Sericin Digestion by Fibroinase, a Cathepsin L-like Cysteine Proteinase, of Bombyx mori Silk Gland

Abstract

In order to obtain conclusive evidence that fibroinase, a cathepsin L-like cysteine proteinase, of silk gland digests sericin, in addition to fibroin, three sericin fractions, s-A, s-M and s-P, were prepared from cocoons by the method of Takasu et al. (2002) and subjected to enzymatic digestion by fibroinase purified from silk gland of day one B. mori pupa. Time course study showed clearly that all the sericin fractions were digested in a time dependent manner and that complete digestion was attained by 3h. Sericin fragments from each sericin fraction digested for 3h were subjected to reverse phase column chromatography. Mass of each sericin fragment was determined by MALDI TOF mass spectrometry to check the purity of each peak fraction, followed by peptide sequencing by protein sequencer. Some sequences of sericin fragments corresponded portions of the sequence of sericin protein deduced from the nucleotide sequence of sericin gene (Garel et al., 1997). By alignment of an amino acid sequence of a sericin fragment with the sericin protein sequence, estimation of cleavage sites of sericin by fibrionase of silk gland was possible. It was concluded that fibroinase of silk gland digests sericin as well as fibroin (Watanabe et al., 2006a). This is because fibroinase of silk gland is a cathepsin L-like cysteine proteinase, which is generally believed to show broad substrate specificity. Fibroinase of silk gland is highly likely the proteinase that is responsible for digestion of fibroin and sericin in the luminal contents of silk gland in the early pupa and at each molt period during larval development.