Signal-recognition-particle (SRP) assembly has a crucial role in targeting secretory proteins to the rough endoplasmic reticulum (ER) membrane. SRP is required for the cotranslational protein translocation for ER import and preferentially recognizes strongly hydrophobic signal sequences. It is involved in targeting the nascent chain-ribosome (RNC) complex to the ER and is proposed to participate in the arrest of nascent chain elongation during membrane targeting. SRP54 binds to the signal sequence of presecretory protein when they emerge from the ribosomes. SRP54 interacts with the scR1 RNA and mediates the association of the resulting SRP-RNC complex with the signal recognition particle receptor (SR) via its alpha subunit SRP101. Both, SRP54 and SRP101, are locked in their GTP bound forms in the SRP-RNC-SR complex, which dissociates upon transferring the signal sequence to the protein-conducting channel (translocon). After signal sequence transfer, SRP54 and SRP101 act as reciprocal GTPase-activating proteins (GAPs), thereby resolving their association