Fibroinase and its physiological inhibitors involved in the regulation of silk gland development in the silkworm, Bombyx mori.

Fibroinase, a cathepsin L-like cysteine protease, was previously identified in the silk gland of the silkworm, Bombyx mori. It shows high degradation activity during the pre-pupa period, when the silk gland undergoes apoptosis and remodeling. Here, we recombinantly expressed pro-fibroinase and activated it in vitro. Fibroinase showed optimal hydrolytic activity at pH 4.0 and its optimum temperature was about 42 °C. One physiological inhibitor, B. mori cysteine protease inhibitor (BCPI) was found, which showed strong inhibitory activity against fibroinase. The inhibitory reaction was caused by the formation of a non-covalent complex; this is in contrast to a previously reported mode of fibroinase inhibition by Serpin18. Expression profiles and immunolocalization analysis demonstrated that fibroinase was involved in silk gland development by degrading silk proteins and apoptosis/remodeling of silk glands at specific points. Furthermore, the comparison of the temporal expression of fibroinase and its inhibitors, BCPI and Serpin18, indicated that these inhibitors were involved in the silk gland development by regulating the activity of fibroinase from the fifth instar until the early spinning stage. These findings improve our understanding of the mechanism of protease regulation and its inhibitors in silk gland development.