Prohormone processing in Xenopus oocytes: characterization of cleavage signals and cleavage enzymes

Novel gene encoding precursor protein consisting of possible several neuropeptides expressed in brain and frontal ganglion of the silkworm, Bombyx mori

A novel gene (BmK5) expressed in the central nervous system of the silkworm, Bombyx mori, was isolated using a cDNA subtraction method. BmK5 was first cloned as a candidate regulator of diapause hormone release from subesophageal ganglion via corpus cardiacum-corpus allatum into the hemolymph; however, subsequent analyses revealed that the gene expression patterns in brain-subesophageal ganglion complexes did not differ between diapause and nondiapause egg producers. The deduced amino acid sequence showed the characteristics of secretory protein precursor or nuclear localization protein. Immunohistochemical experiments with an anti-BmK5 antibody revealed that BmK5 precursor protein exists in the cytoplasm of specific cells of brain and frontal ganglion, but not in the nuclei. In addition, a peptide (GSGTKVGGAGAATKVVTKSGS-NH(2)) possibly processed from the BmK5 precursor protein was immunohistochemically detected in the axons connecting the anti-BmK5 antibody-positive cells to the neurohemal organ, corpus cardiacum-corpus allatum. These results suggest that BmK5 encodes a precursor of the novel neurosecretory protein and that several mature peptides are released into the hemolymph via the corpus cardiacum-corpus allatum, although the functions of these peptides are yet unclear.

Cleavage in the {gamma}-subunit of the epithelial sodium channel (ENaC) plays an important role in the proteolytic activation of near-silent channels

The mechanisms by which proteases activate the epithelial sodium channel (ENaC) are not yet fully understood. We investigated the effect of extracellular proteases on rat ENaC heterologously expressed in Xenopus laevis oocytes. Application of trypsin increased ENaC whole-oocyte currents by about 8-fold without a concomitant increase in channel surface expression. The stimulatory effect of trypsin was preserved in oocytes expressing alphagamma-ENaC, but was abolished in oocytes expressing alphabeta-ENaC. Thus, the gamma-subunit appears to be essential for channel activation by extracellular proteases. Site-directed mutagenesis of a putative prostasin cleavage site in the extracellular loop of the gamma-subunit revealed that mutating the 181Lys residue to alanine (gammaK181A) increases ENaC baseline whole-oocyte currents, decreases channel surface expression, and largely reduces the stimulatory effect of extracellular proteases (trypsin, chymotrypsin and human neutrophil elastase). In single-channel recordings from outside-out patches we demonstrated that the gammaK181A mutation essentially abolishes the activation of near-silent channels by trypsin, while a stimulatory effect of trypsin on channel gating is preserved. This apparent dual effect of trypsin on channel gating and on the recruitment of near-silent channels was confirmed by experiments using the beta518C mutant ENaC which can be converted to a channel with an open probability of nearly one by exposure to a sulfhydryl reagent. Interestingly, the gammaK181A mutation results in the spontaneous appearance of a 67 kDa fragment of the gamma-subunit in the plasma membrane which can be prevented by a furin inhibitor and also occurs after channel activation by extracellular trypsin. This suggests that the mutation promotes channel cleavage and activation by endogenous proteases. This would lower the pool of near-silent channels and explain the constitutive activation and reduced responsiveness of the mutant channel to extracellular proteases. We conclude that the mutated site (K181A) affects a region in the gamma-subunit of ENaC that is functionally important for the activation of near-silent channels by extracellular proteases.

Subtilisin-like proprotein convertase activity is necessary for left–right axis determination in Xenopus neurula embryos

Signaling by members of TGF-beta superfamily requires the activity of a family of site-specific endopeptidases, known as Subtilisin-like proprotein convertases (SPCs), which cleave these ligands into mature, active forms. To explore the role of SPCs in lateral plate mesoderm (LPM) differentiation in Xenopus, two SPC inhibitors, decanoyl-Arg-Val-Lys-Arg-chloromethylketone (Dec-RVKR-CMK) and hexa-arginine, were injected into the left and right LPM of Xenopus neurulae. Left-side injection caused heart-specific left-right reversal, and this phenotype was rescued by co-injection of mature Nodal protein. In contrast, right-side injection caused left-right reversal of both the heart and gut. Tailbud embryos were less sensitive to SPC inhibitors than neurula embryos. Injection of inhibitors into either side of neurula embryos completely abolished expression of the left-LPM-specific genes, Xnr-1, antivin, and pitx2. SPC1 enzyme (Furin) was injected into the left or right LPM of mid-neurula embryos to determine the effect of enhancing SPC activity. Left-side injection of SPC1 did not cause a significant left-right reversal of the internal organs. However, right-side injection of SPC1 strongly induced the expression of Xnr-1 and pitx2 in the right LPM, and caused 100% left-right reversal of both the heart and gut. These results suggest that moderate level of SPC activity in the right LPM of the neurulae is necessary for proper left-right specification. Taken together, SPC enzymatic activity must be present in both LPMs for expression of the left-handed genes and left-right axis determination of the heart and gut in Xenopus embryos.

