A <i>N</i> ‐Acetyl‐D‐Glucosamine‐Binding C‐Type Lectin in the Cutaneous Mucus of Chub Mackerel, <i>Scomber japonicus</i> , Agglutinates Pathogenic Bacteria

ABSTRACT A C‐type lectin with specificity for N‐acetyl‐ d ‐glucosamine was purified from the cutaneous mucus of chub mackerel ( Scomber japonicus ) by affinity chromatography. The lectin comprises 159 amino acid residues and contains an unusual substitution within the conserved Ca 2+ ‐binding motif, in which Trp–Asn–Asp is replaced by Trp–Ser–Asp. Reverse transcription–PCR analysis demonstrated that the lectin gene is specifically expressed in the skin, and immunohistochemistry localized the protein to the outer epidermal layer, predominantly in squamous cells. Agglutinating activity was observed against L. garvieae , but not against any of the other bacterial species tested. An approximately 25‐kDa bacterial protein was identified as a potential ligand, although its identity remains unresolved. Collectively, these findings indicate that this lectin (SjCL) functions as a mucosal immune molecule and may contribute to the first line of defense against bacterial pathogens in teleost fish.