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Introduction
N-linked glycosylation is an essential process in protein modification. This form of glycosylation is important in the folding and sorting of proteins in the endoplasmic reticulum (ER) and the interaction between proteins and cells. [3] In humans, N-linked glycosylation is catalyzed co-translationally by an enzyme complex called oligosaccharyltransferase complex A (OST-A) in the rough ER. This means that the peptide chain is glycosylated by this complex as it is synthesized by the ribosome and enters the ER lumen through translocon protein Sec61.[4] As suggested by the name, this enzyme complex transfers the high mannose fourteen-sugar chain from a lipid-linked oligosaccharide donor containing dolichol pyrophosphate to the peptide chain containing the Asn-X-Thr (N-X-T) sequence, where X is any amino acid but not Proline.[3] In addition, this enzyme complex is also part of the glycosyltransferase-C (GT-C) fold, which is a protein that has a transmembrane helical domain and a mix of α/β soluble domains.[5] On this page, the structure of the OST-A and its components; its mechanism, and diseases associated with this complex are discussed.
Structure
The human OST-A complex is a transmembrane protein that has 27 transmembrane helices integrated into the endoplasmic reticulum (ER) outer membrane with soluble domains on both the cytosolic side and the luminal side of the membrane (Ramirez et. al. 2019).[6] However, most of the functional sites of the complex are found on the luminal side. The OST-A complex consists of three sub-complexes with a total of nine subunits. All subunits have a transmembrane domain and soluble domains. The subcomplex I consists of two subunits: transmembrane protein 258 (TMEM258) and robophorin-1 (RPN-1). The subcomplex II consists of four subunits: STT3A, OST 4 kDa subunit (OST4), keratinocyte-associated protein 2 (KCP2), and DC2. Lastly, the subcomplex III consists of three subunits: defender against cell death 1 (DAD1), OST 48 kDa subunit (OST48), and ribophorin-2 (RPN-2) (Mohanty et. al. 2020).[7] The transmembrane domains of TMEM258 and RPN-1 are also in close proximity to a protein called malectin, which is believed to be involved in quality control in protein synthesis (Ramirez et. al. 2019).[6] In addition, the OST-A complex is associated with a translocon protein in the ER membrane called Sec61. The C-terminal of the RPN-1 subunit also forms a 4-helix bundle that specifically binds to the ribosome in the cytosol (Ramirez et. al. 2019).[6]
Active Site
Function
Disease
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