A peptidoglycan recognition protein involved in immune recognition and immune defenses in Ruditapes philippinarum

Peptidoglycan recognition proteins (PGRPs) are important pattern recognition receptors in the innate immune system of invertebrates. In the study, a short PGRP (designed as RpPGRP) was identified and characterized from the manila clam Ruditapes philippinamm. The open reading frame of RpPGRP encoded a polypeptide of 249 amino acids with a calculated molecular mass of 27.2 kDa and an isoelectric point of 6.62. Multiple alignments and phylogenetic analysis strongly suggested that RpPGRP was a new member of the PGRP superfamily. In non stimulated clams, RpPGRP exhibited different tissue expression pattern, and highly expressed in hepatopancreas and hemocytes. Expression of RpPGRP transcripts was significantly up -regulated in hemocytes of clams post Vibrio crnguillartzn or Micrococcus luteus challenge. The recombinant RpPGRP (rRpPGRP) exhibited high affinity to PGN, LPS and zymosan in a concentration-dependent manner. With a broad spectrum of bacterial binding activities, rRpPGRP exhibited strong agglutination activity to Escherichia colt, Vibrio splendidus, V. anguiliarum and M. luteus. Furthermore, rRpPGRP exhibited Zn2+ -dependent amidase activity and catalyzed the degradation of insoluble PGN. Especially, rRpPGRP exhibited significant antibacterial activity against E. coil and M. luteus. Moreover, the biofilm formation of E. coil could be inhibited after rRpPGRP incubation in the presence of Zn2+. This inhibitory effect of rRpPGRP might attribute to its amide bactericidal activity. Taken together, rRpPGRP played important roles in PGRP-mediated immune defense mechanisms, especially by recognizing antigens and eliminating bacteria