DIFFERENTIAL RECOGNITION BY PROTEINS OF ALPHA-GALACTOSYL RESIDUES ON ENDOTHELIAL-CELL SURFACES

The binding of proteins to cell surface carbohydrates contributes to c ell-cell interactions in development, immunity, and various physiologi c processes. Such interactions are presumably dictated not only by the chemical structure of the carbohydrate but also reflect the propertie s of the protein and the microenvironment in which the protein-carbohy drate interaction occurs. To explore the factors influencing the recog nition of cell surface carbohydrates by proteins, the extent to which three classes of proteins-anti-Gal alpha 1-3Gal IgM found in higher pr imates, Griffonia simplicifolia type I lectin, isolectin B-4 (GS-IB4), and alpha-galactosidase-interact with Gal alpha 1-3Gal on porcine cel l surfaces was tested. Although the Gal alpha 1-3Gal residues expresse d on porcine endothelial cells and recognized by anti-Gal alpha 1-3Gal IgM and GS-IB4 were both sensitive to cleavage by alpha-galactosidase , the sites recognized by GS-IB4 were more sensitive to cleavage than sites recognized by anti-Gal alpha 1-3Gal IgM. Cross-blocking studies an porcine cell surfaces revealed that a significant proportion of ant i-Gal alpha 1-3Gal IgM bound to sites not recognized by GS-IB4; howeve r, anti-Gal alpha 1-3Gal IgM and GS-IB4 recognized the same sites on s olubilized membrane proteins and model compounds. These results sugges t that features of the cell surface such as the three-dimensional arra ngement of Gal alpha 1-3Gal and characteristics of the protein such as size and valency play critical roles in specific interactions on cell surfaces.