GTP-BINDING AND HYDROLYSIS BY THE SIGNAL RECOGNITION PARTICLE DURING INITIATION OF PROTEIN TRANSLOCATION

THE signal recognition particle (SRP) consists of one RNA and six prot ein subunits1,2. The N-terminal domain of the 54K subunit contains a p utative GTP-binding site, whereas the C-terminal domain binds signal s equences and SRP RNA3-7. Binding of SRP to the signal sequence as it e merges from the ribosome creates a cytosolic targeting complex contain ing the nascent polypeptide chain, the translating ribosome, and SRP8. This complex is directed to the endoplasmic reticulum membrane as a r esult of its interaction with the SRP receptor9-11, a membrane protein composed of two subunits, SRalpha and SRbeta, each of which also cont ains a GTP-binding domain12,13. In the presence of GTP, SRP receptor b inding to SRP causes the latter to dissociate from both the signal seq uence and the ribosome13,14. GTP is then hydrolysed so that SRP can be released from the SRP receptor and returned to the cytosol15. Here we show that the 54K subunit (M(r) 54,000) of SRP (SRP54) is a GTP-bindi ng protein stabilized in a nucleotide-free state by signal sequences a nd that the SRP receptor both increases the affinity of SRP54 for GTP and activates its GTPase. We propose that nucleotide-mediated conforma tional changes in SRP54 regulate the release of signal sequences and t he docking of ribosomes at the endoplasmic reticulum.