Import into the endoplasmic reticulum

This chapter discusses import into the endoplasmic reticulum. Proteins destined to the endoplasmic reticulum (ER), the Golgi complex, the vacuole, and the plasma membrane are first imported into the ER before being targeted to their final destination. Import into the ER is therefore a common step in the synthesis of a large set of proteins predetermined to various compartments of the cell. The mechanism by which efficient segregation is obtained has been studied mainly using mammalian and yeast systems. The synthesis of proteins that are imported into the ER begins on cytosolic ribosomes. Generally, soon after translation has started, the growing polypeptide chain and the ribosome are targeted to the ER membrane and translocation occurs cotranslationally. This signal is recognized by a ribonucleoprotein particle that mediates the targeting of the nascent chain and the ribosome to the ER membrane. Interaction with the membrane involves an ER-located receptor, the signal recognition particle (SRP) receptor, or docking protein. © 1995, Academic Press, Inc.