Silk Gland Fibroinase. A Molecular Tool for Species Specificity Studies and an Industrial Tool for Enzymatic Degumming of Silk

Abstract

　　Silk gland fibroinase is cathepsin L-like cysteine proteinase obtainable from insect silk gland. At each larval molt period, it is secreted into the lumen of silk gland and digests both fibroin and sericin. It thus ensures silk gland cells in the next larval instar to synthesize and secrete fibroin and sericin into the lumen of silk gland. In early pupal stage, fibroinase secreted into the lumen of remaining silk gland digests remaining fibroin and sericin to help apoptosis of silk gland. In the feeding larvae, fibroinase is localized in lysosomes in silk gland cells to digest proteins and organelles there.

　　Fibroinase was purified first from the domesticated silkworm, Bombyx mori and then from wild silkmoth, Samia cynthia ricini. These fibroinases were different in some properties. N-terminal amino acid sequence was slightly different. It may be related to molecular structure of its substrate, fibroin. S. cynthia ricini fibroin is consisted of alternate sequences of poly-Ala and repeated Gly-Ala motif. B. mori fibroin is consisted of repeated Gly-Ala motif only. In the fifth instar larval period, S. cynthia ricini fibroinase increases its activity one day after onset of spinning and reaches maximum at end of spinning. B. mori fibroinase increases its activity before onset of spinning and reaches maximum at larval-pupal ecdysis. Maximum absolute activity of S. cynthia ricini fibroinase is 38 times higher than that of B. mori fibroinase. Silk gland of S. cynthia ricini degenerates to disappear after one day of end of spinning. B. mori silk gland remains after two days of larvalpupal ecdysis. Thus silk gland fibroinase can be a molecular tool to study species specificity of silk spinning insects.

　　Fibroinase can beautifully digest sericin in addition to fibroin as seen in the lumen of silk gland at each larval molt period. Fibroinase can be utilized as a new agent for enzymatic degumming of silk. Silkworms instructed this wisdom through fibroinase study of more than 20 years.