Proprotein processing activity and cleavage site selectivity of the Kex2-like endoprotease PACE4

Generation and characterization of non-competitive furin-inhibiting nanobodies

The PC (proprotein convertase) furin cleaves a large variety of proproteins and hence plays a major role in many pathologies. Therefore furin inhibition might be a good strategy for therapeutic intervention, and several furin inhibitors have been generated, although none are entirely furin-specific. To reduce potential side effects caused by cross-reactivity with other proteases, dromedary heavy-chain-derived nanobodies against catalytically active furin were developed as specific furin inhibitors. The nanobodies bound only to furin but not to other PCs. Upon overexpression in cell lines, they inhibited the cleavage of two different furin substrates, TGFβ (transforming growth factor β) and GPC3 (glypican 3). Purified nanobodies could inhibit the cleavage of diphtheria toxin into its enzymatically active A fragment, but did not inhibit cleavage of a small synthetic peptide-based substrate, suggesting a mode-of-action based on steric hindrance. The dissociation constant of purified nanobody 14 is in the nanomolar range. The nanobodies were non-competitive inhibitors with an inhibitory constant in the micromolar range as demonstrated by Dixon plot. Furthermore, anti-furin nanobodies could protect HEK (human embryonic kidney)-293T cells from diphtheria-toxin-induced cytotoxicity as efficiently as the PC inhibitor nona-D-arginine. In conclusion, these antibody-based single-domain nanobodies represent the first generation of highly specific non-competitive furin inhibitors.

The conditioned medium from a stable human GDF3-expressing CHO cell line, induces the differentiation of PC12 cells

Members of the transforming growth factor-β (TGF-β) superfamily have significant roles in the regulation of a wide variety of physiological processes. In our present work, phylogenetic tree analysis showed that human GDF3 (Growth and differentiation factor 3) and human GDF1 formed a subgroup of closely related molecules. Through quantitative real-time PCR analysis in different human tissues, GDF1 and GDF3 expression level had a big difference in brain. GDF3 could activate downstream signaling through associating with ALK7 (Activin receptor-like kinase 7) in a Cripto-dependent fashion. A CHO cell line stably transfected with the encoding sequence of GDF3, named CHO-GDF3, was established. Western blotting analysis demonstrated that GDF3 protein could be secreted into the medium from CHO cells and immunofluorescence experiment showed that GDF3 was mainly distributed in cytoplasm of the stable cell line, the primary hippocampal neurons, and brain tissues. Furthermore, the conditioned medium from CHO-GDF3 could reduce PC12 cell growth and induce cell differentiation. All these findings bring new insights into the functional study of GDF3.

Molecular Validation of PACE4 as a Target in Prostate Cancer

Prostate cancer remains the single most prevalent cancer in men. Standard therapies are still limited and include androgen ablation that initially causes tumor regression. However, tumor cells eventually relapse and develop into a hormone-refractory prostate cancer. One of the current challenges in this disease is to define new therapeutic targets, which have been virtually unchanged in the past 30 years. Recent studies have suggested that the family of enzymes known as the proprotein convertases (PCs) is involved in various types of cancers and their progression. The present study examined PC expression in prostate cancer and validates one PC, namely PACE4, as a target. The evidence includes the observed high expression of PACE4 in all different clinical stages of human prostate tumor tissues. Gene silencing studies targeting PACE4 in the DU145 prostate cancer cell line produced cells (cell line 4-2) with slower proliferation rates, reduced clonogenic activity, and inability to grow as xenografts in nude mice. Gene expression and proteomic profiling of the 4-2 cell line reveals an increased expression of known cancer-related genes (e.g., GJA1, CD44, IGFBP6) that are downregulated in prostate cancer. Similarly, cancer genes whose expression is decreased in the 4-2 cell line were upregulated in prostate cancer (e.g., MUC1, IL6). The direct role of PACE4 in prostate cancer is most likely through the upregulated processing of growth factors or through the aberrant processing of growth factors leading to sustained cancer progression, suggesting that PACE4 holds a central role in prostate cancer.

Proprotein convertases process Pmel17 during secretion

Pmel17 is a melanocyte/melanoma-specific protein that traffics to melanosomes where it forms a fibrillar matrix on which melanin gets deposited. Before being cleaved into smaller fibrillogenic fragments the protein undergoes processing by proprotein convertases, a class of serine proteases that typically recognize the canonical motif RX(R/K)R↓. The current model of Pmel17 maturation states that this processing step occurs in melanosomes, but in light of recent reports this issue has become controversial. We therefore addressed this question by thoroughly assessing the processing kinetics of either wild-type Pmel17 or a secreted soluble Pmel17 derivative. Our results demonstrate clearly that processing of Pmel17 occurs during secretion and that it does not require entry of the protein into the endocytic system. Strikingly, processing proceeds even in the presence of the secretion inhibitor monensin, suggesting that Pmel17 is an exceptionally good substrate. In line with this, we find that newly synthesized surface Pmel17 is already quantitatively cleaved. Moreover, we demonstrate that Pmel17 function is independent of the sequence identity of its unconventional proprotein convertase-cleavage motif that lacks arginine in P4 position. The data alter the current view of Pmel17 maturation and suggest that the multistep processing of Pmel17 begins with an early cleavage during secretion that primes the protein for later functional processing.

