The galectin-3-binding protein of Cynoglossus semilaevis is a secreted protein of the innate immune system that binds a wide range of bacteria and is involved in host phagocytosis.

Galectin-3 binding protein (G3BP) is a secreted glycoprotein that binds galectin-3 and is involved in various pathological conditions including cancer and viral infection. In fish, G3BP-like sequences have been identified in very few species and their biological properties are entirely unknown. In this work, we reported for the first time the identification and analysis of a teleost G3BP, CsG3BP, from half-smooth tongue sole (Cynoglossus semilaevis). CsG3BP is composed of 565 amino acids and possesses a Scavenger Receptor Cysteine-rich (SRCR) domain, the latter containing six conserved cysteine residues that were predicted to form three intramolecular disulfide bridges. Expression of CsG3BP was detected in a wide range of tissues and upregulated by bacterial and megalocytivirus infection in a time-dependent manner. Immunoblot analysis detected CsG3BP in the culture medium of peripheral blood leukocytes (PBL) and in serum following bacterial stimulation. Purified recombinant CsG3BP (rCsG3BP) exhibited bacterial binding ability in a dose-dependent manner. In contrast, the mutant forms of CsG3BP that bear deletion of the SRCR domain or serine substitutions at three cysteine residues involved in disulfide bond formation lost the capacity of bacterial interaction. rCsG3BP displayed a certain substrate preference and bound more effectively to Gram-negative bacteria than to Gram-positive bacteria. Further study showed that when the CsG3BP produced by PBL was blocked by anti-rCsG3BP antibodies, the phagocytic activity of the cells was significantly reduced. Taken together, these results indicate that CsG3BP is a secreted protein that probably plays a role in innate immune defense by binding to bacterial cells via the SRCR domain and thereby facilitating host phagocytosis.