A Radioisotope-free Oligosaccharyltransferase Assay Method

Glycosylation of asparagine residues is widespread in Eukarya, and occurs in virtually all Archaea and some eubacterial species. A membrane-bound enzyme, oligosaccharyltransferase, catalyzes the transfer of an oligosaccharide chain from a sugar donor (lipid-linked oligosaccharide, LLO) to an asparagine residue in the consensus sequence, Asn-X-Ser/Thr (X ≠ Pro), in proteins. The in vitro oligosaccharyl transfer assay reaction mixture contains a detergent-solubilized oligosaccharyltransferase (OST), a sugar donor LLO, and a sugar acceptor peptide. Previous assay methods are problematic, in terms of the use of radioactive compounds and the cumbersome separation procedures using lectin binding or two-phase partitioning. Here, we describe a new oligosaccharyl transfer assay method, which is radioisotope-free and relies on a different separation mechanism. The glycopeptide products are separated from unreacted peptides by SDS-PAGE. A fluorescent dye is attached to the peptide substrate during custom peptide synthesis. The fluorescent imaging of the SDS-PAGE gels ensures high sensitivity and quantitative performance. The user-friendly PAGE format is particularly suitable for presentation in scientific papers. For illustrative applications, time-course and peptide library experiments are shown.