Science doi:10.1126/science.aar5140 (2018)

Oligosaccharyltransferase (OST) catalyzes the key step during N-linked glycosylation—the transfer of the precursor glycan from its lipid carrier to the Asn side chains of substrate proteins. Whereas eukaryotes contain a multisubunit OST localized to the endoplasmic reticulum, which recognizes the Asn-X-Ser/Thr motif in secretory proteins, bacteria encode a plasma-membrane-bound single-subunit OST (ssOST) that is specific for substrates with an Asp-X-Asn-X-Ser/Thr sequon. Wild et al. isolated intact OST from yeast and determined the 3.3-Å cryo-EM structure of the nanodisc-reconstituted complex. The structure revealed that the active site of the OST catalytic subunit STT3 is surface exposed, allowing unhindered substrate access. In addition, differences in the shape and charge distribution between the catalytic sites of STT3 and bacterial ssOST explain the broader substrate specificity of eukaryotic OST. The other seven subunits of the yeast OST complex have scaffolding functions, and their luminal domains may serve as docking platforms for interaction partners and promote substrate binding. The authors also fitted their structure into an earlier published lower-resolution cryo-tomography map of the mammalian ribosome–translocon–OST supercomplex, showing how the nascent polypeptide chain emerging from the luminal exit of the translocon can access the OST active site. The first high-resolution eukaryotic OST structure provides mechanistic insights into N-linked glycosylation and will promote further structural and functional studies.