The 30kP protease A responsible for 30-kDa yolk protein degradation of the silkworm, Bombyx mori: cDNA structure, developmental change and regulation by feeding

Disruption of egg-specific protein causes female sterility in Bombyx mori

Yolk proteins are the main source of nutrients during embryonic and early larval development in oviparous animals. Therefore, vitellogenesis is crucial for reproduction. The silkworm, Bombyx mori, is a model lepidopteran insect in which there are three yolk proteins: vitellin, 30-kDa protein, and egg-specific protein (Esp). In this study, we explored the gene function of Esp through transgenic clustered regularly interspaced palindromic repeats (CRISPR) / CRISPR-associated protein 9 technology-mediated mutations in the silkworm. We found that Esp mutation resulted in female sterility but had no effect on male fertility. Female mutants could lay eggs after mating, but the eggs were smaller and lighter colored than those laid by wild-type females. The most important finding is that the eggs laid by female mutants did not hatch. Furthermore, we observed stable inheritance of female sterility caused by Esp mutation through successive generations. Thus, Esp encodes a yolk protein that is crucial for female reproductive success and is a potential target for pest control.

Bioactivity of Bacillus thuringiensis (Bacillales: Bacillaceae) on Diatraea saccharalis (Lepidoptera: Crambidae) eggs

BACKGROUND

Bacillus thuringiensis (Bt) is a Gram-positive bacterium that synthesizes specific protein toxins, which can be exploited for control of various insect pests, including Diatraea saccharalis, a lepidopteran that severely damages sugarcane crops. Although studies have described the effects of Bt in the larval phases of D. saccharalis, few have examined its effect on insect eggs. Herein, we studied the entomopathogenic potential of Bacillus thuringiensis serovar Aizawai GC-91 (Bta) during D. saccharalis embryo development with the aim of understanding the entomopathogenic mechanism and developing new biological control techniques for target insects.

RESULTS

Bta concentrations of 5, 10 and 20 g L^-1 demonstrated the strongest bioactivity, reducing D. saccharalis egg viability by 28.69%, 33.91% and 34.98%, respectively. The lethal concentrations (LCs) were estimated as: LC₅₀ = 28.07 g L^-1 (CI 95% = 1.89-2.38) and LC₉₀ = 65.36 g L^-1 (CI 95% = 4.19-5.26). Alterations in egg coloration, melanization and granule accumulation were observed at 24 h, persisting until 144 h. The embryo digestive systems were severely damaged, including narrowing of the intestinal lumen, vesiculations and degenerated cells, causing embryonic death.

CONCLUSION

The toxicity caused by Bta in D. saccharalis embryos demonstrated its potential as a biological control agent and as a sustainable alternative for integrated management of D. saccharalis infestation. © 2020 Society of Chemical Industry.

A midgut-specific serine protease, BmSP36, is involved in dietary protein digestion in the silkworm, Bombyx mori

Serine proteases play important roles in digestion and immune responses during insect development. In the present study, the serine protease gene BmSP36, which encodes a 292-residue protein, was cloned from the midgut cells of Bombyx mori. BmSP36 contains an intact catalytic triad (H57, D102 and S195) and a conserved substrate-binding site (G189, H216 and G226), suggesting that it is a serine protease with chymotrypsin-like specificity. The temporal and spatial expression patterns of BmSP36 indicated that its messenger RNA and protein expression mainly occurred in the midgut at the feeding stages. Western blotting, immunofluorescence and liquid chromatography-tandem mass spectrometry analyses revealed secretion of BmSP36 protein from epithelial cells into the midgut lumen. The transcriptional and translational expression of BmSP36 was down-regulated after starvation but up-regulated after refeeding. Moreover, expression of the BmSP36 gene could be up-regulated by a juvenile hormone analogue. These results enable us to better define the potential role of BmSP36 in dietary protein digestion at the feeding stages during larval development.

