Identification of a Ficolin-like Serum Lectin of the Common Carp as a Novel Homologue of Mammalian Microfibrillar-Associated Protein 4

Serum lectins in vertebrates play crucial roles in innate immunity as recognition mole-cules for pathogen-associated molecular patterns (PAMPs). In mammals, two major lectins, mannose-binding lectin (MBL) and ficolin, both containing N-terminal collagen-like domains, activate the lectin pathway of complement. While MBL and ficolin recognize distinct PAMPs, their counterparts in teleost are less understood. To date, MBL and ga-lactose-binding lectin (GalBL) have been identified in teleost, but the presence of ficolin remains unclear. In this study, we purified a 31-kDa serum lectin from common carp that displayed carbohydrate-binding specificity similar to that of mammalian ficolin. Unex-pectedly, this lectin lacked an N-terminal collagenous domain and showed highest similarity to mammalian microfibril-associated glycoprotein 4 (MFAP4). Biochemical analyses revealed that carp MFAP4-like protein forms a hexamer in serum, specifically binds GlcNAc and GalNAc, and recognizes the fish pathogen Vibrio anguillarum. The binding was competitively inhibited by GlcNAc but not by EDTA, indicating Ca²⁺-independent recognition. These findings suggest that MFAP4 functions as a novel serum lectin in teleost fish, serving as a recognition molecule for bacterial pathogens in innate immunity.