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Structure. 2014 Apr 8;22(4):590-601. doi: 10.1016/j.str.2014.02.013. Epub 2014 Mar 27.

Structural basis of substrate specificity of human oligosaccharyl transferase subunit N33/Tusc3 and its role in regulating protein N-glycosylation.

Author information

1
ETH Zürich, Department of Biology, Institute of Molecular Biology and Biophysics, Otto-Stern-Weg 5, 8093 Zurich, Switzerland.
2
Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, UK.
3
ETH Zürich, Department of Biology, Institute of Microbiology, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland.
4
ETH Zürich, Department of Biology, Institute of Molecular Biology and Biophysics, Otto-Stern-Weg 5, 8093 Zurich, Switzerland. Electronic address: rudi@mol.biol.ethz.ch.

Abstract

N-linked glycosylation of proteins in the endoplasmic reticulum (ER) is essential in eukaryotes and catalyzed by oligosaccharyl transferase (OST). Human OST is a hetero-oligomer of seven subunits. The subunit N33/Tusc3 is a tumor suppressor candidate, and defects in the subunit N33/Tusc3 are linked with nonsyndromic mental retardation. Here, we show that N33/Tusc3 possesses a membrane-anchored N-terminal thioredoxin domain located in the ER lumen that may form transient mixed disulfide complexes with OST substrates. X-ray structures of complexes between N33/Tusc3 and two different peptides as model substrates reveal a defined peptide-binding groove adjacent to the active site that can accommodate peptides in opposite orientations. Structural and biochemical data show that N33/Tusc3 prefers peptides bearing a hydrophobic residue two residues away from the cysteine forming the mixed disulfide with N33/Tusc3. Our results support a model in which N33/Tusc3 increases glycosylation efficiency for a subset of human glycoproteins by slowing glycoprotein folding.

PMID:
24685145
DOI:
10.1016/j.str.2014.02.013
[Indexed for MEDLINE]
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