Fluorescent labeling agents change binding profiles of glycan-binding proteins

Interactions of glycan-binding proteins (GBPs) with glycans are essential in cell adhesion, bacterial/viral infection, and cellular signaling pathways. Experimental characterization of these interactions based on glycan microarrays typically involves (1) labeling GBPs directly with fluorescent reagents before incubation with the microarrays, or (2) labeling GBPs with biotin before the incubation and detecting the captured GBPs after the incubation using fluorescently labeled streptavidin, or (3) detecting the captured GBPs after the incubation using fluorescently labeled antibodies raised against the GBPs. The fluorescent signal is mostly measured ex situ after excess fluorescent materials are washed off. In this study, by using a label-free optical scanner for glycan microarray detection, we measured binding curves of 7 plant lectins to 24 glycans: four beta 1-4-linked galactosides, three beta 1-3-linked galactosides, one beta-linked galactoside, one alpha-linked N-acetylgalactosaminide, eight alpha 2-3-linked sialosides, and seven alpha 2-6-linked sialosides. From association and dissociation constants deduced by global-fitting the binding curves, we found that (1) labeling lectins directly with fluorescent agents change binding profiles of lectins, in some cases by orders of magnitude; (2) those lectin-glycan binding reactions characterized with large dissociation rates, though biologically relevant, are easily missed or deemed insignificant in ex situ fluorescence-based assays as most captured lectins are washed off before detection. This study highlights the importance of label-free real-time detection of protein-ligand interactions and the potential pitfall in interpreting fluorescence-based assays for characterization of protein-glycan interactions.