Different oligosaccharide processing of the membrane‐integrated and the secretory form of gp 80 in rat liver.

Rat liver synthesizes a glycoprotein with Mr of 80.000 (gp 80) which is partly inserted into the plasma membrane and partly secreted into the serum. The membrane‐integrated and the secretory form of this glycoprotein have an identical peptide pattern, but different N‐linked glycans. Whereas gp 80 from the serum is glycosylated with complex‐type oligosaccharides, gp 80 from the plasma membrane has high mannose glycans. Phase separation with Triton X‐114 showed that membrane‐integrated gp 80 contains hydrophobic portions, whereas secretory gp 80 has hydrophilic properties. Intracellular transport and oligosaccharide processing of gp 80 were studied in vivo in the endoplasmic reticulum, the Golgi apparatus and plasma membranes of rat liver and in serum using pulse‐chase labeling with L‐[35S]methionine and immunoprecipitation. Peak labeling of gp 80 was reached in the endoplasmic reticulum 10 min after the pulse, in the Golgi apparatus 20 min later, and in the plasma membrane after 2 h; in the serum the specific radioactivity was steadily increasing during the experiment. Gp 80 of the endoplasmic reticulum was completely sensitive to endo‐beta‐N‐glucosaminidase H (endo H), but simultaneously occurred in the Golgi apparatus in an endo H‐sensitive and endo H‐resistant form. The endo H‐sensitive form was transported to the plasma membrane, the endo H‐resistant species secreted into the serum. Conversion from the endo H‐sensitive to the endo H‐resistant form was completed within 10 min after transfer of gp 80 to the Golgi apparatus.(ABSTRACT TRUNCATED AT 250 WORDS)