Which machinery recognizes the ER signal sequence during protein targeting?

The signal-recognition particle (SRP) plays a crucial role in the protein targeting process to the endoplasmic reticulum (ER). When a nascent polypeptide emerges from the ribosome and contains an ER signal sequence, the SRP recognizes and binds to this signal sequence. This interaction halts further translation temporarily, allowing the ribonucleoprotein complex to direct the entire ribosome-polypeptide complex to the ER membrane.

Once the complex is properly positioned at the ER, the SRP interacts with the SRP receptor on the ER membrane, facilitating the transfer of the ribosome to a protein translocator complex. This process is essential for the proper insertion and translocation of proteins into the ER, where they undergo further modifications and folding. The specificity and efficiency of the SRP in recognizing the signal sequence are critical for ensuring that proteins destined for secretion or membrane localization are accurately delivered to the ER in a timely manner.