Molecular characterization and immune analysis of a defensin from freshwater pearl mussel, Hyriopsis schlegelii

Abstract

Defensin is one of the most important antimicrobial peptides(AMPs) in bivalves and is involved in the immune responses. Herein, we reported the isolation and initial characterization of a defensin from the freshwater pearl mussel, Hyriopsis schlegelii (designated as Hs-defn) and its immune response for the first time. The full length Hs-defn cDNA was 410bp long and consisted of a 51bp 5'UTR, a 161bp 3'UTR with a mRNA instability motif (AATAAA) and an open reading frame(ORF) of 198bp encoding a polypeptide of 65 amino acids. The deduced amino acid sequence of Hs-defn shared certain common features of AMPs, such as α-helical structure, net positive charge, lower molecular mass and high hydrophobic residue ratio. Homologue searching against GenBank database suggested that Hs-Defn was also phylogenetically close to the defensin isoforms identified from other mollusks. Furthermore, predicted with homology-modeling method, the three-dimensional structure of Hs-Defn demonstrated a significant similarity with other known defensins such as oyster C. gigas and insect P. terraenovae defensins. Phylogenetic analysis results further revealed that Hs-defn is a member of invertebrate defensin family.Results of tissue specific analysis showed that the expression level of Hs-defn mRNA was the highest in haemocytes, moderate in gill, hepatopancrease, mantle and intestine, the lowest in foot, adductor muscle, gonad and kidney. After pathogenic Aeromonas hydrophila stimulation, the Hs-defn mRNA expression was significantly up-regulated at 2h and 12h in haemocytes, gill, mantle and hepatopancrease. These results implicated that Hs-Defn was a constitutive and inducible acute-phase protein which was perhaps involved in the immune defense of pearl mussel.