The signal recognition particle (SRP) is a ribonucleoprotein complex that mediates the targeting of proteins to the endoplasmic reticulum (ER). The complex consists of a 7S (or 7SL) RNA and 6 different proteins, and signal recognition particle 54 (SRP54) is one of them. SRP54 binds to the signal sequence of presecretory protein as they emerge from the translating ribosomes, and then transfers them to translocating chain-associating membrane protein (TRAM).