The rough ER is the site of synthesis of many different classes of membrane protein. Some of these proteins remain in the ER, whereas others are sorted to compartments such as the Golgi apparatus, lysosomes, and the plasma membrane (PM). One measure of the difficulty of the sorting problem is the degree of “purification” that must be achieved during transport from the ER to the other compartments. For example, if membrane proteins bound for the PM represented 90% of all the proteins in the ER, then only a small degree of purification would be needed (and the sorting problem would appear relatively easy). On the other hand, if PM proteins represented only 0.01% of the proteins in the ER, a very large degree of purification would be required (and the sorting problem would appear much more difficult).

What is the magnitude of the sorting problem? What fraction of membrane proteins in the ER are destined for other compartments?