Antiapoptotic activity of 30 kDa lipoproteins family from fat body tissue of silkworm, Bombyx mori

The family of 30 kDa lipoproteins (LP1-5) is abundant in silkworm pupa fat body (FB) and hemolymph. One of its members, the 29 kDa protein decreased in concentration from peripheral (PP) FB tissue but was sustained in perivisceral (PV) FB tissue at the time of apoptosis. This study investigated the correlation of the 30 kDa proteins with FB apoptosis. Two protein fractions were purified, a 29 and a 30/31 kDa protein fraction, and they were used to test for activity against actinomycin D-induced apoptosis in the FB tissues. Concentrations as little as 50 μg/mL of the 29 kDa protein fraction efficiently inhibited apoptosis. Less antiapoptotic activity was detected for the higher MW fraction; DNA fragmentation was observed in FB tissue treated with 50 μg/mL of the 30/31 kDa fraction. The viability of the cells in the 29 kDa protein-supplemented culture was 40% higher than in the 31 kDa protein-supplemented culture. However, the 30 kDa lipoproteins were not able to prevent scheduled FB degeneration during silkworm metamorphosis. Thus, it is hypothesized that the antiapoptotic 29 kDa protein needs to be proteolytically degraded by a regulatory mechanism to allow programmed cell death of FB tissue.

Crystal structure of Bombyx mori lipoprotein 6: comparative structural analysis of the 30-kDa lipoprotein family

The 30-kDa lipoprotein (LP) family of mulberry silkworm comprises major hemolymph proteins specific to the fifth instar larvae. The family consists of 46 members, 24 of which are referred to as typical 30-kDa LPs. To date, two crystal structures of 30-kDa LPs from Bombyx mori have been described (Bmlp3 and Bmlp7). Here, we present the crystal structure of Bmlp6, another 30-kDa LP member. Bmlp6 is comprised of two domains characteristic of this family, the VHS-type N-terminal domain and β-trefoil C-terminal domain. The structures of the three 30-kDa LPs have been compared and a number of differences are noted, including loop conformation, the surface electrostatic potential, and the potential binding cavities. We discuss the observed structural differences in the light of the potential different roles of the particular 30-kDa LP members in silkworm physiology.

Transcriptome analysis of silkworm, Bombyx mori, during early response to Beauveria bassiana challenges

Host–pathogen interactions are complex processes and it is a central challenge to reveal these interactions. Fungal infection of silkworm, Bombyx mori, may induce a variety of responsive reaction. However, little is known about the molecular mechanism of silkworm immune response against the fungal infection. To obtain an overview of the interaction between silkworm and an entomopathogenic fungus Beauveria bassiana, Digital Gene Expression profiling, a tag based high-throughput transcriptome sequencing method, was employed to screen and identify differentially expressed genes (DEGs, FDR≤0.001, ∣log₂ratio∣≥1) of silkworm larvae during early response against B. bassiana infection. Total 1430 DEGs including 960 up-regulated and 470 down-regulated ones were identified, of which 627 DEGs can be classified into GO categories by Gene Ontology (GO) analysis. KEGG pathways analysis of these DEGs suggested that many biological processes, such as defense and response, signal transduction, phagocytosis, regulation of gene expression, RNA splicing, biosynthesis and metabolism, protein transport etc. were involved in the interaction between the silkworm and B. bassiana. A number of differentially expressed fungal genes were also identified by mapping the sequencing tags to B. bassiana genome. These results provided new insights to the molecular mechanism of silkworm immune response to B. bassiana infection.

Two crystal structures of Bombyx mori lipoprotein 3 - structural characterization of a new 30-kDa lipoprotein family member

