Molecular Characterization of Complement Component 3 (C3) in the Pearl Oyster Improves Our Understanding of the Primitive Complement System in Bivalve.

As the central component in the complement system, complement component 3 (C3) plays essential roles in both the innate and adaptive immune responses. Here, a C3 gene (designated as ) was obtained from the pearl oyster by RT-PCR and rapid amplification of cDNA ends (RACE). The cDNA consists of 5,634 bp with an open reading frame (ORF) of 5,193 bp encoding a protein of 1,730 amino acids with a 19 residue signal peptide. The deduced pf-C3 protein possessed the characteristic structural features present in its homologs and contained the A2M_N_2, ANATO, A2M, A2M_comp, A2M_recep, and C345C domains, as well as the C3 convertase cleavage site, thioester motif, and conserved Cys, His, and Glu residues. Phylogenetic analysis revealed that pf-C3 is closely related to the C3s from other mollusks. mRNA was expressed in all examined tissues including gill, digestive gland, adductor muscle, mantle and foot, while the highest expression was found in the digestive gland. Following the challenge with , expression was significantly induced in hemocytes. Luciferase reporter assays indicated that pf-C3a could activate the NF-B signal pathway in HEK293T cells. Further knockdown of by specific siRNA could significantly reduce the phagocytosis of by hemocytes . These results would help increase understanding of the function of C3 in the invertebrate immune system and therefore provide new insights into the roles of the primitive complement system in invertebrates.