Multipronged ESI-MS Approach for Studying Glycan-Binding Protein Interactions with Glycoproteins

A multipronged electrospray ionization mass spectrometry (ESI-MS) approach for investigating glycan-mediated interactions between water-soluble glycan-binding proteins (GBPs) and glycoproteins (GPs) is described. First, the catch-and-release (CaR)-ESI-MS assay, carried with ion mobility separation prior to GBP "release" (i.e., CaR^IMS-ESI-MS), is employed to rapidly identify GBP-GP binding in solution. The apparent affinity ( K_a) of the GBP for the GP is then measured using the competitive proxy ligand-ESI-MS binding assay. Finally, N-glycans, enzymatically released as free oligosaccharides from the GP, are screened against the GBP using ESI-MS to identify the glycans that are recognized by the GBP. Measurements performed at multiple GBP concentrations allow for the affinities of released N-glycans (grouped as compositional isomers) to be ranked. Implementation of the approach is illustrated using the known interactions between a C-terminal domain fragment of human galectin-3 (hGal-3C) and three human serum GPs, α-1-acid glycoprotein (AGP), haptoglobin phenotype 1-1 (Hp1-1) and α-2-macroglobulin (α2M). Specific binding of hGal-3C to each GP was successfully detected by CaR^IMS-ESI-MS; no binding was detected for a noninteracting reference protein, which served as a negative control. The K_a measured by proxy ligand-ESI-MS for AGP, Hp1-1 and α2M (4 × 10⁵ M^-1, 2 × 10⁵ M^-1 and 3 × 10⁵ M^-1, respectively) are consistent with the reported apparent affinities of hGal-3 for serum GPs and their N-glycans. Screening the N-glycan libraries of each of the GPs against hGal-3C identified ligands corresponding to a total of 20 different saccharide compositions with sialylated bi-, tri-, and tetra-antennary structures. The results of binding measurements performed at two different hGal-3C concentrations revealed that all of the N-glycan ligands exhibit affinities ≥10⁴ M^-1.