Structural basis of carbohydrate recognition by lectin II from Ulex europaeus, a protein with a promiscuous carbohydrate-binding site

Protein-carbohydrate interactions are the language of choice for intercellu lar communication. The legume lectins form a large family of homologous pro teins that exhibit a wide variety of carbohydrate specificities. The legume lectin family is therefore highly suitable as a model system to study the structural principles of protein-carbohydrate recognition. Until now, struc tural data are only available for two specificity families: Man/Glc and Gal /GalNAc. No structural data are available for any of the fucose or chitobio se specific lectins. The crystal structure of Ulex europaeus (UEA-II) is the first of a legume l ectin belonging to the chitobiose specificity group. The complexes with N-a cetylglucosamine, galactose and fucosylgalactose show a promiscuous primary binding site capable of accommodating both N-acetylglucos amine or galacto se in the primary binding site. The hydrogen bonding network in these compl exes can be considered suboptimal, in agreement with the low affinities of these sugars. Ln the complexes with chitobiose, lactose and fucosyllactose this suboptimal hydrogen bonding network is compensated by extensive hydrop hobic interactions in a Glc/GlcNAc binding subsite. UEA-II thus forms the f irst example of a legume lectin with a promiscuous binding site and illustr ates the importance of hydrophobic interactions in protein-carbohydrate com plexes. Together with other known legume lectin crystal structures, it show s how different specificities can be grafted upon a conserved structural fr amework. (C) 2000 Academic Press.