Inhibition of furin by serpin Spn4A from Drosophila melanogaster

The serpin gene Spn4 from Drosophila melanogaster encodes multiple isoforms with alternative reactive site loops (RSL). Here, we show that isoform Spn4A inhibits human furin with an apparent kassoc of 5.5 x 10(6) M(-1) s(-1). The serpin forms SDS-stable complexes with the enzyme and the RSL of Spn4A is cleaved C-terminally to the sequence -Arg-Arg-Lys-Arg/ in accord with the recognition/cleavage site of furin. Immunofluorescence studies show that Spn4A is localized in the endoplasmic reticulum (ER), suggesting that the inhibitor is an interesting tool for investigating the cellular mechanisms regulating furin and for the design of agents controlling prohormone convertases.     

Tip of another iceberg: Drosophila serpins

Serpins are serine protease inhibitors with a conserved structure that have been identified in nearly all species and act as suicide substrates by binding covalently to their target proteases. Serpins regulate various physiological processes and defence mechanisms. In humans, several serpin mutations are linked to diseases. The genome of Drosophila melanogaster encodes 29 serpins and even more serine proteases. To date, three serpins have been investigated in detail. Spn27A controls the Toll pathway during early development and is involved in defence reactions in adult flies. SPN42DaA is an inhibitor of furin, a subtilisin-like convertase that is required for pro-protein maturation. Spn43Ac controls the Toll pathway during the immune response. In each case, Drosophila genetics has shed new light on the function of these serine protease inhibitors.     

Serpin mechanism of hepatitis C virus nonstructural 3 (NS3) protease inhibition: induced fit as a mechanism for narrow specificity

Hepatitis C virus (HCV) nonstructural 3 (NS3) serine protease disrupts important cellular antiviral signaling pathways and plays a pivotal role in the proteolytic maturation of the HCV polyprotein precursor. This recent discovery has fostered the search for NS3 protease inhibitors. However, the enzyme's unusual induced fit behavior and peculiar molecular architecture have imposed considerable obstacles to the development of small molecule inhibitors. In this article, we demonstrate that such unique induced fit behavior and the chymotrypsin-like catalytic domain can provide the structural plasticity necessary to generate protein-based inhibitors of the NS3 protease. We took advantage of the macromolecular scaffold of a Drosophila serpin, SP6, which intrinsically supports chymotrypsin-like enzyme inhibition, to design a novel class of potent and selective inhibitors. We show that altering the SP6 reactive site loop (RSL) resulted in the development of the first effective (K(i) of 34 nm) and selective serpin, SP6(EVC/S), directed at the NS3 protease. SP6(EVC/S) operates as a suicide substrate inhibitor, and its partitioning between the complex-forming and proteolytic pathways for the NS3 protease is HCV NS4A cofactor-dependent and -specific. Once bound to the protease active site, SP6(EVC/S) partitions with equal probability to undergo proteolysis by NS3 at the C-terminal site of the engineered RSL, (P(6))Glu-Ile-(P(4))Val-Met-Thr-(P(1))Cys- downward arrow -(P(1)')Ser, or to form a covalent acyl-enzyme complex characteristic of cognate protease-serpin pairs. Our results also reveal a novel cofactor-induced serpin mechanism of enzyme inhibition that could be explored for developing effective and selective inhibitors of other important induced fit viral proteases of the Flaviviridae family such as the West Nile virus NS3 endoprotease.     

A novel Drosophila serpin that inhibits serine proteases

Serpins define a large protein family in which most members function as serine protease inhibitors. Here we report the results of a search for serpins in Drosophila melanogaster that are potentially required for oogenesis or embryogenesis. We cloned and sequenced ovarian cDNAs that encode six distinct proteins having extensive sequence similarity to mammalian serpins, including residues important in the serpin inhibition mechanism. One of these new serpins in recombinant form inactivates, and complexes with, trypsin-like proteases in vitro. To our knowledge, these results represent the first evidence for a serpin in Drosophila that functions as a serine protease inhibitor.     

