Directing Proteins to the Rough Endoplasmic Reticulum

All proteins destined for the ER contain a unique hydrophobic signal sequence at their N-terminal. This sequence has a positively charged N-terminal region, a central hydrophobic region, and polar amino acids at the C-terminal end. When the signal sequence emerges from the ribosome during translation, it serves as a targeting address that directs the ribosome and nascent protein to the rough ER membrane for translocation.

The signal recognition particle (SRP) is a ribonucleoprotein complex that binds the hydrophobic signal sequence as soon as it emerges from the ribosome. SRP's signal sequence binding pocket is lined with methionines, which have flexible side chains that accommodate hydrophobic regions with different amino acid compositions. This binding provides a fidelity check ensuring only correct cargo intended for the rough ER is recognized and transported.

When SRP binds the ribosome-nascent chain (RNC) complex, its translation pause domain blocks the elongation factor binding site on the ribosome, arresting translation. Simultaneously, SRP undergoes a conformational change that exposes a receptor binding site. This pause prevents the protein from being synthesized further until the ribosome reaches the ER membrane, ensuring proper targeting and translocation.

The SRP-RNC complex uses a GTP-dependent interaction to dock at the SRP receptor present on the ER membrane. Once docked, the SRP-SRP receptor complex undergoes a conformational change that unloads the ribosome and target polypeptide chain onto an adjacent translocon channel. GTP hydrolysis then dismantles the SRP-SRP receptor complex, recycling the components for the next ER protein targeting cycle.

The signal recognition particle contains three key functional domains: a signal sequence binding pocket that recognizes hydrophobic targeting sequences, a translation pause domain that arrests ribosomal elongation, and a GTP-binding domain that enables docking at the SRP receptor. In mammals, SRP comprises six polypeptides and a 7S RNA molecule, with the GTP-linked SRP and SRP receptor forming a heterodimer with two active GTP hydrolysis sites.

Ribosome units assembled for protein synthesis in the cytosol are structurally and functionally identical to those on the rough ER. The ER signal sequence recruits ribosomes from the cytosolic pool to the rough ER membrane. After translation completes, the ribosome units are released back to the cytosol, allowing continuous recycling of ribosomal particles between the ER and cytosol for efficient protein synthesis.

The ER signal sequence contains a cleavage site at its C-terminal end that enables removal of the signal peptide from the mature protein. Once the nascent polypeptide is translocated into the ER lumen through the translocon channel, signal peptidase cleaves the signal sequence from the protein. This cleavage allows the mature protein to fold and undergo post-translational modifications within the ER.