STT3B
Description
The STT3B (STT3 oligosaccharyltransferase complex catalytic subunit B) is a protein-coding gene located on chromosome 3.
Dolichyl-diphosphooligosaccharide—protein glycosyltransferase subunit STT3B is an enzyme that in humans is encoded by the STT3B gene.
== Function == The STT3B protein contains a highly immunogenic minor histocompatibility antigen epitope of 9 amino acids, B6(dom1). Like ITM1 (MIM 601134), STT3B is homologous to yeast STT3, an oligosaccharyltransferase essential for cell proliferation.
STT3B is a catalytic subunit of the oligosaccharyl transferase (OST) complex, responsible for the initial transfer of a specific glycan (Glc(3)Man(9)GlcNAc(2)) from a lipid carrier, dolichol-pyrophosphate, to an asparagine residue within an Asn-X-Ser/Thr sequence in nascent polypeptide chains. This transfer initiates protein N-glycosylation. N-glycosylation takes place during protein synthesis (cotranslationally) and the OST complex associates with the Sec61 complex, a channel-forming complex involved in protein translocation across the endoplasmic reticulum (ER). The complex's full activity requires all subunits, with STT3B containing the active site and binding pockets for the acceptor peptide and donor lipid-linked oligosaccharide (LLO). STT3B is present in a specific subset of OST complexes, mediating both cotranslational and post-translational N-glycosylation. These complexes are essential for efficient post-translational glycosylation and, although less efficient than STT3A-containing complexes for cotranslational glycosylation, STT3B complexes can glycosylate certain nascent sites inaccessible to STT3A. Furthermore, STT3B-containing complexes function post-translationally, modifying skipped glycosylation sites in unfolded proteins. STT3B is involved in the ER-associated degradation (ERAD) pathway, which degrades misfolded ER proteins through ubiquitin-dependent mechanisms. STT3B mediates N-glycosylation of unfolded proteins, marking them for recognition and degradation by the ERAD pathway. STT3B also mediates glycosylation of the disease variant AMYL-TTR 'Asp-38' of TTR at 'Asn-118', leading to its degradation.
STT3B is also known as CDG1X, SIMP, STT3-B.
