Functional analysis of the transmembrane domain and activation cleavage of human corin: design and characterization of a soluble corin

Corin is a cardiac transmembrane serine protease. In cell-based studies, corin converted pro-atrial natriuretic peptide (pro-ANP) to mature ANP, suggesting that corin is potentially the pro-ANP convertase. In this study, we evaluated the importance of the transmembrane domain and activation cleavage in human corin. We showed that a soluble corin that consists of only the extracellular domain was capable of processing recombinant human pro-ANP in cell-based assays. In contrast, a mutation at the conserved activation cleavage site, R801A, abolished the function of corin, demonstrating that the activation cleavage is essential for corin activity. These results allowed us to design, express, and purify a mutant soluble corin, EKsolCorin, that contains an enterokinase recognition sequence at the activation cleavage site. Purified EKsolCorin was activated by enterokinase in a dose-dependent manner. Activated EK-solCorin had hydrolytic activity toward peptide substrates with a preference for Arg and Lys residues in the P-1 position. This activity of EKsolCorin was inhibited by trypsin-like serine protease inhibitors but not inhibitors of chymotrypsin-like, cysteine-, or metallo-proteases. In pro-ANP processing assays, purified active EKsolCorin converted recombinant human pro-ANP to biologically active ANP in a highly sequence-specific manner. The pro-ANP processing activity of EKsolCorin was not inhibited by human plasma. Together, our data indicate that the transmembrane domain is not necessary for the biological activity of corin but may be a mechanism to localize corin at specific sites, whereas the proteolytic cleavage at the activation site is an essential step in controlling the activity of corin.     

Localization of the mosaic transmembrane serine protease corin to heart myocytes.

Corin cDNA encodes an unusual mosaic type II transmembrane serine protease, which possesses, in addition to a trypsin-like serine protease domain, two frizzled domains, eight low-density lipoprotein (LDL) receptor domains, a scavenger receptor domain, as well as an intracellular cytoplasmic domain. In in vitro experiments, recombinant human corin has recently been shown to activate pro-atrial natriuretic peptide (ANP), a cardiac hormone essential for the regulation of blood pressure. Here we report the first characterization of corin protein expression in heart tissue. We generated antibodies to two different peptides derived from unique regions of the corin polypeptide, which detected immunoreactive corin protein of approximately 125-135 kDa in lysates from human heart tissues. Immunostaining of sections of human heart showed corin expression was specifically localized to the cross striations of cardiac myocytes, with a pattern of expression consistent with an integral membrane localization. Corin was not detected in sections of skeletal or smooth muscle. Corin has been suggested to be a candidate gene for the rare congenital heart disease, total anomalous pulmonary venous return (TAPVR) as the corin gene colocalizes to the TAPVR locus on human chromosome 4. However examination of corin protein expression in TAPVR heart tissue did not show evidence of abnormal corin expression. The demonstrated corin protein expression by heart myocytes supports its proposed role as the pro-ANP convertase, and thus a potentially critical mediator of major cardiovascular diseases including hypertension and congestive heart failure.     

Processing of pro-atrial natriuretic peptide by corin in cardiac myocytes

Corin is a type II transmembrane serine protease abundantly expressed in the heart. In a previous study using transfected 293 cells, we showed that corin converted pro-atrial natriuretic peptide (pro-ANP) to atrial natriuretic peptide (ANP), suggesting that corin is likely the pro-ANP convertase. Because other serine proteases such as thrombin and kallikrein had previously also been shown to cleave pro-ANP in vitro, it remained to demonstrate that corin is indeed the endogenous pro-ANP convertase in cardiomyocytes. In this study, we examined pro-ANP processing in a murine cardiac muscle cell line, HL-5. Northern analysis showed that corin mRNA was present in HL-5 cells. In HL-5 cells transfected with a plasmid expressing pro-ANP, recombinant pro-ANP was converted to mature ANP as determined by Western analysis, indicating the presence of the endogenous pro-ANP convertase in these cells. The processed recombinant ANP was shown to be active in an enzyme-linked immunosorbent assay-based cGMP assay in baby hamster kidney cells. The processing of recombinant pro-ANP in HL-5 cells was highly sequence-specific, because mutation R98A, but not mutations R101A and R102A, in pro-ANP prevented the conversion of pro-ANP to ANP. Expression of recombinant wild-type corin enhanced the processing of pro-ANP in HL-5 cells. In contrast, overexpression of active site mutant corin S985A or transfection of oligonucleotide small interfering RNA duplexes directed against the mouse corin gene completely inhibited the processing of recombinant pro-ANP in HL-5 cells. These results indicate that corin is the physiological pro-ANP convertase in cardiac myocytes.     

