Structural insights into the unique inhibitory mechanism of the silkworm protease inhibitor serpin18

Structural Characterization and Physiological Role of Bombyx mori Fibroinase in the Silk Gland Development

Silkworm is a highly valuable insect that produces silk through secretion by a silk gland. Within this gland, a type of cathepsin L protease called Fibroinase was identified as an enzyme for hydrolyzing the primary components of silk, including fibroin and sericin. Here, we determined the crystal structure of Fibroinase fromBombyx mori at a resolution of 1.56 Å. Comparative structural analysis revealed that Fibroinase adopted a similar structural pattern with papain-type cathepsin, consisting of an N-terminal domain and a C-terminal domain. The interface between the domains forms a substrate-binding cleft, where the E64 inhibitor noncovalently binds in a novel manner. Additionally, computational simulations combined with biochemical analysis allowed us to define the binding mode and inhibition mechanism of physiological inhibitor Bombyx cysteine protease inhibitor (BCPI) with Fibroinase. Moreover, the expression profiles and RNA interference of Fibroinase indicated its critical role in removing silk proteins in the silk gland lumen and the destruction of silk gland tissue during the larval-pupal metamorphosis. These findings enhance our understanding of the structural and biochemical features of Fibroinase and its inhibitors, while also providing evidence for the physiological role of Fibroinase in silk gland development.

Analysis of histomorphometric and proteome dynamics inside the silk gland lumen of Bombyx mori revealed the dynamic change of silk protein during the molt stage

Silkworms spin different silks at different growth stages for specific purposes. The silk spun before the end of each instar is stronger than that at the beginning of each instar and cocoon silk. However, the compositional changes in silk proteins during this process are unknown. Consequently, we performed histomorphological and proteomic analyses of the silk gland to characterize changes from the instar end to the next instar beginning. The silk glands were collected on day 3 of third- and fourth-instar larvae (III-3 and IV-3) and the beginning of fourth-instar larvae (IV-0). Proteomic analysis identified 2961 proteins from all silk glands. Silk proteins P25 and Ser5 were significantly more abundant in III-3 and IV-3 than in IV-0, and many cuticular proteins and protease inhibitors increased significantly in IV-0 compared with III-3 and IV-3. This shift may cause mechanical property differences between the instar end and beginning silk. Using section staining, qPCR, and western blotting, we found for the first time that silk proteins were degraded first and then resynthesized during the molting stage. Furthermore, we revealed that fibroinase mediated the changes of silk proteins during molting. Our results provide insights into the molecular mechanisms of silk proteins dynamic regulation during molting.

Genome-Wide Characterization and Comparative Genomic Analysis of the Serpin Gene Family in Microsporidian Nosema bombycis

Microsporidia are ubiquitous in the environment, infecting almost all invertebrates, vertebrates, and some protists. The microsporidian Nosema bombycis causes silkworms pébrine disease and leads to huge economic losses. Parasite secreted proteins play vital roles in pathogen-host interactions. Serine protease inhibitors (serpins), belonging to the largest and most broadly distributed protease inhibitor superfamily, are also found in Microsporidia. In this study, we characterized 19 serpins (NbSPNs) in N. bombycis; eight of them were predicted with signal peptides. All NbSPN proteins contain a typical conserved serpin (PF00079) domain. The comparative genomic analysis revealed that microsporidia serpins were only found in the genus Nosema. In addition to N. bombycis, a total of 34 serpins were identified in another six species of Nosema including N. antheraeae (11), N. granulosis (8), Nosema sp. YNPr (3), Nosema sp. PM-1 (3), N. apis (4), and N. ceranae (5). Serpin gene duplications in tandem obviously occurred in Nosema antheranae. Notably, the NbSPNs were phylogenetically clustered with serpins from the Chordopoxvirinae, the subfamily of Poxvirus. All 19 NbSPN transcripts were detected in the infected midgut and fat body, while 19 NbSPN genes except for NbSPN12 were found in the transcriptome of the infected silkworm embryonic cell line BmE-SWU1. Our work paves the way for further study of serpin function in microsporidia.