Xenopus egg extracts: a model system to study proprotein convertases

The Xenopus egg extract translation system has proved an ideal tool with which to study the biosynthesis of the prohormone convertases. It provides a robust coupled translation/translocation system capable of efficient translocation of any protein containing an N-terminal signal sequence into the lumen of its microsomal membranes, with cotranslational cleavage of the signal peptide. Its main advantage over rival in vitro translation systems is that it will also carry out posttranslational modification of proteins, such as N-glycosylation, and, in the case of the proprotein convertases, support autocatalytic proregion removal. The egg extract also contains an endogenous, acidic pH optimum enzyme activity, suggestive of a proprotein convertase, that can undertake limited proteolysis of precursors containing multibasic processing sites.

Proprotein convertase genes in Xenopus development

Proprotein convertases (PCs) are a family of serine endoproteases that proteolytically activate many precursor proteins within various secretory pathway compartments. Loss-of-function studies have demonstrated a critical role for these proteases in embryonic patterning and adult homeostasis, yet little is known about how substrate selectivity is achieved. We have identified Xenopus orthologs of three PCs: furin, PC6, and PC4. In addition to previously described isoforms of PC6 and furin, four novel splice isoforms of PC6, which are predicted to encode constitutively secreted proteases, and a putative transmembrane isoform of PC4 were identified. Furin and PC6 are expressed in dynamic, tissue-specific patterns throughout embryogenesis, whereas PC4 transcripts are restricted primarily to germ cells and brain in adult frogs.

Structure and expression of Furin mRNA in the ovary of the medaka, Oryzias latipes

A cDNA for furin was cloned from the ovary of the medaka, Oryzias latipes, by a combination of cDNA library screening, 5'-rapid amplification of cDNA ends (RACE), and 3'- RACE. The cDNA sequence codes for a protein of 814 amino acid residues highly homologous to other vertebrate furins, Ca(2+)-dependent serine proteases belonging to the subtilysin-like proprotein convertase family. The medaka preprofurin consists of a leader sequence, a propeptide with autoactivation sites, a Kex2-like catalytic domain, a P domain, a cysteine-rich domain, a putative transmembrane domain, and a cytoplasmic domain. The catalytic triad residues (Asp-164, His-205, and Ser-379) were all conserved. Furin mRNA was expressed in many tissues of this, including the ovary. In the ovary, the greatest expression of furin mRNA occurred in oocytes of small growing follicles, as demonstrated by Northern blotting, RT-PCR, and in situ hybridization analysis. Temporary and spatial expression patterns of the medaka fish furin were similar to those of stromelysin-3 and MT5-MMP during oocyte growth and postnatal development.

Evolution of the prohormone convertases: identification of a homologue of PC6 in the protochordate amphioxus

Many of the protein precursors traversing the secretory pathway undergo cleavage at multibasic sites to generate their bioactive forms. The proprotein convertases (PCs), a family of subtilisin-like proteases, are the major endoproteases that serve this function. Genes encoding seven distinct members of this family have so far been characterized in vertebrates: furin, PC2, PC1/PC3, PC4, PACE4, PC5/PC6 and PC7/PC8/LPC. Multiple PC genes have also been cloned from a number of invertebrates, including Drosophila melanogaster and Caenorhabditis elegans. These findings suggest that gene duplication and diversification of the PCs have occurred throughout metazoan evolution. To investigate the structural and functional changes which have occurred during vertebrate development, we have analyzed the expression of PC genes in the protochordate amphioxus. We have previously shown that amphioxus express homologous PC2 and PC1/PC3 genes [Proc. Natl. Acad. Sci. USA 92 (1995) 3591]. Here we report the characterization of amphioxus cDNAs encoding proteases with a high degree of similarity to mammalian PC6. Three cDNAs encoding three PC6 isoforms differing only in their carboxy-terminal sequences were found, derived by alternative splicing. Two isoforms appear to be soluble enzymes, whereas the third contains a transmembrane hydrophobic segment and thus is likely to be membrane-bound. All three variants contain many repeats of a cysteine-rich motif that is found in several other PC family members. Thus, amphioxus, like the vertebrates, expresses two types of PCs, e.g., PC2 and PC1/PC3 which function in the regulated secretory pathway in neuroendocrine cells, and the more widely expressed PC6 which functions mainly in the constitutive pathway.

Differentiation-induced insulin secretion from nonendocrine cells with engineered human proinsulin cDNA

To investigate the effects of differentiation on insulin secretion from engineered nonendocrine cells, modified human proinsulin cDNA (INS/fur) was transfected to THP-1 monocyte and C2C12 myoblast cell lines. When THP-1 was differentiated into macrophages with phorbol ester, the insulin secretion rate was increased by 3.1-fold. This increase in insulin secretion is accompanied by a 17.6-fold increase in the processing efficiency of the modified human proinsulin and by a 3.5-fold increase in the abundance of furin mRNA. In addition, differentiation of C2C12 into myotubes, which can be induced by changing the serum, showed a 9.9-fold increase in insulin secretion and was accompanied by a 1.6-fold increase in the abundance of furin mRNA. The involvement of posttranslational processing and the exocytotic process in differentiation-induced insulin secretion could lead to the possibility of regulation of insulin secretion from genetically engineered cells.

