Biogenesis of plasma membrane glycoproteins in hepatoma tissue culture cells.

Total membrane glycoproteins were labeled by growing hepatoma tissue culture (HTC) cells in the presence of [Wlfucose, and externally disposed plasma membrane glycoproteins and proteins on the same cells were labeled by reduction of oxidized galactose residues with tritiated sodium borohydride or by lactoperoxidase-catalyzed iodination. The latter labeling methods show that the plasma membrane contains a set of fucose-labeled glycoproteins that is externally disposed. However, the fucose jncorporated into membrane glycoproteins does not fractionate solely with the plasma membrane. Rather, a significant amount of the fucose-labeled glycoprotein fractionates with an internal membrane system(s) of this cell. The fucosecontaining glycoproteins in the different cell fractions were compared by electrophoresis in dodecyl sulfate-polyacrylamide gels; both the internal membrane system and the plasma membrane contain a similar, and probably identical, set of fucose-labeled glycoproteins as resolved by this method. Furthermore, many of these proteins are present in the same relative proportion to each other in both the plasma membrane and the internal membrane system(s) of HTC cells. The glycoproteins are released from both membrane fractions by the same concentration of deoxycholate, a concentration higher than that needed to release microsomal contents. Autoradiographic analysis of thin sections of HTC cells grown in the presence of [3H]fucose confirm the presence of fucose-containing glycoproteins in an internal membrane compartment. Pulse-chase experiments with fucose as precursor show that part of the membrane-bound glycoprotein in the internal pool(s) is transient and transferred to the plasma membrane within 3 h after fucosylation, while another part remains associated with the internal membrane system for a considerable length of time. Based both on these results and our previous studies on the turnover of HTC cell membrane proteins, it is proposed