Two subunits of the insect 26/29-kDa proteinase are probably derived from a common precursor protein

We previously identified the 26/29-kDa proteinase in the hemocytes of Sarcophaga peregrina (flesh fly) that appears to participate in elimination of foreign proteins in this insect. Here, we report the cDNA cloning of this proteinase. The cDNA encodes a protein which includes both the 26- and 29-kDa subunit, strongly suggesting that the both subunits are derived from a single precursor protein. The 26- and 29-kDa subunit located at the amino-terminal and carboxyl-terminal of the precursor protein. The 29-kDa subunit itself appeared to be a proteinase, for this subunit had 52% sequence identity with Sarcophaga cathepsin L, while 26-kDa subunit had no significant similarity. We also showed that 26/29-kDa proteinase was insensitive to specific inhibitors of cathepsin L. These results indicate that this proteinase is a novel member of the papain family. We isolated similar cDNAs from Drosophila melanogaster and Periplaneta americana (cockroach), suggesting that this proteinase is conserved in a wide variety of insects and participates in their defense mechanisms.