have fairly strong preferences for certain locations over others. If position preferences were for patches containing small amounts of food, the sensitivity to amount available was increased so that when location was compensated for, a pattern of optimal foraging was evident. The importance of environmental constraints in producing optimal behavior and the relation of the observed behavior to laboratory findings are discussed.

Discrimination learning in a foraging situation

Rats were allowed to forage in a simulated natural environment made up of eight food sources (patches) each containing a fixed number of pellets. Two of the eight contained an extra supply of peanuts. The peanut patches were signaled by an olfactory/visual cue located at the bottom of the ladder leading to the patch. In successive phases the number of sessions per day, height of the patches, and availability of peanuts were manipulated. Subjects showed evidence of discrimination learning under these conditions, although the degree of discriminatory behavior varied as a function of environmental manipulations. Assessment of behavior within foraging sessions showed that subjects systematically changed their patterns of utilization of patches across time. Sampling or exploration, as well as food reinforcement, seem implicated in these results.

Calcium-dependent plasma membrane repair requires m- or mu-calpain, but not calpain-3, the proteasome, or caspases

Mechanically damaged plasma membrane undergoes rapid calcium-dependent resealing that appears to depend, at least in part, on calpain-mediated cortical cytoskeletal remodeling. Cells null for Capns1, the non-catalytic small subunit present in both m- and mu-calpains, do not undergo calcium-mediated resealing. However, it is not known which of these calpains is needed for repair, or whether other major cytosolic proteinases may participate. Utilizing isozyme-selective siRNAs to decrease expression of Capn1 or Capn2, catalytic subunits of mu- and m-calpains, respectively, in a mouse embryonic fibroblast cell line, we now show that substantial loss of both activities is required to compromise calcium-mediated survival after cell scrape-damage. Using skeletal myotubes derived from Capn3-null mice, we were unable to demonstrate loss of sarcolemma resealing after needle scratch or laser damage. Isolated muscle fibers from Capn3 knockout mice also efficiently repaired laser damage. Employing either a cell line expressing a temperature sensitive E1 ubiquitin ligase, or lactacystin, a specific proteasome inhibitor, it was not possible to demonstrate an effect of the proteasome on calcium-mediated survival after injury. Moreover, several cell-permeant caspase inhibitors were incapable of significantly decreasing survival or inhibiting membrane repair. Taken together with previous studies, the results show that m- or mu-calpain can facilitate repair of damaged plasma membrane. While there was no evidence for the involvement of calpain-3, the proteasome or caspases in early events of plasma membrane repair, our studies do not rule out their participation in downstream events that may link plasma membrane repair to adaptive remodeling after injury.

Fetuin A stabilizes m-calpain and facilitates plasma membrane repair

Yeast two-hybrid experiments identified alpha(2)-Heremans-Schmid glycoprotein (human fetuin A) as a binding partner for calpain domain III (DIII). The tandem DIIIs of calpain-10 interacted under the most selective culture conditions, but DIIIs of m-calpain, calpain-3, and calpain-5 also interacted under less stringent selection. DIIIs of mu-calpain, calpain-6, and the tandem DIII-like domains of the Dictyostelium Cpl protein did not interact with alpha(2)-Heremans-Schmid glycoprotein in the yeast two-hybrid system. Bovine fetuin A stabilized proteolytic activity of purified m-calpain incubated in the presence of mm calcium chloride and prevented calcium-dependent m-calpain aggregation. Consistent with the yeast two-hybrid studies, fetuin A neither stabilized mu-calpain nor prevented its aggregation. Confocal immunofluorescence microscopy of scratch-damaged L6 myotubes demonstrated accumulation of m-calpain at the wound site in association with the membrane repair protein, dysferlin. m-Calpain also co-localized with fluorescein-labeled fetuin A at the wound site. The effect of fetuin A on calpain-mediated plasma membrane resealing was investigated using fibroblasts from Capns1(-/-) and Capns1(+/+) mouse embryos. Capns1 encodes the small noncatalytic subunit that is required for the proteolytic function of m- and mu-calpains. Thus, Capns1(-/-) fibroblasts do not express these calpains in active form. Fetuin A increased resealing of scrape-damaged wild-type fibroblasts but not Capns1(-/-) fibroblasts. These studies identify fetuin A as a potential extracellular regulator of m-calpain at nascent sites of plasma membrane wounding.

Detergent-resistant membrane subfractions containing proteins of plasma membrane, mitochondrial, and internal membrane origins

HEK293 cell detergent-resistant membranes (DRMs) isolated by the standard homogenization protocol employing a Teflon pestle homogenizer yielded a prominent opaque band at approximately 16% sucrose upon density gradient ultracentrifugation. In contrast, cell disruption using a ground glass tissue homogenizer generated three distinct DRM populations migrating at approximately 10%, 14%, and 20% sucrose, named DRM subfractions A, B, and C, respectively. Separation of the DRM subfractions by mechanical disruption suggested that they are physically associated within the cellular environment, but can be dissociated by shear forces generated during vigorous homogenization. All three DRM subfractions possessed cholesterol and ganglioside GM1, but differed in protein composition. Subfraction A was enriched in flotillin-1 and contained little caveolin-1. In contrast, subfractions B and C were enriched in caveolin-1. Subfraction C contained several mitochondrial membrane proteins, including mitofilin and porins. Only subfraction B appeared to contain significant amounts of plasma membrane-associated proteins, as revealed by cell surface labeling studies. A similar distribution of DRM subfractions, as assessed by separation of flotillin-1 and caveolin-1 immunoreactivities, was observed in CHO cells, in 3T3-L1 adipocytes, and in HEK293 cells lysed in detergent-free carbonate. Teflon pestle homogenization of HEK293 cells in the presence of the actin-disrupting agent latrunculin B generated DRM subfractions A-C. The microtubule-disrupting agent vinblastine did not facilitate DRM subfraction separation, and DRMs prepared from fibroblasts of vimentin-null mice were present as a single major band on sucrose gradients, unless pre-treated with latrunculin B. These results suggest that the DRM subfractions are interconnected by the actin cytoskeleton, and not by microtubes or vimentin intermediate filaments. The subfractions described may prove useful in studying discrete protein populations associated with detergent-resistant membranes, and their potential interactions in cell signaling.

Calpain is required for the rapid, calcium-dependent repair of wounded plasma membrane

Mammalian cells require extracellular calcium ion to undergo rapid plasma membrane repair seconds after mechanical damage. Utilizing transformed fibroblasts from calpain small subunit knock-out (Capns1-/-) mouse embryos, we now show that the heterodimeric, typical subclass of calpains is required for calcium-mediated survival after plasma membrane damage caused by scraping a cell monolayer. Survival of scrape-damaged Capns1-/- cells was unaffected by calcium in the scraping medium, whereas more Capns1+/+ cells survived when calcium was present. Calcium-mediated survival was increased when Capns1-/- cells were scraped in the presence of purified m- or mu-calpain. Survival rates of scraped Capns1+/+, HFL-1, or Chinese hamster ovary cells were decreased by the calpain inhibitor, calpeptin, or the highly specific calpain inhibitor protein, calpastatin. Capns1-/- cells failed to reseal following laser-induced membrane disruption, demonstrating that their decreased survival after scraping resulted, at least in part, from failed membrane repair. Proteomic and immunologic analyses demonstrated that the known calpain substrates talin and vimentin were exposed at the cell surface and processed by calpain following cell scraping. Autoproteolytic activation of calpain at the scrape site was evident at the earliest time point analyzed and appeared to precede proteolysis of talin and vimentin. The results indicate that conventional calpains are required for calcium-facilitated survival after plasma membrane damage and may act by localized remodeling of the cortical cytoskeleton at the injury site.

