Purification and characterization of furin, a Kex2-like processing endoprotease, produced in Chinese hamster ovary cells.

Furin, a mammalian homolog of the yeast Kex2 protease, is associated with Golgi membranes and is involved in cleavage of precursor proteins at sites marked by the Arg-X-Lys/Arg-Arg (RXK/RR) motif. We have recently shown that a furin mutant lacking the transmembrane domain can be secreted from cDNA-transfected cells with proteolytic activity for the fluorogenic peptide t-butoxycarbonyl-Arg-Val-Arg-Arg-4-methylcoumarin-7- amide. In this study, we purified and characterized the recombinant furin from the conditioned medium of these cells. Furin was purified as a mixture of 83- and 81-kDa forms and a 96-kDa form. The differences in molecular mass were not due to differences in molecular mass were not due to differences in glycosylation. Moreover, all forms had the same NH2-terminal sequence beginning at the residue after the Arg-Ala-Lys-Arg sequence. These data suggest that the three different forms may be produced by differential COOH-terminal processing of a furin molecule and that mature furin may be autocatalytically produced. Both enzyme preparations showed a pH optimum at 7.0, required Ca2+ for the activity, and showed essentially the same inhibitor profile. These properties resembled those of the Kex2 protease. Both preparations efficiently cleaved fluorogenic peptides with an RXK/RR sequence and moderately cleaved a peptide with an RXXR sequence, but did not cleave dibasic peptides. The sequence requirements determined in vitro were compatible with those determined by expression studies in cultured cells. These data unequivocally demonstrate that furin is an endogenous cellular protease responsible for cleavage of precursor proteins mainly at RXK/RR sites.     

Arg-X-Lys/Arg-Arg motif as a signal for precursor cleavage catalyzed by furin within the constitutive secretory pathway

Many peptide hormones are produced from larger precursors by endoproteolysis at pairs of basic amino acids (e.g. Lys-Arg and Arg-Arg) within the regulated secretory pathway in endocrine cells. However, many other secretory and membrane proteins appear to be produced from precursors through cleavage at multiple, rather than paired, basic residues within the constitutive secretory pathway in non-endocrine cells. By surveying various precursors processed constitutively, we noticed that most of them have the consensus sequence, Arg-X-Lys/Arg-Arg (RXK/RR), at the cleavage site. When expressed in endocrine and non-endocrine cells, a precursor with the RXKR sequence was cleaved in both types of cells, whereas that with the Lys-Arg pair was cleaved only in the endocrine cells. When the RXKR precursor was coexpressed with furin and PC3, both of which are mammalian homologues of the yeast precursor-processing endoprotease Kex2, in non-endocrine cells, enhancement of the precursor cleavage by furin but not by PC3 was observed. By contrast, when the Lys-Arg precursor was coexpressed with the two mammalian proteases in endocrine cells with no endogenous processing activity at dibasic sites, it was cleaved only by PC3. These results indicate that the basic pair and the RXK/RR sequence are the signals for precursor cleavages catalyzed by PC3 within the regulated secretory pathway and by furin within the constitutive pathway, respectively.     

Sequence requirements for precursor cleavage within the constitutive secretory pathway.

We have recently demonstrated that the Arg-X-Lys/Arg-Arg sequence is a signal for precursor cleavage catalyzed by furin, a mammalian homologue of the yeast precursor-processing endoprotease Kex2, within the constitutive secretory pathway. In this study, we further examined sequence requirements for the constitutive precursor cleavage by expression of various prorenin mutants with amino acid substitutions around the native Lys-Arg cleavage site in Chinese hamster ovary cells. The results delineate the following sequence rules that govern the constitutive precursor cleavage. (a) A basic residue (Lys or Arg) at the 4th (position -4) or 6th (position -6) residue upstream of the cleavage site besides basic residues at positions -1 and -2 is necessary. (b) At position -2, a Lys residue is more preferable than Arg. (c) At position -4, an Arg residue is more preferable than Lys. (d) At position 1, a hydrophobic aliphatic amino acid is not suitable.     

Human furin is a calcium-dependent serine endoprotease that recognizes the sequence Arg-X-X-Arg and efficiently cleaves anthrax toxin protective antigen.

