Protein Translocation Machinery on the ER Membrane

The Sec61 complex is a heterotrimeric protein-conducting channel composed of three transmembrane proteins: Sec61α, Sec61β, and Sec61γ. It forms the core of the translocon and contains a vertical channel with a plug-like α helix that opens and closes to allow hydrophilic protein segments to translocate into the ER lumen. The complex also features a lateral gate that opens like a clamshell to transfer hydrophobic domains of transmembrane proteins into the ER membrane.

Sec62 and Sec63 are integral transmembrane proteins that form a subcomplex near the Sec61 channel. They primarily assist in post-translational translocation of secretory proteins by functioning in the lateral gate mechanism. These proteins aid in the exit of signal sequences and transmembrane domains into the lipid bilayer, facilitating the proper insertion and translocation of proteins that are synthesized after the ribosome has released them.

The oligosaccharyltransferase (OST) complex is an integral translocon component that catalyzes the addition of complex sugar molecules to specific asparagine residues on polypeptides, forming glycoproteins. This post-translational modification occurs as nascent proteins pass through the translocon, enabling proper protein folding and function. The OST complex works in coordination with other translocon components to ensure efficient protein processing.

The signal peptidase complex transiently associates with the Sec61 channel under specific conditions to cleave the signal peptide from the polypeptide chain. This cleavage occurs co-translationally or post-translationally as the protein is being translocated into the ER lumen. By removing the signal sequence, the signal peptidase helps direct proteins to the rough endoplasmic reticulum and enables proper protein maturation and localization.

The translocon-associated protein (TRAP) complex is a hetero-tetrameric constitutive subunit positioned behind the Sec61 channel. It assists in binding signal peptides with low hydrophobicity to the Sec61 channel, helping to stabilize and properly orient these peptides during translocation. The TRAP complex ensures efficient recognition and processing of diverse signal sequences entering the translocon.

In higher eukaryotes like mammals, the Sec61 channel is capped in its resting state to prevent leakage of ions such as calcium into the cytosol. The plug-like α helix within the vertical channel opens and closes as required to allow protein translocation while maintaining the integrity of the ER membrane. This gating mechanism balances the need for selective protein transport with the requirement to maintain proper ion gradients across the ER.

Several proteins transiently associate with the translocon to support protein folding and quality control. Lectin chaperones like calnexin and malectin help correctly fold incoming polypeptides, while sensor molecules such as IRE1 detect misfolded proteins. The signal recognition particle receptor (SR) interacts with the Sec61 channel to deliver the ribosome-nascent peptide complex during active translocation, ensuring coordinated and efficient protein synthesis and translocation.