Structural Research of Oligosaccharyltransferases (OST)
Oligosaccharyltransferases (OST) is a transmembrane protein complex that adds sugars to specific Asn residues of newly formed peptides in the lumen of the endoplasmic reticulum. This process, known as N-glycosylation, is involved in signal transduction and intercellular communication. Research surfaces that OST is a potential target for the treatment of drug-resistant tumors. Therefore, in-depth study of OST is a priority for glycobiological and clinical research.
Advances in OST research
Human cells contain two distinct OST complexes, OST-A and OST-B, which both contain six subunits but have different catalytic subunits, STT3A or STT3B. STT3A is primarily responsible for the cotranslational glycosylation of newly generated polypeptides entering the lumen of the endoplasmic reticulum, whereas STT3B mediates post-translational glycosylation. Bacteria exhibit different glycan structures on asparagine residues. They have single-subunit OST (ssOST) enzymes that can transfer a wider range of glycans.
Analysis of OST-A and OST-B structures
The purified OST-A and OST-B complexes are analyzed using a single particle cryoelectron microscope with a resolution of 3.5 Å. There are differences in the protein surfaces where STT3A and STT3B interact with DC2 or MAGT1. In the absence of binding to ribosomes, the C-terminus domain of riboflavin I also forms cytoplasmic four-helix bundles in OST-A. Hydrophobic grooves formed by TM6 and TM11 are present on the surface of STT3B, and the grooves contain binding molecules characterized by polypropylene chains, as well as phosphate groups in contact with divalent ions and conserved arginine side chains.
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Figure 1. Cryo-EM maps of OST-A and OST-B. (Ramírez AS, et al., 2019)
| Protein | Organism | Method | Resolution | PDB Entry ID |
| C. jejuni pglb C-terminal domain | Campylobacter jejuni RM1221 | X-ray diffraction | 2.8 Å | 3AAG |
| O29867_ARCFU in the apo form | Archaeoglobus fulgidus DSM 4304 | X-ray diffraction | 3.413 Å | 3WAK |
| O29867_ARCFU complex with Zn and sulfate | Archaeoglobus fulgidus DSM 4304 | X-ray diffraction | 2.501 Å | 3WAJ |
| O29867_ARCFU tethered with an acceptor peptide containing the NVT sequon via a disulfide bond | Archaeoglobus fulgidus DSM 4304 | X-ray diffraction | 3.5 Å | 5GMY |
| Yeast Ost6p containing oligosaccharyltransferase complex | Saccharomyces cerevisiae S288C | Cryo-EM single particle analysis | 3.46 Å | 7OCI |
| Oligosaccharyltransferase PglB | Campylobacter lari | X-ray diffraction | 3.4 Å | 3RCE |
| AglB in complex with an inhibitory peptide and a dolichol-phosphate | Archaeoglobus fulgidus DSM 4304 | X-ray diffraction | 2.7 Å | 7E9S |
| PglB in complex with an acceptor peptide and a lipid-linked oligosaccharide analog | Campylobacter jejuni | X-ray diffraction | 2.7 Å | 5OGL |
| PglB in complex with an inhibitory peptide and a reactive lipid-linked oligosaccharide analog | Campylobacter lari RM2100 | X-ray diffraction | 3.401 Å | 6GXC |
| C-terminal globular domain of AfAglB-L, O29867_ARCFU as a MBP fusion | Archaeoglobus fulgidus DSM 4304 | X-ray diffraction | 1.897 Å | 3WAI |
| C-terminal globular domain of AfAglB-S2, AF_0040, O30195_ARCFU | Archaeoglobus fulgidus DSM 4304 | X-ray diffraction | 1.94 Å | 3VU0 |
| C-terminal globular domain of PhAglB-L, O74088_PYRHO | Pyrococcus horikoshii OT3 | X-ray diffraction | 2.7 Å | 3VU1 |
| Oligosaccharyl transferase complex | Saccharomyces cerevisiae S288C | Cryo-EM single particle analysis | 3.5 Å | 6C26 |
| Oligosaccharyltransferase complex OST-A | Homo sapiens | Cryo-EM single particle analysis | 3.5 Å | 6S7O |
| Oligosaccharyltransferase complex OST-B | Homo sapiens | Cryo-EM single particle analysis | 3.5 Å | 6S7T |
| 3-O-sulfotransferase (3-OST-1) with bound PAP and heptasaccharide substrate | Mus musculus | X-ray diffraction | 1.844 Å | 3UAN |
| Oligosaccharyltransferase complex with lipid-linked oligosaccharide bound | Saccharomyces cerevisiae | Cryo-EM single particle analysis | 3 Å | 8AGB |
| Oligosaccharyltransferase (OST) complex | Saccharomyces cerevisiae S288C | Cryo-EM single particle analysis | 3.3 Å | 6EZN |
| Oligosaccharyltransferase complex with lipid-linked oligosaccharide and non-acceptor peptide bound | Saccharomyces cerevisiae | Cryo-EM single particle analysis | 3.1 Å | 8AGC |
| Oligosaccharyltransferase complex with acceptor peptide bound | Saccharomyces cerevisiae | Cryo-EM single particle analysis | 2.8 Å | 8AGE |
| Ost4 in DPC micelles | Saccharomyces cerevisiae YJM789 | SOLUTION NMR | / | 6XCR |
| Ost4V23D, a critical mutant of Ost4, in DPC micelles | Saccharomyces cerevisiae YJM789 | SOLUTION NMR | / | 6XCU |
Table 1. Structural research of oligosaccharyltransferases (OST).
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References
- Ramírez AS, et al. Cryo-electron microscopy structures of human oligosaccharyltransferase complexes OST-A and OST-B. Science. 2019. 366(6471):1372-1375.
- Harada Y, et al. Oligosaccharyltransferase: A Gatekeeper of Health and Tumor Progression. Int J Mol Sci. 2019. 20(23):6074.
- Napiórkowska M, et al. Structure of bacterial oligosaccharyltransferase PglB bound to a reactive LLO and an inhibitory peptide. Sci Rep. 2018. 8(1):16297.