Up-regulated serpin gene involved in Cry1Ac resistance in Helicoverpa armigera

Insect resistance to Bacillus thuringiensis (Bt) is a critical limiting factor for applying the Bt crops. Some studies indicated that decreased protoxin activation because of lower enzymatic activities of trypsin and chymotrypsin and increased expression of serpin might involve in Bt resistance. Our previous study identified an endogenous serpin could inhibit the midgut proteases to activate Cry1Ac and reduce the insecticide activity to Helicoverpa armigera. We hypothesis that up-regulated serpin involve in resistance via inhibiting enzymatic activities of trypsin and chymotrypsin to decrease protoxin activation. Herein, we found the serpin-e gene relative expression in midgut was significantly higher in the LF30 resistant strain than that in the susceptible strain during all developmental stages. Importantly, RNAi-mediated silencing of serpin-e gene expression caused 4.46-fold mortality changes in LF30 strain, but the trypsin and chymotrypsin proteases activities were only changed 0.79-fold and 2.22-fold. In addition, although proteases activities were significantly lower in LF30 strain than that in the susceptible strain, the resistance ratios of LF30 to Cry1Ac protoxin and to activated Cry1Ac toxin were no difference. The results indicated serpins caused insect resistance to Cry1Ac protoxins partly through inhibiting the trypsin and chymotrypsin proteases activities, but it also existed other mechanisms in LF30.

SPINK7 Recognizes Fungi and Initiates Hemocyte-Mediated Immune Defense Against Fungal Infections

Serine protease inhibitors of Kazal-type (SPINKs) were widely identified in vertebrates and invertebrates, and played regulatory roles in digestion, coagulation, and fibrinolysis. In this study, we reported the important role of SPINK7 in regulating immune defense of silkworm, Bombyx mori. SPINK7 contains three Kazal domains and has 6 conserved cysteine residues in each domain. Quantitative real-time PCR analyses revealed that SPINK7 was exclusively expressed in hemocytes and was upregulated after infection with two fungi, Saccharomyces cerevisiae and Candida albicans. Enzyme activity inhibition test showed that SPINK7 significantly inhibited the activity of proteinase K from C. albicans. Additionally, SPINK7 inhibited the growth of three fungal spores, including S. cerevisiae, C. albicans, and Beauveria bassiana. The pathogen-associated molecular patterns (PAMP) binding assays suggested that SPINK7 could bind to β-D-glucan and agglutinate B. bassiana and C. albicans. In vitro assays were performed using SPINK7-coated agarose beads, and indicated that SPINK7 promoted encapsulation and melanization of agarose beads by B. mori hemocytes. Furthermore, co-localization studies using immunofluorescence revealed that SPINK7 induced hemocytes to aggregate and entrap the fungi spores of B. bassiana and C. albicans. Our study revealed that SPINK7 could recognize fungal PAMP and induce the aggregation, melanization, and encapsulation of hemocytes, and provided valuable clues for understanding the innate immunity and cellular immunity in insects.

A Comprehensive Phylogenetic Analysis of the Serpin Superfamily

Serine protease inhibitors (serpins) are found in all kingdoms of life and play essential roles in multiple physiological processes. Owing to the diversity of the superfamily, phylogenetic analysis is challenging and prokaryotic serpins have been speculated to have been acquired from Metazoa through horizontal gene transfer due to their unexpectedly high homology. Here, we have leveraged a structural alignment of diverse serpins to generate a comprehensive 6,000-sequence phylogeny that encompasses serpins from all kingdoms of life. We show that in addition to a central “hub” of highly conserved serpins, there has been extensive diversification of the superfamily into many novel functional clades. Our analysis indicates that the hub proteins are ancient and are similar because of convergent evolution, rather than the alternative hypothesis of horizontal gene transfer. This work clarifies longstanding questions in the evolution of serpins and provides new directions for research in the field of serpin biology.

Genome of the webworm Hyphantria cunea unveils genetic adaptations supporting its rapid invasion and spread

BACKGROUND

The fall webworm Hyphantria cunea is an invasive and polyphagous defoliator pest that feeds on nearly any type of deciduous tree worldwide. The silk web of H. cunea aids its aggregating behavior, provides thermal regulation and is regarded as one of causes for its rapid spread. In addition, both chemosensory and detoxification genes are vital for host adaptation in insects.

