Catalytic subunit of the oligosaccharyl transferase (OST) complex that catalyzes the initial transfer of a defined glycan (Glc(3)Man(9)GlcNAc(2) 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 (PubMed:19167329, PubMed:31296534, PubMed:31831667, PubMed:39509507). 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) (PubMed:19167329, PubMed:31296534, PubMed:31831667, PubMed:39509507). All subunits are required for a maximal enzyme activity. This subunit contains the active site and the acceptor peptide and donor lipid-linked oligosaccharide (LLO) binding pockets (PubMed:19167329, PubMed:31296534, PubMed:31831667, PubMed:39509507). STT3B is present in a small subset of OST complexes (OST-B) and mediates both cotranslational and post-translational N-glycosylation of target proteins: STT3B-containing complexes are required for efficient post- translational glycosylation and while they are less competent than STT3A-containing complexes for cotranslational glycosylation, they have the ability to mediate glycosylation of some nascent sites that are not accessible for STT3A (PubMed:19167329, PubMed:22607976, PubMed:31296534, PubMed:39509507). STT3B-containing complexes also act post-translationally and mediate modification of skipped glycosylation sites in unfolded proteins (PubMed:19167329, PubMed:22607976, PubMed:39509507). Plays a role in ER-associated degradation (ERAD) pathway that mediates ubiquitin-dependent degradation of misfolded endoplasmic reticulum proteins by mediating N-glycosylation of unfolded proteins, which are then recognized by the ERAD pathway and targeted for degradation (PubMed:19167329, PubMed:22607976). Mediates glycosylation of the disease variant AMYL-TTR 'Asp-38' of TTR at 'Asn- 118', leading to its degradation (PubMed:19167329, PubMed:22607976). {ECO:0000269|PubMed:19167329, ECO:0000269|PubMed:22607976, ECO:0000269|PubMed:31296534, ECO:0000269|PubMed:31831667, ECO:0000269|PubMed:39509507}. This is the function of Ensembl gene identifier ENSG00000163527 (STT3B, STT3 oligosaccharyltransferase complex catalytic subunit B).