Structural Biology Research Center, IMSS, KEK^* Departments of Endocrinology, Faculty of Medicine, Kagawa University, Japan^** Departments of Department of Applied Biological Chemistry, Faculty of Agriculture, Shizuoka University, Japan^*** Department of Applied Biological Chemistry, Faculty of Agriculture, Shizuoka University, Japan^****
○Masamichi Nagae^* Nozomu Nishi^** Takeomi Murata^*** Taichi Usui^**** Takanori Nakamura^** Soichi Wakatsuki^* Ryuichi Kato^*

The galectins are a family of β-galactoside-binding animal lectins with a conserved carbohydrate recognition domain (CRD). They show high affinities for small β-galactosides and various binding specificities for complex glycoconjugates. The specific carbohydrate recognition is thought to be essential for their proper function. Galectin-9 has two tandem CRDs with a short linker peptide, and we report here the crystal structures of mouse and human galectin-9 N-terminal CRDs (NCRDs) in the presence of the N-acetyllactosamine dimer (LN2, Galβ1-4GlcNAcβ1-3Galβ1-4GlcNAc). The structure of mouse galectin-9 NCRD forms the unique dimer, which is quite different from the canonical 2-fold symmetric dimer seen for galectin-1 and 2. We also observed that the mouse galectin-9 NCRD interacts with each other in solution as indicated by surface plasmon resonance measurements. On the other hand, human galectin-9 NCRD exists as monomer in crystals and the amino acid residues on the mouse galectin-9 NCRD dimer interface are not conserved. The recognition mechanisms of the LN2 molecule are quite different from each other. Dimeric mouse galectin-9 NCRD recognizes β-galactoside residue at the non-reducing end and cooperatively interacts with LN2 molecules. In the crystals of human galectin-9 NCRD in complex with LN2, there are two different types of β-galactoside recognition by the human galection-9 NCRD: either the first or the third residue from the non-reducing end.. A non-conserved asparagine residue in human galectin-9 NCRD allows two different types of carbohydrate recognition. We will discuss the structural difference in same proteins from the different species.