The -galactomannan Davanat binds galectin-1 at a site different from the conventional galectin carbohydrate binding domain

Galectins are a sub-family of lectins, defined by their highly conserved beta-sandwich structures and ability to bind to beta-galactosides, like Gal beta1-4 Glc (lactose). Here, we used (15)N-(1)H HSQC and pulse field gradient (PFG) NMR spectroscopy to demonstrate that galectin-1 (gal-1) binds to the relatively large galactomannan Davanat, whose backbone is composed of beta1-4-linked d-mannopyranosyl units to which single d-galactopyranosyl residues are periodically attached via alpha1-6 linkage (weight-average MW of 59 kDa). The Davanat binding domain covers a relatively large area on the surface of gal-1 that runs across the dimer interface primarily on that side of the protein opposite to the lactose binding site. Our data show that gal-1 binds Davanat with an apparent equilibrium dissociation constant (K(d)) of 10 x 10(-6) M, compared to 260 x 10(-6) M for lactose, and a stiochiometry of about 3 to 6 gal-1 molecules per Davanat molecule. Mannan also interacts at the same galactomannan binding domain on gal-1, but with at least 10-fold lower avidity, supporting the role of galactose units in Davanat for relatively strong binding to gal-1. We also found that the beta-galactoside binding domain remains accessible in the gal-1/Davanat complex, as lactose can still bind with no apparent loss in affinity. In addition, gal-1 binding to Davanat also modifies the supermolecular structure of the galactomannan and appears to reduce its hydrodynamic radius and disrupt inter-glycan interactions thereby reducing glycan-mediated solution viscosity. Overall, our findings contribute to understanding gal-1-carbohydrate interactions and provide insight into gal-1 function with potentially significant biological consequences.