Molecular characterization and expression patterns of the big defensin gene in freshwater mussel (Hyriopsis cumingii)

Antimicrobial peptides (AMPs), of which big defensins are examples, are an important component of the natural defenses of most living organisms, and possess remarkable microbicidal activities. In the present study, using expressed-sequence tag sequences from a cDNA library and RACE, the full-length cDNA sequence of the big defensin gene from the triangle-shell pearl mussel, Hyriopsis cumingii, (HcBD), was cloned. The gene consists of a 5'-untranslated region (UTR) of 166 bp, a 3'-UTR region of 96 bp, and an open reading frame of 342 bp that encodes 113 amino acids, consisting of a 23 amino acid signal peptide and a mature peptide of 90 amino acids with a molecular mass of 12.5 kDa. Amino acid sequence analysis showed that the sequence contained a transmembrane domain and a hydrophobic region. The full-length amino acid sequence showed the highest similarity to an amphioxus (Branchiostoma floridae) sequence (64%), and lower similarities to other known defensins (α-, β-, and θ-defensins, and insect defensins). Expression of HcBD was relatively high in the mantle and blood, lower in other tested tissues, and absent in gill and foot tissues. Real-time quantitative PCR was used to investigate HcBD expression in various tissues at different time points after injection of Aeromonas hydrophila. At 4 h post-inoculation, HcBD expression in the mantle, liver, intestine, gill, and foot was greater than in the control, with the greatest expression at 72 h, while at 24 h, expression in the liver, intestine, gill, and foot were at their lowest levels. These results suggest that HcBD might play an important role in the host immune response. This study enriches the basic research on the big defensin family of antimicrobial peptides and lays foundations for further research on antimicrobial peptide expression and relevance to disease defense.