Helianthus tuberosus lectin reveals a widespread scaffold for mannose-binding lectins

Abstract Background: Heltuba, a tuber lectin from the Jerusalem artichoke Helianthus tuberosus , belongs to the mannose-binding subgroup of the family of jacalin-related plant lectins. Heltuba is highly specific for the disaccharides Man α 1–3Man or Man α 1–2Man, two carbohydrates that are particularly abundant in the glycoconjugates exposed on the surface of viruses, bacteria and fungi, and on the epithelial cells along the gastrointestinal tract of lower animals. Heltuba is therefore a good candidate as a defense protein against plant pathogens or predators. Results: The 2.0 A resolution structure of Heltuba exhibits a threefold symmetric β -prism fold made up of three four-stranded β sheets. The crystal structures of Heltuba in complex with Man α 1–3Man and Man α 1–2Man, solved at 2.35 A and 2.45 A resolution respectively, reveal the carbohydrate-binding site and the residues required for the specificity towards α 1–3 or α 1–2 mannose linkages. In addition, the crystal packing reveals a remarkable, donut-shaped, octahedral assembly of subunits with the mannose moieties at the periphery, suggesting possible cross-linking interactions with branched oligomannosides. Conclusions: The structure of Heltuba, which is the prototype for an extended family of mannose-binding agglutinins, shares the carbohydrate-binding site and β -prism topology of its galactose-binding counterparts jacalin and Maclura pomifera lectin. However, the β -prism elements recruited to form the octameric interface of Heltuba, and the strategy used to forge the mannose-binding site, are unique and markedly dissimilar to those described for jacalin. The present structure highlights a hitherto unrecognized adaptability of the β -prism building block in the evolution of plant proteins.