Identification and characterization of a serpin from Eimeria acervulina

Serpins are serine protease inhibitors that are widely distributed in metazoans but have not been previously characterized in Eimeria spp. A serpin from Eimeria acervulina was cloned, expressed and characterized. Random screening of an E.acervulina sporozoite cDNA library identified a single clone (D14) whose coding region shared high similarity to consensus structure of serpins. Clone D14 contained an entire open reading frame (ORF) consisting of 1,245 nts that encode a peptide 413 amino acids in length with a predicted molecular weight of 45.5 kDa and containing a signal peptide 28 residues in length. By Western blot analysis, polyclonal antiserum to the recombinant serpin (rbSp) recognized a major 55 kDa protein band in unsporulated oocysts and in oocysts sporulated up to 24 hr (fully sporulated). The anti-rbSp detected bands of 55 kDa and 48 kDa in sporozoites (SZ) and merozoites (MZ) respectively. Analysis of MZ secretion products revealed a single protein of 48 kDa which may correspond to secreted serpin. By immuno-staining the serpin was located in granules distributed throughout both the SZ and MZ but granules appeared to be concentrated in the parasite's anterior. Analysis of the structure predicts that the E. acervulina serpin should be an active inhibitor. However, rbSp was without inhibitory activity against common serine proteases. By Western blot analysis the endogenous serpin in MZ extracts did not form the expected high molecular weight complex when coincubated with either trypsin or subtilisin. The results demonstrate that E. acervulina contains a serpin gene and expresses a protein with structural properties similar to an active serine protease inhibitor. Although the function of the E. acervulina serpin remains unknown the results further suggest that serpin is secreted by the parasite where it may be involved in cell invasion and other basic developmental processes.     

Engineering of a macromolecular scaffold to develop specific protease inhibitors

The specific inhibition of serine proteases, which are crucial switches in many physiologically important processes, is of value both for basic research and for therapeutic applications. Ecotin, a potent macromolecular inhibitor of serine proteases of the S1A family, presents an attractive scaffold to engineer specific protease inhibitors because of its large inhibitor-protease interface. Using synthetic shuffling in combination with a restricted tetranomial diversity, we created ecotin libraries that are mutated at all 20 amino acid residues in the binding interface. The efficacy of these libraries was demonstrated against the serine protease plasma kallikrein (Pkal). Competitive phage display selection yielded a Pkal inhibitor with an apparent dissociation equilibrium constant (K(i)*) of 11 pM, whereas K(i)* values for related proteases (such as Factor Xa (FXa), Factor XIa (FXIa), urokinase-type plasminogen activator (uPA), thrombin, and membrane-type serine protease 1 (MT-SP1)) were four to seven orders of magnitude higher. The adaptability of the scaffold was demonstrated by the isolation of inhibitors to two additional serine proteases, MT-SP1/matriptase and Factor XIIa.     

Isolation, characterization, and recombinant expression of multiple serpins from the cat flea, Ctenocephalides felis

Several clones encoding serine protease inhibitors were isolated from larval and adult flea cDNA expression libraries by immunoscreening and PCR amplification. Each cDNA contained an open reading frame encoding a protein of approximately 45 kDa, which had significant sequence similarity with the serpin family of serine protease inhibitors. The thirteen cDNA clones isolated to date encode serpin proteins, which share a primary structure that includes a nearly identical constant region of about 360 amino acids, followed by a C-terminal variable region of about 40-60 amino acids. The variable C-terminal sequences encode most of the reactive site loop (RSL) and are generated by mutually exclusive alternative exon splicing, which may confer unique protease selectivity to each serpin. Utilization of an alternative exon splicing mechanism has been verified by sequence analysis of a flea serpin genomic clone and adjacent genomic sequences. RNA expression patterns of the cloned genes have been examined by Northern blot analysis using variable region-specific probes. Several putative serpins have been overexpressed using the cDNA clones in Escherichia coli and baculovirus expression systems. Two purified baculovirus-expressed recombinant proteins have N-terminal amino acid sequences identical to the respective purified native mature flea serpins indicating that appropriate N-terminal processing occurred in the virus-infected insect cells.     

Anti-hemostatic effects of a serpin from the saliva of the tick Ixodes ricinus

Serpins (serine protease inhibitors) are a large family of structurally related proteins found in a wide variety of organisms, including hematophagous arthropods. Protein analyses revealed that Iris, previously described as an immunomodulator secreted in the tick saliva, is related to the leukocyte elastase inhibitor and possesses serpin motifs, including the reactive center loop (RCL), which is involved in the interaction between serpins and serine proteases. Only serine proteases were inhibited by purified recombinant Iris (rIris), whereas mutants L339A and A332P were found devoid of any protease inhibitory activity. The highest Ka was observed with human leukocyte-elastase, suggesting that elastase-like proteases are the natural targets of Iris. In addition, mutation M340R completely changed both Iris substrate specificity and affinity. This likely identified Met-340 as amino acid P1 in the RCL. The effects of rIris and its mutants were also tested on primary hemostasis, blood clotting, and fibrinolysis. rIris increased platelet adhesion, the contact phase-activated pathway of coagulation, and fibrinolysis times in a dose-dependent manner, whereas rIris mutant L339A affected only platelet adhesion. Taken together, these results indicate that Iris disrupts coagulation and fibrinolysis via the anti-proteolytic RCL domain. One or more other domains could be responsible for primary hemostasis inhibition. To our knowledge, this is the first ectoparasite serpin that interferes with both hemostasis and the immune response.     

