N-linked glycosylation and homeostasis of the endoplasmic reticulum.

As a major site of protein biosynthesis, homeostasis of the endoplasmic reticulum is critical for cell viability. Asparagine linked glycosylation of newly synthesized proteins by the oligosaccharyltransferase plays a central role in ER homeostasis due to the use of protein-linked oligosaccharides as recognition and timing markers for glycoprotein quality control pathways that discriminate between correctly folded proteins and terminally malfolded proteins destined for ER associated degradation. Recent findings indicate how the oligosaccharyltransferase achieves efficient and accurate glycosylation of the diverse proteins that enter the endoplasmic reticulum. In metazoan organisms two distinct OST complexes cooperate to maximize the glycosylation of nascent proteins. The STT3B complex glycosylates acceptor sites that have been skipped by the translocation channel associated STT3A complex.