Evolution of ASABF (Ascaris suum antibacterial factor)-type antimicrobial peptides in nematodes: Putative rearrangement of disulfide bonding patterns

ASABF (Ascaris suum antibacterial factor)-type antimicrobial peptides are defensin-like cysteine-rich peptides that are widely distributed in the phylum Nematoda. In known members of ASABF-type antimicrobial peptides, an array consisting of eight cysteine residues is completely conserved. In this study, we report a novel member ASABF-6Cys-α, which contains only six cysteine residues, in the pig round worm A. suum. The two cysteine residues deleted in ASABF-6Cys-α were not identical to a pair of half-cystine forming a disulfide bridge in ASABF-α, suggesting a rearrangement of disulfide bonding patterns. Gene organization and phylogenetic analyses suggested that ASABF-6Cys-α was generated from an ancestral ASABF gene after the divergence of Ascaridida from Rhabditida. ASABF-6Cys-α transcripts dramatically increased after bacterial injection, suggesting that ASABF-6Cys-α may contribute to immunity in nematodes.