A novel peptidoglycan recognition protein containing a goose-type lysozyme domain from the Pacific oyster, Crassostrea gigas

Peptidoglycan recognition protein (PGRP) is considered an essential molecule for effective immunity in invertebrates by its detection and clarification of invading bacteria. Bivalve mollusks also possess PGRP systems for self-defense, however, their functions in bivalves remain to be understood. In the present study, cDNA of a novel PGRP was identified from the Pacific oyster, Crassostrea gigas, using EST-based RACE PCR. This novel PGRP is homologous to short PGRPs and the presence of a signal peptide was predicted. The PGRP is classified into the short PGRP group, although its molecular weight was estimated as 54 kDa, close to that of long PGRP groups. A conserved domain search detected amidase_2/PGRP and goose-type (g-type) lysozyme domains in this PGRP structure, and thus this novel PGRP was designated as CgPGRP-L. Catalytic residues for PGRP and g-type lysozyme are well conserved, suggesting that CgPGRP-L may have both binding and lytic functions against bacteria. Reverser transcription PCR (RT-PCR) detected CgPGRP-L mRNA expression in circulatory hemocytes, and quantitative real-time RT-PCR revealed that its expression increased after Marinococcus halophilus and Vibrio tubiashii exposure. These results indicate that CgPGRP-L is expressed in hemocytes by bacterial invasion, and then may play roles of a short PGRP and bacterio-lytic lysozyme.