What happens in the ER lumen?

The ER lumen is a major site of protein folding: this requires ER chaperones. Two major ER modifications: Folding proteins may become N-glycosylated and/or disulfide-bonded depending on whether they have the right amino acid sequence motifs for these events. If a protein does not fold properly, it will fail a quality control check carried out by ER chaperones. Misfolded proteins are ejected from the ER in a process called retro-translocation, and are then degraded in the cytosol.