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Galectins are a family of carbohydrate-binding proteins that share a conserved sequence and affinity for ␤-galactosides. Some, such as galectin-1, are isolated as dimers and have a single carbohydrate recognition domain (CRD) in each monomer, whereas others, such as galectin-4, are isolated as monomers and have two CRDs in a single polypeptide chain. In the course of studying mouse colon mRNA for galectin-4, we detected a related mRNA that encodes a new galectin that also has two CRDs in a single peptide chain. The new galectin, galectin-6, lacks a 24-amino acid stretch in the link region between the two CRDs that is present in galectin-4. Otherwise, these two galectins have 83% amino acid identity. Expression of both galectin-4 and galectin-6 is confined to the epithelial cells of the embryonic and adult gastrointestinal tract. Galectin-4 is expressed at about equal levels in colon and small intestine but much less in stomach, whereas galectin-6 is expressed at about equal levels throughout the gastrointestinal tract.

Two ␤-galactoside-binding proteins were found to be prominently expressed in the human colon adenocarcinoma T84 cell line. Cloning and sequencing of one, a 36-kDa protein, identified it as the human homolog of galectin-4, a protein containing two carbohydrate binding domains and previously found only in the epithelial cells of the rat and porcine alimentary tract. The other, a 29-kDa protein, is galectin-3, containing a single carbohydrate binding domain, previously found in a number of different cell types including human intestinal epithelium. Despite the marked similarities in the carbohydrate binding domains of these two galectins, their cellular distribution patterns are strikingly different and vary with cellular conditions. In confluent T84 cells, galectin-4 is mostly cytosolic and concentrated at the basal membrane, whereas galectin-3 tends to be concentrated in large granular inclusions mostly at the apical membrane. In subconfluent T84 cells, each galectin is distributed to specific domains of lamellipodia, with galectin-4 concentrated in the leading edge and galectin-3 more proximally. Such different localization of galectins-4 and -3 within T84 cells implies different targeting mechanisms, ligands, and functions. The localization of galectin-4 suggests a role in cell adhesion which is also supported by the ability of immobilized recombinant galectin-4 to stimulate adhesion of T84 cells.

A monomeric rat beta-galactoside-binding lectin previously purified from extracts of rat lung has been localized to erythrocytes, and the cDNA encoding it has been isolated from a rat reticulocyte cDNA library. The deduced amino acid sequence of the cDNA predicts a protein with a M(r) of 16,199, with no evidence of a signal peptide. The deduced sequence is identical to the sequences of seven proteolytic peptides derived from the purified lectin. Peptide analysis by mass spectrometry indicates that the N-terminal methionine is cleaved and that serine 2 is acetylated. The lectin shares all the strictly conserved amino acid residues of other members of the mammalian galectin family and is designated galectin-5 (GenBank accession number L36862). Galectin-5 is a weak agglutinin of rat erythrocytes, despite its monomeric structure. The gene encoding galectin-5 (LGALS5) has been mapped in mouse to chromosome 11, approximately 50 centimorgans from the centromere and 1.8 +/- 1.8 centimorgans from the polymorphic marker D11Mit34n, a region syntenic with human chromosome 17q11.

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Two cDNA clones were isolated by immunoscreening a human hepatoma cDNA library with an antiserum that bound specifically to a human soluble (-galactoside-binding lectin with Mr of t14,000. The deduced amino acid sequences of the inserts of these two clones show considerable homology with each other, the sequence of chicken skin P-galactoside-binding lectin, and eight peptides derived from purified human lung lectin of Mr =14,000. However, the sequence differences between the two hepatoma clones as well as among each clone and the lung peptides suggest that at least three variants of the gene encoding this lectin are expressed in human tissue.