Molecular cloning and characterization of perlucin from the freshwater pearl mussel, Hyriopsis cumingii

Perlucin is an important functional protein that regulates shell and pearl formation. In this study, we cloned the perlucin gene from the freshwater pearl mussel Hyriopsis cumingii, designated as Hcperlucin. The full-length cDNA transcribed from the Hcperlucin gene was 1460 bp long, encoding a putative signal peptide of 20 amino acids and a mature protein of 141 amino acids. The mature Hcperlucin peptide contained six conserved cysteine residues and a carbohydrate recognition domain, similar to other members of the C-type lectin families. In addition, a "QPS" and an invariant "WND" motif near the C-terminal region were also found, which are extremely important for polysaccharide recognition and calcium binding of lectins. The mRNA of Hcperlucin was constitutively expressed in all tested H. cumingii tissues, with the highest expression levels observed in the mantle, adductor, gill and hemocytes. In situ hybridization was used to detect the presence of Hcperlucin mRNA in the mantle, and the result showed that the mRNA was specifically expressed in the epithelial cells of the dorsal mantle pallial, an area known to express genes involved in the biosynthesis of the nacreous layer of the shell. The significant Hcperlucin mRNA expression was detected on day 14 post shell damage and implantation, suggesting that the Hcperlucin might be an important gene in shell nacreous layer and pearl formation. The change of perlucin expression in pearl sac also confirmed that the mantle transplantation results in a new expression pattern of perlucin genes in pearl sac cells that are required for pearl biomineralization. These findings could help better understanding the function of perlucin in the shell and pearl formation.