Corin Mutation R539C from Hypertensive Patients Impairs Zymogen Activation and Generates an Inactive Alternative Ectodomain Fragment

Corin is a cardiac transmembrane serine protease that regulates blood pressure by activating natriuretic peptides. Corin variants have been associated with African Americans with hypertension and heart disease. Here, we report a new mutation in exon 12 of the CORIN gene identified in a family of patients with hypertension. The mutation resulted in R539C substitution in the Fz2 (Frizzled-2) domain of the corin propeptide region. We expressed and characterized the corin R539C mutant in HEK293 cells. As determined by Western blot analysis, the R539C mutation did not alter corin expression in transfected cells but impaired corin zymogen activation. In a pro-atrial natriuretic peptide processing assay, the corin mutant had reduced activity and exhibited a dominant-negative effect on wild-type corin. In addition, the R539C mutation altered corin ectodomain shedding, producing an alternative ∼75-kDa fragment that was biologically inactive. Using protease inhibitors and the catalytically inactive corin mutant S985A, we showed that the ∼75-kDa fragment was generated by corin autocleavage. We constructed a series of mutants by replacing single or double Arg residues in the corin propeptide and identified Arg-530 in the Fz2 domain as the alternative autocleavage site. Our results show that the corin mutation R539C identified in hypertensive patients impairs corin zymogen activation and causes an alternative autocleavage that reduces corin activity. These data support that human CORIN gene mutations causing impaired corin activity may be an underlying mechanism in hypertension.     

PCSK6对可溶性corin的活化作用研究

目的：研究前蛋白转化酶枯草溶菌素6（proprotein convertase subtilisin/kextin type 6,PCSK6）对可溶性corin的活化作用,以及PCSK6作用后的可溶性corin是否具有生物学活性。方法：以野生型corin质粒为模板,通过PCR扩增出不含跨膜区的corin基因片段,并将目的基因克隆至真核表达载体pSEC得到重组质粒pSECsolCorin,测序验证。分别将可溶性corin、PCSK6及心钠肽前体（pro-atrial natriuretic peptide,pro-ANP）的表达载体转染人胚胎肾（human embryonic kidney,HEK）293细胞,收取上清液。然后将可溶性corin上清液与不同体积的PCSK6上清液共同孵育,免疫印迹（Western blot）方法检测corin蛋白的活化情况。再将PCSK6作用后的可溶性corin与pro-ANP上清液共孵育,检测可溶性corin的生物学活性。结果：构建了可溶性corin的表达质粒,并在HEK293细胞成功表达可溶性corin蛋白。可溶性corin的上清液与pcsk6上清液共同孵育后可检测到活化条带,并可使pro-ANP活化为ANP。结论：PCSK6同样可以剪切活化可溶性corin蛋白,活化后的corin具有生物学活性。为可溶性corin用于心衰的临床治疗提供了重要基础。     

Upregulation of corin gene expression in hypertrophic cardiomyocytes and failing myocardium

