Michael A. Gitt

Strikingly Different Localization of Galectin-3 and Galectin-4 in Human Colon Adenocarcinoma T84 Cells GALECTIN-4 IS LOCALIZED AT SITES OF CELL ADHESION

Two β-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.

Multiple soluble vertebrate galactoside-binding lectins

All vertebrates synthesize soluble galactoside-binding lectins. Many are expressed at high levels in the embryo and at lower levels in the adult, whereas others show an inverse pattern of expression. Most lectins tend to be concentrated in one or a number of specific cell types. In the past few years, the multiplicity of these lectins has become more apparent. For example, in Xenopus laevis 3 galactoside-binding lectins, 2 with a preference for alpha-galactosides, have been purified and partially characterized. They have subunit molecular weights ranging from 16,000 to 69,000. More detailed studies have been done in mammals. For example, rat lung contains 3 soluble beta-galactoside-binding lectins, RL-14.5, RL-18 and RL-29, with subunit molecular weights, respectively, of 14,500, 18,000 and 29,000. A notable feature of these lectins is that, although they all bind lactose about equally well, their carbohydrate-binding sites are actually quite different, as shown by competitive binding studies with a range of complex mammalian glycoconjugates. Human lung also contains several beta-galactoside-binding lectins, including HL-14, HL-22 and HL-29 with subunit molecular weights, respectively, of 14,000, 22,000 and 29,000. They too show significant differences in their carbohydrate-binding sites when analyzed with naturally occurring mammalian glycoconjugates. Sequencing of purified lectins and cDNA clones indicates that at least 4 distinct genes code for what appears to be a family of HL-14. Heterogeneity is also indicated from isoelectric focusing studies which resolve at least 6 acidic forms of HL-14 and 5 acidic forms of HL-29.(ABSTRACT TRUNCATED AT 250 WORDS)