Purification and properties of the Dictyostelium calpain-like protein, Cpl

Calpains are intracellular, cysteine proteases found in plants, animals, and fungi. There is emerging evidence that they are important mediators of cell adhesion and motility in animal cells. Because the cellular slime mold, Dictyostelium discoideum, is a genetically tractable model for cell adhesion and motility, we have investigated whether a calpain-like protein is expressed in this organism. Contig 13130 (Sanger Institute Dictyostelium sequencing project) was identified as a three-exon gene that encodes a calpain-like protein. Using a custom peptide antibody to assay for the presence of this putative protein, we identified Dictyostelium calpain-like protein (Cpl) and purified it to near homogeneity. Cpl is a 72278 Da cytosolic protein. Weak caseinolytic activity inhibitable by cysteine protease inhibitors was copurified with Cpl immunoreactivity, and purified Cpl appeared to undergo autoproteolysis upon transfer to inhibitor-free buffer. The major cleavage, generating a 51291 Da form, occurred after Pro 189. The Cpl domain structure resembles mammalian calpain 10, comprising an N-terminal catalytic domain followed by tandem calpain D-III domains. The putative catalytic domain appears to possess His and Gln active site residues, instead of the canonical His and Asn residues in calpains. The active site Cys has not yet been identified, and definitive proof of a proteolytic function awaits further study. Its phylogenetic distribution in D. discoideum and several protists suggests that the calpain D-III domain evolved early in eukaryotic cells.

Overexpression of a calpastatin transgene in mdx muscle reduces dystrophic pathology

Reduced sarcolemmal integrity in dystrophin-deficient muscles of mdx mice and Duchenne muscular dystrophy (DMD) patients has been reported to result in altered calcium homeostasis. Previous studies have shown a correlative relationship between calcium-dependent protease (calpain) activity in dystrophic muscle and muscle necrosis, but have not tested whether calpain activation precedes cell death or is a consequence of it. To test a causal relationship between calpain activation and muscle cell death in dystrophin deficiency, mdx mice were generated that overexpress a calpastatin transgene in muscle. Calpastatin (CS) is a specific, endogenous inhibitor of m- and micro -calpains that does not inhibit calpain 3 (p94). CS overexpression on a C57/BL 10 background produced no phenotype. Transgenic (Tg) mice crossed with mdx mice were tested for pathological indicators of necrosis, regeneration and membrane damage. Two lines of mice were examined, with different levels of CS overexpression. Both lines of Tg/mdx mice showed reductions in muscle necrosis at 4 weeks of age. These mice had fewer as well as smaller lesions. In addition, one line of mice had significantly less regeneration, indicating a reduction in previous necrosis. The extent of improvement correlated with the level of CS protein expression. Membrane damage, as assessed by procion orange and creatine kinase assays, was unchanged, supporting the idea that calpains act downstream of the primary muscle defect. These data suggest that calpains play an active role in necrotic processes in dystrophic muscle and that inhibition of calpains might provide a good therapeutic option for treatment of DMD.

Calpain inhibition decreases the growth rate of mammalian cell colonies

The calpains, a family of calcium-requiring intracellular proteases, are proposed regulators of cell proliferation. However, ablation of the calpain small subunit gene necessary for function of the conventional calpains did not result in decreased rate of proliferative growth of mouse stem cells under routine culture conditions. To address the reasons for this discrepancy, Chinese hamster ovary cell lines were established that overexpress the calpain inhibitor protein, calpastatin, under control of the ecdysone congener, ponasterone A. Overexpression of calpastatin in these cell lines resulted in a decreased growth of isolated colonies adhering to tissue culture plates. However, when cells were plated at higher density, calpastatin overexpression had no influence on proliferative growth rate. Growth of colonies in soft agar was not inhibited by calpastatin overexpression. Cell adhesion, cell de-adhesion, and cell motility all appeared to be normal after calpastatin overexpression. Differential display analysis was initiated to detect possible alteration of gene expression upon calpastatin overexpression. Analysis of approximately 3000 differential display PCR signals resulted in identification of one band that was underexpressed. Northern blot analysis confirmed a decreased amount of approximately 1 kb mRNA in cells overexpressing calpastatin. Sequence analysis identified a putative protein, Csr, containing a region homologous to two ubiquitin transferases and a putative cation channel protein.

Participation of the conventional calpains in apoptosis

The conventional calpains, m- and micro-calpain, are suggested to be involved in apoptosis triggered by many different mechanisms. However, it has not been possible to definitively associate calpain function with apoptosis, largely because of the incomplete selectivity of the cell permeable calpain inhibitors used in previous studies. In the present study, Chinese hamster ovary (CHO) cell lines overexpressing micro-calpain or the highly specific calpain inhibitor protein, calpastatin, have been utilized to explore apoptosis signals that are influenced by calpain content. This approach allows unambiguous alteration of calpain activity in cells. Serum depletion, treatment with the endoplasmic reticulum (ER) calcium ATPase inhibitor thapsigargin, and treatment with calcium ionophore A23187 produced apoptosis in CHO cells, which was increased in calpain overexpressing cells and decreased by induced expression of calpastatin. Inhibition of calpain activity protected beta-spectrin, but not alpha-spectrin, from proteolysis. The calpains seemed not to be involved in apoptosis triggered by a number of other treatments. Calpain protected against TNF-alpha induced apoptosis. In contrast to previous studies, we found no evidence that calpains proteolyze I kappa B-alpha in TNF-alpha-stimulated cells. These studies indicate that the conventional calpains participate in some, but not all, apoptotic signaling mechanisms. In most cases, they contributed to apoptosis, but in at least one case, they were protective.

Regulation of cell migration by the calcium-dependent protease calpain

Integrin receptors play an important role during cell migration by mediating linkages and transmitting forces between the extracellular matrix and the actin cytoskeleton. The mechanisms by which these linkages are regulated and released during migration are not well understood. We show here that cell-permeable inhibitors of the calcium-dependent protease calpain inhibit both beta1 and beta3 integrin-mediated cell migration. Calpain inhibition specifically stabilizes peripheral focal adhesions, increases adhesiveness, and decreases the rate of cell detachment. Furthermore, these inhibitors alter the fate of integrin receptors at the rear of the cell during migration. A Chinese hamster ovary cell line expressing low levels of calpain I also shows reduced migration rates with similar morphological changes, further implicating calpain in this process. Taken together, the data suggest that calpain inhibition modulates cell migration by stabilizing cytoskeletal linkages and decreasing the rate of retraction of the cell's rear. Inhibiting calpain-mediated proteolysis may therefore be a potential therapeutic approach to control pathological cell migration such as tumor metastasis.