Lefty proteins exhibit unique processing and activate the MAPK pathway

Lefty polypeptides, novel members of the transforming growth factor-beta (TGF-beta) superfamily, are involved in the formation of embryonic lateral patterning. Members of the TGF-beta superfamily require processing for their activation, suggesting cleavage to be an essential step for lefty activation. Transfection of different cell lines with lefty resulted in expression of a 42-kDa protein, which was proteolytically processed to release two polypeptides of 34 and 28 kDa. Since members of the proprotein convertase (PC) family cleave different TGF-beta factors and are involved in the establishment of embryonic laterality, we studied their role in lefty processing. Cotransfection analysis showed that PC5A processed the lefty precursor to the 34-kDa form in vivo, whereas furin, PACE4, PC5B, and PC7 had a limited activity. None of these PCs showed activity in the processing of the lefty polypeptide to the 28-kDa lefty form. The mutation of the consensus sequences for PC cleavage in the lefty protein allowed the lefty cleavage sites to be identified. Mutations of the sequence RGKR to GGKG (amino acids 74-77) and of RHGR to GHGR (amino acids 132-135) prevented the proteolytic processing of the lefty precursor to the 34- and 28-kDa forms, respectively. To identify the biologically active form of lefty, we studied the effect of lefty treatment on pluripotent P19 cells. Lefty did not induce Smad2 or Smad5 phosphorylation, Smad2/Smad4 heterodimerization, or nuclear translocation of Smad2 or Smad4, but activated the MAPK pathway in a time- and dose-dependent fashion. Further analysis showed the 28-kDa (but not the 34-kDa) polypeptide to induce MAPK activity. Surprisingly, the 42-kDa lefty protein was also capable of inducing MAPK activity, indicating that the lefty precursor is biologically active. The data support a molecular model of processing as a mechanism for regulation of lefty signaling.

Primary structure requirements for Xenopus nodal-related 3 and a comparison with regions required by Xenopus nodal-related 2

Transforming growth factor-beta superfamily members play important roles in the early development of animals. Activin and the Xenopus nodal related proteins 1, 2, and 4 induce muscle actin from Xenopus ectodermal explants, whereas the bone morphogenetic proteins 4 and 7 induce ectoderm to differentiate as epidermis. Bone morphogenetic proteins are antagonized by soluble binding proteins such as noggin and chordin, which leads to expression of neural cell adhesion molecule in animal caps. The transforming growth factor-beta superfamily member Xenopus nodal-related 3 also induces the neural cell adhesion molecule through inhibition of bone morphogenetic proteins. Therefore, whereas Xenopus nodal-related 2 and 3 share a high amount of sequence homology, they lead to very different cell fates. This study investigates the functional domains that distinguish the activities of these two factors. It was found that mutually exclusive regions of nodal-related 2 and 3 were required for activity. The central region of the mature domain is required for nodal-related 2 to induce muscle actin, whereas the N- and C-terminal ends of the mature domain are required for nodal-related 3 to induce neural cell adhesion molecule. These results help to define the minimal domains required for the unique activities of these factors.

Recombinant human factor X: high yield expression and the role of furin in proteolytic maturation in vivo and in vitro

Factor X/Xa plays a pivotal role in the coagulation cascade and exhibits a therapeutic potential for the treatment of factor X-deficient as well as FVIII and FIX inhibitor patients. This report describes the establishment of Chinese hamster ovary cell clones expressing recombinant human factor X up to 120 microg/mL x day and 78 microg/10(6) cells x day, that is to 100-fold higher levels than reported previously. Although propeptide removal and single chain precursor to light and heavy chain processing as well as vitamin K-dependent gamma-carboxylation became impaired at these expression levels, up to 25% of the recombinant human factor X produced was active. This represents the highest functional activity ever reported for a vitamin K-dependent protein at such an expression level. Expression of recombinant human factor X in Chinese hamster ovary cells lacking the endoprotease Furin revealed that propeptide removal still occurred, whereas single chain precursor to light/heavy chain processing was abolished. This suggests that a protease different from Furin mediates propeptide removal, a unique finding compared with the other vitamin K-dependent coagulation factors. In contrast, exposure of incompletely processed rFX molecules to soluble recombinant Furin in vitro mediated both of these cleavage reactions despite the absence of a typical argP4-xP3-lys/argP2-argP1 Furin cleavage site in the propeptide, indicating relaxed specificity in vitro. Concomitantly with the degree of processing, the functional activity of recombinant human factor X increased. Interestingly, Furin was shown to even perform correct N-terminal proteolytic trimming of FX molecules truncated amino-terminal to the P3 residue in vitro. Depending on the absence or presence of warfarin in the culture media, as well as on the processing state, four distinct recombinant human factor X light chain isoforms were observed and their structure characterized. One of these light chain forms correlated with the functional activity. Finally, the distribution of the individual light chain isoforms suggests that gamma-carboxylation may be a prerequisite for propeptide removal.

