Amino acid sequence of a novel Kunitz-type chymotrypsin inhibitor from hemolymph of silkworm larvae, Bombyx mori

KPI5 Is Involved in the Regulation of the Expression of Antibacterial Peptide Genes and Hemolymph Melanization in the Silkworm, Bombyx mori

Kunitz-type protease inhibitors (KPIs) are ubiquitously found in many organisms, and participate in various physiological processes. However, their function in insects remains to be elucidated. In the present study, we characterized and functionally analyzed silkworm KPI5. Sequence analysis showed that KPI5 contains 85 amino acids with six conserved cysteine residues, and the P1 site is a phenylalanine residue. Inhibitory activity and stability analyses indicated that recombinant KPI5 protein significantly inhibited the activity of chymotrypsin and was highly tolerant to temperature and pH. The spatio-temporal expression profile analysis showed that KPI5 was synthesized in the fat body and secreted into the hemolymph. In vivo induction analysis showed that the expression of KPI5 in the fat body was significantly upregulated by pathogen-associated molecular patterns (PAMPs). Binding assays suggested that KPI5 can bind to pathogens and PAMPs. In vitro pathogen growth inhibition assay and encapsulation analysis indicated that KPI5 can neither kill pathogenic bacteria directly nor promote the encapsulation of agarose beads by silkworm hemocytes. Recombinant protein injection test and CRISPR/Cas9-mediated knockdown showed that KPI5 promotes the expression of antimicrobial peptides (AMPs) in the fat body. Moreover, the survival rate of individuals in the KPI5 knockdown group was significantly lower than that of the control group after pathogen infection. Phenoloxidase (PO) activity assays showed that KPI5 significantly inhibited the hemolymph PO activity and melanization induced by PAMPs. These findings suggested that KPI5 plays a dual regulatory role in innate immunity by promoting the expression of antimicrobial peptides in the fat body and inhibiting hemolymph melanization. Our study furthers the understanding of the function of insect KPIs and provides new insights into the regulatory mechanism of insect immune homeostasis.

A serpin (CvT-serpin15) of teratocytes contributes to microbial-resistance in Plutella xylostella during Cotesia vestalis parasitism

BACKGROUND

Parasitic wasps are an important group of entomophagous insects for pest control. As parasitic wasps often lay eggs on or into their associated hosts, parasitoids evolve to utilize several factors including venom, polydnavirus (PDV) to alter host physiology for successful parasitism. Some taxa of endoparasitoids produce teratocytes, which are a type of cell that is released into host insects when wasp eggs hatch. Teratocytes display multifunction in parasitism such as host nutritional exploration, immune and developmental regulation, by secreting plenty of proteins into host hemocoel.

RESULTS

A serpin (CvT-serpin15) secreted by teratocytes was characterized. QPCR results showed the expressional level of CvT-serpin15 was upregulated following bacterial challenges. Enzyme activity experiment indicated the recombinant CvT-serpin15 protein could interfere with the growth of Gram-positive bacteria Staphylococcus aureus. The survival rate assay demonstrated CvT-serpin15 increased survival rate of Plutella xylostella infected by S. aureus.

CONCLUSION

CvT-serpin15 secreted by teratocytes would boost the host immune system when pathogens invade host hemocoel during parasitism, and ultimately protect the development of wasp larva from bacterial infection. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Isolation and characterization of a protease inhibitor from Acacia karroo with a common combining loop and overlapping binding sites for chymotrypsin and trypsin

By using affinity and reversed-phase HPLC (RP-HPLC) chromatographies two chymotrypsin-trypsin inhibitors were isolated from seeds of Acacia karroo, a legume of the subfamily Mimosoideae. The primary structure of one of these inhibitors, named AkCI/1, was determined. The inhibitor consists of two polypeptide chains, 139 and 44 residues respectively, which are linked by a single disulfide bridge. The amino acid sequence of AkCI/1 is homologous to and showed more than 60% sequence similarity with other protease inhibitors isolated earlier from the group of Mimosoideae. AkCI/1 inhibits both chymotrypsin (EC 3.4.21.1) and trypsin (EC 3.4.21.4) in a 1:1M ratio with Ki values of 2.8 × 10(-12)M and 1.87 × 10(-12)M, respectively. The P1-P1' residues for trypsin were identified as Arg68-Ile69 by selective hydrolysis of the inhibitor at this site, with bovine trypsin and human trypsin IV. The cleavage did not affect the inhibition of trypsin, but fully abolished the chymotrypsin inhibitory activity of AkCI/1. This finding together with our studies on competition of the two enzymes for the same combining loop suggests that the same loop has to contain the binding sites for both proteases. The most likely P1 residue of AkCI/1 for chymotrypsin is Tyr67.