Specificities of cell permeant peptidyl inhibitors for the proteinase activities of mu-calpain and the 20 S proteasome

Cell-permeant peptidyl aldehydes and diazomethylketones are frequently utilized as inhibitors of regulatory intracellular proteases. In the present study the specificities of several peptidyl inhibitors for purified human mu-calpain and 20 S proteasome were investigated. Acetyl-LLnL aldehyde, acetyl-LLM aldehyde, carbobenzyloxy-LLnV aldehyde (ZLLnVal), and carbobenzyloxy-LLY-diazomethyl ketone produced half-maximum inhibition of the caseinolytic activity of mu-calpain at concentrations of 1-5 x 10(-7) M. In contrast, only ZLLnVal was a reasonably potent inhibitor of the caseinolytic activity of 20 S proteasome, producing 50% inhibition at 10(-5) M. The other inhibitors were at least 10-fold less potent, producing substantial inhibition only at near saturating concentrations in the assay buffer. Further studies with ZLLnVal demonstrated that its inhibition of the proteasome was independent of casein concentration over a 25-fold range. Proteolysis of calpastatin or lysozyme by the proteasome was half-maximally inhibited by 4 and 22 microM ZLLnVal, respectively. Thus, while other studies have shown that ZLLnVal is a potent inhibitor of the hydrophobic peptidase activity of the proteasome, it appears to be a much weaker inhibitor of its proteinase activity. The ability of the cell permeant peptidyl inhibitors to inhibit growth of the yeast Saccharomyces cerevisiae was studied because this organism expresses proteasome but not calpains. Concentrations of ZLLnVal as high as 200 microM had no detectable effect on growth rates of overnight cultures. However, yeast cell lysates prepared from these cultures contained 2 microM ZLLnVal, an amount which should have been sufficient to fully inhibit hydrophobic peptidase activity of yeast proteasome. Degradation of ubiquitinylated proteins in yeast extracts by endogenous proteasome was likewise sensitive only to high concentrations of ZLLnVal. The higher sensitivity of the proteinase activity of calpains to inhibition by the cell permeant inhibitors suggests that calpain-like activities may be targets of these inhibitors in animal cells.

Cardiac high molecular weight calmodulin binding protein contains calpastatin activity

A high molecular weight calmodulin binding protein (HMWCaMBP) was previously identified and purified from bovine heart cytosolic fraction [Sharma, R.K. (1990) J. Biol. Chem. 265, 1152-1157]. In this study, we report the biological function of this protein. HMWCaMBP was subjected to peptide mapping and three peptides were sequenced. Two of the three peptide sequences were shown to be highly homologous to the calpain inhibitor, calpastatin. However, the third peptide did not show homology to any known proteins. The Western blot analysis of HMWCaMBP and purified calpastatin from bovine cardiac muscle showed immunoreactivity with polyclonal antibody raised against HMWCaMBP. Furthermore, HMWCaMBP inhibited calpain II and calpain I activities in a dose dependent fashion. Our data based on sequence homology, amino acid analysis, antibody reactivity and calpain inhibition suggests that HMWCaMBP is homologous to calpastatin and may be a CaM-binding form of calpastatin.

Evidence for participation of a calpain-like cysteine protease in cell cycle progression through late G1 phase

Recent studies have demonstrated that cell-permeant protease inhibitors arrest human fibroblasts in late G1. The target for the inhibitors has been claimed to be either the proteasome, or a calpain-like cysteine protease activity. In the present investigation, the progression of serum-stimulated WI-38 fibroblasts into S-phase was partially inhibited by the cell-permeant general inhibitor of cysteine proteases, E64d, but not by its non-permeant anolog, E64c. Exposure of fibroblasts in late G1 to the proteasome inhibitor, lactacystin, produced only a modest inhibition of progression into S-phase, and did not influence the extensive inhibition produced by the calpain-selective inhibitor, ZLLY-DMK. ZLLnV-CHO and ZLLL-CHO, which are reportedly selective for the proteasome, were less potent than ZLLY-DMK as inhibitors of S-phase progression. These results argue for the involvement of a calpain-like protease acting in late G1 to allow transit into S-phase.

Inhibition of the growth of WI-38 fibroblasts by benzyloxycarbonyl-Leu-Leu-Tyr diazomethyl ketone: evidence that cleavage of p53 by a calpain-like protease is necessary for G1 to S-phase transition

The effect of a calpain-selective cell permeant inhibitor, benzyloxycarbonyl Leu-Leu-Tyr diazomethylketone (ZLLY-CHN2), on the serum-stimulated growth of WI-38 human fibroblasts has been investigated. Only cell permeant protease inhibitors with activity against calpains prevented progression into S-phase. Protein blotting experiments indicated that p53 immunoreactivity increased in late G1 cells treated with ZLLY-CHN2. The content of p21Waf1/Cip1 CDK inhibitor also increased, providing a mechanism for the observed failure to enter S-phase. Further studies indicated that p53 could be degraded by a ZLLY-CHN2-sensitive protease immediately prior to S-phase, but that proteolysis did not occur after this critical time point. Chelation of extracellular Ca2+ by addition of EGTA inhibited the p53 degradation. Consistent with proteolysis of p53 in late G1 phase, mu-calpain immunoreactivity transiently accumulated in cell nuclei at this time. ZLLY-CHN2 did not appear to increase p53 mRNA in WI-38 cells. Purified mu-calpain required only 1 to 3 microM Ca2+ to proteolyze p53 in WI-38 cell lysates. These results indicate that ZLLY-CHN2 inhibits progression of WI-38 cells into S-phase by inactivating a calpain-like protease that is responsible for proteolysis of constitutively expressed p53 in late G1.

Calpain subunits remain associated during catalysis

The Ca2+ dependent cysteine proteases, calpains are heterodimers containing a large (ca. 80 kDa) catalytic subunit and a 25-30 kDa small subunit. Whether calpains remain dimers while catalyzing hydrolysis of protein substrates has been controversial. Now by doing subunit co-immunoprecipitation, we provide direct evidence to resolve this argument. In the presence of Ca2+ concentrations which permit catalytic activity, both subunits of either m- or mu-calpain are co-immunoprecipitated by monoclonal antibodies directed against a single subunit. Furthermore, both subunits can be co-immunoprecipitated during calpain-catalyzed proteolysis of the substrate casein. These results indicate that both major calpain isozymes maintain their heterodimeric form during the catalytic cycle. Thus, small subunit might have a direct role in regulating the physiologic function of either major calpain isozyme.

