Characterization of a thioester-containing protein from the pearl oyster Pinctada fucata and its role in phagocytosis

Thioester-containing proteins (TEPs) superfamily is known to play important innate immune functions in vertebrates and invertebrates, and involved in recognition, and killing of invading pathogens. Here, a TEP gene (designated as Pf-TEP) was obtained from the pearl oyster Pinctada fucata by RT-PCR and rapid amplification of cDNA ends (RACE). The Pf-TEP cDNA consists of 4335 bp with an open reading frame (ORF) of 3639 bp encoding a protein of 1212 amino acids. The deduced TEP protein possessed the characteristic structure features present in its homologs and contained the A2M_N_2, A2M, A2M_comp, A2M_recep, and thioester domain, as well as the conserved Cys, His, and Glu residues. Phylogenetic analysis revealed that Pf-TEP is closely related to the TEPs from other invertebrates. Pf-TEP mRNA was expressed in all examined tissues including gill, digestive gland, adductor muscle, mantle and hemocyte, while the highest expression was found in gill. Following challenge with Vibrio alginolyticus, Pf-TEP expression was significantly induced in hemocytes. Further knockdown of Pf-TEP by specific siRNA could significantly reduce the phagocytosis of V. alginolyticus by hemocytes in vitro. These results would help increase understanding of TEP existed as an ancient complement-like molecular and therefore provide new insights into the roles of TEP in the invertebrate immune system.