What does the SRP bind to in order to initiate the translocation process?

The signal recognition particle (SRP) plays a crucial role in the translocation process of proteins destined for the endoplasmic reticulum (ER). It binds to the ribosomal subunit along with the nascent polypeptide chain during translation. However, its primary function is to direct the ribosome-nascent polypeptide complex to the ER membrane for translocation.

Once the SRP recognizes and binds to the signal sequence of the growing polypeptide chain, it pauses translation and facilitates the transport of the entire ribosomal complex to the ER membrane. At this point, the SRP interacts with an ER membrane protein known as the SRP receptor. This specific binding to the receptor protein on the ER is essential for the SRP to deliver the ribosomal complex to the translocon, where the next steps of translocation into the ER lumen occur.

Thus, the binding of the SRP to the receptor protein on the ER is a critical step in initiating protein translocation, ensuring that proteins are properly directed into the ER for folding and processing.