Calpain inhibitors and serine protease inhibitors can produce apoptosis in HL-60 cells

Recent investigations indicate that proteolysis is an important event in generation of the apoptosis phenotype. Although various proteases have been suggested to be candidates for this proteolysis, the results from different laboratories are inconsistent. In the present studies, HL-60 cells were treated with cycloheximide to investigate proteases involved in apoptosis. The calpain inhibitors benzyloxycarbonyl-Leu-Leu-Tyr diazomethylketone and acetyl-Leu-Leu-Nle aldehyde were not capable of preventing apoptosis induced by cycloheximide. In the absence of cycloheximide, these two inhibitors could initiate apoptosis in HL-60 cells. The thiol protease inhibitor benzyloxycarbonyl-Leu-Val-Gly diazomethylketone neither prevented nor produced apoptosis. The serine protease inhibitors 3,4-dichloroisocoumarin (DCI) and tosyl-Phe chloromethylketone (TPCK) also induced apoptosis in the absence of cycloheximide. On the other hand, the latter two inhibitors decreased cycloheximide-induced apoptosis, assessed either by cell morphologic changes or DNA ladder generation. Benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone and iodoacetamide, inactivators of interleukin 1beta-converting enzyme (ICE)-like proteases, did not produce apoptosis and inhibited the induction of apoptosis by cycloheximide, calpain inhibitors, or serine protease inhibitors. These results are consistent with the ICE-like proteases having a central role in proteolysis during apoptosis, while calpain-like proteases and the serine proteases sensitive to DCI or TPCK are not required for generation of the apoptosis phenotype in HL-60 cells.

The major calpain isozymes are long-lived proteins. Design of an antisense strategy for calpain depletion in cultured cells

Calpains are intracellular Ca2+-dependent proteases that are thought to participate in Ca2+-associated signal transduction pathways. It has been proposed that calpains are activated by an autoproteolytic mechanism. If this is true one would expect a relatively short half-life for calpain protein in cells. To test this hypothesis, WI-38 human diploid fibroblasts were pulse-labeled with [35S]methionine, and calpain was immunoprecipitated at various times after chasing with nonradioactive methionine to determine residual radioactivity. The results demonstrated that the two major calpain isozymes, m-calpain and micro-calpain, had metabolic half-lives of approximately 5 days. Calpains were long-lived proteins in several human cell lines, A-431, HeLa, VA-13, C-33A, and TE2 cells. In addition, calpastatin, the calpain-specific inhibitor protein, also had a long metabolic half-life. These observations suggest that the model for calpain activation by autoproteolysis requires re-investigation. Based on a knowledge of calpain metabolic stability, a protocol was devised for chronic exposure of WI-38 cells and HeLa cells to a calpain small subunit antisense oligodeoxyribonucleotide. Depletion of calpain small subunit after 5 or more days of treatment led to inhibition of cell proliferation that could be reversed by removal of antisense oligodeoxyribonucleotide from the culture medium. Together with previous studies, these results indicate a requirement for calpains in mammalian cell proliferation.

Isolation of a Chinese hamster ovary cell clone possessing decreased mu-calpain content and a reduced proliferative growth rate

A Chinese hamster ovary cell line (CHOp) was cultured in the presence of benzyloxycarbonyl-Leu-Leu-Tyr diazomethyl ketone (ZLLY-CHN2), to select for resistance to this cell-permeant calpain inhibitor. A clone isolated after several courses of exposure (SHI cells) demonstrated decreased sensitivity to ZLLY-CHN2 toxicity and a decreased growth rate. SHI cells also possessed less mu-calpain isozyme relative to CHOp cells, as determined by activity measurement or by protein immunoblotting. Activities of m-calpain, calpastatin, cathepsin B, cathepsin L, and glycogen phosphorylase were not altered. SHI mu-calpain was partially purified by sequential chromatography on Bio-Gel A-1.5m and DEAE-Sepharose. Its chromatographic behavior in either system was the same as for CHOp mu-calpain. Further studies with the partially purified SHI and CHOp mu-calpain fractions failed to distinguish any difference in Ca2+ requirement or in sensitivity to inhibition by calpastatin or ZLLY-CHN2 for these enzymes. These experiments suggest that SHI cells underproduce a form of mu-calpain which is very similar to, if not identical with, CHOp mu-calpain. SHI cells displayed a population doubling time of 29 h compared with 19 h for CHOp cells. The decreased growth rate of SHI cells was the result of a prolonged G1 phase. Introduction of purified human mu-calpain into SHI cells by electroporation transiently restored the growth rate and also increased toxicity associated with exposure to ZLLY-CHN2. SHI cells should be a valuable model in further studies to delineate the function of mu-calpain in cell proliferative growth.

Inhibition of growth of human TE2 and C-33A cells by the cell-permeant calpain inhibitor benzyloxycarbonyl-Leu-Leu-Tyr diazomethyl ketone

Calpains are Ca(2+)-requiring, nonlysosomal proteases which are thought to participate in some aspects of intracellular Ca(2+)-signal transduction. However, their exact physiologic function has not yet been established. Addition of the cell-permeant, irreversible calpain inhibitor, ZLLY-CHN2, to human TE2 or C-33A cells inhibited growth, as assessed either by mitochondrial MTT reductase activity or by direct cell counting. Inhibition of growth produced by a 24-h exposure to 50 microM ZLLY-CHN2 was reversed upon substituting growth medium without inhibitor. Homogenates produced from cells cultured in the presence of ZLLY-CHN2 displayed decreased calpain and Ca(2+)-independent proteolytic activities. Protein immunoblot analysis showed that cell cultures which had lost 80% of their calpain activity still retained full calpain immunoreactivity. Therefore, inhibition by ZLLY-CHN2 appeared to result in accumulation of irreversibly inactivated calpain within the cells. Homogenates from cells cultured in the presence of 20 or 50 microM ZLVG-CHN2, a cell-permeant inhibitor with little activity against calpains, had decreased Ca(2+)-independent proteolytic activity, but demonstrated no decrease in calpain activity. ZLVG-CHN2 did not inhibit cell growth under these conditions. Growth of Saccharomyces cerevisiae cells, which do not appear to express calpain-like proteases, was not inhibited by including 50 microM ZLLY-CHN2 in the culture medium. These results indicate that calpains participate in the social regulation of cell growth in multicellular organisms.

Selective nuclear transport of mu-calpain

To study nuclear transport of purified calpains in an in vitro system, A431 cells were permeabilized with digitonin, and fluorescein-labeled calpains were introduced under conditions known to facilitate energy-dependent nuclear transport of proteins. Fluorescein-mu-calpain was transported into nuclei in an ATP-dependent fashion. The calpain-specific inhibitor protein, calpastatin, could not block mu-calpain translocation. Fluorescein-calpastatin and fluorescein-m-calpain were poorly transported at best. In the presence of rat liver cytosolic factors, accumulation of nuclear mu-calpain was maximum at approximately 1 microM Ca2+, and no transport was observed at 0.3 microM Ca2+. Rat erythrocyte and HeLa cell extracts supported transport in the absence of Ca2+.

