Physarum polycephalum hemagglutinin I is a 104-residue protein that is secreted to extracellular space. The crystal structure of hemagglutinin I has a β-sandwich fold found among lectin structures, such as legume lectins and galectins. Interestingly, the β-sandwich of hemagglutinin I lacks a jelly roll motif and is essentially composed of two simple up-and-down β-sheets. This up-and-down β-sheet motif is well conserved in other lectins derived from animals, plants, bacteria, and viruses. It is more noteworthy that the up-and-down β-sheet motif includes many residues that make contact with the target carbohydrates. Our NMR data demonstrate that hemagglutinin I lacking a jelly roll motif also binds to its target glycopeptide. Taken together, the up-and-down β-sheet motif provides a fundamental scaffold for the binding of legume lectin-like proteins to the target carbohydrates, and the structure of hemagglutinin I suggests a minimal carbohydrate recognition domain
