Glycosylation of serine residues by a uridine diphosphate-xylose: Protein xylosyltransferase from mouse mastocytoma

Abstract

A preparation from an ascitic mast cell tumor catalyzes the transfer of xylose from UDP-xylose- 14C to serine residues of an endogenous protein acceptor. The formation of xylosyl-serine linkages, such as occur in native heparin, is based on the following evidence. Though not esterified, the radioactivity could readily be released by mild alkali; xylitol- 14C was the only radioactive product obtained by treatment with 0.5 n NaOH in the presence of NaBH 4. Pronase digestion of the transfer product released a radioactive glycosyl-amino acid identified as 14C-xylosyl-serine by coelectrophoresis with authentic material by co-chromatography on paper and on Dowex-50, and by co-chromatography of the DNP derivatives. The transfer takes place in the absence of protein synthesis, suggesting that glycosylation of proteins begins only after assembly of the polypeptide. The mastocytoma contains a second xylosyltransferase, which links xylose to an unidentified acceptor, to give a relatively alkali-stable xyloside.