Glucocorticoid hormones regulate the post-translational maturation and sorting of cell surface and extracellular mouse mammary tumor virus (MMTV) glycoproteins in M1.54 cells, a stably infected rat hepatoma cell line. Exposure to monensin significantly reduced the proteolytic maturation and externalization of viral glycoproteins resulting in a stable cellular accumulation of a single 70,000-Mr glycosylated polyprotein (designated gp70). Cell surface- and intracellular-specific immunoprecipitations of monensin-treated cells revealed that gp70 can be localized to the cell surface only in the presence of 1 microM dexamethasone, while in uninduced cells gp70 is irreversibly sequestered in an intracellular compartment. Analysis of oligosaccharide processing kinetics demonstrated that gp70 acquired resistance to endoglycosidase H with a half-time of 65 min in the presence or absence of hormone. In contrast, gp70 was inefficiently galactosylated after a 60-min lag in uninduced cells while rapidly acquiring this carbohydrate modification in the presence of dexamethasone. Furthermore, in the absence or presence of monensin, MMTV glycoproteins failed to be galactosylated in hormone-induced CR4 cells, a complement-selected sorting variant defective in the glucocorticoid-regulated compartmentalization of viral glycoproteins to the cell surface. Since dexamethasone had no apparent global effects on organelle morphology or production of total cell surface-galactosylated species, we conclude that glucocorticoids induce the localization of cell surface MMTV glycoproteins by regulating a highly selective step within the Golgi apparatus after the acquisition of endoglycosidase H-resistant oligosaccharide side chains but before or at the site of galactose attachment.

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