The distribution and function characterization of the i type lysozyme from Apostichopus japonicus

Lysozyme is a very important component of the innate immune system and a key molecule that protects against bacterial infection. Sea cucumber i-type lysozyme (Aj-iLys) has been shown to possess multiple functions. In this study, we investigated the function and characterization of Aj-iLys in detail. Spatial distribution analysis showed that Aj-iLys was constitutively expressed in all tested tissues, with dominant expression in the tentacles and respiratory trees. Challenge with the pathogen V. splendidus and LPS stimulation both significantly up-regulated the mRNA expression of Aj-iLys. More importantly, inhibition of Aj-iLys expression by mRNA interference resulted in significant promotion of coelomocyte apoptosis during LPS challenge in vitro. The results indicated that Aj-iLys serves as an important innate immunity factor and plays a key defense role during host-pathogen interactions in sea cucumbers. From the radius of the antimicrobial zone, it was determined that the non-fusion Aj-iLys exerted a remarkable inhibitive effect on tested bacteria in vitro. Functional investigation revealed that Aj-iLys also exhibited isopeptidase activity based on its ability to hydrolyze l-Glutamic acid γ-(4-nitroanilide) in vitro to produce p-NA, which is an analogue of the isopeptide bond. The optimal catalytic conditions for the isopeptidase activity were 37 °C, pH 6.5, and the optimum ionic strength was about 0.050 mol/L.