Cryo–electron microscopy structures of human oligosaccharyltransferase complexes OST-A and OST-B

A division of labor for glycosylation Glycosylation is a ubiquitous modification of eukaryotic secreted proteins. Asparagine-linked chains of sugars are appended to many substrates as they are translocated into the endoplasmic reticulum. Ramírez et al. solved cryo–electron microscopy structures of two human oligosaccharyltransferase complexes, OST-A and OST-B. The catalytic subunits bind partner proteins that direct glycosylation of specific substrates either cotranslationally (OST-A) or on fully folded proteins (OST-B). High-resolution views of the active site and bound substrates in one of the complexes reveal important features of the human enzymes. Science , this issue p. 1372