RESULTS

Here, a high-quality genome of H. cunea was obtained. Silk-web-related genes were identified from the genome, and successful silencing of the silk protein gene HcunFib-H resulted in a significant decrease in silk web shelter production. The CAFE analysis showed that some chemosensory and detoxification gene families, such as CSPs, CCEs, GSTs and UGTs, were expanded. A transcriptome analysis using the newly sequenced H. cunea genome showed that most chemosensory genes were specifically expressed in the antennae, while most detoxification genes were highly expressed during the feeding peak. Moreover, we found that many nutrient-related genes and one detoxification gene, HcunP450 (CYP306A1), were under significant positive selection, suggesting a crucial role of these genes in host adaptation in H. cunea. At the metagenomic level, several microbial communities in H. cunea gut and their metabolic pathways might be beneficial to H. cunea for nutrient metabolism and detoxification, and might also contribute to its host adaptation.

CONCLUSIONS

These findings explain the host and environmental adaptations of H. cunea at the genetic level and provide partial evidence for the cause of its rapid invasion and potential gene targets for innovative pest management strategies.

Unravelling the fibrillation mechanism of ovalbumin in the presence of mercury at its isoelectric pH

The intriguing resemblances of amyloid fibrils and spider silk in protein aggregation diseases have instigated the exploration of identical structural features if any in their oligomeric pathways. The serpin group protein, ovalbumin, on defolding in HgCl₂ shares commonness to the micellar pathway of spidroins for their aggregation in response to a pH trigger. The structural feature changes from monomer to worm like fibril with a shift in the primary protein pH to slightly acidic pH (4.5), and then proceeds through a secondary nucleation pathway to ‘hillock’ and ‘hydra’ like protofibrils rich in β-sheet and random coil conformers upon exposure to mercury. The findings are backed by atomic force microscopy, confocal Raman spectroscopy and fluorescence measurements. Unlocking such structural features can favorably assist in the design of therapeutics.

Mercuric chloride triggered ovalbumin aggregation pathway and its resemblance to Nephila clavipes.

Bmserpin2 Is Involved in BmNPV Infection by Suppressing Melanization in Bombyx mori

Melanization, an important defense response, plays a vital role in arthropod immunity. It is mediated by serine proteases (SPs) that convert the inactive prophenoloxidase (PPO) to active phenoloxidase (PO) and is tightly regulated by serine protease inhibitors (serpins) which belong to a well distributed superfamily in invertebrates, participating in immune mechanisms and other important physiological processes. Here, we investigated the Bmserpin2 gene which was identified from a transcriptome database in response to Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Quantitative real-time polymerase chain reaction (qRT-PCR) results showed that Bmserpin2 was expressed in all tissues, with maximum expression in fat body. Upon BmNPV infection, the expression of Bmserpin2 was up-regulated in P50 (susceptible strain) and BC9 (resistant strain) in haemocytes, fat body and the midgut. However, up-regulation was delayed in BC9 (48 or 72 h), in contrast to P50 (24 h), after BmNPV infection. Meanwhile, Bmserpin2 could delay or inhibit melanization in silkworm haemolymph. Significant increased PO activity can be observed in Bmserpin2-depleted haemolymph under NPV infection. Furthermore, the viral genomic DNA copy number was decreased in Bmserpin2-depleted haemolymph. We conclude that Bmserpin2 is an inducible gene which might be involved in the regulation of PPO activation and suppressed melanization, and have a potential role in the innate immune system of B. mori.

Fibroinase and its physiological inhibitors involved in the regulation of silk gland development in the silkworm, Bombyx mori

Fibroinase, a cathepsin L-like cysteine protease, was previously identified in the silk gland of the silkworm, Bombyx mori. It shows high degradation activity during the pre-pupa period, when the silk gland undergoes apoptosis and remodeling. Here, we recombinantly expressed pro-fibroinase and activated it in vitro. Fibroinase showed optimal hydrolytic activity at pH 4.0 and its optimum temperature was about 42 °C. One physiological inhibitor, B. mori cysteine protease inhibitor (BCPI) was found, which showed strong inhibitory activity against fibroinase. The inhibitory reaction was caused by the formation of a non-covalent complex; this is in contrast to a previously reported mode of fibroinase inhibition by Serpin18. Expression profiles and immunolocalization analysis demonstrated that fibroinase was involved in silk gland development by degrading silk proteins and apoptosis/remodeling of silk glands at specific points. Furthermore, the comparison of the temporal expression of fibroinase and its inhibitors, BCPI and Serpin18, indicated that these inhibitors were involved in the silk gland development by regulating the activity of fibroinase from the fifth instar until the early spinning stage. These findings improve our understanding of the mechanism of protease regulation and its inhibitors in silk gland development.

