Characterization of toxicity and structure of PirAB(vc)-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(vc)-like and PirB(vc)-like proteins, are encoded by two adjacent genes on a non-pVH plasmid from a Vibrio campbellii strain. Herein, PirAB(vc)-like protein caused neither abnormalities nor death in shrimp postlarvae (Litopenaeus vannamei); furthermore, typical AHPND clinical signs were not observed. PirA(vc)-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(vc)-like resemble Cry toxin domain II (receptor-binding domain) and domain I (pore-forming domain), respectively. PirA(vc)-like and PirB(vc)-like proteins are structurally similar to PirA and PirB, respectively. Subtle structural differences between PirA(vc)-like protein and PirA appear to be involved in ligand-binding and binary protein complex formation. The difference in virulence of PirAB(vc)-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(vc)-like proteins and their relationship with PirAB.