Identification and characterization of a serine protease inhibitor (PtSerpin) in the swimming crab Portunus trituberculatus

Serine protease inhibitors (Serpins) play a key role in diverse immune biological processes. A serine protease inhibitor (Serpin), namely PtSerpin, was identified from the haemocyte cDNA library of swimming crab Portunus trituberculatus. The full-length PtSerpin cDNA was 1593 bp, including an open reading frame (ORF) of 1227 bp encoding a polypeptide of 408 amino acids with estimated molecular mass of 45.048 kDa and theoretical isoelectric point of 7.23. Predicted tertiary structure of PtSerpin contained three β-sheets and nine α-helices. Multiple sequence alignment revealed that deduced amino acid sequence of PtSerpin shared the highest similarity with serpin SPI from green mud crab Scylla paramamosain (SpSerpin). Phylogenetic analysis supported PtSerpin and SpSerpin were closely related to serpins from Penaeus monodon and Daphnia pulex while other decapods formed a separate group. Although the mRNA transcripts of PtSerpin could be detected in all the examined tissues, the higher levels were present in haemocytes and gills which are the major organs respond to pathogenic microorganism. After challenged by Vibrio alginolyticus, Micrococcus luteus and Pichia pastoris, the temporal expression of PtSerpin gene in haemocytes showed different activation times against bacteria and fungi within the experimental period of 72 h. These findings suggest that PtSerpin is involved in the antibacterial defense mechanism of P. trituberculatus crab.