FmLC6: An ultimate dual-CRD C-type lectin from Fenneropenaeus merguiensis mediated its roles in shrimp defense immunity towards bacteria and virus

C-type lectins are a member of pattern recognition receptors (PRRs) that can interact with pathogen-associated molecular patterns of invading microorganisms by using their conserved motifs in carbohydrate recognition domain (CRD). The binding can trigger various immune responses in both direct and indirect mechanisms. Hereby, an ultimate C-type lectin with dual CRDs each of which containing a different motif was identified from hepatopancreas of Fenneropenaeus merguiensis (mentioned as FmLC6). The full-length cDNA of FmLC6 consisted of 1148 bp comprising one 1005 bp open reading frame (ORF) encoding a signal peptide and a mature protein of 317 residues. FmLC6 was composed of two CRDs with a highly conserved QPD (Gln-Pro-Asp) motif and one variant EPQ (Glu-Pro-Gln) motif for illustrating the carbohydrate binding affinity. The transcription of FmLC6 was detected only in hepatopancreas of normal shrimp. After injection with pathogens or immunostimulants, the expression of FmLC6 was significantly up-regulated and reached the highest level at 12 h post-injection except with lipoteichoic acid challenge. The FmLC6 expression was severely suppressed by knockdown based-silencing. This gene silencing with co-injection by Vibrio parahaemolyticus caused increasing in cumulative mortality and reduction of the median lethal time. Purified recombinant proteins of an entire ORF and two individual CRDs of FmLC6 produced in Escherichia coli could induce a broad spectrum of microbial agglutination with calcium dependence. The agglutination induced by rFmLC6, rCRD1 and rCRD2 was suppressed by galactose plus mannose, galactose and mannose, respectively which this event was confirmed by the inhibition of hemagglutination. All three recombinant proteins possessed ability to inhibit the bacterial growth with a dose-response. Purified rFmLC6 could bind directly to white spot syndrome virus particles and also its recombinant proteins including VP15, VP39A and VP28 with different affinity. Altogether, these results indicate that FmLC6 acts as a PRR to recognize invading microorganisms and leads to mediating the immune response to cooperation in pathogenic elimination via the binding, agglutination and antimicrobial activity.