Identification, expression analysis, and antibacterial activity of NK-lysin from common carp Cyprinus carpio

Abstract Natural killer lysin(NK- lysin), produced by cytotoxic T lymphocytes and natural killer cells, is a cationic antimicrobial peptide that has a broad antimicrobial spectrum, including bacteria, viruses, and parasites. Nevertheless, the implication of NK- lysinin the protection against bacterial infection is not aware in common carp. In this study, six different NK- lysingenes ( nkl1 , nkl2 , nkl3 , nkl4 , nkl5 and nkl6 ) were identified in the common carpgenome. Each of the mature peptides of common carpNK- lysinhas six well-conserved cysteine residues, and shares a Saposin B domain, characteristic of saposin-like protein (SALIP) family. The gene nkl1 contains 5 extrons and 4 introns, and nkl2 , nkl3 , nkl4 or nkl5 contains 4 extrons and 3 introns, however, the nkl6 has 3 extrons and 2 introns. By quantitative real-time PCR, nkl2 transcripts were predominantly expressed in spleen of healthy common carp, while elevated mainly in gill and spleen upon Aeromonas hydrophilainfection. The recombinant NK- lysin-2 purified from Pichia pastorisshows antibacterial activity against Staphylococcus aureus (Gram-positive), and Escherichia coli M15, Aeromonas hydrophila, as well as Edwardsiella tarda(Gram-negative), the latter two are important pathogens of aquaculture. Our results indicate that NK- lysinin common carpmight play an important role in fish immune response by enhancing antibacterial defense against bacterial pathogens.