The 30-kDa family of lipoproteins from insect hemolymph has been the focus of a number of studies over the last few years. Recently, four crystal structures of Bombyx mori lipoprotein 7 have been determined. Here we report two crystal structures of another member of the 30-kDa lipoprotein family, Bombyx mori lipoprotein 3 (Bmlp3). The protein was isolated from its natural source, mulberry silkworm hemolymph. It crystallized in two different crystal forms, Bmlp3-p21 (space group P21) and Bmlp3-c2 (space group C2). The crystal structures were solved by molecular replacement using the coordinates of Bmlp7 as a starting model. The crystals of Bmlp3-p21 diffracted X-rays to 2.4 Å resolution and of Bmlp3-c2 to 2.1 Å resolution. Bmlp3 has an overall fold characteristic of 30-kDa lipoproteins, with a VHS-type N-terminal domain and β-trefoil C-terminal domain. Structural comparison of Bmlp3 and Bmlp7 shows that the loops present in the C-terminal domain are flexible and participate in dimer formation. Additionally, new putative binding sites of Bmlp3 have been analyzed in detail and the electrostatic potential of the protein surface at physiological pH 7.4 conditions has been calculated. The results of these calculations are the starting point for an explanation of the recently reported cell-penetrating properties of the 30-kDa lipoproteins.

The synthesis, transportation and degradation of BmLP3 and BmLP7, two highly homologous Bombyx mori 30K proteins

The 30K proteins comprise about 35% of the total embryo yolk proteins and function as storage proteins during embryonic development of the domesticated silkworm Bombyx mori. The most abundant components of hemolymph are 30K proteins in the early and middle pupal stages. In the present study, the 30K protein BmLP7 was purified from larval hemolymph by chromatography. We prepared the antibody of this protein and found that it could bind to both BmLP3 and BmLP7. We used western blotting to analyze the dynamic change of BmLP3 and BmLP7 proteins in the hemolymph during development and found their concentration decreased dramatically from day 4 pupae, which appears to be linked to their accumulation in the oocyte for forming yolk granule since then. We found BmLP3 and BmLP7 proteins reduced significantly in day 10 eggs (the day before hatching). The crude extract of the newly hatched larvae showed proteolytic activity against BmLP3 and BmLP7 and immunohistochemistry showed BmLP3 and BmLP7 were degraded in the embryonic gut lumen in day 10 eggs. These systematic studies of BmLP3 and BmLP7 reveal their synthesis, transportation and degradation, which could represent the experience of all 30K proteins.

A proteomic view on the developmental transfer of homologous 30 kDa lipoproteins from peripheral fat body to perivisceral fat body via hemolymph in silkworm, Bombyx mori

BACKGROUND

A group of abundant proteins of ~30 kDa is synthesized in silkworm larval peripheral fat body (PPFB) tissues and transported into the open circulatory system (hemolymph) in a time-depended fashion to be eventually stored as granules in the pupal perivisceral fat body (PVFB) tissues for adult development during the non-feeding stage. These proteins have been shown to act anti-apoptotic besides being assigned roles in embryogenesis and defense. However, detailed protein structural information for individual PPFB and PVFB tissues during larval and pupal developmental stages is still missing. Gel electrophoresis and chromatography were used to separate the 30 kDa proteins from both PPFB and PVFB as well as hemolymph total proteomes. Mass spectrometry (MS) was employed to elucidate individual protein sequences. Furthermore, 30 kDa proteins were purified and biochemically characterized.

RESULTS

One- and two-dimensional gel electrophoresis (1/2D-PAGE) was used to visualize the relative changes of abundance of the 30 kDa proteins in PPFB and PVFB as well as hemolymph from day 1 of V instar larval stage to day 6 of pupal stage. Their concentrations were markedly increased in hemolymph and PVFB up to the first two days of pupal development and these proteins were consumed during development of the adult insect. Typically, three protein bands were observed (~29, 30, 31 kDa) in 1D-PAGE, which were subjected to MS-based protein identification along with spots excised from 2D-gels run for those proteomes. Gas phase fragmentation was used to generate peptide sequence information, which was matched to the available nucleotide data pool of more than ten highly homologous insect 30 kDa lipoproteins. Phylogenetic and similarity analyses of those sequences were performed to assist in the assignment of experimentally identified peptides to known sequences. Lipoproteins LP1 to LP5 and L301/302 could be matched to peptides extracted from all bands suggesting the presence of full length and truncated or modified protein forms in all of them. The individual variants could not be easily separated by classical means of purification such as 2D-PAGE because of their high similarity. They even seemed to aggregate as was indicated by native gel electrophoresis. Multistep chromatographic procedures eventually allowed purification of an LP3-like protein. The protein responded to lipoprotein-specific staining.

