Final answer:
The two key elements that guide ER signal sequences to the ER membrane are the signal recognition particle (SRP), which binds to the nascent protein's signal peptide, and the SRP receptor on the RER membrane. Stop-transfer sequences also play a crucial role in embedding proteins into the membrane.
Step-by-step explanation:
Guidance of ER Signal Sequences
Proteins that are destined for the endoplasmic reticulum (ER) harbor signal sequences that direct their localization. The two critical elements that guide the ER signal sequences to the ER membrane are:
- Signal recognition particle (SRP): An SRP binds to the hydrophobic signal peptide that emerges from a ribosome as a protein is synthesized. The SRP is a complex that temporarily halts translation until it can find the appropriate receptor on the ER membrane.
- SRP Receptor: Once the ribosome-SRP complex reaches the rough endoplasmic reticulum (RER), it binds to the SRP receptor on the ER membrane, where translation resumes and the protein is translocated into the lumen of the RER or into the membrane.
Stop-transfer sequences are also vital, acting as halting points during the translocation process to embed the protein into the lipid bilayer, giving rise to integral membrane proteins. These sequences trap the protein within the hydrophobic interior of the membrane, and multiple stop-transfer sequences can contribute to proteins that span the membrane several times.
After translation and ER localization, the signal peptide is usually cleaved off by a signal peptidase, which further processes the protein by removing these N-terminal residues.