UuPGRP, a peptidoglycan recognition protein of Urechis unicinctus, promotes immune defense against bacterial infection.

Peptidoglycan recognition proteins (PGRPs) are evolutionarily conserved innate immune receptors with pivotal roles in regulating host defense responses, yet the functional characterization of PGRPs in marine benthic invertebrates, especially in Echiurioidea and Annelida, remains limited. In this study, we identified and characterized a short-type PGRP from the U. unicinctus, designated UuPGRP, to elucidate its role in antimicrobial immunity. Sequence analysis revealed that UuPGRP consists of 232 amino acid residues, including a conserved C-terminal PGRP domain. Phylogenetic and sequence identity analyses demonstrated that UuPGRP shares 40.72%-59.09% sequence identity with PGRPs from other species. Tissue distribution profiling in healthy U. unicinctus showed widespread expression of UuPGRP across multiple tissues, with the highest expression level in anal sac. Notably, following infection with the Vibrio anguillarum, UuPGRP exhibited significant tissue-specific transcriptional regulation. Functional analysis revealed that rUuPGRP interacts with a broad range of Gram-positive and -negative bacteria in a dose-dependent manner. Moreover, rUuPGRP mediated Zn2+-dependent bacterial agglutination, which can be abrogated by EDTA. Importantly, rUuPGRP enhanced reactive oxygen species (ROS) production in hemocytes, suggesting its ability to modulate oxidative burst. Finally, in vivo clearance assays showed that rUuPGRP significantly accelerated the elimination of V. anguillarum from infected U. unicinctus, directly confirming its contribution to pathogen clearance. Collectively, these findings demonstrate that UuPGRP functions as an important immune effector, mediating bacterial recognition and amplifying immune responses via ROS modulation and agglutination. This work expands our understanding of PGRP-mediated immunity in non-model organisms, shedding light on their adaptive strategies for survival in microbe-rich benthic environments.