Autolytic activation mechanism of Bombyx acid cysteine protease (BCP)

Acid cysteine proteinase in the eggs of the silkmoth, Bombyx mori, exists as an inactive proenzyme. This 47‐kDa pro‐BCP1 zymogen molecule can be processed in vitro into an enzymatically active 39‐kDa BCP molecule (11, 12). In this current study, the maximum rate of processing in vitro was achieved at approximately pH 4.0, at a temperature of 37 °C under reducing conditions. The rate of conversion was not affected by increasing concentrations of pro‐BCP. We prepared immobilized BCP bound to AH‐Sepharose and examined the activation. Immobilized pro‐BCP was autolysed, although the rate of processing was slow, indicating that the reaction might be an intramolecular one. Kinetic experiments suggest that the mechanism is likely to involve a stepwise reaction, in which pro‐BCP is converted to an active enzyme through intermediate forms releasing small peptides stepwise. The results suggest that autocatalytic cleavage (intramolecular) is a major processing step in the early stage of pro‐BCP activation.