High levels of plasma atrial natriuretic peptides (ANP) are associated with pathological conditions such as congestive heart failure (CHF). Recently, we have identified a cardiac serine protease, corin, that is the pro-ANP convertase. In this study, we examined the regulation of corin gene expression in cultured hypertrophic cardiomyocytes and in the left ventricular (LV) myocardium of a rat model of heart failure. Quantitative RT-PCR analysis showed that both corin and ANP mRNA levels were significantly increased in phenylephrine (PE)-stimulated rat neonatal cardiomyocytes in culture. The increase in corin mRNA correlated closely with the increase in cell size and ANP mRNA expression in the PE-treated cells (r = 0.95, P < 0.01; r = 0.92, P < 0.01, respectively). The PE-treated cardiomyocytes had an increased activity in converting recombinant human pro-ANP to biologically active ANP, as determined by a pro-ANP processing assay and a cell-based cGMP assay. In a rat model of heart failure induced by ligation of the left coronary artery, corin mRNA expression in the noninfarcted LV myocardium was significantly higher than that of control heart tissues from sham-operated animals, when examined by Northern blot analysis and RT-PCR at 8 wk. These results indicate that the corin gene is upregulated in hypertrophic cardiomyocytes and failing myocardium. Increased corin expression may contribute to elevation of ANP in the setting of cardiac hypertrophy and heart failure.     

Ectodomain shedding and autocleavage of the cardiac membrane protease corin

Corin is a cardiac membrane protease that activates natriuretic peptides. It is unknown how corin function is regulated. Recently, soluble corin was detected in human plasma, suggesting that corin may be shed from cardiomyocytes. Here we examined soluble corin production and activity and determined the proteolytic enzymes responsible for corin cleavage. We expressed human corin in HEK 293 cells and detected three soluble fragments of ～180, ～160, and ～100 kDa, respectively, in the cultured medium by Western blot analysis. All three fragments were derived from activated corin molecules. Similar results were obtained in HL-1 cardiomyocytes. Using protease inhibitors, ionomycin and phorbol myristate acetate stimulation, small interfering RNA knockdown, and site-directed mutagenesis, we found that ADAM10 was primarily responsible for shedding corin in its juxtamembrane region to release the ～180-kDa fragment, corresponding to the near-entire extracellular region. In contrast, the ～160- and ～100-kDa fragments were from corin autocleavage at Arg-164 in frizzled 1 domain and Arg-427 in LDL receptor 5 domain, respectively. In functional studies, the ～180-kDa fragment activated atrial natriuretic peptide, whereas the ～160- and ～100-kDa fragments did not. Our data indicate that ADAM-mediated shedding and corin autocleavage are important mechanisms regulating corin function and preventing excessive, potentially hazardous, proteolytic activities in the heart.     

Distinct Roles of N-Glycosylation at Different Sites of Corin in Cell Membrane Targeting and Ectodomain Shedding

Corin is a membrane-bound protease essential for activating natriuretic peptides and regulating blood pressure. Human corin has 19 predicted N-glycosylation sites in its extracellular domains. It has been shown that N-glycans are required for corin cell surface expression and zymogen activation. It remains unknown, however, how N-glycans at different sites may regulate corin biosynthesis and processing. In this study, we examined corin mutants, in which each of the 19 predicted N-glycosylation sites was mutated individually. By Western analysis of corin proteins in cell lysate and conditioned medium from transfected HEK293 cells and HL-1 cardiomyocytes, we found that N-glycosylation at Asn-80 inhibited corin shedding in the juxtamembrane domain. Similarly, N-glycosylation at Asn-231 protected corin from autocleavage in the frizzled-1 domain. Moreover, N-glycosylation at Asn-697 in the scavenger receptor domain and at Asn-1022 in the protease domain is important for corin cell surface targeting and zymogen activation. We also found that the location of the N-glycosylation site in the protease domain was not critical. N-Glycosylation at Asn-1022 may be switched to different sites to promote corin zymogen activation. Together, our results show that N-glycans at different sites may play distinct roles in regulating the cell membrane targeting, zymogen activation, and ectodomain shedding of corin.     

