Serum GlcNAc-binding IgM of fugu (Takifugu rubripes) suppresses the growth of fish pathogenic bacteria: A novel function of teleost antibody

• A GlcNAc-binding protein, designated as fGBP-78, was purified from sera of fugu.
• The fGBP-78 is a heteromer of 78- and 25-kDa subunits.
• The fGBP-78 showed inhibitory effects on the growth of pathogenic bacteria.
• The fGBP-78 is revealed to be an IgM molecule.

N-acetyl-d-glucosamine (GlcNAc) is one of the components of peptidoglycan, a biopolymer in the bacterial cell wall. We purified a novel GlcNAc-binding protein, designated as fGBP-78, from sera of fugu (Takifugu rubripes). The fGBP-78 is a heteromer of 78- and 25-kDa subunits. Moreover, fGBP-78 exerted remarkable inhibitory effects on the growth of both Gram-positive and Gram-negative bacteria, including ones virulent for marine fish species as well as non-pathogenic Escherichia coli. These results suggest that fGBP-78 contributes to bacterial clearance in fugu. Furthermore, the nanoLC–MS/MS and Western blotting analyses reveal that the 78-kDa subunit is the fugu IgM heavy chain. In addition, the molecular mass of the other subunit (25 kDa) was equal to that of the Ig light chain. Overall, results indicate that fGBP-78 is an IgM molecule presumably acts as a natural antibody. This paper reports a novel function of teleost IgM as a significant suppresser against bacterial growth.