The effects of reactive site location on the inhibitory properties of the serpin alpha(1)-antichymotrypsin

The large size of the serpin reactive site loop (RSL) suggests that the role of the RSL in protease inhibition is more complex than that of presenting the reactive site (P1 residue) to the protease. This study examines the effect on inhibition of relocating the reactive site (Leu-358) of the serpin alpha(1)-antichymotrypsin either one residue closer (P2) or further (P1') from the base of the RSL (Glu-342). alpha(1)-Antichymotrypsin variants were produced by mutation within the P4-P2' region; the sequence ITLLSA was changed to ITLSSA to relocate the reactive site to P2 (Leu-357) and to ITITLS to relocate it to P1' (Leu-359). Inhibition of the chymotrypsin-like proteases human chymase and chymotrypsin and the non-target protease human neutrophil elastase (HNE) were analyzed. The P2 variant inhibited chymase and chymotrypsin but not HNE. Relative to P1, interaction at P2 was characterized by greater complex stability, lower inhibition rate constants, and increased stoichiometry of inhibition values. In contrast, the P1' variant inhibited HNE (stoichiometry of inhibition = 4) but not chymase or chymotrypsin. However, inhibition of HNE was by interaction with Ile-357, the P2 residue. The P1' site was recognized by all proteases as a cleavage site. Covalent-complexes resistant to SDS-PAGE were observed in all inhibitory reactions, consistent with the trapping of the protease as a serpin-acyl protease complex. The complete loss in inhibitory activity associated with lengthening the Glu-342-reactive site distance by a single residue and the enhanced stability of complexes associated with shortening this distance by a single residue are compatible with the distorted-protease model of inhibition requiring full insertion of the RSL into the body of the serpin and translocation of the linked protease to the pole opposite from that of encounter.     

Protease inhibitors and proteolytic signalling cascades in insects

Proteolytic signalling cascades control a wide range of physiological responses. In order to respond rapidly, protease activity must be maintained at a basal level: the component zymogens must be sequentially activated and actively degraded. At the same time, signalling cascades must respond precisely: high target specificity is required. The insects have a wide range of trapping- and tight-binding protease inhibitors, which can regulate the activity of individual proteases. In addition, the interactions between component proteases of a signalling cascade can be modified by serine protease homologues. The suicide-inhibition mechanism of serpin family inhibitors gives rapid turnover of both protease and inhibitor, but target specificity is inherently broad. Similarly, the TEP/macroglobulins have extremely broad target specificity, which suits them for roles as hormone transport proteins and sensors of pathogenic virulence factors. The tight-binding inhibitors, on the other hand, have a lock-and-key mechanism capable of high target specificity. In addition, proteins containing multiple tight-binding inhibitory domains may act as scaffolds for the assembly of signalling complexes. Proteolytic cascades regulated by combinations of different types of inhibitor could combine the rapidity of suicide-inhibitors with the specificity lock-and-key inhibitors. This would allow precise control of physiological responses and may turn out to be a general rule.     

Two proteases defining a melanization cascade in the immune system of Drosophila

The melanization reaction is used as an immune mechanism in arthropods to encapsulate and kill microbial pathogens. In Drosophila, the serpin Spn27A regulates melanization apparently by inhibiting the protease that activates phenoloxidase, the key enzyme in melanin synthesis. Here, we have described the genetic characterization of two immune inducible serine proteases, MP1 and MP2, which act in a melanization cascade regulated by Spn27A. MP1 is required to activate melanization in response to both bacterial and fungal infection, whereas MP2 is mainly involved during fungal infection. Pathogenic bacteria and fungi may therefore trigger two different melanization cascades that use MP1 as a common downstream protease to activate phenoloxidase. We have also shown that the melanization reaction activated by MP1 and MP2 plays an important role in augmenting the effectiveness of other immune reactions, thereby promoting resistance of Drosophila to microbial infection.     