Immunohistochemical distribution of the prohormone convertase PC5-A in rat brain

Prohormone convertase 5 is an endoprotease of the kexin/subtilisin-like family, which has been postulated to play a role in the proteolytic maturation of a variety of pro-peptides in the mammalian brain. In order to gain insight into the functional role of prohormone convertase 5 in the central nervous system, the regional, cellular and subcellular distributions of the enzyme were investigated by immunohistochemistry in rat brain using an N-terminal-directed specific antibody shown previously to recognize both the mature and unprocessed forms of the enzyme. Throughout the brain, prohormone convertase 5 immunoreactivity was concentrated within nerve cell bodies and proximal dendrites. No prohormone convertase 5 immunoreactivity was associated with astrocytes, as confirmed by the absence of prohormone convertase 5 immunolabeling in cells immunopositive for the glial protein S-100alpha. Within neurons, prohormone convertase 5 immunoreactivity was concentrated within the Golgi apparatus, as revealed immunohistochemically within the same sections using antibodies against the medial cisternae protein MG-160. It was also present within small vesicular-like elements distributed throughout the cytoplasm of perikarya and dendrites, but not of axons, as confirmed by its lack of co-localization with the synaptic terminal marker Dynamin-1. These results suggest that prohormone convertase 5 is active within early compartments of the neuronal regulated secretory pathway and that it is unlikely to be released with its processed substrates. At the regional level, prohormone convertase 5-immunoreactive perikarya were distributed extensively throughout the forebrain. The most numerous and intensely labeled were detected in the olfactory bulb, cerebral cortex, globus pallidus, endopeduncular and subthalamic nuclei, septum, diagonal band of Broca, magnocellular and medial preoptic areas, supraoptic and arcuate nuclei of the hypothalamus, and anterodorsal, laterodorsal, paraventricular and reticular nuclei of the thalamus. Moderate to dense neuronal labeling was also evident in the olfactory tubercle, caudate-putamen, claustrum, bed nucleus of the stria terminalis, substantia innominata, hippocampus, amygdala, and remaining thalamic and hypothalamic nuclei. This widespread distribution suggests that prohormone convertase 5 is involved in the processing of a variety of neuropeptide and/or neurotrophin precursors in mammalian brain.

Comparison of substrate specificities against the fusion glycoprotein of virulent Newcastle disease virus between a chick embryo fibroblast processing protease and mammalian subtilisin-like proteases

The fusion (F) protein precursor of virulent Newcastle disease virus (NDV) strains has two pairs of basic amino acids at the cleavage site, and its intracellular cleavage activation occurs in a variety of cells; therefore, the viruses cause systemic infections in poultry. To explore the protease responsible for the cleavage in the natural host, we examined detailed substrate specificity of the enzyme in chick embryo fibroblasts (CEF) using a panel of the F protein mutants at the cleavage site expressed by vaccinia virus vectors, and compared the specificity with those of mammalian subtilisin-like proteases such as furin, PC6 and PACE4 which are candidates for F protein processing enzymes. It was demonstrated in CEF cells that Arg residues at the -4, -2 and -1 positions upstream of the cleavage site were essential, and that at the -5 position was required for maximal cleavage. Phe at the +1 position was also important for efficient cleavage. On the other hand, furin and PC6 expressed by vaccinia virus vectors showed cleavage specificities against the F protein mutants consistent with that shown by the processing enzyme of CEF cells, but PACE4 hardly cleaved the F proteins including the wild type. These results indicate that the proteolytic processing enzymes of poultry for virulent NDV F proteins could be furin and/or PC6 but not PACE4. The significance of individual contribution of the three amino acids at the -5, -2 and +1 positions to cleavability was discussed in relation to the evolution of virulent and avirulent NDV strains.