Previous work demonstrated that human furin is a predominantly Golgi membrane-localized endoprotease that can efficiently process precursor proteins at paired basic residues (-Lys-Arg- or -Arg-Arg-) in transfected cells. Anion-exchange chromatography of culture supernatant from cells expressing a soluble truncated form of human furin resulted in a greatly enriched preparation of the endoprotease (approximately 70% pure as determined by protein staining). Enzymatic studies show that furin is a calcium-dependent (K0.5 = 200 microM) serine endoprotease which has greater than 50% of maximal activity between pH 6.0 and 8.5. The inhibitor sensitivity of furin suggests that it is similar to, yet distinct from, other calcium-dependent proteases. Evidence that furin may require a P4 Arg in fluorogenic peptide substrates suggested that this enzyme might cleave the protective antigen (PA) component of anthrax toxin at the sequence -Arg-Lys-Lys-Arg-. Indeed, PA was cleaved by purified furin at the proposed consensus site (-Arg-X-Lys/Arg-Arg decreases-) at a rate (8 mumol/min/mg total protein) 400-fold higher than that observed with synthetic peptides. In addition, the processing of mutant PA molecules with altered cleavage sites suggests that furin-catalyzed endoproteolysis minimally requires an -Arg-X-X-Arg- recognition sequence for efficient cleavage. Together, these results support the hypothesis that furin processes protein precursors containing this cleavage site motif in the exocytic pathway and in addition, raises the possibility that the enzyme also cleaves extracellular substrates, including PA.     

Cloning and functional expression of a novel endoprotease involved in prohormone processing at dibasic sites.

We cloned and sequenced a cDNA from a library of mouse pituitary AtT-20 cells which are known to cleave an endogenous and various foreign prohormones at dibasic sites. This cDNA encodes a novel 753-residue protein, named PC3, which is structurally related to the yeast Kex2 protease involved in precursor cleavage at dibasic sites and to recently identified mammalian Kex2-like proteins, furin and PC2. Among examined cell lines and tissues, PC3 mRNA was only detected in AtT-20 cells. The substrate specificity of PC3 expressed in mammalian cells was similar to that observed in AtT-20 cells. We conclude that PC3 is a resident prohormone processing endoprotease in AtT-20 cells.     

Proteolytic cleavage of wild type and mutants of the F protein of human parainfluenza virus type 3 by two subtilisin-like endoproteases, furin and Kex2.

The fusion (F) protein of human parainfluenza virus type 3 contains the tribasic cleavage site R-T-K-R, which was altered by site-directed mutagenesis. Wild-type F protein and various mutants were expressed by recombinant vaccinia viruses. The endogenous endoprotease present in CV-1 cells cleaves F variants containing the furin recognition motif R-X-K/R-R but not variants containing the dibasic site K-R or a single R at the cleavage site. A similar cleavage pattern was obtained when the subtilisin-like endoproteases Kex2 and furin were coexpressed with the wild type and mutants of the F protein. Peptidylchloromethylketone inhibitors mimicking basic cleavage sites prevent cleavage of the precursor Fo by the endogenous protease only when the furin-specific motif is present in the peptidyl portion. The data support the concept that furin is a cellular protease responsible for the activation of the F protein of human parainfluenza virus type 3.     

Consensus sequence for precursor processing at mono-arginyl sites. Evidence for the involvement of a Kex2-like endoprotease in precursor cleavages at both dibasic and mono-arginyl sites.

Many peptide hormones and neuropeptides are produced from larger, inactive precursors through endoproteolysis at sites usually marked by paired basic residues (primarily Lys-Arg and Arg-Arg), or occasionally by a monobasic residue (primarily Arg). Based upon data concerning processing of prorenin and its mutants around the native Lys-Arg cleavage site expressed in mouse pituitary AtT-20 cells, we present the following sequence rules that govern mono-arginyl cleavages: (a) a basic residue at the fourth (position -4) or the sixth (position -6) residue upstream of the cleavage site is required, (b) at position -4, Arg is more favorable than Lys, and (c) at position 1, a hydrophobic aliphatic residue is not suitable. These rules are compatible with those proposed by comparison of precursor sequences around mono-arginyl cleavage sites. We also provide evidence that precursor cleavages at mono-arginyl and dibasic sites can be catalyzed by the same Kex2-like processing endoprotease, PC1/PC3.     

Intracellular trafficking and activation of the furin proprotein convertase: localization to the TGN and recycling from the cell surface.

