Cloning and sequence of a cDNA for a highly basic protease from the digestive juice of the silkworm, Bombyx mori

A serine protease of the silkworm, Bombyx mori, with an isoelectric point of pH 10-11 and a pH optimum for succinyl-Leu-Leu-Val-Tyr-MCA degrading activity of about 10, was found in a 0.33 M NaCl-eluted fraction obtained from cation-exchange chromatography of digestive juice. The activity of the enzyme was strongly inhibited by chymostatin and PMSF, indicating that the protease is a chymotrypsin-like serine protease. The N-terminal amino acid sequence of the protease was determined, and a full-length cDNA clone (0.92 kbp) which was isolated from a midgut cDNA library was sequenced. The cDNA encodes a pre-proenzyme of 284 amino acids with a pro-segment of 50 amino acids and mature protein of 234 amino acids. From its primary structure, the predicted molecular mass of the mature protein is 24.5 kDa. A sequence comparison of the Bombyx highly basic protease with other serine proteases revealed that this enzyme is a mammalian-type serine protease with a catalytic triad consisting of His45, Asp92 and Ser186. A large number of Arg residues are encoded by the cDNA which may be responsible for its stability and/or function in the alkaline condition, by remaining charged at high pH.