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

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 alpha 1-3Man or Man alpha 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.

The 2.0 A resolution structure of Heltuba exhibits a threefold symmetric beta-prism fold made up of three four-stranded beta sheets. The crystal structures of Heltuba in complex with Man alpha 1-3Man and Man alpha 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 alpha 1-3 or alpha 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.

The structure of Heltuba, which is the prototype for an extended family of mannose-binding agglutinins, shares the carbohydrate-binding site and beta-prism topology of its galactose-binding counterparts jacalin and Maclura pomifera lectin. However, the beta-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 beta-prism building block in the evolution of plant proteins.