Dissecting glycoprotein quality control in the secretory pathway.

In the early secretory pathway, asparagine-linked glycosylation facilitates the conformational maturation of diverse polypeptides by promoting their physical engagement with the glycoprotein-folding machinery. Misfolded glycoproteins are selectively eliminated from the endoplasmic reticulum by a stringent process of conformation-based quality control. Recent studies indicate that a small ensemble of oligosaccharide-processing enzymes and lectins use the asparagine-linked appendage to orchestrate the selective disposal of numerous transport-defective glycoproteins from the early secretory pathway. The glycan-based disposal system functions as an evolutionarily conserved terminal checkpoint in eukaryote genome expression. That the mechanisms by which glycoprotein substrates are recruited for degradation diverge at the level of signal recognition reflects a previously unappreciated component of cellular differentiation in higher eukaryotes.