title: Novel glycosylation pathways of retroviral envelope proteins identified with avian reticuloendotheliosis virus.
abstract: Previously, we identified two mature glycoproteins, gp90, the surface glycoprotein, and gp20, the transmembrane protein, from avian reticuloendotheliosis virus and an avian reticuloendotheliosis virus env gene-encoded intracellular polyprotein gPr77env, but the precise relationship of gPr77env to the mature envelope proteins was not determined (W.-P. Tsai, T.D. Copeland, and S. Oroszlan, Virology 155:567-583, 1986). In the present study, using metabolic labeling of viral proteins with [35S]cysteine, radioimmunoprecipitation, and carbohydrate structure analysis, we have identified a higher-molecular-weight endo-H-resistant env gene-encoded polyprotein designated gPr115env in addition to the endo-H-sensitive gPr77env. It appears that gPr77env is the primary polyprotein precursor, modified with mannosyloligosaccharides that are processed into sialic-acid-rich extraordinarily large complex-type carbohydrates (up to 17 kilodaltons for each N-linked site) on the gp90 domain but not on the gPr22 domain. In this process, gPr77env is converted into the apparently endo-H-resistant secondary polyprotein, gPr115env, which is rapidly processed into gp90 and gPr22. The proteolytic processing which occurs only after the appearance of an endo-H resistant precursor is now clearly demonstrated for a retrovirus. Some important aspects of carbohydrate structure, including the site-specific glycosylation, as well as the intracellular location and nature of the potential enzyme involved in the proteolytic cleavage of gPr115env are discussed.
authors.affiliation: Laboratory of Molecular Virology and Carcinogenesis, NCI-Frederick Cancer Research Facility, Maryland 21701.
authors.forename: W P
authors.initials: WP
authors.lastname: Tsai
authors.forename: S
authors.initials: S
authors.lastname: Oroszlan
mesh_terms: D000818:Animals; D001345:Autoradiography; D002241:Carbohydrates / Q000032:analysis; D002460:Cell Line; D004591:Electrophoresis, Polyacrylamide Gel; D006023:Glycoproteins / Q000032:analysis; D006026:Glycoside Hydrolases / Q000378:metabolism; D006031:Glycosylation; D007118:Immunoassay; D011498:Protein Precursors / Q000032:analysis; D001356:Reticuloendotheliosis virus / Q000378:metabolism*; D012190:Retroviridae / Q000378:metabolism*; D012191:Retroviridae Proteins / Q000378:metabolism*; D014759:Viral Envelope Proteins / Q000378:metabolism*
chemical_list: D002241:Carbohydrates; D006023:Glycoproteins; D011498:Protein Precursors; D012191:Retroviridae Proteins; D014759:Viral Envelope Proteins; D006026:Glycoside Hydrolases