Lectin activity with specificity for mannose and glucose has been detected in the seed of Platypodium elegans, a legume from the Dalbergiae tribe. The gene of the lectin PELa has been cloned and the resulting 261 amino acid protein belongs to the legume lectin family with similarity with Pterocarpus angolensis agglutinin (PAL) from the same tribe. The recombinant lectin has been expressed in E. coli and refolded from inclusion bodies. Analysis of specificity by Glycan Array evidenced a very unusual preference for complex type N-glycans with asymmetrical branches. A short branch consisting of one mannose residue is preferred on the 6- arm of the N-glycan, while extension by GlcNAc, Gal and NeuAc are favorable on the 3-arm. Affinities have been obtained by microcalorimetry using symmetrical and asymmetrical Asn- linked heptasaccharide prepared by semi-enzymatic method. Strong affinity of 5 µM was obtained for both ligands. Crystal structures of PELa complexed with branched trimannose and symmetrical complex type Asn-linked heptasaccharide have been solved at 2.1 and 1.65 Å resolution respectively. The lectin adopts the canonical dimeric organization of legume lectins. The trimannose bridges the binding sites of two neighbouring dimers, resulting in the formation of infinite chains in the crystal. The Asn-linked heptasaccharide binds with the 6-arm in the primary binding site and extensive additional contacts on both arms. The GlcNAc on the 3-arm is bound in a constrained conformation that may rationalize the higher affinity that is observed on chips for oligosaccharide with shorter 3-arm that do not present this monosaccharide.
