Nearly all resident proteins of the organelles along the secretory pathway, as well as
proteins that are expressed at the cell surface or secreted from the cell, are first co-
translationally translocated into the lumen of the endoplasmic reticulum (ER) as unfolded
polypeptide chains. Immediately after entering the ER, they are often modified with N-linked
glycans, are folded into the appropriate secondary and tertiary structures, which are
stabilized by disulfide bonds, and finally in many cases are assembled into multimeric …