The proprotein convertases

Stability and global fold of the mouse prohormone convertase 1 pro-domain

We have purified the mouse prohormone convertase 1 (PC1) pro-domain expressed in Escherichia coli cells and demonstrated, using a number of biophysical methods, that this domain is an independent folding unit with a T(m) of 39 degrees C at a protein concentration of 20 microM and pH 7.0. This differs significantly from similar pro-domains in bacteria and human furin, which are unfolded at 25 degrees C and require the catalytic domain in order to be structured [Bryan et al. (1995) Biochemistry 34, 10310-10318; Bhattacharjya et al. (2000) J. Biomol. NMR 16, 275-276]. Using heteronuclear NMR spectroscopy, we have determined the backbone (1)H, (13)C, and (15)N assignments for the pro-domain of PC1. On the basis of (1)H/(13)C chemical shift indices, NOE analysis, and hydrogen exchange measurements, the pro-domain is shown to consist of a four-stranded beta-sheet and two alpha-helices. The results presented here show that both the bacterial pro-domain in complex with subtilisin and the uncomplexed mouse PC1 pro-domain have very similar overall folds despite a lack of sequence homology. The structural data help to explain the location of the secondary processing sites in the pro-domains of the PC family, and a consensus sequence for binding to the catalytic domain is proposed.

Key peptide processing enzymes are expressed by breast cancer cells

The expression of the three key peptide processing enzyme families, represented by CPE, PAM, and PC1/3 plus PC2, were examined in MCF-7 and ZR-75-1 breast cancer cell lines. Both of these cell lines express vasopressin receptors as well as the vasopressin gene, but the processing of vasopressin gene-related proteins appears to be limited. Products of the expected size for, CPE, PAM and PC1/PC3 could be amplified by reverse transcription-polymerase chain reaction (RT-PCR) from both cell lines. Cloning and sequencing of these RT-PCR products revealed that each enzyme mRNA had a structure identical to that published for the human form of the respective enzyme. Western analysis provided evidence that mRNAs for these enzymes are translated into proteins. Alternatively, PC2 mRNA was identified to be present in MCF-7 cells both by RT-PCR and Western blot analysis, but could not be demonstrated for ZR-75-1 cells. Our findings suggest that the key processing enzymes needed to generate active vasopressin and other neuropeptide growth factors are present in breast cancer cells.

Peptide repertoire of human cerebrospinal fluid: novel proteolytic fragments of neuroendocrine proteins

Polypeptides in human cerebrospinal fluid (CSF), isolated by phase separation in chloroform-methanol-water and reversed-phase HPLC, were characterised by sequence analysis and mass spectrometry. This identified the presence of peptide fragments of testican, neuroendocrine specific protein VGF, neuroendocrine protein 7B2, chromogranin B/secretogranin I, chromogranin A, osteopontin, IGF-II E-peptide and proenkephalin. The majority of these fragments were generated by proteolysis at dibasic sites, suggesting that they are derived by activities related to prohormone convertase(s). Several of the fragments have previously not been detected, and their functions in CSF or elsewhere are unknown. A characteristic feature of all these fragments is a very high content of acidic residues, in particular glutamic acid. In addition to the fragments of neuroendocrine proteins, endothelin-binding receptor-like protein 2, ribonuclease 1, IGF-binding protein 6, albumin, alpha1-acid glycoprotein 1, prostaglandin-H2 D-isomerase, apolipoprotein A1, transthyretin, beta2-microglobulin, ubiquitin, fibrinopeptide A, and C4A anaphylatoxin were found.

Profiling serine hydrolase activities in complex proteomes

Serine hydrolases represent one of the largest and most diverse families of enzymes in higher eukaryotes, comprising numerous proteases, lipases, esterases, and amidases. The activities of many serine hydrolases are tightly regulated by posttranslational mechanisms, limiting the suitability of standard genomics and proteomics methods for the functional characterization of these enzymes. To facilitate the global analysis of serine hydrolase activities in complex proteomes, a biotinylated fluorophosphonate (FP-biotin) was recently synthesized and shown to serve as an activity-based probe for several members of this enzyme family. However, the extent to which FP-biotin reacts with the complete repertoire of active serine hydrolases present in a given proteome remains largely unexplored. Herein, we describe the synthesis and utility of a variant of FP-biotin in which the agent's hydrophobic alkyl chain linker was replaced by a more hydrophilic poly(ethylene glycol) moiety (FP-peg-biotin). When incubated with both soluble and membrane proteomes for extended reaction times, FP-biotin and FP-peg-biotin generated similar "maximal coverage" serine hydrolase activity profiles. However, kinetic analyses revealed that several serine hydrolases reacted at different rates with each FP agent. These rate differences were exploited in studies that used the biotinylated FPs to examine the target selectivity of reversible serine hydrolase inhibitors directly in complex proteomes. Finally, a general method for the avidin-based affinity isolation of FP-biotinylated proteins was developed, permitting the rapid and simultaneous identification of multiple serine peptidases, lipases, and esterases. Collectively, these studies demonstrate that chemical probes such as the biotinylated FPs can greatly accelerate both the functional characterization and molecular identification of active enzymes in complex proteomes.

Hypothalamo-pituitary-adrenocortical dysregulation in aging F344/Brown-Norway F1 hybrid rats

Hypothalamo-pituitary-adrenocortical (HPA) axis aging was studied in young (3 mo), middle aged (15 mo) and aged (30 mo) F344/Brown Norway hybrid rats. This strain was selected to obviate HPA-relevant pathologies found in other aging models. Aged, unstressed rats showed enhanced central HPA drive, marked by elevated ACTH release and decreased pituitary proopiomelanocortin and corticotropin-releasing factor receptor 1 (CRH-R1) mRNAs. Acute corticosterone responses to spatial novelty were exacerbated in aged rats; however, responses to restraint or hypoxia were not affected. Chronic stress exposure also differentially increased HPA drive in aged animals, marked by elevated paraventricular nucleus CRH peptide levels and pituitary proopiomelanocortin mRNA. Plasma ACTH and pituitary POMC and CRH-R1 mRNA expression in middle-aged rats were intermediate those of young and aged animals. Middle-aged animals responded to chronic stress with disproportionate increases in CRH mRNA levels, and increased corticosterone secretion following hypoxia but not novelty. The results suggest a gradual increase in HPA tone across the aging process, culminating in marked hyperresponsivity to both acute and chronic stress in senescence.

Distinct roles for the N- and C-terminal regions in the cytotoxicity of pierisin-1, a putative ADP-ribosylating toxin from cabbage butterfly, against mammalian cells

Pierisin-1 is an 850-aa cytotoxic protein found in the cabbage butterfly, Pieris rapae, and has been suggested to consist of an N-terminal region with ADP-ribosyltransferase domain and of a C-terminal region that might have a receptor-binding domain. To elucidate the role of each region, we investigated the functions of various fragments of pierisin-1. In vitro expressed polypeptide consisting of amino acid residues 1-233 or 234-850 of pierisin-1 alone did not show cytotoxicity against human cervical carcinoma HeLa cells. However, the presence of both polypeptides in the culture medium showed some of the original cytotoxic activity. Introduction of the N-terminal polypeptide alone by electroporation also induced cell death in HeLa cells, and even in the mouse melanoma MEB4 cells insensitive to pierisin-1. Thus, the N-terminal region has a principal role in the cytotoxicity of pierisin-1 inside mammalian cells. Analyses of incorporated pierisin-1 indicated that the entire protein, regardless of whether it consisted of a single polypeptide or two separate N- and C-terminal polypeptides, was incorporated into HeLa cells. However, neither of the terminal polypeptides was incorporated when each polypeptide was present separately. These findings indicate that the C-terminal region is important for the incorporation of pierisin-1. Moreover, presence of receptor for pierisin-1 in the lipid fraction of cell membrane was suggested. The cytotoxic effects of pierisin-1 were enhanced by previous treatment with trypsin, producing "nicked" pierisin-1. Generation of the N-terminal fragment in HeLa cells was detected after application of intact entire molecule of pierisin-1. From the above observations, it is suggested that after incorporation of pierisin-1 into the cell by interaction of its C-terminal region with the receptor in the cell membrane, the entire protein is cleaved into the N- and C-terminal fragments with intracellular protease, and the N-terminal fragment then exhibits cytotoxicity.

