THE PRIMARY STRUCTURE AND CARBOHYDRATE SPECIFICITY OF A BETA-GALACTOSYL-BINDING LECTIN FROM TOAD (BUFO-ARENARUM HENSEL) OVARY REVEAL CLOSERSIMILARITIES TO THE MAMMALIAN GALECTIN-1 THAN TO THE GALECTIN FROM THE CLAWED FROG XENOPUS-LAEVIS

The detailed characterization of a galectin from the toad (Bufo arenar um Hensel) ovary in its primary structure, carbohydrate specificity, a nd overall biochemical properties has provided novel information perta ining to structural and evolutionary aspects of the galectin family. T he lectin consists of identical single-chain polypeptide subunits comp osed of 134 amino acids (calculated mass, 14,797 daltons), and its N-t erminal residue, alanine, is N-acetylated. When compared to the sequen ces of known galectins, the B. arenarum galectin exhibited the highest identity (48% for the whole molecule and 77% for the carbohydrate rec ognition domain (CRD)) with the bovine spleen galectin-1, but surprisi ngly less identity (38% for the whole molecule and 47% for the CRD) wi th a galectin from Xenopus laevis skin (Marschal, P., Herrmann, J., Le ffler, H., Barondes, S. H., and Cooper, D. N. W. (1992) J. Biol. Chem. 267, 12942-12949). Unlike the X. laevis galectin, the binding activit y of the B. arenarum galectin for N-acetyllactosamine, the human blood group A tetrasaccharide and Gal beta 1,3GalNAc relative to lactose, w as in agreement with that observed for the galectin-1 subgroup and tho se galectins having ''conserved'' (type I) CRDs (Ahmed, H., and Vasta, G. R. (1994) Glycobiology 4, 545-549). Moreover, the toad galectin sh ares three of the six cysteine residues that are conserved in all mamm alian galectins-1, but not in the galectins from X. laevis, fish, and invertebrates described so far. Based on the homologies of the B. aren arum galectin with the bovine spleen galectin-1 and X. laevis skin gal ectin, it should be concluded that within the galectin family the corr elation between conservation of primary structure and phylogenetic dis tances among the source species may not be a direct one as proposed el sewhere (Hirabayashi, J., and Kasai, K. (1993) Glycobiology 3, 297-304 ). Furthermore, galectins with conserved (type I) CRDs, represented by the B. arenarum ovary galectin, and those with ''variable'' (type II) CRDs, represented by the X. laevis 16-kDa galectin, clearly constitut e distinct subgroups in the extant amphibian taxa and may have diverge d early in the evolution of chordate lineages.