Processing of the fibrillin-1 carboxyl-terminal domain

To investigate the processing and general properties of the fibrillin-1 carboxyl-terminal domain, three protein expression constructs have been developed as follows: one without the domain, one with the domain, and one with a mutation near the putative proteolytic processing site. The constructs have been expressed in two eukaryotic model systems, baculoviral and CHO-K1. Post-translational modifications that normally occur in fibrillin-1, including glycosylation, signal peptide cleavage, and carboxyl-terminal processing, occur in the three constructs in both cell systems. Amino-terminal sequencing of secreted protein revealed leader sequence processing at two sites, a primary site between Gly-24/Ala-25 and a secondary site of Ala-27/Asn-28. Processing of the carboxyl-terminal domain could be observed by migration differences in SDS-polyacrylamide gel electrophoresis and was evident in both mammalian and insect cells. Immunological identification by Western blotting confirmed the loss of the expected region. The failure of both cell systems to process the mutant construct shows that the multi-basic sequence is the site of proteolytic processing. Cleavage of the fibrillin-1 carboxyl-terminal domain occurred intracellularly in CHO-K1 cells in an early secretory pathway compartment as demonstrated by studies with secretion blocking agents. This finding, taken with the multi-basic nature of the cleavage site and observed calcium sensitivity of cleavage, suggests that the processing enzyme is a secretory pathway resident furin-like protease.

Regulation of bone morphogenetic protein activity by pro domains and proprotein convertases

Bone morphogenetic proteins (BMPs) are derived from inactive precursor proteins by endoproteolytic cleavage. Here we show that processing of Nodal and Myc-tagged BMP4 is significantly enhanced by SPC1/Furin or SPC4/PACE4, providing direct evidence that regulation of BMP signaling is likely to be controlled by subtilisin-like proprotein convertase (SPC) activities. Nodal processing is dramatically enhanced if two residues adjacent to the precursor cleavage site are substituted with amino acids found at the equivalent positions of Activin, demonstrating that structural constraints at the precursor cleavage site limit the processing efficiency. However, in transfection assays, mature Nodal is undetectable either in culture supernatants or in cell lysates, despite efficient cleavage of the precursor protein, suggesting that mature Nodal is highly unstable. Domain swap experiments support this conclusion since mature BMP4 or Dorsalin are also destabilized when expressed in conjunction with the Nodal pro domain. By contrast, mature Nodal is stabilized by the Dorsalin pro domain, which mediates the formation of stable complexes. Collectively, these data show that the half-life of mature BMPs is greatly influenced by the identity of their pro regions.

A role for PACE4 in the proteolytic activation of anthrax toxin protective antigen

Several bacterial protein toxins require activation by eukaryotic proteases. Previous studies have shown that anthrax toxin protective antigen (PA), Pseudomonas exotoxin A (PE), and diphtheria toxin (DT) are cleaved by furin C-terminal to the sequences RKKR, RQPR, and RVRR, respectively. Because furin-deficient cells retain some sensitivity to PA and DT, it is evident that other cellular proteases can activate these toxins. Whereas furin has been shown to require arginine residues at positions -1 and -4 for substrate recognition, another protease with an activity which could substitute for furin in toxin activation, the furin-related protease PACE4, requires basic residues in the -1, -2, and -4 positions of the substrate sequence. To examine the relative roles of furin and PACE4 in toxin activation, we used furin-deficient CHO cells (FD11 cells) transfected with either the furin (FD11/furin cells) or PACE4 (FD11/PACE4 cells) gene. Mutant PA proteins containing the cleavage sequence RAAR or KR were cytotoxic toward cells expressing only PACE4. In vitro cleavage data demonstrated that PACE4 can recognize RAAR and, to a much lesser extent, KR and RR. When extracts from PACE4-transfected cells were used as a source of proteases, PACE4 had minimal activity, indicating that it had been partially inactivated or did not remain associated with the cell membranes. Cleavage of iodinated PA containing the sequence RKKR or RAAR was detected on the surface of all cell types tested, but cleavage of a dibasic sequence was detected only intracellularly and only in cells that expressed furin or PACE4. The data provide evidence that PACE4 is present at the exterior of cells, that it plays a role in the proteolytic activation of anthrax toxin PA, and that PACE4 can activate substrates at the sequence RAAR or KR.

Identification of the cleavage sites in the alpha6A integrin subunit: structural requirements for cleavage and functional analysis of the uncleaved alpha6Abeta1 integrin

