Kinetics of glycosylation and intracellular transport of sialoglycoproteins in mouse liver.

We have obtained evidence in vivo for the intracellular site of sialylation of glycoproteins and have determined the kinetics of intramembranous transport of newly synthesized sialoglycoproteins. Radioactivity of sialoglycoproteins was determined at different times in homogenate, smooth and rough endoplasmic reticulum, Golgi apparatus, and plasma membrane fractions of liver either from slices which had been incubated with radioactive N-acetylneuraminic acid or from mice which had been injected with radioactive sugar. At short times after labeling, the Golgi apparatus contained the highest specific activity of radioactive sialoglycoproteins and over half of the total cellular radioactive sialoglycoproteins. By 15 to 25 min, most of the radioactivity in the Golgi apparatus had decreased with a concomitant increase of radioactivity in the plasma membrane. Radioactivity in the smooth and rough endoplasmic reticulum accounted for less than 20% of the total cellular radioactivity throughout. Control experiments indicated that leakage of soluble, labeled sialoglycoproteins from membrane vesicles and adsorption of them onto vesicles was minimal. In contrast to the above studies, labeling for short times with radioactive mannose, showed, as expected from studies in vitro, most of the radioactivity in the rough endoplasmic reticulum. At longer times, the radioactivity in the rough endoplasmic reticulum decreased, with an increase in the plasma membrane. Throughout, the radioactivity in the smooth endoplasmic reticulum was low.