Genomic structure and expression analysis of the gene encoding a silkworm basic Kunitz-type chymotrypsin inhibitor

Kunitz-type chymotrypsin inhibitor CIb1 of silkworm Bombyx mori is a basic peptide consisting of 62 amino acid residues. To elucidate the mechanisms of transcriptional regulation of CIb1 gene expression, we cloned it for genomic structure analysis. CIb1 cDNA was used as a probe to screen a BAC sub-library. One positive clone containing the upstream sequences was isolated and the sequence result showed that CIb1 gene consists of three exons spaced by two introns. In the 5'-flanking region, consensus TATA and CCAAT boxes were identified. Other binding sites for transcription factors such as NF-kappaB, GATA, C/EBP, COUP-TF/HNF-4, RORalpha1, SRY, and HOXA3 were also detected. Southern blot analysis suggested a single copy of CIb1 gene in the silkworm genome. Northern blot analysis indicated that the expression of CIb1 gene is transcriptionally regulated during development and is apparently tissue-specific. The CIb1 mRNA was detected in fat body, ovary, trachea, and skin. We furthermore investigated the CIb1 expression profiles after LPS and E. coli injection. The fluctuations of CIb1 transcript in challenged larvae confirm our proposal that CIb1 is an immune responsible gene. According to our data, we discussed the transcriptional factors putatively responsible for the physiological role of CIb1 in the silkworm hemolymph.

Insect silk contains both a Kunitz-type and a unique Kazal-type proteinase inhibitor

Insect silk is made up of structural fibrous (fibroins) and sticky (sericins) proteins, and contains a few small peptides of hitherto unknown functions. We demonstrate that two of these peptides inhibit bacterial and fungal proteinases (subtilisin, proteinase K and pronase). These 'silk proteinase inhibitors' 1 and 2 (SPI 1 and 2) are produced in the middle section of the silk-secreting glands prior to cocoon spinning and their production is controlled at transcription level. The full length cDNA of pre-SPI 1 contains 443 nucleotides and encodes a peptide of 76 amino-acid residues, of which 20 make up a signal sequence. The mature SPI 1 (6056.7 Da, 56 residues) is a typical thermostable Kunitz-type proteinase inhibitor with Arg in P1 position. The cDNA of pre-SPI 2 consists of 260 nucleotides and yields a putative secretory peptide of 58 amino-acid residues. The functional SPI 2 (3993 Da, 36 residues) is a single-domain Kazal-type proteinase inhibitor with unique structural features: free segment of the N-terminus is reduced to a single amino-acid residue, lack of CysI and CysV precludes formation of the A-ring and provides increased flexibility to the C-ring, and absence of several residues around the normal position of CysV shortens and changes the alpha helix segment of the protein. The structure reveals that the length and arrangement of the B-ring, including exposure of the P1 residue, and the position of the C-terminus relative to the B-loop, are essential for the activity of the Kazal-type inhibitors.

Complete genomic sequence of the Amsacta moorei entomopoxvirus: analysis and comparison with other poxviruses

The genome of the genus B entomopoxvirus from Amsacta moorei (AmEPV) was sequenced and found to contain 232,392 bases with 279 unique open reading frames (ORFs) of greater than 60 amino acids. The central core of the viral chromosome is flanked by 9.4-kb inverted terminal repeats (ITRs), each of which contains 13 ORFs, raising the total number of ORFs within the viral chromosome to 292. ORFs with no known homology to other poxvirus genes were shown to constitute 33.6% of the viral genome. Approximately 28.6% of the AmEPV genome encodes homologs of the mammalian poxvirus colinear core genes, which are found dispersed throughout the AmEPV chromosome. There is also no significant gene order conservation between AmEPV and the orthopteran genus B poxvirus of Melanoplus sanguinipes (MsEPV). Novel AmEPV genes include those encoding a putative ABC transporter and a Kunitz-motif protease inhibitor. The most unusual feature of the AmEPV genome relates to the viral encoded poly(A) polymerase. In all other poxviruses this heterodimeric enzyme consists of a single large and a single small subunit. However, AmEPV appears to encode one large and two distinct small poly(A) polymerase subunits. AmEPV is one of the few entomopoxviruses which can be grown and manipulated in cell culture. The complete genomic sequence of AmEPV paves the way for an understanding and comparison of the molecular properties and pathogenesis between the entomopoxviruses of insects and the more intensively studied vertebrate poxviruses.