Molecular basis for oviductin-mediated processing from gp43 to gp41, the predominant glycoproteins of Xenopus egg envelopes

Acquisition of fertilizability in Xenopus coelomic eggs is correlated with the conversion from coelomic to vitelline envelope during passage of the eggs through the pars recta portion of oviduct. The conversion includes processing of a major envelope constituent gp43 of coelomic envelopes to gp41 of vitelline envelopes by a trypsin-type protease, oviductin, which is secreted from the pars recta. Our recent sequencing analyses [Kubo et al., (1997): Dev Growth Diff 39:405-411] strongly suggested that the N-terminal portion of gp41 is exposed as a result of oviductin digestion. In this study, a monoclonal antibody specific to the predicted N-terminus of gp41 was raised by immunizing mice with a synthetic N-terminal hexapeptide (QLPVSP) coupled to keyhole limpet hemocyanin. The antibody specifically reacted to gp41, but not to gp43, indicating that Gln62 is exposed as the N-terminal amino acid of gp41 by oviductin-mediated cleavage of gp43 at Arg61 in GSR61. The C-terminal sequencing of gp43 and gp41 indicated that Arg373 in GSR373 as the C-terminus of gp41 is generated by cleavage of three amino acid (WNQ) residues from the C-terminus of gp43. The resulting polypeptide moiety of gp41 has a molecular mass of 33900 Da with 312 amino acid residues. We propose that oviductin possessing the substrate specificity of GSR simultaneously digests gp43 at Arg residues in GSR61 and GSR373 to generate the N- and C-terminus of gp41, respectively.

Ascaridia galli fatty acid-binding protein, a member of the nematode polyprotein allergens family

A fatty acid-binding protein from the nematode Ascaridia galli was characterized. The gene was isolated and recombinantly expressed in Escherichia coli. According to the deduced amino acid sequence A. galli fatty acid-binding protein (AgFABP) belongs to the family of nematode polyprotein allergens, as shown by Western blotting and PCR analysis with genomic DNA and cDNA. Both native and recombinant proteins bind fatty acids and retinoids with high affinity. The fluorescent fatty acid analogue 11-[(5-dimethylaminonaphthalene-1-sulfonyl)amino] undecanoic acid (DAUDA) shows substantial changes in its emission spectrum when bound to AgFABP; this binding is reversed by fatty acids such as oleate. Moreover, changes of the intrinsic fluorescence of retinol and retinoic acid confirm retinoid binding activity of AgFABP. Fluorescence titration experiments with DAUDA indicate stoichiometric binding to a single binding site per monomer unit with affinities (Kd) of 1.6 and 1.8 x 10(-7) m for native and the recombinant protein, respectively. The apparent binding affinities of the nonfluorescent ligands were calculated in displacement experiments with DAUDA and values in the same range were obtained for myristic, palmitic, oleic, linoleic, arachidonic and retinoic acid. Additionally, the binding affinity of AgFABP for oleate and palmitate was determined by direct and indirect radiochemical analysis and the values obtained were similar to those from the fluorescent experiments. Both proteins show a preference for the binding of long-chain saturated and unsaturated fatty acids, but not for short chain (C3-C12) and branched fatty acids, cholesterol and tryptophan.

Identification and characterization of spodoptera frugiperda furin: a thermostable subtilisin-like endopeptidase

Spodoptera frugiperda (Sf9) cells are widely employed for high-level expression of heterologous recombinant genes from baculovirus vectors. Using a plasmid library encoding cDNA of Sf9 cells we have identified here the Spodoptera frugiperda analog of the proprotein convertase furin which plays an important role in posttranslational protein processing. Spodoptera frugiperda furin (Sfurin) is closest related to Drosophila melanogasterfurin with which it shares an extended cysteine-rich domain, whereas mammalian furin shows high homology only in the catalytic domain. Mammalian furin and Sfurin were further compared by expression from baculovirus vectors. Substrate specificity and inhibitor profiles are identical for Sfurin and mammalian furin, whereas calcium-dependence, pH-optimum, and thermostability differ. Cleavage of recombinant influenza virus hemagglutinin was significantly enhanced in Sf9 cells after overexpression of Sfurin.

BMP-4 is proteolytically activated by furin and/or PC6 during vertebrate embryonic development

Bone morphogenetic protein-4 (BMP-4) is a multifunctional developmental regulator. BMP-4 is synthesized as an inactive precursor that is proteolytically activated by cleavage following the amino acid motif -Arg-Ser-Lys-Arg-. Very little is known about processing and secretion of BMPs. The proprotein convertases (PCs) are a family of seven structurally related serine endoproteases, at least one of which, furin, cleaves after the amino acid motif -Arg-X-Arg/Lys-Arg-. To examine potential roles of PCs during embryonic development we have misexpressed a potent protein inhibitor of furin, alpha1-antitrypsin Portland (alpha1-PDX) in early Xenopus embryos. Ectopic expression of alpha1-PDX phenocopies the effect of blocking endogenous BMP activity, leading to dorsalization of mesoderm and direct neural induction. alpha1-PDX-mediated neural induction can be reversed by co-expression of downstream components of the BMP-4 signaling pathway. Thus, alpha1-PDX can block BMP activity upstream of receptor binding, suggesting that it inhibits an endogenous BMP-4 convertase(s). Consistent with this hypothesis, alpha1-PDX prevents cleavage of BMP-4 in an oocyte translation assay. Using an in vitro digestion assay, we demonstrate that four members of the PC family have the ability to cleave BMP-4, but of these, only furin and PC6B are sensitive to alpha1-PDX. These studies provide the first in vivo evidence that furin and/or PC6 proteolytically activate BMP-4 during vertebrate embryogenesis.

