The translocon: more than a hole in the ER membrane?

In eukaryotes, the vast majority of secreted and integral membrane proteins are targeted to the membrane of the endoplasmic reticulum (ER) early during translation. These polypeptides are then either transported across or inserted into the ER membrane at sites termed translocons. As protein translocation occurs through an aqueous pore, the minimal requirement for a translocon is a passive structure that provides a passage-way across the membrane. However, recent data suggest that the translocon is a complex structure that orchestrates the localization, orientation, maturation and possibly degradation of nascent chains.