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 polyacrytamide get electrophoresis of reaction 1 product. The labeled componem is probably the polypeptide subunit of the endogenous acceptor which is being glucosytated. 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 ammoacyl residue, thus forming an O-glucosidic linkage. ³H-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. [ABSTRACT FROM AUTHOR]