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

Background: Heltuba, a tuber lectin from the Jerusalem artichoke Helianthus tuberosus, belongs to the mannose-binding subgroup of the family of jacali n-related plant lectins, Heltuba is highly specific for the disaccharides M an alpha.1-3Man or Man alpha 1-2Man, two carbohydrates that are particularl y abundant in the glycoconjugates exposed on the surface of viruses, bacter ia and fungi, and on the epithelial cells along the gastrointestinal tract of lower animals. Heltuba is therefore a good candidate as a defense protei n against plant pathogens or predators. Results: The 2.0 Angstrom resolution structure of Heltuba exhibits a threef old symmetric beta-prism fold made up of three four-stranded beta sheets. T he crystal structures of Heltuba in complex with Man alpha 1-3Man and Man a lpha 1-2Man, solved at 2.35 Angstrom and 2.45 Angstrom resolution respectiv ely, 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 possibl e cross-linking interactions with branched oligomannosides. Conclusions: The structure of Heltuba, which is the prototype for an extend ed family of mannose-binding agglutinins, shares the carbohydrate-binding s ite and beta-prism topology of its galactose-binding counterparts jacalin a nd Maclura pomifera lectin, However, the beta-prism elements recruited to f orm 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 adap tability of the beta-prism building block in the evolution of plant protein s.