Difficulty: Easy
Correct Answer: Asn-Xaa-(Ser or Thr)
Explanation:
Introduction / Context:
N-linked glycosylation is a major co-/post-translational modification in eukaryotes where an oligosaccharide is attached to the side chain amide nitrogen of asparagine. Recognizing the canonical sequence motif helps in predicting glycoprotein sites and understanding folding, trafficking, and function of membrane and secretory proteins.
Given Data / Assumptions:
Concept / Approach:
The well-established motif is Asn-Xaa-Ser/Thr, often written as N-X-S/T. The central position (Xaa) can be almost any residue except proline because proline imposes conformational constraints that reduce enzyme recognition. While not every NXS/T site is glycosylated, most verified N-glycans occur within this sequence context.
Step-by-Step Solution:
Verification / Alternative check:
Biochemical mapping and mass spectrometry of glycopeptides repeatedly validate N-X-S/T as the dominant ER cotranslational consensus motif; mutation of any position often abolishes glycosylation.
Why Other Options Are Wrong:
Common Pitfalls:
Confusing O-glycosylation (on Ser/Thr) with N-glycosylation (on Asn), or assuming every N-X-S/T site is necessarily occupied—context and structure also matter.
Final Answer:
Asn-Xaa-(Ser or Thr)
Discussion & Comments