Abstract
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.
Highlights
Serine proteases (SPs), account for almost one-third of all proteases, and serve as inevitable components in catalyzing hydrolytic reactions both intra- and extracellularly
BLAST searches for serpin genes were conducted in N. vitripennis genome, which revealed a similar number of serpins (9) (Supplementary Table S3)
PpSPI4 contains nearly 80 residues between helix I and strand 5 A while PpSPI10 loses the structure from helix C to strand 3 C
Summary
Serine proteases (SPs), account for almost one-third of all proteases, and serve as inevitable components in catalyzing hydrolytic reactions both intra- and extracellularly. The action of SPs is tightly regulated by their inhibitors, including serine protease inhibitors (SPIs)[5,6,7]. Canonical inhibitors can be divided into about 20 protein families such as Kazal, trypsin inhibitor-like (TIL), Pacifastin and Kunitz serine protease inhibitors[5]. It is widely studied that serpins take effect in inhibiting Spätzle-processing enzyme in Toll signaling pathway and PPO activation cascade[16]. In Manduca sexta, serpin-4 (MsSRPN4) and serpin-5 (MsSRPN5) are inhibitors of hemolymph proteinase-1(MsHP-1) and hemolymph proteinase-6 (MsHP6), and function in protease cascades of PPO pathway. There are fewer molecular studies on canonical SPIs. A Kazal-type serine protease inhibitor named oryctin inhibits α-chymotrypsin, endopeptidase K, subtilisin and elastase, indicating it may be related to digestion[22]. Since all of the SPIs play significant roles in several biological processes, identification of these regulator molecules is necessary for better understanding of the biochemical and molecular functions of SPIs
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