A novel L-rhamnose-binding lectin participates in defending against bacterial infection in zebrafish

L-rhamnose-binding lectin (RBL), which is a class of animal lectins independent of Ca2+, can specifically bind L- rhamnose or D-galactose. Although several lectins in zebrafish have been reported, their functional mechanisms have not been fully uncovered. In this study, we discovered a novel L-rhamnose binding lectin (DrRBL) and studied its innate immune function. The DrRBL protein contains only one carbohydrate-recognition domain (CRD), which includes two strictly conserved motifs, YGR and DPC. DrRBL was detected in all tested tissues and was present at high levels in the spleen, hepatopancreas and skin. After Aeromonas hydrophila challenge, the DrRBL mRNA level was significantly upregulated. Additionally, DrRBL was secreted into the extracellular matrix. Recombinant DrRBL (rDrRBL) could significantly inhibit the growth of gram-positive/negative bacteria, bind to several bacteria and cause obvious agglutination. The rDrRBL protein could combine with polysaccharides, such as PGN and LPS, rather than LTA. A more detailed study showed that rDrRBL could combine with mono-saccharides, such as mannose, rhamnose and glucose, which are important components of PGN and LPS. However, rDrRBL could not bind to ribitol, which is an important component of LTA. The DrRBL deletion mu-tants, DrRBL Delta 144-150 and DrRBL Delta 198-200, were also constructed. DrRBL Delta 144-150 (ANYGRTD deficient) showed weak bacterial inhibiting ability. However, DrRBL Delta 198-200 (DPC deficient) showed weak agglutination ability. These results suggest that the DPC domain is important for agglutination. The conserved domain ANYGRTD is essential for inhibiting bacterial growth.

Automated summary

In this study, a novel L-rhamnose binding lectin (DrRBL) was discovered and its innate immune function was studied. DrRBL can inhibit the growth of gram-positive/negative bacteria, bind to several bacteria and cause agglutination. However, it cannot bind to an important component of LTA. The DPC domain is important for agglutination, while the conserved domain ANYGRTD is essential for inhibiting bacterial growth.