Processing of proendothelin-1 at the C-terminus of big endothelin-1 is essential for proteolysis by endothelin-converting enzyme-1 in vivo

Production of endothelin-1 is thought to be a three-step process consisting of an initial proteolytic cleavage of the proendothelin-1 precursor to big endothelin-1-Lys-Arg, C-terminal trimming by a carboxypeptidase and further processing of the big endothelin-1 peptide to endothelin-1 by endothelin-converting enzyme (ECE). To further clarify the mechanism of processing in the biosynthesis of endothelin-1, we introduced a point mutation into endothelin-1 cDNA to replace the Arg in the -4 position of the recognition motifs of furin-like convertase in human preproendothelin-1 (Arg49 or Arg89) by Gly. When mutant cDNAs were expressed in Chinese hamster ovary (CHO)-K1 cells, they failed to be processed at the mutated processing signal, suggesting that the Arg-Ser-Lys-Arg motifs of preproendothelin-1 are recognized by CHO-K1 furin-like convertase. Co-transfection with ECE-1 cDNA revealed that cleavage at Arg52 is not essential for cleavage by ECE-1, but that cleavage at Arg92 is critical. Although a high-molecular-mass form of endothelin-1 is produced by processing by ECE-1 without cleavage at Arg52, it did not evoke Ca2+ transient in endothelinA-receptor-expressing cells. In conclusion, prior cleavage at Arg92 by furin-like convertase is absolutely necessary for cleavage by ECE-1 at Trp73 to produce mature endothelin-1.

Cell type-specific sorting of neuropeptides: a mechanism to modulate peptide composition of large dense-core vesicles

The CNS of Lymnaea stagnalis contains two populations of egg-laying hormone (ELH)-producing neurons that differ in size and topology. In type I neurons, all peptides located C-terminally from the cleavage site Arg-Ser-Arg-Arg180-183 are sorted into secretory large dense-core vesicles (LDCV), whereas N-terminal-located peptides accumulate in a distinct type of vesicle, the large electrondense granule (LEG). Via immunoelectron microscopy, we now show that the second population of ELH-producing neurons, type II neurons, lack LEG and incorporate all proELH-derived peptides into LDCV. This finding provides the first example of a cell type-specific sorting of neuropeptides into LDCV. Furthermore, we provide evidence that LEG are formed through a differential condensation process in the trans-Golgi network and that these bodies are ultimately degraded. Analysis of the endoprotease composition of the two types of proELH-producing neurons suggests that the formation of LEG, and consequently the retention of N-terminal peptides from the secretory pathway, requires the action of a furin-like protein.

Cell type-specific sorting of neuropeptides: a mechanism to modulate peptide composition of large dense-core vesicles

The CNS of Lymnaea stagnalis contains two populations of egg-laying hormone (ELH)-producing neurons that differ in size and topology. In type I neurons, all peptides located C-terminally from the cleavage site Arg-Ser-Arg-Arg180-183 are sorted into secretory large dense-core vesicles (LDCV), whereas N-terminal-located peptides accumulate in a distinct type of vesicle, the large electrondense granule (LEG). Via immunoelectron microscopy, we now show that the second population of ELH-producing neurons, type II neurons, lack LEG and incorporate all proELH-derived peptides into LDCV. This finding provides the first example of a cell type-specific sorting of neuropeptides into LDCV. Furthermore, we provide evidence that LEG are formed through a differential condensation process in the trans-Golgi network and that these bodies are ultimately degraded. Analysis of the endoprotease composition of the two types of proELH-producing neurons suggests that the formation of LEG, and consequently the retention of N-terminal peptides from the secretory pathway, requires the action of a furin-like protein.

Expression of tyrosine-sulfated secretory proteins in Xenopus laevis oocytes. Differential export of constitutive and regulated proteins

Xenopus laevis oocytes were used to study the tyrosine sulfation and secretion of exogenous proteins. Secretogranin II (SgII), a tyrosine-sulfated protein found in secretory granules of a wide variety of endocrine cells and neurons, became tyrosine-sulfated by the oocytes when expressed by injection of poly(A)-rich RNA isolated from the neuroendocrine cell line PC12. The same result was observed when SgII was expressed from cloned SgII cRNA, showing that its tyrosine sulfation did not require the coexpression of exogenous tyrosylprotein sulfotransferase (TPST) but occurred by means of the endogenous oocyte TPST. Sulfophilin, an artificial protein consisting of 12 repeats of a heptapeptide tyrosine-sulfation site, was highly sulfated upon injection of its RNA, indicating the presence of TPST levels sufficient for stoichiometric sulfation of appropriate reporter proteins. Comparison of the secretion of [35S]sulfate-labelled SgII with that of sulfophilin and an exogenous heparan sulfate proteoglycan (HSPG), two proteins delivered to the cell surface by the constitutive pathway of secretion, revealed striking differences. The majority of sulfophilin and the HSPG was found in the medium, whereas that of SgII was found intracellularly. Prolactin, another secretory granule protein, showed the same secretion behaviour as SgII. These results show that oocytes express TPST and that these cells secrete constitutive and regulated secretory proteins in a differential manner.

