Characterization of a novel WAP domain-containing protein with antibacterial immunity from sea cucumber Apostichopus japonicus

In various species, Whey acidic protein domain-containing proteins (WD proteins) are important molecules in the innate immune systems. In this study, a novel WD protein (AjSWD) was cloned from Apostichopus japonicus, its antibacterial function and mechanism were further studied. The AjSWD protein contained one WAP domain (10–55 aa) and one KAZAL domain (99–147 aa). AjSWD mRNA was detected in all chosen tissues, with highest expression level found in the body wall and lower expression level in other detected tissues. Then following the challenge of Vibrio splendidus, we found AjSWD mRNA and protein expression in coelomocytes and body wall tissues increased to significantly high levels. On antibacterial function research, recombinant AjSWD and its WAP domain (AjSWD-WAP) both exhibited obvious bacterial agglutination, PAMPs binding activities and antibacterial activities. Then the antibacterial mechanisms of rAjSWD and rAjSWD-WAP were identified as disruption of bacterial cell membranes. Furthermore, after V. splendidus challenge, survival rate of juvenile A. japonicus was effectively improved by both rAjSWD and rAjSWD-WAP. Additionally, rAjSWD-WAP exhibited higher antibacterial efficacy compared to rAjSWD. For conclusion, this study suggested that rAjSWD and rAjSWD-WAP possessed significant antibacterial properties, and the AjSWD gene plays an important role in the innate immunity of sea cucumber A. japonicus.