Processing of beta-secretase by furin and other members of the proprotein convertase family

The amyloid peptide is the main constituent of the amyloid plaques in brain of Alzheimer's disease patients. This peptide is generated from the amyloid precursor protein by two consecutive cleavages. Cleavage at the N terminus is performed by the recently discovered beta-secretase (Bace). This aspartyl protease contains a propeptide that has to be removed to obtain mature Bace. Furin and other members of the furin family of prohormone convertases are involved in this process. Surprisingly, beta-secretase activity, neither at the classical Asp(1) position nor at the Glu(11) position of amyloid precursor protein, seems to be controlled by this maturation step. Furthermore, we show that Glu(11) cleavage is a function of the expression level of Bace, that it depends on the membrane anchorage of Bace, and that Asp(1) cleavage can be followed by Glu(11) cleavage. Our data suggest that pro-Bace could be active as a beta-secretase in the early biosynthetic compartments of the cell and could be involved in the generation of the intracellular pool of the amyloid peptide. We conclude that modulation of the conversion of pro-Bace to mature Bace is not a relevant drug target to treat Alzheimer's disease.

The testicular germ-cell protease PC4 is also expressed in macrophage-like cells of the ovary

PC4 is a serine protease primarily expressed in spermatids. We have produced PC4-deficient mice carrying an insertion of the bacterial gene for beta galactosidase under the PC4 gene promoter. Male mice lacking PC4 (-/-) exhibit severely reduced fertility. Surprisingly, the fertility of female mice is also significantly diminished in these mutants (Mbikay et al., 1997. Proc. Natl. Acad. Sci. USA 94, 6842-6846). The aim of this study was to determine the site of PC4 expression in mouse ovaries. Using a histoenzymatic assay for beta-galactosidase, we show that the PC4 promoter can drive strong expression of this enzyme in the theca-interstitium and in degenerating corpora lutea of +/- ovaries. We also demonstrate that PC4 transcripts can be detected by RT-PCR in the ovaries of +/- and +/+ mice, but not in those of -/- mice. The cells expressing PC4 were macrophage-like, since they expressed the macrophage markers CD11b and F4/80, as well as interleukin 1beta and tumor necrosis factor alpha (TNFalpha). Expression of PC4 was also detected in the mouse macrophage RAW264.7 cell line. Interestingly, TNFalpha transcripts were 3-fold more abundant in ovarian macrophage-like cells from -/- mice than in those from +/+ mice, suggesting a constitutive state of activation of the mutant cells. An inverse relationship between PC4 expression and macrophage activation was also observed in RAW264.7 cells. When these cells were activated using bacterial lipopolysaccharide, the level of PC4 transcripts decreased, while that of TNFalpha increased. These observations identify PC4 as an enzyme that could influence ovarian physiology by affecting macrophage functions.

Implication of the proprotein convertases furin, PC5 and PC7 in the cleavage of surface glycoproteins of Hong Kong, Ebola and respiratory syncytial viruses: a comparative analysis with fluorogenic peptides

Fluorogenic peptides encompassing the processing sites of envelope glycoproteins of the infectious influenza A Hong Kong virus (HKV), Ebola virus (EBOV) and respiratory syncytial virus (RSV) were tested for cleavage by soluble recombinants of the proprotein convertases furin, PC5 and PC7. Kinetic studies with these intramolecularly quenched fluorogenic peptides revealed selective cleavages at the physiological dibasic sites. The HKV peptide is cleaved by both furin and PC5 with similar efficacy; in comparison, PC7 cleaves this substrate poorly. In contrast with the basic tetrapeptide insertion within the haemagglutinin sequence of HKV, two other dipeptide insertions revealed a poorer cleavage with a similar rank order of potency. These results demonstrate that the N-terminal RERR insertion to the wild-type avian RKKR downward arrow sequence is functionally significant, and suggest that the approx. 5-fold increase in cleavage efficacy contributes to the high infectivity of the H5N1 virus subtype. With regard to RSV peptide processing, PC7 is twice as effective as PC5 and furin. The EBOV peptide was processed with similar efficiency by the three enzymes. Our observations that all of these cleavages can be effectively inhibited by a plant andrographolide derivative at 250 microM or less might aid in the design of potent convertase inhibitors as alternative antiviral therapies.

Glucose-regulated insulin expression in diabetic rats

Retroviral vectors encoding glucose-responsive promoters driving furin expression may provide an amplified, glucose-regulated secretion of insulin. We constructed LhI*TFSN virus to encode a glucose-regulatable transforming growth factor alpha promoter controlling furin expression with a viral LTR promoter driving constitutive expression of furin-cleavable human proinsulin. Autologous BB rat vascular smooth muscle cells transduced with LhI*TFSN virus and cultured in 1.7 and 16.7 mM glucose secreted 50.7 +/- 3.2 and 136.0 +/- 11.0 microU (mean +/- SD) of insulin per 10(6) cells per day, respectively. After the onset of diabetes spontaneously diabetic congenic DR lyp/lyp BB rats received stomach implants containing 2 x 10(6) LhI*TFSN-transduced primary rat vascular smooth muscle cells. In eight treated rats there was a major reduction in insulin requirement to as low as 25% of pretreatment level for up to 3 months and one rat became insulin free without hypoglycemia. Intraperitoneal glucose tolerance tests (IPGTTs) in diabetic rats receiving control implants did not show the characteristic decline in blood glucose of normal rats after glucose administration. In contrast, diabetic rats receiving LhI*TFSN-transduced cells showed significant clearances of blood glucose. These data suggest clinically significant levels of glucose-regulated insulin delivery from implanted vascular smooth muscle cells transduced with LhI*TFSN vector.

Impaired prohormone convertases in Cpe(fat)/Cpe(fat) mice

A spontaneous point mutation in the coding region of the carboxypeptidase E (CPE) gene results in a loss of CPE activity that correlates with the development of late onset obesity (Nagert, J. K., Fricker, L. D., Varlamov, O., Nishina, P. M., Rouille, Y., Steiner, D. F., Carroll, R. J., Paigen, B. J., and Leiter, E. H. (1995) Nat. Genet. 10, 135-142). Examination of the level of neuropeptides in these mice showed a decrease in mature bioactive peptides as a result of a decrease in both carboxypeptidase and prohormone convertase activities. A defect in CPE is not expected to affect endoproteolytic processing. In this report we have addressed the mechanism of this unexpected finding by directly examining the expression of the major precursor processing endoproteases, prohormone convertases PC1 and PC2 in Cpe(fat) mice. We found that the levels of PC1 and PC2 are differentially altered in a number of brain regions and in the pituitary. Since these enzymes have been implicated in the generation of neuroendocrine peptides (dynorphin A-17, beta-endorphin, and alpha- melanocyte-stimulating hormone) involved in the control of feeding behavior and body weight, we compared the levels of these peptides in Cpe(fat) and wild type animals. We found a marked increase in the level of dynorphin A-17, a decrease in the level of alpha-melanocyte-stimulating hormone, and an alteration in the level of C-terminally processed beta-endorphin. These results suggest that the impairment in the level of these and other peptides involved in body weight regulation is mainly due to an alteration in carboxypeptidase and prohormone convertase activities and that this may lead to the development of obesity in these animals.

