Cathepsin K belongs to the family of cysteine cathepsins. It is well known that the cysteine cathepsins participate in various physiological processes and host immune defense in mammals. However, in teleost fish, the function of cathepsin K is very limited. In the present study, a cathepsin K homologue (SsCTSK) from the teleost black rockfish (Sebastes schlegelii) was identified and examined at expression and functional levels. In silico analysis showed that three domains, including signal peptide, cathepsin propeptide inhibitor I29 domain, and functional domain Pept_C1, are existed in SsCTSK. SsCTSK also possesses a peptidase domain with three catalytically essential residues (Cys25, His162 and Asn183). Phylogenetic profiling indicated that SsCTSK was evolutionally close to the cathepsin K of other teleost fish. Expression of SsCTSK occurred in multiple tissues and was induced by bacterial infection. Purified recombinant SsCTSK (rSsCTSK) exhibited apparent maximal peptidase activity at 45 °C, and its enzymatic activity was remarkably declined in the presence of the cathepsin inhibitor E-64. Moreover, rSsCTSK possesses the ability to bind with PAMPs and bacteria. Finally, knockdown of SsCTSK expression facilitated bacterial invasion in black rockfish. Collectively, these results indicated that SsCTSK functions as a cysteine protease and may serves as a target for pathogen manipulation of host defense system.

中文翻译：

---

组织蛋白酶K同源物的鉴定和表征，该同源物与黑色石鱼Sebastes schlegelii中的病原菌相互作用。

组织蛋白酶K属于半胱氨酸组织蛋白酶家族。众所周知，半胱氨酸组织蛋白酶参与各种生理过程并在哺乳动物中具有免疫防御作用。但是，在硬骨鱼中，组织蛋白酶K的功能非常有限。在本研究中，从硬骨黑岩鱼（Sebastes schlegelii）中鉴定了组织蛋白酶K同源物（SsCTSK），并在表达和功能水平上进行了检查。在计算机分析中显示，SsCTSK中存在三个域，包括信号肽，组织蛋白酶前肽抑制剂I29域和功能域Pept_C1。SsCTSK还具有带有三个催化必需残基（Cys25，His162和Asn183）的肽酶结构域。系统发育分析表明，SsCTSK在进化上接近于其他硬骨鱼的组织蛋白酶K。SsCTSK的表达发生在多个组织中，并由细菌感染诱导。纯化的重组SsCTSK（rSsCTSK）在45°C时表现出最大的肽酶活性，并且在组织蛋白酶抑制剂E-64的存在下其酶活性显着下降。此外，rSsCTSK具有与PAMP和细菌结合的能力。最后，SsCTSK表达的敲低促进了细菌在黑石鱼中的入侵。总的来说，这些结果表明SsCTSK起半胱氨酸蛋白酶的作用，并且可以作为宿主防御系统的病原体操纵的靶标。此外，rSsCTSK具有与PAMP和细菌结合的能力。最后，SsCTSK表达的敲低促进了细菌在黑石鱼中的入侵。总的来说，这些结果表明SsCTSK起半胱氨酸蛋白酶的作用，并且可以作为宿主防御系统的病原体操纵的靶标。此外，rSsCTSK具有与PAMP和细菌结合的能力。最后，SsCTSK表达的敲低促进了细菌在黑石鱼中的入侵。总的来说，这些结果表明SsCTSK起半胱氨酸蛋白酶的作用，并且可以作为宿主防御系统的病原体操纵的靶标。