Identification of four small molecular mass proteins in the silk of Bombyx mori

Characterization of silk spun by the embiopteran, Antipaluria urichi

Silks are renowned for being lightweight materials with impressive mechanical properties. Though moth and spider silks have received the most study, silk production has evolved in many other arthropods. One insect group that has been little investigated is Embioptera (webspinners). Embiopterans produce silk from unique tarsal spinning structures during all life stages. We characterize the molecular and mechanical properties of Antipaluria urichi (Embioptera) silk through multiple approaches. First, we quantify the number of silk secretory structures on their forelimbs and the tensile properties of Antipaluria silk. Second, we present silk protein (fibroin) transcripts from an embiopteran forelimb protarsomere cDNA library. We describe a fibroin that shares several features with other arthropod silks, including a subrepetitive core region, a non-repetitive carboxyl-terminal sequence, and a composition rich in glycine, alanine, and serine. Despite these shared attributes, embiopteran silk has several different tensile properties compared to previously measured silks. For example, the tensile strength of Antipaluria silk is much lower than that of Bombyx mori silk. We discuss the observed mechanical properties in relation to the fibroin sequence, spinning system, and embiopteran silk use.

Silks produced by insect labial glands

Insect silks are secreted from diverse gland types; this chapter deals with the silks produced by labial glands of Holometabola (insects with pupa in their life cycle). Labial silk glands are composed of a few tens or hundreds of large polyploid cells that secrete polymerizing proteins which are stored in the gland lumen as a semi-liquid gel. Polymerization is based on weak molecular interactions between repetitive amino acid motifs present in one or more silk proteins; cross-linking by disulfide bonds may be important in the silks spun under water. The mechanism of long-term storage of the silk dope inside the glands and its conversion into the silk fiber during spinning is not fully understood. The conversion occurs within seconds at ambient temperature and pressure, under minimal drawing force and in some cases under water. The silk filament is largely built of proteins called fibroins and in Lepidoptera and Trichoptera coated by glue-type proteins known as sericins. Silks often contain small amounts of additional proteins of poorly known function. The silk components controlling dope storage and filament formation seem to be conserved at the level of orders, while the nature of polymerizing motifs in the fibroins, which determine the physical properties of silk, differ at the level of family and even genus. Most silks are based on fibroin beta-sheets interrupted with other structures such as alpha-helices but the silk proteins of certain sawflies have predominantly a collagen-like or polyglycine II arrangement and the silks of social Hymenoptera are formed from proteins in a coiled coil arrangement.

Expression, purification and characterization of a three-domain Kazal-type inhibitor from silkworm pupae (Bombyx mori)

Serine protease inhibitors are essential for host physiological and immunological activities in insects. Analyzing the amino-acid sequence of a cDNA coding for a serine protease inhibitor in Bombyx mori (BmSPI), we found that BmSPI contained three homologous domains with a conserved sequence of C-X(3)-C-X(9)-C-X(6)-Y-X(7)-C-X(3)-C-X(11)-C similar to that of Kazal-type serine protease inhibitors, suggesting BmSPI as a new member of the Kazal-type serine protease inhibitor family. To characterize the three-domain Kazal-type inhibitor from silkworm pupae, the recombinant protein was expressed in Escherichia coli BL21 (DE3) Star. After purification with affinity and reversed-phase chromatographies, the recombinant BmSPI with a molecular mass of 33.642 Da was shown to be a specific subtilisin A inhibitor. Further studies indicated that the K(i) value of the recombinant BmSPI was 3.35 nM and the inhibitor seemed to form a 1:1 complex with subtilisin A. This is a first description of the structure and characterization of Kazal-type inhibitor with three domains cloned from silkworm pupae, B. mori.