CONCLUSIONS

In B. mori larvae and pupae, 30 kDa lipoproteins LP1 to LP5 and L301/302 were detected in PPFB and PVFB tissue as well as in hemolymph. The concentration of these proteins changed progressively during development from their synthesis in PPFB, transport in hemolymph to storage in PVFB. While the 30 kDa proteins could be reproducibly separated in three bands electrophoretically, the exact nature of the individual protein forms present in those bands remained partially ambiguous. The amino acid sequences of all known 30 kDa proteins showed very high homology. High-resolution separation techniques will be necessary before MS and other structural analysis can shed more light on the complexity of the 30 kDa subproteome in B. mori. A first attempt to that end allowed isolation of a B. mori LP3-like protein, the complete structure, properties and function of which will now be elucidated in detail.

Comparative analysis of proteome maps of silkworm hemolymph during different developmental stages

Background

The silkworm Bombyx mori is a lepidopteran insect with four developmental stages: egg, larva (caterpillar), pupa, and adult. The hemolymph of the silkworm is in an open system that circulates among all organs, and functions in nutrient and hormone transport, injury, and immunity. To understand the intricate developmental mechanisms of metamorphosis, silkworm hemolymph from different developmental stages, including the 3^rd day of fifth instar, the 6^th day of fifth instar, the 3^rd day of pupation, the 8^th day of pupal stage and the first day of the moth stage, was investigated by two-dimensional electrophoresis and mass spectrometry.

Results

Two-dimensional polyacrylamide gel electrophoresis showed that from the larval to moth stages, silkworm hemolymph proteins changed markedly. Not only did major proteins such as SP1, SP2, and the 30 K lipoprotein change, but other proteins varied greatly at different stages. To understand the functions of these proteins in silkworm development, 56 spots were excised from gels for analysis by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). We identified 34 proteins involved in metamorphosis, programmed cell death, food digestion, metabolism, and nutrient storage and transport. Most proteins showed different expression at different stages, suggesting functions in development and metamorphosis. An abundance of proteins related to immunity were found, including hemolin, prophenoloxidase, serine proteinase-like protein, paralytic peptide-binding protein, and protease inhibitor.

Conclusions

Proteomics research not only provides the opportunity for direct investigation of protein expression patterns, but also identifies many attractive candidates for further study. Two-dimensional maps of hemolymph proteins expressed during the growth and metamorphosis of the silkworm offer important insights into hemolymph function and insect metamorphosis.

Genome-wide identification and expression analysis of serine proteases and homologs in the silkworm Bombyx mori

Background

Serine proteases (SPs) and serine proteases homologs (SPHs) are a large group of proteolytic enzymes, with important roles in a variety of physiological processes, such as cell signalling, defense and development. Genome-wide identification and expression analysis of serine proteases and their homologs in the silkworm might provide valuable information about their biological functions.

Results

In this study, 51 SP genes and 92 SPH genes were systematically identified in the genome of the silkworm Bombyx mori. Phylogenetic analysis indicated that six gene families have been amplified species-specifically in the silkworm, and the members of them showed chromosomal distribution of tandem repeats. Microarray analysis suggests that many silkworm-specific genes, such as members of SP_fam12, 13, 14 and 15, show expression patterns that are specific to tissues or developmental stages. The roles of SPs and SPHs in resisting pathogens were investigated in silkworms when they were infected by Escherichia coli, Bacillus bombysepticus, Batrytis bassiana and B. mori nucleopolyhedrovirus, respectively. Microarray experiment and real-time quantitative RT-PCR showed that 18 SP or SPH genes were significantly up-regulated after pathogen induction, suggesting that SP and SPH genes might participate in pathogenic microorganism resistance in B. mori.

