Cysteine proteinase from the eggs of the silkmoth, Bombyx mori: identification of a latent enzyme and characterization of activation and proteolytic processing in vivo and in vitro

When an acid cysteine proteinase, which had been purified from the eggs of silkmoth, Bombyx mori, was incubated at pH 3.6, enzymatic activity appeared after a few minutes, lag period, indicating that the purified cysteine proteinase was a latent form. SDS-polyacrylamide gel electrophoresis showed that after the incubation the latent form of the enzyme (47 kDa) disappeared and the active (39 kDa) form of the enzyme appeared, suggesting that the latent form was processed to the active form under acidic conditions (pH 3.6). The NH2-terminal 22-residue sequence of the active form was determined. The conversion of the latent form to the active form was completely blocked by E-64, which is a specific inhibitor of cysteine proteinases. The results strongly suggest that the processing might be autocatalytic. The latent form (47 kDa) disappeared in the silkmoth eggs during embryonic development and concomitantly with its disappearance, the 39-kDa form appeared, indicating that in vivo the enzyme is activated in a similar manner to that observed in in vitro experiments.