Identification of endogenous binding proteins for the lectin discoidin-I in Dictyostelium discoideum

Recent biochemical and genetic evidence has shown that the endogenous lectin discoidin-I is involved in intercellular adhesion during development of the cellular slime mold Dictyostelium discoideum. We have prepared discoidin-I affinity columns and used them to isolate the lectin receptors. By using the cell surface radioiodination method, 11 discoidin-I binding proteins were identified in wild-type NC4 cells by gel electrophoresis, with apparent molecular weights of 95,000, 85,000, 78,000, 72,000, 60,000, 33,000, 31,000, 28,000, 25,000, 21,000, and 15,000. Only three (gp33, gp31, and gp28) were under developmental regulation. The amount of gp31 increased 8- to 10-fold during aggregation, and it was the predominant discoidin-I binding protein synthesized at the aggregation stage. Discoidin-I binding proteins derived from aggregation stage cells were potent inhibitors of discoidin-I in a hemagglutination assay in vitro. The same preparation was found to promote aggregation of cells bearing discoidin-I on the surface, suggesting a multivalent interaction.