Role of glycosylation in corin zymogen activation

The cardiac serine protease corin is the pro-atrial natriuretic peptide convertase. Corin is made as a zymogen, which is activated by proteolytic cleavage. Previous studies showed that recombinant human corin expressed in HEK 293 cells was biologically active, but activated corin fragments were not detectable, making it difficult to study corin activation. In this study, we showed that recombinant rat corin was activated in HEK 293 cells, murine HL-1 cardiomyocytes, and rat neonatal cardiomyocytes. In these cells, activated corin represented a small fraction of the total corin molecules. The activation of recombinant rat corin was inhibited by small molecule trypsin inhibitors but not inhibitors for matrix metalloproteinases or cysteine proteases, suggesting that a trypsin-like protease activated corin in these cells. Glycosidase digestion showed that rat and human corin proteins contained substantial N-glycans but little O-glycans. Treatment of HEK 293 cells expressing rat corin with tunicamycin prevented corin activation and inhibited its pro-atrial natriuretic peptide processing activity. Similar effects of tunicamycin on endogenous corin activity were found in HL-1 cells. Mutations altering the two N-glycosylation sites in the protease domain of rat corin prevented its activation in HEK 293 and HL-1 cells. Our results indicate that N-linked oligosaccharides play an important role in corin activation.     

N-glycosylation modulates the cell-surface expression and catalytic activity of corin

N-Glycosylation may influence the subcellular localization and biological activity of the pro-ANP convertase, corin. In HEK293-corin cells, the inhibition of N-glycosylation, with tunicamycin, reduced the cell-surface expression of murine corin, but did not alter the total expression. Therefore, tunicamycin treatment likely caused the intracellular accumulation of non-glycosylated corin. Tunicamycin treatment also significantly reduced corin activity (pro-ANP cleavage) in these cells. We developed an assay to measure the effect of N-glycosylation on corin activity, independent of its effect on corin localization. We determined that the reduction in corin activity was due to a direct effect of N-glycosylation, and was not secondary to the effect of N-glycosylation on corin cell-surface expression. Our data provide evidence that N-glycosylation is essential for the cell-surface expression of murine corin and modulates its functional activity. N-Glycosylation represents a possible mechanism for the regulation of native corin on the surface of cardiomyocytes.     

The role of tryptophan residues in the autoprocessing of prosubtilisin E

Subtilisin E, a serine protease from Bacillus subtilis, requires an N-terminal propeptide for its correct folding. The propeptide is autocleaved and digested by the subtilisin domain upon proper folding. Here we investigated the individual roles of the three Trp residues within the subtilisin domain (Trp106, Trp113 and Trp241) on propeptide processing, enzymatic activity and stability of subtilisin. When the propeptide processing was examined by SDS-PAGE after refolding by rapid dilution, the mutation at either position Trp106 or Trp113 was found to significantly delay the propeptide processing, while the mutation at Trp241 had no effect. Far-UV circular dichroism (CD) spectra of the mutants revealed that the mutations at the three positions did not affect appreciably the alpha-helix content of subtilisin. Secondary structure thermal unfolding monitored by CD spectroscopy revealed that none of the tryptophan residues had any significant effect on the stability of mature subtilisin. The enzymatic activity measurements showed that only Trp106 plays a major role in the enzymatic activity of subtilisin E. These results demonstrate that both Trp106 and Trp113 play a specific role in propeptide processing and enzymatic activity, while Trp241 plays no considerable role on any of these activities.     

