Transport of Proteins Across the Endoplasmic Reticulum Membrane

The biosynthesis of many eukaryotic proteins requires their transport across the endoplasmic reticulum (ER) membrane. The process can be divided into two phases: (i) a targeting cycle, during which, by virtue of their signal sequences, nascent polypeptides are directed to translocation sites in the ER and (ii) the actual transfer of proteins across the membrane. The first phase has been well characterized, whereas the latter until recently was completely unresolved. Key components of the translocation apparatus have now been identified and it seems likely that they form a protein-conducting channel in the ER membrane. The transport process is similar to the process of protein export in bacteria.