9.B.142.3.17
OSTA/B (OST-A/OST-B) complex of 11 subunits.  Most subunits are shared by both complexes except that OSTA has STT3A while OSTB has STT3B, and OSTA has DC2 while OSTB has two parologous Mg²⁺ porter proteins, MAGT1 (TC# 1.A.76.1) and TUSC3 (TC# 1.A.76.2). The oligosaccharyl transferase (OST) complex catalyzes the initial transfer of a defined glycan (Glc₃Man₉GlcNAc₂ in eukaryotes) from the lipid carrier dolichol-pyrophosphate to an asparagine residue within an Asn-X-Ser/Thr consensus motif in nascent polypeptide chains, the first step in protein N-glycosylation. N-Glycosylation occurs cotranslationally, and the complex associates with the Sec61 complex at the channel-forming translocon complex that mediates protein translocation across the endoplasmic reticulum (ER) membrane. Ribophorin I associates with a subset of membrane proteins after their integration at the Sec61 translocon (Wilson et al. 2005). All subunits are required for maximal enzyme activity. The SST3 subunit contains the active site and the acceptor peptide and donor lipid-linked oligosaccharide (LLO) binding pockets. STT3A is present in the majority of OST complexes and mediates cotranslational N-glycosylation of most sites on target proteins, while STT3B-containing complexes are required for efficient post-translational glycosylation and mediate glycosylation of sites that have been skipped by STT3A (Ruiz-Canada et al. 2009). High-resolution cryo-EM structures of human OST-A and OST-B have been described (Ramírez et al. 2019). Although the two complexes 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. An acceptor peptide and dolichylphosphate are bound to STT3B, but only dolichylphosphate is bound in STT3A, suggesting distinct affinities of the two OST complexes for protein substrates (Ramírez et al. 2019).