What is a signal recognition particle (SRP)?

A signal recognition particle (SRP) is a ribonucleoprotein complex that plays a crucial role in directing newly synthesized proteins to the endoplasmic reticulum (ER). The SRP recognizes and binds to a specific signal peptide that emerges from the ribosome during protein synthesis. This interaction halts translation temporarily, preventing the nascent polypeptide from being completely synthesized in the cytosol.

Once the SRP binds to the signal peptide, it transports the ribosome-protein complex to the ER membrane, where it interacts with the SRP receptor. Once docked, the SRP is released, allowing translation to continue and the nascent protein to be translocated into the ER lumen or membrane. This process is essential for the synthesis of proteins that will function in the secretory pathway, including those destined for secretion, incorporation into the cell membrane, or delivery to lysosomes.

The other options, while related to cellular processes, do not accurately describe the function of the SRP. The ribosome is responsible for synthesizing proteins but is distinct from the SRP. Enzymes involved in digestion pertain to a different cellular function, and molecules that transport lipids do not encapsulate the SRP's role in protein targeting