Characterization of toxicity and structure of PirAB -like proteins that are structurally almost identical to shrimp AHPND-causing PirAB toxin.

PirAB is a binary toxic protein that causes acute hepatopancreatic necrosis disease (AHPND) in shrimp. Their closest homologs, PirA -like and PirB -like proteins, are encoded by two adjacent genes on a non-pVH plasmid from a Vibrio campbellii strain. Herein, PirAB -like protein caused neither abnormalities nor death in shrimp postlarvae (Litopenaeus vannamei); furthermore, typical AHPND clinical signs were not observed. PirA -like protein corresponds to Cry toxin domain III (ligand-binding domain) and likely binds to N-acetylgalactosamine. The C-terminal and N-terminal of PirB -like resemble Cry toxin domain II (receptor-binding domain) and domain I (pore-forming domain), respectively. PirA -like and PirB -like proteins are structurally similar to PirA and PirB, respectively. Subtle structural differences between PirA -like protein and PirA appear to be involved in ligand-binding and binary protein complex formation. The difference in virulence of PirAB -like and PirAB may result from the specific binding of the protein complex to distinct host receptors. These results shed light on the potential functions and host receptors of PirAB -like proteins and their relationship with PirAB.