Identification of two novel genes specifically expressed in the D-group neurons of the terrestrial snail CNS

A search for genes specifically expressed in the giant interneurons of parietal ganglia of the snail Helix lucorum yielded, among others, two genes named HDS1 and HDS2. According to data obtained by Northern hybridization and whole-mount in situ hybridization, both genes are neurospecific and expressed almost exclusively in the peptidergic D-group neurons (Sakharov, 1974) located in the right parietal ganglion. In situ hybridization of the HDS1 and HDS2 probes with CNS of several related species of the Helicoidea superfamily identified in all cases similarly located homologous groups of neurons. Sequencing of the near full-length cDNA copies of the HDS1 and HDS2 genes revealed open reading frames 107 and 102 amino acids long for HDS1 and HDS2, respectively. Both putative proteins contain a hydrophobic leader peptide and putative recognition sites for furin-like and PC-like endopeptidases. Predicted amino acid sequences of the HDS1 and HDS2 proteins were found to be moderately homologous to each other, as well as to the LYCP preprohormone expressed by the light yellow cells of the freshwater snail Lymnaea stagnalis. These results confirm an earlier hypothesis that the D-group of the Helix family and the light yellow cells of Lymnaea stagnalis represent homologous neuronal groups. Our data suggest that the HDS1 and HDS2 genes encode precursors of secreted molecules, most likely neuropeptides or neurohormones.

BSTI, a trypsin inhibitor from skin secretions of Bombina bombina related to protease inhibitors of nematodes

From skin secretions of the European frog Bombina bombina, a new peptide has been isolated that contains 60 amino acids, including 10 cysteine residues. Its sequence was determined by automated Edman degradation and confirmed by analysis of the cDNA encoding the precursor. A search in the databanks demonstrated that the pattern of cysteine residues in this skin peptide is similar to the ones found in protease inhibitors from Ascaris and in a segment of human von Willebrand factor. The 3D structure of the trypsin inhibitor from Ascaris suum could be used as a template to build a model of the amphibian peptide. In addition, we have demonstrated that this constituent of skin secretion is indeed an inhibitor of trypsin and thrombin, with K(i) values in the range of 0.1 to 1 microM. The new peptide was thus named BSTI for Bombina skin trypsin/thrombin inhibitor.

A second mutant allele of furin in the processing-incompetent cell line, LoVo. Evidence for involvement of the homo B domain in autocatalytic activation

Furin is a Golgi membrane-associated endoprotease that is involved in cleavage of various precursor proteins predominantly at Arg-X-Lys/Arg-Arg sites. Furin itself is synthesized as an inactive precursor, which is activated through intramolecular autocatalytic cleavage at an Arg-X-Lys-Arg site. We previously found that human colon carcinoma LoVo cells have a frameshift mutation within the homo B domain of furin and thereby lack processing activity toward Arg-X-Lys/Arg-Arg sites. In this study, however, we identified a second furin mutation in this cell line. The mutation, a replacement of a conserved Trp residue within the homo B domain with Arg, results in lack of processing activity of the mutant furin. The combination of both mutations can account for the recessive nature of the processing incompetence of LoVo cells. Immunofluorescence analysis with three distinct anti-furin monoclonal antibodies revealed that neither furin mutant underwent the autocatalytic activation or left the endoplasmic reticulum for the Golgi. These data indicate that the homo B domain as well as the catalytic domain is required for autocatalytic activation of furin.

Differences between the catalytic properties of recombinant human PC2 and endogenous rat PC2

Human prohormone convertase PC2 was expressed in Xenopus oocytes and its properties were compared with those of the Type-2 endopeptidase of rat insulin secretory granules, previously identified as PC2 [Bennett, Bailyes, Nielson, Guest, Rutherford, Arden and Hutton (1992) J. Biol. Chem. 267, 15229-15236]. Recombinant PC2 had the same substrate specificity as the Type-2 endopeptidase, cleaving at the CA-junction (Lys64, Arg65) of human des-31,32-proinsulin to generate insulin; little activity was found toward human des-64,65-proinsulin or proinsulin itself. Recombinant PC2 was maximally active in 5-7 mM Ca2+ (K0.5 = 1.6 mM) whereas the Type-2 endopeptidase was maximally active in 0.5-1 mM Ca2+ (K0.5 = 40 microM). Both enzymes had a pH optimum of 5.0-5.5 but the Type-2 endopeptidase was active over a wider pH range. Two molecular forms of recombinant PC2 (71 kDa and 68 kDa) were found, both had an intact C-terminus but differed by the presence of the propeptide. The endogenous PC2 comprised several overlapping forms (size range 64-68 kDa), approximately two-thirds of which lacked C-terminal immunoreactivity. Part of the size difference between recombinant and endogenous PC2 was attributable to differences in N-glycosylation. The different post-translational proteolytic modifications of recombinant and endogenous PC2 did not account for the different pH and Ca2+ sensitivities shown by the enzymes. A modulating effect of carbohydrate on enzyme activity could not be excluded.

