Crystallization and preliminary X-ray diffraction analysis of a protease-resistant mutant form of human galectin-8

Crystallization of Galectin-8 Linker Reveals Intricate Relationship between the N-terminal Tail and the Linker

Galectin-8 (Gal-8) plays a significant role in normal immunological function as well as in cancer. This lectin contains two carbohydrate recognition domains (CRD) connected by a peptide linker. The N-terminal CRD determines ligand binding specificity, whereas the linker has been proposed to regulate overall Gal-8 function, including multimerization and biological activity. Here, we crystallized the Gal-8 N-terminal CRD with the peptide linker using a crystallization condition that contains Ni²⁺. The Ni²⁺ ion was found to be complexed between two CRDs via crystal packing contacts. The coordination between Ni²⁺ and Asp25 plays an indirect role in determining the structure of β-strand F0 and in influencing the linker conformation which could not be defined due to its dynamic nature. The linker was also shortened in situ and crystallized under a different condition, leading to a higher resolution structure refined to 1.08 Å. This crystal structure allowed definition of a short portion of the linker interacting with the Gal-8 N-terminal tail via ionic interactions and hydrogen bonds. Observation of two Gal-8 N-terminal CRD structures implies that the N-terminal tail and the linker may influence each other’s conformation. In addition, under specific crystallization conditions, glycerol could replace lactose and was observed at the carbohydrate binding site. However, glycerol did not show inhibition activity in hemagglutination assay.

X-ray structure of a protease-resistant mutant form of human galectin-8 with two carbohydrate recognition domains

Galectin-8 is a tandem-repeat-type β-galactoside-specific animal lectin possessing N-terminal and C-terminal carbohydrate recognition domains (N-CRD and C-CRD, respectively), with a difference in carbohydrate-binding specificity, involved in cell-matrix interaction, malignant transformation, and cell adhesion. N-CRD shows strong affinity for α2-3-sialylated oligosaccharides, a feature unique to galectin-8. C-CRD usually shows lower affinity for oligosaccharides but higher affinity for N-glycan-type branched oligosaccharides than does N-CRD. There have been many structural studies on galectins with a single carbohydrate recognition domain (CRD), but no X-ray structure of a galectin containing both CRDs has been reported. Here, the X-ray structure of a protease-resistant mutant form of human galectin-8 possessing both CRDs and the novel pseudodimer structure of galectin-8 N-CRD in complexes with α2-3-sialylated oligosaccharide ligands were determined. The results revealed a difference in specificity between N-CRD and C-CRD, and provided new insights into the association of CRDs and/or molecules of galectin-8.