α‐Glucan synthesis on a protein primer, uridine diphosphoglucose: protein transglucosylase I

It was found that the DEAE‐cellulose‐treated UDP‐Glc:protein transglucosylase I catalyzing the first step (reaction 1) in the formation of α‐glucan bound to protein in potato tuber is not only specific for the glucosyl donor but also for the endogenous acceptor. A single radioactive 38‐kDa macromolecular component appeared during denaturing polyacrylamide gel electrophoresis of reaction 1 product. The labeled component is probably the polypeptide subunit of the endogenous acceptor which is being glucosylated. The radioactivity incorporated in reaction 1 product was isolated from a protease digest as a low‐molecular‐mass glucopeptide fraction. A β‐elimination reaction carried out in the presence of a reducing agent demonstrated that only one glucosyl moiety is transferred from UDP‐Glc to the aminoacyl residue, thus forming an O ‐glucosidic linkage. 3 H‐labeled sodium borohydride showed that serine and threonine are involved in the peptide bond to glucose. Ion‐exchange chromatography on DEAE‐cellulose, affinity chromatography on concanavalin‐A—Sepharose, gel filtration on Sephacryl S‐300 and sucrose density gradient centrifugation failed to separate the enzyme catalyzing reaction 1 from the endogenous acceptor.