What is the primary function of a signal recognition particle (SRP)?

The primary function of a signal recognition particle (SRP) is to guide nascent polypeptides to the endoplasmic reticulum (ER) for translation. SRP is a ribonucleoprotein that recognizes and binds to a specific signal peptide sequence on the emerging polypeptide chain as it is synthesized by ribosomes. Once the SRP binds to the signal peptide, it pauses translation and directs the ribosome-polypeptide complex to the ER membrane.

Upon reaching the ER, SRP interacts with the SRP receptor on the membrane, facilitating the transfer of the ribosome and nascent protein to a translocon—a complex that allows the polypeptide to enter the ER lumen or integrate into the membrane. This entire process ensures that proteins destined for secretion or membrane localization are properly synthesized and translocated into the appropriate cellular compartment.

The other choices describe functions that are not related to the role of SRP. For instance, degrading proteins in lysosomes pertains to proteolytic processes, while facilitating membrane fusion typically involves different components like SNARE proteins. Lastly, receptors that bind to specific proteins on the cell surface pertain to signaling and communication rather than protein traffic to the ER.