Silkworm Sericins: Compounds of Different Properties Produced in Different Stages and Silk Gland Parts

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 the five silkworm sericins. Morphological observations between cocoon and non-cocoon silk showed that non-cocoon silk had more sericin than cocoon silk. In addition, we found that the hydrophilicity and adhesive strength of native sericin increases gradually from the inner to outer layer. This research enhances our understanding of various sericins from cocoon silk and non-cocoon silk with regard to their expression patterns, hydrophilicity, and adhesive performances.