Glycosylation's active site—finally

Beginning on page 715, Nilsson et al. drive home the lesson that persistence pays. Using established techniques on a system that has frustrated similar efforts for years, the authors identified the active site of the mammalian oligosaccharyltransferase (OST) complex. In the endoplasmic reticulum, OST carries out N-linked glycosylation, one of the most common and least understood eukaryotic protein modifications. Previous efforts to identify the active site of OST using peptide substrates, or even to determine which of the proteins in the OST complex contains the active site, have produced conflicting results.

The authors designed nascent polypeptide chains with cryptic glycosylation sites incorporating photoreactive probes. As the cryptic site translocates through the endoplasmic reticulum membrane, it can be cross-linked first to components of the translocon pore, and then to the OST. Strikingly, only one OST...