The legume lectins are a large group of homologous carbohydrate-binding proteins of non-immune origin that are found mainly in the seeds of most leguminous plants. These lectins act by deciphering specific glycocodes encoded in the structure of glycans. The interaction between lectins and carbohydrates plays a biological role in cellular processes such as cell communication, host defence, fertilization, development, parasitic infection, tumour metastasis and plant defence against herbivores and pathogens (Vijayan & Chandra, 1999
; Gallego del Sol et al.
). Lectins are ubiquitous in animals, plants and microorganisms. Over 250 three-dimensional structures of lectins from diverse sources are available and legume lectins represent a significant proportion of these proteins. Lectins share a common structural fold but they differ in their carbohydrate specificities. An outstanding feature of the group of legume lectins is that although all the monomers have similar tertiary structures, they show different modes of quaternary association (Brinda et al.
). This structural feature deserves more in-depth investigation in order to identify possible structural aspects that may dictate the determinants of the quaternary association of legume lectins.
Bianchet and coworkers characterized fucose-binding lectins as proteins that bind fucose and share a specific sequence motif whose function is associated with immune recognition in vertebrates and invertebrates (Bianchet et al.
). Fucose-binding lectins are widespread among microorganisms, animals and plants, including Pseudomonas aeruginosa
lectin (PA-IIL), Anguilla anguilla
agglutinin (AAA), Morone saxatilis
agglutinin (MsaFBP32), Dicentrarchus labrax
agglutinin, Chromobacterium violaceum
lectin (CV-IIL), Ralstonia solanacearum
lectin (RS-IIL), Ulex europaeus
agglutinin (UEA-I) and Lotus tetragonolobus
agglutinin (LTA) (Bianchet et al.
; Mitchell et al.
; Vandonselaar & Delbaere, 1994
; Konami et al.
; Zinger-Yosovich et al.
; Odom & Vasta, 2006
; Cammarata et al.
Fucose-binding lectins have been widely studied because the molecular-recognition properties of glycoproteins and glycolipids containing l
-fucose are often characterized in terms of their interactions with fucolectins. Moreover, there have been several reports that l
-fucosyl oligosaccharides are found in most common human cancers, particularly in adenocarcinomas and neuroblastomas. Several pieces of evidence suggest that fucosyl oligosaccharides are important cell-surface recognition determinants (Hakomori, 1984
; Santer et al.
; Cheng et al.
LTA and UEA-I are two homologous fucolectins from the Leguminosae family. Although they possess the same primary specificity for fucose, they display considerable diversity in their carbohydrate binding to fucosylated oligosaccharides. UEA-I recognizes the H-type 2 determinant [α-l
-GlcNAc] but not the Lea
determinants. However, LTA recognizes Lex
and other different divalent l
-fucosyl carbohydrates (Cheng et al.
). Despite their similarity and apparently conserved sequences (41% amino-acid identity), the above lectins possess different biological activities. Furthermore, a previous study of LTA and other legume lectins with unknown structures have shown that the LTA structure should have a quaternary association type that differs from those of known legume lectins (Brinda et al.
). Thus, structural studies of LTA will help us to understand its binding to carbohydrates, which is directly responsible for its biological activity, and to elucidate its quaternary association.
LTA is a member of the legume family (Leguminosae, Papilionoideae, Loteae) of lectins, which have been widely used to explore the properties of membranes from both normal and transformed cells (Shirahama et al.
; Mansour et al.
). The primary structure of LTA has been previously determined and is a glycoprotein containing 240 amino-acid residues with a molecular weight of 26 273.17 Da. Furthermore, the affinity of LTA for l
-fucosyl oligosaccharides has been investigated by nuclear magnetic resonance and electron microscopy (Cheng et al.
; Haselhorst et al.
). In order to establish the crystal structure of this new member of the fucose-binding lectins, we carried out a crystallization study and preliminary X-ray diffraction analysis of LTA, a fucolectin from Lotus tetragonolobus
seeds, with the aim of solving its native quaternary structure (Pereira & Kabat, 1974a
; Konami et al.