Identification of glycoproteins, using lectins as probes, in plasma membranes from Dictyostelium discoideum and human erythrocytes.

The glycoproteins of plasma membranes from axenically grown Dictyostelium discoideum and human red blood cells (O+) were characterized according to their apparent molecular weights in sodium dodecyl sulfate-polyacrylamide gels and their ability to bind lectins. This was achieved by diffusing each of several fluorescein-conjugated lectins into sodium dodecyl sulfate-polyacrylamide gels which contained the purified plasma membranes. Subsequent to identifying fluorescent bands the gels were stained with Coomassie blue to relate the lectin receptors to known proteins. In D. discoideum plasma membranes over 25 macromolecules, ranging in apparent molecular weight from 8,000 to 95,000, were identified with fluorescent wheat germ agglutinin. The binding of wheat germ agglutinin was specific and reflected the presence of receptors containing N-acetylD-glucosamine. Some of these receptors are distinct from those which bind concanavalin A (West, C. M. & McMahon, D. (1977) J. Cell Biol. 74, 264-273). The concanavalin A receptors and all but one of the wheat germ agglutinin receptors were shown to be glycoproteins since they were hydrolyzed by proteolytic treatment. In contrast, Ricinus communis agglutinins 60 and 120, soybean agglutinin and Ulex europeus agglutinin I, and lectins with specificities directed toward the other common simple sugars of the plasma membrane failed to label any glycoproteins in the sodium dodecyl sulfate-polyacrylamide gel. The absence of glycoproteins recognized by the R. communis agglutinins on the plasma membrane was confirmed by conjugating a mixture of the two lectins labeled with fluorescein isothiocyanate and microspheres, and assaying for binding to cells in a fluorescent or scanning electron microscope. Consequently, D. discoideum plasma membrane glycoproteins seem to be restricted to classes which bind concanavalin A and/or wheat germ agglutinin. This pattern persisted during