C-type lectins are a superfamily of proteins that can bind pathogen-associated molecular patterns (PAMPs) and microorganisms through the recognition of carbohydrates, thus they are directly involved in innate defense mechanisms as part of the acute-phase response to infection. In this study, the cDNA of a novel C-type lectin (designated as AiCTL-7) was cloned from bay scallop Argopecten irradians by expression sequence tag (EST) analysis and rapid amplification of cDNA ends (RACE) approach. The full-length cDNA of AiCTL-7 was of 651 bp containing a 525 bp open reading frame which encoded a signal peptide of 15 residues and a conserved carbohydrate-recognition domain (CRD) of 174 residues with the EPD and WSD motifs instead of the invariant EPN and WND motifs for determining the carbohydrate-binding specificity and constructing Ca(2+)-binding site 2 in vertebrates. The deduced amino acid sequence of AiCTL-7 CRD shared homology not only with the CRDs of C-type lectins in mollusks, but also with the fish lectin CRDs. The mRNA transcripts of AiCTL-7 were mainly detected in the tissue of hepatopancreas and also marginally detectable in kidney, gonad, hemocytes, heart and adductor of health scallop. After challenge with fungi Pichia pastoris GS115 and Gram-negative bacteria Listonella anguillarum, the relative expression level of AiCTL-7 was up-regulated significantly in hepatopancreas and hemocytes. The CRD of AiCTL-7 was recombined and expressed in Escherichia coli, and the recombinant protein (rAiCTL-7) aggregated P. pastoris remarkably in a Ca(2+)-dependent manner, and this agglutination could be inhibited by d-mannose, but not by d-galactose or β-1,3-glucan. However, rAiCTL-7 displayed no obvious agglutinating activity against L. anguillarum. These results collectively indicated that AiCTL-7 was involved in the primitive acute-phase response to microbial invasion as an important pattern recognition receptor (PRR) in the innate immune system of scallops.
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