The middle X residue influences cotranslational N-glycosylation consensus site skipping

Asparagine (N)-linked glycosylation is essential for efficient protein folding in the endoplasmic reticulum (ER) and anterograde trafficking through the secretory pathway. N-Glycans are attached to nascent polypeptides at consensus sites, N-X-T/S (X ≠ P), by one of two enzymatic isoforms of the oligosaccharyltransferase (OST), STT3A or STT3B. Here, we examined the effect of the consensus site X and hydroxyl residue on the distributions of co- and post-translational N-glycosylation of a type I transmembrane glycopeptide scaffold. Using rapid radioactive pulse-chase experiments to resolve co-translational (STT3A) and post-translational (STT3B) events, we determined that NXS consensus sites containing large hydrophobic and negatively charged middle residues are frequently skipped by STT3A during protein translation. Post-translational modification of the cotranslationally skipped sites by STT3B was similarly hindered by the middle X residue, resulting in hypoglycosylation of NXS sites containing large hydrophobic and negatively charged side chains. In contrast, NXT consensus sites (barring NWT) were efficiently modified by the cotranslational machinery, reducing STT3B's role in modifying consensus sites skipped during protein translation. A strong correlation between cotranslational N-glycosylation efficiency and the rate of post-translational N-glycosylation was determined, showing that the OST STT3A and STT3B isoforms are similarly influenced by the hydroxyl and middle X consensus site residues. Substituting various middle X residues into an OST eubacterial homologous structure revealed that small and polar consensus site X residues fit well in the peptide binding site whereas large hydrophobic and negatively charged residues were harder to accommodate, indicating conserved enzymatic mechanisms for the mammalian OST isoforms.