Post-translational Translocation of Proteins to the RER

Post-translational protein translocation is a mechanism where proteins are completely synthesized by free ribosomes in the cytosol before entering the ER lumen. Unlike cotranslational translocation, these fully translated proteins are kept unfolded by cytosolic chaperones until they reach the ER membrane, where they cross through the Sec61 channel and enter the lumen for processing and folding.

Cytosolic chaperones, including Hsp40 and Hsp70 molecules, immediately bind to the newly synthesized polypeptide chain to prevent premature folding. This binding keeps the signal sequence exposed and available for recognition by the Sec61 channel, ensuring the protein remains in an unfolded state suitable for translocation across the ER membrane.

The Sec62/Sec63 complex partners with the Sec61 channel during post-translational translocation. Sec62 has strong affinity for the ER signal sequence on cytosol-translated proteins, while Sec63 hydrolyzes ATP to enable BiP chaperone binding. Together, they actively pull the protein into the ER lumen through a Brownian ratcheting mechanism.

BiP is a lumenal chaperone that binds to incoming polypeptides after they enter the ER. The Sec62/Sec63 complex hydrolyzes ATP to induce conformational changes in BiP, allowing it to clasp the polypeptide. Multiple BiP molecules sequentially attach and pull the protein deeper into the lumen, actively driving translocation.

As the polypeptide enters the ER lumen, the signal peptidase enzyme located in the ER membrane cleaves off the signal sequence. This cleavage occurs after the protein has begun crossing the membrane and helps process the protein for its final destination and function within the ER or secretory pathway.

After translocation completes, nucleotide exchange factors replace all ADP molecules bound to BiP with ATP. This nucleotide exchange resets BiP to its ATP-bound state, preparing it for the next cycle of protein translocation and maintaining the efficiency of the post-translational translocation machinery.

Post-translationally translocated proteins have distinctive signal sequences that differ by organism. In yeast, these sequences are relatively less hydrophobic than cotranslational signal sequences. In mammalian systems, post-translational target proteins characteristically have short, positively charged N-terminal sequences that facilitate recognition and translocation.