Molecular cloning and cellular localization of a furin-like prohormone convertase from the atrial gland of Aplysia

Prohormone convertases (PCs) are Ca(2+)-dependent subtilisin-related endoproteases that have been implicated in the post-translational processing of prohormones and other proproteins. Furin is an ubiquitously expressed PC that has been shown to hydrolyze a wide variety of precursor proteins in secretory pathways. We have screened an Aplysia atrial gland cDNA library using a furin probe prepared by polymerase chain reaction (PCR) and have isolated an Aplysia furin-related 6.7-kb cDNA partial clone that was truncated on the 5' end. The remaining 5' atrial gland furin nucleotide sequence was obtained by two stages of reverse transcription PCR. The final composite nucleotide sequence of the atrial gland furin cDNA was 7,837 bp in length. This sequence encoded a putative preproendoprotease (Afurin2) of 824 amino acid residues that was related to other eukaryotic furins, and showed a high sequence identity with a recently reported Aplysia nervous system furin-like sequence. In situ hybridization demonstrated extensive expression of Afurin2 in atrial gland secretory cells. The cDNA clone contained a relatively long 3' untranslated region of 5,230 nucleotides that included a microsatellite repeat region (TG)n. The characterized Aplysia Afurin2 is a candidate PC that may play an important role in the processing of egg-laying hormone (ELH)-related precursors in the secretory cells of the atrial gland. In addition, comparative structural studies of Afurin2, together with previously reported localization studies, argue for the occurrence of a furin-like convertase within secretory granules.

The Dfur2 gene of Drosophila melanogaster: genetic organization, expression during embryogenesis, and pro-protein processing activity of its translational product Dfurin2

The gene structure and expression of the Dfur2 gene of Drosophila melanogaster, which encodes the subtilisin-like serine endoprotease Dfurin2, was studied. The Dfur2 gene is very compact in contrast to the related Dfur1 gene, which has an estimated size of over 100 kbp. The 6-kb Dfur2 mRNA is encoded by 16 exons dispersed over a genomic region of about 9 kbp. The exon/intron organization shows conservation of intron positions not only in comparison with Dfur1, but also with the related mammalian genes FUR, PC1/PC3, PC2, and PC4. This conservation supports the hypothesis that all genes belonging to the family of subtilisin-like pro-protein processing enzymes are evolutionary related by descent from a common ancestral gene. In primer extension experiments, Dfur2 transcription initiation sites were identified in the presumed Dfur2 promoter region. This region was found to contain general RNA polymerase II promoter elements like a potential TATA box, a potential CAP signal, and several potential CCAAT boxes. Also, several sequence motifs putatively corresponding to binding sites for Drosophila transcription factors like zeste, bicoid, and engrailed were found to be present. RNA in situ hybridization experiments on Drosophila embryos revealed presumably maternal Dfur2 expression until the syncytial blastoderm (stage 5 of embryogenesis), no expression during gastrulation (stage 9), transient expression in a subset of neurons in the central nervous system of stage 12-13 embryos, and, from stage 13 onwards, expression in the developing tracheal tree. In a vaccinia expression system, the endoprotease Dfurin2 not only cleaved wild-type precursor of von Willebrand factor (pro-vWF) with pro-region cleavage site R-S-K-R decreases, but also, although to a lesser extent, pro-vWF mutants in which the P2 (vWFK-2A) or P4 (vWFR-4A) basic residue with respect to the pro-region cleavage site had been mutated. This cleavage specificity resembles that of human furin. The cleavage of pro-vWF by Dfurin2 shows that the previously reported lack of cleavage of the precursor of the beta A-chain of activin-A by Dfurin2 in this vaccinia expression system is substrate determined.

Processing specificity and biosynthesis of the Drosophila melanogaster convertases dfurin1, dfurin1-CRR, dfurin1-X, and dfurin2

Pro-protein and pro-hormone convertases are subtilisin/kexin-like enzymes implicated in the activation of numerous precursors by cleavage at sites mostly composed of pairs of basic amino acids. Six members of this family of enzymes have been identified in mammals and named furin (also called PACE), PC1 (also called PC3), PC2, PACE4, PC4, and PC5 (also called PC6). Multiple transcripts are produced for all the mammalian convertases, but only in the cases of PC4, PACE4, and PC5 does differential splicing result in the modification of the C-terminal sequence of these enzymes. A similar molecular diversity is also observed for the convertases of Hydra vulgaris, Caenorhabditis elegans, and Drosophila melanogaster. In the third species, two genes homologous to human furin called Dfur1 and Dfur2 have been identified. The Dfur1 gene undergoes differential splicing to generate three type I membrane-bound proteins called dfurin1, dfurin1-CRR, and dfurin1-X, which differ only in their C-terminal sequence. By using recombinant vaccinia viruses that express each of the dfurin proteins, we investigated the potential effect of the C-terminal domain on their catalytic specificities. For this purpose, these enzymes were coexpressed with the precursors pro-7B2, pro-opiomelanocortin, and pro-dynorphin in a number of cell lines, and the processed products obtained were characterized. Our studies demonstrate that these proteases display cleavage specificities similar to that of mammalian furin but not to that of PC2. In contrast, we noted significant differences in the biosynthetic fates of these convertases. All dfurins undergo rapid removal of their transmembrane domain within the endoplasmic reticulum, resulting in the release of several truncated soluble forms. However, in the media of cells containing secretory granules, such as GH4C1 and AtT-20, dfurin1-CRR and dfurin2 predominate over dfurin1, whereas dfurin1-X is never detected. While pro-segment removal occurs predominantly in the trans-Golgi network for all the dfurins, in the presence of brefeldin A, only dfurin1-CRR and dfurin2 can undergo partial zymogen cleavage. The conclusions drawn from the results of this study may well be applicable to the mammalian convertases PC4, PACE4, and PC5, which also display C-terminal sequence heterogeneity.

