Oligosaccharyltransferase

Oligosaccharyltransferase or OST (EC 2.4.1.119) is a membrane protein complex that transfers a 14-sugar oligosaccharide from dolichol to nascent protein. It is a type of glycosyltransferase. 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. This sequence is called a glycosylation sequon. The reaction catalyzed by OST is the central step in the N-linked glycosylation pathway.

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Location

OST is a component of the translocon in the endoplasmic reticulum (ER) membrane. The active site of OST is located about 4 nm from the lumenal face of the ER membrane ^[1]. It usually acts during translation as the nascent protein is entering the ER, but this cotranslational glycosylation is nevertheless called a posttranslational modification. A few examples have been found of OST activity after translation is complete ^[2], ^[3]. Current opinion is that post-translational activity may occur if the protein is poorly folded or folds slowly ^[3].

Structure and function

Yeast OST is composed of eight different membrane-spanning proteins in three subcomplexes^[4], ^[5]. These octomers do not form higher order oligomers, and three of the eight proteins are glycosylated themselves^[6], ^[4]. OST in mammals is known to have a similar composition^[7].

OST is thought to require many subunits because it must:^[8]

1) Position itself near the translocon pore.

2) Recognize and bind oligosaccharyldolichol.

3) Scan the nascent protein in order to recognize and bind sequons.

4) Move these two large substrates into their proper locations and conformations.

5) Activate the Asn amide nitrogen atom for the actual transfer of oligosaccharide.

6) Release its substrates.

Congenital disorders of glycosylation (CDG) types

CDG syndromes are genetic disorders of the glycosylation pathway. They are labelled "Type I" if the defective gene is for an enzyme involved in the assembly or transfer of the Glc₃Man₉GlcNAc₂-dolichol precursor. They are labelled "Type II" if the defective step occurs after the action of OST in the N-linked glyocsylation pathway or involves O-linked glycosylation.^[9]

References

1. ^  Determination of the distance between the oligosaccharyltransferase active site and the endoplasmic reticulum membrane, I. Nilsson , G. von Heijne, Journal of Biological Chemistry 269, 5798-5801, 1993.
2. ^  Enzymatic conversion of proteins to glycoproteins, D.D. Pless, W.J. Lennarz, Proceedings of the National Academy of Sciences of the USA 74, 134-138, 1977.
3. ^ ^a  Glycosylation of the hepatitis C virus envelope protein E1 occurs posttranslationally in a mannsylphophoryldolichol-deficient CHO mutant cell line, S. Duvet et al, Glycobiology 12, 95-101, 2002.
4. ^ ^a  The oligosaccharyltransferase complex from yeast, R. Knauer, L. Lehle, Biochimica et Biophysica Acta 1426, 259-273, 1999.
5. ^  Oligosaccharyl transferase: gatekeeper to the secretory pathway, R.E. Dempski, B. Imperiali, Current Opinion in Cell Biology 6, 844-850, 2002.
6. ^  Allosteric regulation provides a molecular mechanism for preferential utilization of the fully assembled dolichol-linked oligosaccharide by the yeast oligosaccharyltransferase, D. Karaoglu, D.J. Kelleher, R. Gilmore, Biochemistry 40, 12193-12206, 2001.
7. ^  Photocross-linking of nascent chains to the STT3 subunit of the oligosaccharyltransferase complex, I. Nilsson et al, Journal of Cell Biology 161, 715-725, 2003.
8. ^  Protein glycosylation: the clash of the titans, B. Imperiali, Accounts of Chemical Research 30, 452-459, 1997.
9. ^  Congenital disorders of glycosylation: review of their molecular bases, clinical presentations, and specific therapies, T. Marquardt, J. Denecke, European Journal of Pediatrics 162, 359-179, 2003 http://www.cdgs.com/CDG_Overview_2003.pdf.