The alpha6A and alpha6B integrin subunits are proteolytically cleaved during biosynthesis into a heavy chain (120 kDa) that is disulphide-linked to one of two light chains (31 or 30 kDa). Analysis of the structure of the alpha6A subunit on the carcinoma cell line T24 and human platelets demonstrated that the two light chains of alpha6 are not differentially glycosylated products of one polypeptide. Rather they possess different polypeptide backbones, which presumably result from proteolytic cleavage at distinct sites in the alpha6 precursor. Mutations were introduced in the codons for the R876KKR879, E883K884, R890K891 and R898K899 sequences, the potential proteolytic cleavage sites, and wild-type and mutant alpha6A cDNAs were transfected into K562 cells. The mutant alpha6A integrin subunits were expressed in association with endogenous beta1 at levels comparable to that of wild-type alpha6Abeta1. A single alpha6 polypeptide chain (150 kDa) was precipitated from transfectants expressing alpha6A with mutations or deletions in the RKKR sequence. Mutations in the EK sequence yielded alpha6A subunits that were cleaved once into a heavy and a light chain, whereas alpha6A subunits with mutations in one of the two RK sequences were, like wild-type alpha6A, cleaved into one heavy and two light chains. Thus a change in the RKKR sequence prevents the cleavage of alpha6. The EK site is the secondary cleavage site, which is used only when the primary site (RKKR) is intact. Microsequencing of the N-termini of the two alpha6A light chains from platelets demonstrated that cleavage occurs after Arg879 and Lys884. Because alpha6(RKKG), alpha6(GKKR) and alpha6(RGGR) subunits were not cleaved it seems that both the arginine residues and the lysine residues are essential for cleavage of RKKR. alpha6A mutants with the RKKR sequence shifted to the EK site, in such a way that the position of the arginine residue after which cleavage occurs corresponds exactly to Lys884, were partly cleaved, whereas alpha6A mutants with the RKKR sequence shifted to other positions in the alpha6A subunit, including one in which it was shifted two residues farther than the EK cleavage site, were not cleaved. In addition, alpha6A mutants with an alpha5-like cleavage site, i.e. arginine, lysine and histidine residues at positions -1, -2 and -6, were not cleaved. Thus both an intact RKKR sequence and its proper position are essential. After activation by the anti-beta1 stimulatory monoclonal antibody TS2/16, both cleaved and uncleaved alpha6Abeta1 integrins bound to laminin-1. The phorbol ester PMA, which activates cleaved wild-type and mutant alpha6Abeta1, did not activate uncleaved alpha6Abeta1. Thus uncleaved alpha6Abeta1 is capable of ligand binding, but not of inside-out signalling. Our results suggest that cleavage of alpha6 is required to generate a proper conformation that enables the affinity modulation of the alpha6Abeta1 receptor by PMA.

Processing of wild-type and mutant proinsulin-like growth factor-IA by subtilisin-related proprotein convertases

Insulin-like growth factor I (IGF-I) is required for normal embryonic development and postnatal growth. Like most hormones and growth factors, IGF-I is synthesized as a proprotein that is converted to the mature form by endoproteolysis. Processing of pro-IGF-I to mature IGF-I occurs by cleavage within the unique pentabasic processing motif Lys-X-X-Lys-X-X-Arg71-X-X-Arg-X-X-Arg77. We have previously shown that human embryonic kidney 293 cells process pro-IGF-IA at Arg71 to generate IGF-I-(1-70) and at Arg77 to produce IGF-I-(1-76). Cleavage at each of these sites requires upstream basic residues, indicating that subtilisin-related proprotein convertases (SPCs) may be involved. In order to investigate the identity of the endogenous enzymes involved in maturation of pro-IGF-IA, we have expressed wild-type and mutant pro-IGF-IA in 293 cells and in the furin-deficient Chinese hamster ovary cell line, RPE.40. We have also co-expressed these constructs with SPCs that are thought to play a role in processing precursor proteins in the constitutive pathway: furin, PACE4, PC6A, PC6B, and LPC. The results show that furin is most active at cleaving wild-type and mutant pro-IGF-IA and can cleave these precursors at multiple sites within the pentabasic motif. PC6A and LPC are less active than furin but cleave only at Arg71. PACE4 and PC6B have very little activity on pro-IGF-IA precursors. Wild-type pro-IGF-IA was correctly processed to mature IGF-I in 10 of 10 cell lines that were tested. Since furin, PC6A, and LPC are known to have a broad pattern of tissue distribution and we have demonstrated expression of LPC in RPE.40 cells, our results suggest that these SPCs may be responsible for the endogenous pro-IGF-IA processing activity observed in a wide variety of cell lines.

The role of eukaryotic subtilisin-like endoproteases for the activation of human immunodeficiency virus glycoproteins in natural host cells

Proteolytic activation of the precursor envelope glycoproteins gp160 of human immunodeficiency virus type 1 (HIV-1) and gp140 of HIV-2, a prerequisite for viral infection, results in the formation of gp120/gp41 and gp125/gp36, respectively. Cleavage is mediated by cellular proteases. Furin, a member of the eukaryotic subtilisin family, has been shown to be an activating protease for HIV. Here, we compared the presence of furin and other mammalian subtilisins in lymphatic cells and tissues. Northern blot analyses revealed that furin and the recently discovered protease LPC/PC7 were the only subtilisin-like enzymes transcribed in such cells. Furin was identified as an enzymatically active endoprotease present in different lymphocytic, as well as monocytic, cell lines. When expressed from vaccinia virus vectors, the proprotein convertases were correctly processed, transported, and secreted into the media and enzymatically active. Coexpression of different subtilisins with the HIV envelope precursors revealed that furin and LPC/PC7 are able to cleave HIV-1 gp160. Moreover, both enzymes proteolytically processed the envelope precursor of HIV-2. gp140 was also cleaved to some extent by PC1, which is not, however, present in lymphatic cells. Furin- and LPC/PC7-catalyzed cleavage of HIV-1 gp160 resulted in biologically active envelope protein. In conclusion, among the known members of the subtilisin family, only furin and LPC/PC7 fulfill the requirements of a protease responsible for in vivo activation of HIV envelope glycoproteins.

