Cathepsin B of Cynoglossus semilaevis: identification, expression, and activity analysis

Cathepsin B (EC 3.4.22.1) is a member of the papain family cysteine protease and in mammals is known to be involved in protein degradation and other biological functions. However, very little is known about the function of cathepsin B in fish. In this study, we identified and analyzed a cathepsin B homologue (CsCatB) from tongue sole (Cynoglossus semilaevis, Pleuronectiformes), an economic fish species cultured in China. CsCatB is composed of 322 amino acid residues and shares 70-81.3% overall sequence identities with its counterpart in teleosts and humans. CsCatB possesses typical cathepsin B structural features including the propeptide region and the papain family cysteine protease domain, the latter containing the four catalytic residues (Q101, C107, H277, and N297) that are conserved in lower and higher vertebrates. Quantitative real time RT-PCR analysis showed that CsCatB expression occurred in multiple tissues and was positively regulated by bacterial infection and by immunization with a subunit vaccine. Recombinant CsCatB purified from Escherichia coli exhibited apparent protease activity, which was optimal at 35 °C and pH 5.5. In contrast, a mutant CsCatB bearing glutamic acid substitution at H277 was dramatically reduced in proteolytic activity. These results indicate that CsCatB is a biologically active protease that is likely to be involved in host immune response during bacterial infection and vaccination.