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Lectins are carbohydrate-binding proteins present in all organisms. Some cytoplasmic lectins from fruiting bodies of dikaryotic fungi are toxic against a variety of parasites and predators. We have isolated, cloned and expressed a novel, single domain lectin from Macrolepiota procera, designated MpL. Determination of the crystal structure revealed that MpL is a ricin B-like lectin with a β-trefoil fold. Biochemical characterization, site-directed mutagenesis, co-crystallization with carbohydrates, isothermal titration calorimetry and glycan microarray analyses show that MpL forms dimers with the carbohydrate-binding site at the α-repeat, with the highest specificity for terminal N-acetyllactosamine and other β-galactosides. A second putative carbohydrate-binding site with a low affinity for galactose is present at the γ-repeat. In addition, a novel hydrophobic binding site was detected in MpL with specificity for molecules other than carbohydrates. The tissue specific distribution of MpL in the stipe and cap tissue of fruiting bodies and its toxicity towards the nematode Caenorhabditis elegans indicate a function of MpL in protecting fruiting bodies against predators and parasites.Nucleotide sequence data have been deposited in the DDBJ/EMBL/GenBank databases under accession numbers HQ449738 and HQ449739. Structural data have been deposited in the Protein Data Bank under accession codes 4ION, 4IYB, 4IZX and 4J2S.Lectins from higher fungi frequently possess toxic effects against a variety of organisms and are involved in defense against predators and parasites. The newly characterized ricin B-like lectin MpL from Macrolepiota procera is highly toxic against Caenorhabditis elegans. It forms dimers with the carbohydrate binding site at the α-repeat showing the highest specificity for terminal N-acetyllactosamine and other β-galactosides.