Isolation and characterization of novel inducible serine protease inhibitors from larval hemolymph of the greater wax moth Galleria mellonella

Three inducible serine protease inhibitors (ISPI-1, 2, 3) have been purified from larval hemolymph of greater wax moth larvae, Galleria mellonella, and characterized at a molecular level. These inhibitors were synthesized after larvae were injected with a yeast polysaccharide, zymosan preparation. ISPI-1,2,3 were active against various serine proteases including trypsin and toxic proteases released by the entomopathogenic fungus Metarhizium anisopliae. Precipitation by trichloroacetic acid and heat, followed by FPLC and HPLC separation steps were used for purification of the protease inhibitors from cell-free hemolymph samples. The molecular masses of purified proteins were determined by MS to be 9.2 kDa (ISPI-1), 6.3 kDa (ISPI-2) and 8.2 kDa (ISPI-3) with isoelectric points ranging between 7.2 and 8.3. The N-terminal amino-acid sequences of ISPI-1 and ISPI-3 are not similar to other known proteins, whereas that of ISPI-2 exhibits extensive similarity to known Kunitz-type protease inhibitors.

Serine proteinase inhibitors in arthropod immunity

Arthropod hemolymph contains proteins with serine proteinase inhibitory activity. These inhibitors may exist in plasma or in hemocyte granules. Serine proteinase inhibitors from the Kazal, Kunitz, alpha-macroglobulin, and serpin families have been identified in arthropod hemolymph and have been characterized biochemically. Two new families of low molecular weight serine proteinase inhibitors have recently been discovered: one in silkworms (the Bombyx family) and another in locusts and a crayfish. The serine proteinase inhibitors in arthropod hemolymph are likely to function in protecting their hosts from infection by pathogens or parasites. Some may inhibit fungal or bacterial proteinases. Others probably have roles in regulating endogenous proteinases involved in coagulation, prophenol oxidase activation, or cytokine activation.

Primary structure and possible functions of a trypsin inhibitor of Bombyx mori

A protein with a low molecular mass of 6027 was purified from cocoon shell of silkworm, Bombyx mori. Two-dimensional polyacrylamide gel electrophoresis (2D/PAGE) resolved this protein into a single spot with pI 4.3 and Mr 6000. Amino acid sequence analysis revealed that this protein consists of 55 amino acids, six of these being cysteine residues and is highly homologous to bovine pancreatic trypsin inhibitor-type inhibitors. The 6-kDa protein is heat stable and acid stable and inhibits bovine trypsin by forming a low-dissociation complex with trypsin in a 1 : 1 molar ratio (Ki = 2.8 x 10-10), but does not alpha-chymotrypsin. This cocoon shell-associated trypsin inhibitor (CSTI) was thus concluded to belong to the bovine pancreatic trypsin inhibitor class. CSTI was developmentally regulated in the silk gland at the final stage of larval growth, and its specific distribution in the middle silk gland, an organ in which silk proteins are stored during the final larval instar, occurred before the onset of spinning. This inhibitor protects the tryptic degradation of fibroin light (L) chain in vitro. These results suggest that this trypsin inhibitor may play an important part on regulating proteolytic activity in the silk gland or protecting silk proteins from degradation during histolysis.

Identification of an aprotinin antiviral domain

Digestion of the proteinase inhibitor aprotinin, by clostripain, a cysteine proteinase, yielded five oligopeptide fragments. Two fragments exhibited both antiviral and antibacterial activities, two fragments only antiviral activity, and one fragment showed no antimicrobial activity. One of the former oligopeptides showed antiviral activity against human herpes simplex virus type 1 and bovine parainfluenza virus type 3. It consisted of the hexapeptide Y-F-Y-N-A-K corresponding to amino acids 21-26 of intact aprotinin. An identical synthetic peptide had the same antiviral spectrum as the natural hexapeptide, exhibited no antibacterial activity, and was also devoid of trypsin inhibiting activity. Intact aprotinin, in contrast, is ineffective against human herpes simplex virus 1 and bovine parainfluenza virus 3 but possesses antibacterial properties against several bacterial species [(1992) J. Appl. Bact. 72, 180-187].

Insect immunity: two proteinase inhibitors from hemolymph of Locusta migratoria

Two protease inhibitors were isolated from the plasma of Locusta migratoria and sequenced. They were 35 and 36 amino acids long and revealed very little similitude for the protease inhibitors isolated from other arthropods. They inhibit the proPhenoloxidase Phenoloxidase proteolytic activation cascade in hemocyte extracts of the same insect. This inhibiting activity resulted in a lower production of PO, a key enzyme for the defence mechanism in arthropods. Both peptides however showed a strong in vitro inhibiting activity toward alpha-chymotrypsin and elastase, LMCI I inhibits the human leukocyte enzyme while LMCI II mostly the pancreatic one, a difference explainable on the basis of the active site sequence changes.