Proteases and Their Involvement in the Infection and Immobilization of Nematodes by the Nematophagous Fungus Arthrobotrys oligospora

The nematophagous fungus Arthrobotrys oligospora produced extracellular proteases when grown in a liquid culture, as revealed by measuring the hydrolysis of the chromogenic substrate Azocoll. The extracellular protease activity was inhibited by phenylmethylsulfonyl fluoride (PMSF) and other serine protease inhibitors and partly inhibited by the aspartate protease inhibitor pepstatin and by a cysteine protease inhibitor [l-trans-epoxysuccinyl-leucylamide-(4-guanidino)-butane, or E-64]. Substrate gel electrophoresis showed that the fungus produced several different proteases, including multiple serine proteases. The function of proteases in the infection of nematodes was examined by treating the fungus with various protease inhibitors. None of the inhibitors tested affected the adhesion of nematodes to the traps, but incubating trap-bearing mycelium with a serine protease inhibitor, PMSF, antipain, or chymostatin, or the metalloprotease inhibitor phenanthroline significantly decreased the immobilization of nematodes captured by the fungus. Inhibitors of cysteine or aspartic proteases did not affect the immobilization of captured nematodes. The effects of PMSF on the immobilization of nematodes were probably due to serine proteases produced by the fungus, since the effects were observed when unbound inhibitor was washed away from the fungus before the nematodes were added to the system. No effects were observed when the nematodes only were pretreated with PMSF.     

Mouse DESC1 is located within a cluster of seven DESC1-like genes and encodes a type II transmembrane serine protease that forms serpin inhibitory complexes

We report the identification and functional analysis of a type II transmembrane serine protease encoded by the mouse differentially expressed in squamous cell carcinoma (DESC) 1 gene, and the definition of a cluster of seven homologous DESC1-like genes within a 0.5-Mb region of mouse chromosome 5E1. This locus is syntenic to a region of human chromosome 4q13.3 containing the human orthologues of four of the mouse DESC1-like genes. Bioinformatic analysis indicated that all seven DESC1-like genes encode functional proteases. Direct cDNA cloning showed that mouse DESC1 encodes a multidomain serine protease with an N-terminal signal anchor, a SEA (sea urchin sperm protein, enterokinase, and agrin) domain, and a C-terminal serine protease domain. The mouse DESC1 mRNA was present in epidermal, oral, and male reproductive tissues and directed the translation of a membrane-associated 60-kDa N-glycosylated protein with type II topology. Mouse DESC1 was synthesized in insect cells as a zymogen that could be activated by exposure to trypsin. The purified activated DESC1 hydrolyzed synthetic peptide substrates, showing a preference for Arg in the P1 position. DESC1 proteolytic activity was abolished by generic inhibitors of serine proteases but not by other classes of protease inhibitors. Most interestingly, DESC1 formed stable inhibitory complexes with both plasminogen activator inhibitor-1 and protein C inhibitor that are expressed in the same tissues with DESC1, suggesting that type II transmembrane serine proteases may be novel targets for serpin inhibition. Together, these data show that mouse DESC1 encodes a functional cell surface serine protease that may have important functions in the epidermis, oral, and reproductive epithelium.     

Protease inhibitors in bacteria: an emerging concept for the regulation of bacterial protein complexes?

Serine protease inhibitors (serpins), the antagonists of serine proteases, were unknown in the bacterial kingdom until recently. Kang et al. in this issue of Molecular Microbiology report the cloning and functional analysis of the three serpin genes from the thermophilic anaerobic bacterium Clostridium thermocellum. Two of the serpins contain a dockerin module for location in the extracellular hydrolytic multienzyme complex, the cellulosome. The susceptibility of cellulosome to proteolytic degradation and the presence of a serine protease in the same complex provoke speculation that protease inhibitor/protease pairs could play hitherto unrecognized roles in protein stability and regulation in bacteria.     

The structure of a Michaelis serpin-protease complex.

Serine protease inhibitors (serpins) regulate the activities of circulating proteases. Serpins inhibit proteases by acylating the serine hydroxyl at their active sites. Before deacylation and complete proteolysis of the serpin can occur, massive conformational changes are triggered in the serpin while maintaining the covalent linkage between the protease and serpin. Here we report the structure of a serpin-trypsin Michaelis complex, which we visualized by using the S195A trypsin mutant to prevent covalent complex formation. This encounter complex reveals a more extensive interaction surface than that present in small inhibitor-protease complexes and is a template for modeling other serpin-protease pairs. Mutations of several serpin residues at the interface reduced the inhibitory activity of the serpin. The serine residue C-terminal to the scissile peptide bond is found in a closer than usual interaction with His 57 at the active site of trypsin.     