Silkworm serpin32 functions as a negative-regulator in prophenoloxidase activation

The extracellular serine protease cascade is an essential component of insect humoral immunity. Serine protease inhibitors (serpins) play an important regulatory role in the process of insect immunity by regulating the serine protease cascade pathway. We aimed to clarify the function of Bmserpin32 in this study. First, we performed homologous sequence alignment and phylogenetic analysis of Bmserpin32. Bmserpin32 was found to share 64% amino acid sequence identity with Manduca sexta serpin7, an immunomodulatory protein. Bmserpin32 cDNA was cloned, and the recombinant Bmserpin32 protein was expressed in Escherichia coli and purified by nickel-nitrilotriacetic acid affinity and gel filtration chromatography. The activity assay showed that Bmserpin32 had significant inhibitory activity against trypsin. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry and site-directed mutagenesis combined with activity assays indicated that the cleavage site of Bmserpin32 is between Arg³⁵⁹ and Ile³⁶⁰. After infection with E. coli or Micrococcus luteus, the expression level of Bmserpin32 in immune-related tissues was significantly upregulated. In addition, Bmserpin32 could delay or inhibit the melanization of hemolymph by inhibiting the activation of prophenoloxidase in larval hemolymph. Furthermore, a physiological target of Bmserpin32 was identified as the clip protease, BmPAP3, an apparent ortholog of M. sexta propenoloxidase-activating protease-3. Our observations enable a better understanding of the physiological role of Bmserpin32 in regulating melanization in silkworm.

Dominant-negative SERPING1 variants cause intracellular retention of C1 inhibitor in hereditary angioedema

Hereditary angioedema (HAE) is an autosomal dominant disease characterized by recurrent edema attacks associated with morbidity and mortality. HAE results from variations in the SERPING1 gene that encodes the C1 inhibitor (C1INH), a serine protease inhibitor (serpin). Reduced plasma levels of C1INH lead to enhanced activation of the contact system, triggering high levels of bradykinin and increased vascular permeability, but the cellular mechanisms leading to low C1INH levels (20%-30% of normal) in heterozygous HAE type I patients remain obscure. Here, we showed that C1INH encoded by a subset of HAE-causing SERPING1 alleles affected secretion of normal C1INH protein in a dominant-negative fashion by triggering formation of protein-protein interactions between normal and mutant C1INH, leading to the creation of larger intracellular C1INH aggregates that were trapped in the endoplasmic reticulum (ER). Notably, intracellular aggregation of C1INH and ER abnormality were observed in fibroblasts from a heterozygous carrier of a dominant-negative SERPING1 gene variant, but the condition was ameliorated by viral delivery of the SERPING1 gene. Collectively, our data link abnormal accumulation of serpins, a hallmark of serpinopathies, with dominant-negative disease mechanisms affecting C1INH plasma levels in HAE type I patients, and may pave the way for new treatments of HAE.

Identification and partial characterization of a novel serpin from Eudiplozoon nipponicum (Monogenea, Polyopisthocotylea)

Background: Serpins are a superfamily of serine peptidase inhibitors that participate in the regulation of many physiological and cell peptidase-mediated processes in all organisms (e.g. in blood clotting, complement activation, fibrinolysis, inflammation, and programmed cell death). It was postulated that in the blood-feeding members of the monogenean family Diplozoidae, serpins could play an important role in the prevention of thrombus formation, activation of complement, inflammation in the host, and/or in the endogenous regulation of protein degradation.

Results: In silico analysis showed that the DNA and primary protein structures of serpin from Eudiplozoon nipponicum (EnSerp1) are similar to other members of the serpin superfamily. The inhibitory potential of EnSerp1 on four physiologically-relevant serine peptidases (trypsin, factor Xa, kallikrein, and plasmin) was demonstrated and its presence in the worm’s excretory-secretory products (ESPs) was confirmed.

Conclusion: EnSerp1 influences the activity of peptidases that play a role in blood coagulation, fibrinolysis, and complement activation. This inhibitory potential, together with the serpin’s presence in ESPs, suggests that it is likely involved in host-parasite interactions and could be one of the molecules involved in the control of feeding and prevention of inflammatory responses.

