Final Answer:
When a protein is synthesized with an ER signal sequence, the Signal Recognition Particle (SRP) binds to the ribosome.
Step-by-step explanation:
The Signal Recognition Particle (SRP) is a crucial component in the co-translational targeting of proteins to the endoplasmic reticulum (ER). Proteins destined for the ER contain a specific signal sequence that emerges during translation. When this signal sequence is synthesized, the SRP recognizes it and binds to the ribosome-mRNA-nascent chain complex. This binding temporarily halts translation and targets the entire complex to the ER membrane. The SRP interacts with the SRP receptor on the ER membrane, facilitating the transfer of the nascent chain into the ER for further processing and folding. This co-translational process ensures the proper localization of proteins within the cell.