Furin is a membrane-associated endoprotease that efficiently cleaves precursor proteins on the C-terminal side of the consensus sequence, Arg-X-Lys/Arg-Arg1, and has been proposed to catalyze these reactions in both exocytic and endocytic compartments. To study its biosynthesis and routing, a furin construct (designated fur/f) containing the FLAG epitope tag inserted on the C-terminal side of the enzyme's autoproteolytic maturation site was used. Introduction of the epitope tag had no effect on the expression, proteolytic maturation or activity of furin. Analysis of the localization of fur/f by immunofluorescence microscopy showed that its staining pattern largely overlapped with those of several Golgi-associated markers. Treatment of cells with brefeldin A caused the fur/f distribution to collapse around the microtubule organizing center, indicating that furin is concentrated in the trans-Golgi network (TGN). Immunoelectron microscopy showed unequivocally that furin resides in the TGN where it colocalized with TGN38. In agreement with its proposed activity in multiple compartments, antibody uptake studies showed that fur/f cycles between the cell surface and TGN. Furthermore, targeting to the TGN requires sequences in the cytoplasmic tail of the enzyme. Pulse-chase and immunofluorescence analyses demonstrated that proregion removal occurs in the endoplasmic reticulum and that cleavage may be required for exist from this compartment. Finally, we show that proregion removal is necessary but not sufficient for enzyme activation.     

A furin-defective cell line is able to process correctly the gp160 of human immunodeficiency virus type 1.

Furin, a subtilisin-like mammalian endoprotease, is thought to be responsible for the processing of many proprotein precursors of cellular and viral origin, including gp160 of human immunodeficiency virus type 1, which share the consensus processing site motif, Arg-X-Lys/Arg-Arg, for protease recognition (for reviews, see P. J. Barr, Cell 66:1-3, 1991, and Y. Nagai, Trends Microbiol. 1:81-87, 1993). To confirm and extend the concept that gp160 is processed by furin, we used here a cell line, LoVo, which was recently demonstrated to be furin defective. Unexpectedly, LoVo cells were found to process gp160 as efficiently as normal cell lines do, hence being able to fuse with CD4-expressing HeLa cells and to produce fully infectious virions. On the other hand, the same cell line was almost totally incapable of processing Newcastle disease virus fusion glycoprotein with a similar oligobasic cleavage recognition motif, providing a strong case for furin-mediated processing. Our present study thus raises a further need to search for and identify the proteinases involved in human immunodeficiency virus type 1 gp160 processing rather than supporting the notion that furin is responsible.     

Positional and additive effects of basic amino acids on processing of precursor proteins within the constitutive secretory pathway

We have recently shown that the Arg/Lys-X-Lys/Arg-Arg or Arg/Lys-X-X-X-Lys/Arg-Arg sequence serves as a signal for cleavage of precursor proteins within the constitutive secretory pathway, and this cleavage is catalyzed by furin, a mammalian homolog of the yeast Kex2 protease. In this study, we further examined sequence requirements for the constitutive precursor cleavage. Based on the data concerning cleavage efficiencies of various prorenin mutants with amino acid substitution(s) around the native cleavage site expressed in CHO cells, we revised the sequence rules that govern the constitutive cleavage as follows: (i) the Arg residue at position −1 is essential; (ii) in addition to the Arg at position −1, at least two out of the three basic residues at positions −2, −4, and −6 are required for efficient cleavage (the presence of all the three basic residues results in most efficient cleavage); (iii) at position +1, a hydrophobic aliphatic amino acid is not suitable.     

Identification of the fourth member of the mammalian endoprotease family homologous to the yeast Kex2 protease. Its testis-specific expression.

We used the polymerase chain reaction to identify a mouse testis cDNA that represented another member of a growing class of mammalian endoproteases involved in the processing of precursor proteins. This cDNA encoded a 655-residue protein, designated PC4, containing a bacterial subtilisin-like catalytic domain closely related to those of the recently characterized precursor-processing endoproteases, furin, PC1/PC3, PC2, and Kex2. Within this domain, the amino acid sequence of PC4 was 70, 58, 55, and 45% identical with those of mouse furin, mouse PC1/PC3, mouse PC2, and yeast Kex2, respectively. Northern blot analysis indicated that the PC4 mRNA was detectable only in the testes after the 20th day of postnatal development. Moreover, this message was mainly expressed in the round spermatids. These data suggest that PC4 represents a prime candidate for a precursor-processing endoprotease in the testicular germ cells and that its gene expression is regulated during spermatogenesis.     