PACE4: a subtilisin-like endoprotease with unique properties

PACE4 is one of the neuroendocrine-specific mammalian subtilisin-related endoproteases believed to function in the secretory pathway. The biosynthesis and secretion of PACE4 have been studied using transfected neuroendocrine and fibroblast cell lines. as well as primary pituitary cultures. ProPACE4 (approx. 106 kDa) is cleaved intracellularly before secretion of PACE4 (approx. 97 kDa); the N-terminal propeptide cleavage is accelerated in a truncated form of PACE4 lacking the Cys-rich C-terminal region (PACE4s). Neither PACE4 nor PACE4s is stored in regulated neuroendocrine secretory granules, whereas pro-opiomelanocortin-derived peptides and prohormone convertase I enter the regulated secretory pathway efficiently. The relatively slow cleavage of the proregion of proPACE4 in primary anterior pituitary cells, followed by rapid secretion of PACE4, is similar to the results for proPACE4 in transfected cell lines. The enzyme activity of PACE4 is distinct from furin and prohormone convertases, both in the marked sensitivity of PACE4 to inhibition by leupeptin and the relative insensitivity of PACE4 to inhibition by Ca2+ chelators and dithiothreitol; PACE4 is not inhibited by the alpha1-antitrypsin Portland variant that is very potent at inhibiting furin. The unique biosynthetic and enzymic patterns seen for PACE4 suggest a role for this neuroendocrine-specific subtilisin-like endoprotease outside the pathway for peptide biosynthesis.

Endoprotease activities other than furin and PACE4 with a role in processing of HIV-I gp160 glycoproteins in CHO-K1 cells

We addressed the question of whether furin is the endoprotease primarily responsible for processing the human immunodeficiency virus type I (HIV-I) envelope protein gp160 in mammalian cells. The furin-deficient Chinese hamster ovary (CHO)-K1 strain RPE.40 processed gp160 as efficiently as wild-type CHO-K1 cells in vivo. Although furin can process gp160 in vitro, this processing is probably not physiologically relevent, because it occurs with very low efficiency. PACE4, a furin homologue, allowed processing of gp160 when both were expressed in RPE.40 cells. Further, PACE4 participated in the activation of a calcium-independent protease activity in RPE.40 cells, which efficiently processed the gp160 precursor in vitro. This calcium-independent protease activity was not found in another furin-deficient cell strain, 7.P15, selected from the monkey kidney cell line COS-7.

Human lactase-phlorizin hydrolase is not processed by furin, PC1/PC3, PC2, PACE4 and PC5/PC6A of the family of subtilisin-like proprotein processing proteases

Human lactase-phlorizin hydrolase (LPH, EC 3.2.1.23/62) is synthesized as a single-chain precursor glycoprotein (pro-LPH) with a relative molecular mass of just over 200 kDa. Maturation to the mature enzyme (m-LPH, 160 kDa) occurs after passage of pro-LPH through the Golgi complex and involves the proteolytic removal of a 849 amino acid propeptide. The role of this propeptide as well as its removal is not fully understood and the proteolytic enzyme or enzymes involved are unknown. We studied the potential role of five different members of the family of subtilisin-like proprotein processing proteases in the maturation process of human LPH using a vaccinia virus based coexpression system in pig kidney PK(15) cells. Infected/transfected PK(15) cells expressed full-length pro-LPH but no maturation to m-LPH was observed. Coexpression of human pro-LPH with human furin, human PC1/PC3, human PC2, human PACE4 and mouse PC6A in PK(15) cells did not result in maturation of the enzyme. Cleavage and secretion of von Willebrand factor precursor (pro-vWF) was used as a positive control. None of the five proprotein processing proteases tested were capable of cleaving human pro-LPH, strongly suggesting that they are not involved in the maturation of this enzyme.

