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We have established the amino acid sequence of the fi-D-galactoside binding lectin from the electric eel and the sequences of several peptides from a similar lectin isolated from human placenta. These sequences were compared with the published sequences of peptides derived from the (8-Dgalactoside binding lectin from human lung and with sequences deduced from cDNAs assigned to the P-D-galactoside binding lectins from chicken embryo skin and human hepatomas. Significant homologies were observed. One of the highly conserved regions that contains a tryptophan residue and two glutamic acid residues is probably part of the f8-D-galactoside binding site, which, on the basis of spectroscopic studies of the electric eel lectin, is expected to contain such residues. The similarity of the hydropathy profiles and the predicted secondary structure of the lectins from chicken skin and electric eel, in spite of differences in their amino acid sequences, strongly suggests that these proteins have maintained structural homologies during evolution and together with the other .3-Dgalactoside binding lectins were derived from a common ancestor gene.Embryonic and differentiating tissues of various vertebrates from teleosts (1, 2) to amphibians (3), birds (4, 5), and mammals (6-9) contain a B-D-galactoside binding protein (lectin) that agglutinates trypsinized rabbit erythrocytes and is specifically inhibited by p-D-galactopyranosyl -D-thiogalactopyranoside and lactose. Several of these lectins have been isolated and characterized and found to share common properties such as subunit molecular weight, saccharide specificity, and the requirement for the presence of reducing agents during the purification procedure (1, 10). In some cases, they possess similar amino acid compositions (4, 10). The fact that several /3-D-galactoside binding lectins from phylogenetically distinct species cross react immunologically further indicates that these proteins possess common structural determinants that have been maintained during evolution (4,(12)(13)(14).To determine the extent of the structural homology between various ,-D-galactoside binding lectins, we have undertaken to establish the amino acid sequences of P-Dgalactoside binding lectins from the electric eel (15) and human placenta (9). We compare here the established amino acid sequences of these proteins to the sequences deduced from the cDNAs encoding the j3-D-galactoside binding lectins from embryonic chicken skin (16) and from human hepatomas (17) and to the partial amino acid sequence of the human lung lectin (17).The homologies observed suggest that these proteins are derived from a common ancestor gene and have maintained through evolution part of the structure of the ,3-D-galactoside binding site putatively assigned to residues 70-76. MATERIALS AND METHODSTissues. Live electric eels, Electrophorus electricus, were obtained from Worldwide Paramount Aquarium (Ardsley, NY). They were decapitated, and the main electric organ was cut in small cubes and frozen at -20'C. Fresh hum...

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