Galectins are members of a genetically related family of ␤-galactoside-binding lectins. At least eight distinct mammalian galectins have been identified. More distantly related, but still conserving amino acid residues critical for carbohydrate-binding, are galectins in chicken, eel, frog, nematode, and sponge. Here we report that galectins are also expressed in a species of fungus, the inky cap mushroom, Coprinus cinereus. Two dimeric galectins are expressed during fruiting body formation which are 83% identical to each other in amino acid sequence and conserve all key residues shared by members of the galectin family. Unlike most galectins, these have no N-terminal post-translational modification and no cysteine residues. We expressed one of these as a recombinant protein and studied its carbohydrate-binding specificity using a novel nonradioactive assay. Binding specificity has been well studied for a number of other galectins, and like many of these, the recombinant C. cinereus galectin shows particular affinity for blood group A structures. These results demonstrate not only that the galectin gene family is evolutionarily much older than previously realized but also that fine specificity for complex saccharide structures has been conserved. Such conservation implies that galectins evolved to perform very basic cellular functions, presumably by interaction with glycoconjugates bearing complex lactoside carbohydrates resembling blood group A.Galectins are animal lectins that are related in amino acid sequence and specifically bind to ␤-galactoside carbohydrates such as lactose (1). Members of this gene family all include a conserved carbohydrate-binding domain but vary in inclusion of other domains and in tissue expression patterns. More distantly related, but conserving critical amino acid residues involved in carbohydrate-binding, are galectins in chicken, eel, frog, nematode, and sponge (2).Although galectins have been studied for 20 years now, physiological functions for these proteins have not yet been clearly established. Their affinity for oligosaccharides found on glycoconjugates on cell surfaces or in extracellular matrix has suggested that galectins function extracellularly by binding to such ligands. Indeed, certain galectins have particular affinity for specific glycoprotein ligands, such as polylactosamine chains on laminin (1, 3). When added to cells or overexpressed after transfection, galectins can have major effects on cell adhesion, proliferation, apoptosis, metastasis, and immune function (1-6). However, evidence has also been presented for intracellular functions of galectins, for instance in message splicing (7) or as nuclear proteins (8). Therefore, efforts are being directed at exploring the evolutionary origin of galectins in the hope that their functions will be easier to define in simple model organisms.Here we report that a species of fungus, the inky cap mushroom, Coprinus cinereus, expresses two lectins related in sequence and carbohydrate-binding specificity to other galectins. T...

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