Biosynthesis of membrane glycoproteins in rat hepatoma tissue culture cells.

The early steps in the biosynthesis of glycoproteins associated with the plasma membranes of rat hepatoma tissue culture cells has been analyzed. By measuring the effect of tunicamycin on the incorporation of [3H] mannose and [3H] fucose into cell glycoproteins, it was determined that an interval of about 1 h was required to transfer the glycoprotein from site of mannosylation to the site of fucosylation. This result was corroborated by an analysis of the time required for the appearance of either mannose or fucose-labeled glycoproteins at the cell surface. The separation of membrane glycoproteins by a two-dimensional gel system allowed the visualization of the modifications leading to both size and charge heterogeneity of these proteins. By following the changes in electrophoretic mobility introduced into membrane glycoproteins during a chase period after a pulse labeling, the time course of these molecular alterations could be estimated. Several glycoproteins have apparently higher rates of synthesis than the bulk of membrane-associated glycoproteins. Most of these glycoproteins were released within 2 h after biosynthesis from the intracellular membrane fraction and appear after 3 h in the medium. In addition to the glycoproteins that contain both mannose and fucose and that show a high degree of charge heterogeneity, there are other membrane-bound species that are not noticeably modified by the incorporation of fucose or sialic acids. These glycoproteins could represent constituents limited to the internal membrane system of the HTC cell.