Preferential degradation of the terminal carbohydrate moiety of membrane glycoproteins in rat hepatoma cells and after transfer to the membranes of mouse fibroblasts.

Abstract

Glycoproteins in the plasma membrane of rat hepatoma cells were labeled at their externally exposed tyrosine residues with 131I and at their galactose and sialic acid residues with 3H. The degradation of both isotopes in the total cell protein fraction, in glycoproteins purified by concanavalin A, and in glycoproteins separated on two-dimensional gels was determined. Similarly, the total cellular membrane glycoproteins were metabolically labeled with [35S]methionine and [3H]fucose. The fate of both incorporated labels was followed by lectin chromatography or by precipitation of the proteins with specific antibodies followed by electrophoretic gel separation. In both labeling experiments, the carbohydrate markers were lost from the ligand-recognized fraction with similar kinetics as from the total cell protein fraction. In some glycoprotein species which were separated by two-dimensional gel electrophoresis, the polypeptide portion exhibited up to a twofold slower rate of degradation relative to that of the carbohydrate moiety. This difference is most pronounced in carbohydrate-rich glycoproteins. To corroborate this finding, double-labeled membrane glycoproteins were incorporated into reconstituted phospholipid vesicles which were then transferred via fusion into the plasma membrane of mouse fibroblasts. Both the polypeptide and carbohydrate moieties of the transferred membrane glycoproteins were degraded with the same relative kinetics as in the original hepatoma cells. The rate of degradation is mostly a function of the structural properties of the membrane components as shown by the preservation of metabolically stable fucogangliosides of Reuber H-35 hepatoma cells transferred onto the fibroblasts. The technique of insertion of membrane components into the plasma membrane of another cell should assist in the elucidation of the exact route and mechanism of membrane protein destruction.