Molecular cloning, characterization, expression and activity analysis of cathepsin L in Chinese mitten crab, Eriocheir sinensis

Cathepsins, a superfamily of hydrolytic enzymes produced and enclosed within lysosomes, function in immune response in vertebrates; however, their function within the innate immune system of invertebrates remains largely unknown. Therefore, we investigated the immune functionality of cathepsin L (catL) in Chinese mitten crab (Eriocheir sinensis), a commercially important and disease vulnerable aquaculture species. The full-length catL cDNA (1274 bp) was cloned via PCR based upon an initial expressed sequence tag (EST) isolated from a hepatopancreatic cDNA library. The catL cDNA contained a 978 bp open reading frame (ORF) that encoded a putative 325 amino acid (aa) protein. Comparisons with other reported invertebrate and vertebrate sequences revealed conserved gene structure and enzyme active sites common among papain-like cysteine proteases, and high percent identity among other invertebrate cathepsins. CatL mRNA expression in E. sinensis was (a) tissue-specific, with the highest expression observed in hepatotpancreas, gill, stomach, and hemocytes, and (b) responsive in hemocytes to a Vibrio anguillarum challenge, the catL expression level and enzyme activity both with peak exposure observed 8 h post-injection. Collectively, data demonstrate the successful isolation of catL from the Chinese mitten crab, and its involvement in the innate immune system of an invertebrate.