Identification, Phylogeny and Expressional Profiles of Peptidoglycan Recognition Protein (PGRP) Gene Family in Sinonovacula constricta

Peptidoglycan recognition protein (PGRP) plays a vital role in invertebrate innate immunity system as a specific pattern recognition receptor for peptidoglycan. Bivalves possess various PGRP systems for self-defense; however, it has not been characterized in razor clam Sinonovacula constricta. In this study, eight PGRP coding sequences were identified and analyzed from S. constricta genome, which are designated as ScPGRP-S1, ScPGRP-S2, ScPGRP-S3, ScPGRP-S4, ScPGRP-S5, ScPGRP-S6, ScPGRP-S7, ScPGRP-S8. The results of molecular evolutionary analyses showed that all eight ScPGRP genes were highly conserved and exhibited a typical PGRP/amidase_2 domain as PGRP genes in other mollusks. Moreover, the presence of signal peptides was predicted in ScPGRP-S2, ScPGRP-S3 and ScPRP-S6, while a transmembrane structure only existed in ScPGRP-S6. Notably, a tertiary structure analysis indicated that no disulfide bond was observed in ScPGRP-S5 and ScPGRP-S7. The mRNA transcripts analysis of ScPGRPs revealed that the high expression patterns of ScPGRP-S1 and ScPGRP-S4 were found in mantle, adductor muscle and foot, while those of ScPGRP-S2, ScPGRP-S3 and ScPGRP-S6 were observed in hepatopancreas. Furthermore, the temporal expression profiles of ScPGRPs in the hepatopancreas were analyzed by qPCR after Gram-negative Vibrio parahaemolyticus and Gram-positive Staphylococcus aureus challenges. The mRNA expressions of ScPGRP-S2, ScPGRP-S3 and ScPGRP-S6 could be induced by V. parahaemolyticus and S. aureus. Overall, our findings indicated that ScPGRPs were involved in the immune defense against invaders, which constituted a comprehensive understanding of the potential role of PGRP genes in S. constricta.