15.7:
Post-translational Translocation of Proteins to the RER
Some proteins are completely synthesized by free ribosome particles in the cytosol before entering the ER lumen. This mechanism is called post-translational protein translocation.
After translation, the cytosolic chaperones immediately bind to the polypeptide chain to prevent it from folding prematurely.
Once the chaperone-bound protein reaches the ER membrane, its signal sequence is recognized by a recognition site within the Sec61 channel.
Then, the protein slowly begins to glide down the channel and emerges at the luminal end, where it recruits ATP-bound BiP chaperone.
The heterodimeric Sec62/Sec63 complex hydrolyzes the ATP to induce a conformational change in the BiP molecule, allowing it to clasp the polypeptide.
New BiP molecules keep attaching to the incoming polypeptide, actively pulling it down into the ER lumen.
As the polypeptide enters the lumen, its signal sequence is cleaved off by the signal peptidase located in the ER membrane.
On completion of translocation, the nucleotide exchange factors replace all the ADPs bound to the BiP molecules with ATPs, preparing them for the next cycle of protein translocation.
15.7:
Post-translational Translocation of Proteins to the RER
A sizable fraction of proteins destined for ER are first synthesized in the cell cytosol and then transported across the ER membrane–a process called post-translational translocation. Similar to cotranslationally translocated proteins, these proteins also use the Sec translocon complex to enter the ER lumen.
Targeting proteins to the ER
Hsp40 and Hsp70 chaperone molecules bind the translated proteins in the cytosol to prevent their folding. The chaperone binding helps to keep the signal sequence present on the target proteins available for interaction with the recognition site on the Sec61 channel.
The ER signal sequence of proteins translocating post-translation has characteristic features that distinguish them from cotranslationally translocated proteins. For example, in yeast, the signal sequence of post-translationally translocated proteins is relatively less hydrophobic compared to the signal sequence of cotranslationally translocating proteins. In contrast, in mammalian systems, the target proteins have a characteristic short positively charged N-terminal.
Translocon partners for post-translational translocation
All proteins enter the ER lumen using the Sec61 channel but the channel partners vary depending on whether it is a cotranslational or a post-translational translocation process. Sec62 and Sec63 play an important role during post-translational translocation. Sec62 has a strong affinity for the ER signal sequence present on the cytosol-translated proteins, while Sec63 has a specialized domain that hydrolyzes the ATP to allow the BiP chaperone to bind to the incoming protein. BiP chaperone is a lumenal chaperone that keeps the protein loosely folded or unfolded till it has attained the appropriate secondary structure. Together, the Sec62/63 complex and the BiP chaperone are involved in actively pulling the protein into the ER lumen by a Brownian ratcheting mechanism.
Other functions of the Sec proteins
Besides their central role in translocating proteins, Sec proteins are essential in other cell functions and diseases. For example, Sec62 is involved in the stress recovery of cells, while the Sec61 channel can double up as a passive ER calcium leak channel. Additionally, mutations and overexpression of the SEC genes are linked to diabetes, cancer, and various kidney and liver disorders in humans.
Suggested Reading
- Johnson, Nicholas, Katie Powis, and Stephen High. "Post-translational translocation into the endoplasmic reticulum." Biochimica et Biophysica Acta (BBA)-Molecular Cell Research 1833, no. 11 (2013): 2403-2409.