Identification and characterization of a novel defensin from Asian green mussel Perna viridis

Defensin is one of the most diversified groups of antimicrobial peptides in invertebrate. In the present study, a novel defensin member referred as Pv-Def was identified and characterized from Asian green mussel Perna viridis. Using in silico survey of several EST databases released from diverse tissues of P. viridis, a single peptide referred as Pv-Def was predicted as defensin homologue with Mytilus counterparts. Further analysis on gene structure revealed that Pv-Def was 1001 nt in length and consisted of 3 exons and 2 introns. The precursor of Pv-Def was composed of a signal peptide of 19 amino acids and a mature peptide of 45 amino acids. The mature Pv-Def peptide contains 6 cysteines which formed 3 disulfide bonds at ²⁷C₁- ⁵⁴C₄, ⁴⁰C₂- ⁶⁰C₅ and ⁴⁴C₃- ⁶²C₆. Like most of the defensin family members, mature Pv-Def peptide included an alpha helix and 2 beta strands. Pv-Def showed significantly tissue-specific expression pattern, while highest transcription level was observed in hepatopancreas, which was about 900 folds to that in hemocytes. Moreover, the expression of Pv-Def mRNA in hemocytes was significantly and accurately up-regulated at different time intervals by Vibrio parahaemolyticus challenge. Interestingly, phylogenetic analysis suggested that the Pv-Def possesses closest relationships with arthropods counterparts rather than other mollusk defensins. To our knowledge, this is the first time that a defensin member was reported in Asian green mussel P. viridis.