20-Hydroxyecdysone-upregulated proteases involved in Bombyx larval fat body destruction

During insect larval-pupal metamorphosis, the obsolete larval organs and tissues undergo histolysis and programmed cell death to recycle cellular materials. It has been demonstrated that some cathepsins are essential for histolysis in larval tissues, but the process of tissue destruction is not well documented. Fat body, the homologous organ to mammalian liver and adipose tissue, goes through a distinct destruction process during larval-pupal transition. Herein, we found that most of the Bombyx proteases - including Bombyx cathepsin B (BmCatB) (BmCatLL-2), Bombyx cathepsin D (BmCatD), Bombyx cathepsin L like-1 (BmCatLL-1) and -2(BmCatLL-2), Bombyx fibroinase (BmBcp), Bombyx matrix metalloprotease (BmMmp), Bombyx A disintegrin and metalloproteinase with thrombospondin motifs 1 (BmAdamTS-1), Bombyx A disintegrin and metalloproteinase with thrombospondin motifs like (BmAdamTS L) and Bombyx cysteine protease inhibitor (Bmbcpi)- were expressed highly in fat body during feeding and metamorphosis, with a peak occurring during the nonfeeding moulting or prepupal stage, as well as being responsive to 20-hydroxyecdysone (20E). The aforementioned protease genes expression was upregulated by injection of 20E into the feeding larvae, while blocking 20E signalling transduction led to downregulation. Western blotting and immunofluorescent staining of BmCatB and BmBcp confirmed the coincident variation of their messenger RNA (mRNA) and protein level during the development and after the treatments. Moreover, BmCatB, BmBcp, BmMmp and BmAdamTS-1 RNA interference all led to blockage of larval fat body destruction. Taken together, we conclude that 20E regulates larval fat body destruction by upregulating related protease gene expression and protein levels during larval-pupal transition.

Identification and characterization of serine protease inhibitors in a parasitic wasp, Pteromalus puparum

Serine protease inhibitors (SPIs) regulate protease-mediated activities by inactivating their cognate proteinases, and are involved in multiple physiological processes. SPIs have been extensively studied in vertebrates and invertebrates; however, little SPI information is available in parasitoids. Herein, we identified 57 SPI genes in total through the genome of a parasitoid wasp, Pteromalus puparum. Gene structure analyses revealed that these SPIs contain 7 SPI domains. Depending on their mode of action, these SPIs can be categorized into serpins, canonical inhibitors and alpha-2-macroglobulins (A2Ms). For serpins and canonical inhibitors, we predicted their putative inhibitory activities to trypsin/chymotrypsin/elastase-like enzymes based on the amino acids in cleaved reactive sites. Sequence alignment and phylogenetic tree indicated that some serpins similar to known functional inhibitory serpins may participate in immune responses. Transcriptome analysis also showed some canonical SPI genes displayed distinct expression patterns in the venom gland and this was confirmed by quantitative real-time PCR (qPCR) analysis, suggesting their specific physiological functions as venom proteins in suppressing host immune responses. The study provides valuable information to clarify the functions of SPIs in digestion, development, reproduction and innate immunity.

Characterization and expression analysis of serpinB3, the first clade B serine protease inhibitor in Pacific white shrimp, Litopenaeus vannamei

Currently, about nine serpin clades (A-I) were preferentially observed in higher animals and clustered on the basis of function. Of these, eight clades contain extracellular proteins, while clade B contains predominantly intracellular proteins. In the present study, the first clade B serpin (named LvserpinB3) was identified from the Pacific white shrimp Litopenaeus vannamei. LvserpinB3 encoded a 412-amino acid protein with a 19-amino acid signal peptide and a serpin domain. Moreover, a transmembrane helix (TMHs) was predicted to be located on the N-terminal of LvserpinB3. Alignment with the cDNA sequence indicated that the genomic LvserpinB3 gene contains 2 exons and 1 intron. The P1-P1' scissile bond of the core feature reactive center loop (RCL) represented for Arginine-Isoleucine (RI), which was in accordance with PmserpinB3, Msserpin-4, -5 and -7. The highest mRNA expression level of LvserpinB3 was detected in hepatopancreas. A significant decrease of LvserpinB3 was detected in hepatopancreas at 6 h post Vibrio anguillarum injection, and later on, the expression of LvserpinB3 was remarkably elevated at 24 h post bacterial challenge. Suppression of LvserpinB3 in vivo by double-stranded RNA (dsRNA) mediated RNA interference (RNAi) led to a significant increase in the transcripts of LvSP1 (Serine protease 1), LvPPAE2 (Prophenoloxidase-activating Enzyme 2) and cumulative mortality. Furthermore, rLvserpinB3 protein was expressed and purified in vitro for the prophenoloxidase inhibition assay. The rLvserpinB3 protein can strongly impede the extent of proPO cascade. All above imply that LvserpinB3 might be an inhibitor for prophenoloxidase-activating system.