Identification of a human insulinoma cDNA encoding a novel mammalian protein structurally related to the yeast dibasic processing protease Kex2

We have identified a human insulinoma cDNA (PC2) that encodes a protein homologous to the precursor processing Kex2 endoprotease of yeast by using a polymerase chain reaction to detect and amplify conserved sequences within the catalytic site. The 638-residue amino acid sequence of PC2 begins with a cleavable signal peptide, indicating that it enters the secretory pathway, and contains a 282-residue domain that is homologous to the catalytic modules of both Kex2 and the related bacterial subtilisins. Within this region 49 and 27% of the amino acids are identical to those in the aligned Kex2 and subtilisin BPN' sequences, respectively, and the catalytically essential Asp, His, and Ser residues are all conserved. Northern blot analysis revealed the presence of 2.8- and 5.0-kilobase hybridizing bands in mRNA from the insulinoma. The PC2 protein also shows great similarity to the incomplete NH2-terminal sequence of the human furin gene product, a putative membrane-inserted receptor-like molecule. We propose that PC2 is a member of a family of mammalian Kex2/subtilisin-like proteases that includes members involved in a number of specific proteolytic events within cells, including the processing of prohormones.     

Expression of the Saccharomyces cerevisiae Kex2p endoprotease in inset cells. Evidence for a carboxy-terminal autoprocessing event.

The pheromone-processing Kex2p endoprotease of Saccharomyces cerevisiae has been difficult to characterize due to its low level of expression in yeast cells. To overcome this problem, we have overexpressed Kex2p using the baculovirus/insect cell expression system. Spodoptera frugiperda Sf9 insect cells infected with a recombinant baculovirus, containing the complete KEX2 gene which encodes the Kex2p protease (814 amino acids), accumulate an 120-kDa functional form of the enzyme. The inhibition profile of the insect-cell-derived endoprotease is similar to that of the yeast enzyme. The recombinant infected insect cells also secrete into the medium about half of the total Kex2p activity produced. Deleting the carboxyl-terminal tail and the transmembrane domain of Kex2p (Kex2 delta p, 666 amino acids) does not measurably interfere with the enzyme characteristics and results in the secretion of up to 90% of the total enzyme activity. The truncated form, Kex2 delta p, of the endoprotease accumulates in the cell supernatant to 6.7 x 10(5) U/l. The molecular mass of the secreted forms for both the wild-type Kex2p and Kex2 delta p is the same (70 kDa) and is 50-kDa lower than the intracellular form. This result implicates a processing event which gives rise to shorter extracellular forms of both the wild-type Kex2p and Kex2 delta p and which trims their carboxy termini upsteam of amino acid 666. This processing event requires the integrity of the Ser385 of the Kex2p active site.     

Human fur gene encodes a yeast KEX2-like endoprotease that cleaves pro- beta-NGF in vivo

Extracts from BSC-40 cells infected with vaccinia recombinants expressing either the yeast KEX2 prohormone endoprotease or a human structural homologue (fur gene product) contained an elevated level of a membrane-associated endoproteolytic activity that could cleave at pairs of basic amino acids (-LysArg- and -ArgArg-). The fur-directed activity (furin) shared many properties with Kex2p including activity at pH 7.3 and a requirement for calcium. By using antifurin antibodies, immunoblot analysis detected two furin translation products (90 and 96 kD), while immunofluorescence indicated localization to the Golgi apparatus. Coexpression of either Kex2p or furin with the mouse beta- nerve growth factor precursor (pro-beta-NGF) resulted in greatly enhanced conversion of the precursor to mature nerve growth factor. Thus, the sequence homology shared by furin and the yeast KEX2 prohormone processing enzyme is reflected by significant functional homology both in vitro and in vivo.     

Activation of human furin precursor processing endoprotease occurs by an intramolecular autoproteolytic cleavage.

Human furin is a calcium-dependent serine endoprotease that can efficiently cleave many precursor proteins on the carboxyl side of the consensus cleavage sequence, -Arg-X-Lys/Arg-Arg-, both in vivo and in vitro. Analysis of furin proteins in extracts of cells infected with a vaccinia recombinant expressing human furin show that the enzyme is present as two prominent forms of 90 and 96 kDa. Because the structurally related bacterial subtilisins require endoproteolytic removal of the NH2-terminal pro-region by an autocatalytic intramolecular cleavage, we speculated that the size heterogeneity in the furin doublet similarly may result from a proteolytic removal of an NH2-terminal pro-region. Here we report identification of the 90-kDa furin NH2 terminus and, based on the reported sequence of the furin cDNA, demonstrate that this furin protein is derived from a larger precursor by an endoproteolytic cleavage on the COOH-terminal side of a consensus furin cleavage site, -Arg-Thr-Lys-Arg107-. Expression of mutant furin molecules containing an altered cleavage site (Arg104----Ala or Arg107----Gly) resulted in the production of only the 96-kDa furin protein. Assays of furin-dependent cleavage of a protein substrate in vitro showed that proteolytic activity was associated with the 90-kDa and not the 96-kDa furin protein, demonstrating that removal of the NH2-terminal pro-region is required for furin activity. Expression of a third furin construct containing a mutation of the active site aspartate (Asp153----Asn) similarly resulted in the expression of only the 96-kDa protein, suggesting that furin activation occurs by an autoproteolytic cleavage. Finally, the production of 90-kDa furin from either site-directed furin mutant could not be potentiated by overexpressing active furin, suggesting that the autoproteolytic activation was an intramolecular event.     

