Genome-wide identification of ApeC domain-containing protein and revealing the function of ChACP-1.10 in oyster mucosal immunity

The Apextrin C-terminal (ApeC) domain-containing protein (ACP), a poorly studied gene family that is found exclusively in invertebrates, plays critical roles in mucosal immunity. However, the evolution and immunological mechanisms of ACP in oysters remain elusive. In this study, a total 148 ACP genes were identified in six oyster species genomes and were classified into five subgroups based on their domain architectures and phylogenetic analysis. Tandem duplications maybe play a crucial role in the expansion and diversification of the oyster ACP gene family. Transcriptomic analysis revealed that many CgACPs presented increased expression at 12 h and/or 24 h postinfection in the digestive gland upon exposure to Vibrio parahaemolyticus. qRT-PCR showed that C. hongkongensis ChACP-1.10 mRNA significantly increased 220-fold in the gills and modestly increased 2.6-fold in the hemolymph after 3 h post-V. coralliilyticus infection, respectively. These results suggest that ChACP-1.10 may play a potential role in oyster mucosal immunity. Structural analysis revealed that the ChACP-1.10-ApeC domain, a distinct β-tripod fold, is capable of binding three Ca²⁺ ions in three long Loop (DXEDX₄N). The recombinant protein ChACP-1.10 agglutinates and binds bacteria and fungi in a Ca²⁺-dependent manner. ChACP-1.10 functions not only as a PRR binding PGN, but also as an immune effector that enhances the mRNA expression of ChDefensins and ChIL-17s. Overall, this work provides insight into oyster mucosal immunity and health aquaculture as well as the function and evolution of the ACP gene family.