Identification and characterization of sericin5 reveals non-cocoon silk sericin components with high ß-sheet content and adhesive strength.

Sericins are glue proteins on the surface of silk fibers. Four sericins have been characterized in silkworm, namely sericin1 (Ser1), sericin2 (Ser2), sericin3 (Ser3), and sericin4 (Ser4). In this study, we report a novel sericin, sericin5 (Ser5), which exists only in non-cocoon silk. We describe the sequence, exon-intron structure, and translation products of Ser5 in Bombyx mori. The Ser5 gene is approximately 22-kb long and comprises 16 exons. Ser5 protein has a size of 260 kDa, as determined by SDS-PAGE, western blot, and LC-MS/MS. Immunofluorescence analysis revealed that Ser5 co-localizes with Ser1 in the sericin layer. The expression pattern of Ser5 was detected at the transcriptional and translational levels. We systematically analyzed and compared the amino acid composition, repeat regions, and hydrophilicity of silkworm sericins. Morphological observations showed that non-cocoon silk had more sericin than cocoon silk. Circular dichroism spectra revealed that non-cocoon silk sericin contained more ß-sheet structures than cocoon silk sericin. In addition, we found that the hydrophilicity and adhesive strength of native sericin increases gradually from the inner layer to the outer layer. This research enhances our understanding of various sericins from cocoon silk and non-cocoon silk with regard to their expression patterns, hydrophilicity, secondary structure and adhesive performances. STATEMENT OF SIGNIFICANCE: Sericin is a natural biomaterial with diverse biological properties, which has long been used as tissue engineering and biomedical applications. However, the composition and distribution of sericins in different kinds of silk are still uncertain, and the properties difference between sericins have not yet been reported. Our study makes a significant contribution to the literature as it identifies the sequence, composition, hydrophilicity and adhesive property of sericins. Moreover, it provides key insights into the structure-function and function-distribution relationships associated with sericins. We believe that this study will arouse the interest to the readership of your journal as it identifies the new complete sequence of sericin and revealed the composition and properties of sericin, thus highlighting their future potentials applications in both the biomaterial and technical fields.

Insights into the structure and composition of mineralized hard cocoons constructed by the oriental moth, Monema (Cnidocampa) flavescens Walker.

Animals widely use minerals and organic components to construct biomaterials with excellent properties, such as teeth, bones, molluscan shells and eggshells. The larvae of the oriental moth, Monema (Cnidocampa) flavescens Walker, secrete silk proteins that combine closely with calcareous minerals to construct a hard cocoon, which is completely different from the mineral-free Bombyx mori cocoon. The cocoons of oriental moths are likely to be the hardest among the cocoons constructed by insect species. The cocoons of oriental moths were found to be mainly composed of calcium oxalates and Asx/Ser/Gly-rich cocoon proteins, but the types of calcium oxalates and cocoon proteins remain to be elucidated. In this study, we provide an in-depth explanation of the inorganic and organic components in the oriental moth cocoon. Microscopy and imaging technologies revealed that the cocoon is composed of mineral crystals, silk fibers and other organic matter. X-ray diffraction and infrared spectral analyses showed that the mineral crystals in the oriental moth cocoon were mainly CaC2H2O4·H2O. ICP-OES analysis suggested that the mineral crystals in the cocoons were mainly CaC2H2O4·H2O. LC-MS/MS-based proteomics allowed us to identify 467 proteins from the oriental moth cocoon, including 252 uncharacterized proteins, 87 enzymes, 36 small molecule binding proteins, and 5 silk proteins. Among the uncharacterized proteins, 25 of which were Asn-rich proteins because they contained a high proportion of Asn residues (19.1%-41.4%). Among the top 20 cocoon proteins with the highest abundance, 9 of which were Asn-rich proteins. The qPCR was used to investigate the expression patterns of the major cocoon protein-coding genes. Three fibroins and three Asn-rich proteins were expressed only in the silk gland but not in other tissues. The expression of Asn-rich proteins in the silk gland gradually increased from the anterior silk gland to the posterior silk gland. These findings provide important references for understanding the formation mechanism and mechanical properties of mineralized hard cocoons constructed by oriental moths.