In this report, we describe a new lectin from the fungus Rhizopus stolonifer that agglutinates rabbit red blood cells. Agglutinating activity was detected in the extract of mycelium-forming spores cultured on agar plates but not in the mycelium-forming no spores from liquid medium. This lectin, which we designated R. stolonifer lectin (RSL), was isolated by affinity chromatography with porcine stomach mucin-Sepharose. SDS-polyacrylamide gel electrophoresis and mass spectral analysis showed that RSL is ∼4.5 kDa, whereas gel filtration indicated a mass of 28 kDa. This indicates that the lectin is a hexamer of noncovalently associated RSL monomers. RSL activity was very stable, since it was insensitive to heat treatment at 70 °C for 10 min. Analysis of RSL binding specificity by both microtiter plate and precipitation assays showed that N-glycans with L-fucose linked to the reducing terminal GlcNAc residues are the most potent inhibitors of RSL binding, whereas N-glycans without α(1-6)-linked fucose residues are ∼100-fold weaker inhibitors of binding. Oligosaccharides with α(1-2, -3, and -4) linkages showed no inhibition of binding in these assays. In a mirror resonance biosensor assay, high affinity binding was observed between RSL and the glycopeptide of bovine γ-globulin, which has N-glycans with α(1-6)-linked fucose residues. However, RSL showed only a weak interaction with the glycopeptide of quail ovomucoid, which lacks fucose residues. Finally, capillary affinity electrophoresis studies indicated that RSL binds strongly to N-glycans with α(1-6)-linked fucose residues. Together, these results show that RSL recognizes the core structure of N-glycans with α(1-6)-linked L-fucose residues. This specificity could make RSL a valuable tool for glycobiological studies.
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