Structural insights into what glycan arrays tell us about how glycan-binding proteins interact with their ligands

Screening of glycan arrays represents a powerful, high-throughput approach to defining oligosaccharide ligands for glycan-binding receptors, commonly referred to as lectins. Correlating results from such arrays with structural analysis of receptor–ligand complexes provide one way to validate the arrays. Using examples drawn from the family of proteins that contain C-type carbohydrate-recognition domains, this review illustrates how information from the arrays reflects the way that selectivity and affinity for glycan ligands is achieved. A range of binding profiles is observed, from very restricted binding to a small set of structurally similar ligands to binding of broad classes of ligands with related terminal sugars and even to failure to bind any of the glycans on an array. These outcomes provide insights into the importance of multiple factors in defining the selectivity of these receptors, including the presence of conformationally defined units in some oligosaccharide ligands, local and extended interactions between glycans and the surfaces of receptors, and steric factors that exclude binding of some ligands

Targeted glycoproteomic identification of cancer cell glycosylation

GalMBP is a fragment of serum mannose-binding protein that has been modified to create a probe for galactose-containing ligands. Glycan array screening demonstrated that the carbohydrate-recognition domain of GalMBP selectively binds common groups of tumor-associated glycans, including Lewis-type structures and T antigen, suggesting that engineered glycan-binding proteins such as GalMBP represent novel tools for the characterization of glycoproteins bearing tumor-associated glycans. Blotting of cell extracts and membranes from MCF7 breast cancer cells with radiolabeled GalMBP was used to demonstrate that it binds to a selected set of high molecular weight glycoproteins that could be purified from MCF7 cells on an affinity column constructed with GalMBP. Proteomic and glycomic analysis of these glycoproteins by mass spectrometry showed that they are forms of CD98hc that bear glycans displaying heavily fucosylated termini, including Lewisx and Lewisy structures. The pool of ligands was found to include the target ligands for anti-CD15 antibodies, which are commonly used to detect Lewisx antigen on tumors, and for the endothelial scavenger receptor C-type lectin, which may be involved in tumor metastasis through interactions with this antigen. A survey of additional breast cancer cell lines reveals that there is wide variation in the types of glycosylation that lead to binding of GalMBP. Higher levels of binding are associated either with the presence of outer-arm fucosylated structures carried on a variety of different cell surface glycoproteins or with the presence of high levels of the mucin MUC1 bearing T antigen