Enzymatic conversion of proteins to glycoproteins (lipid-linked saccharides/protein unfolding/oligosaccharide-lipid/Asn-X-Ser or Asn-X-Thr tripeptide/glycosyl transferase)

The enzymatic transfer of the oligosaccharide moiety from an oligosaccharide-lipid to denatured forms of three secretory proteins-ovalbumin, a-lactalbumin, and ri- bonuclease A-has been demonstrated utilizing a membrane fraction from hen oviduct. Based on a survey of 10 proteins de- natured by sulfitolysis, the presence of the tripeptide sequence -Asn-X_Ser- (X represents a variable amino acid) appears to be necessary but not sufficient for the protein to serve as acceptor in vitro. The results of this investigation also suggest that un- folding of the polypeptide chain is required in order to expose sites for carbohydrate attachment. Studies in this laboratory have established that membranes prepared from hen oviduct catalyze synthesis of an oligosac- charide-lipid, and transfer of its oligosaccharide moiety to en- dogenous, membrane-bound protein acceptors (1-3). The participation of similar lipid-linked saccharides in the synthesis