Analysis of bacteria-challenged wild silkmoth, Antheraea mylitta (lepidoptera) transcriptome reveals potential immune genes

Background

In the recent years a strong resemblance has been observed between the insect immune system and the mammalian innate immune mechanisms suggesting their common origin. Among the insects, only the dipterans (Drosophila and various mosquito species) have been widely investigated for their immune responses towards diverse pathogens. In the present study we constructed and analysed the immune transcriptome of the lepidopteran Antheraea mylitta, an economically important Indian tasar silkmoth with a view to unravel the potential immune-related genes and pathways.

Results

An expressed sequence tag (EST) library was constructed from mRNA obtained from fat bodies of A. mylitta larvae that had been challenged by infection with Escherichia coli cells. We identified 719 unique ESTs from a total of 1412 sequences so generated. A third of the transcriptome showed similarity with previously characterized immune-related genes that included both the known and putative immune genes. Of the four putative novel defence proteins (DFPs) annotated by PSI-BLAST three showed similarity to extracellular matrix proteins from vertebrates implicated in innate immunity, while the fourth was similar to, yet distinct from, the anti-microbial protein cecropin. Finally, we analysed the expression profiles of 15 potential immune-related genes, and the majority of them were induced more prominently with E. coli compared to Micrococcus luteus. We also identified several unknown proteins, some of which could have probable immune-related functions based on the results of the ProDom analysis.

Conclusion

The present study has identified many potential immune-related genes in A. mylitta some of which are vertebrate homologues and others are hitherto unreported putative defence proteins. Several genes were present as members of gene families, as has also been observed in other insect species.

A five-domain Kazal-type serine proteinase inhibitor from black tiger shrimp Penaeus monodon and its inhibitory activities

A novel five-domain Kazal-type serine proteinase inhibitor, SPIPm2, identified from the hemocyte cDNA library of black tiger shrimp Penaeus monodon was successfully expressed in the Escherichia coli expression system. The expressed recombinant SPIPm2 (rSPIPm2) as inclusion bodies was solubilized with a sodium carbonate buffer, pH10, and purified by gel filtration chromatography. The molecular mass of rSPIPm2 was determined using MALDI-TOF mass spectrometry to be 29.065 kDa. The inhibitory activities of rSPIPm2 were tested against trypsin, alpha-chymotrypsin, subtilisin and elastase. The inhibitor exhibited potent inhibitory activities against subtilisin and elastase, weak inhibitory activity against trypsin, and did not inhibit chymotrypsin. Tight-binding inhibition assay suggested that the molar ratios of SPIPm2 to subtilisin and elastase were 1:2 and 1:1, respectively. The inhibition against subtilisin and elastase was a competitive type with inhibition constants (Ki) of 0.52 and 3.27 nM, respectively. The inhibitory activity of SPIPm2 against subtilisin implies that, in shrimp, it may function as a defense component against proteinases from pathogenic bacteria but the elastase inhibitory function is not known.

Structurally unique recombinant Kazal-type proteinase inhibitor retains activity when terminally extended and glycosylated

Recombinant derivatives of the Kazal-type serine proteinase inhibitor GmSPI2 (36 amino acid residues), which is a component of insect silk, were prepared in the expression vector Pichia pastoris. The rhSPI2 had a C-terminal hexahistidine tag attached to the GmSPI2 sequence, rtSPI2 was extended with GluAlaAla at the N-terminus, and rfSPI2 included this N-terminal extension and a C-terminal tail of 22 residues (myc epitope and hexahistidine). A portion of the secreted rfSI2 was O-glycosylated with a trimannosyl or hexamannosyl. The native inhibitor was active slightly on trypsin and highly on subtilisin and proteinase K. The extended C-terminus in rhSPI2 and rfSPI2 enhanced activity on the two latter enzymes and rendered rfSPI2 active on elastase and pronase, but abolished the inhibition of trypsin. The glycosylation of rfSPI2 reduced its inhibitory activity to a level comparable with the native inhibitor. The rtSPI2 with tripeptide extension at the N-terminus and no C-terminal modification was clearly less active than the native inhibitor. None of the tested compounds inhibited alpha-chymotrypsin and the non-serine proteinases.