Evidence that furin is an authentic transforming growth factor-beta1-converting enzyme

Transforming growth factor (TGF)-beta1 plays an essential role in cell growth and differentiation. It is also considered as a gatekeeper of immune homeostasis with gene disruption leading to autoimmune and inflammatory diseases. TGF-beta1 is produced as an inactive precursor polypeptide that can be efficiently secreted but correct proteolytic cleavage is an essential step for its activation. Assessment of the cleavage site has revealed a unique R-H-R-R sequence reminiscent of proprotein convertase (PC) recognition motifs and has previously demonstrated that this PC-like cleavage site is correctly cleaved by furin, a member of the PC family. Here we report that among PC members, furin more closely satisfies the requirements needed to fulfill the role of a genuine TGF-beta1 convertase. Even though six members of the PC family have the ability to cleave TGF-beta1, ectopic expression of alpha(1)-antitrypsin Portland (alpha(1)-AT-PDX), a potent furin inhibitor, blocked 80% of TGF-beta1 processing mediated by endogenous enzymes as demonstrated in an in vitro digestion assay. Genetic complementation of a furin-deficient LoVo cell line with the wild-type gene restores the production of mature and bioactivable TGF-beta1. Moreover, both furin and TGF-beta are coordinately expressed and regulated in vitro and in vivo in the hematopoietic and immune system, an important tissue target. These results demonstrate for the first time that furin is an authentic and adaptive TGF-beta1-converting enzyme whereas other members of the PC family might substitute or supplement furin activity. Our study advances our comprehension of the complexity of the TGF-beta system and should facilitate the development of therapeutically useful TGF-beta inhibitors.

A predicted alpha -helix mediates targeting of the proprotein convertase PC1 to the regulated secretory pathway

The proprotein convertase PC1 is a protease whose activity is largely confined to the dense core secretory granules of neuroendocrine cells. Efficient processing of PC1 substrates in granules requires a mechanism that will both limit the activity of the enzyme to these organelles and promote its targeting to the nascent secretory granules. In the current study, we provide evidence that targeting of PC1 to secretory granules is mediated by alpha-helical structures in its C-terminal tail and, at least in part, is dependent on interactions with specific components of the secretory granule membrane.

Dynamic palmitoylation of lymphoma proprotein convertase prolongs its half-life, but is not essential for trans-Golgi network localization

Proprotein convertases are responsible for the endoproteolytic activation of proproteins in the secretory pathway. The most recently discovered member of this family, lymphoma proprotein convertase (LPC), is a type-I transmembrane protein. Previously, we have demonstrated that its cytoplasmic tail is palmitoylated. In this study, we have identified the two most proximal cysteine residues in the cytoplasmic tail as palmitoylation sites. Substitution of either cysteine residue by alanine interfered with palmitoylation of the other. Palmitoylation of LPC was found to be sensitive to the protein palmitoyltransferase inhibitor tunicamycin but not cerulenin. It was also insensitive to the drugs brefeldin A, monensin and cycloheximide, indicating that the modification occurs in a late exocytic or endocytic compartment. Turnover of palmitoylated LPC is significantly faster (t(1/2) approximately 50 min) than that of the LPC polypeptide backbone (t(1/2) approximately 3 h), suggesting that palmitoylation is reversible. Abrogation of palmitoylation reduced the half-life of the LPC protein, but did not affect steady-state localization of LPC in the trans-Golgi network. Finally, LPC could not be detected in detergent-resistant membrane rafts. Taken together, these results suggest that dynamic palmitoylation of LPC is important for stability, but does not function as a dominant trafficking signal.

Glucose-dependent insulin release from genetically engineered K cells

Genetic engineering of non-beta cells to release insulin upon feeding could be a therapeutic modality for patients with diabetes. A tumor-derived K-cell line was induced to produce human insulin by providing the cells with the human insulin gene linked to the 5'-regulatory region of the gene encoding glucose-dependent insulinotropic polypeptide (GIP). Mice expressing this transgene produced human insulin specifically in gut K cells. This insulin protected the mice from developing diabetes and maintained glucose tolerance after destruction of the native insulin-producing beta cells.

Identification of a novel consensus sequence at the cleavage site of the Lassa virus glycoprotein

The Lassa virus glycoprotein consists of an amino-terminal and a carboxy-terminal cleavage fragment designated GP-1 and GP-2, respectively, that are derived by proteolysis from the precursor GP-C. The membrane-anchored GP-2 obtained from purified virions of the Josiah strain revealed the N-terminal tripeptide GTF(262) when analyzed by Edman degradation. Upstream of this site, GP-C contains the tetrapeptide sequence RRLL(259), which is conserved in all Lassa virus isolates published to date. Systematic mutational analysis of vector-expressed GP-C revealed that the motif R-X (L/I/V)-L(259) (where X stands for L, I, or V) is essential for cleavage of the peptide bond between leucine(259) and glycine(260). This cleavage motif is homologous to the consensus sequence recognized by a novel class of cellular endoproteases which have so far not been implicated in the processing of viral glycoproteins.

Expression and localization of prohormone convertase 1/3 (SPC3) in porcine ovary

Tissue distribution and cellular localization of PC1/3 mRNA in porcine tissues were examined by ribonuclease protection assay and in situ hybridization. PC1/3 mRNA was detected mainly in the corpus luteum of pregnant sow and brain. Within the ovary, PC1/3 and relaxin transcripts colocalized within large luteal cells. Levels of PC1/3 transcripts in corpora lutea increased as gestation advanced, parallel with an observed increase in relaxin transcripts. A role for PC1/3 in proprotein processing in the ovary is discussed.

Polyarginines are potent furin inhibitors

The ubiquitous serine endoprotease furin has been implicated in the activation of bacterial toxins and viral glycoproteins as well as in the metastatic progression of certain tumors. Although high molecular mass bioengineered serpin inhibitors have been well characterized, no small nontoxic nanomolar inhibitors have been reported to date. Here we describe the identification of such inhibitors using positional scanning amidated and acetylated synthetic l- and d-hexapeptide combinatorial libraries. The results indicated that l-Arg or l-Lys in all positions generated the most potent inhibitors. However, further investigation revealed that the peptide terminating groups hindered inhibition. Consequently, a series of non-amidated and acetylated polyarginines was synthesized. The most potent inhibitor identified, nona-l-arginine, had a K(i) for furin of 40 nm. The K(i) values for the related convertases PACE4 and prohormone convertase-1 (PC1) were 110 nm and 2.5 microm, respectively. Although nona-l-arginine was cleaved by furin, the major products after a 6-h incubation at 37 degrees C were hexa- and hepta-l-arginines, both of which retained the great majority of their potency and specificity against furin. Hexa-d-arginine was as potent and specific a furin inhibitor as hexa-l-arginine (K(i) values of hexa-d-arginine: 106 nm, 580 nm, and 13.2 microm for furin, PACE4, and PC1, respectively). PC2 was not inhibited by any polyarginine tested; indeed, PC2 showed an increase in activity of up to 140% of the control in the presence of l-polyarginines. Data are also presented that show extended subsite recognition by furin and PC2. Whereas N-terminal acetylation was found to reduce the inhibitory potency of the l-hexapeptide LLRVKR against furin 8-fold, C-terminal amidation reduced the potency < 2-fold. Conversely, N-terminal acetylation increased the potency against PC2 nearly 3-fold, whereas C-terminal amidation of the same peptide increased the potency by a factor of 1.6. Our data indicate that non-acetylated, poly-d-arginine-derived molecules may represent excellent lead compounds for the development of therapeutically useful furin inhibitors.

