Pathogenicity and protein analysis of photorhabdus insect-related (Pir) toxin PirAB revealed PirAB(vp) is a host-specific toxin

摘要

Acute hepatopancreatic necrosis disease (AHPND) is an emerging disease in shrimp aquaculture which directly caused by binary photorhabdus insect-related (Pir) toxins PirAB(vp) from Vibrio parahaemolyticus. To delineating the evolutionary origin and structure variation of PirAB, the protein sequence of PirAB(vp) resembles from bacteria and insects were retrieved from the public database. The evolutionary relationship of PirAB(vp) and resembles were analyzed phylogenetically. I-TASSER server was used to predict the protein-structure of PirA/B in investigated species. The cell-free supernatant obtained from the broth of V. parahaemolyticus and crude and heterologously expressed PirAB(vp) proteins were respectively administrated to shrimp (Litopenaeus vannamei), crab (Eriocheir sinensis), European seabass (Dicentrarchus labrax), bloodworm (Nereis succinea) and mosquito (Aedes albopictus) to confirm the host range of toxin. Results showed that majority of PirAB located in a genomic island with low GC content. Phylogenetic analysis of PirB from bacteria and insects divided them into three lineages, namely Marine, Soli and Insect lineage respectively, with the majority belonging to Soli lineage. Overall, resembles of PirAB(vp) were structurally similar to the PirAB(vp), but harbored three highly variable regions in receptor binding region and ligand binding sites, respectively. Shrimps bioassayed with cell-free supernatant of V. parahaemolyticus, crude and heterologously expressed PirAB(vp) proteins resulted in 76.5%, 68.7% and 86.6% mortality, respectively. However, almost no mortality was found in crab, bloodworm, mosquito and European seabass. In summary, results indicated that PirB(vp) and its homologs have significant sequence divergence and remarkable variability in the receptor binding sites, which might explain why PirAB(vp) are shrimp-specific toxins. Homology analysis of proteins revealed that there were three evolutionary directions for PirAB and the emergence of PirAB(vp) was an ancient event that not associated with the aquaculture practices of shrimp. This study enhanced our understanding of the evolution of pirAB, which provided new insights into the origins of AHPND. However, why only shrimp is highly susceptible to PirAB(vp) remains mysteries and need to solve in the future research.