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 Structures elucidate the basis for substrate specificity in two glycosyltransferase complexes. Oligosaccharyltransferase (OST) catalyzes the transfer of a high-mannose glycan onto secretory proteins in the endoplasmic reticulum. Mammals express two distinct OST complexes that act in a cotranslational (OST-A) or posttranslocational (OST-B) manner. Here, we present high-resolution cryo–electron microscopy structures of human OST-A and OST-B. Although they have similar overall architectures, structural differences in the catalytic subunits STT3A and STT3B facilitate contacts to distinct OST subunits, DC2 in OST-A and MAGT1 in OST-B. In OST-A, interactions with TMEM258 and STT3A allow ribophorin-I to form a four-helix bundle that can bind to a translating ribosome, whereas the equivalent region is disordered in OST-B. We observed an acceptor peptide and dolichylphosphate bound to STT3B, but only dolichylphosphate in STT3A, suggesting distinct affinities of the two OST complexes for protein substrates.