Figure 12–40 A ribosome (green) bound to the ER protein translocator (blue). (a) a side-view reconstruction of the complex from electron microscopic images. (B) a view of the translocator seen from the er lumen. The translocator contains sec61, accessory proteins, and detergent used in the preparation. Domains of accessory proteins extend across the membrane and form the lumenal bulge. (C) a schematic drawing of a membrane-bound ribosome attached to the translocator, indicating the location of the tunnel in the large ribosomal subunit through which the growing polypeptide chain exits from the ribosome. The mrna (not shown) would be located between the small and large ribosomal subunits. (adapted from J.f. ménétret et al., J. Mol. Biol. 348:445–457, 2005. With permission from academic press.)
In eukaryotic cells, four Sec61 complexes form a large translocator assembly that can be visualized on ER-bound ribosomes after detergent solubilization of the ER membrane (Figure 12–40). It is likely that this assembly includes other membrane complexes that associate with the translocator, such as enzymes that modify the growing polypeptide chain, including oligosaccharide transferase and the signal peptidase. The assembly of a translocator with these accessory components is called the translocon.
Translocation across the er membrane Does not always require ongoing polypeptide Chain elongation
As we have seen, translocation of proteins into mitochondria, chloroplasts, and peroxisomes occurs post-translationally, after the protein has been made and released into the cytosol, whereas translocation across the ER membrane usually occurs during translation (co-translationally). This explains why ribosomes are bound to the ER but not to other organelles.
Some completely synthesized proteins, however, are imported into the ER, demonstrating that translocation does not always require ongoing translation. Post-translational protein translocation is especially common across the yeast ER membrane and the bacterial plasma membrane (which is thought to be evolutionarily related to the ER). To function in post-translational translocation, the ER translocator needs accessory proteins that feed the polypeptide chain into the pore and drive translocation (Figure 12–41). In bacteria, a translocation motor protein, the SecA ATPase, attaches to the cytosolic side of the translocator, where it undergoes cyclic conformational changes driven by ATP hydrolysis. Each time an ATP is hydrolyzed, a portion of the SecA protein inserts into the pore of the translocator, pushing a short segment of the passenger protein with it. As a result of this ratchet mechanism, the SecA ATPase progressively pushes the polypeptide chain of the transported protein across the membrane.
Eukaryotic cells use a different set of accessory proteins that associate with the Sec61 complex. These proteins span the ER membrane and use a small domain on the lumenal side of the ER membrane to deposit an hsp70-like chaperone protein (called BiP, for binding protein) onto the polypeptide chain as it emerges from the pore into the ER lumen. ATP-dependent cycles of BiP binding and release drive unidirectional translocation, as described earlier for the mitochondrial hsp70 proteins that pull proteins across mitochondrial membranes.
Proteins that are transported into the ER by a post-translational mechanism are first released into the cytosol, where they bind to chaperone proteins to prevent folding, as discussed earlier for proteins destined for mitochondria and chloroplasts.
in single-pass Transmembrane proteins, a single internal er signal sequence remains in the lipid Bilayer as a membrane-spanning α helix
The ER signal sequence in the growing polypeptide chain is thought to trigger the opening of the pore in the Sec61 protein translocator: after the signal sequence is released from the SRP and the growing chain has reached a sufficient length, the