A study of intracellular transport of secretory glycoproteins in rat liver.

To study the transport of secretory glycoproteins in the endoplasmic reticulum of rat liver, the distribution of nascent glycoproteins in the membrane and luminal fraction of rough and smooth microsomes has been examined after a short-time incorporation of radioactive glucosamine in vivo. 50--60% of the radioactivity was associated with the membranes of rough and smooth microsomes, whereas about 10% of the serum albumin was found in the same fractions. The relative amount of radioactivity in the membranes was the same whether the luminal content of the microsomal vesicles was released by sonication, French press, Triton X-100, Brij 35 or sodium deoxycholate. The distribution of labeled glycoproteins between the membrane and luminal fraction of rough and smooth microsomes did not change during the time interval of 15--120 min after administration of the isotope. The similarity of the labeling patterns obtained after sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis indicated that the same set of glycoproteins were located in the lumen and the membrane of rough and smooth microsomes. A specific precipitation of nascent glycoproteins from both the membrane and luminal fractions of rough and smooth microsomes were obtained with rabbit antiserum against rat serum. The nascent glycoproteins associated with the membranes were not released by high ionic strength or treatment with mercaptoethanol. A slow exchange between [14C]glucosamine-labeled glycoproteins in the lumen and membrane fraction was, however, found.