Identification and molecular analysis of a novel C-type lectin from Scophthalmus maximus

C-type lectins are calcium-dependent carbohydrate-binding proteins that play important roles in innate immunity. In this study, a C-type lectin homologue (SmLec1) was identified from turbot (Scophthalmus maximus) and analyzed at expression and functional levels. The open reading frame of SmLec1 is 504 bp, with a 5'-untranslated region (UTR) of 101 bp and a 3'-UTR of 164 bp. The deduced amino acid sequence of SmLec1 shares 34%-38% overall identities with the C-type lectins of several fish species. In silico analysis identified in SmLec1 conserved C-type lectin features, including a carbohydrate-recognition domain, four disulfide bond-forming cysteine residues, and the mannose-type carbohydrate-binding motif. In addition, SmLec1 possesses a putative signal peptide sequence and is predicted to be localized in the extracellular. Expression of SmLec1 was highest in liver and responded positively to experimental challenges with fish pathogens. Recombinant SmLec1 (rSmLec1) purified from yeast was able to agglutinate the Gram-negative fish pathogen Listonella anguillarum but not the Gram-positive pathogen Streptococcus iniae. The agglutinating ability of rSmLec1 was abolished in the presence of mannose and ethylenediaminetetraacetic acid and by elevated temperature (65 degrees C). Further analysis showed that rSmLec1 could stimulate kidney lymphocyte proliferation and enhance the killing of bacterial pathogen by macrophages. Taken together, these results suggest that SmLec1 is a unique mannose-binding C-type lectin that possesses apparent immunomodulating property and is likely to be involved in host defense against bacterial infection.