Maturation of HIV envelope glycoprotein precursors by cellular endoproteases

The entry of enveloped viruses into its host cells is a crucial step for the propagation of viral infection. The envelope glycoprotein complex controls viral tropism and promotes the membrane fusion process. The surface glycoproteins of enveloped viruses are synthesized as inactive precursors and sorted through the constitutive secretory pathway of the infected cells. To be infectious, most of the viruses require viral envelope glycoprotein maturation by host cell endoproteases. In spite of the strong variability of primary sequences observed within different viral envelope glycoproteins, the endoproteolytical cleavage occurs mainly in a highly conserved domain at the carboxy terminus of the basic consensus sequence (Arg-X-Lys/Arg-Arg downward arrow). The same consensus sequence is recognized by the kexin/subtilisin-like serine proteinases (so called convertases) in many cellular substrates such as prohormones, proprotein of receptors, plasma proteins, growth factors and bacterial toxins. Therefore, several groups of investigators have evaluated the implication of convertases in viral envelope glycoprotein cleavage. Using the vaccinia virus overexpression system, furin was first shown to mediate the proteolytic maturation of both human immunodeficiency virus (HIV-1) and influenza virus envelope glycoproteins. In vitro studies demonstrated that purified convertases directly and specifically cleave viral envelope glycoproteins. Although these studies suggested the participation of several enzymes belonging to the convertases family, recent data suggest that other protease families may also participate in the HIV envelope glycoprotein processing. Their role in the physiological maturation process is still hypothetical and the molecular mechanism of the cleavage is not well documented. Crystallization of the hemagglutinin precursor (HA0) of influenza virus allowed further understanding of the molecular interaction between viral precursors and the cellular endoproteases. Furthermore, relationships between differential pathogenicity of influenza strains and their susceptibility to cleavage are molecularly funded. Here we review the most recent data and recent insights demonstrating the crucial role played by this activation step in virus infectivity. We discuss the cellular endoproteases that are implicated in HIV gp160 endoproteolytical maturation into gp120 and gp41.

Cysteine array matrix metalloproteinase (CA-MMP)/MMP-23 is a type II transmembrane matrix metalloproteinase regulated by a single cleavage for both secretion and activation

Matrix metalloproteinases characterized so far are either secreted or membrane anchored via a type I transmembrane domain or a glycosylphosphatidylinositol linkage. Lacking either membrane-anchoring mechanism, the newly discovered CA-MMP/MMP-23 was reported to be expressed as a cell-associated protein. In this report, we present evidence that CA-MMP is expressed as an integral membrane zymogen with an N-terminal signal anchor, and secreted as a fully processed mature enzyme. We further demonstrate that L(20)GAALSGLCLLSALALL(36) is required for this unique membrane localization as a signal anchor and its secretion is regulated by a proprotein convertase motif RRRR(79) sandwiched between its pro- and catalytic domains. Thus, CA-MMP is a type II transmembrane MMP that can be regulated by a single proteolytic cleavage for both activation and secretion, establishing a novel paradigm for protein trafficking and processing within the secretory pathway.

Neuropeptide amidation: cloning of a bifunctional alpha-amidating enzyme from Aplysia

One of the most common mechanisms of posttranslational modifications to generate biologically active (neuro)peptides is the process of peptide alpha-amidation. The only enzyme known to catalyze this important modification is peptidylglycine alpha-amidating monooxygenase (PAM): a (bifunctional) zymogen, giving rise to a monooxygenase (PHM) and a lyase (PAL). The highly peptidergic central nervous system and endocrine system of the marine mollusk Aplysia has homologs of various mammalian peptide processing enzymes, including furin, Afurin2, prohormone convertase 1 (PC1), PC2, carboxypeptidase E (CPE) and CPD. Previously, it has been shown that the abdominal ganglion of Aplysia, which contains approximately 800 peptidergic bag cell neurons, contains the highest specific alpha-amidating activity. We have identified and cloned multiple overlapping central nervous system and bag cell cDNAs that encode a predicted 748-residue protein that is a member of the PAM family. The protein sequence contains the contiguous sequence of the catalytic domains of PHM and PAL, clearly demonstrating the existence of bifunctional Aplysia PAM, the first invertebrate PAM zymogen with an organization similar to that in vertebrates. None of the characterized clones encoded the so-called exon A domain between the PHM and PAL domains. Furthermore, in a specific search by reverse transcription-polymerase chain reaction of RNA from multiple tissues we could only detect exon A-less transcripts. PAM expression was detected in the central nervous system, and in several endocrine and exocrine organs. Aplysia PAM is a candidate prohormone processing enzyme that plays an important role in the processing of Aplysia prohormones in the secretory pathway.

Proteolytic activation of recombinant pro-memapsin 2 (pro-beta-secretase) studied with new fluorogenic substrates

Memapsin 2 (beta-secretase), a membrane-anchored aspartic protease, is involved in the cleavage of beta-amyloid precursor protein to form beta-amyloid peptide. The primary structure of memapsin 2 suggests that it is synthesized in vivo as pro-memapsin 2 and converted to memapsin 2 by an activating protease [Lin et al. (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 1456-1460]. To simulate this activation mechanism and to produce stable mature memapsin 2 for kinetic/specificity studies, we have investigated the activation of recombinant pro-memapsin 2 by several proteases with trypsin-like specificity. Clostripain, kallikrein, and trypsin increased the activity of pro-memapsin 2. Clostripain activation was accompanied by the cleavage of the pro region to form mainly two activation products, Leu(30p)- and Gly(45p)-memapsin 2. Another activation product, Leu(28p)-memapsin 2, was also purified. Kinetics of the activated memapsin 2 were compared with pro-memapsin 2 using two new fluorogenic substrates, Arg-Glu(5-[(2-aminoethyl)amino]naphthalene-1-sulfonic acid (EDANS))-Glu-Val-Asn-Leu-Asp-Ala-Glu-Phe-Lys(4-(4-dimethylaminophe nyl azo)benzoic acid (DABCYL))-Arg and (7-methoxycoumarin-4-yl)acetyl (MCA))-Ser-Glu-Val-Asn-Leu-Asp-Ala-Glu-Phe-Lys(2,4-dinitrophenyl (DNP)). These results establish that the activity of pro-memapsin 2 stems from a part-time and reversible uncovering of its active site by its pro region. Proteolytic removal of part of the pro-peptide at Leu(28p) or Gly(45p), which diminishes the affinity of the shortened pro-peptide to the active site, results in activated memapsin 2. These results also suggest that Glu(33p)-memapsin 2 observed in the cells expressing this enzyme [Vassar et al. (1999) Science 286, 735-741; Yan et al. (1999) Nature 402, 533-537] is an active intermediate of in vivo activation, or that the peptide Glu(33p)-Arg(44p) may serve a regulatory role.

Cathepsin-B fusion proteins misroute secretory protein partners such as the proprotein convertase PC2-7B2 complex toward the lysosomal degradation pathways

A general strategy is presented for the dominant negative reduction in the levels of heterodimeric soluble proteins within the secretory pathway through fusion of one of its partners C-terminal to the lysosomal enzyme cathepsin B (CB). Stable transfectants of CB-7B2 chimeras in AT20 cells result in a drastic reduction of the endogenous levels of its partner, the proprotein convertase PC2. This dominant negative suppressive effect requires active CB. It was partially reversed by NH(4)Cl, the cell-permeable CB inhibitor CA-074Me, but not by the proteasome inhibitor Lactacystin, suggesting the potential participation of the lysosomal/endosomal degradative pathway in this process.

N-arginine dibasic convertase (nardilysin) isoforms are soluble dibasic-specific metalloendopeptidases that localize in the cytoplasm and at the cell surface

N-arginine (R) dibasic (NRD) convertase (nardilysin; EC 3.4.24.61), a metalloendopeptidase of the M16 family, specifically cleaves peptide substrates at the N-terminus of arginines in dibasic motifs in vitro. In rat testis, the enzyme localizes within the cytoplasm of spermatids and associates with microtubules of the manchette and axoneme. NRD1 and NRD2 convertases, two NRD convertase isoforms, differ by the absence (isoform 1) or presence (isoform 2) of a 68-amino acid insertion close to the active site. In this study, we overexpressed both isoforms, either by vaccinia virus infection of BSC40 cells or transfection of COS-7 cells. The partially purified enzymes exhibit very similar biochemical and enzymic properties. Microsequencing revealed that NRD convertase is N-terminally processed. Results of immunocytofluorescence, immunoelectron microscopy and subcellular fractionation studies argue in favour of a primary cytosolic localization of both peptidases. Although the putative signal peptide did not direct NRD convertase into microsomes in an in vitro translation assay, biotinylation experiments clearly showed the presence of both isoforms at the cell surface. In conclusion, although most known processing events at pairs of basic residues are achieved by proprotein convertases within the secretory pathway, NRD convertase may fulfil a similar function in the cytoplasm and/or at the cell surface.