Different sensitivities of native and oxidized forms of Na+/K(+)-ATPase to intracellular proteinases

Inactivation of Na+/K(+)-ATPase by partially reduced oxygen metabolites has been implicated in ischemia-reperfusion injury to heart and other organs. Because oxidation of many proteins makes them more susceptible to degradation by intracellular proteinases, we studied the effects of several such proteinases on native and H2O2-oxidized preparations of Na+/K(+)-ATPase from canine kidney (containing alpha 1 isoform of the catalytic subunit) and rat axolemma (containing alpha 2 and alpha 3 isoforms). Lysosomal cathepsin D degraded the native and the oxidized preparations at acid pH, but it was significantly more effective against the oxidized forms. m-Calpain had little or no effect on the native Na+/K(+)-ATPase preparations, but it digested the oxidized alpha-subunits of the axolemma and the kidney enzymes. mu-Calpain's effects were similar to those of m-calpain. Multi-catalytic proteinase which is known to degrade a large number of oxidized proteins, did not affect the native or the oxidized forms of Na+/K(+)-ATPase. The findings suggest that (a) during oxidative stress there may be accelerated degradation of the oxidatively damaged Na+/K(+)-ATPase, either through internalization and transport to lysosomes, or by the action of calpains at the membrane; and (b) those isoforms of the enzyme that are more sensitive to oxidants are more susceptible to degradation by the above processes.

Inhibition of nonlysosomal calcium-dependent proteolysis by glycine during anoxic injury of rat hepatocytes

BACKGROUND/AIMS

The mechanism by which glycine protects against hepatocyte death during anoxia remains unclear. Nonlysosomal proteolysis, including calpain proteolysis, has been implicated as a mechanism of lethal cell injury. However, the effect of glycine on nonlysosomal proteolysis is unknown. The aim of this study was to ascertain if glycine cytoprotection is associated with inhibition of nonlysosomal proteolysis.

METHODS

Rat hepatocyte suspensions were rendered anoxic using an anaerobic chamber. Cell viability was measured by propidium iodide fluorometry. Nonlysosomal protease activity was quantitated by the release of trichloroacetic acid-soluble free amines or tyrosine. Calpain protease activity was measured using a fluorogenic substrate.

RESULTS

Glycine and alanine (but not valine) markedly improved cell viability during anoxia in a concentration-dependent manner. During anoxia, the majority of nonlysosomal proteolysis (60%) was dependent on extracellular Ca2+. Glycine only inhibited that portion of nonlysosomal proteolysis that was dependent on extracellular Ca2+. Amino acids inhibited the anoxia-stimulated increase in calpain protease activity with the same specificity and concentration-dependence observed for cytoprotection. Glycine was more potent in directly inhibiting purified m-calpain as compared with mu-calpain protease activity.

CONCLUSIONS

Glycine may exert its cytoprotective activity during lethal anoxic hepatocyte injury, in part by inhibiting Ca(2+)-dependent degradative, nonlysosomal proteases, including calpains.

Distinction between places and paths in rats' spatial representations

Rats were trained on a 3-dimensional, 4-arm radial maze. In Experiment 1, Ss trained to climb to the single goal platform chose fewer novel routes to the goal than Ss trained to climb to the 4 spatially distinct platforms. In Experiment 2 a reinforcement contingency was imposed, requiring a novel route choice on each trial to receive reinforcement. Learning to associate route choice with reinforcement outcome was much more difficult for Ss tested with the single goal than for Ss tested with the 4 distinct goals. In Experiment 3 a partitioned central platform group learned the reinforcement contingency as quickly as the Ss given 4 spatially distinct platforms. In Experiment 4, distinctive floor inserts did not affect performance relative to no inserts.

m-Calpain requires DNA for activity on nuclear proteins at low calcium concentrations

m-Calpain (calpain II, m-CANP), which normally requires millimolar Ca2+ for activity in vitro, was capable of proteolyzing a number of matrix proteins in isolated rat liver nuclei at Ca2+ concentrations as low as 3 microM (Mellgren, R. L. (1991) J. Biol. Chem. 266, 13920-13924). Treatment of nuclei with deoxyribonuclease I eliminated the activity of m-calpain at low Ca2+ concentrations, while ribonuclease A and phospholipase C had no effect. Addition of DNA to DNase-treated nuclei restored m-calpain activity at low Ca2+. RNA had little if any effect. Eukaryotic and prokaryotic DNA were equally effective, and synthetic polydeoxyribonucleotides were also activators. m-Calpain did not bind to a DNA-cellulose column in the presence of 200 microM Ca2+, and m-calpain preincubated in the presence of DNA and 200 microM Ca2+ was not activated at low Ca2+ concentrations following removal of the DNA. DNA did not alter the Ca2+ requirement for m-calpain-catalyzed cleavage of casein. These results demonstrate that the Ca2+ requirement for proteolysis of nuclear matrix proteins by m-calpain can be dramatically decreased in the presence of DNA. Activation did not seem to be a result of DNA binding directly to calpain but appeared to require interaction of DNA, calpain, and calpain substrates in the nuclear matrix.

A comparison of the intracellular distribution of mu-calpain, m-calpain, and calpastatin in proliferating human A431 cells

Little is known about the relative intracellular localizations of the calcium-dependent proteases, calpains, and their naturally occurring inhibitor, calpastatin. In the present study, the intracellular localization of mu-calpain, m-calpain, and calpastatin was studied at the light microscopic level in proliferating A431 cells. Highly specific antibodies against the three antigens revealed distinct staining patterns in interphase and mitotic cells. Most notably, calpastatin in interphase cells was localized near the nucleus in tube-like, or large granular structures, while the calpains were more uniformly distributed through the cytoplasm in either a fibrillar form (mu-calpain) or a diffuse or fine granular form (m-calpain). The distribution patterns of the two calpain isozymes were distinctly different during mitosis. m-Calpain was concentrated at the mitotic spindle poles and midbody, while mu-calpain appeared to accumulate at the cell membrane and the spindles. Four other human cell lines as well as normal human monocytes were examined to determine if the calpains-calpastatin segregation patterns are common to other cells or are unique to the A431 line. With the exception of abundant nuclear mu-calpain in the C-33A cervical carcinoma, the segregation of the proteins was similar to that of A431. These studies indicate that calpains may be localized at regions which are relatively poor in calpastatin content. Proteins at these sites may be susceptible to calpain-catalyzed cleavage.

