A novel peptidoglycan recognition protein involved in the prophenoloxidase activation system and antimicrobial peptide production in Antheraea pernyi

Pattern recognition receptors (PRRs) are employed in insects to defend against infectious pathogens by triggering various immune responses. Peptidoglycan recognition proteins (PGRPs), a vital family of PRRs, are widely distributed and highly conserved from vertebrates to invertebrates. To date, five PGRP genes have been identified in Antheraea pernyi, but their biochemical roles still remain unknown. In this study, we focused on the immune functions of PGRP-SA in A. pernyi (ApPGRP-SA), which was confirmed to be immune-related according to its significantly up-regulated expression level post microbial injection. In addition, the binding properties of ApPGRP-SA were investigated using a recombinant protein produced in a prokaryotic expression system, revealing that rApPGRP-SA displayed a multi-binding ability to various microbes, including the Gram-positive bacteria Staphylococcus aureus and Micrococcus luteus, Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, and fungus Candida albicans, together with their surface pathogen associated molecular patterns (PAMPs). Further studies showed that after recognition, the mixture of rApPGRP-SA/PAMP remarkably stimulated prophenoloxidase (PPO) activation in the hemolymph of A. pernyi in vitro, while the ds-PGRP-SA-treated hemolymph exhibited a lower sensitivity to PAMPs in comparison to the native sample. Moreover, the transcriptional level of the three antimicrobial peptides was also decreased in PGRP-SA knock-down larvae in response to immune-challenge. In summary, we conclude that ApPGRP-SA is a novel identified PGRP in A. pernyi that might act as a broad-spectrum pattern recognition receptor and is involved in the PPO activation system as well as antimicrobial peptide production.