Cathepsin S of Sciaenops ocellatus: Identification, transcriptional expression and enzymatic activity.

Cathepsin S is a member of cysteine cathepsins and belongs to the cathepsin L-like family. In mammals, it is known to participate in various physiological processes and host immune defense. In teleost fish, the function of cathepsin S is less investigated. In the present work, we characterized a cathepsin S homologue (SoCatS) from red drum (Sciaenops ocellatus), a commercially valuable fish in Chinese mariculture. Like all cathepsin S, SoCatS possesses a peptidase domain with four catalytically essential residues (Gln140, Cys146, His285, and Asn305) conserved in the cathepsin S of different organisms. SoCatS shares 60-90% overall sequence identities with known teleost cathepsin S. Phylogenetic profiling indicated that SoCatS is evolutionally close to the cathepsin S of other teleost fish, especially Miichthys miiuy, a member of Sciaenidae family like red drum. SoCatS expression was detected in various tissues and was enhanced by bacterial infection. Purified recombinant SoCatS exhibited apparent peptidase activity with maximum at 50°C and pH 7.5. This activity depended on the catalytic residue Cys146 and was severely reduced by the cathepsin inhibitor E-64. Our results suggest that SoCatS functions as a cysteine protease which is probably involved in the antibacterial immunity of red drum.