Analysis of mutations in alleles of the fur gene from an endoprotease-deficient Chinese hamster ovary cell strain

RPE.40 mutant cells differ from wild-type Chinese hamster ovary (CHO-K1) cells in their increased resistance to Pseudomonas exotoxin A and their inability to process the insulin proreceptor and certain viral envelope proproteins. Northern analysis revealed that RPE.40 cells maintained a substantially lower steady-state level of 4.0 kb fur mRNA than did CHO-K1 cells. Analysis of fur cDNAs showed that RPE.40 cells were diploid at the fur locus, and RPE.40 cells had a Cys (TGC) to Tyr (TAC) mutation in codon 196 of one allele (allele I). Approximately 25-30% of the CHO-K1 cells were also heterozygous (Tyr/Cys) at codon 196, and pre-mRNAs transcribed from the second allele (allele II) in RPE.40 cells were defectively spliced. All other pre-mRNAs were correctly spliced. Rapid turnover of defectively spliced transcripts may account for the reduced steady-state level of fur mRNA observed in RPE.40 cells. Our results provide a mechanistic basis for the endoprotease-deficient phenotype of RPE.40 cells.

Homology modelling of the catalytic domain of human furin. A model for the eukaryotic subtilisin-like proprotein convertases

A model is presented for the three-dimensional structure of the catalytic domain of the human serine proteinase furin and its interaction with model substrates. This homology model is based on the crystal structures of subtilisin BPN' and thermitase in complex with the inhibitor eglin, and it also applies to other members of the eukaryotic subtilisin-like proprotein convertases. Predictions are made of the general protein fold, inserted loops, disulfide bonds, Ca(2+)-binding sites and salt bridges. A detailed prediction of the substrate-binding region attempts to explain the basis of specificity for multiple basic residues preceding the cleavage site. Specific acidic residues in the S1, S2 and S4 subsites of the substrate-binding region of furin are identified which appear to be of particular importance, while residues of the S2', S3, S5 and S6 subsites may also contribute to substrate binding. Based on this model, protein engineering can be employed not only to test the predicted enzyme-substrate interactions, as demonstrated for human furin, but, equally importantly, to design proprotein convertases with a desired specificity, or to design novel substrates or inhibitors.

Isolation and in situ localization of a cDNA encoding a Kex2-like prohormone convertase in the nematode Caenorhabditis elegans

1. A cDNA that encodes a Kex2-like prohormone convertase (PC) containing an active site similar to that of mammalian PC2 has been isolated from C. elegans. Total RNA was isolated from a mixed population of strain BA713 worms. After poly-(A)-selection and reverse transcription, degenerate/nested polymerase chain reactions (PCR) were performed using primers based on conserved regions within the active sites of the known vertebrate and invertebrate endoproteases. 2. Two distinct 300-bp PCR products that shared homologies with the active sites of known Kex2-like endoproteases were isolated. These two PCR products were used to screen a C. elegans cDNA library. 3. The complete cDNA for a Kex2-like endoprotease, designated CELPC2, was isolated and determined to be 2527 bp in length. This size was confirmed by northern analysis. The deduced amino acid sequence for the CELPC2 cDNA is very similar to the known Kex2-like endoproteases, especially at conserved regions within the active sites, but not identical to any one of them. The strongest structural homology was to vertebrate and invertebrate PC2 sequences. 4. In situ hybridization suggests that CELPC2 is synthesized primarily in cells associated with the circumpharyngeal nerve ring and the dorsorectal ganglion.

Processing of a fusion protein by endoprotease in COS-1 cells for secretion of mature peptide by using a chimeric expression vector

The subtilisin-related proprotein convertase furin is expressed in various mammalian tissues. Expecting that COS-1 cells have a furin-like endoprotease, we constructed a fusion expression vector for production of a recombinant foreign protein having no signal peptide or a protein in truncated form into secreted mature protein. A cDNA fragment encoding N-terminal procalcitonin (pro-CT) of human calcitonin precursor was inserted into the mammalian expression vector pME18S. We used PCR techniques to generate four kinds of cDNAs encoding the C terminus of the pro-CT with Arg residues at P4 (Arg-Xaa-Lys-Arg), P6 (Arg-Xaa-Xaa-Xaa-Lys-Arg), or both (Arg-Xaa-Arg-Xaa-Lys-Arg), in addition to the Lys-Arg motif at the cleavage site, in order to determine the conditions for efficient processing in nonendocrine cells, such as COS-1 cells. The cDNA coding for the Fc fragment of human immunoglobulin G1 was fused in-frame to the cDNA encoding pro-CT at its C terminus. Upon transfection of the chimeric plasmids into COS-1 cells, almost all of the fusion protein with the Arg residues at both P4 and P6 were processed into secreted Fc product, even without cotransfection of furin. These results indicate that COS-1 cells have a furin-like endoprotease and suggest that pro-CT, with the Arg residues at both P4 and P6, can be used as a carrier peptide for expression of a foreign protein having no signal peptide or a protein in truncated form in COS-1 cells.