Human corin isoforms with different cytoplasmic tails that alter cell surface targeting

Corin is a cardiac serine protease that activates natriuretic peptides. It consists of an N-terminal cytoplasmic tail, a transmembrane domain, and an extracellular region with a C-terminal trypsin-like protease domain. The transmembrane domain anchors corin on the surface of cardiomyocytes. To date, the function of the corin cytoplasmic tail remains unknown. By examining the difference between human and mouse corin cytoplasmic tails, analyzing their gene sequences, and verifying mRNA expression in hearts, we show that both human and mouse corin genes have alternative exons encoding different cytoplasmic tails. Human corin isoforms E1 and E1a have 45 and 15 amino acids, respectively, in their cytoplasmic tails. In transfected HEK 293 cells and HL-1 cardiomyocytes, corin isoforms E1 and E1a were expressed at similar levels. Compared with isoform E1a, however, isoform E1 was more active in processing natriuretic peptides. By cell surface labeling, glycosidase digestion, Western blotting, and flow cytometry, we found that corin isoform E1 was activated more readily as a result of more efficient cell surface targeting. By mutagenesis, we identified a DDNN motif in the cytoplasmic tail of isoform E1 (which is absent in isoform E1a) that promotes corin surface targeting in both HEK 293 and HL-1 cells. Our data indicate that the sequence in the cytoplasmic tail plays an important role in corin cell surface targeting and zymogen activation.     

NARC-1/PCSK9 and its natural mutants: zymogen cleavage and effects on the low density lipoprotein (LDL) receptor and LDL cholesterol

The discovery of autosomal dominant hypercholesterolemic patients with mutations in the PCSK9 gene, encoding the proprotein convertase NARC-1, resulting in the missense mutations suggested a role in low density lipoprotein (LDL) metabolism. We show that the endoplasmic reticulum-localized proNARC-1 to NARC-1 zymogen conversion is Ca2+-independent and that within the zymogen autocatalytic processing site SSVFAQ [downward arrow]SIP Val at P4 and Pro at P3' are critical. The S127R and D374Y mutations result in approximately 50-60% and > or =98% decrease in zymogen processing, respectively. In contrast, the double [D374Y + N157K], F216L, and R218S natural mutants resulted in normal zymogen processing. The cell surface LDL receptor (LDLR) levels are reduced by 35% in lymphoblasts of S127R patients. The LDLR levels are also reduced in stable HepG2 cells overexpressing NARC-1 or its natural mutant S127R, and this reduction is abrogated in the presence of 5 mm ammonium chloride, suggesting that overexpression of NARC-1 increases the turnover rate of the LDLR. Adenoviral expression of wild type human NARC-1 in mice resulted in a maximal approximately 9-fold increase in circulating LDL cholesterol, while in LDLR-/- mice a delayed approximately 2-fold increase in LDL cholesterol was observed. In conclusion, NARC-1 seems to affect both the level of LDLR and that of circulating apoB-containing lipoproteins in an LDLR-dependent and -independent fashion.     

Molecular modeling of D151Y and M391T mutations in the LDL receptor

The low-density lipoprotein receptor (LDLR) is a key regulator of cholesterol homeostasis, and defects in the function of LDLR result in familial hypercholesterolemia (FH). In the present study, we performed structural analyses of two novel LDLR mutations, D151Y and M391T. Both mutations occurred in conserved residues of LDLR. The D151Y mutation, in the ligand binding domain, caused an elimination of a hydrogen bond in the calcium binding site, higher solvent accessibility and a loss of negative charge in the Y151 residue. On the other hand, the M391T mutation, in the β-propeller of the epidermal growth factor (EGF) precursor homology domain, caused an additional hydrogen bond to form, higher solvent accessibility and a distortion of the β-strand. These data suggest that the irregular structures of the mutated LDLRs are likely to cause the functional defect that contributes to the pathology of FH.     

