Characterization of a recombinant C-type lectin, rCEL-IV, expressed in Escherichia coli cells using a synthetic gene

A C-type lectin (MrLec) with high expression in intestine is involved in innate immune response of Macrobrachium rosenbergii

C-type lectins (CTLs) are pattern-recognition proteins that play an important role in innate immunity of vertebrates and invertebrates. In this study, a lectin cDNA named MrLec was cloned and characterized from giant freshwater prawns (Macrobrachiun rosenbergii). The full-length cDNA of MrLec was 1431 bp, which contained an open reading frame of 1041 bp that encoded a protein with 346 amino acids. MrLec was found to contain a typical signal peptide of 18 amino acids and a single carbohydrate-recognition domain with 121 amino acids. The phylogenetic analysis showed that MrLec was grouped with vertebrates and had 57% identity with C-type lectin 3 from Marsupenaeus japonicas. Tissue expression analysis showed that MrLec was ubiquitously distributed at a high level in the intestine, with lower expression levels in the hemocytes, heart, hepatopancreas, gill and stomach. Vibrio parahaemolyticus infection induced the upregulation of MrLec in the gills and intestine. For the white spot syndrome virus (WSSV) challenge, MrLec in gills was upregulated at 24, 36 and 48 h. In intestine, MrLec also went up at 36 and 48 h WSSV challenge. Recombinant MrLec can agglutinate (Ca²⁺-dependent) and bind both Gram-negative and Gram-positive bacteria. rMrLec could attach to lipopolysaccharide and peptidoglycan in a dose-dependent manner. These results indicated possible MrLec involvement in the immune response of giant freshwater prawns.

Two C-type lectins from shrimp Litopenaeus vannamei that might be involved in immune response against bacteria and virus

C-type lectins play crucial roles in innate immunity to recognize and eliminate pathogens efficiently. In the present study, two C-type lectins from shrimp Litopenaeus vannamei (designated as LvLectin-1 and LvLectin-2) were identified, and their expression patterns, both in tissues and toward pathogen stimulation, were then characterized. The full-length cDNA of LvLectin-1 and LvLectin-2 was 567 and 625 bp, containing an open reading frame (ORF) of 471 and 489 bp, respectively, and deduced amino acid sequences showed high similarity to other members of C-type lectin superfamily. Both two C-type lectins encoded a single carbohydrate-recognition domain (CRD). The motif of Ca(2+) binding site 2 in CRD, which determined carbohydrate-binding specificity, was QPN (Gln(122)-Pro(123)-Asn(124)) in LvLectin-1, but QPD (Gln(128)-Pro(129)-Asp(130)) in LvLectin-2. Two C-type lectins exhibited similar tissue expression pattern, for their mRNA were both constitutively expressed in all tested tissues, including hepatopancreas, muscle, gill, hemocytes, gonad and heart, furthermore they were both mostly expressed in hepatopancreas, though the expression level of LvLectin-2 was much higher than LvLectin-1. The expression level of two C-type lectins mRNA in hemocytes varied greatly after the challenge of Listonella anguillarum or WSSV. After L. anguillarum challenge, the expression of both C-type lectins were significantly (P<0.01) up-regulated compared with blank group, and LvLectin-1 exhibited higher level than LvLectin-2; while after the stimulation of WSSV, the expression of LvLectin-2 was significantly up-regulated at 6 h (P<0.01) and 12 h (P<0.05), but the expression level of LvLectin-1 down-regulated significantly (P<0.01) to 0.4-fold at 6 and 12 h post-stimulation. The results indicated that the two C-type lectins might be involved in immune response toward pathogen infection, and they might perform different recognition specificity toward bacteria or virus.

Galactose recognition by a tetrameric C-type lectin, CEL-IV, containing the EPN carbohydrate recognition motif

CEL-IV is a C-type lectin isolated from a sea cucumber, Cucumaria echinata. This lectin is composed of four identical C-type carbohydrate-recognition domains (CRDs). X-ray crystallographic analysis of CEL-IV revealed that its tetrameric structure was stabilized by multiple interchain disulfide bonds among the subunits. Although CEL-IV has the EPN motif in its carbohydrate-binding sites, which is known to be characteristic of mannose binding C-type CRDs, it showed preferential binding of galactose and N-acetylgalactosamine. Structural analyses of CEL-IV-melibiose and CEL-IV-raffinose complexes revealed that their galactose residues were recognized in an inverted orientation compared with mannose binding C-type CRDs containing the EPN motif, by the aid of a stacking interaction with the side chain of Trp-79. Changes in the environment of Trp-79 induced by binding to galactose were detected by changes in the intrinsic fluorescence and UV absorption spectra of WT CEL-IV and its site-directed mutants. The binding specificity of CEL-IV toward complex oligosaccharides was analyzed by frontal affinity chromatography using various pyridylamino sugars, and the results indicate preferential binding to oligosaccharides containing Galβ1-3/4(Fucα1-3/4)GlcNAc structures. These findings suggest that the specificity for oligosaccharides may be largely affected by interactions with amino acid residues in the binding site other than those determining the monosaccharide specificity.