Human galectin-3 (Mac-2 antigen): Defining molecular switches of affinity to natural glycoproteins, structural and dynamic aspects of glycan binding by flexible ligand docking and putative regulatory sequences in the proximal promoter region

Background: Human galectin-3 (Mac-2 antigen) is a cell-type-specific multifunctional effector owing to selective binding of distinct cell-surface glycoconjugates harboring β-galactosides. The structural basis underlying the apparent preferences for distinct glycoproteins and for expression is so far unknown. Methods: We strategically combined solid-phase assays on 43 natural glycoproteins with a new statistical approach to fully flexible computational docking and also processed the proximal promoter region in silico. Results: The degree of branching in N-glycans and clustering of core 1 O-glycans are positive modulators for avidity. Sialylation of N-glycans in α2-6 linkage and of core 1 O-glycans in α2-3 linkage along with core 2 branching was an unfavorable factor, despite the presence of suited glycans in the vicinity. The lectin-ligand contact profile was scrutinized for six natural di- and tetrasaccharides enabling a statistical grading by analyzing flexible docking trajectories. The computational analysis of the proximal promoter region delineated putative sites for Lmo2/c-Ets-1 binding and new sites with potential for RUNX binding. General significance: These results identify new features of glycan selectivity and ligand contact by combining solid-phase assays with in silico work as well as of reactivity potential of the promoter.