AsnXSerThrSequenz

AsnXSerThrSequenz, also known as the N-X-S/T sequon, refers to the minimal consensus sequence for N-linked glycosylation in eukaryotic secretory and membrane proteins. It consists of asparagine (N) followed by any amino acid except proline (X), and then serine or threonine (S/T): N-X-S/T.

Function and biology

Glycosylation at this sequon is mediated by the oligosaccharyltransferase (OST) complex, which attaches a preassembled oligosaccharide to the amide nitrogen of the Asn residue in the endoplasmic reticulum during protein synthesis (co-translationally). This modification typically occurs on proteins that are destined for secretion or reside in the cell surface, and is usually found on extracellular or lumenal domains rather than cytosolic regions. The sequon provides the recognition motif for the initial glycosylation event, which can influence protein folding, stability, trafficking, and quality control.

Limitations and context

While the N-X-S/T motif is necessary for N-linked glycosylation, it is not always sufficient. Local protein structure, accessibility, and neighboring residues influence whether a given sequon is glycosylated, and occupancy can vary between tissues and conditions. A proline residue at the X position frequently disrupts recognition and prevents glycosylation at that site.

Applications

In practice, the motif is used to predict potential N-glycosylation sites in protein sequences, often complemented by experimental validation such as mass spectrometry. Predictive tools and glycoproteomics databases help annotate sites, but experimental confirmation is typically required to establish true occupancy.