Acquisition of the functional properties of the transferrin receptor during its biosynthesis

The properties of the newly synthesized and partially glycosylated forms of the transferrin receptor were examined to determine which co- and post-translational modifications are necessary for the acquisition of transferrin binding activity and transport of the receptor to the cell surface. The nascent transferrin receptor containing core-glycosylated asparagine-linked oligosaccharides does not possess complete intersubunit disulfide bonds, sediments predominantly as a monomer in sucrose density gradients, and shows reduced binding to transferrin-agarose. Within 20-30 min after synthesis, the transferrin receptor acquires the ability to bind to a transferrin-linked affinity column. Intersubunit disulfide bond formation occurs slowly throughout the transit of the receptor to the cell surface. These results indicate that core glycosylation of the receptor may be necessary but is not sufficient for the acquisition of the ability of the receptor to bind transferrin and that intersubunit disulfide bond formation is a post-translational event. Inhibition of complex carbohydrate synthesis by either swainsonine (1 micrograms/ml) or deoxynojirimycin (4 mM) does not inhibit the ability of this receptor to form intersubunit disulfide bonds or to be transported to the cell surface. The partially glycosylated receptor, however, does show an approximately 3-fold reduced affinity for transferrin.