Mapping of the calpain proteolysis products of the junctional foot protein of the skeletal muscle triad junction

The Ca2+ activated neutral protease calpain II in a concentration-dependent manner sequentially degrades the junctional foot protein (JFP) of rabbit skeletal muscle triad junctions in either the triad membrane or as the pure protein. This progression is inhibited by calmodulin. Calpain initially cleaves the 565 kDa JFP monomer into peptides of 160 and 410 kDa, which is subsequently cleaved to 70 and 340 kDa. The 340 kDa peptide is finally cleaved to 140 and 200 kDa or its further products. When the JFP was labeled in the triad membrane with the hydrophobic probe 3-(trifuoromethyl) 3-(m)[125I]iodophenyl) diazirine and then isolated and proteolysed with calpain II, the [125I] was traced from the 565 kDa parent to Mr 410 kDa and then to 340 kDa, implying that these large fragments contain the majority of the transmembrane segments. A 70-kDa fragment was also labeled with the hydrophobic probe, although weakly suggesting an additional transmembrane segment in the middle of the molecule. These transmembrane segments have been predicted to be in the C-terminal region of the JFP. Using an ALOM program, we also predict that transmembrane segments may exist in the 70 kDa fragment. The JFP has eight PEDST sequences; this finding together with the calmodulin inhibition of calpain imply that the JFP is a PEDST-type calpain substrate. Calpain usually cleaves such substrates at or near calmodulin binding sites. Assuming such sites for proteolysis, we propose that the fragments of the JFP correspond to the monomer sequence in the following order from the N-terminus: 160, 70, 140 and 200 kDa. For this model, new calmodulin sequences are predicted to exist near 160 and 225 kDa from the N-terminus. When the intact JFP was labeled with azidoATP, label appeared in the 160 and 140 kDa fragments, which according to the above model contain the GXGXXG sequences postulated as ATP binding sites. This transmembrane segment was predicted by the ALOM program. In addition, calpain and calpastatin activities remained associated with triad component organelles throughout their isolation. These findings and the existence of PEDST sequences suggest that the JFP is normally degraded by calpain in vivo and that degradation is regulated by calpastatin and calmodulin.

Proteolysis of nuclear proteins by mu-calpain and m-calpain

Purified calpains are capable of proteolyzing several high Mr nuclear proteins and solubilizing a histone H1 kinase activity from rat liver nuclei upon exposure to 10(-6) - 10(-5) M Ca2+. Major nuclear substrates displayed apparent molecular masses of 200, 130, 120, and 60 kDa on Coomassie Blue-stained SDS-PAGE gels. The nuclear proteins and the H1 kinase were released from Triton-treated nuclei following incubation with buffer containing 0.5 M NaCl. They therefore appeared to be internal nuclear matrix proteins. The nuclear H1 kinase activity solubilized by incubation with m-calpain was eluted in the void volume of a Bio-Gel A-1.5m column, indicating an apparent mass greater than 1,500 kDa. Treatment of the calpain-solubilized kinase with 0.5 M NaCl dissociated it to a form having an apparent mass of 300 kDa (Stokes radius = 5.6 nm), suggesting that the 300-kDa (Stokes radius = 5.6 nm), nuclei by calpain treatment as a large complex containing other internal matrix proteins. Purified human erythrocyte mu-calpain was capable of proteolyzing the nuclear matrix proteins at 10(-6) M Ca2+. In contrast, human erythrocyte multicatalytic protease complex produced little cleavage of the nuclear proteins. Proteolysis of nuclear proteins by either mu-calpain or m-calpain was inhibited by calpastatin. These experiments suggest a physiologic role for the calpains in the turnover of nuclear proteins.

Interaction of human erythrocyte multicatalytic proteinase with polycations

The multicatalytic proteinase from human erythrocytes (macropain, proteasome) is a large enzyme composed of at least six distinct subunits ranging in molecular masses from 20 to 30 kDa. As its name implies, this proteinase appears to contain multiple catalytic sites with differing specificities toward peptide substrates. Several polycationic substances, including polylysines, polyarginine, protamine and histone H1 markedly stimulated caseinolytic activity of the proteinase. Activation was instantaneous, and involved increasing the Vmax of the proteinase for casein. Prolonged preincubation with polylysine at 37 degrees C resulted in autolytic inactivation of the proteinase. The polylysine concentrations required for half-maximal activation or autolytic inactivation were the same. A 23 kDa subunit of the proteinase disappeared at the same rate as loss of catalytic activity, and with the same pH dependence and polylysine concentration dependence. These results suggest that polylysine perturbs the structure of the multicatalytic proteinase, resulting in increased catalytic activity toward substrates; and, with prolonged exposure, allowing autoproteolytic inactivation to occur. The 23 kDa subunit appeared to be required for expression of caseinolytic activity, and may therefore be a catalytic subunit of the complex having activity against casein.

Calpain II-dependent solubilization of a nuclear protein kinase at micromolar calcium concentrations

Incubation of isolated, Triton X-100 washed rat liver nuclei with purified bovine myocardial calpain II resulted in solubilization of a histone H1 kinase activity. The release of kinase from nuclei could be prevented by including the calpain inhibitors leupeptin or calpastatin in the incubation. Titration with Ca2+/EGTA buffers indicated that the calpain-dependent release of the kinase was half-maximal at approximately 3 microM Ca2+. In contrast, calpain II required at least 50 microM Ca2+ to produce detectable proteolysis of soluble or membrane-associated substrates. These results suggest that the cell nucleus is a site of calpain II activation and function.

The grid-blot: a procedure for screening large numbers of monoclonal antibodies for specificity to native and denatured proteins

This report describes a procedure referred to as a grid-blot for simultaneously testing up to 30 monoclonal antibodies for specificity with an equivalent number of different proteins on a single sheet of nitrocellulose paper. Only 150 microliters of hybridoma culture supernatant is required for the screening and the entire procedure can be completed in less than five hours. This assay was developed to quickly identify those hybridoma cultures producing antibodies that preferentially recognize the native form of a protein and those that also recognize the SDS denatured form and were optimal for use in Western blots. Monoclonal antibodies raised against two distinct proteins, myofibril C-protein (120 antibodies) and the catalytic subunit of cyclic-AMP dependent protein kinase (240 antibodies) were tested. The grid-blot results indicated that 85 of the C-protein antibodies and 55 of the catalytic subunit antibodies were monospecific. Only 4 of the C-protein and 9 catalytic subunit antibodies showed a preferential staining for the appropriate native protein. The antibodies that stained the denatured protein most intensely in the grid-blot corresponded with those that produced the best immunostain in the Western blot. Finally, a version of the grid-blot was found to be an efficient means of determining antibody isotypes.

The binding of large calpastatin to biologic membranes is mediated in part by interaction of an amino terminal region with acidic phospholipids

Animal cells contain a non-lysosomal proteolytic system which degrades various protein substrates in the presence of calcium ion. The calcium-dependent proteinases (calpains) co-exist in cells with a specific protein inhibitor called calpastatin. Distribution of this inhibitor to different subcellular sites could be important in overall regulation of the calpains. Previously, myocardial calpastatin was shown to be present in preparations of sarcoplasmic reticulum and sarcolemma. In the present work, we show that purified bovine myocardial calpastatin binds to the acidic phospholipids, phosphatidylinositol and phosphatidylserine, but not to the neutral phospholipids, phosphatidylcholine and phosphatidylethanolamine. Large forms of calpastatin from canine myocardium and rabbit liver also were bound to phosphatidylinositol. Smaller forms of calpastatin present in the preparations did not bind to acidic phospholipids. Bovine large calpastatin was subjected to CNBr digestion, and a phospholipid-binding fragment representing approximately one-sixth of the intact protein mass was purified. Amino acid sequence analysis indicated that the phospholipid-binding fragment was derived from the amino terminus of the large calpastatin.

