Oligosaccharyltransferase

The sugar Glc3Man9GlcNAc2 (where Glc=Glucose, Man=Mannose, and GlcNAc=N-acetylglucosamine) is attached to an asparagine (Asn) residue in the sequence Asn-X-Ser or Asn-X-Thr where X is any amino acid except proline.

A lipid-linked core-oligosaccharide is assembled at the membrane of the endoplasmic reticulum and transferred to selected asparagine residues of nascent polypeptide chains by the oligosaccharyl transferase complex.

[3] The active site of OST is located about 4 nm from the lumenal face of the ER membrane.

[4] It usually acts during translation as the nascent protein is entering the ER, but this cotranslational glycosylation is nevertheless called a posttranslational modification.

They are labelled "Type I" if the defective gene is for an enzyme involved in the assembly or transfer of the Glc3Man9GlcNAc2-dolichol precursor.

ER Translocon complex. [ 2 ] Many protein complexes are involved in protein synthesis. The actual production takes place in the ribosomes (grey and light blue). Through the ER translocon (green: Sec61, blue: TRAP complex, and red: oligosaccharyl transferase complex) the newly synthesized protein is transported across the membrane (gray) into the interior of the ER. Sec61 is the protein-conducting channel and the OST adds sugar moieties to the nascent protein.