What does SRP bind to upon recognizing the ER signal sequence?

The signal recognition particle (SRP) is essential for directing the synthesis of certain proteins to the endoplasmic reticulum (ER). When SRP binds to the ER signal sequence of a nascent polypeptide, it subsequently binds to the SRP receptor located in the membrane of the rough ER.

This interaction is critical for the translocation of the ribosome and the nascent protein into the ER lumen. The SRP binding to the SRP receptor serves as a docking mechanism that facilitates the continuation of translation through the translocon, which is the channel through which the protein enters the ER. This ensures that proteins destined for secretion or for use in the ER and beyond are correctly directed to their appropriate cellular locations.

In this context, the receptor in the rough ER membrane is specifically structured to recognize and bind the SRP, making it pivotal in the process of protein trafficking. The other options do not play relevant roles in this phase of protein synthesis and trafficking.