Slow diffusion of lactose out of galectin-3 crystals monitored by X-ray crystallography: possible implications for ligand-exchange protocols

Galectin-3 is a multifunctional carbohydrate-binding protein that has roles in cancer progression. In addition to carbohydrate-dependent extracellular functions, galectin-3 participates in carbohydrate-independent intracellular signalling pathways, including apoptosis, via protein-protein interactions, some of which engage the carbohydrate-binding groove. When ligands bind within this site, conformational rearrangements are induced and information on unliganded galectin-3 is therefore valuable for structure-based drug design. Removal of cocrystallized lactose from the human galectin-3 carbohydrate-recognition domain was achieved via crystal soaking, but took weeks despite low affinity. Pre-soaking to remove lactose enabled the subsequent binding of cryoprotectant glycerol, whereas when the lactose was not removed a priori the glycerol could not displace it in the short cryosoaking time frame. This slow diffusion of lactose out of the crystals contrasts with the entrance of glycerol, which takes place within minutes. The importance of the removal of incumbent ligands prior to attempts to introduce alternative ligands is indicated, even for proteins exhibiting low affinity for ligands, and has significance for ligand exchange in structure-based drug design.