Molecular Cloning And Functional Characterization Of A Novel Complement-like Component AiC1qDC-1 From Bay Scallop Argopecten Irradians

As the first component of complement classical pathway, C1q plays an essential role in recognizing pathogens and activating the whole complement system. Being the possible evolutionary trace of C1q in invertebrates, it is speculated that the C1q-domain-containing (C1qDC) proteins should also have important function in immune defence of invertebrates against pathogens. In this study, a novel C1qDC protein from bay scallop Argopecten irradians (AiC1qDC-1) was identified and characterized by using bioinformatics methods and molecular biotechniques including EST, RACE, RT-PCR and recombinant DNA.The full-length cDNA of AiC1qDC-1 was composed of 733 bp, encoding 178 amino acids including a signal peptide of 19 residues and a typical C1q domain of 137 residues containing all eight invariant amino acids in human C1qDC proteins and seven aromatic residues essential for effective packing of the hydrophobic core of AiC1qDC-1. The C1q domain of AiC1qDC-1, which possessed the typical ten-stranded ?-sandwich fold with a jelly-roll topology common to all C1q family members, showed high homology not only to those of C1qDC proteins in mollusk but also to those of C1qDC proteins in human. The AiC1qDC-1 transcripts were mainly detected in the tissue of hepatopancreas and also marginally detectable in adductor, heart, mantle, gill and hemocytes. There was a significant up-regulation in the relative expression level of AiC1qDC-1 in hepatopancreas and hemocytes of the scallops challenged by fungi Pichia pastoris GS115, Gram-negative bacteria Listonella anguillarum and Gram-positive bacteria Micrococcus luteus. The recombinant C1q domain of AiC1qDC-1 protein displayed no obvious agglutination against M. luteus and L. anguillarum, but it aggregated P. pastoris remarkably. This agglutination could be inhibited by D-mannose and PGN but not by LPS, glucan or D-galactose.These results indicated that AiC1qDC-1 functioned as a pattern recognition receptor in the immune defense of scallops against pathogens. The C1q domain of AiC1qDC-1 shared structural and functional homology with those of complement C1q in vertebrates, indicating that AiC1qDC-1 might be an ancestor form of C1q in invertebrates. The present study contributed to understanding the innate immune mechanism of mollusk and provided new clues for illuminating the evolution of complement system.