Identification and functional characterizations of serpin8, a potential prophenoloxidase-activating protease inhibitor in Pacific white shrimp, Litopenaeus vannamei

Serpins have been characterized from varieties of organisms by their inhibitory roles on serine or cysteine proteases. However, research for the functional study of serpins in crustacean is relatively small. To fully clarify the immune characterizations of serpin, a novel serpin (named Lvserpin8) encoding 414 amino acids with a 19-amino acid signal peptide and a serpin domain was identified from the Pacific white shrimp Litopenaeus vannamei. Sequence analysis indicated that the genomic Lvserpin8 gene contains 5 exons and 4 introns. The P1 residues of the predicted scissile bond in the reactive center loop (RCL) region represented for Lysine (Lys), which is in accordance with Pmserpin8, Dmserpin27A, Ofserpin3, Bmserpin3 and Msserpin3. Quantitative results showed that high mRNA expression of Lvserpin8 was detected in hepatopancreas and testis. Notably, a significant increase of Lvserpin8 was appeared post injection of Vibrio anguillarum, and Micrococcus lysodeikticus. Moreover, Lvserpin8 was knocked down in vivo by double-stranded RNA (dsRNA) mediated RNA interference (RNAi). Suppression of Lvserpin8 led to a significant increase in the transcripts of LvPPAE2 (Prophenoloxidase-activating Enzyme 2) and cumulative mortality. What's more, recombinant Lvserpin8 protein (rLvserpin8) displayed inhibition roles on trypsin activity, and prophenoloxidase activation. Taken together, the results implied that Lvserpin8 may be involved in shrimp innate immunity via the inhibition of prophenoloxidase-activating proteases.

1.45 Å resolution structure of SRPN18 from the malaria vector Anopheles gambiae

Serine protease inhibitors (serpins) in insects function within development, wound healing and immunity. The genome of the African malaria vector, Anopheles gambiae, encodes 23 distinct serpin proteins, several of which are implicated in disease-relevant physiological responses. A. gambiae serpin 18 (SRPN18) was previously categorized as non-inhibitory based on the sequence of its reactive-center loop (RCL), a region responsible for targeting and initiating protease inhibition. The crystal structure of A. gambiae SRPN18 was determined to a resolution of 1.45 Å, including nearly the entire RCL in one of the two molecules in the asymmetric unit. The structure reveals that the SRPN18 RCL is extremely short and constricted, a feature associated with noncanonical inhibitors or non-inhibitory serpin superfamily members. Furthermore, the SRPN18 RCL does not contain a suitable protease target site and contains a large number of prolines. The SRPN18 structure therefore reveals a unique RCL architecture among the highly conserved serpin fold.

Serpins in arthropod biology

Serpins are the largest known family of serine proteinase inhibitors and perform a variety of physiological functions in arthropods. Herein, we review the field of serpins in arthropod biology, providing an overview of current knowledge and topics of interest. Serpins regulate insect innate immunity via inhibition of serine proteinase cascades that initiate immune responses such as melanization and antimicrobial peptide production. In addition, several serpins with anti-pathogen activity are expressed as acute-phase serpins in insects upon infection. Parasitoid wasps can downregulate host serpin expression to modulate the host immune system. In addition, examples of serpin activity in development and reproduction in Drosophila have also been discovered. Serpins also function in host-pathogen interactions beyond immunity as constituents of venom in parasitoid wasps and saliva of blood-feeding ticks and mosquitoes. These serpins have distinct effects on immunosuppression and anticoagulation and are of interest for vaccine development. Lastly, the known structures of arthropod serpins are discussed, which represent the serpin inhibitory mechanism and provide a detailed overview of the process.