A cysteine proteinase with properties similar to mammalian cathepsins has been found in silkmoth eggs. Enzyme activity in the ovary increased abruptly 5-6 days after larval-pupal ecdysis. Activity in the mature ovary is quite high, and high activity is maintained throughout embryonic development. In a previous paper, we reported that the enzyme was synthesized in the ovary. To define the site of synthesis in more detail, follicle cells were isolated from ovaries and cultured in a medium containing 14C-amino acids. Isolated follicle cells synthesized and secreted cysteine proteinase into the medium. Immunohistochemistry also showed that the major site of synthesis is the follicle cells. Cysteine proteinase is able to degrade vitellin (Vn) and egg specific protein (ESP) in vitro in a profile similar to that found in vivo. The results are discussed in relation to the degradation mechanism of yolk proteins during early embryonic development.
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