The C 1q-domain-containing (C1qDC) proteins are a family of proteins characterized by a globular C1q (gC1q) domain in the C-terminus.They are involved in various processes of vertebrates and supposed to be an important pattern recognition receptor in innate immunity of invertebrates.In this study, a new aC1q-domain-containing protein (AiC 1qDC-2) gene was cloned from Argopecten irradians by rapid amplification of cDNA ends (RACE) approaches and expressed sequence tag (EST) analysis.The full-length cDNA of AiC1qDC-2 was composed of 1062 bp, encoding a signal peptide of 18 residues, a coiled coil of 56 residues and a typical gC1q domain of 138 residues.The AiC1qDC-2 transcripts were mainly detected in the tissue of mantle, gill, heart and hemocytes by fluorescent quantitative real-time PCR (Fig.1).In the microbial challenge experiment, there was a significant up-regulation in the relative expression level of AiC1qDC-2 in hemocytes of the scallops challenged by fungi Pichia pastoris GS115, Gram-positive bacteria Micrococcus luteus and Gram-negative bacteria Listonella anguillarum (Fig.2).PAMPs binding assay with the polyclonal antibody specific for AiC1qDC-2 proved that the rAiC1qDC-2 could bind not only LPS, PGN and PolyIC (Fig.3A), but also mannan, yeast-glucan and β-1,3-glucan in vitro (Fig.3B).Further the recombinant AiC1qDC-2 (rAiC1qDC-2) protein exhibited obvious agglutination against Gram-negative bacteria Escherichia coli and Listonellan anguillarum, Gram-positive bacteria Bacillus subtilis and fungi Pichia pastoris GS115 (Fig.4).These results indicated that AiC 1 qDC-1 functioned as a pattern recognition receptor in the immune defense of scallops against pathogens and provided clues for illuminating the evolution of the complement classical pathway.