Serine proteinase inhibitor profiles in the hemolymph of a wide range of insect species

1. The inhibition of trypsin, chymotrypsin, neutrophil elastase and cathepsin G, and pancreatic elastase by the hemolymph of 14 insect species in six orders has been investigated. 2. All samples showed great diversity in terms of both total proteinase inhibitory capacity and specificity. 3. The highest total inhibitory capacity was found in the larval hemolymph of species in the beetle family Tenebrionidae and the lowest in that of an adult coreid bug, Acanthocephala femorata.

Evolutionary origin of a Kunitz-type trypsin inhibitor domain inserted in the amyloid beta precursor protein of Alzheimer's disease

The Kunitz-type protease inhibitor is one of the serine protease inhibitors. It is found in blood, saliva, and all tissues in mammals. Recently, a Kunitz-type sequence was found in the protein sequence of the amyloid beta precursor protein (beta APP). It is known that beta APP accumulates in the neuritic plaques and cerebrovascular deposits of patients with Alzheimer's disease. Collagen type VI in chicken also has an insertion of a Kunitz-type sequence. To elucidate the evolutionary origin of these insertion sequences, we constructed a phylogenetic tree by use of all the available sequences of Kunitz-type inhibitors. The tree shows that the ancestral gene of the Kunitz-type inhibitor appeared about 500 million years ago. Thereafter, this gene duplicated itself many times, and some of the duplicates were inserted into other protein-coding genes. During this process, the Kunitz-type sequence in the present beta APP gene diverged from its ancestral gene about 270 million years ago and was inserted into the gene soon after duplication. Although the function of the insertion sequences is unknown, our molecular evolutionary analysis shows that these insertion sequences in beta APP have an evolutionarily close relationship with the inter-alpha-trypsin inhibitor or trypstatin, which inhibits the activity of tryptase, a novel membrane-bound serine protease in human T4+ lymphocytes.

Patchwork-structure serpins from silkworm (Bombyx mori) larval hemolymph

A new serpin (serine proteinase inhibitor), having antichymotryptic activity, was isolated from silkworm, Bombyx mori, larval hemolymph and was named silkworm antichymotrypsin II (sw-AchyII). Amino-acid-sequence analysis of sw-AchyII revealed that it consisted of 375 amino acids without cysteine or glycosylated residues. sw-AchyII formed an SDS-undissociable complex with alpha-chymotrypsin, but this complex was broken down at pH 12.5 into alpha-chymotrypsin and sw-AchyII in which the reactive site was cleaved. Amino-acid-sequence analysis after cleavage identified in P1-P1' residue at the reactive site of sw-AchyII as Phe340-Met341. The amino acid sequence from the amino terminus to residue 336 was completely identical to the corresponding region of sw-AT [Takagi, H., Narumi, H., Nakamura, K. & Sasaki, T. (1990) J. Biochem. (Tokyo) 108, 372-378]. The degree of similarity between sw-AchyII and silkworm antitrypsin (sw-AT) from residue 337 to the carboxy terminus was only 46%. Reactive sites of both serpins were in the variable regions.

Purification and characterization of a chymotrypsin Kunitz inhibitor type of polypeptide from the venom of cobra (Naja naja naja)

A chymotrypsin Kunitz inhibitor type of polypeptide has been isolated from the venom of Naja naja naja by reverse phase HPLC and cation exchange FPLC. It is present in a considerably lower amount than that of the corresponding trypsin inhibitor. The primary structure, determined by sequence analysis of the whole molecule and its tryptic peptides, has 57 residues with an apparent molecular mass of 6.2 kDa. The main contact site with the protease (P1) has a Phe, showing the specificity of the inhibitor. Of residues considered functionally important in Kunitz-type inhibitors, Gly-36 is replaced by Ser in a segment of weak contacts with the protease.

The reactive site of silkworm hemolymph antichymotrypsin is located at the COOH-terminal region of the molecule

The complex of silkworm larval hemolymph antichymotrypsin (Mr = 43,000) and C-chain of bovine alpha-chymotrypsin was obtained. This complex showed two NH2-terminal amino acid sequences identical to those of intact silkworm antichymotrypsin and C-chain of alpha-chymotrypsin, respectively. Alkali treatment of the complex brought about its dissociation and the separated inhibitor component (Mr = 36,000) had an NH2-terminal amino acid sequence identical to that of intact silkworm antichymotrypsin. These results suggest that the reactive site of this inhibitor is located at the COOH-terminal region of the molecule and that the nature of association of this inhibitor and alpha-chymotrypsin is an acyl-bond.