An improved purification procedure for calpastatin, the inhibitor protein specific for the intracellular calcium-dependent proteinases, calpains

The specific inhibitor protein (calpastatin) for the calcium-dependent intracellular proteinases (calpains) is an important regulator of these enzymes. In this communication we describe a one day procedure for purifying 3 to 5 mg of calpastatin from a kilogram of bovine myocardium. This represents a substantial improvement over previously described methods, and should facilitate future studies of calpastatin structure and function. A key, novel step in the purification was dye-matrix chromatography on an Affi-Gel Blue column. Contrary to previous indications, calpastatin purified by the new method did not contain significant amounts of carbohydrate. However, the presence of covalently bound phosphate in purified bovine myocardial calpastatin was confirmed and co-migration of phosphate and calpastatin activity was demonstrated on Bio-Gel A-1.5m chromatography. Thus, it is possible that calpastatin function is regulated by phosphorylation.

The influence of stimulated peritoneal feeder cells and mitogens upon antibody secreting hybridomas

Peritoneal exudate cells from mice injected with immunostimulatory agents were evaluated for their ability to promote hybridoma growth. Peritoneal cells from mice receiving peritoneal injections of either Freund's incomplete adjuvant or pristane, seven days prior to harvesting, produced the greatest number of antibody-producing hybridomas. Freund's incomplete adjuvant produced 16 fold more peritoneal cells than unstimulated mice, thus reducing the number of mice needed to supply feeder cells for the hybridoma cultures. In separate experiments a number of B-lymphocyte stimulating lectins and factors were tested for their ability to promote hybridoma growth. 2-mercaptoethanol (25 microM) routinely increased the number of antibody producing hybridomas by 5 to 15 fold. 2-mercaptoethanol had a varying ability to increase the numbers of hybridoma colonies. The cloning efficiency, rate of cell growth and antibody production of hybridoma cell lines, previously produced in the absence of 2-mercaptoethanol could also be increased when this reducing agent was added to the culture medium.

Myocardial calpain 2 is inhibited by monoclonal antibodies specific for the small, noncatalytic subunit

Calpains (EC 3.4.22.17) are nonlysosomal intracellular proteinases which require calcium ion for activity. The calpains are heterodimers composed of a large catalytic subunit and a small subunit which may have a regulatory function during the catalytic cycle. However, whether calpains remain in the dimeric form or dissociate upon exposure to calcium is controversial. To resolve this issue, two monoclonal antibodies which specifically recognize the small calpain subunit were prepared using bovine calpain 2 heterodimer as the antigen. Both antibodies, designated P-1 and P-2, were capable of inhibiting bovine or canine calpain 2, and partially purified human erythrocyte calpain 1. However, neither could produce full inhibition. Further studies with P-1 and bovine calpain 2 indicated that the antibody decreased the calcium requirement for the proteinase. The Km for casein was increased and the Vmax was decreased. The addition of P-1 to the assay mixture several minutes after initiation of proteolytic activity resulted in a rapid inhibition. The P-1 antibody was also capable of decreasing the ability of the protein inhibitor of calpains (calpastatin) to inhibit bovine calpain 2. These studies indicate that the small subunit remains bound to the large subunit during catalysis and may influence its activity.

On the mechanism of binding of calpastatin, the protein inhibitor of calpains, to biologic membranes

Bovine myocardial calpastatin, the endogenous inhibitor of the calcium-dependent proteinases, calpains, could bind to sarcoplasmic reticulum preparations at neutral pH and low ionic strength. Even in the presence of 100 to 200 mM KCl, 4 to 5 micrograms of calpastatin was bound per mg of membrane. Although calpastatin is found associated with bovine myocardial sarcolemma, neither canine nor human erythrocyte calpastatins were present in isolated erythrocyte membrane preparations. The bovine myocardial calpastatin, but not human erythrocyte calpastatin, could associate with purified phospholipid vesicles at low ionic strength. Thus, phospholipids appear to be involved in the binding of calpastatin to membranes.

A sarcolemma-associated inhibitor is capable of modulating calcium-dependent proteinase activity

Purified bovine myocardial sarcolemma vesicles were shown to contain calcium-dependent proteinase inhibitor protein by direct assay and by immunoblot analysis following gel electrophoresis (Western blotting). Calcium-dependent proteinase (calpain, EC 3.4.22.17) was not detected in the sarcolemma vesicles. The inhibitor protein was not solubilized when the vesicles were ruptured by repetitive freezing and thawing. However, a large amount of latent inhibitor activity was exposed after freezing and thawing the sarcolemma, and the inhibitor was much more susceptible to removal by 1.0 M NaCl or proteolysis following this treatment. Since the vesicles were predominantly right-side-out, the latter observations suggested that the inhibitor was associated with the cytoplasmic face of the sarcolemma. The endogenous inhibitor was capable of protecting sarcolemmal protein kinase C from proteolytic conversion to soluble protein kinase M by type I or type II calcium-dependent proteinase. Thus, the inhibitor is probably important in controlling calcium-dependent proteolysis of sarcolemmal proteins.

Calcium-dependent proteases: an enzyme system active at cellular membranes?

Proteases having a neutral pH optimum and an absolute requirement for calcium ion are found in virtually all mammalian cells. Association of calcium-dependent proteases and a specific inhibitor protein with biological membranes seems to be an important regulatory feature of this proteolytic system, and it is likely that membranes are preferred sites for calcium-dependent protease action. Several recent hypotheses for the physiological function of calcium-dependent proteolysis are consistent with a membrane-associated protease action. Calcium-dependent proteases may participate in cell membrane fusion: the proteolysis of membrane proteins, which is required for the efficient fusion of erythrocytes, may be catalyzed by these enzymes. There is also evidence for the involvement of calcium-dependent proteolysis in postsynaptic membrane remodeling in the hippocampus after long-term potentiation. Although the relationship of the proteolysis to synaptic function is not known, it could have important physiological or pathophysiological consequences. Finally, it has recently been suggested that calcium-dependent proteolysis may be a physiologically significant mechanism for activating membrane-associated protein kinase C after exposure of some cell types to phorbol esters or other mitogens. Further pursuit of these hypotheses may reveal a novel role for intracellular calcium-regulated proteolysis in membrane-associated cell functions.

Isolated bovine myocardial sarcolemma and sarcoplasmic reticulum vesicles contain tightly bound calcium-dependent protease inhibitor

Bovine myocardial sarcolemma and sarcoplasmic reticulum vesicle preparations contained calcium-dependent protease inhibitor protein. No inhibitor was detected in mitochondrial membranes. The membrane-bound inhibitor co-purified with the marker enzymes for sarcolemma and sarcoplasmic reticulum, Na+,K+-ATPase and Ca2+,K+-ATPase respectively, on isopycnic ultracentrifugation through linear sucrose density gradients. Sarcolemma and sarcoplasmic reticulum vesicles contained about 1 mg of inhibitor per g of membrane protein. However, about one-half of the inhibitor in sarcoplasmic reticulum vesicles was not tightly associated with the membrane. The membrane-bound inhibitor may function to modulate calcium-dependent proteolytic cleavage of sarcolemmal or sarcoplasmic reticulum-associated proteins.

