Identification and functional characterization of a novel antistasin/WAP-like serine protease inhibitor from the tropical sea cucumber, Stichopus monotuberculatus

A novel antistasin/WAP-like serine protease inhibitor, named as StmAW-SPI, was identified from sea cucumber (Stichopus monotuberculatus) and functionally characterized in this study. The full-length cDNA of StmAW-SPI is 1917 bp in length with a 72 bp 5'-untranslated region (UTR), a 294 bp 3'-UTR and a 1551 bp open reading frame (ORF) encoding a protein of 516 amino acids with a deduced molecular weight of 54.56 kDa. The StmAW-SPI protein has 5-fold internal repeats (IRs) of antistasin domain and 6-fold IRs of WAP domain. For the gene structure, StmAW-SPI contains 10 exons separated by 9 introns. The StmAW-SPI mRNA expression pattern was determined using quantitative real-time PCR. The highest level of StmAW-SPI was found in the intestine, followed by coelomocytes, gonad, body wall and respiratory tree. The StmAW-SPI expressions were significantly up-regulated after polyriboinosinic polyribocytidylic acid [Poly (I:C)] or lipopolysaccharides (LPS) challenge in in vitro experiments performed in primary coelomocytes. In addition, the serine protease inhibitory activity and bacterial protease inhibitory activity of StmAW-SPI were examined, and the antibacterial activity was also demonstrated in this study. Our study, as a whole, suggested that StmAW-SPI might play a critical role in the innate immune defense of sea cucumber against microbial infections, by not only inactivating the serine protease but also inhibiting the growth of pathogens.