Characterization and comparison of two C-type lectins with novel key motifs from mud crab Scylla paramamosain

As an important pattern recognition receptor (PRR) in the innate immune system, C-type lectin plays an important role in the innate immune process of invertebrates. Two C-type lectins Sp CTL-C and Sp CTL-D were characterized from mud crab ( Scylla paramamosain ). The predicted Sp CTL-C and Sp CTL-D proteins both contain a single carbohydrate-recognition domain (CRD) with key motif Gln-Pro-Ala (QPA) and Met-Pro-Ala (MPA), respectively. Sp CTL-C and Sp CTL-D transcripts distributed in all examined tissues, and the expression level was the highest in hepatopancreas. As PRR, the purified recombinant proteins r Sp CTL-C and r Sp CTL-D have high affinity for three kinds of pathogen-associated molecular patterns (PAMPs): β-glucan, lipopolysaccharide, and peptidoglycan. Besides, r Sp CTL-D can bind to all nine microorganisms tested, while r Sp CTL-C can bind to seven microorganisms except for Staphylococcus aureus and Micrococcus luteus. Both r Sp CTL-C and r Sp CTL-D showed agglutination activity towards fungi Pichia pastoris and Saccharomyces cerevisiae . However, r Sp CTL-C and r Sp CTL-D exhibited different antimicrobial activities: r Sp CTL-D has a certain inhibitory effect on the growth of Vibrio fluvialis and M. luteus , while r Sp CTL-C has no obvious inhibitory activity. The results show that r Sp CTL-C and r Sp CTL-D had better phagocytosis-promoting effect on M. luteus than the negative control. Meanwhile, both r Sp CTL-C and r Sp CTL-D had certain encapsulation-promoting activity. Collectively, two C-type lectins with novel key motifs make an important impact as PRR in immune response towards pathogens. At the same time, they play different functions in the innate immunity of mud crab S. paramamosain .