Molecular characterization and expression analysis of four cathepsin L genes in the razor clam, Sinonovacula constricta

Cathepsin L (CTSL) is a lysosomal cysteine protease involved in immune responses in vertebrates. However, few studies exist regarding the role of cathepsin L in bivalves. In this study, we isolated and characterized four cathepsin L genes from the razor clam Sinonovacula constricta, referred to as CTSL1, CTSL2, CTSL3 and CTSL4. These four genes contained typical papain-like cysteine protease structure and enzyme activity sites with ERWNIN-like and GNFD-like motifs in the proregion domain and an oxyanion hole (Gln) and a catalytic triad (Cys, His and Asn) in the mature domain. Expression analysis of the four transcripts revealed a tissue-specific pattern with high expression of CTSL1 and CTSL3 in liver and gonad tissues and high expression of CTSL2 and CTSL4 in liver and gill tissues. During the developmental stages, the four transcripts showed the highest expression in the juvenile stage; however, CTSL3 had a much higher expression level than the other three transcripts during embryogenesis. The four transcripts showed significant changes in expression as early as 4 h or 8 h after infection with Vibrio anguillarum. The fact that bacterial infection can induce expression of the four CTSL transcripts suggests that these transcripts are important components of the innate immunity system of the clam.