Research Highlights
Oligosaccharyltransferases: A sweet assembly line
Functional Glycomics (12 February 2009) | doi:10.1038/fg.2009.8Standfirst
Two isoforms of the oligosaccharyltransferase active site subunit cooperate to glycosylate polypeptides in the endoplasmic reticulum (ER) – one adds carbohydrate modifications to proteins as they are translated whereas the other is responsible for post-translational glycosylation of sites overlooked by the first enzyme.
As polypeptides are synthesized in the secretory pathway, most are modified by the addition of an oligosaccharide at a glycosylation site defined by the amino acid sequon asparagine-X-serine/threonine. Oligosaccharyltransferases (OST) catalyze this transport of oligosaccharide from donor intermediates to nascent polypeptides as they enter the ER lumen through a translocon channel. Each OST is made up of seven or eight subunits, including the active site subunit STT3, which is often found in two isoforms – STT3A and STT3B. Whether STT3A and STT3B are both necessary for glycosylation or whether they play a redundant role has long remained unclear. In a study appearing in Cell, Reid Gilmore and colleagues now reveal that full polypeptide glycosylation is sequential and requires both isoforms.
To examine the role of STT3A and STT3B in glycosylation,the authors tested three polypeptide substrates. They found that lack of STT3A, or lack of both STT3A and STT3B, triggered increased expression of BiP, an ER chaperon protein upregulated by the unfolded protein response pathway – the cellular response that helps relieve ER stress caused by an accumulation of unfolded proteins. Since proper glycosylation facilitates protein folding, they concluded that most glycosylation sites are modified by STT3A.
Despite its importance in costranslational glycosylation, however, STT3A was not required for posttranslational modification. As a nascent polypeptide passes through an ER translocon, STT3A is usually the first OST isoform it encounters. Yet STT3A can only glycosylate sequons located 65-75 residues from the large ribosomal. In instances where a sequon is found close to the C terminus, the authors found that STT3B steps in to glycosylate those sites either co- or posttranslationally, assuring that no sequon is missed.
Based on these findings, Reid Gilmore and colleagues propose a new model for protein glycosylation in the secretory pathway. Rather than finding just a single OST positioned near each translocon as previously thought, this study suggests that nascent polypeptides meet multiple OST complexes as they emerge in the ER lumen. This duplication of OST function provides quality control by assuring that proteins are properly glycosylated and folded before they leave the secretory pathway.
Original research paper:
- Ruiz-Canada C., et al. Cotranslational and posttranslational N-glycosylation of polypeptides by distinct mammalian OST isoforms. Cell 136, 272–283 (2009). doi: 10.1016/j.cell.2008.11.047. | Article |
