The B Subunit of PirAB Toxin Secreted from Vibrio parahaemolyticus Causing AHPND Is an Amino Sugar Specific Lectin

Vibrio parahaemolyticus (Vp) is the etiological agent of the acute hepatopancreatic necrosis disease (AHPND) in Penaeus vannamei shrimp. Vp possesses a 63-70 kb conjugative plasmid that encodes the binary toxin PirA/PirB. The 250 kDa PirAB complex was purified by affinity chromatography with galactose-sepharose 4B and on a stroma from glutaraldehyde-fixed rat erythrocytes column, as a heterotetramer of PirA and PirB subunits. In addition, recombinant pirB (rPirB) and pirA (rPirA) were obtained. The homogeneity of the purified protein was determined by SDS-PAGE analysis, and the yield of protein was 488 ng/100 μg of total protein of extracellular products. The PirAB complex and the rPirB showed hemagglutinating activity toward rat erythrocytes. The rPirA showed no hemagglutinating capacity toward the animal red cells tested. Among different mono and disaccharides tested, only GalNH₂ and GlcNH₂ were able to inhibit hemagglutination of the PirAB complex and the rPirB. Glycoproteins showed inhibitory specificity, and fetuin was the glycoprotein that showed the highest inhibition. Other glycoproteins, such as mucin, and glycosaminoglycans, such as heparin, also inhibited the activity. Desialylation of erythrocytes enhanced the hemagglutinating activity. This confirms that Gal or Gal (β1,4) GlcNAc are the main ligands for PirAB. The agglutinating activity of the PirAB complex and the rPirB is not dependent on cations, because addition of Mg²⁺ or Ca²⁺ showed no effect on the protein capacity. Our results strongly suggest that the PirB subunit is a lectin, which is part of the PirA/PirB complex, and it seems to participate in bacterial pathogenicity.