Plasma Membrane Tetraspanin CD81 Complexes with Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) and Low Density Lipoprotein Receptor (LDLR), and Its Levels Are Reduced by PCSK9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is an important factor in plasma cholesterol regulation through modulation of low density lipoprotein receptor (LDLR) levels. Naturally occurring mutations can lead to hyper- or hypocholesterolemia in human. Recently, we reported that PCSK9 was also able to modulate CD81 in Huh7 cells. In the present study, several gain-of-function and loss-of-function mutants as well as engineered mutants of PCSK9 were compared for their ability to modulate the cell surface expression of LDLR and CD81. Although PCSK9 gain-of-function D374Y enhanced the degradation both receptors, D374H and D129N seemed to only reduce LDLR levels. In contrast, mutations in the C-terminal hinge-cysteine-histidine-rich domain segment primarily affected the PCSK9-induced CD81 degradation. Furthermore, when C-terminally fused to an ACE2 transmembrane anchor, the secretory N-terminal catalytic or hinge-cysteine-histidine-rich domain domains of PCSK9 were able to reduce CD81 and LDLR levels. These data confirm that PCSK9 reduces CD81 levels via an intracellular pathway as reported for LDLR. Using immunocytochemistry, a proximity ligation assay, and co-immunoprecipitation, we found that the cell surface level of PCSK9 was enhanced upon overexpression of CD81 and that both PCSK9 and LDLR interact with this tetraspanin protein. Interestingly, using CHO-A7 cells lacking LDLR expression, we revealed that LDLR was not required for the degradation of CD81 by PCSK9, but its presence strengthened the PCSK9 effect.     

General function of N-terminal propeptide on assisting protein folding and inhibiting catalytic activity based on observations with a chimeric thermolysin-like protease

Pro-aminopeptidase processing protease (PA protease) is a thermolysin-like metalloprotease produced by Aeromonas caviae T-64. The N-terminal propeptide acts as an intramolecular chaperone to assist the folding of PA protease and shows inhibitory activity toward its cognate mature enzyme. Moreover, the N-terminal propeptide strongly inhibits the autoprocessing of the C-terminal propeptide by forming a complex with the folded intermediate pro-PA protease containing the C-terminal propeptide (MC). In order to investigate the structural determinants within the N-terminal propeptide that play a role in the folding, processing, and enzyme inhibition of PA protease, we constructed a chimeric pro-PA protease by replacing the N-terminal propeptide with that of vibriolysin, a homologue of PA protease. Our results indicated that, although the N-terminal propeptide of vibriolysin shares only 36% identity with that of PA protease, it assists the refolding of MC, inhibits the folded MC to process its C-terminal propeptide, and shows a stronger inhibitory activity toward the mature PA protease than that of PA protease. These results suggest that the N-terminal propeptide domains in these thermolysin-like proteases may have similar functions, in spite of their primary sequence diversity. In addition, the conserved regions in the N-terminal propeptides of PA protease and vibriolysin may be essential for the functions of the N-terminal propeptide.     

Corin, a mosaic transmembrane serine protease encoded by a novel cDNA from human heart.

A novel cDNA has been identified from human heart that encodes an unusual mosaic serine protease, designated corin. Corin has a predicted structure of a type II transmembrane protein and contains two frizzled-like cysteine-rich motifs, seven low density lipoprotein receptor repeats, a macrophage scavenger receptor-like domain, and a trypsin-like protease domain in the extracellular region. Northern analysis showed that corin mRNA was highly expressed in the human heart. In mice, corin mRNA was detected by in situ hybridization in the cardiac myocytes of the embryonic heart as early as embryonic day (E) 9.5. By E11.5-13.5, corin mRNA was most abundant in the primary atrial septum and the trabecular ventricular compartment. Expression in the heart was maintained through the adult. In addition, mouse corin mRNA was also detected in the prehypertrophic chrondrocytes in developing bones. By fluorescent in situ hybridization analysis, the human corin gene was mapped to 4p12-13 where a congenital heart disease locus, total anomalous pulmonary venous return, had been previously localized. The unique domain structure and specific embryonic expression pattern suggest that corin may have a function in cell differentiation during development. The chromosomal localization of the human corin gene makes it an attractive candidate gene for total anomalous pulmonary venous return.     

