A novel mannose-binding lectin homolog identified in the large yellow croaker (Larimichthys crocea) with evident binding and agglutinating activities

Lectins are a large group of carbohydrate-binding proteins that widely participate in innate immune response, however, our understanding of its subfamily mannose-binding lectins (MBLs) is still in paucity for commercially important fish species such as the large yellow croaker (Larimichthys crocea). In this study, a novel MBL homolog (designated as galactose-binding lectin, LcGalBL-1, due to its galactose preference) was identified in L. crocea through transcriptomic screening. In addition to bioinformatic characterization, expression profiling of LcGalBL-1 in different tissues and upon pathogenic infection were also evaluated. Furthermore, the binding and agglutinating activities of LcGalBL-1 to various bacteria, carbohydrates, and/or heterologous blood cells (or hemocytes) in the presence and absence of Ca²⁺ were assessed using the recombinant LcGalBL-1 (rLcGalBL-1). Our data illustrated that LcGalBL-1, a protein with 235 amino acids, was a Ca²⁺-dependent galactose-binding lectin, harboring typical collagen-like region (CLR) and QPD motif-characterized carbohydrate recognition domain (CRD). The LcGalBL-1 was found to be highly homologous to those reported in other teleosts, especially its close relatives. Moreover, the LcGalBL-1 was constitutionally expressed in all the tissues investigated (predominantly in liver and heat kidney) and was shown to be significantly induced upon pathogenic bacterial challenge. Additionally, the rLcGalBL-1 displayed extensive binding activities towards various bacteria and carbohydrates as well as strong agglutinating activities against gram-positive/negative bacteria and blood cells of donor species. Collectively, our findings suggest that LcGalBL-1 is a constitutive and inducible acute-phase immune molecule that protects L. crocea from a broad-spectrum of pathogens.