Non-canonical interactions between plant proteins and lectins cause false positives in lectin blots

Abstract

Lectins are proteins that specifically recognize and non-covalently bind to soluble carbohydrates or to the carbohydrate moieties of glycoproteins or glycolipids. Historically,lectin-blot analysis has been widely used as a tool for structural characterization of many mammalian glycoconjugates. In the present study, we demonstrate that the application of this technique to screen sugar moieties of plant proteins results in numerous false positives. Plants lack the enzyme machinery necessary to perform sialylation, however many bands appear upon probing of N. benthamiana L. leaf proteins with Maacia amurensis agglutinin (MAA) that recognizes specifically N-linked or core 2 O-linked glycans containing Neu5Ac/Gc-α2,3Galβ-1,4GlcNAc/Glc and O-linked glycans containing the trisaccharide Neu5Ac-3Galβ1-3GalNAc. The non-canonical binding is a direct result of sample preparation for SDS PAGE, because native proteins do not show an affinity to MAA-agarose resin. Moreover, inhibition with known hapten fails to prevent binding of MAA to plant proteins in lectin blots. We also provide evidence that interactions of a hydrophobic nature contribute, at least in part, to the non-specific binding, and that other lectins – Sambucus nigra agglutinin (SNA) and Vicia villosa agglutinin (VVA) – also bind non-specifically to plant proteins. In conclusion, lectin blot analysis of plant proteins should always be verified by probing the binding specificity with a known hapten inhibitor alongside appropriate mammalian glycoprotein controls. Alternatively, non-specific binding can be avoided if lectin affinity chromatography of the native plant proteins is performed prior to lectin blots analysis.