Catalytic subunit of the polycation-stimulated protein phosphatase. Effect of proteolysis on polycation stimulation

The phosphorylase phosphatase activity of the holoenzyme form of phosphatase 2A isolated from extracts of porcine renal cortex or bovine heart was stimulated 600% and 500%, respectively, by the addition of histone H1. After conversion of the phosphatase to the catalytic subunit form by treatment with ethanol at room temperature, histone H1 stimulated activity by about 150% only. Purification of the catalytic subunit from porcine renal cortex yielded two forms of the enzyme which were separated by heparin-Sepharose chromatography. These forms were designated peak 1 and peak 2 according to their order of elution from the column. Peak 1 catalytic subunit was stimulated by more than 400% by histone H1, whereas peak 2 was stimulated by about 50% only. Based on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, peak 2 had a slightly higher Mr value than peak 1 (35,500 vs. 35,000). Incubation of the peak 2 phosphatase with trypsin converted it to a form that was similar to peak 1 with respect to Mr and stimulation by histone H1. When the catalytic subunit of phosphatase 2A was purified from bovine heart only one form was obtained. Bovine heart enzyme was similar to renal peak 2 in that it had an apparent Mr of 35,500 and was only slightly stimulated by histone H1. Treatment of the bovine heart catalytic subunit with trypsin, chymotrypsin or type 2 Ca2+-dependent proteinase decreased the apparent Mr by about 500 and increased histone H1 stimulation to about 500%. Thus, when a small peptide was removed by proteolysis, histone H1 stimulation of the 'nicked' catalytic subunit was similar to that obtained with the holoenzyme.

Isolation and characterization of an inhibitor-sensitive and a polycation-stimulated protein phosphatase from rat liver nuclei

Two protein phosphatases were isolated from rat liver nuclei. The enzymes, solubilized from crude chromatin by 1 M NaCl, were resolved by column chromatography on Sephadex G-150, DEAE-Sepharose and heparin-Sepharose. The phosphorylase phosphatase activity of one of the enzymes (inhibitor-sensitive phosphatase) was inhibited by heat-stable phosphatase inhibitor proteins and also by histone H1. This phosphatase had a molecular weight of approx. 35,000 both before and after 4 M urea treatment. Its activity was specific for the beta-subunit of phosphorylase kinase. Pretreatment with 0.1 mM ATP inhibited the enzyme only about 10%, and it did not require divalent cations for activity. On the basis of these properties, this nuclear enzyme was identified as the catalytic subunit of phosphatase 1. The other phosphatase (polycation-stimulated phosphatase) was insensitive to inhibition by inhibitor 1, and it was stimulated 10-fold by low concentrations of histone H1 (A0.5 = 0.6 microM). This enzyme had a molecular weight of approx. 70,000 which was reduced to approx. 35,000 after treatment with 4 M urea. It dephosphorylated both the alpha- and beta-subunits of phosphorylase kinase. The enzyme was inhibited more than 90% by preincubation with 0.1 mM ATP and did not require divalent cations for activity. On the basis of these properties, this nuclear enzyme was identified as phosphatase 2A.

Purification and characterization of the polycation-stimulated protein phosphatase catalytic subunit from porcine renal cortex

The predominant form of phosphorylase phosphatase activity in porcine renal cortical extracts was a polycation-stimulated protein phosphatase. This activity was present in extracts in a high-molecular-weight form which could be converted to a free catalytic subunit by treatment with ethanol, urea, or freezing and thawing in the presence of beta-mercaptoethanol. The catalytic subunit of the polycation-stimulated phosphatase was purified by chromatography on DEAE-Sephacel, heparin-Sepharose, and Sephadex G-75. The phosphatase appeared to be homogeneous on SDS-polyacrylamide gel electrophoresis. The enzyme had an apparent Mr of 35 000 on gel filtration and SDS-polyacrylamide gel electrophoresis. The purified phosphatase could be stimulated by histone H1, protamine, poly(D-lysine), poly(L-lysine) or polybrene utilizing phosphorylase a as the substrate. It preferentially dephosphorylated the alpha-subunit of phosphorylase kinase. The phosphatase was highly sensitive to inhibition by ATP. These results suggest that the renal polycation-stimulated phosphatase catalytic subunit is very similar to or identical with the skeletal muscle phosphatase form which has been previously designated phosphatase-2Ac.

Proteolysis of the calcium-dependent protease inhibitor by myocardial calcium-dependent protease

Bovine heart peak II calcium-dependent protease was capable of hydrolyzing its specific inhibitor protein at high molar ratios of protease to inhibitor. The proteolysis was inhibited by leupeptin and required millimolar calcium. Thus, it appeared to be attributable to the calcium-dependent protease and not to possible contaminating proteases in the purified preparations of inhibitor or calcium-dependent protease. Incubation of the purified inhibitor with the calcium-dependent protease produced a discrete pattern of inhibitor fragments on Western blots developed with an inhibitor-specific monoclonal antibody. Traces of similar or identical lower molecular weight immunoreactive material could be observed in Western blots of bovine heart extracts, and the immunoreactivity present as these lower molecular weight forms could be increased by incubation of the extracts with calcium ion. These results suggest that the inhibitor can be proteolyzed to low molecular weight forms which can be detected in cardiac tissue extracts, and that calcium-dependent protease(s) may be responsible for this phenomenon.

Subcellular localization of bovine heart calcium-dependent protease inhibitor

Four monoclonal antibodies for a calcium-dependent protease inhibitor protein were produced by fusing Sp2/0 myeloma cells with spleen cells from a Balb/C mouse immunized with purified bovine heart inhibitor. Each of the monoclonal antibodies was highly specific for the inhibitory protein as revealed by electro-blot analysis. The antibodies recognized different antigenic sites on CNBr peptides prepared from the purified inhibitor protein. Immunofluorescent microscopy of sections from bovine heart ventricles treated with each of the antibodies demonstrated the same fluorescent pattern. Fluorescence was observed at or near the sarcolemma of the myocytes, and along the Z-discs of relaxed myofibrils within the myocytes. Contracted myofibrils did not appear to bind antibody. Immunostaining of glycerinated relaxed cardiac myofibrils revealed staining at the Z-discs. One of the antibodies could also stain the Z-disc region of bovine skeletal muscle myofibrils.

Dephosphorylation of phosphorylase kinase by a histone H1-stimulated phosphoprotein phosphatase

A histone H1-stimulated protein phosphatase isolated from rabbit skeletal muscle dephosphorylated [32P]phosphorylase kinase. The rate of dephosphorylation was markedly increased by 5-50 micrograms histone H1/ml. Only the alpha and alpha' subunits were dephosphorylated in the absence or presence of histone H1. This is consistent with previous results suggesting that the H1-stimulated phosphatase is a type-2 protein phosphatase. The present studies suggest that the histone H1-stimulation is the result of a direct interaction of the histone with the phosphatase.