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.