Folding of viral glycoproteins in the endoplasmic reticulum.

The endoplasmic reticulum (ER) is a subcellular compartment specialized in folding and assembly of newly synthesized polypeptides. The polypeptides expressed in the ER include all secretory proteins produced in the cell, lumenal or membrane-bound proteins of the endocytic/vacuolar and secretory compartments and transmembrane proteins that operate at the plasma membrane. In the lumen of the ER, molecular chaperones and folding factors facilitate the maturation of newly synthesized proteins. In a process defined as ER-quality control, they also warrant that only properly structured and assembled products leave the ER and are transported to their target organelles and compartments. If proper maturation fails, the aberrant products are degraded. Quality control in the ER is essential to prevent exit of improperly regulated or not-functional products that could lead to harmful effects. The mechanisms of protein folding and quality control in the ER are far from being fully understood. They are fundamental for the life of cells and organisms, but they are also linked to important human hereditary diseases in which mutated gene products are retained in the ER and degraded (e.g., cystic fibrosis and hereditary lung emphysema).