Ribosome-Sec61 translocon complex

The mechanism by which the translocon channel opens and closes is controversial at this time. Some evidence suggests that a protein within the ER lumen blocks the translocon pore when a ribosome is not bound to the cytosolic side of the translocon. Other observations, however, indicate that Sec61 complexes may normally reside in the ER membrane in an unassembled state and that the gating process involves the assembly of a translocon channel at the site where the ribosome and nascent chain are brought to the membrane by the SRP and SRP receptor. 

---