Mutational analysis of bli-4/kpc-4 reveals critical residues required for proprotein convertase function in C. elegans

Kex2/subtilisin-like proteinase activity is required for the production of the adult cuticle in the nematode Caenorhabditis elegans. Deletion of the carboxy termini of four of the bli-4/kpc-4 convertase isoforms results in blistering of the adult cuticle. The blisters vary in severity (expressivity) and are not evident in all individuals (reduced penetrance). We have isolated 13 bli-4/kpc-4 mutants that arrest development in late embryogenesis. Using a PCR-based heteroduplex technique, we have identified nucleotide changes responsible for eight of these lethal mutations. The lesions reside within the first 12 exons that are shared by all of the bli-4/kpc-4 gene products, with the majority of mutations clustered within the protease domain. This finding suggests that the protease domain represents a large mutable target. Among these mutations, allele h384 represents a molecular null mutant in which the catalytically essential serine residue (Ser415) is replaced by phenylalanine. Novel missense mutations that change the identity of amino acids evolutionary conserved in all kex2/subtilisin-convertases highlight critical residues essential for activity. We examined the functional activity of BLI-4/KPC-4 products expressed from several lethal mutants by testing their effect on the variable penetrance of blistering exhibited by the e937 allele. We found that the combination of a bli-4/kpc-4 lethal mutation in trans to the bli-4(e937) mutation was sufficient to cause severe blistering in heteroallelic progeny, even in the presence of a known dominant suppressor.

Cutting edge: introduction of an endopeptidase cleavage motif into a determinant flanking region of hen egg lysozyme results in enhanced T cell determinant display

The choice of which determinants of a whole Ag will be presented on cell surface MHC class II molecules after uptake and processing by APC is the result of the interplay between structural characteristics of the Ag and the processing machinery of the APC. In this study, we demonstrate that introduction of a dibasic motif adjacent to a subdominant determinant enhances the presentation of this determinant from the whole molecule. This is the first report showing that a single amino acid substitution in a whole Ag, designed to introduce an endopeptidase recognition site, enhances display of class II-restricted determinants, most likely by creating a peptide chain cleavage in the antigenic molecule. Our findings have important implications for the understanding of immunodominance and for vaccine design.

Applications of plastic embedding to electron microscopic immunocytochemistry and in situ hybridization in observations of production and secretion of peptide hormones

Plastic embedding has been used to localize various antigens in conjunction with immunohistochemistry. Peptide hormones have been among the antigens that have been studied extensively. Recent application of water-soluble plastics such as LR White and Lowicryl has extended the ranges of detectable antigens and enabled the observation of antigen-antigen or mRNA-antigen combinations. This review article deals with technical aspects, procedures, and applications in endocrine cells.

VGF: a novel role for this neuronal and neuroendocrine polypeptide in the regulation of energy balance

Insight into the mechanisms of action of neurotrophic growth factors has been obtained through the identification and characterization of gene products that are regulated or modified at the transcriptional, translational, and/or posttranslational level in response to neurotrophin treatment. VGF (non-acronymic) was identified approximately 15 years ago as a nerve growth factor (NGF)-regulated transcript in rat PC12 pheochromocytoma cells. Subsequent studies have demonstrated that neurotrophins such as NGF and brain-derived neurotrophic factor induce vgf gene expression relatively rapidly in PC12 cells and cultured cortical neurons, respectively, in comparison to less robust regulation by epidermal growth factor (EGF) and insulin, growth factors which do not trigger the neuronal differentiation of PC12 cells. vgf gene expression is stimulated in vitro by NGF and the ras/map kinase signaling cascade through a CREB-dependent mechanism, while in vivo, VGF mRNA levels are regulated by neuronal activity, including long-term potentiation, seizure, and injury. Both the mRNA and encoded approximately 68-kDa protein (VGF) are selectively synthesized in neuroendocrine and neuronal cells. The predicted VGF sequence is rich in paired basic amino acid residues that are potential sites for proteolytic processing, and VGF undergoes regulated release from dense core secretory vesicles. Although VGF mRNA is synthesized widely, by neurons in the brain, spinal cord, and peripheral nervous system, its expression is particularly abundant in the hypothalamus. In addition, VGF peptides are found in hypophysial, adrenal medullary, gastrointestinal, and pancreatic endocrine cells, suggesting important neuroendocrine functions. Recent analysis of VGF knockout mice indeed demonstrates that VGF plays a critical role in the control of energy homeostasis. VGF knockout mice are thin, small, hypermetabolic, hyperactive, and relatively infertile, with markedly reduced leptin levels and fat stores and altered hypothalamic pro-opiomelanocortin, neuropeptide Y, and agouti-related peptide expression. Coupled with the demonstration that VGF mRNA levels are induced in the normal mouse hypothalamic arcuate nuclei in response to fasting, important central and peripheral roles for VGF in the regulation of metabolism are suggested. Here we review previous studies of VGF in the broader context of its newly recognized role in the control of energy balance and propose several models and experimental approaches that may better define the mechanisms of action of VGF.

Interaction of Drosophila melanogaster prohormone convertase 2 and 7B2. Insect cell-specific processing and secretion

The prohormone convertases (PCs) are an evolutionarily ancient group of proteases required for the maturation of neuropeptide and peptide hormone precursors. In Drosophila melanogaster, the homolog of prohormone convertase 2, dPC2 (amontillado), is required for normal hatching behavior, and immunoblotting data indicate that flies express 80- and 75-kDa forms of this protein. Because mouse PC2 (mPC2) requires 7B2, a helper protein for productive maturation, we searched the fly data base for the 7B2 signature motif PPNPCP and identified an expressed sequence tag clone encoding the entire open reading frame for this protein. dPC2 and d7B2 cDNAs were subcloned into expression vectors for transfection into HEK-293 cells; mPC2 and rat 7B2 were used as controls. Although active mPC2 was detected in medium in the presence of either d7B2 or r7B2, dPC2 showed no proteolytic activity upon coexpression of either d7B2 or r7B2. Labeling experiments showed that dPC2 was synthesized but not secreted from HEK-293 cells. However, when dPC2 and either d7B2 or r7B2 were coexpressed in Drosophila S2 cells, abundant immunoreactive dPC2 was secreted into the medium, coincident with the appearance of PC2 activity. Expression and secretion of dPC2 enzyme activity thus appears to require insect cell-specific posttranslational processing events. The significant differences in the cell biology of the insect and mammalian enzymes, with 7B2 absolutely required for secretion of dPC2 and zymogen conversion occurring intracellularly in the case of dPC2 but not mPC2, support the idea that the Drosophila enzyme has specific requirements for maturation and secretion that can be met only in insect cells.

Neuropeptides--an overview

The present article provides a brief overview of various aspects on neuropeptides, emphasizing their multitude and their wide distribution in both the peripheral and central nervous system. Interestingly, neuropeptides are also expressed in various types of glial cells under normal and experimental conditions. The recent identification of, often multiple, receptor subtypes for each peptide, as well as the development of peptide antagonists, have provided an experimental framework to explore functional roles of neuropeptides. A characteristic of neuropeptides is the plasticity in their expression, reflecting the fact that release has to be compensated by de novo synthesis at the cell body level. In several systems peptides can be expressed at very low levels normally but are upregulated in response to, for example, nerve injury. The fact that neuropeptides virtually always coexist with one or more classic transmitters suggests that they are involved in modulatory processes and probably in many other types of functions, for example exerting trophic effects. Recent studies employing transgene technology have provided some information on their functional role, although compensatory mechanisms in all probability could disguise even a well defined action. It has been recognized that both 'old' and newly discovered peptides may be involved in the regulation of food intake. Recently the first disease-related mutation in a peptidergic system has been identified, and clinical efficacy of a substance P antagonist for treatment of depression has been reported. Taken together it seems that peptides may play a role particularly when the nervous system is stressed, challenged or afflicted by disease, and that peptidergic systems may, therefore, be targets for novel therapeutic strategies.