Evidence that differentiates between precursor cleavages at dibasic and Arg-X-Lys/Arg-Arg sites.

It is well known that precursor cleavage at paired basic amino acids (e.g., Lys-Arg, Arg-Arg) within the regulated secretory pathway is one of the key steps to produce bioactive peptides. On the other hand, we have recently shown that precursors with an Arg residue at the fourth residue upstream of the cleavage site besides the basic pair, i.e. with the Arg-X-Lys/Arg-Arg (RXK/RR) motif, are cleaved within the constitutive secretory pathway. To discriminate between the precursor cleavage at RXK/RR sites within the constitutive pathway and that at dibasic sites within the regulated pathway, we examined the effects of drugs affecting the secretory process, intracellular Ca2+ depletion, and a protease inhibitor on these cleavages. Chloroquine (a weak base), depletion of intracellular Ca2+ by A23187 (a Ca2+ ionophore), and the Pittsburgh-type mutant of alpha 1-protease inhibitor differentially affected these two cleavages. Brefeldin A, which impedes protein transport from the endoplasmic reticulum to the Golgi complex, inhibited both cleavages. Colchicine (an anti-microtubular drug) had no discernible effect on either cleavage. These observations support the notion that the precursor cleavages at dibasic and RXK/RR sites occur in different subcellular compartments, and are catalyzed by different processing endoproteases.     

Kex2-like proteolytic activity in adrenal medullary chromaffin granules.

This study demonstrates the presence of boc-Gln-Arg-Arg-MCA cleaving activity in bovine chromaffin granule membranes that resembles yeast Kex2 proteolytic activity. The chromaffin granule boc-Gln-Arg-Arg-MCA cleaving activity, like Kex2 proteolytic activity, shows calcium dependence, optimum activity at pH 7.5-8.2, inhibition by serine protease inhibitors, and preference for cleavage at the COOH-terminal side of Arg-Arg and Lys-Arg, over Lys-Lys, paired basic residues. Potent inhibition by the active-site directed inhibitor [D-Tyr]-Glu-Phe-Lys-Arg-CK (20 microM) provided further evidence for dibasic residue cleavage site specificity. These results are the first report of endogenous mammalian Kex2-like proteolytic activity that may be related to PC1/PC3 and PC2 enzymes, the newly discovered mammalian homologues of Kex2 protease. It will be important to determine the role of this Kex2-like proteolytic activity in processing the precursors of adrenal medullary neuropeptides.     

Sequence requirements for prohormone processing in mouse pituitary AtT-20 cells. Analysis using prorenins as model substrates

Although cleavage of peptides at sites marked by paired basic amino acids is a common feature of prohormone processing, little is known about the properties of endoprotease(s) responsible for cleavage of the precursor. To examine the cleavage specificity of a processing endoprotease, we have altered the Lys-Arg cleavage site of human prorenin to Arg-Arg, Lys-Lys and Arg-Lys by site-directed mutagenesis, and expressed the native and mutated precursors in mouse pituitary AtT-20 cells which are known to process foreign prohormones, including prorenin, at paired basic sites during the regulated secretory process. All native and mutated human prorenins were sorted into the regulated secretory pathway. The mutated precursor with Arg-Arg instead of the Lys-Arg native pair was processed at about half the efficiency of the native one, while the Lys-Lys and Arg-Lys mutants were not processed. Rat prorenin, which naturally has a Lys-Lys pair, was not processed in the cells. In addition, mouse Ren2 prorenin, which has a Ser residue next to the Lys-Arg pair, but not mouse Ren1 prorenin, which has a Pro residue next to the pair, was processed. These results suggest that the Arg residue at the COOH side of the basic pair is essential for cleavage of prorenins by a processing enzyme during the regulated secretory process in AtT-20 cells, although the NH2-side Lys residue also plays a role. The results also demonstrate that the processing enzyme cannot cleave the Arg-Pro peptide bond.