Conclusion

Silkworm SP and SPH genes were identified. Comparative genomics showed that SP and SPH genes belong to a large family, whose members are generated mainly by tandem repeat evolution. We found that silkworm has species-specific SP and SPH genes. Phylogenetic and microarray analyses provide an overview of the silkworm SP and SPHs, and facilitate future functional studies on these enzymes.

Interplay between acid phosphatase and cysteine proteases in mediating vitellin degradation during early embryogenesis of Periplaneta americana

In this work, we characterized the activities of two classes of proteases and AcP during early embryogenesis of Periplaneta americana. AcP activity was first detected at day 6 and reached a maximum level at day 10 of development. Using phosphoamino acids, phosphatase activity was shown to be directed only against phosphotyrosine at day 6 while at day 10 it was also active against phosphoserine. In parallel, two classes of proteases were detected and located within yolk granules: a clan CA-cysteine protease, which was inhibited by E-64, insensitive to CA 074 and activated by acidic pH at day 3; and a neutral serine protease, which was inhibited by aprotinin at day 6. Assays of vitellin (Vt) degradation evidenced that incubations at neutral pH induced slight proteolysis, while the incubations at acidic pH did not result in Vt degradation. However, pre-incubations of Vt with AcP increased the levels of Vt acidic proteolysis and this could be inhibited by the addition of phosphatase inhibitors. On the other hand, the same pre-incubations showed no effects on the profile of degradation at neutral pH. We propose that AcP and cysteine protease cooperate to assure Vt breakdown during early embryogenesis of P. americana.

Metalloproteases and egg-hatching in Pediculus humanus, the body (clothes) louse of humans (Phthiraptera: Insecta)

To investigate the biochemical components of egg-hatch in the body louse, Pediculus humanus, egg-shell-washings (ESW) were collected during the first 2 h post-hatching and analysed by gelatin SDS-PAGE. These ESW contained proteases with molecular mass in the range of 25–100 kDa; the most abundant proteases were ~25 kDa. The 3 main regions of protease activity in the one-dimensional gelatin SDS-PAGE gels resolved to at least 23 distinct regions of protease activity when analysed by two-dimensional gelatin SDS-PAGE, with iso-electric points spread over the entire 3 to 10 pH range. Mechanistic characterization indicated that the ESW contained proteases of the metallo-class, inhibited by both 1,10-phenanthroline and EDTA. Several protease inhibitors were tested for their ability to inhibit louse egg-hatch in vitro. The metalloprotease inhibitor 1,10-phenanthroline and the aminopeptidase inhibitor bestatin significantly inhibited (P<0·05) louse egg-hatch (100% and 58%, respectively). The presence of metalloproteases at the time of egg-hatch and the inhibition of egg-hatch in P. humanus by metalloprotease inhibitors suggests a crucial role for these proteases in the hatching of this medically important parasite.

Molecular properties and tissue distribution of 30K proteins as ommin-binding proteins from diapause eggs of the silkworm, Bombyx mori

We previously reported the purification of an ommin-binding protein (OMBP) from an acid-methanol extract of diapause eggs of the silkworm and that OMBP reacted with the anti-30K proteins antiserum. In order to clarify the relationship between OMBP and the 30K proteins, we attempted to determine the sequence of the N-terminal amino acid of OMBP, which was separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). We observed ten protein spots of various isoelectric points; the spots corresponded with 30 kDa. Based on the sequence of the N-terminal amino acid (20 residues), the spots belonged to two kinds of 30K proteins (6G1 and 19G1), which are known as the major plasma proteins in the larval hemolymph of the silkworm. The proteins are expected to attach to polysaccharide because they reacted with concanavalin A and elderberry bark lectin. Immunohistochemical observations clarified that the proteins were localized in yolk granules and serosa in the diapause egg. These results suggest that OMBP is composed of 30K proteins which were modified with polysaccharides. In addition, the expression of 30K proteins mRNA was observed at early embryonic stage in diapause eggs by RT-PCR analysis. The 30K proteins as OMBP may play an important role in the transport and accumulation of tryptophan metabolites and ommochrome during the formation of serosa.