Mutational analysis of the insulin-like growth factor I prohormone processing site

Insulin-like growth factor I (IGF-I) is a mitogenic peptide that is produced in most tissues and cell lines and plays an important role in embryonic development and postnatal growth. IGF-I is initially synthesized as a prohormone precursor that is converted to mature IGF-I by endoproteolytic removal of the carboxyl-terminal E-domain. Regulation of the conversion of proIGF-I to mature IGF-I is a potential mechanism by which the biological activity of this growth factor might be modulated. Endoproteolysis of the IGF-I prohormone occurs at the unique pentabasic motif Lys-X-X-Lys-X-X-Arg71-X-X-Arg-X-X-Arg. Recently, a family of enzymes which cleave prohormone precursors at sites containing multiple basic residues has been discovered. The goals of this study were 1) to determine which basic residues in the pentabasic proIGF-I processing site were necessary for proper cleavage and 2) to examine the role that subtilisin-related proprotein convertase 1 (SPC1/furin) might play in proIGF-I processing. We have shown that an expression vector coding for an epitope-tagged proIGF-I directs synthesis and secretion of mature IGF-I-(1-70), extended IGF-I-(1-76), proIGF-I, and N-glycosylated proIGF-I in human embryonic kidney 293 cells. Extended IGF-I-(1-76) is produced by cleavage at Arg77 and requires both Arg74 (P4) and Arg77 (P1). Cleavage at Arg77 does not occur in the SPC1-deficient cell lines RPE.40 and LoVo, suggesting that processing at this site is mediated by SPC1. Mature IGF-I-(1-70) is produced by cleavage at Arg71 and requires both Lys68 (P4) and Arg71 (P1). Lys65 in the P7 position is important for efficient cleavage. SPC1 is not required for processing at Arg71 since this cleavage occurs in RPE.40 and LoVo cells. These data suggest the existence of a processing enzyme which is specific for the Lys-X-X-Arg motif of proIGF-I.

Processing of transforming growth factor beta 1 precursor by human furin convertase

Proteolytic processing of the transforming growth factor beta precursor (pro-TGF beta) is an essential step in the formation of the biologically active TGF beta homodimeric protein (TGF beta). The 361-amino-acid precursor pro-TGF beta 1 has within its primary structure the R-H-R-R processing signal found in many constitutively secreted precursor proteins and potentially recognized by members of the mammalian convertase family of endoproteases. To determine whether cleavage of pro-TGF beta 1 can be achieved by the furin convertase in vitro, purified precursor was incubated in the presence of a truncated/secreted form of the enzyme. Immunoblots showed that the 55-kDa pro-TGF beta 1 was converted into the 44 and 12.5 kDa bands corresponding to the pro-region and the mature monomer, respectively. Treatment of pro-TGF beta 1 with furin resulted in a 5-fold increase in the production of biologically active TGF beta 1. Furthermore, when expressed in the furin-deficient LoVo cells, no processing of pro-TGF beta 1 was observed. In contrast, efficient processing was observed when pro-TGF beta was coexpressed with the furin convertase. Collectively, these results provide evidence that in our experimental systems the TGF beta 1 precursor is efficiently and correctly processed by human furin thus permitting release of the biologically active peptide.

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.

Amyloid precursor protein is not processed by furin, PACE 4, PC1/3, PC2, PC4 and PC5/6 of the furin family of proprotein processing enzymes

Proteolytic cleavage of the amyloid precursor protein (APP) has previously been shown to release its extracellular domain into the medium. The identification of the responsible proteinase(s), termed secretase(s), is a high priority in ongoing Alzheimer research. This is hampered by the unusual characteristics of these enzyme(s) and by the fact that they cleave only membrane associated APP. We report here, using a vaccinia virus based expression system, that pig kidney PK(15) cells express full-length, membrane bound APP695, but that secretion of APP is low. This heterologous expression system allows to assay candidate secretases in a cellular context by simple co-transfection of the APP and candidate secretase cDNA containing plasmids. Eight different members of the mouse and human furin family of proprotein processing enzymes were tested in this assay, but none of them enhanced the secretion of APP. Secretion of von Willebrand's factor was used as a positive control.

Proteolytic activation of bacterial toxins by eukaryotic cells is performed by furin and by additional cellular proteases

Before intoxication can occur, anthrax toxin protective antigen (PA), Pseudomonas exotoxin A (PE), and diphtheria toxin (DT) must be activated by proteolytic cleavage at specific amino acid sequences. Previously, it was shown that PA and DT can be activated by furin. In Chinese hamster ovary (CHO) cells, wild-type (RKKR) and cleavage site mutants of PA, each administered with a modified form of anthrax toxin lethal factor (the N terminus of lethal factor fused to PE domain III), had the following potencies: RKKR (wild type) (concentration causing 50% cell death [EC50] = 12 ng/ml) > or = RAAR (EC50 = 18 ng/ml) > FTKR (EC50 = 24 ng/ml) > STRR (EC50 = 49 ng/ml). In vitro cleavage of PA and cleavage site mutants of PA by furin demonstrated that native PA (RKKR) and PA with the cleavage sequence RAAR are substrates for furin. To characterize eukaryotic proteases that play a role in activating bacterial toxins, furin-deficient CHO cells were selected after chemical mutagenesis. Furin-deficient cells were resistant to PE, whose cleavage site, RQPR, constitutes a furin recognition site and to all PA cleavage site mutants, but were sensitive to DT (EC50 = 2.9 ng/ml) and PA (EC50 = 23 ng/ml), whose respective cleavage sites, RKKR and RVRR, contain additional basic residues. Furin-deficient cells that were transfected with the furin gene regained sensitivity to PE and PA cleavage site mutants. These studies provide evidence that furin can activate the three toxins and that one or more additional proteases contribute to the activation of DT and PA.

