Sec61 translocon, membrane protein

The most well understood pathway is the one that delivers secretory and membrane proteins to the endoplasmic reticulum (ER) membrane in eukaryotic cells and to the inner membrane in bacteria. In both kinds of cells, the pivotal role is played by the so-called Sec61 (in eukaryotes) or SecYEG (in prokaryotes) translocon, a multisubunit translocation channel that provides a conduit for soluble proteins to cross the membrane. The same translocon also serves to integrate membrane proteins into the lipid bilayer. 

The mechanism for translocating bacterial proteins across the Inner membrane shares several key features with the translocation of proteins into the ER of eukaryotic cells. First, translocated proteins usually contain an N-termlnal hydrophobic signal sequence, which is cleaved by a signal peptidase. Second, bacterial proteins pass through the Inner membrane In a channel, or translocon, composed of proteins that are structurally similar to the eukaryotic Sec61 complex. Third, bacterial cells express two proteins, Ffh and its receptor (FtsY), that are homologs of the SRP and SRP receptor, respectively. In bacteria, however, these latter proteins appear to function mainly In the insertion of hydrophobic membrane proteins Into the Inner membrane. Indeed, all bacterial proteins that are translocated across the inner membrane do so only after their synthesis In the cytosol is completed but before they are folded Into their final conformation. 

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. 

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