Comparative characterization of two forms of recombinant human SPC1 secreted from Schneider 2 cells

SPC1 (furin/PACE), an enzyme belonging to the S8 group of serine endoproteases, is a type I integral membrane protein that catalyzes the processing of a multitude of precursor proteins. We report here the use of transfected Drosophila melanogaster Schneider 2 cells to produce milligram amounts of two forms of recombinant human SPC1. In order to investigate the role of the cysteine-rich region (CRR) of SPC1, we compared the biochemical and enzymatic properties of hSPC1/714 that has the C-terminal tail and transmembrane region of the native enzyme removed with that of hSPC1/585 which had, in addition, the CRR deleted. Two stable cell lines were established. The S2-hSPC1/714 line secreted a major form of apparent molecular weight of 83 kDa and a minor form of 80 kDa whereas the S2-hSPC1/585 line secreted a single 59-kDa protein. PNGase F treatment of the different forms demonstrated that the enzymes were glycosylated. Automated NH(2)-terminal sequencing revealed that all purified forms resulted from processing at the expected zymogen activation site. Removal of the CRR resulted in a broadening of the enzyme's pH range, a shift of K(0.5) for Ca(2+), and a shorter enzymatic half-life when compared to the longer form, which suggest that the CRR of hSPC1 may help in stabilizing the enzyme's proteolytic activity. The use of this high-level expression system will meet the demand for material necessary to perform biochemical and structural studies that are needed to further our understanding of this and other SPCs at the molecular level.

A look at the Caenorhabditis elegans Kex2/Subtilisin-like proprotein convertase family

Significant advances have recently been made in our understanding of the mechanisms of activation of proteins that require processing. Often this involves endoproteolytic cleavage of precursor forms at basic residues, and is carried out by a group of serine endoproteinases, termed the proprotein convertases. In mammals, seven different convertases have been identified to date. These act in both the regulated secretory pathway for the processing of prohormones and proneuropeptides and in the constitutive secretory pathway, in which a variety of proproteins are activated endoproteolytically. The recently completed sequence of the nematode Caenorhabditis elegans genome affords a unique opportunity to examine the entire proprotein convertase family in a multicellular organism. Here we review the nature of the family, emphasising the structural features, characteristic of the four nematode genes, that supply all of the necessary functions unique to this group of serine endoproteinases. Studies of the C. elegans genes not only provide important information about the evaluation of this gene family but should help to illuminate the roles of these proteins in mammalian systems. BioEssays 22:545-553, 2000.

Identification of INSL6, a new member of the insulin family that is expressed in the testis of the human and rat

A new member of the insulin gene family (INSL6) was identified from an Expressed Sequence Tag database through a search for proteins containing the insulin family B-chain cysteine motif. Human and rat INSL6 encoded polypeptides of 213 and 188 amino acids, respectively. These orthologous sequences contained the B-chain, C-peptide, and A-chain motif found in other members of the insulin family. Human INSL6 was 43% identical to human relaxin H2 in the B- and A-chain regions. As with other family members, human and rat INSL6 had predicted dibasic sequences at the junction of the C-peptide and A-chain. Human INSL6 sequence had an additional dibasic site near the C-terminus of the A-chain. The presence of a single basic residue at the predicted junction of the B-chain and C-peptide suggests that multiple prohormone convertases are required to produce the fully mature hormone. INSL6 was found to be expressed at high levels in the testis as determined by Northern blot analysis and specifically within the seminiferous tubules in spermatocytes and round spermatids as detected by in situ hybridization analysis. Radiation hybrid mapping placed the human INSL6 locus at chromosome 9p24 near the placenta insulin-like homologue INSL4 and the autosomal testis-determining factor (TDFA) locus.

Conversion and storage of somatostatin are established before response to secretagogue stimuli in P19 neurons

In mature neurons, neuropeptides are synthesized via limited proteolysis of propolypeptides by convertases. The bioactive peptides are then stored in secretory granules until they are released extracellularly upon the induction of a fusion between granules and the plasma membrane, in response to secretagogues. We used the mouse P19 embryonic carcinoma cells as a model to determine if the capacities to convert and store neuropeptides and to secrete them in a regulated fashion are established coordinately during neuronal differentiation. We have previously shown that both undifferentiated P19 cells and their neuronal derivatives express the largely distributed furin, PACE4 and PC5 convertases, whereas only neuronal derivatives express the neuroendocrine convertase PC2. In addition, undifferentiated cells displayed furin- rather than PC2-like converting capacities. The present work demonstrates that day 8 P19 neurons mainly convert prosomatostatin (proSS) to somatostatin-14 (SS-14) using HPLC and radioimmunoassay (RIA) analyses, indicating that P19 cells acquire PC2-like converting capacities as a consequence of neuronal differentiation. SS-14 was predominantly intracellular in neuronal cells which were shown to express several granins, markers of granules, by Western blotting. However, cell membrane depolarization with 50 mM K+, a general secretagogue stimulus, evoked the release of SS-14 by day 12, but not by day 8, P19 neurons. The results thus demonstrate that capacities to convert and store neuropeptides can be established before coupling of stimulus-secretion during neuronal differentiation.

Damage-induced neuronal endopeptidase (DINE) is a unique metallopeptidase expressed in response to neuronal damage and activates superoxide scavengers

We isolated a membrane-bound metallopeptidase, DINE (damage-induced neuronal endopeptidase), by differential display PCR using rat normal and axotomized hypoglossal nuclei. The most marked properties of DINE were neuron-specific expression and a striking response to axonal injury in both the central nervous system and peripheral nervous system. For instance, cranial and spinal nerve transection, ischemia, corpus callosum transection, and colchicine treatment increased DINE mRNA expression in the injured neurons, whereas kainate-induced hyperexcitation, immobilization, and osmotic stress failed to up-regulate DINE mRNA. Expression of DINE in COS cells partially inhibited C2-ceramide-induced apoptosis, probably because of the activation of antioxidant enzymes such as Cu/Zn-superoxide dismutase, Mn-superoxide dismutase, and glutathione peroxidase through the proteolytic activity of DINE. These data provide insight into the mechanism of how injured neurons protect themselves against neuronal death.

The PC6B cytoplasmic domain contains two acidic clusters that direct sorting to distinct trans-Golgi network/endosomal compartments

The mammalian proprotein convertases (PCs) are a family of secretory pathway enzymes that catalyze the endoproteolytic maturation of peptide hormones and many bioactive proteins. Two PCs, furin and PC6B, are broadly expressed and share very similar cleavage site specificities, suggesting that they may be functionally redundant. However, germline knockout studies show that they are not. Here we report the distinct subcellular localization of PC6B and identify the sorting information within its cytoplasmic domain (cd). We show that in neuroendocrine cells, PC6B is localized to a paranuclear, brefeldin A-dispersible, BaCl(2)-responsive post-Golgi network (TGN) compartment distinct from furin and TGN38. The 88-amino acid PC6B-cd contains sorting information sufficient to direct reporter proteins to the same compartment as full-length PC6B. Mutational analysis indicates that endocytosis is predominantly directed by a canonical tyrosine-based motif (Tyr(1802)GluLysLeu). Truncation and sufficiency studies reveal that two clusters of acidic amino acids (ACs) within the PC6B-cd contain differential sorting information. The membrane-proximal AC (AC1) directs TGN localization and interacts with the TGN sorting protein PACS-1. The membrane-distal AC (AC2) promotes a localization characteristic of the full-length PC6B-cd. Our results demonstrate that AC motifs can target proteins to distinct TGN/endosomal compartments and indicate that the AC-mediated localization of PC6B and furin contribute to their distinct roles in vivo.