In vivo expression of mutant preproendothelins: hierarchy of processing events but no strict requirement of Trp-Val at the processing site

Endothelin-1 (ET-1), a 21-residue vasoconstrictor peptide, originates in human cells from a 212-amino acid precursor (preproET-1). Big ET-1, an intermediate form of 38 amino acids, is generated by cleavage at basic-pair residues of proET-1, while a specific "ET-converting enzyme" was proposed to process the unusual Trp-Val site at positions 21 and 22 of big ET-1. We have previously shown that expression of synthetic RNA encoding human preproET-1 in Xenopus oocytes results in secretion of putative ET-1 and big ET-1. Here, to further dissect the processing pathway of preproET-1, we designed and expressed in oocytes a set of preproET-1 mutants. Four mutants affecting the Trp-Val site always originated putative ET-1(s) at levels comparable to the wild type, suggesting that there is only a conformational requirement for cleavage at this site. An Arg-->Ile mutation at the basic-pair site after the C terminus of big ET-1 fully inhibited the formation of both big ET-1 and ET-1, indicating that processing at this site is an early event and that big ET-1 is an obligate intermediate for the synthesis of ET-1 in vivo. Also, a truncated mutant bearing a stop codon after the C terminus of the big ET-1 sequence was totally stable and further processed into mature big ET-1 and ET-1, indicating that the second part of the precursor is not necessary for maturation.

Expression of mutant ELH prohormones in AtT-20 cells: the relationship between prohormone processing and sorting

Posttranslational processing of many proteins is essential to the synthesis of fully functional molecules. The ELH (egg-laying hormone) prohormone is cleaved by endoproteases in a specific order at a variety of basic residue processing sites to produce mature peptides. The prohormone is first cleaved at a unique tetrabasic site liberating two intermediates (amino and carboxy) which are sorted to different classes of dense core vesicles in the bag cell neurons of Aplysia. When expressed in AtT-20 cells, the ELH prohormone is also first cleaved at the tetrabasic site. The amino-terminal intermediate is then sorted to the constitutive pathway, and a portion of the carboxy-terminal intermediate is sorted to the regulated pathway. Here, we use mutant constructs of the ELH prohormone expressed in AtT-20 cells to examine the relationship between prohormone processing and consequent sorting. Prohormone which has a dibasic site in place of the tetrabasic site is processed and sorted similarly to wild type. Furthermore, mutant prohormone which lacks the tetrabasic site is processed at an alternative site comprising three basic residues. In these mutant prohormones, mature ELH is still produced and stored in dense core vesicles while amino-terminal products are constitutively secreted. However, deletion of the tetrabasic and tribasic sites results in the rerouting of the amino-terminal intermediate products from the constitutive pathway to the regulated secretory pathway. Thus, in the ELH prohormone, the location of the proteolytic processing events within the secretory pathway and the order of cleavages regulate the sorting of peptide products.

Processing endopeptidase deficiency in neurohypophysial secretory granules of the diabetes insipidus (Brattleboro) rat

The homozygote Brattleboro rat exhibits a hereditary diabetes insipidus due to a deficiency of vasopressin, the antidiuretic hormone. It has previously been shown that in this animal a single nucleotide deletion in the provasopressin gene leads to a mutant precursor with a C-terminal amino acid sequence different from that of the wild-type. However the N-terminal region including the hormone moiety, the processing signal as well as the first two-thirds of the neurophysin is entirely preserved and absence of maturation has to be explained by an additional cause. We show here that the neurohypophysis of the homozygote Brattleboro rat, in contrast to the adenohypophysis, displays a significant decrease in the Lys-Arg processing endopeptidase activity when compared to the heterozygote or the wild-type Wistar. It is suggested that hypothalamic vasopressinergic neurons of the homozygote Brattleboro rat display a deficiency in the processing enzyme in contrast to the oxytocinergic neurons in which processing of prooxytocin is normal.

Isolation and sequence of a cDNA encoding the precursor of a bombesinlike peptide from brain and early embryos of Xenopus laevis

A cDNA encoding the precursor of a bombesinlike peptide was isolated from brain of Xenopus laevis. The predicted end product resembles neuromedin B, which was originally isolated from mammalian spinal cord. The mRNA for this precursor was also present in gastrointestinal tract and in ovaries. Moreover, it could be detected in early embryos (stage 2 and stage 10) of X. laevis. These findings suggest novel roles for peptides of the bombesin family in oocyte maturation and early amphibian development.

Proinsulin processing by the subtilisin-related proprotein convertases furin, PC2, and PC3

Experiments using recombinant vaccinia viruses expressing rat proinsulin I coinfected into COS-7 cells with recombinant vaccinia virus expressing human furin, human PC2, mouse PC3 (subtilisin-related proprotein convertases 1-3, respectively), or yeast Kex2 indicate that in this system both Kex2 and furin produce mature insulin, whereas PC2 selectively cleaves proinsulin at the C-peptide-A-chain junction. This is a property consistent with its probable identity with the rat insulinoma granule type II proinsulin processing activity as described by Davidson et al. [Davidson, H. W., Rhodes, C. J. & Hutton, J. C. (1988) Nature (London) 333, 93-96]. PC3 generates mature insulin but cleaves preferentially at the proinsulin B-chain-C-peptide junction. This pattern of cleavage by PC3 is similar, but not identical, to that of the highly B-chain-C-peptide junction-selective type I activity as described by Davidson et al., perhaps due to the presence of a P4 arginine residue near the C-peptide-A-chain junction unique to the rat proinsulins. These results along with data presented on the expression of both PC2 and PC3 in islet beta cells strongly support the conclusion that these proteases are involved in the conversion of proinsulin to insulin in vivo.