Characterization of a novel CUB domain containing protein with immune functions in Mytilus galloprovincialis.

CUB (Complement/Uegf/Bmp) domain is a structural motif present in many proteins with important functions in tissue development and immunity. CUB-domain-containing proteins (CDCPs) have been identified as immune-related molecules from bivalves. However, the knowledge about the role of CDCP in bivalves' immunity is still limited. In this study, a novel CDCP containing a signal peptide and only one CUB domain was characterized from Mytilus galloprovincialis and named Mg-CUB here. The tissue distribution and expression profiles of Mg-CUB gene after microbes' stimulation were determined. In addition, Mg-CUB was recombinantly expressed and the immune-related functions of rMg-CUB protein were tested. Mg-CUB exhibits a wide distribution in all tested tissues with the highest expression level was observed in adductor muscle followed by hemocytes. Infection with bacteria significantly up-regulated the expression level of Mg-CUB in the hemocytes, suggesting that Mg-CUB might be involved in host immune defense response. Recombinant Mg-CUB (rMg-CUB) exhibited relatively stronger antimicrobial activity against Gram-negative bacteria, as well as binding capacity and agglutination ability for the tested bacteria. In addition, rMg-CUB also showed binding ability with lipopolysaccharide and peptidoglycan. Injection of rMg-CUB protein significantly increased the expression level of TLR4, MyD88, and NF-κB in the hemocytes, indicating a potential role of Mg-CUB in the activation of the related signaling pathway. This study provided valuable data for understanding the biological roles of CUB domain as both immune effector and pattern recognition receptor (PRR) in the innate immune system of mollusks.