Regulation of pancreatic PC1 and PC2 associated with increased glucagon-like peptide 1 in diabetic rats

The pancreatic processing enzymes, PC1 and PC2, convert proinsulin to insulin and convert proglucagon to glucagon and glucagon-like peptide 1 (GLP-1). We examined the effect of streptozotocin (STZ) treatment on the regulation of these enzymes and the production of insulin, glucagon, and GLP-1 in the rat. Pancreatic PC1 and PC2 mRNA increased >2-fold and >4-fold, respectively, in rats receiving intraperitoneal STZ (50 mg/kg) daily for 5 days. Immunocytochemistry revealed that, although pancreatic islet cells in the STZ-treated rats were sparse and atrophic PC1, PC2, glucagon, and GLP-1 immunoreactivity increased dramatically in the remaining islet cells. Heightened PC1 and PC2 expression was seen in cells expressing glucagon but not in insulin-expressing cells. Furthermore, in STZ-treated rats, bioactive GLP-1(7-36 amide) accumulated in pancreatic extracts and serum 3- and 2.5-fold, respectively, over control animals. This treatment also caused a 2-fold increase in the ratio of amidated forms of GLP-1 immunoreactivity to total glucagon immunoreactivity in the pancreas but did not affect the ratio of proinsulin to insulin. We conclude that hyperglycemic rats have an increased expression of prohormone converting enzymes in islet alpha cells, leading to an increase in amidated GLP-1, which can then exert an insulinotropic effect on the remaining beta cells.

The pro-alpha3(V) collagen chain. Complete primary structure, expression domains in adult and developing tissues, and comparison to the structures and expression domains of the other types V and XI procollagen chains

The low abundance fibrillar collagen type V is widely distributed in tissues as an alpha1(V)(2)alpha2(V) heterotrimer that helps regulate the diameters of fibrils of the abundant collagen type I. Mutations in the alpha1(V) and alpha2(V) chain genes have been identified in some cases of classical Ehlers-Danlos syndrome (EDS), in which aberrant collagen fibrils are associated with connective tissue fragility, particularly in skin and joints. Type V collagen also exists as an alpha1(V)alpha2(V)alpha3(V) heterotrimer that has remained poorly characterized chiefly due to inability to obtain the complete primary structure or nucleic acid probes for the alpha3(V) chain or its biosynthetic precursor, pro-alpha3(V). Here we provide human and mouse full-length pro-alpha3(V) sequences. Pro-alpha3(V) is shown to be closely related to the alpha1(V) precursor, pro-alpha1(V), but with marked differences in N-propeptide sequences, and collagenous domain features that provide insights into the low melting temperature of alpha1(V)alpha2(V)alpha3(V) heterotrimers, lack of heparin binding by alpha3(V) chains and the possibility that alpha1(V)alpha2(V)alpha3(V) heterotrimers are incorporated into heterotypic fibrils. In situ hybridization of mouse embryos detects alpha3(V) expression primarily in the epimysial sheaths of developing muscles and within nascent ligaments adjacent to forming bones and in joints. This distribution, and the association of alpha1(V), alpha2(V), and alpha3(V) chains in heterotrimers, suggests the human alpha3(V) gene COL5A3 as a candidate locus for at least some cases of classical EDS in which the alpha1(V) and alpha2(V) genes have been excluded, and for at least some cases of the hypermobility type of EDS, a condition marked by gross joint laxity and chronic musculoskeletal pain. COL5A3 is mapped to 19p13.2 near a polymorphic marker that should be useful in analyzing linkage with EDS and other disease phenotypes.

A protein-based therapeutic for human cytomegalovirus infection

Current antiviral strategies target viral gene products. Although initially successful, their severe toxicity and susceptibility to circumvention by the generation of drug-resistant variants limit their usefulness. By contrast, the central role of the host cell serine endoprotease furin in the proteolytic activation of numerous pathogens points to the endoprotease as a strategic target for therapeutics. Herein, we show that the production of infectious human cytomegalovirus is dramatically reduced by exogenous addition of a bioengineered serpin, alpha(1)-PDX. This protein is a potent and selective furin inhibitor (K(i) = 0.6 nM) and is 10-fold more effective than currently used antiherpetic agents in cell-culture models. The requirement of furin for the processing of envelope glycoproteins from many pathogenic viruses and for the activation of several bacterial toxins suggests that selective inhibitors of furin have potential as broad-based anti-pathogens.

Coordinate regulation of neuroendocrine convertase PC2 and peptide 7B2 in P19 neurons

Convertases are proteases responsible for the bioactivation of many proteins and peptides having a potential role in ontogenesis. As a model to study regulation of convertases in embryo, we use the P19 embryonal carcinoma cell line, which can differentiate into various cell types. The expression of convertase PC2 and its specific binding peptide 7B2 are co-induced during neuronal differentiation of P19 cells. We investigated the possibility that expression of both proteins may be coregulated by T3 and dexamethasone, activators of nuclear receptors, isobutylmethylxanthine, and dibutyryl cAMP, activators of protein kinase A, and phorbol 12-myristate 13-acetate, an activator of protein kinase C. Western blotting results show that expression of PC2 and 7B2 can be upregulated by modulators of the protein kinases, and upregulation needs not be strictly stoichiometric.

LeSBT1, a subtilase from tomato plants. Overexpression in insect cells, purification, and characterization

The cDNA of a tomato subtilase designated LeSBT1 was cloned from a tomato flower cDNA library. The deduced amino acid sequence indicated for LeSBT1 the structure of a prepro-protein targeted to the secretory pathway by virtue of an amino-terminal signal peptide. LeSBT1 was expressed in the baculovirus/insect cell system and a processed 73-kDa form of LeSBT1, lacking both signal peptide and prodomain, was purified to homogeneity from culture supernatants. This 73-kDa LeSBT1, however, lacked proteolytic activity. Zymogen activation to yield 68-kDa LeSBT1 required the additional processing of an amino-terminal autoinhibitory peptide in a strictly pH-dependent manner. Mature 68-kDa LeSBT1 showed highest activity at acidic pH consistent with its presumed localization in the apoplast of the plant cell. In comparison to other plant subtilases, LeSBT1 exhibited a narrower substrate specificity in that it cleaves only polypeptide substrates preferentially but not exclusively carboxyl-terminal of glutamine residues. The possible involvement of LeSBT1 in selective proprotein processing is discussed with reference to the related mammalian proprotein convertases.

Biosynthesis and enzymatic characterization of human SKI-1/S1P and the processing of its inhibitory prosegment

Biochemical and enzymatic characterization of the novel human subtilase hSKI-1 was carried out in various cell lines. Within the endoplasmic reticulum of LoVo cells, proSKI-1 is converted to SKI-1 by processing of its prosegment into 26-, 24-, 14-, 10-, and 8-kDa products, some of which remain tightly associated with the enzyme. N-terminal sequencing and mass spectrometric analysis were used to map the cleavage sites of the most abundant fragments, which were confirmed by synthetic peptide processing. To characterize its in vitro enzymatic properties, we generated a secreted form of SKI-1. Our data demonstrate that SKI-1 is a Ca(2+)-dependent proteinase exhibiting optimal cleavage at pH 6.5. We present evidence that SKI-1 processes peptides mimicking the cleavage sites of the SKI-1 prosegment, pro-brain-derived neurotrophic factor, and the sterol regulatory element-binding protein SREBP-2. Among the candidate peptides encompassing sections of the SKI-1 prosegment, the RSLK(137)- and RRLL(186)-containing peptides were best cleaved by this enzyme. Mutagenesis of the latter peptide allowed us to develop an efficiently processed SKI-1 substrate and to assess the importance of several P and P' residues. Finally, we demonstrate that, in vitro, recombinant prosegments of SKI-1 inhibit its activity with apparent inhibitor constants of 100-200 nM.

