Cotranslationele eiwittranslocatie

Most proteins enter the ER lumen while still under synthesis — a process called cotranslational translocation.

First, the SRP-SRP receptor complex carrying the ribosome-nascent chain complex unloads it on the Sec61 channel.

Once the ribosome aligns with the channel, the signal sequence at the N terminal of the polypeptide chain latches onto a signal sequence recognition site.

As the translation continues,  the polypeptide chain forms a loop pushing open the channel plug for entry into the ER lumen.

While the polypeptide descends into the lumen, the signal sequence exits through the lateral gate of the Sec61 channel to enter the adjacent signal peptidase complex, where it is cleaved off.

The channel plug remains pushed open as the protein passively descends into the ER lumen.

After complete protein translocation into the lumen, the channel plug bounces back to block the protein from returning to the cytosol.

Inside the ER lumen, resident ER chaperones like BiP immediately bind the translocated protein and assist in protein folding.