Characterization and functional analysis of tandem threonine containing C-type lectin (Thr-Lec) from the ridgetail white prawn Exopalaemon carinicauda

As an important pattern-recognition receptor (PRR), C-type lectins (CTLs) play significant roles in recognizing microbes and battle against pathogenic microorganism in innate immunity. In this study, two tandem threonine containing CTLs (designated as EcThr-LecA and EcThr-LecB) were identified from Exopalaemon carinicauda. The full-length cDNA of EcThr-LecA and EcThr-LecB consisted of 1521 and 1518 bp with 1251 and 1242 bp open reading frame encoding a protein with 412 and 413 amino acids, respectively. The genome structure of EcThr-LecA included 10 exons and 9 introns, and the sequences of intron6 and intron7 were variable. The nucleotide sequence of intron2 in EcThr-LecB was specific and different with that of EcThr-LecA. EcThr-LecA and EcThr-LecB proteins were predicted to have a signal peptide, two conserved carbohydrate recognition domain (CRD), and tandem threonine region. The expression levels of EcThr-LecA and EcThr-LecB in the intestine were significantly up-regulated after Vibrio parahaemolyticus and white spot syndrome virus (WSSV) challenge. RNA interference (RNAi) was used to explore the effects of EcThr-LecB silencing on the mRNA expression of anti-lipopolysaccharide factor (ALF), crustin (CRU), and lysozyme (LYSO). Knock down of EcThr-LecB could evidently down-regulate the expression of eight different antibacterial peptides (AMPs), including EcALF2, EcCRU1, EcCRU3, EcCRU4, EcLYSO1, EcLYSO2, EcLYSO3, and EcLYSO4, whereas make no effect on the transcription of EcALF1, EcALF3, EcCRU2, and EcLYSO5. The recombinant two CRD domains and tandem threonine region (RLecB) of EcThr-LecB could bind diverse bacteria, lipopolysaccharide, and peptidoglycans in vitro. In addition, RLecB could accelerate the clearance of V. parahaemolyticus in vivo. The present data indicated that new-found tandem threonine containing CTLs in E. carinicauda may act as PRR to participate in the innate immune defense against pathogens by the recognition of non-self, regulation of AMPs, and clearance of invaders.