Maturation and specificity of Plasmodium falciparum subtilisin-like protease-1, a malaria merozoite subtilisin-like serine protease

Plasmodium falciparum subtilisin-like protease-1 (PfSUB-1) is a protein belonging to the subtilisin-like superfamily of serine proteases (subtilases). PfSUB-1 undergoes extensive posttranslational proteolytic processing. The primary translation product is converted in the parasite endoplasmic reticulum to p54. This is further processed to p47, which accumulates in secretory organelles within the merozoite. Here, we present a detailed study of this processing. In vitro translated PfSUB-1 showed no capacity to undergo autocatalytic processing. However, parasite extracts contain a protease that cleaves the in vitro translated proprotein between Asp(219) and Asn(220) to form two products of 31 (p31) and 54 kDa; the latter was indistinguishable from authentic p54 and remained complexed with p31 in a noncovalent interaction characteristic of that between a subtilase prodomain and its cognate catalytic domain. Cross-linking studies showed that this complex also exists in the parasite. Expression of PfSUB-1 in recombinant baculovirus also resulted in processing to p54. Mutation of the predicted active site serine abolished processing. Recombinant p54 was secreted in a complex with p31, and could be further converted to p47 in vitro. Conversion required calcium, was an intramolecular autocatalytic process, and involved a second cleavage between Asp(251) and Ala(252). A decapeptide based on sequence flanking Asp(219) was efficiently cleaved by recombinant PfSUB-1. We conclude that PfSUB-1 is a subtilase with an unusual substrate specificity and that it is activated by two autocatalytic processing steps.

Delta-peptide is the carboxy-terminal cleavage fragment of the nonstructural small glycoprotein sGP of Ebola virus

In the present study we have investigated processing and maturation of the nonstructural small glycoprotein (sGP) of Ebola virus. When sGP expressed from vaccinia virus vectors was analyzed by pulse-chase experiments using SDS-PAGE under reducing conditions, the mature form and two different precursors have been identified. First, the endoplasmic reticulum form sGP(er), full-length sGP with oligomannosidic N-glycans, was detected, sGP(er) was then replaced by the Golgi-specific precursor pre-sGP, full-length sGP containing complex N-glycans. This precursor was finally converted by proteolysis into mature sGP and a smaller cleavage fragment, Delta-peptide. Studies employing site-directed mutagenesis revealed that sGP was cleaved at a multibasic amino acid motif at positions 321 to 324 of the open reading frame. Cleavage was blocked by RVKR-chloromethyl ketone. Uncleaved pre-sGP forms a disulfide-linked homodimer and is secreted into the culture medium in the presence of the inhibitor as efficiently as proteolytically processed sGP. In vitro treatment of pre-sGP by purified recombinant furin resulted in efficient cleavage, confirming the importance of this proprotein convertase for the processing and maturation of sGP. Delta-peptide is also secreted into the culture medium and therefore represents a novel nonstructural expression product of the GP gene of Ebola virus. Both cleavage fragments contain sialic acid, but only Delta-peptide is highly O-glycosylated.

Leukolysin/MMP25/MT6-MMP: a novel matrix metalloproteinase specifically expressed in the leukocyte lineage

A novel matrix metalloproteinase (MMP) was identified from leukocytes and found to be specifically expressed by peripheral blood leukocytes among 29 different tissues examined. Named leukolysin, it encodes for 562 residues with a conserved MMP structure, i.e., pre-, pro-, catalytic, hinge- and hemopexin-like domains, but also a RXK/RR motif, known for its role in MMP zymogen activation, and a C-terminal hydrophobic segment. Overall, leukolysin displays the strongest homology to the newly identified MT-MMP subgroup with 45% and 39% identities to MT4- and MT1-MMPs vs 30% and 31.5% to MMP1 and 3 respectively. Unlike MT4-MMP whose proteolytic activity remains undefined, a C-terminally truncated leukolysin is expressed as a strong gelatinolytic species at 28 kDa which is derived from a cell-associated 34 kDa proenzyme, presumably by furin or proprotein convertase mediated removal of the propeptide (approximately 6 kDa). By green fluorescent protein (GFP) tagging, the intracellular proenzyme is localized to granules throughout the cell, suggesting that activation occur immediately prior to secretion. Taken together, leukolysin may be part of the proteolytic arsenal deployed by leukocytes during inflammatory responses. Molecular cloning of a novel MMP: MMP:25

The prosegments of furin and PC7 as potent inhibitors of proprotein convertases. In vitro and ex vivo assessment of their efficacy and selectivity

All proprotein convertases (PCs) of the subtilisin/kexin family contain an N-terminal prosegment that is presumed to act both as an intramolecular chaperone and an inhibitor of its parent enzyme. In this work, we examined inhibition by purified, recombinant bacterial prosegments of furin and PC7 on the in vitro processing of either the fluorogenic peptide pERTKR-MCA or the human immunodeficiency virus envelope glycoprotein gp160. These propeptides are potent inhibitors that display measurable selectivity toward specific proprotein convertases. Small, synthetic decapeptides derived from the C termini of the prosegments are also potent inhibitors, albeit less so than the full-length proteins, and the C-terminal P1 arginine is essential for inhibition. The bacterial, recombinant prosegments were also used to generate specific antisera, allowing us to study the intracellular metabolic fate of the prosegments of furin and PC7 expressed via vaccinia virus constructs. These vaccinia virus recombinants, along with transient transfectants of the preprosegments of furin and PC7, efficiently inhibited the ex vivo processing of the neurotrophins nerve growth factor and brain-derived neurotrophic factor. Thus, we have demonstrated for the first time that PC prosegments, expressed ex vivo as independent domains, can act in trans to inhibit precursor maturation by intracellular PCs.

Purification and characterization of a cobalt-activated carboxypeptidase from the hyperthermophilic archaeon Pyrococcus furiosus

A novel metallocarboxypeptidase (PfuCP) has been purified to homogeneity from the hyperthermophilic archaeon, Pyrococcus furiosus, with its intended use in C-terminal ladder sequencing of proteins and peptides at elevated temperatures. PfuCP was purified in its inactive state by the addition of ethylenediaminetetraacetic acid (EDTA) and dithiothreitol (DTT) to purification buffers, and the activity was restored by the addition of divalent cobalt (K, = 24 +/- 4 microM at 80 degrees C). The serine protease inhibitor phenylmethylsulfonyl fluoride (PMSF) had no effect on the activity. The molecular mass of monomeric PfuCP is 59 kDa as determined by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and 58 kDa by SDS-PAGE analysis. In solution, PfuCP exists as a homodimer of approximately 128 kDa as determined by gel filtration chromatography. The activity of PfuCP exhibits a temperature optimum exceeding 90 degrees C under ambient pressure, and a narrow pH optimum of 6.2-6.6. Addition of Co2+ to the apoPfuCP at room temperature does not alter its far-UV circular dichroism (CD) or its intrinsic fluorescence spectrum. Even when the CoPfuCP is heated to 80 degrees C, its far-UV CD shows a minimal change in the global conformation and the intrinsic fluorescence of aromatic residues shows only a partial quenching. Changes in the intrinsic fluorescence appear essentially reversible with temperature. Finally, the far-UV CD and intrinsic fluorescence data suggest that the overall structure of the holoenzyme is extremely thermostable. However, the activities of both the apo and holo enzyme exhibit a similar second-order decay over time, with 50% activity remaining after approximately 40 min at 80 degrees C. The N-blocked synthetic dipeptide, N-carbobenzoxy-Ala-Arg (ZAR), was used in the purification assay. The kinetic parameters at 80 degrees C with 0.4 mM CoCl2 were: Km, 0.9 +/- 0.1 mM; Vmax, 2,300 +/- 70 U mg(-1); and turn over number, 600 +/- 20 s(-1). Activity against other ZAX substrates (X = V, L, I, M, W, Y, F, N, A, S, H, K) revealed a broad specificity for neutral, aromatic, polar, and basic C-terminal residues. This broad specificity was confirmed by the C-terminal ladder sequencing of several synthetic and natural peptides, including porcine N-acetyl-renin substrate, for which we have observed (by MALDI-TOF MS) stepwise hydrolysis by PfuCP of up to seven residues from the C-terminus: Ac-Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Leu-Val-Tyr-Ser.