Processing and secretion of rat alpha 1-microglobulin-bikunin expressed in eukaryotic cell lines

The precursor protein alpha 1-microglobulin-bikunin was cleaved to the same degree whether expressed in CHO cells or in mutated CHO cells, RPE.40 cells, suggested to lack a functional form of the intracellular protease furin. Thus, alpha 1-microglobulin-bikunin probably is not cleaved in vivo by furin. However, simultaneous overexpression of the precursor and furin in COS, CHO and RPE.40 cells increased the cleavage, suggesting that compartmentalisation and concentrations of protease and precursor are important for the cleavage, besides the in vitro specificity. Expression of alpha 1-microglobulin and bikunin alone gave different protein patterns of SDS-PAGE as compared to expression of the precursor and subsequent cleavage, suggesting that the precursor protein is important for the post-translational handling of alpha 1-microglobulin and bikunin.

Proprotein-processing endoproteases PC6 and furin both activate hemagglutinin of virulent avian influenza viruses

Among the proprotein-processing subtilisin-related endoproteases, furin has been a leading candidate for the enzyme that activates the hemagglutinin (HA) of virulent avian influenza viruses. In the present study, we examined the cleavage activity of two other recently isolated ubiquitous subtilisin-related proteases, PACE4 and PC6, using wild-type HA of A/turkey/Ireland/1378/83 (H5N8) and a series of its mutant HAs. Vaccinia virus-expressed wild-type HA was not cleaved in human colon adenocarcinoma LoVo cells, which lack active furin. This processing defect was corrected by the expression of furin and PC6 but not of PACE4 and a control wild-type vaccinia virus. PC6 showed a sequence specificity similar to that with the endogenous proteases in cultured cells. When LoVo cells were infected with a virulent avian virus, A/turkey/Ontario/7732/66 (H5N9), only noninfectious virions were produced because of the lack of HA cleavage. However, when the cells were coinfected with vaccinia virus that expressed either furin or PC6, the avian virus underwent multiple cycles of replication, indicating that both furin and PC6 specifically cleave the virulent virus HA at the authentic site. These data suggest that PC6, as well as furin, can activate virulent avian influenza viruses in vivo, implying the presence of multiple HA cleavage enzymes in animals.

Cleavage of proalbumin peptides by furin reveals unexpected restrictions at the P2 and P'1 sites

Proalbumin is the principal substrate of the in situ hepatic convertase. Here we investigated the specificity of furin using synthetic peptides based on the N-terminal sequence of human proalbumin. The propeptide was rapidly cleaved from the normal ((-6)RGVFRR(-1)DAHKSEAVW(+9)) peptide but as expected, there was no cleavage of the proalbumin Lille analogue with a -2 His (-2H). Surprisingly, the effect of this substitution could not be corrected by introducing a -4 Arg (-4R-2H). In contrast, the peptide -4R-2A was an excellent substrate being cleaved five times faster than normal, indicaabting that His is not allowed as an P2 residue. Replacement of the -4 Val by Glu supported the expected importance of a positive charge at P4 as the cleavage rate dropped to 10% of normal after this substitution. The -6 Arg makes a small contribution to cleavage, its replacement by Ala decreased the cleavage rate to 60% of normal. The Lys-Arg propeptide was almost as good a substrate as the normal Arg-Arg peptide, but the introduction of a Lys at P1 totally abolished processing. The exclusion of P'1 positive charges would be an important requirement for preventing aberrant cleavage in the middle of tetrabasic sequences.

Furin-mediated proprotein processing activity: involvement of negatively charged amino acid residues in the substrate binding region

Furin, which is encoded by the recently discovered FUR gene, appears to be the first known mammalian member of the subtilisin family of serine proteases with cleavage selectivity for paired or multiple basic residues. A consensus cleavage sequence, Arg-X-Lys/Arg-Arg has been proposed. Most likely, furin is primarily involved in the processing of precursors of proteins that are secreted via the constitutive secretory pathway. Homology modelling of the catalytic domain of this protein suggested that negatively charged amino acid residues near or in the substrate binding region might contribute to the observed specificity for substrate segments with paired and multiple basic amino acid residues. To investigate this hypothesis, furin mutants were generated in which negatively charged residues, predicted to be located near or in the substrate binding pockets and involved in interactions with basic residues of the substrate, were replaced by neutral residues. Analysis of processing by these furin mutants of wild-type and cleavage mutants of pro-von Willebrand factor (pro-vWF) revealed that particular negatively charged residues are critical for specific cleavage activity.