Evidence for posttranslational O-glycosylation of fetuin.
Fetuin, a major glycoprotein in the serum of fetal calves that contains three N-linked and three O-linked carbohydrate side chains, was found to be synthesized in the liver with an 18 amino acid signal peptide, Met-X-X-X-X-Leu-Leu-X-Cys-Leu-Ala-X-Leu-X-X-Cys-X-X, and to undergo cotranslational N-glycosylation. In order to examine O-glycosylation, fetuin peptidyl-tRNA was purified from liver and analyzed for O-linked carbohydrate by quantitating the released [3H]GalNAcitol produced after beta-elimination in the presence of NaB3H4. Within the limits of the assay, less than 1.3% of the O-linked chains had been initiated. Additionally, rough microsomes were used to program a cell-free protein synthesis system. A radiolabeled fetuin intermediate was isolated by immunoprecipitation and shown to contain N-linked carbohydrate by binding to concanavalin A and by susceptibility to cleavage by endoglycosidase H. However, this fetuin intermediate was not detectably bound (less than 1%) by GalNAc-specific lectins, which were shown to bind asialoagalactofetuin. These results suggest that O-glycosylation of fetuin is a posttranslational event.
D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D002474:Cell-Free System; D005333:Fetus; D006023:Glycoproteins / Q000235:genetics*; D006027:Glycosides; D008099:Liver / Q000378:metabolism; D014176:Protein Biosynthesis; D011499:Protein Processing, Post-Translational; D012333:RNA, Messenger / Q000235:genetics; D011817:Rabbits; D012156:Reticulocytes / Q000378:metabolism; D000509:alpha-Fetoproteins / Q000235:genetics*
D006023:Glycoproteins; D006027:Glycosides; D012333:RNA, Messenger; D000509:alpha-Fetoproteins