The Acrolepiopsis assectella silk cocoon: kairomonal function and chemical characterisation

Two soluble sericin-like polypeptides, B1 and B2, from leek moth (Acrolepiopsis assectella) cocoons trigger host-acceptance behaviour in the parasitoid, Diadromus pulchellus (Proc. Roy. Soc. London B 269 (2002) 1879). We found that these polypeptides were particularly cysteine-rich and lost their ability to trigger host-acceptance behaviour after being denatured and purified. This suggests that inter-disulphide bonds and the secondary structure of B1 and B2 are important for their biological activity. We also isolated six insoluble polypeptides (or polypeptides of low solubility) from A. assectella cocoons. At least four of these polypeptides triggered host-acceptance behaviour. The strongest responses were observed with P22, a light-chain fibroin or a seroin-peptide, and P100, a sericin-like polypeptide that is probably more strongly associated with the silk core than are B1 and B2. In conclusion, several polypeptides from different parts of the A. assectella silk-cocoon (the insoluble core and coating of the silk thread) are able to elicit host-acceptance behaviour in D. pulchellus females. These polypeptides belong to different silk protein families and are used as kairomones by this specialist parasitoid.

The construction of an EST database for Bombyx mori and its application

To build a foundation for the complete genome analysis of Bombyx mori, we have constructed an EST database. Because gene expression patterns deeply depend on tissues as well as developmental stages, we analyzed many cDNA libraries prepared from various tissues and different developmental stages to cover the entire set of Bombyx genes. So far, the Bombyx EST database contains 35,000 ESTs from 36 cDNA libraries, which are grouped into approximately 11,000 nonredundant ESTs with the average length of 1.25 kb. The comparison with FlyBase suggests that the present EST database, SilkBase, covers >55% of all genes of Bombyx. The fraction of library-specific ESTs in each cDNA library indicates that we have not yet reached saturation, showing the validity of our strategy for constructing an EST database to cover all genes. To tackle the coming saturation problem, we have checked two methods, subtraction and normalization, to increase coverage and decrease the number of housekeeping genes, resulting in a 5-11% increase of library-specific ESTs. The identification of a number of genes and comprehensive cloning of gene families have already emerged from the SilkBase search. Direct links of SilkBase with FlyBase and WormBase provide ready identification of candidate Lepidoptera-specific genes.

Correlation between fibroin amino acid sequence and physical silk properties

The fiber properties of lepidopteran silk depend on the amino acid repeats that interact during H-fibroin polymerization. The aim of our research was to relate repeat composition to insect biology and fiber strength. Representative regions of the H-fibroin genes were sequenced and analyzed in three pyralid species: wax moth (Galleria mellonella), European flour moth (Ephestia kuehniella), and Indian meal moth (Plodia interpunctella). The amino acid repeats are species-specific, evidently a diversification of an ancestral region of 43 residues, and include three types of regularly dispersed motifs: modifications of GSSAASAA sequence, stretches of tripeptides GXZ where X and Z represent bulky residues, and sequences similar to PVIVIEE. No concatenations of GX dipeptide or alanine, which are typical for Bombyx silkworms and Antheraea silk moths, respectively, were found. Despite different repeat structure, the silks of G. mellonella and E. kuehniella exhibit similar tensile strength as the Bombyx and Antheraea silks. We suggest that in these latter two species, variations in the repeat length obstruct repeat alignment, but sufficiently long stretches of iterated residues get superposed to interact. In the pyralid H-fibroins, interactions of the widely separated and diverse motifs depend on the precision of repeat matching; silk is strong in G. mellonella and E. kuehniella, with 2-3 types of long homogeneous repeats, and nearly 10 times weaker in P. interpunctella, with seven types of shorter erratic repeats. The high proportion of large amino acids in the H-fibroin of pyralids has probably evolved in connection with the spinning habit of caterpillars that live in protective silk tubes and spin continuously, enlarging the tubes on one end and partly devouring the other one. The silk serves as a depot of energetically rich and essential amino acids that may be scarce in the diet.