The novel bovine serpin endopin 2C demonstrates selective inhibition of the cysteine protease cathepsin L compared to the serine protease elastase, in cross-class inhibition

Molecular cloning revealed the unique serpin endopin 2C that demonstrates selective inhibition of cathepsin L compared to papain or elastase. Endopin 2C, thus, functions as a serpin with the property of cross-class inhibition. Endopin 2C possesses homology in primary sequence to endopin 2A and other isoforms of endopins related to alpha1-antichymotrypsin, yet endopin 2C differs in its target protease specificity. Recombinant endopin 2C showed effective inhibition of cathepsin L with a stoichiometry of inhibition (SI) of 1/1 (molar ratio of inhibitor/protease), with the second-order rate constant, k(ass), of 7.2 x 10(5) M(-1) s(-1). Less effective endopin 2C inhibition of papain and elastase occurred with k(ass) association rate constants of approximately 1 x 10(4) M(-1) s(-1) with high SI values. Endopin 2C formed SDS-stable complexes with cathepsin L, papain, and elastase that are typical of serpins. These results are among the first to demonstrate stable serpin complexes with target cysteine proteases. Interactions of endopin 2C with cathepsin L and elastase were indicated by protease cleavage of the RSL region between P1-P1' residues of Thr-Ser. The hydrophobic Phe residue in the P2 position of the RSL region is consistent with the specificity of cathepsin L for hydrophobic residues in the P2 position of its substrate cleavage site. The NH2-terminal signal sequence of endopin 2C, like that of cathepsin L, predicts their colocalization to subcellular organelles. These findings demonstrate endopin 2C as a novel serpin that possesses cross-class inhibition with selectivity for inhibition of cathepsin L.     

Effect of reactive site loop elongation on the inhibitory activity of C1-inhibitor

The serine protease inhibitor C1-Inhibitor (C1-Inh) inhibits several complement- and contact-system proteases, which play an important role in inflammation. C1-Inh has a short reactive site loop (RSL) compared to other serpins. RSL length determines the inhibitory activity of serpins. We investigated the effect of RSL elongation on inhibitory activity of C1-Inh by insertion of one or two alanine residues in the RSL. One of five mutants had an increased association rate with kallikrein, but was nevertheless a poor inhibitor because of a simultaneous high stoichiometry of inhibition (>10). The association rate of the other variants was lower than that of wild-type C1-Inh. These data suggest that the relatively weak inhibitory activity of C1-Inh is not the result of its short RSL. The short RSL of C1-Inh has, surprisingly, the optimal length for inhibition.     

Extending the Cellulosome Paradigm: the Modular Clostridium thermocellum Cellulosomal Serpin PinA Is a Broad-Spectrum Inhibitor of Subtilisin-Like Proteases

Clostridium thermocellum encodes a cellulosomal, modular, and thermostable serine protease inhibitor (serpin), PinA. PinA stability but not inhibitory activity is affected by the Fn(III) and Doc(I) domains, and PinA is a broad inhibitor of subtilisin-like proteases and may play a key role in protecting the cellulosome from protease attack.     

Novel scabies mite serpins inhibit the three pathways of the human complement system

Scabies is a parasitic infestation of the skin by the mite Sarcoptes scabiei that causes significant morbidity worldwide, in particular within socially disadvantaged populations. In order to identify mechanisms that enable the scabies mite to evade human immune defenses, we have studied molecules associated with proteolytic systems in the mite, including two novel scabies mite serine protease inhibitors (SMSs) of the serpin superfamily. Immunohistochemical studies revealed that within mite-infected human skin SMSB4 (54 kDa) and SMSB3 (47 kDa) were both localized in the mite gut and feces. Recombinant purified SMSB3 and SMSB4 did not inhibit mite serine and cysteine proteases, but did inhibit mammalian serine proteases, such as chymotrypsin, albeit inefficiently. Detailed functional analysis revealed that both serpins interfered with all three pathways of the human complement system at different stages of their activation. SMSB4 inhibited mostly the initial and progressing steps of the cascades, while SMSB3 showed the strongest effects at the C9 level in the terminal pathway. Additive effects of both serpins were shown at the C9 level in the lectin pathway. Both SMSs were able to interfere with complement factors without protease function. A range of binding assays showed direct binding between SMSB4 and seven complement proteins (C1, properdin, MBL, C4, C3, C6 and C8), while significant binding of SMSB3 occurred exclusively to complement factors without protease function (C4, C3, C8). Direct binding was observed between SMSB4 and the complement proteases C1s and C1r. However no complex formation was observed between either mite serpin and the complement serine proteases C1r, C1s, MASP-1, MASP-2 and MASP-3. No catalytic inhibition by either serpin was observed for any of these enzymes. In summary, the SMSs were acting at several levels mediating overall inhibition of the complement system and thus we propose that they may protect scabies mites from complement-mediated gut damage.