Function of the central domain of streptokinase in substrate plasminogen docking and processing revealed by site-directed mutagenesis

The possible role of the central beta-domain (residues 151-287) of streptokinase (SK) was probed by site-specifically altering two charged residues at a time to alanines in a region (residues 230-290) previously identified by Peptide Walking to play a key role in plasminogen (PG) activation. These mutants were then screened for altered ability to activate equimolar "partner" human PG, or altered interaction with substrate PG resulting in an overall compromised capability for substrate PG processing. Of the eight initial alanine-linker mutants of SK, one mutant, viz. SK(KK256.257AA) (SK-D1), showed a roughly 20-fold reduction in PG activator activity in comparison to wild-type SK expressed in Escherichia coli (nSK). Five other mutants were as active as nSK, with two [SK(RE248.249AA) and SK(EK281.282AA), referred to as SK(C) and SK(H), respectively] showing specific activities approximately one-half and two-thirds, respectively, that of nSK. Unlike SK(C) and SK(H), however, SK(D1) showed an extended initial delay in the kinetics of PG activation. These features were drastically accentuated when the charges on the two Lys residues at positions 256 and 257 of nSK were reversed, to obtain SK(KK256.257EE) [SK(D2)]. This mutant showed a PG activator activity approximately 10-fold less than that of SK(D1). Remarkably, inclusion of small amounts of human plasmin (PN) in the PG activation reactions of SK(D2) resulted in a dramatic, PN dose-dependent rejuvenation of its PG activation capability, indicating that it required pre-existing PN to form a functional activator since it could not effect active site exposure in partner PG on its own, a conclusion further confirmed by its inability to show a "burst" of p-nitrophenol release in the presence of equimolar human PG and p-nitrophenyl guanidino benzoate. The steady-state kinetic parameters for HPG activation of its 1:1 complex with human PN revealed that although it could form a highly functional activator once "supplied" with a mature active site, the Km for PG was increased nearly eightfold in comparison to that of nSK-PN. SK mutants carrying simultaneous two- and three-site charge-cluster alterations, viz., SK(RE24249AA:EK281.282AA) [SK(CH)], SK(EK272.273AA;EK281.282AA) [SK(FH)], and SK(RE248.249AA;EK272.273AA:EK281.282AA+ ++) [SK(CFH)], showed additive/synergistic influence of multiple charge-cluster mutations on HPG activation when compared to the respective "single-site" mutants, with the "triple-site" mutant [SK(CFH)] showing absolutely no detectable HPG activation ability. Nevertheless, like the other constructs, the double- and triple-charge cluster mutants retained a native like affinity for complexation with partner PG. Their overall structure also, as judged by far-ultraviolet circular dichroism, was closely similar to that of nSK. These results provide the first experimental evidence for a direct assistance by the SK beta-domain in the docking and processing of substrate PG by the activator complex, a facet not readily evident probably because of the flexibility of this domain in the recent X-ray crystal structure of the SK-plasmin light chain complex.     

The DEG15 serine protease cleaves peroxisomal targeting signal 2-containing proteins in Arabidopsis

Two distinct peroxisomal targeting signals (PTSs), the C-terminal PTS1 and the N-terminal PTS2, are defined. Processing of the PTS2 on protein import is conserved in higher eukaryotes. Recently, candidates for the responsible processing protease were identified from plants (DEG15) and mammals (TYSND1). We demonstrate that plants lacking DEG15 show an expressed phenotype potentially linked to reduced beta-oxidation, indicating the impact of protein processing on peroxisomal functions in higher eukaryotes. Mutational analysis of Arabidopsis (Arabidopsis thaliana) DEG15 revealed that conserved histidine, aspartic acid, and serine residues are essential for the proteolytic activity of this enzyme in vitro. This indicates that DEG15 and related enzymes are trypsin-like serine endopeptidases. Deletion of a plant-specific stretch present in the protease domain diminished, but did not abolish, the proteolytic activity of DEG15 against the PTS2-containing glyoxysomal malate dehydrogenase. Fluorescence microscopy showed that a DEG15-green fluorescent protein fusion construct is targeted to peroxisomes in planta. In vivo studies with isolated homozygous deg15 knockout mutants and complemented mutant lines suggest that this enzyme mediates general processing of PTS2-containing proteins.