Carbohydrate-Binding Specificity of Human Galectins: An Overview by Frontal Affinity Chromatography

Abstract

To understand the biological functions of lectins, it is important to investigate their sugar-binding specificity. Although galectins are characterized as β-galactoside-binding proteins comprising evolutionarily conserved amino-acid sequences, they have significantly divergent specificities depending on their individual carbohydrate-recognition domains (CRDs). Of the various methods available to analyze lectin-glycan interactions, frontal affinity chromatography is unique in that it provides a quantitative set of dissociation constants (K d's) between immobilized lectins and a panel of (>100) fluorescently labeled oligosaccharides. In this article, we provide an overview of the features of galectin specificities with a focus on human galectins based on published data. From the data obtained, comprehensive features of individual CRDs can be systematically understood in terms of branching, and 3′-modifications including sialylation, sulfation, αGal/GalNAc substitutions, β1-3Gal extension, and N-acetyllactosamine repeats. Additionally, we analyze evolutionarily more distant galectin molecules of non-human origins. These findings provide not only basic knowledge but also useful information for their applications: e.g., for engineering superior galectins improved in their specificity and affinity and developing galectin-targeted drugs. A. Introduction: Transforming Lectinomics Lectins are a group of carbohydrate-binding proteins with diverse structures and specificities. Recent analysis revealed that the number of protein families of lectins and those involving lectin domains is almost 50 (1). This is surprising considering that the number of representative lectin families at the end of 20th century was only 10: R-type lectins (ricin B chain-like lectins) of β-trefoil fold, L-type lectins (legume lectin-like lectins) of β-sandwich fold (jellyroll), GNA (Galanthus nivalis agglutinin)-related lectins of β-prism II fold, jacalin-related lectins (mJRLs and gJRLs) of β-prism I fold, hevein-like lectins (chitin-binding lectins) of hevein-type cystineknot motif, C-type lectins (Ca-dependent-type animal lectins) of Ctype α/β fold, galectins (β-galactoside-binding lectins), siglecs (sialic acid-binding immunoglobulin-like lectins), hyaladhesins (hyaluronan-binding proteins) of C-type α/β fold, and pentraxins (a class of pattern recognition receptors involved in innate immunity) of β-sandwich fold (jellyroll). The latest members of new lectin families include the 17-kDa α-D-galactose-binding lectin from the Mediterranean mussel Mytilus galloprovincialis termed MytiLec 1) (2) and a 15.5-kDa mannose-specific lectin from the oyster Crassostrea gigas termed CGL1 (3). Lectin domains are also known as carbohydraterecognition domains (CRDs) in hydrolytic enzymes, typically belonging to the R-type lectin family (4). They are termed "carbohydrate-binding modules" (CBMs) in the framework of Carbohydrate-Active enZymes (CAZy), with family numbers CBM 1-84 (http:// www.cazy.org/Carbohydrate-Binding-Modules.html). Lectin domains are also fo...

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